Effects of anion channel antagonists in canine colonic myocytes: Comparative pharmacology of Cl^-, Ca²⁺ and K⁺ currents

1. Volume-Sensitive, Outwardly Rectifying (VSOR) Cl^- currents were measured in canine colonic myocytes by whole-cell patch clamp. Decreasing extracellular osmolarity 50 milliosmoles l^-1 activated current that was carried by Cl^- and 5-7 times greater in the outward direction. 2. Niflumic acid, an inhibitor of Ca²⁺-activated Cl^- channels, did not inhibit VSOR Cl^- current. Glibenclamide, an antagonist of CFTR, and anthracene-9-carboxylate (9-AC) inhibited current less than 25% at 100 μM. 3. DIDS (4,4-diisothiocyanato-stilbene-2,2'disulphonate) inhibited VSOR Cl^- current more potently than SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonate). IC₅₀s were 0.84 and 226 μM, respectively. 4. VSOR Cl^- current was strongly inhibited by tamoxifen ([Z]-1-[p-dimethylaminoethoxy-phenyl]-1,2-diphenyl-1-butene), an anti-oestrogen compound (IC₅₀ = 0.57 μM). 5. Gd³⁺ antagonized VSOR Cl^- current more potently than La³⁺. The IC₅₀ for Gd³⁺ was 23 μM. In contrast, 100 μM La³⁺ inhibited current only 35 ± 7%. 6. Antagonists of VSOR Cl^- current had non-specific effects. These compounds blocked voltage-dependent K⁺ and Ca²⁺ currents in colonic myocytes. Tamoxifen (10 μM) and DIDS (10 μM) inhibited L-type Ca²⁺ current 87 ± 7 and 31 ± 5%, respectively. Additionally, in the presence of 300 nM charybdotoxin, tamoxifen (1 μM) and DIDS (10 μM) inhibited delayed rectifier K⁺ current 38 ± 8 and 10 ± 2%, respectively. 7. The pharmacology of VSOR Cl^- channels overlaps with voltage-dependent cation channels. DIDS and tamoxifen inhibited VSOR Cl^- equally. However, because DIDS had much less effect on L-type Ca²⁺ and delayed rectifier K⁺ channels than did tamoxifen, it might be useful in experiments to investigate the physiological and pathophysiological role of this conductance in whole tissues.