Extracellular pH modulates the Ca2+ current activated by depletion of intracellular Ca2+ stores in human macrophages

Intracellular Ca2+ (Ca(i)) signaling following the binding of surface receptors activates a Ca2+ permeable plasma membrane conductance which has been shown to be associated with store depletion in a number of cell types. We examined the activation of this conductance in human monocyte-derived macrophages (HMDMs) using whole-cell voltage-clamp techniques coupled with fura-2 microfluorimetry and characterized the importance of external pH (pHo) as a modulator of current amplitude. Current activation was observed following experimental maneuvers designed to deplete intracellular Ca(2+)-stores including: (i) dialysis of the cell with 100 microM inositol 1,4,5-triphosphate (IP3), (ii) intracellular dialysis with high concentrations of the Ca2+ buffers EGTA and BAPTA, or (iii) exposure of the cell to the Ca(2+)-ATPase inhibitor thapsigargin (1 microM). Currents associated with store depletion were inwardly rectifying with kinetics, inactivation, and selectivity that appeared similar irrespective of the mode of activation. Currents were Ca2+ selective with a selectivity sequence of Ca2+ > Sr2+ >> Mg2+ = Mn2+ = Ni2+. The Ca2+ influx current was modulated by changes in pHo; modulation was not produced as a consequence of changes in internal pH (pHi). External acidification led to a reversible reduction in current amplitude with a pKa at pH 8.2. Changes in pHo alone failed to induce current activation. These observations are consistent with a scheme by which changes in pHo, as would be encountered by macrophages at sites of inflammation, could change the time course and magnitude of the Cai transient associated with receptor activation by regulating the influx of Ca2+ ions.