BRADYKININ B-2 RECEPTORS AND COUPLING MECHANISMS IN THE SMOOTH-MUSCLE OF THE GUINEA-PIG TAENIA CECI

1 In the smooth muscle of the guinea-pig taenia caeci, bradykinin produces a relaxation followed by a contraction. In the presence of hexamethonium and guanethidine, both these phases of the response were insensitive to tetrodotoxin (100 nM), omega-conotoxin GVIA (100 nM) and ibuprofen (1 mu M), suggesting that they are due to a direct action on the smooth muscle. 2 The B-1 receptor-selective agonist, [des-Arg(9)]-BK (1-100 mu M), was inactive in the taenia caeci, and the B-1 receptor-selective antagonist, [Leu(8),des-Arg(9)]-BK (1-10 mu M), did not inhibit either phase of the bradykinin-induced response. The B-2 receptor-selective antagonist, D-Arg-[Hyp(3),Thi(5),D-Tic(7),Oic(8)]-BK (Hoe 140) (30-300 nM), inhibited both the bradykinin-induced relaxation and contraction with a similar affinity (apparent pK(B) estimates of 8.5 +/- 0.1 and 8.4 +/- 0.1 respectively). 3 In a depolarizing high-K+-solution, bradykinin produced concentration-related contractions, though of diminished magnitude; but no relaxation was observed in such media. In Krebs solution, the Ca2+-activated K+-channel blocker, apamin (10 nM), abolished relaxant responses. These observations suggest that contraction results both from membrane potential-dependent, and membrane potential-independent, mechanisms; whereas relaxant responses result entirely from membrane potential-dependent mechanisms. Contractile responses obtained in the high K+-solution were inhibited by D-Arg-[Hyp(3),Thi(5),D-Tic(7),Oic(8)]-BK with an apparent pK(B) value of 8.4 +/- 0.1. 4 In a Ca2+-free, EGTA-containing medium, relatively high concentrations of bradykinin (> 100 nM) produced transient contractions, suggesting that a component of the contractile response results from release of Ca2+ from an intracellular store. This intracellular Ca2+ store could be refilled in the presence of extracellular Ca2+. The B-1 receptor antagonist, [Leu(8),des-Arg(9)]-BK (10 mu M), did not inhibit this bradykinin-induced contraction, whereas the B-2 receptor antagonist, D-Arg-[Hyp(3),Thi(5),D-Tic(7),Oic(8)]-BK (100 nM) markedly attenuated it (P < 0.001; n = 6). 5 Bradykinin (10 nM - 100 mu M) significantly elevated tissue levels of total [H-3]-inositol phosphates in the presence of Li+, after incubation with myo-[H-3]-inositol. The B-1 receptor-selective angonist, [des-Arg(9)]-BK (100 mu M) did not stimulate [H-3]-inositol phosphate formation, and the B-1 receptor-selective antagonist, [Leu(8),des-Arg(9)]-BK, did not inhibit the formation of [H-3]-inositol phosphates in response to a submaximal concentration of bradykinin (10 mu M; P > 0.05). Two B-2 receptor antagonists, D-Arg-[Hyp(3),D-Phe(7)]-BK, and D-Arg-[Hyp(3),Thi(5),D-Tic(7),Oic(8)]-BK, inhibited bradykinin-induced accumulation of total [H-3]-inositol phosphates with apparent pK(B) estimates of 5.4 +/- 0.3 and 8.4 +/- 0.1, respectively. 6 These data suggest that in the guinea-pig taenia caeci, the five aspects of the action of bradykinin studied (the relaxant and the contractile elements of the biphasic mechanical response, the contractile response in a depolarizing high-K+ solution medium and zero-Ca2+ media, and stimulation of phosphatidylinositol turnover), all result from activation of B-2 receptors. A possible causal relationship is suggested between these B-2 receptor-mediated membrane potential-dependent, and -independent events, and their roles in excitation contraction coupling.