Indirect and direct studies indicate that a variety of proteins filtered at the glomerulus are absorbed by the proximal tubule by luminal endocytosis and are hydrolyzed by lysosomal enzymes. Studies on isolated, perfused tubular segments have not demonstrated contraluminal uptake of albumin or insulin. Evidence for transport of intact protein molecules across tubular cells is inconclusive, and additional studies are required to resolve this problem. Experimental findings suggest that the kidney plays an important role in albumin homeostasis. In health, greater than 90% of the small amount of albumin filtered by the glomerulus is absorbed by the proximal tubule and digested therein, which accounts for approximately 10% of total albumin catabolism. Only trace amounts of albumin appear in the urine. In disease, large loads of filtered albumin probably saturate the absorptive-digestive capacity of the proximal tubule and albuminuria ensues. In this circumstance, the kidneys may account for 30–40% of total albumin catabolism.

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