Chapter 7

In mid-afternoon, a coworker brings 43-year-old JM to the emergency department because he is unable to continue picking vegetables. His gait is unsteady and he walks with support from his colleague. JM has difficulty speaking and swallowing, his vision is blurred, and his eyes are filled with tears. His coworker notes that JM was working in a field that had been sprayed early in the morning with a material that had the odor of sulfur. Within 3 hours after starting his work, JM complained of tightness in his chest that made breathing difficult, and he called for help before becoming disoriented.

How would you proceed to evaluate and treat JM? What should be done for his coworker?

Acetylcholine-receptor stimulants and cholinesterase inhibitors make up a large group of drugs that mimic acetylcholine (cholinomimetic agents) (Figure 7–1). Cholinoceptor stimulants are classified pharmacologically by their spectrum of action, depending on the type of receptor—muscarinic or nicotinic—that is activated. Cholinomimetics are also classified by their mechanism of action because some bind directly to (and activate) cholinoceptors whereas others act indirectly by inhibiting the hydrolysis of endogenous acetylcholine.

###### Figure 7–1

The major groups of cholinoceptor-activating drugs, receptors, and target tissues. ACh, acetylcholine.

### Spectrum of Action of Cholinomimetic Drugs

Early studies of the parasympathetic nervous system showed that the alkaloid muscarine mimicked the effects of parasympathetic nerve discharge; that is, the effects were parasympathomimetic. Application of muscarine to ganglia and to autonomic effector tissues (smooth muscle, heart, exocrine glands) showed that the parasympathomimetic action of the alkaloid occurred through an action on receptors at effector cells, not those in ganglia. The effects of acetylcholine itself and of other cholinomimetic drugs at autonomic neuroeffector junctions are called parasympathomimetic effects and are mediated by muscarinic receptors. In contrast, low concentrations of the alkaloid nicotine stimulated autonomic ganglia and skeletal muscle neuromuscular junctions but not autonomic effector cells. The ganglion and skeletal muscle receptors were therefore labeled nicotinic. When acetylcholine was later identified as the physiologic transmitter at both muscarinic and nicotinic receptors, both receptors were recognized as cholinoceptor subtypes.

Cholinoceptors are members of either G protein–linked (muscarinic) or ion channel (nicotinic) families on the basis of their transmembrane signaling mechanisms. Muscarinic receptors contain seven transmembrane domains whose third cytoplasmic loop is coupled to G proteins that function as transducers (see Figure 2–11). These receptors regulate the production of intracellular second messengers and modulate certain ion channels via their G proteins. Agonist selectivity is determined by the subtypes of muscarinic receptors and G proteins that are present in a given cell (Table 7–1). When expressed in cells, muscarinic receptors form dimers or oligomers that are thought to function in receptor movement between the endoplasmic reticulum and plasma membrane. Conceivably, agonist or antagonist ligands could signal by changing the ...

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