Abstract
Since it became clear that K$^+$ shifts with exercise are extensive
and can cause more than a doubling of the extracellular K$^+$
(K$^+$(s)) as reviewed here, it has been suggested that these
shifts may cause fatigue through the effect on muscle excitability
and action potentials (AP). The cause of the K$^+$ shifts is
a transient or long-lasting mismatch between outward repolarizing
K$^+$ currents and K$^+$ influx carried by the Na$^+$-K$^+$
pump. Several factors modify the effect of raised K$^+$(s)
during exercise on membrane potential (E(m)) and force production.
1) Membrane conductance to K$^+$ is variable and controlled 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 stabilize the E(m) during brief periods
of large K$^+$ shifts. 2) The Na$^+$-K$^+$ pump contributes
with a hyperpolarizing current. 3) Cell swelling accompanies muscle
contractions especially in fast-twitch muscle, although little in
the heart. This will contribute considerably to the lowering of intracellular
K$^+$ (K$^+$(c)) and will attenuate the exercise-induced
rise of intracellular Na$^+$ (Na$^+$(c)). 4) The rise
of Na$^+$(c) is sufficient to activate the Na$^+$-K$^+$
pump to completely compensate increased K$^+$ release in the
heart, yet not in skeletal muscle. In skeletal muscle there is strong
evidence for control of pump activity not only through hormones,
but through a hitherto unidentified mechanism. 5) Ionic shifts within
the skeletal muscle t tubules and in the heart in extracellular clefts
may markedly affect excitation-contraction coupling. 6) Age and state
of training together with nutritional state 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 fatigue in skeletal muscle, whereas in the heart,
except during ischemia, the K$^+$ balance is controlled much
more effectively.
- 11015618
- acid-base
- aging,
- animals,
- atpase,
- body
- channels,
- compartments,
- contraction,
- equilibrium,
- exertion,
- extracellular
- fatigue,
- fluid
- fluid,
- gov't,
- humans,
- intracellular
- ion
- isoforms,
- membrane
- muscle
- muscle,
- myocardium,
- non-u.s.
- organ
- potassium
- potassium,
- potentials,
- protein
- research
- sarcolemma,
- skeletal,
- space,
- specificity,
- support,
- transport,
- {n}a$^{+}$-{k}$^{+}$-exchanging
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