Characterization of Membrane-Bound Adenosinetriphosphatase Activity of Sarcolemma-Enriched Fraction from Vascular Smooth Muscle

Abstract

Membrane-bound adenosinetriphosphatase (ATPase) activities of the sarcolemma- enriched fraction from bovine aorta were characterized. The membranes, isolated by a sucrose density gradient method, were enriched about 31-fold in sodium- and potassiumstimulated, magnesium-dependent ATPase (Na,K-ATPase) activity, and about 8-fold in 5'- nucleotidase activity compared to the homogenate, suggesting that the isolated membranes were substantially enriched with the sarcolemma. The membranes exhibited about 31, 33 and 42 μmol Pi/mg protein/h of Na,K-ATPase, magnesium-dependent ATPase and calciumdependent ATPase activities, respectively, in the presence of 4 mmol/1 ATP. The sarcolemmaenriched membranes required considerably high concentrations of well-known inhibitors for Na,K-ATPase such as vanadate (more than 1 μmol/1), lanthanum (more than 1 mmol/1) and calcium (10 mmol/1), to induce a significant inhibition in the Na,K-ATPase activity. Treatments of the membrane with physical disruptions and sodium dodecyl sulfate or deoxycholate reduced the total Na,K-ATPase activity, and did not expose fully the ouabain sensitivity of the Na,K-ATPase. These results indicate that there are marked differences in the properties of the ATPase between vascular smooth muscle sarcolemma and cardiac sarcolemma.

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“…The ATPases activity estimated according to the method of Takeo and Sakanashi, [24]. The activities of total, Na + /K + , Mg 2+ and Ca 2+ ATPases were determined using appropriate inhibitors of ATP; the inorganic phosphate liberated was estimated by the method of Fiske and Subbarow, [25].…”