Identification of two components in the natriuretic response to saline loading in the rat

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Abstract
Studies were undertaken in rats to determine whether the natriuresis induced by acute extracellular fluid volume expansion, once established, could be reversed by reducing renal artery pressure (RAP) to 70 mmHg and returning glomerular filtration rate (GFR) to preexpansion levels. RAP was reduced at the onset, after 30 min, and after 45 min of saline loading (24 ml/h). An immediate reduction in RAP blocked the natriuresis (UNaV [absolute Na excretion] = 0.6 .+-. 0.3 .mu.eq/min). In the 30 and 45-min studies a significant natriuresis developed prior to clamping, but the influence of the reduction in RAP and GFR on Na excretion differed markedly. After 30 min, this maneuver virtually abolished the natriuresis (UNaV = 0.3 .+-. 0.1 .mu.eq/min), whereas after 45 min Na excretion remained elevated (UNaV = 3.4 .+-. 0.6 .mu.eq/min). The difference in response persisted when identical saline loads were administered prior to aortic constriction in the 2 groups. The natriuretic response to acute volume expansion in the rat apparently had 2 components. The 1st increased Na excretion promptly, but was reversed by reducing RAP and returning GFR to control levels. The 2nd evolved more slowly and resulted in a natriuresis that was resistant to a reduction in RAP and GFR.