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Heart transplantation is, for certain types of cardiovascular disease, the only form of treatment resulting in patient survival. Its clinical application is, however, limited by the shortage of donor organs. This shortage is largely due to the inability to consistently preserve adequate myocardial function over prolonged ischemic periods. It is the goal of this research to provide information which may contribute to techniques for heart preservation, thus improving graft survival following preservation and transplantation. Current methods for myocardial preservation generally involve the arrest and immersion of the heart in cold cardioplegic solution, the composition of which is designed to provide for the reduced metabolic demands of the cold, arrested muscle. These methods have extended the preservation period to approximately 6 hours; however, hearts cannot be held longer than this period because, although metabolism has been slowed by hypothermia, alterations take place which compromise functional recovery upon reperfusion. A variety of perfusates and perfusion techniques have been developed to protect the myocardium from the damage thought to occur as a consequence of ischemic storage of the isolated heart. However, a consistently successful technique for long-term preservation of the heart remains undefined. A growing body of knowledge has led to the hypothesis that injury to the microcirculation may result in myocardial ischemia during preservation and decreased contractile function following preservation. To test this hypothesis, standard Langendorff techniques for the measurement of left ventricular function were combined with biochemical, histological and morphological techniques to determine: (1) whether loss of microvascular function occurs in isolated hearts hypothermically perfused with an oxygenated solution; (2) the impact of microvascular dysfunction during the preservation period on the functional recovery of hearts; and (3) which mechanisms contribute to decreased microvascular function during preservation. This experimental approach will allow for characterization of the role of the microvasculature in decreased contractility of preserved hearts and will provide information regarding the contribution of specific mechanisms to the compromised contractility of preserved hearts. Systematic evaluation of mechanisms thought to be responsible for decreased contractility of isolated hearts could contribute to improved myocardial preservation techniques that can be applied to clinical transplantation.

Despite the development and use of specialised cardiac preservation solutions, the quality of the donor heart may still be compromised by its obligatory exposure to periods of ischaemia (both cold and warm) followed by reperfusion upon reintroduction of the recipient circulation. This is reflected in Transplant Registry data showing increased primary allograft failure as a function of increasing ischaemic time. The research described in this thesis is designed to further the understanding of the mechanisms by which the donor heart may be adapted to these prolonged periods of ischaemia and reperfusion by the activation of endogenous pro-survival signalling pathways by the addition of pharmacological agents to Celsior, a clinical preservation solution. Studies were conducted in an isolated working rat heart model of donor heart preservation. The first study investigated the cardioprotective effects of a novel inhibitor of poly(ADP-ribose) polymerase 1, INO-1153. Maximum protective effect (after a 6 hour storage period) was observed when the PARP inhibitor was administered prior to cardiac arrest and storage and when the agent was added to the Celsior cardioplegic / storage solution. This protective affect was associated with activation of the Akt signalling pathway and could be prevented by inhibition of Akt phosphorylation and activation. The second study examined functional protection and pro-survival signalling pathway activation in hearts arrested and stored for 6 hours in Celsior supplemented with glyceryl trinitrate (an exogenous source of nitric oxide) and Cariporide (an inhibitor of sodium hydrogen exchange). Here, cardiac protection was accompanied by activation of the ERK 1/2 pro-survival pathway as well as a decrease in apoptosis. The third study examined the cardioprotective effect of supplementation of Celsior with all three agents after an extended (10 hour) period of hypothermic storage. Significant recovery of function was only observed in the triply supplemented hearts, being accompanied by activation of both the Akt and ERK pathways. These studies demonstrate for the first time the feasibility of recruitment of endogenous pro-survival pathways as an approach to increasing the post-storage function of the donor heart. Importantly, for the logistics of clinical transplantation, these pathways can be recruited by addition of appropriate pharmacological agents to the arresting and storage solution.