Contents

📒 Nicholls 2016

The Pancreatic β-Cell: A Bioenergetic Perspective1

Sciwheel

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canonical pathway of GSIS

Experimental method

https://user-images.githubusercontent.com/40054455/97670727-d2a57080-1ac1-11eb-9836-05e0fd11b072.png
Contrasting time courses of glucose elevation and insulin secretion rate seen in vivo

In vivo: smooth rise and subsequent recovery. In oral glucose tolerance test, plasma glucose slowly increases from 5.5 to 9.5 mM over 60 min and returns to basal over the subsequent 3 h (Figure 2A). In healthy subjects, the rate of insulin secretion, facilitated by the secretion of gut-derived incretins (130), closely mirrors the glucose concentration.

The majority of in vitro studies employing isolated islets, dissociated primary cells or insulinoma cell lines, have employed a simplified protocol involving a step increase in extracellular glucose, typically from 2.8–3 to 11–16 mM or even higher. Islets show a biphasic release of insulin, with an initial rapid and transient release followed by a slowly accelerating secretion, particularly at supramaximal glucose . The two phases are commonly referred to as first (or triggering) and second (or amplification) phases

BIOENERGETIC BASIS OF GLUCOSE-STIMULATED INSULIN SECRETION

Beta-cells are responsive to physiological range of glucose due to

  • upstream delivery : the low-affinity high-capacity glucose transporter (GLUT-2) (Km 15–20 mM) & hexokinase IV (glucokinase, GK) in beta cells.
  • downstream dissipative pathways: high endogenous mitochondrial proton leak and ATP utilizing pathways. (which differs from hepatocytes)

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Simplistic hydraulic models of the responses of muscle and β-cell mitochondria to altered metabolic states

The mitochondria in muscle and muscle-derived cells are notable for possessing an endogenous proton leak that is not only low, but also “nonohmic”, i.e., highly potential-dependent. Any drop in Δp, and hence ΔGp, is immediately sensed by the glycolytic pathway to increase the flux to restore Δp and ATP levels. In beta-cells, over the range 140-100 mV, the endogenous proton leak was roughly ohmic and exerted a high level of control over respiration and Δp

The activity of muscle contration is controlled by flux (ATP turnover) rather than ATP/ADP ratio, and the opposite in beta-cells.

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Mitochondrial respiration in INS-1 cells

ΔpH increased from −0.13 units in 2.5 mM glucose to −0.51 units in 16.7 mM glucose, probably as a consequence of Ca2+ uptake into the matrix. Thus, even a quantitative estimate (e.g. in microscopy) of Δψ hyperpolarization cannot be equated with a change in Δp unless the ΔpH component is also determined.

Importance of Mitochondrial Content and Morphology

Live-cell fluorescent imaging shows that the organelles are highly dynamic and motile, undergoing continuous fusion and fission, and that these processes are critical for mitochondrial quality control and function.

Fusion is mediated by three GTP-utilizing proteins: Mfn1 and Mfn2 on the outer membrane and Opa1 on the inner membrane. Fission is mediated by Fis1 and Mff on the outer membrane and Drp1.

Fissioned daughters that showed an abnormally low Δψ did not reenter the fusion/fission cycle, but were removed by autophagy.

Las et al.2 made the important observation that 48-h exposure of INS-1 cells to the supraphysiological unbound fatty acid concentrations frequently employed (∼5:1 palmitate to albumin molar ratio, see Figure 8) caused fragmentation of the mitochondrial network and defective apoptosis.

Some Relevant Equilibrium Thermodynamic Relationships

In typical cell cytoplasm (37°, pH 7.1, 0.9 mM $[Mg^{2+}]$), the apparent equilibrium constant K' for ATP synthesis has been calculated to be ∼ $3.7 \times 10^{-6} M^{-1}$

The overall H+/ATP stoichiometry for synthesis and export 3.67 (2.67 from ATP synthase and 1 from ANT).

https://user-images.githubusercontent.com/40054455/97675033-8100e400-1ac9-11eb-8fbd-30c47160b57c.png
equilibria thermodynamic relationships

Phosphate

In elevated glucose, β-cells decrease their matrix and cytosolic free Pi by 50 and 60%, respectively.

