Effect of Hydrocortisone on Pituitary ACTH and Adrenal Weight in the Rat. ∗ †

Summary

Pituitary ACTH and adrenal weight were concurrently determined, in the rat, during treatment with hydrocortisone (3 and 6 mg/100 g, daily). An immediate and slight depletion of pituitary ACTH was followed by rise to a peak of 133% of the initial level, reached after 4 hours and tapering off in the next 20 hours. A sharp decrease was observed thereafter. Less than 10% of the original concentration was found after 24 days of the 6 mg regimen. No ACTH could be detected after 55 days, at the 3 mg dosage level. This regimen (3 mg) induced an exponential reduction of the adrenal weight to hypophysectomy level within 32–55 days. From available indices of ACTH release under comparable conditions, it is inferred that the initial post-injection fall of pituitary ACTH is related to the ACTH-releasing effect of the injection per se, and its subsequent rise, to the inhibitory effect of the steroid on further release. Sustained and complete blockade of release during the chronic phase of the treatment is suggested by the rate of adrenal weight loss. The concomitant disappearance of pituitary ACTH is imputed to inactivation of the stored hormone in the absolute or relative absence of synthesis.