Cholinergic Agonists

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Chapter 32 Pharmacology

acetylcholinesterase

enzyme responsible for the immediate breakdown of acetylcholine when released from the nerve ending; prevents overstimulation of cholinergic receptor sites

Alzheimer Disease

degenerative disease of the cortex with loss of acetylcholine-producing cells and cholingergic receptors; characterized by progressive dementia

cholinergic

responding to acetylcholine; refers to receptor sites stimulated by acetylcholine, as well as neurons that release acetylcholine

miosis

constriction of the pupil; relieves intraocular pressure in some types of glaucoma

myasthenia gravis

autoimmune disease characterized by antibodies to cholinergic receptor sites, leading to destruction of the receptor sites and decreased response at the neuromuscular junction; it is progressive and debilitating, leading to paralysis

nerve gas

irreversible acetylcholinerase inhibitor used in warfare to cause paralysis and death by prolonged muscle contraction and parasympathetic crisis

parasympathomimetic

mimicking the effects of the parasympathetic nervous system, leading to bradycardia, hypotension, pupil constriction, increased GI secretions and activity, increased bladder tone, relaxation of sphincters, and bronchoconstriction

Bethanechol (Prototype Summary)

-Indications: Acute postoperative or postpartum nonobstructive urinary retention; neurogenic atony of the bladder with retention.
-Actions: Acts directly on cholinergic receptors to mimic the effects of acetylcholine; increases tone of detrusor muscles and causes emptying of the bladder
-Route: Oral; Onset: 30-90 min; Peak: 60-90 min; Duration: 1-6 h
-Adverse effects: Abdominal discomfort, salivation, nausea, vomiting, sweating, flushing

bethanechol (Duvoid, Urecholine)

Dosage/Route: 10-50 mg PO b.i.d. to q.i.d.
Indications: Treatment of nonobstructive postoperative and postpartum urinary retention, neurogenic bladder atony in adults and children >8yr; diagnosis and treatment of reflux esophagitis in adults, and used orally in infants and children for treatment of esophageal reflux

carbachol (Miostat)

Dosage/Route: 1-2 drops (gtt) in affected eye(s) as needed, up to three times a day
Indications: Induction of miosis to relieve increased intraocular pressure of glaucoma; allows surgeons to perform certain surgical procedures

cevimeline (Evoxac)

Dosage/Route: 30 mg PO t.i.d.
Indications: Treatment of symptoms of dry mouth in patients with Sjogren syndrome

pilocarpine (Pilocar, Salagen)

Dosage/Route: 1-2 gtt in affected eye(s) as needed, up to six times per day; 5-10 mg t.i.d. with meals if treating Sjogren syndrome
Indications: Induction of miosis to relieve increased intraocular pressure of glaucoma; allows surgeons to perform certain surgical procedures; treatment of symptoms of dry mouth in patients with Sjogren syndrome

Muscarinic Receptors

-Receptors that are stimulated by muscarine (plant alkaloid from mushrooms-reason wild mushrooms can have toxic effects)
-Found in visceral effector organs
-Found in sweat glands
-Found in some vascular smooth muscle

Stimulation of Muscarinic Receptors

Pupil constriction-miosis
Increased GI motility
Increased salivation
Increased urinary bladder constriction urination
Decreased heart rate

Nicotinic Receptors

-Located in the CNS, adrenal medulla, autonomic ganglia, and neuromuscular junction

-Stimulation causes:
Muscle contraction, Autonomic response, Release of norepinephrine and epinephrine from the adrenal medulla

Cholinergic Drugs

Definition: Chemicals that act at the same site as the neurotransmitter acetylcholine (ACh)

Actions: Often called parasympathomimetic drugs because their action mimics that of the PSNS
Since they are not limited to a specific site, cholinergics are associated with many undesirable systemic effects

Direct-acting cholinergic agonists

-Occupy receptor sites for ACh on the membranes of the target organs
-Cause increased stimulation of the cholinergic receptors.

Indirect-acting cholinergic agonists

-React with the enzyme acetylcholinesterase, preventing it from breaking down the ACh that was released from the nerve
-Cause increased stimulation of the ACh receptor sites

Bethanechol (Duvoid, Urecholine)

-Direct-Acting Cholinergic Agonists
-Treats urinary retention and neurogenic bladder atony
-Diagnoses and treats reflux esophagitis

Carbachol (Miostat) and pilocarpine (Pilocar)

-Direct-Acting Cholinergic Agonists
-Induce miosis or pupil constriction
-Relieve intraocular pressure of glaucoma
-For use in certain surgical procedures

What are the actions and indications of Direct-Acting Cholinergic Agonists?

-Actions: Act at cholinergic receptors in the peripheral nervous system to mimic the effects of ACH and parasympathetic stimulation

-Indications: Increase the tone of the detrusor muscle of the bladder and relax the bladder sphincter

What are the contraindications of Direct-Acting Cholinergic Agonists?

-Any condition exacerbated by parasympathetic effects: bradycardia, hypotension
-Peptic ulcer disease (avoid increasing secretions)
-Intestinal obstruction or recent GI surgery
-Asthma (protective SNS mechanism is overriden)
-Bladder obstruction
-Epilepsy and parkinsonism

What are the adverse reactions and drug-to-drug interactions of Direct-Acting Cholinergic Agonists?

Adverse reactions: Nausea, vomiting, cramps, diarrhea, increased salivation, and involuntary defecation,Bradycardia, and hypotension,Urinary urgency, Flushing and increased sweating
Drug-to-drug interaction: Acetylcholinesterase inhibitors => increased effect

What are Indirect-Acting Cholinergic Agonists?

-Do not react directly with ACh receptor sites
-React chemically with acetylcholinesterase in the synaptic cleft to prevent it from breaking down Ach
-As a result, ACh released from the presynaptic nerve accumulates, stimulating the ACh receptors
-Bind reversibly to acetylcholinesterase, so effects pass with time

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