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  • 1
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    Pharmacological enhancement of leg and muscle microvascular blood flow does not augment anabolic responses in skeletal muscle of young men under fed conditions
    American Physiological Society ; 2014
    In:  American Journal of Physiology-Endocrinology and Metabolism Vol. 306, No. 2 ( 2014-01-15), p. E168-E176
    Details
    In: American Journal of Physiology-Endocrinology and Metabolism, American Physiological Society, Vol. 306, No. 2 ( 2014-01-15), p. E168-E176
    Abstract: Skeletal muscle anabolism associated with postprandial plasma aminoacidemia and insulinemia is contingent upon amino acids (AA) and insulin crossing the microcirculation-myocyte interface. In this study, we hypothesized that increasing muscle microvascular blood volume (flow) would enhance fed-state anabolic responses in muscle protein turnover. We studied 10 young men (23.2 ± 2.1 yr) under postabsorptive and fed [iv Glamin (∼10 g AA), glucose ∼7.5 mmol/l] conditions. Methacholine was infused into the femoral artery of one leg to determine, via bilateral comparison, the effects of feeding alone vs. feeding plus pharmacological vasodilation. We measured leg blood flow (LBF; femoral artery) by Doppler ultrasound, muscle microvascular blood volume (MBV) by contrast-enhanced ultrasound (CEUS), muscle protein synthesis (MPS) and breakdown (MPB; a-v balance modeling), and net protein balance (NPB) using [1,2- 13 C 2 ]leucine and [ 2 H 5 ]phenylalanine tracers via gas chromatography-mass spectrometry (GC-MS). Indexes of anabolic signaling/endothelial activation (e.g., Akt/mTORC1/NOS) were assessed using immunoblotting techniques. Under fed conditions, LBF (+12 ± 5%, P 〈 0.05), MBV (+25 ± 10%, P 〈 0.05), and MPS (+129 ± 33%, P 〈 0.05) increased. Infusion of methacholine further enhanced LBF (+126 ± 12%, P 〈 0.05) and MBV (+79 ± 30%, P 〈 0.05). Despite these radically different blood flow conditions, neither increases in MPS in response to feeding (0.04 ± 0.004 vs. 0.08 ± 0.01%/h, P 〈 0.05) nor improvements in NPB (−4.4 ± 2.4 vs. 16.4 ± 5.7 nmol Phe·100 ml leg −1 ·min −1 , P 〈 0.05) were affected by methacholine infusion (MPS 0.07 ± 0.01%/h ; NPB 24.0 ± 7.7 nmol Phe·100 ml leg −1 ·min −1 ), whereas MPB was unaltered by either feeding or infusion of methacholine. Thus, enhancing LBF/MBV above that occurring naturally with feeding alone does not improve muscle anabolism.
    Type of Medium: Online Resource
    ISSN: 0193-1849 , 1522-1555
    URL: Article
    DOI: 10.1152/ajpendo.00440.2013
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2014
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  • 2
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    Exploring mechanistic links between extracellular branched-chain amino acids and muscle insulin resistance: an in vitro approach
    American Physiological Society ; 2020
    In:  American Journal of Physiology-Cell Physiology Vol. 319, No. 6 ( 2020-12-01), p. C1151-C1157
    Details
    In: American Journal of Physiology-Cell Physiology, American Physiological Society, Vol. 319, No. 6 ( 2020-12-01), p. C1151-C1157
    Abstract: Branched-chain amino acids (BCAAs) are essential for critical metabolic processes; however, recent studies have associated elevated plasma BCAA levels with increased risk of insulin resistance. Using skeletal muscle cells, we aimed to determine whether continued exposure of high extracellular BCAA would result in impaired insulin signaling and whether the compound sodium phenylbutyrate (PB), which induces BCAA metabolism, would lower extracellular BCAA, thereby alleviating their potentially inhibitory effects on insulin-mediated signaling. Prolonged exposure of elevated BCAA to cells resulted in impaired insulin receptor substrate 1/AKT signaling and insulin-stimulated glycogen synthesis. PB significantly reduced media BCAA and branched-chain keto acid concentrations and increased phosphorylation of AKT [+2.0 ± 0.1-fold; P 〈 0.001 versus without (−)PB] and AS160 (+3.2 ± 0.2-fold; P 〈 0.001 versus −PB); however, insulin-stimulated glycogen synthesis was further reduced upon PB treatment. Continued exposure of high BCAA resulted in impaired intracellular insulin signaling and glycogen synthesis, and while forcing BCAA catabolism using PB resulted in increases in proteins important for regulating glucose uptake, PB did not prevent the impairments in glycogen synthesis with BCAA exposure.
