The evaluation of urinary protein patterns in a stone-forming animal model using two-dimensional polyacrylamide gel...

The exact role of urinary proteins in kidney stone formation remains an area of controversy. Some investigators believe that urinary proteins are selectively incorporated within urinary calculi and as such have an active role in stone formation. Other investigators believe that urinary proteins are nonspecifically adsorbed into urinary crystals and thus have only a passive role in stone formation. In the current investigation a previously described stone-forming animal model (hyperoxaluric rat) was utilized along with an animal model for renal tubular injury (gentamicin nephrotoxicity) in an effort to clarify the role of urinary proteins in kidney stone formation. Urine specimens were collected before and after the induction of stone formation and after the induction of renal tubular injury. The purified proteins from each urine specimen were separated in two dimensions by electrophoresis resulting in a characteristic "map" of protein spots for each urine specimen. All animals in the stone-forming group had pathologic evidence of early stone formation (diffuse intranephronic calculosis). Early stone formation was consistently associated with a reduction in the excretion of low molecular weight urinary proteins (30,000 dalton and less than 20,000 dalton ranges). Alcian blue staining confirmed the presence of matrix within clumps of intranephronic calcium oxalate crystals. Renal tubular injury was associated with an increase in the excretion of low molecular weight proteins (approximate 20,000 dalton range) consistent with classical tubular proteinuria. These results suggest that low molecular weight urinary proteins are selectively incorporated within the crystalline structure of the stone early during its formation.