Effects of arylaminobenzoate-type chloride channel blockers on equivalent short-circuit current in rabbit colon

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Abstract
Arylaminobenzoates were examined in rabbit colon mounted in an Ussing chamber. The open-circuit transepithelial voltage (V te) and resistance (R te) were measured and the equivalent short-circuit current (I SC=V te/ R te) was calculated. After serosal (s) and mucosal (m) addition of indomethacin (1 μmol/l) I SC was −71±11 (n = 118) μA/cm2. Amiloride (0.1 mmol/l, m) inhibited this current and reversed the polarity to + 32±4 (n=118) μA/cm2. In the presence of amiloride and indomethacin, prostaglandin E2 (1 μmol/l, s), known to induce Cl− secretion, generated an I SC of -143 ± 8 (n = 92) μA/cm2. The arylaminobenzoate and Cl− channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) reduced I SC reversibly with a half-maximal inhibition (IC50) at approximately 0.35 mmol/l and 0.2 mmol/l for mucosal and serosal application respectively. To test whether the poor effect was caused by mucus covering the luminal surface, dose/response curves of the mucosal effect were repeated after several pretreatments. Acidic pH on the mucosal side reduced IC50 to approximately 0.1 mmol/l. A similar effect was observed after N-acetyl-l-cysteine (m) preincubation. Pretreatment with N-acetyl-l-cysteine (m) and carbachol (s), in order to exhaust mucus secretion, and l-homocysteine (m) were more effective and reduced IC50 to approximately 50 μmol/l. To test whether this effect of NPPB was caused by non-specific effects, the two enantiomers of 5-nitro-2-(+/−1-phenylethylamino)-benzoate were tested of which only the (+) form inhibited the Cl− conductance in the thick ascending limb of the loop of Henle (TAL). In the present study the (+) enantiomer inhibited significantly more strongly than the (−) form. This suggests that the inhibitory effect of NPPB, even though it requires rather high concentrations, is probably due to Cl− channel inhibition. For other arylaminobenzoates the sequence of potencies was different from that determined for the TAL. The present data indicate that substances that have been designed to block the Cl− conductance of the TAL segment also inhibit reversibly but with much lower affinity the PGE2-induced Cl− secretion in rabbit colon.