Plasmalemmal H+ extruders in mammalian alveolar macrophages

The distribution of plasmalemmal V-type H+-pumps (V-ATPase) among mammalian macrophages (mphi) is uncertain and, hence, the functional significance of mphi plasmalemmal V-ATPase is unclear. This study investigated the role of V-ATPase in the regulation of intracellular pH (pH(i)) by resident alveolar mphi from sheep, pigs, dogs and rabbits. The fluorescent probe 2',7'-biscarboxyethyl-5,6-carboxyfluorescein was used to monitor baseline pH(i) and the rate of pH(i) recovery (dpH(i)/dt) from intracellular acid-loads (NH4Cl prepulse). Baseline pH(i) was 7.1-7.2. In sheep, pig and dog studies, 10 muM bafilomycin A(1) (a selective V-ATPase inhibitor) caused a rapid fall in baseline pH(i) (0.15-0.20 units); baseline values were unaffected by 0.1 mM amiloride (a Na+ transport inhibitor). V-ATPase activity (bafilomycin-sensitive component of dpH(i)/dt) was solely responsible for pH(i) recovery from intracellular acid-loads at acid-loaded pH(i) values >6.8-6.9. Na+/H+ exchange (amiloride-sensitive component of dpH(i)/dt) was detected only at acid-loaded pH(i) values <6.8. The activity of both H+ extruders increased at lower pH(i) values, albeit the Na+/H+ exchanger was more pH-sensitive than was V-ATPase. In rabbit studies, 10 μM bafilomycin A(1) and 1 mM N-ethylmaleimide (a nonspecific H+-pump inhibitor) produced similar falls in baseline mφ pH(i), but had significantly larger effects than did the selective V-ATPase inhibitor concanamycin A (≤15 μM). The findings suggest that plasmalemmal V-ATPase activity plays a major role in pH(i) regulation by alveolar mφ of sheep, pigs, dogs and rabbits. (C) 2002 Elsevier Science Inc. All rights reserved.