Role of Cytosolic Ca²⁺ in Impaired Sensitivity to Glucose of Rat Pancreatic Islets Exposed to High Glucose In Vitro

Sustained exposure to high concentrations of glucose selectively impairs the ability of pancreatic islets to secrete insulin in acute glucose stimulation. In order to evaluate the interrelationship between impaired insulin secretion and the dynamics of the cytosolic free Ca²⁺ level ([Ca²⁺]i), we have investigated the effect of high glucose exposure on both [Ca²⁺]i dynamics in single rat β-cells and insulin release from rat pancreatic islets. Islets cultured at a high glucose concentration (16.7 mM) for 24 h showed significant reductions of the 16.7 mM GSIR compared with islets cultured at a normal glucose concentration (5.5 mM) (3.38 ± 0.24 vs. 4.26 ± 0.34%, respectively, P < 0.05). The capacity of glucose to raise the [Ca²⁺]i level also was significantly reduced in the β-cells maintained for 24 h at 16.7 mM glucose (P < 0.001). An additional culture in the medium with 5.5 mM glucose for 16 h restored both the GSIR and the [Ca²⁺]i response of islets cultured at high glucose. On the other hand, insulin release and [Ca²⁺]i rise in response to 20 mM L-Arg were well preserved. These observations confirm that exposure of pancreatic β-cells to high glucose concentrations induces a selective reduction of the GSIR and, further, shows that this impaired response is reversibly restored by an additional culture with normal glucose. We also suggest that the inability of glucose to provoke a [Ca²⁺]i rise, which is observed in the β-cells exposed to high glucose, may be responsible for the selective impairment of the GSIR.