Contribution of an electrogenic sodium pump to the response of sinoatrial node cells to acetylcholine.

ACh (10-7) - 10(-6) g/ml) induced hyperpolarization in pacemaker cells in the sinoatrial (SA) node of the rabbit heart varied inversely with the level of the maximal diastolic potential of the pacemaker cells. The hyperpolarization induced by ACh was completely abolished at 4 degrees -6 degrees C. Ouabain (10(-6) g/ml) markedly depressed or abolished the hyperpolarizing response. The response was also depressed in K-free solution, but not abolished. In Ca-free or low-Na solution, the sino atrial node cell responded well to ACh, showing marked hyperpolarization. Present results indicate that the hyperpolarization induced by ACh is at least in part due to an electrogenic Na pump activated by ACh.