Kinetic analysis of cytosolic pH regulation in alveolar macrophages: V-ATPase-mediated responses to a weak acid.
Three different mechanisms interact to control the cytosolic pH (pHi) of alveolar macrophages (M phi), namely, plasmalemmal vacuolar-type H(+)-ATPase (V-ATPase), Na+/H+ exchange, and Na(+)-independent HCO3-/Cl- exchange. To investigate the activity of plasmalemmal V-ATPase in alveolar M phi, we developed a nonlinear mathematical model of pHi regulation that incorporates the biophysical determinants of pHi and the fluxes of individual acid-base equivalents. The model was used to analyze the acid-base responses of rabbit alveolar M phi to a weak acid (propionic acid) under conditions that favored V-ATPase-mediated effects (presence of 1 mM amiloride and nominal absence of CO2). The pHi was measured using the fluorescent probe, 2',7'-biscarboxethyl-5,6-carboxyfluorescein. M phi exposure to propionic acid caused a rapid fall in pHi. Recovery of pHi after acid loading varied directly with the magnitude of the acid load. Mathematical analyses showed that pHi recovery was hindered by persistent influx of propionic acid driven, in turn, by transporter-mediated H+ extrusion and propionate efflux. Eventually, a new steady state was established in which propionate and H+ were cycled out of the M phi and propionic acid was recycled into the cell. As a consequence, model predictions of the rate of V-ATPase-mediated H+ efflux were almost twice that estimated from experimental determinations of the initial rate of pHi recovery.