• M3 muscarinic receptors;
  • M5 receptors;
  • atypical receptor;
  • ciliary muscle;
  • melanin;
  • urinary bladder smooth muscle
  • The pharmacological characteristics of muscarinic receptors mediating contraction of dog isolated ciliary muscle were determined and compared to those mediating contraction of dog urinary bladder smooth muscle.

  • (+)-Cis-dioxolane induced concentration-dependent contractions of ciliary muscle (pEC50=7.18±0.07, Emax=453±64 mg, n=19) and urinary bladder isolated smooth muscle (pEC50=6.55±0.07, Emax=11±1 g, n=19). These responses were antagonized by several muscarinic receptor antagonists (pKb values for the ciliary muscle and the bladder smooth muscle, respectively): atropine (8.25±0.14 and 9.21±0.09), pirenzepine (6.31±0.13 and 6.70±0.25), tolterodine (7.97±0.14 and 8.68±0.12), oxybutynin (7.40±0.08 and 7.88±0.12), zamifenacin (6.46±0.19 and 7.69±0.11), S-secoverine (6.66±0.14 and 8.13±0.07), AQ-RA 741 (6.16±0.15 and 7.08±0.23), p-F-HHSiD (7.10±0.27 and 7.35±0.07) and responses were not antagonized by PD 102807 (up to 100 nM).

  • In urinary bladder smooth muscle, the profile of antagonist pKB values correlated significantly with pKi values at human recombinant m3 muscarinic receptors, suggesting that M3 muscarinic receptors mediated the response. In the ciliary muscle, a significant (P<0.01) correlation was obtained with human recombinant m3 and m5 receptors.

  • Darifenacin displayed insurmountable antagonism at receptors in the bladder. At receptors in the ciliary muscle, it exhibited two phases of antagonism, comprising an initial low affinity (pKB<6) component and a high affinity phase (pKB>8).

  • The role of pigmentation in the atypical behaviour of darifenacin was examined. In blue coloured eyes, darifenacin produced apparent surmountable, competitive antagonism of the responses to (+)-cis-dioxolane (pKB=8.76±0.07). The antagonist profile obtained in this tissue suggested the involvement of a site which has the pharmacological attributes of the M5 receptor.

  • We suggest that the dog urinary bladder contracts in response to M3 muscarinic receptor activation. Contraction of the brown-eyed dog ciliary muscle is more complex and may include involvement of at least two receptors, possibly the M5 and M3 receptor, whereas blue-eyed dog ciliary muscle may involve a single population of M5 muscarinic receptors.

British Journal of Pharmacology (2001) 132, 835–842; doi:10.1038/sj.bjp.0703901