Mitochondrial dysfunction in uraemic cardiomyopathy : Technetium 99m sestamibi washout as a novel marker of mitochondrial function

Callan, Paul

Medicine
September 2018

Thesis or dissertation


Rights
© 2018 Paul Callan. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
Abstract

Introduction
Cardiovascular disease is the most common cause of morbidity and mortality in patients with chronic kidney disease. Uraemic cardiomyopathy describes a classical phenotype of left ventricular hypertrophy, diastolic impairment, myocardial fibrosis and altered response to ischaemic injury. Mitochondrial dysfunction plays a key role in the development and progression of this condition. Technetium 99m sestamibi washout is a novel tool that can be used for the non-invasive assessment of cardiac mitochondrial function.

Methods
Clinical studies
We undertook a retrospective analysis of the utility of myocardial SPECT imaging in patients with end stage renal failure referred for kidney transplantation. We assessed its ability to predict mortality and cardiovascular events.
A study of early vs delayed SPECT imaging using Technetium 99m sestamibi was performed in healthy subjects to develop a protocol for measuring Technetium washout in clinical studies.

Animal studies
Uraemia was induced in rats using a 5/6ths nephrectomy model. Ex-vivo measurement of cardiac function was performed using a Langendorff perfusion mode after 12 weeks, followed by in-vitro measurement of mitochondrial respiration and enzymatic activity of respiratory chain complexes.
Myocardial single positron emitted computed tomography (SPECT) was performed on isolated rat hearts using a modified perfusion rig, and perfused with buffer containing Technetium 99m sestamibi tracer. Technetium washout from the myocardium was recorded. Mitochondrial uncoupling agents were added to assess its effect on Technetium washout.
In vivo myocardial SPECT imaging was performed on rats to develop a protocol for measuring Technetium washout in pre-clinical studies.

Results
Clinical studies
The presence of resting perfusion defects or moderate to severe stress perfusion defects on myocardial SPECT scans performed in patients with end stage renal failure were independent predictors of adverse cardiovascular outcomes. The scan findings did not add to the risk prediction models based on clinical risk factors alone, highlighting the need for more robust tools to assess cardiovascular prognosis.
Evaluation of early Technetium kinetics in clinical practice found marked variability in early Technetium washout within the first hour. Inducible cardiac ischaemia was associated with higher washout rates.

Animal studies
A rat model of uraemic cardiomyopathy by subtotal nephrectomy was characterised, which identified a reduction in respiratory complex II/III enzyme activity, and an increase in state 4 mitochondrial respiration in the presence of glutamate and malate, when compared with controls.
An isolated perfused heart system for assessing ex vivo Technetium sestamibi washout was developed. Washout rates increased significantly with the addition of mitochondrial uncoupling agent CCCP, indicating that Technetium retention is dependent upon an intact mitochondrial membrane potential. We developed a method of measuring Technetium washout in vivo, and demonstrated stable Technetium retention in control rats.

Conclusion
We have developed a robust model for assessing Technetium 99m sestamibi washout both in an isolated heart model and in vivo. It has the potential to be utilised as a tool to increase our understanding of the role of mitochondrial dysfunction in the progression of uraemic cardiomyopathy, and to evaluate novel treatments for targeted at myocardial metabolism.

Publisher
Hull York Medical School, The University of Hull and The University of York
Supervisor
Tweddel, Ann; Seymour, Anne-Marie
Qualification level
Doctoral
Qualification name
MD
Language
English
Extent
3 MB
Identifier
hull:17282
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