Chronotropic effect of alkali metals on spontaneously beating right atria

The alkali metal ions lithium, potassium, rubidium and cesium depress the rate of spontaneous beating of isolated rabbit right atria. At low concentrations (2 to 4 mM) the negative chronotropic effect was in the order: Cs greater than Rb greater than K or Li; at a higher concentration (12 mM) it was Rb or K greater than Cs or Li. Force of contraction was also depressed by potassium and cesium at all levels, but was stimulated by lithium and by low levels of rubidium (2 mM). Lithium had little chronotropic effect up to relatively high concentrations, decreasing spontaneous beating rate to 80% of control at 100 mM LiCl. The significant positive inotropic effect at 2 mM LiCl (to 120% of control) increased (to 180% of control) at 40 mM LiCl. Rate of beating was significantly depressed by 0.2 mM CsCl. The chronotropic effect of cesium was biphasic; the decrease in SBR at 2 and 4 mM cesium was greater than the negative chronotropic effect at 10 mM CsCl. The effect of rubidium (above 4 mM) closely resembled that of increased potassium in decreasing spontaneous beating rate and contractile force.