Citations

Abstract Background An accurate discrete spot scanning (DSS) beam delivery time (BDT) model is essential, particularly as 4D dynamic dose accumulation calculations rely on machine‐specific time models for reliable interplay evaluation. Purpose To derive a BDT model for a synchrotron‐based Hitachi Probeat PBS proton system from oscilloscope measurements of key BDT structures, including multi‐energy extraction (MEE) characteristics. Methods Key BDT structures were characterized using oscilloscope measurements, supplemented by delivery log files where appropriate. Spot‐to‐spot scanning trajectories were visualized using a CROSSmini 2D strip ionization chamber (Liverage Biomedical Inc, Taiwan) via the dose‐driven continuous scanning (DDCS) mode. The MEE design and MEE charge recapture efficiencies were also characterized. In addition, the impact of the number of delivered spots on both the extractable fraction of charge and MEE recapture efficiency was investigated. Results An improved synchrotron‐based BDT model was derived that incorporates dead times between radiofrequency knockout (RFK) and high‐speed steering magnet (HSST) signals. Spot‐to‐spot movement follows a “hockey‐stick”‐like behavior where the effective spot‐to‐spot move time is governed by the slower axis. An energy‐dependent variation in the number of MEE layers deliverable per spill was also observed, with fewer MEE layers per spill (1–3) was observed in low and high energy ranges compared to the mid‐energy range (4–5), deviating from the nominal vendor value of five MEE layers per spill per block. A reduction in the extractable fraction of charge was observed with an increasing number of spots per spill, while increasing the number of delivered spots in the preceding layer significantly reduced the MEE recapture efficiency. Conclusion MEE characteristics should be incorporated into synchrotron‐based time models for accurate beam delivery modeling. The methodology presented here also provides a framework for centers seeking to derive machine‐specific delivery time models for their proton systems.

ABSTRACT The coexistence of obesity and insulin resistance is associated with elevated plasma amino acid concentrations. However, it remains unclear whether adiposity or insulin resistance is the stronger determinant of plasma amino acid dysregulation in this setting. Twenty-two adults (10 women, 12 men) spanning a broad range of body mass index (BMI) and insulin resistance underwent a 75-g oral glucose tolerance test (OGTT) after an overnight fast. Plasma glucose, insulin, and amino acid concentrations were measured serially, and insulin resistance/sensitivity was estimated from OGTT-derived glucose and insulin responses, using the homeostasis model assessment of insulin resistance (HOMA-IR) and the Matsuda insulin sensitivity index (Matsuda-ISI). Principal component analysis (PCA) of fasting plasma amino acid concentrations showed no clear separation by obesity or insulin resistance classifications. In contrast, PCA of OGTT-stimulated plasma amino acid concentrations revealed clearer clustering by BMI, fat mass, and waist circumference, whereas separation by HOMA-IR and Matsuda-ISI was less distinct. Importantly, regression analyses showed that BMI, fat mass, and waist circumference were significant predictors of OGTT-stimulated, but not fasting, amino acid responses, with waist circumference accounting for the greatest proportion of the variance in branched-chain amino acid responses during the OGTT (R 2 = 0.54). In conclusion, measures of adiposity, particularly total fat mass and waist circumference, accounted for a greater proportion of the variance in plasma amino acid responses under physiologically stimulated conditions than indices of insulin resistance. These findings support the view that plasma amino acid concentrations reflect adiposity-related metabolic alterations more strongly than insulin resistance.

ABSTRACT Background In healthy individuals, exercise and amino acid availability act synergistically to stimulate muscle anabolism. However, this interaction may be impaired in individuals with obesity. Objective We examined whether acute aerobic exercise alters amino acid-stimulated muscle protein synthesis during immediate postexercise recovery in adults with obesity. Methods Sixteen sedentary adults with a body mass index >30 kg/m 2 underwent a stable-isotope tracer infusion to measure mixed-muscle protein fractional synthesis rate (FSR) in the basal/fasted state and during an amino acid infusion, either with or without prior aerobic exercise. Participants were randomly assigned to receive either amino acid infusion alone (AA) or 45 min of cycling exercise at ∼65% heart rate reserve immediately before amino acid infusion (EX + AA). Results Amino acid infusion increased muscle protein FSR in the AA group ( P < 0.0001) but not in the EX + AA group ( P > 0.05), and the amino acid-stimulated increase in FSR was 78% lower in EX + AA than in AA ( P < 0.01). The amino acid infusion increased ( P < 0.05) plasma amino acid concentrations in both groups. However, during the amino acid infusion, plasma concentrations of essential and branched-chain amino acids, including leucine, were lower in EX + AA than in AA ( P < 0.05). Moreover, across participants, absolute changes in muscle protein FSR were positively associated with plasma leucine concentrations during the amino acid infusion ( P < 0.05). Conclusions These findings show that, in individuals with obesity, acute aerobic exercise markedly attenuates amino acid-stimulated muscle protein synthesis during early postexercise recovery. These findings have important implications for designing nutritional strategies to optimize muscle anabolism in this population.

