Abstract
Since acetylcholine (ACh) was identified as a neurotransmitter at parasympathetic nerve terminals by pioneering pharmacologists such as O. Schmiedeberg, R. Hunt, O. Loewi and H.H. Dale, muscarinic acetylcholine receptors (mACh-R) serving as a transducer of muscarinic action have been assumed to exist. After many tries to identify the mACh-R, it's existence was established by the group of S.H. Snyder, who employed binding assays with the radioligand 3H-QNB. The presence of a neuronal (M1) and a peripheral (M2) mACh-R was suggested from the action of an M1-specific agonist, McN-A-343; and this observation was followed by the discovery that the antagonist pirenzepine had higher affinity for M1 than for M2. Later, peripheral mACh-Rs were further subclassified in two types by the heart-specific action of gallamine and the different affinities of AF-DX116 and 4-DAMP. At present, three subtypes, M1 (neuronal), M2 (heart) and M3 (other peripheral organs), can be pharmacologically distinguished by affinity differences. On the other hand, purification of mACh-R and analysis by gene technology revealed the presence of five mACh-R mRNAs (m1-m5), which were expressed in various organs with different abundances. These subtypes couple with subcellular muscarinic responses through different GTP-binding proteins. The connection between the subtypes, GTP-binding proteins and responses is not fully understood yet. Our studies showed that in guinea pig heart, in which only m2 mRNA is expressed, muscarinic agonists recognize two subgroups (M2α and M2β) with different affinities. One couples with the inhibition of adenylate cyclase, and the other couples with PI turnover through different GTP-binding proteins. These results indicate that the subtypes can be subclassified further with posttranslational protein modification.
