Do we have drugs that mimic ACh?
Yes and they are called parasympathomimetic drugs which stimulate the muscarinic receptor.
Bethanechol (Urecholine) – First we have a parasympathomimetic drug called bethanechol. What would a parasympathomimetic drug do to the GI tract? And what would it do in regards to urination? (Think about it!)
It would stimulate GI motility and bladder contraction.
This drug is typically reserved in a hospital setting after surgery. Post-operatively, the autonomic nervous system doesn’t work too well. If you take a stethoscope to the GI tract you don’t hear anything. The patient may get constipated and urination may be problematic. So in this case a parasympathomimetic may be used to assist after surgery.
Pilocarpine (Pilocar) – This is a short acting eye drop used to treat glaucoma by lowering intra-ocular pressur. Remember, parasympathomimetics are useful for treating glaucoma.
Is there such a drug called Acetylcholine?
No. We can’t administer ACh to a patient because it’s not-selective and it will go to ALL of the muscarinic receptors. Think about what would happen if I stimulated every muscarinic receptor in your body. You’d be tearing (lacrimation). You’d be drooling (saliva secretion). Your stomach would feel like a furnace (stomach acid secretion). Your intestinal tracts will be cramping (peristalsis).
Contraindications: When do we need to be careful with parasympathomimetics?
Asthmatics: We don’t want to give it to people prone to asthma due to the broncho-constriction.
Cardiac insufficiency: If someone has cardiac insufficiency and poor cardiac output, we don’t want to lower their heart rate.
Peptic Ulcers: If a person is prone to peptic ulcers, we don’t want to give them this because it increases stomach acid secretion.
Eye Drop Precaution: There are ways to block absorption of eye drops so if you put it on the eye it won’t go in the blood vessels: If you put pressure on the inner corner of the eye, the drug can’t drain and go into the blood stream. Can I use the ophthalmic drop in an asthmatic? Yes, as long as you use the precaution.