Hypophyseal regulation of hepatic histidase during postnatal development and adulthood I. Pituitary suppression of histidase activity

Abstract

Hypophysectomy at 3 weeks of age effects precocious development augmentation of hepatic histidase activity in both the maturing male and female rat; pituitary excision likewise augments this enzymic activity in adult males and in ovariectomized but not in gonad-bearing, adult females. This enhanced histidase activity following hypophysectomy is entirely blocked when liver protein synthesis is inhibited by administration of l-ethionine. Attempts to ascertain the identity of the histidase repressive factor(s)_have implicated the anterior pituitary as their site of origin. Since hypophysectomy of castrated-adrenalectomized males results in elevation in histidase activity, it is thought that the hypophysis secretes principle(s) suppressive to hepatic histidase which are supplementary to and distinct from trophic influences on the testis and adrenal, which themselves also contribute to suppression of histidase under certain conditions. In hypophysectomized animals lacking target glands corresponding to particular pituitary trophic hormones, follicle-stimulating hormone and luteinizing hormone, alone and in combination, and thyrotrophic hormone are without effect, whereas growth hormone and adrenocorticotrophic hormone both suppress hepatic histidase activity. These, and possibly other as yet unidentified, hypophyseal repressive influences contribute to the establishment of the characteristic postnatal developmental pattern and to the maintenance of adult levels of hepatic histidase in the rat.