Type I and type III collagen mRNA levels in kidney regions of old and young rats.

Interstitial fibrosis is a common feature of renal aging. The steady-state levels of type I and type III collagen mRNAs as well as DNA, protein and collagen deposition were investigated in the cortex, inner and outer medulla of aged (22 months old) rats in comparison to young (5 months old) controls. Our data show that the cortex and outer medulla of old rats expressed significantly higher percentage of type I collagen mRNA compared to the respective regions in the young rat kidneys. Moreover, within the group of the old rats, the cortex expressed significantly higher percentage of type I collagen mRNA compared to the inner medulla whereas in the group of the young rats the expression was similar in all kidney regions. The ratio of extracellular collagen to DNA was significantly higher in the cortex, inner and outer medulla of old compared to young rats. The ratio of collagen to total protein, although showing a similar age-related difference, attained statistical significance in the cortex only. Thus, the present study indicates a close relationship between the expression of the mRNA for type I collagen, the major structural constituent of fibrotic tissues, and the deposition of collagen in both the cortex and outer medulla of the kidney. Moreover, the clear differences found between old and young rat kidneys can serve as markers for renal aging and might explain at least some of the kidney impairments caused by fibrosis during senescence.