Investigation into the effects of age and diet induced obesity on cardiac contractility and its effect on drug responses

  • Ralfe Alibhai Ribeiro

    Student thesis: Doctoral ThesisDoctor of Philosophy


    Cardiac contractility is often defined as the ability of the cardiac muscle to generate power and results from a number of mechanisms. Contractile work is also the trigger that causes an increase or decrease in ATP generation, via one of two main processes: fatty acid (FA) oxidation or carbon substrate metabolism. These two mechanisms are crucial in contraction and any changes can lead to cardiac dysfunction. To this effect, both age and obesity have been previously shown to play a significant role in cardiac dysfunction, primarily via changes to contractility and ATP generation and in pathways involving the citric acid cycle and uncoupling proteins. The primary aim of this thesis was to investigate cardiac contractility in rodent models of aging and high fat diet-induced obesity (HFD) in the presence and absence of inotropic drugs.
    The first part of this project was a physiological investigation into the effects of ageing (3, 6 and 18-month murine models) and high fat diet-induced obesity (6 and 18-month lean and obese murine models) on cardiac function. The second part of the project was a pharmacological investigation into the effects of inotrope-induced changes on cardiac contractility in ageing and high-fat diet-induced obesity (using the same models). To achieve this, three techniques were used: the Langendorff isolated heart model, the papillary muscle work-loop assay used to assess power output and western blotting protein analysis of Pyruvate dehydrogenase E1-α subunit (PDH) and Uncoupling protein 3 (UCP3).
    Cardiac function of the whole heart diminished with age, showing significant differences for the Rare pressure product (RPP) on the 18-month models when compared to the 3 and 6-month models (p<0.05), but the cardiac muscles also suffer time and age-dependant decreases in performance, when comparing the 6-month models to the 3-month models. 
    This translated to reductions in power output (p<0.05) and total net-work done. In addition to this, and in agreement with previous literature, the results from the western blots on PDH have shown decreased phosphorylation in an age-dependant fashion and indicate that there is, in fact, a decrease in FA oxidation (p<0.0001 when comparing between the 6 and 18-month models with the 3-month model). The UCP3 results corroborate these findings and suggest that there is a decrease in the rate of FA oxidation, in an age-dependant fashion (p<0.0001 when comparing between the 6 and 18-month models with the 3-month model).
    HFD functional changes were also recorded, as the 6-month lean heart showed significant decreases in the absolute values for the haemodynamic parameters when compared to the 6-month HFD model (p<0.0001); the 18-month model showed a significant increase compared to the 6-month lean model, but not when compared to the 6-month HFD model, which aligns with previous literature which associated these changes with hypertrophic changes to the heart. On the muscle assay, significant decreases in power output, fatigue and total net-work done were recorded when comparing between lean and HFD muscles (p<0.01). Finally, the pPDH and UCP3 were shown to be effective biomarkers of obesity, as the formers levels decreased in the presence of the high-fat diet (p<0.0001), while the latter increased significantly. The measured increase in UCP3, specifically, is indicative of increased FA availability and its consequent usage.
    On the ageing pharmacological chapter, a phenomenon known as β-receptor desensitization was present, as the LVDP, HR and RPP for the 18-month dobutamine-treated hearts were shown to have age-dependant impairments, when compared to the 3 and 6-month models (p<0.0001). As for the western blot assay, it was found that PDH was reduced in an age-dependant fashion for all three drug-treated hearts, with seemingly no changes in expression between them (p<0.0001 when comparing between the 6 and 18-month models with the 3-month model).
    This seems to indicate that FA oxidation and ATP production are being affected, but not detected in the isolated heart models (other than on the Dobutamine-treated hearts). As for UCP3, interestingly no change was recorded for atenolol or itraconazole across different ages, but a significant decrease was recorded for the 6 and 18-month dobutamine treated hearts when compared to the 3-month heart (p<0.0001). It is therefore possible that there is a correlation between the observed β-receptor desensitization and the UCP3 levels, even though the exact link between the two is unclear.
    On the isolated heart model, between the lean and HFD atenolol-treated hearts, but both the Dobutamine and Itraconazole data showed severe impairment and exacerbation (respectively) in their inotropic effects on LVDP, HR and RPP, when treating the HFD models and comparing them to the lean models (p<0.01 for each parameter for Dobutamine and p<0.0001 for each parameter for Itraconazole). PDH was significantly changed for the HFD hearts treated with Dobutamine (p<0.0001) and Itraconazole (p<0.0001), with little to no change in our atenolol treated hearts. UCP3 on the other hand showed no significance between the dobutamine and atenolol treated HFD and lean hearts but showed a significant decrease for the itraconazole HFD treated hearts when compared to the lean hearts (p<0.001).
    In conclusion, these results showed that both age and HFD cause functional changes in the heart in both absence and presence of drug treatments and their combined synergistic effect can cause further detrimental changes on different haemodynamic parameters. In addition, PDH activity, specifically, seems to be heavily linked to the inotropic effect of both Dobutamine and Itraconazole in the presence of obesity, but further work will still be necessary in order to confirm this observation.
    Date of AwardApr 2021
    Original languageEnglish
    Awarding Institution
    • Coventry University
    SponsorsInoCardia Ltd
    SupervisorJason Tallis (Supervisor), Mayel Gharanei (Supervisor), Mike Dodd (Supervisor) & Helen Maddock (Supervisor)

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