- Email: [email protected]
Sequential changes in plasma progesterone, total oestrogens and corticosteroids in the cow throughout pregnancy and around parturition
Br. vet.
3.
(1990) . 146, 24
SEQUENTIAL CHANGES IN PLASMA PROGESTERONE, TOTAL OESTROGENS AND CORTICOSTEROIDS IN THE COW THROUGHOUT PREGNANCY AND AROUND PARTURITION
H . M . EISSA and M . S . EL-BELELY Department of Surgery and Theriogenology, Faculty of Veterinary Medicine, Cairo University, Giza, Egypt
SUMMARY Blood samples were taken thrice weekly for 2 weeks from 33 pregnant cows in nine groups, 1 month apart, and twice daily for 8 days commencing 1 week before parturition from eight cows . Plasma progesterone and total oestrogens were quantified by validated radioimmunoassays and total corticosteroids by a competitive protein binding technique . During the first 3 months of pregnancy, no remarkable changes were observed for the three hormones studied . From the fourth month on, concentrations of progesterone and total corticosteroids decreased significantly (P<0-05) and remained fairly constant until the ninth month . Values of total oestrogens increased significantly (P< 0 . 05) from the fourth to the sixth month and thereafter remained unchanged . In the periparturient cows, progesterone levels decreased linearly (P<0 . 01) during the 3 days preceding delivery . Concentrations of total oestrogens rose sharply from day -5 and peaked at calving . Values of total corticosteroids increased dramatically on the sixth day and also at 24 hours before parturition . These results suggest that, contrary to earlier studies, plasma corticosteroids could have a primary key role for triggering parturition in cows .
INTRODUCTION Although there have been several studies concerning the concurrent changes of progesterone, oestrogens and total corticosteroids in the blood of cows during late pregnancy and around parturition, results are inconsistent and not clearly expressed (Randel & Erb, 1971 ; Smith et al., 1973 ; Arije, Wiltbank & Hopwood, 1974 ; Hunter et al., 1977 ; Seren et al., 1977 ; Heshmat & Shehata, 1982) . However, in most of these studies only a few animals with infrequent sampling were used . This paper was designed to investigate the pattern of concentrations of plasma progesterone, total oestrogens and corticosteroids throughout the whole length of gestation and around parturition in greater detail than has hitherto been reported .
HORMONE CHANGES DURING PREGNANCY
25
MATERIALS AND METHODS Animals and sampling
German black pied cows in the Clinic of Cattle Diseases and the Clinic of Obstetrics and Gynaecology of Cattle at the High School of Veterinary Medicine, Hanover, were used . Thirty-three pregnant cows were chosen . Based on the recorded date of mating, confirmed by rectal findings, these animals were allotted to nine groups, 1 month apart, each group including three to five animals (Fig . 1) . Blood samples were collected from each pregnant animal thrice weekly for 2 weeks . 4800
12
4200
E
rn
10 3600
v ó
oo
8 3000
oo ó
6
E
c c 4 d N
0- L
i
1 0
1 2 3 4 5 6 7 8 9 -7 (4) (3) (3) (5) (3) (4) (3)(5)(3)(8) Month of pregnancy
-6 (8)
-5 (8)
-4 (8)
-3 (8)
-2 (8)
-1 (8)
0 (8)
+1 (8)
Days before (-) and after (+) calving
Fig. 1 .
Mean±sE plasma concentrations of progesterone ( •- •-•) , total oestrogens (X- X) and corticosteroids (•- --0) throughout pregnancy and around parturition . Values in the lefthand part of the figure are the means of different groups of cows, each sampled six times . Figures in parentheses are numbers of animals .
