We note with interest the recent commentary entitled What is the role of albumin in proteinuric glomerulopathies by B. Rippe [1] whose content is closely related to our paper. It is important to bring to the attention of the reader that there are other publications in the literature, which would not support the views expressed by Rippe.
The underpinning concept for Rippe's views on glomerular permselectivity and renal handling of albumin is that there exist endothelial sieve plugs made up of polyanionic material. It is proposed that such plugs have a profound influence on albumin permeability as they electrostatically repel negatively charged albumin, thereby giving rise to apparently its low sieving coefficient. This is a modification of an old concept in nephrologythat of negative charge selectivity. Accurate physicochemical studies that have been published in the 19601970s, together with recent evidence on glomerular permselectivity have established that charge selectivity does not exist and the concept is a flawed one.
Direct evidence of an electrostatic repulsion of albumin by any polyanion under physiological conditions has never been demonstrated. Efforts to mimic this electrostatic exclusion of albumin from polyanion environments have demonstrated that the exclusion is only size based, independent of ionic strength [24]. Accurate measurements of albumin distribution in cartilage, which has the highest concentration of immobilized polyanions in the body, have failed to demonstrate the role of electrostatic exclusion [5,6]. It may be argued that interaction of albumin with polyanions in solution in a test tube is too simplistic a model for an endothelial plug and that a specific pore arrangement may present the anionic charges in a different manner. This has been tested in recent studies, which conclusively demonstrated that when one uses stable negatively charged probes including negatively charged Ficoll and negatively charged dextran or starch then charge selectivity does not exist [79].
Theories developed to explain charge selectivity have not been tested, but only adopted [10]. The adoption requires the use of an untested adjustable parameter, the apparent charge of the glomerular capillary wall to account for any anomalies.
Rippe's examples of evidence of charge selectivity all come from indirect experiments in physiological systems or complicated in vitro systems. This is where much of the problem lies, as invariably all factors that may influence albumin handling have not been delineated and we are still only coming to terms with these. For example, in many of the studies cited by Rippe there has been no recognition that albumin undergoing transcapillary exchange may undergo degradation and that material filtered may not be detected in the urine [7]. Albumin transport will be severely underestimated. Other experimental systems have demonstrated to be of questionable value. Ohlson et al. [11] have attempted to measure the glomerular sieving coefficient of albumin in a low temperature perfusion system. The low temperature perfusion was used with the intention of inhibiting all cellular processes. The system is complicated. These investigators have perfused rat kidneys in situ with a perfusate at 8°C while the rat was maintained at 37°C. Apart from the low perfusate temperature, the experimental conditions are quite different from the in vivo state as the perfusate contained human albumin at a concentration of 18 mg/ml (50% normal) and the glomerular filtration rate was only 1020% of normal in spite of the presence of vasodilators furosemide and nitroprusside. The investigators found differences between the excretion of albumin and uncharged Ficoll. The results were used to support the negative charge selectivity concept (they did not compare the fractional clearance of Ficoll with negatively charged Ficoll). The major outstanding issue of potential temperature dependent interactions governing urinary excretion of albumin and other charged proteins in the system that would normally not occur at 37°C was not investigated. It now appears that results in this system are problematic. A recent in vivo study of the fractional clearance of negatively charged Ficoll as compared with Ficoll revealed that negative charge selectivity did not existin fact for high radii the negatively charged Ficoll was facilitated in its transport [9].
Finally, some comment should be made of cited evidence of glomerular permselectivity as determined by the use of electron dense probes through the use of the electron microscope. A good deal of caution should accompany the interpretation of these types of studies particularly when conclusions concerning transglomerular transport are made. There is no a priori relationship between localization of the electron-dense probe and fractional clearance, as localization takes no account of the amount of material that has actually undergone transglomerular transport. Also, the ultrastructural localization is performed under non-equilibrium conditions, whereas fractional clearance is a steady state measurement. While such localization may mean genuine transport restriction, it may also represent a binding interaction. Overall, it is very difficult to use a static picture of the kidney to understand dynamic events such as transglomerular transport.
Conflict of interest statement. None declared.
1Department of Biochemistry and Molecular Biology Monash University 2Nephrology Section Department of Medicine Monash Medical Centre Clayton Victoria Australia Email: wayne.comper{at}med.monash.edu.au
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