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

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Abstract
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 i.v. injection of several sex hormones upon its activity was investigated. The basal arylamidase activity considerably higher in the neurohypophysis as compared to the adenohypophysis. The injection of 10 .mu.g 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 i.v. administration of LH, and 16 h after the injection of estradiol and progesterone into diestrous 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 i.v. application of 10 .mu.g LH into ovariectomized rats resulted in an increase of pituitary L-cystine arylamidase by about 30%. The injection of testosterone, estradiol and .sbd. to a lesser degree .sbd. progesterone into adult male rats caused a significant increase of enzyme activity in the hypophysis. Progesterone proved to be ineffective in the hypothalamus. In contrast to the hypothalamus there is no sex-specific response of the LH-RH-degrading enzyme in the pituitary. This enzyme system seems to play an important part in the mechamisms regulating gonadotropin release in the pituitary.