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The purpose of these experiments was to examine the effects of norepinephrine and propranolol on distal Purkinje fibers of the canine heart. Previous studies have indicated that action potential duration is normally greatest in distal Purkinje fibers 3-4 mm from the Purkinje-muscle junction. In such preparations, premature beats initiated either proximal or distal to the area of maximal action potential duration may propagate with delay or block. Our experiments were performed on the right bundle branch and the distal Purkinje fibers of dog hearts and used standard microelectrode techniques. Premature beats with a coupling interval comparable to the duration of action potentials at the area of maximal duration were conducted with delay. Earlier beats were confined to fibers proximal to the area of maximal action potential duration. Norepinephrine (1 x 10-5 M ) increased action potential duration in some fibers, consistently prolonged the functional refractory period of distal Purkinje fibers, and produced a marked delay in the propagation of early premature beats. Propranolol (0.25-1.0 mg/liter) shortened action potential duration, shortened the functional refractory period of distal fibers, and reduced or abolished conduction delay and block of early premature beats. These observations suggest that the unique properties of the Purkinje-muscle junction may contribute to the genesis of arrhythmias and that this region is an important site of action of propranolol. The action of propranolol on the Purkinje-muscle junction was dose dependent.

Adrenergic Influences on the Distal Purkinje System of the Canine Heart