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Restriction of placental and fetal growth in the guinea-pig
Placenta(1993), 14, 125-135
Current Topic: Restriction of Placental and Fetal Growth in the Guinea-Pig A N T H O N Y M. CARTER DepartmentofPhysiology, Universityof Odense, Winslowparken19, DK-5000 Odense, Denmark
Ligation of the uterine artery in mid-pregnancy is a simple procedure that restricts the subsequent growth of the placentae and fetuses. It was introduced for the rat by Wigglesworth (1964) and first applied to the guinea-pig by Lafeber (1981). The guinea-pig fetus weighs > 100 g at term (68 days) and even the growth-restricted fetus is amenable to surgical intervention, permitting study of circulatory adaptations to intrauterine growth restriction (IUGR) and measurements of placental transfer. The limitations of the model are set by fetal size and by the necessity of performing acute studies under anaesthesia. Therefore, it is important to recognize that there are alternative models of IUGR in sheep that allow measurements to be made on the unanaesthetized fetus. Each horn of the guinea-pig uterus is supplied by an arterial arcade formed by the ovarian and uterine arteries (Egund and Carter, 1974). Ligation of one uterine artery results in the loss of all fetuses from the ipsilateral horn in one-third of all animals (Detmer and Carter, 1992). In the remainder, some or all of the fetuses survive, supported by the blood supply from the ovarian artery. The extent to which growth restriction occurs is variable, both within and between litters. Because asymmetrical growth restriction is a significant feature of IUGR, we have chosen to select fetuses with an increased ratio of brain weight to liver weight. With the cut-offset arbitrarily at a ratio of 0.85, 35 per cent of fetuses are growth restricted, with at least one such fetus occurring in 48 per cent of experiments. A different result is obtained if the fetuses are classified by body weight, which some authors use to divide the material into subgroups with different degrees of growth restriction (Lafeber, Rolph and Jones, 1984). The primary insult entails a reduction of maternal placental blood flow (Jones and Parer, 1983). If the uterine artery is ligated at 30-32 days gestation, restriction of placental size is demonstrable within 14 days, whereas the limitation on fetal growth becomes apparent between 45 and 55 days and term (Jansson, Thordstein and Kjellmer, 1986). There is delayed maturation of most tissues, with some intriguing differential effects. The weight of the liver and spleen falls to a greater extent than body weight, but the heart and kidneys are reduced in proportion to, and the brain and adrenal glands less than, fetal body weight (Lafeber et al, 1984). This asymmetric pattern of growth has been likened byJansson (1990) to type II human IUGR (Evans and Lin, 1984) and is seen in ovine models of IUGR that rely upon a reduction in fetal or maternal placental blood flow. Alternative methods used to manipulate fetal growth are reviewed elsewhere (Owens, Owens and Robinson, 1989; Carter, 1991). In sheep, carunclectomy prior to conception limits placental and fetal growth 0143-4004/93/020125 + 11 $08.00/0
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throughout gestation; this procedure was introduced by Alexander (1964) and is extensively reviewed by Owens, Owens and Robinson (1989). Restriction of maternal placental blood flow entails a reduction in the supply of oxygen and substrate to the placenta. Because the guinea-pig placenta operates as a countercurrent exchanger, in which blood leaving the fetal capillaries equilibrates with blood entering the arterial side of the maternal labyrinth, there is little room for compensatory improvement in placental exchange. Under normal circumstances the placenta is about 65 to 75 per cent as efficient as an ideal countercurrent exchanger (Moll and Kastendieck, 1977; Schrrder, Brrries and Leichtweiss, 1991); the difference can be attributed to the presence of non-countercurrent components and to mismatching of fetal and matemal blood flows ( Q / Q # 1). Schrrder, Brrries and Leichtweiss (1991) recently measured Q / Q ratios in small pieces of placenta (mean weight 19 mg). The mean value was close to 1, but Q / Q ranged from 0.4 to 2.5 even in the two-thirds of the placenta without extreme Q/Qratios. Although Rankin (1976) has proposed a mechanism for the regulation of Q/Qratios in the placenta, there is no firm evidence that better matching occurs in response to reduced oxygen delivery, and Q/Qratios have yet to be examined in the placenta of the IUGR guinea-pig fetus.
PLACENTAL OXYGEN TRANSFER The factors affecting placental exchange of respiratory gases have been reviewed elsewhere (Longo, 1987; Carter, 1989; Wilkening and Meschia, 1992). At any value of Po2 achieved in umbilical venous blood, delivery of oxygen to the fetus will depend on the haemoglobin flow rate, i.e. the product of fetal placental blood flow and haemoglobin concentration, and on the oxygen affinity of fetal haemoglobin. In growth-restricted guinea-pigs fetal placental blood flow is reduced in proportion to placental mass (Carter and Detmer, 1990). This is hardly surprising, as placental blood flow normally accounts for one-third of the biventricular cardiac output (Girard et al, 1983). To pump an even greater fraction through the umbilical circuit, the growth-restricted fetus would have to maintain a large cardiac output relative to its body size. Yet myocardial mass and body mass decrease to about the same extent. In addition, theoretical considerations indicate that a deviation in the mean Q/Qratio from 1 will reduce the efficiency of diffusional exchange (Schrrder et al, 1991). Lafeber, Rolph and Jones (1984) reported that packed cell volume was elevated in the IUGR fetus and considered this finding to be indicative ofhypoxia. However, measurements of haemoglobin concentration yield conflicting results, even in the same laboratory (Widmark et al 1990, 1991). One explanation could be the inability of the fetus to mount an appropriate haematopoietic response to hypoxia in IUGR due to the disproportionate reduction in liver mass. We have been unable to confirm the compensatory increase in extramedullary haematopoiesis described by Lafeber, Rolph and Jones (1984) and have found a significant reduction in the number of haematopoietic colonies on the right side of the liver (Ernst, Salafia and Carter, unpublished data). In guinea-pigs, the high oxygen affinity of fetal blood is attributable no.'. to a specific fetal haemoglobin, but to the low 2,3-diphosphoglycerate (DPG) content of the fetal red cells (Bard and Shapiro, 1979; Jelkmann and Bauer, 1980; Carter and Gr~nlund, 1985). Recently we measured the intra-erythrocytic DPG concentration in IUGR fetuses and found it to be even lower than in control fetuses (Table 1).
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Table 1. Comparison of haemoglobin and 2,3-diphosphoglycerate (DPG) concentrations in normal and growth-restricted guinea-pig fetuses in late gestation (mean _+ s.e.m.). Control fetuses Number of samples Fetal Weight (g) Brain wt./liver wt. Haemoglobin (mmol/l) DPG (mmolA) DPG mol per mol Hb4
89 0.64 2.32 0.61 0.27
13 _+3 + 0.02 _+ 0.07 "4-0.03 _+0.02
Growth-restricted fetuses 7 50 _+ 5*** 1.24 _ 0.12"** 2.29 _+ 0.09 0.43 +_ 0.06** 0.20 _+ 0.02*
Growth restricted against controls *P < 0.02, **P < 0.01, ***P < 0.001.
Gaseous exchange in the placenta is limited by the oxygen diffusing capacity, including the diffusing capacity of the placental membrane (Din) and the diffusing capacities of the maternal and fetal capillary blood. Growth restriction in guinea-pigs, induced by maternal hypoxia, was associated with a 63 per cent increase in placental diffusing capacity (Gilbert et al, 1979). About one-third of this increase could be ascribed to an increase in D m secondary to increased surface area and decreased diffusion distance and the remainder to increased haemoglobin concentrations and vascular volumes (Bacon et al, 1984). Placental diffusing capacity has not been examined in the uterine artery ligation model of IUGR. A considerable fraction of the oxygen extracted from the maternal circulation is not available to the fetus, since it is used to support placental metabolism. Data on placental oxygen consumption is not available for the guinea-pig. In sheep, the placenta accounts for c. 40 per cent of uterine oxygen consumption (Wilkening and Meschia, 1983).
PLACENTAL TRANSFER OF GLUCOSE, AMINO ACIDS AND NEFA Uterine artery ligation reduces the amount ofsubstrate conveyed to the gravid uterus. There is a compensatory increase in the fractional extraction of glucose (Jones et al, 1988; Jansson and Persson, 1990), yet the fetus is hypoglycaemic (Jones, Lafeber and Roebuck, 1984; Widmark et al, 1990), as may occur in the growth-restricted human fetus (Economides and Nicolaides, 1989). Studies with isolated perfused placentae from growth-restricted guineapig fetuses indicate that the transport capacity per unit of placental mass is no different from that in normal pregnancies (Jones et al, 1988), a conclusion that is supported by in vivo measurements of the weight-specific placental transfer capacity for [3H]-methylglucose (Jansson and Persson, 1990).Jones et al (1988) estimated that the placenta normally utilizes about two-thirds of the glucose extracted from the maternal circulation and calculated that this fraction would increase further in IUGR, substantially reducing glucose availability to the fetus (Figure 1). They arrived at this conclusion by applying measurements of unidirectional glucose flux and placental glucose consumption, obtained in the doubly perfused isolated placenta, to the in vivo situation. In the sheep carunclectomy model of IUGR, the fetal share of uterine glucose consumption increases (Owens, Falconer and Robinson, 1987b). Placental transfer capacity for a-aminoisobutyric acid (AIB) is reduced in growthrestricted guinea-pigs, by 36 per cent on day 50 and 22 per cent on day 63 of gestation, with corresponding decreases in fetal uptake (Jansson and Persson, 1990). Fetal uptake of AIB is
Placenta (1993), VoL 14
128 Mother
Placenta
Fetus
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3150 (3.5)
80 460 (89'4)
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~
26000(100)
22828 (87.8)
52 (0.2)
3120 (12)
Flux across one placenta and to one fetus = ~mol glucose/min (Percentage of flux)
Figure 1. Estimated glucose supply to and consumption by the normal and growth-restricted guinea-pig fetus and placenta at 60-62 days gestation. Flux rates are expressed in nmol- min 1 (percentages in parentheses). Uterine artery ligation reduces the glucose supply to the placenta. Placental glucose consumption falls in proportion to placental mass, but accounts for a greater share of the total flux, thereby further limiting the amount of glucose available to the fetus. From Jones et al, 1988.
