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Heitman K, Alexander MS, Faul C. Skeletal Muscle Injury in Chronic Kidney Disease-From Histologic Changes to Molecular Mechanisms and to Novel Therapies. Int J Mol Sci 2024; 25:5117. [PMID: 38791164 PMCID: PMC11121428 DOI: 10.3390/ijms25105117] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2024] [Revised: 05/03/2024] [Accepted: 05/06/2024] [Indexed: 05/26/2024] Open
Abstract
Chronic kidney disease (CKD) is associated with significant reductions in lean body mass and in the mass of various tissues, including skeletal muscle, which causes fatigue and contributes to high mortality rates. In CKD, the cellular protein turnover is imbalanced, with protein degradation outweighing protein synthesis, leading to a loss of protein and cell mass, which impairs tissue function. As CKD itself, skeletal muscle wasting, or sarcopenia, can have various origins and causes, and both CKD and sarcopenia share common risk factors, such as diabetes, obesity, and age. While these pathologies together with reduced physical performance and malnutrition contribute to muscle loss, they cannot explain all features of CKD-associated sarcopenia. Metabolic acidosis, systemic inflammation, insulin resistance and the accumulation of uremic toxins have been identified as additional factors that occur in CKD and that can contribute to sarcopenia. Here, we discuss the elevation of systemic phosphate levels, also called hyperphosphatemia, and the imbalance in the endocrine regulators of phosphate metabolism as another CKD-associated pathology that can directly and indirectly harm skeletal muscle tissue. To identify causes, affected cell types, and the mechanisms of sarcopenia and thereby novel targets for therapeutic interventions, it is important to first characterize the precise pathologic changes on molecular, cellular, and histologic levels, and to do so in CKD patients as well as in animal models of CKD, which we describe here in detail. We also discuss the currently known pathomechanisms and therapeutic approaches of CKD-associated sarcopenia, as well as the effects of hyperphosphatemia and the novel drug targets it could provide to protect skeletal muscle in CKD.
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Affiliation(s)
- Kylie Heitman
- Division of Nephrology and Section of Mineral Metabolism, Department of Medicine, Heersink School of Medicine, The University of Alabama at Birmingham, Birmingham, AL 35294, USA;
| | - Matthew S. Alexander
- Division of Neurology, Department of Pediatrics, The University of Alabama at Birmingham and Children’s of Alabama, Birmingham, AL 35294, USA
- Center for Exercise Medicine, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
- Department of Genetics, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
- Civitan International Research Center, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
- Center for Neurodegeneration and Experimental Therapeutics, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - Christian Faul
- Division of Nephrology and Section of Mineral Metabolism, Department of Medicine, Heersink School of Medicine, The University of Alabama at Birmingham, Birmingham, AL 35294, USA;
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Chande S, Caballero D, Ho BB, Fetene J, Serna J, Pesta D, Nasiri A, Jurczak M, Chavkin NW, Hernando N, Giachelli CM, Wagner CA, Zeiss C, Shulman GI, Bergwitz C. Slc20a1/Pit1 and Slc20a2/Pit2 are essential for normal skeletal myofiber function and survival. Sci Rep 2020; 10:3069. [PMID: 32080237 PMCID: PMC7033257 DOI: 10.1038/s41598-020-59430-4] [Citation(s) in RCA: 13] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2019] [Accepted: 01/29/2020] [Indexed: 01/25/2023] Open
Abstract
Low blood phosphate (Pi) reduces muscle function in hypophosphatemic disorders. Which Pi transporters are required and whether hormonal changes due to hypophosphatemia contribute to muscle function is unknown. To address these questions we generated a series of conditional knockout mice lacking one or both house-keeping Pi transporters Pit1 and Pit2 in skeletal muscle (sm), using the postnatally expressed human skeletal actin-cre. Simultaneous conditional deletion of both transporters caused skeletal muscle atrophy, resulting in death by postnatal day P13. smPit1-/-, smPit2-/- and three allele mutants are fertile and have normal body weights, suggesting a high degree of redundance for the two transporters in skeletal muscle. However, these mice show a gene-dose dependent reduction in running activity also seen in another hypophosphatemic model (Hyp mice). In contrast to Hyp mice, grip strength is preserved. Further evaluation of the mechanism shows reduced ERK1/2 activation and stimulation of AMP kinase in skeletal muscle from smPit1-/-; smPit2-/- mice consistent with energy-stress. Similarly, C2C12 myoblasts show a reduced oxygen consumption rate mediated by Pi transport-dependent and ERK1/2-dependent metabolic Pi sensing pathways. In conclusion, we here show that Pit1 and Pit2 are essential for normal myofiber function and survival, insights which may improve management of hypophosphatemic myopathy.
