Ovulatory response, ovarian LH/hCG receptors and serum concentrations of LH, insulin and steroid hormones in immature Zucker rats after PMSG treatment

Abstract Immature female ‘fatty’ (fa/fa) and lean Zucker rats (40 days of age) were injected with a single dose of 4 IU pregnant mare serum gonadotropin (PMSG) to induce ovulation. Rats were killed at 0, 6, 12, 24, 36, 48, 60, 72 and 96 h after PMSG treatment. Ovulatory response was measured by counting the oocytes flushed from the oviducts and ovarian luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptors by receptor assay. Serum luteinizing hormone (LH), insulin, estradiol-17β, progesterone and androgens were measured by radioimmunoassay (RIA). No differences in the numbers of recovered oocytes between fatty and lean rats were observed; however, all ova obtained from fatty rats until 48 h after PMSG injection were abnormal. The ovarian LH/hCG receptor content in fatty rats was greater at 24, 36 and 72 h after PMSG than in lean rats. Serum LH concentrations surged at 36 h in fatty and 6 h in lean rats. No differences in peak values were observed between the two groups. Serum insulin levels in fatty rats were higher between 12 and 96 h than in lean rats. Serum estradiol-17β values in fatty rats were lower at 0 and 12 h after PMSG treatment compared with lean rats and showed a surge at 36 h. No differences in serum progesterone and only minor differences in androgens between fatty and lean rats were observed. These observations provided further evidence of reproductive abnormalities in fatty Zucker rats including high incidence of defective oocytes, decreased serum estradiol-17β levels and consistently elevated serum insulin concentrations.