In addition, eight pregnant cows were selected 10-15 days before the estimated time of parturition . Blood samples were collected from these animals twice daily at 7 a .m. and 6 p .m. Breeding records (gestation period ranged from 279 to 282 days) and daily observations for symptoms of calving (loosening of the sacrosciatic ligaments, enlargement of the mammary gland and vulva and discharge of mucus) were used to estimate the expected time of calving . The time of collection of all the samples was subsequently standardized from the time of parturition taken as day 0 . Blood samples collected during the 7 days before parturition and on the day of calving were taken for hormone analysis . Blood collection
Jugular vein blood samples (20 ml) for progesterone and oestradiol analyses were collected by venepuncture . Tail vein blood samples (5 ml) for corticosteroid determination were obtained prior to jugular blood sampling in order to minimize the effects
26
BRITISH VETERINARY JOURNAL, 146, 1
of stress on blood corticosteroid level . All collections were made over 3 months from January to March . Blood samples were drawn into heparinized tubes and centrifuged immediately at 3000 g for 20 min . Plasma was stored at -20° C until assayed . Hormonal assays
Tritiated progesterone (1,2,6,7_ 3 H-progesterone, 80-100 Ci/mmol) and oestrogen (SE1- 3 H-oestradiol-17 1g-AK-6, 80-120 Ci/mmol) were obtained from the Institute of Physiology, High Veterinary School, Hanover . The labelled corticosteroid (1,2,6,7-3 Hhydrocortisone, 90-115 Ci/mmol) was purchased from Firma NEN Chemicals GmbH, Dreieichenhain . The plasma progesterone concentration was determined by the RIA technique described by Linneweber (1981) . The antibody used to assay progesterone cross-reacted strongly (96%) only with 3a-dihydroprogesterone . The sensitivity of the assay, defined as the smallest concentration significantly (P< 0 . 05) distinguishable from zero, was 0 . 1 ng/ ml plasma . The average intra- and inter-assay coefficients of variation were 6 . 8% (n -8) and 8 . 2% (n-15), respectively . Concentration of plasma total oestrogens was assayed by the method reported by Da Silva (1979) . The antibody for total oestrogens cross-reacted 100% with oestrone, 68 . 5% with 17p-oestradiol, 23 . 6% with 17a-oestradiol and 10% with oestriol . The sensitivity of the assay was 10 pg/ml plasma . Intra- and inter-assay coefficients of variation averaged 7 . 2% (n=10) and 8 . 6% (n=18), respectively . The plasma level of corticosteroids was measured by a competitive protein binding assay employed by Murphy (1967) as modified by Weiss & Scherzinger (1973) and Eissa (1985) . The antibody to corticosteroids cross-reacted 96% with cortisone, 91% with corticosterone and 85% with cortisol . The sensitivity of the assay, intra- and inter-assay coefficients of variation were 0 . 1-0 . 2 ng/ml plasma, 10 . 5% (n ° 33) and 10% (n=25), respectively Statistical analyses
For each pregnant animal sampled thrice weekly over 2 weeks, mean values of hormone concentrations for the six samples were calculated for statistical analyses . This was because there were negligible changes in concentrations of progesterone, total oestrogens or corticosteroids among times of blood sampling . The effect of pregnancy on hormone concentrations was determined by one-way analysis of variance . If the F-value was significant, differences in means amongst groups were evaluated by the Studentized range Qmethod and were considered to be significant if P< 0 . 05 (Snedecor & Cochran, 1976) . Hormone concentrations in samples taken twice daily from the cows approaching parturition were analysed by analysis of variance for repeated measurements (Winer, 1971) . RESULTS The mean concentrations (± SE) of plasma progesterone, total oestrogens and corticosteroids throughout pregnancy and around parturition are shown in Fig . 1 . Pregnancy influenced concentrations of progesterone (P<0-05), total oestrogens (P<0 . 01) and corticosteroids (P<0 . 01) . Plasma concentrations of progesterone fluctuated between 8 . 9 and 9 . 7 ng/ml (9 . 4±0 . 2 ng/ml) until the third month of pregnancy,
HORMONE CHANGES DURING PREGNANCY
27
decreasing significantly (P<0-05) to 6 . 9 ±0 . 8 ng/ml at the fourth month and remaining below 6 . 5 ng/ml until late pregnancy . Concentrations of plasma total oestrogens were low for the first 3 months of pregnancy (24-8±3-1 pg/ml), then increased progressively (P<0-05) up to the sixth month (178 . 2 ± 20 .0 pg/ml) and remained fairly constant thereafter. Concentrations of corticosteroids in plasma of pregnant cows were high from I month after mating (8.0 ± 1 .0 ng/ml) to the third month of pregnancy (8 . 3 ± 0 . 8 ng/ ml). After a statistically significant drop (P< 0 . 05) at the fourth month (4 . 3 ± 0 .7 ng/ml), plasma corticosteroid concentrations remained around 3 ng/ml from the fifth to the ninth month of pregnancy . In periparturient cows, mean plasma progesterone concentrations increased progressively from 5 .7 ± 0 . 4 ng/ml on day -7 to 7 . 5 ± 1 . 0 ng/ml on day -5, then decreased significantly (P<0 . 01) and linearly from 6 . 9± 1 . 0 ng/ml on day -3, reaching baseline values at and immediately after parturition . Values of plasma total oestrogens started to increase sharply (P< 0 .01) from below 200 pg/ml on day -6 . 5 to 2003 .0 ± 70 .3 pg/ml on day -5 and continued to increase steeply, peaking on the day of calving (4604.0 ± 25 . 6 pg/ml) . This was followed by an abrupt decline to 198 . 3 ± 16 . 5 pg/ml 12 hours after parturition . Plasma concentrations of corticosteroids showed no differences among most of the prepartal periods (overall mean 5 . 8 ± 0 . 8 ng/ml), with only two sharp increases. The first increase (10 . 7 ± 1 . 1 ng/ml) was short and abrupt, occurring on day -6. The onset of the second rise on day -1 was followed by a linear and gradual increase until day of calving (8 . 6 ± 0 . 4 ng/ml) where concentrations began to decrease . These two peaks of plasma corticosteroids were observed in all eight animals studied .
