Ammonia Excretion In Fish Research Articles

Did Acidic Stress Resistance in Vertebrates Evolve as Na+ /H+ Exchanger-Mediated Ammonia Excretion in Fish?

How vertebrates evolved different traits for acid excretion to maintain body fluid pH homeostasis is largely unknown. The evolution of Na+ /H+ exchanger (NHE)-mediated NH4+ excretion in fishes is reported, and the coevolution with increased ammoniagenesis and accompanying gluconeogenesis is speculated to benefit vertebrates in terms of both internal homeostasis and energy metabolism response to acidic stress. The findings provide new insights into our understanding of the possible adaptation of fishes to progressing global environmental acidification. In human kidney, titratable H+ and NH4+ comprise the two main components of net acid excretion. V-type H+ -ATPase-mediated H+ excretion may have developed in stenohaline lampreys when they initially invaded freshwater from marine habitats, but this trait is lost in most fishes. Instead, increased reliance on NHE-mediated NH4+ excretion is gradually developed and intensified during fish evolution. Further investigations on more species will be needed to support the hypothesis. Also see the video abstract here https://youtu.be/vZuObtfm-34.

Is ammonia excretion affected by gill ventilation in the rainbow trout Oncorhynchus mykiss?

Ammonia (NH3 + NH4+) is the major nitrogenous waste in teleost fish. NH3 is also the third respiratory gas, playing a role in ventilatory control. However it is also highly toxic. Normally, ammonia excretion through the gills occurs at about the same rate as its metabolic production, but the branchial transport mechanisms have long been controversial. An influential review in this journal has claimed that ammonia excretion in fish is probably limited by diffusion rather than by convection, so that increases in ventilation would have negligible effect on the rate of ammonia excretion. Why then should elevated plasma ammonia stimulate ventilation? The diffusion-limitation argument was made before the discovery of Rhesus (Rh) glycoproteins and the associated metabolon in the gills, which serve to greatly increase branchial ammonia permeability under conditions of ammonia loading. Therefore, we hypothesized here that (i) in accord with the diffusion-limitation concept, changes in ventilation would not affect the rate of ammonia excretion under conditions where branchial Rh metabolon function would be low (resting trout with low plasma ammonia levels). However, we also hypothesized that (ii) in accord with convective limitation, changes in ventilation would influence the rate of ammonia excretion under conditions where diffusion limitation was removed because branchial Rh metabolon function would be high (ammonia-loaded trout with high plasma ammonia levels). We used variations in environmental O2 levels to manipulate ventilation in trout under control or ammonia-loaded conditions – i.e. hyperventilation in moderate hypoxia or hypoventilation in moderate hyperoxia. In accord with hypothesis (i), under resting conditions, ammonia excretion was insensitive to experimentally induced changes in ventilation. Ammonia-loading by NH4HCO3 infusion for 30h + increased the gill mRNA expressions of two key metabolon components (Rhcg2, V-H+-ATPase or HAT), together with a 7.5-fold increase in plasma ammonia concentration and a 3-fold increase in ammonia excretion rate. In accord with hypothesis (ii), in these fish, hypoxia-induced increases in ventilation elevated the ammonia excretion rate and lowered plasma ammonia, while hyperoxia-induced decreases in ventilation reduced the ammonia excretion rate, and elevated plasma ammonia concentration. We conclude that under conditions of natural ammonia loading (e.g. meal digestion, post-exercise recovery), diffusion-limitation is removed by Rh metabolon upregulation, such that the stimulation of ventilation by elevated plasma ammonia can play an important role in clearing the potentially toxic ammonia load.

Ammonia production, excretion, toxicity, and defense in fish: a review.

