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The water and ion balance of an aquatic animal is important for optimal physiological functioning, which is within rather narrow limits (Schmidt-Nielsen, 1997). The balance is strike by ionic and osmotic regulation of animal’s body fluid with respect to its external environment.
Introduction The water and ion balance of an aquatic animal is important for optimal physiological functioning, which is within rather narrow limits (Schmidt-Nielsen, 1997). The balance is strike by ionic and osmotic regulation of animal’s body fluid with respect to its external environment. Osmoregulation is the process of regulating organism’s osmotic pressure of body fluid by movement of solutes across biological membrane to control osmotic gradient. The process generally involves ionic and osmotic regulation by (1) uptake of ions and (1) excretion of metabolic waste products, toxic compounds, excess water and salt ion. Aquatic vertebrate has adapted different balancing strategies to survive different water environment. There are two main strategies for osmotic regulation. Firstly, osmoconformer fish, like marine elasmobranch and cyclostomes, strive to remain isosmotic gradient with the medium. Secondly, osmoregulator fish, like the teleost, is able to regulate osmotic concentration of body fluid regardless of external concentration changes. Fish gill epithelium, kidney, guts are the main sites of osmoregulation for gas exchange, ionic regulation, acid-base balance, and nitrogenous waste excretion (Evans, 1987). Epithelial tissues possess ion transporters that allow osmoregulation to occur. The type of transporters are: Na+/K+ and H+ ATPase, various single ion channels, electroneutral cotransporters (Na+-K+-2Cl- and K+-Cl- cotransporters) and, electroneutral exchangers (Na+/H+, NH4+/ H+ and Cl-/ HCO3- exchangers). Osmoregulatory process is controlled by hormones, which regulate activity of ion transporters at epithelium cells (Bern and Madsen, 1992). The fish gill lamellae is made up of mitochondria-rich chloride cells amongst mitochondria-rich and low-mitochondrial-content pavement cells. Both mitochondria-rich chloride and pavement cells participate in transport functions of the gills. Water salinity governs the direction of ions and water transport in the gills. As teleost fishes maintain the NaCl content of their body fluids at approximately 40% that of seawater, marine teleost is hyperosmotic and freshwater teleost is hyposmotic to their respective environment (Evans, 1987). Freshwater fish, being hyperosmotic to its environment, need to take up ion from the environment to maintain body fluid osmolity against steep electrochemical gradients. Pavement cells absorb Na+ using ion channel and enhanced using H+ ATPase to create decrease pH, which increases the electrochemical driving force for Na+ transport. Chloride cell imports Cl-/ HCO3- exchanger, with Ca2+ uptake through ion channel. Both types of cells requires carbonic anhydrase to produce HCO3- for changing pH or as counter ions. On the contrary, marine fish, being hyposmotic to seawater, need to prevent influx of ions and excessive water loss. Marine teleost achieves osomoregulation by drinking seawater constantly to replace water loss and excreting excess ions through Cl- ion channels and paracellular transport of Na+ (Moyes, 2006). Interestingly, diadromous, catadromous and anadromous fish species that need to adapt between seawater and freshwater environment in their life history, undergo smoltification to remodel ion transporter types in the epithelium at the gills, kidney and gut using specific hormones for difference in environment osmolarity (Moyes, 2006). Copper Heath (1984) found that freshwater fish exposed to copper experienced decreased plasma osmolality as the uptake of sodium and chloride ions is greatly reduced by high concentration of Cu in freshwater fish gills (McDonald et. al., 1988). The osmoregulation is upset by the rapid decrease in plasma osmolality. Reduction is tested in vivo that enzyme Na+/K+ ATPase in the gill tissue is inhibited (Lorz and McPherson, 1976). Coho salmon was able to adapt to prolonged and gradual increase in concentration of copper, starting in sublethal concentration (Buckley et al., 1982). Marine For anadromous Coho salmon (Onchorhynchus kisutch), chronic exposure to Cu during migration result in organism becoming unable to osmoregulate because of reduction in activity of Na+/K+ ATPase in the gills, which is activated during migration to seawater (Lorz and McPherson, 1976). That might reduce survival of the salmon during migration. Zinc Studies of Spry and Wood (1984 and 1985) showed that non-lethal dose of zinc causes drop in intracellular calcium and efflux of sodium ions lead to osmoregulatory malfunction over long period of time and to a physiological adaptation. Due to the efflux of sodium ions, the uptake of ions from the water resulted in a raise in the rainbow trout’s gills ATPase activity (Watson and Beamish, 1980). In seawater killifish, the zinc (and copper) restrains chloride transport at concentration of 4x10-5 M and only the ATPase molecules at the basal area of the chloride cells (Wilson and Taylor, 1993). Hence, that explains the low sensitivity of seawater-adapted