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Department of General Physiology and Biophysics, Panum Institute, University of Copenhagen, Denmark.

1. Inhibition of 36Cl- efflux in frog muscle by the stilbene disulphonates (SD) SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonate) and DNDS (4,4'-dinitro-stilbene-2,2'-disulphonate) depends on the external pH, while the blocking of Cl- conductance with SITS is independent of pH. The 36Cl- efflux inhibition with DNDS has been studied in thin depolarized muscle fibre bundles in order to examine whether this difference is consistent with one transporter or reflects the existence of two mutually independent conductive and non-conductive SD-sensitive Cl- pathways. 2. The 36Cl- efflux response to a sudden inward KCl gradient was studied. At high pH the efflux decreased as predicted for dominant conductive Cl- single-filing, and at low pH the efflux increased in agreement with dominant non-conductive Cl- antiport. DNDS inhibition resulted in the same response, an efflux decrease, at both high and low pH, suggesting a selective reduction of the non-conductive contribution at low pH. 3. The inhibition of 36Cl- efflux as a function of the DNDS concentration at an external pH of 6.9 showed complex kinetics, which could be described as the sum of two Michaelis-Menten functions with different half-inhibition concentrations of DNDS. 4. The results support the hypothesis that the stilbene disulphonate-sensitive Cl- transport is mediated by two separate transporters rather than one.