Novel bUT-B2 urea transporter isoform is constitutively activated

Tickle P, Thistlethwaite A, Smith CP, Stewart GS. Novel bUT-B2 urea transporter isoform is constitutively activated. Am J Physiol Regul Integr Comp Physiol 297: R323-R329, 2009. First published May 27, 2009; doi:10.1152/ajpregu.00199.2009.-Our previous studies have detailed a novel facilitative UT-B urea transporter isoform, bUT-B2. Despite the existence of mouse and human orthologs, the functional characteristics of UT-B2 remain undefined. In this report, we produced a stable MDCK cell line that expressed bUT-B2 protein and investigated the transepithelial urea flux across cultured cell monolayers. We observed a large basal urea flux that was significantly reduced by known inhibitors of facilitative urea transporters; 1,3 dimethylurea (P<0.001, n=17), thionicotinamide (P<0.05, n=11), and phloretin (P<0.05, n=9). Pre-exposure for 1 h to the antidiuretic hormone vasopressin had no effect on bUT-B2-mediated urea transport (NS, n=3). Acute vasopressin exposure for up to 30 min also failed to elicit any transient response (NS, n=9). Further investigation confirmed that bUT-B2 function was not affected by alteration of intracellular cAMP (NS, n=4), intracellular calcium (NS, n=3), or protein kinase activity (NS, n=4). Finally, immunoblot data suggested a possible role for glycosylation in regulating bUT-B2 function. In conclusion, this study showed that bUT-B2-mediated transepithelial urea transport was constitutively activated and unaffected by known regulators of renal UT-A urea transporters.