Voltage-activated fast currents of frog optic tectum neurones are blocked by ouabain or K(+)-free solution.

Frog optic tectum neurones in vitro (under whole cell patch clamp with a Cs(+)-filled electrode) generated, in response to voltage steps negative to -80 mV from -60 mV holding potential, transient inward currents dependent on extracellular Na+ and blocked by tetrodotoxin (TTX). Depolarizing steps from the same holding potential induced conventional, fast Na+ currents followed by a transient outward cationic current (Icat). Voltage-activated Na+ currents (induced by negative or positive steps) and Icat were blocked by ouabain or K(+)-free solution with comparable timecourse. It is suggested that transient inward and outward currents of frog tectal neurones were at least partly dependent on the rapid operation of a membrane cation transporter.