Control of right atrial pressure at constant cardiac output suppresses volume natriuresis in anesthetized rats

The blood volume of anesthetized rats was expanded acutely by 33% with donor blood while a caval snare was gradually tightened so that right atrial pressure (RAP) was prevented from rising (n = 6). In control experiments (n = 5) an aortic snare was used to hold mean arterial blood pressure near the values found in the experimental series. However, RAP was allowed to change freely and increased by 1.6 +/- 0.4 mmHg (1 mmHg = 133.322 Pa) during volume expansion. When the two groups were compared, there were no significant differences between their mean arterial blood pressures (near 110 mmHg) or in their cardiac outputs (near 0.25 mL X min-1 X g body weight-1). There were, however, significant differences between their renal responses to the volume load. When RAP was free to change, the rate of volume excretion (V) increased to 30 +/- 15 (SEM) microL X min-1 X g kidney weight-1 (KW) from its control value of 3.49 +/- 0.31 and the rate of sodium excretion (UNaV) increased to 3.59 +/- 0.20 muequiv X min-1 X g KW-1 from its preinfusion value of 0.42 +/- 0.10. When RAP was not allowed to increase during volume loading, V and UNaV did not change from their respective preinfusion values (2.99 +/- 0.46 microL X min-1 X g KW-1 and 0.35 +/- 0.10 muequiv X min-1 X g KW-1). The results imply that during acute blood volume expansion increased central vascular pressure is a prerequisite for the homeostasis of body water and salt.