Medical Physiology 2005

Epithelial Transport Problem Set

Cholera is caused by uncontrolled secretion of salts into the intestines, with concomitant osmotic water secretion, leading to massive diarrhea. The primary cause of the disorder is active transport of Cl^- via chloride channels in the apical (luminal) membrane of intestinal secretory cells.

1. Explain why epithelial active transport of Cl^- leads to the concomitant secretion of large quantities of salt-containing fluid? The level of NaCl secreted exceeds that of KCl. Why?

Oral administration of water or salt-containing solutions is ineffective in treating cholera. However, oral administration of World Health Organization Oral Rehydration Solution (WHO-ORS) is effective in rehydrating cholera patients. The composition of WHO-ORS is as follows (in mM): Na⁺ 90, K⁺ 20, Cl^- 80, and citrate⁼ 20 and glucose 111.

2. Why is WHO-ORS effective in rehydrating the patients, while other exclusively salt-containing solutions not effective (Hint: the citrate has nothing to do with the answer)?

3. In severe cases of cholera, foods (notably, carbohydrates and proteins) should not be given concomitantly with WHO-ORS; the foods should be avoided during the first 4 hours of therapy until the patient regains much of the initial fluid and salt loss. Can you come up with a plausible reason why one should avoid solid foods during the initial stages of therapy? (Hint: consider the process of digestion, where large macromolecules [proteins, starches] are broken down to smaller molecules [amino acids, monosaccharides] within the intestinal lumen prior to absorption.)