Interactions of norepinephrine and strophanthidin in cardiac Purkinje fibers.
The actions and interactions between strophanthidin and norepinephrine on electrical and mechanical events were studied in cardiac canine Purkinje fibers perfused in vitro. The results obtained show that norepinephrine (but not strophanthidin) shifts phase 1 and the beginning of the plateau to a more positive potential, an effect reduced by beta-receptor blockade. Norepinephrine increases the contractile force to the maximal value sooner than strophanthidin. In low Ca solution, the inotropic action of strophanthidin far exceeds that of norepinephrine. Norepinephrine does not cause spontaneous discharge in the driven fibers, whereas strophanthidin eventually induces fast spontaneous rhythms. When norepinephrine is given in the presence of strophanthidin (or vice versa) spontaneous activity is induced which consists either of a slow rhythm (characterized by large action potentials) or of a fast rhythm (characterized by small action potentials). The latter is typically induced by strophanthidin alone but occurs sooner in the presence of both agents. Propranolol prevents the onset of the slow rhythm but not that of the fast rhythm. It is concluded that norepinephrine and strophanthidin increase force by different mechanisms and that the potentiation between these agents in causing spontaneous discharge may involve an enhancement of diastolic depolarization(slow rhythm) or of the oscillatory potential (fast rhythm).
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D003971:Diastole / Q000187:drug effects; D004285:Dogs; D004347:Drug Interactions; D005260:Female; D006329:Heart Conduction System / Q000187:drug effects*; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology*; D011433:Propranolol / Q000494:pharmacology; D011690:Purkinje Fibers / Q000187:drug effects*; D013327:Strophanthidin / Q000494:pharmacology* / Q000633:toxicity; D013997:Time Factors
D013327:Strophanthidin; D011433:Propranolol; D002118:Calcium; D009638:Norepinephrine