Protein Binding of Corticosteroids in Undiluted Rat Plasma1

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Abstract
Data obtained from limited ultrafiltration of undiluted rat plasma at 37 C were analyzed by a nonlinear, weighted, least-squares curve-fitting program for the IBM 7090 computer, using multiple mass-action theory. This approach to the binding of corticosteroids by plasma macromolecules obtains values for the apparent physicochemical parameters of binding and simultaneously provides data in a form convenient for physiological interpretation. Rat plasma was found to bind corticosteroids as if it contained 2 macromolecular systems: one resembling the human transcortin system and the other resembling the albumin system. The concentration of the transcortin-like system was increased slightly by estrogen treatment, and considerably by adrenalectomy. Binding was decreased by corticosteroid treatment. The transcortin- like system was present in 2-fold greater amounts in adult females than in adult males, but in lesser, equal amounts in weanling males and females. Cortisol and corticosterone each interfered with the binding of the other. Progesterone, at physiological levels, did not interfere with the binding of corticosterone. Cortisol was bound more extensively in rat plasma than in human plasma, and more extensively in human than in dog plasma. In rat plasma corticosterone was more extensively bound than was cortisol. (Endocrinology79: 884, 1966)