Pharmacology of intestines
There are four types of drugs which are used in constipation. They are also called purgative, laxative, aperient, or evacuants. They all act mainly on the colon.
These are little absorbed and encourage osmosis in the colon (which physiologically allows absorption) with increase in bulk and reduction in viscosity in order to produce a fluid stool. Agents include inorganic salts such as magnesium hydroxide and lactulose, which is a synthetic dissacharide
Fecal softeners (emollients)
These are gentle softeners which disallow hard feces and make the feces soft and are useful in conditions like fistula-in-ano or fissure-in-ano in which hard fecal matter may have caused the tear in the first place. They include docusate sodium and liquid parafiin.
These drugs stimulate gut motility and peristalsis and may cause abdominal cramps. They are contraindicated in pregnancy and intestinal obstruction. They include bisacodyl, sodium picosulphate, gluycerol senna, and danthron. it also include anthraquinone group such as senna, danthron, cascara, aloes and rhiubarb.
Stool bulking agents
These are fibers which absorb water in the large intestine and are insoluble and therefore not digestible by enzymes in the small intestine. They absorb water and therefore increase volume at the same time as reducing viscosity of intestinal content. They produce soft and bulky stool and encourage normal reflexes in the bowel. They include bran, ispaghula, methylcellulose and sterculia.
7-8 liters of fluid are secreted into intestine daily. Absorption of sodium in the intestine is influenced by the following phenomena
Sodium glucose coupled entry: glucose in the small intestine stimulate the absorption of sodium and water follows together with chloride ions in the solvent drag phenomenon.
Sodium ion coupled entry: Na+ and Cl- enter the epithelial cells either alone or as double exchange with H+ or Cl- Na- with H+ or Cl- with OH- or HCO3-. Hence oral rehydration solutions should contain sodium, chloride and bicarbonate.
Treatment of diarrhea:
Oral rehydration therapy This contains the oral rehydration salts as follows: sodium chloride, potassium chloride, sodium citrate and anhydrous glucose useful in diarrhea and dehydration .
Antimotility drugs: They act on gut muscles to delay motility in order to allow more time for absorption. In gut infections they may not be necessary. You have codeine, diphenoxylate and loperamide
Increase in viscosity of gut drugs: Kaolin and ispaghula
Drugs used in inflammatory bowel diseases (ulcerative colitis): Aminosalicylates such as sulfasalazine, mesalazine