structure and function correlations in electrophysiology

Question 1

functional consequences of differential gap jucntion/ion channel distribution

SAN
Atrium
His-purkinje
AVN
Ventricle

Answer 1

SA Node
• Prominent Phase 4 for pacemaker activity1
• If encoding cDNA transcripts 140 times higher than ventricle2
• Longest APD in the centre to prevent entrance of ectopics3
• More atrial like at periphery4
• Cx45

Atrium
• APD ¯ from crista terminalis to the pectinate muscles to “stream” the impulses from the SAN to the AVN.5,6
• Cx43/ Cx40

His-Purkinje
Cx43/ Cx40/ Cx45

AV node
• Low excitability limits max. no. of impulses to prevent rapid ventricular rates7
• Cx45

Ventricle
• No significant phase 4 depolarisation
• Endo-epi AP differences give rise to ECG T wave8
• Cx43

Question 2

Importance of Refractoriness Heterogeneity in the Enhanced Vulnerability to Atrial Fibrillation Induction Caused by Tachycardia-Induced Atrial Electrical Remodeling

Answer 2

Fareh et al. Circulation. 1998;98:2202–2209.
• Atrial fibrillation leads to electrophysiological changes
• What is the effect of non-uniform changes?
• Pacemaker implanted into adult mongrel dogs
• 12 underwent rapid pacing (400bpm)/ 12 left inactivated for 24 hrs
• Chest was opened and a 240 bipolar electrode array sutured to atrial epicardium
• Linear conduction velocities were measured
• Effective refractory period measured by premature extrastimuli
• Rapid atrial pacing in dogs ® Pathological heterogeneous ion channel changes ® Heterogeneous refractory period ® short ERP adjacent to long ERP ® conduction block ­ AF vulnerability

Question 3

Altered Pattern of Connexin40 Distribution in Persistent
Atrial Fibrillation in the Goat
Van der Velden et al. J Cardiovasc Electrophysiol.1998 Jun;9(6):596-607

Gap junctional remodeling in relation to stabilization of atrial fibrillation in the goa

Answer 3

• Gap junctional remodelling occurs in atrial fibrillaton
• What is the timing and localisation of these changes?
• 36 goats were implanted with pacemakers to deliver 50 Hz bursts lasting 2s on detection of SR
• Maintained in AF for up to 16 weeks
• On sacrifice RAA and LAA were fixed and labelled for Cx40 and Cx43
• Atrial fibrillation is associated with heterogenous loss of Cx40 in a goat model

Question 4

Remodelling of cardiac repolarization: how homeostaticresponses can lead to arrhythmogenesis
Multiple ion channel changes occur in congestive heart failure:

Answer 4

• Late INa due to slow inactivation, contributes to APD prolongation.
• Sarcoplasmic reticulum (SR) Ca2+ stores reduced, SERCA downregulated, slow relaxation of the systolic Ca2+ transient and diastolic Ca2+ may be increased.
• NCX is upregulated and may compensate for cytosolic Ca2+ removal impairments
• Changes in NCX may affect repolarization
• Voltage-dependent K+ currents are downregulated leading to APD prolongation

Question 5

Disturbed Connexin43 Gap Junction Distribution Correlates With the Location of Reentrant Circuits in the Epicardial Border Zone of Healing Canine Infarcts That Cause Ventricular Tachycardia

Answer 5

• Gap junctional changes occur after infarct
• Does the localisation of these changes affect conduction properties?
• MI induced in 6 mongrel dogs by LAD ligation
• 292 bipolar electrode array sutured onto epicardial LV over infarct area for mapping of conduction
• Tissue underlying the array was fixed and stained for Cx43 in sections from epicardium to endocardium
• Immunohistochemistry for Cx43
• Abnormalities in gap junction distribution in the canine healing infarct border zone correlates with re-entrant arrhythmias

Question 6

Characterization of Conduction in the Ventricles of Normal and Heterozygous Cx43 Knockout Mice Using Optical Mapping

Answer 6

• Gap junctions are a pathway for cell to cell conduction
• Does a lack of Cx43 lead to slow conduction?
• 14 WT and 7 Cx43+/- were used
• Hearts were removed and Langendorff perfused with oxygenated Tyrodes
• Pacing was carried out at CL 80 – 150ms
• Fluorescence intensity recordings were taken after staining with voltage sensitive dye Di-4-ANEPPS
• CV were calculated
• Things aren’t always as it seems it should be.
• Cx43 KO does not slow conduction.

Question 7

Microscopic conduction in cultured strands of neonatal rat heart cells measured with voltage-sensitive dyes.

Answer 7

• Neonatal rat myocytes were cultured in 1-d strands
• Fluorescence intensity recordings were taken after staining with voltage sensitive dye RH-237

Conduction in a 1-d chain of myocytes is regular

Question 8

Anisotropic conduction block and reentry in neonatal rat ventricular myocyte monolayers

Answer 8

• Cells in whole heart are elongated
• What happens if cultured cells are patterned in this way?
• Neonatal rat ventricular myocytes were seeded on culture dishes either direct or after abrasion
• Fluorescence intensity recordings were taken after staining with voltage sensitive dye RH-237
• Conduction in a 2-d layer of round cells is isotropic
• Conduction in a 2-d layer of elongated cells is isotropic

Question 9

fibre orientation in the whole heart

Answer 9

• Fibre orientation is complex in the whole heart
• There is a difference in orientation from endo ® epi
• This leads to a specific pattern of activation

Question 10

Total Excitation of the Isolated Human Heart
Durrer Circulation. 1970;41:899-912
• What is the pattern of excitation in the normal human heart?

Answer 10

• 7 hearts were excised from patients who died from cerebral conditions
• Epicardial and intramural activity recorded from 660 electrode array or 20 multi-electrode needles

Pattern of activation was calculated

Functional effect of changes in anisotropy and fibre direction in a 2D monolayer
• Conduction block in an anisotropic monolayer can lead to re-entry

Change in fibre direction can be a point of initiation of re-entry

Question 11

Functional consequences of dissociation of fibre layers in atrial fibrillation
Time course and mechanisms of endo-epicardial electrical dissociation during atrial fibrillation in the goat

Answer 11

Eckstein et al Cardiovascular Research (2011) 89, 816–824
• Muscle bundles are dissociated in atrial fibrillation
• Is activation between layers of the atrial wall seperated?
• Goats were induced into atrial fibrillation for 3 weeks or 6 months (7 in each group)
• A double-sided eno/epi 256 channel mapping array was placed on the left atrium
• Double-sided activation maps were calculated
• Endo- and epi-cardial layers can be dissociated in atrial fibrillation
• This leads to fibrillatory wavefronts

Question 12

Functional consequences in changes in fibre orientation after infarction
Towards predictive modelling of the electrophysiology of the heart

Answer 12

• Fibre orientation is disrupted in the infarct border zone due to scar
• This is a substrate for re-entry
• Vigmond et al Exp Physiol Volume 94, Issue 5, pages 563–577, May 2009
• Cellular location and expression of ion channels and gap junctional channels are specific to the normal functioning of each cell type in different regions of the heart.
• Abnormal localisation/ expression of channels can be pro-arrhythmic
• Specific tissue structure and orientation allows for normal conduction propagation through the heart
• Loss of tissue structure can be pro-arrhythmic