EVIDENCE FOR A CAPSAICIN-SENSITIVE, TACHYKININ-MEDIATED, COMPONENT IN THE NANC CONTRACTION OF THE RAT URINARY-BLADDER TO NERVE-STIMULATION

1 In the presence of atropine (1 mu M) and guanethidine (3 mu M), electrical field stimulation (EFS) of the rat isolated urinary bladder for 30 s induced a frequency-dependent (1-30 Hz) nonadrenergic non-cholinergic (NANC) triphasic contraction characterized by a peak response (within 10 s from onset of stimulation), a late response (determined as the tension developed at the end of the stimulation period) and a prolonged post-stimulus 'off' response. The latter peaked at 2-6 min from the end of the stimulation period. At 10 Hz, the amplitude of the three responses averaged 89 +/- 6, 76 +/- 6 and 18 +/- 3% of the response to 40 mM KCI, respectively. Tetrodotoxin (1 mu M) abolished all contractile responses to EFS. 2 In capsaicin-pretreated bladder strip's (10 mu M for 15 min) the amplitude of the peak response to EFS (1-30 Hz for 30 s) was unchanged, the 'late' response to EFS was significantly reduced as compared to controls, and the post-stimulus response was absent, being replaced by a transient relaxation. 3 When varying train duration from 1 to 120 s at a frequency of 10 Hz, the differences between control and capsaicin-treated strips became evident for periods of stimulation > 10 s. 4 The tachykinin NK1 receptor antagonist, SR 140,333 (0.1-1 mu M) had no effect on the peak response to EFS (10 Hz for 30 s) while it decreased significantly the late response at both concentrations tested (16 +/- 3 and 33 +/- 3% inhibition). At 1 mu M, SR 140,333 also significantly reduced (29 +/- 9% inhibition) the peak of the post-stimulus contraction. The tachykinin NK2 receptor antagonist, MEN 10,627 (0.1-1 mu M) had no significant effect on the peak response to EFS (10 Hz for 30 s), and decreased the late response at 1 mu M Only (32 +/- 4% inhibition). MEN 10,627 inhibited the post-stimulus response at both concentrations tested and almost abolished it at 1 mu M. 5 The combined administration of SR 140,333 and MEN 10,627 (1 mu M each) produced a small reduction (22 +/- 3% inhibition) of the peak response to EFS, a marked reduction (48 +/- 3% inhibition) of the late response and the abolition of the post-stimulus response which was replaced by a post-stimulus relaxation as observed in capsaicin-pretreated strips. 6 SR 140,333 (0.1 and 1.O mu M) produced a large rightward shift in the concentration-response curve to the NK1 receptor agonist, [Sar(9)]substance P sulphone (apparent pK(B) 8.97 +/- 0.14), without affecting the response to the NK2 receptor-selective agonist, [beta Ala(8)]neurokinin A (4-10). MEN 10,627 (0.1 and 1 mu M) produced a large rightward shift of the concentration-response curve to [beta Ala(8)]neurokinin A (4-10) (apparent pK(B) 8.95 +/- 0.16) without affecting the response to [Sar(9)]substance P sulphone. SR 140,333 and MEN 10,627 (1.O mu M each) did not affect the contraction produced by exogenous ATP (1 mM). 7 These findings provide evidence that the NANC contraction of the rat isolated urinary bladder to transmural nerve stimulation has two components, which are sharply differentiated by blockade of the efferent function of sensory nerves following in vitro capsaicin administration. The first component, probably mediated by endogenous ATP, is fully activated during short periods of nerve activity (< 10 s) and does not involve capsaicin-sensitive nerve afferents. The second component, which is capsaicin-sensitive and tachykinin-mediated, is evident as a late 'on' response during nerve stimulation and as a post-stimulus 'off' response for periods of stimulation > 10 s. Activation of both NK1 and NK2 receptors contributes to the capsaicin-sensitive responses.