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

Abstract

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 NH4+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 NH(3), NH4+ 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.

Keywords: ammonia; ammonia excretion; ammonia toxicity; ammonia transporter; fish; nitrogen metabolism.

Figure 1

The catabolism of excess amino acids through the process of transdeamination in liver cells releases ammonia and α-ketoglutarate (αKG) from glutamate (Glu) catalyzed by glutamate dehydrogenase (GDH) in the mitochondria. To avoid the disruption of the H⁺ gradient set up across the inner mitochondrial membrane by the electron transport system, ammonia produced within the mitochondrial matrix can exit the mitochondrion as $\text{N}{\text{H}}_4^{+}$ through a putative $\text{N}{\text{H}}_4^{+}$transporter (AMT) or a Na⁺/H⁺ exchanger (e.g., NHE6) in the liver cells of ammonotelic fishes. Alternately, ammonia can exit through an aquaporin channel (e.g., AQP8) as NH₃, accompanied with proton (H⁺) transport through an H⁺-ATPase. Ammonia can also be detoxified through the mitochondrial glutamine synthetase (GS) to glutamine (Gln), which is proton-neutral, before permeating the inner mitochondrial membrane to support various anabolic reactions (e.g., purine and pyrimidine syntheses). In ureogenic fishes that possess a functional ornithine-urea cycle, Gln synthesized within the mitochondria can be converted to carbamoyl phosphate (CP) catalyzed by carbamoyl phosphate synthetase III (CPS III) and subsequently to citrulline catalyzed by ornithine transcarbamylase (OTC) in the mitochondrial matrix of liver cells. Citrulline, being proton-neutral, can permeate the inner mitochondrial membrane without disrupting the H⁺ gradient. Some fishes (e.g., Bostrychus sinensis) possess a high activity of cytosolic GS in the liver (and the muscle), which would facilitate the detoxification of both endogenous and exogenous ammonia to glutamine in the cytosol, with glutamate being supplied from intestinal cells to support increased glutamine synthesis.

Figure 2

A hypothetical scheme of ammonia-induced astrocyte swelling resulting from (a) the permeation of NH₃ and ${\mathbf{\text{NH}}}_4^{+}$ through the blood–brain barrier with or without the aid of transport proteins (NKA, Na⁺/K⁺-ATPase; NKCC, Na⁺:K⁺:2Cl⁻-cotransporter; Rh channels, Rhesus glycoprotein channels) from the blood to the brain, (b) an increase in glutamine (Gln) synthesis from ${\mathbf{\text{NH}}}_4^{+}$ and glutamate (Glu) catalyzed by glutamine synthetase (GS) in the cytosol, (c) the entry of Gln into the mitochondria, (d) the breakdown of Gln by mitochondrial glutaminase (Glnase) and the release of ${\mathbf{\text{NH}}}_4^{+}$ in the mitochondrial matrix, (e) an increase in the production of reactive oxygen species (ROS), (f) the induction of mitochondrial permeability transition (MPT), (g) the occurrence of oxidative/nitrosative stress (ONS) in the cell, and (h) the activation of aquaporin channels (e.g., AQP4), leading to the influx of water and resulting in swelling and astroglial dysfunction.

Figure 3

The catabolism of excess amino acids through transdeamination in the liver mitochondrial matrix releases α-ketoglutarate (αKG) and ${\mathbf{\text{NH}}}_4^{+}$ through the deamination of glutamate catalyzed by glutamate dehydrogenase (GDH). Some $\text{N}{\text{H}}_4^{+}$ would dissociate to NH₃ and H⁺ in the matrix which has a more alkaline pH than the inter-membranous space and the cytosol. The permeation of NH₃ through aquaporin channels (e.g., aquaporin 8, APQ8) or the phospholipid bilayer would disrupt the H⁺ gradient set up by the electron transport system (ETS) across the inner mitochondrial membrane and uncouple ETS from oxidative phosphorylation.

Figure 4

The roles of Rhesus glycoproteins (Rhbg, Rhcg), in conjunction with those of Na⁺/K⁺-ATPase (NKA), Na⁺/H⁺ exchanger 2 (NHE2), Na⁺/H⁺ exchanger 3 (NHE3), and proton-ATPase (H⁺-ATPase), in ammonia excretion through the gills of freshwater fishes.

Figure 5

The roles of Rhesus glycoproteins (Rhbg, Rhcg1, and Rhcg2), in conjunction with those of Na⁺/K⁺-ATPase (NKA), Na⁺:K⁺:2Cl⁻-cotransporter (NKCC), and Na⁺/H⁺ exchanger 2 (NHE2), in ammonia excretion through the gills of marine fishes.