Immunological characterization and localization of the Na⁺/Ca2⁺- exchanger in bovine retina

The sodium/calcium exchanger was purified from bovine retinal rod outer segment membranes and used for the immunization of New Zealand White rabbits. A polyclonal antibody was produced which was found to bind specifically to the 230 kDa Na+/Ca2+-exchanger protein as assessed by Western blotting. The antibody did not bind to the high-molecular- weight “rim protein,” thereby demonstrating that this protein is distinct from the rod outer segment of Na+/Ca2+-exchanger. We used the polyclonal antibody for immunohistochemically localizing the exchange protein in bovine retina. Fluorescent light microscopy revealed intensive immunolabeling of the photoreceptor outer segments, whereas other retinal cell layers exhibited minimal binding. Using the electron microscopic immunogold method, we found specific antibody binding to the extracellular side of rod outer segment plasma membrane. Rod disk membranes, rod inner segments, and cone photoreceptors displayed no significant labeling. We therefore conclude that the Na+/Ca2+-exchanger is localized primarily in the rod outer segment plasma membrane, the most appropriate localization considering its proposed role in the process of vertebrate phototransduction