Characteristics of sodium reabsorption in the loop of Henle and distal tubule

This study was performed to define the characteristics of the relation between sodium delivery to and sodium reabsorption in the superficial loop of Henle (loop segment) and distal convoluted tubule (distal segment). This relation is examined in 3 experimental groups: when delivery was modestly increased with minimal changes in the extracellular fluid (ECF) volume; when delivery was significantly increased in association with acute progressive isotonic ECF volume expansion; and when delivery was significantly decreased by partial aortic constriction during progressive isotonic ECF volume expansion. In the loop segment, in all groups, absolute sodium reabsorption changed directly with sodium delivery, but the relation between delivery and absolute reabsorption was such that fractional reabsorption varied in inverse proportion to sodium delivery. In the 1st group the increases in delivery were minimal and absolute reabsorption in the distal segment paralleled delivery; fractional reabsorption remained constant. In the 2nd and 3rd group, absolute reabsorption varied directly but not proportionately with the large changes in delivery; fractional reabsorption therefore varied inversely to delivery. (28 references)