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Affiliation(s)
- Sampada Chande
- Department of Internal Medicine, Section Endocrinology, Yale University School of Medicine, New Haven, CT, USA
| | - Daniel Caballero
- Department of Internal Medicine, Section Endocrinology, Yale University School of Medicine, New Haven, CT, USA
| | - Bryan B Ho
- Department of Internal Medicine, Section Endocrinology, Yale University School of Medicine, New Haven, CT, USA
| | - Jonathan Fetene
- Department of Internal Medicine, Section Endocrinology, Yale University School of Medicine, New Haven, CT, USA
| | - Juan Serna
- Department of Internal Medicine, Section Endocrinology, Yale University School of Medicine, New Haven, CT, USA
| | - Dominik Pesta
- Department of Cellular&Molecular Physiology, Yale University School of Medicine, New Haven, CT, USA
- German Diabetes Center, Düsseldorf, Germany, University of Washington, Box 355061, Foege Hall Seattle, WA, 98195, USA
| | - Ali Nasiri
- Department of Cellular&Molecular Physiology, Yale University School of Medicine, New Haven, CT, USA
| | - Michael Jurczak
- Department of Internal Medicine, Section Endocrinology, Yale University School of Medicine, New Haven, CT, USA
- Department of Medicine, Division of Endocrinology, University of Pittsburgh, University of Washington, Box 355061, Foege Hall Seattle, WA, 98195, USA
| | - Nicholas W Chavkin
- Department of Bioengineering, University of Washington, Box 355061, Foege Hall Seattle, WA, 98195, USA
| | - Nati Hernando
- Institute of Physiology, University of Zürich, Switzerland and National Center of Competence in Research NCCR Kidney.CH, Zürich, Switzerland
| | - Cecilia M Giachelli
- Department of Bioengineering, University of Washington, Box 355061, Foege Hall Seattle, WA, 98195, USA
| | - Carsten A Wagner
- Institute of Physiology, University of Zürich, Switzerland and National Center of Competence in Research NCCR Kidney.CH, Zürich, Switzerland
| | - Caroline Zeiss
- Comparative Medicine, Yale University School of Medicine, New Haven, CT, USA
| | - Gerald I Shulman
- Department of Internal Medicine, Section Endocrinology, Yale University School of Medicine, New Haven, CT, USA
- Department of Cellular&Molecular Physiology, Yale University School of Medicine, New Haven, CT, USA
| | - Clemens Bergwitz
- Department of Internal Medicine, Section Endocrinology, Yale University School of Medicine, New Haven, CT, USA.
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K⁺ and Rb⁺ Affinities of the Na,K-ATPase α₁ and α₂ Isozymes: An Application of ICP-MS for Quantification of Na⁺ Pump Kinetics in Myofibers. Int J Mol Sci 2018; 19:ijms19092725. [PMID: 30213059 PMCID: PMC6165224 DOI: 10.3390/ijms19092725] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2018] [Revised: 09/10/2018] [Accepted: 09/10/2018] [Indexed: 11/21/2022] Open
Abstract
The potassium affinities of Na,K-ATPase isozymes are important determinants of their physiological roles in skeletal muscle. This study measured the apparent K+ and Rb+ affinities of the Na,K-ATPase α1 and α2 isozymes in intact, dissociated myofibers obtained from WT and genetically altered mice (α1S/Sα2R/R and skα2−/−). It also validates a new method to quantify cations in intact, dissociated myofibers, using inductively coupled plasma mass spectrometry (ICP-MS). Our findings were that: (1) The extracellular substrate sites of Na,K-ATPase bind Rb+ and K+ with comparable apparent affinities; however; turnover rate is reduced when Rb+ is the transported ion; (2) The rate of Rb+ uptake by the Na,K-ATPase is not constant but declines with a half-time of approximately 1.5 min; (3) The apparent K+ affinity of the α2 isozymes for K+ is significantly lower than α1. When measured in intact fibers of WT and α1S/Sα2R/R mice in the presence of 10 µM ouabain; the K1/2,K of α1 and α2 isozymes are 1.3 and 4 mM, respectively. Collectively, these results validate the single fiber model for studies of Na,K-ATPase transport and kinetic constants, and they imply the existence of mechanisms that dynamically limit pump activity during periods of active transport.
