Dynamic changes in pituitary responses to luteinizing hormone releasing hormone after ovariectomy in the rat.

Changes in pituitary responses to pulses of LH releasing hormone (LH-RH) after ovariectomy in the rat have been investigated with an in-vitro perifusion system. On the third day after ovariectomy there was a large increase in the responsiveness of the pituitary gland to LH-RH compared with days 1 and 2 and this preceded the first significant rise in circulating concentrations of LH. Exaggerated responses were observed on all subsequent days tested (days 4, 6, 10, 18 and 28) although the size of the response on day 10 was significantly lower compared with days 6, 18 or 28. It is suggested that the early phase of increased pituitary responsiveness to LH-RH results from a rise in pituitary LH-RH receptors, which increases both the synthesis of LH and the response to exogenous LH-RH. The reduced LH response, measured on day 10, may correlate with an increase in the endogenous secretion of LH-RH and an imbalance between LH synthesis and secretion at this time.