Dihydroxyacetone-induced Oscillations in Cytoplasmic Free Ca2+ and the ATP/ADP Ratio in Pancreatic β-Cells at Substimulatory Glucose

Abstract

Glucose stimulation of pancreatic ␤-cells causes oscillatory influx of Ca 2؉ , leading to pulsatile insulin secretion. We have proposed that this is due to oscillations of glycolysis and the ATP/ADP ratio, which modulate the activity of ATP-sensitive K ؉ channels. We show here that dihydroxyacetone, a secretagogue that feeds into glycolysis below the putative oscillator phosphofructokinase, could cause a single initial peak in ] i oscillations with a period of minutes, which correspond to oscillations in insulin secretion (1-3). Similar oscillations in insulin secretion are observed in vivo and are abnormal or diminished in type 2 diabetes (4 -6); loss of oscillations may reduce the effectiveness of insulin (7,8) and thus contribute to the development of the disease. In addition to this K ATP channel-mediated action to raise [Ca 2ϩ ] i , there is also a K ATP channel-independent stimulation of secretion by glucose (9, 10), which may involve the ATP/ADP ratio (9) or lipid signals (11). Because oscillations in insulin secretion can be seen under conditions in which [Ca 2ϩ ] i is not changing, it has been proposed that the underlying oscillator is metabolic rather than ionic (12). In particular, we have proposed that the basis of these oscillations in the pancreatic ␤-cell is the spontaneous oscillations of glycolysis and the ATP/ADP ratio due to pulsed action of the muscle isoform of phosphofructokinase (PFK-M), which is present in ␤-cells (13). As demonstrated previously in muscle extracts, such oscillatory behavior can result from autocatalytic activation of PFK-M by its product, fructose 1,6-bisphosphate (Fru-1,6-P 2 ) (14, 15). The occurrence of glycolytic oscillations in the ␤-cell is supported by observations of similar oscillations in [Ca 2ϩ ] i , glucose 6-phosphate, and the ATP/ADP ratio (16,17), as well as oscillatory release of lactate from perifused islets (18).As a further test of this hypothesized role of glycolytic oscillations, we examined the actions of the trioses dihydroxyacetone (DHA) and glyceraldehyde (GA) that on phosphorylation feed into glycolysis downstream of the phosphofructokinase reaction. The trioses can also serve as secretagogues (19 -21), though they have not been shown to cause oscillations in [Ca 2ϩ ] i or insulin secretion. Of the two, GA is the more potent and the more widely studied. However, there is concern that it may not be a suitable glucose mimic in that it causes strong cellular acidification, most likely because of direct metabolism by glyceraldehyde-3-phosphate dehydrogenase and perhaps phosphoglycerate kinase to produce glyceric acid (22). We therefore focused on DHA, although some experiments were also performed with GA for comparison. Furthermore, we previously showed in the glycolyzing muscle extract system that the addition of DHA-P could trigger a pulse of PFK activity by

Ca2+ controls slow NAD(P)H oscillations in glucose‐stimulated mouse pancreatic islets

“…The phase relationship observed in our steady-state recordings is consistent with a scenario where metabolic oscillations, via regulation of the K ATP channel, induce slow oscillations of [Ca 2+ ] c and, ultimately, insulin secretion. Metabolic oscillations in the β-cell have been proposed to arise from intrinsic glycolytic mechanisms involving the allosteric feedback-activation of phosphofructokinase (Tornheim, 1997;Juntti-Berggren et al 2003). That glycolytic flux is oscillatory has been demonstrated by recordings of oscillations in islet glucose consumption (Jung et al 2000) and islet lactate release (Chou et al 1992).…”

“…Reports of metabolic oscillations in β-cells and islets are consistent with this (Longo et al 1991;Jung et al 2000;Ortsater et al 2000;Ainscow & Rutter, 2002); however, the basis of such oscillations is debated. One possibility is that they are a consequence of enzymatic feedback mechanisms inherent to glycolysis (Tornheim, 1997;Juntti-Berggren et al 2003). However, the picture may be more complex, as increases in cytosolic Ca 2+ concentration ([Ca 2+ ] c ) evoked by glucose, or other agonists that elevate [Ca 2+ ] c , are rapidly relayed to the mitochondria of β-cells in primary islets (Ainscow & Rutter, 2001) and insulin secreting cell lines (Rutter et al 1993;Kennedy et al 1996;Nakazaki et al 1998).…”

Rapid Flip-flop of Oleic Acid across the Plasma Membrane of Adipocytes

“…We performed new experiments to investigate the possibility that FA-induced decreases in pH in in cells with active metabolism and membrane transport of other metabolites and ions are because of some secondary pH change caused by a FAinduced metabolic event, as with glucose (66). The observed fast decreases in pH in PMV indicate that the cytosolic compartment and its participation in metabolism of FA are not essential for the decrease in pH after addition of FA.…”

BLX-1002, a novel thiazolidinedione with no PPAR affinity, stimulates AMP-activated protein kinase activity, raises cytosolic Ca2+, and enhances glucose-stimulated insulin secretion in a PI3K-dependent manner

“…Pancreatic islet cells attached to coverslips were incubated in KRBH buffer in the presence of 0.1% BSA at 3 mM glucose for 30 min at 37°C. NAD(P)H fluorescence was monitored in single cells at an excitation wavelength of 366 nm, a dichroic mirror at 400 nm, and an emission band-pass filter at 450 -470 nm (30). During the experiments, cells were perifused with KRBH buffer at 3 mM glucose.…”