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Plasma osteocalcin concentrations in cattle under various pathophysiological conditions
131
Plasma osteocalcin concentrations in cattle under various pathophysiological conditions
M.-J. Davicco, V. Coxam, R. Roux and J.-P. Barlet M,ne,nlMelubolirmoni*,IN&4 Theu. F.63122.cqvrar.France (Received 14hdy 1989) (Accepted 20 April 1990)
Summary Plasmaortecalcin
(BGP)
conantradon5
were meaured
using an homologous radioimmunoassay in
plasma sampler (n = 6-14 per group) from fetal and newborn calves, 16month.old and pregnant lactadng cows.
The highestvalues (nM) (62 ? 3) weremeasured
lowest (1.5 f 3) in P-year-old pregnant and lactating cow
heifers and bulb,
in fetal calves and the
No significant relationship could be dernan-
waled between plasma BGP and somatomedm C. or 1,25(OH),D or calcium concentrations. In eight normally calving cows, parturition was followed by a pragrersive mcreax in plasma BGP concentration, maximum 3daysaftercalving.
It returned toprepartum YLIBCI I0dayslarer.
hypocalcaemw and paretic cows, hypocalcaemia occurrmg wthin
lnsixparturient
12 h following calving was associated
with a prompt and very rran6wd increase in plasma BGP concentrations. These results indicate that OSteocakin might play a role in the regulation of bone metabolism in cattle.
Key wards: Bone; Cattle; Hypocalcaemra; Gsteocalcl”
Introduction
In the bovine PS in other mammalian species skeletal growth is affected by genetic, hormonal and nutritiona: factors. Little is known on the mechanism of bone growth, mineralization and bone metabolism because, until recently, no specific marker of bone formation was available. The occurrence of a vitamin K-dependent calcium binding protein, containing y-carboxyglutamic acid (Gla) was reported in bovine and swordfish bone and called bone-Gla-protein or BGP by Price et al. [l], and by Hauschka et al. [Z] in chicken bone (termed osteocalcin). BGP is a 49-r&due protein specifically synthesized by the osteoblasts. It circulates in the blood where it can be measured by radioimmunoassay [3]. This measurement provides a Correspondence to: I.-P. Bark,
Mmeral Metabobsm Unit, INRA
Weix. F-63122CeyraL
France.
132 marker of skeletal growth [4] and bone metabolism [5,6], specific for osteoblastic bone formation [7,8]. Because of lactation and the marked drain of calcium (Ca) into the milk, the cow has active and responsive bone homeostatic mechanisms. A failure in these control mechailisms at parturition and the initiation of lactation results in the development of d clinicai ayudtome ciralacterixd by severe hypocalcrmis and hypopi,ospbatrmia, associated with muscular weakness, paresis and death if appropriate treatment is not rapidly initiated [9]. Investigations into the cause and prevention of the syndrome have shown ttat FTH synthesis and secretion is normal [10,11], but trabecular and Haversian bone surfaces are inactive [12]. Using an homologous radioimmunoassay, we have measured plasma BGP concentrations in cattle, more espeilaZy Lnhypocalcemicparturient cows.
Animals and Methods Animalsand treatments Fourteen 9-year-old Holstein parturient cows (6th parturition) weighing 694 + 9 kg were used. Each cow was fed maize silage and grain concentrate allowing a daily intake of 110 g calcium and 95 g inorganic phosphorus. Jugular blood samples were collected daily at 8 a.m. before feeding on each animal, before and after calving, and 3 months later, when cows were both pregnant and lactating. Blood samples were also collected on ten 265-day-old-fetuses (four males and six females obtained from ten slaughtered pregnant cows), on twelve S-day-old calves (six males and six females), on six 16-month-old heifers and on six bulls of the same age. Blood was collected in heparinized tubes by puncture of an external jugular vein. In paretic cows, sampling occurred just before treatment (calcium gluconate intravenous injection) for paresis. Plasma was collected after centrifugation and kept frozen until analysis.
