PLASMA-LH AND FSH AFTER ESTRADIOL, NORGESTOMET AND GN-RH TREATMENT IN OVARIECTOMIZED BEEF HEIFERS

Plasma luteinizing hormone (LH) first decreased then increased in a preovulatory-like fashion after administration of estradiol-17β (E) in ovariectomized heifers. However, in heifers implanted with norgestomet (N), a synthetic progestogen, an injection of E caused no preovulatory-like surge in plasma LH; only the suppression in plasma LH was noted. Injections of luteinizing hormone releasing hormone/follicle-stimulating hormone releasing hormone (Gn-RH) increased serum LH when given alone and in combination with either E, N, or N plus E. There was no evidence that the exogenous steroids blunted the ability of the pituitary to release LH in response to exogenous Gn-RH. Estradiol administered alone or in combination with N depressed plasma FSH. The implant of N alone had no significant effect on plasma FSH. Injections of Gn-RH stimulated only small increases in plasma FSH compared to LH, and no increase in plasma FSH was observed after E; rather, E alone or in combination with N depressed plasma FSH. The progestational block of E-induced LH release appeared to be the result of reduced endogenous secretion of gonadotropin releasing hormone because N did not blunt the ability of the pituitary to release LH in response to exogenous Gn-RH. © 1990.