Mechanistic insight into the functional and toxic effects of Strophanthidin in the failing human myocardium.

Cardiac glycosides are characterized by a narrow therapeutic range with Ca2+-overload and arrhythmias occurring at higher concentrations. Data on cardiac glycosides in isolated failing human myocardium are scarce and the frequency-dependent actions and toxicity of Strophanthidin have not yet been characterized. To determine inotropic responses and toxicity of Strophanthidin in failing human myocardium. Experiments were performed in trabeculae from 64 end-stage failing hearts. Developed force, and intracellular [Ca2+]i and [Na+]i were recorded with Strophanthidin (0.01 to 1 micromol/L; 37 degrees C, 1 Hz) and compared to interventions with distinct mechanisms of action (elevated [Ca2+]o, Isoproterenol, and EMD57033). The effects of Strophanthidin on force-frequency behaviour were also assessed. Strophanthidin exerted concentration-dependent positive inotropic effects. These were paralleled by increases in intracellular [Na+] as well as increasing [Ca2+]i-transients and SR-Ca2+-load. At high concentrations (>0.5 micromol/L), Strophanthidin caused afterglimmers and aftercontractions, with declining developed force despite further increasing [Ca2+]i-transients. The force-frequency-relationship and diastolic function at higher pacing rates was worsened by Strophanthidin in a concentration-dependent manner. Strophanthidin toxicity was dependent on concentration, calcium load, beating rate and beta-adrenergic receptor activation. Our data support the view that low doses, heart rate control and additional beta-adrenergic receptor blockade are essential in the use of cardiac glycosides in heart failure.