ENHANCED CHLORIDE REABSORPTION IN THE LOOP OF HENLE IN DAHL SALT-SENSITIVE RATS

This study examines the nephron segments contributing to the blunted pressure-natriuretic response in Dahl salt-sensitive rats. Urine and late proximal and early distal tubular fluid samples were collected from 16-20-week-old, inbred Dahl salt-sensitive (DS/Jr) and salt-resistant (DR/Jr) rats, and Dahl salt-sensitive (DS) and salt-resistant (DR) rats from the Brookhaven colony, that were maintained from birth on a low (0.3%) sodium chloride diet. Urine flow and sodium and chloride excretions were 65% less in the DS/Jr than in the DR/Jr rats when their kidneys were perfused at an equal renal perfusion pressure of approximately 150 mm Hg. The percentages of the filtered load of water and chloride remaining at the end of the proximal tubule were significantly greater in DS/Jr rats than in DR/Jr rats; however, the percentages of the filtered load of water and chloride reaching the early distal tubule were significantly less, by 29% and 77%, respectively. Fractional reabsorption of water and chloride in the loop of Henle of DS/Jr rats was twice that observed in DR/Jr rats. Similar results were obtained in DS and DR rats of the Brookhaven strain. Urine flow and sodium and chloride excretions were 60% lower in DS than in DR rats at a renal perfusion pressure of 135 mm Hg. Proximal tubular reabsorption of water and chloride was similar in DS and DR rats; however, the percentages of the filtered load of water and chloride reabsorbed in the loop of Henle were greater in DS than in DR rats. These results indicate that enhanced reabsorption of water and chloride in the loop of Henle contributes to the resetting of the pressure-natriuretic relations in DS and DS/Jr rats. In addition, a reduced glomerular filtration rate in DS/Jr rats, which impairs their ability to excrete sodium at normal levels of renal perfusion pressure, may explain the tendency of DS/Jr rats to develop mildly elevated arterial pressures even when fed a low salt diet.