The in Vitro Red Blood Cell Uptake of C'4-cortisol; Studies of Plasma Protein Binding of Cortisol in Normal and Abnormal

The investigative work of Daughaday (1), Sandberg, Slaunwhite and Antoniades (2), and Slaunwhite and Sandberg (3) has established the presence of a plasma protein that plays an important role in the transport of blood cortisol.1 This protein, called transcortin or corticosteroid-binding globulin, has been shown to have great affinity, but low capacity, for binding cortisol and certain other corticosteroids. In dialysis and ultrafiltration experiments on human plasma containing concentrations of cortisol less than 15 ,ug per 100 ml, only a small percentage of cortisol is found to be freely diffusible (1, 4, 5). This is attributed to the firm binding of the transcortin-cortisol complex. At higher concentrations of plasma cortisol, transcortin becomes saturated, and other plasma proteins, mainly albumin, assume a greater role in the binding equilibrium. Since albumin binds cortisol less firmly than transcortin, the diffusible fraction of plasma cortisol increases (1-4). While much investigative work has centered around plasma factors in blood cortisol transport, relatively little attention has been given to the red cells. In this investigation the percentage of blood cortisol associated with the red blood cells in vivo and in an in vitro system has been studied. During the course of these studies it was found that the in vitro distribution of C14-cortisol between plasma and red blood cells could be used to measure the cortisol-binding properties of plasma proteins. Although there are other methods of study, this procedure is advantageous in that the conditions