CHARACTERISTICS OF THE ADRENAL ASCORBIC ACID RESPONSE TO ADRENOCORTICOTROPHIC HORMONE (ACTH) IN THE RAT¹

Abstract

The rate of ascorbic acid release from the adrenals of intact rats in response to endogenous ACTH secretion has been systematically determined following unilateral adrenalectomy, sham-adrenalectomy, cannulation of femoral artery and vein, sciatic nerve stimulation, and the subcutaneous administration of Lepinephrine. The adrenal activation in response to ACTH administered to hypophysectomized rats has also been investigated. Examination of the kinetics of the adrenal ascorbic acid response to either endogenous or exogenous ACTH reveals that regardless of the type of experimental procedure applied as a stress the adrenal ascorbic acid changes occur at a uniform rate. Calculation of the relative rates of adrenal ascorbic acid depletion and repletion indicates that the rate of ascorbic acid loss from the rat adrenal is about 120 times more rapid than the rate of recovery. A linear relationship between the log dose and response can be reliably demonstrated for doses of ACTH up to 5 µg. when the usual one hour interval is employed between injection of ACTH and adrenal gland removal. Doses of ACTH greater than 5 µg., or severe stresses (i.e. unilateral adrenalectomy) require longer time intervals to attain maximal adrenal ascorbic acid depletion. The range of the dose-response curve for adrenal ascorbic acid depletion may be extended appreciably by using a twohour interval between ACTH injection and removal of the second adrenal gland. Four hours after unilateral adrenalectomy an increase in weight of the remaining adrenal can be demonstrated. This rapid increase in adrenal weight is not the result of a true tissue hypertrophy but is the result of an accumulation of fluid in the remaining adrenal. The reliability and limitations of the adrenal ascorbic acid index for quantitative ACTH estimations are discussed.