Influence of anoxia on glucose metabolism in pancreatic islets: Lack of correlation between fructose-1,6-diphosphate and apparent glycolytic flux

Abstract

Summary. When equilibrated.with O2-CO 2 (95:5), pancreatic islets of non-inbred ob/ob-mice eXhibited a sigmoidal dependence of 3H20 production on D-(5-aH)-glucose cone, entration; the rate was most sensitive to changes of glucose concentration around 5mM and tended to be maximum above about 15 mM glucose. 3H20 production from more than 5 mM D-(5-3H)-glncose was about twice as fast as the production of 14CO z from equimolar D-(U-14Q) -glucose. Islets equilibrated with N2-CO 2 (95:5) did not exhibit a sigmoidal dose-response curve for 3H20 production, the process being inhibited by anoxia at glucose concentrations above 5mM. Pieces of exoerine pancreas had a slower aerobic 3HzO production than the islets and showed a clear enhancement of the process during anoxia. In comparison with oxygenated islets, anoxic islets exhibited decreased concentrations of glucose-6-phosphate and increased concentrations of fructose-l,6-diphosphate. The concomitant inhibition of glycolytic flux may be due to a low lactate dehydrogenase activity in islets yielding a slow reoxidation of NADH and a slow phosphoglyceraldehyde oxidation under anaerobic conditions.Key words: Anoxia, glucose metabolism, pancreatic islets, Pasteur effect.Stimulation of insulin release by glucose may result from the production of a metabolic signal in the pancreatic B-cells [1][2][3][4]. The understanding of insulin secretion may thus be promoted by insight into the mechanisms that regulate glucose metabolism in these cells. In most other tissues glycolysis is faster during anaerobic than under aerobic conditions. This effect of oxygen, the Pasteur effect, is probably due to the phosphofructokinase being inhibited by ATP and by a fall of ADP [5]. Matschinsky et al. [6] reported the inhibitory influence of ATP on phosphofructokinase in islet extracts. We have investigated the effects of anoxia on glycolysis in the B-cell-rich islets of non-inbred ob/ob-mice by measuring the transmembrane transport of glucose, the islet contents of glucose-6-phosphate and fructose-l,6-diphosphate and the production of 3H20 from D-(5-3H)glucose. Materials and Methods GeneralFresh islets were isolated by free-hand microdissection [7] of pancreatic glands from adult, non-inbred ob/ob-mice from the Ume~ colony; the isolation was performed at 2 ~ with the glands suspended in KrebsRinger bicarbonate buffer equilibrated with O2-CO 2 (95:5). In a few experiments pieces of exocrine pancreas were taken for comparison with the islets.Krebs-Ringer bicarbonate buffer equilibrated with O2-COz (95:5) or N2-CO2 (95:5) was used as basal medium in subsequent incubations at 37 ~ . Isolated islets were subjected to preliminary incubation for 30 min at 37 ~ in the absence or presence (5 raM) of glucose. Incubation was then continued in basal medium supplemented with D-glucose as required; the sugar was added as pure a-D-glucose, but this was done long. before the incubation to allow equlibration of the a and/3 anomers. The detailed incubation procedure depended on the aim of the experiment as follo...

B cell insensitivity in a rat model of non-insulin-dependent diabetes. Evidence for a rapidly reversible effect of previous hyperglycemia.

“…The exaggerated response to this secretagogue in STZ, which has previously been documented (5) (21). Evidence that such a Pasteur effect is operative also in pancreatic islets is lacking (22), but cannot be ruled out. Interference with the signal for secretion normally exerted by exogenous glucose could also arise through the breakdown of glycogen.…”

Glycerol 3-phosphate-induced ATP production in intact mitochondria from pancreatic B-cells

“…In the glucose-stimulated B-cell there is a diminished content of NAD+ which may limit glycolysis at the glyceraldehyde-3-phosphate dehydrogenase level and restrict the supply of pyruvate for mitochondrial oxidation [29]. For Ca2+ to fulfil the alleged stimulatory effect through increased ATP (a) availability an efficient coupling of the free Ca2+ level to the oxidation of other mitochondrial fuels might be necessary.…”

Chemical Oxygenation of Pancreatic Tissue Prior to Islet Isolation and Transplantation

“…Since approximately 10% of the normal metabolic activity is still operative in ischemic tissue stored at 4°C, hypothermic organ perfusion and subsequent immersion in various preservation solutions such as University of Wisconsin solution (UWS) or Histidine-Tryptophan-Ketoglutarate (HTK) do not completely prevent irreversible pancreas injury once a critical period of cold ischemia is exceeded (Lakey et al, 1995). This can be explained by the specific preference of islets for the respiratory pathway of glucose breakdown providing more than 95% of the total islet ATP production (Erecinska et al, 1992) if an adequate supply oxygen for cellular energy generation is provided (Hellman et al, 1975, Malaisse et al, 1988, Sekine et al, 1994, Tamarit-Rodriguez et al, 1998. Although islets represent only 1% of the pancreatic tissue, they receive more than 12% of the total pancreatic blood flow (Lifson et al, 1980, Jansson andHellerstrom, 1983).…”

“…Nevertheless, among other organs the exocrine pancreas has the capability to produce low amounts of energy during ischemia utilizing the Pasteur effect i.e. the increased anaerobic generation of ATP via glycolysis (Hellman et al, 1975). The increased production of lactate causes intracellular acidosis which is one of the main potential triggers of the premature intracellular auto-activation of trypsinogen in acinar cells (Gorelick and Otani, 1999).…”