Effect of External Potassium and Acetylstrophanthidin on Conduction Velocity of Isolated Canine Purkinje Fibers

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Abstract
Isolated long Purkinje fibers were perfused in oxygenated Tyrode''s solution. The conduction velocity of action potentials driven from one end of a fiber at cycle lengths of 300 and 1000 ms was measured using Ag wire surface electrodes. Action potentials were recorded using glass microelectrodes. Tyrodes''s solutions of different K concentrations were used and preparations were equilibrated for 10 min. Prior to acetylstrophanthidin, maximum conduction velocities reaching 2.3 m/s were recorded at (K)o = 6 mM. The conduction velocity was not directly proportional to maximum dV/dt [change in ventricular pressure with time] or action potential amplitude at (K)o between 2 and 6 mM; maximum dV/dt and action potential were greater although conduction velocity was decreased. In this range of (K)o membrane excitability may be an important factor. Acetylstrophanthidin (10-7 and 5 .times. 10-8 g/ml for slowly and rapidly driven preparations, respectively) increased diastolic depolarization and slowed conduction at (K)o less than 6 mM, but it did not increase automaticity or decrease conduction at (K)o over 6 mM. The added effect of high K on digitalis-induced slowing of conduction, reported in whole animal experiments, was not observed.