5.9: Direct-Acting Cholinergic Agonists: Pharmacological Actions

Direct-acting cholinergic agonists exert their pharmacological actions by mimicking the effects of acetylcholine on postsynaptic muscarinic receptors to generate parasympathetic responses. These agents elicit a range of physiological responses, including cardiovascular effects. For example, activation of muscarinic receptors induces bradycardia, decreased cardiac output, reduced peripheral resistance, and consequent hypotension. In the eye, stimulation of M₃ receptors leads to smooth muscle contraction in the iris, resulting in miosis. Simultaneously, the contraction of the ciliary muscle adjusts the lens curvature and focal length, improving near vision. These actions help regulate aqueous humor outflow and intraocular pressure, making them beneficial in treating glaucoma.

Furthermore, muscarinic agonists induce smooth muscle contractions in the urinary and respiratory systems while enhancing peristalsis in the gastrointestinal tract. Synthetic direct-acting agents capable of crossing the blood-brain barrier elicit central nervous system effects, including cortical stimulation, tremor, and hypothermia. Nicotinic agonists activate receptors at neuromuscular junctions, leading to depolarization, increased ion permeability, and subsequent muscle contraction.

Knowledge of the uses and side effects of cholinergic agonists is crucial in managing the various physiological processes for which they are prescribed.

Direct-acting cholinergic agonists mimic acetylcholine to stimulate the parasympathetic nervous system, controlling pupil constriction, heart rate, and gut peristalsis.

For instance, muscarinic agonists can decrease heart rate and dilate blood vessels, leading to changes in blood pressure.

In the eyes, these drugs activate M₃ receptors in the iris sphincter muscle, resulting in pupilar constriction.

Additionally, muscarinic agonists can stimulate contraction of the ciliary muscle in the eye, adjusting lens curvature and focal length to help focus on near objects.

Muscarinic agonists also stimulate smooth muscle contractions of the urinary and respiratory systems and increase gut peristalsis.

As synthetic direct-acting agents can cross the blood-brain barrier, they can produce CNS effects, such as cortical stimulation, tremor, and hypothermia.