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Cardiac and Hepatic Metabolism Modulation using L-carnitine injection: 1H Magnetic Resonance Spectroscopy Study1University of Oxford, Oxford, United Kingdom
Synopsis
This study investigated the in vivo effects on acetylcarnitine levels in the heart and the liver after an injection of L-carnitine. Carnitine acts as a buffer of acetyl-CoA units in the mitochondria, as well as facilitating transport of fatty acids. We show that an injection of L-carnitine can alter cardiac lipid levels in the in vivo healthy human. However, even though L-carnitine injection shows high cardiac variability in acetylcarnitine levels, hepatic variability of acetylcarnitine levels were reduced. Further studies are needed to confirm the dose and time-frame of a response in acetylcarnitine levels after L-carnitine injections.
Introduction
L-carnitine is critical for transportation of fatty acids to the mitochondria, it also modulates the intracellular acetyl-CoA/CoA ratio, which is important for controlling the balance between carbohydrate and lipid oxidation[1]. Reduced levels of plasma and cardiac L-carnitine have been reported in patients with cardiovascular disease[2], diabetes[3],[4] and chronic liver disease[5]. Supplementation with L-carnitine has been shown to improve the outcomes in patients with cardiovascular disease who do not have primary carnitine deficiency[6], to reduce plasma lipid levels, cytokines and improve histological scores in patients with non-alcoholic fatty liver disease (NAFLD)[7],[8]. L-carnitine buffers acetyl-CoA within the mitochondria through the generation of acetylcarnitine. Recently, it has been shown that tissue acetylcarnitine can be non-invasively assessed in vivo in skeletal and cardiac muscle, using long echo time 1H-MRS [9]. Therefore, the aim of this study was to explore the effect of post-intravenous injection of L-carnitine on the acetylcarnitine resonance and lipid metabolism in vivo in the liver and the heart using long echo time 1H-MRS in young healthy volunteers.Method
Five healthy volunteers were fasted overnight and refrained from coffee and tea in the morning. Baseline cardiac and liver 1H-MRS was acquired on a 3T system (Prisma, Siemens Healthineers) using a 30-channel phased array coil and a spine coil. Lipid quantification: Five water-suppressed and one non-water-suppressed breath held acquisitions were performed in the cardiac interventricular septum (5 measurements each, TE:10 ms, TR:4 s, voxel: 32×18×22 mm3), and three water-suppressed and one non-water-suppressed breath held acquisitions were performed in the posterior part of the right liver lobe using a single-voxel STEAM sequence (5 measurements each, TE: 10 ms, TR: 4 s, voxel: 20×20×20 mm3). Acetylcarnitine quantification: Five breath held acquisitions using a water-suppression cycled PRESS acquisition [10] (6 measurements each, TE:90 ms, TR: 4 s, voxel: 32×25×22 mm3) in the heart and a free-breathing STEAM acquisition (64 averages, TE:90 ms, TR: 4 s, voxel: 30×30×30 mm3) in the liver.Following baseline MRS, volunteers were given an intravenous injection of L-carnitine (50 mg/kg body weight) and two hours post-injection another set of cardiac and liver spectra was acquired (Figure 1).Fat fraction was calculated using the OXSA toolbox in Matlab[11]. The same software was used for fitting multiple resonances of fat (methyl (d=0.9 ppm), methylene (d=1.3 ppm), allylic (d=2.0 ppm) diacyl (d=2.8 ppm)), and acetylcarnitine. These resonances were then normalized to the unsuppressed water signal. Both hepatic and cardiac spectra were frequency-aligned (using the residual water signal), phased and averaged before fitting with the OXSA toolbox.
Results
Results were obtained from five volunteers (n = 5, age = 30 ± 4 yr, BMI = 23 ± 2 kg/m2). Lipid peaks in both heart and liver had high variability across all lipid peaks. There was, however, a significant reduction in the allylic peak in the heart after injection of L-carnitine (27 %, p < 0.05, t-test) (Figure 2).After L-carnitine injection, acetylcarnitine levels in the heart were variable (Figure 3A). Total fat fraction did not change significantly (Figure 3B), even though the unsaturated allylic fat was reduced after L-carnitine injection in the heart (Figure 2AB). Total fat fraction was not correlated with acetylcarnitine levels (R2=0.22, p = 0.21) in the heart (Figure 3C).
In contrast hepatic acetylcarnitine levels (example spectra illustrated on Figure 4) had reduced variability after injection of L-carnitine compared to before(F-test, p = 0.031, Figure 5A).
Total hepatic fat fraction showed no trend (Figure 5B) and had no correlation with acetylcarnitine levels (Figure 5C). Despite no significant change in total hepatic lipid content, acetylcarnitine levels in the liver correlated positively with the allylic fat peak (R2=0.59, p = 0.009) and with the methylene fat peak (R2=0.43, p = 0.04).
Discussion and Conclusion
This study shows the characterization of in vivo metabolism of fatty acids and acetylcarnitine levels from healthy human livers and hearts in response to an injection of L-carnitine. There was a significant reduction in the cardiac allylic fat peak after intravenous injection of L-carnitine and despite high variability, the individual cardiac lipid peaks were within a reported normal range (0.3-0.6 %)[12]. Even though L-carnitine had no significant effect on lipid levels in the liver, hepatic acetylcarnitine was correlated with both methylene and allylic fat peak in the liver [13], [14]. L-carnitine injection resulted in reduced variability in hepatic acetylcarnitine levels in vivo. A previous study showed that intravenous injection of L-carnitine in healthy volunteers led to a 17 % elevation of whole-body glucose utilization [15], thereby impairing fatty acid metabolism, which may explain the different response from lipid metabolism seen in our study. Further studies are needed to validate that the timing of the MRS acquisition after L-carnitine injection and to test if the dose is sufficient for biological modulation.Acknowledgements
This study was funded by DS (Oxfordshire Health Services Research reference: 2019/1333). LV would like to acknowledge the support of the Slovak Grant Agencies VEGA [2/0003/20] and APVV [#19–0032]. Furthermore, we would like to acknowledge the Novo Nordisk fellowship (DS) and mentor Mette Skalshøj Kjær for her expertise on liver metabolism.References
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