Agonist-induced activation of chloride channels in smooth muscle cells is one of the mechanisms suggested to produce membrane depolarization resulting in calcium entery through voltage-dependent calcium channels and muscle contraction (Bolton, 1979; Van Helden, 1988). This study aimed to investigate whether chloride channels are involved in contractions initiated by the thromboxane A₂ mimetic U46619 and by 5-hydroxy tryptamine in bovine pulmonary arteries.

Bovine lungs were obtained fresh from the local abattoir. Ring segments of conventional arteries (0.3-0.5cm in diameter, dissected from the 3^rd and 4^th arterial generations) were mounted in 10 ml organ baths suspended between stainless steel hooks in Krebs-Henseleit buffer (37 °C) under a tension of 2 g and gassed with a mixture of O₂:CO₂ 95%/5% v/v. Tissues were allowed to equilibrate for 1 hour before the addition of drugs. All tissues were first contracted with to 60 mM KCl. After washing, cumulative concentration response curves (CRCs) were constructed either to U46619 or to 5HT in the absence or presence of chloride channel blockers. The chloride channel blockers used were 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 50 µM), 9-Anthracene Carboxylic acid (9-AC, 500 µM) and Di-sodium 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS, 500 µM); all were dissolved in DMSO. Results are expressed as a percentage of the potassium chloride- (60 mM) induced contraction and are the means ± S.E.M. Statistical analysis was carried out using Student's t-test and p < 0.05 is considered significant.

5-HT (1 nM - 300 µM) induced concentration-dependent contractions, which were unaffected by NPPB, 9-AC or DIDS, pEC₅₀ and maximum response (R_max.), control, 5.57 ± 0.09, 189 ± 8 % n=7; NPPB, 5.8 ± 0.1, 166 ± 9 % n=5; 9-AC, 5.5 ± 0.08, 181 ± 7 % n=4; DIDS, 5.7 ± 0.06, 177 ± 5 % n=4.

U46619 (100 pM -10 µM) induced concentration-dependent contractions, (NPPB, 9-AC and DIDS caused a rightward shift of the concentration-response curve, pEC₅₀ control, 7.6 ± 0.07, n=16; NPPB, 6.5 ± 0.03, P<0.001, n=6; 9-AC, 6.6 ± 0.09, P<0.001, n=5; DIDS 6.61 ± 0.03, P<0.001, n=5. NPPB and 9-AC but not DIDS reduced the maximum response. R_max, control, 169 ± 5%; NPPB, 91 ± 2%, P<0.001; 9-AC, 138 ± 8%, P<0.01; DIDS, 169 ± 3%, P>0.9.

The reduction of contractile response to U46619 but not to 5HT with chloride channel blockers suggests that chloride channels are involved in U46619- but not 5-HT-mediated contraction.

Bolton, T.B. (1979) Physiol. Rev., 59, 606-92.
Van Helden D.F. (1988) J. Physiol., 401, 489-501.