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 plum 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 (less than or equal to 10 muM) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (1-100 muM), 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 muM) 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-vitro 2-(3-phenylpropylamine) benzoic acid (NPPB; 10-100 muM) also elicited concentration-related inhibition of HS-induced contractions, producing a decrease to 35.9 +/- 11.3% of the control at 100 muM. 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.