Determinants of intramyocellular lipid concentrations in rat hindleg muscle

Abstract

The investigation of intramyocellular lipids (IMCLs) with proton MR spectroscopy ( 1 H-MRS) in humans has recently received increasing attention. IMCL levels correlate with insulin resistance and are affected by diet and exercise, making IMCL an interesting marker for metabolic investigations. In the present in vivo study, the feasibility of using 1 H MRS for the detection of IMCL in rats is demonstrated, and the influence of various factors, such as age, gender, muscle type, and rat strain, on IMCL levels is systematically analyzed. In healthy Wistar and Sprague Dawley (SD) rats, the highest ratios of IMCL/tCr were found in young rats, and IMCL/tCr decreased with increasing age. In addition, IMCL concentration was clearly influenced by gender and muscle type. Insulin-resistant, male, obese, Zucker diabetic fatty (ZDF) rats showed significantly higher IMCL levels than Wistar or SD rats. Proton MR spectroscopy ( 1 H-MRS) has been applied successfully to human muscle in vivo, both in investigations of muscle lipid metabolism in healthy volunteers, and in studies of patients with muscular diseases. The most relevant spectroscopic signals of muscle tissue stem from total creatin (tCr, 3.05 ppm) and from the methylene and methyl protons of lipid resonances (1.0 -1.6 ppm (1,2)).In human muscle, localized MRS can identify two sets of proton resonances from fatty acyl chains-one from intramyocellular (IMCL) and one from extramyocellular (EMCL) lipids (1), which are separated by a frequency shift of 0.2 ppm. The frequency shift has been explained by magnetic susceptibility differences between these two compartments, and the anisotropic spatial arrangement of muscle fibers. The chemical shift of the EMCL resonance has been shown to be orientation-dependent (3), which is most likely due to the arrangement of adipocytes along muscle fibers. IMCL, on the other hand, is stored in the cytoplasm of muscle cells within spherical droplets (4), resulting in no spatial dependence of their chemical shift on the main magnetic field direction (3). This orientation dependency is essential for the separation of EMCL and IMCL by MRS. The best separation between the two signals is achieved when the investigated muscle lies roughly parallel to the main magnetic field B 0 (3).Volume-selective 1 H-MRS is currently the only methodology that enables noninvasive monitoring of IMCL levels in vivo. The main reason for the recently increased interest in the measurement of IMCL levels stems from the good correlation between the IMCL concentration and insulin resistance (5-7). Insulin resistance is the key feature of type 2 diabetes, but may also be found in other common disorders, such as obesity, dyslipidemia, and arterial hypertension. Recent studies indicate that measurements of IMCL levels may also increase our knowledge of the pathogenesis of noninsulin-dependent type 2 diabetes mellitus (8). Other investigators have analyzed the role of IMCL as an energy depot, and the effects of diet and exercise on IMCL levels (9 -12).Currently, it is still...

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

“…However, the voxel position used by these researchers is in a region close to the skin that contains hardly any type I and only a few type IIa muscle fibers (personal observation during histological analyses) and therefore can best be compared with our voxel position 1. When the IMCL content reported in our study is normalized to the tCr signal instead of expressed as a percentage of the water signal, then voxel position 1 contains ϳ30% of the IMCL signal of the SOL (data not shown), which is in good agreement with the results of Neumann-Haefelin et al (6,7).…”

“…When fat storage in rat skeletal muscle is studied, the IMCL is usually normalized to the tCr signal (3,6,7,16,17). In human studies, however, the IMCL is often expressed as a percentage of the water signal originating from the same voxel, since the tCr signal is small compared to the IMCL signal and is not easy to quantify at commonly used clinical field strengths (18).…”

“…Neumann-Haefelin et al (6) described various determinants of IMCL concentrations in rat hindleg muscles, including age, strain, gender, and muscle type, and emphasized the need to take these factors into account when studying diseases such as insulin resistance and diabetes. Our results point to the voxel position when applying localized single-voxel 1 H MRS as another determinant of the IMCL content in rodent muscle.…”

“…Using 1 H MRS, Neumann-Haefelin et al (6) showed that the IMCL content in rat depends on age, strain, and gender, which complicates comparisons among different rat groups and different studies. They also showed that the rather oxidative soleus muscle (SOL) has a higher IMCL content than the predominantly glycolytic tibialis anterior (TA) (6,7).…”

Muscle-Type Specific Intramyocellular and Hepatic Lipid Metabolism During Starvation in Wistar Rats

“…Measurement of tCr in soleus, tibialis anterior, and EDL was performed as described previously (33). Linear detection was achieved between 0 and 50 mol/ml for both analytes, with r 2 values of Ͼ0.999 for Cr and Cr-P.…”