- Email: [email protected]
Effect of prostaglandins E2 and F2α on umbilical blood flow and fetal hemodynamics
EFFECT OF PROSTAGLANDINS E2 AND F ~ O N UMBILICAL BLOOD FLOW AND FETAL HEMODYNAMICS Miles J. Navy , George Piaseckl, and Beniamln T. Jackson Division of Perinatal Medicine, Oregon Regional Primate Research Center, Beaverfon, and the University of Oregon Medical School, Portland, Oregon 97201; and the Surgical Service, Boston, V. A. Hospltal, and the Department of Surgery, Boston University Medical Center, Boston, Massachusetts 02118. ABSTRACT The hemodynamic effects of PGF~, PGE2, and norepinephrlne injected into the umbilical arterial circulation were compared in nine fetal lambs in utero. Umbilical blood flow was measured with radioactive microspheres and an e-lenamagnetic flow transducer implanted on the distal aorta of the fetus after l lgation of external lilac arteries and other accessible distal aortic branches. PGF2r~ and norepinephrine increased fetal arterial pressure and umbilical blood flow w~ile umbilical vascular resistance increased slightly (PGF2~) or not at all (norepinephrine). PGE2 increased fetal arterial pressure, decreased umbilical blood flow, and exerted a profound active vasoconstrictor effect on the fetal placental bed. Our data taken together with the observations of others suggest that prostaglandins may play a role in the circulatory adaptations of the fetus at birth and that PGE2 in high concentrations is likely to have deleterious hemodynamlc consequences in the fetus in utero. INTRODUCTIO N In 1938 van Euler observed that a crude prostaglandin preparation (probably a mixture of PGE1 and PGE2) increased the resistance of isolated human placental vessels to perfuslon (1). More recent in vitro experiments indicate that PGE2 and PGF2_ contract isolated human umbili-E-al-6~"eries and veins (2) and sheep umbilical vessels (3). High concentrations of E and F prostaglandins in the umbilical cord (4) raise the question whether these substances are significant in regulating umbilical blood flow in the fetus, in the physiological mechanism of cord constriction after delivery, and in mediating circulatory changes in the neonate at birth. Moreover, prostaglandins administered to the mother may gain access to the fetus and thus Publication No. 707 from the Oregon Regional Primate Research Center. Supported in part by grant No. HD-06159 from the NICHD, grant No. HD-05969 from the NICHD, and funds from the John A. Hartford Foundation, Inc., New York. We thank Dr. John Pike of the Upjohn Company, Kalamazoo, Michigan, for g i f t s of authentic prostaglandins. :V
Address reprint requests to: Dr. M. J. Navy, Oregon Regional Primate Research Center, Beaverton, Oregon 97005.
Accepted February 6, 1974. PROSTAGLANDINS MARCH 25, 1974
VOL. 5 NO. 6
543
PROSTAGLANDINS
influence its circulation and oxygenation (5). Reportsof the effects of vasoactive substances on isolated cord vessels, as noted above, may not be relevant to in vivo circumstances because of differing experimental conditions and becaus~ihe um-~-~llcal artery and vein in vivo contribute less than 25% of the total placental vascular resistance (6). Since the effects of prostaglandins on the umbilical circulation in the intact fetus have not previously been reported, we studied the hemodynamlc effects of PGE2 and PGF2e~administered to the fetal lamb in utero and paid special attention to the measurement of umbilical blood flow and b"m'~t'~al vascular resistance. MATERIALS AND METHODS Nine pregnant ewes of mixed breed were studied at 115 to 135 days gestatlon (term, 150 days). Anesthesia was induced with intravenous sodium thiamylal 15 mg/kg. After tracheal intubation, a semiclosed rebreathing system with halothane and oxyg en was used for anesthesia. After the maternal abdomen had been opened with a midline incision, medlcal-grade polyvinyt catheters (Bolab, Reading, Mass.), 0.034 inch I.D. x 0.048 inch O . D . , were inserted into a major umbilical vein by retrograde cannulatlon from a peripheral placental cotyledon (7). To avoid the problem of spasm of the umbilical vessels during dlrect appllcation of electromagnetic flow transducers to the cord, we used a new approach to measure umbilical blood flow in the unexteriorized sheep fetus. A marsupializing incision was performed in the left lumbar region of the fetus according to the general principles described by Jackson and Egdahl (8). The dissection was carried retroperitoneally to the distal abdominal aorta of the fetus. The external lilac arteries were llgated as were all other accessible distal aortic branches. In most cases only the umbilical arteries, the small middle sacral, and two small hypogastrlc branches were left intact (Fig. 1). A cuffed extraluminal flow transducer was placed around the fetal aorta at the iliac bifurcation, and a Statham model 0-5000 nonoccluslve zero electromagnetic flowmeter was used to measure blood flow. Actual placental blood flow was determined as a percent of distal aortic blood flow by intraaortic injection of nuclide-labeled microspheres (50 microns in diameter) at the level of the transducer. Tissueswere analyzed for radioactive microspheres as prevlously described (9,10). Our experiments indicated that approximately 90% (mean 91%, range 85 to 98°/c) of distal aortic blood flow measured by electromagnetic flowrneter was distributed to the placenta. The degree of variability depended on the completeness of the surgical ligation of small distal branches of the aorta. Umbilical blood flow (~ umb) in each experiment was then calculated as follows: 6 umb = C~ distal aorta x
CPM placenta ...... C'PMfetus + CPMplacenta
Fetal arterial pressure measurements and arterlal blood samples were obtained by cannulatlng the fetal aorta via the left renal artery with a polyvlnyl catheter. Continuous measurements of fetal arterial and umbilical venous, as well as of maternal femoral arterial pressures, were obtained with Sanborn pressure transducers and recorded on a Hewlett-Packard 7788A multichannel direct writing recorder. Fetal heart rate was obtained by a heart rate computer from a standard limb lead fetal ECG or directly from the arterial pulse tracing. Placental
544
MARCH 25, 1974
VOL. 5 NO. 6
PROSTAGI~ANDINS
Fig. I. Excised distal aorta of the fetal lamb. The left renal artery has been cannulated. The external iliac arteries and the lumbar arteries have been ligated. The umbilical arteries, two small hypogastric branches, and a middle sacral vessel remain patent. (umbillcal) vascular resistance was calculated as placental perfusion pressure (mean fetal arterial pressure minus umbilical venous pressure [mm Hg] divided by umbilical blood flow ~nl/min/kg of fetus1). Amniotic fluid pressure was measured |ndependently. Zero reference levels for all fetal pressure transducers were placed at the midpoint of the uterus. Fetal and maternal arterial blood gases and pH were analyzed wlth Instrumentation Laboratory's microelectrodes and pH gas analyzer model 114. After completlon of the surgical procedure, minimum concentratlons of halothane (less than 1%) were maintained to keep the maternal and fetal preparations in a steady state. A control period was established during which fetal and maternal cardiorespiratory parameters were unchangedfor at least 10 minutes. Thereafter we injected intraaortically into the fetus solutions of PGF2~THAM (Upjohn) 100 to 300 l.g/kg of fetus, or PGE2, t to 100 pg/kg, and as a basis for comparison, noreplnephrlne (Levophed bltartrate, Winthrop, dose in terms of base) 1 to 2 pg,/kg. All injections, including those of vehicle control solutions, were made in small volumes (0.5 to 1.5 ml). Prostaglandlns E2 and F~,-THAM were stored in liquid nitrogen in powdered Form or dissolved in 95% alcohol (PGE2, t mg/ml) or in saline (PGF~-THAM 1 mg/ml). On the days of the experiments, the stock solutions were stored in the freezer and diluted with saline before fetal iniectlons.
