The role of calcium in overdrive suppression of canine cardiac Purkinje fibers.

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Abstract
The role of calcium in the electrical and mechanical changes induced by overdrive was studied in canine cardiac Purkinje fibers. The following results were obtained: (1) hyperpolarization and pause induced by a "short" (1-minute) overdrive are smaller in a low and larger in a high calcium solution; (2) overdriving at a constant ratio of overdrive to spontaneous rate only reduces the difference in pause duration in different calcium concentrations; (3) after overdrive, the initial diastolic depolarization is flatter and the late diastolic depolarization is steeper the control in low calcium and the opposite changes occur in high calcium; (4) the threshold for the first beat after overdrive is more negative in low than high calcium; (5) with "long" (2-minute or more) overdrives, hyperpolarization of comparable magnitude is seen during overdrive in low and high calcium; (6) the decline of hyperpolarization after overdrive is faster in high calcium unless the fiber is driven at slow rate ("postdrive"); (7) the contractile force after overdrive is greater than before overdrive in low and normal calcium and smaller than before overdrive in high calcium, and this difference is less during postdrive; (8) hyperpolarization and pause are reduced in a low sodium solution. It is concluded that calcium modifies overdrive suppression in several respects (hyperpolarization, slope of diastolic depolarization, duration of the pause and threshold), but is only one of the several factors involved in determining overdrive hyperpolarization and suppression.