Dependence of the chronotropic effects of calcium, magnesium and sodium on temperature and cycle length in isolated rabbit atria.

Ca (1.1--5.5 mM) acts positive chronotropic on isolated right atria of the rabbit at 38 degrees C. At 34 degrees C, the chronotropic effect was also positive but smaller than at 38 degrees C. At 30 degrees C, the chronotropic effect is variable. Negative and positive chronotrpic effects were observed. At all three temperatures, the magnitude of the effect depended on the cycle length. Mg (0.6--6.0 mM) has a negative chronotropic effect. This effect was not dependent on the cycle length. The relationship between prolongation of the cycle length and the Mg concentration turned out to be linear. The chronotropic effects of Ca and Mg were simply algebraically additive. On the average, Na (77.4--154.8 mM) has no significant chronotropic effect. However, it acts positive chronotropic on fast preparations, although it acts negative chronotropic on slow preparations. So the chronotropic effect of Na depends on the cycle length also.