In isolated neurons of visceral ganglia of snail Helix pomatia a slow early outward current (IA) was studied using a two-microelectrode voltage clamp technique. 2. The time of activation and inactivation of IAS at -40 mV were 90-120 msec and 3-5 sec respectively. The removal of inactivation at -120 mV took 2-5 min. 3. The reversal potential of the IAS was about -80 mV in normal saline and was sensitive to the external potassium concentration, changing about 35 mV per fivefold change in potassium over the range from 4 to 20 mM. The results suggest that IA were due to K+. 4. The IA persisted in Ca2+ -free medium and in the presence of Ca2+ -channels blockers, e.g., Cd2+. 5. The IA were blocked by 1-10 microM extracellular 4-aminopyridine, 1 mM of tetraethylammonium ions, 1 mM of Ba2+, but one was resistant to 1 mM Cs+. 6. 4-aminopyridine had a dual effect on the IA. It blocked the normal current, and then appeared to increase the inactivated currents.
