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[Negative chronotropic effect of calcium on the sinus node of rabbits].

The chronotropic effect of [Ca~(2+)]_0 was studied in the isolated rabbit sinus nodeby means of the microelectrode recording technique. The results were as follows:(1) The chronotropic effect of [Ca~(2+)]_0 depended on the sinus cycle length; in prepara-tions with long cycle length (400ms), increase in [Ca~(2+)]_0 from 1.1 to 5.5 mMproduced a biphasic chronotropic effect, whereas in preparations with short cyclelength (400ms) high [Ca~(2+)]_0 produced a negative chronotropic effect. (2) Atropineand propranolol did not apparently affect the chronotropic action of calcium. (3)When [Ca~(2+)]_0 was increased, the dominant pacemaker cells showed decreases inthe amplitude of the action potential, the maximum diastolic potential and themaximum rate of depolarization, but the slope of diastolic depolarization was in-creased with an upward shift of the take-off potential and lengthening of actionpotential duration (APD). (4) At a still higher level of [Ca~(2+)]_0 (4.4mM), theeffective refractory period (ERP), ERP /APD radio and the threshold at the endingpoint of ERP (P0.01) were increased. The results suggest that the negative chronotropic effect of high [Ca~(2+)]_0 onthe sinus node does not seem to be mediated through sympathetic and parasym-pathetic nerves, but rather through the direct action of the Ca~(2+) upon the electricactivity of the pacemaker cells in the sinus node.