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Articles

Activity of the Electrogenic Pump in Chara corallina as Inferred from Measurements of the Membrane Potential, Conductance, and Potassium Permeability ¹

Section of Botany, Genetics and Development, Division of Biological Sciences, Cornell University, Ithaca, New York 14853

The effects of various inhibitors on the membrane potential, resistance, and K⁺ permeability of Chara corallina were measured, providing evidence that there is an electrogenic pump in the membrane. It was found that: (a) 5.0 µM carbonyl cyanide m-chlorophenyl hydrazone depolarizes the membrane potential and increases the membrane resistance. This inhibition is faster in the dark than in the light but the extent of inhibition is the same in both cases. (b) Fifty µM dicyclohexylcarbodiimide increases the resistance and the K⁺ permeability and depolarizes the membrane to a diffusion potential mainly controlled by K⁺. (c) Forty µM diethylstilbestrol and 0.1 mM 2,4-dinitrophenol increase the resistance and depolarize the potential to a value given by the Goldman diffusion equation. (d) Both 3-(3,4-dichlorophenyl)-1,1-dimethylurea and darkness (at pH 6) cause the membrane resistance to increase but neither has a large effect on the potential. 3-(3,4-dichlorophenyl)-1,1-Dimethylurea increases K⁺ permeability while darkness decreases it.

In all cases, the increase in resistance is interpreted as an inhibition of conductance through the electrogenic pump. As a consequence of this inhibition, the electrogenic component of the membrane potential is reduced, depolarizing the membrane. The electrogenic pump may be an H⁺-ATPase in the plasmalemma.

5,5-Dimethyloxazolidine-2,4-dione at 5.0 mM decreases the membrane resistance, by lowering the internal pH providing more substrate for the pump. La³⁺ decreased cation permeability and depolarized the membrane but, since it had little effect on the membrane resistance, it probably does not affect the electrogenic pump.

¹ This research was supported by National Science Foundation Grant PCM75-15277.