Na⁺/H⁺ antiporter in lacrimal acinar cell basal-lateral membranes

The first step in the formation of lacrimal gland fluid is believed to depend on transport systems which couple a flux of Cl^- ions to the passive influx of Na⁺ ions across the acinar cell basal-lateral plasma membrane. The transport systems which mediate these fluxes have not yet been characterized, but a review of previous studies (Parod and Putney, Am J Physiol 239:G106, 1980) raises the possibility that Na⁺/H⁺ antiporters might represent a major pathway for Na⁺ influx. This conclusion is of interest, because antiporter mediated Na⁺ fluxes can, potentially, drive net Cl^- fluxes. We have now examined a sample of basal-lateral membrane vesicles from rat exorbital lacrimal gland to verify the presence of a Na⁺/H⁺ antiporter activity. Imposition of an outward H⁺ gradient caused a 4.4-fold increase in the ²²Na influx rate, while imposition of an outward Na⁺ gradient accelerated H⁺ uptake as determined by changes in acridine orange absorbance. All transport experiments were done in the presence of valinomycin and symmetrical K⁺ concentrations, eliminating the possibility of conductive Na⁺ or H⁺ fluxes driven by diffusion potentials. The pH gradient dependent Na⁺ influx was completely inhibited by 1 mM amiloride, indicating that it was mediated by a Na⁺/H⁺ antiporter similar to those described on other tissues. Comparison of the density distributions of Na⁺/H⁺ antiport and standard membrane marker enzyme activities confirmed that the antiporter was primarily localized to the basal-lateral membranes.