UREA TRANSPORT BY HEPATOCYTES AND RED-BLOOD-CELLS OF SELECTED ELASMOBRANCH AND TELEOST FISHES

Although urea transport is receiving increased attention in mammalian systems, very little is known about urea transport in fish tissues. Th is study examined mechanisms of urea transport in red blood cells and hepatocytes from the lesser spotted dogfish (Scyliorhinus canicula), A tlantic stingray (Dasyatis sabina), turbot (Scopthalmus maximus), redf ish (Scianops ocellatus), gulf toadfish (Opsanus beta) and oyster toad fish (Opsanus tau). Urea appeared to be passively distributed in both tissues (i.e. there was no difference between plasma and tissue urea c oncentrations). Additionally, a number of in vitro experiments examini ng [C-14]urea flux were performed. In red blood cells from all species except redfish, urea transport occurred via simple passive diffusion, but redfish red blood cells showed a small (25%) phloretin-sensitive uptake component. In hepatocytes of the two elasmobranch species (dogf ish and stingray), urea efflux was also by simple passive diffusion. H owever, urea efflux in toadfish (both O. beta and O. tau) hepatocytes exhibited a marked phloretin-sensitivity, and O. beta hepatocytes were used in further experiments with other inhibitors and treatments. Ure a transport in O. beta had a relatively high specificity for urea comp ared with the urea analogues acetamide, thiourea and N-methylurea, was unaffected by phloridzin and extracellular Na+ removal, and was not i nhibited by physiological levels of glucose (0.5-10 mmoll(-1)). A phlo retin-sensitive glucose transport, that was not inhibited by physiolog ical levels of urea, was discovered in O. beta hepatocytes. The result s are discussed in terms of patterns of species distribution and simil arities between urea and glucose transport.