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

1. In voltage-clamped 'motor-giant' neurones of the crayfish Orconectes limosus a depolarizing voltage step clicits a transient inward current carried by Na which is followed by an early and a delayed outward current. 2. 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. 3. 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. 4. Ionophoretic injections of EGTA did not depress the early outward current. 5. Short ionophoretic injections of Na into the neurone increased the outward current elicited by a depolarization but did not affect the leakage current. 6. 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. [ABSTRACT FROM AUTHOR]