Quantitative evaluations have been made of the chief anions and cations in plasma, urine, and pericardial fluid taken both from freshly captured goosefish and from those undergoing "laboratory diuresis." Measurements included: Na, K, Ca, Mg, Cl, SO4, PO4, protein, HCO3, NH3, pH, titratable acidity, freezing point depression, creatine, trimethylamine oxide, and plasma volume. The total patterns of electrolyte distribution in these body fluids are presented.

The morphologically undifferentiated aglomerular tubule acts as a barrier to the free diffusion of monovalent electrolytes, while transporting actively the divalent ions, especially Mg.

Urine taken from freshly captured fish is hypotonic to plasma, low in electrolyte, and as much as 50 per cent of its total osmolarity is accounted for by nitrogenous components. Of these creatine is transported most actively by the renal tubule cells.

With the onset of diuresis immediately after capture, plasma osmolarity slowly rises and urine suddenly becomes isotonic with plasma as chloride floods into the urine. The active movement of Mg continues during diuresis and urine/plasma concentration ratios of 100 or more are sustained for days while the animals are kept in the laboratory. Na follows chloride and never reaches 50 per cent of plasma values, and K never appears in urine in more than mere traces.

Electrolytes in this system are viewed as not being in true equilibrium but rather as constituting a biological steady state with the distribution across renal cells being maintained against passive diffusion by the expenditure of cellular energy.

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