α-Melanocyte-Stimulating Hormone Stimulation of Aldosterone Secretion in Hypophysectomized Rats*

Intact and hypophysectomized male rats on low and high sodium diets were treated with sc infused ACTH and alpha MSH, and the levels of aldosterone and corticosterone were determined in truncal blood. Plasma aldosterone levels were lower in hypophysectomized animals on the low sodium diet than in intact animals on the low sodium diet. Alpha MSH (8 micrograms/day) restored plasma levels of aldosterone to normal in hypophysectomized rats on the low sodium diet. ACTH (6 micrograms/day) did not cause significant changes in plasma levels of aldosterone in hypophysectomized animals. ACTH restored plasma corticosterone to normal in hypophysectomized rats, whereas alpha MSH had no effect on corticosterone. Alpha MSH did not increase plasma aldosterone levels in intact rats. These data suggest that alpha MSH may be important in the regulation of aldosterone secretion in the rat and that zona glomerulosa responsiveness to alpha MSH in vivo is increased by hypophysectomy. The mechanisms of alpha MSH action on glomerulosa cells are different from those of ACTH.