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pharm
2/18/98

today we're talking about the pharmacology 
of intestinal secretion and absorption. probably this is all in the 
notes. you have to know GI physiology to talk about GI pharmacology. 


first, recall from histology that there's a functional separation of 
secretion and absorption. the enterocytes are 'born' in the crypts as 
secretory cells, and they migrate up on to the villi (in small intestine) 
as mature absorptive cells. the villous absorptive cells and crypt 
secretory cells are the key players. 

all the absorptive and secretory 
mechanisms are thought to be powered by the Na/K ATPase pump which pumps 
sodium out of the cell in exchange for potassium, powered by ATP. this is 
where the energy goes into making things work. everything else is 
passive, or carrier mediated, not using energy.

remember the inside of 
the cell stays a bit negative since it isn't a 1:1 Na/K exchange. so 
sodium wants to get into the cell. it can get in by diffusing into the 
cell from the lumen, but that's slow. it can get in via a carrier system 
which takes in Na+ and Cl- in a cotransport mechanism (actually is Na+ 
exchange for H+ and Cl- exchange for bicarb, but don't worry about that 
right now, it looks like Na+ and Cl- are entering the cell. Na+ is then 
pumped out, and water follows out. 

another mechanism going on here is a 
combination of Na+ absorption and something else we call a substrate - 
can be glucose, AAs, small peptides - many carrier mechanisms, some carry 
more than one substrate, some more specific than others - very efficient 
mechanism, but requires substrate to occur, so only happens after meals 
and so forth whereas the other mechanism always occurs.

secretion is 
pretty similar - there's an Na/K exchange pump on the serosal side as 
with the absorptive cell. the tight junctions in proximal small intestine 
aren't that tight, get tighter as you go distal. the Na/Cl cotransport 
isn't lumenal like with absorptive cell - it's closer to serosal side. 
sodium can cycle from the pump on the serosal side into the cotransport 
mechanism.in any case, Cl- starts to accumulate in the cell. on the 
lumenal side are gates which can open, allowing Cl- to diffuse out,which 
it does pretty effectively across its electrochemical gradient. 
therefore, sodium and water will diffuse through the tight junction to 
balance out the Cl- which has gone out of the cell. 

internal control - 
intracellular control - 
one of the big controls that's been studied 
really well is the cAMP control - it's produced by interaction of 
membranes with things outside. you get an increase in cAMP due to adenyl 
cyclase, which seems to be on serosal side of cell. when you increase 
cAMP it stimulates changes in protein kinases, which affect the cell 
wall, turning off the Na+Cl- absorption. in crypt cells, increasing cAMP 
opens the Cl- gate at the lumenal side. so increased cAMP stops 
absorption and stimulates secretion. cAMP does other things - there are 
also on the serosal side of the absorptive cell some Ca++ channels which 
are stimulated to open, allowing a little Ca++ influx into cell, which 
acts on ER to cause release of intracellular Ca++ stores, so 
intracellular Ca++ rises, and that causes cell wall changes, calmodulin 
activation, activation of protein kinases - ultimately this stops 
absorption and stimulates secretion, like cAMP does. cAMP also releases 
Ca++ from the ER, so these mechanisms work kind of hand in hand to 
stimulate each other (ooh, kinky. just think about all those 
intracellular mechanisms in your guts, sitting there stimulating each 
other,hand in hand!).

another mechanism on the lumenal side of the cell 
is a cGMP mechanism, where you get cGMP produced, and prostaglandins 
being made in the cell wall. when you increase these byproducts of this 
mechanism in the cell you again change the membrane to stop absorption 
and start secretion. 

the enteric nervous system is important- there are 
about 100 million neurons in it, about as complex as the spinal cord. it 
runs up and down the whole GI tract. myenteric and submucosal plexi are 
important. interactions through vagus and other connections are 
important. when you read about this system you hear lots of stuff and it 
can be confusing. he's going to try to simplify this.

