Background: Leukocyte adhesion to the vascular endothelium is mediated by endothelial adhesion molecules binding to their leukocyte ligands. The endothelial adhesion molecules are embedded in the glycocalyx, a dense network of membrane-bound proteoglycans and glycoproteins. In health, the glycocalyx contributes to the anti-inflammatory regulation of the vascular wall, effectively ‘hiding’ the adhesion molecules from the circulating leukocytes. However, in pathological conditions associated with glycocalyx loss such as diabetes this protective mechanism is lost, possibly contributing to the associated microangiopathies e.g. diabetic retinopathy. Indeed, the early stages of retinopathy are characterised by increased leukocyte adhesion, contributing to inflammation and vessel blockage. Thus, strategies to maintain glycocalyx integrity may be beneficial in the treatment of diabetic retinopathy. It is possible that diabetes-associated glycocalyx loss is due to reduced bioavailability of H2S. We have shown that plasma H2S levels are significantly lower in individuals with type 2 diabetes compared with sex/age/BMI matched controls, and that reduced H2S levels correlate with microvascular dysfunction. However, the direct effects of H2S on the glycocalyx are unknown. Aim: To investigate (a) the effects of the slow release H2S donor GYY4137 on hyperglycaemia-induced changes to the glycocalyx and leukocyte adhesion and (b) potential mechanisms of action of GYY4137, specifically endothelial KATP channel activation. Methods: Bovine retinal endothelial cells (BREC) were exposed to 5.6mM glucose (NG) and 25mM glucose (HG) ± GYY4137 for 24h. A cell based fluorescence assay was used to study glycocalyx changes. Glibenclamide (Glib), gliclazide (Glic) and cromakalim (Crm) were used to test involvement of endothelial KATP channels. Leukocyte adhesion to BREC was measured at a shear stress of 1dyn/cm2. Data are presented as mean±SD. Results: HG-induced glycocalyx degradation was reversed by GYY4137 [HG 93±5% vs HG + GYY 107±8%, p<0.001, n=12; control,100%)]. Crm, Glib or Glic had no effect on glycocalyx changes. HG enhanced leukocyte adhesion and this was attenuated by H2S [HG 348±110% vs HG GYY 124±47%; p=0.028, n=4; NG control,100%]. The H2S mediated decrease in adhesion was not inhibited by Glib or Glic suggesting no involvement of endothelial KATP channel activation. H2S added simultaneously with HG to BREC pre-treated with Crm did not affect leukostasis. Conclusions: Glycocalyx degradation and increased leukocyte adhesion induced by HG were reversed by GYY4137, highlighting the therapeutic potential for H2S donors in hyperglycaemic conditions.