Mechanisms of impulse generation in isolated cells from the rabbit sinoatrial node

The contribution to spontaneous activity of three currents INa, If, and Isi was investigated in isolated spontaneously active SA nodal cells. It was demonstrated that isolated cells have electrophysiological properties similar to those of cells in the intact node. Evidence for the contributory role of INa to the upstroke of the action potential was obtained from membrane responsiveness curves and from the observation that Vmax was strongly reduced after the addition of 9 microM TTX to the bathing solution. The relative role of If and Isi as depolarizing diastolic membrane currents was investigated by relating the maximal density of each of these currents to the corresponding DDR during spontaneous activity. Both currents If and Isi-peak appear to be linearly related to DDR, indicating their contributory role to diastolic depolarization. Although the cells studied were heterogeneous with regard to the density of If and Isi we found no evidence for their separation into distinct groups of pacemaker cell types.