Activation of K+-Channel in Membrane Excitation of Nitella axilliformis

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Abstract
Two processes of the K+ channel activation in plasma membrane excitation are suggested for Nitella axilliformis. One is related to the repolarizing process in the action potential and the other to the after-hyperpolarization (AH). Extra- and intracellular tetraethylammonium (TEA+) and extracellular Co2+ prolonged the action potential, indicating involvement of K+ channel activation in the repolarizing process of the action potential. The following findings showed that AH is caused by K+ channel activation. First, AH was inhibited by extracellular K+ and Rb+ but not by Na+ and Li+. Second, it was not inhibited by intracellular TEA+ but by extracellular TEA+. Third, the membrane conductance increased during AH. Generation of AH was dependent on the level of the resting membrane potential [(Em)rest] which is affected by the activity of the electrogenic H+ pump. AH was generated, when (Em)rest was more positive than a critical value, which was supposed to be the equilibrium potential for K+ across the plasma membrane. Since extracellular Ca2+ competed with extracellular TEA+ and Co2+ in prolonging the action potential, and sometimes in inhibiting AH, Ca2+ may be involved in the K+ channel activation.