Nitrogen Metabolism and Excretion in Bullfrog (Rana catesbelana) Tadpoles and Adults Exposed to Elevated Environmental Ammonia Levels

Abstract

The pattern of nitrogen excretion during different stages of anuran development has been well documented, but, unlike for teleost fish, very little is known about how water quality influences nitrogen metabolism and excretion. The purpose of this study was to determine the effects of elevated environmental ammonia concentrations on ammonia and urea excretion and metabolism in premetamorphic and adult bullfrogs (Rana catesbeiana). Tadpoles were exposed to 1 mmol/L NH₄Cl for 48 h or 7 d, whereas adult bullfrogs were exposed to 1 and 5 mmol/L NH₄CI (at waterpH 8. 1 or 9.2) for 48 h. In both tadpoles and adult frogs, there was an initial period, lasting 24-48 h, in which the ammonia diffusion gradient was reversed (from water to animal) and ammonia was taken up by the animal. Ammonia excretion rates were eventually reestablished (days 2-3), despite continued exposure to elevated external ammonia levels. Subsequent to the inhibition of ammonia excretion, urea excretion rates increased in both tadpoles (100%-250%) and adult bullfrogs (65%-70%). The activity of the key regulatory enzyme in the ornithine-urea cycle, carbamoylphosphate synthetase I, was relatively low in tadpole liver tissue and was not induced precociously by ammonia exposure. Carbamoyl phosphate synthetase I activity was comparatively high in adult bullfrogs but was not altered by external ammonia. Likewise, hepatic uricase, allantoinase, allantoicase, and glutamine synthetase activities were not affected by ammonia exposure in both tadpoles and adult bullfrogs. These findings indicate that both ammonotelic tadpoles and ureotelic adult bullfrogs respond to environmental ammonia by increasing the rate of urea excretion without increasing the activities of ureagenic enzymes.