Corticosteroidal regulation of the glucocorticoid-binding capacity of rat heart cytoplasmic fraction.

The effects were studied of variations in blood corticosteroid levels on the glucocorticoid-binding capacities of two types of complex-forming substances, whose presence in the myocardium had been established in previous studies. The hormone binding was assessed by the method of adsorption of free forms of steroids on activated dextrancoated charcoal with application of Scatchard's analysis. The binding capacity was tested both in the presence of endogenous steroids and in cytoplasmic fraction made free of the hormones. After adrenalectomy, experimental stress, and administration of exogenous glucocorticoids the concentration of the complex-forming substance I changed in parallel with changing blood transcortin levels. This finding corroborates the concept that the complex-forming substance I is an intercellular (and possibly partially also intracellular) pool of transcortin or a transcortin-like protein; this pool regulates the accessibility of glucocorticoids to the cytoreceptor proper. The intracellular concentration of the complex-forming substance II (performing the function of receptor) changed only little with varying corticosteroid levels, and variations in its binding capacity evidently resulted from shifts in the equilibrium between the "occupied" and "free" findings sites as well as in the distribution of this protein between the cytoplasmic and nuclear fractions.