Comment on: cloning and characterization of porcine aquaporin 1 water channel expressed extensively in the gastrointestinal system.

antigens in erythrocytes and subsequently cloned from kidney complementary DNA libraries and shown to possess water transporting activity. Although AQP1 has been found to be important in osmotic water movement across cell membranes of many epithelial and endothelial barriers, its expression in the gastrointestinal system is exclusively limited to microvascular endothelia (Figure 1) and other aquaporin isoforms are known to be expressed in the epithelial cell lining. These aquaporins include AQP3, AQP4, AQP5, AQP8, AQP9 and AQP10. It is therefore clearly established that multiple aquaporin isoforms are present along the gastrointestinal system; some of these are classical aquaporins (i.e., AQP1, AQP4, AQP5, AQP8) and are probably mainly involved in water homeostasis, whereas others are aquaglyceroporins (i.e., AQP3, AQP7, AQP9, AQP10) and are potentially involved in the facilitated movement of small uncharged organic molecules, such as glycerolas well as water. In the fluid-transporting epithelia of the kidney nephron, AQP1 is permeated by water, driven by osmotic gradients and AQP1 is abundant in the apical and basolateral membranes of renal proximal tubules and descending thin limbs and plays a key role in setting up and maintaining the countercurrent multiplication system. Although AQP1 is known to be present in a number of extrarenal tissues, such as the ciliary body of the eye and the choroid plexus in the brain, it has not been reported in normal epithelial cells lining the gastrointestinal system. The presence of AQP1 in gastrointestinal epithelial cells has only been reported in tumors of the colon, where it has been reported to contribute to tumor angiogenesis and the formation of high interstitial fluid pressures and high vascular permeability of tumor microvessels. Jin et al conclude that porcine AQP1 (pAQP1) is the first porcine aquaporin to be identified by means of molecular biology techniques. This is a valid and correct statement. However, the functional data they provide is from transfected CHO cells and red blood cells. The functional data presented by Jin et al may not be used to support the statement that pAQP1 plays a key role in fluid transport in epithelial and endothelial structures of the pig gut. Furthermore, the immunohistochemical and Northern blot data presented by the authors do not prove that the observed differences in pAQP1 mRNA and protein abundance in porcine liver, small intestine, colon and salivary glands are due to differential and LETTERS TO THE EDITOR