Many fishes are ammonotelic but some species can detoxify ammonia to glutamine or urea. Certain fish species can accumulate high levels of ammonia in the brain or defense against ammonia toxicity by enhancing the effectiveness of ammonia excretion through active transport, manipulation of ambient pH, or reduction in ammonia permeability through the branchial and cutaneous epithelia. Recent reports on ammonia toxicity in mammalian brain reveal the importance of permeation of ammonia through the blood–brain barrier and passages of ammonia and water through transporters in the plasmalemma of brain cells. Additionally, brain ammonia toxicity could be related to the passage of glutamine through the mitochondrial membranes into the mitochondrial matrix. On the other hand, recent reports on ammonia excretion in fish confirm the involvement of Rhesus glycoproteins in the branchial and cutaneous epithelia. Therefore, this review focuses on both the earlier literature and the up-to-date information on the problems and mechanisms concerning the permeation of ammonia, as NH3, or proton-neutral nitrogenous compounds, across mitochondrial membranes, the blood–brain barrier, the plasmalemma of neurons, and the branchial and cutaneous epithelia of fish. It also addresses how certain fishes with high ammonia tolerance defend against ammonia toxicity through the regulation of the permeation of ammonia and related nitrogenous compounds through various types of membranes. It is hoped that this review would revive the interests in investigations on the passage of ammonia through the mitochondrial membranes and the blood–brain barrier of ammonotelic fishes and fishes with high brain ammonia tolerance, respectively.

Rhesus glycoprotein gene expression in the mangrove killifishKryptolebias marmoratusexposed to elevated environmental ammonia levels and air

The mechanism(s) of ammonia excretion in the presence of elevated external ammonia are not well understood in fish. Recent studies in other organisms have revealed a new class of ammonia transporters, Rhesus glycoprotein genes (Rh genes), which may also play a role in ammonia excretion in fish. The first objective of this study was to clone and characterize Rh genes in a fish species with a relatively high tolerance to environmental ammonia, the mangrove killifish Kryptolebias marmoratus (formerly Rivulus marmoratus). We obtained full-length cDNAs of three Rh genes in K. marmoratus: RhBG (1736 bp), RhCG1 (1920 bp) and RhCG2 (2021 bp), which are highly homologous with other known Rh gene sequences. Hydropathy analysis revealed that all three Rh genes encode membrane proteins with 10-12 predicted transmembrane domains. RhBG, RhCG1 and RhCG2 are highly expressed in gill tissue, with RhBG also present in skin of K. marmoratus. Exposure to elevated environmental ammonia (2 mmol l(-1) NH(4)HCO(3)) for 5 days resulted in a modest (+37%) increase in whole-body ammonia levels, whereas gill RhCG2 and skin RhCG1 mRNA levels were upregulated by 5.8- and 7.7-fold, respectively. RhBG mRNA levels were also increased in various tissues, with 3- to 7-fold increases in the liver and skeletal muscle. In a separate group of killifish exposed to air for 24 h, RhCG1 and RhCG2 mRNA levels were elevated by 4- to 6-fold in the skin. Thus, the multifold induction of Rh mRNA levels in excretory tissues (gills and skin) and internal tissues in response to conditions that perturb normal ammonia excretion suggests that RhBG, RhCG1 and RhCG2 may be involved in facilitating ammonia transport in this species. Furthermore, the findings support earlier studies demonstrating that the skin is an important site of ammonia excretion in K. marmoratus.

Exposure to brackish water, upon feeding, leads to enhanced conservation of nitrogen and increased urea synthesis and retention in the Asian freshwater stingray Himantura signifer

