An early outward transient K+ current that depends on a preceding Na+ current and is enhanced by insulin

A whole-cell early transient outward current occurs in rat myoballs if and only if there is an immediatly preceding current of large amplitude through the voltage-gated, tetrodotoxin-inhibitable Na⁺ channel. This early outward transient is a K⁺ current, designated I_K(Na⁺). Under the conditions in which I_K(Na⁺) appears, simultaneous measurement of voltage and current, under voltage clamp, demonstrates that there is transient voltage escape to depolarized levels, peaking at about the time of peak inward Na⁺ current arid resembling an action potential. I_K(Na⁺) was never seen in the absence of this breach of the voltage clamp, suggesting that I_K(Na⁺) might be an artefact due to transient depolarization from the clamp. However, when the voltage escape was mimicked by voltage commands under conditions in which the Na⁺ channel was not activated, there was no I_K(Na). Insulin increased or produced I_K(Na⁺) even though insulin had no effect on I_Na or on the delayed rectifier K⁺ current or on the escape from voltage clamp. It is concluded that there is a population of rat myoballs in which there is an early outward K⁺ current that requires an immediately preceding current through the voltagegated tetrodotoxin-inhibitable Na⁺ channel and is enhanced by insulin.