Effects of bunazosin, a selective alpha 1-blocking agent, and propranolol used alone and in combination on canine ventricular refractoriness and its dispersion during myocardial ischemia

Differences in refractory-period response of canine subendocardium and subepicardium to bunazosin, an alpha1-adrenoceptor antagonist, and propranolol during myocardial ischemia

Our objective was to investigate the effects of alpha1- or beta-adrenoceptor blockers on endocardial and epicardial refractory-period changes during myocardial ischemia in alpha-chloralose-anesthetized dogs. The first and second diagonal branches of the left anterior descending coronary artery were ligated. The refractory period was determined by an S1-S2 extrastimulus method. Dogs were treated with the alpha1-blocker bunazosin (0.1-0.2 mg/kg, i.v.; n = 16), the beta-blocker propranolol (0.2 mg/kg, i.v.; n = 15), or saline (n = 11). Dogs that developed ventricular tachycardia/fibrillation (VT/VF) during the experiment were excluded from the statistical assessment in refractory periods. In all groups, coronary ligation produced a significant shortening of the refractory period of ischemic epicardial tissue (p < 0.05) but only minimal shortening of ischemic endocardial refractory periods, resulting in an increased difference in repolarization time between the endo- and epicardial sites. Treatment with bunazosin ameliorated this ischemia-related shortening of refractory periods at both the endo- and epicardial sites, with a greater effect seen epicardially (p < 0.05), resulting in values similar to those in the nonischemic tissue. Treatment with propranolol prolonged refractory periods more in the epicardial (p < 0.01) than in endocardial sites, exacerbating the disparity in the refractory period between the endo- and epicardial sites (p < 0.05). Propranolol also prolonged the refractory period of nonischemic tissue (p < 0.05 and p < 0.01 in endo- and epicardial sites, respectively), resulting in a significant difference between the ischemic and normal myocardium at the endocardial site (p < 0.05). Results suggest that the alpha1-blocker bunazosin reduces the refractory-period disparity between the ischemic and normal myocardium without increasing the disparity between the endo- and epicardial surfaces, whereas propranolol produces a greater disparity.

Effects of dofetilide on electrical dispersion and arrhythmias in post-infarcted anesthetized dogs

An increase in dispersion of myocardial refractoriness has been shown to coincide with a greater risk of inducible ventricular arrhythmias. We compared the dispersion of electrophysiologic parameters and antiarrhythmic effects of dofetilide (0.03, 0.1, 0.3 and 1 mg/kg i.v.) in post-infarcted anesthetized dogs. Animals were tested for inducibility of arrhythmias using a programmed electrical stimulation (PES) protocol, and divided into inducible (I) and non-inducible (NI) groups. In addition, myocardial vulnerability was measured using ventricular fibrillation thresholds (VFT), as well as susceptibility to sudden cardiac death (SCD). Dofetilide significantly increased ventricular effective refractory periods (ERP) and monophasic action potential durations (APD) in a dose-dependent manner. The standard deviation of ERP, which was used as an index of dispersion of refractoriness, increased from sham (control value of 5.4 +/- sd 2.5 ms), non-inducible (control value of 11.0 +/- 5.5 and 8.0 +/- 3.7 ms for vehicle and dofetilide groups, respectively) and inducible states (control value of 17.3 +/- 6.2 and 21.6 +/- 7.1 ms for vehicle and dofetilide groups, respectively). However, dofetilide treatment did not alter dispersion of refractoriness over the dose range studied. Dofetilide did not significantly increase inducibility in the NI group (2 out of 8 [25%] compared to 0 out of 9 [0%] in vehicle treated animals). In the I group, dofetilide (0.3 mg/kg) treated animals converted 2 out of 7 (29%) to NI, and 5 out of 7 (71%; significant at p < 0.05) to a NI or non sustained ventricular tachycardia. There were no significant changes in VFT following the last dose of dofetilide given. Dofetilide did not significantly affect SCD survival (88% and 29% in the NI and I group, respectively) relative to vehicle (66% and 50% in the NI and I group, respectively). Although infarct sizes were significantly greater in the I groups, there was no difference between vehicle and dofetilide animals within these groups. In conclusion, dofetilide increased ERP and APD values, but did not affect dispersion of refractoriness. Thus, changes in dispersion of refractoriness may be used as a marker for inducibility in untreated animals, but it did not predict the antiarrhythmic effects observed with dofetilide.