Fatiguing high-intensity intermittent exercise depresses maximal Na+-K+-ATPase activity in human skeletal muscle assessed using a novel NADH-coupled assay

Abstract

The Na⁺-K⁺-ATPase is a critical regulator of ion homeostasis during contraction, buffering interstitial K⁺ accumulation, which is linked to muscle fatigue during intense exercise. Within this context, we adopted a recently reported methodology to examine exercise-induced alterations in maximal Na⁺-K⁺-ATPase activity. Eighteen trained healthy young males completed a repeated high-intensity cycling protocol consisting of three periods (EX1-EX3) of intermittent exercise. Each period comprised 10 × 45-s cycling at ~ 105% W_max and a repeated sprint test. Muscle biopsies were sampled at baseline and after EX3 for determination of maximal in vitro Na⁺-K⁺-ATPase activity. Blood was drawn after each period and in association with a 2-min cycling test at a standardized high intensity (~ 90% W_max) performed before and after the session to assess plasma K⁺ accumulation. Further, a 5-h recovery period with the ingestion of carbohydrate or placebo supplementation was implemented to explore potential effects of carbohydrate availability before sampling a final biopsy and repeating all tests. A ~ 12% reduction in maximal Na⁺-K⁺-ATPase activity was demonstrated following EX3 compared to baseline (25.2 ± 3.9 vs. 22.4 ± 4.8 μmol·min^-1·g^-1 protein, P = 0.039), which was sustained at the recovery time point (~ 15% decrease compared to baseline to 21.6 ± 5.9 μmol·min^-1·g^-1 protein, P = 0.008). No significant effect of carbohydrate supplementation was observed on maximal Na⁺-K⁺-ATPase activity after recovery (P = 0.078). In conclusion, we demonstrate an exercise-induced depression of maximal Na⁺-K⁺-ATPase activity following high-intensity intermittent exercise, which was sustained during a 5-h recovery period and unrelated to carbohydrate availability under the present experimental conditions. This was shown using a novel NADH coupled assay and confirms previous findings using other methodological approaches.

Keywords: Carbohydrate; Excitability; Excitation–contraction coupling; Fatigue; Glycogen; Potassium.

Fig. 1

Overview of the main experimental day with muscle sampling (biopsy symbols highlighted in red denoting the samples used for maximal Na⁺-K⁺-ATPase activity measurements), blood sampling, repeated sprint ability (RSA) and the 2-min cycling test at standardized intensity for measurements of K⁺ release before and subsequent to three periods (EX1-EX3) of high-intensity intermittent exercise (10 × 45-s bouts with 135 s recovery) followed by randomization to low or high carbohydrate (CHO) fluid supplementation

Fig. 2

Representative trace and overview of the maximal Na⁺-K⁺-ATPase activity assay

Fig. 3

Average workload (A) and ratings of perceived exertion (B) during each of the three high-intensity exercise periods (EX1-EX3), as well as mean repeated sprint ability (RSA) pre-exercise, post EX1-EX3 and at the recovery time point (Rec) for the pooled sample (C). Data are presented as means ± SD, n = 18. * denotes significant difference from baseline/EX1; # denotes significant difference from EX1 and EX2; P ≤ 0.05

Fig. 4

Muscle glycogen concentrations measured pre-exercise, post-exercise and after a 5-h recovery period with PLA and CHO supplementation. Data are presented as means ± SD, n = 18. # denotes significant between-group difference; P ≤ 0.05

Fig. 5

Maximal in vitro Na⁺-K⁺-ATPase activity measured pre-exercise, post-exercise and after a 5-h recovery period for A) pooled data for carbohydrate and placebo groups with individual values and B) data summarized for the carbohydrate and placebo groups. Data are presented as means ± SD, n = 15–18. * denotes significant difference from baseline; P ≤ 0.05

Fig. 6

A) Plasma K⁺ concentration at baseline (Pre), after each period of high-intensity intermittent exercise (EX1-EX3), as well as after 5 h of recovery (Rec); and B) plasma K⁺ levels before (Pre) and after (Post) the 2-min cycling test at a standardized high intensity performed at baseline, post-exercise and after recovery (Rec). Data are presented as means ± SD. * denotes significant difference from baseline and recovery in (A) and for corresponding post values in (B); # denotes significant difference from EX1 in (A) and significant difference from corresponding pre values in (B); P ≤ 0.05

Fig. 7

Baseline relationships between maximal in vitro Na⁺-K⁺-ATPase activity and participant characteristics. A) Na⁺-K⁺-ATPase activity and the sum of Na⁺-K⁺-ATPase subunit isoforms α1, α2 and β₁, B) Na⁺-K⁺-ATPase activity and VO_2max, C) Na⁺-K⁺-ATPase activity and repeated sprint ability (mean of 5 sprints relative to body mass) and D) Na⁺-K⁺-ATPase activity and % myosin heavy chain 1 (MHC). n = 18; P ≤ 0.05

Fig. 8

Maximal in vitro Na⁺-K⁺-ATPase activity post-exercise (A) and at the recovery time point (B) in relation to the repeated sprint ability at the corresponding time point normalized to baseline performance. n = 15–16; P ≤ 0.05