Effects of chloride channel blockers on hypotonicity-induced contractions of the rat trachea.

1: We have investigated the inhibitory effects of blockers of volume-activated (Cl_vol) and calcium-activated (Cl_Ca) chloride channels on hypotonic solution (HS)-induced contractions of rat trachea, comparing their effects with those of the voltage-dependent calcium channel (VDCC) blocker nifedpine. 2: HS elicited large, stable contractions that were partially dependent on the cellular chloride gradient; a reduction to 41.45±7.71% of the control response was obtained when extracellular chloride was removed. In addition, HS-induced responses were reduced to 26.8±5.6% of the control by 1?µM nifedipine, and abolished under calcium-free conditions, indicating a substantial requirement for extracellular calcium entry, principally via VDCCs. 3: The established Cl_vol blockers tamoxifen (?10?µM) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (1-100?µM), at concentrations previously reported to inhibit Cl_vol in smooth muscle, did not significantly inhibit HS-induced contractions. 4: In contrast, the recognized Cl_Ca blocker niflumic acid (NFA; 1-100?µM) produced a reversible, concentration-dependent inhibition of HS responses, with a reduction to 36.6±6.4% of control contractions at the highest concentration. The mixed Cl_vol and Cl_Ca blocker, 5-nitro 2-(3-phenylpropylamine) benzoic acid (NPPB; 10-100?µM) also elicited concentration-related inhibition of HS-induced contractions, producing a decrease to 35.9±11.3% of the control at 100?µM. 5: Our results show that HS induces reversible, chloride-dependent contractions of rat isolated trachea that were inhibited by NFA and NPPB, while exhibiting little sensitivity to recognized blockers of Cl_vol. The data support the possibility that opening of calcium-activated chloride channels under hypotonic conditions in respiratory smooth muscle may ultimately lead to VDCC-mediated calcium entry and contraction.British Journal of Pharmacology (2004) 141, 367-373. doi:10.1038/sj.bjp.0705615 [ABSTRACT FROM AUTHOR]