| Voltage-gated sodium current (INa) plays a crucial role in initiating action potentials and propagating excitation impulse in the heart. The present study was designed to determine whether the biophysical properties, drug sensitivity of INa are different in atrial and ventricular myocytes from rat hearts using a whole-cell patch voltage clamp technique, RT-PCR and western blot.We found that density of fast inward INa (at -35mV) was about 40% greater in left atrial myocytes (LA,-38.67±4.66 pA/pF, n=17) and right atrial myocytes (RA,-38.81±2.42 pA/pF, n=16) than that in left ventricular myocytes (LV,-23.59±0.85pA/pF, n=48) and right ventricular myocytes (RV,-22.25±1.53 pA/pF, n=29, p<0.01). Activation and inactivation time constant (tau) of INa are smaller in LA/RA myocytes (0.26±0.03ms, n=11; 0.42±0.02ms, n=12) than that in LV/RV myocytes (0.70±0.07ms, n=21; 0.66±0.02ms,n=25, p<0.05). Also, the resting state inactivation is fater in LA (taul and tau2:27.40±1.57ms, 380.5±46.63ms, n=13) and RA(34.89±4.60ms,361.53±58.40ms, n=12) than that in LV(42.69±1.50ms,317.59±27.53ms, n=12) and RV (47.45±0.46ms,315.4±12.95ms, n=19, p<0.05).However, recovery of INa from inactivation was slower in LA/RA myocytes (recovery tau: 8.45±0.69ms, n=13; 8.75±0.54ms, n=12) than that in LV/RV myocytes (tau:5.60±0.55ms, n=12; 5.40±0.24ms, n=26, p<0.01).Interestingly, inhibition of INa by the class I antiarrhythmic drug flecainide was greater in atrial myocytes (IC5o=4.13μM) than that in ventricular myocytes (IC5o=13.15μM), whereas mexiletine had no significant difference in inhibiting INa in atrial (IC5o=25.98μM) and ventricular (IC50=26.81μM) myocytes. Moreover, flecainide negatively shifted stedy-state inactivation potential of INa more in atrial myocytes than that in ventricular meocytes.RT-PCR and western blot analysis revealed that the SCN5A and SCN3B expressions were greater in LA/RA myocytes, while SCN4B expression was greater in LV/RV cell, and no observation of SCN2B in rat heart.Our results demonstrate that heterogeneity of INa in current density and biophysical properties are present in rat atrial and ventricular myocytes. The class I antiarrhythmic drug flecainide, but not mexiletine, more sensitive to inhibition of atrial INa, which is likely the ionic mechanism underlying flecainide (not mexiletine) exhibits an anti-atrial fibrillation effect. Also, the different subunits expressions may contribute to atrial-selective sodium blocker. Whether atrial sodium channel can be a target for developing selective anti-atrial fibrillation remains to be further studied. The transient outward K+ current (Ito) is believed to play an important role in human atrial repolarization. Inhibition of Ito prolongs the action potential duration in human atrium. The present study was to investigate the potential effect of Allitridi on the cloned human transient outward K+ channel (Kv4.3) stably expressed in HEK293 using a whole-cell patch voltage-clamp technique.It was found that Allitridi inhibited Kv4.3 channel current in a concentration-dependent manner, and the IC50 for inhibiting Kv4.3 current was 13.06μM. Allitridi reduces Ito inactivation time constant in a voltage-dependent and dose-dependent manner (an open channel blocker property). In addition, allitridi (10μM) negatively shifted the availability potential of Kv4.3 current (from-41.54±0.17 mV of control to-48.44±0.17 mV with allitridi, n=8, P<0.05). However, allitridi accelerated Ito recovery from inactivation (recovery tau from 131.84±27.82 ms to 87.02±20.22ms, n=8, P<0.05).These results indicate that Allitridi inhibit Kv4.3 channel by binding to an open channel. Molecular determinants of allitridi for blocking Kv4.3 channel are under investigation. |
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