DISCUSSION The present results indicated that plasma progesterone concentrations remained fairly constant from 4 to 9 months of pregnancy. These findings were comparable to those reported by Randel & Erb (1971), Robertson (1972), Ginther et at (1974) and Terblanche & Labuschagne (1981) . The higher plasma values of progesterone during the first 3 months of pregnancy might be associated with the formation of accessory luteal tissue arising from an ovulation which frequently occurs within this pregnancy period (Robertson, 1972). Moreover, the early bovine conceptus could exert a significant and positive luteotrophic effect (Bulman & Lamming, 1978) . The finding that levels of plasma total oestrogens increased steeply from the fourth to sixth month and then fluctuated within a narrow range until late pregnancy was in agreement with the findings of Eraldo, Leopard & Gainani (1974), Seren, Bono & Bolelli (1976) and Choi, Möstl & Rechberger (1983) . In our study, concentrations of plasma corticosteroids were markedly higher in the early than in mid and late pregnancy periods . The results conflict with those reported by Erb, Randel & Callahan (1971) and Randel & Erb (1971) who found no significant differences in plasma corticosteroids throughout pregnancy . The discrepancy might be due to differences in frequency of sampling . Elevated concentrations of plasma oestrogens during the 5 days preceding delivery in conjunction with decreased values of plasma progesterone 2-3 days prepartum have also been described by Seren et at (1977), Möstl et at (1981) and Heshmat & Shehata (1982) . In contrast, Arije et al. (1974), Hunter et al. (1977), Guilbault (1985) and Williams &
28
BRITISH VETERINARY JOURNAL, 146, 1
Gross (1986) found a marked elevation in blood oestrogens simultaneously with a gradual decrease in blood progesterone over the last 30 days or so of gestation . Plasma concentrations of corticosteroids peaked at day -6 and again on the day of parturition . The corticosteroid profile recorded in this study disagrees with the conflicting reports on blood corticosteroid concentrations in prepartal cows ; concentrations have been found not to vary during the entire prepartal period (Hunter et al, 1977 ; Seren et al, 1977), to increase markedly between 3 and 1 days prepartum (Arije et al., 1974 ; Susan Hudson et al., 1976 ; Heshmat & Shehata, 1982), or to increase sharply on the day of calving only (Smith et al, 1973) . Some of these discrepancies could be due in part to insufficient blood sampling or the low number of animals studied . Also, the method of blood collection can influence the profile of corticosteroids (Hunter et al., 1977) . The prepartal rise in blood corticosteroids reported by Smith et al (1973), Arije et al (1974), Susan Hudson et al (1976) and Heshmat & Shehata (1982) coincided only with the second increase encountered in this study at 24 hours before calving. These authors attributed the elevated blood corticosteroids to the stress of delivery . On the basis of a comparative analysis of the data presented above, we have formulated an hypothesis about the physiological factors which induce parturition in the cow . A fundamental role is certainly played by the abrupt increase in plasma corticosteroids on the sixth day before parturition . This corticosteroid peak seems to shut off the source of progesterone in the corpus luteum as well as the adrenals, whether directly or indirectly, and must be related to production of a luteolysin, interruption of luteotrophic support or both (Evans & Wagner, 1976) . Furthermore, the increased corticosteroid level at this time stimulates increased secretion of placental oestrogens, resulting in increased release of prostaglandins by the endometrium (Pimentel et al, 1986) . Rising concentrations of placental oestrogens also create a large oestrogen to progesterone ratio . In advance of parturition, this ratio became critical for stimulating the development of oxytocin receptors (Wendorf, Lawyer & First, 1983) . In this way the inhibiting action exerted by progesterone on uterine motility would cease, thereby allowing prostaglandins and oxytocin to act on the myometrium, so that birth may ensue .
