INDUCTION OF UREOGENESIS IN PERFUSED LIVER OF A FRESH-WATER TELEOST, HETEROPNEUSTES-FOSSILIS, INFUSED WITH DIFFERENT CONCENTRATIONS OF AMMONIUM-CHLORIDE

The induction pattern of urea cycle enzymes and the rate of urea-N exc retion were studied with relation to ammonia load in the perfused live r of a freshwater ammoniotelic teleost, Heteropneustes fossilis, when infused with different concentrations of ammonium chloride for 60 min. Both urea-N excretion and uptake of ammonia by the perfused liver wer e found to be a saturable process, The V-max of urea-N excretion (0.45 mu mol/g liver/min) was obtained at ammonium chloride addition of 1.1 8 mu mol/g liver/min. The maximum induction of carbamyl phosphate synt hetase (ammonia dependent), similar to 200%, and of ornithine transcar bamylase, similar to 120%, was seen by the addition of 0.58 mu mol/g l iver/min, and for argininosuccinate synthetase and argininosuccinate l yase of similar to 150% and 115%, respectively, by the addition of 2.8 mu mol/g liver/min of ammonium chloride. However, arginase activity d id not alter in any of the concentrations of ammonium chloride added. An increase of ammonia load of 3-5 mu mol/g wet wt from the physiologi cal level in the perfused liver was sufficient to initiate and to caus e maximum induction of most of the urea cycle enzymes activitty. These results further confirm the capacity of transition from ammoniotelism to ureotelism in this unique freshwater air-breathing teleost to tole rate a very high ambient ammonia.