The lipids as a structural element of the cell membrane, lipids regulate metabolic homeostasis through various mechanisms, thus they are essential to maintaining homeostasis. However, lipids can also have deleterious effects on glucose metabolism and insulin sensitivity. When animals receive a HFD and obesity occurs, the body's normal metabolism slows dramatically, and adaptive mechanisms often fail to give the body appropriate feedback. The resulting influx of lipids from adipose tissue exceeds its storage capacity, leading to the accumulation of harmful lipids in skeletal muscle, which is thought to be an important factor in insulin resistance [17, 18]. Most phospholipids exist in skeletal muscle membranes and are important regulators of skeletal muscle mitochondrial respiratory function [19, 20]. Relevant studies have shown that increased phospholipids in skeletal muscle may lead to changes in mitochondrial metabolism or increase in mitochondrial membrane remodeling, making phospholipids play a role in the inflammation of skeletal muscle. Further, excess lipids have harmful effects on the body through the action of the endoplasmic reticulum. In the case of a HFD or chronic nutritional stress, lipid synthesis in the endoplasmic reticulum is disordered, which interrupts the calcium signal [19]. Lysophosphatidic acid is a kind of lipid with strong biological activity produced by the ATX enzyme. High-fat-induced changes in LPA concentrations have been associated with the development of obesity-related damage to glucose homeostasis in addition to inflammatory disease [21, 22]. Previous studies showed that excessive endogenous LPA production in HFD mice inhibited glucose tolerance, which may lead to the deterioration of glucose tolerance [23, 24]. In this study, PC and PE (18:1) in the HFD rabbit group were significantly up-regulated, consistent with previous studies, and could be used as a potential biomarker for inflammation. However, different levels of lysophospholipids (LPC (15:1), LPC (16:1), LPC (17:1)) were significantly down-regulated compared with the SND rabbit group, contrary to previous studies. These results suggest that a HFD may not have adverse effects on LPA metabolism in rabbit skeletal muscle. In other words, not all high-fat diets have harmful effects, but they do promote skeletal muscle growth and development to some extent.
Carnitine is a type β fatty acid produced during the esterification of fatty acids. It plays a vital role in the regulation of muscle oxidative metabolism. Ninety five percent of medium-chain acylcarnitines (MCACs) and long-chain acylcarnitines (LCACs), and intermediate accumulation of fatty acid oxidation in skeletal muscle may lead to insulin resistance. There is increasing evidence that LCACs refers to the metabolic syndrome caused by excess energy [25]. Rats fed with HFD reduced LCACs activity, aggravated the circulation of glucose-fatty acid, and resulted in metabolic stiffness caused by diet [26]. Hexanoylcarnitine, propionylcarnitine and propionylcarnitine are the LCAC fatty acid derivatives of carnitine [27]. Hexanoylcarnitine was markedly increased in the HFD rabbit group than in the SND rabbit group in this study. This indicates that a LCACs increase in skeletal muscle can cause a disturbance in fatty acid-glucose circulation. This metabolism change of LCACs is helpful to improve our understanding of a potential physiological mechanism for skeletal muscle dysfunction in HFD rabbits. However, propionylcarnitine and DL-Carnitine decreased in the HFD group. High levels of propionylcarnitine and DL-Carnitine may serve as a marker of vitamin B12 deficiency, be associated with orofacial clefts, and have beneficial effects on cardiac metabolism, myocardial microvasculature function during ischemia, and coronary blood flow[28]. These two LCACs may not be harmful to the body. Although previous studies have shown that increased concentrations of these LCACs are beneficial to the body, the mechanism of the influence of low concentrations of LCACs on the body is still unclear and needs further study. Longer studies are needed to investigate if reductions of propionylcarnitine and DL-Carnitine levels could also decrease insulin sensitivity. The effect of LCACs on skeletal muscle metabolism in rabbits was described only by metabonomic methods in this paper, thus further studies on other aspects are needed.