ACKNOWLEDGEMENT Hormonal assays were conducted at the Clinic of Obstetrics and Gynaecology of Cattle, High Veterinary School, Hanover, West Germany, utilizing laboratory facilities under the direction of Prof. Dr E . Grunert .
REFERENCES ARIDE, C . R ., WILTBANK, J. N . & HOPWOOD, M . L . (1974) . Journal of Animal Science 39,338 . BULMAN, D .C .& LAMMING, G. E . (1978) . Journal of Reproduction and Fertility 34,447. CHOI, H . S., MOSTL, E . & RECHBERGER, H . (1983) . Wiener tierärztliche Monatsschrift 70, 250. DA SILVA, A. E . (1979). Inaugural Dissertation, Hanover . EISSA, H . M . (1985) . Inaugural Dissertation, Hanover . ERALDO, S ., LEOPARD, A . & GAINANI, R . (1974) . Atti della Societa Italiana delle Science Veterinarie 28, 339 . ERB, R . E., RANDEL, R . D . & CALLAHAN, C . J . (1971). Journal of Animal Science 32 (Suppl . 1), 80.
HORMONE CHANGES DURING PREGNANCY
29
EVANS, L. E . & WAGNER, W . C . (1976) . Acta Endocrinologica 81, 385 . GINTHER, 0 . J ., NUTZ, N ., WENTWORTH, B. C. & TYLER, W . J . (1974) . Proceedings of the Society for Experimental Biology and Medicine 146, 354. GUILBAULT, L . A . (1985) . Dissertation Abstracts International 45, 2827-B . HESHMAT, H. A. & SHEHATA, Y . M . (1982) . Indian Journal of Animal Science 52, 1173 . HUNTER, J . T ., FAIRCLOUGH, R . J ., PETERSON, A . J . & WELCH, R . A . S . (1977) . Acta Endocrinologica 84, 653 . LINNEWEBER, F . (1981) . Inaugural Dissertation, Hanover. MOSTL, E ., MÖSTL, K ., CHOI, H . S ., STOCKL, H . K . & BAMBERG, E . (1981) . Journal of Endocrinology 89, 251 . MURPHY, B . E . P . (1967) . Journal of Clinical Endocrinology and Metabolism 27, 463 . PIMENTEL, C. A., VIJAYKUMAR, R ., WESTON, P . G ., PIMENTEL, S . M ., WAGNER, W . C . & HIXON, J . E . (1986) . American Journal of Veterinary Research 47, 1972 . RANDEL, R. D . & ERB, R . E . (1971) . Journal of Animal Science 33,115 . ROBERTSON, H . A. (1972) . Canadian Journal of Animal Science 52, 645 . SEREN, E ., BONO, G . & BOLELLI, G . (1976) . Archivio Veterinario Italiano 27, 155 . SEREN, E ., BONO, G ., DE FANTI, C . & MATTEUZZI, A. (1977) . Archivio Veterinario Italiano 28, 152 . SMITH, V . G ., EDGERTON, L . A ., HAFS, H . D . & CONVEY, E . M. (1973) . Journal of Animal Science 36, 391 . SNEDECOR, G . W . & COCHRAN, W . G . (1976) . Statistical Methods, 6th edn, p . 258 . Ames, Iowa : Iowa State Univ . Press . SUSAN HUDSON, S ., MULLFORD, M., WHITTLESTONE, W . G . & PAYNE, E . (1976) . Journal of Dairy Science 59, 744. TERBLANCHE, H . M . & LABUSCHAGNE, J . M . (1981) . Journal of the South African Veterinary Association 52,187 . WEISS, Y . & SCHERZINGER, E . (1973) . Zentralblatt für Veterinärmedizin A20, 661 . WENDORF, G . L ., LAWYER, M . S . & FIRST, N . L . (1983). Journal of Reproduction and Fertility 68, 281 . WILLIAMS, W . F. & GROSS, T . S . (1986) . Biology of Reproduction 34 (Suppl. 1), 77. WINER, B . J . (1971) . Statistical Principles in Experimental Design, 2nd edn, p. 105 . New York-London : McGraw-Hill Book Co . (Accepted for publication 6 March 1989)
3.
