Restoration of the LH surge and ovulation by insulin in alloxan-diabetic immature rats treated with pregnant mare's serum gonadotrophin

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Abstract
Immature alloxan-diabetic rats injected with pregnant mare''s serum gonadotropin (PMSG) do not ovulate and the LH [luteinizing hormone] surge is absent. The effects of several insulin treatments on the LH surge and ovulation were examined in alloxan-diabetic rats. Rats were made diabetic by injection of alloxan on day 24 of age. Only those rats with fasting blood glucose concentrations exceeding 180 mg/100 ml on day 27 were considered diabetic. PMSG was injected on day 30. Rats received insulin either by injection (2, 3, 4 or 6 IU/100 g/day; Ultralente twice daily) or by s.c. implants (Alzet osmotic minipumps; 1.8 or 2.4 IU/day). None of the diabetic rats without insulin treatment ovulated. Some of the animals in each insulin treatment group ovulated, however, the percentage of animals ovulating was highly variable from experiment to experiment when the insulin was given by injection. When insulin was administered by osmotic minipump, the results were more consistent, with at least 60% of the rats ovulating in each experiment. LH surges were found on the afternoon of presumed pro-estrus (day 32) in diabetic insulin-treated rats which ovulated. In confirmation of previous results, rats without insulin treatment did not have LH surges. Although the site of insulin action was not determined, the LH surge mechanism in the immature PMSG-treated rat apparently is insulin-dependent.