also reduced in natural runts (Hayter et al, 1964). Interestingly, guinea-pigs on a low protein diet, which causes a moderate reduction in fetal and placental mass, exhibit a compensatory increase in placental transfer capacity for AIB and a consequent increase in the weightspecific uptake of this amino acid (Young and Widdowson, 1975). Although fetal and placental handling of other amino acids has not been studied in IUGR, it is probably correct to conclude that there is an impaired supply of amino acids to the fetus (Jansson and Persson, 1990). However, fetal plasma concentrations are elevated rather than depressed because of a sharp fall in the utilization of amino acids by the tissues (Jones, 1989). The other main substrate for the guinea-pig fetus is non-esterified fatty acid. NEFA is rapidly transferred from maternal to fetal blood, sequestered by the liver, and converted to triacylglycerols (Jones, 1976; Thomas and Lowy, 1984). In a study of hepatic lipid metabolism in IUGR, we gave a maternal I.V. infusion of [1-14C]-palmitic acid and measured plasma concentrations of the label in umbilical venous blood collected after 15, 30 and 45 min (Detmer, Carter and Thomas, in press). A slower rise towards steady state was seen in the IUGR fetuses (Figure 2). The possibility that this is due to a reduction in transfer capacity for NEFA deserves further investigation.
Carter: Fetal Growth Restriction
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Figure2. Incorporation ofradiolabel into the blood of growth-restricted (IUGR) and control (C) fetuses at 15, 30 and 45 min during infusion of [1-n4C]-palmitate into a maternal vein. A reduction in placental transfer capacity for NEFA could account for the slower rise towards steady state seen in the IUGR fetuses. Shading refers to incorporation of [14C] into NEFA ([]), triacytglycerols (R), cholesterol (m), cholesterol esters (11), and unidentified material ([3). Error bars refer to total label. From Detmer, Carter and Thomas, 1992.
FETAL RESPONSES TO THE REDUCED OXYGEN SUPPLY Fetal oxygen consumption (vo2) has been measured in guinea-pig fetuses (Girard et al, 1983), but no data is available for the growth-restricted fetus. In the sheep carunclectomy model of IUGR, fetal vo2 is reduced to the same extent as fetal weight. However, there is a smaller margin of safety between supply of and demand for oxygen (Owens, Falconer and Robinson, 1987a), glucose (Owens, Falconer and Robinson, 1987b), and other substrates. The concept of a margin of safety in fetal oxygen supply, based on the relative insensitivity of fetal vo2 to acute changes in placental blood flow and fetal oxygen delivery, has recently been challenged. When the relationship between vo2 andpo2 of the sheep fetus was studied over a wide range ofpo2 values, the data suggested that there is no margin of safety in oxygen supply for a significant fraction of fetal tissues (Asakura, Ball and Power, 1990). Moreover, hypoxia has been shown to reduce the VOz of fetal guinea-pig skeletal muscle cells in monolayer culture, for which vo2 correlates with po2 (Braems and Jensen, 1991). Oxygen consumption by the brain is maintained even at extremely low values of po2 (J6hannsson and Siesj6, 1975; Jones et al, 1977). Therefore, if the IUGR fetus is hypoxic, brain sparing would have to occur either by increasing cerebral blood flow to maintain oxygen delivery or by increasing fractional oxygen extraction. The fetus normally responds to a fall in arterial oxygen content ([O2]a) with an increase in cerebral perfusion (Carter and Gu, 1988). This relationship can be difficult to demonstrate in the IUGR fetus (Detmer, Gu and Carter, 1991), but a recent data set (Figure 3) suggests a leftward and downward shift of the regression line describing the relationship between brain blood flow and [Oz]a. Any shortfall in the oxygen supply must be made up by increased extraction of oxygen from the blood, and an inevitable result of increased extraction will be a fall in mean capillarypOz and tissue po2.
Placenta (1993), Vol. 14
130 4
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Figure 3. Relation between brain blood flow (CBF) and arterial O2 content ([O2]a) in guinea-pig fetuses at 62-63 days gestation. Measurements were made in 12 normal fetuses (CBF = 5.2- (1/[02]a)--0.5, r = 0.82, P < 0.001) and 11 IUGR fetuses (CBF = 2.5- (1/[02]a)--0.2, r - 0.83, P < 0.01). Carter and Detmer (unpublished data).
This will slow some oxygen-consuming reactions, which are extremely sensitive to changes in poz, even though their contribution to total oxygen consumption is insignificant. This category of reactions includes some that are essential to the synthesis of neurotransmitters, such as the hydroxylation of tryptophan (Davis et al, 1973), and serotonin levels fall in the fetal rat forebrain following uterine artery ligation (Thordstein and Hedner, 1992). An inadequate blood flow response to hypoxaemia may act through such mechanisms to delay the maturation of the brain in IUGR fetuses (Lafeber, Rolph and Jones, 1984) and affect the cerebral integrity of the growth-restricted newborn guinea-pig (Thordstein and Kjellmer, 1988). Arterial blood pressure is maintained in IUGR fetuses, but there is a reduction in heart rate and thus in the rate-pressure product (Detmer, Gu and Carter, 1991). Normally, guinea-pig fetuses adapt to low levels of oxygenation by increasing the rate ofperfusion of the myocardium, but in IUGR fetuses the data were scattered and a significant correlation could not be shown (Detmer, Gu and Carter, 1991). The lower rate pressure product of IUGR fetuses suggests a decline in myocardial oxygen consumption. ECG wave form analysis indicates that aerobic myocardial metabolism is maintained in normoxaemic (Widmark et al, 1990) but not in hypoxaemic IUGR fetuses (Widmark et al, 1991). This has been ascribed to a decreased myocardial blood flow reserve and a blunted sympathoadrenal response (Widmark et al, 1991). These factors could affect development of the heart muscle and its innervation. Heart rate is depressed in growth-restricted guinea-pig neonates (Thordstein and Kjellmer, 1988) and the ability to respond to acute stresses such as hypoxia is impaired in newborn IUGR rats (Shaul, Cha and Oh, 1989). Adrenal weight is reduced in the growth-restricted guinea-pig fetus, but there is a significant increase in the adrenal perfusion rate (Table 2). During normal fetal growth, plasma ACTH and cortisol concentrations rise between 55 and 60 days of gestation (Jones and Roebuck, 1980), and a high rate of cortical perfusion may be required for steroidogenesis. ACTH and cortisol levels do rise in growth-restricted fetuses, but plasma concentrations are about half of those normally seen (Jones, Lafeber and Roebuck, 1984). The observed
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Table 2. Comparison of the weight of the adrenal glands and their rate of perfusion in normal and growth-restricted guinea-pig fetuses in late gestadon (means _+ s.e.m). Data from Carter and Demaer (1990). Control fetuses Number of fetuses Fetal weight (g) Brain wt./liver wt. Adrenal weight (mg) Adrenal blood flow (rnl/min per g)
10 89 + 4 0.56 _+ 0.03 35.8 4- 2.2 2.4 _ 0.5
Growth-restricted fetuses 5 57 ___3*** 1.00 4- 0.05*** 21.2 + 3.6** 5.6 4- 1.1"*
Growth restricted against controls **P < 0.01, *** P < 0.001.
increase in adrenal perfusion in IUGR presumably serves to ensure adequate oxygen delivery to the cortex and may involve an autoregulatory component, since plasma ACTH is not increased, as might otherwise be expected in chronic hypoxaemia (Jones, Lafeber and Roebuck, 1984). Plasma catecholamine concentrations are elevated in IUGR guinea-pig fetuses (Widmark et al, 1990) and there is precocious neural regulation of the adrenal medulla following uterine artery ligation in rats (Shaul, Cha and Oh, 1989). It is conceivable that maturational changes in sympathoadrenal function serve to explain the rink between low birth weight and subsequent hypertension in man (Barker et al, 1989; Gennser, Rymark and Isberg, 1988), which is reproducible following uterine artery ligation in guinea-pigs (Persson and Jansson, 1992). The disproportionate reduction in size of the liver in asymmetric IUGR is intriguing. The guinea-pig fetus lacks a ductus venosus (Harman and Herbertson, 1938; Girard et al, 1983; Carter, 1984), and the entire umbilical venous return perfuses the liver, which thus is presented with blood of a higher oxygen and substrate content than that which perfuses the brain and myocardium. In the fetal sheep, hepatic demand for oxygen is met by a very high rate of blood flow and a remarkably low oxygen extraction, and hepatic oxygen uptake is linearly related to oxygen delivery (Bristow et al, 1983). If the growth-restricted guinea-pig fetus likewise is unable to increase oxygen extraction, to compensate for the fall in umbilical blood flow and oxygen delivery, this may place a constraint upon liver growth. The blood leaving the hepatic veins is an important source of oxygen for other tissues (Rudolph, 1983). Because the fetal liver responds to a reduced oxygen delivery with a slower rate of growth, rather than by increasing oxygen extraction, other organs--notably the brain--can be spared. It should be noted that the absence of a ductus venosus does not preclude preferential distribution of the umbilical venous return to the heart and brain. Everett & Johnson (1950) demonstrated that approximately three-quarters of the blood derived from the umbilical vein passes through the foramen ovale to the left side of the heart. Consideration of the intralobular distribution of flows and liver mass bolsters the case for a relationship between liver growth and oxygen supply. The right lobe of the liver derives its blood supply from the portal vein and the hepatic artery, with only a minor contribution from the umbilical vein (Carter and Detmer, 1990; Carter, Detmer and Egund, 1992). In IUGR, increased oxygen extraction by other tissues probably lowers the oxygen content of the blood reaching the right lobe from these vessels. This might exert an additional effect on the growth of the right lobe, which accounts for a relatively smaller proportion of total fiver weight (Carter and Detmer, 1990). In addition, there is an increased tendency to fatty degeneration
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and decreased haematopoietic activity in the right liver lobe of IUGR fetuses (Ernst, Salafia and Carter, unpublished data). These observations are of interest in a clinical context, since non-siderotic degeneration of the right side of the liver is a frequent finding in infants that die in the perinatal period (Gruenwald, 1949; Daamen and Schaberg, 1969).
