Relationship between renal fluid and Mg secretion in a glomerular marine teleost

Renal tubular fluid secretion has been studied in seawater-acclimated winter flounder, Pseudopleuronectes americanus. Although this animal has a filtration kidney and a relatively high glomerular filtration rate (GFR = 1.35 ml . kg-1 . h-1), apparently more than half of the final urine volume was secreted fluid, and net fluid secretion was frequently observed. The dominant divalent ion excreted in the urine was Mg, of which 98% was secreted. A strong correlation (r = 0.98) was seen between secreted Mg and the rate of tubular fluid secretion. The calculated concentration of Mg in secreted fluid was 169 mM, a figure that was substantiated by values obtained for urine Mg concentration when urine flow was due almost entirely to tubular fluid secretion. Alterations in urine flow rate reflected changes in the rate of Mg secretion; however, no correlation was seen between GFR and secreted Mg, which may indicate independence of renal portal blood flow and glomerular blood flow. No relationship between Mg secretion and sodium reabsorption was apparent. These observations support the hypothesis that Mg secretion together with accompanying anions (Cl and SO4) accounts entirely for tubular fluid secretion in the winter flounder.