    Type of Medium: Online Resource
    ISSN: 0363-6143 , 1522-1563
    URL: Article
    DOI: 10.1152/ajpcell.00377.2020
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2020
    detail.hit.zdb_id: 1477334-X
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  • 3
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    Intake of low-dose leucine-rich essential amino acids stimulates muscle anabolism equivalently to bolus whey protein in older women at rest and after exercise
    American Physiological Society ; 2015
    In:  American Journal of Physiology-Endocrinology and Metabolism Vol. 308, No. 12 ( 2015-06-15), p. E1056-E1065
    Details
    In: American Journal of Physiology-Endocrinology and Metabolism, American Physiological Society, Vol. 308, No. 12 ( 2015-06-15), p. E1056-E1065
    Abstract: Dysregulated anabolic responses to nutrition/exercise may contribute to sarcopenia; however, these characteristics are poorly defined in female populations. We determined the effects of two feeding regimes in older women (66 ± 2.5 yr; n = 8/group): bolus whey protein (WP-20 g) or novel low-dose leucine-enriched essential amino acids (EAA) [LEAA; 3 g (40% leucine)]. Using [ 13 C 6 ]phenylalanine infusions, we quantified muscle (MPS) and albumin (APS) protein synthesis at baseline and in response to both feeding (FED) and feeding plus exercise (FED-EX; 6 × 8 knee extensions at 75% 1-repetition maximum). We also quantified plasma insulin/AA concentrations, whole leg (LBF)/muscle microvascular blood flow (MBF), and muscle anabolic signaling by phosphoimmunoblotting. Plasma insulinemia and EAA/aemia were markedly greater after WP than LEAA ( P 〈 0.001). Neither LEAA nor WP modified LBF in response to FED or FED-EX, whereas MBF increased to a similar extent in both groups only after FED-EX ( P 〈 0.05). In response to FED, both WP and LEAA equally stimulated MPS 0–2 h ( P 〈 0.05), abating thereafter (0–4 h, P 〉 0.05). In contrast, after FED-EX, MPS increased at 0–2 h and remained elevated at 0–4 h ( P 〈 0.05) with both WP and LEAA. No anabolic signals quantifiably increased after FED, but p70 S6K1 Thr 389 increased after FED-EX (2 h, P 〈 0.05). APS increased similarly after WP and LEAA. Older women remain subtly responsive to nutrition ± exercise. Intriguingly though, bolus WP offers no trophic advantage over LEAA.
    Type of Medium: Online Resource
    ISSN: 0193-1849 , 1522-1555
    URL: Article
    DOI: 10.1152/ajpendo.00481.2014
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2015
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  • 4
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    Application of deuterium oxide (D 2 O) to metabolic research: just D 2 O it? Depends just how you D 2 O it!
    American Physiological Society ; 2015
    In:  American Journal of Physiology-Endocrinology and Metabolism Vol. 308, No. 9 ( 2015-05-01), p. E847-E847
    Details
    In: American Journal of Physiology-Endocrinology and Metabolism, American Physiological Society, Vol. 308, No. 9 ( 2015-05-01), p. E847-E847
    Type of Medium: Online Resource
    ISSN: 0193-1849 , 1522-1555
    URL: Article
    DOI: 10.1152/ajpendo.00581.2014
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2015
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  • 5
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    The impact of delivery profile of essential amino acids upon skeletal muscle protein synthesis in older men: clinical efficacy of pulse vs. bolus supply
    American Physiological Society ; 2015
    In:  American Journal of Physiology-Endocrinology and Metabolism Vol. 309, No. 5 ( 2015-09-01), p. E450-E457
    Details
    In: American Journal of Physiology-Endocrinology and Metabolism, American Physiological Society, Vol. 309, No. 5 ( 2015-09-01), p. E450-E457
    Abstract: Essential amino acids (EAA) are responsible for skeletal muscle anabolic effects after nutrient intake. The pattern of appearance of EAA in blood, e.g., after intake of “slow” or “fast” protein sources or in response to grazing vs. bolus feeding patterns, may impact anabolism. However, the influence of this on muscle anabolism is poorly understood, particularly in older individuals. We determined the effects of divergent feeding profiles of EAA on blood flow, anabolic signaling, and muscle protein synthesis (MPS) in older men. Sixteen men (∼70 yr) consumed EAA either as a single dose (bolus, 15 g; n = 8) or as small repeated fractions (pulse, 4 × 3.75 g every 45 min; n = 8) during 13 C 6 phenylalanine infusion. Repeated blood samples and muscle biopsies permitted measurement of fasting and postprandial plasma EAA, insulin, anabolic signaling, and MPS. Muscle blood flow was assessed by contrast-enhanced ultrasound (Sonovue). Bolus achieved rapid insulinemia (12.7 μiU/ml 25-min postfeed), essential aminoacidemia (∼3,000 μM, 45–65 min postfeed), and mTORC1 activity; pulse achieved attenuated insulin responses, gradual low-amplitude aminoacidemia (∼1,800 μM 80–195 min after feeding), and undetectable mTORC1 signaling. Despite this, equivalent anabolic responses were observed: fasting FSRs of 0.051 and 0.047%/h (bolus and pulse, respectively) increased to 0.084 and 0.073%/h, respectively. Moreover, pulse led to sustainment of MPS beyond 180 min, when bolus MPS had returned to basal rates. We detected no benefit of rapid aminoacidemia in this older population despite enhanced anabolic signaling and greater overall EAA exposure. Rather, apparent delayed onset of the “muscle-full” effect permitted identical MPS following low-amplitude-sustained EAA exposure.
