An Electrophysiologic Correlate of Ouabain Inotropy in Canine Cardiac Muscle

Action potentials were recorded via intracellular microelectrodes, together with recordings of isometric-contractile tension, from isolated trabecular muscles of canine ventricle. The effects of administration of ouabain solution in concentrations varying from 1.7 x 10 -7 to 5 x 10 -4 M were recorded. The early inotropic effects of ouabain (increase in contractile force) were accompanied by changes in configuration of the action potential identical to those associated with interval-dependent potentiation in the control state, that is, abbreviation of phase 2 and lengthening of phase 3, without significant change in duration of the action potential. The progressive administration of ouabain, while associated with contractile enhancement of the regularly stimulated contraction was also accompanied by a loss of the contractile increment associated with poststimulation potentiation; this loss of interval-dependent potentiation corresponds to a loss of interval-dependent alteration of the action potential. Thus, both interval-dependent contractile potentiation and ouabain inotropy correlate consistently with phase 2 abbreviation of the accompanying action potentials.