MARCH 25, 1974
V O L 5 NO. 6
545
©
b~
¢'b
b-
48:L 1.4 59~ 3.3 <0.05
44 + 1.3 58+ 1.3 <0.001
Control PGE 2 P
Control Norepinephrine P
9 + 1.4 12+ 1.7 <0.0,1
11 + 0.9 11 :k 1.1 n.s.
9 + 0.9 12+ 1.1 <0.001
Umbilical venous pressure mm Hg
129 ± 14.5 170+ 20.0 <0.001
131 + 18.5 67+ 15.3 <0.01
132+ 19.1 170+ 19.0 <0.01
Umbilical blood flow ml/kg/mln
0.144 + 0.015 0 . 1 4 3 + 0.014 n.s.
0.158+ 0.018 0.659+ 0.195 <0.05
0 . 1 5 0 + 0.018 0.168+ 0.020 <0.05
Umbilical vascular resistance mm Hg/ml/kg/mln
181 :k 14.5 187:k 7.4 n.s.
184+ 10.6 168:~ 14.6 n.s.
191 + 10.2 184+ 8.5 n.s.
Heart rate beats/min
PGF~, 100 - 300 ~ / k g (n = 7); PGE2, 20 - 100 ~ / k g (n = 8); norepinephrlne, 1 - 2 ~/kg-~'n = 8). Hemodynamic parameters were analyzed at the time of maximal change in mean arterial pressure. All data represent means ~- SEM.
44:~ 1.2 64:~ 1.5 <0.001
Control PGF2e P m
Arterial Pressure mm Hg
Fetal Hemodynamic Responses to PGF2~, PGE2r and Noreplnephrine
TABLE I
C~
©
PROSTAGLANDINS
In most experiments, the effects of prostaglandins and noreplnephrine were compared in the same fetal preparations but the order in which the substances were tested was alternated. Sufficient time was allowed between iniections for the pressures and flows to return to control values. Control values for fetal hemodynamic parameters were taken as the mean and the standard deviations at 1-minute intervals in the 5 minutes preceding the experimental iniectlon of drug. The fetal hemodynamic responses after iniection of prostaglandins were analyzed at 1-minute intervals and also at the time of maximal deviation from the control period for each parameter. Means, standard deviations, and Student's t-tests for paired comparisons were calculated on an Olivetti Programma calculat~ with standard programs. RESULTS The effects of PGF~c~ PGE2, and norepinephrine on fetal arterial pressure, umbilical blood flow, fetal heart rate, and umbilical vascular resistance are shown in Table I and in Figure 2. %
ARTERIAL
160 [
PRESSURE
140 I
PGF~o~
PGEz
t20I_
140 r
I
L~BILICAL
120 I i
6O L 120 r HEART RATE I00 ~
~
............
I
80 t. 180 [ UMBILICAL 160 VASCULAR RESISTANCE I40
I
120 k
TIME-minutes
Fig. 2. Mean effects of PGF2~(100-300 pg/kg) and PGE2 (20-75 1.g/kg) expressed as percentages of initial control values on arterial pressure, umbilical blood flow, heart rate, and umbilical vascular resistance in fetal lambs. The symbol on left of each parameter represents ~: 2 S.D. of 1-minute time intervals during a 5-minute control period. M A R C H 25, 1974
VOL. 5 NO. 6
547
PROSTAGLANDINS
PGF2~in doses of 100 to 300 pg/kg injected into the distal fetal aorta resulted in an average rise in the mean fetal arterial pressure and umbilical blood flow o~ 48% and 38°~ respectively. PGF 2 caused a small (11%) but statistically significant (P <0.05) increase in umbillca~" vascular resistance (Fig. 2; Table I). In all exper~ents, maximal changes in fetal arterial pressure and umbilical blood flow occurred about 2 minutes after the injection of PGF2c,and were followed by a gradual decline to control levels within 15 minutes after injection. A representative tracing of the initial blood pressure and umbilical blood flow response to PGF2c~ is shown in Figure 3. In the dose ranges studied, PGF2~ had no significant effect on fetal heart rate but it tended to increase slightly. There was a small but significant elevation of umbilical venous pressure with PGF2c~
FCR-S-0 8O
o
0
I1 t
PGF,o, 130h~g/Kg
Fig. 3. Effects of PGF2cv(130 ~ / k g ) injected into the distal aorta on arterial pressui'e, umbilical blood flow, and umbilical venous pressure of a 2.3 kg fetal lamb. The iniectlon of PGE~ into the distal aorta of the fetus evoked hemodynamic effects which were qualitatively and quantitatively different from the effects of PGF2~(FIg. 2; Fig. 4). In all experiments, PGE2 caused a transitory decrease (wlth~n 20 seconds) in fetal arterial pressure which was followed by a prolonged rise. These effects were not produced by the iniectlon of alcohol vehicle alone. The blphasic changes in fetal arterial pressure were accompanied by an immediate decrease in umbilical blood flow which was sustained for 8 to 10 minutes (Fig. 2).
548
MARCH 25, 1974
VOL, 5 NO. 6
PROSTAGLA.NDINS
.
80
~_
--
.
iii
.
.
iiii~i ~
.
::'frill! ! ~!IIT~
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
2o 0
200 E ~ ~ l l i h t i i !
li!!it:.
:j
o ~-i!;~Li~
z~
FCR-S-8
~ i
:L
m
ii,
o t
Fig. 4.
PGE~90 #g/Kg
Effects of PGE 2 (90 Fg/kg) iniected into the distal aorta on arterial pressure, umbilical blood flow, and umbilical venous pressure of a 2.3 kg fetal lamb.
There were no statistically significant changes in fetal heart rate although we noted a bradycardla after PGE2 iniections which tended to return to control values as umbilical blood flow recovered.