The white-edge freshwater whip ray Himantura signifer is ammonotelic in freshwater, but retains the capacities of urea synthesis and ureosmotic osmoregulation to survive in brackish water. The first objective of this study was to examine whether exposure to brackish water would lead to increases in food intake, and/or conservation of nitrogen in H. signifer upon daily feeding. Results obtained showed that a progressive increase in ambient salinity, from 1 per thousand to 15 per thousand over a 10-day period, did not lead to an increase in daily food intake. However, there were significant reductions in daily rates of ammonia and urea excretion in H. signifer during salinity changes, especially between day 5 (in 10 per thousand water) and day 10 (in 15 per thousand water) when compared to those of the control kept in 1 per thousand water. Consequently, there was a significant decrease in the percentage of nitrogen (N) from the food being excreted as nitrogenous waste (ammonia-N+urea-N) during this period. On day 10, the tissue urea contents in fish exposed to 15 per thousand water were significantly greater than those of fish kept in 1 per thousand water, and the excess urea-N accumulated in the former fish could totally account for the cumulative deficit in excretion of urea-N+ammonia-N during the 10-day period. Thus, it can be concluded that H. signifer is N-limited, and conserved more N from food when exposed to brackish water. The conserved N was converted to urea, which was retained in tissues for osmoregulation. The second objective of this study was to elucidate whether the retention of the capacity of N conservation in H. signifer would lead to an accumulation of urea in fish exposed to not only 15 per thousand water, but also 1 per thousand water, upon feeding. For fish pre-acclimated to 1 per thousand water or 15 per thousand water for 10 days and then fasted for 48 h, the rate of ammonia excretion in fish exposed to 15 per thousand water was consistently lower than that of fish exposed to 1 per thousand water, throughout the 36-h post-feeding period. In addition, the hourly rate of urea excretion in the former was significantly lower than that of the latter between hours 12 and 36. There were postprandial increases in ammonia contents in the muscle, liver, stomach, intestine, brain and plasma of fish kept in 1 per thousand water; but postprandial increases in ammonia occurred only in the liver and brain of fish exposed to 15 per thousand water, and the magnitudes of increases in the latter were smaller than those in the former. Indeed, postprandial increases in tissue urea contents occurred in both groups of fish, but the greatest increase in urea content was observed in the muscle of fish exposed to 15 per thousand water. Taken together, these results indicate that H. signifer in freshwater could be confronted with postprandial osmotic stress because of its capacity of conserving N and increasing urea synthesis upon feeding.

Influence of dietary phosphorus levels on growth, metabolic response and body composition of juvenile silver perch ( Bidyanus bidyanus)

A growth trial was conducted to estimate the phosphorus requirement of juvenile silver perch. Effects of dietary phosphorus levels on the selected minerals in plasma, vertebrae and whole body, liver lipid classes and postprandial accumulated ammonia excretion of the fish were also examined. Eight semipurified (casein–gelatin based) diets were formulated to contain 42% crude protein, 15.74 MJ digestible energy/kg diet and phosphorus (monobasic sodium phosphate) levels ranging from 0.24% to 1.08%. Each diet was fed to triplicate groups of 12 fish (initial average weight of 2.27 g) over 8 weeks. At the end of the trial, percent weight gain increased significantly ( P < 0.05) with increasing dietary phosphorus from 0.24% to 0.72%; the value slightly decreased thereafter. A similar trend was observed in the feed efficiency. The result of broken line regression indicated that phosphorus requirement of juvenile silver perch for maximal growth was satisfied with a diet containing 0.71% phosphorus (about 0.45 mg phosphorus per MJ digestible energy). The amount of postprandial ammonia excretion in fish fed lower dietary phosphorus levels was higher than that of fish fed diets with sufficient phosphorus. Lipid contents in the whole body and liver were higher in fish received lower dietary phosphorus levels. A similar tendency was also found in triacyglycerols, while phosphatidylcholine and phosphatidylethanolamine levels were significantly higher in fish fed diets with sufficient phosphorus. Plasma inorganic phosphorus concentrations increased with increasing dietary phosphorus levels and reached a plateau within 0.72–1.08% levels, but dietary treatment had no effects on plasma calcium, magnesium or zinc, and the activity of plasma alkaline phosphatase was higher in fish fed diets with insufficient phosphorus. The ash contents of the fish body and vertebrae were lower in fish fed lower dietary phosphorus levels. Calcium, phosphorus and magnesium concentrations in the whole body and vertebrae increased with increasing dietary phosphorus levels, and reached a plateau or even slightly decreased at phosphorus levels from 0.72% to 1.08%. However, concentrations of zinc in both body and vertebrae were not affected by dietary treatments. Signs of phosphorus deficiency were characterized by poor growth, loss of appetite, dark coloration, lower physical activity, poor bone mineralization and an increase in lipid content of fish body and liver.