Studies on the Site of Action of Vasopressin in Inducing Adrenocorticotropin Secretion*

Hypophysectomized rats bearing three transplanted pituitaries under the kidney capsule responded to synthetic lysine vasopressin or pitressin with a significant elevation of plasma corticosterone, whereas hypophysectomized rats with no grafts did not. This response was completely abolished by pretreatment of animals with dexamethasone but was unaltered by central hypothalamic destruction. Corticotropin-releasing factor content of the hypothalamic median eminence, hypophyseal stal-, or pars nervosa of the posterior pituitary of intact rats was unchanged 5 or 10 min after ip injection of vasopressin compared to the basal level. We conclude that vasopressin and dexamethasone act directly on the adenohypophysis in vivo to exert their stimulatory or inhibitory effect on ACTH secretion.