FETAL ENDOCRINE RESPONSES T O THE REDUCED SUBSTRATE SUPPLY It has been suggested that a reduction in the supply of amino acids may limit fetal growth by restraining protein synthesis (Owens, Owens and Robinson, 1989). Therefore, tissues with the highest rates of protein turnover, including the liver, may be more vulnerable to intrauterine malnutrition. In support of this, a significant reduction in a-aminonitrogen and in most essential amino acids was demonstrable in blood obtained by cordocentesis from human fetuses that were small for gestational age (Cefin et al, 1990). Although there is a diminished supply of amino acids in the growth-restricted guinea-pig fetus (Jansson and Persson, 1990), and a fall in plasma a-aminonitrogen (Lafeber, 1981), concentrations of some amino acids are elevated rather than depressed because of a sharp fall in the capacity of the tissues to metabolize amino acids (Jones, 1989). The metabolic changes in IUGR can largely be ascribed to changes in fetal endocrine state. The predominant difference is hypoinsulinaemia (Jones, Lafeber and Roebuck, 1984), which is consistent with the hypoglycaemia and may be responsible for the observed decrease in fetal glucose consumption (Jones, 1989). The extent of the reduction in plasma insulin concentration is closely related to fetal size (Jones, Lafeber and Roebuck, 1984), supporting the contention that insulin is the major growth-promoting hormone of the fetus (Fowden, 1989). Glucagon levels fall markedly in the last half of gestation, but growth restriction slows the decline (Jones, Lafeber and Roebuck, 1984). However, deposition of glycogen in the liver is maintained in the face of a rise in the insulin/glue.agon ratio (Lafeber, Rolph and Jones, 1984). Indeed, despite the small size of the liver in IUGR fetuses, hepatic stores of glycogen are unchanged in relation to body weight, due to a significant increase in glycogen concentration (Lafeber, Rolph and Jones, 1984). This observation has been linked to a rise in insulin-like growth factor II (IGF II) and seen to support the view that IGF II is a major gluconeogenetic hormone in the fetus (Jones et al, 1987; Jones, 1989). Insulin is an important regulator of fetal IGF secretion and the effects of insulin on fetal growth may be mediated by altered IGF I release (Bassett et al, 1989). In growth-restricted guinea-pig fetuses, IGF I follows a pattern similar to insulin. Sulphation-promoting activity falls even more sharply (Jones et al, 1987). These changes are consistent with slowing of DNA, RNA and protein synthesis in tissues of the IUGR fetus. In addition, plasma cortisol concentrations are much lower in the IUGR fetus, and plasma levels of T3, T4 and rT3 fall sharply (Jones, Lafeber and Roebuck, 1984), resulting in a diminished thyroid drive to maturation. Thus, the supply and utilization ofsubstrates is central to the regulation of fetal growth and there is a prima facie case for the existence of pathways that communicate placental nutritional state and perfusion to the fetus (Jones, 1989). This would explain why the ratio of fetal to placental weight is maintained following uterine artery ligation (Jones and Parer, 1983). The sheep placenta has high affinity type 1 IGF receptors and removes IGF I from the fetal circulation (Bassett et al, 1990). When the concentration of IGF I in fetal plasma falls below a critical level, placental uptake of IGF I ceases (Iwamoto, Chernausek and Murray,
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1991). T h u s the possibility exists o f a feedback loop in which placental f u n c t i o n d e t e r m i n e s substrate availability, w h i c h d e t e r m i n e s I G F I release, which in t u r n affects placental f u n c t i o n (Bassett et al, 1989). T h e decrease in plasma levels o f I G F I s e e n in the g u i n e a - p i g fetus following u t e r i n e artery ligation is consistent with this i n t e r e s t i n g hypothesis.
ACKNOWLEDGEMENTS The author's studies of intrauterine growth restriction have been supported generously by the Danish Medical Research Council.
REFERENCES Alexander, G. (1964) Studies on the placenta of the sheep (Ovis aries L.): Effect of surgical reduction in the number of caruncles. Journal of Reproduction and Fertility, 7, 307-322. Asakura, H., Ball, K. T. & Power, G. G. (1990) Interdependence of arterial POz and Oz consumptionin the fetal sheep. Journal of DevelopmentalPhysiology, 13, 205-213. Bacon, B.J., Gilbert, R. D., Kaufmann, P., Smith, A. D., Trevino, F. T. & Longo, L. D. (1984) Placental anatomy and diffusing capacity in guinea pigs followinglong-term maternal hypoxia. Placenta, 5, 475-488. Bard, H. & Shapiro, M. (1979) Perinatal changes of 2,3-diphosphoglycerate and oxygen affinity in mammals not having fetal type hemoglobins. PediatricResearch, 13, 167-169. Barker, D. J. P., Osmond, C., Golding, J., Kuh, D. & Wadsworth, M. E.J. (1989) Growth in utero, blood pressure in childhood and adult life, and mortality from cardiovascular disease. British MedicalJournal, 298, 564-567. Bassett, N. S., Gluckman, P. D., Breier, B. H. & Ball, K. T. (1989) Gro~h hormone and insulin-like growth factors iri the fetus: regulation and role in fetal growth. InAdvances in Fetal Physiology:Reviews in Honor ofG. C. Liggins (Ed.) Gluckman, P. D., Johnston, B. M. & Nathanielsz, P. W. pp. 303-314. Ithaca, New York: Perinatology Press. Bassett, N. S., Breier, B. H., Hodgkinson, S. C., Davis, S. R., Henderson, H. V. & Gluckman, P. D. (1990) Plasma clearance of radiolabelled IGF-1 in the late gestation ovine fetus. Journal of DevelopmentPhysiology, 14, 73-79. Braems, G. & Jensen, A. (1991) Hypoxia reduces oxygen consumption of fetal skeletal muscle cells in monolayer culture.Journal of DevelopmentalPhysiology, 16, 209-215. Bristow, J., Rudolph, A. M., Itskowitz, J. & Barnes, R. (1983) Hepatic oxygen and glucose metabolism in the fetal lamb. Journal of Clinical Investigation, 71, 1047-1061. Carter, A. M. (1984) The blood supply to the abdominal organs of the fetal guinea-pig.Journal of Developmental Physiology, 6, 407-416. Carter, A. M. (1989) Factors affecting gas transfer across the placenta and the oxygen supply to the fetus.Journalof DevelopmentalPhysiology, 12, 305-322. Carter, A. M. (1991) Placental blood flow and fetal oxygen supply in animal models of intra-uterine growth retardation. In Placenta:BasicResearchfor ClinicaIApplication (Ed.) Soma, H. pp. 23-31. Basel: Karger. Carter, A. M. & Detmer, A. (1990) Blood flow to the placenta and lower body in the growth-retarded guinea pig fetus.Journal of DevelopmentalPhysiology, 13, 261-269. Carter, A. M., Detmer, A. & Egund, N. (1992) Contributionof the umbilical and portal veins to the hepatic blood supply of the guinea pig fetus. An angiographic study. LaboratoryAnimal Sdence, 42, 174-179. Carter, A. M. & Grnnlund, J. (1985) Influence of 2,3-diphosphoglycerate (DPG) concentration in maternal red cells on the transptacental exchange of respiratory gases. In The PhysiologicalDevelopmentof the Fetus and Newborn (Ed.) Jones, C. T. & Nathanielsz, P. W. pp. 47-50. London: Academic Press. Carter, A. M. & Gu, W. (1988) Cerebral blood flow in the fetal guinea-pig.Journal ofDevelopmentalPhysiology, 10, 123-129. Cetin, I., Corbetta, C., Sereni, L. P., Marconi, A. M., Bozzetti, P., Pardi, G. & Battaglia, F. C. (1990) Umbilical amino acid concentrations in normal and growth-retarded fetuses sampled in utero by cordocentesis. AmericanJournal of Obstetricsand Gynecology, 162, 253-261. Daamen, C. B. F. & Schaberg, A. (1969) Hemilateral liver degeneration.Journal of Pathology, 97, 29-34. Davis, J. N., Carlsson, A., MacMillan, V. & Siesj6, B. K. (1973) Brain tryptophan hydroxylation: Dependence on arterial oxygen tension. Science, 182, 72-74. Detmer, A. & Carter, A. M. (1992) Factors influencing the outcome of ligating the uterine artery and vein in a guinea pig model of intrauterine growth retardation. ScandinavianJournal ofLaboratoryAnimal Science, 19, 9-16. Detmer, A., Carter, A. M. & Thomas, C. R. (1992) The metabolism of free fatty acids and triacylglycerolsby the fetal liver in a guinea pig model of intrauterine growth retardation. PediatricResearch, 32, 441-446.
