Effects Of Urapidil On The Fast Soudium Ionic Channel Currents In Sd Tat Ventricular Myocytes

Objective: Arrhythmia as a common cardiovascular disease may have serious effect on haemodynamics, always complicating with other cardiovascular diseases (CVD) sometimes cause sudden death, seriously threated to people's health. However, up to now many antiarrhythmic drugs (AADs) have the arrhythmogenic effects. The CAST trail shows that some typeⅠAADs can increase the mortality of myocardial infarction. The activation of sympathoadrenal system plays a very important role in the development of CVD, and there is no exception of arrhythmia. But ever since a long time ago,β-AR block is given a widespread interest because of its beneficial effect in CVD. Whileα-AR block's role has been ignored. In fact,α-AR block has special benefit in the treatment of hypertension and heart failure. As early in the 1953, Leimdorfer first reported thatα-AR block phentolamine has the effect of antiarhythmia. Recently some animal experiments show that alpha and beta adrenalin system take a synergistic effect in the development of arrhythmia. But detail mechanism of the action has not been explained.The sympathoadrenal system plays a very important role in the development of arrhythmia at the early time of acute myocardial ischemia. If some people have the acute myocardial ischemia, their active of sympathetic nervous system and the release of catecholamine will be obviously increased. When the adrenergic nerve is cut, the rate of malignant arrhythmia with early myocardial ischemia such as ventricular tachycardia and ventricular fibrillation will be decreased. Butβ-AR block can not degrade the incidence rate of malignant arrhythmia with early myocardial ischemia. The activity ofα-AR may have an important arrhythmogenic effect at the early time of myocardial ischemia. But the concrete mechanism of action is not clearly. Ion channel is the conductive foundation of cardiac electromyographic signal. The relationship between arrhythmia and ion channel are very close. The present study is designed firstly to separate the ventricular myocytes and then examine the effect of fast sodium ionic channel with or without urapidil by the whole cell patch-clamp.Methods:1 Experimental animalsFifty healthy SD rats of either sex weighing 150-250g were used in the study which came from Hebei Medical University.2 Cell isolationSingle ventricular myocytes were enzymatically( typeⅠcollagenase ) dissociated and isolated.3 Cell selectionChose the cells which had clear structure, obvious transverse striation and baculiform. Cells like these can be sealed and ruptured easily.4 Group and medicationThese cells were divided to three groups: normal control group which had no urapidil, low concentration group which had 5×10^-5mmol/L urapidil, high concentration group which had 5×10^-4mmol/L urapidil. Every group had 15 cells which were perfused with different concentration urapidil, the floe rate was 2ml/min, about 3-5 minutes.5 Current recordingThe INa current was recorded by means of patch clamp whole-cell recording teachniques under voltage clamp. The INa currents from group of different concentration urapidil were compared with the normal control group. The current recording program is controlled by the pulse+pulsesefit software. The signal is amplified by the EPC-9 patch clamp amplifier.6 StatisticsData were expressed as mean value±standard deviation ( x±s). The statistical analysis was performed using ANOVY. The variance within groups was analyzed by t test (SPSS,versin 17.0), A value of P < 0.05 was considered statistically significant.Results:1 Effect of urapidil on INa current density-voltage relationship (I-V) curve of ventricular myocytesThe INa (I-V) curves and activation curves were voltage-dependent and showed as"V"type current properties. They were activated at–70mV, reached the maximum at–40mV and reversed at +50mV. The INa (I-V) curves of intervention group was moved up especially in the high concentration group (5×10^-4)mmol/L). This was not accompanied by a shift in the current density-voltage relationship configuration and did not change their active, peak and reverse potentials.Peak INa current density (at -40mV) was significantly reduced in low- concentration of urapidil (5×10 - 5)mmol/L) (-29.86±1.37pA/pF,) and high-concentration of urapidil (5×10 - 4)mmol/L) (-14.44±8.21pA/pF,) compared with the control group (-48.74±2.66pA/pF),P<0.05.2 Effect of urapidil on the INa steady-state inactivation curve of ventricular myocytesThe steady-state inactivation curves of INa were voltage-dependent and showed as"reversed S"type. The INa steady-state inactivation curves showed that the membrane potentials for half-maximal inactivation (V0.5) were shifted markedly in the upward direction (that is, to more negative potentials) in high- concentration of urapidil (5×10^-4mmol/L) (–109.12±1.58mV)and low-concentration of urapidil (5×10^-5mmol/L) (–96.04±1.85mV) compared with the control group (–76.41±2.84mV), P<0.05.3 Effect of urapidil on the INa recovery from inactivation curve of ventricular myocytesWhen the pulse time is 25ms ,the INa recover 79.64% in the control group , 53.27% in low-concentration of urapidil (5×10^-5mmol/L), 43.50% in high-concentration of urapidil (5×10^-4mmol/L). When the pulse time is 85ms, the INa recover 99.82% in the control group , 80.00% in low-concentration of urapidil (5×10^-5mmol/L), 69.91% in high-concentration of urapidil (5×10^-4 mmol/L). When the pulse time is 145ms, the INa recover 100.00% in the control group , 90.90% in low-concentration of urapidil (5×10^-5mmol/L), 86.90% in high-concentration of urapidil (5×10^-4mmol/L). It showed that the rate of recovery from inactivation of INa were significantly slower in high-concentration of urapidil (5×10^-4mmol/L) and low-concentration of urapidil (5×10^-5mmol/L) compared with the control group, (P<0.05).Conclusions:1 The experiment showed that INa current densities in different concentration urapidil were significant suppressed. There were a series of changes in the ventricular myocytes after they were given urapidil, such as reduction of INa current density, shift in upward direction of steady-state INa inactivation curves, and slow INa recovery rate from inactivation. These changes may underlie the altered electrical activity (decrease in the velocity and amplitude of phase 0 of action potential) and abnormal transmembrane action potentials contributing to inhabit ventricular arrhythmias.2 The experiment indicated that different concentration of urapidil had the different effect. The high-concentration urapidil group had stronger power. This phenomenon showed that the pharmacologic action of urapidil was concentration dependent.3 Neither the mechanism of arrhythmia was single, nor the effect of ion channel was isolated. There should be further study about urapidil to reveal whether it has the effect of antiarrhythmia and the direct mechanism.