Assay procedure Plasma BGP concentrations were measured by homologous radioimmunoassay (RIA) [6] using the Osteocalcin Assay System from 3ris (Gif sur Yvette, France). In our experimental conditions the sensitivity of the method was 8 pmol.l-‘. The intra- and interassay precisionswxe 6 and 9%. respectively. Plasma somatomedin C (IGF,) concentrations were determined by RIA after extraction [13]. In order to dissociate and separate IGF, from its carrier protein, calf plasma was mixed with 4 vol. 0.5 M-hydrochloric acid and incubated in stoppered glass tubes at room temperature 1141. After incubation, an ODS-siiica extraction was performed (Sed Pak C,, Cartridges; Waters Associates, Milford, MA, USA). IGF, being identical in human and bovine species [15], the efficiency of IGF, extraction in bovine plasma was examined by adding pure recombinant human IGF, (Ciba-Geigy. Basel, Switzerland) to OS-ml portions of calf plasma. The RIA doseresponse curves of these samples paralleled those of pure IGF, alone, but only 88 + 2% of the added IGF, could be detected. Thus the results were corrected according
133 to this extraction efficiency. In our experimental conditions intra- and interassay variations were 7 and ll%, respectively. The minimum detectable amount wasless than 4pmol.l-‘. Plasma 1,25(OH),D concentrations were determined using a previously described radiocompetitive protein-binding assay [16]. Ca concen.rations in plasma were measured by atomic absorption spectrophotometry ;i’er~in Elmer400). Inorganic phosphorus was determined by calorimetry (Technicon Au~aanalyxx). Results are expressed as mean f SEM. This statistical signifiance of differences observed between groups was calculated using the Mann-Whitney U-test.
Plasma BGP concentrations (nM) measured in ten 265-day-old fetuses (62 + 3) were higher than those measured in their dams (16 + 2; P < O.Ol), in six 16-monthold heifers (27 i: 1; P < O.Dl), in six 16-month-old bulls (34 + 3; P < 0.01) and in six &day-old male (49 + 4; P c 0.01) or female (53 k 6; P <: 0.01) calves. Sex had no significant influence on plasma BGP concentrations in fetal (male 61 f 2; n = 4; female 64 f 3; II = 6) or in 8-day-old calves. However, in the 16-month-old animals, plasma BGP level was higher in bulls than in heifers (P< 0.05). No significant relationship could be established either between plasma BGP and IGF, concentrations
134 or between BGP and 1,25(OH),D, or between BGP and Ca, or between BGP and inorganic phosphorus (not shown). Highest plasma IGF,, l,ZS(OH),D and Ca plasma concentrations were measured in &month-old cattle. calves and ietusoa, respectively (Fig. 1). Among the 14 parturient cows. six developed clinical symptoms (muscular tremors, slight paresis, hypoesthesia, hypothermia) and biochemical evidence (hypocalcaemia and hypophosphataemia; Fig. 2) of parturient paresis between the 10th and the 12th hour following calving. They were treated by calcium gluconate intravenous injection (0.5 mg Ca per kg body weight) and each animal quickly recovered. No relapse was observed. A slight but significant (P < 0.05) decrease in plasma Ca
and inorganic phosphora concentrations at the time of calving was also observed in control parturient EOWS(Fig. 2). Plasma BGP concemrations were similar in both groups of cows until calving time. They climbed up to 74 f 3 at the time of Ca intravenous injection in paretic cows and returned to control values 12 h later, after
260 .$
“~
v;;:i .2
.I
I ’
CI\L”ING
2
3
4 .-cl DAYS
Fig. I. %sma osteocalcin(BGP). calcium(Ca) and inorganicphosphorus(P) concentrationsin eight normallycalvmgcomrolcows(dotted lines)and in six panurientpareticcows(fulllines)(mean? SEM; ‘P < 0.05;“P 4 0.01,comparisonbetweenthe IWOgroupsofcows).
135 which they did not significantly vary. fn control Icows plasma BGP concentrations significantly increased 48 h after calving (P < 0.01, comparison with values at calving iirnr). They remained significantly higher than in paretic COWS untl!dnj S z&x calving (Fig. 2).
