Drugs Acting on the Intestines
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Drugs Increasing Smooth Muscle Contractility
These drugs are used for conditions that are associated with hypomotility, such as megacolon, oesophageal reflux, atony and ileus.
Drugs Acting on 5-HT4 Receptors
An examples is metoclopramide that is an agonist at serotonin and dopamine receptors, this results in an increased release of acetylcholine and so increases gut motility.
Muscarinic Receptor Agonists
Examples are carbachol and bethanecol. They should be used with slight caution as their resultant contractions can be painful.
Laxatives and Purgatives
These drugs should mainly be used to treat constipation, but bulk luxatives can be used to help control diarrhoea.
Lubricant Laxatives
An example is liquid paraffin that is given orally. They supposedly coat and lubricate the contents of the gastrointestinal tract and help soften the faeces, thus making them easier to pass. The use of liquid paraffin is under debated between some clinicians as some feel that it works just as well as water, but has the added risk of causing a very serious pneumonia if the paraffin enters the trachea. On the other hand most equine vets in practice feel that it is a useful tool to help resolve impaction colics.
Bulk Laxatives
Examples are bran, ispaghula and stericula, and they are administered orally. They work by absorbing water and so swell, this stimulates stretch receptors in th gastrointestinal tract and waves of peristalsis are stimulated. They are usedful in controlling diarrhoea because of their absorbant properties.
NB Never use in dehydrated animals as animals will loose water when these laxatives are in use. Please remember to offer the animal plenty of drinking water.
Osmotic Laxatives
An example is lactulose, which is given orally. It works by drawing water into the intestinal lumen, this stretches the intestinal walls resulting in increased peristaltic waves.
NB Never use in dehydrated animals as animals will loose water when these laxatives are in use. Please remember to offer the animal plenty of drinking water.
Stimulant Laxatives
An example is bisacodyl and is given orally. This works by having an irritant effect on the intestinal mucosa. This stimulates enteric nerves, resulting in increased peristalsis and increased water and electrolyte secretion into the lumen.
Enemas
This is where warm water and mineral oil or sodium citrate are administered into the rectum and lower bowels of an animal. This helps evacuate the bowels of an animal before radiographs or surgery.
Drugs Decreasing Smooth Muscle Contractility
These drugs are used to treat conditions associated with smooth muscle spasms or diarrhoea.
Muscarinic Receptor Antagonist
Examples are hyoscine, atropine and propantheline. Buscopan is a preparation commonly used in large animal medicine that contains hyoscine and the analgesic, dipyrone. They are all administered by injection.
They work by competitively inhibiting the effect of acetylcholine on the intestinal smooth muscle muscarinic receptors. This results in decreased motility and intestinal mucus secretions. It has no effect on fluid secretions. It has a lack of selectivity and so any side effects experienced are due to it's actions on muscarinic receptors at other sites around the body.
It is used to help reduce smooth muscular spasms in the gastrointestinal tract.
Opioid Receptor Agonists
Examples are loperamide, diphenoxylate and codeine, which are administered orally.
They work by mimicking the action of endogenous endorphins and enkephalins. This results in an increase of transit time of ingesta through the gastrointestinal tract. This allows more time for absorption of fluid; combining this action with it's ability to decrease intestinal fluid secretions it is often a good treatment for diarrhoea.
Loperamide works for prolonged periods on the gastrointestinal tract, as it undergoes enterohepatic recycling. Though it is poorly absorbed and won't cross the blood-brain barrier and so will have no CNS side-effects.
Diphenoxylate is better absorbed than loperamide but doesn't have any effects on the CNS.
Care should be used when given to cats as they cause hyperexcitability on overdose.
Other Drugs
Adsorbents
Examples are bismuth salts, kaolin and pectin, which are all given orally. They are used to treat diarrhoea and work by coating the mucus membrane and absorb any chemicals or poisons and so hopefully protect the intestinal mucosa from damage.
Other Diarrhoea Treatments
Sulphasalasine an active 5-amino salicylate and glucocorticoids sometimes used in some chronic cases of diarrhoea.
Pancreatin
This is given to dogs with exocrine pancreatic insuffiency. Most preparations of it contain enyzmes with protease, lipase and amylase activity to help with the digestion of protein, fat and starch.