afferent end of 
the enteric NS starts - on the villi, near the end of them, are special 
cells, the primary receptors being enterochromaffin cells, which are 
special epithelial cells that have some villi on them, resembling the 
usual epi cells there, but these are special sensory cells. you may have 
increased cAMP in the cell, or increased cGMP in the cell, in response to 
some sensory stimulus, causing release of 5HT/serotonin, which is 
released around the cell, and which stimulates afferent nerve endings via 
a local increase in prostaglandins which opens calcium channels in the 
afferent nerve endings. a bunch of afferent endings (vagal et al) are up 
there in the villus. the local afferent goes to myenteric plexus and 
synapses there. they synapse with interneurons that go to the submucosal 
plexus, and in the submucosal plexus they synapse with effector neurons 
that then go to many places, including right near the crypt cells, where 
they release some mediators in the area of the crypt cells. so as far as 
NT in this area - in myenteric plexus are ACH, which is important there, 
and substance P is also important there. the ACH receptors there are 
nicotinic. in the submucosal plexus ACH is again probably most important, 
or best understood anyway, and again receptors are nicotinic. then when 
you get down to the crypt cell itself, the two important NTs are ACH and 
VIP (vasoactive intestinal polypeptide). so the VIP and prostaglandins 
cause increases in cAMP in crypt cell, and ACH causes opening of Ca++ 
channel. these act to stop absorption and start secretion. 

many 
inhibitory mechanisms here too. as far as the myenteric plexus goes 
inhibitors include somatostatin and enkephalins, which basically come 
from other enteric NS neurons in the area that synapse there, and also 
norepi, which is coming centrally from the central neurons, also 
inhibitory here. the same things inhibit in the submucosal plexus, and at 
the crypt cell itself it isn't quite as clear, but they may also work at 
this level - somatostatin and norepi seem to inhibit the calcium gate. 
the norepi receptors here are A2 receptors at both plexi and the crypt 
cells (probably). 

so it's fairly complex. to make it more complex, 
consider the effector neurons not only release mediators right by crypt 
cells, but they also have branches controlling blood supply to the 
villus, and to the crypt area, and they also have receptors that supply 
little fibrils that are in the villus, which are poorly understood but 
seem to be contractile, and if they contract, they pull the villus down 
and if they relax,the villus gets longer. contraction, then, inhibits 
absorption, and relaxation promotes/favors absorptoin. so by stimulating 
these contractile fibers, the enteric NS can bypass all these other 
mechanisms and still control absorption/secretion this way. as far as 
blood supply goes, obviously this is also important and is also 
controlled by enteric NS. 

if you stimulate enterochromaffin cells, they 
release serotonin, cause local prostaglandin production and neuron fires, 
stimulating interneuron, which stimulates effector neurons, which modify 
cAMP in crypt cell, starting secretion
so if you stimulate villus, you 
cause secretion. also cause villus to contract, reduce blood supply, and 
reduce absorption. note that also some of the afferents turn into 
efferents but this is kind of difficult to control pharmacologically.


many physiologic stimuli feed into this. ingesta in the lumen will 
stimulate the enterochromaffin cells (depends on pH, and osmolarity, and 
so forth). if osmolarity is high, you want to dilute it. cells are 
stimulated, you stimulate secretion, pump fluid into lumen, osmolarity 
drops, things return to normal, and you absorb. also, there can be 
efferents elsewhere that are stretch receptors - if you stretch the lumen 
of the bowel, you turn off absorption and start secretion also. there's 
also probably often a low leevel of prostaglandins, a physiologic level, 
at ends of villus,that mediate this too.

pathogens take advantage of 
this mechanism to cause diarrhea. E.coli has enterotoxins - a heat labile 
and heat stable enterotoxin.the labile toxin - remember, if it's going to 
cause disease, e.coli has to attach to epi cells. the pilus on the 
bacteria will bind onto the epi cell, and if it does this, and makes 
toxin, the toxin will get onto the enterochromaffin cells. heat labile 
toxin increases cAMP in these cells. the toxin also never comes off, so 
as long as that cell lives, its going to be under toxin influence (but 
will slough soon). heat stable toxin causes serotonin release, stimulates 
the previously discussed mechanism. so you get a secretory diarrhea. 


