Tachykinin antagonists and capsaicin‐induced contraction of the rat isolated urinary bladder: evidence for tachykinin‐mediated cotransmission

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Abstract
1 The possible involvement of tachykinins (TKs) in the contraction produced by capsaicin in the rat isolated urinary bladder was addressed on the hypothesis that co-release of substance P (SP) and neurokinin A (NKA) occurs from sensory nerve terminals. 2 A low concentration of SP (30 nm) produced a rapid contraction which faded to baseline within 10 min. A low concentration of NKA (10 nm) produced a slowly developing contraction which was still evident at 10 min. Capsaicin (1 μm) produced a rapid phasic response and a tonic response (late response to capsaicin). Co-administration of SP and NKA mimicked the response to capsaicin more than each TK alone. 3 Fading of the response to SP was not caused by receptor desensitization and was partially prevented by peptidase inhibitors. 4 Spantide (3 μm) selectively antagonized the SP-induced contraction while L-659,877 (3–10 μm) or MEN 10,376 (10–30 μm) which are NK2 receptor selective antagonists selectively blocked the response to NKA. Co-administration of spantide and L-659,877 inhibited the response to both SP and NKA by an amount not greater than that produced by each antagonist alone. 5 Spantide selectively reduced the peak response to capsaicin, while leaving the late response unaffected. L-659,877 (3 μm) and MEN 10,376 (10 μm) selectively inhibited the late response to capsaicin while, at higher concentrations, also reduced the peak response to capsaicin. Co-administration of spantide and L-659,877 reduced the peak response to capsaicin more than that produced by each antagonist alone. 6 Bombesin (10 nm) produced a tonic contraction similar to that induced by NKA. The response to bombesin was not affected by spantide, L-659,877 or MEN 10,376. 7 P2x purinoceptor desensitization by repeated administration of α,β-methylene ATP depressed the twitch response to electrical stimulation of postganglionic nerves but did not affect the peak or the late response to capsaicin. 8 We conclude that multiple TKs are coreleased by capsaicin in the rat bladder and mediate the capsaicin-induced contraction by activating both NK1 and NK2 receptors. Endogenous TK with preferential affinity for the NK1 receptor (putatively SP) are selectively involved in the peak response to capsaicin while endogenous TK with preferential affinity for the NK2 receptor (putatively NKA) are selectively involved in the late response to capsaicin and partly contribute to the peak response. These findings provide pharmacological evidence for tachykinin-mediated cotransmission in the rat urinary bladder. ATP is unlikely to be involved in the efferent function of capsaicin-sensitive sensory nerves in the rat bladder.