BRADYKININ ACTING ON B-2 RECEPTORS CONTRACTS COLON CIRCULAR MUSCLE-CELLS BY IP3 GENERATION AND ADENYLATE-CYCLASE INHIBITION

Receptor subtypes and intracellular signaling events involved in brady kinin-evoked contraction of colonic circular muscle are unknown. We st udied the roles of inositol trisphosphate (IP3) and cyclic AMP generat ion and the selectivity for B-1 and B-2 receptors in guinea pig colon. Bradykinin induced concentration-dependent contraction of circular mu scle strips with an EC(50) of 2 x 10(-8) M that was inhibited by the B -2 antagonist D-Arg(0)-(Hyp(3),Thi(5,8),D-Phe(7))-bradykinin but not t he B-1 antagonist des-Arg(9)-[Leu(8)]bradykinin. The B-1 agonist des-A rg(9)-bradykinin did not evoke contraction or relaxation. Bradykinin i nduced concentration-dependent shortening of isolated myocytes from ci rcular muscle with an EC(50) of 2 x 10(-11) M that was inhibited by th e B-2 but not the B-1 antagonist, confirming the myogenic nature of th e bradykinin receptors. Persistence of myocyte contraction in a calciu m-free medium with EGTA confirmed the lack of dependence on extracellu lar calcium. In colon muscle tissue, bradykinin evoked concentration-d ependent IP3 generation with an EC(50) of 10(-7) M and a maximal level of 58 +/- 17 pmol/mg of protein at 10(-4) M that was inhibited by the B-2 but not the B-1 antagonist. Bradykinin, acting on B-2 receptors, inhibited cyclic AMP formation after forskolin (10(-5) M) with an EC(5 0) of 3 x 10(-8) M and maximal inhibition of 48% at 10(-5) M. In concl usion, bradykinin induces colon muscle contraction via myogenic non-B- 1 receptors, which are likely of the B-2 subtype, with phosphoinositid e turnover activation and adenylate cyclase inhibition, This concurren t activation of a stimulatory pathway and blunting of an inhibitory pa thway may enhance the contractile efficiency of bradykinin on colonic myocytes.