INFLUENCES OF AGE AND REPRODUCTIVE STATUS ON OVARIAN OVULATORY RESPONSIVENESS TO GONADOTROPIN STIMULATION

Reproductive aging in the female rat is associated with the gradual lo ss of regular ovulatory function, decreased fertility, and smaller lit ter sizes. In the present study, we assessed ovarian ovulatory respons iveness to exogenous gonadotropin stimulation in young and middle-aged cyclic females and in middle-aged acyclic persistent-estrous (PE) rat s. The ovulatory response to human chorionic gonadotropin (hCG) was do se-dependent in both young and middle-aged cyclic rats, with the perce ntages of rats ovulating and the numbers of ova shed per rat increasin g with the dose of hCG administered. At the highest dose tested (10 ml U hCG/g bw), the range in ovulation rates among middle-aged cyclic rat s (0-18 ova shed/rat) was greater than that in young animals (12-18 ov a/rat). However, there were no statistically significant differences i n either the percentages of females ovulating or in the mean ovulation rates between young and middle-aged cyclic groups. In contrast to the normal ovulatory responses observed in most middle-aged cyclic animal s, response to hCG was markedly impaired in PE females of the same age . Middle-aged PE rats consistently failed to ovulate in response to a dose of hCG (10 mlU/g bw), which elicited high ovulation rates in youn g rats. At an even higher dose (20 mlU/g bw), only minimal ovulatory r esponses were observed (1.8 +/- 0.8 ova/rat; 80% of rats ovulating). T hus, most middle-aged regularly cyclic females maintain a similar ovul atory responsiveness to hCG as young rats, suggesting that decreased o vulation rates during aging may be related to attenuated preovulatory LH surges. However, impaired ovulatory responses were observed in midd le-aged PE females, indicating altered ovarian function in acyclic ani mals, which may contribute to their anovulatory state.