Characterization of kinin receptors modulating neurogenic contractions of the mouse isolated vas deferens

1 This study analyses the receptors mediating the effects of bradykinin (BK) and analogues on neurogenic twitch contractions of the mouse isolated vas deferens evoked, in the presence of captopril (3 μ m ), by electrical field stimulation with trains of 4 rectangular 0.5 ms pulses of supramaximal strength, delivered at a frequency of 10 Hz every 20 s. 2 BK (0.1–300 nM) induced a graded potentiation of twitches, with an EC 50 (geometric mean and 95% confidence limits) of 4.5 nM (1.7–11.6) and an E max of 315 ± 19 mg per 10 mg of wet tissue ( n = 6). Similar results were obtained in tissues challenged with Lys‐BK, [Hyp 3 ]‐BK, Met, Lys‐BK and the selective B 2 receptor agonist [Tyr(Me) 8 ]‐BK (0.1–300 nM). 3 The selective B 2 receptor antagonists, Hoe 140 (1–10 nM) and NPC 17731 (3–30 nM), caused graded rightward shifts of the curve to BK‐induced twitch potentiation, yielding apparent pA2 values of 9.65 ± 0.09 and 9.08 ±0.13, respectively, and Schild plot slopes not different from 1. Both antagonists (100 nM) failed to modify similar twitch potentiations induced by substance P (3 nM) or endothelin‐1 (1 nM). Preincubation with the selective B 1 receptor antagonist, [Leu 8 des‐Arg 9 ‐BK (1 μm), increased the potentiating effect of BK on twitches at 30–300 nM. 4 In contrast to BK, the selective B 1 receptor agonist, [des‐Arg 9 ‐BK (0.3–1000 nM) reduced the amplitude of twitches in a graded fashion, with an IC 50 of 13.7 nM (10.4–16.1) and an I max of 175 ± 11 mg ( n = 4). The twitch depression induced by [des‐Arg 9 ]‐BK (300 nM) was not affected by Hoe 140 (30 nM) or NPC 17731 (100 nM), but was abolished by the selective B 1 receptor antagonist, [Leu 8 des‐Arg 9 ‐BK (1 μm), which did not modify the twitch inhibitory effect of clonidine (1 nM) or morphine (300 nM). 5 In non‐stimulated preparations, BK (100 nM) also potentiated, in a Hoe 140‐sensitive (10 nM) manner, the contractions induced by ATP (100 μm), but not by noradrenaline (10 μm), whereas [des‐Arg 9 ]‐BK (300 nM) did not modify the contractions induced by either agonist. 6 It is concluded that the mouse vas deferens expresses both B 1 and B 2 receptors, which modulate sympathetic neurotransmission in opposing ways. Neurogenic contractions are inhibited by stimulation of possibly prejunctional B 1 receptors, whereas activation of B 2 receptors increases twitch contractions, in part by amplifying the responsiveness of the smooth muscle cells to the sympathetic co‐transmitter ATP.