Fish, excretion

Ammonia is highly toxic. Fish excrete NHj directly birds convert NHj to uric acid. Higher vertebrates convert NHj to urea. 

Ammonia (NH3) is a relatively strong base, and at physiological pH values it is mainly present in the form of the ammonium ion NH4 (see p. 30). NH3 and NH4 are toxic, and at higher concentrations cause brain damage in particular. Ammonia therefore has to be effectively inactivated and excreted. This can be carried out in various ways. Aquatic animals can excrete NH4 directly. For example, fish excrete NH4 via the gills (ammonotelic animals). Terrestrial vertebrates, including humans, hardly excrete any NH3, and instead, most ammonia is converted into urea before excretion ureotelic animals). Birds and reptiles, by contrast, form uric acid, which is mainly excreted as a solid in order to save water uricotelic animals). 

FIGURE 22-45 Catabolism of purine nucleotides. Note that primates as uric acid from purine degradation. Similarly, fish excrete much more... 

In humans and many other vertebrates, ammonia arising from deamination reactions or other sources is excreted in the form of urea. These animals are called ureo-telic. Fish excrete nitrogen in the form of ammonium ions and are therefore ammonotelic. Animals that need to conserve water excrete their nitrogen in the form of crystalline uric acid. They are uricotelic, or purinotelic. One often finds animals that convert uric acid to allantoin via uric acid oxidase (Figure 20.8). Allan-toin is more water soluble than uric acid. Uric acid oxidase is absent from primates. 

The endogenous repletion of water in the sea Lets nitrogenous excretion proceed quite easily For ammonia though toxic is quickly washed away So fish excrete (4 times) the ammonotelic way (repeat). 

For example, fish excrete the ammonia produced by the decomposition of proteins directly into the watery environment in which they live. Birds, who consume less water by gram of weight than do most other animals, Urea is transported from the liver to the kidneys in the bloodstream. By the time it leaves the body in urine, its concentration is sixty to seventy times its concentration in the bloodstream. 1... 

Fish excrete excess nitrogen as ammonia, and birds excrete it as uric acid. Mammals excrete it as urea. 

Aquatic organisms, such as fish and invertebrates, can excrete compounds via passive diffusion across membranes into the surrounding medium and so have a much reduced need for specialised pathways for steroid excretion. It may be that this lack of selective pressure, together with prey-predator co-evolution, has resulted in restricted biotransformation ability within these animals and their associated predators. The resultant limitations in metabolic and excretory competence makes it more likely that they will bioacciimiilate EDs, and hence they may be at greater risk of adverse effects following exposure to such chemicals. 

Foods that acidify the urine (cranberries, plums, primes, meats, cheeses, egjsJ3, fish, and grains) may increase excretion and decrease the effectiveness of chloroquine... 

Alcohol sulfates are easily metabolized by mammals and fishes either by oral or intraperitoneal and intravenous administration. Several labeled 35S and 14C alcohol sulfates have been used to determine their metabolism in experiments with rats [336-340], dogs [339], swines [341], goldfish [342], and humans [339]. From all of these studies it can be concluded that alcohol sulfates are absorbed in the intestine of mammals and readily metabolized by to and p oxidation of the alkyl chain and excreted in the urine and feces, but are also partially exhaled as carbon dioxide. Fishes absorb alcohol sulfates through their gills and metabolize them in a similar way to that of mammals. 

Fish show generally low HMO activities that are not strongly related to body weight. This may reflect a limited requirement of fish for metabolic detoxication they are able to efficiently excrete many compounds by diffusion across the gills. The weak relationship of HMO activity to body weight is probably because fish are poikilotherms and should not, therefore, have an energy requirement for the maintenance of body temperature that is a function of body size. In other words, the rate of intake of xenobiotics with food is unlikely to be strongly related to body size. 

Metabolism is the main mechanism of loss in terrestrial vertebrates, but is less important in fish, which can achieve excretion by diffusion into ambient water. 

Huang KC, SF Collins (1962) Conjugation and excretion of aminobenzoic acid isomers in marine fishes. J Cell Comp Physiol 60 49-52. 

Hunn JB, RA Schoettger, WA Willford (1968) Turnover and urinary excretion of free and acetylated MS 222 by rainbow trout, Salmo gairdneri. J Fish Res Bd Can 25 215-231. 

Fingerlings exposed to radiosilver-110m for 57 days then transferred to clean freshwater for 28 days After 57 days the whole body BCF was 2.7 with about 70% of total radiosilver concentrated in the liver (BCF for liver was 282) during depuration, 23% of whole fish radiosilver was excreted but concentration in liver was unchanged 35... 

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