| Objective: Arrhythmias remain a leading cause of mortality and morbidity. Exporing the mechanism of the arrhythmias and seeking the safe and effective antiarrhythmic therapy become the research hot spots. Previous studies have described the important regional differences in the electrophysiology and electropharmacolgy of ventricular myocardium in mammalian hearts, which are termed as electrophysiological heterogeneity. Sodium current plays a important role in electrophysiological heterogeneity. Ibutilide is a new class III antiarrhythmic agent. It has antifibrillatory effects in atrial fibrillation and flutter and in ventricular tachyarrhytmias. But the cellular mechanism is still uncertain, especially its effects in ventricular tachyarrhythmias. The aim of the present study is to explore the cellular basis of antiventricular arrhythmias effects of Ibutilide and to explore the relationship between Ibutilide and INa heterogeneity of the three types ventricular myocytes.Methods: The experimental animals were healthy Newsland pure white rabbits weighing 2.0 to 2.5 kg with either sex, which were bought from HeBei medical university and were qualified for experiments. The three types myocytes were isolated ensymatiedly from the left ventricular wall of the rabbits. The INa in epicardial, endocardial and midmyocardial cells were recorded using patch clamp techniques in the whole-cell configuration from the three cellular subtypes of the left ventricular free wall. To explore the relationship of Ibutilide and the three types ventricular myocytes, the different solution of Ibutilide were used: 10-7M and 10-6M. The experiments were performed at room temperature. The data were expressed as mean±SD, statistical analysis was performed using one-way analysis of variance (ANOVA) with SPSS 10.0 software package to test for the significance of the effects of Ibutilide on Na+ channel activity and kinetics. A value of P<0.05 was considered significant.Result1 The change of INa heterogeneity among the normal and Ibutilide groups1.1 The INa current density-voltage relationship (I-V) curves were voltage-dependent, they were activated at -70mV, reached the maximum at -20mV and reversed at +60mV. In the normal group, the Peak INa current density of the M, Endo and Epi were -31.53±3.76 pA/pF (n=13 cells),-15.22±2.67 pA/pF (n=12 cells)and -13.00±3.45pA/pF (n=10 cells). The Peak INa current density in M cells was larger than in Epi and Endo, and significant differences could be found, P<0.01. There was no difference between Epi and Endo. In the Ibutilide (10-7M) group, the Peak INa c urrent density of the Epi, Endo and M were -7.82±2.99 pA/pF (n=10 cells),-9.64±3.42 pA/pF (n=10 cells) and -11.32±2.33 pA/pF (n=13 cells),There were no difference among the three types. The Peak INa current density of the Epi,Endo and M in the Ibutilide (10-6M) group were -6.70±3.21 pA/pF (n=10 cells),-6.02±2.58 pA/pF (n=10 cells)and-9.63±1.49 pA/pF (n=13 cells), There was no difference among M, Epi and Endo, P>0.05.1.2 The INa steady-state inactivation curves of the Epi, M and Epi were voltage-dependent. In the normal group the V0.5 of the INa steady-state inactivation curves about M, Epi and Endo were -102.36±8.74mV (n=11cells),-97.17±15.53 mV (n=11 cells) and -93.07±13.79mV (n=10 cells). The V0.5 of M cells was significantly different compared with the other two groups and significant differences could be found, P<0.05. There was no difference between Epi and Endo, P>0.05. In the Ibutilide (10-7M) group the V0.5 of the INa steady-state inactivation curves of M, Epi and Endo were -107.23±6.85 mV (n=11 cells),-99.14±7.74 mV (n=11 cells)and -102.01±7.32 mV (n=11 cells). There was no difference among M, Epi and Endo,P>0.05. In the Ibutilide (10-6M) group the V0.5 of the INa steady-state inactivation curves about M, Epi and Endo were-124.56±8.22 mV (n=11 cells),-108.15±6.92 mV (n=11 cells)and-99.83±7.91 mV (n=11 cells). The V0.5 of the INa steady-state inactivation curves of three subtypes of cells was n o difference compared with each other,P>0.05.1.3 There was no significant difference in the INa recovery from inactivation of three subtypes of cells in each group. The three subtypes of cells in Ibutilide group recovered slowly than the normal group.2 The effects of Ibutilide on INa of the subtypes of left ventricular myocardial cells2.1 INa I-V curves: Compared the Peak INa current density of M cell among the three groups,there were differences between every two groups, P<0.05; Compared the Peak INa current density of Endo cell among the three groups, there were differences between the normal group and Ibutilide groups. For the two Ibutilide groups, there was no diference; Compared the Peak INa current density of Epi cell among the three groups, our rusullts were the same as Endo cell.2.2 The INa steady-state inactivation curves: Compared the INa steady-state inactivation curves of M cells among the three groups, there were differences between the normal group and Ibutilide groups. For the two Ibutilide groups, there was no difference; Compared the INa steady-state inactivation curves of Endo cells among the three groups, there were differences between every two groups, P<0.05; Compared the curves of Epi cells among the three groups,there were differences between the normal group and Ibutilide groups.For the two Ibutilide groups, there was no diffe rence.2.3 The INa recovery from inactivation curves: Compared the INa recovery from inactivation curves of M, Endo and Epi cells among three groups in turn, there were differences between every two groups, P<0.05.Conclusions1 There was heterogeneity in the INa of Epi, M and Endo.The Peak INa current density in M cells was larger than those in epicardial and endocardial cells. M cells inactivated faster than epicardial and endocardial cells. There were no significant differences in their recovery curves from inactivation among the three types of myocytes.2 The study indicated Ibutilide can suppress significantly the heterogeneity of Epi, M and Endo, which may underlie the made electrical activity (in the velocity and amplitude of pase 0 of action potential, transmembrane action potentials) tend to the same degree. The changes contribute to inhibit reentrant ventricular arrhythmias.3 The effects of Ibutilide on left ventricular myocardial were concentration-depended. Under some condition, with a higher concentration Ibutilide will suppress sodium current in a greater extent.
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