Inward potassium transport systems in skeletal muscle derived cells are highly sensitive to oxidant exposure

Strenuous physical exercise causes a remarkable perturbation of K+ homeostasis in skeletal muscle tissue. Potassium efflux is crucial for a number of physiological control processes; however, exercise-induced perturbation of K+ homeostasis in skeletal muscle is suggested to be implicated in the generation of muscle fatigue. Physical exercise is also known to induce oxidative stress; a possible contribution of oxygen free radicals to the development of muscle fatigue has been hypothesized. To reveal the dose-dependent effect of oxidant exposure on inward and outward K+ (86RbCl) transporting systems, skeletal muscle derived L6 cells were treated with different concentrations of tert-butylhydroperoxide (TBOOH). We document the responses of (1) the ouabain-sensitive component of K+ influx (Na+,K+ pump), (2) bumetanide-sensitive ouabain-insensitive component of K+ influx (Na+,K+, 2Cl- cotransporter), (3) ouabain- and bumetanide-insensitive component of K+ influx (passive permeability of the cell membrane to inward K+), (4) ouabain-insensitive component of K+ efflux, and (5) passive leakage component of K+ efflux following exposure of L6 cells to oxidant treatment. Even very low doses of TBOOH (25 mumol) caused powerful activation of the Na+,K+ pump. Following TBOOH treatment, activity of the Na+,K+,2Cl- cotransporter was remarkably inhibited. Such a treatment also significantly decreased the permeability of the cell membrane to inward flux of K+ (passive influx). Thus, we observed that even very low doses of oxidant had remarkable specific effects on the different components of K+ influx in the skeletal muscle derived cells. However, K+ efflux mechanisms appeared to be rather insensitive to the extracellular oxidant challenge.