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- Berts, Alf, et al.
(author)
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Oscillatory Ca2+ signaling in somatostatin-producing cells from the human pancreas
- 1997
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In: Metabolism. - 0026-0495 .- 1532-8600. ; 46:4, s. 366-369
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Journal article (peer-reviewed)abstract
- Oscillatory Ca2+ signaling was studied in human somatostatin-releasing pancreatic δ cells identified by immunostaining. A ratiometric fura-2 technique was used for measuring cytoplasmic concentrations of Ca2+ and Sr2+ in δ cells exposed to the respective cation. Rhythmic activity in terms of slow (frequency, 0.1 to 0.4 per minute) oscillations from close to the basal level was seen in the presence of 3 to 20 mmol/L glucose during superfusion with medium containing 2.6 to 5 mmol/L Ca2+ or 5 mmol/L Sr2. These oscillations could be transformed into a sustained increase by decreasing extracellular Ca2+ or adding 1 mmol/L tolbutamide or 20 nmol/L glucagon. Addition of glucagon to a medium containing 20 mmol/L glucose resulted in the generation of short (< 30 seconds) transients, which disappeared upon exposure to 100 nmol/L of the intracellular Ca2+-adenosine triphosphatase (ATPase) inhibitor thapsigargin. When analyzing small aggregates of islet cells, it became evident that oscillatory activity in δ cells can be synchronous with that in adjacent non—δ cells. It is concluded that secretion of pancreatic somatostatin in man involves Ca2+ signaling similar to that regulating the pulsatile release of insulin.
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| 2. |
- Berts, Alf, et al.
(author)
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Suppression of Ca2+ oscillations in glucagon-producing alfa2-cells by insulin/glucose and amino acids
- 1996
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In: Biochimica et Biophysica Acta. - 0006-3002 .- 1878-2434. ; 1310:2, s. 212-216
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Journal article (peer-reviewed)abstract
- The cytoplasmic Ca2+ concentration ([Ca2+]i) was continuously monitored in single glucagon-producing α2-cells isolated from the mouse pancreas and later identified by immunostaining. Up to 60% of the α2-cells exhibited spontaneous [Ca2+]i oscillations (frequency 0.1–0.3/min) in a medium containing 3 mM glucose. In originating from a basal level of 60–100 nM, reaching peak values of 300–400 nM and promptly disappearing after blocking voltage-dependent Ca2+ channels with methoxyverapamil, the oscillations resembled those in insulin-releasing β-cells stimulated by glucose. The oscillatory activity was suppressed when combining elevation of glucose to 20 mM with the addition of 2–2000 ng/ml insulin. Whereas 10 mM of l-arginine or l-glycine transformed the oscillations into sustained elevation of [Ca2+];, there was no response to 1 mM tolbutamide or 0.1–1 mM γ-aminobutyric acid. The observations that α2-cells differ from islet cells secreting insulin and somatostatin in responding to adrenaline with mobilisation of intracellular calcium can be used for their rapid identification. It is suggested that the oscillations reflect periodic entry of Ca2+ due to variations of the membrane potential.
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| 3. |
- Chow, R H, et al.
(author)
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Coincidence of early glucose-induced depolarization with lowering of cytoplasmic Ca2+ in mouse pancreatic beta-cells
- 1995
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In: Journal of Physiology. - 0022-3751 .- 1469-7793. ; 485:3, s. 607-617
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Journal article (peer-reviewed)abstract
- 1. The temporal relationship between the early glucose-induced changes of membrane potential and cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in insulin-releasing pancreatic beta-cells. 2. The mean resting membrane potential and [Ca2+]i were about -70 mV and 60 nM, respectively, in 3 mM glucose. 3. Elevating the glucose concentration to 8-23 mM typically elicited a slow depolarization, which was paralleled by a lowering of [Ca2+]i. When the slow depolarization had reached a threshold of -55 to -40 mV, there was rapid further depolarization to a plateau with superimposed action potentials, and [Ca2+]i increased dramatically. 4. Imposing hyperpolarizations and depolarizations of 10 mV from a holding potential of -70 mV had no detectable effect on [Ca2+]i. Furthermore, glucose elevation elicited a decrease in [Ca2+]i even at a holding potential of -70 mV. 5. Step depolarizations induced [Ca2+]i transients, which decayed with time courses well fitted by double exponentials. The slower component became faster by a factor of about 4 upon elevation of glucose, suggesting involvement of ATP-dependent Ca2+ sequestration or extrusion of [Ca2+]i. 6. Glucose stimulation increased the size and accelerated the recovery of carbachol-triggered [Ca2+]i transients, and thapsigargin, an intracellular Ca(2+)-ATPase inhibitor, counteracted the glucose-induced lowering of [Ca2+]i, indicating that calcium transport into intracellular stores is involved in glucose-induced lowering of [Ca2+]i. 7. The results support the notion that in beta-cells, nutrient-induced elevation of ATP leads initially to ATP-dependent removal of Ca2+ from the cytoplasm, paralleled by a slow depolarization due to inhibition of ATP-sensitive K+ channels. Only after depolarization has reached a threshold do action potentials occur, inducing a sharp elevation in [Ca2+]i.