Iniectlons of 20 to 50 pg/kg of PGE2 produced no significant alterations in umbilical venous pressure, and as a consequence we calculated an 80°/~ increase in umbilical vascular resistance at the time of maximal decrease in umbilical blood flow. W~thln 10 minutes after these iniectlons , the increase in vascular resistance gradually declined to 40% above control values. The curvilinear nature of the . relation between the dose of PGE2 and its effect on umbilical vascular resistance is shown in Figure 5. Repeated injections in the same animal led to tachyphylaxis. In the event of placental vasoconstriction and hind quarter vasodilatatlon, our measurements of umbilical blood flow after injection of PGE 2 would be overestimated, which would result in a calculated placental vascular resistance lower than the actual value. However, the magnitude of such an error is likely to be small in view of the nearly complete llgatlon of the distal aortic branches. Table I and Figure 6 present data on the hemodynamlc effects of noreplnephrlne injected into the distaJfetal aorta. In doses of 1 to 2 Fg/kg, noreplnephrlne did not produce a significant increase in the umbillcat vascular resistance, an indication that, unlike the prostaglandlns, norepinephrine had no significant vaso-
MARCH 25, 1974 VOL. 5 NO. 6
549
PROSTAGLANDINS
I000
900
800
0
¢= o
700
r, >o
600
500
o
400
300
200
IOO
o
,o 20 ~o
~
4'o so go PGE2
I
?o 80
I
90
I
,oo
pg/kg
Fig. 5. Relationship between the dose of PGER iniected into the distal fetal aorta and the increase in placental vascular resistance expressed as percentages of initial control values (control is 100°/c). The curve of best fit is derived from pooled data obtained from 8 individual animals. constrictor effect on fetal placental vessels in the doses studied. In two experiments, the effects of noreplnephrlne on fetal hemodynamics were compared with those of PGF2crand PGE2 after the administration of 3 to 10 t-g of the alpha-adrenerglc blocker dibenamlne. The adequacy of the blockade was established by the absence of changes in fetal heart rate or blood pressure after graded injections of noreplnephrine. Subsequent injections of PGE2 (50 i.g/kg) and PGF~, (200 pg/kg) elicited the same hemodynamlc responses in the fetuses with adrenergic blockade as had been noted without blockade.
550
MARCH 25, 1974
VOL. 5 NO. 6
PROSTAGLANDINS FCR-S-IO
J
i
60
sec
ii:t:
l:xJ~;l:i!|i[~
;I
t Fig. 6.
!?[
I
li't
NOREPINEPHRIN[ 2#g/Kg
Effects of norepinephrine (2 pg/kg) iniected into the distal aorta on arterial pressure, umbilical blood flow, and umbilical venous pressure of a 2.5 kg fetal lamb.
In the control periods before injection of prostaglandins, the mean fetal arterial pH was 7 . 2 9 ± 0.06 S.D., and the mean Po2 was 21 t: 3.0ram Hg S.D. There was a tendency for fetal arterial Po2 to fall coincident with the large and sometimes prolonged reductions in umbilical blood flow elicited by PGE2, but we obtained no systematic data to indicate the effects of prostaglandins on fetal oxygenation per se. Judging by the intraamniotic fluid pressure, prostaglandins injected i n t o ~ - f ~ t a t umbilical circulation had no observable effect on maternal uterine tonus. The mean amnlotic fluid pressure was 4.0 :k 1.4 mm Hg S.D. before prostaglandins and 3.5:E 1.2 mm Hg S.D. after prostaglandln injection. DISCUSSION As far as we know, thls is the first study to use a direct method for measuring umbilical blood flow which demonstrates in the intact fetus the effect of prostaglandins on placental vascular resistance. The most striking results of thls investigation were that intraaortic injections of PGE 2 increased fetal arterial pressure, decreased umbilical blood flow, and exerted a profound active vasoconstrictor effect on the placental vascular bed. PGF2r¢ produced a larger elevation in fetal arterial pressure, but this was associated wlth a nearly parallel increase in umbilical blood flow and only a small increase in placental vascular resistance.
MARCH 25, 1974
VOL. 5 NO. 6
551
PROSTAGLANDINS
The effect of PGF~ in increasing both fetal arterial pressure and (to a nearly equivalent degree) i~lacental blood flow suggests either systemic vasoconstrlction or an increase in cardlac output as a major feature of the fetal pressor response to this substance. Our results are in keeping with the finding that PGF compounds have a moderate hypertensive and peripheral vasoconstrictor action in adult animals of several species (11). However, Assali and coworkers (12) observed no effect on fetal blood pressure after the intravenous iniectlon of 0.5 to 100 pg/kg of PGF~in an exteriorized fetal lamb. The apparent discrepancy between these find.'~ngs and ours may be due to the known varlability in the dose responses of individual animals to prostaglandlns or to tachyphylaxls after repeated injections of PGFz~In the experiment of Assail. Furthermore, differences in the route of admlnlstration to the fetus of prostaglandins will influence the effective dose reaching the placental bed. The failure of adrenergic blockade to alter significantly the fetal response to PGF2c~in our study indicates that the vasoactive effects of this compound in the fetus are not mediated through stimulation of alphaadrenergic receptors. This is consistent with reported results in adult animals (13). The finding that PGE2 exerts a more potent vasoconstrictor effect on the fetal placental circulation than does PGF~, contrasts with the in vitro observations of others that both prostaglandlns were eqdally reactive in con'~'rsc'ET~'gisolated human and sheep umbilical artery preparations (3,14). Our results, however, presumably reflect the effect of prostaglandlns on the small resistance vessels of the placenta more than they do that of an effect on the umbilical cord vessels per se. Since it is known from studles on other vascular beds (e.g., the c o r o n a r y - ~ c u l a t i o n ) that the large and the small arteries sometimes react differently to smooth muscle agonists (t5), a comparison of our work with the studies of major umbilical arteries may not be appropriate. The fetal hemodynamlc effects of PGE2 and PGF2c~ injected into the distal aorta differed in several other respects. We noted an in[tlal decrease in fetal arterial pressure after the injection of PGE2 but not after PGF2,# noreplnephrine, or vehicle injections alone. This transient vasodepressor effect of PGE2 was maximal within 20 seconds of iniectlon and was then followed by the evolution of the vasopressor effect. The mechanism of the blphaslc effect of PGE2 on fetal arterial pressure is not entirely clear. We interpret the initial decrease in fetal arterial pressure as resulting from systemic vasodilatation by PGE2since circulation tlmes are rapid in the fetus (16) and because we calculate a slight increase in placental vascular resistance even in the first 30 seconds after PGE2 injection. PGE compounds are known to be systemic vasodilators in different species (17) and to have a hypotensive effect when injected into the ascending aorta of the mature fetal lamb (18) but at the same time to cause constriction of the blood vessels of the nasal mucosa of dogs (19) and the isolated aorta and coronary arteries of rabbits (20). However, in the absence of measurements of regional blood flow, we can draw no clear conclusion about systemic vasodilatation in the fetus beyond the first minute after PGE2 injectlon. Since the placenta receives nearly 50% of the fetal cardiac output (6), placental vasoconstriction may obscure the effect of systemic vasodilatatlon on fetal arterial pressure. The biphasic nature of the PGE2 effect on fetal arterial pressure suggests the possibillty that development of placental vasoconstriction has a longer latency than does systemic vasodilatatlon and raises the question as to whether or not the
