Effect of acetylstrophanthidin on myocardial function and K+ and Ca2+ exchange in newborn rabbit

The effects of acetylstrophanthidin (AS) on the mechanical function, tissue potassium content, and tissue calcium content were studied in the isolated, arterially perfused newborn and adult rabbit septum. Newborn and adult muscles were perfused with Krebs-Henseleit solutions containing 0.25, 0.5, 1.0, 1.5, 2.5, or 5.0 X 10(-6) M AS. 42K and 47Ca were used in the measurement of tissue uptakes of these ions. The inotropic effect of AS in the newborn was equal to that in the adult for AS concentrations less than 1.0 X 10(-6) M but was greater (P less than 0.01) than that in the adult at higher concentrations. In muscles perfused with solutions containing AS concentrations higher than 0.5 X 10(-6) M in the adult and 1 X 10(-6) M in the newborn, contractility increased initially and then decreased (mechanical toxicity). Mechanical toxicity in the adult occurred sooner than in the newborn. A decrease in tissue potassium and an increase in tissue calcium were observed immediately after AS infusion in the two age groups. The rates of tissue potassium loss and calcium gain in the newborn were not significantly different from those in the adult. Mechanical toxicity in the newborn was less than that in the adult for an equal amount of tissue potassium loss and/or tissue calcium gain. These data indicate that, in the isolated arterially perfused septal preparation, the newborn myocardium tolerates larger concentrations of AS, resulting in a greater inotropic effect which is observed before the onset of toxicity.