Adrenocorticotropic activity in the rat assessed by in vivo and in vitro indices

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Abstract
Several indices of adrenocorticotropic (ACTH) activity were compared in order to establish the index most suitable for assay purposes, particularly of ACTH in blood. In hypophysectomized rats, the effects of ACTH on adrenal ascorbic acid, cholesterol, and steroid formation in vitro were studied. In intact rats, the effects of formalin on these variables as well as on the adrenal and plasma corticosterone levels and hypophyseal and blood ACTH activity were measured. Adrenal corticosterone as well as steroid formation in vitro increased very rapidly after stimulation by ACTH. In hypophysectomized rats, after intravenous ACTH, significant increases were observed after 5 min. In normal rats, 3 min after the injection of formalin, significant increases of steroid formation in vitro and ACTH activity were observed. The in vitro technique is suitable for the study of changes in ACTH activity. ACTH increases the fraction of corticosterone found in the total amount of corticoid secreted by rat adrenals in vitro.