Fig. 2. Involvement of AQPs in epithelial fluid transport. (A) Reduced trans-epithelial water permeability in the kidney collecting duct impairs urinary concentrating ability by preventing osmotic equilibration of luminal fluid. (B) Reduced water permeability in glandular epithelium impairs active, near-isosmolar fluid transport by slowing osmotic water transport into the acinar lumen, producing hypertonic secretion.