Changes in salinity and ionic compositions can act as environmental signals to induce a reduction in ammonia production in the African lungfish Protopterus dolloi

The slender African lungfish, Protopterus dolloi, does not aestivate in a subterranean mud cocoon, but is capable of aestivating inside a layer of dried mucus on land during drought. In this study, we aimed to elucidate if a slight increase in salinity in association with changes in the ionic composition could act as signals for P. dolloi to decrease endogenous ammonia production, in preparation for aestivation when the external medium dries up. Specimens of P. dolloi exposed to 3 per thousand water for 6 days exhibited consistently lower daily urea excretion rate than the freshwater control. This led to significant decreases in the cumulative total nitrogenous wastes excreted on days 3, 5 and 6. On day 6, there were decreases in urea contents in various tissues and organs. Taken together, these results suggest that there was a decrease in the rate of urea synthesis, the magnitude of which was greater than the decrease in the rate of urea excretion, and therefore resulted in decreases in internal urea contents. A decrease in the rate of urea synthesis should result in a decrease in the rate of glutamine utilization, and subsequently led to the accumulations of glutamine and/or ammonia. However, there were no changes in contents of glutamine and ammonia in various tissues and organs in the experimental animals. A logical explanation for this is that there must be a simultaneous reduction in ammonia production; if not, ammonia would accumulate due to the decrease in rate of urea synthesis. Since fish were unfed during the experiment, endogenous ammonia must be derived mainly from amino acid catabolism. Therefore, these results suggest that a suppression of amino acid catabolism occurred in specimens exposed to 3 per thousand for 6 days. The differences in effects of freshwater and 3 per thousand water on endogenous ammonia production could not be due to food deprivation because both groups of fish were fasted for the same period. Because control and experimental fish were kept in water and because there were no changes in the wet mass of the fish and blood osmolality before and after the experiment, dehydration did not occur. Furthermore, both groups of fish have comparable blood pH, pO2 and pCO2 on day 6 as they had free access to air, and therefore CO2 retention could be eliminated as the initiating factor of suppressed endogenous ammonia production. In conclusion, our results suggest that P. dolloi could respond to increases in salinity and changes in ionic composition in the external medium by suppressing ammonia production in preparation for aestivation when the water dries up.