DeFilippis, Eleanna

Skeletal muscle is a heterogenous tissue composed of fibers (i.e., cells) with distinct metabolic and contractile properties. Muscle fibers are classified into 2 separate groups: Type 1 fibers which are primarily oxidative, and Type 2 fibers (further subdivided to Types 2a and 2x) which are predominantly glycolytic. Acute exercise induces transcriptional changes in skeletal muscle. However, these changes have been studied at the whole muscle level. The effects of exercise on differential gene expression in distinct muscle fiber types in response to acute exercise remains unclear. In this study, we evaluated gene expression in Type 1, 2a, and 2x muscle fibers in response to acute aerobic exercise. Muscle biopsies were collected from 5 (31±3 yrs) apparently healthy individuals before and 3hrs following an acute 45 min session of cycling exercise performed at 65% of maximal heart rate. Individual muscle fibers (n=138) were isolated from the muscle biopsy sample manually under a microscope. Half of the fiber was used to characterize it as Type 1, 2a, or 2x based on its myosin heavy chain isoform expression content using single fiber SDS-PAGE. The other half of the fiber underwent RNA sequencing. The effects of exercise on fiber types were analyzed using subject specific pseudo-bulk differential expression before and after exercise (Type 1 n=28, 2a n=28, and 2x n=12). Gene counts were normalized as counts per million (CPM) and log2 transformed with a pseudocount of +1. The top 200 differentially expressed genes (DEG’s) were then used to calculate fold changes between fiber types at baseline and in response to exercise. After exercise, 73.5% of DEG’s were upregulated in Type 1 fibers, 52% were upregulated in Type 2a fibers, and 60.5% were downregulated in 2x fibers, relative to baseline. Type 1 fibers showed the greatest increases in gene expression (notably, TPM1 log2FC = 2.46, ATP2A1 log2FC = 3.23, and UQCRB log2FC=0.39) and displayed transcriptional changes consistent with oxidative phosphorylation, calcium signaling, and cytoskeletal remodeling pathways. Type 2a fibers displayed modest increases (ATP2A1 log2FC = 0.34, TNNC2 log2FC = 0.21 and ENO3 log2FC=0.35) and an overall upregulation of glycogenic/gluconeogenesis pathways. Conversely, Type 2x fibers exhibited broad transcriptional suppression and downregulation of calcium signaling pathways. In conclusion, acute aerobic exercise induces gene expression in muscle that is fiber type-specific. Type 1 fibers upregulate genes related to oxidative phosphorylation and calcium signaling, whereas Type 2a fibers upregulate genes related to glycolysis. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Obesity is associated with altered skeletal muscle protein metabolism. While plasma amino acids (AA) stimulate protein synthesis in muscle, it remains unclear how aerobic exercise modifies this response in obesity. In this study, we evaluated basal and amino acid-stimulated skeletal muscle protein fractional synthesis rate (FSR) in humans with obesity, induced by a primed, constant AA infusion (15% Clinisol; 4 mg·kg FFM-1·min-1), immediately after a single bout of acute aerobic exercise. We studied two groups of subjects with obesity: Group 1: AA infusion alone for 240 mins (AA; N = 8; BMI: 34.9 ± 1 kg/m 2 ; body fat percent: 35.2 ± 2) and group 2: acute aerobic exercise followed by AA infusion for 240 mins (AA+EX; N=8; BMI: 36.1 ± 2 kg/m 2 ; body fat percent: 42.4 ± 2). Subjects in the AA+EX group performed 45 minutes of cycling on a stationary bike at 65% VO2 max, and their amino acid infusion started immediately after the exercise ended. A d10-leucine tracer was infused for the entire duration of the study to measure mixed-muscle protein FSR (MMP-FSR). Muscle biopsies from the vastus lateralis were taken at baseline, 45 mins after the start of the amino acid infusion and at the end of the infusion study to determine MMP-FSR. Baseline MMP-FSR values did not differ between groups (p > 0.05). Participants receiving the AA infusion alone exhibited a significant increase in MMP-FSR from baseline to post-infusion (0.060 ± 0.005 to 0.119 ± 0.011 %/hr; p < 0.0001), whereas the AA+EX group showed no significant change from baseline (0.067 ± 0.004 to 0.084 ± 0.009 %/hr; p > 0.05). Plasma insulin concentrations increased significantly in response to the amino acid infusion (p < 0.0001) but did not differ between groups (p > 0.05). Baseline amino acid concentrations were not different between groups (p > 0.05). Among the plasma amino acids (μmol/L) changed, baseline plasma leucine (AA:137 ± 