Stimulation of ureogenesis in the perfused liver of an Indian air-breathing catfish, Clarias batrachus, infused with different concentrations of ammonium chloride

Abstract

The pattern of changes of activity of the urea cycle enzymes and the rate of urea-N excretion were studied in the perfused liver of an Indian air-breathing ureogenic walking catfish, Clarias batrachus. The liver was perfused with different concentrations of NH4Cl for a period of 60 min to determine the role of ammonia for stimulation of hepatic ureogenesis and the threshold level of ammonia loading needed to cause such stimulation. Both the urea-N excretion and the ammonia uptake by the perfused liver were found to be a saturable process. Ammonia accumulated significantly in the liver infused with 1.25 μmoles g liver −1 min−1 of NH4Cl, followed by a maximum accumulation of about 28.5 μmoles g wet wt−1 with the infusion of 5.08 μmoles g liver−1 min−1. The Vmax of the urea-N excretion (0.47 μmol g liver−1 min−1) was obtained with the addition of 5.08 μmoles g liver−1 min−1 of NH4Cl. Both the tissue and the specific activity of the urea cycle enzymes, except ornithine transcarbamylase and arginase, were stimulated significantly with the infusion of either 1.25 or 5.08 μmoles g liver−1 min−1 of NH4Cl. Maximum stimulation of tissue activity of carbamoyl phosphate synthetase (about 120%) was seen with the infusion of 5.08 μmol g liver−1 min−1, and for argininosuccinate synthetase (about 135%), and argininosuccinate lyase (about 50%) with the infusion of 10.81 μmol g liver−1 min−1 of NH4Cl. Higher accumulation of ammonia of about 10–15 μmol g wet wt−1 from the physiological level in the perfused liver while infusing with NH4Cl was suggested to be one of the major causes of stimulation of ureogenesis. The presence of such physiological adaptive strategy is probably necessary in this unique group of air-breathing walking catfish to survive under hyper-ammonia stress in their normal habitat or while living outside water or while burrowing inside mud.