Nitrogenous waste excretion by the larvae of a phylogenetically ancient vertebrate: the sea lamprey (Petromyzon marinus)

Larval sea lampreys (Petromyzon marinus) (ammaocoetes) excreted significant quantities of urea, which composed 15-20% of the total nitrogenous waste e xcreted. Compared with teleosts of similar size, ammonia and urea excretion rates (J(Amm) and J(Urea), respectively) in ammocoetes were relatively low , reflecting the low metabolic rate of these burrow-dwelling suspension fee ders. Analyses bf:liver enzymes indicated that ammocoetes had all the enzym es necessary to produce urea via uricolysis, but not those of the ornithine -urea cycle (OUC). Further, exposure to 2 mmol.L-1 total ammonia for 5 d wa s accompanied by a 3-fold elevation of J(Urea) but did not lead to greater OUC activity. Internal ammonia levels increased markedly, however, exceedin g 2000 mu mol.L-1 in plasma and 5000 mu mol.L-1 in muscle after the 5-d exp osure period. This high resistance to internal ammonia accumulation was rel ated to the very high glutamine synthetase activities measured in ammocoete brains. The excretion and production of urea by ammocoetes demonstrates fo r the first time that agnathans are capable of producing physiologically re levant amounts of urea. Given the ancient origins and conserved evolution o f lampreys, these observations also suggest that at least some of the early jawless vertebrates were able to produce and excrete urea.