Direct cardiac effects of a novel vesamicol receptor ligand, m-iodobenzyl-trozamicol, assessed in the canine isolated, blood-perfused heart preparations

Constitutional rho-kinase regulates atrioventricular nodal conduction and ventricular repolarization of the canine heart

Given the limited information, physiological roles of Rho-kinase in the cardiac conduction system and ventricular repolarization process were assessed in comparison with those in the coronary vascular tone. A specific Rho-kinase inhibitor Y-27632 was administered to the nutrient coronary artery of the canine isolated, blood-perfused atrioventricular node preparation under the monitoring of the ventricular monophasic action potentials. Administration of Y-27632 moderately suppressed the atrioventricular nodal conduction, slightly but significantly accelerated the repolarization process, and potently increased the coronary blood flow, whereas it hardly affected the intraventricular conduction. The estimated concentrations of Y-27632 causing the currently observed effects were enough to inhibit Rho-kinase. These results suggest that constitutional Rho-kinase functions to moderately facilitate the atrioventricular nodal conduction, slightly delay ventricular repolarization process, and significantly increase the coronary vascular tone.

Effects of a typical I(Kr) channel blocker sematilide on the relationship between ventricular repolarization, refractoriness and onset of torsades de pointes

The effects of a typical I(Kr) channel blocker sematilide on the relationship between ventricular repolarization, refractoriness and onset of torsades de pointes (TdP) were studied using the canine isolated, blood-perfused ventricular septum preparation with monophasic action potential (MAP) recording. Intracoronary infusion of sematilide (10-100 microg/min) prolonged the repolarization phase and effective refractory period, the extent of which was greater in the former than in the latter, resulting in prolongation of terminal repolarization process. Prolonging the basic pacing cycle length from 400 to 600 ms and/or increasing the drug doses enhanced each of these actions. Reverse use-dependence was obvious in the drug-induced prolongation of MAP duration, but it was less clear in the effective refractory period. More importantly, during sematilide infusion, in preparations paced at longer basic cycle length of 600 - 2000 ms, TdP-like polymorphic ventricular tachycardia was repeatedly induced by an extra-stimulus applied on the terminal repolarization phase, which indicates the appearance of electrically vulnerable period. Prolonging the basic pacing cycle length and/or increasing the drug doses prolonged this electrically vulnerable period in parallel with the terminal repolarization phase. These results suggest that prolongation of the terminal repolarization process by sematilide would enhance the chance of conduction slowing at less complete repolarization levels, which may be associated with a high incidence of TdP induction.

Molecular mechanisms of cardiostimulatory effects of sildenafil

To better understand the molecular mechanisms of the previously described cardiostimulatory action of the phosphodiesterase type-5 (PDE5) inhibitor sildenafil, we first evaluated its effects on cyclic AMP level in the canine ventricular membrane preparation. Sildenafil (10 micromol/L) significantly increased the net cyclic AMP production rate, the potency of which was similar to that of 3-isobutyl-1-methylxanthine (IBMX). Next, we assessed the inhibitory effect of sildenafil on PDE of bovine heart. Sildenafil (> or = 1 micromol/L) as well as IBMX significantly decreased the cyclic AMP hydrolyzing speed of PDE. These results suggest that a supra-therapeutic concentration of sildenafil may directly inhibit cyclic AMP hydrolyzing PDEs in the heart, although indirect inhibition of PDE3 via the "cross-talk" pathway cannot be totally excluded.