552
MARCH 25, 1974
VOL. 5 NO. 6
PROSTAGLANDINS
pressor effect of PGE2 is mediated at least in part by an indirect rather than direct mechanism of action. Hemodynamic responses to PGE2 were essentially unaltered after adrenerglc blockade, as was the case for PGF~, indicating that placental vasoconstriction by PGE2 is not mediated by alpha-a-drenergic receptors. The lack of an effect of noreplnephrlne on placental vascular resistance further supports this conclusion. However, a profound reduction in umbilical blood flow (as produced by larger doses of PGE2) may lead to fetal hypoxia and catecholamlne release and thereby contribute to the elevation in fetal arterial pressure. Although the dosesof prostaglandins used in this study are considerably higher than levels assayed in maternal or fetal plasma at birth (21), it is known that E and F prostaglandlns are present in high concentrations in the umbilical cord (4) and that prostaglandlns are reJeased in tissues by mechanical and pharmacologic stimuli (22,23). Therefore it is reasonable to speculate that prostaglandlns released locally in placental vessels may produce circulatory effects similar to those demonstrated in this study. Recent evidence indicates that PGF compounds contract the ductus arteriosus of the fetal calf in vitro (24), that PGE compounds dilate the pulmonary vascular bed (25), and thaTth~-Fprostaglandlns have effects on the ductus arteriosus which vary with the concentration of oxygen in the environment (26). Our data taken together with the observations of others suggest that prostaglandins may play a role in the circulatory adaptations of the fetus at birth, perhaps in conjunction with the peptide bradyklnin (27). Likewise, prostaglandins may have hemodynamic consequences in the fetus after maternal infusions or upon direct administration into the amnlotlc cavity. Beazley and coworkers (5) have shown placental transfer of PGF2~ or its radioactive metabolites during human pregnancy. Similar data are lacking with respect to PGE2 nor is it certain that prostaglandlns cross the placenta in a vasoactive form. High concentrations of 15-hydroxyprostaglandin dehydrogenase in the human placenta may protect the fetus from circulating prostaglandlns (28). Moreover, prostaglandln degradation by lung and liver has been demonstrated in fetuses near term (18). On the other hand, PGE2 administered directly into the amniotic cavity (as for induction of labor or abortion) could have a direct effect on placental vessels and thus a potentially adverse influence upon fetal oxygenation or fetoplacental endocrine function. REFERENC ES 1. Euler, U. S. von. Action of adrenaline, acetylchollne and other substances on nerve-free vessels (human placenta). J. Physiol. Lond. 93:129, 1938. 2. Hilller, K., and S. M. Mo Karim. Effects of prostaglandins El, E2, FI~ , F2c~ on isolated human umbilical and placental blood vessels. J. Obstet. Gynaec. Brit. Cwlth. 75:667, 1968. . Dyer, D. C. Comparison of the constricting actions produced by serotonin and prostaglandins on isolated sheep umbilical arteries and veins. Gynec. Invest. 1:204, 1970. 4. Karim, S. M. M. The identification of prostaglandlns in human umbillcal cord. Brit. J. Pharmacol. Chemother. 29:230, 1972.
MARCH 25, 1974
VOL. 5 NO, 6
553
PROSTAGLANDINS
. Beazley, J. M . , H. C. Brummer, and A. Kuriak. Distribution of 9-H 3prostaglandln F2~in pregnant and non-pregnant subjects. J. Obstet. Gynaec. Brit. Cwlth. 79:800, 1972. 6. Dawes, G. S, Umbilical blood flow, in Respiratory Gas Exchange and Blood Flow in the Placenta (L. D. Longo and H. Bartels, Editors), U. S. Department of Health, Education and Welfare, Bethesda, Md., 1972, p. 107. 7. Meschia, G., J. R. Cotter, C. S. Breathnach, and D. H. Barron. The hemoglobin, oxygen, carbon dioxide and hydrogen ion concentrations in the umbilical bloods of sheep and goats as sampled via indwelling plastic catheters. Quart. J. Exp. Physiol. 50:185, 1965. .
Jackson, B. T., and R. H. Egdahl. The performance of complex fetal operations in utero without amnlotic fluid toss or other disturbances of fetal-maternal relationships. Surgery48:564, 1960.
9. Rudolph, A. M . , and M. A. Heymann. The circulation of the fetus in utero. Methods for studying distribution of blood flow, cardiac output and organ blood flow. Circ. Res. 21:163, 1967. 10. Novy, M. J., and M. J. Cook. Redistribution of blood flow by prostaglandin F~,in the rabbit ovary. Am. J. Obstet. Gynec. 117:381, t973. 11. Nakano, J., and B. Cole. Effects of prostaglandlns E1 and F2~on systemic, pulmonary, and splanchnlc circulation in dogs. ,am. J. Physiol. 217:222, 1969. 12. Oakes, G., M. Mofld, C. R. Brlnkman III, and N. S. ~sali. Insensitivity of the sheep to prostaglandlns. Proc. Soc. Exp. Biol. Medo 142:194, 1973. 13. White, R. P., J. A. Heaton, and I. C. Denton. Pharmacological comparison of prostaglandln F2c~, serotonin and noreplnephrine on cerebrovascular tone of monkey. Europ. J. Pharmacol. 15:300, 1971. 14. Altura, B. M . , D. Malaviya, C. F. Reich, and L. R. Orkln. Effects of vasoactlve agents on isolated human umbilical arteries and veins. Am. J. Physiol. 222:345, 1972. 15. Zuberbuhler, R. C., and D. F. Bohr. Responsesof coronary smooth muscle to catecholamines. Circ. Res. 16:431, 1965. 16. Novy, M. J., and J. Metcalfe. Measurements of umbilical blood flow and vascular volume by dye dilution. Am. J. Obstet. Gynec. 106:899, 1970. 17. Nakano, J. Cardiovascular actions, in The Prostaglandlns, Vol. 1 (P. W. Ramwell, Editor), Plenum Press, New York, 1973, p. 239. 18. Olley, P. M . , F. Coceanl, and G. Kent. Inactivation of prostaglandin El by lungs of the foetal lamb. Experientla, in press.
554
MARCH 25, 1974
VOL. 5 NO. 6
PROSTAGLANDINS
19. Jackson, B. T., and R. Stovall. Vasoconstriction of nasal blood vessels induced by prostaglandins, in Prostaglandln Symposium of the Worcester Foundation for Experimental Biology (P. W. Ramwell and J. E. Shaw, Editors), Interscience Publishers, New York, 1968, pp. 329-334. 20. Strong, C. G., and D. V. Bohr. Effects of prostaglandins El, E2, A1, and FI~ on isolated vascular smooth muscle. Am. J. Physiol. 213:724, 1967. 21. Craft, I. Lo, R. Scrivener, and C. J. Dewhurst. Prostaglandin F2~ levels in the maternal and fetal circulations in late pregnancy. J. Obstet. Gynaec. Brit. Cwlth. 80:616, 1973. 22. Gustavll, B. Labour: a delayed menstruation ? Lancet 2:1149, 1972. 23. McGiff, J. C., N. A. Terragno, K. U. Malik, and A. J. Lonlgro. Release of a prostaglandln E-like substance from canine kidney by bradyklnln. Circ. Res. 31:36, 1972. 24. Elllott, R. B., and M. B. Starling. The effect of prostaglandin F2c~ in the closure of the ductus arterlosus. Prostaglandlns 2:399, 1972.
