The role of M 2 ‐muscarinic receptors in mediating contraction of the pig urinary bladder in vitro

In urinary bladder, M 2 ‐muscarinic receptors predominate, but it is the smaller population of M 3 ‐receptors which mediate detrusor contraction. This study examines the M 2  : M 3 ratio and the role of M 2 ‐receptors in contraction of pig urinary bladder. Competition experiments with [ 3 H]‐QNB determined the ratio of M 2  : M 3 . In functional studies, affinity values (pK B ) for 4‐DAMP, darifenacin and methoctramine were calculated. Similar experiments were performed on tissues following selective M 3 ‐inactivation (incubation with 40 n M 4‐DAMP mustard in the presence of 1 μ M methoctramine to protect M 2 ‐receptors), precontraction with 50 m M KCl and relaxation with isoprenaline (30 μ M ) or forskolin (1 μ M ). In competition binding, displacement of [ 3 H]‐QNB by 4‐DAMP, darifenacin and methoctramine best fitted a two‐site model suggesting a predominant (70–80%) population of M 2 ‐receptors. On normal detrusor in vitro , 4‐DAMP and methoctramine caused surmountable antagonism of responses to carbachol with pK B values of 9.37±0.07 and 6.05±0.05 respectively. Darifenacin caused unsurmountable antagonism, the apparent pK B value being 8.61±0.10. In tissues where the M 3 ‐receptors had been inactivated and cyclic AMP levels elevated, 4‐DAMP and darifenacin were less potent, with apparent pK B values of 8.72±0.08 and 6.74±0.07. In contrast, methoctramine was more potent, the apparent pK B value increasing significantly to 6.86±0.06. These data suggest that the pig bladder possesses a similar muscarinic receptor population to the human bladder and that the M 3 ‐receptor subtype mediates contraction of the normal detrusor muscle. However an involvement of M 2 ‐receptors in contraction can be observed following pharmacological manipulation of the receptor population.British Journal of Pharmacology (2000) 131 , 1482–1488; doi: 10.1038/sj.bjp.0703719