Rat splenic lymphocytes contain specific, saturable glucocorticoid receptors. When suspensions of these cells are incubated at 0°C in the presence of [3H]-triamcinolone acetonide ([3H]-TA). 80% of receptor-bound steroid is found associated with the cytoplasm, whereas at 37°C, 80% of [3H]-TA binding is associated with the nucleus. The equilibrium dissociation constant for interaction of [3H]-TA with the cytoplasmic receptor at 0°C is 5.6 nM while that for nuclear binding at 37°C is 0.85 nM, suggesting an increase in receptor affinity for [3H]-TA at physiological temperatures. Specificity studies show a high degree of glucocorticoid specificity at 37°C both in cytoplasm and nucleus, with little competition observed for either progesterone (P) or cortexolone (S). At 0°C, however, P and S compete with [3H]-TA for receptor binding as effectively as dexamethasone. Estradiol-17β, testosterone, 5α-dihydrotestosterone and cortisone do not compete under any of these conditions. When cells are incubated at 0°C in the presence of 0.02 μM [3H]-TA and concentrations of P and S ranging from 0.04 to 2.56 μM, [3H]-TA binding is inhibited at steroid concentrations as low as 0.1 μM. If these same cells are then warmed to 37°C [3H]-TA binding in cytoplasm and nucleus is inhibited only at P and S concentrations ≥ 1 μM. Thus, there is, concomitant with increasing temperature, an increase in affinity and specificity of the receptor for active glucocorticoids. This specificity change is regarded as evidence for a conformational change in the hormone-receptor complex coincident with activation. © 1979.
