Ammonia transport mediated by urea transporter A isoforms

Urea transporters (UTs) are a family of urea-selective channel proteins that play an essential role in the urine-concentrating mechanism of the mammalian kidney. In addition to urea, UT-A2 and UT-A3 - the N- and C-terminal regions of full-length UT-A1, respectively - and UT-B transport water, and human UT-B transports water and ammonia (NH3). However, UT-A-mediated NH3 transport has not been evaluated. Given that regulated renal NH3/NH4+ transport by renal epithelial cells is essential to acid-base homeostasis and considering UT-A2 and UT-A3 localization in the inner medulla, where the transport of urea, water, and NH3 is important, it is plausible that UT-A-mediated NH3 transport could be physiologically relevant. The present study characterized the urea, water, and NH3 transport properties and solute pathways of murine UT-A2, UT-A3, and UT-B heterologously expressed in Lithobates catesbeianus oocytes. Control and UT-expressing oocytes were evaluated for surface protein expression through lysine-biotinylation and immunoblotting. Urea uptake was measured using radiolabeled urea, water permeability was assessed using video microscopy, and NH3 transport was monitored using a surface pH microelectrode. All UT-encoding cRNAs were translated, glycosylated, and inserted into the oocyte membrane. Wild-type UT-expressing oocytes displayed significantly higher urea, water, and NH3 transport than day-matched water-injected control cells. Pre-treating the oocytes with phloretin or mutating the urea pore threonines (Thr177 and Thr339 human UT-B numbering) to valines (Val) attenuated UT-mediated urea, water and NH3 transport to control oocyte values. Our study showed for the first time that UT-A2 and UT-A3 increase the membrane NH3 permeability. Thus, besides the critical role of UTs in urinary concentration, these proteins may also impact acid-base homeostasis and contribute to other processes associated with health and disease.