The larval muscle fibers of Drosophila show four outward K+ currents in addition to the inward Ca2+ current in voltage-clamp recordings. The Shaker (Sh) and the slowpoke (slo) mutations, respectively, eliminate the voltage-activated fast K+ current (IA) and the Ca2(+)-activated fast K+ current (ICF). Quinidine specifically blocks the voltage-activated delayed K+ current (IK) at micromolar concentrations. We used Sh, slo and quinidine to remove specifically one or more K+ currents, so as to study physiological properties of these currents not previously characterized, and to examine their role in membrane excitability. A linear relationship was observed between the peak ICF and the peak ICa at different membrane potentials. ICF inactivated considerably during a 140 ms pulse to +20 mV. Recovery from inactivation was not complete for up to 2 s at the holding potential of −50 mV, which is much slower than the recovery of Ca2+ current from inactivation. In addition to IA and ICF, two delayed K+ currents are also observed in these fibers, the voltage-activated IK and the Ca2(+)-activated ICS. Near the end of a 500 ms depolarizing pulse, both IA and ICF are inactivated. Ca2(+)-free and 20 mmol l-1 Ca2+ saline were used to examine the tail currents of the remaining IK and ICS. The tail currents of ICS were slower than those of IK and reversed between −30 and −50 mV in different fibers. We further studied the dose-dependence of the blockade of IK by quinidine, which did not indicate a simple one-to-one binding mechanism. Current-clamp recordings from normal, Sh, slo and the double-mutant Sh;slo fibers suggested that ICF plays a stronger role than IA in repolarization of the larval muscle membrane. Elimination of ICF facilitates the occurrence of action potentials. Further elimination of IK prolonged the action potentials to several hundred milliseconds.

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