Abstract
Transmembrane potentials were recorded from single cells of the isolated rabbit sinoatrial node by microelcctrodes filled with a mixture of isoproterenol or noradrenaline and KCl. The electrode was used for recording the membrane potential and also for the iontophoretic application of the amines. Intracellularly applied isoproterenol caused a significant increase in the slope of diastolic depolarization, resulting in tachycardia. However, the extent of these changes was markedly less than that obtained when the amine was extracellularly applied by the iontophoretic technique, and the latency for inducing the changes was longer. These effects were abolished by 10-6 M propranolol applied to the bathing media. There were regional differences in the responsiveness of S-A nodal cells to intracellularly applied isoproterenol. An area from which true pacemaker action potentials were most often recorded, tended to be most sensitive to the amine. Noradrenaline applied intracellularly caused an increase in the slope of diastolic depolarization and a tachycardia only when the preparations were treated with 10-6 M cocaine. These results suggest that adrenergic beta receptors are on the outside of the membrane of S-A nodal cells.