Creatine pool

To estimat4e cytosolic ATP/ADP ratio: since the PCr/Cr pool is purely cytoplasmic and largely unbound, estimations are free from the compartmentalization complications inherent in whole cell adenine nucleotide determinations

Roles of Adenylate Kinase (ADK) and AMP

Mammalian cells possess two isoforms: AK1 is cytosolic, while AK2 is located in the mitochondrial intermembrane space. The enzymes operate close to equilibrium in the cell; thus the concentration of AMP in the cytosol may be expressed as a function of that of ATP together with the ATP/ADP ratio.

The ADP concentration is much more responsive to changes in Δp, while AMP, assuming equilibrium via adenylate kinase, changes by several thousandfold.

As glucose was increased to 30 mM, ADP fell exponentially to <5 μM, while ATP remained close to 2 mM. A cytosolic ATP/ADP ratio of 400 suggested that the mitochondria in these cells were fully polarized in high glucose, with a Δp greater than 170 mV.

AMP kinase

The action of the kinase is to inhibit ATP-consuming reactions, such as protein synthesis, and facilitate those associated with ATP generation.

Meformin, the drug for T2DM, acts as an AMPK activator in hepatocytes, but a mild complex I inhibitor in beta-cells.

Adenine Nucleotide Pool Responses to the Low-to-High Glucose Transition

“Resting” ATP/ADP rstio ~5 in beta-cells. An isolated study monitoring the PCr/Cr ratio in creatine-supplemented β-HC9 cells indicated a cytosolic ATP/ADP ratio of ∼200 in high glucose.

Adenine Nucleotide Interactions With the KATP Channel

MgATP closes the channel, but ADP will block the action of ATP.

https://user-images.githubusercontent.com/40054455/97676011-f3be8f00-1aca-11eb-9164-f62ff13059cf.png
the β-cell KATP channel

CALCIUM INTERACTIONS WITH ENDOPLASMIC RETICULUM AND MITOCHONDRIA

An activation of metabolism in response to an elevation in mitochndrial calcium is a general mitochondrial phenomenon, activating PDH, IDH3, and KGDH.

In the cytosol Mg-ATP/Pi exchanger and ARALAR, the glutamate/aspartate carrier are activated by cytosolic calcium.

Lactate dehydrogenase

The absence, or very low expression, of LDH and the MCT in primary β-cells.

The presence of two alternative fates for pyruvate, reduction to lactate and mitochondrial oxidation, would disturb the coupling between glucose and ΔGp.

OSCILLATORY PARAMETERS

The human pancreas contains ∼1,000,000 islets, each with ∼1,000 β-cells containing some 10,000 secretory granules. In response to elevated glucose, insulin release detected in the portal vein oscillates with a period of 5–6 min.

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Patterns of oscillations

Electrical activity is synchronized throughout the islet: Gap Junctions

Model Building

The most comprehensive current mathematical construction, the Dual Oscillator Model 3, proposes interactions between three modules: glycolysis, the mitochondrion, and the plasma membrane.


  1. Nicholls DG. The Pancreatic β-Cell: A Bioenergetic Perspective. Physiol Rev. 2016;96(4):1385–447. ↩︎

  2. Las G, Serada SB, Wikstrom JD, Twig G, Shirihai OS. Fatty acids suppress autophagic turnover in β-cells. J Biol Chem. 2011 Dec 9;286(49):42534–44. PMC3234912 ↩︎

  3. Ren J, Sherman A, Bertram R, Goforth PB, Nunemaker CS, Waters CD, et al. Slow oscillations of KATP conductance in mouse pancreatic islets provide support for electrical bursting driven by metabolic oscillations. Am J Physiol Endocrinol Metab. 2013 Oct 1;305(7):E805-17. PMC3798703 ↩︎