THE IN VITRO RED BLOOD CELL UPTAKE OF C14-CORTISOL; STUDIES OF PLASMA PROTEIN BINDING OF CORTISOL IN NORMAL AND ABNORMAL STATES*

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Abstract
The RBC uptake of cortisol, or the per cent of blood cortisol associated with the red blood cells, has been studied in vivo and in vitro. In normal subjects the RBC uptake ranges from 3 to 31% and is dependent upon the binding of cortisol by plasma proteins and the plasma cortisol concentration. At a plasma cortisol concentration of 10 [mu]g per 100 ml, the RBC uptake is 3 + 3% (mean + SD). With increasing plasma cortisol concentration, the RBC uptake increases along a sigmoid curve to a maximal value of 31+ 3% at plasma cortisol concentrations above 80 [mu]g per 100 ml. The in vitro RBC uptake of C14-cortisol, when correlated with the plasma cortisol concentration, serves as simple, but sensitive and relatively physiologic, method for measuring the cortisol-binding properties of plasma proteins. Using this method increased plasma protein binding is observed in pregnancy, estrogen treatment, and hypothyroidism; decreased plasma protein binding occurs in some subjects with hyperthyroidism and metastatic carcinoma. The finding of increased plasma protein binding in hypothyroidism is of interest in that such a phenomenon conceivably might result in a decreased rate of cortisol movement from blood stream to tissues, thereby decreasing the tissue concentration of this hormone and its rate of degradation.

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