[Basics of intestinal electrolyte transport (author's transl)]

In the small and large intestine Na and Cl are absorbed via active transport systems, whereas K appears to distribute passively between the lumen and the blood side of the epithelium. Three types of Na influx mechanisms across the luminal cell membrane of the epithelium may be distinguished: 1. In the gall bladder and the small intestine Na and Cl influx are coupled in an obligatory one for one fashion, hence this influx mechanism is electrically neutral. This coupled NaCl uptake process in the luminal cell membrane is inhibited by a rise in intracellular cAMP levels. 2. In the colon Na influx is not dependent on Cl influx. Due to the partition of cations and anions the Na uptake process is electrogenic, i.e. the electrical potential difference across the luminal cell membrane is decreased. This type of Na influx is blocked by the diuretic amiloride and enhanced by aldosterone. The electrogenic Na influx mechanism is also present in the ileum together with the neutral NaCl influx mechanism. Active Cl absorption in the colon is electrically neutral, most likely Cl is exchanged for HCO3 at the luminal membrane. Stimuli which increase cellular cAMP or Ca cause electrogenic Cl secretion. 3. In the ileum exists an additional Na influx mechanism which is dependent on the presence of certain sugars (glucose) or amino acids (alanine) on the luminal side of the epithelium. This process is inhibited by phlorizin. The finding of glucose-stimulated Na absorption has proven to be therapeutically useful: the rate of intestinal fluid loss can be decreased in certain forms of diarrhea by oral administration of electrolyte solutions containing glucose.