Pancreatic β-cell dysfunction, expression of iNOS and the effect of phosphodiesterase inhibitors in human pancreatic islets of type 2 diabetes

Aims Induction of inducible nitric oxide synthase (iNOS) in pancreatic islets leads to exaggerated nitric oxide (NO) production associated with dysfunctional beta-cells. We examined insulin secretion, iNOS expression and its relationship to the cAMP system in islets from human type 2 diabetes. Methods Insulin, glucagon and cAMP were analysed by RIA; iNOS or phosphodiesterase (PDE) expression by quantitative PCR (qPCR), Western blot and confocal microscopy; cell viability by MTS. Results Diabetic islets displayed impaired insulin and glucagon responses to glucose, disturbed cAMP generation and high inducible nitric oxide synthase (iNOS) mRNA and protein expression. Confocal microscopy showed iNOS protein expression in diabetic islets being confined to insulin, glucagon and somatostatin cells. Culture of diabetic islets at 5.5 mmol/l glucose with dibutyryl-cAMP (Bt2-cAMP) for 24 h was accompanied by marked suppression of iNOS mRNA, reduced nitrite production and increased insulin secretion. Diabetic islets displayed marked increase in PDE3A and PDE3B mRNA expression. Short-time incubation of diabetic islets showed, among the PDE inhibitors tested, cilostazol being most favourable to increase insulin secretion. Diabetic islets were most susceptible to long-term (72 h) culture at high glucose (20 mmol/l) reacting with increased apoptosis. Bt2-cAMP and the PDE inhibitors cilostazol, milrinone and IBMX efficiently increased cell viability at high glucose during culture. Defective glucose-stimulated insulin release upon induction of iNOS was restored by iNOS inhibitor aminoguanidine. Conclusion Our results suggest that in islets from type 2 diabetes, stimulatory effects in certain cAMP-compartments induced by PDE inhibitors might play a central role in the suppression of iNOS, resulting in increased beta-cell viability and improved secretory response to glucose.