In rabbit alveolar macrophages, recovery of intracellular pH (pH(i)) from acid loads to pH(i) values greater than or equal to 6.8 at an extracellular pH (pH(o)) of 7.4 (nominal absence of CO2-HCO3-) is insensitive to amiloride, an inhibitor of Na+-H+ exchange, and abolished by bafilomycin A(1), an inhibitor of vacuolar-type H+-ATPase [A. Bidani, S. E. S. Brown, T. A. Heming, R. Gurich, and T. D. Dubose, Jr. Am. J. Physiol. 257 (Cell Physiol. 26): C65-C76, 1989; A. Bidani and S. E. S. Brown. Am. J. Physiol. 259 (Cell Physiol. 28): C586-C598, 1990]. To further evaluate the roles of Na+-H+ exchange and H+-ATPase activity in pH(i) regulation in rabbit alveolar macrophages, we have investigated the effects of amiloride and bafilomycin over a greater range of pH(i) (6.3-7.0) and pH(o) (5.0-7.4). The results indicate that rabbit alveolar macrophages possess H+-ATPase and a Na+-H+ antiporter, both of which are activated by decrements in pH(i). However, in all cases, H+-ATPase activity exclusively determined basal pH(i) and was the principal mechanism (> 50%) for pH(i) recovery from intracellular acid loads. The pH(i) set point for activation of Na+-H+ exchange was similar to 6.8 at pH(o) of 7.4 and similar to 6.5 at pH(o) of 6.8. Na+-H+ exchange did not contribute significantly to pH(i) recovery at acid-loaded pH(i) above these set points. At pH(o) of 7.4 and pH(i) greater than or equal to 6.8, pH(i) recovery displayed an activation energy of similar to 11,000 kcal/mol and temperature coefficient of similar to 2.1, which are consistent with an energy-dependent process (i.e., H+ pump). Taken together, these data indicate that the plasmalemmal vacuolar-type H+ pump is the primary mechanism for regulation of pH(i) in rabbit alveolar macrophages. Na+-H+ exchange plays only a minor role, if any, in pH(i) regulation in the physiological range.
