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

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 µg/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. These results suggest (1) that pituitary responsiveness to GHRH changes during the estrous cycle and (2) that ovarian secretion mediates partially the age increase of the response by a mechanism not related with changes in pituitary GH content. In addition, the effects of gonadectomy on the effectiveness of GHRH depend on the time elapsed between suppression of gonadal secretion and the administration of GHRH.