(1990) . 146, 24
SEQUENTIAL CHANGES IN PLASMA PROGESTERONE, TOTAL OESTROGENS AND CORTICOSTEROIDS IN THE COW THROUGHOUT PREGNANCY AND AROUND PARTURITION
H . M . EISSA and M . S . EL-BELELY Department of Surgery and Theriogenology, Faculty of Veterinary Medicine, Cairo University, Giza, Egypt
SUMMARY Blood samples were taken thrice weekly for 2 weeks from 33 pregnant cows in nine groups, 1 month apart, and twice daily for 8 days commencing 1 week before parturition from eight cows . Plasma progesterone and total oestrogens were quantified by validated radioimmunoassays and total corticosteroids by a competitive protein binding technique . During the first 3 months of pregnancy, no remarkable changes were observed for the three hormones studied . From the fourth month on, concentrations of progesterone and total corticosteroids decreased significantly (P<0-05) and remained fairly constant until the ninth month . Values of total oestrogens increased significantly (P< 0 . 05) from the fourth to the sixth month and thereafter remained unchanged . In the periparturient cows, progesterone levels decreased linearly (P<0 . 01) during the 3 days preceding delivery . Concentrations of total oestrogens rose sharply from day -5 and peaked at calving . Values of total corticosteroids increased dramatically on the sixth day and also at 24 hours before parturition . These results suggest that, contrary to earlier studies, plasma corticosteroids could have a primary key role for triggering parturition in cows .
INTRODUCTION Although there have been several studies concerning the concurrent changes of progesterone, oestrogens and total corticosteroids in the blood of cows during late pregnancy and around parturition, results are inconsistent and not clearly expressed (Randel & Erb, 1971 ; Smith et al., 1973 ; Arije, Wiltbank & Hopwood, 1974 ; Hunter et al., 1977 ; Seren et al., 1977 ; Heshmat & Shehata, 1982) . However, in most of these studies only a few animals with infrequent sampling were used . This paper was designed to investigate the pattern of concentrations of plasma progesterone, total oestrogens and corticosteroids throughout the whole length of gestation and around parturition in greater detail than has hitherto been reported .
HORMONE CHANGES DURING PREGNANCY
25
MATERIALS AND METHODS Animals and sampling
German black pied cows in the Clinic of Cattle Diseases and the Clinic of Obstetrics and Gynaecology of Cattle at the High School of Veterinary Medicine, Hanover, were used . Thirty-three pregnant cows were chosen . Based on the recorded date of mating, confirmed by rectal findings, these animals were allotted to nine groups, 1 month apart, each group including three to five animals (Fig . 1) . Blood samples were collected from each pregnant animal thrice weekly for 2 weeks . 4800
12
4200
E
rn
10 3600
v ó
oo
8 3000
oo ó
6
E
c c 4 d N
0- L
i
1 0
1 2 3 4 5 6 7 8 9 -7 (4) (3) (3) (5) (3) (4) (3)(5)(3)(8) Month of pregnancy
-6 (8)
-5 (8)
-4 (8)
-3 (8)
-2 (8)
-1 (8)
0 (8)
+1 (8)
Days before (-) and after (+) calving
Fig. 1 .
Mean±sE plasma concentrations of progesterone ( •- •-•) , total oestrogens (X- X) and corticosteroids (•- --0) throughout pregnancy and around parturition . Values in the lefthand part of the figure are the means of different groups of cows, each sampled six times . Figures in parentheses are numbers of animals .
