Na+-Ca2+ exchange in sarcolemmal membrane vesicles of dog mesenteric artery

The kinetic characteristics of a Na+-Ca2+ exchange system in the cell membrane of vascular smooth muscle were explored in vitro in isolated sarcolemmal membrane vesicles of dog mesenteric artery. Na+-loaded vesicles rapidly accumulated Ca2+ when an outwardly directed Na+ concentration gradient was created by suspension of the vesicles in a Na+-free medium. This Ca2+ uptake process was reversible depending on the direction and the magnitude of the Na+ concentration gradient across the membrane of the vesicles. Low temperature, monensin, and external Na+ drastically decreased Ca2+ uptake in Na+-loaded vesicles. Monovalent cations K+, Rb+, Li+, and Cs+ did not substitute for Na+ in the exchange process. Divalent cations Ba2+, Cd2+, Mg2+, Mn2+, and Sr2+ inhibited Ca2+ uptake in Na+-loaded vesicles. The order of potency of these divalent cations and concentration which produced 50% inhibition (IC50, microM) were Cd2+(38) greater than Sr2+(110) greater than Ba2+(405) greater than Mn2+(500) greater than Mg2+(greater than 2,500). The trivalent cation La3+ also inhibited Ca2+ uptake (IC50 = 0.175 microM). The apparent Km for free Ca2+ in vesicles loaded with 150 mM NaCl was 2.64 +/- 0.5 microM, and the apparent maximum velocity was 14.8 +/- 1.9 nmol.min-1.mg protein-1. The half of the apparent maximum rate (K0.5) of Ca2+ uptake was observed at 45.5 mM Na+ when loaded internally in the vesicles. Valinomycin in the presence of K+ increased the magnitude of Ca2+ uptake by 16% in Na+-loaded vesicles, indicating that the process may be electrogenic. These data indicate the existence and operation of a specific carrier-mediated Na+-Ca2+ exchange system in sarcolemmal membrane vesicles isolated from a small blood vessel.