THE REGULATORY FUNCTION OF A PITUITARY LH-RH-DEGRADING ENZYME SYSTEM IN THE FEEDBACK CONTROL OF GONADOTROPHINS

ABSTRACT It had previously been shown that a luteinizing hormone-releasing hormone (LH-RH)-inactivating enzyme in the rat hypothalamus, L-cystine arylamidase, is in all probability involved in the short-loop feedback mechanism of LH. Since this enzyme system was also shown to be present in the pituitary, the effect of the iv injection of several sex hormones upon its activity was investigated. The basal arylamidase activity was found to be considerably higher in the neurohypophysis as compared to the adenohypophysis. The injection of 10 μg LH brought about a significant rise in arylamidase activity (+60 %) only in the latter. There was a dose-dependent increase in the pituitary enzyme activity 2 h after the iv administration of LH, and 16 h after the injection of oestradiol and progesterone into dioestrous rats. Contrary to previous findings obtained with hypothalamic homogenates, the effect of LH upon the enzyme in the pituitary proved to be not dependent on the presence of sex steroids, as the iv application of 10 μg LH into ovariectomized rats resulted in an increase of pituitary L-cystine arylamidase by about 30 %. The injection of testosterone, oestradiol and - to a lesser degree - progesterone into adult male rats caused a significant increase of enzyme activity in the hypophysis. Progesterone proved to be ineffective in the hypothalamus. This indicates that in contrast to the hypothalamus there is no sex-specific response of the LH-RH-degrading enzyme in the pituitary. It is concluded that this enzyme system seems to play an important part in the mechanisms regulating gonadotrophin release in the pituitary.

SHORT-LOOP FEEDBACK MECHANISM OF LUTEINIZING HORMONE: LH STIMULATES HYPOTHALAMIC L-CYSTINE ARYLAMIDASE TO INACTIVATE LH-RH IN THE RAT HYPOTHALAMUS

ABSTRACT The effect of the intravenous administration of several hormones on L-cystine arylamidase in the hypothalamus of the rat was examined. The injection of 10 μg LH into adult male and female rats was followed within 90 min by an increase of the enzyme activity by 50 %, while infantile animals were not affected. When prepuberal rats were pre-treated sc with testosterone and oestrogen respectively, the iv injection of LH brought about a rise in hypothalamic arylamidase activity. When ovariectomized rats were tested, no response was seen after LH injection unless the animals had been pre-treated with oestrogen and progesterone. The iv injection of 0.1 μg oestradiol-17β into female, and of 0.5 μg testosterone into male intact mature rats also resulted in an increase of hypothalamic enzyme activity. The maximal increase in enzyme activity was seen 16 h after steroid treatment. As it had previously been shown that L-cystine arylamidase inactivates LH-RH, it may be assumed that this enzyme is involved in the short-loop feedback of LH. This assumption is based on the observation that an elevation of plasma LH brings about an activation of the enzyme system, which subsequently leads to increased inactivation of LH-RH in the hypothalamus. This mechanism seems to depend on the presence of certain plasma levels of oestrogen in female, and of testosterone in male animals.

INACTIVATION OF LUTEINIZING HORMONE RELEASING HORMONE BY RAT HYPOTHALAMIC L-CYSTINE ARYLAMIDASE

ABSTRACT The rate of hydrolysis of several aminoacyl-4-nitroanilides by rat hypothalamic arylamidases was investigated. The activity of these enzymes which were mainly found in the 105 000 × g supernatant fraction of homogenates of the hypothalamus and other parts of the brain was shown to depend upon the presence of metal ions and free thiol groups, and to be inhibited by puromycin. As previous investigations had shown that Cys-NA is an appropriate substrate for measuring hormone effects on hypothalamic arylamidases, L-cystine arylamidase and its interaction with various peptide hormones were examined in detail. It could conclusively be shown that this enzyme interacts particularly with oligopeptides. Its activity was competitively inhibited by TRF, oxytocin, lysine vasopressin, and LH-RH. It was also shown that the biological activity of LH-RH and its inhibitory effect on the hydrolysis of L-cystine-bis-p-nitroanilide decreased when it was incubated for various periods of time with the 105 000 × g supernatant of rat hypothalamus homogenate.

STIMULATION BY STEROIDS AND AGE-DEPENDENT CHANGES OF L-CYSTINE ARYLAMIDASE ACTIVITY IN THE HYPOTHALAMUS OF THE RAT

ABSTRACT L-cystine arylamidase activity was found to be high in hypothalamic homogenates from 10 day old rats, and then to decrease significantly until sexual maturity was reached (P < 0.01). This enzyme activity could be stimulated by treating 10 to 35 day old male rats with 100 μg of testosterone. Contrary to this, L-cystine arylamidase activity could not be stimulated in female animals by the injection of 7 μg of ethinyl-oestradiol before Day 45 of life. These data are interpreted as meaning that the L-cystine arylamidase activity plays a rôle in the processes regulating sexual maturation.