Comparative evaluation of fractional excretion of sodium following saline infusion in transplanted kidneys and in isolated perfused kidneys in conditions of previous high or low dietary sodium intake.

The increase of fractional excretion of sodium following the infusion of isotonic saline solution is quantitatively the same in dog kidneys perfused by a whole animal or by a pump-oxygenator machine at identical arterial and venous pressures. The equal response found in both experimental conditions demonstrates that the dilution of the blood, and primarily the decrease of post-glomerular plasma oncotic pressure, represents the only extrarenal humoral factor of significant importance in the reduction of fractional reabsorption of sodium following large saline infusion. The renal response to hemodilution is reduced similarly in both types of experiments when the dogs have been previously submitted to a dietary deprivation of sodium. The decreased sensitivity of the kidney to the stimulus of hemodilution appears as a major determinant of the poor natriuretic response to saline loading of sodium-deprived animals.