8, AA+EX: 144 ± 10; p > 0.05) increased significantly in both groups during AA infusion but was higher in the AA (424 ± 15) than the AA+EX (226 ± 13) group (p< 0.0001). Similarly, baseline plasma valine (AA: 226 ± 7, AA+EX: 234 ± 19; p > 0.05) increased significantly in both groups during AA infusion but was higher in the AA (553 ± 28) than the AA+EX (456 ± 23) group (p< 0.002). In contrast baseline arginine (AA: 59 ± 5, AA+EX: 71±11; p > 0.05), although it increased significantly in both groups during AA infusion, was lower in the AA (197 ± 3) than the AA+EX (266 ± 11) group (p< 0.0001). Despite marked increases in plasma amino acid and insulin concentrations, the AA+EX group exhibited a blunted muscle protein synthesis response to amino acid stimulation compared with the AA group. In conclusion, aerobic exercise blunts the amino acid-stimulated increase in muscle anabolism in the immediate period after exercise in humans with obesity. This research was supported by the National Institutes of Health/ National Institute of Diabetes and Digestive and Kidney Diseases. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Skeletal muscle is a highly plastic tissue, adapting its structure and metabolism in response to various stimuli like exercise. This study aimed to determine the plasticity of skeletal muscle DNA methylation in response to an acute exercise bout in volunteers with a range of insulin sensitivities. We studied 14 sedentary participants (7 male/7 female): age = 34.5 ± 3.1 years, body mass index = 29.3 ± 1.9 kg/m2 and HbA1c = 5.3 ± 0.1 %. We performed VO2peak tests to determine exercise capacity (VO2peak = 25.5 ± 1.6 mL per Kg per minute) and euglycemic hyperinsulinemic clamps (Rd value = 5.9 ± 0.8 mg per Kg per minute) with vastus lateralis muscle biopsies before and 30 minutes after a single 48-minute exercise bout. We extracted DNA from the muscle biopsies and performed reduced representation bisulfite sequencing. After performing quality control on the sequencing data, 1,034,947 sites were included for downstream analyses. MethylSig analysis revealed 17,030 differentially methylated cytosines (DMC), 9,020 increased and 8,010 decreased. To identify biologically meaningful DMCs, we used a mean change of >10%, which resulted in 1,179 and 1,436 increased and decreased sites, respectively. Pathway analysis of the increased DMCs revealed an enrichment in efferocytosis, leukocyte transendothelial migration, and FOXO-mediated transcription of oxidative stress pathways. In addition, pathway analysis of the decreased DMCs revealed an enrichment in metabolic (type 2 diabetes/ PI3K-Akt signaling), actin cytoskeleton, integrin/extracellular matrix and calcium signaling pathways. Our study reveals that an acute exercise bout induces alterations in the DNA methylation of skeletal muscle, impacting specific pathways in individuals with varying degrees of insulin sensitivity. Notably, our findings underscore the remarkable plasticity of skeletal muscle tissue. This research was funded by the National Institutes of Health, grant number R01DK094013 (Dawn Coletta). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Changes in circulating insulin-like growth factor 1 (IGF-1) implicate regulation of skeletal muscle metabolism, including protein and glucose metabolism. Previous studies have shown that an acute bout of exercise increases serum IGF-1 concentrations in healthy, lean humans. However, exercise-induced changes in serum IGF-1 concentrations in humans with obesity remain unknown. We compared changes in plasma IGF-1 during and after an acute bout of endurance exercise between humans with and without obesity. Eight subjects with obesity (i.e., OB: BMI = 34.48 ± 2.96) and eight subjects without obesity (i.e., LN: BMI = 24.45 ± 2.43 kg/m2) exercised for 45 min in a cycle ergometer at 65% of their maximum oxygen uptake, after an overnight 10-h fast. Blood draws were taken immediately before exercise, at 15 and 30 minutes (i.e., during exercise), and at 55, 75, 95, and 115 minutes after the start of the exercise. Serum concentrations of total IGF-1 and insulin were determined using commercially available ELISA assays (Alpco 22-IGFHU-E01 and 80-INSHU-E10.1, respectively). Plasma glucose concentrations were determined using an automatic analyzer (YSI glucose analyzer). One-way repeated measures analyses of variance were carried out to detect significant changes over time within each group. Independent t tests were used to determine group differences at each time point. Alpha level was set at p ≤ 0.05 and data are reported as mean ± SD. There were no