Alterations in sarcolemmal adrenergic receptors due to intracellular Ca2+ overload in the myocardium

To investigate the effect of an overload of intracellular Ca2+ on the adrenergic receptors, rat hearts perfused with Ca(2+)-free medium for 5 or 10 minutes were reperfused (Ca(2+)-paradox) with a medium containing 1.25 mM Ca2+. The status of beta-adrenergic and alpha-adrenergic receptors in a purified sarcolemmal preparation was studied by measuring the specific binding of [3H]dihydroalprenolol and [3H]prazosin, respectively. Reperfusion with Ca(2+)-containing medium increased the density of both beta- and alpha-adrenoceptors. While the hearts perfused with Ca(2+)-free medium for 5 minutes did not show any change in the Kd values upon reperfusion, the hearts perfused with Ca(2+)-free medium for 10 minutes showed an increase in the Kd value for beta-receptors and a decrease in the Kd value for alpha-receptors. The alterations in adrenergic receptors upon reperfusion in the hearts perfused with Ca(2+)-free medium for 5 minutes were not evident if the medium contained a low concentration of Na+ (35 mM) or was maintained at a low temperature (21 degrees C) to prevent the occurrence of an overload of intracellular Ca2+. These results suggest that an overload of intracellular Ca2+ might play an important role in changing the characteristics of adrenergic receptors in diseased hearts.