Prolongation of diestrus after removal of pups in lactating rats.
The recurrence of diestrus following removal of pups on day 10 of lactation was studied in lactating rats. Pituitary responsiveness to LHRH, and plasma LH, prolactin and progesterone levels were determined after removal of pups. Diestrous periods following romoval of pups were 8.61±0.36 and 3.33±0.21 days in the females which had been nursing 12 and 2 pups, respectively. Seventy-eight % of animals that had nursed 12 pups exhibited typical vaginal smear of pro-estrus on day 2 or 3 of post-weaning, but none of these animals ovulated on the following day. The number of follicles over 550μm in diameter rapidly in-creased from day 1 (1.7±1.2) to day 2 of post-weaning (7.6±1.9) in females that had nursed 12 pups. On day 2 or 3 of post-weaning obvious LH surge occurred in females that had nursed 2 pups, but not in animals that had nursed 12 pups. The rise of plasma LH levels following LHRH injection was less and the ovulatory responsiveness to LHRH injection was lower in females nursing 12 pups than those in rats nursing 2 pups on day 2 or 3 of post-weaning. High plasma progesterone levels were maintained until day 6 of post-weaning in animals that had nursed 12 pups, while concentration decreased to the basal level within 18 h after removal of pups in females nursing 2 pups. In accordance with the high level of plasma progesterone, the fluctuation of prolactin levels was observed following removal of pups in animals that had nursed 12 pups. It was suggested that the prolongation of the diestrous period following removal of pups in the rats nursing a large litter was attributable to the reduced responsiveness of pituitary to LHRH and to the maintenance of functional corpora lutea formed at post-partum ovulation.