Background Among central and peripheral factors contributing to exercise intolerance (EI) in heart failure (HF), the extent to which skeletal muscle (SM) energy metabolic abnormalities occur and contribute to EI and increased fatigability in HF patients with reduced or preserved ejection fraction (HFrEF and HFpEF, respectively) are not known. An energetic plantar flexion exercise fatigability test and magnetic resonance spectroscopy were used to probe the mechanistic in vivo relationships between SM high-energy phosphate concentrations, mitochondrial function and EI in HFrEF and HFpEF patients and in healthy controls. Methods and Results Resting SM high-energy phosphate concentrations and ATP flux rates were normal in HFrEF and HFpEF patients. Fatigue occurred at similar SM energetic levels in all subjects, consistent with a common SM “energetic limit”. Importantly, HFrEF NYHA class II–III patients with EI and high fatigability exhibited significantly faster rates of exercise-induced high-energy phosphate decline than did HFrEF patients with low fatigability (NYHA class I), despite similar left ventricular ejection fractions. HFpEF patients exhibited severe EI, the most rapid rates of high-energy phosphate depletion during exercise, and impaired maximal oxidative capacity. Conclusions Symptomatic fatigue during plantar flexion exercise occurs at a common energetic limit in all subjects. HFrEF and HFpEF patients with EI and increased fatigability manifest early, rapid exercise-induced declines in SM high-energy phosphates and reduced oxidative capacity as compared to healthy and low fatigability HF patients, suggesting SM metabolism is a potentially important target for future HF treatment strategies.

Morbidity and mortality from heart failure (HF) are high, and current risk stratification approaches for predicting HF progression are imperfect. Adenosine triphosphate (ATP) is required for normal cardiac contraction, and abnormalities in creatine kinase (CK) energy metabolism, the primary myocardial energy reserve reaction, have been observed in experimental and clinical HF. However, the prognostic value of abnormalities in ATP production rates through CK in human HF has not been investigated. Fifty-eight HF patients with nonischemic cardiomyopathy underwent 31P magnetic resonance spectroscopy (MRS) to quantify cardiac high-energy phosphates and the rate of ATP synthesis through CK (CK flux) and were prospectively followed for a median of 4.7 years. Multiple-event analysis (MEA) was performed for HF-related events including all-cause and cardiac death, HF hospitalization, cardiac transplantation, and ventricular-assist device placement. Among baseline demographic, clinical, and metabolic parameters, MEA identified four independent predictors of HF events: New York Heart Association (NYHA) class, left ventricular ejection fraction (LVEF), African-American race, and CK flux. Reduced myocardial CK flux was a significant predictor of HF outcomes, even after correction for NYHA class, LVEF, and race. For each increase in CK flux of 1 μmol g−1 s−1, risk of HF-related composite outcomes decreased by 32 to 39%. These findings suggest that reduced CK flux may be a potential HF treatment target. Newer imaging strategies, including noninvasive 31P MRS that detect altered ATP kinetics, could thus complement risk stratification in HF and add value in conditions involving other tissues with high energy demands, including skeletal muscle and brain.

Hays AG, Iantorno M, Soleimanifard S, Steinberg A, Schär M, Gerstenblith G, Stuber M, Weiss RG. Coronary vasomotor responses to isometric handgrip exercise are primarily mediated by nitric oxide: a noninvasive MRI test of coronary endothelial function. Am J Physiol Heart Circ Physiol 308: H1343-H1350, 2015. First published March 27, 2015 doi:10.1152/ajpheart.00023.2015.-Endothelial cell release of nitric oxide (NO) is a defining characteristic of nondiseased arteries, and abnormal endothelial NO release is both a marker of early atherosclerosis and a predictor of its progression and future events. Healthy coronaries respond to endothelial-dependent stressors with vasodilatation and increased coronary blood flow (CBF), but those with endothelial dysfunction respond with paradoxical vasoconstriction and reduced CBF. Recently, coronary MRI and isometric handgrip exercise (IHE) were reported to noninvasively quantify coronary endothelial function (CEF). However, it is not known whether the coronary response to IHE is actually mediated by NO and/or whether it is reproducible over weeks. To determine the contribution of NO, we studied the coronary response to IHE before and during infusion of N G -monomethyl-L-arginine (L-NMMA, 0.3 mg·kg Ϫ1 ·min Ϫ1 ), a NO-synthase inhibitor, in healthy volunteers. For reproducibility, we performed two MRI-IHE studies ϳ8 wk apart in healthy subjects and patients with coronary artery disease (CAD). Changes from rest to IHE in coronary cross-sectional area (%CSA) and diastolic CBF (%CBF) were quantified. L-NMMA completely blocked normal coronary vasodilation during IHE [%CSA, 12.9 Ϯ 2.5 (mean Ϯ SE, placebo) vs. Ϫ0.3 Ϯ 1.6% (L-NMMA); P Ͻ 0.001] and significantly blunted the increase in flow [%CBF, 47.7 Ϯ 6.4 (placebo) vs. 10.6 Ϯ 4.6% (L-NMMA); P Ͻ 0.001]. MRI-IHE measures obtained weeks apart strongly correlated for CSA (P Ͻ 0.0001) and CBF (P Ͻ 0.01). In conclusion, the normal human coronary vasoactive response to IHE is primarily mediated by NO. This noninvasive, reproducible MRI-IHE exam of NO-mediated CEF promises to be useful for studying CAD pathogenesis in low-risk populations and for evaluating translational strategies designed to alter CAD in patients. endothelial function; coronary artery disease; magnetic resonance imaging ENDOTHELIAL CELL RELEASE of nitric oxide (NO) is a defining characteristic of nondiseased vascular tissue; it inhibits platelet aggregation, attenuates inflammation, decreases cellular proliferation, and induces local vascular smooth muscle vasodilation (4). Most classic and novel cardiovascular risk factors converge to impair endothelial function, including dyslipidemia, insulin resistance, inflammation, tobacco abuse, and oxidative stress, as well as hemodynamic and genetic factors (4, 33). Critically, endothelial dysfunction is a marker for subclinical disease, an independent predictor of adverse cardiovascular events, and a potential target for medical interventions (21,24,28).Traditionally, coronary endothelial function (CEF) is assessed by the directi...

