SUMMARY
Marine elasmobranchs retain urea and other osmolytes, e.g. trimethylamine oxide (TMAO), to counterbalance the osmotic pressure of seawater. We investigated whether a renal urea transporter(s) would be regulated in response to dilution of the external environment. A 779 bp cDNA for a putative skate kidney urea transporter (SkUT) was cloned, sequenced and found to display relatively high identity with facilitated urea transporters from other vertebrates. Northern analysis using SkUT as a probe revealed three signals in the kidney at 3.1, 2.8 and 1.6 kb. Upon exposure to 50% seawater, the levels of all three SkUT transcripts were significantly diminished in the kidney (by 1.8- to 3.5-fold). In response to environmental dilution, renal tissue osmolality and urea concentration decreased, whereas water content increased. There were no significant differences in osmolyte and mRNA levels between the dorsal–lateral bundle and ventral sections of the kidney. Taken together, these findings provide evidence that the downregulation of SkUT may play a key role in lowering tissue urea levels in response to external osmolality.
