Concentration-dependent block of sodium current in guinea pig ventricular myocytes by a class III antiarrhythmic agent, MS-551

Effect of Nifekalant, a Class III Anti-Arrhythmic Agent, on Ca2+ Waves in Rat Intact Trabeculae

BACKGROUND

Nifekalant, a class III anti-arrhythmic agent, has been used clinically at serum concentrations of 1-10 micromol/L in patients with ventricular arrhythmias. However, the effect of nifekalant on triggered arrhythmias has not yet been established.

METHODS AND RESULTS

Trabeculae were dissected from the right ventricles of 16 rat hearts. The force was measured using a silicon strain gauge, the membrane potential using ultra-compliant microelectrodes, and the regional intracellular Ca2+ ([Ca2+]i) using electrophoretically microinjected fura-2 and an image intensified CCD camera at a sarcomere length of 2.1 microm. Rapid cooling contractures (RCCs) were measured to estimate the Ca2+ content in the sarcoplasmic reticulum. Ca2+ waves and aftercontractions were measured after the induction of reproducible Ca2+ waves. Nifekalant at 1, 10 and 250 micromol/L increased significantly the action potential duration, the peak [Ca2+]i, the developed force and the amplitude of RCCs in a concentration-dependent manner (stimulus interval = 2 s, [Ca2+]o = 0.7 mmol/L, 26.0+/-0.2 degrees C). Nifekalant at 10 and 250 micromol/L increased significantly the velocity of Ca2+ waves with an enhancement of the aftercontractions (stimulus interval = 0.5 s for 7.5 s, [Ca2+]o = 1.8+/-0.1 mmol/L, 22.3+/-0.5 degrees C).

CONCLUSIONS

Nifekalant, even at a therapeutic concentration, can increase muscle contraction, but may worsen triggered arrhythmias because of the acceleration of Ca2+ waves under Ca2+-overloaded conditions.

Electropharmacological and Proarrhythmic Effects of a Class III Antiarrhythmic Drug Nifekalant Hydrochloride Assessed Using the In Vivo Canine Models

Cardiovascular effects of Nifekalant were examined using halothane-anesthetized dogs, and its proarrhythmic potential was estimated with chronic complete atrioventricular block dogs. Nifekalant was intravenously administered to the halothane-anesthetized dogs in three doses of 0.03, 0.3, and 3 mg/kg/10 minutes with a pause of 20 minutes (n = 6). The low dose hardly affected any of the cardiovascular parameters. The middle dose, a clinically recommended antiarrhythmic dose, decreased the total peripheral resistance, increased the cardiac output, and prolonged the ventricular repolarization phase and effective refractory period. The high dose increased the left ventricular contraction, transiently decreased the mean blood pressure, and enhanced the atrioventricular conduction, besides potentiation of the changes induced by the middle dose. Increment in the repolarization phase by the high dose was greater than that in the refractoriness, leading to increase of ventricular electrical vulnerability. To the atrioventricular block animals, clinically relevant antiarrhythmic dose of 3 mg/kg p.o. of Nifekalant and its 10-times-higher dose were administered. The high dose prolonged QT interval leading to torsades de pointes in all animals (n = 5), which was not detected by the clinical dose (n = 5). These results suggest that antiarrhythmic dose of Nifekalant can be used safely; however, caution should be paid for patients complicating bradycardia and/or a risk of elevated plasma drug concentration.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.