Physiological relevance of aquaporins: luxury or necessity?

Aquaporins are members of a large family of pore-forming intrinsic membrane proteins, the MIP family. Based on their permeability properties they are now further subdivided into aquaporins, with real water-selective pores, an d aquaglyceroporins with slightly less selective pores. Aquaporins are expr essed in a large variety of tissues throughout the body but in most situati ons it is not clear whether their presence is necessary for the proper phys iological function of these tissues. This review focuses on recent insight into the physiological relevance of aquaporins gained from studying aquapor in knockout mouse models and from diseases, on new surprising findings rela ted to gating and selectivity, and on the consequences of tetramerization f or routing and the genetics of nephrogenic diabetes insipidus. The active f luid transport in proximal tubules and in salivary glands is seriously comp romised by aquaporin deletion. This is in contrast to lung, airways and sto mach, where active fluid transport proceeds unhindered in the face of great ly reduced water permeabilities due to aquaporin deletion. Therefore, aquap orins seem to be a necessity at extreme high rates of active fluid transpor t but appear to be more of a luxury at medium or low fluid transport rates.