Dependence of saline induced natriuresis upon exposure of the kidney to the physical effects of extracellular fluid volume expansion

Abstract

In many previous studies, the natriuresis induced by saline loading has been demonstrated to persist even though glomerular filtration rate (GFR) has been decreased to below pre expansion levels by a reduction in renal artery pressure. In such studies, however, the kidney has been exposed to the effects of volume expansion for varying periods of time before renal artery pressure was controlled. The present experiments were designed to evaluate whether this period of exposure induces critical changes in intrarenal factors that are responsible for the natriuresis. Experiments were carried out in rats, in which renal artery pressure was decreased to 70 mm Hg either at the onset of saline loading (immediate clamping experiments) or after 45 min of saline loading had elapsed (delayed clamping experiments). In the delayed clamping experiments, consonant with previous studies, mean sodium excretion, 3.2μeq/min, remained markedly increased above control, despite a reduction in GFR to 91% of the hydropenic control value. In contrast, when renal artery pressure was comparably reduced at the onset of saline loading mean sodium excretion was only trivally increased, 0.4μeq/min, although GFR increased to 140% of the hydropenic control value.