Entrainment and Synchrony of Insulin Pulses

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Entrainment and Synchrony of Insulin Pulses

• Morten Gram Pedersen
• Post. Doc.Department of Information Engineering
• University of Padova

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• The pancreatic ß-cell responds to glucose with
  oscillations in glucose consumption, oxygen
  consumption, NAD(P)H, Ca2 insulin secretion
  (period 3-8 min.)
• Secretion from islets, pancreas and plasma
  insulin levels oscillate on a similar timescale
• Disrupted in early development of Type II
  diabetes
• Millions of islets must be synchronized
• Likely that metabolic oscillations underlie by
  inducing periodic fluctuations of K(ATP) channel
  conductance via the ATP/ADP ratio

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Glycolytic oscillations (Jung et al., JBC 2000)
Extra cellular glucose
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Lack of synchrony between islets?(Valdeolmillos
et al., 1996)
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Entrainment of insulin pulses in rat (Sturis et
al. 1994)
Pancreas (normal/diabetic)
25 islets (normal/diabetic)
Glucose constant 7mM
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... and of calcium and NAD(P)H in mouse islets
Ca2
NAD(P)H Autofluorescence
(D. Luciani 2004, Ph.D. Thesis, DTU)
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Entrainment in humans by glucose
infusions(Pørksen et al., 2000 Pørksen 2002)
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... but also pulsatile insulin in constant
glucose
Pancreas, constant glucose 7mM. No significant
period by auto-correlation(Sturis et al., 1994)
Islets constant glucose (Ritzel et al., 2006)
Glucose clamp (Song et al., 2000)
Insulin / glucose
Insulin secretion
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Nerve signals?
Pancreatic nerve / release from connected piece
of pancreas (20 islets) (Sha et al., 2001)

• Blocking and stimulating nerve signals perturb
  insulin secretion in the pancreas
• Oscillations in pancreatic ganglia in cat with
  period 7 min (King et al., 1989)
• Led to the idea of an intra-pancreatic pacemaker
• Several experiments (Stagner Samols, 1985 Sha
  et al., 2000) were done using TTX in preparations
  from dog ? inhibits nerve signals AND ?
  interferes with electrical activitity in ß-cells
  (Na-channels important in dog)
• Delay gt30 min before TTX affected (?) pulsatile
  insulin in rat pancreas (Na-channels of little
  importance in rat ß-cells) (Opara Go, 1991)

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The b-cell model (Bertram et al. 2007)

• Consists of a three combined models
• Glycolysis
• Mitochondria
• Electrical activity / Ca2 dynamics
• Coupled through ATP
• Glycolysis leads to ATP production
• ATP inhibits the glycolytic enzyme PFK
• ATP closes K(ATP)-channels, stimulating Ca2
  influx
• Ca2 inhibits ATP production and increases
  consumption
• Subsystems oscillate
• Glycolysis slow oscillations due to PFK
• Electrical / Ca2 subsystem Bursting

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Entrainment

• Entrainment is a general property of forced
  nonlinear oscillators

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Synchronization of dispersed islets through
glucose oscillation
Pedersen et al., Biophys. J. 2005
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Modes of entrainment

• 12 entrainment (general nm)1 forcing period
  2 endogenous periods

Ca2
NAD(P)H
Insulin / glucose
?m
Insulin secretion
Sturis et al., 1994(pancreas)
Ritzel at al., 2006(islets)
D. Luciani, PhD thesis 2004 (islet)
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12 entrainment in vivo (Mao et al.,1999)
Healthy
Diabetic
Glucose
Insulin
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12 entrainment, Ca2 patterns
Different patterns can reflect different glucose
sensitivity (VGK max GK rate)
D. Luciani, PhD thesis 2004
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12 entrainment still synchronizes pulses
Insulin / glucose
Insulin / glucose
Insulin / glucose
Insulin secretion
Insulin secretion
Insulin secretion
Ritzel at al., 2006
Glc
Model (5 islets with different Vgk)
Avg Ca2
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Role of Ca2 for entraiment of metabolism

• Experiment (D. Luciani)Diazoxide (no Ca2
  influx) changes 12 entrainment to apparent 11
  pattern
• ModelLower Ca2 ? increased ATP ? lower PFK
  rate ? glycolysis no longer oscillatory ?
  forcing moves fixpoint

Dz
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In vivo synchronization

• Plasma insulin oscillates in vivo
• Millions of islets must be synchronized
• Mechanisms
• Intra-pancreatic neural network
• Mutual entrainment through glucose oscillations
  (globally coupled nonlinear oscillators)

Insulin
Insulin
Glucose
Glucose oscillations
Glucose
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In vivo synchronization
Full with liver Broken with constant glucose
(no liver)
Glucose (red)
Insulin (blue)
Full with liver Broken with constant glucose
(no liver)
M.G. Pedersen, R. Bertram, A. Sherman Biophys.
J. (2005).
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Conclusions

• Model where insulin pulses are a result of
  glycolytic oscillations in interaction with
  mitochondria and calcium dynamics
• Entrainable to external glucose fluctuations
  (inter-islet synchronization) 11 and 12
• 12 patterns can reflect heterogeneity wrt.
  glucose sensitivity
• Feedback Ca2 ? ATP ? glycolysis can explain 12
  ? 11 in diazoxide
• Could provide the in vivo synchronization
  mechanism through feedback from the liver
  needs very large glucose oscillations (model
  improvement?)
• Role of nerve signals in vivo?
• Other synchronization mechanisms?
• Why are diabetics unentrainable?
• Importance for insulin signaling?

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Acknowledgments

• Supported by the EU BioSim Network and Villum
  Kann Rasmussen Foundation
• Collaborators
• Dan S. Luciani
• Richard Bertram
• Arthur Sherman