Prevention of ischemic ventricular tachycardia of Purkinje origin: role for alpha(2)-adrenoceptors in Purkinje?

Recent studies have shown the presence of postjunctional alpha (2)-adrenerg ic receptors on canine Purkinje fibers but not muscle cells. Stimulation of these receptors results in prolongation of the action potential duration a nd the Purkinje relative refractory period. We studied the effect of alpha (2)-adrenergic agonists on inducible ischemic ventricular tachycardia (VT) of both Purkinje fiber and myocardial origin. Open-chest dogs in whom VT wa s induced with extrastimuli after occlusion of the anterior descending coro nary artery were studied. A mapping system, incorporating Purkinje signals, characterized the mechanisms of VT. The alpha (2)-adrenergic agonists clon idine (0.5-4.0 mug/kg) or UK 14,304 (4-5 mug/kg) versus saline were given i ntravenously after reproducibility of inducible sustained monomorphic VT ha d been demonstrated. Eighteen dogs were given clonidine, eleven of which ha d focal Purkinje VT. Of these 11 dogs, clonidine blocked VT induction in 9 (81.9%) and rendered VT nonsustained in 1 (9.1%), and VT remained inducible in 1 dog (9.1%), although this was focal midmyocardial VT only. In the sev en dogs with VT of myocardial origin, six (85.6%) remained inducible with c lonidine, whereas one dog (14.4%) had only nonsustained VT after clonidine. Of the six dogs, UK 14,304 blocked VT induction in four (66.6%) and render ed VT nonsustained in one (16.7%), and VT remained inducible in one dog (16 .7%). In four dogs with VT of myocardial origin, VT remained inducible. In the eight control dogs that were given saline, focal Purkinje VT was repeat edly inducible. Pharmacological stimulation of postjunctional alpha (2)-adr enoceptors on Purkinje fibers may selectively prevent induction of VT of Pu rkinje fiber origin in the ischemic canine ventricle.