Translocation of glucokinase in pancreatic beta-cells during acute and chronic hyperglycemia

Glucokinase (GK) plays a key role in the regulation of glucose-induced insulin secretion, and questions have been raised about its relationship to the glucose transporter GLUT2 and its function in diabetes. This study examined the location of immunostained GK and GLUT2 in beta-cells using confocal microscopy. On double stained sections from pancreases of normal fed rats, GLUT2 Texas Red staining was restricted to the plasma membrane, and GK fluorescein isothiocyanate staining was found in a limited area of cytoplasm that was perinuclear with slight extension toward the apical pole. The GK staining occupied 8.6 +/- 1.7% of total cytoplasmic area and was almost never adjacent to the GLUT2 staining of the plasma membrane. To determine whether the GK staining pattern is altered by metabolic perturbation, normal rats were made acutely hyperglycemic with iv glucose injections; after 20 min the GK staining changed from being localized to become diffusely distributed throughout the cytoplasm. To examine the influence of chronic hyperglycemia, rats were subjected to 90% partial pancreatectomy (Px), which produced glucose levels of 10.9-20.8 mM. When studied 6 or 14 days after Px, those rats with glucose levels greater than 17.7 mM had an altered GK staining pattern that was variable; in some beta-cells GK staining was diffuse and in others the localized staining pattern was preserved. GLUT2 staining was reduced overall, but variability between cells was observed, unlike the more uniform reductions seen with hyperglycemia of longer duration. Other rats received islet transplants to prevent hyperglycemia after Px; their GK and GLUT2 staining patterns were normal. These findings indicate that GK is translocated in association with acute and chronic hyperglycemia. The translocation of this key enzyme for glucose recognition by beta-cells may lead to altered rates of insulin secretion during acute perturbations of fuel provision and in the diabetic state.