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Abstract

Isolated cells from rat distal colon were investigated with the patch-clamp technique. In cell-attached and cell-excised patches (inside-out) single chloride channels with outward-rectifying properties were observed. In excised patches the single-channel conductance was 47 + 5 pS at positive and 22 + 2 pS at negative clamp potentials (n = 6). The Cl⁻ channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 10 μM) induced fast closing events, whereas 10 μM of 3′,5-dichlorodiphenylamine-2-carboxylic acid (DCDPC) had no effect when applied to the cytosolic side. Quinine in the bath inhibited the Cl⁻ channel by reducing its single-channel amplitude and increased open channel noise. With 0.1 mM the current amplitude decreased by 54% and with 1 mM quinine by 67%. Ca<sup>2+<sup>-dependent nonselective cation channels where observed after excision of the membrane patch. This channel was completely and reversibly inhibited by 100 μM DCDPC. Application of 1 mM quinine to the bath induced flickering and reduced the open-state probability from 0.94 to 0.44. In summary, besides its well established effects on K⁺ channels, quinine also inhibits nonselective cation channels and chloride channels by inducing fast closing events.