Reabsorptive response of renal tubules to elevated sodium and chloride concentrations in plasma

Abstract

Sustained and relatively constant levels of hypernatremia and hyperchloremia were obtained by infusing dogs with 150 ml of 5% or 10% sodium chloride solutions for 30 minutes and following this with infusions of 0.85% sodium chloride at the rate of 5 ml/min. Inulin clearances and serum and urinary sodium and chloride values were determined during a number of periods when plasma sodium and chloride were at normal or near normal values and compared with similar determinations made after hypertonic saline infusions when plasma sodium and chloride were at high but relatively steady levels. For any given rate of glomerular filtration more sodium chloride was reabsorbed at high plasma salt levels than at normal or near normal levels. Moreover, the rate of reabsorption increases as plasma sodium and chloride increase. The enhanced tubular reabsorptive activity toward sodium chloride at a time when a large surfeit of salt exists appears paradoxical when viewed as a consequence of active transport processes. On the other hand, these results are precisely what would be predicted if passive reabsorption of glomerular filtrate occurred in an area of the tubule freely permeable to salt as well as water.