The Influence of Progesterone and Estradiol on the Acute Changes in Pulsatile Luteinizing Hormone Release Induced by Ovariectomy on Diestrus Day 1 in the Rat

Abstract

The aim of this study was to determine whether the rapid increases in LH pulse amplitude and frequency that occur within 24 h after ovariectomy (ovx) on diestrus day 1 (Dl) were due to the removal of progesterone (P) and/or estradiol (E). Initial studies demonstrated that plasma levels of E and P were 18.2 ± 1.2 pg/ml and 34.1 ± 3.2 ng/ml, respectively, between the evening of Dl and the morning of D2 in our colony of intact rats. Immediately after ovx and jugular venous cannulation on the morning of Dl, rats were implanted either with empty Silastic capsules or capsules capable of restoring physiological levels of E and P to the control values reported above. These rats were continuously bled (75 μl/6 min) for 3 h 1 day after ovx for analysis of pulsatile LH release, and then additional plasma samples were gathered for determination of E and P levels. Rats with empty capsules had decreased levels of E and P and increases in mean blood LH levels, LH pulse amplitude, and pulse frequency. Animals with E capsules had physiological levels of E and decreased levels of P, but no suppression of the acute post-ovx increase in pulsatile LH release. In contrast, animals with P capsules had physiological plasma levels of P, decreased levels of E, and a marked reduction in the acute LH response to ovx. This suppression was due entirely to a decrease in LH pulse amplitude, as pulse frequency was not altered. Rats with E and P capsules had physiological levels of these hormones, which resulted in an even greater reduction in the acute LH response to ovx. This suppression was due to decreases in both LH pulse amplitude and pulse frequency. The effect of P on LH pulse amplitude was centrally mediated, since the in vitro response to LHRH of anterior pituitary fragments from Pimplanted rats was the same as that of anterior pituitary fragments taken from rats with empty capsules.

These studies demonstrate that the acute increase in LH pulse amplitude that occurs within 24 h after ovx on Dl is due to the absence of a central inhibitory effect of ovarian P, while the rapid increase in LH pulse frequency is due to the loss of both ovarian E and P. Thus, during the rat estrous cycle, the decrease in LH pulse amplitude occurring from Dl to D2 may well be due to the central inhibitory effect of P, while the stable LH pulse frequency between Dl and D2 may be due to the restraining or braking effect of E and P combined upon the firing rate of the LHRH pulse generator. (Endocrinology114: 1605, 1984)