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| 9. |
- Dezaki, Katsuya, et al.
(author)
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Ghrelin Attenuates cAMP-PKA Signaling to Evoke Insulinostatic Cascade in Islet beta-Cells
- 2011
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In: Diabetes. - : American Diabetes Association. - 0012-1797 .- 1939-327X. ; 60:9, s. 2315-2324
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Journal article (peer-reviewed)abstract
- OBJECTIVE-Ghrelin reportedly restricts insulin release in islet beta-cells via the G alpha(i2) subtype of G-proteins and thereby regulates glucose homeostasis. This study explored whether ghrelin regulates cAMP signaling and whether this regulation induces insulinostatic cascade in islet beta-cells. RESEARCH DESIGN AND METHODS-Insulin release was measured in rat perfused pancreas and isolated islets and cAMP production in isolated islets. Cytosolic cAMP concentrations ([cAMP](i)) were monitored in mouse MIN6 cells using evanescent-wave fluorescence imaging. In rat single beta-cells, cytosolic protein kinase-A activity ([PKA](i)) and Ca(2+) concentration ([Ca(2+)](i)) were measured by DR-II and fura-2 microfluorometry, respectively, and whole cell currents by patch-clamp technique. RESULTS-Ghrelin suppressed glucose (8.3 mmol/L)-induced insulin release in rat perfused pancreas and isolated islets, and these effects of ghrelin were blunted in the presence of cAMP analogs or adenylate cyclase inhibitor. Glucose-induced cAMP production in isolated islets was attenuated by ghrelin and enhanced by ghrelin receptor antagonist and anti-ghrelin antiserum, which counteract endogenous islet-derived ghrelin. Ghrelin inhibited the glucose-induced [cAMP](i) elevation and [PKA](i) activation in MIN6 and rat beta-cells, respectively. Furthermore, ghrelin potentiated voltage-dependent K(+) (Kv) channel currents without altering Ca(2+) channel currents and attenuated glucose-induced [Ca(2+)](i) increases in rat beta-cells in a PKA-dependent manner. CONCLUSIONS-Ghrelin directly interacts with islet beta-cells to attenuate glucose-induced cAMP production and PKA activation, which lead In activation of Kv channels and suppression of glucose-induced [Ca(2+)](i) increase and insulin release.
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| 10. |
- Dyachok, Oleg, 1965-, et al.
(author)
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Glucose-induced cyclic AMP oscillations regulate pulsatile insulin secretion
- 2008
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In: Cell Metabolism. - : Cell Press. - 1550-4131 .- 1932-7420. ; 8:1, s. 26-37
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Journal article (peer-reviewed)abstract
- Cyclic AMP (cAMP) and Ca2+ are key regulators of exocytosis in many cells, including insulin-secreting β-cells. Glucose-stimulated insulin secretion from β cells is pulsatile and involves oscillations of the cytoplasmic Ca2+ concentration ([Ca2+]i), but little is known about the detailed kinetics of cAMP signalling. Using evanescent-wave fluorescence imaging we found that glucose induces pronounced oscillations of cAMP in the sub-membrane space of single MIN6-cells and primary mouse β-cells. These oscillations were preceded and enhanced by elevations of [Ca2+]i. However, conditions raising cytoplasmic ATP could trigger cAMP elevations without accompanying [Ca2+]i rise, indicating that adenylyl cyclase activity may be controlled also by the substrate concentration. The cAMP oscillations correlated with pulsatile insulin release. Whereas elevation of cAMP enhanced secretion, inhibition of adenylyl cyclases suppressed both cAMP oscillations and pulsatile insulin release. We conclude that cell metabolism directly controls cAMP, and that glucose-induced cAMP oscillations regulate the magnitude and kinetics of insulin exocytosis.
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