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Title: Age-associated changes to calcium handling proteins across the whole heart
Author: Hatch, Fiona Sara
Awarding Body: University of Hull
Current Institution: University of Hull
Date of Award: 2012
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Age is a major risk factor for the development of cardiac problems. The ageing process causes significant changes to cardiac structure and function. These are linked with the elderly heart’s diminished ability to respond to cardiac demand and associated with the increased frequency of myocardial dysfunction and arrhythmias. A key factor involved in such problems is the regulation of intracellular calcium ions (Ca²⁺) which is involved in contraction of the heart and the upstroke of the sinoatrial (SA) node pacemaker. Dysfunction in the regulation of intracellular Ca²⁺ increases the probability of an arrhythmogenic episode and would be indicative of progressive SA node, atrial and ventricular dysfunction. The current study sought to investigate the effects of ageing on the expression of Ca²⁺ handling proteins. The ageing process was analysed in rats at 6 (young-adult), 12 (adult) and 24 months old (elderly). Heart samples were assessed by western blot, qPCR, immunocytochemistry and electrophysiology. The expression of L-type calcium channels (Caᵥ1.2), ryanodine receptors (RYRs), sarcoendoplasmic reticulum ATPase (SERCA), phospholamban (PLB), sodium-calcium exchangers (NCX) and plasma membrane Ca²⁺ ATPase pumps (PMCA) were analysed in the SA node, right atrium, left atrium, left ventricle and right ventricle. The effect of ageing of the SA node was assessed via recordings of intrinsic heart rate in response to isoprenaline, a β-adrenergic stimulator, and nifedipine. With age response to isoprenaline was diminished and SR Ca²⁺ cycling was established as crucial for pacemaker activity. Application of cylopiazonic acid profoundly reduced the response to isoprenaline in the younger age groups but had no effect in the elderly, elucidating the age-associated reduced role of SERCA2a. The decline in protein expression of SERCA2a, RYR2 and Caᵥ1.2 supported dysfunctional spontaneous activity in the elderly SA node. In addition there was an age related increase in sensitivity to nifedipine which highlighted the importance of Caᵥ1.2 channels and the compensatory up-regulation of NCX1. Age-associated changes in the crista terminalis (CT) and right atrium (RA) muscle were investigated to determine whether the relationship between SA node dysfunction and age can be marked by alterations to surrounding tissue. With advancing age, CT tissue exhibited increased Caᵥ1.2 and RYR2 expression and decreased SERCA2a expression. These are considered key components in generating arrhythmogenic episodes. The LA contained significantly higher levels of Ca²⁺ transport proteins compared with the RA. The combined increase of Caᵥ1.2, RYR2 and SERCA2a activity in the LA, greatly augments SR Ca²⁺ content and the probability of SR ‘leakage’ leading to an arrhythmia. In contrast the RA exhibited dramatic Ca²⁺ remodelling with decreased expression of all Ca²⁺ handling proteins. The expression of Ca²⁺ handling proteins were assessed in the left ventricle (LV) and right ventricle (RV); the former was further distinguished into the LV endocardium and epicardium. SERCA2a activity was significantly lower in the RV compared with the LV. Within the elderly age group RYR2, SERCA2a, PMCA4a all decreased in expression, whist NCX1 had elevated levels in the LV and RV. The changes in the LV were further investigated across the myocardial wall and found RYR2, Caᵥ1.2 and NCX1 PLB to be elevated in the young endocardium. Heterogeneity varied with age; SERCA2a activity and RYR2 protein was increased in the endocardium, but overall disparity across the myocardium was reduced. This emphasised the effect of ageing across the LV wall which may contribute to the endocardium’s increased susceptibility to arrhythmias. Overall this study reported age-associated changes to the expression of Ca²⁺ handling proteins which will alter the Ca²⁺ transient and AP propagation in the ventricles, atria and spontaneous activity in the SA node. Although these changes may be adaptive mechanisms to maintain contractile function, increased SR Ca²⁺ content or inadequate Ca²⁺ removal would predispose to arrhythmias. Therefore, although ageing is not considered a disease, it remains the most prevalent risk factor as a predisposition to disease. Data in this study has clearly linked 'ageing' with altered intracellular Ca²⁺ regulation in all regions of the heart.
Supervisor: Jones, Sandra A.; Seymour, Anne-Marie Sponsor: University of Hull
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Biological sciences