Gonadotropin-Releasing Hormone, Estradiol, and Inhibin Regulation of Follicle- Stimulating Hormone and Luteinizing Hormone Surges: Implications for Follicle Emergence and Selection in Heifers1

Mechanisms regulating gonadotropin surges and gonadotro-pin requirements for follicle emergence and selection were studied in heifers. Experiment 1 evaluated whether follicular inhibins regulate the preovulatory luteinizing hormone (LH)/ follicle-stimulating hormone (FSH) surges elicited by gonado-tropin-releasing hormone (GnRH) injection (Hour 0) and the subsequent periovulatory FSH surge. Treatments included control (n 6), steroid-depleted bovine follicular fluid (bFF) at Hour 4 (n 6), and bFF at Hour 6 (n 6). Gonadotropins in blood were assessed hourly from Hours 6 to 36, and follicle growth tracked by ultrasound. Consistent with inhibin indepen-dence, bFF at Hour 4 did not impact the GnRH-induced preovulatory FSH surge, whereas treatment at Hour 6 delayed onset of the periovulatory FSH surge and impeded growth of