Effect of Hypophysectomy on Immobilization-Induced Elevation of Tyrosine Hydroxylase and Phenylethanolamine-N-methyl Transferase in the Rat Adrenal

The depletion of adrenal epinephrine observed after immobilization stress in normal rats is even greater in hypophysectomized rats. The activities of adrenal tyrosine hydroxylase (TH) and phenylethanolamine-N-methyl transferase (PNMT), enzymes in the epinephrine biosynthetic pathway, are decreased following hypophysectomy. After repeated immobilization TH levels in hypophysectomized rats increase significantly but do not reach control levels. In hypophysectomized rats PNMT levels remain markedly decreased with repeated immobilization. When ACTH is administered to hypophysectomized rats before each period of immobilization there is less depletion of adrenal epinephrine and levels of TH and PNMT approach those found in sham-hypophysectomized immobilized rats. When dexamethasone is given prior to immobilization there is again less depletion of adrenal epinephrine and an increase in PNMT levels but no change in TH levels. Thyroxine does not increase epinephrine or enzyme levels in hypophysectomized immobilized rats. Adrenal denervation completely blocks the immobilization- induced increase in TH in hypophysectomized rats. When ACTH is given before immobilization there is a marked increase in TH activity in the intact adrenal and a small but significant increase in TH in the denervated gland. PNMT levels increase in intact and denervated adrenals of hypophysectomized rats given ACTH prior to immobilization. Increases in TH levels induced by immobilization stress require both intact neuronal and pituitary-adrenal systems. PNMT levels appear to be controlled by both systems, but the pituitaryadrenocortical system appears to be more important.