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Reyngoudt H, Turk S, Carlier PG. 1 H NMRS of carnosine combined with 31 P NMRS to better characterize skeletal muscle pH dysregulation in Duchenne muscular dystrophy. NMR IN BIOMEDICINE 2018; 31:e3839. [PMID: 29130550 DOI: 10.1002/nbm.3839] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/09/2017] [Revised: 09/06/2017] [Accepted: 09/07/2017] [Indexed: 05/18/2023]
Abstract
In recent years, quantitative nuclear magnetic resonance imaging and spectroscopy (NMRI and NMRS) have been used more systematically as outcome measures in natural history and clinical trial studies for Duchenne muscular dystrophy (DMD). Whereas most of these studies have emphasized the evaluation of the fat fraction as an assessment for disease severity, less focus has been placed on metabolic indices measured by NMRS. 31 P NMRS in DMD reveals an alkaline inorganic phosphate (Pi ) pool, originating from either leaky dystrophic myocytes or an increased interstitial space. 1 H NMRS, exploiting the pH-sensitive proton resonances of carnosine, an intracellular dipeptide, was used to distinguish between these two hypotheses. NMR data were obtained in 23 patients with DMD and 14 healthy subjects on a 3-T clinical NMR system. Both 31 P and 1 H NMRS data were acquired at the level of the gastrocnemius medialis muscle. A multi-slice multi-echo imaging acquisition was performed for the determination of water T2 and fat fraction in the same region of interest. Whereas nearly all patients with DMD showed an elevated pH compared with healthy controls when using 31 P NMRS, 1 H NMRS-determined pH was not systematically increased. As expected, the carnosine-based intracellular pH was never found to be alkaline in the absence of a concurrent Pi -based pH elevation. In addition, abnormal intracellular pH, based on carnosine, was never associated with normal water T2 values. We conclude that, in one group of patients, both 1 H and 31 P NMRS showed an alkaline pH, originating from the intracellular compartment and reflecting ionic dysregulation in dystrophic myocytes. In the other patients with DMD, intracellular pH was normal, but an alkaline Pi pool was still present, suggesting an extracellular origin, probably revealing an expanded interstitial volume fraction, often associated with fibrotic changes. The data demonstrate that 1 H NMRS could serve as a biomarker to assess the normalization of intramyocytic pH and sarcolemmal permeability following therapy inducing dystrophin expression in patients with DMD.
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Affiliation(s)
- Harmen Reyngoudt
- NMR Laboratory, Institute of Myology, Paris, France
- CEA, DRF, IBFJ, MIRCen, Paris, France
| | - Suna Turk
- NMR Laboratory, Institute of Myology, Paris, France
- CEA, DRF, IBFJ, MIRCen, Paris, France
| | - Pierre G Carlier
- NMR Laboratory, Institute of Myology, Paris, France
- CEA, DRF, IBFJ, MIRCen, Paris, France
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Pirkmajer S, Chibalin AV. Na,K-ATPase regulation in skeletal muscle. Am J Physiol Endocrinol Metab 2016; 311:E1-E31. [PMID: 27166285 DOI: 10.1152/ajpendo.00539.2015] [Citation(s) in RCA: 74] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/31/2015] [Accepted: 05/02/2016] [Indexed: 12/17/2022]
Abstract
Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.
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Affiliation(s)
- Sergej Pirkmajer
- Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia; and
| | - Alexander V Chibalin
- Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden
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Shobeiri N, Adams MA, Holden RM. Phosphate: an old bone molecule but new cardiovascular risk factor. Br J Clin Pharmacol 2014; 77:39-54. [PMID: 23506202 PMCID: PMC3895346 DOI: 10.1111/bcp.12117] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2012] [Accepted: 02/21/2013] [Indexed: 12/24/2022] Open
Abstract
Phosphate handling in the body is complex and involves hormones produced by the bone, the parathyroid gland and the kidneys. Phosphate is mostly found in hydroxyapatite. however recent evidence suggests that phosphate is also a signalling molecule associated with bone formation. Phosphate balance requires careful regulation of gut and kidney phosphate transporters, SLC34 transporter family, but phosphate signalling in osteoblasts and vascular smooth muscle cells is likely mediated by the SLC20 transporter family (PiT1 and PiT2). If not properly regulated, phosphate imblanace could lead to mineral disorders as well as vascular calcification. In chronic kidney disease-mineral bone disorder, hyperphosphataemia has been consistently associated with extra-osseous calcification and cardiovascular disease. This review focuses on the physiological mechanisms involved in phosphate balance and cell signalling (i.e. osteoblasts and vascular smooth muscle cells) as well as pathological consequences of hyperphosphataemia. Finally, conventional as well as new and experimental therapeutics in the treatment of hyperphosphataemia are explored.
