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Electron microscopic cytochemical localization of Ca‐ATPase in the rod outer segments of the toad Bufo marinus

Transmission electron microscopy and ultracytochemistry were employed in an attempt to localize the enzyme calcium adenosine triphosphatase (Ca‐ATPase) in the rod outer segments (ROS) of the toad retina. Utilizing a one‐step incubation procedure, Ca‐ATPase was identified as an electron‐dense precipitate in the intradiskal spaces of the rod disks (vesicles) of the ROS. Analytical microscopy identified the reaction product as lead phosphate. The formation of the reaction product was dependent on the presence of ATP (the substrate) and calcium ions. However, calcium ions could be substituted for by magnesium ions. In addition, the reaction was vanadate sensitive. The latter is known to inhibit Ca‐ATPase activity. Such data appear to indicate the presence of a Ca‐Mg‐ATPase in association with the rod disks. Since cyclic guanosine monophosphate (cyclic GMP), rather than calcium ions, is currently believed to be the primary intracellular messenger associated with phototransduction, the presence of an ROS Ca‐ATPase may indicate other functions for this cation in the physiology and biochemistry of the visual process. Ca‐ATPase might play a role in directional calcium fluxes between intracellular compartments.