The dependence of membrane potential on extracellular chloride concentration in mammalian skeletal muscle fibres.

1. The steady-state intracellular membrane potential of fibres in thin bundles dissected from mouse extensor digitorum longus or soleus muscles or rat sternomastoid muscles was measured with 3 M-KCl glass micro-electrodes. The steady-state membrane potential was found to depend on the extracellular concentrations of Na, K and Cl ions. 2. The resting membrane potential (3.5 mM-[K]o, 160 mM-[Cl]o) was -74 +/- 1 mV (mean +/- S.E.) and a reduction in [Cl]o to 3.5 mM caused a reversible steady-state hyperpolarization to -94 +/- 1 mV (mean +/- S.E.). 3. The steady-state membrane potentials recorded in fibres exposed to different [K]o and zero [Cl]o were consistent with potentials predicted by the Goldman, Hodgkin & Katz (GHK) equation for Na and K. The results of similar experiments done with Cl as the major external anion could not be fitted by the same equation. 4. The GHK equation for Na, K and Cl did fit data obtained from fibres in solutions containing different [K]o with Cl as the major external anion if the intracellular Cl concentration was allowed to be out of equilibrium with the steady-state membrane potential. 5. It is suggested that an active influx of Cl ions controls the intracellular Cl concentrations in these fibres and hence maintains the Cl equilibrium potential at a depolarized value with respect to the resting membrane potential. 6. The steady-state membrane potential of rat diaphragm fibres was independent of [Cl]o and it seems likely that the intracellular Cl concentration of these fibres is not controlled by active Cl transport.