Table 1.
Muscarinic receptors, second messengers and tissue responses
| Receptor subtype | Second messenger | Organ/tissue | Response |
| M1 | IP3/DAG↑ | autonomic ganglion including myenteric plexus | depolarization, slow EPSP increased gastric acid secretion via vagus nerve |
| M2 | cAMP↓ and direct coupling to K+ channel | vas deferens (rabbit) brain (cerebral cortex) canine saphenous vein heart - | reduced twitch height and NA release binding sites contraction |
| atrium | reduced rate (negative chronotropy) | ||
| sinu-atrial node prejunctional at autonomic nerve endings | reduced rate (negative chronotropy) reduced NA or Ach release | ||
| ileum smooth muscle | predominant binding sites - uncertain function | ||
| M3 | IP3/DAG↑ | smooth muscle -gut urinary bladder trachea iris circular muscle blood vessels- | contraction minor binding sites |
| endothelium smooth muscle glands - | release of NO and vasodilatation contraction | ||
| oxyntic cells (gastric acid) salivary glands pancreatic β cells | increased acid secretion salivation insulin release | ||
| M4 | cAMP↓ | guinea-pig uterus | contraction |
| rabbit anococcygeus muscle | NO-dependent relaxation | ||
| rabbit lung | binding sites | ||
| M5 | IP3/DAG↑ | brain regions | binding sites after all others occluded |
| Abbreviations: Ach, acetylcholine; cAMP, cyclic adenosine 3',5'-monophosphate; DAG, 1,2-diacylglycerol; EPSP, slow depolarization; IP3, inositol 1,4,5-triphosphate; NA, noradrenaline; NO, nitric oxide. | |||