Effect of ouabain on inhibition of Na⁺, K⁺-ATPase activity by digoxin. Guinea pig heart membrane preparations were incubated at 37°C with various concentrations of digoxin in the presence or absence of different ouabain concentrations. Na⁺, K⁺-ATPase activity was determined by the release of P_i, measured using the Malachite Green assay. Values are expressed as mean ± SE (n = 5). *Value lower than that obtained in the absence of ouabain (P < 0.05).

Effect of wortmannin on digoxin (A)- and ouabain (B)-induced cardiotoxicity in vivo. Digoxin (500 µg·kg⁻¹·h⁻¹) or ouabain (300 µg·kg⁻¹·h⁻¹) was infused into anaesthetized guinea pigs in the presence or absence of wortmannin (100 µg·kg⁻¹·h⁻¹). The time of onset of first arrhythmia and VT/VF and death was calculated from the start of the cardiac steroid infusion. The values are expressed as the mean ± SE (n = 6). *Value higher than that in the presence of digoxin alone (P < 0.05). VF, ventricular fibrillation; VT, ventricular tachycardia.

Effect of ouabain on digoxin-induced cardiotoxicity in vivo. Digoxin (500 µg·kg⁻¹·h⁻¹) was infused into anaesthetized guinea pigs in the presence or absence of ouabain (Oua) at 91 ng·kg⁻¹·h⁻¹ or 182 ng·kg⁻¹·h⁻¹. Time of onset of three end points [time to first arrhythmia (1st arrhythmia), time to first VT/VF and time to death] were calculated from the start of the cardiac steroid infusion. The values are expressed as the mean ± SE (n = 4–8). *Value higher than that obtained in the presence of digoxin alone (P < 0.05). **Value lower than that obtained in the presence of digoxin alone (P < 0.05). †Value lower than that obtained in the presence of digoxin with ouabain 91 ng·kg⁻¹·h⁻¹ (P < 0.05). VF, ventricular fibrillation; VT, ventricular tachycardia.

Differences between ouabain- and digoxin-induced changes in blood pressure (A) and heart rate (B) in guinea pigs. Digoxin (500 µg·kg⁻¹·h⁻¹) or ouabain (300 µg·kg⁻¹·h⁻¹) was infused into anaesthetized guinea pigs. The lowest blood pressure and heart rate values just before the onset of first arrhythmia and VT/VF were calculated as the percentage of the values obtained during the control period (before the start of the cardiac steroid infusion). The values are expressed as the mean ± SE (n = 4–8). *Values higher than control values (P < 0.05). **Values lower than control values (P < 0.05). †value lower than that obtained for digoxin (P < 0.05) and not different from control values. ††Value higher than that obtained for digoxin (P < 0.05) and not different from control values. VF, ventricular fibrillation; VT, ventricular tachycardia.

Effect of ouabain on digoxin-induced blood pressure (A) and heart rate (B) fluctuations in vivo. Digoxin (500 µg·kg⁻¹·h⁻¹) was infused into anaesthetized guinea pigs in the presence or absence of ouabain (91 ng·kg⁻¹·h⁻¹). The lowest values of blood pressure and heart rate just before onset of first arrhythmia and VT/VF were calculated as the percentage of the values obtained during the control period (before the start of the cardiac steroid infusion). The values are expressed as the mean ± SE (n = 4–8). *Values higher than control values (P < 0.05). **Values lower than control values (P < 0.05). †Value lower than that obtained for digoxin alone (P < 0.05) and not different from control values. VF, ventricular fibrillation; VT, ventricular tachycardia.

Effect of ouabain on bufalin- and digoxin-induced toxicity in heart muscle preparations. Papillary muscles and left atria were electrically stimulated to induce contration; the right atria beat spontaneously. Twitch force was measured (gF, gram force). Following a control period, bufalin or digoxin was added to the bath in the presence or absence of ouabain. (A) Representative experiment showing delay of arrythmia in papillary muscle in the presence of ouabain (0.4 µmol·L⁻¹) caused by bufalin (0.4 µmol·L⁻¹). (B) Time elapsed from drug administration to first arrythmia. Bufalin at 0.4 µmol·L⁻¹ for papillary muscle and 225 nmol·L⁻¹ for atria was added in the presence or absence of ouabain at 0.4 µmol·L⁻¹ and 30 nmol·L⁻¹ (for papillary muscle and atria, respectiviely). Preparations that did not develop arrhythmia were given an arbitrary value of 90 min. (C) Maximal inotropic effects of papillary muscles (PM), left atria (LA) and right atria (RA) following bufalin (Buf) administration in the presence or absence of ouabain (Oua) at concentrations as in (B). (D) Representative experiment showing the postponement of arrythmia in papillary muscle caused by 3.5 µmol·L⁻¹ digoxin in the presence of 0.4 µmol·L⁻¹ ouabain. (E) Time elapsed from drug administration to first arrythmia. Digoxin at 0.4 µmol·L⁻¹ was added in the presene or absence of ouabain at 0.4 µmol·L⁻¹. Preparations that did not develop arrhythmia were given an arbitrary value of 90 min. (F) Maximal inotropic effect on papillary muscle following bufalin or digoxin administration in the presence or absence of ouabain at concentrations as in (E). The values are expressed as the mean ± SE (n = 5–12). *Value higer than that obtained in the presence of bufalin alone (P < 0.05). **Value lower than that obtained in the presence of digoxin alone (P < 0.05). +dT/dt, upward force slope; −dT/dt, downward force slope.