    Type of Medium: Online Resource
    ISSN: 0193-1849 , 1522-1555
    URL: Article
    DOI: 10.1152/ajpendo.00112.2015
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2015
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  • 6
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    Targeted genotype analyses of GWAS-derived lean body mass and handgrip strength-associated single-nucleotide polymorphisms in elite master athletes
    American Physiological Society ; 2020
    In:  American Journal of Physiology-Regulatory, Integrative and Comparative Physiology Vol. 319, No. 2 ( 2020-08-01), p. R184-R194
    Details
    In: American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, American Physiological Society, Vol. 319, No. 2 ( 2020-08-01), p. R184-R194
    Abstract: Recent large genome-wide association studies (GWAS) have independently identified a set of genetic loci associated with lean body mass (LBM) and handgrip strength (HGS). Evaluation of these candidate single-nucleotide polymorphisms (SNPs) may be useful to investigate genetic traits of populations at higher or lower risk of muscle dysfunction. As such, we investigated associations between six SNPs linked to LBM or HGS in a population of elite master athletes (MA) and age-matched controls as a representative population of older individuals with variable maintenance of muscle mass and function. Genomic DNA was isolated from buffy coat samples of 96 individuals [consisting of 48 MA (71 ± 6 yr, age-graded performance 83 ± 9%) and 48 older controls (75 ± 6 yr)]. SNP validation and sample genotyping were conducted using the tetra-primer amplification refractory mutation system (ARMS). For the three SNPs analyzed that were previously associated with LBM ( FTO, IRS1, and ADAMTSL3), multinomial logistic regression revealed a significant association of the ADAMTSL3 genotype with %LBM ( P 〈 0.01). For the three HGS-linked SNPs, neither GBF1 nor GLIS1 showed any association with HGS, but for TGFA, multinomial logistic regression revealed a significant association of genotype with HGS ( P 〈 0.05). For ADAMTSL3, there was an enrichment of the effect allele in the MA ( P 〈 0.05, Fisher’s exact test). Collectively, of the six SNPs analyzed, ADAMTSL3 and TGFA showed significant associations with LBM and HGS, respectively. The functional relevance of the ADAMTSL3 SNP in body composition and of TGFA in strength may highlight a genetic component of the elite MA phenotype.
    Type of Medium: Online Resource
    ISSN: 0363-6119 , 1522-1490
    URL: Article
    DOI: 10.1152/ajpregu.00110.2020
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2020
    detail.hit.zdb_id: 1477297-8
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  • 7
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    The vitamin D receptor regulates mitochondrial function in C2C12 myoblasts
    American Physiological Society ; 2020
    In:  American Journal of Physiology-Cell Physiology Vol. 318, No. 3 ( 2020-03-01), p. C536-C541
    Details
    In: American Journal of Physiology-Cell Physiology, American Physiological Society, Vol. 318, No. 3 ( 2020-03-01), p. C536-C541
    Abstract: Vitamin D deficiency has been linked to a reduction in skeletal muscle function and oxidative capacity; however, the mechanistic bases of these impairments are poorly understood. The biological actions of vitamin D are carried out via the binding of 1α,25-dihydroxyvitamin D 3 (1α,25(OH) 2 D 3 ) to the vitamin D receptor (VDR). Recent evidence has linked 1α,25(OH) 2 D 3 to the regulation of skeletal muscle mitochondrial function in vitro; however, little is known with regard to the role of the VDR in this process. To examine the regulatory role of the VDR in skeletal muscle mitochondrial function, we used lentivirus-mediated shRNA silencing of the VDR in C2C12 myoblasts (VDR-KD) and examined mitochondrial respiration and protein content compared with an shRNA scrambled control. VDR protein content was reduced by ~95% in myoblasts and myotubes ( P 〈 0.001). VDR-KD myoblasts displayed a 30%, 30%, and 36% reduction in basal, coupled, and maximal respiration, respectively ( P 〈 0.05). This phenotype was maintained in VDR-KD myotubes, displaying a 34%, 33%, and 48% reduction in basal, coupled, and maximal respiration ( P 〈 0.05). Furthermore, ATP production derived from oxidative phosphorylation (ATP Ox ) was reduced by 20%, suggesting intrinsic impairments within the mitochondria following VDR-KD. However, despite the observed functional decrements, mitochondrial protein content, as well as markers of mitochondrial fission were unchanged. In summary, we highlight a direct role for the VDR in regulating skeletal muscle mitochondrial respiration in vitro, providing a potential mechanism as to how vitamin D deficiency might impact upon skeletal muscle oxidative capacity.
