title: Evidence against a Ca(2+)-induced potentiation of dehydrogenase activity in pancreatic beta-cells.
abstract: Pancreatic beta-cells respond to an unchanging stimulatory glucose concentration with oscillations in membrane potential (Vm), cytosolic Ca(2+) concentration ([Ca(2+)]c), and insulin secretion. The underlying mechanisms are largely ascertained. Some particular details, however, are still in debate. Stimulus-secretion coupling (SSC) of beta-cells comprises glucose-induced Ca(2+) influx into the cytosol and thus into mitochondria. It is suggested that this activates (mitochondrial) dehydrogenases leading to an increase in reduction equivalents and ATP production. According to SSC, a glucose-induced increase in ATP production would thus further augment ATP production, i.e. induce a feed-forward loop that is hardly compatible with oscillations. Consistently, other studies favour a feedback mechanism that drives oscillatory mitochondrial ATP production. If Ca(2+) influx activates dehydrogenases, a change in [Ca(2+)]c should increase the concentration of reduction equivalents. We measured changes in flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) autofluorescence in response to changes in glucose concentration or glucose-independent changes in [Ca(2+)]c. The FAD signal was altered by glucose but not by alterations in [Ca(2+)]c. NAD(P)H was increased by glucose but even decreased by Ca(2+) influx evoked by tolbutamide. The mitochondrial membrane potential ΔΨ was hyperpolarized by 4 mM glucose. As adding tolbutamide then depolarized ΔΨ, we deduce that Ca(2+) does not activate mitochondrial activity but by contrast even inhibits it by reducing the driving force for ATP production. Inhibition of Ca(2+) influx reversed the Ca(2+)-induced changes in ΔΨ and NAD(P)H. The results are consistent with a feedback mechanism which transiently and repeatedly reduces ATP production and explain the oscillatory activity of pancreatic beta-cells at increased glucose concentrations.
authors.affiliation: Department of Pharmacology, Institute of Pharmacy, University of Tübingen, Auf der Morgenstelle 8, 72076, Tübingen, Germany.
authors.forename: Gisela
authors.initials: G
authors.lastname: Drews
authors.affiliation: Department of Pharmacology, Institute of Pharmacy, University of Tübingen, Auf der Morgenstelle 8, 72076, Tübingen, Germany.
authors.forename: Cita
authors.initials: C
authors.lastname: Bauer
authors.affiliation: Department of Pharmacology, Institute of Pharmacy, University of Tübingen, Auf der Morgenstelle 8, 72076, Tübingen, Germany.|Institute of Pharmaceutical and Medicinal Chemistry, University of Münster, Corrensstraße 48, 48149, Münster, Germany.
authors.forename: Armin
authors.initials: A
authors.lastname: Edalat
authors.affiliation: Institute of Pharmaceutical and Medicinal Chemistry, University of Münster, Corrensstraße 48, 48149, Münster, Germany.
authors.forename: Martina
authors.initials: M
authors.lastname: Düfer
authors.affiliation: Department of Pharmacology, Institute of Pharmacy, University of Tübingen, Auf der Morgenstelle 8, 72076, Tübingen, Germany. peter.krippeit-drews@uni-tuebingen.de.
authors.forename: Peter
authors.initials: P
authors.lastname: Krippeit-Drews
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D038181:FMN Reductase / Q000378:metabolism; D005182:Flavin-Adenine Dinucleotide / Q000378:metabolism; D005947:Glucose / Q000494:pharmacology; D007004:Hypoglycemic Agents / Q000494:pharmacology; D050417:Insulin-Secreting Cells / Q000187:drug effects* / Q000201:enzymology*; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D008810:Mice, Inbred C57BL; D010088:Oxidoreductases / Q000378:metabolism*; D014044:Tolbutamide / Q000494:pharmacology
chemical_list: D007004:Hypoglycemic Agents; D005182:Flavin-Adenine Dinucleotide; D014044:Tolbutamide; D010088:Oxidoreductases; D038181:FMN Reductase; D005947:Glucose; D002118:Calcium
keywords: Beta-cell; Calcium; Dehydrogenase; Flavin adenine dinucleotide (FAD); Mitochondrial membrane potential; Nicotinamide adenine dinucleotide (phosphate) (NAD(P)H); Oscillations