Cytochemical Localization of a "basic" ATPase to Canine Myocardial Surface Membrane

Enzymatic properties of a canine cardiac muscle microsomal fraction were determined to localize in situ a "basic," divalent cation dependent adenosine triphosphatase (ATPase) by ultrastructural cytochemistry. The microsomal fraction had a buoyant density of 1.08--1.13 (20--30% [w/w] sucrose) and hydrolyzed adenosine triphosphate in the presence of Mg2+, Ca2+, Mn2+, or Co2+, but not in that of Sr2+ or Ni2+, under conditions that inhibited interfering (Na+ + K+)-ATPase and sarcoplasmic reticulum Ca2+-ATPase activities. "Basic" ATPase was localized in paraformaldehyde-fixed tissue in a medium containing Mg2+ or a high Ca2+ concentration (4 mM). A free Pb2+ concentration of less than 1 microM was used to capture enzymatically released phosphate anions. Electron-dense lead precipitates were present at the plasmalemma, T-system, and intercalated disc membranes with the exception of the nexus. These studies suggest that "basic" ATPase activity is associated with surface membrane structures of canine cardiac muscle.