Modulation of the macrophage respiratory burst by an acidic environment: the critical role of cytoplasmic pH regulation by proton extrusion pumps

Within the acidic milieu of an abscess or tumor, macrophages must be able to maintain their cytoplasmic pH (pHi) close to the physiologic range to ensure optimal cell function. Our recent studies have demonstrated that a proton-extrusion mechanism with the characteristics of an H+ adenosine triphosphatase mediates pHi recovery in acid-loaded macrophages. These studies were designed to examine the role of these H+ pumps in maintaining cell function in an acidic extracellular environment. Peritoneal macrophages were tested for superoxide production in response to phorbol myristate acetate at either physiologic or acidic extracellular pH (pHo; 7.35 or 6.70, respectively). pHi was measured with the fluorescent dye 2',7'-biscarboxy-ethyl-5(6)-carboxy-fluorescein. Bafilomycin A1, a specific H+ adenosine triphosphatase inhibitor, was used to examine the contribution of the H+ pump to pHi regulation and cell function. At pHo 7.35, bafilomycin A1 had no effect on pHi or phorbol myristate acetate-stimulated superoxide production. However, at pHo 6.70, bafilomycin A1 reduced pHi to 6.61 +/- 0.01 versus 6.79 +/- 0.01 in control cells (p less than 0.001) and caused a concomitant reduction in superoxide production to 4.8 +/- 1.2 versus 13.0 +/- 1.2 nmol/10(6) cells/40 min in control cells (p less than 0.001). To determine whether the observed reduction in superoxide formation was the result of the pHi reduction, superoxide production was measured in cells whose pHi was pharmacologically clamped at various levels according to the K+/nigericin method. Lowering pHi from 6.80 to 6.60 caused a significant reduction in superoxide production from 13.1 +/- 1.8 to 7.5 +/- 0.9 nmol/10(6) cells/40 min (p less than 0.01). Thus H+ extrusion pumps are important to maintenance of macrophage pHi at low pHo, permitting continued superoxide production under these conditions. By keeping pHi close to the physiologic range, these pumps serve to optimize cell function in an acidic extracellular environment.