The influence of ammonia on the transition to ureotelism in Xenopus laevis

Toads of the species Xenopus laevis were immersed in 5 mM NH4Cl or 5 mM NaCl for periods of up to 28 days. In stronger solutions of NH4Cl, toads died within one hour. The rate of excretion of ammonia and urea and the activities of carbamyl phosphate synthetase, ornithine transcarbamylase, arginase and glutamate dehydrogenase in liver and kidney were measured. In toads in 5 mM NH4Cl, the excretion of urea was increased about ten-fold, and there were increases in the activities of carbamyl phosphate synthetase and glutamate dehydrogenase in the liver. The activities of ornithine transcarbamylase and arginase in the liver, and of glutamate dehydrogenase in the kidney, were unaffected. In toads in 5 mM NaCl, there was no change in the rate of urea excretion or in the activities of any of the enzymes assayed. It was concluded that a rise in tissue ammonia levels is followed by alterations in the metabolic pathways of the toads which result in a decrease in those levels. Thus, the transition to ureotelism in X. laevis is triggered by an increase in tissue ammonia levels. Because there was no response to immersion in 5 mM NaCl, it is concluded that a slight increase in the osmotic pressure of the external medium has no effect, and that urea is of little significance in osmotic regulation in this toad.