Effects of SITS on chloride permeation in Xenopus skeletal muscle.

Abstract

Small signal cable analysis has been used to estimate the resting membrane chloride conductance of sartorius muscle fibres of Xenopus laevis. Muscles were bathed in solutions in which potassium was replaced by rubidium at pH's 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's by the addition of 4-acetamido-4′-isothiocyano-2,2′-stilbene disulphonic acid (SITS). Rubidium conductance was determined as the membrane conductance in a solution in which potassium was replaced by rubidium and chloride was replaced by sulphate. Rubidium conductance was not pH sensitive. Subtraction of rubidium conductance indicated that 0.2 mM SITS reduced chloride 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 chloride (2.5 mM potassium) the membrane potential of the muscle became insensitive to changes of chloride concentration in the bathing medium.It is concluded that SITS exerts a blocking effect on chloride conductance and permeability in Xenopus muscal membrane by binding to a site in the chloride channel that is not itself sensitive to the external pH.