Determination of occupied cytoplasmic glucocorticoid receptor sites by an exchange assay in rat muscles.

We have established an assay to determine total cytoplasmic glucocorticoid receptor sites in heart, vastus and levator-ani-bulbocavernosus muscles from rats. We have tested the exchange of endogenously bound hormone with labeled triamcinolone acetonide by performing long incubation at 0-4 degrees C. This procedure appeared to produce an adequate exchange of exogenously added unlabeled corticosterone in the range of 16 nM (normal unstressed concentration of corticosterone in plasma) with vastus and levator ani-bulbocavernosus muscles. Beyond that level, the displacement of unlabeled corticosterone by labeled triamcinolone acetonide was less efficient. By contrast, in heart and especially in liver cytosols, the exchange was adequate even in the presence of high concentrations of corticosterone (64 and 460 nM respectively). Using the exchange assay conditions (20 h of incubation at 0-4 degrees C with 16 nM of labeled triamcinolone acetonide), we found no significant difference between glucocorticoid receptor levels in intact male and female rats. In both sexes, these glucocorticoid receptor values were not affected by gonadectomy, but increased more than 2-fold after adrenalectomy. No seasonal variations were recorded in intact or adrenalectomized rats.