Body fluid expansion is not essential for salt-induced hypertension in SS/Jr rats

To evaluate the importance of volume in the development of hypertension in inbred Dahl salt-sensitive rats (SS/Jr), we measured the changes in blood p ressure (BP) that occurred with oral intake of food (salt) and water in rat s whose body weight was permitted to increase versus those in which body we ight was maintained constant with a servo-control system. We hypothesized t hat if volume expansion is essential in the development of hypertension, th en EP would not increase if body weight was held constant. We found that or al presentation of chow containing 4% salt to SS/Jr rats caused BP to incre ase 32.2 +/- 2.9 mmHg over 4 days when body weight was controlled at its in itial value. Plasma sodium increased from 142.0 to 145.2 meq/l during 4 day s of high salt. Neither plasma volume, hematocrit, nor central venous press ure changed significantly on the high-sail diet. In contrast, the inbred Da hl salt-resistant rats (SR/Jr) did not increase their BP during body weight control when given 4% salt. This demonstrates that volume expansion is not an obligatory step in the pressure response to increased salt in SS/Jr rat s. Our results obtained with oral presentation of salt, in contrast to intr avenous, represent a physiological evaluation of the significance of volume changes in response to dietary salt because no potential regulatory reflex es have been bypassed.