The role of anions in pH regulation of Leishmania major promastigotes.

The pH regulation of Leishmania major promastigotes was studied as a function of the ionic composition of the medium and in response to acid and alkali load. Intracellular pH (pHi) was monitored by on-line ratio fluorescence using the fluorescence-dependent pH indicator 2',7'-bis-(carboxyethyl)-5,6-carboxyfluorescein (BCECF). In Cl(-)-based medium (pH 7.4, 30 degrees C), the steady state pHi was maintained at 6.75 +/- 0.01. Only a minor (< or = 0.07 +/- 0.02 unit) decrease in steady state pHi was observed when parasites were treated with H(+)-ATPase inhibitors such as vanadate, N-ethylmaleimide, or bafilomycin. After treatment with the impermeant anion transport blocker DIDS, or in the presence of the reduced analog H2DIDS, pHi decreased by > or = 0.2 unit. In gluconate-based medium, however, pHi gradually decreased to 6.53 +/- 0.05 and showed a swift but time-dependent recovery (alkalinization) when Cl- or other halides or nitrate were restored to the medium. That recovery was also inhibited by pretreating cells with DIDS or exposing them to H2DIDS. The findings provide evidence for Cl- transport mechanisms that support a pHi regulatory process which is operative in acidic-neutral cytoplasmic milieu. Under alkali load induced by weak base treatment, parasites undergo a rapid alkalinization which was followed first by a fast but limited acidification and subsequently by a slower but more robust acidification (recovery) to reach a pHi of 6.85 +/- 0.05. The recovery of pHi was markedly reduced in the presence of H2DIDS and/or in the absence of Cl- in the medium. Based on these results and on the fact that the natural parasite environment is both alkaline and rich in HCO3-/CO3(2-) ions, we propose (Cl-)o-(HCO3-)i or (Cl-)o-(OH-)i exchange as the major mechanism of regulatory cell acidification which is operative upon cell alkalinization. The possibility that similar pH regulatory mechanisms are operative in Leishmania promastigotes in both acidic and alkaline conditions is considered. The putative pH regulatory mechanisms might serve as potential targets for therapeutic intervention.