Regulation of Na+,K+-ATPase isoforms and phospholemman (FXYD1) in skeletal muscle fibre types by exercise training and cold-water immersion in men

Little is understood about the fibre type-dependent regulation of Na+,K+-ATPase (NKA) isoforms by exercise training in humans. The main aim of this study was therefore to assess the impact of a period of repeated exercise sessions on NKA-isoform protein abundance in different skeletal muscle fibre types in men. Post-exercise cold-water immersion (CWI) has been reported to increase oxidative stress, which may be one mechanism underlying increases in NKA-isoform expression. Thus, a second aim was to evaluate the effect of CWI on training-induced modulation of NKA-isoform abundance. Vastus lateralis muscle biopsies were obtained from nineteen men at rest before (Pre) and after (Post) six weeks of intense interval cycling, with training sessions followed by passive rest (CON, n=7) or CWI (10°C; COLD, n=5). Training increased (p<0.05) the abundance of NKAα1 and NKAβ3 in both type I and type II fibres, NKAβ1 in type II fibres, but was without effect on NKAα2 and NKAα3 (p>0.05). Furthermore, training decreased FXYD1 protein content in type I fibres, which abolished its fibre type-specific expression detected at Pre (p<0.05). CWI was without impact on the responses to training (p>0.05). These results highlight that NKA isoforms are regulated in a fibre type-dependent fashion in response to intense training in humans. This may partly explain the improvement in muscle Na+/K+ handling after a period of intense training. CWI may be performed without adversely or favourably affecting training-induced changes in NKA-isoform abundance. Summary in key points It is unclear how Na+,K+-ATPase (NKA) isoforms are regulated in different skeletal muscle fibre types by exercise training in humans, and the effect on phospholemman (FXYD1) protein abundance in different fibre types remains to be elucidated. We investigated the impact of six weeks of training on NKA-isoform protein abundance (α1-3, β1-3 and FXYD1) in type I and II muscle fibres in men. We show that intense interval training selectively increases the protein content of NKA α1 and β3 in both fibre types, β1 in type II fibres, and decreases FXYD1 in type I fibres. These results suggest the favourable impact of intense training on human muscle Na+/K+ regulation could be attributable, in part, to fibre type-dependent modulation of NKA-isoform abundance. Given that cold exposure has been shown to modulate cellular redox state, which has been linked to increased NKA expression, we also investigated the effect of exercise training plus cold-water immersion (CWI) on the fibre type-specific responses of NKA isoforms and FXYD1. We found that CWI was without effect on the responses to training. Abbreviations AMPKβ2, 5’ AMP-activated protein kinase subunit β2; CaMKII, Ca2+-calmodulin-dependent protein kinase isoform 2; COLD, cold-water immersion group; CON, control group; Ct, cycle threshold; CV, coefficient of variation; CWI, cold-water immersion; EDL, extensor digitorum longus; FXYD1, phospholemman isoform 1; HSP70, heat-shock protein 70; GXT, graded exercise test; K+, potassium; Km, Michaelis–Menten constant; MHC, myosin heavy chain; Na+, sodium; NF-1, neurofibromatosis type 1; NKA, Na+,K+-ATPase; ROS, reactive oxygen species; SDS-PAGE, sodium dodecyl sulphate polyacrylamide gel electrophoresis; SERCA1, sarco/endoplasmic reticulum Ca2+-ATPase isoform 1; VO2peak, maximum oxygen uptake.