Renal function and sodium balance in conscious Dahl S and R rats

Renal transplantation studies have indicated that some form of renal dysfunction underlies the development of hypertension in Dahl salt-sensitive (S) rats. In the present study, we compared renal hemodynamic and tubular function of conscious Dahl S and salt-resistant (R) rats. Prehypertensive Dahl S rats had a blunted natriuretic response to an intravenous isotonic sodium chloride load compared with the responses of normotensive Dahl R or hypertensive Dahl S rats. This difference was probably not related to a generalized defect in renal tubular handling of sodium and water, since prehypertensive Dahl S rats excreted quantities of sodium comparable to those of R or hypertensive S rats when infused with hypertonic sodium chloride solutions. Dahl S rats also elevated free water clearance and lowered urine osmolality similar to R rats when challenged with a hypotonic saline load. Renal blood flows and glomerular filtration rates were similar in prehypertensive Dahl S, hypertensive Dahl S, and Dahl R rats. The possible link between sodium retention and the development of hypertension in Dahl S rats was examined further by measuring the changes in sodium and water balance, extracellular fluid volume (ECV), and blood pressure after exposure to an 8% sodium chloride diet. No differences could be detected in the salt and water balances of Dahl S and R rats exposed to a high-salt diet for 14 days. ECV increased significantly by 10% in Dahl S rats on the 1st day of a high-salt diet, whereas no change was observed in Dahl R animals. However, significant differences in ECV in Dahl S and R rats were not detected on any day of the study. Blood pressures of Dahl S rats increased significantly by 9 mmHg on the 1st day of the high-salt diet and continued to increase to a peak of 172 ± 4 mmHg by day 14 of the study. Blood pressure remained unaltered in Dahl R rats during the study. These results indicate that conscious prehypertensive Dahl S rats may retain sodium relative to Dahl R rats under at least one but not all experimental conditions. However, when placed on a high-salt diet, Dahl S rats rapidly develop hypertension, whereas Dahl R rats do not, even though they retain similar quantities of sodium.