The production of renal calcinosis by magnesium deficiency in rainbow trout (Salmo gairdneri).

1. Replicate groups of rainbow trout (Salmo gairdneri) were given one of five experimental diets (diets 1-5) for 16 weeks. The diets contained different amounts of calcium, phosphorus and magnesium and were prepared so that there were three levels of Ca (g/kg): 14 (diet l) , 26 (diets 2 and 3) and 40 (diets 4 and 5) , Ca:P being approximately 1 : I in all diets. Diets 1, 2 and 4 had basal Mg levels (not more than 0.063 g/kg) whereas diets 3 and 5 contained supplementary Mg (1.0 g/kg). 2. Weight gains of the trout given diets containing supplementary Mg were twice those of trout given diets with basal levels of Mg. At both dietary Mg concentrations weight gain was unaffected by the dietary Ca level. 3. Serum Mg levels were significantly reduced in those trout given diets without supplementary Mg. The serum Ca level in those trout given the lowest concentration of Ca in their diet (14 gjkg, diet 1) was significantly greater than in those given higher amounts of Ca in their diets. Serum P levels were not significantly different with any of the experimental diets. 4. The renal Ca concentration was increased in trout given diet 3 (26 g Ca/kg; basal Mg levels). No further increase in renal Ca concentration occurred in trout given diet 5 (40 g Ca/kg; basal Mg levels). With diets containing supplementary Mg renal Ca levels were increased at a dietary Ca level of 40 g/kg but not at a dietary Ca level of 26 g/kg. Renal Mg and P concentrations were not significantly different between any of the dietary treatments. 5. Renal calculi were demonstrated by light and electron microscopy in tubules of those trout given diets 3 and 5 (basal Mg; 26 and 40 g Ca/kg respectively). Electron-probe micro-analysis showed that these calculi contained or comprised tricalcium phosphate. 6. The skeletal muscle of Mg-deficient trout contained significantly more sodium than that of normal trout. It is suggested that this is indicative of an increase in extracellular fluid in the muscle of Mg-deficient trout.