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INSERM Unité 323, Faculté de Médecine Necker Enfants-Malades, Paris, France.

1. A small-conductance Cl- channel that is stimulated by ATP and protein kinase A has been identified in the basolateral membranes of cortical thick ascending limbs (CTALs) of the mouse nephron. The present study uses the cell-attached and inside-out variants of the patch-clamp technique to investigate the pH sensitivity of this channel. 2. The open-state probability (Po) was dependent upon the internal pH in inside-out patches. Expressed as a percentage of the Po value at pH 7.2, Po increased to about 180% at pH 7.6, and decreased to 25% at pH 6.8. Po was close to zero at pH 6.4. The internal pH had no effect on the channel unit conductance. 3. The effect of pH on the CTAL Cl- channel was assessed in intact cells using NH4Cl to acidify the intracellular compartment. Experiments with the pH-sensitive fluorescent dye 2',7'-(carboxyethyl)-5'(6')-carboxy fluorescein penta-acetoxymethyl ester (BCECF) indicated that 1 mmol l-1 NH4Cl acidified the cytoplasm by 0.15 pH units and 5 mmol l-1 NH4Cl by 0.34 pH units. These concentrations of NH4Cl reduced the activity of the CTAL Cl- channel by 24 and 82% in cell-attached patches, showing that moderate changes in internal pH substantially altered the activity of this channel. NH4+ had no direct effect on channel activity. 4. Inhibition at low pH is a newly discovered property of small-conductance Cl- channels in epithelia, which might help discriminate between types of Cl- channel.