Abstract
Since intracellular Na$^+$ activity (aiNa) is one important determinant
of Na$^+$ K$^+$-pump rate as well as excitability and the
finely tuned contractility, it is surprising that the relation between
aNa and pump rate reported by different authors has kQ5 varying between
10 and 40 mmol L-1. Other data also point to a variable relation
between pump rate and aiNa. During stimulation of isolated rat soleus
muscles at 2 Hz, ouabain-sensitive 86Rb uptake was increased in spite
of the intracellular Na$^+$ remaining unaltered. In isolated
cardiomyocytes, a transient Na$^+$. K$^+$-pump current was
observed upon activation by extracellular K$^+$ in spite of good
control of aiNa. Na$^+$-loaded, isolated, sheep cardiac Purkinje
fibres initially hyperpolarized over a period of up to 1 min upon
activation of the Na$^+$, K$^+$ pump with no detectable change
of aiNa. These examples are compatible with the existence of a micro-environment
close to the membrane where diffusion is slower than in the rest
of the cytosol, so that local aiNa may fluctuate or gradients may
develop as visualized by Wendt-Gallitelli at al. (1993). We conclude
that the reported relationships between Na$^+$, K$^+$, pump
rate and aiNa in intact cells probably underestimate the true affinity
of the Na$^+$, K$^+$ pump for Na$^+$ due to a functional
diffusion barrier beneath the sarcolemma, and also because of incomplete
cell dialysis in whole-cell voltage clamp experiments. The Na$^+$,
K$^+$ pump seems to be preferentially supplied with Na$^+$
from the outside through neighbouring channels and transporters.
- 8729681
- animals,
- atpase,
- gov't,
- humans,
- kinetics,
- muscle,
- myocardium,
- non-u.s.
- rats,
- research
- skeletal,
- sodium,
- support,
- {n}a$^{+}$-{k}$^{+}$-exchanging
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