The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system Nevertheless, substantial questions about the evolution of these mechanisms and control remain

https://people.clas.ufl.edu/devans/files/PRVGill.pdf

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

Developmental Plasticity and Evolution. In: Mary Jane West-Eberhard, Editor, Oxford University Press (2003) Pp. xi+794. Price $50.00 paper.

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Methods Of Enzymatic Analysis

Preface. Selected Atomic Weights. Customary Metric Conversion Factors. 1. Water Quality and Aquaculture: Preliminary Considerations. 2. Ecology of Aquaculture Ponds. 3. Water Quality Requirements. 4. Water Use. 5. Liming. 6. Fertilization. 7. Aeration. 8. Water Circulation. 9. Turbidity and Appearance of Water. 10. Aquatic Weed Control. 11. Off-Flavors and Harmful Algae. 12. Pollution. 13. Chemical, Physical, and Biological Treatments. 14. Waste Management. 15. Measurement of Water Quality. 16. Sustainability and Environmental Issues. References. Index.

Pond aquaculture water quality management

Ammonia toxicity in fish

There are diverse physiological functions of nitrogen end products in different animal groups, including excretion, acid-base regulation, osmoregulation and buoyancy. Animals excrete a variety of nitrogen waste products, but ammonia, urea and uric acid predominate. A major factor in determining the mode of nitrogen excretion is the availability of water in the environment. Generally, aquatic animals excrete mostly ammonia, whereas terrestrial animals excrete either urea or uric acid. Ammonia, urea and uric acid are transported across cell membranes by different mechanisms corresponding to their different chemical properties in solution. Ammonia metabolism and excretion are linked to acid-base regulation in the kidney, but the role of urea and uric acid is less clear. Both invertebrates and vertebrates use nitrogen-containing organic compounds as intracellular osmolytes. In some marine invertebrates, NH4+ is sequestered in specific compartments to increase buoyancy.

Nitrogen excretion : three end products, many physiological roles

Ammonia excretion at the gills of fish has been studied for 80 years, but the mechanism(s) involved remain controversial. The relatively recent discovery of the ammonia-transporting function of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in bacteria, yeast and plants, and the occurrence of these genes and glycosylated proteins in fish gills has opened a new paradigm. We provide background on the evolution and function of the Rh proteins, and review recent studies employing molecular physiology which demonstrate their important contribution to branchial ammonia efflux. Rhag occurs in red blood cells, whereas several isoforms of both Rhbg and Rhcg occur in many tissues. In the branchial epithelium, Rhcg appears to be localized in apical membranes and Rhbg in basolateral membranes. Their gene expression is upregulated during exposure to high environmental ammonia or internal ammonia infusion, and may be sensitive to synergistic stimulation by ammonia and cortisol. Rhcg in particular appears to be coupled to H(+) excretion and Na(+) uptake mechanisms. We propose a new model for ammonia excretion in freshwater fish and its variable linkage to Na(+) uptake and acid excretion. In this model, Rhag facilitates NH(3) flux out of the erythrocyte, Rhbg moves it across the basolateral membrane of the branchial ionocyte, and an apical "Na(+)/NH (+)(4) exchange complex" consisting of several membrane transporters (Rhcg, V-type H(+)-ATPase, Na(+)/H(+) exchanger NHE-2 and/or NHE-3, Na(+) channel) working together as a metabolon provides an acid trapping mechanism for apical excretion. Intracellular carbonic anhydrase (CA-2) and basolateral Na(+)/HCO (-)(3) cotransporter (NBC-1) and Na(+)/K(+)-ATPase play indirect roles. These mechanisms are normally superimposed on a substantial outward movement of NH(3) by simple diffusion, which is probably dependent on acid trapping in boundary layer water by H(+) ions created by the catalysed or non-catalysed hydration of expired metabolic CO(2). Profitable areas for future investigation of Rh proteins in fish are highlighted: their involvement in the mechanism of ammonia excretion across the gills in seawater fish, their possible importance in ammonia excretion across the skin, their potential dual role as CO(2) transporters, their responses to feeding, and their roles in early life stages prior to the full development of gills.

