Molecular Physiology of Water Balance.

To the Editor: In their review article on water balance, Knepper et al. (April 2 issue)1 discuss water channels (aquaporins) in renal tubular cells. They omit mention of the critical role played by aquaporin-1 in microvascular endothelia. In the renal microvasculature, endothelial aquaporin-1 mediates the osmotic water efflux across descending vasa recta and is required for regulation of medullary blood flow, maintenance of the medullary interstitial gradient, and urinary concentrating ability.2 Aquaporin-1 is also highly expressed in the endothelium lining peritoneal capillaries, where it facilitates osmotically driven water transport during peritoneal dialysis. Peritoneal dialysis is a technique of renal-replacement therapy that is increasingly used worldwide to restore homeostasis, including water balance, in patients with end-stage renal disease.3 Studies in experimental models have shown that aquaporin-1 mediates up to 50% of ultrafiltration during peritoneal dialysis with crystalloid (glucose-based) solutions.4 In these models, pharmacologic regulation of aquaporin-1, either through induction with glucocorticoids4 or gating with furosemide-based compounds,5 enhances aquaporin-1–mediated water transport across the peritoneal membrane. These data show the important role of endothelial water channels in water homeostasis.