The positive inotropic effect of ouabain on atria and ventricle strips of cycloheximide-pretreated (20 mg/kg i.p. 3 hr before sacrifice) guinea-pigs and rats was significantly reduced or absent. Similar results were obtained when actinomycin (8 X 10(-6)M) was added to the perfusion medium of rat ventricle strips for 30 min before ouabain. These results suggest that either the synthesis of a short-lived protein is required for the inotropic effect of ouabain, or that at least part of ouabain's inotropic effect is due to the stimulated synthesis of some protein(s).
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