Abstract
Albumin is filtered through the glomerulus with a sieving coefficient of 0.00062, which results in approximately 3.3 g of albumin filtered daily in human kidneys. The proximal convoluted tubule reabsorbs 71%, the loop of Henle and distal tubule 23%, and collecting duct 3% of the glomerular filtered albumin, thus indicating that the kidney plays an important role in protein metabolism. Dysfunction of albumin reabsorption in the proximal tubules, due to reduced megalin expression, may explain the microalbuminuria in early-stage diabetes. Meanwhile, massive nonselective proteinuria is ascribed to various disorders of the glomerular filtration barrier, including podocyte detachment, glomerular basement membrane rupture, and slit diaphragm dysfunction in focal segmental glomerulosclerosis, membranous nephropathy, and other glomerulonephritis. Selective albuminuria associated with foot process effacement and tight junction-like slit alteration is observed in the patients with minimal-change nephrotic syndrome, and the albumin uptake is enhanced in the podocyte cell body, possibly mediated by albumin receptors in the low-dose puromycin model. The role of enhanced podocyte albumin transport needs to be investigated to elucidate the mechanism of the selective albuminuria in minimal-change disease.
Highlights
The kidneys are responsible for maintaining the homeostasis of body fluids by the regulation of water balance, electrolyte balance, acid-base balance, and excretion of uremic toxins, and production of various hormones such as renin, erythropoietin, and activation of vitamin D3
This paper describes the mechanisms and pathways of glomerular albumin filtration and the amount of tubular reabsorption of albumin along the nephron in normal and pathological conditions based on our previous micropuncture studies
The glomerular-sieving coefficient of albumin is 0.00062, and the kidney plays an important role in protein metabolism
Summary
The kidneys are responsible for maintaining the homeostasis of body fluids by the regulation of water balance, electrolyte balance, acid-base balance, and excretion of uremic toxins, and production of various hormones such as renin, erythropoietin, and activation of vitamin D3. This paper describes the mechanisms and pathways of glomerular albumin filtration and the amount of tubular reabsorption of albumin along the nephron in normal and pathological conditions based on our previous micropuncture studies. The concept that glomerular albumin filtration is restricted by the size and charge barriers of the glomerular basement membrane, and by the fine pores of the slit diaphragm, is widely accepted. Glomerular albumin filtration could be performed by the diffusion of albumin back and forth across the GBM [3], how albumin molecules can diffuse out across the effaced podocyte foot processes entirely covering the basement membrane in minimal-change nephrotic syndrome remains unclear. This paper discusses the ultrastructural morphological changes of the glomerular filtration barrier in various glomerular diseases and proposes a new mechanism of glomerular albumin filtration in minimal-change nephrotic syndrome
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