Reversal of Electrogenic Cl−Pump inAcetabulariaIncreases Level and32P Labelling of ATP

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Abstract
In order to test the prediction that the Cl− ATPase in the plasmalemma of Acetabularia is reversible, the content and 32P labelling of ATP in centrifuged, cytoplasm-depleted stalk segments has been measured upon incubation in 32P containing media with normal (c. 500 mM) and zero Cl−concentration. Increases in content and labelling of ATP could be demonstrated when the preparations were exposed to zero external Cl− (1.1 M sorbitol, isotonic with the control medium–emdash Erdschreiber solution). These effects are reduced in 1: 1 mixtures of 1.1 M sorbitol with 0.55 M Nal or Na-benzenesulphonate, the latter two of which are known to diminish the number of active Cl− pumps. Increased ATP content upon exposure to sorbitol is not observed at 5 °C, when the Cl- pump is known to be strongly inhibited. These results provide strong evidence for the hypothesis that the electrogenic Cl− pump is a reversible, primary Cl− transporting ATPase. The relationship of these results to the transmembrane potential difference measured under the same conditions is not yet clear.