Differences in cytoplasmic pH and the sensitivity to acid load between prespore cells and prestalk cells of Dictyostelium

The relationship between cytoplasmic pH (pH)) and prestalk/prespore differentiation in the cellular slime mould Dictyostelium discoideum was investigated by: (1) comparing the pHi of prestalk and prespore cells, and (2) examining the effects on pH; of weak acids, weak bases and proton-pump inhibitors. These substances are known to affect the spore-to-stalk ratio. The pHi of dissociated slug cells was measured by microfluorometry using a pH-sensitive fluorescence dye, carboxyfluorescein. There was a slight but significant difference in pHi between prestalk cells (7’38 ± 0·09) and prespore cells (7·49 ± 0·03), and prestalk cells were more sensitive to acid load than prespore cells. In addition, weak acids and proton-pump inhibitors, which promote prestalk differentiation, significantly lowered pHi whereas weak bases, which induce prespore differentiation, elevated pHi. However, differentiation inducing factor (DIF), an endogenous inducer of stalk differentiation, caused only a slight and transient decrease in pHi. These results suggest two alternative possibilities: (1) a small and transient decrease in pHi is sufficient to induce prestalk differentiation, or (2) there is a mediator other than pHi that links the action of DIF and the pathway choice.