Effects of Rb and Cs on the electrogenic Na-pump in rabbit sinoatrial node cells

The effects of Rb and Cs on the electrogenic Na-pump of the rabbit sinoatrial node cell were studied, using conventional microelectrode techniques. Hyperpolarizations induced by K or its substitutes were recorded from the preparations which were perfused previously with an extracellular K-free solution. The K-induced hyperpolarizations after K-depletion depended linearly on K-concentration in the range of 1.35 to 10.8 mM. When 2 mM Rb was used as an activator, a subthreshold oscillation appeared and the amplitude of the spontaneous action potentials in the Rb solution was larger than with the standard solution. As the concentration of Rb increased, the Rb-activated hyperpolarization increased, but the amplitude of the action potential decreased. In the case of Cs, the induced hyperpolarization was smaller than with K or Rb, and a transient recovery of pacemaker activity and gradual hyperpolarization preceded the true recovery in the standard solution. Since these hyperpolarization effects were not observed after the application of ouabain-containing K-free solution, the hyperpolarizations were considered to be electrogenic. A small hyperpolarization induced by activation of the external site of the Na-pump appears to be necessary for the immediate recovery of automatism from arrest at low resting potentials.