Alpha-MSH induces vasodilatation and exerts cardioprotection via the heme-oxygenase pathway in rat hearts

K

Anions and arrhythmias in experimental heart disease

Ischaemic heart disease can cause sudden cardiac death from ventricular arrhythmias occuring as a consequence of ischaemia itself, or as a consequence of subsequent reperfusion of ischaemic tissue. Current agents designed to prevent these arrhythmias do not provide adequate protection. Conventionally, antiarrhythmic agents are subdivided into four classes on the basis of actions on 3-adrenoceptors, or on membrane currents carried by cations. We have explored the modulation of anions as a possible new approach to the prevention of sudden cardiac death. Using an isolated rat heart model, we showed that substitution of the chloride anion by nitrate protects against ischaemia- and reperfusion-induced arrhythmias in a concentration-dependent manner without deleterious haemodynamic consequences. Studies of the site of action demonstrated that (i) protection against ischaemia-induced VF resulted largely from an action in the ischaemic zone, and (ii) protection against reperfusion-induced VF resulted principally from an action occurring during reperfusion and within the reperfused tissue. Significant reductions in ventricular arrhythmias were produced by anions with membrane permeabilities greater than chloride and the 'anion, methylsulphate, which is less permeable than chloride was proarrhythmic. These findings strongly support the hypothesis that altered membrane permeability may contribute to the antiarrhythmic activity of some anion surrogates. In conclusion, anions appear to play a hitherto unrecognized role in arrhythmogenesis in ischaemia and reperfusion. Their manipulation represents a novel target for antiarrhythmic agents

Biochemical adaptations in cardiac hypertrophy

Cardiac hypertrophy is the adaptive response of the heart to chronic overload. The metabolic adaptations that occur during hypertrophy are initially beneficial, but can ultimately deteriorate into heart failure. The mechanisms underlying this are unknown. Evidence of impaired energy reserve, which may be caused by changes in the profile of substrate use, has been implicated in the transition of compensatory hypertrophy to heart failure. The work of this thesis characterises the alterations in substrate utilisation that occur in the heart, secondary to pressure-overload induced cardiac hypertrophy, the their implications on heart function.Pressure-overload hypertrophy was induced surgically in male Sprague- Dawley rats by inter-renal ligation. 13C-NMR spectroscopy was performed on extracts from hypertrophied and control hearts perfused with 13C-labelled substrate mixtures to determine the profile of substrate utilisation. Nine weeks pressure- overload achieved a moderate hypertrophy, evidenced by a 10-15% increase in heart mass to tibia length. The hypertrophied hearts showed an increased reliance on glucose and endogenous substrate contribution to TCA cycle oxidation for the production of ATP (15.0% versus 11.0%) compared to control hearts.Prolonged fifteen weeks pressure-overload resulted in further metabolic changes including impaired long-chain fatty acid oxidation and the accumulation of long-chain acylcarnitines. Alteration in substrate utilisation preceded any change in heart function and is strong evidence to suggest that impaired substrate delivery at the level of the mitochondria in cardiac hypertrophy plays an important role in the development of heart failure and is not a secondary phenomenon. At high workloads both hypertrophied and control hearts, showed similar profiles of substrate use, with glucose being the predominant substrate utilised for TCA cycle oxidation. At high workloads, hypertrophied hearts initially exhibited significantly higher mechanical function, but was not sustained, suggesting that physiological changes were becoming detrimental. This study highlights that sequential metabolic adaptations occur during the development of hypertrophy and precede any functional abnormality, providing potential prognostic markers

Effect of increased left ventricle mass on ischemia assessment in electrocardiographic signals: rabbit isolated heart study