25. Hauge, A., P. K. M. Lunde, and B. A. Waaler. Effects of prostaglandln El and adrenaline on the pulmonary vascular resistance (PVR) in isolated rabbit lungs. Life Scl. 6:673, 1967. 26. Coceanl, F., and Po M. Olley. The response of the ductus arterlosus to prostaglandlns. Canad. J. Physiol. Pharmacol. 51:220, 1973. 27. Melmon, K. L., M. J. Cline, T. Hughes, and A. S. Nies. Kinlns: possible mediators of neonatal circulatory changes in man. J. Clln. Invest. 47:1295, 1968. 28. Jarabak, J. Human placental 15-hydroxyprostaglandln dehydrogenase. Proc. Nat. Acad. Sci. USA69:533, 1972.
M A R C H 25, 1974
VOL. 5 NO. 6
555
Address reprint requests to: Dr. M. J. Navy, Oregon Regional Primate Research Center, Beaverton, Oregon 97005.
Accepted February 6, 1974. PROSTAGLANDINS MARCH 25, 1974
VOL. 5 NO. 6
543
PROSTAGLANDINS
influence its circulation and oxygenation (5). Reportsof the effects of vasoactive substances on isolated cord vessels, as noted above, may not be relevant to in vivo circumstances because of differing experimental conditions and becaus~ihe um-~-~llcal artery and vein in vivo contribute less than 25% of the total placental vascular resistance (6). Since the effects of prostaglandins on the umbilical circulation in the intact fetus have not previously been reported, we studied the hemodynamlc effects of PGE2 and PGF2e~administered to the fetal lamb in utero and paid special attention to the measurement of umbilical blood flow and b"m'~t'~al vascular resistance. MATERIALS AND METHODS Nine pregnant ewes of mixed breed were studied at 115 to 135 days gestatlon (term, 150 days). Anesthesia was induced with intravenous sodium thiamylal 15 mg/kg. After tracheal intubation, a semiclosed rebreathing system with halothane and oxyg en was used for anesthesia. After the maternal abdomen had been opened with a midline incision, medlcal-grade polyvinyt catheters (Bolab, Reading, Mass.), 0.034 inch I.D. x 0.048 inch O . D . , were inserted into a major umbilical vein by retrograde cannulatlon from a peripheral placental cotyledon (7). To avoid the problem of spasm of the umbilical vessels during dlrect appllcation of electromagnetic flow transducers to the cord, we used a new approach to measure umbilical blood flow in the unexteriorized sheep fetus. A marsupializing incision was performed in the left lumbar region of the fetus according to the general principles described by Jackson and Egdahl (8). The dissection was carried retroperitoneally to the distal abdominal aorta of the fetus. The external lilac arteries were llgated as were all other accessible distal aortic branches. In most cases only the umbilical arteries, the small middle sacral, and two small hypogastrlc branches were left intact (Fig. 1). A cuffed extraluminal flow transducer was placed around the fetal aorta at the iliac bifurcation, and a Statham model 0-5000 nonoccluslve zero electromagnetic flowmeter was used to measure blood flow. Actual placental blood flow was determined as a percent of distal aortic blood flow by intraaortic injection of nuclide-labeled microspheres (50 microns in diameter) at the level of the transducer. Tissueswere analyzed for radioactive microspheres as prevlously described (9,10). Our experiments indicated that approximately 90% (mean 91%, range 85 to 98°/c) of distal aortic blood flow measured by electromagnetic flowrneter was distributed to the placenta. The degree of variability depended on the completeness of the surgical ligation of small distal branches of the aorta. Umbilical blood flow (~ umb) in each experiment was then calculated as follows: 6 umb = C~ distal aorta x
CPM placenta ...... C'PMfetus + CPMplacenta
Fetal arterial pressure measurements and arterlal blood samples were obtained by cannulatlng the fetal aorta via the left renal artery with a polyvlnyl catheter. Continuous measurements of fetal arterial and umbilical venous, as well as of maternal femoral arterial pressures, were obtained with Sanborn pressure transducers and recorded on a Hewlett-Packard 7788A multichannel direct writing recorder. Fetal heart rate was obtained by a heart rate computer from a standard limb lead fetal ECG or directly from the arterial pulse tracing. Placental
544
MARCH 25, 1974
VOL. 5 NO. 6
PROSTAGI~ANDINS
Fig. I. Excised distal aorta of the fetal lamb. The left renal artery has been cannulated. The external iliac arteries and the lumbar arteries have been ligated. The umbilical arteries, two small hypogastric branches, and a middle sacral vessel remain patent. (umbillcal) vascular resistance was calculated as placental perfusion pressure (mean fetal arterial pressure minus umbilical venous pressure [mm Hg] divided by umbilical blood flow ~nl/min/kg of fetus1). Amniotic fluid pressure was measured |ndependently. Zero reference levels for all fetal pressure transducers were placed at the midpoint of the uterus. Fetal and maternal arterial blood gases and pH were analyzed wlth Instrumentation Laboratory's microelectrodes and pH gas analyzer model 114. After completlon of the surgical procedure, minimum concentratlons of halothane (less than 1%) were maintained to keep the maternal and fetal preparations in a steady state. A control period was established during which fetal and maternal cardiorespiratory parameters were unchangedfor at least 10 minutes. Thereafter we injected intraaortically into the fetus solutions of PGF2~THAM (Upjohn) 100 to 300 l.g/kg of fetus, or PGE2, t to 100 pg/kg, and as a basis for comparison, noreplnephrlne (Levophed bltartrate, Winthrop, dose in terms of base) 1 to 2 pg,/kg. All injections, including those of vehicle control solutions, were made in small volumes (0.5 to 1.5 ml). Prostaglandlns E2 and F~,-THAM were stored in liquid nitrogen in powdered Form or dissolved in 95% alcohol (PGE2, t mg/ml) or in saline (PGF~-THAM 1 mg/ml). On the days of the experiments, the stock solutions were stored in the freezer and diluted with saline before fetal iniectlons.
MARCH 25, 1974
V O L 5 NO. 6
545
©
b~
¢'b
b-
48:L 1.4 59~ 3.3 <0.05
44 + 1.3 58+ 1.3 <0.001
Control PGE 2 P
Control Norepinephrine P
9 + 1.4 12+ 1.7 <0.0,1
11 + 0.9 11 :k 1.1 n.s.