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Affiliation(s)
- Navid Shobeiri
- Department of Biomedical and Molecular Sciences, Queen's University, Kingston, ON, Canada
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Hu MC, Shi M, Zhang J, Quiñones H, Griffith C, Kuro-o M, Moe OW. Klotho deficiency causes vascular calcification in chronic kidney disease. J Am Soc Nephrol 2011; 22:124-36. [PMID: 21115613 PMCID: PMC3014041 DOI: 10.1681/asn.2009121311] [Citation(s) in RCA: 694] [Impact Index Per Article: 53.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2009] [Accepted: 09/03/2010] [Indexed: 12/12/2022] Open
Abstract
Soft-tissue calcification is a prominent feature in both chronic kidney disease (CKD) and experimental Klotho deficiency, but whether Klotho deficiency is responsible for the calcification in CKD is unknown. Here, wild-type mice with CKD had very low renal, plasma, and urinary levels of Klotho. In humans, we observed a graded reduction in urinary Klotho starting at an early stage of CKD and progressing with loss of renal function. Despite induction of CKD, transgenic mice that overexpressed Klotho had preserved levels of Klotho, enhanced phosphaturia, better renal function, and much less calcification compared with wild-type mice with CKD. Conversely, Klotho-haploinsufficient mice with CKD had undetectable levels of Klotho, worse renal function, and severe calcification. The beneficial effect of Klotho on vascular calcification was a result of more than its effect on renal function and phosphatemia, suggesting a direct effect of Klotho on the vasculature. In vitro, Klotho suppressed Na(+)-dependent uptake of phosphate and mineralization induced by high phosphate and preserved differentiation in vascular smooth muscle cells. In summary, Klotho is an early biomarker for CKD, and Klotho deficiency contributes to soft-tissue calcification in CKD. Klotho ameliorates vascular calcification by enhancing phosphaturia, preserving glomerular filtration, and directly inhibiting phosphate uptake by vascular smooth muscle. Replacement of Klotho may have therapeutic potential for CKD.
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Affiliation(s)
- Ming Chang Hu
- Charles and Jane Pak Center for Mineral Metabolism and Clinical Research, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390-8885, USA.
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Jin C, Zoidis E, Ghirlanda C, Schmid C. Dexamethasone and cyclic AMP regulate sodium phosphate cotransporter (NaPi-IIb and Pit-1) mRNA and phosphate uptake in rat alveolar type II epithelial cells. Lung 2009; 188:51-61. [PMID: 19806400 DOI: 10.1007/s00408-009-9183-1] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2009] [Accepted: 09/14/2009] [Indexed: 11/25/2022]
Abstract
Alveolar epithelial type II (AT II) cells need phosphate (Pi) for surfactant synthesis. The Na-dependent (Na(d)) Pi transporters NaPi-IIb and Pit-1 are expressed in lung, but their expression, regulation, and function in AT II cells remain unclear. We studied NaPi-IIb and Pit-1 mRNA expression in cultured AT II cells isolated from adult rat lung, their regulation by agents known to enhance surfactant production, dexamethasone (dex) and dibutyryl cyclic AMP (cAMP), and the effects of dex and cAMP on Na(d) Pi uptake by this cell type. By Northern analysis, cultured AT II cells expressed both NaPi-IIb (4.8 and 4.0 kb) and Pit-1 (4.3 kb) mRNA. Treatment with 100 nmol/l dex for 24 h decreased the expression of both mRNAs (to 0.48 +/- 0.06 and 0.77 +/- 0.05, respectively, as compared to control), while 0.1 mmol/l cAMP stimulated NaPi-IIb (1.94 +/- 0.22) but not Pit-1 mRNA (0.90 +/- 0.05, compared to vehicle-treated cells). NaPi-IIb and Pit-1 proteins could not be identified by western analysis of plasma membrane preparations of cultured AT II cells. AT II cells take up Pi in a Na(d) manner. Uptake was slightly (to 0.78-fold of the control) decreased by 100 nmol/l dex but not affected by 0.1 mmol/l cAMP treatment. Although NaPi-IIb mRNA expression was maintained to some extent by AT II cells kept in primary culture, Pi uptake was more closely related to Pit-1 mRNA expression.
