A pharmacological analysis of receptor subtypes and the mechanisms mediating the biphasic response induced by kinins in the rat stomach fundus in vitro.

Bradykinin (BK), des-Arg9-BK (DABK) and related kinins caused biphasic response (BR) in rat stomach fundus (RSF) precontracted with BaCl2. The B2 receptor antagonist HOE 140 (3-30 nM) produced parallel rightward shifts of the contractile concentration-response curve (CRC) for BK, yielding a pA(2) value of 9.07 +/- 0.27 and slope of 0.99, but caused only discrete rightward shift of the relaxant CRC for BK, leaving the BR CRC to DABK unaffected. The B1 receptor antagonist des-Arg9-NPC 17761 (10 nM to 1 microM) caused graded rightward shifts of the relaxant (but not the contractile) CRC to DABK, yielding a pA2 value of 8.35 +/- 0.05 and slope of 0.59, but had no effect on BK-induced BR. BK- and DABK- (100 nM) induced relaxation were almost suppressed by apamin (1 microM) or by nifedipine (1 nM), but were unaffected by nitric oxide synthase inhibitors, methylene blue, lipo and cyclooxygenase inhibitors, selective receptor antagonist for histamine (H1 and H2), nicotine, platelet activating factor, tachykinins (both NK1 and NK2, calcitonin gene-related peptide, vasoactive intestinal peptide and ganglion blocking agent. BK- and DABK-mediated relaxations were reduced in Ca2+ -free medium plus EGTA, although BK-mediated contraction was more resistant. Escherichia coli endotoxin treatment (10 microgram /rat), 24 hr before, potentiated DABK-induced relaxation, but not contraction, and reduced BK-mediated relaxation (P < .05). It is concluded that RSF express both B1 and B2 receptors. BK-induced contraction involves activation of B2 receptors, although DABK-induced relaxation is mediated by B1 receptors. Both B1 and B2 receptors in RSF are constitutive, but LPS treatment caused induction of B1 and down-regulation of B2 receptors. Finally, kinin-mediated relaxation in RSF are coupled to activation of Ca2+ activated K+ channels, and rely on Ca2+ influx via L-type voltage-sensitive channels.