Insulin-like growth factors are growth-promoting peptides structurally related to insulin which possess autocrine and paracrine activities. IGF-I is mainly synthesized in hepatocytes, but little is known about its paracrine action on hepatocyte-adjacent perisinusoidal hepatic stellate cells, the principal matrix producing precursor cell type in the liver. IGF-I might stimulate proliferation and phenotypical transformation of hepatic stellate cells into myofibroblasts, the cell type responsible for the excessive production of connective tissue elements during fibrogenesis. In this study we investigated the expression and function of the IGF-I receptor during transformation of isolated and cultured hepatic stellate cells. The respective stages of transformation of hepatic stellate cells were defined by determination of cellular smooth muscle iso-alpha-actin and retinyl-palmitate content, respectively. IGF-I receptor protein decreased stage-dependently down to 0.5 at the 4th day and about 0.17 at the 8th day. The number of IGF-I receptors was determined to be initially 1.3 x 10(5)/cell. Their quantity decreased to 0.8 x 10(5) sites/cell (4th day) down to 0.5 x 10(5) sites/cell at the 7th day and remained constant thereafter at 0.7 x 10(5) sites/cell. Dissociation constants (KD) for IGF-I range from 0.32-0.57 nmol/l showing constantly high receptor affinity. Northern blot analyses revealed distinct upregulation of IGF-I receptor mRNA level during culture. It is concluded that hepatocyte-generated IGF-I and/or IGF-I binding proteins are candidate mediators of hepatic stellate cell activation during the initial period of transformation to myofibroblasts. After completion of transformation the cell becomes relatively refractory to the action of IGF-I as judged from receptor density.