H+ secretion induced by hypertonic stress in the cellular slime mold Dictyostelium discoideum

We found that Dictyostelium amebas shrank about 50% in 10 min after a change in extracellular osmolarity from 70 mosM to 170-370 mosM. Cells shrunk by the addition of 0.1 M mannitol showed a partial recovery of cell volume in 2 h when the culture medium contained salts. A full recovery of cell volume was observed when 0.1 or 0.2 osM NaCl or KCl was added instead of mannitol. No recovery of cell volume was observed after the addition of 0.1 M mannitol or glycine when the extracellular medium did not contain any salt. An elevation in extracellular pH of starved Dictyostelium cells, which is probably due to ammonia production, was suppressed by hypertonic stress. This hypertonic stress-induced suppression of the elevation in extracellular pH was observed only when the extracellular medium contained salt. We also found that cells with artificially elevated intracellular pH withstood hypertonic stress much better than control cells. These results suggest that Dictyostelium cells under hypertonic conditions secrete protons and take up charged extracellular osmolytes in exchange. The uptake of charged osmolytes induces the elevation of intracellular osmotic strength and the recovery of cell volume. The elevation of intracellular pH, which is presumably induced by the proton secretion, plays a role in protecting cells from lysis.