bile acids cause diarrhea b/c they irritate the tips of the villi, 
causing increased prostaglandins, increased firing of neurons, increased 
secretion and reduced absorption.

salmonella attaches to tip of villus 
too - has receptors that work best there - but only attaches long enough 
to get in.then causes inflammatory rxn, bradykinins and prostaglandins 
are produced - also turns on same mechanism, resulting in secretory 
diarrhea. so part of pathophysiology of salmonella uses same pathway as 
e.coli. 

what about food allergies? you eat something, things are 
absorbed in villus, processed in submucosa, recognized by immune system, 
get local inflammation which causes afferent neuron to fire - remember, 
myenteric plexi are all connected. if you stimulate one in the proximal 
SI you cause stimulation of the whole mess of them. so you eat something 
you're allergic to, and you can have diarrhea 10min later because your 
enteric NS causes your colon to start secreting. same with salmonella and 
e.coli - the toxin doesn't have to getto the colon to cause the diarrhea 
- the enteric NS stimulation will do it.

sometimes colics reflux - they 
put a tube in, and fluid comes out. why? distension stimulates fluid 
secretion. if distension is inappropriate, fluid will continue to be 
secreted. to stop that you have to get rid of the distension.

horses 
with peritonitis have diarrhea - why? receptors on serosa synapse with 
myenteric plexus also. they cause the same secretory thing to happen. 
horses with peritonitis have low volume, liquid diarrhea - not quite the 
same as calves with e.coli, which is high volume, so there are 
modifications to the processes, but basically they use similar pathways.


if pathogens can do it,why can't we?

how do laxatives work?
sometimes,we 
want secretion - if we have impaction or something, or dry stuff in GI 
tract. one thing you can do, you can give mineral oil - this will go into 
lumen and act as an osmotic agent to hold water in lumen. if there isn't 
enough water in lumen, it will stimulate secretion. epsom salts too. they 
do this by changing osmolarity. other laxatives - hopefully no one here 
has used castor oil, but it is an old laxative - in the lumen, it 
irritates the mucosa and causes inflammation- if you take a small dose, 
you get inflammation that increases local prostaglandins and secretion 
via the mechanism previously described. in equine medicine we can use 
castor oil to reproduce a fatal enteritis, it's that irritating! 
another 
drug - DSS - dioctyl sodium sulfasuccinate. this was actually at first a 
wetting agent/industrial solvent, and someone got the idea to use it as a 
laxative. it gets into hard things and softens them. they gave it to 
people and animals and found that it does liquefy feces. but it won't 
liquefy a fecal mass. it dissolves tight junctions, and dissolves the 
basement membrane. when put into the lumen, some epi cells will come off, 
causing inflammation, and so forth, causing secretion as previously 
described. so a little DSS produces diarrhea. a lot of DSS is bad - 
overdose causes lots of inflammation and secretion, hypovolemia and 
death. 

enteroquinones - dantron - all those drugs stimulate 
prostaglandins and secretion. 

lots of laxatives take advantage of this 
system, firing afferent neurons.

most of our problems are stopping 
secretion, it's easy to start it but we need ways to stop it. some cells 
are secreting, some absorbing - we want a net result of absorption. one 
way to cause this, and this is used successfully in cholera (which has an 
e.coli like toxin) where you don't block this pathway, but you use the 
sodium substrate cotransport - one way to turn around a secretory 
diarrhea is to give the patient a fluid with enough AA, glucose, sodium, 
to cause absorption in excess of the secretion. this can rehydrate the 
patient despite the diarrhea.