9 + 0.9 12+ 1.1 <0.001
Umbilical venous pressure mm Hg
129 ± 14.5 170+ 20.0 <0.001
131 + 18.5 67+ 15.3 <0.01
132+ 19.1 170+ 19.0 <0.01
Umbilical blood flow ml/kg/mln
0.144 + 0.015 0 . 1 4 3 + 0.014 n.s.
0.158+ 0.018 0.659+ 0.195 <0.05
0 . 1 5 0 + 0.018 0.168+ 0.020 <0.05
Umbilical vascular resistance mm Hg/ml/kg/mln
181 :k 14.5 187:k 7.4 n.s.
184+ 10.6 168:~ 14.6 n.s.
191 + 10.2 184+ 8.5 n.s.
Heart rate beats/min
PGF~, 100 - 300 ~ / k g (n = 7); PGE2, 20 - 100 ~ / k g (n = 8); norepinephrlne, 1 - 2 ~/kg-~'n = 8). Hemodynamic parameters were analyzed at the time of maximal change in mean arterial pressure. All data represent means ~- SEM.
44:~ 1.2 64:~ 1.5 <0.001
Control PGF2e P m
Arterial Pressure mm Hg
Fetal Hemodynamic Responses to PGF2~, PGE2r and Noreplnephrine
TABLE I
C~
©
PROSTAGLANDINS
In most experiments, the effects of prostaglandins and noreplnephrine were compared in the same fetal preparations but the order in which the substances were tested was alternated. Sufficient time was allowed between iniections for the pressures and flows to return to control values. Control values for fetal hemodynamic parameters were taken as the mean and the standard deviations at 1-minute intervals in the 5 minutes preceding the experimental iniectlon of drug. The fetal hemodynamic responses after iniection of prostaglandins were analyzed at 1-minute intervals and also at the time of maximal deviation from the control period for each parameter. Means, standard deviations, and Student's t-tests for paired comparisons were calculated on an Olivetti Programma calculat~ with standard programs. RESULTS The effects of PGF~c~ PGE2, and norepinephrine on fetal arterial pressure, umbilical blood flow, fetal heart rate, and umbilical vascular resistance are shown in Table I and in Figure 2. %
ARTERIAL
160 [
PRESSURE
140 I
PGF~o~
PGEz
t20I_
140 r
I
L~BILICAL
120 I i
6O L 120 r HEART RATE I00 ~
~
............
I
80 t. 180 [ UMBILICAL 160 VASCULAR RESISTANCE I40
I
120 k
TIME-minutes
Fig. 2. Mean effects of PGF2~(100-300 pg/kg) and PGE2 (20-75 1.g/kg) expressed as percentages of initial control values on arterial pressure, umbilical blood flow, heart rate, and umbilical vascular resistance in fetal lambs. The symbol on left of each parameter represents ~: 2 S.D. of 1-minute time intervals during a 5-minute control period. M A R C H 25, 1974
VOL. 5 NO. 6
547
PROSTAGLANDINS
PGF2~in doses of 100 to 300 pg/kg injected into the distal fetal aorta resulted in an average rise in the mean fetal arterial pressure and umbilical blood flow o~ 48% and 38°~ respectively. PGF 2 caused a small (11%) but statistically significant (P <0.05) increase in umbillca~" vascular resistance (Fig. 2; Table I). In all exper~ents, maximal changes in fetal arterial pressure and umbilical blood flow occurred about 2 minutes after the injection of PGF2c,and were followed by a gradual decline to control levels within 15 minutes after injection. A representative tracing of the initial blood pressure and umbilical blood flow response to PGF2c~ is shown in Figure 3. In the dose ranges studied, PGF2~ had no significant effect on fetal heart rate but it tended to increase slightly. There was a small but significant elevation of umbilical venous pressure with PGF2c~
FCR-S-0 8O
o
0
I1 t
PGF,o, 130h~g/Kg
Fig. 3. Effects of PGF2cv(130 ~ / k g ) injected into the distal aorta on arterial pressui'e, umbilical blood flow, and umbilical venous pressure of a 2.3 kg fetal lamb. The iniectlon of PGE~ into the distal aorta of the fetus evoked hemodynamic effects which were qualitatively and quantitatively different from the effects of PGF2~(FIg. 2; Fig. 4). In all experiments, PGE2 caused a transitory decrease (wlth~n 20 seconds) in fetal arterial pressure which was followed by a prolonged rise. These effects were not produced by the iniectlon of alcohol vehicle alone. The blphasic changes in fetal arterial pressure were accompanied by an immediate decrease in umbilical blood flow which was sustained for 8 to 10 minutes (Fig. 2).
548
MARCH 25, 1974
VOL, 5 NO. 6
PROSTAGLA.NDINS
.
80
~_
--
.
iii
.
.
iiii~i ~
.
::'frill! ! ~!IIT~
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
2o 0
200 E ~ ~ l l i h t i i !
li!!it:.
:j
o ~-i!;~Li~
z~
FCR-S-8
~ i
:L
m
ii,
o t
Fig. 4.
PGE~90 #g/Kg
Effects of PGE 2 (90 Fg/kg) iniected into the distal aorta on arterial pressure, umbilical blood flow, and umbilical venous pressure of a 2.3 kg fetal lamb.
There were no statistically significant changes in fetal heart rate although we noted a bradycardla after PGE2 iniections which tended to return to control values as umbilical blood flow recovered.
Iniectlons of 20 to 50 pg/kg of PGE2 produced no significant alterations in umbilical venous pressure, and as a consequence we calculated an 80°/~ increase in umbilical vascular resistance at the time of maximal decrease in umbilical blood flow. W~thln 10 minutes after these iniectlons , the increase in vascular resistance gradually declined to 40% above control values. The curvilinear nature of the . relation between the dose of PGE2 and its effect on umbilical vascular resistance is shown in Figure 5. Repeated injections in the same animal led to tachyphylaxis. In the event of placental vasoconstriction and hind quarter vasodilatatlon, our measurements of umbilical blood flow after injection of PGE 2 would be overestimated, which would result in a calculated placental vascular resistance lower than the actual value. However, the magnitude of such an error is likely to be small in view of the nearly complete llgatlon of the distal aortic branches. Table I and Figure 6 present data on the hemodynamlc effects of noreplnephrlne injected into the distaJfetal aorta. In doses of 1 to 2 Fg/kg, noreplnephrlne did not produce a significant increase in the umbillcat vascular resistance, an indication that, unlike the prostaglandlns, norepinephrine had no significant vaso-
MARCH 25, 1974 VOL. 5 NO. 6
549
PROSTAGLANDINS
I000
900
800
0
¢= o
700
r, >o
600
500
o
400
300
200
IOO
o
,o 20 ~o
~
4'o so go PGE2
I
?o 80
I
90
I
,oo
pg/kg
Fig. 5. Relationship between the dose of PGER iniected into the distal fetal aorta and the increase in placental vascular resistance expressed as percentages of initial control values (control is 100°/c). The curve of best fit is derived from pooled data obtained from 8 individual animals. constrictor effect on fetal placental vessels in the doses studied. In two experiments, the effects of noreplnephrlne on fetal hemodynamics were compared with those of PGF2crand PGE2 after the administration of 3 to 10 t-g of the alpha-adrenerglc blocker dibenamlne. The adequacy of the blockade was established by the absence of changes in fetal heart rate or blood pressure after graded injections of noreplnephrine. Subsequent injections of PGE2 (50 i.g/kg) and PGF~, (200 pg/kg) elicited the same hemodynamlc responses in the fetuses with adrenergic blockade as had been noted without blockade.
