One proposed role of glomerular mesangial cells is the regulation of glomerular blood flow by contraction. Alterations in the contractile activity of mesangial cells could lead to alterations in glomerular hemodynamics and then to glomerular injury. In this study, the effects of glucose and insulin on the contractile response of cloned homogeneous cultures of rat glomerular mesangial cells to angiotensin II were examined. Cells were cultured in normal-glucose medium (D-glucose at 200 mg/dl) and normal-glucose medium with added insulin (4 microgram/ml). To mimic the diabetic state, cells were cultured in high-glucose medium (D-glucose at 550 mg/dl) and high-glucose medium with added insulin. The media contained 20% fetal calf serum. Cells were grown for at least 1 wk in medium prior to contraction experiments. All clones of mesangial cells grown in the presence of additional insulin, in either normal- or high-glucose media, underwent contraction when treated with angiotensin II (0.001-10 microM). Seventy-five percent of the cells contracted. Not one contracted cell was seen in cultures grown without insulin in the medium, even when exposed to 10 microM angiotensin II. From these data, it appears that insulin may be required for the contractile response of mesangial cells to angiotensin II. Loss of contractile activity by mesangial cells in low- or no-insulin conditions (e.g., juvenile diabetes mellitus) could lead to a marked increase in glomerular blood flow, ultimately resulting in glomerulosclerosis.