Myofibroblast and α₁(III) collagen expression in experimental tubulointerstitial nephritis. Despite the importance of tubulointerstitial fibrosis as a predictor of renal function in patients with primary glomerular disease, the identity of the cell(s) that is the source of interstitial collagen production remains unknown. The present study was performed to identify the site of α₁(III) production during the development of tubulointerstitial fibrosis. We studied a model of experimental tubulointerstitial nephritis associated with puromycin aminonucleoside (PAN) nephrosis. There was a twofold increase in renal cortical α₁(III) mRNA expression coincident with the onset of tubulointerstitial myofibroblasts infiltration in rats with PAN nephrosis beginning on day 6, which increased to a fivefold difference by day 10. There were 60.8 ± 40.3 myofibroblast/mm² within the renal tubulointerstitium of rats with PAN nephrosis on day 6 that peaked at 240.2 ± 11.1 myofibroblast/mm² on day 14, which then declined to 43.7 ± 9.8 myofibroblast/mm² by day 21. By combining in situ hybridization with immunohistochemistry, α₁(III) mRNA expression was colocalized to cells that labeled for α-smooth muscle actin identifying them as myofibroblasts. Interestingly, the major site of α₁(III) mRNA expression shifted to tubuloepithelial cells with the waning of myofibroblast infiltration on day 21. To determine if PDGF-BB induced myofibroblasts to synthesize α₁(III) mRNA, we examined kidneys from rats that had been treated with PDGF-BB (5 mg/kg/day). α₁(III) mRNA expression also localized to cells that labeled for α-smooth muscle actin. These data demonstrate the cellular source of α₁(III) production within the renal tubulointerstitium following injury, and suggest that PDGF-BB may be mediating this production.