550
MARCH 25, 1974
VOL. 5 NO. 6
PROSTAGLANDINS FCR-S-IO
J
i
60
sec
ii:t:
l:xJ~;l:i!|i[~
;I
t Fig. 6.
!?[
I
li't
NOREPINEPHRIN[ 2#g/Kg
Effects of norepinephrine (2 pg/kg) iniected into the distal aorta on arterial pressure, umbilical blood flow, and umbilical venous pressure of a 2.5 kg fetal lamb.
In the control periods before injection of prostaglandins, the mean fetal arterial pH was 7 . 2 9 ± 0.06 S.D., and the mean Po2 was 21 t: 3.0ram Hg S.D. There was a tendency for fetal arterial Po2 to fall coincident with the large and sometimes prolonged reductions in umbilical blood flow elicited by PGE2, but we obtained no systematic data to indicate the effects of prostaglandins on fetal oxygenation per se. Judging by the intraamniotic fluid pressure, prostaglandins injected i n t o ~ - f ~ t a t umbilical circulation had no observable effect on maternal uterine tonus. The mean amnlotic fluid pressure was 4.0 :k 1.4 mm Hg S.D. before prostaglandins and 3.5:E 1.2 mm Hg S.D. after prostaglandln injection. DISCUSSION As far as we know, thls is the first study to use a direct method for measuring umbilical blood flow which demonstrates in the intact fetus the effect of prostaglandins on placental vascular resistance. The most striking results of thls investigation were that intraaortic injections of PGE 2 increased fetal arterial pressure, decreased umbilical blood flow, and exerted a profound active vasoconstrictor effect on the placental vascular bed. PGF2r¢ produced a larger elevation in fetal arterial pressure, but this was associated wlth a nearly parallel increase in umbilical blood flow and only a small increase in placental vascular resistance.
MARCH 25, 1974
VOL. 5 NO. 6
551
PROSTAGLANDINS
The effect of PGF~ in increasing both fetal arterial pressure and (to a nearly equivalent degree) i~lacental blood flow suggests either systemic vasoconstrlction or an increase in cardlac output as a major feature of the fetal pressor response to this substance. Our results are in keeping with the finding that PGF compounds have a moderate hypertensive and peripheral vasoconstrictor action in adult animals of several species (11). However, Assali and coworkers (12) observed no effect on fetal blood pressure after the intravenous iniectlon of 0.5 to 100 pg/kg of PGF~in an exteriorized fetal lamb. The apparent discrepancy between these find.'~ngs and ours may be due to the known varlability in the dose responses of individual animals to prostaglandlns or to tachyphylaxls after repeated injections of PGFz~In the experiment of Assail. Furthermore, differences in the route of admlnlstration to the fetus of prostaglandins will influence the effective dose reaching the placental bed. The failure of adrenergic blockade to alter significantly the fetal response to PGF2c~in our study indicates that the vasoactive effects of this compound in the fetus are not mediated through stimulation of alphaadrenergic receptors. This is consistent with reported results in adult animals (13). The finding that PGE2 exerts a more potent vasoconstrictor effect on the fetal placental circulation than does PGF~, contrasts with the in vitro observations of others that both prostaglandlns were eqdally reactive in con'~'rsc'ET~'gisolated human and sheep umbilical artery preparations (3,14). Our results, however, presumably reflect the effect of prostaglandlns on the small resistance vessels of the placenta more than they do that of an effect on the umbilical cord vessels per se. Since it is known from studles on other vascular beds (e.g., the c o r o n a r y - ~ c u l a t i o n ) that the large and the small arteries sometimes react differently to smooth muscle agonists (t5), a comparison of our work with the studies of major umbilical arteries may not be appropriate. The fetal hemodynamlc effects of PGE2 and PGF2c~ injected into the distal aorta differed in several other respects. We noted an in[tlal decrease in fetal arterial pressure after the injection of PGE2 but not after PGF2,# noreplnephrine, or vehicle injections alone. This transient vasodepressor effect of PGE2 was maximal within 20 seconds of iniectlon and was then followed by the evolution of the vasopressor effect. The mechanism of the blphaslc effect of PGE2 on fetal arterial pressure is not entirely clear. We interpret the initial decrease in fetal arterial pressure as resulting from systemic vasodilatation by PGE2since circulation tlmes are rapid in the fetus (16) and because we calculate a slight increase in placental vascular resistance even in the first 30 seconds after PGE2 injection. PGE compounds are known to be systemic vasodilators in different species (17) and to have a hypotensive effect when injected into the ascending aorta of the mature fetal lamb (18) but at the same time to cause constriction of the blood vessels of the nasal mucosa of dogs (19) and the isolated aorta and coronary arteries of rabbits (20). However, in the absence of measurements of regional blood flow, we can draw no clear conclusion about systemic vasodilatation in the fetus beyond the first minute after PGE2 injectlon. Since the placenta receives nearly 50% of the fetal cardiac output (6), placental vasoconstriction may obscure the effect of systemic vasodilatatlon on fetal arterial pressure. The biphasic nature of the PGE2 effect on fetal arterial pressure suggests the possibillty that development of placental vasoconstriction has a longer latency than does systemic vasodilatatlon and raises the question as to whether or not the
552
MARCH 25, 1974
VOL. 5 NO. 6
PROSTAGLANDINS
pressor effect of PGE2 is mediated at least in part by an indirect rather than direct mechanism of action. Hemodynamic responses to PGE2 were essentially unaltered after adrenerglc blockade, as was the case for PGF~, indicating that placental vasoconstriction by PGE2 is not mediated by alpha-a-drenergic receptors. The lack of an effect of noreplnephrlne on placental vascular resistance further supports this conclusion. However, a profound reduction in umbilical blood flow (as produced by larger doses of PGE2) may lead to fetal hypoxia and catecholamlne release and thereby contribute to the elevation in fetal arterial pressure. Although the dosesof prostaglandins used in this study are considerably higher than levels assayed in maternal or fetal plasma at birth (21), it is known that E and F prostaglandlns are present in high concentrations in the umbilical cord (4) and that prostaglandlns are reJeased in tissues by mechanical and pharmacologic stimuli (22,23). Therefore it is reasonable to speculate that prostaglandlns released locally in placental vessels may produce circulatory effects similar to those demonstrated in this study. Recent evidence indicates that PGF compounds contract the ductus arteriosus of the fetal calf in vitro (24), that PGE compounds dilate the pulmonary vascular bed (25), and thaTth~-Fprostaglandlns have effects on the ductus arteriosus which vary with the concentration of oxygen in the environment (26). Our data taken together with the observations of others suggest that prostaglandins may play a role in the circulatory adaptations of the fetus at birth, perhaps in conjunction with the peptide bradyklnin (27). Likewise, prostaglandins may have hemodynamic consequences in the fetus after maternal infusions or upon direct administration into the amnlotlc cavity. Beazley and coworkers (5) have shown placental transfer of PGF2~ or its radioactive metabolites during human pregnancy. Similar data are lacking with respect to PGE2 nor is it certain that prostaglandlns cross the placenta in a vasoactive form. High concentrations of 15-hydroxyprostaglandin dehydrogenase in the human placenta may protect the fetus from circulating prostaglandlns (28). Moreover, prostaglandln degradation by lung and liver has been demonstrated in fetuses near term (18). On the other hand, PGE2 administered directly into the amniotic cavity (as for induction of labor or abortion) could have a direct effect on placental vessels and thus a potentially adverse influence upon fetal oxygenation or fetoplacental endocrine function. REFERENC ES 1. Euler, U. S. von. Action of adrenaline, acetylchollne and other substances on nerve-free vessels (human placenta). J. Physiol. Lond. 93:129, 1938. 2. Hilller, K., and S. M. Mo Karim. Effects of prostaglandins El, E2, FI~ , F2c~ on isolated human umbilical and placental blood vessels. J. Obstet. Gynaec. Brit. Cwlth. 75:667, 1968. . Dyer, D. C. Comparison of the constricting actions produced by serotonin and prostaglandins on isolated sheep umbilical arteries and veins. Gynec. Invest. 1:204, 1970. 4. Karim, S. M. M. The identification of prostaglandlns in human umbillcal cord. Brit. J. Pharmacol. Chemother. 29:230, 1972.
