Effect of thoracic cava obstruction on response of proximal tubule sodium reabsorption to saline infusion.

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Abstract
The effect of acute partial obstruction of the thoracic inferior vena cava (TIVC) on the response of the proximal tubule to isotonic saline infusion has been evaluated in dogs loaded with steroid and vasopressin. Micropuncture collection from the same proximal convolutions during hydropenia and during saline infusion with TIVC obstruction was carried out. Despite the infusion of saline, inulin tubule fluid to plasma (TF/P) increased significantly (mean 22%) after TIVC obstruction, above values found in hydropenia. Natriuresis and diuresis did not occur. There was a fall in glomerular filtration rate (GFR) averaging 30%. After release of TIVC obstruction natriuresis and diuresis ensued despite variable changes in GFR, and inulin TF/P decreased significantly in all instances to a level averaging 18% below that found in hydropenia. Elevation of renal venous pressure by acute partial obstruction of the abdominal inferior vena cava (AIVC) did not prevent natriuresis and diuresis after isotonic saline infusion, and inulin TF/P fell significantly (mean 21%) from the values found in hydropenia. There was no further appreciable change with release of AIVC obstruction. These studies indicate that acute parial TIVC obstruction prevents and reverses the depression of proximal tubule sodium reabsorption that follows saline loading, and that this effect is independent of changes in filtration rate, renal venous pressure, or circulating levels of adrenal steroid or vasopressin.

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