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Detmer, A., Gn, W. & Carter, A. M. (1991) The blood supply to the heart and brain in the growth retarded guinea pig fetus. Journal of DevelopmentalPhysiology, 15, 153-160. Economides, D. L. & Nicolaides, K. H. (1989) Blood glucose and oxygen tension levels in small-for-gestationalage fetuses. AmericanJournal of Obstetricsand Gynecology, 160, 385-389. Egund, N. & Carter, A. M. (1974) Uterine and placental circulation in the guinea-pig: an angiographic study. Journal of Reproduction & Fertility, 40, 401--410. Evans, M. I. & Lin, C. C. (1984) Retarded fetal growth. In Intrauterine growth retardation: Pathophysiology and ClinicalManagement (Ed.) Lin, C. C. & Evans, M. I. pp. 55-57. New York: McGraw-Hill. Everett, N. B. & Johnson, R.J. (1950) Use of radioactive phosphorus in studies of fetal circulation. American Journal of Physiology, 162, 147-152. Fowden, A. L. (1989) The endocrine regulation of fetal metabolism and growth. In Advances in Fetal Physiology: Reviews in Honor ofG. C. Liggins (Ed.) Gluckman, P. D., Johnston, B. M. & Nathanielsz, P. W. pp. 229-243. Ithaca, New York: Perinatology Press. Gennser, G., Rymark, P. & Isberg, P. E. (1988) Low birth weight and risk of high blood pressure in adulthood. British MedicalJournal, 296, 1498-1500. Gilbert, R. D., Cummings, L. A., Jaehau, M. R. & Longo, L. D. (1979) Placental diffusing capacity and fetal development in exercising or hypoxic guinea pigs.Journal ofApplied Physiology, 46, 828-834. Girard, H., Klappstein, S., Bartng, I. & Moll, W. (1983) Blood circulation and oxygen transport in the fetal guinea pig. Journal of DevelopmentalPhysiology, 5, 181-193. Gruenwald, P. (1949) Degenerative changes in the right half of the liver resulting from intra-uterine anoxia. AmericanJournal of Clinical Pathology, 19, 801-813. Harman, M. T. & Herbertson, J. E. (1938) Concerning the postnatal obliteration of the umbilical vein and arteries, the vitelline vein and artery, and the ductus arteriosus in the guinea pig. Transactionsof the KansasAcademy of Science, 41,369-376. Hayter, C.J., Hutchinson, E. A., Karvonen, M.J. & Young, M. (1964) Placental transport of a-amino isobutyric acid in the unanaesthetized guinea-pig. Journal of Physiology, 175, 11P-13P. Iwamoto, H. S., Chernuasek, S. D. & Murray, M. A. (1991) Regulation of plasma insulin-like growth factor (IGF-I) by oxygen and nutrients in fetal sheep. 2rid International IGF Symposium Abstract, A48. Jansson, T. (1990) Placentalbloodflow regulationand substratetransferin intrauterinegrowth retardation.An experimental study in the conscious,chronicallycatheterizedguineapig. MD Thesis, University of G6teborg. Jansson, T. & Persson, E. (1990) Placental transfer of glucose and amino acids in intrauterine growth retardation: Studies with substrate analogs in the awake guinea pig. Pediatric Research, 28, 203-208. Jansson, T., Thordstein, M. & KjeUmer, I. (1986) Placental blood flow and fetal weight following uterine artery ligation. Biology of the Neonate, 49, 172-180. Jelkmann, W. & Bauer, C. (1980) 2,3 -DPG levels in relation to red cell enzyme activities in rat fetuses and hypoxic newborns. PfliigersArchiv, 389, 61-68. J6hannsson, H. & SiesjiJ, B. K. (1975) Cerebral blood flow and oxygen consumption in the rat in hypoxic hypoxia. Acta PhysiologicaScandinavica, 93, 269-276. Jones, C. T. (1976) Lipid metabolism and mobilization in the guinea pig duringpregnancy. BiochemicalJournal, 156, 357-365. Jones, C. T. (1989) Observations on the control of prenatal growth in the guinea pig. InAdvances in FetalPhysiology: Reviews in Honor ofG. C. Liggins (Ed.) Gluckman, P. D., Johnston, B. M. & Nathanielsz, P. W. pp. 287-302. Ithaca, New York: Perinatology Press. Jones, C. T., Harding, J. E., Gu, W. & Lafeber, H. N. (1988) Placental metabolism and endocrine effects in relation to the control of fetal and placental growth. In The Endocrine Controlofthe Fetus (Ed.) Kiinzel, W. &Jensen, A. pp. 213-222. Heidelberg: Springer Verlag. Jones, C. T., Lafeber, H. N. & Roebuck, M. M. (1984) Studies on the, growth of the fetal guinea pig. Changes in plasma hormone concentration during normal and abnormal growth. Journal of Developmental Physiology, 6, 461-472. Jones, C. T., Lafeber, H. N., Price, D. A. &Parer, J. T. (1987) Studies on the growth of the fetal guinea pig. Effects of reduction in uterine blood flow on the plasma sulphation-promoting activity and on the concentration of insulin-like growth factors-I and -II. Journal of Developmental Physiology, 9, 181-201. Jones, C. T. &Parer, J. T. (1983) The effect of alterations in placental blood flow on the growth of and nutrient supply to the fetal guinea-pig.Journal of Physiology, 343,525-537. Jones, C. T. & Roebuck, M. M. (1980) The development of the pituitary-adrenal axis in the guinea pig. Acta Endocrinologica, 94, 107-116. Jones, Jr, M. D., Sheldon, R. E., Peeters, L. L., Mesehia, G., Battaglia, F. C. & Makowski, E. L. (1977) Fetal cerebral oxygen consumption at different levels of oxygenation. Journal ofApplied Physiology, 43, 1080-1084. Lafeber, H. N. (1981) Experimental intra-uterine growth retardation in the guinea pig. MD Thesis, Rotterdam. Lafeber, H. N,, Rolph, T. P. &Jones, C. T. (1984) Studies on the growth of the fetal guinea pig. The effects of ligation of the uterine artery on organ growth and development. Journal of DevelopmentalPhysiology, 6, 441-459. Longo, L. D. (1987) Respiratory gas exchange in the placenta. In Handbook of Physiology. Section 3. The Respiratory System. VolumeIV. Gas Exchange, pp. 351-401. Bethesda: American Physiological Society.
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Moll, W. & Kastendieek, E. (1977) Transfer of N20 , CO and HTO in the artificially perfused guinea-pig placenta. RespirationPhysiology, 29, 283-302. Owens, J. A., Owens, P. C. & Robinson, J. A. (1989) Experimental fetal growth retardation: metabolic and endocrine aspects. In Advances in Fetal Physiology: Reviews in Honor of G. C. Liggins (Ed.) Gluckman, P. D., Johnston, B. M. & Nathanielsz, P. W. pp. 263-286. Ithaca, New York: Perinatology Press. Owens,J. A., Faleoner, J. & Robinson,J. S. (1987a) Effect of restriction of placental growth on oxygen delivery to and consumption by the pregnant uterus and fetus. Journal of DevelopmentalPhysiology, 9, 137-150. Owens, J. A., Faleoner, J. &Robinson, J. S. (1987b) Effect of restriction of placental growth on fetal and uteroplacental metabolism. Journal of DevelopmentalPhysiology, 9, 225-238. Persson, E. & Jansson, T. (1992) Low birth weight is associated with elevated adult blood pressure in the chronically catheterized guinea-pig. Acta PhysiologicaScandinavica, 145, 195-196. Rankin, J. H. G. (1976) A role for prostaglandins in the regulation of the placental blood flows. Prostaglandins, 11, 343-353. Rudolph, A. M. (1983) Hepatic and ductus venosus blood flows during fetal life. Hepatology, 3,254-258. Sehriider, H.J., B6rries, M. & Leiehtweiss, H.-P. (1991) Distribution of flows and transfer efficiency in the isolated guinea pig placenta. In Placenta: BasicResearchfor ClinicalApplication (Ed.) Soma, H. pp. 11-22. Basel: Karger. Shaul, P. W., Cha, C. M. & Oh, W. (1989) Neonatal sympathoadrenal response to acute hypoxia: Impairment after experimental intrauterine growth retardation. PediatricResearch, 25, 466-472. Thomas, C. R. & Lowy, C. (1984) Contribution of circulating maternal lipids to fetal tissues in the guinea pig. Journal of DevelopmentalPhysiology, 6, 143-151. Thordstein, M. & Hedner, T. (1992) Cerebral and adrenal monoamine metabolism in the growth retarded rat fetus under normoxia and hypoxia. PediatricResearch, 31, 131-137. Thordstein, M. & Kjellmer, I. (1988) Cerebral tolerance of hypoxia in growth retarded and appropriately grown newborn guinea pigs. PediatricResearch, 24, 633-638. Widmark, C., Jansson, T., Lindeerantz, K. & Ros~n, K. G. (1990) ECG wave form, short term heart rate variability and plasma catecholamine concentrations in intrauterine growth-retarded guinea-pig fetuses. Journal of DevelopmentalPhysiology, 13,289-293. Widmark, C., Jansson, T., Lindeerantz, K. & Ros6n, K. G. (1991) ECG waveform, short term heart rate variability and plasma catecholamine concentrations in response to hypoxia in intrauterine growth retarded guinea-pig fetuses. Journal of DevelopmentalPhysiology, 15, 161-168. Wigglesworth, J. S. (1964) Experimental growth retardation in the foetal rat. Journal of Pathology and Bacteriology, 88, 1-13. Wilkening, R. B. & Mesehia, G. (1983) Fetal oxygen uptake, oxygenation, and acid-base balance as a function of uterine blood flow. AmericanJournal of Physiology, 244, H749-H755. Wilkening, R. B. & Meschia, G. (1992) Comparative physiology of placental oxygen transport. Placenta, 13, 1-15. Young, M. & Widdowson, E. M. (1975) The influence of diets deficient in energy, or in protein, on conceptus weight, and the placental transfer of a non-metabolisable amino acid in the guinea pig. Biologyof the Neonate, 27, 184-191.
