Bradykinin B₂ receptors and coupling mechanisms in the smooth muscle of the guinea‐pig taenia caeci

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), ω‐conotoxin GVIA (100 nm) and ibuprofen (1 μm), suggesting that they are due to a direct action on the smooth muscle.

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), ω‐conotoxin GVIA (100 nm) and ibuprofen (1 μm), suggesting that they are due to a direct action on the smooth muscle.

The B₁ receptor‐selective agonist, [des‐Arg₉]‐BK (1–100 μm), was inactive in the taenia caeci, and the B₁ receptor‐selective antagonist, [Leu8, des‐Arg₉‐BK (1–10 μm), did not inhibit either phase of the bradykinin‐induced response. The B₂ receptor‐selective antagonist, D‐Arg‐[Hyp³, Thi⁵, D‐Tic⁷, Oic⁸]‐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).

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‐[Hyp³, Thi⁵, D‐Tic⁷, Oic⁸]‐BK with an apparent pK_B value of 8.4 ± 0.1.

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 B₁ receptor antagonist, [Leu⁸, des‐Arg⁹‐BK (10 μm), did not inhibit this bradykinin‐induced contraction, whereas the B₂ receptor antagonist, D‐Arg‐[Hyp³, Thi⁵, D‐Tic⁷, Oic⁸]‐BK (100 nm) markedly attenuated it (P < 0.001; n = 6).

Bradykinin (10 nm‐100 μm) significantly elevated tissue levels of total [³H]‐inositol phosphates in the presence of Li⁺, after incubation with myo‐[³H]‐inositol. The B₁ receptor‐selective agonist, [des‐Arg⁹‐BK (100 μm) did not stimulate [³H]‐inositol phosphate formation, and the B₁ receptor‐selective antagonist, [Leu⁸, des‐Arg⁹‐BK, did not inhibit the formation of [³H]‐inositol phosphates in response to a sub‐maximal concentration of bradykinin (10 μm; P > 0.05). Two B₂ receptor antagonists, D‐Arg‐[Hyp³, D‐Phe⁷]‐BK and D‐Arg‐[Hyp³, Thi⁵, D‐Tic⁷, Oic⁸]‐BK, inhibited bradykinin‐induced accumulation of total [³H]‐inositol phosphates with apparent pK_B estimates of 5.4 ± 0.3 and 8.4 ± 0.1, respectively.

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 phos‐phatidylinositol turnover), all result from activation of B₂ receptors. A possible causal relationship is suggested between these B₂ receptor‐mediated membrane potential‐dependent, and ‐independent events, and their roles in excitation contraction coupling.

DOI: 10.1111/j.1476-5381.1994.tb17033.x

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