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Background & Introduction: Heart transplantation is the most effective treatment for end-stage heart failure (HF) but is hindered due to an inadequate availability of donor organs. Left ventricular assist devices (LVADs) have been shown to be a suitable alternative and primarily used as a ‘bridge to transplantation’ wherein the failing heart can be supported by mechanical circulatory assistance until a suitable donor organ becomes available. In 4-9% of patients, the LVAD also acts as ‘bridge to recovery’, since it induces substantial functional improvement that allows LVAD explantation, without the need of transplantation. The rate for explantation of the LVAD remains low and the functional improvement observed, a transient phenomenon. LVADs cause mechanical unloading, which produces functional, structural, signalling and molecular changes in HF. Development of myocardial unloading-induced atrophy, time- dependent myocyte contractile dysfunction and excitation-contraction (EC) coupling changes may have detrimental consequences. However, when mechanical unloading is combined with pharmacological therapy, including the β2-AR agonist clenbuterol, an improved ‘bridge to recovery’ rate of 75% can be achieved. The effects of clenbuterol on functional, structural, signalling and molecular changes in a normal and failing hearts, during mechanical unloading, are unknown. This thesis investigates some of the key effects of mechanical unloading and β2-AR agonist stimulation with clenbuterol for HF treatment, based on the following hypotheses which have been individually addressed in Chapters 3, 4 and 5 respectively. • Chronic administration of clenbuterol alters myocardial structure and function and affects calcium handling in normal rat hearts. • Mechanical left ventricular unloading and the consequent left ventricular atrophy results in altered whole-heart and cellular function in non- failing/normal rat hearts. • Clenbuterol treatment during mechanical unloading of a normal rat heart normalises whole-heart and cellular function. • Clenbuterol has an additional benefit when combined with mechanical unloading in the treatment of failing rat hearts. Methods: Clenbuterol was administered by osmotic minipumps. Mechanical un- loading was achieved by heterotopic abdominal heart transplantation. Heart failure was induced by left coronary artery ligation in rats. In-vivo whole heart function was assessed by echocardiography and ex-vivo function by pressure- volume relationship. Only LV myocytes were isolated and studied using optical, fluorescence and electrophysiological techniques. Calcium handling protein expression was assessed by Western blotting. Theory and methodology of the single-cell studies is outlined, together with validation experiments and the necessary assumptions. Results: Data obtained and their interpretations are presented in three chapters according to the proposed hypotheses. The results show that clenbuterol with, without mechanical unloading, or on its own can affect both whole-heart and cellular function in normal hearts. The treatment of failing hearts with clenbuterol, alone or in combination with mechanical unloading, improves LV function at whole-heart and cellular level by effects on cell morphology, EC coupling and myofilament sensitivity to calcium. The details of individual experiments and their interpretation are discussed in the respective chapter. General Discussion: This thesis supports the use of clenbuterol in the strategy to improve recovery in HF patients treated with LVADs and also begins to elucidate some of the possible cellular mechanisms responsible for the improvement in LV function. The questions remaining unanswered are discussed and possible future experiments that could be performed to address them are described. |