Background-Energy metabolism is essential for myocellular viability. The high-energy phosphates adenosine triphosphate (ATP) and phosphocreatine (PCr) are reduced in human myocardial infarction (MI), reflecting myocyte loss and/or decreased intracellular ATP generation by creatine kinase (CK), the prime energy reserve of the heart. The pseudo-first-order CK rate constant, k, measures intracellular CK reaction kinetics and is independent of myocyte number within sampled tissue. CK flux is defined as the product of

A lethal and extensively characterized familial form of hypertrophic cardiomyopathy (HC) is due to a point mutation (Arg403Gln) in the cardiac β-myosin heavy-chain (MHC) gene. Although this is associated with abnormal energy metabolism and progression to heart failure in an animal model, in vivo cardiac energetic shave not been characterized in patients with this mutation. Noninvasive phosphorus saturation transfer magnetic resonance spectroscopy was used to measure the adenosine triphosphate (ATP)supplied by the creatine kinase (CK) reaction and phosphocreatine (PCr), the heart’s primary energy reserve, in 9 of 10 patients from a single kindred with HC caused by Arg403GIn mutation, and 17 age-matched healthy controls. Systolic and diastolic function was assessed by echocardiography in all 10 HC patients. HC patients had impairment of diastolic function as well as mild systolic dysfunction, when assessed using global systolic longitudinal strain. Myocardial [PCr] was significantly decreased by 24% in patients (7.1±2.3μmol/g) compared to controls (9.4±1.2μmol/g; p=0.003). The pseudo-first-order CK rate-constant was 26% lower (0.28±0.15 vs. 0.38±0.07s−1, p=0.035) and the forward CK flux was 44% lower (2.0±1.4 vs3.6±0.9 μmol/g/s, p=0.001) than controls. The contractile abnormalities did not correlate with metabolic indices. In conclusion, myocardial PCr and CK ATP delivery are significantly reduced in patients with HC due to Arg403Gln mutation, akin to prior results from mice with the same mutation. Lack of a relation between energetic and contractile abnormalities suggests the former are due to the sarcomeric mutation and not a late consequence of mechanical dysfunction.

