ON THE GLOMERULAR MECHANISM OF THE RENAL PROTEIN EXCRETION - STUDIES IN THE ISOLATED PERFUSED RAT-KIDNEY

The glomerular permeability of the isolated perfused kidney is manifold higher than in vivo, although structural changes at the glomerular filtration barrier are not described. It has been proposed that in vivo high molecular weight plasma proteins are concentrated at the glomerular endothelium forming an additional filtration barrier ('concentration polarization'). To test this hypothesis, isolated rat kidneys were perfused with substrate-enriched Krebs-Henseleit solutions containing albumin (50 g/l) or albumin/globulin (40/10 g/l), or with plasma. The pO2 was 675 mm Hg. Furthermore, experiments were carried out with the addition of erythrocytes with and without a reduced O2 supply (pO2 35 mm Hg). Independent of the composition of the perfusate a continuously increasing glomerular permeability was observed immediately after perfusion was started. After about 10 min, a steady state value for the glomerular permeability was reached. The addition of globulin to the perfusate or perfusion with plasma did not prevent the initial increase of the permeability. However, after addition of 5% erythrocytes to the perfusate the increase of permeability was much less pronounced. Neither the reduction of the O2 supply nor the addition of erythrocytes to a higher concentration than 5% had any further effect on the glomerular permeability. These data show that concentration polarization of high molecular weight proteins does not take place at the endothelium of the glomerulus. Independent of their ability to carry O2 erythrocytes play an important role in the glomerular filtration process.