The glomerular filter: an imperfect barrier is required for perfect renal function.

PURPOSE OF REVIEW: There is currently a major debate on the mechanisms of albuminuria, and this review appraises recent studies in this area.

RECENT FINDINGS: The traditional view of albuminuria is that it is the result of damage to an essentially impermeable glomerular barrier. However, over the years, critical evidence for this traditional model has been shown to be flawed. An alternative explanation has evolved in which the glomerular filter governs albumin permeability by size selectivity alone. This means that the filter offers a significant barrier to albumin, but it is imperfect - the barrier leaks albumin. The virtue of this leakage is that it endows the filter an in-built anticlogging mechanism. The filtered albumin, if not rescued, would be excreted at nephrotic levels in the urine. There is evidence that proximal tubular cells participate in retrieving most of this filtered albumin to return it back to the blood supply intact. A small amount of the filtered albumin is not retrieved but directed toward lysosomal degradation, and the peptide products are exocytosed into the tubular lumen and excreted.

SUMMARY: In acquired and chemically induced kidney disease, albuminuria is the result of dysfunction in proximal tubular cell processing of albumin rather than alterations in glomerular permeability.