fish to zinc and copper (very high concentration needed to be lethal). Chromium Young coho salmon in freshwater was exposed to 1-4weeks of chromium at the concentration of 0.5 mg Cr/L and released into seawater (Sugatt, 1980). The young fish survived the first week but there was high mortality on the second week. When control fish was compared with Cr- exposed fish, Cr-exposed has larger and longer increase in osmolality, hence the process of adaptation to seawater was hindered by Cr. Similar to copper, Cr inhibit the enzyme Na+/K+ ATPase in the gill tissue (Lorz and McPherson, 1976). Aluminum The effect of Al depends on the pH and calcium content in the water. When exposed to Al acutely to Al at pH 4 to 4.5, the rainbow and brook trout experienced inhibition of Na+/K+ ATPase in the gills reduced sodium and chloride uptake with increased passive efflux of ions (Staurnes et al.,1984; Booth et al., 1988). When pH is increased to 4.5 to 5, the trout was known to have more adverse effect at pH4.8 where it experience hypoxic condition coupled with loss of ions (Malt and Weber, 1988). Interestingly, upon acclimation with chronic exposure to Al, trout has physiological and histological adaptations that allow the gill cells to increase have greater association with calcium and lead to reduce Al binding at the surface of the cells (Heath, 1995). Cadmium As mentioned earlier, marine fish rely on the gills to excrete excess sodium chloride. However, the in vitro metabolic rate of this tissue can be decreased by 50% without any effect on the plasma electrolytes (Thurberg and Dawson, 1974). Therefore, cadmium seems to affect other osmoregulatory organs such the kidneys rather than the gills. A relatively high concentration is required to change the concentration of plasma sodium and chloride in marine fishes. How cadmium affects the kidneys of marine fish was studied by Larsson et al. in 1981 where he investigated the effect of cadmium exposure on the flounder, Platichthys flesus, a species of fish living in brackish water. The environment the flounder lives in contains a higher concentration of magnesium but a lower concentration of potassium and calcium than the blood plasma. Therefore, there is a net diffusion of magnesium into the blood plasma and a net diffusion of potassium and calcium ions out. Excess magnesium is normally excreted by the kidney (Heath, 1997). A rise in magnesium concentration in the plasma in response to cadmium is an indicator of the kidney’s failure to excrete magnesium. The kidney is therefore a target organ of cadmium and this heavy metal causes histological damage to the kidney. Cadmium affects the kidney’s ability to regulate magnesium concentration of the plasma and in turn affects osmoregulation. On the other hand, calcium metabolism in freshwater fish is more drastically affected by cadmium than in marine or brackish waters fish. Within a week of exposure to 0.3 ppm cadmium, the plasma calcium of a species of the rainbow trout dropped by approximately 50% (Roch and Maly, 1979). This concentration of cadmium causes only a small drop in the plasma calcium of brackish water fish. Cadmium causes a decrease in plasma calcium by inhibiting the influx of calcium at the gills (Reid and McDonald, 1988; Verbost et al., 1987). Cadmium displaces calcium from protein carriers in the gill epithelium. The target organ of cadmium in freshwater fish is essentially the gill in contrast to the kidneys in marine fish. However, continued exposure to cadmium induces freshwater fish to increase prolactin secretion which allows for the recovery of plasma calcium Therefore, exposure to modest amounts of cadmium will not affect calcium regulation or osmoregulation to large extent. Cadmium generally causes most harm to the early life stages of fish (Benoit et al.,1976; Rombough and Garside, 1982). Exposure of larval fish to cadmium causes a reduction in uptake rate of sodium, potassium calcium ions and water from the environment. Ion regulation and osmoregulation may thus be drastically altered. Mercury Mercury causes osmoregulation to alter in freshwater and marine fish to different degrees, greater in marine species than freshwater species. When a freshwater fish, brook trout was exposed to methylmercury at very low concentrations, there was an increase in plasma sodium and chloride. On the other hand, another freshwater fish, rainbow trout suffers a decrease in plasma sodium and chloride when exposed to higher concentrations of mercury. Osmoregulatory failure may be one of the causes of death from mercury poisoning in freshwater fish. Mercury cause osmoregulatory failure by causing an increased permeability of the gills to water (Heath, 1997). Lead Freshwater fish, rainbow trout when exposed to lead in brackish water, show a dose-dependent rise in plasma potassium. This potassium, from experimental results on the effect of lead on mammalian erythrocytes cells, is inferred to have leaked out from tissue cells. The lead is thought to inhibit the Na,K ATPase which controls the relative concentrations of potassium between the body fluids and the cytoplasm of body cells (Heath, 1997). However, this increase in plasma potassium has little effect on the regulation of sodium by the gills as the fish was in an environment isotonic