Effect of vasopressin on the induction of adrenal tyrosine hydroxylase and phenylethanolamine N-methyltransferase

Abstract

We have assessed the effect of arginine vasopressin (AVP) on adrenal tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) activities. Both enzymes show marked increases after systemic administration of AVP in the range of 66 and 100 μg/day. To determine whether the pituitary gland plays a role in these inductions, the effect of AVP (66 μg per day, given divided into 3 doses for 4 days) on the adrenal enzymes was studied in hypophysectomized rats. These animals showed induction of TH but not PNMT. This indicates that a pituitary factor(s) mediates the increase in PNMT caused by AVP. Adrenal TH activity was measured after the injection of AVP (1 or 2 μg per rat) into the lateral ventricle: there was a statistically significant increase in TH. TH was not induced in the denervated adrenal gland of rats administered AVP systemically. These findings suggest that AVP may act centrally to induce the enzyme. The continuous s.c. infusion of AVP by osmotic minipump at the rate of 1 μg/day for 6 days led to a striking increase in adrenal TH activity. However, PNMT did not increase significantly. It can be concluded that different mechanisms are involved in the induction of adrenal TH and PNMT caused by AVP. A neural mechanism is involved in TH induction, whereas PNMT induction requires release of a pituitary factor, presumably ACTH, but innervation of the adrenal is not needed for it. Moreover, the inductions of these two enzymes are differentially sensitive to the concentration of circulating AVP.