On the Existence of a Na†-Ca2† Exchange System in the Cell Membrane of Vascular Smooth Muscle

Abstract

Ca2+ uptake was studied in Na+-loaded cell membrane vesicles isolated from blood vessels of several species. A rapid uptake of Ca2+ was observed in an isotonic KCl medium when there was an outwardly directed Na+ concentration gradient created across the membrane. Elimination of the Na+ concentration gradient by including Na+ or the sodium ionophore monensin in the assay medium resulted in drastic reduction of Ca2+ uptake in the vesicles. This Ca2+ uptake was found to be specific for Na+ since other monovalent cations did not substitute Na+. Employing this approach, a Na+-Ca2+ exchange system was demonstrated in dog mesenteric artery, rat mesenteric artery, rat aorta, and rabbit aorta. The apparent Km for free Ca2+ of this process in these smooth muscles varies from 1.61 to 2.72 microM and the apparent maximum velocity varies from 7 to 16 nmol/min/mg protein. The results of the study with isolated membrane vesicles indicate the existence of a Na+-Ca2+ exchange in the cell membrane of vascular smooth muscle.