ROLE OF CALCIUM-ION IN THE EXCITABILITY AND ELECTROGENIC PUMP ACTIVITY OF THE CHARA-CORALLINA MEMBRANE .1. EFFECTS OF LA-3+, VERAPAMIL, EGTA, W-7, AND TFP ON THE ACTION-POTENTIAL

The cytoplasmic streaming of the normal internodel cell of giant alga Chara stops transiently at about the peak of action potential. Application of La3+ or verampamil (a calcium channel blocker) or removal of external Ca2+ by EGTA caused a partial depolarization of the resting potential, partial decrease of the membrane conductance and a marked decrease of the amplitude of action potential. Under these conditions, the conductance at the peak of action potential reduced markedly and the streaming of cytoplasm did not cease during action potential (excitation-cessation (EC) uncoupling). The effects of Ca2+ channel blockers could not be removed by addition of CaCl2 to the external medium. In contrast, the effect of EGTA on the excitability could be removed to a greater extent and the cytoplasmic streaming ceased at about the peak of action potential by the addition of Ca2+ externally. Application of calmodulin antagonists W-7 or TFP caused similar effects on the action potential and on the cytoplasmic streaming.