Depolarization and Action Potential Duration in Cardiac Purkinje Fibers

The effect of depolarization on the duration of the action potential was studied in short cardiac Purkinje fibers. Membrane potential was altered by overdriving pacemaker fibers showing spontaneous diastolic depolarization (dog), by injecting constantcurrent pulses through an intracellular microelectrode (sheep), or by voltage clamping (dog). Test action potentials were elicited at the end of 50-300-msec depolarizations by extracellular stimuli in sheep fibers and by cathodal intracellular stimuli in dog fibers. In both species the duration of the test action potential was unchanged by subthreshold depolarization. In fibers showing spontaneous diastolic depolarization, action potential duration was not affected by the take-off potential within the range between -80 and -60 mv. In dog fibers depolarized from the threshold level (about -60 mv), action potential duration was shortened by 14-36%. After stronger depolarization (up to -10 mv), short action potentials could be elicited during the first 100-150 msec. This period coincided with the flow of net inward current. After longer suprathreshold clamps when the net membrane current reversed its polarity, the fibers were not excitable, and the clamp termination was followed by exponential decay of the membrane potentials. Our findings indicate that changes in take-off potential within the range between resting membrane potential and -60 my do not alter action potential duration.