What is the role of albumin in proteinuric glomerulopathies?

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Abstract
Albumin is a 69 kDa plasma protein (molecular radius ∼36 Å) with a variety of functions, including maintenance of plasma oncotic pressure, buffering of plasma acid–base changes and functioning as a transport protein for hormones, fatty acids, phospholipids, ions and heavy metals [1]. Albumin was early identified as critical in regulating endothelial permeability, especially in studies on single perfused microvessels [2]. Later, plasma acidic glycoproteins, especially orosomucoid, were found to be (additional) crucial factors in maintaining normal microvascular permeability, particularly to negatively charged macromolecules (e.g. albumin) [3]. The critical role of albumin and orosomucoid in maintaining vascular permeability has also been demonstrated for glomerular capillaries [4,5]. Paradoxically, in Nagase analbuminaemic rats (NARs), a mutant of the Sprague–Dawley strain devoid of circulating albumin, the clearance of proteins (albumin and IgG) to the interstitium was, however, only moderately increased [6]. Furthermore, in a recent study, the glomerular permeability to macromolecules was only slightly altered in this rat strain [7].