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.

Abstract BACKGROUND Gaseous hydrogen sulfide (H2S), a by-product of cysteine metabolism, inhibits cancer cell behavior and growth in glioblastoma (GBM) models. H2S production capacity (HPC) is decreased in human GBM specimens compared to non-tumor controls. Thus, boosting HPC is a novel strategy for GBM treatment. Suppression of thyroid hormone signaling increases production of H2S. Hypothesis: methimazole-induced hypothyroidism will increase tumor HPC to enhance chemotherapy efficacy in GBM. GOAL Proof of concept that methimazole, by reducing thyroid hormone signaling, can increase HPC in patients with recurrent/progressive GBM (rGBM). PRIMARY ENDPOINTS 10% increase in HPC and a 1.5-fold increase in peripheral blood sulfhydration signaling. METHODS This window of opportunity trial evaluates safety, HPC in plasma and tumor, and efficacy of methimazole + chemotherapy in patients with rGBM. rGBM patients planned for a resection begin methimazole 15 mg/d 5-7 days pre-op with investigator’s choice chemotherapy added 10-28 days post-op. Patients receive combination until progression. Pre- and post-op plasma and resected tumor tissue are assayed for HPC and proteins relevant to H2S production. Key eligibility: rGBM for whom a clinically-indicated resection is planned; normal thyroid function with no history of thyroid dysfunction. Patients may have received unlimited prior regimens including bevacizumab. RESULTS To date 15 patients (8 male), median age 56 years, enrolled. The trial was amended to increase methimazole to 25 mg/d; 3 patients have received this dose. Median TSH rose from 1.3 mU/ml to 20.5 at cycle 6. Free T3 and T4 did not fall significantly. Median plasma HPC rose 3-fold while tumor HPC did not rise significantly. No treatment-related grade 3-5 toxicities. Efficacy: PFS6 – 15% and OS12 – 21%. CONCLUSIONS Induction of subclinical hypothyroidism is feasible and safe in rGBM with significant increases in plasma but not tumor HPC. Further tumor correlative data for the dose-escalated methimazole will be presented.

Individuals with obesity and endurance exercise-trained athletes both exhibit excess lipid content in their skeletal muscle compared with healthy, sedentary individuals, yet they experience vastly different health outcomes. Lipids taken up from the circulation contribute to lipid stored in muscle in both populations. Differences in the muscle uptake of plasma non-esterified fatty acids (NEFA) and fatty acids derived from plasma triacylglycerol (TG) between individuals with obesity and endurance-trained athletes have not been systematically examined. In athletes, muscle actively regulates the uptake of TG-derived fatty acids through upregulation of the activity of muscle lipoprotein lipase—the enzyme responsible for the intravascular hydrolysis of TG, a phenomenon evident in the fasting state. In contrast, in individuals with obesity, skeletal muscle functions as a passive recipient of TG-derived fatty acids, an event that becomes quantitively more important when the plasma TG concentrations increase during the postprandial state. Considerable differences in the muscle uptake of plasma NEFA between athletes and individuals with obesity are less evident. These observations indicate mechanistic differences in the regulation of plasma TG-derived fatty acids uptake in muscle between individuals with obesity and endurance-trained athletes in shaping the excess lipid content in their muscles. Moreover, this evidence highlights the need for targeting a reduction in plasma TG in the postprandial state when aiming to attenuate lipid accumulation in muscle in the pathophysiology of obesity.

Abstract Context Humans with obesity and insulin resistance exhibit lipid accumulation in skeletal muscle, but the underlying biological mechanisms responsible for the accumulation of lipid in the muscle of these individuals remain unknown. Objective We investigated how plasma insulin modulates the extraction of circulating triglycerides (TGs) and nonesterified fatty acids (NEFAs) from ingested and endogenous origin in the muscle of lean, insulin-sensitive humans (Lean-IS) and contrasted these responses to those in humans with obesity and insulin resistance (Obese-IR). Methods The studies were performed in a postprandial state associated with steady-state plasma TG concentrations. The arterio-venous blood sampling technique was employed to determine the extraction of circulating lipids across the forearm muscle before and after insulin infusion. We distinguished the kinetics of TGs and NEFAs from ingested origin from those from endogenous origin across muscle by incorporating stable isotope-labeled triolein in the ingested fat. Results Insulin infusion rapidly suppressed the extraction of plasma TGs from endogenous but not ingested origin in the muscle of the Lean-IS, but this response was absent in the muscle of the Obese-IR. Furthermore, in the muscle of the Lean-IS, insulin infusion decreased the extraction of circulating NEFAs from both ingested and endogenous origin; however, this response was absent for NEFAs from ingested origin in the muscle of the Obese-IR subjects. Conclusion Partitioning of circulating lipids away from the skeletal muscle when plasma insulin increases during the postprandial period is impaired in humans with obesity and insulin resistance.

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.