Digoxin and exercise effects on skeletal muscle Na+,K+-ATPase isoform gene expression in healthy humans

In muscle, digoxin inhibits Na⁺,K⁺-ATPase (NKA) whereas acute exercise can increase NKA gene expression, consistent with training-induced increased NKA content. We investigated whether oral digoxin increased NKA isoform mRNA expression (qPCR) in muscle at rest, during and post-exercise in 10 healthy adults, who received digoxin (DIG, 0.25 mg per day) or placebo (CON) for 14 days, in a randomised, double-blind and cross-over design. Muscle was biopsied at rest, after cycling 20 min (10 min each at 33%, then 67% ${\dot{V}}_{O_2\mathrm{peak}}$), then to fatigue at 90% ${\dot{V}}_{O_2\mathrm{peak}}$ and 3 h post-exercise. No differences were found between DIG and CON for NKA α_1–3 or β_1–3 isoform mRNA. Both α₁ (354%, P = 0.001) and β₃ mRNA (P = 0.008) were increased 3 h post-exercise, with α₂ and β_1–2 mRNA unchanged, whilst α₃ mRNA declined at fatigue (−43%, P = 0.045). In resting muscle, total β mRNA (∑(β₁+β₂+β₃)) increased in DIG (60%, P = 0.025) and also when transcripts for each isoform were normalised to CON then either summed (P = 0.030) or pooled (n = 30, P = 0.034). In contrast, total α mRNA (∑(α₁+α₂+α₃), P = 0.348), normalised then summed (P = 0.332), or pooled transcripts (n = 30, P = 0.717) did not differ with DIG. At rest, NKA α_1–2 and β_1–2 protein abundances were unchanged by DIG. Post-exercise, α₁ and β_1–2 proteins were unchanged, but α₂ declined at 3 h (19%, P = 0.020). In conclusion, digoxin did not modify gene expression of individual NKA isoforms at rest or with exercise, indicating NKA gene expression was maintained consistent with protein abundances. However, elevated resting muscle total β mRNA with digoxin suggests a possible underlying β gene-stimulatory effect.

Highlights

• What is the central question of this study?

  Na⁺,K⁺-ATPase (NKA) in muscle is important for Na⁺/K⁺ homeostasis. We investigated whether the NKA-inhibitor digoxin stimulates increased NKA gene expression in muscle and exacerbates NKA gene responses to exercise in healthy adults.

• What is the main finding and its importance?

  Digoxin did not modify exercise effects on muscle NKA α_1–3 and β_1–3 gene transcripts, which comprised increased post-exercise α₁ and β₃ mRNA and reduced α₃ mRNA during exercise. However, in resting muscle, digoxin increased NKA total β isoform mRNA expression. Despite inhibitory-digoxin or acute exercise stressors, NKA gene regulation in muscle is consistent with the maintenance of NKA protein contents.