ACETYLCHOLINE is the prototypical cholinergic agent. It functions as a neurotransmitter at all cholinergic sites in the body; because of its unique pharmacokinetic properties, it has never been used in medical therapeutics; the discussion which follows is for academic exercise.
Pharmacokinetics
Acetylcholine is poorly absorbed from the gastric mucosa; therefore it is ineffective if given orally. The recommended way of administration is parenteral. In the blood it is rapidly hydrolyzed by the enzyme cholinesterase into acetic acid and choline; this makes its duration of action very short and unreliable for therapeutic purposes.
Pharmacodynamics
As mentioned earlier it has two types of actions: nicotinic and muscarinic; the muscarinic actions are of main interest and are discussed below.
- Cardiovascular system
Heartà slow heart rate
Blood vesselsà vasodilator
Blood pressureà falls because of the effect on the heart and blood revels
- Gastrointestinal tract
It stimulates the tone and motility of the Gl tract but the sphincters will be relaxed
- Urinary tract
It stimulates the detrusor muscle and relaxes the internal urethral sphincter resulting in evacuation of bladder
- Bronchioles
It increase bronchial secretion and brings about bronchoconstriction
- Eye– It has two effects- miosis and accommodation for near objects because of stimulation of the constrictor pupillae and ciliary muscles respectively.
- Exocrine glands– it stimulates salivary, gastric, bronchial, lachrymal and sweat gland secretions.
SYNTHETIC CHOLINE ESTERS. These are synthetic derivatives of choline and include metacholine, carbachol and betanechol. These drugs have the following advantages over acetylcholine:
- They have longer duration of action,
- They are effective orally as well as parenterally, and
- They are relatively more selective in their actions.
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