Voltage‐clamp analysis of the potassium current that produces a negative‐going action potential in Ascaris muscle.

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Abstract
A voltage clamp was developed for the pharyngeal muscle of the nematode A. lumbricoides and used to analyze the K current that produces a negative-going, regenerative action potential in this muscle. Depolarizing voltage steps elixit a sustained inward current, returning the membrane voltage to the resting level evokes a strong, transient, outward current. This outward current reverses direction at the same voltage as that reached by the negative-going spike and is identified as the negative spike current. The negative spike current decays with a time constant of 30 ms at voltages more negative than -30 mV. This inactivation of the negative spike conductance is removed by holding the membrane at potentials more positive than -15 mV. The time constant for removal of inactivation decreases from more than 300 ms at -15 mV to about 30 ms at + 10 mV. When inactivation was removed, the negative spike conductance is turned on by stepping to potentials more negative than -15 mV. Although the reversal potential for this current depends strongly on [K+]o [outside concentration of K+] (42 mV/decade), the potential at which the conductance is turned on is independent of [K+]o. External Na+ seems to facilitate the negative spike current. Reduction of [Na+]O reduces its conductance and shifts the reversal potential to more positive values. External Rb+ and Cs+ show voltage-dependent blocking of this current. This K current is different from all the K currents studied previously; it is analogous to the classical NA current of nerve and muscle, except for an inversion of the voltage dependencies.