DEPENDENCE OF THE CHRONOTROPIC EFFECTS OF CALCIUM, MAGNESIUM AND SODIUM ON TEMPERATURE AND CYCLE LENGTH IN ISOLATED RABBIT ATRIA

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Abstract
Ca (1.1-5.5 mM) acts positive chronotropic on isolated right atria of the rabbit at 38.degree. C. At 34.degree. C, the chronotropic effect was positive but smaller than at 38.degree. C. At 30.degree. C, the chronotropic effect is variable. Negative and positive chronotropic effects were seen. At all 3 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. Na acts positive chronotropic on fast preparations, but acts negative chronotropic on slow preparations. The chronotropic effect of Na depends on the cycle length.