We have succeeded in obtaining cultures of pure rat islet capillary endothelial cells. These multiply in vitro and exhibit the same antigenic phenotype as expressed in situ: von Willebrand factorhigh, Ox43 (rat endothelial marker)weak, and Ox2 (thymocyte and brain endothelium marker)high. This phenotype differs from both exocrine endothelium stained in situ and rat aorta endothelial cells cultured in vitro under identical conditions. Islet and aorta endothelial cells were cultured in the presence of various glucose concentrations. Nitrite and citrulline concentrations in culture supernatants were measured as an indirect quantification of nitric oxide formation. In islet endothelia, both nitrite and citrulline levels were found to be strongly glucose-dependent, with high levels at high glucose concentrations and vice versa, in contrast to aorta endothelial cells, where no glucose effect was found. Shifting islet endothelial cultures from high to low glucose levels or the reverse led to a slow decrease or increase in nitrite and citrulline formation with several cell generations needed to reach steady levels. Adding a combination of the cytokines interleukin-1 beta, tumor necrosis factor-alpha, and interferon-gamma to both endothelial cell cultures led to a dramatic increase of nitric oxide formation. Again with islet but not with aorta endothelial cells a modulating effect by glucose concentrations was found. Reverse-transcription-polymerase chain reaction with specific primers demonstrated the presence of constitutively expressed nitric oxide synthase-RNA in the islet capillary endothelial cells and confirmed the glucose effect. In addition, we found that cytokines indeed induce the expression of inducible synthase messenger RNA in both endothelial cells, which was not found in the absence of cytokines. Electron paramagnetic resonance spectroscopy of islet endothelial cells confirmed intracellular synthesis of nitric oxide in the presence of cytokines. In conclusion, we here for the first time provide evidence that constitutive nitric oxide synthase is also expressed in capillary endothelium and that cytokine challenge leads to the expression of the inducible isoform in these cells.