Potentiation of a transient outward current by Na+ influx in crayfish neurones

In voltage-clamped "motor-giant" neurones of the crayfish Orconectes limosus a depolarizing voltage step elicits a transient inward current carried by Na+ which is followed by an early and a delayed outward current. The early outward current is reduced if the Na+ current is suppressed by tetrodotoxin or the removal of external Na+. It is also abolished if the K+ channel blocking agents tetraethylammonium and 3,4-diaminopyridine are applied to the neurone. The outward current was not depressed if Li+ was substituted for Na+ in the external solution or if the Na-K pump was inhibited by ouabain or the removal of external K+. Ionophoretic injections of EGTA did not depress the early outward current. Short ionophoretic injections of Na+ into the neurone increased the outward current elicited by a depolarization but did not affect the leakage current. It is suggested that the influx of Na+ leads to a transient increment of the Na+ concentration near K+ channels and that internal Na+ ions exert an activating or modulating effect on K+ channels.