In addition, eight pregnant cows were selected 10-15 days before the estimated time of parturition . Blood samples were collected from these animals twice daily at 7 a .m. and 6 p .m. Breeding records (gestation period ranged from 279 to 282 days) and daily observations for symptoms of calving (loosening of the sacrosciatic ligaments, enlargement of the mammary gland and vulva and discharge of mucus) were used to estimate the expected time of calving . The time of collection of all the samples was subsequently standardized from the time of parturition taken as day 0 . Blood samples collected during the 7 days before parturition and on the day of calving were taken for hormone analysis . Blood collection
Jugular vein blood samples (20 ml) for progesterone and oestradiol analyses were collected by venepuncture . Tail vein blood samples (5 ml) for corticosteroid determination were obtained prior to jugular blood sampling in order to minimize the effects
26
BRITISH VETERINARY JOURNAL, 146, 1
of stress on blood corticosteroid level . All collections were made over 3 months from January to March . Blood samples were drawn into heparinized tubes and centrifuged immediately at 3000 g for 20 min . Plasma was stored at -20° C until assayed . Hormonal assays
Tritiated progesterone (1,2,6,7_ 3 H-progesterone, 80-100 Ci/mmol) and oestrogen (SE1- 3 H-oestradiol-17 1g-AK-6, 80-120 Ci/mmol) were obtained from the Institute of Physiology, High Veterinary School, Hanover . The labelled corticosteroid (1,2,6,7-3 Hhydrocortisone, 90-115 Ci/mmol) was purchased from Firma NEN Chemicals GmbH, Dreieichenhain . The plasma progesterone concentration was determined by the RIA technique described by Linneweber (1981) . The antibody used to assay progesterone cross-reacted strongly (96%) only with 3a-dihydroprogesterone . The sensitivity of the assay, defined as the smallest concentration significantly (P< 0 . 05) distinguishable from zero, was 0 . 1 ng/ ml plasma . The average intra- and inter-assay coefficients of variation were 6 . 8% (n -8) and 8 . 2% (n-15), respectively . Concentration of plasma total oestrogens was assayed by the method reported by Da Silva (1979) . The antibody for total oestrogens cross-reacted 100% with oestrone, 68 . 5% with 17p-oestradiol, 23 . 6% with 17a-oestradiol and 10% with oestriol . The sensitivity of the assay was 10 pg/ml plasma . Intra- and inter-assay coefficients of variation averaged 7 . 2% (n=10) and 8 . 6% (n=18), respectively . The plasma level of corticosteroids was measured by a competitive protein binding assay employed by Murphy (1967) as modified by Weiss & Scherzinger (1973) and Eissa (1985) . The antibody to corticosteroids cross-reacted 96% with cortisone, 91% with corticosterone and 85% with cortisol . The sensitivity of the assay, intra- and inter-assay coefficients of variation were 0 . 1-0 . 2 ng/ml plasma, 10 . 5% (n ° 33) and 10% (n=25), respectively Statistical analyses
For each pregnant animal sampled thrice weekly over 2 weeks, mean values of hormone concentrations for the six samples were calculated for statistical analyses . This was because there were negligible changes in concentrations of progesterone, total oestrogens or corticosteroids among times of blood sampling . The effect of pregnancy on hormone concentrations was determined by one-way analysis of variance . If the F-value was significant, differences in means amongst groups were evaluated by the Studentized range Qmethod and were considered to be significant if P< 0 . 05 (Snedecor & Cochran, 1976) . Hormone concentrations in samples taken twice daily from the cows approaching parturition were analysed by analysis of variance for repeated measurements (Winer, 1971) . RESULTS The mean concentrations (± SE) of plasma progesterone, total oestrogens and corticosteroids throughout pregnancy and around parturition are shown in Fig . 1 . Pregnancy influenced concentrations of progesterone (P<0-05), total oestrogens (P<0 . 01) and corticosteroids (P<0 . 01) . Plasma concentrations of progesterone fluctuated between 8 . 9 and 9 . 7 ng/ml (9 . 4±0 . 2 ng/ml) until the third month of pregnancy,
HORMONE CHANGES DURING PREGNANCY
27
decreasing significantly (P<0-05) to 6 . 9 ±0 . 8 ng/ml at the fourth month and remaining below 6 . 5 ng/ml until late pregnancy . Concentrations of plasma total oestrogens were low for the first 3 months of pregnancy (24-8±3-1 pg/ml), then increased progressively (P<0-05) up to the sixth month (178 . 2 ± 20 .0 pg/ml) and remained fairly constant thereafter. Concentrations of corticosteroids in plasma of pregnant cows were high from I month after mating (8.0 ± 1 .0 ng/ml) to the third month of pregnancy (8 . 3 ± 0 . 8 ng/ ml). After a statistically significant drop (P< 0 . 05) at the fourth month (4 . 3 ± 0 .7 ng/ml), plasma corticosteroid concentrations remained around 3 ng/ml from the fifth to the ninth month of pregnancy . In periparturient cows, mean plasma progesterone concentrations increased progressively from 5 .7 ± 0 . 4 ng/ml on day -7 to 7 . 5 ± 1 . 0 ng/ml on day -5, then decreased significantly (P<0 . 01) and linearly from 6 . 9± 1 . 0 ng/ml on day -3, reaching baseline values at and immediately after parturition . Values of plasma total oestrogens started to increase sharply (P< 0 .01) from below 200 pg/ml on day -6 . 5 to 2003 .0 ± 70 .3 pg/ml on day -5 and continued to increase steeply, peaking on the day of calving (4604.0 ± 25 . 6 pg/ml) . This was followed by an abrupt decline to 198 . 3 ± 16 . 5 pg/ml 12 hours after parturition . Plasma concentrations of corticosteroids showed no differences among most of the prepartal periods (overall mean 5 . 8 ± 0 . 8 ng/ml), with only two sharp increases. The first increase (10 . 7 ± 1 . 1 ng/ml) was short and abrupt, occurring on day -6. The onset of the second rise on day -1 was followed by a linear and gradual increase until day of calving (8 . 6 ± 0 . 4 ng/ml) where concentrations began to decrease . These two peaks of plasma corticosteroids were observed in all eight animals studied .
