These studies evaluated the contribution of insulin to the development of the abnormal mesangial matrix that characterizes diabetic nephropathy and is common to mesangial cells in culture. Glomeruli were isolated from a single rat and divided into two aliquots. In one set (SI-MC), the insulin contained in the medium was only that contributed by the fetal calf serum (20%). For the other set, the tissue culture medium was supplemented with 1 microM insulin (SI+MC). Mesangial cell outgrowths from each condition were isolated, cloned, and propagated. At passage 4, mesangial cells were characterized by morphology and cell markers, and compared in terms of composition and appearance of the secreted extracellular matrix. SI-MC grew in nests of cells surrounded by a thin layer of matrix that was rich in collagen IV. In contrast, mesangial cells supplemented with insulin aggregated into macroscopic "hillocks" rich in collagens I and III as described previously. Insulin (1 microM) or IGF-I (0.1 microM) was subsequently added to the medium of SI-MC. Insulin, but not IGF-I, induced a change in culture morphology and collagen accumulation characteristic of SI+MC. In contrast to SI+MC, SI-MC express insulin receptors and at physiologic concentrations insulin is a more potent stimulator of MC proliferation than is IGF-I. Insulin-induced changes in the collagenous composition of the accumulated ECM were directionally correlated with the rate of collagen I synthesis measured by biosynthetic labeling experiments and collagens III and IV as determined by ELISA. These data demonstrate that insulin alters the phenotype of mesangial cells in culture and their expression of interstitial and basement membrane collagens. These observations implicate insulin as a factor in the pathogenesis of mesangial matrix accumulation in diabetic nephropathy. Furthermore, a method for culturing mesangial cells that accumulate an extracellular matrix that is similar in composition to normal mesangial matrix provides a new model system for future studies of mesangial cell biology.