other drugs that help turn off secretion - 
most successful drug probably loperamide. also called immodium. this is 
pretty good stuff.it's an opiate, stimulates opiate receptors, but almost 
80% of it stays in the GI tract, and it doesn't cross the BBB. you don't 
get addicted to it. it's reversible by naloxone, so an overdose can be 
corrected. almost all the effect is via opiate receptors. the drug 
concentrates in the villus and myenteric plexus. any that is absorbed 
goes through enterohepatic circulation. it appears to work in myenteric 
plexus, submucosal plexus, and maybe right near enterocyte. it will stop 
diarrhea produced by castor oil- probably by blocking effect at myenteric 
plexus. also blocks the effect of e.coli enterotoxins. it's fairly 
effective so if you're traveling you should try it. it is easier to pack 
than pepto bismol, which also works ok. it stops the local prostaglandin 
increase. it also prevents diarrhea caused by oral prostaglandin 
administration. it will also block diarrhea from bile salts, many of the 
diarrheas we see. loperamide not only blocks secretion, it also seems to 
increase absorption by affecting blood flow to the villus and blocking 
the effect on the myotubules so they don't contract. this makes 
loperamide very unique.

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one drug bovine practicioners 
found to be good for diarrhea was banamine. this worked for e.coli 
diarrhea. this drug is an NSAID that inhibits prostaglandins.
so NSAIDs 
like flunixin meglamine (banamine) can work, but the toxicity of these 
drugs increases w/dehydration so you have to be careful.

another drug 
that works is pepto-bismol or bismuth subsalicylate. this drug has been 
used since the turn of the century. for many years the manufacturer 
didn't care how or if it worked. then it was sold to proctor and gamble 
who studied it.
it is first of all bactericidal. it deposits on the 
bacteria and causes lysis. it will kill e.coli. another thing it does is 
to bind to the receptor sites and interfere with attachment of bacteria 
and toxin. definitely prevents toxin attachment. it also blocks - as a 
subsalicylate, with a bismuth substitution, it's an anti-prostaglandin 
drug - modifies prostaglandin levels. sometimes it inhibits them, 
sometimes it stimulates them, but it tends to normalize them back to a 
physiologic level. not clear how. before we knew about helicobacter 
pylori and ulcers, we knew that giving peptobismol to people with ulcers 
helped to heal them - this was due to bacterial lysis. sometimes the drug 
works miracles, sometimes not. but you don't want to keep using it after 
the diarrhea stops, may cause constipation. there may also be other 
effects of the drug,but we haven't finished studying it yet.

another 
group of drugs is A2 adrenergic agonists - the first one tried if you 
remember we heard that norepi will block the effect of ACH at the crypt 
cell level, and may block transmission in submucosal plexus. at crypt 
cell, seems to keep Ca++ channel closed. this seems to be A2 effect, and 
klonidine, an A2 agonist, will decrease diarrhea (and make you drowsy and 
depressed, so not great for animals). telazoline has fewer side effects 
but still can have serious side effects. the A2 receptors in gut are sl 
different from cardiovascular A2 receptors, so they may be able to get 
gut specific drugs in the future.they're working on it.

another drug 
that has shown promise is somatostatin, which we knwo causes inhibition 
at myenteric and submucosal plexus and at mucosal cell itself. there is a 
synthetic version of somatostatin to get around the problem of the short 
half life, this is made by sandoz corporation, called sandostatin. this 
drug will block diarrhea. people with VIP producing tumors get diarrhea - 
this drug will block it. it's expensive though. most of the diarrhea 
mechanisms we know about seem to go through the reflex that ends up 
releasing VIP. the fact that this drug works so well in patients that 
have lots of VIP indicates that this drug works at the mucosal cell where 
the VIP works.

calcium chnnel blockers may work at level of crypt cells 
and neurons. 
chloride blocking drugs, if you could block the Cl- channel 
on the secretory cell that would be great.

last drug to discuss is 
atropine - many people think of using this to treat GI problems...you 
need to realize that the same neurons which cause secretion also 
innervate the muscles in the GI tract and modify their action - one 
stimulus can cause motility changes along with secretion. atropine would 
work at the muscarinic site of ACH receptor on the secretory cell serosal 
side. most diarrheas aren't mediated through ACh, but by VIP. but 
atropine will prevent the muscle contraction, so you hve an animal 
secreting a lot of fluid now having an ileus. in horses, you give them a 
paralytic ileus they don't recover froma nd they die. it doesn't block 
the right pathways and adversely affects motility so you shouldn't use 
atropine for diarrhea. the only exception to this would be heat stable 
E.coli toxin diarrhea, which may work through ACH mechanism.

