Effects of intra-peritoneal injection with NH4Cl, urea, or NH4Cl+urea on nitrogen excretion and metabolism in the African lungfish Protopterus dolloi

This study aimed to (1) determine if ammonia (as NH4Cl) injected intraperitoneally into the ureogenic slender African lungfish, Protopterus dolloi, was excreted directly rather than being converted to urea; (2) examine if injected urea was retained in this lungfish, leading to decreases in liver arginine and brain tryptophan levels, as observed during aestivation on land; and (3) elucidate if increase in internal ammonia level would affect urea excretion, when ammonia and urea are injected simultaneously into the fish. Despite being ureogenic, P. dolloi rapidly excreted the excess ammonia as ammonia within the subsequent 12 h after NH4Cl was injected into its peritoneal cavity. Injected ammonia was not detoxified into urea through the ornithine-urea cycle, probably because it is energetically intensive to synthesize urea and because food was withheld before and during the experiment. In addition, injected ammonia was likely to stay in extracellular compartments available for direct excretion. At hour 24, only a small amount of ammonia accumulated in the muscle of these fish. In contrast, when urea was injected intraperitoneally into P. dolloi, only a small percentage (34%) of it was excreted during the subsequent 24-h period. A significant increase in the rate of urea excretion was observed only after 16 h. At hour 24, significant quantities of urea were retained in various tissues of P. dolloi. Injection with urea led to an apparent reduction in endogenous ammonia production, a significant decrease in the hepatic arginine content, and a significantly lower level of brain tryptophan in this lungfish. All three phenomena had been observed previously in aestivating P. dolloi. Hence, it is logical to deduce that urea synthesis and accumulation could be one of the essential factors in initiating and perpetuating aestivation in this lungfish. Through the injection of NH4Cl + urea, it was demonstrated that an increase in urea excretion occurred in P. dolloi within the first 12 h post-injection, which was much earlier than that of fish injected with urea alone. These results suggest that urea excretion in P. dolloi is likely to be regulated by the level of internal ammonia in its body. J. Exp. Zool. 303A: 272-282, 2005. (c) 2005 Wiley-Liss, Inc.