Addition of purified orosomucoid preserves the glomerular permeability for albumin in isolated perfused rat kidneys.

The serum protein, orosomucoid has been shown to be essential for the maintenance of normal capillary permeability in several different organs, including the kidney. Thus, the clearance of albumin was found to be almost fivefold higher in the absence of orosomucoid in a previous study on isolated rat kidneys, perfused with either of two commercially available human albumin solutions of similar composition, but differing in their content of orosomucoid (0.21 g l-1 vs. < 0.005 g l-1). The following experiments were performed in order to verify the hypothesis that this effect on glomerular permselectivity was due to orosomucoid per se and not to other ingredients in the two solutions. Both kidneys of 12 rats were isolated and perfused with identical albumin solutions without orosomucoid, but with the addition of purified orosomucoid (0.25 g l-1) to one of the kidneys. No significant differences in vascular resistance, urine flow or glomerular filtration rate (GFR), which was found to be 27 +/- 2 ml min-1 100 g-1, were observed between the two groups of kidneys. The fractional clearance of albumin (theta) was initially similar for both kidneys (0.0022 +/- 0.0002). In the absence of orosomucoid, theta gradually increased to 0.0076 +/- 0.0013 after 1 h of perfusion compared to 0.0040 +/- 0.0006 for the kidneys with orosomucoid added to the perfusate (P < 0.001, n = 12). We conclude that the plasma glycoprotein orosomucoid indeed plays an important role in regulating the dynamic properties of the glomerular capillary wall by reducing the permeability towards macromolecules such as albumin.