Alterations in net glucose uptake and in the pancreatic B-cell GLUT2 transporter induced by diazoxide and by secretory stimuli

Abstract

The pancreatic B-cell GLUT2 transporter and glucose metabolism were examined in isolated rat islets subjected to treatments affecting insulin secretion. Diazoxide was used to inhibit, while glipizide or depolarization of the plasma membrane with a high extracellular K + concentration were used to stimulate insulin release in short-term experiments. Islet GLUT2 and insulin were determined by quantitative immunohistochemistry and GLUT2 was also determined by Western blot analysis. Islet net glucose uptake and glucose oxidation were measured using radioactively labelled glucose. Exposure of the islets to diazoxide was associated with a marked increase in the B-cell plasma membrane staining for GLUT2 and increased net glucose uptake. Glucose oxidation was not changed, which may reflect a lowered energy requirement. Conversely, islets subjected to a stimulated insulin secretion with glipizide or a high extracellular K + concentration showed a reduced staining of the GLUT2 transporter. The net glucose uptake and glucose oxidation were also reduced. In islets exposed to the high K + concentration no change in the molecular weight or phosphorylation of GLUT2 was observed but a lesser amount of the transporter was found by Western blot analysis. Thus, GLUT2 and glucose uptake in the pancreatic B-cell are modified by the secretory process, which suggests that changes in the glucose transporter have a functional role in normal B-cell physiology.

KATPchannel-deficient pancreatic β-cells are streptozotocin resistant because of lower GLUT2 activity

“…These changes in fluorescence intensity were observed within 30 min, and the authors argued that this time period was too short to affect the cell membrane GLUT2 protein content. Thus this study and the one by Zhao et al (26) demonstrate that modulating K ATP channel activity in ␤-cells affects the capacity of anti-GLUT2 antibodies to bind to their epitope under immunofluorescence conditions, especially in the COOH terminus. This characteristic appeared to be specific to pancreatic islets because labeling liver GLUT2 with anti-GLUT2(C-term) gave rise to similar fluorescence intensities in wild-type and Kir6.2 Ϫ/Ϫ mice.…”

“…Ϫ/Ϫ than in wild-type islets. Interestingly, glipizide, a K ATP channel blocker, also reduces glucose uptake in pancreatic islets (26). Together these results suggest that the absence of K ATP channel activity reduces GLUT2 activity in pancreatic ␤-cells.…”

“…Zhao et al (26) recently reported that the fluorescence intensity, following labeling with anti-GLUT2(C-term), was dramatically reduced when K ATP channels were acutely blocked with glipizide and increased when the channels were activated with diazoxide. These changes in fluorescence intensity were observed within 30 min, and the authors argued that this time period was too short to affect the cell membrane GLUT2 protein content.…”

“…However, Zhao et al (26) have shown that a prolonged K ϩ -induced depolarization of the cell membrane, which normally occurs when K ATP channels close, also results in a decrease GLUT2 activity. This suggests that the downregulation of GLUT2 activity appears downstream of the K ATP channel.…”

“…Null pancreatic ␤-cell lines for the GLUT2 gene do not transport STZ even if they overexpress GLUT1 (4,5,19). Finally, blocking K ATP channel activity in wild-type pancreatic ␤-cells reduces GLUT2 activity (26). We therefore hypothesized that Kir6.2 Ϫ/Ϫ mice are STZ resistant because STZ is not transported in the cytosol as GLUT2 activity is downregulated in Kir6.2 Ϫ/Ϫ pancreatic ␤-cell.…”