DUAL EFFECT OF 4-AMINOPYRIDINE ON LOW-THR ESHOLD POTASSIUM CHANNELS OF NEURONAL MEMBRANE

The blocking effect of 4-aminopyridine (4AP) on the transient (I-A) an d slow (I-As) low-threshold potassium currents in isolated snail neuro nes was studied using the two microelectrodes voltage clamp method. 4A P in the concentration of 5-200 mkmol/l displayed dual effect both on the I-A and on the I-As. Blocking the normal currents, 4AP appeared to increase the inactivated currents. This resulted in a crossover pheno menon: in the presence of 4AP, the outward current was less than contr ol at the begining of a depolarizing pulse but crossed over during the pulse to become greater than control. Similar crossover phenomenon wa s obtained in the curves of channel reactivation before and after 4AP application. The processes of 4AP-induced block and inactivation of A- and As-channels appear to be mutually exclusive.