THE ACTION OF OUABAIN ON THE FUNCTION OF THE ATRIOVENTRICULAR NODE IN RABBITS

¹ Department of Pharmacology, School of Medicine, University of Washington, Seattle, Washington

A trial preparations isolated from rabbits were used. Transmembrane potentials were recorded from the sinoatrial (S-A) node, right atrium, atrioventricular (A-V) node (AN, N and NH regions) and bundle of His. The contractile force of the right atrium was recorded simultaneously. Transmural stimulation applied at the A-V node produced block of A-V nodal conduction and characteristic changes in configuration of A-V nodal action potentials, which were considered to result from the release of acetylcholine. Three concentrations of ouabain (5 x 10^-8, 2 x 10^-7 and 10^-6 g/ml) were used. The highest concentration of ouabain completely blocked the atrial-A-V nodal and the A-V nodal-His conduction in spontaneously beating preparations. The conduction block was not reversed by atropine (2 x 10^-6 g/ml) or propranololol (2 x 10^-7 g/ml) but was reversed by washout of ouabain. In preparations driven electrically at the right atrium, the minimal interstimulus interval which the A-V node was able to follow was increased by the two lower concentrations of ouabain. In spontaneously beating and electrically driven preparations, the negative dromotropic response to transmural stimulation applied at the A-V node was augmented by the two higher concentrations of ouabain, whereas the negative chronotropic response to the stimulation applied at the S-A node was inhibited by l0^-6 g/ml of ouabain. The maximal diastolic potential and the amplitude of action potentials of the right atrium, S-A node, A-V node (N region) and His bundle were markedly reduced by 10^-6 g/ml of ouabain. At the same concentration the depolarization velocity was slowed, the early repolarization was hastened and the late repolarization was retarded. These changes were partially reversed by washout of ouabain. It is concluded that block of A-V nodal conduction of isolated rabbit heart induced by ouabain is associated with a noncholinergic influence which may be independent of changes in membrane potential of the right atrium, A-V node and bundle of His.