Glucose Modulates Glucose Transporter Affinity, Glucokinase Activity, and Secretory Response in Rat Pancreatic β-Cells

Abstract

Pancreatic islets were cultured for 24 h in medium containing either low (1.4), normal (5.5), or high (16.7 mM) glucose, and then insulin secretion was measured at the end of 1 h incubation at 37 degrees C. Insulin release in the absence of glucose was 64 +/- 20, 152 +/- 11, and 284 +/- 30 pg.islet-1.h-1 (mean +/- SE, n = 6, G1.4 and G16.7 vs. G.5.5, P < 0.05) and the response to 22 mM glucose stimulation was 640 +/- 136, 2460 +/- 276, and 1890 +/- 172 pg.islet-1.h-1, respectively (n = 6, G1.4 vs. G5.5, P < 0.01, G16.7 vs. G5.5, P = 0.065). The 50% maximal response of insulin secretion (increment over baseline) was reached at an average glucose concentration of 9.9 +/- 0.7 mM in islets preexposed to G5.5, and at glucose 13.3 +/- 0.9 and 4.8 +/- 0.4 mM (P < 0.05 in respect to G5.5) in islets preexposed to G1.4 and G16.7, respectively. To investigate the molecular mechanism responsible for this altered glucose sensitivity, we measured, in parallel experiments, the kinetic characteristics of glucose transport, glucokinase, and glucose utilization. Glucose transport was measured by evaluating 3-O-methylglucose uptake. The apparent Km of the low-affinity transporter (GLUT2) was 16.6 +/- 2.4 mM in isolated pancreatic cells cultured at 5.5 mM glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged exposure of human beta cells to elevated glucose levels results in sustained cellular activation leading to a loss of glucose regulation.

“…This is in line with our observations on the biosynthetic activity of the cells. Our data are also comparable with those obtained previously in both in vivo and in vitro models of hyperglycemia (16,17,(31)(32)(33)(34). This study emphasizes that the insulin levels measured at low or at high glucose cannot be taken as an adequate parameter for the secretory activity of the ␤ cells, if they cannot be expressed as a function of the number of ␤ cells and their hormone content.…”

Interaction between genetic and dietary factors determines beta-cell function in Psammomys obesus, an animal model of type 2 diabetes.

“…The glucose-insulin dose-response curve in islets from animals of both P. obesus lines and on either diet revealed a left shift compared with islets from nondiabetic rats (mean EC 5 0 of 5.5-6.3 mmol/l in P. obesus versus 6.8-15 mmol/l in adult rat islets [9,28,29]), indicating that this increased responsiveness to glucose is species related. Superimposed on this genetic trait, an adaptive response of the DP P. obesus to nutritional load (HE diet) could be responsible for further left shift of the E C 5 0 of glucose to 3.7 ± 0.5 mmol/l (Fig.…”

Hexokinase Shift to Mitochondria Is Associated With an Increased Sensitivity to Glucose in Rat Pancreatic Islets

“…Glucose utilization. The utilization of glucose was determined by measuring the formation of 3 H 2 O from (5-3 H)glucose, as previously described (11). In brief, groups of 15 islets were incubated in 40 ul of Krebs-Ringer bicarbonate buffer containing 2 uCi r>(5-3 H)glucose and glucose concentrations ranging from 2.5 to 20 mmol/l.…”

“…Both in vivo and in vitro, after several hours of exposure to high glucose, (3-cells will become desensitized to glucose (1)(2)(3)(4)(5)(6)(7)(8). Before desensitization, however, an increased sensitivity to glucose will occur, indicated by a shift to the left of the dose-response curve of glucose-induced insulin release (9)(10)(11). In the present study, we investigated the molecular mechanism responsible for this latter phenomenon by measuring glucose phosphorylation, a process that plays a crucial role in regulating glucose metabolism in the (3-cell and, as a consequence, in glucose-induced insulin release.…”