We studied the effects of
hypoproteinemia following 12 days of repeated
plasmapheresis and
low-protein diet on
sodium balance, fluid volumes, and renal hemodynamics in six conscious dogs on 50 mmol
sodium intake. Measurements during
hypoproteinemia were obtained during a 5-day recovery period starting 20 h after the final
plasmapheresis session, with continued
low-protein diet. During the
plasmapheresis period
sodium was retained.
Sodium balance became negative on the first recovery day when
plasma protein was 29 +/- 1 g/l (control 60 +/- 2 g/l, P less than 0.01), and plasma
colloid osmotic pressure (COP) was 9 +/- 1 mmHg (control 22 +/- 1 mmHg, P less than 0.01). Subcutaneous fluid COP was lowered from 14 +/- 1 to 4 +/- 1 mmHg (P less than 0.01). Blood volume, plasma
renin activity, and
aldosterone were unchanged. Glomerular filtration rate and effective renal plasma flow were slightly reduced (NS), and filtration fraction was unchanged. After a second
plasmapheresis period in three of the dogs,
plasma protein fell to 26 +/- 1 g/l and COP to 7 +/- 1 mmHg. Now
sodium was retained on the first day after stopping
plasmapheresis, and
renin and
aldosterone were high. The next day, when
plasma protein was again 29 +/- 1 g/l and COP 8 +/- 1 mmHg, these three dogs were able to completely excrete an infusion of 130 mmol
sodium. These data suggest that the level of plasma COP below which dogs on a medium-
sodium intake would retain
sodium averages 8 mmHg, which is considerably lower than generally thought.