Magnitude of myocardial potassium changes during acute alterations in pacing frequency in the in situ pig heart

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Abstract
A transient loss of potassium from cardiac tissue during increments in stimulation frequency has been found in different isolated preparations, but there is no agreement as to the magnitude and time course of this loss. In the present study myocardial potassium balance was determined during changes in heart rate in pigs with an intact circulation. The left azygos vein, which drains into the coronary sinus in this species, was cannulated and a shuntline to the right atrium established. Coronary sinus blood was thus continuously drawn from the shunt by a pump, without admixture of systemic venous blood, and myocardial release and uptake of potassium were determined before, during, and after periods of pacing tachycardia. A transient mean(SEM) loss of potassium (13.0(5.6) μmol·100 g−1 or about 0.25 μmol per beat change in heart rate) occurred during the first 90 s after increasing heart rate by a mean(SEM) of 53(4) beats·min−1. By discontinuing pacing heart rate returned to control values (mean(SEM) −43(7) beats·min−1), and myocardial potassium uptake ensued (mean(SEM) 9.8(3.3) μmol·100g−1 or 0.23 μmol per beat change in heart rate). The peak changes in coronary sinus potassium concentrations occurred 30 s after heart rate changed. The potassium lost during the periods of pacing tachycardia represented only about 0.2% of total myocardial potassium, equivalent to a reduction in intracellular potassium concentration of 0.3 mmol·litre−1. Since intracellular sodium and calcium concentrations are closely linked to the potassium concentration, the observed changes in potassium concentrations, although small, may be related to the positive inotropic effect of pacing tachycardia (the positive staircase phenomenon).