In canine Purkinje fibers firing spontaneously at a cycle length of about 2.7 sec, 2-nicotinamidethyl nitrate (SG-75) (10-6−10-4M) increased the cycle length and in high concentrations (10-5−10-4M) abolished the spontaneous firing. The increase in cycle length was due entirely to that decrease in slope of the diastolic depolarization which was more marked during late diastole than during early diastole. Neither the maximum diastolic potential nor the take-off potential was changed. When the membrane potential failed to reach the take-off potential, it polarized again after the early diastolic depolarization, in some fibers. The depth of the notch was increased and the peak value of the plateau of the action potential was decreased by SG-75, however, the maximum rate of rise of the action potential remained unchanged. Shortening of the action potential duration was evident when Purkinje fibers were stimulated at a cycle length of 2 sec. These fibers remained excitable at 10-4M SG-75 and when stimulated at a cycle length of 2 sec, the action potential occurred with a duration of less than half of control. These results can be interpreted as being due to an increase in the background K current induced by SG-75.
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