Characterisation of muscarinic receptor subtypes in avian smooth muscle

The identity of the muscarinic receptor subtype in the chick ileum was investigated in functional and binding studies. Preliminary studies [Choo, L.-K., Mitchelson, F., Napier, P. 1988. J. Auton. Pharmacol. 8, 259-266] suggested apparent avian and mammalian family differences in the muscarinic receptor profile of ileal smooth muscle. In the current study, further characterisation was undertaken using a greater range of antagonists exhibiting high affinity for specific muscarinic receptor subtypes. Dissociation constants from functional and binding experiments were compared with published values for antagonists at each of the five muscarinic receptor subtypes. Linear regression and correlation analyses revealed the receptor initiating the contractile response was most likely of the muscarinic M(3) receptor subtype as the slope of the linear regression was 1.01+/-0.14 and the corresponding correlation coefficient (r) was 0.95. The mammalian muscarinic M(5) receptor subtype also showed a high correlation with the data giving a slope of 0.89+/-0.27 and r value of 0.76. These findings were in direct contrast to those from binding experiments in which the single binding site detected was of the muscarinic M(2) receptor subtype. The slope of the linear regression was 1.14+/-0.24 with an r value of 0.87. Thus, these results suggest that there exists a high proportion of the muscarinic M(2) receptor subtype within the tissue that does not contribute to the functional response.