Physiological role of ANP under resting and volume expanded conditions in normotensive rats

To study the physiological role of the atrial natriuretic peptide in the control of hemodynamics and renal function, circulating ANP was reduced by surgical bilateral atrial appendectomy (circulating ANP levels: 153 +/- 15 pg/ml for sham animals, and 95 +/- 11 pg/ml for appendectomized rats) and the relationships between central venous pressure (CVP), plasma ANP and renal function were studied. Mean arterial pressure (MAP), heart rate (HR), CVP, plasma ANP, urine volume (UV) and sodium (UVNa) excretion were measured under basal conditions, during and after a volume load of 6 ml iso-oncotic Ringer's solution. Basal hemodynamic variables were not different between sham and appendectomized rats. CVP was significantly higher during the volume load in appendectomized rats whereas other hemodynamics were not different during and after the volume load. Plasma ANP was significantly reduced in appendectomized rats before, during and after the volume load. Basal urine and sodium excretion were not different whereas stimulated urine and sodium excretion were significantly reduced in appendectomized rats. ANP levels were not totally dependent on CVP during volume load and urine and sodium excretion were of late onset, compared to the onset of increased circulating ANP. This study indicates that circulating ANP plays an important role in volume expanded conditions but there is a dissociation between circulating ANP and renal excretion. Our data suggest involvement of a volume shift from intra-to extravascular space.