Current Topic: Restriction of Placental and Fetal Growth in the Guinea-Pig A N T H O N Y M. CARTER DepartmentofPhysiology, Universityof Odense, Winslowparken19, DK-5000 Odense, Denmark
Ligation of the uterine artery in mid-pregnancy is a simple procedure that restricts the subsequent growth of the placentae and fetuses. It was introduced for the rat by Wigglesworth (1964) and first applied to the guinea-pig by Lafeber (1981). The guinea-pig fetus weighs > 100 g at term (68 days) and even the growth-restricted fetus is amenable to surgical intervention, permitting study of circulatory adaptations to intrauterine growth restriction (IUGR) and measurements of placental transfer. The limitations of the model are set by fetal size and by the necessity of performing acute studies under anaesthesia. Therefore, it is important to recognize that there are alternative models of IUGR in sheep that allow measurements to be made on the unanaesthetized fetus. Each horn of the guinea-pig uterus is supplied by an arterial arcade formed by the ovarian and uterine arteries (Egund and Carter, 1974). Ligation of one uterine artery results in the loss of all fetuses from the ipsilateral horn in one-third of all animals (Detmer and Carter, 1992). In the remainder, some or all of the fetuses survive, supported by the blood supply from the ovarian artery. The extent to which growth restriction occurs is variable, both within and between litters. Because asymmetrical growth restriction is a significant feature of IUGR, we have chosen to select fetuses with an increased ratio of brain weight to liver weight. With the cut-offset arbitrarily at a ratio of 0.85, 35 per cent of fetuses are growth restricted, with at least one such fetus occurring in 48 per cent of experiments. A different result is obtained if the fetuses are classified by body weight, which some authors use to divide the material into subgroups with different degrees of growth restriction (Lafeber, Rolph and Jones, 1984). The primary insult entails a reduction of maternal placental blood flow (Jones and Parer, 1983). If the uterine artery is ligated at 30-32 days gestation, restriction of placental size is demonstrable within 14 days, whereas the limitation on fetal growth becomes apparent between 45 and 55 days and term (Jansson, Thordstein and Kjellmer, 1986). There is delayed maturation of most tissues, with some intriguing differential effects. The weight of the liver and spleen falls to a greater extent than body weight, but the heart and kidneys are reduced in proportion to, and the brain and adrenal glands less than, fetal body weight (Lafeber et al, 1984). This asymmetric pattern of growth has been likened byJansson (1990) to type II human IUGR (Evans and Lin, 1984) and is seen in ovine models of IUGR that rely upon a reduction in fetal or maternal placental blood flow. Alternative methods used to manipulate fetal growth are reviewed elsewhere (Owens, Owens and Robinson, 1989; Carter, 1991). In sheep, carunclectomy prior to conception limits placental and fetal growth 0143-4004/93/020125 + 11 $08.00/0
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throughout gestation; this procedure was introduced by Alexander (1964) and is extensively reviewed by Owens, Owens and Robinson (1989). Restriction of maternal placental blood flow entails a reduction in the supply of oxygen and substrate to the placenta. Because the guinea-pig placenta operates as a countercurrent exchanger, in which blood leaving the fetal capillaries equilibrates with blood entering the arterial side of the maternal labyrinth, there is little room for compensatory improvement in placental exchange. Under normal circumstances the placenta is about 65 to 75 per cent as efficient as an ideal countercurrent exchanger (Moll and Kastendieck, 1977; Schrrder, Brrries and Leichtweiss, 1991); the difference can be attributed to the presence of non-countercurrent components and to mismatching of fetal and matemal blood flows ( Q / Q # 1). Schrrder, Brrries and Leichtweiss (1991) recently measured Q / Q ratios in small pieces of placenta (mean weight 19 mg). The mean value was close to 1, but Q / Q ranged from 0.4 to 2.5 even in the two-thirds of the placenta without extreme Q/Qratios. Although Rankin (1976) has proposed a mechanism for the regulation of Q/Qratios in the placenta, there is no firm evidence that better matching occurs in response to reduced oxygen delivery, and Q/Qratios have yet to be examined in the placenta of the IUGR guinea-pig fetus.
PLACENTAL OXYGEN TRANSFER The factors affecting placental exchange of respiratory gases have been reviewed elsewhere (Longo, 1987; Carter, 1989; Wilkening and Meschia, 1992). At any value of Po2 achieved in umbilical venous blood, delivery of oxygen to the fetus will depend on the haemoglobin flow rate, i.e. the product of fetal placental blood flow and haemoglobin concentration, and on the oxygen affinity of fetal haemoglobin. In growth-restricted guinea-pigs fetal placental blood flow is reduced in proportion to placental mass (Carter and Detmer, 1990). This is hardly surprising, as placental blood flow normally accounts for one-third of the biventricular cardiac output (Girard et al, 1983). To pump an even greater fraction through the umbilical circuit, the growth-restricted fetus would have to maintain a large cardiac output relative to its body size. Yet myocardial mass and body mass decrease to about the same extent. In addition, theoretical considerations indicate that a deviation in the mean Q/Qratio from 1 will reduce the efficiency of diffusional exchange (Schrrder et al, 1991). Lafeber, Rolph and Jones (1984) reported that packed cell volume was elevated in the IUGR fetus and considered this finding to be indicative ofhypoxia. However, measurements of haemoglobin concentration yield conflicting results, even in the same laboratory (Widmark et al 1990, 1991). One explanation could be the inability of the fetus to mount an appropriate haematopoietic response to hypoxia in IUGR due to the disproportionate reduction in liver mass. We have been unable to confirm the compensatory increase in extramedullary haematopoiesis described by Lafeber, Rolph and Jones (1984) and have found a significant reduction in the number of haematopoietic colonies on the right side of the liver (Ernst, Salafia and Carter, unpublished data). In guinea-pigs, the high oxygen affinity of fetal blood is attributable no.'. to a specific fetal haemoglobin, but to the low 2,3-diphosphoglycerate (DPG) content of the fetal red cells (Bard and Shapiro, 1979; Jelkmann and Bauer, 1980; Carter and Gr~nlund, 1985). Recently we measured the intra-erythrocytic DPG concentration in IUGR fetuses and found it to be even lower than in control fetuses (Table 1).
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Table 1. Comparison of haemoglobin and 2,3-diphosphoglycerate (DPG) concentrations in normal and growth-restricted guinea-pig fetuses in late gestation (mean _+ s.e.m.). Control fetuses Number of samples Fetal Weight (g) Brain wt./liver wt. Haemoglobin (mmol/l) DPG (mmolA) DPG mol per mol Hb4
89 0.64 2.32 0.61 0.27
13 _+3 + 0.02 _+ 0.07 "4-0.03 _+0.02
Growth-restricted fetuses 7 50 _+ 5*** 1.24 _ 0.12"** 2.29 _+ 0.09 0.43 +_ 0.06** 0.20 _+ 0.02*
Growth restricted against controls *P < 0.02, **P < 0.01, ***P < 0.001.
Gaseous exchange in the placenta is limited by the oxygen diffusing capacity, including the diffusing capacity of the placental membrane (Din) and the diffusing capacities of the maternal and fetal capillary blood. Growth restriction in guinea-pigs, induced by maternal hypoxia, was associated with a 63 per cent increase in placental diffusing capacity (Gilbert et al, 1979). About one-third of this increase could be ascribed to an increase in D m secondary to increased surface area and decreased diffusion distance and the remainder to increased haemoglobin concentrations and vascular volumes (Bacon et al, 1984). Placental diffusing capacity has not been examined in the uterine artery ligation model of IUGR. A considerable fraction of the oxygen extracted from the maternal circulation is not available to the fetus, since it is used to support placental metabolism. Data on placental oxygen consumption is not available for the guinea-pig. In sheep, the placenta accounts for c. 40 per cent of uterine oxygen consumption (Wilkening and Meschia, 1983).
PLACENTAL TRANSFER OF GLUCOSE, AMINO ACIDS AND NEFA Uterine artery ligation reduces the amount ofsubstrate conveyed to the gravid uterus. There is a compensatory increase in the fractional extraction of glucose (Jones et al, 1988; Jansson and Persson, 1990), yet the fetus is hypoglycaemic (Jones, Lafeber and Roebuck, 1984; Widmark et al, 1990), as may occur in the growth-restricted human fetus (Economides and Nicolaides, 1989). Studies with isolated perfused placentae from growth-restricted guineapig fetuses indicate that the transport capacity per unit of placental mass is no different from that in normal pregnancies (Jones et al, 1988), a conclusion that is supported by in vivo measurements of the weight-specific placental transfer capacity for [3H]-methylglucose (Jansson and Persson, 1990).Jones et al (1988) estimated that the placenta normally utilizes about two-thirds of the glucose extracted from the maternal circulation and calculated that this fraction would increase further in IUGR, substantially reducing glucose availability to the fetus (Figure 1). They arrived at this conclusion by applying measurements of unidirectional glucose flux and placental glucose consumption, obtained in the doubly perfused isolated placenta, to the in vivo situation. In the sheep carunclectomy model of IUGR, the fetal share of uterine glucose consumption increases (Owens, Falconer and Robinson, 1987b). Placental transfer capacity for a-aminoisobutyric acid (AIB) is reduced in growthrestricted guinea-pigs, by 36 per cent on day 50 and 22 per cent on day 63 of gestation, with corresponding decreases in fetal uptake (Jansson and Persson, 1990). Fetal uptake of AIB is
Placenta (1993), VoL 14
128 Mother
Placenta
Fetus
Normal 90 000 (100)
3150 (3.5)
80 460 (89'4)
Growth Retarded
~
26000(100)
22828 (87.8)
52 (0.2)
3120 (12)
Flux across one placenta and to one fetus = ~mol glucose/min (Percentage of flux)
Figure 1. Estimated glucose supply to and consumption by the normal and growth-restricted guinea-pig fetus and placenta at 60-62 days gestation. Flux rates are expressed in nmol- min 1 (percentages in parentheses). Uterine artery ligation reduces the glucose supply to the placenta. Placental glucose consumption falls in proportion to placental mass, but accounts for a greater share of the total flux, thereby further limiting the amount of glucose available to the fetus. From Jones et al, 1988.
