The relationship of excitability to conduction velocity in canine Purkinje tissue

The relationship between interelectrode conduction time and 'take-off' potential (TOP) was studied with microelectrode techniques in isolated canine false tendons. Conduction of regular or test beats initiated during phase 4 depolarization or late phase 3 repolarization speeded as TOP decreased. Similarly, beats initiated during digitalis-induced oscillatory afterpotentials demonstrated more rapid conduction at lower TOP. Because of the frequency-coupled nature of the oscillations, conduction times became rate dependent. Phenytoin antagonized digitalis oscillations and reversed speeding of conduction attributable to the oscillations. No uniform relationship between speed of conduction and maximum upstroke velocity of the action potential could be demonstrated in the above experiments or when speed of conduction was varied by changes in concentration of K⁺ or Ca²⁺. However, speed of conduction could be demonstrated to vary directly with changes in excitability as measured by intracellular current injection.