The actions of dehydroemetine on isolated guinea-pig atria. Influence of ouabain and calcium.

1. The mechanism of the inhibitory effect of dehydroemetine on the heart was investigated using (a) the spontaneously contracting isolated guinea-pig atrial preparation and (b), the electrically driven left atrial preparation.2. Dehydroemetine decreased the rate and amplitude of contraction of spontaneously contracting atria.3. Increasing the calcium concentration of the bath fluid to 28.2 mM abolished the effect of dehydroemetine on amplitude of contraction while the effect on rate persisted.4. Pretreatment with dehydroemetine increased the capacity of ouabain to induce arrhythmias in spontaneously contracting atria. Similarly pretreatment with ouabain augmented the inhibitory effect of dehydroemetine on spontaneous atrial activity.5. In the electrically driven left atrial preparation, dehydroemetine caused a decrease in contractile strength which was opposed by ouabail and by increased calcium concentration of the bath fluid.6. The effect of dehydroemetine on spontaneously contracting and on electrically driven atria was mimicked by adding excess potassium to the bath fluid.7. It was concluded that the effects of dehydroemetine on rate and contractility are, to a large extent, independent of one another. The effect on spontaneous rate is consistent with alteration of the potassium permeability of the myocardial cell membrane while the effect on contractility appears to be due to a decrease in the uptake of calcium from the medium by the myocardial cell.