to its blood. Tin compounds which are used in antifouling paints on ships were found to cause no change in the plasma electrolytes (Pinkney et al., 1989). Even though tin compounds were observed in in vitro conditions to inhibit the enzymes Na, K ATPase and Mg ATPase in the striped bass, these enzymes are greatly stimulated by tin in in vivo conditions. This implies that the striped bass, which lives in brackish water may have adapted to the presence of tin and induced more enzyme synthesis. This is synonymous with the lack of effect tin has on the striped bass osmoregulation and ion regulation. The regulation of sodium is very adversely affected by manganese. Body and plasma sodium concentrations fall by up to 50% before death. Therefore, it is highly likely that loss of plasma sodium is a primary mode of death in acute doses of manganese (Heath, 1997). In contrast, iron at high concentrations cause very little change to plasma sodium. Applications Information on the toxicity of heavy metals can be applied to various fields of study, particularly in health and commercial aspects. An interesting feature about fish which allows them to survive in various concentrations of heavy metals pollution is their ability to bioaccumulate heavy metal ions in their bodies. This bioaccumulation has various implications on the survival of the fish themselves and their potential predators including humans. As mentioned earlier, some heavy metals such as copper affect osmoregulation in fish by inhibiting the enzyme Na+/ K+ ATPase. In addition to this inhibition, fish also accumulate copper in their gills (Stagg and Shuttleworth, 1982; Wilson and Taylor, 1993) and in the intestine of marine fish in response to high levels of copper in the water (Grosell et al., 2004). The bioaccumulation of heavy metals in fish’s bodies causes many problems for human consumers as even low levels of heavy metals such as mercury or lead can cause drastic effects to human body. A case study of the Minamata disease, which is a neurological disease caused by severe mercury poisoning, revealed its origin to be in Minamata City in Japan. This disease was caused by human consumption of fish and shellfish in Minamata Bay and Shiranui which had bioaccumulated the extremely poisonous mercury. During the outbreak of the Minamata disease in Minamata City in Japan, little was yet known about the toxicity of heavy metals and how high concentrations could be bioaccumulated by fish and other marine organisms. The knowledge of bioaccumulation is then useful not just to understand how fish survive in high concentrations of heavy metal but to prevent future outbreaks of fatal diseases caused by heavy metals. Fish, unlike human or other mammals, have adapted to living in environment of certain heavy metals concentration through bioaccumulation. It was found that heavy metals, such as cadmium are normally taken up by the viscera, particularly the gut of marine fishes. This is due to the fact that marine fish drink large amounts of saltwater to maintain their plasma osmotic potential. The heavy metals taken up are then accumulated to a certain lethal concentration before it kills them. This adaptation in fish to accumulate heavy metals without killing themselves is rather interesting and the exact mechanism bioaccumulation and its concept could be applied to biomedical research for perhaps a drug for human to safely accumulate certain heavy metals. This would be extremely beneficial for people who constantly have to be exposed to heavy metals due their profession The knowledge of the effect of heavy metals on fish osmoregulation also has potential applications to commercial fish farming and fisheries. Commercial fisheries can develop methods to test heavy metals content in the fish they capture. There are many different forms of fish farming which is the principal form of aquaculture. A type of fish farming that could potentially be affected by heavy metals pollution is the cage system farming method, whereby the cages are placed in open water bodies such as rivers, lakes or open seas. One of the problems of having fish farms in natural water bodies is that any pollutants such as heavy metals present or polluting the water body will also pollute the water of the fish farms. Therefore, to ensure that farmed fish are safe for consumption, it is important that commercial fish farms are informed about the bioaccumulation of heavy metals in fish as well as how fish are affected by each type of heavy metal. Being well-informed about the effects of heavy metals on fish also allows fish farms to research and develop methods of keeping water in the farms free of heavy metals pollution. Recently, as an alternative to open oceans cage systems type of fish farms, recirculating aquaculture system was developed. In this system, water is treated by continuous recycling, whereby there was removal of particulate matter through the conversion on accumulated chemicals to non-toxic ones. With further research on the relationship between osmoregulation in fish and heavy metals pollution, more methods of efficient and safe fish farming could be developed. Conclusion Different heavy metals affect osmoregulation in fish to a different extent. 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