cases:

1. 
one day old TB colt with normal attended birth, stood w/in 30 min of 
foaling, nursed w/in 60 min, seemed normal until 14 hrs of age, when it 
strained with back arched and hindlegs together, indicating straining to 
defecate.  PE WNL except straining and some distress. what might cause 
this? meconium impaction might. persistent cloacal membrane or some kind 
of atresia (but this foal had originally passed some meconium so it 
couldn't be that). could be ruptured bladder, even though we think it's 
straining to defecate, it could be confused; it could be intussusception, 
volvulus, ulcers, hypoxic ischemic encephalopathy (maladjusted foal 
syndrome). they did a digital rectal exam and felt a hard piece of 
meconium stuck in there. that's what this was. so, they used a soapy 
water enema - ivory soap and water per rectum. this will dilate the 
rectum, stimulating stretch receptors, increasing motility and local 
secretion. also, the soap is a bit irritating, and will cause 
prostaglandin production and secretion. soap may also provide 
lubrication. if this doesn't work, some people will try a DSS enema. this 
is more of an irritant. some people will also use mineral oil enema - 
this acts as an osmotic agent and lubricant. also some people will use a 
lubricant enema, which is very gentle and nonirritating. another possible 
enema solution is glycerin - this will cause some local irritation, act 
as an osmotic draw, and cause secretion. 
mother nature deals with this 
problem with colostrum, which has many laxative properties - you can give 
colostrum orally as a laxative. these are the ways we usually try to deal 
with this problem. most of them resolve w/in a few hours. this foal they 
used the soapy water and then lubrication enemas, but 10 hrs later foal 
was still impacted. this isn't a good way of starting life. these foals 
don't nurse well because they hurt. so, they start attacking orally -give 
mineral oil and liquid. or milk of magnesia is also commonly used - Mg 
sulfate, a fairly gentle osmotic agent. if these don't work, and you're 
out to 48 hrs, practicioners sometimes use castor oil, although it's not 
recommended. it will cause GI irritation and active secretion.  "green 
bomb" formula - 2 oz castor oil, 8 oz mineral oil, 4 oz milk of magnesia, 
and some mare's milk. sure, this may get rid of impaction, but now you've 
irritated GI tract so much that these foals can get septic. this is his 
opinion, mind you. another approach to impaction like this is using 
mucomyst/acetylcysteine in a "retention enema" which involves using a 
foley catheter with a balloon at the end, put it into the rectum, blow up 
the balloon (in the pelvis, to avoid perforation), enough so they can't 
squeeze past it. then put in 4% acetylcysteine solution, and leave it in 
for 20 minutes. if you do this, then remove the catheter, this often 
works, we're not sure why. just like the DSS, people may say that the 
disulfide bonds in meconium are broken by the acetylcysteine - but this 
doesn't happen in vitro, so that's unlikely. it seems that it makes the 
outside of the meconium slippery though. maybe it partly works because of 
activation of stretch receptors via foley catheter - but it doesn't work 
as well w/o using the acetylcysteine, so.... one good reason to have them 
retain it 20 min is that it costs about 60 bucks, so you want to make 
sure it works.

2. 2 mo old standardbred filly, came in w/hx of diarrhea 
from 3 wks of age, persisting 5 wks. physiologic diarrhea is pretty much 
ruled out based on age. foal treated with fenbendazole, antibiotics 
(TMP-sulfa) x 2 wks which didn't help. ddx: salmonellosis,other 
infectious disease, parasite eg small strongyles, nutritional (more 
likely), milk intolerance (rare), overproduction of milk by mare, flora 
imbalance (probiotics don't often work, though)
loperamide treatment 
resolved the problem w/in 24 hrs. the problem didn't come back after 
stopping the drug, either. loperamide is a good thing to try in foals of 
this age, because sometimes it will stop the diarrhea for good. another 
drug to try is bismuth subsalicylate/pepto bismol. 

