Physiological and molecular characterization of urea transport by the gills of the Lake Magadi tilapia (Alcolapia grahami)

The Lake Magadi tilapia (Alcolapia grahami) is an unusual fish, excreting a ll its nitrogenous waste as urea because of its highly alkaline and buffere d aquatic habitat. Here, using both physiological and molecular studies, we describe the mechanism of branchial urea excretion in this species. In viv o, repeated short-interval sampling revealed that urea excretion is continu ous, The computed urea permeability of A. grahami gill is 4.74x10(-5)+/-0.3 8x10(-5) cm s(-1) (mean +/- S.E.M., N=11), some 10 times higher than passiv e permeability through a lipid bilayer and some five times higher than that of even the most urea-permeable teleosts studied to date (e.g. the gulf to adfish), Transport of urea was bidirectional, as demonstrated by experiment s in which external [urea] was elevated, Furthermore, urea transport was in hibited by classic inhibitors of mammalian and piscine urea transporters in the order thiourea>N-methylureal>acetamide. A 1700 base pair cDNA for a pu tative Magadi tilapia urea transporter (mtUT) was cloned, sequenced and fou nd to display high homology with urea transporters from mammals, amphibians and other fishes, When cRNA transcribed from mtUT cDNA was injected into X enopus laevis oocytes, phloretin-inhibitable urea uptake was enhanced 3.4-f old relative to water-injected controls, Northern analysis of gill, red blo od cells, liver, muscle and brain using a portion of mtUT as a probe reveal ed that gill is the only tissue in which mtUT RNA is expressed, Magadi tila pia gill pavement cells exhibited a trafficking of dense-cored vesicles bet ween the well-developed Golgi cisternae and the apical membrane. The absenc e of this trafficking and the poor development of the Golgi system in a non -ureotelic relative (Oreochromis niloticus) suggest that vesicle traffickin g could be related to urea excretion in Alcolapia grahami, Taken together, the above findings suggest that the gills of this alkaline-lake-adapted spe cies excrete urea constitutively via the specific facilitated urea transpor ter mtUT.