Functional coupling of Na+/H+ and Na+/Ca2+ exchangers in the alpha 1-adrenoreceptor-mediated activation of hepatic metabolism

Abstract

The purpose of this study was to characterize the role of ions other than Ca2+ in hepatic responses to alpha 1-adrenergic stimulation. We report that the alpha 1-adrenoreceptor activation of hepatic functions is accompanied by extracellular acidification and an increase in intracellular pH. These effects are dependent on extracellular Na+ concentration and are inhibited by the Na+/H+ antiporter blocker 5-(N-ethyl-N-isopropyl) amiloride under conditions that preclude antagonistic effects on agonist binding. Thus, the activation of plasma membrane Na+/H+ exchange is an essential feature of the hepatic alpha-adrenoreceptor-coupled signaling pathway. The following observations indicate that the sustained hepatic alpha 1-adrenergic actions rely on a functional coupling between the plasma membrane Na+/H+ and Na+/Ca2+ exchangers, resulting in the stimulation of Ca2+ influx. 1) Inhibition of the Na+/K(+)-ATPase does not prevent the alpha 1-adrenergic effects. However, alpha 1-adrenoreceptor stimulation fails to induce intracellular alkalinization and to acidify the extracellular medium in the absence of extracellular Ca2+. 2) A non-receptor-induced increase in intracellular Na+ concentration, caused by the ionophore monensin, stimulates Ca2+ influx and increases vascular resistance. 3) Inhibition of Na+/Ca2+ exchange prevents, in a concentration-dependent manner, most of the alpha 1-agonist-induced responses. 4) The actions of Ca(2+)-mobilizing vasoactive peptide receptors or alpha 2-adrenoreceptors, which produce neither sustained extracellular acidification nor release of Ca2+, are insensitive to Na+/H+ exchange blockers.