Dissociation between changes in muscle Na+-K+-ATPase isoform abundance and activity with consecutive days of exercise and recovery
The early plasticity of vastus lateralis Na+-K+-ATPase to the abrupt onset of prolonged submaximal cycling was studied in 12 untrained participants (V̇o2 peak 44.8 ± 2.0 ml·kg−1·min−1, mean ± SE) using a 6-day protocol (3 days of exercise plus 3 days of recovery). Tissue samples were extracted prior to (Pre) and following exercise (Post) on day 1 (E1) and day 3 (E3) and on each day of recovery (R1, R2, R3) and analyzed for changes in maximal protein (βmax) (vanadate-facilitated [3H]ouabain binding), α- and β-isoform concentration (quantitative immunoblotting) and maximal Na+-K+-ATPase activity ( Vmax) (3- O-methylfluorescein K+-stimulated phosphatase assay). For βmax (pmol/g wet wt), an increase ( P < 0.05) of 11.8% was observed at R1 compared with E1-Pre (340 ± 14 vs 304 ± 17). For the α-isoforms α1, α2, and α3, increases ( P < 0.05) of 46, 42, and 31% were observed at R1, respectively. For the β-isoform, β1 and β2 increased ( P < 0.05) by 19 and 28% at R1, whereas β3 increased ( P < 0.05) by 18% at R2. With the exception of α2 and α3, the increases in the isoforms persisted at R3. Exercise resulted in an average decrease ( P < 0.05) in Vmax by 14.3%. No differences were observed in Vmax at E1 - Pre and E3 - Pre or between R1, R2, and R3. It is concluded that 3 days of prolonged exercise is a powerful stimulus for the rapid upregulation of the Na+-K+-ATPase subunit isoforms. Contrary to our hypothesis, the increase in subunit expression is not accompanied by increases in the maximal catalytic activity.