Effect of nicotinamide on atrioventricular conduction in rabbits

The effects of nicotinamide on A-V conduction were studied in anesthetized rabbits and isolated rabbits hearts by recording His bundle electrogram. In anesthetized rabbits, A-H interval was markedly shortened after iv tervals were not changed. This effect of nico tinamide was not prevented by propranolo. Nicotiamied overcame the effect of verapamil interval. When verapamil dosage reached 2.18 mg/kg, nicotinamide failed to overcome it. Concentration of Ca++ in the control perfusate was 2.4 mM. Low Ca++(0.8mM) prolonged the A-H interval, whereas high Ca++(4.0mM) caused a greater prolongation of the interval. After nicotinamide 60 mg was injected into the cannula perfusing the coronary system, the A-H interval was immediately shortened in solution, containing normal Ca++ concentration. Nicotinamide improved the A-V conduction depressed by low Ca++, but aggravated the A-V conduction depressed by high Ca++. These results indicate that nicotinamide accelerates intranodal conduction and has little effect on intraatrial and His-Pirkinje conduction. Nicotinamide may be an activator of the slow channel, increasing slow inward current of A-V nodal cells rather than stimulating β-adrenoreceptor.