Abstract

Magnesium modulates hormone-sensitive adenylate cyclase activation. In the present studies, we have examined the magnesium requirement of beta-adrenergic-stimulated adenylate cyclase activity in permeabilized human lymphocytes. Following isoproterenol pretreatment, under conditions that lead to homologous beta-adrenergic desensitization, the EC50 of magnesium for beta-adrenergic-stimulated adenylate cyclase activity was significantly increased [control; 1.99 (+0.81/-0.57) mM; desensitized; 3.82 (+0.31/-0.29) mM]. Further, when assays were performed at high Mg2+ concentrations following agonist pretreatment, we detected only small and inconsistant reductions in beta-adrenergic-stimulated adenylate cyclase activity and in beta-adrenergic cAMP-dependent protein kinase activity. In contrast, the detection of agonist-induced beta-adrenergic receptor sequestration and alterations in receptor affinity for agonists was not qualitatively affected by changes in magnesium concentrations. The data demonstrate that the functional consequences of beta-adrenergic desensitization may be obscured at high magnesium concentrations. Furthermore, in this system desensitization may be viewed as a beta-receptor-specific reduction in magnesium sensitivity.