Chronotropic effects of calcium and magnesium ions at different temperatures

In the isolated right atrium of rabbit hearts, excess calcium causes an increase of heart rate at 38 degrees C; this effect is absent or even reversed at a temperature of 30 degrees C. From microelectrode studies it seems that when the calcium concentration is increased the pacemaker shifts within the sinoatrial node to cells in which excess calcium causes an acceleration of diastolic depolarization (type A). In fibers where the impulse originates at low calcium (type B), excess calcium causes a deceleration of diastolic depolarization. At low temperature, the effects on type A fibers are completely absent. Excess magnesium has a negative chronotropic effect at both temperatures, mainly by a deceleration of diastolic depolarization in type B fibers.