Effects of High Glucose on Insulin Secretion by Isolated Rat Islets and Purified β-Cells and Possible Role of Glycosylation

Abstract

We investigated the effect of 24 h of exposure to various glucose concentrations on insulin secretion by isolated rat pancreatic islets and purified rat beta-cells. Compared with islets cultured with standard medium (5.5 mM glucose), islets cultured with 16.7 mM glucose showed a higher basal insulin release (means +/- SE, 3.0 +/- 0.5 vs. 0.7 +/- 0.2%, n = 8, P less than .005) and reduced glucose-stimulated insulin secretion (2.4 +/- 0.3 vs. 5.8 +/- 0.4%, n = 8, P less than .005). Similar results were also obtained with purified beta-cells. The effect of high glucose was time dependent (present after 12 h, maximal after 24 h) and reversible: when islets cultured with high glucose were transferred to standard medium, normal responsiveness to glucose was restored within 8 h and normal basal release within 24 h. Mannitol, 3-O-methylglucose, and 2-deoxyglucose were not able to mimic the effects of glucose. Islets or purified beta-cells cultured in the presence of high glucose had a normal response when stimulated with glyburide, dibutyryl cyclic AMP, and isobutylmethylxanthine. Tunicamycin, an inhibitor of N-terminal glycosylation, prevented glucose-induced desensitization when added during 24 h of islet culture with 16.7 mM glucose. Swainsonine, another agent that influences glycosylation, had a similar effect. Our study indicates 1) that 24 h of exposure to high glucose induces a specific and reversible impairment of insulin secretion in response to glucose, 2) that this is a direct effect of glucose on beta-cells, and 3) that islet glucose metabolism and glycosylation processes may play a critical role in determining glucose desensitization.

Metformin restores insulin secretion altered by chronic exposure to free fatty acids or high glucose: a direct metformin effect on pancreatic beta-cells.

“…However, whether metformin also affects glucose and FFA oxidation in other tissues, including the pancreatic ␤-cells, is unknown. This may be a relevant issue because chronic elevation of glucose or FFAs is known to inhibit insulin secretion in isolated pancreatic islets (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21), and these abnormalities occur frequently in diabetic patients and negatively affect ␤-cell function. Moreover, experiments in isolated rat islets indicate that a glucose-FFA cycle (termed the Randle cycle [22]) with a reciprocal relationship between glucose and FFA metabolism is operative in the ␤-cell (23).…”

“…Glucose oxidation was determined by measuring the formation of 14 CO 2 from [U-14 C]glucose (27). Groups of 15 islets were incubated in a plastic cup in 100 µl of KRBB (118 mmol/l NaCl, 4.8 mmol/l KCl, 2.5 mmol/l CaCl 2 , 1.2 mmol/l MgSO 4 , 1.2 mmol/l KH 2 PO 4 , 25 mmol/l NaHCO 3 ) supplemented with 10 mmol/l HEPES (pH 7.4) containing 3 µCi D-[U- 14 C]glucose (specific activity, 302 mCi/mmol) plus nonradioactive glucose to a final concentration of either 1.5 or 16.7 mmol/l. The cups, which were suspended in standard 20-ml glass scintillation vials, were gassed with O 2 :CO 2 (95:5%) and were capped airtight.…”

Role of ATP Production and Uncoupling Protein-2 in the Insulin Secretory Defect Induced by Chronic Exposure to High Glucose or Free Fatty Acids and Effects of Peroxisome Proliferator-Activated Receptor-γ Inhibition

“…In these patients, the mechanisms that cause the progressive ␤-cell failure are currently under investigation: the altered insulin secretory pattern depends, at least in part, on the negative influence of chronic high glucose (5)(6)(7)(8) and/or high free fatty acid (FFA) (9 -12) plasma concentrations (gluco-or lipotoxicity). These metabolites are believed to affect pancreatic ␤-cell function by chronic ␤-cell stimulation and consequent "desensitization" to glucose.…”

Glibenclamide but not other sulphonylureas stimulates release of neuropeptide Y from perifused rat islets and hamster insulinoma cells

“…Although the sulphonylureas continue to be used in the treatment of type 2 diabetes mellitus, long-term clinical studies have reported a secondary failure in increasing insulin secretion (Groop 1992). The cause of this 'secondary-failure' has been the subject of several investigations and have been ascribed to ' -cell exhaustion' (Schauder et al 1977, Gullo et al 1991, Rabuazzo et al 1992) and/or ' -cell desensitization' to glucose (Hoenig et al 1986, Bolaffi et al 1988, Purrello et al 1989 or other secretagogues (Grodsky 1989). An in vitro experiment (Gullo et al 1991) has shown that 24 h exposure of islets to 100 nmol/l glibenclamide did not deplete the insulin content, suggesting that mechanisms other than insulin depletion are operative which contribute to the reduced insulin.…”