Cellular basis for improved left ventricular pump function after digoxin therapy in experimental left ventricular failure

The present study examined left ventricular (LV) and myocyte contractile performance and electrophysiologic variables after long-term digoxin treatment in a model of LV failure. A fundamental therapeutic agent for patients with chronic LV dysfunction is the cardiac glycoside digoxin. However, whether digoxin has direct effects on myocyte contractile function and electrophysiologic properties in the setting of chronic LV dysfunction remains unexplored. Left ventricular and isolated myocyte function and electrophysiologic variables were examined in five control dogs, five dogs after the development of long-term rapid pacing (rapid pacing, 220 beats/min, 4 weeks) and five dogs with rapid pacing given digoxin (0.25 mg/day) during the pacing period (rapid pacing and digoxin). Left ventricular ejection fraction decreased in the dogs with rapid pacing compared with that in control dogs (30 ± 2% vs. 68 ± 3%, p < 0.05) and was higher with digoxin than that in the rapid pacing group (38 ± 3%, p = 0.038). Left ventricular end-diastolic volume increased in the rapid pacing group compared with the control group (84 ± 6 ml vs. 59 ± 7 ml, p < 0.05) and remained increased with digoxin (79 ± 6 ml). Isolated myocyte shortening velocity decreased in the rapid pacing group compared with the control group (37 ± 1 μm/s vs. 59 ± 1 μm/s, p < 0.05) and increased with digoxin compared with rapid pacing (46 ± 1 μm/s, p < 0.05). Action potential maximal upstroke velocity was diminished in the rapid pacing group compared with the control group (135 ± 6 V/s vs. 163 ± 9 V/s, p < 0.05) and increased with digoxin compared with rapid pacing (155 ± 12 V/s, p < 0.05). Action potential duration increased in the rapid pacing group compared with the control group (247 ± 10 vs. 216 ± 6 ms, p < 0.05) and decreased with digoxin compared with rapid pacing (219 ± 12 ms, p < 0.05). In this model of rapid pacing-induced LV failure, digoxin treatment improved LV pump function, enhanced isolated myocyte contractile performance and normalized myocyte action potential characteristics. This study provides unique evidence to suggest that the cellular basis for improved LV pump function with digoxin treatment in the setting of LV failure has a direct and beneficial effect on myocyte contractile function and electrophysiologic measures.