BACKGROUND Physical frailty in older individuals is characterized by subjective symptoms of fatigue and exercise intolerance (EI). Objective abnormalities in skeletal muscle (SM) mitochondrial high-energy phosphate (HEP) metabolism contribute to EI in inherited myopathies; however, their presence or link to EI in the frail older adult is unknown. METHODS Here, we studied 3 groups of ambulatory, community-dwelling adults with no history of significant coronary disease: frail older (FO) individuals (81 ± 2.7 years, mean ± SEM), nonfrail older (NFO) individuals (79 ± 2.0 years), and healthy middle-aged individuals, who served as controls (CONT, 51 ± 2.1 years). Lower extremity SM HEP levels and mitochondrial function were measured with 31 P magnetic resonance (MR) techniques during graded multistage plantar flexion exercise (PFE). EI was quantified by a 6-minute walk (6MW) and peak oxygen consumption during cardiopulmonary testing (peak VO 2 ). RESULTS During graded exercise, FO, NFO, and CONT individuals all fatigued at similar SM HEP levels, as measured by 31 P-MR. However, FO individuals fatigued fastest, with several-fold higher rates of PFE-induced HEP decline that correlated closely with shorter exercise duration in the MR scanner and with 6MW distance and lower peak oxygen consumption on cardiopulmonary testing ( P < 0.001 for all). SM mitochondrial oxidative capacity was lower in older individuals and correlated with rapid HEP decline but less closely with EI. CONCLUSION Several-fold faster SM energetic decline during exercise occurs in FO individuals and correlates closely with multiple measures of EI. Rapid energetic decline represents an objective, functional measure of SM metabolic changes and a potential new target for mitigating frailty-associated physical limitations. FUNDING This work was supported by NIH R21 AG045634, R01 AG063661, R01 HL61912, the Johns Hopkins University Claude D. Pepper Older Americans Independence Center P30AG021334, and the Clarence Doodeman Endowment in Cardiology at Johns Hopkins.

Background Normal endothelial function is a measure of vascular health and dysfunction a predictor of coronary events. Nitric Oxide (NO)-mediated coronary artery endothelial function (CEF), as assessed by vasomotor reactivity during isometric handgrip exercise (IHE), was recently quantified noninvasively with MRI. Because the internal mammary artery (IMA) is often visualized during coronary MRI we propose the strategy of simultaneously assessing systemic and coronary endothelial function noninvasively by MRI during IHE. Methods and Results Changes in cross-sectional area (CSA) and blood flow (BF) in the right coronary artery (RCA) and the IMA in 25 CAD patients and 26 healthy subjects during IHE were assessed using 3T MRI. In 8 healthy subjects a NO synthase inhibitor was infused to evaluate the role of NO in the IMA-IHE response. Inter-observer IMA-IHE reproducibility was good for CSA (R=0.91) and BF (R=0.91). In healthy subjects, CSA and BF of the IMA increased during IHE and these responses were significantly attenuated by L-NMMA (p<0.01 vs. placebo). In CAD patients, the RCA did not dilate with IHE and dilation of the IMA was less than that of the healthy subjects (p=0.01). The BF responses of both the RCA and IMA to IHE were also significantly reduced in CAD patients. Conclusions MRI-detected IMA responses to IHE primarily reflect NO-dependent endothelial function, are reproducible and reduced in CAD patients. Endothelial function in both coronary and systemic (IMA) arteries can now be measured noninvasively with the same imaging technique and promises novel insights into systemic and local factors affecting vascular health.

BackgroundPhosphorus saturation transfer (ST) magnetic resonance spectroscopy can measure the rate of ATP generated from phosphocreatine (PCr) via creatine kinase (CK) in the human heart. Recently, the triple-repetition time ST (TRiST) method was introduced to measure the CK pseudo-first-order rate constant kf in three acquisitions. In TRiST, the longitudinal relaxation time of PCr while γ-ATP is saturated, T1`, is measured for each subject, but suffers from low SNR because the PCr signal is reduced due to exchange with saturated γ-ATP, and the short repetition time of one of the acquisitions. Here, a two-repetition time ST (TwiST) method is presented. In TwiST, the acquisition with γ-ATP saturation and short repetition time is dropped. Instead of measuring T1`, an intrinsic relaxation time T1 for PCr, T1intrinsic, is assumed. The objective was to validate TwiST measurements of CK kinetics in healthy subjects and patients with heart failure (HF).MethodsBloch equation simulations that included the effect of spillover irradiation on PCr were used to derive formulae for T1intrinsic and kf measured by both TRiST and TwiST methods. Spillover was quantified from an unsaturated PCr measurement used in the current protocol for determining PCr and ATP concentrations. Cardiac TRiST and TwiST data were acquired at 3 T from 12 healthy and 17 HF patients.ResultsSimulations showed that both kf measured by TwiST and T1intrinsic require spill-over corrections. In human heart at 3 T, the spill-over corrected T1intrinsic = 8.4 ± 1.4 s (mean ± SD) independent of study group. TwiST and TRiST kf measurements were the same, but TwiST was 9 min faster. Spill-over corrected TwiST kf was 0.33 ± 0.08 s−1 vs. 0.20 ± 0.06 s−1 in healthy vs HF hearts, respectively (p < 0.0001).ConclusionTwiST was validated against TRiST in the human heart at 3 T, generating the same results 9 min faster. TwiST detected significant reductions in CK kf in HF compared to healthy subjects, consistent with prior 1.5 T studies using different methodology.Electronic supplementary materialThe online version of this article (doi:10.1186/s12968-015-0175-4) contains supplementary material, which is available to authorized users.