Amino acids are closely related to the TCA cycle and glucose metabolism. Studies have shown that when dietary lipid exceeds demand, phosphorylation of nitric oxide synthase occurs in the skeletal muscle endothelium, resulting in reduced amino acid metabolism, leading to the accumulation of large amounts of amino acids in skeletal muscle, resulting in blocked synthesis and interruption of mitochondrial oxidation ability, which may cause diseases such as type 2 diabetes and insulin resistance [29]. Other studies reported that endogenous amino acids increase in skeletal muscle is related to diabetes, making amino acids a candidate biomarker for related diseases. Because of high levels of amino acids compete with glucose oxidation at the substrate level and interfere with the transcription of insulin signals, thus reducing the sensitivity of insulin signals and leading to impaired glucose oxidation [30]. Branched chain amino acids (leucine, isoleucine, and valine), aromatic amino acids (phenylalanine, tyrosine and tryptophan), and aliphatic amino acid (lysine) are generated by carboxylic acid by transamination of intermediates dependent on glucagon and insulin secretion to regulate glucose metabolism [31, 32]. Altered levels of four amino acids were detected in rabbit skeletal muscle in this study using an untargeted metabolomics approach, namely leucine, glycine, lysine, histidine and carnosine(a histidine residue). Leucine and carnosine were significantly down-regulated whereas glycine, lysine and histidine were significantly up-regulated in skeletal muscle from the HFD group compared to the SND group. Numerous studies have shown that if the concentration of histidine is too high, a large amount of high nitrogen ammonia will be produced during the decomposition process, changing the anti-oxidation effect of carnosine, leading to accelerated lipid oxidation, and diseases related to insulin resistance. In addition to increase in the production of ammonia, high concentrations of histidine will lead to changes in the concentration of several amino acids; glutamic acid, alanine and glutamine will increase, and branched amino acids (valine, leucine and isoleucine) will decrease, leading to amino acidemia[33, 34]. In this study, histidine and carnosine levels in the HFD group were significantly up-regulated compared to the SND group, leading to significantly up-regulated glutamine levels and significantly down-regulated leucine levels, consistent with previous reports. This suggests that a HFD has specific effects on skeletal muscle metabolism in rabbits, which can be used as potential markers for metabolic diseases. However, contrary to previous research, five consecutive weeks of HFD failed to cause metabolic disorders due to high levels of lysine and glycine in rabbit skeletal muscle, which further ensured the normal operation of TCA circulation, gluconeogenesis and oxidative decomposition of glucose, and maintained the normal growth and development of rabbits. Moreover, previous studies reported that elevated plasma leucine levels were associated with insulin-resistance related diseases, but excessive histidine in skeletal muscle could lead to decreased leucine levels and insulin-resistance related diseases. These contrasting results may be an indication that leucine content in different parts of the body will have a different action mechanism, which needs further study.
Glucocorticoids containing corticosterone, cortisol, and tetrahydrocorticosterone are produced by the adrenal glands [35]. As a substance of cholesterol metabolism, glucocorticoids are an important part of the biological stress response that helps regulate body balance and glucose homeostasis. With changes in diet, bone cells can adversely affect the skeletal system by altering the activity of glucocorticoid levels causing muscle wasting, osteoporosis and insulin resistance [36]. When mice were fed a high-fat diet, lipids were deposited in skeletal muscles, resulting in increased fat decomposition, gluconeogenesis disorder, and reduced stress response. This can lead to an increase in glucocorticoids followed by metabolic syndrome and type 2 diabetes [37]. Conversely, mice fed a high-fat diet for eight weeks had similar levels of corticosterone as mice fed a low-fat diet [38]. In the present study, rabbits in the HFD group had significantly higher concentrations of tetrahydrocorticosterone than rabbits in the SND group, consistent with the above reports, indicating that tetrahydrocorticosterone could be used as a potential marker for related diseases in skeletal muscle. However, corticosterone concentration decreased significantly. Other studies showed that corticosterone, in addition to being a biological pacemaker, controls physiological processes in mammals including humans, and it can also regulate the daily circadian rhythm by adjusting the level of adrenocorticosterone controlled by the suprachiasmatic nucleus. In addition, overall corticosterone levels were higher at night than during the day because rabbits eat 20 to 25% more at night than during the day [39]. Therefore, the results of the current study differ from those of previous studies perhaps due to the decrease of corticosterone levels in skeletal muscle caused by the unique biological characteristics of rabbits and the daytime samples taken in this trial. The level of glucocorticoids in skeletal muscle contributes to our understanding of steroid hormones, but the biological mechanism needs further research.