Chloride efflux measurements in mammalian skeletal muscle

A rapid sampling technique has been used to resolve the components of 36Cl efflux from isolated extensor digitorum longus (EDL) muscles of young rats. Four distinct fluxes with apparent rate constants of 4.40 +/- 0.02 min-1 (k1), 1.30 +/- 0.04 min-1 (k2), 0.24 +/- 0.04 min-1 (k3), and 0.048 +/- 0.05 min-1 (k4) at 30 degrees C were identified. Together, these fluxes accounted for the movement of more than 98% of exchangeable muscle chloride (Cl) ions. The muscle compartment associated with the fastest flux (k1) contained 23% +/- 2% of the total muscle Cl corresponding to the extracellular space as determined with inulin or mannitol. The compartment associated with k2 accounted for 71% of the intracellular volume, and k2 was assumed to represent 36Cl efflux across the surface membrane. The rate constant k2 was temperature-dependent with a Q10 of 1.11 between 5 degrees C and 30 degrees C. Aromatic carboxylic acids known to block sarcolemmal Cl conductance (GCl) specifically lowered k2 by 25% at 30 degrees C, as did replacement of external Cl with nitrate.