Urine composition and kidney tubular function in southern flounder, Paralichthys lethostigma, in seawater.

Abstract

Magnesium, sulfate, and chloride make up approximately 80% of the total millimolar concentration of major electrolytes in the urine of southern flounder in seawater, The remaining major electrolytes present are calcium, sodium, potassium, phosphate, and an unknown anion component. As a rule, more sodium was present when urine flow was very low: phosphate concentration was highly variable but its concentration was not associated with urine flow. Urine composition was independent of the glomerular filtration rate. With one exception, the urine osmolality was substantially less than the millimolar concentration, because of the low activities of the divalent ions present and because some of the urine calcium and phosphate precipitated out of solution. The urine was usually 5–15 mosmoles hypoosmotic to the blood; it occasionally became slightly hyperosmotic to the blood for a brief interval after operative procedures.The average percentage filterabilities of the plasma electrolytes, determined by centrifugal ultrafiltration, were Na, 78.4%; Cl 91.1%; K, 88.0%;Ca, 18.5%; Mg, 54.6%; PO4, 48.7%. Calculation of the quantities of electrolytes filtered, reabsorbed, secreted, and excreted showed that the urine is primarily a product of tubular secretory activity, but that at high filtration rates the absolute quantities of monovalent ions reabsorbed by the tubular epithelium may greatly exceed the quantities of divalent ions secreted. The divalent ion secretory mechanism was found to respond sensitively and specifically to ion loading produced by MgCl2 infusion or exposure to concentrated seawater. The possible location of specific ion reabsorptive and secretory activities and differences in tubular permeability to water is discussed in relation to the known morphology of the kidney.