Detailed quantitative analysis of the effect of left ventricle (LV) hypertrophy on myocardial ischemia manifestation in ECG is still missing. The associations between both phenomena can be studied in animal models. In this study, rabbit isolated hearts with spontaneously increased LV mass were used to evaluate the effect of such LV alteration on ischemia detection criteria and performance. Electrophysiological effects of increased LV mass were evaluated on sixteen New Zealand rabbit isolated hearts under non-ischemic and ischemic conditions by analysis of various electrogram (EG) parameters. To reveal hearts with increased LV mass, LV weight/heart weight ratio was proposed. Standard paired and unpaired statistical tests and receiver operating characteristics analysis were used to compare data derived from different groups of animals, monitor EG parameters during global ischemia and evaluate their ability to discriminate between unchanged and increased LV as well as non-ischemic and ischemic state. Successful evaluation of both increased LV mass and ischemia is lead-dependent. Particularly, maximal deviation of QRS and area under QRS associated with anterolateral heart wall respond significantly to even early phase (the 1st-3rd min) of ischemia. Besides ischemia, these parameters reflect increased LV mass as well (with sensitivity reaching approx. 80%). However, the sensitivity of the parameters to both phenomena may lead to misinterpretations, when inappropriate criteria for ischemia detection are selected. Particularly, use of cut-off-based criteria defined from control group for ischemia detection in hearts with increased LV mass may result in dramatic reduction (approx. 15%) of detection specificity due to increased number of false positives. Nevertheless, criteria adjusted to particular experimental group allow achieving ischemia detection sensitivity of 89–100% and specificity of 94–100%, respectively. It was shown that response of the heart to myocardial ischemia can be successfully evaluated only when taking into account heart-related factors (such as LV mass) and other methodological aspects (such as recording electrodes position, selected EG parameters, cut-off criteria, etc.). Results of this study might be helpful for developing new clinical diagnostic strategies in order to improve myocardial ischemia detection in patients with LV hypertrophy

Atrial Arrhythmogenic Substrates: The Role of Structure and Molecular Remodeling

Atrial tachyarrhythmias, specifically atrial flutter: AFl) and fibrillation: AF), affect over 2.2 million Americans, leading to more hospitalizations than any other cardiac arrhythmia. These arrhythmias are defined by the presence of reentrant circuits of excitation leading to high atrial rates and uncoordinated activation of the ventricles. The underlying mechanisms of AFl/AF have proven complex and, despite a century of research, no one effective treatment has been developed. Surgical ablation and pharmacological therapies are both fraught with risks and potential pro-arrhythmic side effects. Electrical cardioversion, on the other hand, is extremely effective in terminating these arrhythmias, but the high-energy shocks required for termination cause substantial pain to the patient. In this dissertation, we first investigate the underlying molecular and structural mechanisms of AF in two clinically-relevant models - the human and canine hearts. We identify structural and molecular substrates responsible for the generation and maintenance of AFl/AF. We then explore the application of a novel low-voltage defibrillation therapy to a rabbit model of atrial tachyarrhythmias and show significant reductions in the defibrillation threshold for both AF and AFl. We utilize a variety of experimental techniques, such as high throughput quantitative PCR, optical coherence tomography, and optical mapping. Only truly integrative approaches to arrhythmia research, combining a variety of experimental models and techniques, can continue to unravel the complexities of underlying molecular, structural, and electrophysiological mechanisms and develop effective, safe therapies, as we demonstrate in this dissertation with regards to atrial tachyarrhythmias

The influence of age and type 2 diabetes on cardioprotective interventions against myocardial ischaemia-reperfusion injury.

The background of the thesis is based on the conflicting results between bench and bedside regarding the susceptibility to myocardial infarction with old age and diabetes. In laboratories all over the world, strategies have been developed to protect the myocardium from this insult, including the use of ischaemic conditioning (short periods of ischaemia and reperfusion prior to or following lethal ischaemia) or the use of a variety of pharmacological agents. However, surprisingly, translating these effective cardioprotective treatments into the clinic has proved problematic. The main issue seems to be the fact that the experimental investigations have mainly used young, healthy animals while the human patients present often with a number of other risk factors, or comorbidities, such as type 2 diabetes and old age. Therefore the aim of this thesis was to investigate the susceptibility to ischaemia-reperfusion injury and the proficiency of cardioprotective strategies to protect the heart in the setting of ageing and type 2 diabetes. Utilizing a model of type 2 diabetes, the Goto-Kakizaki rat and its normoglycaemic control Wistar rat, within the range of 3 to 18 months of age, the Langendorff isolated heart model and in vivo coronary artery occlusion and reperfusion were employed to investigate the susceptibility to ischaemia-reperfusion injury. Mechanical or pharmacological cardioprotective strategies were also investigated in this setting and the mechanisms of the failed cardioprotection were examined further using in vitro techniques focusing on known pro survival signalling pathways within the myocardium. The ageing diabetic heart demonstrated an increased vulnerability to injury and was less amenable to protection by ischaemic conditioning. Pharmacological agents namely, metformin and sitagliptin appear to differentially protect the diabetic and non-diabetic heart, and this could be due to the underlying intracellular changes associated with ageing and diabetes