The physiological role of Ito has yet to be clarified. The goal of this study is to investigate the possible contribution of the transient outward current (Ito) on the generation of transmembrane action potentials (APs) and the sensitivity of mouse sinoauricular node (SAN) cells to a 4-aminopyridine (4AP) as Ito blocker. The electrophysiological identification of cells was performed in the sinoauricular node artery area (nstrips = 38) of the subendocardial surface using microelectrode technique. In this study, for the first time, it was observed that dependence duration of action potential at the level of 20% repolarization (APD20) level under a 4AP concentration in the pacemaker SAN and auricular cells corresponds to a curve predicted by Hill’s equation. APD20 raised by 70% and spike duration of AP increased by 15–25%, when 4AP concentration was increased from 0.1 to 5.0 mmol/L. Auricular cells were found to be more sensitive to 4AP than true pacemaker cells. This was accompanied by a decrease in the upstroke velocity as compared to the control. Our data and previous findings in the literature lead us to hypothesize that the 4AP-sensitive current participates in the repolarization formation of pacemaker and auricular type cells. Thus, study concerning the inhibitory effects of lidocaine and TTX on APD20 can explain the phenomenon of the decrease in upstroke velocity, which, for the first time, was observed after exposure to 4AP. Duration of AP at the level of 20% repolarization (APD20) under a 4-AP concentration 0.5 mmol/L in the true pacemaker cells lengthen by 60–70% with a control.

Introduction Ventricular tachycardia and/or ventricular fibrillation (VT/VF), a life‐threatening complication of acute myocardial ischemia are based on the interaction between a vulnerable arrhythmogenic substrate and an arrhythmic trigger. Melatonin was proposed as a potential antiarrhythmic medication and was shown to ameliorate the arrhythmogenic substrate. Also, melatonin provides a sympatholytic effect and thereby can attenuate ectopic activity, which provides the triggers for VT/VF. However, little is known about a melatonin effect on the arrhythmic triggering. In the present study, we aimed at evaluation of the melatonin effects on catecholamine synthesis in normal and ischemic myocardium and heart rhythm disturbances during an episode of ischemia and reperfusion. Methods Experiments were done in 30 control and 32 melatonin‐treated (10 mg/kg, daily, for 7 days) male rats. The hearts from 10 control and 10 treated animals were taken for histochemical analysis of sympathetic fibers (glyoxylic acid‐induced fluorescence). Continuous ECG recording was performed in 20 control and 22 treated anesthetized animals before and during an ischemia‐reperfusion episode induced by reversible 5‐min occlusion of the left anterior descending coronary artery. Tyrosine hydroxylase expression in normal and ischemic myocardial regions was assessed by Western blotting analysis in 12 control and 12 melatonin‐treated animals. Results No differences in the state of sympathetic innervation (density and fluorescence intensity) were observed in histochemical analysis. However, Western blotting demonstrated that melatonin treatment suppressed tyrosine hydroxylase expression in normal (p<0.05 vs control) but not in ischemic regions of myocardium. Melatonin‐treated animals had longer RR‐intervals in the baseline state, but this difference progressively disappeared during the period of ischemia due to tachycardic response to ischemia in the treated group (Figure). No differences in the number of extrasystoles were found between the control and treated groups during ischemia and reperfusion periods (Figure). Conclusion Chronic melatonin treatment led to the peripheral sympatholytic effect in myocardium, which was associated with decreased heart rate in preischemic conditions. However, this effect attenuated during ischemia and no differences in heart rate or extrasystolic burden were found between the control and treated animals during ischemia and reperfusion.

Experiments on spontaneously contracting strips from sinoatrial region of the hearts of 2-month-old albino mice showed that cesium (Cs(+)), a blocker of hyperpolarization-activated I(f) current, in a concentration of 1 mM produced the greatest negative chronotropic effect on the duration of diastolic depolarization phase (75%), its rate (59%), and action potential duration (29%). The threshold concentration of Cs(+) was approximately 0.15 mM. In a concentration of about 8.5 mM, spontaneous generation of action potentials stopped. The effect was reversible. Thus, blockade of I(f) current by Cs(+) reduced the rate of action potential generation in cells of mouse sinoatrial node by app. 42% in comparison with controls.