Cholinergic drugs
Cholinergic drugs promote the action of the neurotransmitter acetylcholine. These drugs are also called parasympathomimetic drugs because they produce effects that imitate parasympathetic nerve stimulation.
Mimickers and inhibitors
There are two major classes of cholinergic drugs:
- Cholinergic agonists mimic the action of the neurotransmitter acetylcholine.
- Anticholinesterase drugs work by inhibiting the destruction of acetylcholine at the cholinergic receptor sites.
By directly stimulating cholinergic receptors, cholinergic agonists mimic the action of the neurotransmitter acetylcholine.
They include such drugs as
- bethanechol
- carbachol
- cevimeline
- pilocarpine
Pharmacokinetics (how drugs circulate)
The action and metabolism of cholinergic agonists vary widely and depend on the affinity of the individual drug for muscarinic or nicotinic receptors.
No I.M. or I.V. injections
Cholinergic agonists rarely are administered by I.M. or I.V. injection because they’re almost immediately broken down by cholinesterases in the interstitial spaces between tissues and inside the blood vessels. Moreover, they begin to work rapidly and can cause a cholinergic crisis (a drug overdose resulting in extreme muscle weakness and possibly paralysis of the muscles used in respiration).
Topically, orally, or under the skin
Cholinergic agonists are usually administered:
- topically, with eye drops
- orally
- by subcutaneous (subQ) injection.
SubQ injections begin to work more rapidly than oral doses.
Metabolism and excretion
All cholinergic agonists are metabolized by cholinesterases:
- at the muscarinic and nicotinic receptor sites
- in the plasma (the liquid portion of the blood)
- in the liver.
All drugs in this class are excreted by the kidneys.
Pharmacodynamics (how drugs act)
Cholinergic agonists work by mimicking the action of acetylcholine on the neurons in certain organs of the body called target organs. When they combine with receptors on the cell membranes of target organs, they stimulate the muscle and produce:
- salivation
- bradycardia (a slow heart rate)
- dilation of blood vessels
- constriction of the bronchioles
- increased activity of the GI tract
- increased tone and contraction of the bladder muscles
- constriction of the pupils.
Pharmacotherapeutics (how drugs are used)
Cholinergic agonists are used to:
- treat atonic (weak) bladder conditions and postoperative and postpartum urine retention
- treat GI disorders, such as postoperative abdominal distention and GI atony
- reduce eye pressure in patients with glaucoma and during eye surgery
- treat salivary gland hypofunction caused by radiation therapy or Sjögren’s syndrome.
Drug interactions
Cholinergic agonists have specific interactions with other drugs. Examples include the following:
- Other cholinergic drugs, particularly anticholinesterase drugs (such as ambenonium, edrophonium, neostigmine, physostigmine, and pyridostigmine), boost the effects of cholinergic agonists and increase the risk of toxicity.
- Cholinergic blocking drugs (such as atropine, belladonna, homatropine, methantheline, methscopolamine, propantheline, and scopolamine) reduce the effects of cholinergic drugs.
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