title: Na+/Ca2+ antiport in cultured arterial smooth muscle cells. Inhibition by magnesium and other divalent cations.
abstract: Cultured smooth muscle cells from rat aorta were loaded with Na+, and Na+/Ca2+ antiport was assayed by measuring the initial rates of 45Ca2+ influx and 22Na+ efflux, which were inhibitable by 2',4'-dimethylbenzamil. The replacement of extracellular Na+ with other monovalent ions (K+, Li+, choline, or N-methyl-D-glucamine) was essential for obtaining significant antiport activity. Mg2+ competitively inhibited 45Ca2+ influx via the antiporter (Ki = 93 +/- 7 microM). External Ca2+ or Sr2+ stimulated 22Na+ efflux as would be expected for antiport activity. Mg2+ did not stimulate 22Na+ efflux, which indicates that Mg2+ is probably not transported by the antiporter under the conditions of these experiments. Mg2+ inhibited Ca2+-stimulated 22Na+ efflux as expected from the 45Ca2+ influx data. The replacement of external N-methyl-D-glucamine with K+, but not other monovalent ions (choline, Li+), decreased the potency of Mg2+ as an inhibitor of Na+/Ca2+ antiport 6.7-fold. Other divalent cations (Co2+, Mn2+, Cd2+, Ba2+) also inhibited Na+/Ca2+ antiport activity, and high external potassium decreased the potency of each by 4.3-8.6-fold. The order of effectiveness of the divalent cations as inhibitors of Na+/Ca2+ antiport (Cd2+ greater than Mn2+ greater than Co2+ greater than Ba2+ greater than Mg2+) correlated with the closeness of the crystal ionic radius to that of Ca2+.
mesh_terms: D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001013:Aorta, Thoracic / Q000378:metabolism; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000037:antagonists & inhibitors*; D002413:Cations, Divalent / Q000494:pharmacology*; D008274:Magnesium / Q000494:pharmacology*; D008536:Meglumine / Q000494:pharmacology; D008565:Membrane Proteins / Q000037:antagonists & inhibitors*; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger
chemical_list: D002352:Carrier Proteins; D002413:Cations, Divalent; D008565:Membrane Proteins; D019831:Sodium-Calcium Exchanger; D002104:Cadmium; C053494:2',4'-dimethylbenzamil; D008536:Meglumine; D000584:Amiloride; D012964:Sodium; D008274:Magnesium; D002118:Calcium