significant ( p > 0.05) differences between groups for IGF-1 serum concentrations at baseline (LN = 205.61 ± 40.07 ng/mL, OB = 200.89 ± 80.40 ng/mL) nor during or after exercise. Significant increases from baseline in serum IGF-1 concentrations were detected in the LN group at 15 min (231.30 ± 50.47 ng/mL, p = 0.007) and 30 min (227.94 ± 49.02 ng/mL, p = 0.04) during exercise, followed by significant decreases from baseline at 95 min (189.99 ± 34.73 ng/mL, p = 0.052) and 115 min (196.42 ± 40.39 ng/mL, p = 0.023). No significant ( p > 0.05) differences from baseline were observed in the obese group neither during nor after exercise. Higher serum insulin concentrations were observed in the OB group at baseline (LN = 5.55 ± 2.62 μIU/mL, OB = 11.07 ± 5.76 μIU/mL, p = 0.027) but significant group differences no longer existed ( p > 0.05) during exercise and up to 55 min after exercise; however, insulin concentrations for the OB group were significantly greater than those measured in the LN group at 75 min ( p = 0.028), 95 min ( p = 0.008), and 115 min ( p = 0.007) after exercise. Moreover, a significant increase from baseline was measured 55 min after exercise in the lean group (8.22 ± 3.13 μIU/mL, p = 0.005), whereas no significant changes were observed in the OB groups at any time point ( p > 0.05). No significant ( p > 0.05) group differences were detected for plasma glucose concentrations at any time point, and despite a significant decrease from baseline observed in the LN group at 115 min after exercise (baseline = 83.32 ± 7.66 mg/dL, 115 min = 81.02 ± 7.00 mg/dL, p = 0.044). In conclusion, endurance exercise increases serum IGF-1 concentrations in humans without obesity, but this response is impaired in humans with obesity. National Institute of Health (NIH)\National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Grant/Award Number: R01DK123441. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Lipoprotein Lipase (LPL) hydrolyzes plasma triglycerides into free fatty acids in the circulation for use and/or storage in tissues (i.e., adipose tissue, skeletal muscle). Impaired LPL activity (LPLa) is associated with unfavorable health outcomes, such as elevated plasma triglyceride concentrations. We sought to determine how plasma LPLa relates to body composition measurements in individuals with varying degrees of adiposity. Eighteen subjects (ages 19 to 45 years) with body mass index (BMI) between 18 and 45 kg/m2 had their body composition measured by dual-energy X-ray absorptiometry (DEXA) and insulin sensitivity estimated from an oral glucose tolerance test by using the Matsuda Index (MI). Subjects ingested a bolus of 5g fat (i.e., whipping cream, 0.2 g fat/kg fat free mass), followed by small boluses of fat over the course of a 3-hour experimental period. This experimental protocol allows for the induction of a postprandial state without perturbation of the plasma triglyceride or insulin concentrations. LPLa was measured in the presence of increased plasma insulin induced by intravenous insulin infusion during the last 30 mins of the 3-hour experimental protocol. Insulin was infused at a rate of 0.5 mU/kg/min for subjects with MI-determined insulin sensitivity < 4 and 1 mU/kg/min for those with MI-determined insulin sensitivity > 4, in order to produce comparable levels of plasma insulin concentrations across subjects. To release endothelial-bound LPL, subjects received an injection of intravenous heparin (75 UI/kg) at the end of the plasma insulin infusion. Plasma samples were collected 10 minutes after the heparin infusion and analyzed for LPL concentration (LPLc) and LPLa using commercially available ELISA kits. Person’s correlation was used to evaluate relationships between several body composition parameters and LPLc as well as LPLa. LPLc was not correlated (P >0.05) with BMI (r = 0.03), total lean body mass (r = -0.20), total body fat (r = -0.02), or android fat (r = - 0.08). However, LPLa was positively correlated (P <0.05) with BMI (r = 0.49) and total lean body mass (r = 0.58), as well as total body fat (r = 0.54, P = 0.02) and android fat (r = 0.58, P = 0.01). In conclusion, increased body fat, including android fat, are associated with increased intravascular LPLa in the presence of insulin. Greater insulin-stimulated intravascular LPLa can enhance the release of triglyceride-associated free fatty acids and their uptake by tissues, resulting in insulin resistance as documented in individuals with increased android fat distribution. American Diabetes Association Grant #7-12-CT-40. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.