Regional variations in intramyocellular lipid concentration correlate with muscle fiber type distribution in rat tibialis anterior muscle

Abstract

1 1 H MR spectroscopy (MRS) has proved to be a valuable noninvasive tool to measure intramyocellular lipids (IMCL) in research focused on insulin resistance and type II diabetes in both humans and rodents. An important determinant of IMCL is the muscle fiber type, since oxidative type I fibers can contain up to three times more IMCL than glycolytic type II muscle fibers. Because these different muscle fiber types are inhomogeneously distributed in rodent muscle, in the present study we investigated the distribution of IMCL within the rat tibialis anterior muscle (TA) in vivo using single-voxel 1 H MRS along with the muscle fiber distribution in the TA ex vivo determined from immunohistological assays. IMCL levels in the TA differed by up to a factor of 3 depending on the position of the voxel. Skeletal muscle insulin resistance is one of the earliest detectable aberrations in and a predisposing factor for the development of type II diabetes. Increased content of intramyocellular lipids (IMCL) has been closely associated with decreased whole-body and skeletal muscle insulin sensitivity in both healthy and diabetic humans, as well as in animals (reviewed in Ref. 1). IMCL itself probably does not interfere with the insulin signaling pathway directly; rather, it is considered a surrogate marker of lipid metabolites, such as long chain acyl-CoA, diacylglycerol, and ceramides, that are likely to induce defects in the insulin signaling cascade (2). The good correlation of IMCL with insulin sensitivity led to the suggestion that IMCL could be used as a biomarker for insulin resistance in rat models (3).Skeletal muscle stores lipids not only intracellularly, but also extracellularly within interstitial adipocytes. Lipid extraction analysis of muscle biopsy samples therefore cannot differentiate between IMCL and extramyocellular lipids (EMCL) and hence may overestimate the actual IMCL content (1). The discovery that IMCL and EMCL resonate at different frequencies in the 1 H MR spectrum of human skeletal muscle was an important methodological advance in the study of skeletal muscle lipid metabolism (4). The resonance at 1.28 ppm is independent of the angle between the muscle fibers and the static magnetic field, and therefore is attributed to the CH 2 groups of IMCL because IMCL is dispersed as spherical droplets within the muscle cells. The other lipid CH 2 peak shifts with the angle between the muscle fibers and the field, and is therefore attributed to EMCL, which supposedly is arranged in a sheet-like manner along the muscle fibers. The largest chemical shift difference between the two peaks is 0.2 ppm, which is obtained when the muscle fibers are aligned parallel with the magnetic field (5). Since 1 H MRS is a noninvasive method, it allows repetitive analysis of IMCL in the same area. Thus, 1 H MRS-based IMCL measures are especially useful for determining IMCL levels in human studies, as well as for animal intervention and longitudinal studies.Using 1 H MRS, Neumann-Haefelin et al. (6) showed that the IMCL content in ...

Role of proton MR for the study of muscle lipid metabolism

“…In trained athletes this relationship was reversed and high IMCL predicted high insulin sensitivity. In recent years, 1 H-MRS of IMCL has been increasingly used in animals, in particular to study pharmacological interventions and at higher magnetic field strength (23)(24)(25)(26)(27)(28). It would go beyond the target of this review to go into methodological details of these studies since MR examinations in small animals with voxels of a few microliters pose different methodological challenges than MRS in humans.…”

“…In recent years, 1 H-MR spectra of IMCL have been acquired at field strengths above 3 T, either in humans (52,98,115) or particularly in small animals (23,25,27,28). These studies show the potential of higher field strengths for a separation of IMCL and EMCL, and particularly the reduction of contamination by EMCL that can be achieved by a smaller voxel size.…”

Inductively coupled helmholtz coil on a dedicated imaging platform for the in vivo1H‐MRS measurement of intramyocellular lipids in the hind leg of rats

“…Compared to a standard surface coil, the Helmholtz coil platform provides a homogenous radiofrequency field profile and significantly increased imaging SNR, translating directly into an improved SNR in single voxel MR spectroscopy (ϩ80%) and a decreased experimental time (-43%). In case a decreased measurement time is not needed, an improved SNR will allow the user to achieve a more precise, quantitative analysis of the data (reduced noise), to reduce the voxel's volume in order to assess even smaller regional variations, or to perform chemical shift imaging in the muscle (19,20). In addition to its advantageous imaging and spectroscopy properties, handling of the animal and fixation of the leg are markedly improved using the Helmholtz coil platform.…”

“…By design, surface coils exhibit a hemispheric radiofrequency field profile, which directly translates into a distance-dependent reception profile. However, especially for the measurement of regional differences (12) or spectroscopic imaging (13) in rat muscles, a homogeneous reception profile is highly desirable. In addition, an increase in sensitivity of the receiving coil would allow the user to either shorten the measuring time for higher throughput or reduce the size of the voxel volume for higher spatial specificity within the muscle.…”

Effects of acute exercise on lipid content and dietary lipid uptake in liver and skeletal muscle of lean and diabetic rats

“…Spectra from the 32 LASER-POCE experiments with and without the 13 C editing pulse were added separately, and the difference spectrum was calculated using Matlab (R2009b; Mathworks, Natick, MA). Water and IHCL/IMCL methylene (IHCL-CH 2/IMCL-CH2 signal at 1.3 ppm) peak areas were quantified from the unsuppressed and suppressed spectra, respectively, using the jMRUI software package (81) as described previously (15). Total ( 12 C ϩ 13 C) and 13 C-labeled IHCL/IMCL levels were determined from the LASER-POCE spectra without 13 C editing and the difference spectra, respectively, and are presented as a percentage of the unsuppressed water signal measured in the same voxel.…”