Plasma BGP concentrations measured in ads:: :att!- _,cd in this experiment are in good agreement with those reported in adult cows (15-30 q/ml) [17,18]. Plasma BGP concentrations higher in fetal calves than in their dams have already been reported by others [17] and ourselves (131. Similarly, in the ovine species, sennn EGP (@ml) rapidly and significantly decreased from 535 at birth to 240 at 45 days, 152 at 90 days and 5.9 at 7 years of age [lS]. In the same way, in a wild ruminant, the white-tailed deer, plasma BGP is higher in fawns (37.6 &I) than in adults (16.3 pgll). in which plasma BGP increased during antler mineralization [ZO]. In the human species serum BGP is higher during the growing period than in adults 141,and in healthy adults a negative correlation exists between age and plasma BGP [Zl]. It has been demonstrated that 1,2S(OH),D increased serum levels of EC? in rats, and thus BGP might have a direct or an indirect role in carrying the action of vitamin D on bone 1221. However, in our animals given a Ca-rich diet, no relationship could be demonstrated either between plasma BGP and l,?S(OH)& or between plasma BGP and IGF,, a known stimulator of bone cell replication and matrix synthesis [23]. Both plasma BGP and IGF, concentrations are decreased in growh hormone-deficient children [4]. At present, serumBGP is the most useful bone protein for assisting in the diagnosis and management of high turnover metabolic disease states [24]. The use of plasma BGP measurements to indicate bone formation rate depends upon the clearance of BGP being constant and variations in the plasma concentrations being solely due to variations in osteohlastic activity. BGP metabolic clearance rate is presently unknown in cattle. However data obtained in oophorectomized ewes suggest that most of the variability in plasma BGP is attributable to variations in plasma production rate since the plasma clearance is practically constant 12.51. The mobilization of skeletal reserves appears to contribute significantly to Ca homeostasis near parturition in cows and factors, such as calcitonin 1261,which interfere with rapid mineral resorption result in the development of profound hypocalcaemia [27]. In humans, plasma BGP concentrations increase in high-turnover metabolic hone diseases [7] and decrease in low-turnover osteopathy [S]. Since bone turnover is high in normal prepartal cows [12] and low in hypocalcaemic paretic cows [26,27], this might explain differences after parturition in plasma BGP concentrations measured in normal and paretic cows (Fig. 2). However, the reasons for the sudden increase and decrease in plasma BGP concentrations in paretic cow (Fig. 2) remain obscure. In ewes, BGP plasma production rate increased by 15 days after parturition and achieved a IO-fold elevation at 49 days post parturn [28]. In paretic cows, the increase in plasma BGP concentration cannot be related to Ca glu-
136
conate intravenous injection since blood was collected before Ca treatment. In conclusion, in the bovine species, in adult parturient cows as well as in very young calves, BGP appears to be a good marker of bone remodelling; it may play a role in the regulation of bone metabolism in these animals.
