Induced development of ovulatory follicles during the early stages of lactation by the administration of LH in the rat

To determine whether failure of follicular maturation during the early stages of lactation in rats is due to inadequate LH stimulation, lactating rats nursing eight pups were injected twice daily for 1-3 days (days 2-5 of lactation) with various doses of ovine LH. Follicular maturation was determined by the ability of the follicles to ovulate in response to 10 IU human chorionic gonadotrophin (hCG), endogenous oestradiol-17 beta and inhibin production. Ovulation was not induced in control animals in response to 10 IU hCG given between days 2 and 5 of lactation. On the other hand, an injection of 10 IU hCG could induce ovulation in LH-treated animals, in which 25 and 50 micrograms LH per injection were given s.c. from days 2 to 5 of lactation. Concentrations of oestradiol-17 beta and inhibin activity in ovarian venous plasma increased progressively after the administration of LH, indicating that induced development of ovulatory follicles had occurred. Plasma concentrations of FSH declined in LH-treated animals compared with those in control animals. The decrease in plasma concentrations of FSH was not observed when lactating rats were ovariectomized before the first injection of LH, indicating that ovarian products, probably inhibin, from developing follicles may suppress the secretion of FSH from the pituitary gland. In both LH-treated and control animals, concentrations of prolactin and progesterone remained increased during the period of LH administration. The present results, therefore, suggest that the plasma levels of LH are an important determinant of follicular maturation during lactation in rats.