Abstract: 211

OVARIAN STEROIDOGENIC RESPONSIVENESS TO GONADOTROPIN STIMULATION IN YOUNG AND MIDDLE-AGED FEMALE RATS.

Bey-Ying Chern¹ , Jean Chen¹ , Long-Sheng Hong¹ , Philip S. LaPolt^{1 *}
Dept. of Biology & Microbiology, California State University, Los Angeles, CA 90032 ¹

Middle-aged cyclic (MC) female rats display attenuated progesterone (P) and luteinizing hormone (LH) surges on proestrus. MC rats then enter the acyclic persistent-estrous (PE) state, characterized by further declines in P and LH release. This study examined whether decreased P levels in aging rats reflect altered ovarian sensitivity to gonadotropin stimulation. Young cyclic (Y) and MC proestrous rats received sodium pentobarbitol to block endogenous LH surges, followed by administration of human chorionic gonadotropin (hCG; 2.5, 5.0, or 10 mIU/g bw) at 1600 h. Similar treatments were performed in PE rats. Serial blood samples were obtained between 1600-2300 h to characterize plasma levels of hCG, LH, and P. Injection of hCG resulted in equivalent plasma hCG levels in each treatment group. The minimal hCG dose that induced a significant (p < 0.05) rise in P levels differed between MC (2.5 mIU/g bw), Y (5.0 mIU/g bw) and PE (10 mIU/g bw) rats, indicating differing sensitivities to gonadotropin stimulation. While all three groups displayed increased P production at the highest dose of hCG, P levels were significantly lower in PE females, compared with Y and MC rats. Analysis of ovarian cytochrome P450 side chain cleavage (P450_scc) expression revealed that attenuated P responses in PE rats were related to lower ovarian P450_scc mRNA levels, compared to Y and MC animals. These findings demonstrate that steroidogenic responsiveness to hCG is higher in MC than Y rats, but lower in PE than MC and Y females. Thus, lower proestrous P levels in MC rats reflect attenuated proestrous LH surges, rather than impaired ovarian response to LH. Further declines in P levels in PE rats are associated with decreased gonadotropin responsiveness, attenuated P450_scc expression, and impaired regulation of LH release.

    This abstract is being presented on Monday, August 2 at 8:00 AM to 10:15 AM at CUB 2nd Floor Ballroom.