L Price PA, Gtsuka AS, Poser J, Kristapads 1, Reman N. Characterization of y-catboxyglotamic acidcontaining protein from bone. Pmc Natl Acad Sci USA 1976.73~1447-1451. 2 Hauschka PV, Lian JB, Gallop PM. Direct identification of the calcium binding amino acid 7-c .arboxyglutamate in mineralized tissues. ProcNatl Acad Sci USA 1975;12:3925-3929. 3 Price PA, Nishimoto IK. Radioimmunoassay for the vitamin K-dependent protein of bone and its dinmverv in &ma. Proc Nat1 Acad Sci USA I980:77:2234-2238. 4 Delma.s~D,~Chatelain P, Malaval L, Bonne 0. Serum bone Gla-protein in growth-hormone deticient children. 1 Bone Minera! Res 1986:1:333-338. 5 Price PA, Parthemore JG, Deftos W. New biochemical marker for bane metabolism. J Clin Invest 1980;66:878-883. 6 Delmas PD. Wahner HW, Mann KG, Riggs BL. Assessment of bone :omover in oostmeoooaosal osteoporosis by measuremeot of serum bon;-Gla-protein. J Lab Clin Med 3983102~~70-476. Brown JP. Delmas PD. Malaval L, Edouard C, Chapus MC, Meunier Serum bone Gla-protein: a specific marker for bone formation in postmenopausal osteoporosis. Lance, 19&l;i:I@91-1093, ^ Delmar PD, Malaval L. Arlot M, Meunier PI. Serum bane Gla.protein compared to bone histomorphometry in endocrine diseases. Bone 1985;6:339-341. 9 Kronfeld DS. Partorient hypocalcaemia in dairy cows. Adv Vet Sci Comp Med 397I;I5:333-137. 10 Mayer GP, Ramberg CF. Kmnfeld DS, Buckle RM, Shemood LM, Aurbach GC, Potts IT. Plasma parathyroid hormone concentration in hypocalcaemia partutient rows. Am J Vet Rcs 1%9;30:15871597. II Capen CC, Young DM. The ultrastructure of the parathyroid glands and thyroid parafollicular ceIk of mwswithparturientparesisand hypacakaemia. Lab Invest 1%7;17:717-737. 1% Rowland GN, Capen CC, Young DM, Black HE. Microradiographic evahtafion of bone from cows with experimental hypervitaminosis D. diet-induced hypaakemis, and naturally o;surriog paw rient paresis. CalcifTissue Res 1972;9:179-193. 13 Coxam V, Davicco MI. Dnrdillat C, Opmeer F, Barlet JP. Systemic bate growh factots concentradons in calves during the perinatal period. J Der Pi1yriol1988:10:423-431. 14 Undenvood LE. D’Erm!e JA, Capeland KC, Van Wik, JJ. HurleyT. Handivergen S. Development of eo heteralogous radioimmunoassay for romatotnedin C in sheep blwd. J Endocrinol 1%82;93: 31-39. 15 Honneger AM, Humbel RE. Insulin-like growth factors -I and -II in fetal and adult bovine serum. Purification, primary structures and immunological ctoss reactivities. 3 Biol Chsm 1986;261: 569-575. 16 Riad F, Lefaivre J, Etarlet JP. I .2S-Dihydroxycholecalciferol phospate secretion in cattle. J EndoctinolI9g7:112:427-430. I7 Price PA, Nishimoto SK. Radioimmunoassay for the vitamin K-dependent pro:ein of bone and its discovery in plasma. Proc NatI Acad Sci USA 3980;7732234-2238. 18 Patterson-Allen P, Brantigam CE, Grindeland RE. Asling CW, Callaghan PX. A specific radioimmunoassay for osteocalcin with advantageous species cmssreactivities. Analyt B&hem 1982;120:1-7. 19 Pastoureau P, Merle B, Delmas PD. Specific radioimmunoassay for ovine bone Gla-protein (OStcaealcin). Aeta Eodocrinol 1988:119:152-160. 20 Van der Ecms KL, Brown RD. Gundberg CM. Circulating levels of 1,25dihydmx)ritamin D. alka-
7
PJ.
.cg~la!es salivary
137 line phoiphatax,
hydroxyproline
and oestewalcin
asociated
with ander growth in white-railed
deer. Acta Endocrinol 1988:118:407-414. *I
Garcia-Carasco
M, Gruron M, De Vernejoul
MC, Dane
MA,
Miravet L. Osteocalcin and bone
morpbometric psrameters in adults without bone disease. Calelf TissueInt 1988,42:13-17. 22 Price PA. Bankol SA. 1.25.Dihydroxyvitamin bone protein. B&hem
D, increases senan levels
ofthevitamin
K-dependent
Biophys Res Commun 1981.99:928-935.
