JH, a 63-year-old architect, complains of urinary symptoms to his family physician. He has hypertension, and during the last 8 years, he has been adequately managed with a thiazide diuretic and an angiotensin-converting enzyme inhibitor. During the same period, JH developed the signs of benign prostatic hypertrophy, which eventually required prostatectomy to relieve symptoms. He now complains that he has an increased urge to urinate as well as urinary frequency, and this has disrupted the pattern of his daily life. What do you suspect is the cause of JH’s problem? What information would you gather to confirm your diagnosis? What treatment steps would you initiate?
Cholinoceptor antagonists, like the agonists, are divided into muscarinic and nicotinic subgroups on the basis of their specific receptor affinities. Ganglion blockers and neuromuscular junction blockers make up the antinicotinic drugs. The ganglion-blocking drugs have little clinical use and are discussed at the end of this chapter. Neuromuscular blockers are heavily used and are discussed in Chapter 27. This chapter emphasizes drugs that block muscarinic cholinoceptors.
Five subtypes of muscarinic receptors have been identified, primarily on the basis of data from ligand-binding and cDNA-cloning experiments (see Chapters 6 and 7). A standard terminology (M1 through M5) for these subtypes is now in common use, and evidence—based mostly on selective agonists and antagonists—indicates that functional differences exist between several of these subtypes. The X-ray crystallographic structures of the M1–4 subtypes of muscarinic receptors have been reported. The structures of the M1–4 receptors are very similar in the inactive state with inverse agonist or antagonist bound to the receptor. The binding pocket for orthosteric ligands lies well within the plane of the plasma membrane, and the amino acids composing the site are conserved among muscarinic receptor subtypes. This observation underscores the difficulty in identifying subtype-selective ligands. A structure forming a “lid” separates the orthosteric binding site from an upper cavity termed the “vestibule” (Figure 8–1). The binding site for allosteric ligands is the extracellular vestibule. Among the receptor subtypes, the extracellular vestibule is comprised of different amino acids that provide distinctive sites for binding by selective allosteric modulators. The M1 receptor subtype is located on central nervous system (CNS) neurons, autonomic postganglionic cell bodies, and many presynaptic sites. M2 receptors are located in the myocardium, smooth muscle organs, and some neuronal sites. M3 receptors are most common on effector cell membranes, especially glandular and smooth muscle cells. M4 and M5 receptors are less prominent and appear to play a greater role in the CNS than in the periphery.
Upper portion of the M3 receptor with a “lid” formed by tyrosine (Tyr) residues separating the cavity into an upper portion called the vestibule from the lower portion, with the orthosteric binding site depicted as occupied by tiotropium. The receptor is in black, ...