[Prolonged after-potentials of the myelinated nervous fibres in amphibia after 4-aminopyridine-induced blockage of potassium channels].

Myelinated nerve fibres of Rana ridibunda Pallas were studied by using extra-cellular recording technique. 4-aminopyridine, a blocker of fast potassium channels produced considerable changes in duration of the after-depolarization and increase in total membrane depolarization (evaluated by an action potential area). These effects were time-dependent. The after-depolarization duration under 4-aminopyridine (4-AP) treatment changes in 3 phases: at the beginning of the exposure its duration increased; then the increase stopped; and later duration of the after-depolarization start to decrease. The last phase was often followed by hyperpolarizing after-potential. The interchange of these phases accompanies by statistically significant increase of the action potential area (from 189.201 +/- 28.437 m x ms in normal solution to 489.945 +/- 41.75 m x ms in 5 minutes after 4-AP application). These observations are explained as a result of the increased activation of the slow potassium channels and decreased portion of fast potassium channels involved in membrane repolarization. The decrease in post-tetanic depolarization duration, the appearance of the post-tetanic hyperpolarization at low frequencies and increase of post-tetanic hyperpolarization at higher frequencies of stimulation under 4-AP treatment are dependent on activation of slow potassium channels.