Reversal of Electrogenic Cl- Pump in Acetabularia Increases Level and 32P Labelling of ATP

In order to test the prediction that the Cl^– ATPase inthe plasmalemma of Acetabularia is reversible, the content and³²P labelling of ATP in centrifuged, cytoplasm-depleted stalksegments has been measured upon incubation in ³²P containingmedia with normal (c. 500 mM) and zero Cl^–concentration.Increases in content and labelling of ATP could be demonstratedwhen the preparations were exposed to zero external Cl^–(1.1 M sorbitol, isotonic with the control medium–emdashErdschreiber solution). These effects are reduced in 1: 1 mixturesof 1.1 M sorbitol with 0.55 M Nal or Na-benzenesulphonate, thelatter two of which are known to diminish the number of activeCl^– pumps. Increased ATP content upon exposure to sorbitolis not observed at 5 °C, when the Cl- pump is known to bestrongly inhibited. These results provide strong evidence forthe hypothesis that the electrogenic Cl^– pump is a reversible,primary Cl^– transporting ATPase. The relationship of theseresults to the transmembrane potential difference measured underthe same conditions is not yet clear. Key words: Acetabularia, Electrogenic Cl^– pump, Cl^– ATPase, Cl^– gradient driven ATP synthesis, Reversible ATPase