PITUITARY AND ADRENAL RESPONSIVENESS IN RATS AFTER PROLONGED TREATMENT WITH ACTH

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Abstract
Prolonged exposure to ACTH considerably increased adrenal responsiveness in the rat both in vivo and in vitro. The last of 5 and 14 daily injections each produced a significantly higher blood corticosterone level than did a single injection. In the presence of ACTH added in vitro, adrenal corticosterone production in animals subjected to prolonged treatment with ACTH significantly exceeded the production per unit of weight and unit of time measured in saline-treated animals. Reduced adrenal responsiveness in the stage of resistance, elicited by formalin as a non-specific stress, cannot be invoked as an explanation for the absence of an increase in corticosterone secretion. The conclusion is that after prolonged exposure to non-specific stress there is no longer any ACTH hypersecretion.Twenty-four hours after the last injection of prolonged ACTH treatment there was inhibition of endogenous ACTH release by the pituitary gland, formalin produced no rise in the corticosterone level of the peripheral blood, and operative trauma caused substantially less ascorbic acid depletion than it did in saline-treated controls, although the plasma corticosterone level was normal or below normal.