also reduced in natural runts (Hayter et al, 1964). Interestingly, guinea-pigs on a low protein diet, which causes a moderate reduction in fetal and placental mass, exhibit a compensatory increase in placental transfer capacity for AIB and a consequent increase in the weightspecific uptake of this amino acid (Young and Widdowson, 1975). Although fetal and placental handling of other amino acids has not been studied in IUGR, it is probably correct to conclude that there is an impaired supply of amino acids to the fetus (Jansson and Persson, 1990). However, fetal plasma concentrations are elevated rather than depressed because of a sharp fall in the utilization of amino acids by the tissues (Jones, 1989). The other main substrate for the guinea-pig fetus is non-esterified fatty acid. NEFA is rapidly transferred from maternal to fetal blood, sequestered by the liver, and converted to triacylglycerols (Jones, 1976; Thomas and Lowy, 1984). In a study of hepatic lipid metabolism in IUGR, we gave a maternal I.V. infusion of [1-14C]-palmitic acid and measured plasma concentrations of the label in umbilical venous blood collected after 15, 30 and 45 min (Detmer, Carter and Thomas, in press). A slower rise towards steady state was seen in the IUGR fetuses (Figure 2). The possibility that this is due to a reduction in transfer capacity for NEFA deserves further investigation.
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Figure2. Incorporation ofradiolabel into the blood of growth-restricted (IUGR) and control (C) fetuses at 15, 30 and 45 min during infusion of [1-n4C]-palmitate into a maternal vein. A reduction in placental transfer capacity for NEFA could account for the slower rise towards steady state seen in the IUGR fetuses. Shading refers to incorporation of [14C] into NEFA ([]), triacytglycerols (R), cholesterol (m), cholesterol esters (11), and unidentified material ([3). Error bars refer to total label. From Detmer, Carter and Thomas, 1992.
FETAL RESPONSES TO THE REDUCED OXYGEN SUPPLY Fetal oxygen consumption (vo2) has been measured in guinea-pig fetuses (Girard et al, 1983), but no data is available for the growth-restricted fetus. In the sheep carunclectomy model of IUGR, fetal vo2 is reduced to the same extent as fetal weight. However, there is a smaller margin of safety between supply of and demand for oxygen (Owens, Falconer and Robinson, 1987a), glucose (Owens, Falconer and Robinson, 1987b), and other substrates. The concept of a margin of safety in fetal oxygen supply, based on the relative insensitivity of fetal vo2 to acute changes in placental blood flow and fetal oxygen delivery, has recently been challenged. When the relationship between vo2 andpo2 of the sheep fetus was studied over a wide range ofpo2 values, the data suggested that there is no margin of safety in oxygen supply for a significant fraction of fetal tissues (Asakura, Ball and Power, 1990). Moreover, hypoxia has been shown to reduce the VOz of fetal guinea-pig skeletal muscle cells in monolayer culture, for which vo2 correlates with po2 (Braems and Jensen, 1991). Oxygen consumption by the brain is maintained even at extremely low values of po2 (J6hannsson and Siesj6, 1975; Jones et al, 1977). Therefore, if the IUGR fetus is hypoxic, brain sparing would have to occur either by increasing cerebral blood flow to maintain oxygen delivery or by increasing fractional oxygen extraction. The fetus normally responds to a fall in arterial oxygen content ([O2]a) with an increase in cerebral perfusion (Carter and Gu, 1988). This relationship can be difficult to demonstrate in the IUGR fetus (Detmer, Gu and Carter, 1991), but a recent data set (Figure 3) suggests a leftward and downward shift of the regression line describing the relationship between brain blood flow and [Oz]a. Any shortfall in the oxygen supply must be made up by increased extraction of oxygen from the blood, and an inevitable result of increased extraction will be a fall in mean capillarypOz and tissue po2.
Placenta (1993), Vol. 14
130 4
b
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E 2-
~,, oN oR
1-
(I
I
I
I
I
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1
2
3
4
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Figure 3. Relation between brain blood flow (CBF) and arterial O2 content ([O2]a) in guinea-pig fetuses at 62-63 days gestation. Measurements were made in 12 normal fetuses (CBF = 5.2- (1/[02]a)--0.5, r = 0.82, P < 0.001) and 11 IUGR fetuses (CBF = 2.5- (1/[02]a)--0.2, r - 0.83, P < 0.01). Carter and Detmer (unpublished data).
This will slow some oxygen-consuming reactions, which are extremely sensitive to changes in poz, even though their contribution to total oxygen consumption is insignificant. This category of reactions includes some that are essential to the synthesis of neurotransmitters, such as the hydroxylation of tryptophan (Davis et al, 1973), and serotonin levels fall in the fetal rat forebrain following uterine artery ligation (Thordstein and Hedner, 1992). An inadequate blood flow response to hypoxaemia may act through such mechanisms to delay the maturation of the brain in IUGR fetuses (Lafeber, Rolph and Jones, 1984) and affect the cerebral integrity of the growth-restricted newborn guinea-pig (Thordstein and Kjellmer, 1988). Arterial blood pressure is maintained in IUGR fetuses, but there is a reduction in heart rate and thus in the rate-pressure product (Detmer, Gu and Carter, 1991). Normally, guinea-pig fetuses adapt to low levels of oxygenation by increasing the rate ofperfusion of the myocardium, but in IUGR fetuses the data were scattered and a significant correlation could not be shown (Detmer, Gu and Carter, 1991). The lower rate pressure product of IUGR fetuses suggests a decline in myocardial oxygen consumption. ECG wave form analysis indicates that aerobic myocardial metabolism is maintained in normoxaemic (Widmark et al, 1990) but not in hypoxaemic IUGR fetuses (Widmark et al, 1991). This has been ascribed to a decreased myocardial blood flow reserve and a blunted sympathoadrenal response (Widmark et al, 1991). These factors could affect development of the heart muscle and its innervation. Heart rate is depressed in growth-restricted guinea-pig neonates (Thordstein and Kjellmer, 1988) and the ability to respond to acute stresses such as hypoxia is impaired in newborn IUGR rats (Shaul, Cha and Oh, 1989). Adrenal weight is reduced in the growth-restricted guinea-pig fetus, but there is a significant increase in the adrenal perfusion rate (Table 2). During normal fetal growth, plasma ACTH and cortisol concentrations rise between 55 and 60 days of gestation (Jones and Roebuck, 1980), and a high rate of cortical perfusion may be required for steroidogenesis. ACTH and cortisol levels do rise in growth-restricted fetuses, but plasma concentrations are about half of those normally seen (Jones, Lafeber and Roebuck, 1984). The observed
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131
Table 2. Comparison of the weight of the adrenal glands and their rate of perfusion in normal and growth-restricted guinea-pig fetuses in late gestadon (means _+ s.e.m). Data from Carter and Demaer (1990). Control fetuses Number of fetuses Fetal weight (g) Brain wt./liver wt. Adrenal weight (mg) Adrenal blood flow (rnl/min per g)
10 89 + 4 0.56 _+ 0.03 35.8 4- 2.2 2.4 _ 0.5
Growth-restricted fetuses 5 57 ___3*** 1.00 4- 0.05*** 21.2 + 3.6** 5.6 4- 1.1"*
Growth restricted against controls **P < 0.01, *** P < 0.001.
increase in adrenal perfusion in IUGR presumably serves to ensure adequate oxygen delivery to the cortex and may involve an autoregulatory component, since plasma ACTH is not increased, as might otherwise be expected in chronic hypoxaemia (Jones, Lafeber and Roebuck, 1984). Plasma catecholamine concentrations are elevated in IUGR guinea-pig fetuses (Widmark et al, 1990) and there is precocious neural regulation of the adrenal medulla following uterine artery ligation in rats (Shaul, Cha and Oh, 1989). It is conceivable that maturational changes in sympathoadrenal function serve to explain the rink between low birth weight and subsequent hypertension in man (Barker et al, 1989; Gennser, Rymark and Isberg, 1988), which is reproducible following uterine artery ligation in guinea-pigs (Persson and Jansson, 1992). The disproportionate reduction in size of the liver in asymmetric IUGR is intriguing. The guinea-pig fetus lacks a ductus venosus (Harman and Herbertson, 1938; Girard et al, 1983; Carter, 1984), and the entire umbilical venous return perfuses the liver, which thus is presented with blood of a higher oxygen and substrate content than that which perfuses the brain and myocardium. In the fetal sheep, hepatic demand for oxygen is met by a very high rate of blood flow and a remarkably low oxygen extraction, and hepatic oxygen uptake is linearly related to oxygen delivery (Bristow et al, 1983). If the growth-restricted guinea-pig fetus likewise is unable to increase oxygen extraction, to compensate for the fall in umbilical blood flow and oxygen delivery, this may place a constraint upon liver growth. The blood leaving the hepatic veins is an important source of oxygen for other tissues (Rudolph, 1983). Because the fetal liver responds to a reduced oxygen delivery with a slower rate of growth, rather than by increasing oxygen extraction, other organs--notably the brain--can be spared. It should be noted that the absence of a ductus venosus does not preclude preferential distribution of the umbilical venous return to the heart and brain. Everett & Johnson (1950) demonstrated that approximately three-quarters of the blood derived from the umbilical vein passes through the foramen ovale to the left side of the heart. Consideration of the intralobular distribution of flows and liver mass bolsters the case for a relationship between liver growth and oxygen supply. The right lobe of the liver derives its blood supply from the portal vein and the hepatic artery, with only a minor contribution from the umbilical vein (Carter and Detmer, 1990; Carter, Detmer and Egund, 1992). In IUGR, increased oxygen extraction by other tissues probably lowers the oxygen content of the blood reaching the right lobe from these vessels. This might exert an additional effect on the growth of the right lobe, which accounts for a relatively smaller proportion of total fiver weight (Carter and Detmer, 1990). In addition, there is an increased tendency to fatty degeneration
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and decreased haematopoietic activity in the right liver lobe of IUGR fetuses (Ernst, Salafia and Carter, unpublished data). These observations are of interest in a clinical context, since non-siderotic degeneration of the right side of the liver is a frequent finding in infants that die in the perinatal period (Gruenwald, 1949; Daamen and Schaberg, 1969).
