Alpha-adrenergic effects on relative refractory period in Purkinje system of intact canine left ventricles.

To investigate the electrophysiological effects of alpha-agonists, we studied 23 chloralose-anesthetized sinoaortic and vagotomized dogs, measuring epicardial and endocardial effective refractory period and relative refractory period of the Purkinje system during graded infusions of norepinephrine and phenylephrine. In group 1, epicardial and endocardial effective refractory periods shortened equivalently with norepinephrine. These effects were blocked with metoprolol. In group 2, epicardial and endocardial refractory periods did not prolong with phenylephrine despite addition of metoprolol. In group 3, phenylephrine, after the addition of metoprolol, prolonged the relative refractory period of Purkinje only at the highest phenylephrine dose of 50 micrograms.kg-1.min-1. In group 4, these latter effects were not produced by raising pressure with descending aortic occlusion. However, the effects of phenylephrine were blocked with prazosin (1.0 mg/kg). Taken together, these data demonstrate that alpha-adrenergic influences prolong Purkinje but not muscle refractory period in the intact canine ventricle. The high dose of phenylephrine and resulting hypertension suggest that this may be a pharmacological and not a physiological response.