Kidney and urinary bladder functions of the rainbow trout in Mg and Na excretion

Magnesium concentration in the urinary bladder of marine teleosts is high and [Na] is low. The inverse relationship may be due to the coupling of Mg secretion to Na reabsorption through a common tubular transport system or coupling may be indirect. Unanesthetized sea-water-adapted Salmo gairdneri were infused with saline or MgCl2, and ureteral urine was collected. Over a wide range of Mg excretion rates, tubular Mg secretion and Na reabsorption show no clear correlation as might be expected from an obligatory tubular Mg/Na exchange. Instead ureteral Mg and Na concentrations and excretion rates are positively correlated. These data are not consistent with the presence of a tubular exchange system. When ureteral urine was allowed to remain in the urinary bladder before being analyzed the inverse relationship between [Mg] and [Na] developed because the bladder reabsorbed Na and H2O, thereby concentrating Mg. Hence, tubular Mg/Na exchanges do not produce the inverse Mg/Na relationship in bladder urine. Instead, Mg is secreted in the nephron, while Na and H2O are reabsorbed from the bladder.