Co-localization and polarized distribution of Na,K-ATPase alpha 3 and beta 2 subunits in photoreceptor cells.

Na,K-ATPase plays a central role in the visual sensitivity of photoreceptors by driving the dark current of vision. The alpha 3 and beta 2 isoforms of Na,K-ATPase were previously shown to be the major alpha and beta subunit mRNAs expressed in photoreceptors. Here we compared the distribution of beta-subunits of the enzyme in the retina and kidney, using electron microscopic immunocytochemistry with specific antibodies against alpha 3, beta 1, and beta 2 isoforms as well as with an antibody (Ax2) that binds to alpha 2 and/or alpha 3 isoforms. Both the alpha 3 and beta 2 isoforms were localized to photoreceptor inner segments at highest labeling density between the base of the connecting cilium and the outer limiting membrane (OLM). Quantitative analysis of Ax2 antibody binding to alpha 3 revealed a significant decrease in labeling density below the OLM and above the base of the connecting cilium. Although the beta 2-subunit has been reported to have adhesive functions in glial cells in cerebellum, we detected beta 2 in the photoreceptor, a cell of neural origin, but not in the Mueller cell, the glial cell of the retina. Moreover, anti-beta 2 antibodies bound maximally to portions of photoreceptor cells not involved in cell-cell contact.