EVIDENCE OF A K+-H+-ATPASE IN VASCULAR SMOOTH-MUSCLE CELLS

In earlier studies Na+-K+-adenosinetriphosphatase (ATPase) and Na+-K+-2Cl- cotransport partially accounted for vascular smooth muscle cell (VSMC) K+ (Rb+) uptake. In other cells Rb+ is taken up by a K+-H+-ATPase that is sensitive to NC-1300-B, SCH28080, omeprazole, and N-ethylmaleimide (NEM). This study examines the effects of K+-H+-ATPase inhibitors on VSMC. Rubidium uptake by primary cultures of canine coronary artery (CCA) VSMC or cultured rat aortic (CRA) VSMC line A7r5 was reduced 19-37% by NC-1300-B, SCH28080, or omeprazole. N-ethylmaleimide reduced CCA VSMC K+ content from 1.55 +/- 0.02 to 1.24 +/- 0.06-mu-eq/mg protein. The NC-1300-B-sensitive portion of CRA VSMC Rb+ uptake was not blocked by ouabain (0.1 mM) or bumetanide (0.1 mM), but was reduced by alkalinization with 7.5 mM NH4Cl, and increased by acidification with 7.5 mM Na-acetate. Intracellular pH (pH(i)) of CRA VSMC was reduced 0.14 +/- 0.03 U by NC-1300-B and 0.22 +/- 0.03 U by NEM. pH(i) of CCA VSMC was reduced 0.20 +/- 0.03 U by omeprazole (1 mM) and 0.20 +/- 0.03 U or 0.20 +/- 0.05 U by amiloride in the absence or presence of omeprazole, respectively. Fluorescence of 2',7'-bis(carboxyethyl)-5-(6')-carboxyfluorescein due to excitation at 500:441 nm in rat aortic strips was reduced by 0.21 +/- 0.02 U by omeprazole and 0.22 +/- 0.03 U by K+ removal and increased by 0.21 +/- 0.06 U by K+ repletion. We conclude that VSMC possess a previously unknown Rb+ uptake mechanism. This newly discovered mechanism helps to maintain K+ gradient and pH(i) by extruding H+ in exchange for K+, and is presumably a K+-H+-ATPase similar to those described in other tissues.