The Langendorff perfused murine heart has become an increasingly important research model in cardiovascular physiology and pharmacology. However, the model remains relatively poorly characterised when compared with the widely employed rat preparation. The purpose of the research within this thesis was initially two-fold: 1) to characterise the functional and substrate-dependent properties of the murine model; and 2) to characterise the relationships between glycolysis, ischaemic tolerance and adenosine-mediated cardioprotection in the mouse. Initial studies, confirmed by simultaneous/subsequent work in other laboratories, revealed the frequent occurrence of regular cyclic oscillations in contractile function and coronary flow in glucose-perfused isovolumically contracting hearts. This phenomenon (labelled 'cycling') was unaltered by inhibition of ?-adrenergic receptors, prostaglandins, and nitric oxide synthase. However, A1/A2 adenosine receptor agonism did abolish the oscillations in flow and reduced contractile oscillations by 50%. Importantly, cycling was eliminated by addition of 50 IU/l insulin to perfusion fluid, or provision of 5 mM pyruvate as a co-substrate with glucose. These data suggest that functional 'cycling' in glucose-perfused murine hearts likely occurs as a result of a mismatch between substrate metabolism (energy supply) and myocardial energy demand. It may be that glycolysis with exogenous glucose is insufficient to ensure appropriate matching of myocardial energy supply and demand. For this reason, it is advisable to employ a co-substrate such as pyruvate in studies of murine hearts. Further studies performed within this thesis generally employ this co-substrate addition. Addition of pyruvate as co-substrate removes 'cycling' but is also known to inhibit/modify glycolysis, which may affect ischaemic tolerance and/or cardioprotection mediated by adenosine. Experiments throughout this thesis demonstrated that pyruvate-perfusion improved tolerance to both ischaemia (delayed time to onset of ischaemic contracture; TOC) and reperfusion (reduced diastolic dysfunction and cell death). The delay in TOC as a result of pyruvate-perfusion also suggests that contracture is not solely influenced by anaerobic glycolysis (as outlined in current paradigms). To test the relevance of glycolysis to ischaemic injury hearts were subjected to various forms of glycolytic inhibition. Glycolysis was inhibited by use of 10 mM pyruvate, (iodoacetic acid) IAA treatment, and glycogen depletion by pre-ischaemic substrate-free perfusion (all groups employing pyruvate as sole-substrate). Each form of glycolytic modification resulted in significant delays in TOC, in complete contrast to findings from other models and species. Glycogen depletion also reduced the peak level of contracture. These findings indicate that the mouse is either unique in terms of substrate metabolism and mechanisms of contracture (an unlikely possibility), or raise serious questions regarding current models of contracture development during ischaemia (theorised to be delayed by prolonging anaerobic glycolysis). Modification of glycolysis also altered post-ischaemic outcome, with pyruvate perfusion and glycogen depletion both enhancing functional recoveries. However, IAA treated hearts, despite near-identical ischaemic tolerance (ie contracture development) to pyruvate-perfused hearts, displayed very poor functional recovery, which was below that for all other groups. These data clearly reveal that blocking glycolysis improves tolerance to ischaemia (as evidenced by reduced contracture), provide evidence of dissociation of ischaemic injury or contracture from post-ischaemic recovery, and confirm the key importance of glycolysis in enhancing recovery from ischaemia. Since tolerance to ischaemia/reperfusion was shown to be glycolysis dependent, and since it has been theorised that adenosine protects hearts through modulating glycolysis, the relationships between glycolytic inhibition and adenosine-mediated cardioprotection was tested. In a number of studies, exogenously applied adenosine was shown to protect both glucose- and pyruvate-perfused hearts (supporting no dependence of adenosinergic protection on glycolysis). However, to more equivocally test the role of glycolysis effects of IAA were studied and were shown to markedly limit protection with adenosine. The effects of adenosine during ischaemia were abolished by IAA treatment, and effects on post-ischaemic recovery were reduced (but not eliminated). Similar results were acquired for protection with endogenous adenosine (using iodotubercidin to block adenosine phosphorylation). Collectively, these data reveal that adenosinergic protection during ischaemia depends entirely upon glycolysis while protection during reperfusion likely involves glycolysis dependent and independent processes. However, glycolysis is required for full recovery of function during reperfusion. Further studies assessed the involvement of glycolysis in cardioprotection afforded by transgenic A1 adenosine receptor (A1AR) overexpression. It was found that pyruvate-perfusion provided the same protection as A1AR overexpression, and the two responses (to pyruvate and A1AR overexpression) were not additive. Thus, it is probable that common mechanisms are targeted in both responses (likely glycolysis). Finally, the effects of adenosine and pyruvate on oxidant injury were studied, testing whether interactions between adenosine and pyruvate observed in prior work within this thesis could be explained by alterations in anti-oxidant responses. It was found that adenosine has quite profound anti-oxidant responses in glucose-perfused hearts, with very selective effects on markers of damage. Pyruvate also had some anti-oxidant effects but interestingly it reduced the anti-oxidant effects of adenosine. In conclusion, the work entailed within this thesis demonstrates that the isolated mouse heart model may possess unique properties and should be further characterised by potential users in order to improve its utility, and the reliability of experimental findings (chiefly when studying ischaemia-reperfusion). Other work within thesis demonstrates that modification of glycolysis is important in dictating recovery from ischaemia-reperfusion, and also impacts on adenosine-mediated protection (principally but not exclusively during ischaemia itself). The manner in which glycolysis is modified and contributes to protection remains unclear.
| Identifer | oai:union.ndltd.org:ADTP/195121 |
| Date | January 2005 |
| Creators | Hack, Benjamin Daniel, n/a |
| Publisher | Griffith University. School of Health Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.gu.edu.au/disclaimer.html), Copyright Benjamin Daniel Hack |
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