A GLP-1 receptor agonist enhances islet microvascular flow through nitric oxide-dependent mechanisms in a diabetic...

Impaired regulation of islet microvascular blood flow has been implicated in beta cell dysfunction in diabetes; this dysregulation is linked to endothelial dysfunction with reduced nitric oxide bioavailability. However, dynamic regulation of islet microvascular flow has not been directly examined in vivo. We hypothesised that acute pharmacological activation of the glucagon-like peptide-1 receptor (GLP-1R) rapidly enhances islet microvascular flow through NO-dependent mechanisms under diabetic conditions.

Islet microvascular dynamics were visualised in vivo using intravital two-photon microscopy in mice with hyperglycaemia induced by multiple low doses of streptozocin (STZ), which represents a model of beta cell dysfunction, and in normoglycaemic controls. Acute vascular responses to the GLP-1R agonist liraglutide were quantified by measuring peri-islet vascular volume fraction and erythrocyte velocity, with or without inhibition of nitric oxide synthase using Nω-nitro-L-arginine methyl ester (L-NAME). To assess the physiological relevance of the results, a subset of diabetic mice received chronic liraglutide treatment, followed by analyses of islet hypoxia and beta cell function.

Acute liraglutide administration in the diabetic mice rapidly increased peri-islet vascular volume fraction (p=0.010) and erythrocyte velocity (p=0.003). These vascular responses were completely abolished by treatment with L-NAME, and were absent in normoglycaemic mice, indicating a diabetes-specific, NO-dependent microvascular response. Chronic liraglutide treatment was associated with reduced islet hypoxia and improved glucose-stimulated insulin secretion (p=0.042), supporting the physiological relevance of acute microvascular enhancement.

Acute pharmacological activation of the GLP-1R rapidly enhances NO-dependent islet microvascular flow specifically under diabetic conditions. This previously unrecognised in vivo vascular response indicates dynamic regulation of the islet microenvironment by GLP-1R signalling, and suggests that rapid microvascular adaptation may contribute to preservation of beta cell function during metabolic stress.