Hypogonadism in Heat Stressed Goats: Poor Responsiveness of the Ovary to the Pulsatile LH Stimulation Induced by Hourly Injections of a Small Dose of GnRH

Abstract

An experiment was designed to determine if hypogonadism in heat stressed animals is associated with reduced endocrine function of the pituitary or the ovary. A long day-induced anestrous goat model in which ovulatory follicular growth is known to be induced by hourly injections of a small dose of GnRH was used. Ten anestrous animals, divided into two groups of 5 each, were treated with GnRH (3 ng/kg body weight) for 48 h under two temperature conditions, 25 C (Control) or 38 C (Heat stress). Changes in plasma concentrations of LH and estradiol were monitored at 2 to 4 h-intervals. In addition LH was monitored every 15 min for 16 h starting at 4 h before the first GnRH injection. GnRH treatment resulted in a prompt and sustained increase in plasma estradiol concentrations accompanied by a preovulatory LH surge within 24 h of GnRH treatment in all control animals. Heat stress abolished such ovarian responses in 4 out of 5 animals with the estradiol concentrations remained unchanged or increased slightly (7.7 pg/ml vs 2.0 pg/ ml, P<0.01). Before GnRH treatment, the frequency but not the amplitude of the LH pulse was higher in the heat stressed group (0.75/4 h vs 2.75/4 h, P<0.05). The active pulsatile secretion was associated with slightly lowered estradiol concentration during this period (2.4 pg/ml vs 1.4 pg/ml P<0.05). GnRH-treatment resulted in a synchronized pulsatile secretion of LH in both groups with the frequency of LH pulses being controlled at once every hour (3.67/4 h vs 4.17/4 h). The amplitude of GnRH-induced LH pulse (0.81 ng/ml vs 1.62 ng/ml, P<0.01) as well as the baseline (0.73 ng/ml vs 2.02 ng/ml, P<0.05) and mean concentrations (1.19 ng/ml vs 2.81 ng/ml, P<0.05) of LH were significantly higher in the heat stressed group. These results demonstrate that heat stress abolishes GnRH-induced ovulatory responses in anestrous goats not mediated by a decreases in pituitary LH secretion, but rather by reducing responsiveness of the ovary to LH. © 1995, THE SOCIETY FOR REPRODUCTION AND DEVELOPMENT. All rights reserved.