Download Electrical coupling

Role of gap junctions in ischaemia-induced
arrhythmias and preconditioning
Gap junctions: structure and main functions
6 connexin protein
hemichannel.
2 hemichannels
gap junction.
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
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).
Isoforms in the heart:
• Cx40 in the atria
• Cx40 and Cx45 in the
conduction system
• Cx43 in the working
myocardium of both atria
and ventricles
Electrical coupling in the myocardium
More gap junctions on the cell
poles than on the sides:
Longitudinal conduction of AP
is faster than the transversal
(about 3 times).
Anisotropy of conduction.
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.
Cx43
Cells
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.
Electrical uncoupling during ischaemia;
protection by preconditioning
Changes in the ischaemic heart.
Resistivity
Time
140
140
120
120
100
100
80
80
60
60
40
40
20
20
0
0
0
Phase
delay
And in the preconditioned…
5
10
15
20
25
0
0
0
-1
-1
-2
-2
-3
-3
-4
-4
-5
-5
-6
-6
-7
-7
-8
-8
-9
-9
-10
-10
5
10
15
20
25
…the same protection occurs as against ventricular arrhythmias.
Premature
beats per
minute
90
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0
1
3
5
7
9
11
13
15
17
19
21
23
25
1
3
5
7
9
11
13
15
17
19
21
23
25
Mechanism of uncoupling and arrhythmogenesis
during ischaemia
Uncoupling can be triggered by :
• increase in intracellular Ca2+
• ATP loss
Thus, also by:
Ischaemia.
• intracellular acidification
Electrical uncoupling causes
conduction blocks and reentry circuits…
…thus, it is
arrhythmogenic.
VPBs
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
Lucifer yellow (LY)
MW = 457
Rhodamineconjugated dextrane
MW > 70 000 (RD)
RD: stains injured cells only
LY: spreading depends on GJs
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”)
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.
Thank you for your
attention!