Dynamics and consequences of potassium shifts in skeletal muscle and heartduring exercise

Since it became clear that K+ shifts with exercise are extensive and can ca use more than a doubling of the extracellular [K+] ([K+](s)) as reviewed he re, it has been suggested that these shifts may cause fatigue through the e ffect on muscle excitability and action potentials (AP). The cause of the K + shifts is a transient or long-lasting mismatch between outward repolarizi ng K+ currents and K+ influx carried by the Na+-K+ pump. Several factors mo dify the effect of raised [K+](s) during exercise on membrane potential (E- m) and force production. 1) Membrane conductance to K+ is variable and cont rolled by various K+ channels. Low relative K+ conductance will reduce the contribution of [K+](s) to the E-m. In addition, high Cl- conductance may s tabilize the E-m during brief periods of large K+ shifts. 2) The Na+-K+ pum p contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in th e heart. This will contribute considerably to the lowering of intracellular [K+] ([K+](c)) and will attenuate the exercise-induced rise of intracellul ar [Na+] ([Na+](c). 4) The rise of [Na+](c) is sufficient to activate the N a+-K+ pump to completely compensate increased KC release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for con trol of pump activity not only through hormones, but; through a hitherto un identified mechanism. 5) Ionic shifts within the skeletal muscle tubules an d in the heart in extracellular clefts may markedly affect excitation-contr action coupling. 6) Age and state of training together with nutritional sta te modify muscle K+ content and the abundance of Na+-K+ pumps. We conclude that despite modifying factors coming into play during muscle activity, the K+ shifts with high-intensity exercise may contribute substantially to fat igue in skeletal muscle, whereas in the heart, except during ischemia, the K+ balance is controlled much more effectively.