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

Abstract

Exposure of pancreatic islets of Langerhans to physiological concentrations of glucose leads to secretion of insulin in an oscillatory

Direct Measurements of Oscillatory Glycolysis in Pancreatic Islet β-Cells Using Novel Fluorescence Resonance Energy Transfer (FRET) Biosensors for Pyruvate Kinase M2 Activity

“…The presence of glycolytic oscillations in PKM2 activity implies that PFK1 can induce oscillations in PKM2 activity both by 1) inducing oscillations in substrate flow to the enzyme as well as 2) direct allosteric activation by FBP, although only the latter affects the PKM2 tetramerization state and would be detectable with PKAR. The average oscillatory period, 6.3 Ϯ 0.5 min (n ϭ 30), was similar to what we and others have observed for downstream oscillations of NAD(P)H, Ca 2ϩ , and insulin in mouse islets (8,21). Although islet ␤-cell Ca 2ϩ oscillations are electrically synchronized by gap junctions (51) and metabolically synchronized at the mitochondria (8), these results make it clear that this coupling extends to glycolytic oscillations.…”

“…NAD(P)H is strongly fluorescent in its reduced state ( ex,max Х 365 nm) (53,54). Although NAD(P)H fluorescence has been shown to oscillate in ␤-cells (8,21,27), strictly differentiating the mitochondrial-specific NADH component from the total signal requires two-photon microscopy and makes it difficult to detect oscillations as it reduces the sensitivity of the technique (53,54). In contrast to NAD(P)H, endogenous flavins fluoresce in their oxidized state ( ex,max Х 450 nm) and reflect the level of flavin adenine dinucleotides and mitochondrial flavoproteins.…”

Oscillatory control of insulin secretion

“…Oscillations in NAD(P)H have been observed in single rat β-cells (Pralong et al, 1994) but have been difficult to catch in islets (Panten et al, 1973;Gilon and Henquin, 1992). It was not until recently that NAD(P)H oscillations were convincingly demonstrated in islets (Luciani et al, 2006) (Krippeit-Drews et al, 2000) and with temporal characteristics similar to that of the NAD(P)H oscillations (Luciani et al, 2006). Direct observations of oscillations in ATP have been made both in individual mouse and human β-cells (Ainscow and , and there is evidence for oscillations in the activity of the K ATP channels (Dryselius et al, 1994;Larsson et al, 1996), which are probably the most important ATP targets in stimulussecretion coupling.…”

“…According to one hypothesis oscillations are an inherent property of metabolism (Tornheim, 1997). Another possibility involves feedback effects of Ca 2+ on metabolism (Detimary et al, 1998;Magnus and Keizer, 1998a;Magnus and Keizer, 1998b;Jung et al, 2000;Krippeit-Drews et al, 2000;Kindmark et al, 2001;Luciani et al, 2006;Bertram et al, 2007a).…”

“…The situation is further complicated by the fact that Ca 2+ stimulates metabolism by activating mitochondrial dehydrogenases (McCormack et al, 1990;Civelek et al, 1996;Pitter et al, 2002) and that Ca 2+ may regulate glycolytic enzymes (Jung et al, 2000). Metabolic oscillations may indeed reflect intrinsic oscillatory mechanisms in glucose metabolism, but the oscillations are modified both by stimulatory and inhibitory feedback effects of Ca 2+ (Luciani et al, 2006;Bertram et al, 2007b (Henquin, 2000) or after knocking out functional K ATP channels (Nenquin et al, 2004;Szollosi et al, 2007). This K ATP channel-independent process is mostly considered as an amplification mechanism, which remains functionally silent unless [Ca 2+ ] i is elevated.…”