ATP delivery rates in myocardial hypertrophy

This thesis aims to better understand the metabolic dimension of cardiac adaptation to increased work by studying the creatine kinase (CK) system in participants with pathological hypertrophy secondary to severe aortic stenosis with preserved ejection fraction (SevAS-pEF) or secondary to severe primary mitral regurgitation (MR), participants with hearts adapted to exercise training, and participants with no hypertrophy (NH). It also aims to understand why some hearts transition to failure in the presence of pressure overload, by studying a group with severe aortic stenosis with reduced ejection fraction (SevAS-rEF). Broadly speaking there are two pathways for ATP delivery from sites of production in mitochrondria to sites of use at the myofibrils: facilitated diffusion by the CK system and simple diffusion. This thesis aims to investigate both. Closely related to this, CK also has roles in the temporal buffering of adenine nucleotides and the control of respiratory flux over varying work rates, so more broadly represents a kind of metabolic reserve. CK activity can be assessed on freeze-and-extract biochemical assay. Using 31phosphorus-magnetic resonance spectroscopy (MRS) saturation transfer to estimate the CK rate constant (kf) is the first step toward estimates of in vivo flux. Over 60 intra-operative left ventricular (LV) biopsies in participants with pre-operative 31P-MRS are collected to validate the novel TRiST sequence against biochemically-assessed maximum CK rate. Ch. 3 describes non-invasive and invasive predictors of CK total activity and derives a three-variable linear regression model in 37 triplicate datasets to predict CK total activity from total creatine, kf and LV end-systolic volume index. Ch. 4 studies invasive and non-invasive measures of the CK system in participants with sevAS-pEF, sevAS-rEF, and NH. The chapter argues for the assessment of metabolic reserve as a way of identifying – and explaining – the recently described subset of patients whose ventricles deteriorate sooner in the face of pressure overload. A stepwise fall in median CK total activity is found when arranging groups in order of increasing LV mass index. Non-invasive measures are also assessed in participants with moderate AS, healthy volunteers, and sevAS-pEF participants late post-operatively. The CK system is then studied in participants with severe primary MR and preserved systolic function pre- and post-operatively (Ch. 5), and in participants with physiological hypertrophy or normal, non-trained, hearts at rest and during dobutamine stress (Ch. 6). Ch. 7 relates CK activity with ATP diffusion distance using 3D models built from tomographic stacks of electron microscopy images in a subset of LV biopsies.