UREOGENESIS IN INDIAN AIR-BREATHING TELEOSTS - ADAPTATION TO ENVIRONMENTAL CONSTRAINTS

Most of the Indian air-breathing teleosts are primarily ammoniotelic, but appear to have retained the genes for the urea cycle enzymes, sinc e a full complement of urea cycle enzymes have been reported for many of them. The ability to synthesize urea by these fish is probably due to their amphibious nature, and their normal habitat of swamps, where the water ammonia level may to be quite high, is uninhabitable to any typical freshwater teleosts. One of these air-breathing species, the s inghi catfish (Heteropneustes fossilis), can tolerate very high ambien t total ammonia concentrations (up to 75 mM ammonium chloride) for wee ks without any deleterious effects. Transition from ammoniotelism to u reotelism occurs in some of these species of air-breathing fish when e xposed to apparently stressful conditions such as higher ambient ammon ia, to air, and also when they live in semidry condition inside mud du ring habitat drying. Although the real mechanism(s) of regulation of u reogenesis is not clear in these fish, given available data, it is hyp othesized that the accumulation of ammonia within the body per se unde r the above stressful conditions is likely the internal modulator for enhanced ureogenesis mainly to avoid any build up of ammonia to a leve l that can be toxic to these fish. An active urea cycle is believed to predominate over uricolysis as a source of urea, even though both pat hways are present in these air-breathing fish. The presence of signifi cant levels of both carbamyl phosphate synthetase (CPS), CPS I-like an d CPS III activities, reported in some air-breathing catfishes, may re present intermediate scenarios for a proposed evolutionary transition from CPS III to CPS I, or may play an important physiological adaptive role in the tolerance of these fish to high concentrations of ambient ammonia (C) 1998 Elsevier Science Inc. All rights reserved.