They are not well absorbed by the GI tract because they are hydrolyzed by stomach acid and GI tract, and only acetylcholine is broken down by acetylcholinesterase
Muscarinic Agonists: Muscarine & Oxotremorine
They are parasympathomimetic - slows the heart down, decreased rate and force of contraction, causes NO release to dilate blood vessels, complicated reflexes. Dilation of blood vessels might cause high heart rate (baroreceptor reflex)
Vasodilation requires to have endothelial cell layer in order for tissue relaxation.
Required for vasodilation: Endothelial Derived Relaxing Factor (EDRF = NO, Nitric Oxide, not Nitrous Oxide)
NO causes activation of guanylyl cyclase which produces cGMP. cGMP causes muscle excitation
1st response, non-damaged vessel
2nd response, endothelia damaged
Muscarine Receptor Effects
Contracts the sphincter muscle of iris - Miosis (pin point pupils).
Contracts ciliary muscle - accommodation for near vision - myopia.
Cholinergic effects allow aqueous humour to leave the eye, lowers intraocular pressure - useful for glaucoma