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.
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Department of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, Ohio 45267-0575.
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002413:Cations, Divalent; D002414:Cations, Monovalent; D004285:Dogs; D005260:Female; D007700:Kinetics; D008297:Male; D008638:Mesenteric Arteries / Q000378:metabolism*; D008985:Monensin / Q000494:pharmacology; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D012508:Sarcolemma / Q000187:drug effects / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger
D002352:Carrier Proteins; D002413:Cations, Divalent; D002414:Cations, Monovalent; D019831:Sodium-Calcium Exchanger; D008985:Monensin; D012964:Sodium; D002118:Calcium