5-Hydroxytryptamine receptors that facilitate excitatory neuromuscular transmission in the guinea-pig isolated detrusor muscle

1 In isolated detrusor strips from the guinea-pig urinary bladder, contractile responses to electrical field stimulation were mostly mediated by neurally released acetylcholine (ACh) and adenosine 5'-triphosphate (ATP). 2 5-Hydroxytryptamine (5-HT) produced a concentration-dependent increase in the amplitude of stimulated detrusor strip contractions. The 5-HT concentration-response curve showed a biphasic profile: the high potency phase was obtained at sub-micromolar concentrations (10-300 nM), while the low potency phase in the range 1-30 μM. The maximum response of the first phase was 30% of the total 5-HT response. 3 Like 5-HT, the 5-HT₃ receptor agonist, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT: 0.3-100 μM), the 5-HT₂ receptor agonist, (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI: 30 nM-3 μM) and the 5-HT₄ receptor agonist, 5-methoxytryptamine (5-MeOT: 0.1-30 μM) potentiated, though with lower potency, detrusor contractions. The resulting concentration-response curves were monophasic in nature. 2-Methyl-5-HT had a maximum effect comparable to that of 5-HT. By contrast, the maximal effects of DOI and 5-MeOT were only 20% and 30% of that elicited by 30 μM 5-HT, respectively. 4 The 5-HT₃ receptor antagonist, granisetron (0.3 μM) had no effect on the high potency phase, but caused a rightward parallel shift of the low potency phase of the 5-HT curve (pK(B) =7.3). Granisetron (0.3 μM) antagonized with comparable affinity (pK(B)= 7.1) 5-HT-induced responses after pharmacological isolation of 5-HT₃ receptors with the 5-HT₁/5-HT₂ receptor antagonist, methiothepin (0.3 μM) and the 5-HT₄ receptor antagonist, GR 125487 (30 nM). Granisetron (0.1, 0.3 and 1 μM) competitively antagonized the potentiating effect of 2-methyl-5-HT with an estimated pA₂ of 7.3. 5 Methiothepin (0.3 μM) and the 5-HT(2A) receptor antagonist, ketanserin (0.3 μM) produced a slight inhibition of the first phase of the 5-HT curve. In the presence of ketanserin, an equimolar concentration of methiothepin was ineffective in further reducing the effect of 5-HT. Similarly, the 5-HT₄ receptor antagonist, GR 125487 (30 nM) slightly inhibited the first phase of the 5-HT curve. Conversely, this phase was suppressed when detrusor strips were coincubated with ketanserin (or methiothepin) and GR 125487. 6 In a separate set of experiments, the interactions of 5-HT with either the purinergic or cholinergic components of excitatory neuromuscular transmission were investigated. In the presence of hyoscine (1 μM), 5-HT was mostly effective at sub-micromolar concentrations, while in the presence of the P₂-purinoceptor antagonist, suramin (300 μM), 5-HT-induced potentiation was mainly obtained with micromolar concentrations. 7 Thus, in electrically stimulated detrusor strips from guinea-pig, 5-HT potentiated excitatory neuromuscular transmission by activating at least three separate neural 5-HT receptors. These include the 5-HT(2A) and 5-HT₄ receptors, which mediate the 5-HT high potency phase mainly by activation of purinergic transmission. On the other hand, the potentiating effect caused by micromolar concentrations of 5-HT mostly involves cholinergic transmission and is mediated by the 5-HT, receptors.