Parturition and lactation in the bitch: serum progesterone, cortisol and prolactin

Concurrent changes in serum progesterone, cortisol and prolactin associated with the prepartum period and lactation of 6 suckling and 1 nonsuckling Beagle bitches were determined by radioimmunoassay. Progesterone in each bitch fell sharply prior to parturition and remained low during lactation. Mean progesterone at approximately 120, 36, 20 and 10 h prepartum was 4.5 ± 0.6, 3.1 ± 0.4, 1.2 ± 0.4 and 0.6 ± 0.1 ng/ml, respectively. In 6 of 7 bitches, serum cortisol was elevated above mean prepartum levels (23 ± 1 ng/ml) during the day prior to the onset of parturition, reached peak levels of 63 ± 7 ng/ml at 8-24 h prepartum and fell to 19 ± 4 ng/ml at 8-12 h postpartum. Mean cortisol remained between 22 ± 1 and 27 ± 5 ng/ml during lactation and weaning. During the last week of pregnancy, serum prolactin levels (14-97 ng/ml) were variable within (50 ± 2 to 33 ± 8 ng/ml) and among (25 ± 2 to 80 ± 9 ng/ml) bitches and averaged 40 ± 7 ng/ml. In each bitch, prolactin increased by 195 ± 29% during the 16-56 h prepartum and reach peak levels (117 ± 24 ng/ml) at 8-32 (21 ± 3) h prepartum in 6 bitches and at 24 h postpartum in the remaining bitch. By 36 h after these peaks, prolactin levels were reduced to 37 ± 8 ng/ml before again increasing in response to suckling. Mean prolactin increased during the first week of lactation, peaked at 86 ± 19 ng/ml at 1.5 weeks of lactation, fell slowly to 43 ± 6 ng/ml at 5 weeks, shortly before weaning and then fell abruptly to 13 ± 2 ng/ml following weaning. The hormone changes observed were similar to those reported for several other species and suggest that the bitch may be a useful model for the study of maternal endocrine adjustments.