Open AccessMuscarinic Receptor Binding Sites of the M 4 Subtype in Porcine Lung Parenchyma
Author(s) -
Chelala Jean L.,
Kilani Ahmad,
Miller Martha J.,
Martin Richard J.,
Ernsberger Paul
Publication year - 1998
Publication title -
pharmacology & toxicology
Language(s) - English
Resource type - Journals
eISSN - 1600-0773
pISSN - 0901-9928
DOI - 10.1111/j.1600-0773.1998.tb01469.x
Subject(s) - muscarinic acetylcholine receptor , receptor , biology , endocrinology , medicine , population , muscarinic acetylcholine receptor m3 , muscarinic antagonist , microbiology and biotechnology , biochemistry , environmental health
Muscarinic acetylcholine receptors regulate distal airway resistance and secretion. The subtype expressed in the lung in different species remains uncertain. It has recently become possible to identify the M 4 subtype by careful comparison of antagonist affinities. We characterized the binding of [ 3 H]quinuclidinyl benzilate ([ 3 H]QNB) to muscarinic receptors in cell membranes from lung parenchyma of 2–8 week old pigs in comparison to cloned human M 3 and M 4 receptors expressed in COS cells, to M 2 in rat atria and to M 4 in bovine adrenal medulla. In porcine lung, [ 3 H]QNB bound with high affinity (K d =95±9pM) to a single homogeneous population of muscarinic receptor sites (B max =340±10 fmol/mg protein). Competition studies showed that the affinity (expressed as pK i ) of 3 selective blockers was in close agreement between pig lung and cloned human m 4 (r=0.996). A series of 10 blockers showed affinities closely matching reported values for M 4 receptors of the adrenal medulla (r=0.965). Conversely, affinity values in porcine lung differed significantly (P< 0.05, t‐test) from those determined in parallel with either human cloned M 3 or with rat atria expressing the M 2 subtype. We conclude that pig lung muscarinic receptor binding sites most closely resemble the M 4 subtype, in contrast to the M 3 subtype typical of large airways in this species.