Inhibition of the fast sodium inward current in ventricular cardiomyocytes of rats and guinea pigs by a novel potent sodium channel blocking agent

In enzymatically-dispersed single ventricular cardiomyocytes of adult rats and guinea pigs the inhibition of the cardiac sodium current by a novel Sodium Channel blocking agent (LG 83-6-05, 1-[3-(2-Hydroxy-3-(2-methylpropylamino)-propoxy)-4-methyl-2- thienyl++ ]-3-phenyl-1-propanon hydrochloride) was studied. A single-electrode voltage-clamp system (switch clamping, patch electrodes) was used to measure action potentials as well as ionic currents during voltage-clamp experiments. In addition single channel measurements were performed using the patch-clamp technique. The single cell system enabled us to demonstrate that LG 83-6-05 is an inhibitor of the cardiac sodium current. The substance acts concentration-dependently and belongs to the most potent sodium-channel blocking agents known. It could be shown that the whole-cell sodium inward current is blocked in a frequency-dependent manner (phasic block) and that the steady-state inactivation curve of the sodium current is shifted significantly towards negative potentials, indicating a considerable tonic block at the resting membrane potential. The time constant of the recovery from inactivation of the sodium current as estimated from voltage-clamp experiments is prolonged by a factor of up to 290 (holding potential -110 mV, 2 mumol/l). This prolongation is voltage dependent, faster release from block occurring at more negative potentials. The open state probability of the single cardiac Sodium Channel is reduced in a frequency-dependent manner, whereas its current amplitude remains unchanged during the influence of the substance. The number of channels not available for opening is increased considerably with increasing stimulus frequency. These findings suggest stabilization of the inactivated state of the ionic channel by drug binding.