Physiology, metabolism and redox mechanisms in chronic cardiac volume overload

Abstract

Chronic haemodynamic stress in the heart, induced by volume or pressure overload, contributes to cardiac dysfunction and heart failure and is associated with several human cardiovascular diseases (e.g. valvular heart diseases, arterial hypertension). Volume and pressure overload induce distinct cardiac remodelling responses in humans and mouse models, including different signalling patterns, but the differences between them are incompletely understood. In order to comprehensively study these, murine models of volume overload (aortocaval fistula, Shunt) and pressure overload (transverse aortic constriction, TAC and abdominal aortic banding, AAB) were studied in this work. The overall aims were (a) to apply advanced non-invasive methods to characterize changes in diastolic physiology in the two models; (b) to compare the changes in glucose metabolism that occur during these haemodynamic stress conditions; (c) to study during chronic cardiac volume overload the role of NADPH oxidase-4 (Nox4), a reactive oxygen species (ROS)-generating enzyme recently found to be protective against chronic pressure overload. Assessment of diastolic function in mice following chronic volume and pressure overload using state-of-the-art echocardiography revealed marked differences between the models with respect to left ventricular relaxation and filling. The isovolumic relaxation time (IVRT), left atrial area, E/E’ and reverse longitudinal strain rate were found to be consistent and reproducible parameters to analyze diastolic properties in these haemodynamically different settings. For interpretation, however, the physiological and haemodynamic background needs to be well-understood. A novel methodology of in vivo [U-13C] glucose administration followed by isotopomer analysis using NMR-spectroscopy, as well as expression profiles of metabolic enzymes, revealed fundamental differences in cardiac glucose metabolism following chronic volume or pressure overload in mice. Despite very similar increases in left ventricular hypertrophy between TAC and Shunt, glycolysis, TCA cycle activity, glutamine synthesis and O-GlcNAcylation of proteins were significantly increased only following TAC. These findings together with a nearly unchanged glucose metabolism after Shunt suggest a much more pronounced metabolic complexity during pressure overload and concentric remodelling than during volume overload. Nox4 was found to promote eccentric hypertrophy following two weeks of volume overload, as global Nox4-null mice (Nox4-/-) developed significantly less left ventricular hypertrophy and dilation compared to WT littermates. This was attributed to a Nox4-dependent activation of Akt and its downstream targets S6 ribosomal protein and eIF4E-BP1, which are known to initiate protein synthesis. Despite its role in eccentric remodelling, Nox4 did not seem to alter cardiac function at this point. This study provides novel data on cardiac physiology and metabolism using advanced echocardiographic techniques and a novel in vivo 13C-labelling methodology following volume versus pressure overload in mice. It also identifies a novel Nox4-regulated pathway, which appears to be important for cardiac adaptation during volume overload. These results might be of relevance for future heart failure therapy development.