The effect of digoxin in normal man on the cardiorespiratory response to severe effort

Oral digoxin effects on exercise performance, K+ regulation and skeletal muscle Na+ ,K+ -ATPase in healthy humans

Abstract

We investigated whether digoxin lowered muscle Na⁺,K⁺‐ATPase (NKA), impaired muscle performance and exacerbated exercise K⁺ disturbances. Ten healthy adults ingested digoxin (0.25 mg; DIG) or placebo (CON) for 14 days and performed quadriceps strength and fatiguability, finger flexion (FF, 105%_{peak‐workrate}, 3 × 1 min, fourth bout to fatigue) and leg cycling (LC, 10 min at 33% VO2peak and 67% VO2peak, 90% VO2peak to fatigue) trials using a double‐blind, crossover, randomised, counter‐balanced design. Arterial (a) and antecubital venous (v) blood was sampled (FF, LC) and muscle biopsied (LC, rest, 67% VO2peak, fatigue, 3 h after exercise). In DIG, in resting muscle, [³H]‐ouabain binding site content (OB‐F_ab) was unchanged; however, bound‐digoxin removal with Digibind revealed total ouabain binding (OB+F_ab) increased (8.2%, P = 0.047), indicating 7.6% NKA–digoxin occupancy. Quadriceps muscle strength declined in DIG (−4.3%, P = 0.010) but fatiguability was unchanged. During LC, in DIG (main effects), time to fatigue and [K⁺]_a were unchanged, whilst [K⁺]_v was lower (P = 0.042) and [K⁺]_a‐v greater (P = 0.004) than in CON; with exercise (main effects), muscle OB‐F_ab was increased at 67% VO2peak (per wet‐weight, P = 0.005; per protein P = 0.001) and at fatigue (per protein, P = 0.003), whilst [K⁺]_a, [K⁺]_v and [K⁺]_a‐v were each increased at fatigue (P = 0.001). During FF, in DIG (main effects), time to fatigue, [K⁺]_a, [K⁺]_v and [K⁺]_a‐v were unchanged; with exercise (main effects), plasma [K⁺]_a, [K⁺]_v, [K⁺]_a‐v and muscle K⁺ efflux were all increased at fatigue (P = 0.001). Thus, muscle strength declined, but functional muscle NKA content was preserved during DIG, despite elevated plasma digoxin and muscle NKA–digoxin occupancy, with K⁺ disturbances and fatiguability unchanged.

Key points

The Na⁺,K⁺‐ATPase (NKA) is vital in regulating skeletal muscle extracellular potassium concentration ([K⁺]), excitability and plasma [K⁺] and thereby also in modulating fatigue during intense contractions.

NKA is inhibited by digoxin, which in cardiac patients lowers muscle functional NKA content ([³H]‐ouabain binding) and exacerbates K⁺ disturbances during exercise.

In healthy adults, we found that digoxin at clinical levels surprisingly did not reduce functional muscle NKA content, whilst digoxin removal by Digibind antibody revealed an ∼8% increased muscle total NKA content.

Accordingly, digoxin did not exacerbate arterial plasma [K⁺] disturbances or worsen fatigue during intense exercise, although quadriceps muscle strength was reduced.

Thus, digoxin treatment in healthy participants elevated serum digoxin, but muscle functional NKA content was preserved, whilst K⁺ disturbances and fatigue with intense exercise were unchanged. This resilience to digoxin NKA inhibition is consistent with the importance of NKA in preserving K⁺ regulation and muscle function.

Effect of digoxin on physical performance in healthy man

The effect of digoxin on the maximal oxygen uptake, the heart rate reaction during submaximal and maximal bicycle exercise and the isokinetic skeletal muscle strength in the thigh was investigated in nine well-trained healthy young men. A daily dose of digoxin of 0.50 mg for 2 weeks, giving a steady state serum digoxin concentration of 1.0 +/- 0.2 nmol/l, did not significantly change maximal oxygen uptake or isokinetic muscle strength. However, the heart rate at rest and during exercise, both at submaximal and maximal levels, decreased significantly during digoxin administration.

The effect of digoxin on exercise capacity and exercising cardiac function in cystic fibrosis

The ability of digoxin to increase exercise capacity and stroke volume (SV) during exercise was evaluated in ten patients with cystic fibrosis (CF) ages 12 to 20 years with moderate to severe degrees of airway obstruction but no history of heart failure. A double-blind crossover trial of digoxin versus placebo was carried out. An evaluation of exercise performance was undertaken upon entry into the study, and after each of the one-week periods in which digoxin 0.25 mg/day or placebo was taken. Exercise testing consisted of a progressive exercise test on a cycle ergometer to measure maximum work capacity (Wmax) and a steady state test at 2/3 of the baseline Wmax. During the steady state test, the oxygen consumption and carbon dioxide production were measured and cardiac output (Q) was calculated by the indirect Fick (CO2) method. From Q and heart rate (HR), SV was derived. After digoxin, Wmax was unchanged. On steady state exercise HR was unchanged, but there was a slight but significant fall in Q due to a fall in SV. The decrease in SV was associated with exercising hypoxemia. We conclude that digoxin did not increase exercise capacity or improve exercising cardiac function in patients with moderate to severe airway obstruction due to CF.

Effect of digitalis on left ventricular function in exercising dogs

The effect of ouabain on left ventricular function in nonfailing hearts was assessed in 14 chronically instrumented dogs during graded treadmill exercise. At rest, ouabain increased the maximum first derivative of the left ventricular pressure (dp/dtmax) and stroke volume by 38% and 16%, respectively. No change occurred in end-diastolic left ventricular diameter or peak systolic left ventricular pressure. During exercise, ouabain reduced maximum running speed and limited the increments in heart rate and systolic pressure but did not alter dp/dtmax, stroke volume, or end-diastolic diameter. When atropine and ouabain were given and severe exercise was performed, there were no differences from controls in running speed, heart rate, dp/dtmax, or other parameters. When ouabain and propranolol were given dp/dtmax increased at rest and during exercise, compared with results with propranolol alone. It is concluded that the inotropic effect of ouabain is negligible during strenuous physical activity because of the presence of high levels of sympathetic stimulation. However, during exercise in the presence of beta-adrenergic blockade, increases in myocardial contractility do occur in response to ouabain.

Effect of digitalis on skeletal muscle in man

Although an inotropic effect of digitalis on skeletal muscle has been demonstrated in animals, it has not been shown in man. Digitalis, in previous studies, has failed to improve voluntary exercise performance. In this investigation the strength of nerve-stimulated involuntary thumb adduction was measured before, during and after infusion of ouabain into the brachial artery. With this experimental design, the many uncontrolled factors that govern ordinary exercise tolerance were avoided. Large doses of ouabain (0.5 mg) produced significant augmentation of peak strength of thumb adduction whereas smaller doses (75 mug) more likely to reach the thumb during systemic digitalization produced only suggestive increases in peak contraction strength. In patients previously digitalized for heart failure, the large doses of ouabain did not significantly change contractility. The findings suggest that skeletal muscle is less sensitive than cardiac muscle to ouabain, and that systemic digitalization has a minor effect on skeletal muscle. When the differences between skeletal and cardiac muscle in excitation-contraction coupling are considered, the reduced effect of ouabain on skeletal muscle contraction is compatible with cell membrane locus of action in both tissues.