Fibre type-specific change in FXYD1 phosphorylation during acute intense exercise in humans

The aim of the present study was to examine fibre type-specific Na+K+ pump subunit expression and exercise-induced alterations in phospholemman (FXYD1) phosphorylation in humans. Segments of human skeletal muscle fibres were dissected and fibre typed, and protein expression was determined by Western blotting. The protein expression of the Na+K+ pump 2 isoform was lower in typeI than in typeII fibres (0.63 +/- 0.04 a.u. vs. 1.00 +/- 0.07 a.u., P<0.001), while protein expression of the Na+K+ pump 1 and 1 isoforms was not different. Protein expression of the ATP-dependent potassium channel Kir6.2 was higher in typeI compared with typeII fibres. In both typeI (P<0.01) and typeII fibres (P<0.001) the AB_FXYD1 signal was lower after exercise compared with rest, indicating an increase in unspecific FXYD1 phosphorylation. The FXYD1 serine68 phosphorylation was higher (P<0.001) after exercise compared with rest in typeII fibres (1.90 +/- 0.25 vs. 1.00 +/- 0.08) and not changed in typeI fibres. Total FXYD1 was not expressed in a fibre type-specific manner. Expression of phosphofructokinase was lower (P<0.001) in typeI than in typeII fibres, whereas citrate synthase and 3-hydroxyacyl-CoA dehydrogenase were more abundant (P<0.001) in typeI fibres. In conclusion, FXYD1 phosphorylation at serine68 increased after an acute bout of intense exercise in human typeII fibres, while AB_FXYD1 signal intensity was lower in both typeI and typeII fibres, indicating fibre type-specific differences in FXYD1 phosphorylation on serine63, serine68 and threonine69. This, together with the observation of a higher abundance of the Na+K+ pump 2 isoform protein in typeII fibres, is likely to have importance for the exercise-induced human Na+K+ pump activity in the different fibre types.