Analysis of moricizine block of sodium current in isolated guinea-pig atrial myocytes: Atrioventricular difference of moricizine block

Abstract

The effects of moricizine on Na + channel currents ( I Na) were investigated in guinea-pig atrial myocytes and its effects on I Na in ventricular myocytes and on cloned hH1 current were compared using the whole-cell, patch-clamp technique. Moricizine induced the tonic block of I Na with the apparent dissociation constant ( K d,app) of 6.3 μM at −100 mV and 99.3 μM at −140 mV. Moricizine at 30 μM shifted the h ∞ curve to the hyperpolarizing direction by 8.6±2.4 mV. Moricizine also produced the phasic block of I Na, which was enhanced with the increase in the duration of train pulses, and was more prominent with a holding potential (HP) of −100 mV than with an HP of −140 mV. The onset block of I Na induced by moricizine during depolarization to −20 mV was continuously increased with increasing the pulse duration, and was enhanced at the less negative HP. The slower component of recovery of the moricizine-induced I Na block was relatively slow, with a time constant of 4.2±2.0 s at −100 mV and 3.0±1.2 s at −140 mV. Since moricizine induced the tonic block of ventricular I Na with K d,app of 3.1±0.8 μM at HP=−100 mV and 30.2±6.8 μM at HP=−140 mV, and cloned hH1 with K d,app of 3.0±0.5 μM at HP=−100 mV and 22.0±3.2 μM at HP=−140 mV, respectively, either ventricular I Na or cloned hH1 had significantly higher sensitivity to moricizine than atrial I Na. The h ∞ curve of ventricular I Na was shifted by 10.5±3.5 mV by 3 μM moricizine and that of hH1 was shifted by 5.0±2.3 mV by 30 μM moricizine. From the modulated receptor theory, we have estimated the dissociation constants for the resting and inactivated state to be 99.3 and 1.2 μM in atrial myocytes, 30 and 0.17 μM in ventricular myocytes, and 22 and 0.2 μM in cloned hH1, respectively. We conclude that moricizine has a higher affinity for the inactivated Na + channel than for the resting state channel in atrial myocytes, and moricizine showed the significant atrioventricular difference of moricizine block on I Na. Moricizine would exert an antiarrhythmic action on atrial myocytes, as well as on ventricular myocytes, by blocking Na + channels with a high affinity to the inactivated state and a slow dissociation kinetics.