In muscle fibers from the rat diaphragm, 85% of the resting membrane ion conductance is attributable to Cl−. At 37°C and pH 7.0, GCi averages 2.11 mmho/cm! while residual conductance largely due to K+ averages 0.34 mmho/cm2. The resting GCl exhibits a biphasic temperature dependence with a Q10 of 1.6 between 6°C and 25°C and a Q10 of nearly 1 between 25°C and 40°C. Decreasing external pH reversibly reduced Gcl; the apparent pK for groups mediating this decrease is 5.5. Increasing pH up to 10.0 had no effect on Gcl. Anion conductance sequence and permeability sequence were both determined to be Cl− > Br− a I ≥ CH3SO4−. Lowering the pH below 5.5 reduced the magnitude of the measured conductance to all anions but did not alter the conductance sequence. The permeability sequence was likewise unchanged at low pH. Experiments with varying molar rados of CL and I- indicated a marked interaction between these ions in their transmembrane movement. Similar but less striking interaction was seen between Cl− and Br−. Current-voltage relationships forGCl measured at early time-points in the presence of Rb+ were linear, but showed marked rectification with longer hyperpolarizing pulses (<50 ms) due to a slow time- and voltage-dependent change in membrane conductance to Cl−. This nonlinear behavior appeared to depend on the concentration of Cl− present but cannot be attributed to tubular ion accumulation. Tubular disruption with glycerol lowers apparent GCl but not GK, suggesting that the transverse tubule (T-tubule) system is permeable to Cl− in this species. Quantitative estimates indicate that up to 80% of GC1 may be associated with the T tubules.
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