Defective Stimulus-Secretion Coupling in Islets of Psammomys obesus, an Animal Model for Type 2 Diabetes

Abstract

Psammomys obesus is a model of type 2 diabetes that displays resistance to insulin and deranged β-cell response to glucose. We examined the major signaling pathways for insulin release in P. obesus islets. Islets from hyperglycemic animals utilized twice as much glucose as islets from normoglycemic diabetes-prone or diabetes-resistant controls but exhibited similar rates of glucose oxidation. Fractional oxidation of glucose was constant in control islets over a range of concentrations, whereas islets from hyperglycemic P. obesus showed a decline at high glucose. The mitochondrial substrates α-ketoisocaproate and monomethyl succinate had no effect on insulin secretion in P. obesus islets. Basal insulin release in islets from diabetes-resistant P. obesus was unaffected by glucagon-like peptide 1 (GLP-1) or forskolin, whereas that of islets of the diabetic line was augmented by the drugs. GLP-1 and forskolin potentiated the insulin response to maximal (11.1 mmol/l) glucose in islets from all groups. The phorbol ester phorbol myristic acid (PMA) potentiated basal insulin release in islets from prediabetic animals, but not those from hyperglycemic or diabetes-resistant P. obesus. At the maximal stimulatory glucose concentration, PMA potentiated insulin response in islets from normoglycemic prediabetic and diabetes-resistant P. obesus but had no effect on islets from hyperglycemic P. obesus. Maintenance of islets from hyperglycemic P. obesus for 18 h in low (3.3 mmol/l) glucose in the presence of diazoxide (375 μmol/l) dramatically improved the insulin response to glucose and restored the responsiveness to PMA. Immunohistochemical analysis indicated that hyperglycemia was associated with reduced expression ofα-protein kinase C (PKC) and diminished translocation of λ-PKC. In summary, we found that 1) P. obesus islets have low oxidative capacity, probably resulting in limited ability to generate ATP to initiate and drive the insulin secretion; 2) insulin response potentiated by cyclic AMP—dependent protein kinase is intact in P. obesus islets, and increased sensitivity to GLP-1 or forskolin in the diabetic line may be secondary to increased sensitivity to glucose; and 3) islets of hyperglycemic P. obesus display reduced expression of α-PKC and diminished translocation of λ-PKC associated with impaired response to PMA. We conclude that low β-cell oxidative capacity coupled with impaired PKC-dependent signaling may contribute to the animals' poor adaptation to a high-energy diet.