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A new paradigm for ammonia excretion in aquatic animals: role of Rhesus (Rh) glycoproteins.

Ammonia is toxic to all vertebrates. It can be converted to the less toxic urea, but this is a metabolically expensive process found only in terrestrial vertebrates that cannot readily excrete ammonia and marine fish that use urea as an osmotic filler. Freshwater fish mostly excrete ammonia with only a small quantity of urea. It seems the ornithine cycle for urea production has been suppressed in all freshwater teleosts except for some airbreathers which, when exposed to air, increase urea synthesis via the cycle. Here we show that the tilapia fish Oreochromis alcalicus grahami, the only fish living in Lake Magadi, an alkaline soda lake (pH = 9.6-10) in the Kenyan Rift Valley, excretes exclusively urea and has ornithine-urea cycle enzymes in its liver. A closely related species that lives in water at pH 7.1 lacks these enzymes and excretes mainly ammonia with small amounts of urea produced via uricolysis. It dies within 60 min when placed in water from Lake Magadi. We suggest that urea production via the ornithine-urea cycle permits O. a. grahami to survive the very alkaline conditions in Lake Magadi.

Urea excretion as a strategy for survival in a fish living in a very alkaline environment.

Short-term treatments (3h) designed to change the relative NH 3 (ΔP NH NH3 ) and NH 4 + (ΔNH 4 + ) gradients and sodium transport (J Na ) across the gills were employed to analyse the normal mechanism(s) of branchial ammonia excretion (J Amm net ) in trout acclimated to fresh water of pH ≃ 8.0. Control J Amm net occurred in the absence of, or against, an apparent ΔPNH 3 gradient, while ΔNH 4 + was positive. Severe acid exposure (pH = 4.06) raised ΔPNH 3 and ΔNH 4 + , abolished J Na in , and reduced J Amm net by 28%, while moderate acidity (pH = 6.64), which also elevated ΔPNH 3 had no significant influence on J Na in and J Amm net . Severe alkaline exposure (pH = 9.54) raised ΔNH 4 + , reduced ΔPNH 3 to a very negative value, and decreased J Na in and J Amm net by equimolar amounts, representing 55% and 80% of control levels respectively. Moderate alkalinity (pH = 8.69) had similar effects on ΔPNH 3 and ΔNH 4 + , but reduced J Na in and J Amm net by only 25%. The sodium transport inhibitor amiloride (10 −4 moll −1 in the external water, pH ≃ 8.0) had very similar effects to pH = 4.06 on both J Na in and J Amm net but did not alter ΔPNH 3 or ΔNH 4 + . The results discount the quantitative importance of NH 4 + diffusion and favour a flexible combination of NH 3 diffusion and Na + /NH 4 + exchange as the major mechanisms of (J Amm net ). with the latter dominating under the particular control conditions of the present study.

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An Analysis of Branchial Ammonia Excretion in the Freshwater Rainbow Trout: Effects of Environmental pH change and Sodium Uptake Blockade

Branchial ammonia transport in freshwater teleosts is not well understood. Most studies conclude that NH3 diffuses out of the gill and becomes protonated to NH4+ in an acidified gill boundary layer...

Patricia A. Wright

Papers

Nitrogen excretion : three end products, many physiological roles

A new paradigm for ammonia excretion in aquatic animals: role of Rhesus (Rh) glycoproteins.

Urea excretion as a strategy for survival in a fish living in a very alkaline environment.

An Analysis of Branchial Ammonia Excretion in the Freshwater Rainbow Trout: Effects of Environmental pH change and Sodium Uptake Blockade

Ammonia excretion in rainbow trout (Oncorhynchus mykiss): evidence for Rh glycoprotein and H+-ATPase involvement