Acute oral digoxin in healthy adults hastens fatigue and increases plasma K⁺ during intense exercise, despite preserved skeletal muscle Na⁺,K⁺‐ATPase

We investigated acute effects of the Na⁺,K⁺-ATPase (NKA) inhibitor, digoxin, on muscle NKA content and isoforms, arterial plasma [K⁺] ([K⁺]_a) and fatigue with intense exercise. In a randomised, crossover, double-blind design, 10 healthy adults ingested 0.50 mg digoxin (DIG) or placebo (CON) 60 min before cycling for 1 min at 60% <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>peak</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}{\mathrm{peak}}}}$</mml:annotation></mml:semantics> </mml:math> then at 95% <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>peak</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}{\mathrm{peak}}}}$</mml:annotation></mml:semantics> </mml:math> until fatigue. Pre- and post-exercise muscle biopsies were analysed for [³H]-ouabain binding site content without (OB-F_ab) and after incubation in digoxin antibody (OB+F_ab) and NKA α_1-2 and β_1-2 isoform proteins. In DIG, pre-exercise serum [digoxin] reached 3.36 (0.80) nM [mean (SD)] and muscle NKA-digoxin occupancy was 8.2%. Muscle OB-F_ab did not differ between trials, whereas OB+F_ab was higher in DIG than CON (8.1%, treatment main effect, P = 0.001), whilst muscle NKA α_1-2 and β_1-2 abundances were unchanged by digoxin. Fatigue occurred earlier in DIG than CON [-7.7%, 2.90 (0.77) vs. 3.14 (0.86) min, respectively; P = 0.037]. [K⁺]_a increased during exercise until 1 min post-exercise (P = 0.001), and fell below baseline at 3-10 (P = 0.001) and 20 min post-exercise (P = 0.022, time main effect). In DIG, [K⁺]_a (P = 0.035, treatment effect) and [K⁺]_a rise pre-fatigue were greater [1.64 (0.73) vs. 1.55 (0.73), P = 0.016], with lesser post-exercise [K⁺]_a decline than CON [-2.55 (0.71) vs. -2.74 (0.62) mM, respectively, P = 0.003]. Preserved muscle OB-F_ab with digoxin, yet increased OB+F_ab with unchanged NKA isoforms, suggests a rapid regulatory assembly of existing NKA α and β subunits exists to preserve muscle NKA capacity. Nonetheless, functional protection against digoxin was incomplete, with earlier fatigue and perturbed [K⁺]_a with exercise. KEY POINTS: Intense exercise causes marked potassium (K⁺) shifts out of contracting muscle cells, which may contribute to muscle fatigue. Muscle and systemic K⁺ perturbations with exercise are largely regulated by increased activity of Na⁺,K⁺-ATPase in muscle, which can be specifically inhibited by the cardiac glycoside, digoxin. We found that acute oral digoxin in healthy adults reduced time to fatigue during intense exercise, elevated the rise in arterial plasma K⁺ concentration during exercise and slowed K⁺ concentration decline post-exercise. Muscle functional Na⁺,K⁺-ATPase content was not reduced by acute digoxin, despite an 8.2% digoxin occupancy, and was unchanged at fatigue. Muscle Na⁺,K⁺-ATPase isoform protein abundances were unchanged by digoxin or fatigue. These suggest possible rapid assembly of existing subunits into functional pumps. Thus, acute digoxin impaired performance and exacerbated plasma K⁺ disturbances with intense, fatiguing exercise in healthy participants. These occurred despite the preservation of functional Na⁺,K⁺-ATPase in muscle.