A Radioreceptor Assay for Direct Measurement of Plasma Free Glucocorticoid Activit

An assay for plasma free glucocorticoid receptorbinding activity has been developed with the use of specific glucocorticoid receptors in cultured rat pituitary tumor cells (GC line). Unlike other assays for corticosteroids, this assay measures the summed free (rather than the total) receptor-binding activity of all natural and synthetic steroids present in the sample. Plasma samples (0.5 ml) were incubated with [3H]dexamethasone and GC cells at 37 C for 45 min, and the nuclear bound dexamethasone was then measured. The free glucocorticoid receptor- binding activity was determined by the extent to which steroids present in the sample inhibit binding of the 3H-labeled steroids. Free cortisol concentrations of 0.3–3.6 μg/dl were accurately detected. That the assay measures free but not total steroids and that the free steroid provides the correct index of receptor-occupying capability were verified by two findings: 1) the assay yields similar results when the free cortisol is separated from plasma and then r...