State-dependent expression of pressure diuresis in conscious rats

In 1967, Guyton and Coleman modeled pressure diuresis as the underlying, es sential, long-term mechanism that regulates arterial pressure when sodium i ntake changes. Other mechanisms that influence renal function interact with pressure diuresis to achieve sodium balance and determine the blood pressu re. Increases in sodium intake suppress sodium conserving mechanisms and ac tivate natriuretic mechanisms; decreases in sodium intake have the opposite effect. If the Guyton-Coleman model is correct, then pressure diuresis sho uld be more readily detected in animals on a high-salt diet than in animals on a low-salt diet. We measured spontaneous changes in arterial pressure a nd urine flow in conscious rats fed low-salt (0.4% NaCl) and high-salt (8.0 % NaCl) chow. For 10 rats fed a high-salt diet, arterial pressure and urine flow were positively correlated in 19 of 32 (59%) trials. In 10 rats fed a low-salt diet, a positive correlation was observed in 10 of 33 (30%) trial s. Chi-square analysis revealed that differences in Na+ content of the diet were significantly associated with the probability of a positive relations hip between blood pressure and urine flow. These results support the hypoth esis that the expression of pressure diuresis across time is dependent on t he state of sodium balance.