A Mg2+-independent high-affinity Ca2+-stimulated adenosine triphosphatase in the plasma membrane of rat stomach smooth muscle. Subcellular distribution and inhibition by Mg2+.
Plasma membrane enriched fraction isolated from the fundus smooth muscle of rat stomach displayed Ca2+-stimulated ATPase activity in the absence of Mg2+. The Ca2+ dependence of such an ATPase activity can be resolved into two hyperbolic components with a high affinity (Km = 0.4 microM) and a low affinity (Km = 0.6 mM) for Ca2+. Distribution of these high-affinity and low-affinity Ca2+-ATPase activities parallels those of several plasma membrane marker enzyme activities but not those of endoplasmic reticulum and mitochondrial membrane marker enzyme activities. Mg2+ also stimulates the ATPase in the absence of Ca2+. Unlike the Mg2+-ATPase and low-affinity Ca2+-ATPase, the plasmalemmal high-affinity Ca2+-ATPase is not sensitive to the inhibitory effect of sodium azide or Triton X-100 treatment. The high-affinity Ca2+-ATPase is noncompetitively inhibited by Mg2+ with respect to Ca2+ stimulation. Such an inhibitory effect of Mg2+ is potentiated by Triton X-100 treatment of the membrane fraction. Calmodulin has little effect on the high-affinity Ca2+-ATPase activity of the plasma membrane enriched fraction with or without EDTA pretreatment. Findings of this novel, Mg2+-independent, high-affinity Ca2+-ATPase activity in the rat stomach smooth muscle plasma membrane are discussed with those of Mg2+-dependent, high-affinity Ca2+-ATPase activities previously reported in other smooth muscle plasma membrane preparations in relation to the plasma membrane Ca2+-pump.
D000818:Animals; D001386:Azides / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002147:Calmodulin / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D009130:Muscle, Smooth / Q000201:enzymology*; D011092:Polyethylene Glycols / Q000494:pharmacology; D051381:Rats; D012508:Sarcolemma / Q000201:enzymology*; D013270:Stomach / Q000201:enzymology*; D013347:Subcellular Fractions / Q000201:enzymology
D001386:Azides; D002147:Calmodulin; D011092:Polyethylene Glycols; D000252:Calcium-Transporting ATPases; D008274:Magnesium