MARCH 25, 1974
VOL. 5 NO, 6
553
PROSTAGLANDINS
. Beazley, J. M . , H. C. Brummer, and A. Kuriak. Distribution of 9-H 3prostaglandln F2~in pregnant and non-pregnant subjects. J. Obstet. Gynaec. Brit. Cwlth. 79:800, 1972. 6. Dawes, G. S, Umbilical blood flow, in Respiratory Gas Exchange and Blood Flow in the Placenta (L. D. Longo and H. Bartels, Editors), U. S. Department of Health, Education and Welfare, Bethesda, Md., 1972, p. 107. 7. Meschia, G., J. R. Cotter, C. S. Breathnach, and D. H. Barron. The hemoglobin, oxygen, carbon dioxide and hydrogen ion concentrations in the umbilical bloods of sheep and goats as sampled via indwelling plastic catheters. Quart. J. Exp. Physiol. 50:185, 1965. .
Jackson, B. T., and R. H. Egdahl. The performance of complex fetal operations in utero without amnlotic fluid toss or other disturbances of fetal-maternal relationships. Surgery48:564, 1960.
9. Rudolph, A. M . , and M. A. Heymann. The circulation of the fetus in utero. Methods for studying distribution of blood flow, cardiac output and organ blood flow. Circ. Res. 21:163, 1967. 10. Novy, M. J., and M. J. Cook. Redistribution of blood flow by prostaglandin F~,in the rabbit ovary. Am. J. Obstet. Gynec. 117:381, t973. 11. Nakano, J., and B. Cole. Effects of prostaglandlns E1 and F2~on systemic, pulmonary, and splanchnlc circulation in dogs. ,am. J. Physiol. 217:222, 1969. 12. Oakes, G., M. Mofld, C. R. Brlnkman III, and N. S. ~sali. Insensitivity of the sheep to prostaglandlns. Proc. Soc. Exp. Biol. Medo 142:194, 1973. 13. White, R. P., J. A. Heaton, and I. C. Denton. Pharmacological comparison of prostaglandln F2c~, serotonin and noreplnephrine on cerebrovascular tone of monkey. Europ. J. Pharmacol. 15:300, 1971. 14. Altura, B. M . , D. Malaviya, C. F. Reich, and L. R. Orkln. Effects of vasoactlve agents on isolated human umbilical arteries and veins. Am. J. Physiol. 222:345, 1972. 15. Zuberbuhler, R. C., and D. F. Bohr. Responsesof coronary smooth muscle to catecholamines. Circ. Res. 16:431, 1965. 16. Novy, M. J., and J. Metcalfe. Measurements of umbilical blood flow and vascular volume by dye dilution. Am. J. Obstet. Gynec. 106:899, 1970. 17. Nakano, J. Cardiovascular actions, in The Prostaglandlns, Vol. 1 (P. W. Ramwell, Editor), Plenum Press, New York, 1973, p. 239. 18. Olley, P. M . , F. Coceanl, and G. Kent. Inactivation of prostaglandin El by lungs of the foetal lamb. Experientla, in press.
554
MARCH 25, 1974
VOL. 5 NO. 6
PROSTAGLANDINS
19. Jackson, B. T., and R. Stovall. Vasoconstriction of nasal blood vessels induced by prostaglandins, in Prostaglandln Symposium of the Worcester Foundation for Experimental Biology (P. W. Ramwell and J. E. Shaw, Editors), Interscience Publishers, New York, 1968, pp. 329-334. 20. Strong, C. G., and D. V. Bohr. Effects of prostaglandins El, E2, A1, and FI~ on isolated vascular smooth muscle. Am. J. Physiol. 213:724, 1967. 21. Craft, I. Lo, R. Scrivener, and C. J. Dewhurst. Prostaglandin F2~ levels in the maternal and fetal circulations in late pregnancy. J. Obstet. Gynaec. Brit. Cwlth. 80:616, 1973. 22. Gustavll, B. Labour: a delayed menstruation ? Lancet 2:1149, 1972. 23. McGiff, J. C., N. A. Terragno, K. U. Malik, and A. J. Lonlgro. Release of a prostaglandln E-like substance from canine kidney by bradyklnln. Circ. Res. 31:36, 1972. 24. Elllott, R. B., and M. B. Starling. The effect of prostaglandin F2c~ in the closure of the ductus arterlosus. Prostaglandlns 2:399, 1972.
25. Hauge, A., P. K. M. Lunde, and B. A. Waaler. Effects of prostaglandln El and adrenaline on the pulmonary vascular resistance (PVR) in isolated rabbit lungs. Life Scl. 6:673, 1967. 26. Coceanl, F., and Po M. Olley. The response of the ductus arterlosus to prostaglandlns. Canad. J. Physiol. Pharmacol. 51:220, 1973. 27. Melmon, K. L., M. J. Cline, T. Hughes, and A. S. Nies. Kinlns: possible mediators of neonatal circulatory changes in man. J. Clln. Invest. 47:1295, 1968. 28. Jarabak, J. Human placental 15-hydroxyprostaglandln dehydrogenase. Proc. Nat. Acad. Sci. USA69:533, 1972.
M A R C H 25, 1974
VOL. 5 NO. 6
555