EFFECTS OF PROGESTERONE AND CORTISOL ON THE RELEASE OF GONADOTROPIN-RELEASING-HORMONE FROM THE PERIFUSED PITUITARY STALK-MEDIAN EMINENCE AND ON LUTEINIZING-HORMONE RELEASE FROM THE PITUITARY OF COWS

The direct effects of progesterone and cortisol on the release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) were examined by perifusing tissues from the bovine pituitary stalk-median eminence and the anterior pituitary. These tissues were isolated and perifused for 160 min and 8 h, respectively. To assess the effect of progesterone and cortisol, from 0 to 10 mu M of progesterone or cortisol was added separately to the tissues and the amounts of GnRH and LH assayed in the effluent medium. The release of GnRH from the hypothalamic fragments was significantly reduced within 20 min after the addition of 0.1 and 10 mu M of progesterone when compared with the values for the control group (P<0.05), but GnRH release was not markedly affected by the addition of up to 10 mu M cortisol. Spontaneous release of LH from the anterior pituitary was not markedly affected by the addition of 0.1 or 10 mu M progesterone, although the GnRH-stimulated release of LH was significantly higher than that of the control group (P<0.05). GnRH-stimulated release of LH was not markedly affected by addition of cortisol. These results suggest that high concentrations of progesterone act directly on the bovine pituitary stalk-median eminence to inhibit the release of GnRH, but they enhanced the release of LH from the anterior pituitary in response to the same GnRH stimulus.