23 Canalis E, Centrellr M, Butch W, McCarthy Tf.. Insulin-like growth faclar I mediates selecdve anabolic effects of parathyroid hormone in bone cultures. J Clin Invest 1989;83:60-65. 24 Epstein S. Serum and urinary markers of bone remodeling: assessmentof bone twnover. Endocrine Rev 19!3*:9:437-449. 25 Melick BA. Farrugia W. f&ton
CL, Zuelcb K1. Smg~ins BA, Work JD. The mctaho!ic clearance
rate ofoesteocalcin in sheep. C.&if Tissue fnr 198&42:18526 Barkt
JP. Inhibition
by c.&itanin
190.
of hypercalcaemia induced by I.Zs-dihydrolrycbolecaleiferol.
J
Endocrinol1980;85:63-67. 27 Yarrington JT, Capen CC, B!ack HE, Re R. Poha IT. Gehow B. Experimental partwient hypocalcemia in cow
following prepanal
chemical inhibition of bone resorpdon. Am J Pathol 197683:
569-588. 28 Farqia
W, Fortune CL, Heath J, Caplc IW, Wart JD. Dsteocalcin ar an index of osteablast func-
tion duringandafterovine
pregnancy. Endocrinology 1989;125:1705-1710.
Plasma osteocalcin concentrations in cattle under various pathophysiological conditions
M.-J. Davicco, V. Coxam, R. Roux and J.-P. Barlet M,ne,nlMelubolirmoni*,IN&4 Theu. F.63122.cqvrar.France (Received 14hdy 1989) (Accepted 20 April 1990)
Summary Plasmaortecalcin
(BGP)
conantradon5
were meaured
using an homologous radioimmunoassay in
plasma sampler (n = 6-14 per group) from fetal and newborn calves, 16month.old and pregnant lactadng cows.
The highestvalues (nM) (62 ? 3) weremeasured
lowest (1.5 f 3) in P-year-old pregnant and lactating cow
heifers and bulb,
in fetal calves and the
No significant relationship could be dernan-
waled between plasma BGP and somatomedm C. or 1,25(OH),D or calcium concentrations. In eight normally calving cows, parturition was followed by a pragrersive mcreax in plasma BGP concentration, maximum 3daysaftercalving.
It returned toprepartum YLIBCI I0dayslarer.
hypocalcaemw and paretic cows, hypocalcaemia occurrmg wthin
lnsixparturient
12 h following calving was associated
with a prompt and very rran6wd increase in plasma BGP concentrations. These results indicate that OSteocakin might play a role in the regulation of bone metabolism in cattle.
Key wards: Bone; Cattle; Hypocalcaemra; Gsteocalcl”
Introduction
In the bovine PS in other mammalian species skeletal growth is affected by genetic, hormonal and nutritiona: factors. Little is known on the mechanism of bone growth, mineralization and bone metabolism because, until recently, no specific marker of bone formation was available. The occurrence of a vitamin K-dependent calcium binding protein, containing y-carboxyglutamic acid (Gla) was reported in bovine and swordfish bone and called bone-Gla-protein or BGP by Price et al. [l], and by Hauschka et al. [Z] in chicken bone (termed osteocalcin). BGP is a 49-r&due protein specifically synthesized by the osteoblasts. It circulates in the blood where it can be measured by radioimmunoassay [3]. This measurement provides a Correspondence to: I.-P. Bark,
Mmeral Metabobsm Unit, INRA
Weix. F-63122CeyraL
France.
132 marker of skeletal growth [4] and bone metabolism [5,6], specific for osteoblastic bone formation [7,8]. Because of lactation and the marked drain of calcium (Ca) into the milk, the cow has active and responsive bone homeostatic mechanisms. A failure in these control mechailisms at parturition and the initiation of lactation results in the development of d clinicai ayudtome ciralacterixd by severe hypocalcrmis and hypopi,ospbatrmia, associated with muscular weakness, paresis and death if appropriate treatment is not rapidly initiated [9]. Investigations into the cause and prevention of the syndrome have shown ttat FTH synthesis and secretion is normal [10,11], but trabecular and Haversian bone surfaces are inactive [12]. Using an homologous radioimmunoassay, we have measured plasma BGP concentrations in cattle, more espeilaZy Lnhypocalcemicparturient cows.
