Mapping subcellular distribution of Na+-K+-ATPase in rat parotid gland.

Recent subcellular fractionation studies have raised the possibility that Na+-K+-ATPase might be present in both the apical and the basal-lateral membranes of exocrine gland acinar cells. Analytical fractionation and immunofluorescence microscopy studies of rat parotid glands were performed to confirm this interpretation. The distributions of biochemical markers after analyses based on differential sedimentation, equilibrium density-gradient centrifugation, and partitioning in an aqueous polymer two-phase system defined a total of 15 physically and biochemically distinct membrane populations. Among these populations, it was possible to select one (designated population i) with the characteristics expected of acinar cell basal-lateral plasma membranes. It contained Na+-K+-ATPase enriched 33-fold, and gamma-glutamyl transpeptidase enriched 23-fold with respect to the initial homogenate. A second population (designated population c) had the characteristics expected of acinar cell apical plasma membranes; it contained Na+-K+-ATPase enriched 28-fold, and gamma-glutamyl transpeptidase enriched 53-fold with respect to the initial homogenate. Although the identification of population c remains provisional, immunofluorescence studies verified that Na+-K+-ATPase is present in both the apical and the basal-lateral acinar cell plasma membranes. In view of these results, it is likely that the apical Na+-K+-ATPase would participate in series with basal-lateral sodium- and chloride-entry pathways in driving the secretory electrolyte fluxes.