FETAL ENDOCRINE RESPONSES T O THE REDUCED SUBSTRATE SUPPLY It has been suggested that a reduction in the supply of amino acids may limit fetal growth by restraining protein synthesis (Owens, Owens and Robinson, 1989). Therefore, tissues with the highest rates of protein turnover, including the liver, may be more vulnerable to intrauterine malnutrition. In support of this, a significant reduction in a-aminonitrogen and in most essential amino acids was demonstrable in blood obtained by cordocentesis from human fetuses that were small for gestational age (Cefin et al, 1990). Although there is a diminished supply of amino acids in the growth-restricted guinea-pig fetus (Jansson and Persson, 1990), and a fall in plasma a-aminonitrogen (Lafeber, 1981), concentrations of some amino acids are elevated rather than depressed because of a sharp fall in the capacity of the tissues to metabolize amino acids (Jones, 1989). The metabolic changes in IUGR can largely be ascribed to changes in fetal endocrine state. The predominant difference is hypoinsulinaemia (Jones, Lafeber and Roebuck, 1984), which is consistent with the hypoglycaemia and may be responsible for the observed decrease in fetal glucose consumption (Jones, 1989). The extent of the reduction in plasma insulin concentration is closely related to fetal size (Jones, Lafeber and Roebuck, 1984), supporting the contention that insulin is the major growth-promoting hormone of the fetus (Fowden, 1989). Glucagon levels fall markedly in the last half of gestation, but growth restriction slows the decline (Jones, Lafeber and Roebuck, 1984). However, deposition of glycogen in the liver is maintained in the face of a rise in the insulin/glue.agon ratio (Lafeber, Rolph and Jones, 1984). Indeed, despite the small size of the liver in IUGR fetuses, hepatic stores of glycogen are unchanged in relation to body weight, due to a significant increase in glycogen concentration (Lafeber, Rolph and Jones, 1984). This observation has been linked to a rise in insulin-like growth factor II (IGF II) and seen to support the view that IGF II is a major gluconeogenetic hormone in the fetus (Jones et al, 1987; Jones, 1989). Insulin is an important regulator of fetal IGF secretion and the effects of insulin on fetal growth may be mediated by altered IGF I release (Bassett et al, 1989). In growth-restricted guinea-pig fetuses, IGF I follows a pattern similar to insulin. Sulphation-promoting activity falls even more sharply (Jones et al, 1987). These changes are consistent with slowing of DNA, RNA and protein synthesis in tissues of the IUGR fetus. In addition, plasma cortisol concentrations are much lower in the IUGR fetus, and plasma levels of T3, T4 and rT3 fall sharply (Jones, Lafeber and Roebuck, 1984), resulting in a diminished thyroid drive to maturation. Thus, the supply and utilization ofsubstrates is central to the regulation of fetal growth and there is a prima facie case for the existence of pathways that communicate placental nutritional state and perfusion to the fetus (Jones, 1989). This would explain why the ratio of fetal to placental weight is maintained following uterine artery ligation (Jones and Parer, 1983). The sheep placenta has high affinity type 1 IGF receptors and removes IGF I from the fetal circulation (Bassett et al, 1990). When the concentration of IGF I in fetal plasma falls below a critical level, placental uptake of IGF I ceases (Iwamoto, Chernausek and Murray,
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1991). T h u s the possibility exists o f a feedback loop in which placental f u n c t i o n d e t e r m i n e s substrate availability, w h i c h d e t e r m i n e s I G F I release, which in t u r n affects placental f u n c t i o n (Bassett et al, 1989). T h e decrease in plasma levels o f I G F I s e e n in the g u i n e a - p i g fetus following u t e r i n e artery ligation is consistent with this i n t e r e s t i n g hypothesis.
ACKNOWLEDGEMENTS The author's studies of intrauterine growth restriction have been supported generously by the Danish Medical Research Council.
REFERENCES Alexander, G. (1964) Studies on the placenta of the sheep (Ovis aries L.): Effect of surgical reduction in the number of caruncles. Journal of Reproduction and Fertility, 7, 307-322. Asakura, H., Ball, K. T. & Power, G. G. (1990) Interdependence of arterial POz and Oz consumptionin the fetal sheep. Journal of DevelopmentalPhysiology, 13, 205-213. Bacon, B.J., Gilbert, R. D., Kaufmann, P., Smith, A. D., Trevino, F. T. & Longo, L. D. (1984) Placental anatomy and diffusing capacity in guinea pigs followinglong-term maternal hypoxia. Placenta, 5, 475-488. Bard, H. & Shapiro, M. (1979) Perinatal changes of 2,3-diphosphoglycerate and oxygen affinity in mammals not having fetal type hemoglobins. PediatricResearch, 13, 167-169. Barker, D. J. P., Osmond, C., Golding, J., Kuh, D. & Wadsworth, M. E.J. (1989) Growth in utero, blood pressure in childhood and adult life, and mortality from cardiovascular disease. British MedicalJournal, 298, 564-567. Bassett, N. S., Gluckman, P. D., Breier, B. H. & Ball, K. T. (1989) Gro~h hormone and insulin-like growth factors iri the fetus: regulation and role in fetal growth. InAdvances in Fetal Physiology:Reviews in Honor ofG. C. Liggins (Ed.) Gluckman, P. D., Johnston, B. M. & Nathanielsz, P. W. pp. 303-314. Ithaca, New York: Perinatology Press. Bassett, N. S., Breier, B. H., Hodgkinson, S. C., Davis, S. R., Henderson, H. V. & Gluckman, P. D. (1990) Plasma clearance of radiolabelled IGF-1 in the late gestation ovine fetus. Journal of DevelopmentPhysiology, 14, 73-79. Braems, G. & Jensen, A. (1991) Hypoxia reduces oxygen consumption of fetal skeletal muscle cells in monolayer culture.Journal of DevelopmentalPhysiology, 16, 209-215. Bristow, J., Rudolph, A. M., Itskowitz, J. & Barnes, R. (1983) Hepatic oxygen and glucose metabolism in the fetal lamb. Journal of Clinical Investigation, 71, 1047-1061. Carter, A. M. (1984) The blood supply to the abdominal organs of the fetal guinea-pig.Journal of Developmental Physiology, 6, 407-416. Carter, A. M. (1989) Factors affecting gas transfer across the placenta and the oxygen supply to the fetus.Journalof DevelopmentalPhysiology, 12, 305-322. Carter, A. M. (1991) Placental blood flow and fetal oxygen supply in animal models of intra-uterine growth retardation. In Placenta:BasicResearchfor ClinicaIApplication (Ed.) Soma, H. pp. 23-31. Basel: Karger. Carter, A. M. & Detmer, A. (1990) Blood flow to the placenta and lower body in the growth-retarded guinea pig fetus.Journal of DevelopmentalPhysiology, 13, 261-269. Carter, A. M., Detmer, A. & Egund, N. (1992) Contributionof the umbilical and portal veins to the hepatic blood supply of the guinea pig fetus. An angiographic study. LaboratoryAnimal Sdence, 42, 174-179. Carter, A. M. & Grnnlund, J. (1985) Influence of 2,3-diphosphoglycerate (DPG) concentration in maternal red cells on the transptacental exchange of respiratory gases. In The PhysiologicalDevelopmentof the Fetus and Newborn (Ed.) Jones, C. T. & Nathanielsz, P. W. pp. 47-50. London: Academic Press. Carter, A. M. & Gu, W. (1988) Cerebral blood flow in the fetal guinea-pig.Journal ofDevelopmentalPhysiology, 10, 123-129. Cetin, I., Corbetta, C., Sereni, L. P., Marconi, A. M., Bozzetti, P., Pardi, G. & Battaglia, F. C. (1990) Umbilical amino acid concentrations in normal and growth-retarded fetuses sampled in utero by cordocentesis. AmericanJournal of Obstetricsand Gynecology, 162, 253-261. Daamen, C. B. F. & Schaberg, A. (1969) Hemilateral liver degeneration.Journal of Pathology, 97, 29-34. Davis, J. N., Carlsson, A., MacMillan, V. & Siesj6, B. K. (1973) Brain tryptophan hydroxylation: Dependence on arterial oxygen tension. Science, 182, 72-74. Detmer, A. & Carter, A. M. (1992) Factors influencing the outcome of ligating the uterine artery and vein in a guinea pig model of intrauterine growth retardation. ScandinavianJournal ofLaboratoryAnimal Science, 19, 9-16. Detmer, A., Carter, A. M. & Thomas, C. R. (1992) The metabolism of free fatty acids and triacylglycerolsby the fetal liver in a guinea pig model of intrauterine growth retardation. PediatricResearch, 32, 441-446.
