Contribution of nitric oxide, prostanoids and Ca²⁺-activated K⁺ channels to the relaxant response of bradykinin in the guinea pig bronchus in vitro

Abstract.

In the guinea pig bronchus with epithelium, pre-contracted with histamine, bradykinin (BK), lysyl-BK, prostaglandin E₂ (PGE₂), cromakalim and S-nitroso-N-acetylpenicillamine each caused graded relaxation with mean EC₅₀s of 34 nM, 11 nM, 0.1 nM, 0.3 µM and 3.4 µM, respectively. The addition of NO synthase inhibitors N ^W-nitro-l-arginine (L-NOARG), N ^G-monomethyl-l-arginine or 7-nitroindazole reduced BK-induced relaxation by 41±6%, 59±4% and 51±2%, respectively. The inhibition of BK response caused by L-NOARG was completely reversed by l-, but not by d-arginine. Methylene blue and 6-(phenylamino)-5,8-quinolinedione (LY 83583) inhibited the BK response by 88±5% and 64±4%, while 1H-[1,2,4]oxadiazolo[4,3-a]quinolaxin-1-one (ODQ) had no effect. However, ODQ almost abolished SNAP-induced relaxation.

Indomethacin and the cyclo-oxygenase 2 (COX-2) inhibitor 5,5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulfonyl)phenyl-2(5H)-furanone (DFU) caused graded inhibtion of BK responses with mean IC₅₀s of 60 nM and 0.6 nM, respectively. Addition of tetraethylammonium (TEA), charybdotoxin (ChTx), or iberotoxin (IbTx) inhibited BK-induced relaxation by 76±4%, 30±4% and 99±1%, respectively, but the relaxations of PGE₂ and cromakalim were unaffected. In contrast, 4-aminopyridine, apamin or glibenclamide did not affect BK-induced relaxation.

These results indicate that BK-induced epithelium-dependent relaxation in the guinea pig bronchus is partially mediated by release of NO or by NO-related substances, involving an activation of both cyclo-oxygenase 1 (COX-1) and COX-2 enzymes, through a cyclic guanosine monophosphate (cGMP)-independent mechanism. Furthermore, BK-induced relaxation involves an activation of high-conductance Ca²⁺-activated K⁺ channels highly sensitive to IbTx, and to a lesser extent to ChTx and TEA.