Unaccustomed Eccentric Contractions Impair Plasma K(+) Regulation in the Absence of Changes in Muscle Na(+),K(+)-ATPase Content

The Na(+),K(+)-ATPase (NKA) plays a fundamental role in the regulation of skeletal muscle membrane Na(+) and K(+) gradients, excitability and fatigue during repeated intense contractions. Many studies have investigated the effects of acute concentric exercise on K(+) regulation and skeletal muscle NKA, but almost nothing is known about the effects of repeated eccentric contractions. We therefore investigated the effects of unaccustomed maximal eccentric knee extensor contractions on K(+) regulation during exercise, peak knee extensor muscle torque, and vastus lateralis muscle NKA content and 3-O-MFPase activity. Torque measurements, muscle biopsies, and venous blood samples were taken before, during and up to 7 days following the contractions in six healthy adults. Eccentric contractions reduced peak isometric muscle torque immediately post-exercise by 26±11% and serum creatine kinase concentration peaked 24 h post-exercise at 339±90 IU/L. During eccentric contractions, plasma [K(+)] rose during Set 1 and remained elevated at ∼4.9 mM during sets 4–10; this was despite a decline in work output by Set 4, which fell by 18.9% at set 10. The rise in plasma [K(+)].work(−1) ratio was elevated over Set 2 from Set 4– Set 10. Eccentric contractions had no effect on muscle NKA content or maximal in-vitro 3-O-MFPase activity immediately post- or up to 7 d post-exercise. The sustained elevation in plasma [K(+)] despite a decrease in work performed by the knee extensor muscles suggests an impairment in K(+) regulation during maximal eccentric contractions, possibly due to increased plasma membrane permeability or to excitation-contraction uncoupling.