Coupling of β-Cell Desensitization by Hyperglycemia to Excessive Stimulation and Circulating Insulin in Glucose-Infused Rats

Nondiabetic rats were infused with glucose for 48 h to maintain moderate or marked hyperglycemia (mean blood glucose 13.2 ± 0.7 or 22.8 ± 0.3 mM, respectively). The two levels of hyperglycemia increased plasma insulin levels severalfold but decreased the insulin response to 27 mM glucose by 19 and 95%, respectively, versus saline infusion. Diazoxide (5 mg ± kg⁻¹ · h⁻¹), when continuously infused during the hyperglycemia protocols, completely inhibited the glucose-induced rise in plasma insulin levels. Diazoxide transformed β-cell insensitivity to stimulation: glucose-induced insulin release was thus increased 318% after moderate hyperglycemia and 707% after marked hyperglycemia. These stimulatory effects of diazoxide were reversed by exogenous insulin infusion (8 or 2 U/24 h) in a dose-dependent manner. It is concluded that excessive β-cell stimulation rather than glucotoxicity underlies hyperglycemia-induced β-cell insensitivity. Effects of hyperinsulinemia can form part of the mechanisms whereby excessive stimulation affects β-cell secretion.