From: dyer@ursolaris.spdcc.com (Steve Dyer)
Newsgroups: sci.med.pharmacy
Subject: Re: What is Atropine
Date: 3 Sep 1998 21:06:38 GMT

In article <1998090315381600.LAA28704@ladder01.news.aol.com>,
KB6UDA <kb6uda@aol.com> wrote:
>I have read the definition in the dictionary but still don't understand this
>substance/drug. This is basically a I don't understand what I have read
>question and can U give me some simple examples.

I don't understand what you don't understand. :-)

If you said "I don't understand aspirin", you'd be asked "in what way
don't you understand it?  How it's used?  Where it comes from?  How it
works?" and so forth.  Same situation here.

To understand how a drug like this works, you need a smattering of
neurobiology.  Not a lot, but terms like neurotransmitter, sympathetic
and parasympathetic nervous system should be somewhat familiar.
Cooper, Bloom and Roth's "The Biochemical Basis of Neuropharmacology"
is a good college-level reference.

Atropine is a naturally-occurring alkaloid which blocks the actions
of the neurotransmitter acetylcholine at a certain subclass of specialized
receptors, known as muscarinic receptors.  (The other major type of
acetylcholine receptor is known as the nicotinic receptor, which is found
in sympathetic ganglia and at the motor endplate, where your voluntary
muscles receive signals.)  Muscarinic receptors are found in the brain, and
throughout the parasympathetic nervous system, including the gut, the muscles
in your eye which control the size of your pupil, sweat glands, the salivary
glands, the heart, and the bladder, just to name a few locations.

By the way, these receptors are named for the drugs which were found
to selectively stimulate them.  Acetylcholine stimulates both types
of receptor, but muscarine only stimulates muscarinic receptors and
nicotine only stimulates nicotinic receptors.  Muscarine is a toxic
alkaloid from a certain species of mushroom.  Nicotine is a toxic
alkaloid from tobacco. :-)

By blocking the actions of acetylcholine, atropine can have a variety
of effects, depending on the target organ you're studying.  You've probably
had eye exams which required that you have special eyedrops containing
an atropine-like drug to dilate your pupils.  Your pupils dilate because
atropine blocks the signals which would ordinarily cause the pupil to
close in response to ambient light.  Atropine commonly causes a dry
mouth, because the salivary glands produce saliva in response to signals from
the parasympathetic nervous system.  Blocking the signal by administering
atropine dries you up.  Atropine and other atropine-like drugs can
cause constipation, because the regular peristalsis of the gut is
slowed when you block acetylcholine receptors.  Atropine increases
the heart rate by blocking inhibitory inputs to the heart from the
vagus nerve.

You will sometimes see insecticides such as organophosphates like Malathion
and Diazinon and carbamates like Sevin labelled with the phrase "atropine
is antidotal".  These pesticides are literally no different than "nerve gas";
only less potent.  Acetylcholine released into a synapse is destroyed by the
enzyme acetylcholinesterase--that's how nerve signals which are mediated
via the release of acetylcholine are terminated.  These pesticides inactivate
the enzyme, causing a buildup of the neurotransmitter, which wreaks havoc
on signalling.  Atropine is partially antidotal because it blocks the actions
of acetylcholine at muscarinic receptors.  It's not a complete antidote for
nerve gas poisoning, because you still have too much acetylcholine stimulating
nicotinic receptors at the neuromuscular junction and at sympathetic ganglia.


--
Steve Dyer
dyer@ursa-major.spdcc.com