Glutamine synthetase activity in rat urine as sensitive marker to detect S-3 segment-specific injury of proximal tubule induced by xenobiotics

The possibility of detecting segment-specific injury of the proximal tubule by means of urinary enzymes was investigated in rats. Urinary glutamine sy nthetase, an enzyme exclusively localized in the S-3 segment, and N-acetyl- beta-D-glucosaminidase, prevalently a S-1-S-2, but S-3 enzyme also, were de termined after single treatment with 100 mg/kg body wt. of hexachloro-1:3-b utadiene (HCBD; i.p.), toxic for the S-3 segment, or 25 mg/kg body wt. of p otassium dichromate (s.c.), toxic for the S-1-S-2 segments. Excretion of to tal urinary proteins was also measured. In addition, a dose-response relati onship was determined between three doses (50, 100 and 200 mg/kg body wt.) of HCBD and glutamine synthetase activity in urine. Glutamine synthetase ac tivity, measured according to a new assay for urine based on modification o f methods developed for organs, increased in the urine only when the S-3 se gment of the proximal tubule was damaged, as demonstrated by histological f indings of the kidneys. HCBD caused early excretion of the enzyme related t o the necrosis of the S-3 segment, whereas potassium dichromate caused a sl ight increase only when the resulting lesion to this segment (vacuolization ) began to develop. On the contrary, N-acetyl-beta-D-glucosaminidase activi ty showed the peak of excretion 24 and 34 h after treatment with HCBD or po tassium dichromate, respectively, according to the histological findings of necrosis of the S-3 segment (the former) and vacuolization of the S-1-S-2 segments (the latter). Excretion of total urinary proteins reached the peak 24 h (HCBD) and 48 h (potassium dichromate) after treatment. HCBD at 200 m g/kg body wt. caused a peak of glutamine synthetase activity in urine 10 h after injection, whereas the peak caused by doses of 50 and 100 mg/kg body wt. occurred 24 h following treatment. The peak of enzyme activity in urine significantly increased with the dose. The results suggest that the measur ement of urinary activity of S-3 segment-specific enzyme as glutamine synth etase allows us to detect early S-3 segment-specific injury of the proximal tubule. In addition, the method for urinary enzyme activity appears sensit ive, simple and fast.