Effect of Waterborne and Dietary Cadmium on Plasma Ions of the Teleost Oreochromis mossambicus in Relation to Water Calcium Levels

The effects of cadmium administered via ambient water or food on plasma ions of the Af­ rican freshwater cichlid Oreochromis mossambicus were studied for 2, 4, 14, and 35 days, in low cal­ cium (0.2 mM) and high calcium (0.8 mM) water. In low calcium water, an environmentally relevant concentration of 10 fig/L water-borne cadmium in­ duced a significant and dramatic hypocalcemia on days 2 and 4. Recovery of plasma calcium was ob­ served on days 14 and 35. Hypermagnesemia was observed on day 2 , but normal levels were already found on day 4. In high calcium water adapted fish, the extent of hypocalcemia and hypermagnesemia was less pronounced than in fish from low calcium water. Water-borne cadmium caused no significant changes in plasma phosphate, sodium, potassium, or osmolality. On days 2 and 4, dietary cadmium (averaging 10 fxg Cd/fish/day) caused hypermagne­ semia and hypocalcemia in low calcium wateradapted fish. Recovery was observed on days 4 and 14, respectively. In fish from high calcium water, dietary cadmium caused a significant reduction in plasma calcium on day 4 only; plasma magnesium was unaffected. Hyperphosphatemia was apparent on day 14, irrespective of the water calcium con­ centration. No changes in plasma sodium, potas­ sium, or osmolality were found. The results show that sublethal concentrations of cadmium, administered via the water as well as via the food, affect calcium and magnesium metabo­ lism in tilapia. High water calcium ameliorates the Address correspondence to Dr. S. E. Wendelaar Bonga, Depart­ ment of Animal Physiology, Faculty of Science, University of Nijmegen, 6525 ED Nijmegen, The Netherlands. effects of both water and dietary cadmium on plasma calcium and magnesium levels. Among the various heavy metal pollutants, cad­ mium is frequently present in natural water bodies as a result of discharges from industrial processes or other anthropogenic contamination. The harmful effects of cadmium on mammals and other terres­ trial animals have been widely studied and re­ viewed (Flick et al. 1971; Vallee and Ulmer 1972; Webb 1979; Korte 1983; Foulkes 1986). Aquatic vertebrates such as fish, live in very intimate con­ tact with the environment through their gills. This makes them very susceptible to aquatic pollutants. Since it is well established that freshwater fish take up most of the ions necessary for homeostasis from the water via the gills (Eddy 1982), cadmiuminduced plasma ionic disturbances are apparently caused by impaired uptake and diffusional losses of ions via these organs (Larsson et al. 1981; Giles 1984). Ionic disturbances have also been reported after exposure of fish to sublethal concentrations of heavy metals. For example, changes in the plasma ionic composition have been observed in fish ex­ posed to copper and zinc (Lewis and Lewis 1971; Spry and Wood 1985), mercury (Lock et al. 1981), and chromium (Van der Putte et al. 1983). With re­ spect to cadmium, exposure of rainbow trout to sublethal levels induced hypocalcemia, with re­ duced plasma sodium, potassium, chloride and in­ creased plasma magnesium (Giles 1984). In Euro­ pean flounder, cadmium-induced hypocalcemia and elevated levels of plasma phosphate, magnesium and potassium were observed (Larsson et al. 1981). In addition to water, food could also be a source of cadmium for fish, since it accumulates in aquatic organisms through trophic transfers (Anonymous Water-Borne and Dietary Cd in Fish 569 1971; Williams and Giesy 1978; Coombs 1979). In­ deed, Bryan (1976) concluded that food as a source of Zn, Mn, Co, and Fe for molluscs, crustaceans and fish was more important than water. From various studies on both water-borne and food-containing metals, reviewed by Dallinger et al. (1987), there is evidence that uptake of heavy metals such as Cd, Cu, Co, Pb, Hg, and Zn from food is also the predominant pathway in freshwater Fish. Koyama and Itazawa (1977) reported significant hypocal­ cemia and elevated plasma phosphate levels in cad­ mium-fed carps. Similarly, plaice and thornback ray both accumulated more cadmium from food than from seawater (Pentreath 1977). In general, cadmium concentrations in natural waters are ex­ tremely low and a more important route of cad­ mium uptake by fish may be represented via the gut. Experiments with dietary cadmium may there­ fore yield more representative information for field situations. In this investigation, we have compared the ef­ fects of a sublethal concentra tion of cadmium administered via the water or via the food in the African cichlid fish Oreochromis mossambicus ( t i l a p ia ) . P la s m a ions and o s m o la l i ty w ere determined. Cadmium was administered at sublethal concentrations, in the order of magnitude that may occur in natural waters (^10 [xg Cd/L). In many studies aimed at evaluating the effects of cad­ mium on fishes, high concentrations (>1 mg Cd/L) of cadmium have been used. Hence severe physio­ logical, behavioral and detrimental effects have been reported. Such high concentrations are rarely found in nature, except in cases of spillage or heavily polluted waters. The Working Group on Cadmium Toxicity (EIFAC 1977) has suggested that chronic exposure to low cadmium concentrations is more relevant to understanding the mechanisms in­ volved in the intoxication process in teleost fish. We further studied the influence of relatively low and high calcium concentration of the water on the toxic effects of cadmium. The effects of water hardness (mainly Ca2+ and Mg2+ ions) on heavy metal toxicity have been demonstrated in various species of teleosts (Part et al. 1985). Increased tox­ icity of cadmium to fish in soft water as compared to hard water has been demonstrated in catfish and guppies (K inkade and E rdm an 1975), goldfish (McCarty et al. 1978), striped bass (Palawski et al. 1985), brook trout (Carroll et al. 1979) and rainbow trout (Calamari et al. 1980; Pasco et al. 1986). Sim­ ilar o b se rv a t io n s on te leos ts exposed to zinc, copper and lead (Sinley et al. 1974; Zitko and Carson 1976; Judy and Davies 1979; Lauren and McDonald 1986) indicate a protective role of cal­ l a b l e 1. The concentration of several electrolytes in mM of normal and high calcium water; Ph inorganic phosphate Ca2+ N a + Mg2+ K + Pi Low calcium water High calcium water 0.20 0.80 3.89 3.96 0.24 0.27 0.19 0.17 0.06 0.08 cium against the toxic effects of heavy metals. It was also investigated whether the protective effect of the water-calcium concentration is limited to water-borne cadmium only, or also applies to di­ etary cadmium. Materials and Methods Freshwater acclimated laboratory stock of male Oreochromis mossambicus (tilapia) ranging from 12-14 cm in total length and a body weight of 16 to 25 g were used in the present study. Fishes were maintained in 100 L aquaria with continuous aera­ tion and circulating filtered water, pH 7.4 (360 L/hr, Eheim pumps 1021) at 28°C on a daily 12 hr photoperiod.