DISCUSSION The present results indicated that plasma progesterone concentrations remained fairly constant from 4 to 9 months of pregnancy. These findings were comparable to those reported by Randel & Erb (1971), Robertson (1972), Ginther et at (1974) and Terblanche & Labuschagne (1981) . The higher plasma values of progesterone during the first 3 months of pregnancy might be associated with the formation of accessory luteal tissue arising from an ovulation which frequently occurs within this pregnancy period (Robertson, 1972). Moreover, the early bovine conceptus could exert a significant and positive luteotrophic effect (Bulman & Lamming, 1978) . The finding that levels of plasma total oestrogens increased steeply from the fourth to sixth month and then fluctuated within a narrow range until late pregnancy was in agreement with the findings of Eraldo, Leopard & Gainani (1974), Seren, Bono & Bolelli (1976) and Choi, Möstl & Rechberger (1983) . In our study, concentrations of plasma corticosteroids were markedly higher in the early than in mid and late pregnancy periods . The results conflict with those reported by Erb, Randel & Callahan (1971) and Randel & Erb (1971) who found no significant differences in plasma corticosteroids throughout pregnancy . The discrepancy might be due to differences in frequency of sampling . Elevated concentrations of plasma oestrogens during the 5 days preceding delivery in conjunction with decreased values of plasma progesterone 2-3 days prepartum have also been described by Seren et at (1977), Möstl et at (1981) and Heshmat & Shehata (1982) . In contrast, Arije et al. (1974), Hunter et al. (1977), Guilbault (1985) and Williams &
28
BRITISH VETERINARY JOURNAL, 146, 1
Gross (1986) found a marked elevation in blood oestrogens simultaneously with a gradual decrease in blood progesterone over the last 30 days or so of gestation . Plasma concentrations of corticosteroids peaked at day -6 and again on the day of parturition . The corticosteroid profile recorded in this study disagrees with the conflicting reports on blood corticosteroid concentrations in prepartal cows ; concentrations have been found not to vary during the entire prepartal period (Hunter et al, 1977 ; Seren et al, 1977), to increase markedly between 3 and 1 days prepartum (Arije et al., 1974 ; Susan Hudson et al., 1976 ; Heshmat & Shehata, 1982), or to increase sharply on the day of calving only (Smith et al, 1973) . Some of these discrepancies could be due in part to insufficient blood sampling or the low number of animals studied . Also, the method of blood collection can influence the profile of corticosteroids (Hunter et al., 1977) . The prepartal rise in blood corticosteroids reported by Smith et al (1973), Arije et al (1974), Susan Hudson et al (1976) and Heshmat & Shehata (1982) coincided only with the second increase encountered in this study at 24 hours before calving. These authors attributed the elevated blood corticosteroids to the stress of delivery . On the basis of a comparative analysis of the data presented above, we have formulated an hypothesis about the physiological factors which induce parturition in the cow . A fundamental role is certainly played by the abrupt increase in plasma corticosteroids on the sixth day before parturition . This corticosteroid peak seems to shut off the source of progesterone in the corpus luteum as well as the adrenals, whether directly or indirectly, and must be related to production of a luteolysin, interruption of luteotrophic support or both (Evans & Wagner, 1976) . Furthermore, the increased corticosteroid level at this time stimulates increased secretion of placental oestrogens, resulting in increased release of prostaglandins by the endometrium (Pimentel et al, 1986) . Rising concentrations of placental oestrogens also create a large oestrogen to progesterone ratio . In advance of parturition, this ratio became critical for stimulating the development of oxytocin receptors (Wendorf, Lawyer & First, 1983) . In this way the inhibiting action exerted by progesterone on uterine motility would cease, thereby allowing prostaglandins and oxytocin to act on the myometrium, so that birth may ensue .