134
Placenta (1993), Vol. 14
Detmer, A., Gn, W. & Carter, A. M. (1991) The blood supply to the heart and brain in the growth retarded guinea pig fetus. Journal of DevelopmentalPhysiology, 15, 153-160. Economides, D. L. & Nicolaides, K. H. (1989) Blood glucose and oxygen tension levels in small-for-gestationalage fetuses. AmericanJournal of Obstetricsand Gynecology, 160, 385-389. Egund, N. & Carter, A. M. (1974) Uterine and placental circulation in the guinea-pig: an angiographic study. Journal of Reproduction & Fertility, 40, 401--410. Evans, M. I. & Lin, C. C. (1984) Retarded fetal growth. In Intrauterine growth retardation: Pathophysiology and ClinicalManagement (Ed.) Lin, C. C. & Evans, M. I. pp. 55-57. New York: McGraw-Hill. Everett, N. B. & Johnson, R.J. (1950) Use of radioactive phosphorus in studies of fetal circulation. American Journal of Physiology, 162, 147-152. Fowden, A. L. (1989) The endocrine regulation of fetal metabolism and growth. In Advances in Fetal Physiology: Reviews in Honor ofG. C. Liggins (Ed.) Gluckman, P. D., Johnston, B. M. & Nathanielsz, P. W. pp. 229-243. Ithaca, New York: Perinatology Press. Gennser, G., Rymark, P. & Isberg, P. E. (1988) Low birth weight and risk of high blood pressure in adulthood. British MedicalJournal, 296, 1498-1500. Gilbert, R. D., Cummings, L. A., Jaehau, M. R. & Longo, L. D. (1979) Placental diffusing capacity and fetal development in exercising or hypoxic guinea pigs.Journal ofApplied Physiology, 46, 828-834. Girard, H., Klappstein, S., Bartng, I. & Moll, W. (1983) Blood circulation and oxygen transport in the fetal guinea pig. Journal of DevelopmentalPhysiology, 5, 181-193. Gruenwald, P. (1949) Degenerative changes in the right half of the liver resulting from intra-uterine anoxia. AmericanJournal of Clinical Pathology, 19, 801-813. Harman, M. T. & Herbertson, J. E. (1938) Concerning the postnatal obliteration of the umbilical vein and arteries, the vitelline vein and artery, and the ductus arteriosus in the guinea pig. Transactionsof the KansasAcademy of Science, 41,369-376. Hayter, C.J., Hutchinson, E. A., Karvonen, M.J. & Young, M. (1964) Placental transport of a-amino isobutyric acid in the unanaesthetized guinea-pig. Journal of Physiology, 175, 11P-13P. Iwamoto, H. S., Chernuasek, S. D. & Murray, M. A. (1991) Regulation of plasma insulin-like growth factor (IGF-I) by oxygen and nutrients in fetal sheep. 2rid International IGF Symposium Abstract, A48. Jansson, T. (1990) Placentalbloodflow regulationand substratetransferin intrauterinegrowth retardation.An experimental study in the conscious,chronicallycatheterizedguineapig. MD Thesis, University of G6teborg. Jansson, T. & Persson, E. (1990) Placental transfer of glucose and amino acids in intrauterine growth retardation: Studies with substrate analogs in the awake guinea pig. Pediatric Research, 28, 203-208. Jansson, T., Thordstein, M. & KjeUmer, I. (1986) Placental blood flow and fetal weight following uterine artery ligation. Biology of the Neonate, 49, 172-180. Jelkmann, W. & Bauer, C. (1980) 2,3 -DPG levels in relation to red cell enzyme activities in rat fetuses and hypoxic newborns. PfliigersArchiv, 389, 61-68. J6hannsson, H. & SiesjiJ, B. K. (1975) Cerebral blood flow and oxygen consumption in the rat in hypoxic hypoxia. Acta PhysiologicaScandinavica, 93, 269-276. Jones, C. T. (1976) Lipid metabolism and mobilization in the guinea pig duringpregnancy. BiochemicalJournal, 156, 357-365. Jones, C. T. (1989) Observations on the control of prenatal growth in the guinea pig. InAdvances in FetalPhysiology: Reviews in Honor ofG. C. Liggins (Ed.) Gluckman, P. D., Johnston, B. M. & Nathanielsz, P. W. pp. 287-302. Ithaca, New York: Perinatology Press. Jones, C. T., Harding, J. E., Gu, W. & Lafeber, H. N. (1988) Placental metabolism and endocrine effects in relation to the control of fetal and placental growth. In The Endocrine Controlofthe Fetus (Ed.) Kiinzel, W. &Jensen, A. pp. 213-222. Heidelberg: Springer Verlag. Jones, C. T., Lafeber, H. N. & Roebuck, M. M. (1984) Studies on the, growth of the fetal guinea pig. Changes in plasma hormone concentration during normal and abnormal growth. Journal of Developmental Physiology, 6, 461-472. Jones, C. T., Lafeber, H. N., Price, D. A. &Parer, J. T. (1987) Studies on the growth of the fetal guinea pig. Effects of reduction in uterine blood flow on the plasma sulphation-promoting activity and on the concentration of insulin-like growth factors-I and -II. Journal of Developmental Physiology, 9, 181-201. Jones, C. T. &Parer, J. T. (1983) The effect of alterations in placental blood flow on the growth of and nutrient supply to the fetal guinea-pig.Journal of Physiology, 343,525-537. Jones, C. T. & Roebuck, M. M. (1980) The development of the pituitary-adrenal axis in the guinea pig. Acta Endocrinologica, 94, 107-116. Jones, Jr, M. D., Sheldon, R. E., Peeters, L. L., Mesehia, G., Battaglia, F. C. & Makowski, E. L. (1977) Fetal cerebral oxygen consumption at different levels of oxygenation. Journal ofApplied Physiology, 43, 1080-1084. Lafeber, H. N. (1981) Experimental intra-uterine growth retardation in the guinea pig. MD Thesis, Rotterdam. Lafeber, H. N,, Rolph, T. P. &Jones, C. T. (1984) Studies on the growth of the fetal guinea pig. The effects of ligation of the uterine artery on organ growth and development. Journal of DevelopmentalPhysiology, 6, 441-459. Longo, L. D. (1987) Respiratory gas exchange in the placenta. In Handbook of Physiology. Section 3. The Respiratory System. VolumeIV. Gas Exchange, pp. 351-401. Bethesda: American Physiological Society.
Carter: Fetal Growth Restriction
135
Moll, W. & Kastendieek, E. (1977) Transfer of N20 , CO and HTO in the artificially perfused guinea-pig placenta. RespirationPhysiology, 29, 283-302. Owens, J. A., Owens, P. C. & Robinson, J. A. (1989) Experimental fetal growth retardation: metabolic and endocrine aspects. In Advances in Fetal Physiology: Reviews in Honor of G. C. Liggins (Ed.) Gluckman, P. D., Johnston, B. M. & Nathanielsz, P. W. pp. 263-286. Ithaca, New York: Perinatology Press. Owens,J. A., Faleoner, J. & Robinson,J. S. (1987a) Effect of restriction of placental growth on oxygen delivery to and consumption by the pregnant uterus and fetus. Journal of DevelopmentalPhysiology, 9, 137-150. Owens, J. A., Faleoner, J. &Robinson, J. S. (1987b) Effect of restriction of placental growth on fetal and uteroplacental metabolism. Journal of DevelopmentalPhysiology, 9, 225-238. Persson, E. & Jansson, T. (1992) Low birth weight is associated with elevated adult blood pressure in the chronically catheterized guinea-pig. Acta PhysiologicaScandinavica, 145, 195-196. Rankin, J. H. G. (1976) A role for prostaglandins in the regulation of the placental blood flows. Prostaglandins, 11, 343-353. Rudolph, A. M. (1983) Hepatic and ductus venosus blood flows during fetal life. Hepatology, 3,254-258. Sehriider, H.J., B6rries, M. & Leiehtweiss, H.-P. (1991) Distribution of flows and transfer efficiency in the isolated guinea pig placenta. In Placenta: BasicResearchfor ClinicalApplication (Ed.) Soma, H. pp. 11-22. Basel: Karger. Shaul, P. W., Cha, C. M. & Oh, W. (1989) Neonatal sympathoadrenal response to acute hypoxia: Impairment after experimental intrauterine growth retardation. PediatricResearch, 25, 466-472. Thomas, C. R. & Lowy, C. (1984) Contribution of circulating maternal lipids to fetal tissues in the guinea pig. Journal of DevelopmentalPhysiology, 6, 143-151. Thordstein, M. & Hedner, T. (1992) Cerebral and adrenal monoamine metabolism in the growth retarded rat fetus under normoxia and hypoxia. PediatricResearch, 31, 131-137. Thordstein, M. & Kjellmer, I. (1988) Cerebral tolerance of hypoxia in growth retarded and appropriately grown newborn guinea pigs. PediatricResearch, 24, 633-638. Widmark, C., Jansson, T., Lindeerantz, K. & Ros~n, K. G. (1990) ECG wave form, short term heart rate variability and plasma catecholamine concentrations in intrauterine growth-retarded guinea-pig fetuses. Journal of DevelopmentalPhysiology, 13,289-293. Widmark, C., Jansson, T., Lindeerantz, K. & Ros6n, K. G. (1991) ECG waveform, short term heart rate variability and plasma catecholamine concentrations in response to hypoxia in intrauterine growth retarded guinea-pig fetuses. Journal of DevelopmentalPhysiology, 15, 161-168. Wigglesworth, J. S. (1964) Experimental growth retardation in the foetal rat. Journal of Pathology and Bacteriology, 88, 1-13. Wilkening, R. B. & Mesehia, G. (1983) Fetal oxygen uptake, oxygenation, and acid-base balance as a function of uterine blood flow. AmericanJournal of Physiology, 244, H749-H755. Wilkening, R. B. & Meschia, G. (1992) Comparative physiology of placental oxygen transport. Placenta, 13, 1-15. Young, M. & Widdowson, E. M. (1975) The influence of diets deficient in energy, or in protein, on conceptus weight, and the placental transfer of a non-metabolisable amino acid in the guinea pig. Biologyof the Neonate, 27, 184-191.