The study of urinary protein excretion in male rats as compared with human proteinuria

Male rats are widely used for comparative investigations in renal physiology and pathology. However, they demonstrate a striking physiological proteinuria which must be considered if protein excretion is one parameter in the description of a model. Electrophoretic differentiation of urinary proteins shows that the major portion of male urinary protein consists of low molecular weight proteins which are strain-, sex- and age-dependent. The excretion of these proteins depends mainly on sex hormones and only to a lesser degree on kidney function. The excretion of albumin is a sensitive indicator to detect glomerular permeability changes, mainly because the tubular transport maximum is restricted and because the concentration difference between glomerular ultrafiltrate and plasma is very large, so that a small increase in permeability may cause a greatly elevated albumin excretion. Therefore, selective determination of albumin, e.g., by electrophoretic separation, is indispensable to estimate the renal permeability. Complicating the evaluation of experimental results, spontaneous kidney disease (progressive glomerulonephritis or glomerulosclerosis) develops in aging rats. Therefore, long-term studies in rats should be avoided because this spontaneous kidney disease with concomitant changes in protein excretion might interfere with experimental results.