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 pressure (BP) that occurred with oral intake of food (salt) and water in rats whose body weight was permitted to increase versus those in which body weight was maintained constant with a servo-control system. We hypothesized that if volume expansion is essential in the development of hypertension, then BP would not increase if body weight was held constant. We found that oral presentation of chow containing 4% salt to SS/Jr rats caused BP to increase 32.2 +/- 2.9 mmHg over 4 days when body weight was controlled at its initial value. Plasma sodium increased from 142.0 to 145.2 meq/l during 4 days of high salt. Neither plasma volume, hematocrit, nor central venous pressure changed significantly on the high-salt diet. In contrast, the inbred Dahl 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 rats. Our results obtained with oral presentation of salt, in contrast to intravenous, represent a physiological evaluation of the significance of volume changes in response to dietary salt because no potential regulatory reflexes have been bypassed.