High levels of sodium-calcium exchange in vascular smooth muscle sarcolemmal membrane vesicles.

Membrane vesicles which exhibit high levels of Nai-dependent Ca2+ uptake have been prepared from either porcine or bovine aortic smooth muscle. These membranes are identified as being of sarcolemmal origin by enrichment of marker activities associated with the sarcolemma (e.g., binding of the ligands PN 200-110, iodocyanopindolol, and ouabain). The Vmax of Na-Ca exchange in the two aortic sarcolemmal preparations [0.5-3.5 nmol s-1 (mg of protein)-1] is significantly higher than that previously reported with membrane preparations derived from visceral and vascular smooth muscle and compares favorably with maximal values recorded in cardiac sarcolemmal membrane vesicles [5-20 nmol-1 s-1 (mg of protein)-1] under identical experimental conditions. The Km of Ca2+ (15 +/- 5 microM) and the Km of Na+ (15 +/- 7 mM) are similar values as determined in heart. Aortic and cardiac Na-Ca exchange activities are equivalent in their sensitivity to inhibition by La3+ and two known classes of mechanism-based organic blockers of transport activity (i.e., amiloride analogues and bepridil-like agents). Both also display electrogenic behavior. However, Li+, K+, and choline all inhibit the smooth muscle transporter with markedly greater potency than found in heart, and intravesicular Ca2+ does not affect transport activity in smooth muscle membranes as it does in the cardiac system. When maximal transport velocities are compared, aortic membrane vesicles have 3-6-fold higher Na-Ca exchange than sarcolemmal Ca2+-ATPase Ca2+ transporting capacities.(ABSTRACT TRUNCATED AT 250 WORDS)