Plasmalemmal V-H⁺-ATPases regulate intracellular pH in human lung microvascular endothelial cells

The lung endothelium layer is exposed to continuous CO₂ transit which exposes the endothelium to a substantial acid load that could be detrimental to cell function. The Na⁺/H⁺ exchanger and HCO₃^--dependent H⁺-transporting mechanisms regulate intracellular pH (pH^cyt) in most cells. Cells that cope with high acid loads might require additional primary energy-dependent mechanisms. V-H⁺-ATPases localized at the plasma membranes (pmV-ATPases) have emerged as a novel pH regulatory system. We hypothesized that human lung microvascular endothelial (HLMVE) cells use pmV-ATPases, in addition to Na ⁺/H⁺ exchanger and HCO₃^--based H⁺-transporting mechanisms, to maintain pH^cyt homeostasis. Immunocytochemical studies revealed V-H⁺-ATPase at the plasma membrane, in addition to the predicted distribution in vacuolar compartments. Acid-loaded HLMVE cells exhibited proton fluxes in the absence of Na⁺ and HCO₃^- that were similar to those observed in the presence of either Na⁺, or Na⁺ and HCO₃ ^-. The Na⁺- and HCO₃^--independent pH^cyt recovery was inhibited by bafilomycin A₁, a V-H ⁺-ATPase inhibitor. These studies show a Na⁺- and HCO ₃^--independent pH^cyt regulatory mechanism in HLMVE cells that is mediated by pmV-ATPases.