Medicinal Chemistry

NEUROTRANSMITTERS AND THEIR RECEPTORS 205

Cholinergic impulses Adrenergic impulses

Response Receptor Response

Effector organs type

Lung

Bronchial muscle Contraction β Relaxation

Bronchial glands Stimulation Inhibition

Stomach

Motility and tone Increase β Decrease (usually)

Sphincters Relaxation (usually) α Contraction (usually)

Secretion Stimulation Inhibition

Intestine

Motility and tone Increase α,β Decrease

Sphincters Relaxation (usually) α Contraction (usually)

Secretion Stimulation Inhibition

Liver – β Glycogenolysis

Pancreas

Acini Secretion –

Islets Insulin secretion α Inhibition of insulin secretion

β Insulin secretion

Gallbladder and ducts Contraction Relaxation

Urinary bladder

Detrusor Contraction β Relaxation (usually)

Trigone and sphincter Relaxation α Contraction

Ureter

Motility and tone Increase Increase (usually)

Skin

Pilomotor muscles – α Contraction

Sweat glands Generalized secretion α Slight, localized secretions

Adrenal medulla Secretion of epinephrine –

and norepinephrine

Eye

Radial muscle of iris – α Contraction (mydriasis)

Sphincter muscle of iris Contraction (miosis) –

Ciliary muscle Contraction for near β Relaxation for far vision

vision

As do most neuronal systems, cholinergic receptors show multiplicity, and we
distinguish between nicotinicandmuscarinicreceptors, which differ in many respects.
Whereas acetylcholine (4.1) binds to both types of receptors, the plant alkaloids
nicotine (4.2) and muscarine (4.3) trigger a response only from nicotinic or muscarinic
cholinergic receptors, respectively. Nicotinic receptors are found in all autonomic gan-
glia (i.e., in the sympathetic system as well as the parasympathetic) and at the neuro-
muscular endplate of striated muscle. Muscarinic receptors occur at postganglionic

Table 4.1Continued