AJP - Lung Cellular and Molecular Physiology, Vol 266, Issue 6 642-L648, Copyright © 1994 by American Physiological Society
Characterization of muscarinic receptor subtypes in pig airways: radioligand binding and northern blotting studies
E. B. Haddad, J. C. Mak, A. Hislop, S. G. Haworth and P. J. Barnes
Department of Thoracic Medicine, National Heart and Lung Institute, London, United Kingdom.
This study was undertaken to characterize the muscarinic receptor subtypes
present in adult pig peripheral lung and airway smooth muscle. The binding
of the nonselective muscarinic antagonist [N-methyl-3H]scopolamine
([3H]NMS) to pig airways showed a single class of binding sites with a
maximum density of 172 and 450 fmol/mg protein in peripheral lung and
airway smooth muscle, respectively. Unlike [3H]NMS, the M1-selective
antagonist, [3H]telenzepine, recognized two populations of binding sites in
peripheral lung. Approximately 14% of total [3H]telenzepine binding sites
displayed high affinity [dissociation constant (Kd) = 0.95 nM], whereas the
remaining sites showed low affinity (Kd = 14.2 nM). The high- and the
low-affinity [3H]telenzepine binding sites displayed the pharmacological
profile of M1 and M2 receptors, respectively. Heterogeneity of pig airways
muscarinic receptor was also revealed by competitive binding experiments
against [3H]NMS with the M2-selective antagonist methoctramine. This
compound recognized 70 and 90% of total receptors with high affinity in
airway smooth muscle (Ki = 4.44 nM) and peripheral lung (Ki = 9.82 nM),
respectively. This result suggests that the dominant muscarinic receptor in
pig airways is of the M2 subtype. Northern blot analysis demonstrated the
presence of m1 and m2 mRNAs transcripts in peripheral lung and m2 and m3
mRNAs in airway smooth muscle with no evidence for m4 mRNA.
