A model of potassium ion efflux during exercise of skeletal muscle.

Potassium (K+) is a vasoactive agent and is released from muscle cells during exercise. A simple diffusion model does not predict the time course of K+ efflux during exercise, which decreases as the exercise progresses. We constructed a mathematical model using the concept of an active Na+-K+ ion pump to account for the decreased efflux during and uptake after exercise. Passive fluxes are calculated by the Nernst equation. Active fluxes are constrained to balance these passive fluxes at rest. The pump activity increases as either extracellular K+ or intracellular Na+ concentration increases. To test the model, the venous K+ efflux profile was simulated for direct stimulation (4/s) of the anterior calf mus cles of dogs. The model simulated the K+ release during the stimulation period and [K+] undershoot after the stimulation. The active Na+-K+ ATPase transport concept used in the model was further tested by observing K+ efflux after administration of ouabain. Ouabain infusion decreased K+ uptake during exercise slightly and abolished [K+] undershoot after the stimulation. These experimental data were matched by the model only if a discontinuous effect of ouabain is assumed. This suggests that ouabain may more completely block the sensitivity of the pump to intracellular [Na+] than to extracellular [K+].