Initiation of parturition and lactation in the sow: effects of delaying parturition with medroxyprogesterone acetate.

The synthetic progestagen, medroxyprogesterone acetate (MPA), was administered to sows in late pregnancy with the objective of slightly delaying the time of farrowing and thereby providing more marked associations between hormonal changes and the termination of pregnancy, and the initiation of farrowing and lactation in this species. MPA was administered orally (140 mg, twice daily) to eight sows in late pregnancy on days 112, 113 and 114 of gestation. Parturition was then induced to occur on day 116 by injecting 200 micrograms cloprostenol i.m. on day 115 of gestation. The peripartum changes in the plasma concentrations of progesterone, cortisol, oestradiol-17 beta, relaxin, prolactin, lactose and 13,14-dihydro-15-keto prostaglandin F2 alpha (PGFM) were measured in these sows together with a group of untreated sows. The gestational length for the MPA-treated sows (116.3 +/- 0.3 days, mean +/- S.E.M.) was significantly (P less than 0.01) greater compared with the untreated sows (114.9 +/- 0.3 days). Plasma progesterone declined earlier (P less than 0.05) with respect to the time of parturition in the treated sows compared with the untreated group. With respect to the timing of parturition, the time at which maximal concentrations of relaxin were attained and the timing of the subsequent decline were earlier in the MPA-treated sows. In both groups of sows, the concentration of relaxin increased before the decline in plasma progesterone. In the untreated sows, the concentration of PGFM increased either slightly before or at the same time as the decline in plasma progesterone, whereas in sows treated with MPA, progesterone concentrations began to decline before any significant increase in the plasma concentration of PGFM. The profiles of cortisol, oestradiol-17 beta and PGFM were similar in both groups of sows. In both groups of sows, the timing of the initial increase in the concentration of plasma prolactin coincided with a similar rise in plasma lactose (P less than 0.01). Plasma progesterone either declined earlier or at the same time as the rise in plasma lactose (P less than 0.01) in the treated group of sows only. We conclude that since the prepartum changes in the concentration of progesterone and relaxin occurred before significant changes in the concentration of PGFM in the MPA-treated sows, the nature of the luteolytic factor and the mechanism by which it exerts its action remains obscure. The higher concentration of lactose in the mammary secretion at birth in the MPA-treated sows compared with the untreated group suggested that lactogenesis was initiated earlier with respect to parturition following MPA treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Lactose in plasma during lactogenesis, established lactation and weaning in sows.

The concentration of lactose in plasma was determined in different sows at all phases of their reproductive cycle and related to the compositional changes in mammary secretion during lactogenesis, established lactation and weaning. Lactose was present in low concentrations (3-4 microM) in the blood of virgin sows and pregnant sows up to 107 days of gestation. From day 4 pre-partum to day 1 pre-partum circulating lactose rose gradually to 34.5 +/- 7.7 microM (mean +/- S.E. of mean). Maximal concentrations of 262 +/- 168.4 microM were reached 6 h after parturition. The concentration of lactose in plasma was correlated with the amount of lactose in mammary secretion (r = 0.88, P less than 0.01) at the beginning of farrowing. During established lactation the concentrations of lactose, Na and K in milk, and of lactose in plasma (72-86 microM), were constant. The concentration of lactose in plasma did not vary significantly during periods of suckling, or after stimulation of milk ejection by oxytocin. However, the amount of lactose in plasma rose significantly (P less than 0.02) after the administration of oxytocin if milk ejection was not accompanied by suckling. The mean plasma concentration of lactose began to rise 36 h after weaning to a peak value of 241.8 +/- 53.6 microM at 48 h; thereafter it declined to 10.2 +/- 2.0 microM by 6 days. This study has shown that lactose concentrations in the plasma vary according to the secretory activity of the mammary gland. Its plasma concentration provides an earlier temporal measure of lactogenesis in individual sows than is obtained either from observation or analysis of mammary secretion.

Total and free plasma concentrations of progesterone, cortisol and oestradiol-17 beta during pregnancy, parturition and early lactation in sows.

The total (bound plus free) concentrations of progesterone, 20 alpha-dihydroprogesterone, oestradiol-17 beta and cortisol were determined in the plasma of sows at three stages during pregnancy and more intensively from 5 days pre-partum to 5 days post-partum. The free fractions of progesterone, oestradiol-17 beta and cortisol were measured in the same samples by a rate dialysis method. Up to day 110 of gestation, the amounts of free hormone in plasma did not fluctuate independently of their total concentrations. During farrowing, the total and free concentrations of progesterone and cortisol varied independently of each other, whereas total and free oestradiol-17 beta declined simultaneously. The initiation of parturition was associated with a decrease in circulating total progesterone, and was accentuated by a decrease in the free fraction (P less than 0.005) so that its active free concentration was only 20% of its day 1 pre-partum value. Total and free cortisol concentrations rose rapidly during labour so that at 12-18 h after birth of the first piglet 30% of that cortisol in maternal plasma was free hormone.

Pregnenolone-binding components in the porcine adrenal gland.

The object of this study was to determine whether binding components for pregnenolone, analogous to those described in the adrenal cortex of guinea pigs and rats, were present in the porcine adrenal. A binding component for pregnenolone in the cytosolic fraction of porcine adrenal was demonstrated by sucrose density gradient centrifugation. It banded maximally at 9.6% sucrose (w/w) compared to 12.2% and 12.4% sucrose (w/w) for the plasma-binding component and serum albumin, respectively. At a pregnenolone concentration of 1 X 10(-5) M, specific cytosolic binding of 1 X 10(-8) M [3H]pregnenolone was decreased by 42%. The fractions from sucrose gradients which bound pregnenolone maximally contained 3 beta-hydroxysteroid dehydrogenase-isomerase. The cytosolic supernatant of porcine adrenal gland was resolved by chromatography on hydroxyapatite into eleven fractions, four of which bound added pregnenolone and three of which displayed enzymatic activity. Electrophoretic analysis of the enzymatically active fractions in polyacrylamide gel showed that two of them were of heterogeneous composition, whereas the third, most enzymatically active, fraction consisted principally of one band of high molecular weight.