RESUMO
Single "early" action potentials (AP) or "early" postdepolarization (EAD) were recorded in multicellular stripes working in the regimen of true pacemaker cells located in the basis of the sinoatrial valve of the dace (Leuciscus rutilus) heart after Ni2+ (0.3 mM) were added to superfused solution on the 15 th min. The standard microelectrode technique was used. The appearance of single EAD was preceded by increase of AP spike duration almost 1.5-fold due to increase of the duration of a final repolarization phase and of diastolic depolarization. EADs appeared at different levels of repolarization from -20 to -60 mV. EAD's amplitude was lower than the AP proper. The effect is reversible. Nifedipine (2 microM) decreased by 20% the AP duration to 50% repolarization in the cells of atrial type. In case the stripes were primarily subjected to nifedipine effect and then even after a long procedure of washout, the Ni2+ (0.3 mM) did not cause EADs. The hypothesis has been tested of what is the main reason of single EAD appearance in pacemaker cells under the effect of Ni2+; it is either blocking Ca release from the cell by Ni2+, or reactivation of L-type Ca-channels' conductivity. The analysis of the data obtained leads to the conclusion that 30-50% Na-Ca-exchangers block is the basic reason of single EAD appearance in pacemaker cells.
