RODELA, T.M.*; BALLANTYNE, J.S.; WRIGHT, P.A.; University of Guelph; University of Guelph; University of Guelph: Mitochondrial Urea Transporters: A Teleost versus Elasmobranch Comparison

Urea is an important organic osmolyte in elasmobranches. In teleosts, urea synthesis may be a valuable strategy to detoxify ammonia or urea may be simply the end product of arginine catabolism. Urea is produced via the hepatic ornithine-urea cycle (OUC) within the mitochondria. Rapid equilibration of urea across the mitochondrial membrane would be vital for cell osmoregulation. Hence, the presence of a urea transporter (UT) may be important to facilitate the exit of urea into the cytosol. To test the hypothesis that UTs exist in mitochondrial membranes, hepatic mitochondria were isolated from rainbow trout (Oncorhynchus mykiss) and little skates (Raja erinacea) by differential centrifugation. Characterization of urea transport was performed by exposing the mitochondria to various external urea solutions containing ¹⁴C-urea and measuring influx by a rapid filtration technique. In both skates and trout, urea transport was saturable at low concentrations with a diffusion component at higher urea concentrations. Further analysis of urea uptake of the skate, revealed the presence of a phloretin-sensitive (IC₅₀=0.25 mM), energy-dependent urea transporter (K_m=0.71). Similar analysis in the rainbow trout demonstrated that urea transport occurs by a phloretin-sensitive (IC₅₀=0.01 mM), energy-independent urea transporter (K_m=0.46 mM). These findings provide the first evidence for the presence of UTs in mitochondria from any organism. Differences in the kinetic properties of the skate and trout UTs may relate to their different osmoregulatory strategies.