Osmotically active laxatives

Osmotically active laxatives (C) are soluble but nonabsorbable particles that retain water in the bowel by virtue of their osmotic action. The osmotic pressure (particle concentration) of bowel contents always corresponds to that of the extracellular space. The intestinal mucosa is unable to maintain a higher or lower osmotic pressure of the luminal contents. Therefore, absorption of molecules (e.g., glucose, NaQ) occurs isoosmotically, i.e., solute molecules are followed by a corresponding amount of water. Conversely, water remains in the bowel when molecules cannot be absorbed. 

Comparative studies In a randomized study, patients took Pico-Salax (a small-volume, osmotically active laxative containing sodium picosulfate lOmg -I- magnesium... 

Lactulose is the foundation of pharmacologic therapy to prevent and treat hepatic encephalopathy. It is a non-digestible synthetic disaccharide laxative that is hydrolyzed in the gut to an osmotically-active compound that draws water into the colon and stimulates defecation. Lactulose also lowers colonic pH, which favors the conversion of ammonia (NH3) to ammonium (NHf).48 Ammonium is ionic and cannot cross back into systemic circulation it is eliminated in the feces. Lactulose is usually initiated at 15 to 30 mL two to three times per day and titrated to a therapeutic goal of two to four soft bowel movements daily.20 49 50... 

If drugs are used, the first choice should be a bulk laxative, e.g. isphagula or sterculia gum. Nonabsorbent carbohydrates with osmotic activity also work well but often cause flatulence. Salinic laxatives like polyethylene glycol and magnesium oxide are very effective but often cause stomach upsets like flatulence and abdominal pain. Tegaserod, a 5-HT4 partial agonist, appears to improve the frequency of bowel movements in those with chronic constipation (see Evans et ah, 2007). 

The cathartic effect of sulfate is mainly due to the osmotic activity of unabsorbed sulfate salts in the intestine. The laxative effect that results from sulfate is an osmotic diarrhea. Whether or not this laxative effect occurs depends on the amount of sulfate and other osmotically active materials that are present in the intestines these materials include magnesium, sodium, and some sugars. 

A US EPA health-based advisory for acute effects (absence of laxative effects) of 500 mg of sulfate per liter is recommended. In situations, where the water contains high concentrations of total dissolved solids and/or other osmotically active ions, laxative-like effects may occur if mixed with concentrated infant formula or powdered nutritional supplement therefore, an alternate low-mineral-content water source is advised. Infants are more susceptible to diarrhea water loss than adults because of differences in gastrointestinal structure and function. 

Apart from specific antidotes (if they exist), the treatment of poisonings also calls for symptomatic measures (control of blood pressure and blood electrolytes monitoring of cardiac and respiratory function prevention of toxin absorption by activated charcoal). An important step is early emptying of the stomach by gastric lavage and, if necessary, administration of an osmotic laxative. Use of emetics (saturated NaCl solution, ipecac syrup, apomorphine s.c.) is inadvisable. 

Several quite different mechanisms can lead to diarrhea. In carbohydrate malabsorption, the presence of unabsorbed solutes in the bowel causes an osmotic diarrhea as water enters the bowel from the tissue. By contrast, the diarrhea of most laxative abuse and in VIPomas is due to active secretion of water and electrolytes into the bowel, which is described as secretory diarrhea. Inflammatory bowel diseases (ulcerative colitis and Crohn s disease) cause diarrhea as a consequence of the inflammatory process with loss of fluid into the bowel. 

For rectal administration, the osmotic value may vary within wide limits. An osmotic value corresponding to a 0-1.8 % sodium chloride solution is tolerated. Adjusting the iso-osmotic value of an enema, by addition of, for example, sodium chloride, offers no advantage regarding activity and irritation, and can be omitted. StrrMig hyperosmotic solutions must be avoided in enemas, except for those intended as laxative. A strrMigly hyperosmotic solution, for example a phosphate enema, induces a defecation reflex. The osmotic value of this enema is about seven times higher than that of a normal saline solution. 

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