Early events in plasma membrane of Chara corallina after electrical stimulation. I. Studies under current clamp conditions. Plant Cell Physiol

Events in the early periods of the stimulus-response (current-voltage) relation of Chara cor-allina internodal cell were studied under current clamp conditions. Whenever the internodal cell elicited an action potential by a current stimulus, the voltage exhibited an additional peak soon after application of the stimulus; that is, the peak voltage preceded the threshold for the action potential. The strength-peak voltage relation was almost a straight line when the voltage was less negative than about —50 mV. The electromotive force at the peak voltage varied by 48.4 mV be-tween 0.1 and 1.0 mM of external K+ concentration, and it was within expected range of equilibri-um potential for K+ ions. Even when the action potential was not elicited at a low external Ca2+ concentration, the peak voltage still appeared, keeping the electromotive force within the range of equilibrium potential for K+ ions. We concluded that the K+ channel opens very early after electr-ical stimulation and when the action potential is elicited under a sufficient external Ca2+ concen-tration, the entry of external Ca2+ ions necessary for the opening of the Ca2+ activated Cl~ channel occurs after the peak voltage. Key words: Chara corallina — K+ channel — Peak voltage — Stimulus-response relation — Threshold potential. The action potential in the Chara internodal cell is one of the typical excitation phenomena in plant cells. During excitation, the membrane conductance (G ^ transiently in-creases by 10- to 100-fold, which is mainly due to increases in the conductances of both K+ and Cl ~ ions, i.e., GK an