Ovarian Role in the Modulation of Pituitary Responsiveness to Growth Hormone-Releasing Hormone in Rats

Abstract

The influence of ovarian function on the pituitary responsiveness to growth hormone-releasing hormone (GHRH) was studied by measuring GH levels in serum before and after the injection of GHRH 1-29 NH2 (5 micrograms/kg i.v.) in female Wistar rats under sodium pentobarbital anesthesia (50 mg/kg i.p.). In the first experiment, GHRH was administered to 90-day-old females in different phases of the estrous cycle (in the morning and afternoon of proestrus and in the morning of estrus, diestrus 1 and diestrus 2). In the second experiment, females were ovariectomized or sham-operated on day 23 and analyzed on days 30, 45, 60 and 90. Pituitary growth hormone (GH) content in females ovariectomized or sham-operated on day 23 was also analyzed at different ages. In the third experiment, females ovariectomized or sham-operated on day 83 were tested on day 90. Results showed that (a) pituitary responsiveness to GHRH changes throughout the estrous cycle is maximal in diestrus and minimal in proestrus; (b) GHRH stimulated GH secretion in intact and ovariectomized females on days 45, 60 and 90, but not on day 30; (c) the age increase of GH responsiveness to GHRH administration was slightly reduced in animals ovariectomized on day 23; (e) the age increase of pituitary GH content was similar in control and ovariectomized females; and (d) ovariectomy on day 83 reduced pituitary responsiveness to GHRH more effectively than ovariectomy on day 23.(ABSTRACT TRUNCATED AT 250 WORDS)