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The effects of variation in calcium concentration on the action potential of Purkinje fibers isolated from the dog heart were studied. Action potentials recorded during perfusion with Tyrode solution containing 2.7 mM calcium chloride were compared with those recorded during subsequent perfusion with solutions containing 0.675 (1/4X), 1.35 (1/2X), 5.4 (2X), or 10.8 (4X) mM calcium chloride. In both 1/2X and 1/4X solutions, the time required to repolarize to minus 60 mv and the duration of the action potential were increased. There were significant decreases in the slopes of phases 2 and 3 and the terminal phase of repolarization, while the slope of phase 1 increased. In 2X or 4X solutions, repolarization was speeded mainly by an earlier onset of phase 3. As a result the time to repolarize to minus 60 mv and the duration of the action potential were decreased significantly. The slope of phase 1 decreased significantly in 4X solution. In both high calcium solutions the rate and magnitude of diastolic depolarization increased. It was shown that the rate of the calcium-enhanced diastolic depolarization was dependent on the stimulus rate. The possible role of these changes in transmembrane potential in causing the ventricular arrhythmias following CaCl2 infusion in intact animals is discussed.

Effect of Calcium Concentration on the Transmembrane Potentials of Purkinje Fibers