Impaired secretion of epinephrine in response to insulin among hypophysectomized dogs.

Almost all of the epinephrine secreted into the circulation of mammals is synthesized within the adrenal medulla through the action of the enzyme phenylethanolamine-N-methyl transferase (PNMT). This enzyme catalyzes the transfer of a methyl group from S-adenosylmethionine to the amine group of norepinephrine.1' 2 PNMT activity is stimulated by certain adrenocortical hormones. When rats are hypophysectomized, the activity of the enzyme decreases markedly,3' 4 but can be restored to normal by the administration of small doses of adrenocorticotropin (ACTH) or very large doses of carbohydrateactive steroids.5 Since the adrenal medulla is normally perfused with venous portal blood, which has drained through the adrenal cortex,6 and is thus very rich in steroid hormones, it seems likely that the location of the medulla within the cortical envelope constitutes a physiologic adaptation to maintain normal PNMIIT activity.5 The effect of steroid hormones on PNMT activity is probably mediated by the inductioni of new enzyme.4 It has previously beeni demonstrated that hypophysectomy also lowers the epinephrine content of the rat adrenal.5 We will now show that the removal of the pituitary leads to ani impairmenit in epinephrine secretion, both basally and in response to insulin-induced hypoglycemia. It is suggested that this effect constitutes one mechanism by which hypophysectomy increases the sensitivity of mammals to insulin. lMethods.-Mongrel male dogs were hypophysectomized by the transbuccal route7 and maintained for 3-5 months on daily subcutaneous injections of 12.5 mg cortisone (Table 1). Some of the dogs also received ACTH (40 units/day, s.c.) for 4 weeks prior to the measurement of catecholamine secretioni. Conitrol dogs were unoperated; they were not treated with cortisone because previous studies8 had shown that low doses of such steroids inhibit PNMT activity in rats. Under nembutal anesthesia, one adrenal vein was cannulated as previously described,9 and blood was collected before and at 10-min intervals after the intravenous injection of insulin. Hypophysectomized animals received 0.1 unit of insulin per kg; some of the control dogs were giveni 0.3 unit/kg in an effort to produce decreases of blood glucose comparable to those produced in the hypophysectomized animals (Table 1). Two blood samples of approximately 1 ml each were obtained at each experimental point; the collection of each took 20-60 sec. Onie sample was mixed with sodium fluoride and potassium oxalate and was subsequently assayed for glucose; the other was mixed with heparin, and the plasma was used for determinations of epinephrine and norepinephrine.10 At the end of the collection period, both adrenals were removed, frozen, and subsequently assayed for epinephrine, norepinephrine, and PNMT.5 8 Results.-As had been expected, the adrenal glands of hypophysectomized dogs weighed considerably less than those of control animals or of hypophysectomized animals treated with ACTH (Table 2). The hypophysectomized animals