Interactions of ouabain and noradrenaline in isolated rabbit's atria.

1. The present study was made using sympathetic nerve-atria preparations, conventional atrial preparations and left atria isolated from rabbits in order that interactions of ouabain and noradrenaline on the transmembrane potential of S-A nodal pacemaker fibres, the pacemaker rate and the contractile force of the left atrium could be investigated.2. In pacemaker action potentials, the maximal diastolic potential was decreased, the early repolarization was accelerated and the late repolarization was slowed by toxic concentrations of ouabain. Oscillating potentials of the diastolic pacemaker membrane were produced.3. Stimulation of the postganglionic sympathetic nerve caused an increase in the slope of diastolic depolarization, resulting in an acceleration of the pacemaker rate. In the presence of ouabain, the pacemaker action potential configuration was significantly altered by nerve stimulation in preparations which showed ouabain-induced bradycardia, but the potential configuration was not changed by nerve stimulation in preparations which showed ouabain-induced tachycardia. Cardiac noradrenaline altered the ion permeability of the cardiac pacemaker membrane during diastole, but it had no effect during systole. Sympathetic nerve stimulation transiently corrected the overshoot and the repolarization of atrial action potentials altered by toxic concentrations of ouabain.4. The positive chronotropic effect of sympathetic nerve stimulation was not affected by either therapeutic or toxic concentrations of ouabain. Ouabain shifted the concentration-chronotropic response curve for exogenous noradrenaline to the left, but did not shift the curve for tyramine.5. Relationship between the developed tension and the rate of contraction was altered differently by ouabain and by noradrenaline. The positive inotropic effect of noradrenaline was not significantly altered by therapeutic concentrations of ouabain but it was inhibited by toxic concentrations.