Chloride-thiocyanate interactions in frog muscle anion-conducting channels at pH 5.

Bi-ionic membrane potential measurements and three-microelectrode voltage clamp experiments have been performed on surface fibres of Xenopus laevis sartorius muscle at various mole fractions of SCN- in Cl- in the perfusate, at pH 5. Potassium was replaced in the test solutions by rubidium and/or tetraethylammonium and when the mole fraction of anions was changed the measured membrane potential changed to a new constant (i.e. time-independent) value. Over a mole fraction range of 0.05-0.95 the permeability ratio of thiocyanate to chloride was independent of [SCN-]. When the bathing solution was completely changed from control to one containing thiocyanate the change in membrane potential indicated that the permeability ratio, PSCN/PCl is close to 5.9. Inward voltage clamp currents (chloride efflux) were suppressed in the presence of thiocyanate, the degree of suppression increasing with [SCN-]. Outward currents (anion influx) were not substantially altered, suggesting that it is only the voltage-dependent anion current that is sensitive to SCN-. The results are interpreted as indicating that there is a binding site in the anion-conducting channel, accessible to the extracellular space, that must be occupied by an anion in order for the channel to be "open". But that for ion traverse to be complete, the ion at the binding site must be exchanged. If the site is occupied by thiocyanate, the more strongly bound ion, the thiocyanate blocks the channel. The bi-ionic permeability ratio is thought to reflect the ratio of the binding constants for the anions at that site.