Luteinizing Hormone Responsiveness to Luteinizing Hormone Releasing Hormone in the Adult Guinea-Pig: Direct Ovarian Involvement

Abstract

The effects of intravenous injection of synthetic luteinizing hormone releasing hormone (LH-RH) on the release of LH and FSH have been studied in the adult guinea-pig. In all the experiments the secretion of FSH was apparently unaffected by administration of LH-RH. The release of LH was log dose-dependent over the range 0\m=.\5\p=n-\50 \ g=m\ g LH-RH.During the female cycle, the maximum increment in the concentration of LH after a single i.v. injection of 0\m=.\5 \g=m\g LH-RH decreased progressively from day 1 to day 13. Double or triple injections of 1 \g=m\g LH-RH at 1 h intervals produced no potentiation on day 3 but progressively greater LH responses occurred on days 7, 10 or 13 of the oestrous cycle. Ovariectomy immediately before the first injection of LH-RH on day 7 blocked the potentiated response to subsequent injections, whereas ovariectomy immediately before the second injection still permitted this potentiation. These results suggest that there is direct ovarian involvement in the potentiated response to LH-RH observed during the later part of the guinea-pig oestrous cycle.Infusion of LH-RH (1 \ g=m\ g over 200 min) produced a potentiated release of LH in female guinea-pigs on day 7 (but not on day 3) of the oestrous cycle after a delay of 1\m=.\5h. It is proposed that there are two 'pools' of LH in the pituitary gland of the female guinea-pig and that the second pool is 'activated' consequent upon previous hypophysial stimulation of secretion from an ovary containing adequately developed follicles.Luteinization or ovulation can be induced in guinea-pigs by two injections of LH-RH administered 1 h apart after day 7 of the cycle ( . T. Donovan, personal communication). Animals were therefore treated on days 7, 10 or 13 with two i.v. injections of 1 µg LH-RH (2 µg/ml) each flushed in with 0-5 ml physiological saline) 65 min apart. Blood samples were collected 10, 20 and 30 min before and 7, 15, 25, 35, 45 and 55 min after the first injection and 7, 15, 25, 35, 45, 55, 70, 85, 100, 115, 130, 145, 160, 175 and 190 min after the second injection of LH-RH.

Factors Affecting the Secretion of Luteinizing Hormone in the Ewe

“…(iii) GnRH versus oestrogen in pituitary responsiveness. In rats and guinea pigs, the self-priming effect is small during all stages of the cycle except proestrus, when oestrogen levels are highest (Aiyer et al, 1 9 7 4~; Blake, 1978;ter Haar, 1978;Terasawa et al, 1980;Waring & Turgeon, 1980). During dioestrus and metoestrus, the effect is magnified if the rats are pretreated with oestrogen (Aiyer et al, 1 9 7 4~) .…”

“…Similarly, in ovariectomized ewes or rodents and hypogonadal women, the self-priming effect is small or non-existent except after oestrogen therapy (Aiyer, Sood & Brown-Grant, 1976; Yen, 1977;ter Haar, 1978;Beck et al, 1978;Terasawa et al, 1980;Wright, Findlay & Anderson, 1981 a ) , and bovine pituitary cells show greater self-priming in vitro in the presence of oestradiol (Padmanabhan, Leung & Convey, 1982). However, as has been discussed, oestrogen reduces pituitary responsiveness in the short term, and indeed the self-priming effect in rats is not due to the secretion of oestrogen induced by the first injection of GnRH (Aiyer et al, 1 9 7 4~) .…”

Effect of stimulation or lesioning of the hypothalamus upon gonadotrophin secretion in the guinea‐pig.

“…The fall in the plasma level of LH was less abrupt after the placement of a d.c. lesion than after electrical stimulation of the hypothalamus, perhaps through potentiation of the hypophysial response to gonadotrophin releasing factor by that earlier discharged by hypothalamic excitation (ter Haar, 1978). Because the electrodes were made of platinum, electrochemical stimulation of the hypothalamus by the deposition of iron from steel electrodes (Everett, 1964) was avoided, even though previous experiments aimed at the induction of ovulation by this means had proved unrewarding (Booth & Donovan, 1974a).…”