Role of blood volume expansion in Dahl rat model of hypertension.

Continuous measurement and servo control (SC) of total body weight of unrestrained rats were used to investigate the role of volume expansion in the development of hypertension in Dahl salt-resistant (SR) and Dahl salt-sensitive (SS) rats. A change in sodium intake from 1 to 20 meq/day was associated with an increase in total body weight of 7.2% in both SS and SR rats over 96 h. Plasma sodium (pNa) increased from 145.0 to 147.4 meq/l in both SS (n = 10) and SR (n = 10) rats. Only in the SS rats was the volume expansion associated with an increase in arterial pressure of 27 +/- 3 mmHg. Prevention of the volume expansion by SC blocked the rise in arterial pressure in the SS rats (n = 10) but increased pNa from 143.5 to 152.4 meq/l. Hematocrit fell from 36.6 to 27.5% in both non-SC groups but decreased less in SC groups (35.7 to 32.0%). Plasma volume expansion from 17.6 +/- 0.6 to 25.2 +/- 0.8 ml in non-SC rats was greatly blunted by SC. In non-SC rats, SS (n = 10) and SR (n = 9) rats an increase in salt intake was associated with a rise in cardiac output from 413 +/- 6 to 507 +/- 12 ml.min-1.kg-1 in both groups. These results indicate that fluid retention is required to trigger the rise of pressure in Dahl SS rats.