DYNAMICS OF CHLORIDE AND POTASSIUM CURRENTS DURING THE ACTION-POTENTIAL IN CHARA STUDIED WITH ACTION-POTENTIAL CLAMP

The Cl- and K+ currents underlying the action potential (AP) in the giant alga Chara were directly recorded with the action potential clamp method. An electrically triggered action potential was recorded and repetitively replayed as command voltage to the same cell under voltage clamp. The resulting clamp current was close to zero. Only the initial rectangular current used for stimulation was approximately reproduced by the clamp circuit. Inhibition of Cl- channels with niflumic acid or ethacrynic acid and of K+ channels with Ba2+ evoked characteristic compensation currents because the amplifier had to add the selectively inhibited currents. Integration of the compensation currents revealed a mean flux through Cl- and K+ channels of 3.3 10(-6) and 2.1 10(-6) mole m(-2) AP(-1) respectively. The dynamics of Cl- and K+ channel activation/inactivation were obtained by converting the relevant clamp currents to ionic permeabilities using the Goldman-Hodgkin-Katz current equation. During the AP the Cl- permeability reaches a peak 370 ms, on average, after termination of the stimulating pulse. The following inactivation proceeds 3.6 times slower than the activation. The increase in K+ permeability lags behind the rise in Cl- permeability, reaching a peak approximately 2 s after the latter.