Age-dependent changes in the excretion of urinary proteins by the rat

The excretion of total urinary proteins (TUP) of rats beginning at weaning and extending to 12 months of age was correlated with the output of the sex-dependent alpha2u-globulin and albumin. At puberty, 40 days of age, the excretion of TUP corresponded to the output of alpha2u-globulin. At this age, alpha2u represented 30% of the total while albumin less than 10%. From 100 to 200 days of age, TUP remained constant while the excretion of albumin steadily increased. After 150--180 days of age, the concentrations of alpha2u and albumin in TUP were approximately equal. Thereafter, the excretion of albumin and TUP increased markedly whereas alpha2u excretion remained constant. At 373 days of age, albumin represented over 50--60% of the TUP while alpha2u was only 6--7%. Female rats which excrete little or no alpha2u exhibited a much lower level of proteinuria than the male during the first year. We suggest the existence of two phases of proteinuria in the male rat, namely, an early physiologic or alpha2u-globulinuric phase and a later albuminuric phase during which increasing quantities of plasma proteins, especially albumin, are lost.