Temporal Interrelationships Between Plasma LH, Ovarian Secretion Rates and Peripheral Plasma Progestin Concentrations in the Rat: Effects of Nembutal and Exogenous Gonadotropins

Abstract

The patterns in the ovarian secretion rates of progesterone and 20α-hydroxypregn-4-en-3-one (20α-OHP) have been studied at various time periods during the 4-day rat estrous cycle. These patterns have been compared to the changes which occur in peripheral plasma concentrations of LH and the progestins in the same animal. At proestrus, peak plasma LH concentrations were observed between 1400 and 1600 hr and had decreased to base line values of 10 μg/ml by approximately 1800 hr. No significant increase in ovarian secretion rates of progesterone occurred prior to LH release, although the 20α-OHP was significantly elevated by 1400 hr. Following adenohypophyseal LH release, progesterone secretion rates increased significantly and reached the highest values obtained in this study at 2400 hr. The 20α-OHP did not show a comparable increase at 1700 hr, but thereafter it was elevated above the earlier time periods (1000, 1400, 1700 hr). While similar changes in peripheral plasma progesterone concentrations occurred during proestrus, an important difference was noted at 1400 hr, when the concentration of this steroid was elevated above the values observed at 1000 hr. Ovulationblocking dosages of Nembutal, administered at 1330 hr on proestrus, inhibited the late afternoon increase in ovarian secretion rates and peripheral plasma concentrations of progesterone. Injection of 10 μg LH iv to Nembutaltreated proestrous rats markedly stimulated progesterone secretion rates and produced a modest increase in the 20α-OHP. Neither combinations of LH, prolactin nor FSH produced greater stimulation of ovarian progestin output than that which occurred with LH alone. Administration of 10 μg LH iv to estrous or diestrous animals also resulted in marked stimulation of ovarian secretion rates of progesterone, but only on the second diestrous day were peripheral plasma concentrations elevated above the untreated values. (Endocrinology88: 1437, 1971)