    Type of Medium: Online Resource
    ISSN: 0363-6143 , 1522-1563
    URL: Article
    DOI: 10.1152/ajpcell.00568.2019
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2020
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  • 8
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    “Nutraceuticals” in relation to human skeletal muscle and exercise
    American Physiological Society ; 2017
    In:  American Journal of Physiology-Endocrinology and Metabolism Vol. 312, No. 4 ( 2017-04-01), p. E282-E299
    Details
    In: American Journal of Physiology-Endocrinology and Metabolism, American Physiological Society, Vol. 312, No. 4 ( 2017-04-01), p. E282-E299
    Abstract: Skeletal muscles have a fundamental role in locomotion and whole body metabolism, with muscle mass and quality being linked to improved health and even lifespan. Optimizing nutrition in combination with exercise is considered an established, effective ergogenic practice for athletic performance. Importantly, exercise and nutritional approaches also remain arguably the most effective countermeasure for muscle dysfunction associated with aging and numerous clinical conditions, e.g., cancer cachexia, COPD, and organ failure, via engendering favorable adaptations such as increased muscle mass and oxidative capacity. Therefore, it is important to consider the effects of established and novel effectors of muscle mass, function, and metabolism in relation to nutrition and exercise. To address this gap, in this review, we detail existing evidence surrounding the efficacy of a nonexhaustive list of macronutrient, micronutrient, and “nutraceutical” compounds alone and in combination with exercise in relation to skeletal muscle mass, metabolism (protein and fuel), and exercise performance (i.e., strength and endurance capacity). It has long been established that macronutrients have specific roles and impact upon protein metabolism and exercise performance, (i.e., protein positively influences muscle mass and protein metabolism), whereas carbohydrate and fat intakes can influence fuel metabolism and exercise performance. Regarding novel nutraceuticals, we show that the following ones in particular may have effects in relation to 1) muscle mass/protein metabolism: leucine, hydroxyl β-methylbutyrate, creatine, vitamin-D, ursolic acid, and phosphatidic acid; and 2) exercise performance: (i.e., strength or endurance capacity): hydroxyl β-methylbutyrate, carnitine, creatine, nitrates, and β-alanine.
    Type of Medium: Online Resource
    ISSN: 0193-1849 , 1522-1555
    URL: Article
    DOI: 10.1152/ajpendo.00230.2016
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2017
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  • 9
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    Focal adhesion kinase is required for IGF-I-mediated growth of skeletal muscle cells via a TSC2/mTOR/S6K1-associated pathway
    American Physiological Society ; 2013
    In:  American Journal of Physiology-Endocrinology and Metabolism Vol. 305, No. 2 ( 2013-07-15), p. E183-E193
    Details
    In: American Journal of Physiology-Endocrinology and Metabolism, American Physiological Society, Vol. 305, No. 2 ( 2013-07-15), p. E183-E193
    Abstract: Focal adhesion kinase (FAK) is an attachment complex protein associated with the regulation of muscle mass through as-of-yet unclear mechanisms. We tested whether FAK is functionally important for muscle hypertrophy, with the hypothesis that FAK knockdown (FAK-KD) would impede cell growth associated with a trophic stimulus. C 2 C 12 skeletal muscle cells harboring FAK-targeted (FAK-KD) or scrambled (SCR) shRNA were created using lentiviral transfection techniques. Both FAK-KD and SCR myotubes were incubated for 24 h with IGF-I (10 ng/ml), and additional SCR cells (±IGF-1) were incubated with a FAK kinase inhibitor before assay of cell growth. Muscle protein synthesis (MPS) and putative FAK signaling mechanisms (immunoblotting and coimmunoprecipitation) were assessed. IGF-I-induced increases in myotube width (+41 ± 7% vs. non-IGF-I-treated) and total protein (+44 ± 6%) were, after 24 h, attenuated in FAK-KD cells, whereas MPS was suppressed in FAK-KD vs. SCR after 4 h. These blunted responses were associated with attenuated IGF-I-induced FAK Tyr 397 phosphorylation and markedly suppressed phosphorylation of tuberous sclerosis complex 2 (TSC2) and critical downstream mTOR signaling (ribosomal S6 kinase, eIF4F assembly) in FAK shRNA cells (all P 〈 0.05 vs. IGF-I-treated SCR cells). However, binding of FAK to TSC2 or its phosphatase Shp-2 was not affected by IGF-I or cell phenotype. Finally, FAK-KD-mediated suppression of cell growth was recapitulated by direct inhibition of FAK kinase activity in SCR cells. We conclude that FAK is required for IGF-I-induced muscle hypertrophy, signaling through a TSC2/mTOR/S6K1-dependent pathway via means requiring the kinase activity of FAK but not altered FAK-TSC2 or FAK-Shp-2 binding.
