Temperature Modulation of Glucocorticoid-receptor Affinity in a Hibernator, the European Ground Squirrel, and a Non-Hibernator, the Rat
The time course of the specific binding of triamcinolone acetonide (a synthetic glucocorticoid) to its cytoplasmic receptor (rat and ground squirrel liver cytosol preparations) has been recorded at a series of incubation temperatures, from 0 to 37°C. Experimental results were fitted with a theoretical model accounting for the temperature dependence of complex formation and dissociation. A pronounced instability of the hormone-receptor complex was noted at relatively high incubation temperatures. Binding maxima formed bell-shaped temperature dependence profiles, peaking at a considerably lower temperature in the hibernator. A detailed study of the temperature dependence of the equilibrium dissociation constant (KD) showed that, with both preparations, KD values substantially increased with increasing temperature above 15°C (“positive temperature modulation of affinity”). A U-shaped temperature profile of the dissociation constant (KD), with its minimum at about 10°C, was obtained with preparations from the hibernator. Empirical data agreed completely with theoretical predictions, suggesting that the observed thermal modulation of affinity might be a direct consequence of intrinsic kinetic properties of steroid-receptor interactions. In the case of the rat, however, empirical data points deviated appreciably from model-predictions in the low temperature region, forming a“J”-type plot rather than the predicted U-shape profile. The lowest KD value was situated at about 15°C, five degrees above the hibernator’s KD inversion temperature. In the case of both species, temperatures characterized by the smallest KD values (greatest hormone-receptor affinity) were close to body temperatures representing temperature limits of spontaneous recovery from induced hypothermia.
KeywordsGlucocorticoid Receptor Pyruvate Kinase Ground Squirrel Triamcinolone Acetonide Transition State Theory
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