Infusion of magnesium salts into the body cavity of freshwater-adapted rainbow trout led to elevated plasma magnesium concentrations and to stimulation of renal tubular secretion of magnesium. The majority of the infused load was excreted renally, no net branchial excretion being detected. Magnesium sulphate infusion led to increased tubular secretion of sulphate. Magnesium chloride infusion led to reduced tubular reabsorption of chloride. Magnesium could either be reabsorbed or secreted in control freshwater-adapted trout, apparently as a function of nutritional status. Fish could switch from reabsorption to secretion in response to magnesium loading. It is suggested that freshwater fish eliminate excess dietary magnesium renally.

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