EFFECT OF SUSTAINED EXPANSION OF EXTRACELLULAR FLUID VOLUME UPON FILTRATION RATE, RENAL PLASMA FLOW AND ELECTROLYTE AND WATER EXCRETION IN THE DOG

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Abstract
The extracellular fluid of 6 normal dogs (18 expts.) was expanded 25-50% by the intraven. injn. of a modified Locke''s soln. with minimal alterations in its ionic composition. This expansion was maintained at a constant level for periods up to 8 hrs. by the injn. of fluid having almost exactly the same composition as the urine. Under these conditions the filtration rate, renal plasma flow, electrolyte excretion and urine flow show a 3-phase response which, though it differs quantitatively in different dogs, is qualitatively similar in all and is quantitatively quite reproducible in any one dog. The significant changes in renal hemodynamics after expansion of the extracellular fluid are a marked increase in filtration rate and renal plasma flow (phase I); a return of these functions towards normal (phase 2); and re-elevation of both functions to a maintained value, which considerably exceeds the control level (phase 3). These changes in renal hemodynamics can be attributed only partly, if at all, to blood pressure changes or dilution of the plasma proteins. The initial increase in the filtration rate in phase I is accompanied by a marked increase in Na excretion. In phase 2, however, the Na reabsorption increases, reducing the rate of excretion and leading to a relatively dilute urine. Because of the phasic response in the filtration rate, it can be shown that at identical Na loads (and identical values of PNa and wCF), Na reabsorption in phase 2 and 3 is substantially greater than in phase I, revealing a specific change in tubular activity, sufficient to decrease Na excretion by 50%. During phase 2, despite the continuous admn. of pitressin in physiological doses, water reabsorption falls behind Na reabsorption and the urine is for a time slightly hypotonic, demonstrating that water reabsorption is influenced by other factors in addition to the antidiuretic hormone.

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