Increased synthesis of fibronectin is associated with the development of basement membrane thickening-a characteristic lesion of diabetic microangiopathy, and it may affect the function of vascular cells. Because antisense technology offers the possibility to modulate specific gene expression, we investigated the effect of antisense phosphorothioate oligonucleotides directed against fibronectin mRNA on fibronectin synthesis and cell proliferation in a cell line derived from rat microvascular endothelium and cultured under high (30 mmol/l) glucose conditions. The rat endothelial cells grown in high glucose medium for 5.6 +/- 1.3 days exhibited increased cell proliferation compared to control cells grown in 5 mmol/l glucose (149% of control, p = 0.006). Fibronectin protein and mRNA levels (determined by Western blotting and reverse transcription-polymerase chain reaction) were also increased to 157%, p = 0.012 and 178%, p = 0.034 of control, respectively. However, when cells grown in high glucose medium were transfected with 0.4 mmol/l fibronectin-antisense phosphorothioate oligonucleotides in the presence of cationic liposomes, the cell number, fibronectin protein, and mRNA levels decreased compared to untransfected cells grown in high glucose medium to 102, 69, and 107% of control, respectively. This study shows that fibronectin antisense oligonucleotides targeted to the translation initiation site of the fibronectin transcript specifically reduce fibronectin synthesis in rat endothelial cells and the proliferative effect of high glucose concentrations.