Animals and Methods Animalsand treatments Fourteen 9-year-old Holstein parturient cows (6th parturition) weighing 694 + 9 kg were used. Each cow was fed maize silage and grain concentrate allowing a daily intake of 110 g calcium and 95 g inorganic phosphorus. Jugular blood samples were collected daily at 8 a.m. before feeding on each animal, before and after calving, and 3 months later, when cows were both pregnant and lactating. Blood samples were also collected on ten 265-day-old-fetuses (four males and six females obtained from ten slaughtered pregnant cows), on twelve S-day-old calves (six males and six females), on six 16-month-old heifers and on six bulls of the same age. Blood was collected in heparinized tubes by puncture of an external jugular vein. In paretic cows, sampling occurred just before treatment (calcium gluconate intravenous injection) for paresis. Plasma was collected after centrifugation and kept frozen until analysis.
Assay procedure Plasma BGP concentrations were measured by homologous radioimmunoassay (RIA) [6] using the Osteocalcin Assay System from 3ris (Gif sur Yvette, France). In our experimental conditions the sensitivity of the method was 8 pmol.l-‘. The intra- and interassay precisionswxe 6 and 9%. respectively. Plasma somatomedin C (IGF,) concentrations were determined by RIA after extraction [13]. In order to dissociate and separate IGF, from its carrier protein, calf plasma was mixed with 4 vol. 0.5 M-hydrochloric acid and incubated in stoppered glass tubes at room temperature 1141. After incubation, an ODS-siiica extraction was performed (Sed Pak C,, Cartridges; Waters Associates, Milford, MA, USA). IGF, being identical in human and bovine species [15], the efficiency of IGF, extraction in bovine plasma was examined by adding pure recombinant human IGF, (Ciba-Geigy. Basel, Switzerland) to OS-ml portions of calf plasma. The RIA doseresponse curves of these samples paralleled those of pure IGF, alone, but only 88 + 2% of the added IGF, could be detected. Thus the results were corrected according
133 to this extraction efficiency. In our experimental conditions intra- and interassay variations were 7 and ll%, respectively. The minimum detectable amount wasless than 4pmol.l-‘. Plasma 1,25(OH),D concentrations were determined using a previously described radiocompetitive protein-binding assay [16]. Ca concen.rations in plasma were measured by atomic absorption spectrophotometry ;i’er~in Elmer400). Inorganic phosphorus was determined by calorimetry (Technicon Au~aanalyxx). Results are expressed as mean f SEM. This statistical signifiance of differences observed between groups was calculated using the Mann-Whitney U-test.
Plasma BGP concentrations (nM) measured in ten 265-day-old fetuses (62 + 3) were higher than those measured in their dams (16 + 2; P < O.Ol), in six 16-monthold heifers (27 i: 1; P < O.Dl), in six 16-month-old bulls (34 + 3; P < 0.01) and in six &day-old male (49 + 4; P c 0.01) or female (53 k 6; P <: 0.01) calves. Sex had no significant influence on plasma BGP concentrations in fetal (male 61 f 2; n = 4; female 64 f 3; II = 6) or in 8-day-old calves. However, in the 16-month-old animals, plasma BGP level was higher in bulls than in heifers (P< 0.05). No significant relationship could be established either between plasma BGP and IGF, concentrations
134 or between BGP and 1,25(OH),D, or between BGP and Ca, or between BGP and inorganic phosphorus (not shown). Highest plasma IGF,, l,ZS(OH),D and Ca plasma concentrations were measured in &month-old cattle. calves and ietusoa, respectively (Fig. 1). Among the 14 parturient cows. six developed clinical symptoms (muscular tremors, slight paresis, hypoesthesia, hypothermia) and biochemical evidence (hypocalcaemia and hypophosphataemia; Fig. 2) of parturient paresis between the 10th and the 12th hour following calving. They were treated by calcium gluconate intravenous injection (0.5 mg Ca per kg body weight) and each animal quickly recovered. No relapse was observed. A slight but significant (P < 0.05) decrease in plasma Ca
and inorganic phosphora concentrations at the time of calving was also observed in control parturient EOWS(Fig. 2). Plasma BGP concemrations were similar in both groups of cows until calving time. They climbed up to 74 f 3 at the time of Ca intravenous injection in paretic cows and returned to control values 12 h later, after
260 .$
“~
v;;:i .2
.I
I ’
CI\L”ING
2
3
4 .-cl DAYS
Fig. I. %sma osteocalcin(BGP). calcium(Ca) and inorganicphosphorus(P) concentrationsin eight normallycalvmgcomrolcows(dotted lines)and in six panurientpareticcows(fulllines)(mean? SEM; ‘P < 0.05;“P 4 0.01,comparisonbetweenthe IWOgroupsofcows).
