Nincs diac

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Role of gap junctions in ischaemia-induced
arrhythmias and preconditioning
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Gap junctions structure and main functions

• Two main functions
• Electrical coupling movement of ions, mainly in
  excitable tissues ( electrical synapse).
• Chemical coupling interchange of molecules
  smaller than 1 kDa, in almost every kind of
  tissue.

• Regulation
• Voltage dependent
• Cell metabolites (discussed later)
• Phosphorylation
• Protein synthesis, degradation

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Connexin (Cx) structure and isoforms

• 4 transmembrane domains, with intracellular N
  and C terminals.
• Several types of connexins exist with different
  C-terminal and IC loop length.
• About 20 isoforms known in the human body, named
  for their molecule weight, e.g. Cx32, Cx40(26 -
  60 kDa).

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Electrical coupling in the myocardium
Cx43
Cells

• Aberrations from anisotropy
• Physiological, local aberration in the sinus-
  and AV node, transversal conduction is as fast
  as the longitudinal.
• Pathological prolonged ischaemia or chronic
  heart failure, due to spatial redistribution of
  gap junction channels.

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Assesment of electrical coupling
Electrical impedance total resistivity of a
circuit where alternating current (AC) flows. In
the tissue membranes act both as resistances and
capacitors. AC flows through capacitors, but it
is arrested increase in resistance and
phase delay between current and voltage.
Measurement 4 electrodes, between outers
subthreshold AC between inners measurement of
voltage (resistivity, U/I) and phase. Electrical
uncoupling resistivity increases and phase
shifts further to the negative direction.
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Electrical uncoupling during ischaemia
protection by preconditioning
Resistivity
Time
Phase delay
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Mechanism of uncoupling and arrhythmogenesis
during ischaemia

• Uncoupling can be triggered by
• increase in intracellular Ca2
• ATP loss
• intracellular acidification

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Chemical coupling, permeability

• Interchange of molecules smaller than 1 kDa.
• E.g. ATP, NAD, glucose, amino acids, glutathion,
  small molecule antioxidants, ions ....

How to measure
Double dye-loading of a freshly excised tissue
block
RD stains injured cells only
LY spreading depends on GJs
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Changes in chemical coupling during ischaemia
Normal
25 min ischaemia
Preconditioned
LY
RD

• In reperfusion suddenly opening gap junctions
  allow the share of death signals (e.g. Ca2, Na
  overload)
• Cell death occurring in bands (contraction
  band necrosis)

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Clinical, pharmacological use

• Antiarrhythmic peptides by keeping gap
  junctions open they retain homogenous impulse
  propagation, even under ischaemic conditions
  (effective in animal studies, clinical trials
  have been started).
• In reperfusion prevention of sudden GJ opening
  by drugs reduces cell death, thus infarct size
  (animal studies).

• Preconditioning contribution of GJs has been
  verified, but the detailed mechanism - so far -
  is unknown.
• Our aim to evoke or help preconditioning with
  adequate modulation of gap junctinal coupling.

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Thank you for your attention!