In vitro evidence for modification of rat liver glucocorticoid receptor binding properties and transformation by hyperthermia

Abstract

The physico-chemical parameters of the interaction of [ 3H]triamcinolone acetonide (TA) with the glucocorticoid receptor (GR) and the in vitro activation of glucocorticoid-receptor complexes were studied in liver cytosols of rats exposed to 41°C hyperthermia. A significant reduction in glucocorticoid binding and a slight increase in binding affinity were detected in hyperthermic rats as compared to the controls. The number of binding sites was 0.48 ± 0.02 and 0.73 ± 0.03 pmol/mg protein for heat-treated and control rats, respectively. Differences in equilibrium dissociation constants (0.52 ± 0.08 nM for hyperthermic and 0.94 ±0.13 nM for control animals) were reflected in corresponding differences in dissociation rate constants at 25°C (5.1 × 10[su−4] and 7.5 × 10 −4min −1, respectively), whereas association rate constants were similar. The inactivation kinetics of unoccupied GR at 25°C was the same in both groups. Glucocorticoid-receptor complexes in liver cytosol from hyperthermic and control rats were thermally activated to a similar extent, but the activation rate was significantly lower in the former. Mixing experiments with the control and hyperthermic rat liver cytosols suggest that this impairment of glucocorticoid-receptor complexes activation could be at least in part ascribed to heat stress-related changes in the action of activation modulator(s). Sedimentation behaviour of unactivated and activated [ 3H]TA-receptor complexes of hyperthermic and control rats was identical.