Abstract
Recent studies have revealed that around half of patients living with heart failure (HF) may present with heart failure with preserved ejection fraction (known as HFpEF). This is a condition of diastolic dysfunction in the absence of overt systolic dysfunction and is reported to have comparable clinical outcomes to systolic HF. These patients do not respond to existing therapies for HF, and HFpEF therefore represents a significant clinical burden. Clinical studies have recently highlighted a strong correlation between repolarisation abnormalities and diastolic dysfunction in HF patients, but it remains unclear whether this represents a causal relationship. It is possible that perturbations to ventricular repolarisation may act to disrupt normal intracellular calcium handling, thus contributing to diastolic dysfunction through impaired active ventricular relaxation.The present study has aimed to address this question using a direct approach to determine the relationship between diastolic function and ventricular repolarisation in the whole heart, beginning in healthy Langendorff-perfused guinea pig hearts and progressing to isolated working hearts from a novel deoxycorticosterone acetate (DOCA)-salt guinea pig model of HFpEF. Action potential duration (APD) was prolonged via pharmacological blockade of repolarising currents IKr and IKs to replicate changes to QT interval seen in the clinic, while ex vivo cardiac function was measured with the use of an intraventricular balloon in Langendorff-perfused hearts and a pressure-volume admittance catheter in working hearts. In vivo cardiac function in the guinea pig DOCA-salt model was measured using cardiac ultrasound, while additional characterisation data was obtained from conscious radiotelemetry blood pressure and anaesthetised ECG recordings. Ex vivo characterisation included the assessment of intracellular sodium levels using 23Na NMR spectroscopy in isolated Langendorff-perfused hearts and western blotting to examine the ventricular expression of proteins involved in intracellular calcium handling.
Initial experiments utilised whole-heart optical mapping to demonstrate that APD prolongation results in concomitant prolongation of the underlying calcium transient, providing proof-of-principle for the hypothesis. Cardiac function was then measured in the presence of APD prolongation in healthy hearts or in combination with the pharmacological replication of other pathological changes commonly reported in HF. This was achieved using ouabain to elevate intracellular sodium and cyclopiazonic acid (CPA) to impair SERCA function, while QT interval was measured from ECG recordings and used as a surrogate of ventricular APD. These changes affect intracellular calcium handling and are therefore likely to be relevant to the proposed mechanism by which altered ventricular repolarisation could affect diastolic function. Data revealed mixed results, with APD prolongation resulting in mild diastolic impairment in healthy working hearts and Langendorff-perfused hearts under conditions of SERCA inhibition, but having no effect on diastolic function in working hearts in combination with ouabain and CPA treatment. These studies were followed by the development of a novel DOCA-salt guinea pig model of HFpEF, allowing the relationship between ventricular repolarisation and diastolic function to be assessed in a clinically relevant setting. Extensive characterisation data revealed this model to demonstrate many of the key characteristics of HF, including elevated intracellular sodium and reduced SERCA expression, which previous experiments had aimed to replicate. The presence of diastolic dysfunction in vivo was also confirmed, alongside hypertension and QT prolongation. However, APD prolongation in isolated working hearts from this model did not exacerbate diastolic dysfunction ex vivo.
Findings from the present study indicate that ventricular repolarisation abnormalities are likely not a major contributor to impaired diastolic function in the failing heart.
| Date of Award | 1 May 2021 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Michael Shattock (Supervisor) & James Winter (Supervisor) |