Proteinuria – selected issues

Proteins in the urine of healthy individuals of various species are present in trace, practically immeasurable amounts. Under physiological conditions, the renal glomeruli filter out proteins with a molecular weight below 69 kDa, which are then almost completely reabsorbed in the proximal tubules. Occasionally, as an effect of low temperature, physical exercise, rapid change of body position, high-protein diet, medications, or at the end of pregnancy and in the postnatal period etc., proteins may be present in the urine of healthy individuals in higher amounts. This condition is referred to as physiological proteinuria. Most often, however, proteinuria is a symptom of a kidney disorder and may lead to further damage, eventually to renal failure. Proteinuria may be a result of: (a) increased penetration of proteins, mainly of low molecular weight, through the normal filtration membrane and the inability to absorb the increased amount of proteins in the proximal tubules – overflow proteinuria, (b) increased permeability of the glomerular filtration barrier, most often as a result of its damage – glomerular proteinuria, (c) damage to the renal tubules due to failure of reabsorption mechanisms – tubular proteinuria. Excretion of larger amounts of protein in the urine is always indicative of dysfunction of the kidneys and/or of the urinary tract. Having knowledge on the kind of excreted proteins (in terms of weight/size of the molecules) is very useful in medical and veterinary practice, as it enables early identification of the causes of proteinuria and distinguishing its etiology. In recent years, much attention has been paid to the role of uromodulin as a diagnostic marker of an early phase of renal dysfunction, especially of the tubules. The observations on the interaction of the digestive and excretory systems in the regulation of proteonemia in the postnatal period also seem to be important.