Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a potent fibroblast and epithelial cell mitogen that may be important in wound healing. The aim of this study was to determine its distribution and possible function in segmental renal infarction. At day 1 postinfarction, in situ hybridization showed that HB-EGF mRNA was markedly increased by tubular epithelial cells bordering the infarcted zone. At day 3, typical myofibroblasts expressing alpha-smooth muscle actin (alpha-SMA) were present in large numbers at the peri-ischemic border and, over succeeding days, were also seen within the infarcted area. Some of these cells expressed HB-EGF mRNA by in situ hybridization suggesting possible autocrine stimulation. Endothelial cells appeared to be more resistant to ischemia than tubules because some capillaries at the periphery of the infarct, surrounded by infarcted tubules, also expressed HB-EGF mRNA. The staining intensity of HB-EGF mRNA in individual tubules and endothelial cells was maximal at day 5 after infarction, although Northern blots of tissue from the peri-infarct area only showed significantly increased expression of HB-EGF mRNA at days 1 and 3, perhaps reflecting a smaller area of greater intensity of expression at day 5. Because tubular cells expressing high levels of HB-EGF mRNA were directly apposed to myofibroblasts, an attempt was made to determine whether HB-EGF contributed to upregulation of alpha-SMA by human fibroblasts. Although stimulation of the fibroblast cell line MRC-5 with transforming growth factor-beta1 (TGF-beta1) increased alpha-SMA, HB-EGF reduced expression. HB-EGF also strongly inhibited the increased expression of alpha-SMA due to TGF-beta1. Because HB-EGF is a potent fibroblast mitogen and TGF-beta is usually antiproliferative, this study suggests that HB-EGF may contribute to a local balance between fibroblast proliferation and differentiation into myofibroblasts during scarring.