Effects of SITS on choride permeation in Xenopus skeletal muscle

Abstract

Small signal cable analysis was used to estimate the resting membrane Cl conductance of sartorius muscle fibers of Xenopus laevis. Muscles were bathed in solutions in which K was replaced by Rb at pH 8.8, 7.04 and 5.25. The conductance ratio in these solutions was 12:5:1. The conductance was significantly reduced at all pH by the addition of 4-acetamido-4''-isothiocyano-2,2''-stilbene disulphonic acid (SITS). Rb conductance was determined as the membrane conductance in a solution in which K was replaced by Rb and Cl was replaced by SO4. Rb conductance was not pH sensitive. Subtraction of Rb conductance indicated that 0.2 mM SITS reduced Cl conductance by about 50%, regardless of pH, and 1 mM SITS reduced it by at least 75%. When 2 mM SITS was added to solutions containing 120 and 30 mM Cl (2.5 mM K) the membrane potential of the muscle became insensitive to changes of Cl concentration in the bathing medium. SITS apparently exerts a blocking effect on Cl conductance and permeability in Xenopus muscle membrane by binding to a site in the Cl channel that is not itself sensitive to the external pH.