Cytosolic pH regulation in density-defined subpopulations of bronchoalveolar macrophages

Bronchoalveolar macrophages (m phi) represent a heterogeneous population of morphologically and functionally distinct cells. In mixed populations of bronchoalveolar m phi, cytosolic pH (pHi) regulation has been shown to involve both Na(+)-dependent and -independent mechanisms for H+ extrusion, i.e., passive H+ extrusion in exchange for extracellular Na+ (Na(+)-H+ exchange or NHE) and active H+ extrusion by plasmalemmal vacuolar-type H(+)-ATPase (V-ATPase), respectively. The present studies explored the possibility that individual subpopulations of bronchoalveolar m phi possess distinct ensembles of H+ extrusion mechanisms. Rabbit bronchoalveolar m phi were separated into five density-defined subpopulations using a discontinuous density gradient. Scanning and transmission electron microscopy revealed morphological differences between the subpopulations. The number of plasmalemmal projections and electron-dense inclusions increased with increments in cell density. The subpopulations were also functionally distinct. Fc receptor-mediated phagocytosis increased in the increasing density subpopulations. Despite these differences, all subpopulations displayed Na(+)-dependent and -independent mechanisms for pHi recovery from intracellular acid loads (ammonia prepulse technique). We conclude that NHE and V-ATPase activities were present in each subpopulation. These findings support the use of mixed populations to study pHi homeostasis in bronchoalveolar m phi.