The relation of ovarian steroid levels in young female rats to subsequent estrous cyclicity and reproductive function during aging.

In multiparous rats, the incidence of regular estrous cyclicity and fertility decreases markedly at middle age. However, recent studies have shown that repeated pregnancies or progesterone (P) implants can subsequently cause retired breeder females to maintain regular cyclicity for an extended period of time; these results suggest a P-mediated deceleration of reproductive aging. In the present study, we examined the relation of ovarian steroid levels in young virgin females to their subsequent estrous cyclicity and reproductive function during aging as compared to multiparous females. Beginning at 4 mo of age and continuing to 6 mo of age, regularly cyclic virgin rats received either consecutive P implants (n = 41) or no implants (controls, n = 45) for 3 wk, followed by implant removal for 1 wk. Additional females (n = 72) were mated and allowed to undergo repeated pregnancies at 4, 6 1/2, and 8 mo of age. Blood samples were obtained throughout the estrous cycle (virgin females), during pregnancy (multiparous rats), and on Day 11 of successive treatments with P implants (virgins with P implants) for P, estradiol (E2), and testosterone (T) measurements. Subsequently, regularly cyclic females from all three groups were mated with fertile males to undergo term pregnancies at 10 and 12 mo of age. While the virgin controls showed cyclic increases in P, T, and E2 secretion during their estrous cycles, the P-implanted females had persistently low E2 and high P and T levels during treatment, which indicates an inhibition of ovarian E2 synthesis by P.(ABSTRACT TRUNCATED AT 250 WORDS)