Calcium modulation of action potential duration in canine Purkinje fibers

Microelectrode studies were performed on canine Purkinje fibers to analyze the effects of diastolic interval and extracellular Ca concentration on action potential duration. Durations were measured at -25 mV, -60 mV, and 95% repolarization following a single early stimulus and in the steady state at basic drive frequencies of 6 to 240/min. The relationship obtained between action potential duration and the intervening diastolic period could be satisfactorily described by a rectanglular hyperbola, asymptotically approaching the steady state duration for each frequency. A voltage dependent reduction in action potential duration was introduced at very short diastolic intervals by the decrease in take off potential resulting from incomplete repolarization of the preceding action potential. Increasing the extracellular Ca concentration from 1.8 mM, to 5.4 to 10.8 mM, always decreased the duration at all levels without substantially altering the hyperbolic relationship. Verapamil (0.5 mg/l) tended to abolish the plateau, lengthen phase 3 and terminal repolarization, and reduce the amount of frequency dependent shortening measured at the 95% repolarization level. These data suggest that Ca may exert a dual effect on the shape of the action potential: as a current carrier during the plateau, an as an intracellular modulator of potassium conductance during repolarization.