Introduction: Factors that influence atrioventricular (AV) nodal conduction are complex and not well understood. Multiple studies have been performed to explain the mechanisms responsible for AV nodal conduction but the AV node (AVN) remains a "riddle". With ageing there is an increase in the incidence of AV nodal dysfunction leading to AV block. Methodology: I have performed electrophysiological (EP) and immunohistochemistry experiments on male Wistar-Hanover rats aged 3 months (equivalent to 20 year old humans; n=24) and 2 years (equivalent to 70 year old humans; n=15). AH interval, Wenkebach cycle length (WCL) and AV node effective refractory period (AVNERP) were measured. I used cesium (Cs+ = 2 mM) to block HCN channels responsible for the funny current "If " (and therefore the membrane clock), and ryanodine (2 μM) to block RyR2 channels responsible for Ca2+ release from the sarcoplasmic reticulum (and therefore the Ca2+ clock) in the two age groups. Protein expression in each group (from n=9 young and n=8 old rats) from different regions of the AV conduction axis: inferior nodal extension (INE), compact node (CN), proximal penetrating bundle (PPB) and distal penetrating or His bundle (His) were studied using immunofluorescence and confocal microscopy. The expression of the gap junction channels Cx43 and Cx40 and ion channel’s including HCN4 (responsible for If current), Nav1.5 (major cardiac Na+ channel responsible for INa) and Cav1.3 (L-type Ca2+ channel), and calcium handling proteins, RyR2 and SERCA 2a (involved in Ca2+ release and reuptake from cardiac sarcoplasmic reticulum, SR) were studied. Semi-quantitative signal intensity of these channels was measured using Volocity software. Structural characteristics of the tissue were studied using histology (Masson’s trichome stain and picrosirius red stain for collagen). Statistical analysis was performed with Prism 6.0. Electrophysiological measurements were performed using Spike2.Results: Without drugs to block the If current and Ca2+ release from the SR, there was a significant prolongation of the AH interval (P<0.005), WCL (P<0.005) and AVNERP (P<0.001) with ageing. In young rats (but not old rats), Cs+ prolonged the AH interval (P<0.001), WCL (P<0.01) and AVNERP (P<0.01). Ryanodine prolonged the AH interval (P<0.01) and WCL (P<0.01) in young and old rats. Immunofluorescence revealed that with ageing: Cx43 is downregulated in the PPB and His (P<0.05); Cx40 is upregulated in the INE and CN (P<0.05); HCN4 is downregulated in the His bundle (P=0.05); Nav1.5 is downregulated in the CN and PB (P<0.05); RyR2 is downregulated in the CN and PPB (P<0.05); SERCA2a and Cav1.3 is upregulated in the PPB (P<0.05). Histology confirmed that with ageing that the cells of CN, PPB and His are more loosely packed and irregularly arranged. There is cellular hypertrophy, decrease in the number of nuclei and increase in the collagen content with ageing. The clinical study has shown that elderly patients with syncope and cardiac conduction system disease are at risk of high mortality and recurrent transient loss of consciousness. Conclusion: For the first time, we have shown that both HCN and RyR2 channels play an important role in AV nodal conduction. With ageing the expression of HCN4 and the role of If in AV nodal conduction decreases, whereas the role of Ca2+ clock in AV nodal conduction was unchanged, although the expression of RyR2 and SERCA2a changes. The clinical study suggests that AV nodal disease is associated with significant morbidity and higher mortality among elderly patients who present with transient loss of consciousness.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:684827 |
| Date | January 2016 |
| Creators | Saeed, Yawer |
| Contributors | Garratt, Clifford ; Dobrzynski, Halina |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/structural-and-functional-remodelling-of-the-atrioventricular-node-with-ageing(9a72ee2b-89e0-4d08-a731-22282c10af74).html |
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