C Lygate

Investigating cardiac energetics in heart failure

•u2002 What is the topic of this review? This report reviews the current literature on non‐invasive in vivo methodologies for the assessment of cardiac energetics in the mouse. •u2002 What advances does it highlight? Example applications of magnetic resonance spectroscopy are discussed, including suggestions for how this approach can be advanced.

Myocardial creatine levels do not influence response to acute oxidative stress in isolated perfused heart

Background Multiple studies suggest creatine mediates anti-oxidant activity in addition to its established role in cellular energy metabolism. The functional significance for the heart has yet to be established, but antioxidant activity could contribute to the cardioprotective effect of creatine in ischaemia/reperfusion injury. Objectives To determine whether intracellular creatine levels influence responses to acute reactive oxygen species (ROS) exposure in the intact beating heart. We hypothesised that mice with elevated creatine due to over-expression of the creatine transporter (CrT-OE) would be relatively protected, while mice with creatine-deficiency (GAMT KO) would fare worse. Methods and Results CrT-OE mice were pre-selected for creatine levels 20–100% above wild-type using in vivo 1H–MRS. Hearts were perfused in isovolumic Langendorff mode and cardiac function monitored throughout. After 20 min equilibration, hearts were perfused with either H2O2 0.5 µM (30 min), or the anti-neoplastic drug doxorubicin 15 µM (100 min). Protein carbonylation, creatine kinase isoenzyme activities and phospho-PKCδ expression were quantified in perfused hearts as markers of oxidative damage and apoptotic signalling. Wild-type hearts responded to ROS challenge with a profound decline in contractile function that was ameliorated by co-administration of catalase or dexrazoxane as positive controls. In contrast, the functional deterioration in CrT-OE and GAMT KO hearts was indistinguishable from wild-type controls, as was the extent of oxidative damage and apoptosis. Exogenous creatine supplementation also failed to protect hearts from doxorubicin-induced dysfunction. Conclusions Intracellular creatine levels do not influence the response to acute ROS challenge in the intact beating heart, arguing against creatine exerting (patho-)physiologically relevant anti-oxidant activity.

Metabolic Flux as a Predictor of Heart Failure Prognosis

The creatine kinase (CK) system is thought to play an integral role in maintaining levels of chemical energy in the form of ATP, which is essential for normal cardiac function. In the failing heart, it has long been established that multiple components of CK energy metabolism are commonly impaired and that these correlate with disease severity. A recent study published in Science Translational Medicine adds significantly to this body of evidence by demonstrating that the rate of ATP transfer via CK, measured noninvasively by magnetic resonance spectroscopy, is an independent predictor of adverse clinical outcome in patients with nonischemic cardiomyopathy. This finding invites speculation on the future role of metabolic imaging for risk stratification in patients with heart failure. The authors further assert an implied causal role for energetics in disease progression. Although this is not supported by recent findings in loss-of-function mouse models, there is, nonetheless, a strong argument for the development of novel metabolic therapies for the failing heart.

T2-mapping of ischaemia/reperfusion-injury in the in vivo mouse heart

Introduction Oedema is a key feature of acute ischaemia/reperfusion (IR) injury. As such, it is a diagnostic and potentially therapeutic target, assessable using MRI. To date, its application in the mouse heart is limited due to the challenges associated with low SNR inherent in T2-weighting, miniscule anatomy, and rapid motion. Absolute quantification of transverse relaxation time (T2-mapping) circumvents SNR constraints and may be an alternative to T2weighted imaging. We have therefore measured myocardial T2 in IR-mice and related T2-maps to the histological area-at-risk (AAR)

002 Compensation for impaired myocardial phosphotransfer in guanidinoacetate-N-methyltransferase knockout mice

Guanidinoacetate-N-methyltransferase (GAMT) is a key enzyme in creatine biosynthesis, such that GAMT knockout mice lack phosphocreatine (PCr) as a substrate for energy transfer via the creatine kinase (CK) reaction. Despite undetectable levels of PCr and creatine, GAMT knockout mice exhibit only minor changes in baseline function and impaired contractile reserve, …

018 Guanidinoacetate N-methyltransferase knockout mice exhibit normal left ventricular remodelling, haemodynamics and survival after myocardial infarction despite lack of phosphocreatine

Guanidinoacetate N-methyltransferase (GAMT) catalyses the final step of creatine biosynthesis such that GAMT mice have undetectable levels of phosphocreatine and creatine and accumulation of the precursor (phospho-)guanidinoacetate (PGA). Like phosphocreatine, PGA acts as an energy reservoir, but energy transfer via creatine kinase is 100 times slower. We hypothesised that reduced energy transfer would be detrimental following myocardial infarction (MI). Methods GAMT and wild-type controls received coronary artery ligation or sham operation (n5104), with 3D echocardiography and left ventricular haemodynamics after 6 weeks. Results Sham GAMT mice had reduced pressure-generating capacity compared with wild-type (wt), with left ventricular systolic pressure and dP/dtmax both significantly lower and impaired contractile reserve. Despite this, there was no significant difference in post-MI survival between GAMT and wt. Both GAMT and wt infarct groups exhibited left ventricular dilatation compared with sham controls, and systolic and diastolic functionwas also severely impaired. However, there was no significant difference between GAMT and wt infarct groups for left ventricular systolic pressure, left ventricular end-diastolic pressure, dP/dtmax, or Tau, nor for end-diastolic and endsystolic volumes or ejection fraction. Left ventricular/body weight increased by 30% in GAMT and 27% in wt, indicating a similar degree of left ventricular hypertrophy in response to MI. Conclusions Loss of energy transfer in GAMT mice was not detrimental to left ventricular remodelling, haemodynamics and survival post-MI. As acute reduction of energy transfer in the rat infarct model dramatically reduces survival, this strongly suggests that significant compensatory processes occur in GAMT mice as a result of creatine loss during early life.