Effect of external potassium and acetylstrophanthidin on conduction velocity of isolated canine Purkinje fibers.

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 msec was measured using silver wire surface electrodes. Action potentials were recorded using glass microelectrodes. Tyrode's solutions of different potassium concentrations were used, and preparations were equilibrated for 10 min. Prior to acetylstrophanthidin, maximum conduction velocities reaching 2.3 m/sec were recorded at (K)o = 6 mM. The conduction velocity was not directly proportional to maximum dV/dt or action potential amplitude at (K)o between 2 and 6 mM; maximum dV/dt and action potential amplitude 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 X 10(-8) g/ml for slowly and rapidly driven preparations, respectively) 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 potassium on digitalis-induced slowing of conduction, reported in whole animal experiments, was not observed.