Abstract
Trans membrane potential or ionic current changes may play a role in signal transduction and differentiation in the cellular slime mold Dictyostelium discoideum. Therefore, the contribution of electrogenic ion pumps to the membrane potential of D. discoideum cells was investigated. The (negative) peak-value of the rapid potential transient, seen upon microelectrode impalement, was used to detect membrane potential changes upon changes in the external pH in the range of 5.5 to 8.0. The membrane potential was close to the Nernstian potential for protons over the pH range 5.5 to 7.5. The acid-induced changes in membrane potential were consistent with outward-proton pumping. The maximal membrane potential was at pH 7.5. Furthermore, the proton pump inhibitors diethylstilbestrol, miconazole and zearalenone directly depolarize the membrane. Cyanide and temperature decrease cause membrane depolarization as well. During recovery from cyanide poisoning a H + efflux is present. From these measurements we conclude that the membrane potential of D. discoideum cells is mainly generated by an electrogenic protom pump. Measurements in cells with different extracellular potassium and H + concentrations suggest a role for potassium in the function of the electrogenic proton pump. These results provide a framework for future research towards a possible role for the proton pump in signal transduction and differentiation.
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