    Type of Medium: Online Resource
    ISSN: 0193-1849 , 1522-1555
    URL: Article
    DOI: 10.1152/ajpendo.00541.2012
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2013
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  • 10
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    A validation of the application of D 2 O stable isotope tracer techniques for monitoring day-to-day changes in muscle protein subfraction synthesis in humans
    American Physiological Society ; 2014
    In:  American Journal of Physiology-Endocrinology and Metabolism Vol. 306, No. 5 ( 2014-03-01), p. E571-E579
    Details
    In: American Journal of Physiology-Endocrinology and Metabolism, American Physiological Society, Vol. 306, No. 5 ( 2014-03-01), p. E571-E579
    Abstract: Quantification of muscle protein synthesis (MPS) remains a cornerstone for understanding the control of muscle mass. Traditional [ 13 C]amino acid tracer methodologies necessitate sustained bed rest and intravenous cannulation(s), restricting studies to ∼12 h, and thus cannot holistically inform on diurnal MPS. This limits insight into the regulation of habitual muscle metabolism in health, aging, and disease while querying the utility of tracer techniques to predict the long-term efficacy of anabolic/anticatabolic interventions. We tested the efficacy of the D 2 O tracer for quantifying MPS over a period not feasible with 13 C tracers and too short to quantify changes in mass. Eight men (22 ± 3.5 yr) undertook one-legged resistance exercise over an 8-day period (4 × 8–10 repetitions, 80% 1RM every 2nd day, to yield “nonexercised” vs. “exercise” leg comparisons), with vastus lateralis biopsies taken bilaterally at 0, 2, 4, and 8 days. After day 0 biopsies, participants consumed a D 2 O bolus (150 ml, 70 atom%); saliva was collected daily. Fractional synthetic rates (FSRs) of myofibrillar (MyoPS), sarcoplasmic (SPS), and collagen (CPS) protein fractions were measured by GC-pyrolysis-IRMS and TC/EA-IRMS. Body water initially enriched at 0.16–0.24 APE decayed at ∼0.009%/day. In the nonexercised leg, MyoPS was 1.45 ± 0.10, 1.47 ± 0.06, and 1.35 ± 0.07%/day at 0–2, 0–4, and 0–8 days, respectively (∼0.05–0.06%/h). MyoPS was greater in the exercised leg (0–2 days: 1.97 ± 0.13%/day; 0–4 days: 1.96 ± 0.15%/day, P 〈 0.01; 0–8 days: 1.79 ± 0.12%/day, P 〈 0.05). CPS was slower than MyoPS but followed a similar pattern, with the exercised leg tending to yield greater FSRs (0–2 days: 1.14 ± 0.13 vs. 1.45 ± 0.15%/day; 0–4 days: 1.13 ± 0.07%/day vs. 1.47 ± 0.18%/day; 0–8 days: 1.03 ± 0.09%/day vs. 1.40 ± 0.11%/day). SPS remained unchanged. Therefore, D 2 O has unrivaled utility to quantify day-to-day MPS in humans and inform on short-term changes in anabolism and presumably catabolism alike.
    Type of Medium: Online Resource
    ISSN: 0193-1849 , 1522-1555
    URL: Article
    DOI: 10.1152/ajpendo.00650.2013
    Language: English
    Publisher: American Physiological Society
    Publication Date: 2014
    detail.hit.zdb_id: 1477331-4
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