Effects of acute volume expansion and altered hemodynamics on renal tubular function in chronic caval dogs

It is well established that dogs with chronic partial constriction of the thoracic inferior vena cava develop sodium retention, ascites, and respond poorly to acute saline loading. A group of such chronic caval dogs, and a group of normal controls were studied during hydropenia, and again after acute saline loading by clearance and recollection micropuncture techniques. After volume expansion, the caval dogs excreted 52 muEq/min per kidney of sodium compared with 370 muEq/min per kidney for the normal controls. During hydropenia and after the saline infusions, single nephron filtration rates, fractional reabsorption of sodium within the proximal tubule, and proximal delivery of filtrate to the distal nephron were comparable in both groups of dogs. Micropuncture of distal tubular segments confirmed that the loop of Henle was the major site for salt and water retention in the expanded caval dogs. Alteration of intrarenal hemodynamics by vasodilating one kidney and elevating systemic arterial blood pressure induced a normal natriuretic response in the saline-loaded caval dogs. Proximal tubular function remained unchanged and the loop of Henle appeared to be the major site responsive to these hemodynamic maneuvers. These same experiments in saline-loaded control dogs had no effect on function of the proximal or distal nephron and did not increase urinary excretion of sodium or water. These experiments provide evidence that the loop of Henle is the major site for sodium retention in volume-expanded chronic caval dogs excreting minimal amounts of sodium.