Insulin increases 17β-estradiol production by the dominant follicle of the first postpartum follicle wave in dairy cows

Prolonged anovulation following parturition has a negative impact on fertility in dairy cows. Insulin plays an important role in ovarian function in many species, and is profoundly depressed in dairy cows during early lactation. We hypothesized that hypoinsulinemia during early lactation represents a key indicator of nutritional status, resulting in delayed ovulation. Holstein cows (n = 10) were subjected to either a hyperinsulinemic-euglycemic clamp (INS) or saline infusion (CTL) for 96 h, beginning on day 10 after parturition during the first postpartum follicular wave. Insulin was infused continuously (0.3 mug/kg body weight per h) via a jugular catheter, and euglycemia was maintained by infusion of glucose. Circulating insulin concentrations were elevated 2.6-fold in INS cows compared with CTL cows (0.73 +/- 0.026 vs 0.28 +/- 0.026 ng/ml; P< 0.001). Insulin treatment did not affect (P> 0.05) luteinizing hormone (LH) pulse frequency, pulse amplitude or mean circulating LH. Circulating estradiol was elevated in INS cows (P< 0.01) and circulating testosterone also tended to be higher. The ratio of testosterone to estradiol was not different between treatments for the initial 30 h of infusion, but was significantly reduced thereafter in response to insulin (P< 0.01), suggesting that hyperinsulinemia increased follicular aromatase activity. Insulin treatment also resulted in reduced circulating nonesterified fatty acids, and increased circulating total and free insulin-like growth factor-1 concentrations. Insulin infusion increased estradiol secretion by the dominant follicle of the first postpartum follicular wave in dairy cows, and this effect appears not to be mediated through changes in pulsatile LH release.