Is there a blood-enteric system barrier?

Paul S. Winalski winalski at gemgrp.enet.dec.com
Fri Oct 8 14:13:33 EST 1993


In article <16C60FC1F.ULIBARRI at wsuvm1.csc.wsu.edu>,
ULIBARRI at wsuvm1.csc.wsu.edu (Cathy) writes:
|>I'm posting this for a collaborator of mine.  He is interested in the role of
|>glutamate in enteric functioning, but has been having some trouble finding out
|>some basics.
|> 
|>He writes "Olney et al looked for evidence of neuronal degeneration in neurons
|>of the CNS and retina after injections of exogenous L-glutamate.  They found
|>that formation of a blood-brain-barrier in slightly older animals afforded
|>protection against excitotoxicity.  They did not report enteric (gastro-
|>intestinal) effects.  Is this because there is always a blood-enetric barrier?
|>I am looking for references/information about whether a blood-eneteric barrier
|>exists."
|> 
|>It seems like there must be one because nobody reports GI symptoms in "Chinese
|>Restaurant Syndrome"

I'm not an expert in this area, but here's my understanding of how it all
works:

L-glutamate is one of the 20 amino acids commonly found in all proteins.
Enzymatic digestion of proteins to their component amino acids in the small
intestine thus releases large amounts of L-glutamate, which is absorbed by the
cells of the intestinal lining.  Venous blood flow from the intestines is not
connected directly to the general circulatory system.  Instead it is routed
to the liver via the hepatic portal veins, which branch out to capillaries
in the liver, then back again into veins that join the general circulation.
Thus, all blood flow from the intestines is filtered by the liver before
going back into general circulation.  L-glutamate is normally transported in
a conjugated form in the bloodstream, not as the bare amino acid.  I think
it is the intestinal lining cells that originally absorb L-glutamate from the
intestinal lumen that perform the conjugation and only release L-glutamate
into the portal system in conjugated form.  The liver, the body's "great
detoxifier", also can remove bare L-glutamate from the bloodstream.

"Chinese Restaurant Syndrome" occurs when excessive amounts of L-glutamate are
absorbed by the stomach lining and dumped into the hepatic portal system.
Protein digestion in the stomach usually only breaks down proteins into small
polypeptides, not free amino acids, so the stomach normally doesn't see much
free L-glutamate.  This is why the syndrome only occurs when free L-glutamate
in the form of its monosodium salt is used in excessive amounts as a food
seasoning.  If you swamp the liver's ability to remove L-glutamate from the
hepatic circulation, you will get free L-glutamate in the general blood
circulation.  The consequent neurological effects produce the "Chinese
Restaurant Syndrome".

The hepatic portal circulation is your conjectured blood-enteric barrier.  It
ensures that anything entering the bloodstream from the gut is processed by
the liver before it gets into the general blood cirulation.

I think you don't see toxic effects in enetric neurons from L-glutamate
introduced directly into the general circulation because these neurons are
exposed to free amino acids all the time, as are the liver and intestinal
lining cells.  They have the enzymes necessary to counteract the toxic
effects.

--PSW  



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