Potassium Channels in Chara corallina

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Abstract
Plasmalemma electrical properties were used to investigate K+ transport and its control in internodal cells of Chara corallina Klein ex Willd., em R.D.W. Cell exposure to solutions containing 10 mm KCl caused the potential, normally −250 millivolts (average), to depolarize in two steps. The first step was a 21 millivolt depolarization that lasted from 1 to 40 minutes. The second step started with an action potential and left the membrane potential at −91 millivolts, with a 10-fold reduction in resistance. We suggest that the second step was caused by the opening of K+ -channels in the membrane. This lowered the resistance and provided a current pathway that partially short-circuited the electrogenic pump. Although largely short-circuited, the electrogenic pump was still operating as indicated by: (a) the depolarized potential of −91 millivolts was more negative than Ek (=−42 millivolts in 10 mm K+); (b) a large net K+ uptake occurred while the cell was depolarized; (c) both the electrogenic pump inhibitor, diethylstilbestrol, and the sulfhydryl-reagent N-ethylmaleimide (which increased the passive membrane permeability) further depolarized the potential in 10 mm KCl. A two-phase recovery back to normal cell potentials occurred upon lowering the K+ concentration from 10 to 0.2 mm. The first phase was an apparent Nernst potential response to the change in external K+ concentration. The second phase was a sudden hyperpolarization accompanied by a large increase in membrane resistance. We attribute the second phase to the closing of K+ -channels and the removal of the associated short-circuiting effect on the electrogenic pump, thereby allowing the membrane to hyperpolarize. Further experiments indicated that the K+ -channel required Ca2+ for normal closure, but other ions could substitute, including: Na+, tetraethylammonium, and 2,4,6-triaminopyrimidine. Apparently, K+ -channel conductance is determined by competition between Ca2+ and K+ for a control (gating?) binding site.