MEASUREMENT OF FREE CORTICOSTEROIDS IN RAT PLASMA: PHYSIOLOGICAL VALIDATION OF A METHOD1

Abstract

Levels of free plasma corticosteroids (expressed asµg. Compound B/100 ml. plasma) have been measured routinely in 2 ml. of rat plasma using a method based on fluorescence of Compound B in H₂SO₄. Resting levels are 9 to 15 µg./100 ml. After stress, levels increase up to 500%. After total adrenalectomy or hypophysectomy readings correspond to 3.5–6.5 µg./100 ml. There is no change upon exposure to stress of the adrenalectomized or hypophysectomized animal. Intra-venous injection of 0.1 to 1.0 milliunit of ACTH U.S.P. Standard, 24 hours after hypophysectomy increases the concentrations of corticoids in a linear log-dose response. After adrenalectomy ACTH is without effect. In the hypophysectomized animals, large doses of highly purified FSH, LH, TSH, STH, α-MSH and arginine vasopressin have no effect on the corticoid levels. Considerable stimulation of corticoidogenesis can be observed in the absence of detectable changes in the adrenal ascorbic acid concentrations when the two variables are measured simultaneously in the same hypophysectomized animals.