Acute Lowering of Plasma Oncotic Pressure Increases Filtration Fraction and Sodium Excretion in Conscious Sheep

We examined the effect of acutely lowering the colloid osmotic pressure by removing plasma (36.2 +/- 3.1 ml/kg) and replacing it with Hartmann's solution (93.0 +/- 8.2 ml/kg) in 6 conscious merino sheep. The colloid osmotic pressure was reduced significantly (p < 0.05) from 20.3 +/- 0.9 to 8.5 +/- 2.5 mm Hg 0 h after plasmapheresis and to 15.2 +/- 0.8 mm Hg 20 h after treatment. The filtration fraction increased from 0.16 +/- 0.02 to 0.27 +/- 0.02 at 0 h (p < 0.05 vs. control) and to 0.20 +/- 0.02 at 20 h after treatment (p < 0.05 vs. control). At 0 h after plasmapheresis, there was an increase in both sodium excretion from 187 +/- 69 to 459 +/- 82 mumol.min-1 (p < 0.05) and in fractional sodium excretion from 1.6 +/- 0.4 to 4.4 +/- 1.0% (p < 0.05). The results are consistent with the hypothesis that acute alteration of Starling forces produce clinically significant effects on both glomerular filtration and tubular reabsorption.