UREA PRODUCTION AND TRANSPORT IN TELEOST FISHES

Teleosts appear ta have retained the genes for the urea cycle enzymes. A few species express the full complement of enzymes and are ureoteli c (e.g., Lake Magadi tilapia) or ammoniotelic (e.g., largemouth bass), whereas most species have low or non-detectable enzyme activities in liver tissue and excrete little urea (e.g., adult rainbow trout). It w as surprising, therefore, to find the expression of four urea cycle en zymes during early life stages of rainbow trout. The urea cycle may pl ay a role in ammonia detoxification during a critical time of developm ent. Exposure ro alkaline water (pH 9.0-9.5) or NH4Cl (0.2 mmol/l) inc reased urea excretion by several-fold in trout embryos, free embryos a nd alevin. Urea transport is either by passive simple diffusion or via carried-mediated transport proteins. Molecular studies have revealed that a specialised urea transport protein is present in kidney tissue of elasmobranchs, similar to the facilitated urea transporter found in the mammalian inner medulla of the kidney. COMP BIOCHEM PHYSIOL 119A; 1:47-54, 1998. (C) 1998 Elsevier Science Inc.