ROLE OF ENDOGENOUS DOPAMINE IN THE NATRIURETIC RESPONSE TO VARIOUS DEGREES OF ISOOSMOTIC VOLUME EXPANSION IN RATS

It is recognized that endogenous dopamine (DA) plays an important role in regulation of sodium excretion under certain physiological and experimental conditions. However, the relative contribution of intrarenally produced DA to natriuresis accompanying various degrees of acute volume expansion (VE) still remains to be clarified. In the present study, acute iso-osmotic VEs were performed in pentobarbital-anesthetized rats over a 60 min period. The rats were divided into 3 groups, each received either modest (2.5% body weight), moderate (5% body weight), or large degree of VE (10% body weight), respectively. Acute VE in these three groups evoked pronounced increases in urine output (UV) and urinary sodium excretion (U(Na)V), which were associated with significant increase in urinary DA excretion (U(DA)V). Compared to the modest VE (2.5% body weight), moderate VE (5% body weight) produced larger increases in UV (43 +/- 4.7 vs 29.0 +/- 4.7-mu-l/min, p < 0.05) and U(Na)V (7.8 +/- 0.7 vs 4.7 +/- 1.0-mu-Eq/min, p < 0.05) with slight but significantly greater increase in U(DA)V (1.38 +/- 0.06 vs 1.23 +/- 0.02 ng/min, p < 0.05). Compared to moderate VE (5% body weight), large VE (10% body weight) produced more pronounced increases in UV (91 +/- 14 vs 43 +/- 4.7-mu-l/min, p < 0.01) and U(Na)V (16 +/- 2.3 vs 7.8 +/- 0.7-mu-Eq/min, p < 0.01), however the increase in U(DA)V was similar to that seen during moderate VE. These results suggest that endogenous DA is involved in natriuretic response to various degrees of acute VE. Furthermore, it was discovered that the relative contribution of endogenous DA to overall VE-induced natriuresis is related to some extent to the degree of VE, inasmuch as DA appears to play a greater role in the overall natriuretic response seen during modest to moderate degree of VE.