Subsarcolemmal lipid droplet responses to a combined endurance and strength exercise intervention

Abstract

Muscle lipid stores and insulin sensitivity have a recognized association although the mechanism remains unclear. We investigated how a 12‐week supervised combined endurance and strength exercise intervention influenced muscle lipid stores in sedentary overweight dysglycemic subjects and normal weight control subjects (n = 18). Muscle lipid stores were measured by magnetic resonance spectroscopy (MRS), electron microscopy (EM) point counting, and direct EM lipid droplet measurements of subsarcolemmal (SS) and intramyofibrillar (IMF) regions, and indirectly, by deep sequencing and real‐time PCR of mRNA of lipid droplet‐associated proteins. Insulin sensitivity and VO2max increased significantly in both groups after 12 weeks of training. Muscle lipid stores were reduced according to MRS at baseline before and after the intervention, whereas EM point counting showed no change in LD stores post exercise, indicating a reduction in muscle adipocytes. Large‐scale EM quantification of LD parameters of the subsarcolemmal LD population demonstrated reductions in LD density and LD diameters. Lipid droplet volume in the subsarcolemmal LD population was reduced by ~80%, in both groups, while IMF LD volume was unchanged. Interestingly, the lipid droplet diameter (n = 10 958) distribution was skewed, with a lack of small diameter lipid droplets (smaller than ~200 nm), both in the SS and IMF regions. Our results show that the SS LD lipid store was sensitive to training, whereas the dominant IMF LD lipid store was not. Thus, net muscle lipid stores can be an insufficient measure for the effects of training.

Lipid droplet size and location in human skeletal muscle fibers are associated with insulin sensitivity

“…6 and 7). The latter finding is in line with recent results (3,28,35) and together with a concomitant improved insulin sensitivity in the present study (see Ref. 6) emphasizes a role of subsarcolemmal LDs in insulinmediated glucose uptake.…”

“…LDs are distributed in between the myofibrils (intermyofibrillar LDs) and just beneath the surface membrane (subsarcolemmal LDs). Several studies have suggested that only the subsarcolemmal LDs are associated with insulin resistance in humans (3,9,28,35). However, a causal relationship has been questioned by the lack of association after aerobic training (35) and by the fact that white rat muscles (type 2 fibers) were devoid of subsarcolemmal LDs despite being insulin resistant (24).…”

Myostatin in relation to physical activity and dysglycaemia and its effect on energy metabolism in human skeletal muscle cells

“…Body composition was measured by magnetic resonance imaging (MRI)/MR spectrometry (MRS) (Li et al . ). Plasma concentration of mature myostatin protein (free or latent) was measured with an ELISA kit (Cat# DGDF80, R&D Systems), according to the manufacturer's protocol.…”

Irisin – a myth rather than an exercise-inducible myokine

“…All base positions were of high quality (Phred score >30). Alignment of sequenced cDNA reads to the RefSeqGene database as transcriptome reference and quantitative analysis of the transcripts was done as described by Li et al (2014) 37 .…”