3. calf - 5 day old 
holstein from 40 cow herd that's had some diarrhea problems before. calf 
was on cow x 48 hrs then in calf hutch.during first 2 days in hutch was 
ok. diarrhea began on day 4 of life. PE: could walk around but didn't 
want to get up. dry mms, poor skin turgor, watery &white diarrhea. temp 
102.8. white diarrhea is associated w/e.coli.this was a mix of e.coli and 
rotavirus. 
tx: ?
pepto bismol. kills e.coli, blocks toxin attachment. 
doesn't help w/rotavirus. of course, we haven't gotten the labwork back 
yet so we don't know that. calf needs fluids, too. she's not walking 
around so she's unlikely to drink on her own. they gave her some calf 
electrolyte mixture per os. also milk causes more problems too due to 
electrolyte imbalance, so you have to stop milk temporarily. also, you 
could use some banamine in this calf to stop the prostaglandin part of 
the problem. some people use a dose of oral antibiotics, too, to try 
killing e.coli, although that might enhance resistance. by day 3 this 
calf was better. common practice when starting to refeed is to give 1/2 
electrolytes, 1/2 milk at first. if you keep calf on full milk and let 
them have the diarrhea, they gain more weight and recover at the same 
time, so you won't have a drop in weight.but, if you don't stop the 
diarrhea, farmers get upset. now, if there's a diarrhea problem on the 
farm, the farmer might try to reduce milk and feed more electrolyte 
solution, so calves won't grow well. if you really restrict the protein 
they get, the enterocytes don't work well,causing more diarrhea, and then 
if you stop feeding them it's worse, so you really have to be careful 
when you cut back nutrition in these young calves. what about the 
rotavirus? you can't do much. you might think there's no sense with 
orally rehydrating calves with rotavirus because it kills the 
enterocytes- but rotavirus doesn't kill ALL the enterocytes. only some. 
so you can get oral rehydration. but you also get malabsorption. so the 
more you give orally, the more diarrhea you see. now, is diarrhea bad, or 
is dehydration and wt loss bad? think about that one. 

4. adult 8 yr old 
brood mare, 8 mos in foal. came in after 4 days of colic. kicking, 
rolling. responded well to banamine (NSAID), but still didn't eat, and 
played with the water a lot - she'd put her nose in it and splash it 
around, blow bubbles. when she came in she had HR 60 (high), RR 50 (high) 
and normal temp. mms pink, normal crt. abdomen moderately distended (8 
mos pregnant,though). GI sounds - quiet borborygmi. got loud when she 
showed signs of pain. rectal exam showed large mass on left side of 
abdomen - felt firm - impacted pelvic flexure. so, they had to make sure 
she didn't eat, first of all, but they want her to drink. they could give 
mineral oil, to keep fluid in lumen of gut and  lubricate impacted mass. 
giving anything orally will cause reflex motility, because distended 
stretched stomach will tell the bowel to contract. another thing often 
used is banamine for analgesia - which may stop some secretion, but when 
used to treat pain, you get better gut motility than if you used a 
different pain med that blocked motility. so it's a bit of a trade off. 
another drug used is DSS - but has to be used at right dose, 8-16 oz. 
you;'d use 1-2 gallons of mineral oil. another drug often used for these 
impactions is Mg sulfate, acts as osmotic draw, pulls fluid into GI 
tract. part of the problem with the impaction is that there is less fluid 
in GI tract because horse isn't drinking well, or is losing water. colon 
is often considered a water reserve. but when it gets dehydrated, you 
need to get water to it. you can put animal on high rate of IV fluids, 
which will rehydrate the impaction faster. but what about the fetus? in 
human medicine, they'd never give an NSAID to a pregnant woman like this. 
we do it without seeing problems. maybe in equine placentation these 
drugs aren't well absorbed. the other things are ok. 


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