ACKNOWLEDGEMENT Hormonal assays were conducted at the Clinic of Obstetrics and Gynaecology of Cattle, High Veterinary School, Hanover, West Germany, utilizing laboratory facilities under the direction of Prof. Dr E . Grunert .
REFERENCES ARIDE, C . R ., WILTBANK, J. N . & HOPWOOD, M . L . (1974) . Journal of Animal Science 39,338 . BULMAN, D .C .& LAMMING, G. E . (1978) . Journal of Reproduction and Fertility 34,447. CHOI, H . S., MOSTL, E . & RECHBERGER, H . (1983) . Wiener tierärztliche Monatsschrift 70, 250. DA SILVA, A. E . (1979). Inaugural Dissertation, Hanover . EISSA, H . M . (1985) . Inaugural Dissertation, Hanover . ERALDO, S ., LEOPARD, A . & GAINANI, R . (1974) . Atti della Societa Italiana delle Science Veterinarie 28, 339 . ERB, R . E., RANDEL, R . D . & CALLAHAN, C . J . (1971). Journal of Animal Science 32 (Suppl . 1), 80.
HORMONE CHANGES DURING PREGNANCY
29
EVANS, L. E . & WAGNER, W . C . (1976) . Acta Endocrinologica 81, 385 . GINTHER, 0 . J ., NUTZ, N ., WENTWORTH, B. C. & TYLER, W . J . (1974) . Proceedings of the Society for Experimental Biology and Medicine 146, 354. GUILBAULT, L . A . (1985) . Dissertation Abstracts International 45, 2827-B . HESHMAT, H. A. & SHEHATA, Y . M . (1982) . Indian Journal of Animal Science 52, 1173 . HUNTER, J . T ., FAIRCLOUGH, R . J ., PETERSON, A . J . & WELCH, R . A . S . (1977) . Acta Endocrinologica 84, 653 . LINNEWEBER, F . (1981) . Inaugural Dissertation, Hanover. MOSTL, E ., MÖSTL, K ., CHOI, H . S ., STOCKL, H . K . & BAMBERG, E . (1981) . Journal of Endocrinology 89, 251 . MURPHY, B . E . P . (1967) . Journal of Clinical Endocrinology and Metabolism 27, 463 . PIMENTEL, C. A., VIJAYKUMAR, R ., WESTON, P . G ., PIMENTEL, S . M ., WAGNER, W . C . & HIXON, J . E . (1986) . American Journal of Veterinary Research 47, 1972 . RANDEL, R. D . & ERB, R . E . (1971) . Journal of Animal Science 33,115 . ROBERTSON, H . A. (1972) . Canadian Journal of Animal Science 52, 645 . SEREN, E ., BONO, G . & BOLELLI, G . (1976) . Archivio Veterinario Italiano 27, 155 . SEREN, E ., BONO, G ., DE FANTI, C . & MATTEUZZI, A. (1977) . Archivio Veterinario Italiano 28, 152 . SMITH, V . G ., EDGERTON, L . A ., HAFS, H . D . & CONVEY, E . M. (1973) . Journal of Animal Science 36, 391 . SNEDECOR, G . W . & COCHRAN, W . G . (1976) . Statistical Methods, 6th edn, p . 258 . Ames, Iowa : Iowa State Univ . Press . SUSAN HUDSON, S ., MULLFORD, M., WHITTLESTONE, W . G . & PAYNE, E . (1976) . Journal of Dairy Science 59, 744. TERBLANCHE, H . M . & LABUSCHAGNE, J . M . (1981) . Journal of the South African Veterinary Association 52,187 . WEISS, Y . & SCHERZINGER, E . (1973) . Zentralblatt für Veterinärmedizin A20, 661 . WENDORF, G . L ., LAWYER, M . S . & FIRST, N . L . (1983). Journal of Reproduction and Fertility 68, 281 . WILLIAMS, W . F. & GROSS, T . S . (1986) . Biology of Reproduction 34 (Suppl. 1), 77. WINER, B . J . (1971) . Statistical Principles in Experimental Design, 2nd edn, p. 105 . New York-London : McGraw-Hill Book Co . (Accepted for publication 6 March 1989)