135 which they did not significantly vary. fn control Icows plasma BGP concentrations significantly increased 48 h after calving (P < 0.01, comparison with values at calving iirnr). They remained significantly higher than in paretic COWS untl!dnj S z&x calving (Fig. 2).
Plasma BGP concentrations measured in ads:: :att!- _,cd in this experiment are in good agreement with those reported in adult cows (15-30 q/ml) [17,18]. Plasma BGP concentrations higher in fetal calves than in their dams have already been reported by others [17] and ourselves (131. Similarly, in the ovine species, sennn EGP (@ml) rapidly and significantly decreased from 535 at birth to 240 at 45 days, 152 at 90 days and 5.9 at 7 years of age [lS]. In the same way, in a wild ruminant, the white-tailed deer, plasma BGP is higher in fawns (37.6 &I) than in adults (16.3 pgll). in which plasma BGP increased during antler mineralization [ZO]. In the human species serum BGP is higher during the growing period than in adults 141,and in healthy adults a negative correlation exists between age and plasma BGP [Zl]. It has been demonstrated that 1,2S(OH),D increased serum levels of EC? in rats, and thus BGP might have a direct or an indirect role in carrying the action of vitamin D on bone 1221. However, in our animals given a Ca-rich diet, no relationship could be demonstrated either between plasma BGP and l,?S(OH)& or between plasma BGP and IGF,, a known stimulator of bone cell replication and matrix synthesis [23]. Both plasma BGP and IGF, concentrations are decreased in growh hormone-deficient children [4]. At present, serumBGP is the most useful bone protein for assisting in the diagnosis and management of high turnover metabolic disease states [24]. The use of plasma BGP measurements to indicate bone formation rate depends upon the clearance of BGP being constant and variations in the plasma concentrations being solely due to variations in osteohlastic activity. BGP metabolic clearance rate is presently unknown in cattle. However data obtained in oophorectomized ewes suggest that most of the variability in plasma BGP is attributable to variations in plasma production rate since the plasma clearance is practically constant 12.51. The mobilization of skeletal reserves appears to contribute significantly to Ca homeostasis near parturition in cows and factors, such as calcitonin 1261,which interfere with rapid mineral resorption result in the development of profound hypocalcaemia [27]. In humans, plasma BGP concentrations increase in high-turnover metabolic hone diseases [7] and decrease in low-turnover osteopathy [S]. Since bone turnover is high in normal prepartal cows [12] and low in hypocalcaemic paretic cows [26,27], this might explain differences after parturition in plasma BGP concentrations measured in normal and paretic cows (Fig. 2). However, the reasons for the sudden increase and decrease in plasma BGP concentrations in paretic cow (Fig. 2) remain obscure. In ewes, BGP plasma production rate increased by 15 days after parturition and achieved a IO-fold elevation at 49 days post parturn [28]. In paretic cows, the increase in plasma BGP concentration cannot be related to Ca glu-
136
conate intravenous injection since blood was collected before Ca treatment. In conclusion, in the bovine species, in adult parturient cows as well as in very young calves, BGP appears to be a good marker of bone remodelling; it may play a role in the regulation of bone metabolism in these animals.
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