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Plant and Cell Physiology, 1985, Vol. 26, No. 4 661-668
© 1985

Article

Electrogenic Pump Current and ATP-Dependent H+ Efflux Across the Plasma Membrane of Nitellopsis obtusa

Kazuhiko Takeshige, Teruo Shimmen and Masashi Tazawa

Department of Botany, Faculty of Science, University of Tokyo Hongo, Tokyo 113, Japan

The correlation between the pump current and the ATP-dependent H+ efflux was examined in internodal cells of Nitellopsis obtusa. To control the cytoplasmic pH and ATP concentration, the tonoplast was removed by intracellular perfusion with an EGTA-containing medium. Two groups of perfused cells were prepared, one with 1 mM ATP (+ATP cells) and the other without ATP but with hexokinase and glucose (–ATP cells). The ATP-dependent H+ efflux was calculated as the difference in H+ efflux between the +ATP and –ATP cells. Based on an electrically equivalent circuit model of the plasma membrane, the pump current was calculated from the membrane potentials and the membrane resistances of both +ATP and –ATP cells. When the membrane potential was not too high (–220 mV), the ATP-dependent H+ current (19 mA m–2) was almost equal to the pump current (20 mA m–2) calculated from the electrical data. This indicates that the electrogenic pump current across the plasma membrane of Nitellopsis obtuse was mostly carried by H+. But when the membrane potential was high (–280 mV), the H+ current was lower than the pump current. The possible cause of this discrepancy is discussed.

(Received November 5, 1984; Accepted February 28, 1985)
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