Nerve-mediated contractions of detrusor smooth muscle are mediated through the release of two neurotransmitters, acetylcholine and ATP (Burnstock et al. 1978). These are rapidly broken down in the synapse by specific extracellular enzymes. The breakdown product of ATP, adenosine, reduces contractions in other smooth muscle to reduce contractures (Lynch & Huddart 1991). Adenosine may modulate contraction either by acting on the muscle or the motor-nerve ending and is believed to act through P1-receptors that are divided into four subtypes (A1, A2a, A2b, A3). The site and subtypes of these receptors in detrusor smooth muscle are unclear and this study has addressed these questions.

Detrusor strips were obtained from guinea-pig bladders or human biopsy samples, the latter obtained with local ethical committee permission and informed patient consent. Guinea-pigs were humanely killed. Strips were superfused at 37 °C in HCO₃^–/CO₂-Tyrode solution (pH 7.35). Contractions were evoked either by field-stimulation of embedded motor nerves (3 s train of 0.1 ms pulses, 1-40 Hz, abolished by 1 mM TTX) or direct activation of the muscle with the acetylcholine analogue carbachol (10 mM). All chemicals except ZM241385 were obtained from Sigma UK. Data are expressed as means ± S.D. (n), where n is the number of experiments. Student’s unpaired t test was used to test for significance between data sets (P < 0.05). pEC₅₀ = -log₁₀EC₅₀.

Adenosine (1 mM), the non-selective P1-agonist 5′-(N-ethylcarboxamido)adenosine (NECA) (10 mM) and the A1-selective agonist N⁶-cyclopentyladenosine (CPA) (10 mM) reduced nerve-mediated contractions in guinea-pig tissue. (63 ± 11 % (4), 61 ± 11 % (6), 71 ± 11 % (10) of control, respectively, 8 Hz stimulation). The reduction by CPA was significantly less than that induced by adenosine or NECA. Concentrations were maximal and were determined for each agent previously (pEC₅₀ values were 3.88 ± 0.83 (4), 6.44 ± 0.29 (6) and 7.01 ± 0.86 (4), respectively). The A2a- and A2b-antagonists ZM241385 (100 nM) and alloxazine (1 mM) had no significant effect. Carbachol contractures were not significantly altered by any P1-agonist. There was a frequency-dependent action of P1-agonists, with greater attenuation at low stimulation frequencies. Nerve-mediated contractions with human detrusor strips from stable and unstable bladders were not reduced by any of the above P1-agonists. Carbachol contractures were also unaffected. Force-frequency relationships in the presence of α,β-methylene-ATP (10 mM) reduced contractions to a greater proportion at lower stimulation frequencies.

We interpret these data to signify that adenosine and analogues with A1-activity exert a presynaptic action on guinea-pig detrusor preparations, but there is little effect postsynaptically. In human there was little effect by the P1-receptor agonists pre- and postsynaptically. P1-receptor agonists are more effective at reducing contractions at lower stimulation frequencies that coincide with a proportionally higher release of ATP.

We thank St Peter’s Trust for financial assistance and Pfizer Ltd for the gift of ZM241385.