Adenosine triphosphatase activities of subcellular fractions of normal and ischemic muscles

Activities of sodium- and potassium-dependent ATPase (Na+K ATPase) and dinitrophenol-stimulated ATPase (DNP ATPase) were measured in subcellular fractions of homogenates of normal and ischemic rat striated muscle. The activity of Na+K ATPase in the heavy microsomal fraction did not decrease after tissue ischemia of 3 and 6 hours' duration. The activity of DNP ATPase of the mitochondrial fraction was decreased in the ischemic tissue. Lactic acid inhibition of Na+K ATPase of the microsomal fraction was concomitant with the fall of pH. When the pH was maintained at 7.4, addition of lactic acid to levels as high as 20 m m final concentration did not produce any effect. These results would indicate that the inhibition of the sodium pump during shock may be caused by depression of Na+K ATPase activity due to acidosis as well as to the decreased biosynthesis of ATP, and not by damage to the Na+K ATPase.