Inhibitory effects of estrogen and progesterone on several parameters of pulsatile LH release in the ovariectomized rat.

Recent reports on the effects of progesterone on the frequency of pulsatile LH release in several species have compelled a reevaluation of the regulation of pulsatile LH secretion by gonadal steroids in the rat. Long-term ovariectomized (OVX) adult rats were given Silastic implants filled with estradiol-17 beta in oil, and blood samples were taken every 5 min for 3 h, either 1 or 2 days later. Compared with the control group (empty Silastic capsules), estrogen-treated animals had significantly lower mean plasma LH concentrations, because of a significant increase in the time between pulses, pulse amplitude not being significantly affected. In a second experiment, it was determined that progesterone alone had no significant effect on pulse amplitude, frequency or mean plasma LH concentrations. In a third experiment, OVX animals received estrogen-filled (or empty) capsules on 1 day followed by progesterone-filled (or empty) capsules on the next day, and frequent blood samples were taken on the following day. Two-way analysis of variance showed significant inhibitory effects of estrogen on mean LH levels, pulse amplitude and pulse frequency. Progesterone lowered the pulse amplitude significantly, but had no effect on pulse frequency, unlike its reported effects in other species. There were no significant interactions between estrogen and progesterone on any parameter tested, indicating that there is no synergism between the two steroids under these conditions. We conclude that estrogen is the principal ovarian hormone restraining LH secretion in the long-term OVX rat and that progesterone has only a modest effect, even in the presence of estrogen.