Effects of kinins on isolated stomachs of control and transgenic knockout B2 receptor mice

Abstract

The aim of this study was to investigate the pharmacological profile of the kinin B₁ and B₂ receptors in isolated stomachs from wild-type control and B₂ receptor knockout mice. Isometric contractions evoked by bradykinin (BK) (9 nM) and desArg⁹BK (28 nM) were shown to be different. The contraction induced by desArg⁹BK had a longer duration than that evoked by BK and increased during incubation in vitro in stomachs of wild-type controls, while in the transgenic B₂ receptor knockout mice, the contractions evoked by desArg⁹BK and BK were similar and followed the B₁ receptor agonist pattern. BK but not the carboxypeptidase-resistant analog, [Phe⁸ψ(CH₂-NH)Arg⁹]BK, was found to be active in the stomach of B₂ receptor knockout mice. BK-induced contractions were prevented by mergetpa (a carboxypeptidase M inhibitor) (10 μM) and by a the B₁ receptor antagonist, AcLys[DβNal⁷,Ile⁸]desArg⁹BK (R 715) (0.88 μM), while not being influenced by the B₂ receptor antagonist HOE 140 (0.38 μM). BK and [Phe⁸ψ(CH₂-NH)Arg⁹]BK were potent contractants of the wild-type mice stomach and their effects were not influenced by mergetpa or by the B₁ receptor antagonist: they were reduced by HOE 140. After incubation in vitro for 3–4 hours, the tissues were treated with HOE 140 (4 μM) and FR-173657 (17 μM) to eliminate B₂ receptor function. In these tissues, BK evoked a B₁-like contraction which was inhibited by mergetpa (10 μM) and antagonized by R 715 (8 μM).

The results indicate that BK acts primarily on B₂ receptors. However, after intramural conversion to desArg⁹BK, activation of B₁ receptors of the mice stomach occurs. In the tissues of B₂ receptor knockout mice, BK behaves as a pure B₁ receptor agonist while in stomachs of control animals, the B₂ receptor contribution is overwhelming. After complete blockade of the B₂ receptor, BK is able to evoke B₁-mediated responses similar to those observed in tissues of B₂ receptor knockout mice. It is concluded that the disruption of the B₂ receptor gene eliminates the B₂ receptor without influencing the B₁ receptor system.