Two Populations Of Muscarine Receptors

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


This work provides evidence for two distinct populations of muscarine receptors in mammalian tissues, M1 and M2, and for two states of M2 receptors.The differences between the two receptors are summarized as follows. (1) M1 receptors have higher affinity for most antagonists than do M2 receptors. For example pirenzepine (apparent K(,d) for M1 = 1.65 x 10('-8) M, for M2 = 5.6 x 10('-7) M) shows 35-fold selectivity for M1. Exceptions to this rule are gallamine (apparent K(,d) for M1 = 3 x 10('-8) M, for M2 = 1.0 x 10('-8)-1.3 x 10('-8) M) and pancuronium (apparent K(,d) for M1 = 5.2 x 10('-8), for M2 = 2.3 x 10('-8)) which are 11-fold selective for M2. (2) M2 receptors have higher affinity for most agonists than do M1 receptors. For example carbachol (apparent K(,d) for M1 = 1.8 x 10('-5) M, for M2 = 5.0 x 10('-7) M) shows 36-fold selectivity. While some agonists do not exhibit selectivity (e.g. MCN-A-343-11, apparent K(,d) = 3 x 10('-7)) I did not find any which select for M1. (3) M1 receptors are located in forebrain regions (hippocampus, striatum, and cortex) and glands (e.g. intestinal mucosa and submaxillary glands). M2 receptors are located in brainstem regions (medulla-pons and cerebellum), heart, and smooth muscle (e.g. ileum). (4) In the presence of 1 mM Mn('++), but not EDTA, M2 but not M1 receptors show a high affinity agonist-binding state which disappears in the presence of GppNHp. Alkaline extraction of membranes also prevents the formation of this high affinity state of M2, without affecting M1. These observations imply the presence of ternary complexes of agonist-M2 plus a guanine nucleotide binding protein, which are destabilized by GppNHp. The presumptive coupled form has (TURN)30-fold higher affinity than free M2. (5) M1 but not M2 receptors undergo a 10-fold increase in agonist affinity in the presence of 1 mM Mn('++) plus 0.1 mM GppNHp.Thus M1 and M2 differ in location and affinity for agonists and antagonists and it is probable that they have different mechanisms of action.


Health Sciences, Pharmacology

Link to Full Text