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Affiliation(s)
- Chengluo Jin
- Division of Endocrinology and Diabetology, Department of Internal Medicine, University Hospital, 8091, Zurich, Switzerland
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Francia P, Cosentino F, Schiavoni M, Huang Y, Perna E, Camici GG, Lüscher TF, Volpe M. p66Shc protein, oxidative stress, and cardiovascular complications of diabetes: the missing link. J Mol Med (Berl) 2009; 87:885-91. [DOI: 10.1007/s00109-009-0499-3] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2009] [Revised: 06/16/2009] [Accepted: 06/25/2009] [Indexed: 10/20/2022]
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Scheibye-Knudsen M, Quistorff B. Regulation of mitochondrial respiration by inorganic phosphate; comparing permeabilized muscle fibers and isolated mitochondria prepared from type-1 and type-2 rat skeletal muscle. Eur J Appl Physiol 2008; 105:279-87. [PMID: 18989695 DOI: 10.1007/s00421-008-0901-9] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 10/13/2008] [Indexed: 11/24/2022]
Abstract
ADP is generally accepted as a key regulator of oxygen consumption both in isolated mitochondria and in permeabilized fibers from skeletal muscle. The present study explored inorganic phosphate in a similar regulatory role. Saponin permeabilized fibers and isolated mitochondria from type-I and type-II muscle from male Wistar rats were prepared. Respiration was measured while the medium P(i) concentration was gradually increased. The apparent K(m) values for P(i) were 607 +/- 17 microM and 405 +/- 15 microM (P < 0.0001) for type-I and type-II fibers, respectively. For isolated mitochondria the values were significantly lower than type-1 permeabilized fibers, 338 +/- 130 microM and 235 +/- 30 microM (P < 0.05), but not different with respect to fiber type. The reason for this difference in K(m) values in the permeabilized muscle is unknown, but a similar pattern has been observed for K(m) of ADP. Our data indicate that phosphate may play a role in regulation of oxygen consumption in vitro and in vivo.
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Affiliation(s)
- Morten Scheibye-Knudsen
- Department of Biomedical Sciences, NMR Center, Faculty of Health Sciences,The Panum Institute, The University of Copenhagen, Blegdamsvej 3, 2200 Copenhagen, Denmark.
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Current World Literature. Curr Opin Nephrol Hypertens 2005. [DOI: 10.1097/01.mnh.0000172731.05865.69] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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Abraham KA, Terjung RL. Phosphate uptake in rat skeletal muscle is reduced during isometric contractions. J Appl Physiol (1985) 2004; 97:57-62. [PMID: 14990549 DOI: 10.1152/japplphysiol.01294.2003] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
During contractions, there is a net efflux of phosphate from skeletal muscle, likely because of an elevated intracellular inorganic phosphate (P(i)) concentration. Over time, contracting muscle could incur a substantial phosphate deficit unless P(i) uptake rates were increased during contractions. We used the perfused rat hindquarter preparation to assess [(32)P]P(i) uptake rates in muscles at rest or over a range of energy expenditures during contractions at 0.5, 3, or 5 Hz for 30 min. P(i) uptake rates were reduced during contractions in a pattern that was dependent on contraction frequency and fiber type. In soleus and red gastrocnemius, [(32)P]P(i) uptake rates declined by approximately 25% at 0.5 Hz and 50-60% at 3 and 5 Hz. Uptake rates in white gastrocnemius decreased by 65-75% at all three stimulation frequencies. These reductions in P(i) uptake are not likely confounded by changes in precursor [(32)P]P(i) specific activity in the interstitium. In soleus and red gastrocnemius, declines in P(i) uptake rates were related to energy expenditure over the contraction duration. These data imply that P(i) uptake in skeletal muscle is acutely modulated during contractions and that decreases in P(i) uptake rates, in combination with expected increases in P(i) efflux, exacerbate the net loss of phosphate from the cell. Enhanced uptake of P(i) must subsequently occur because skeletal muscle typically maintains a relatively constant total phosphate pool.
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Affiliation(s)
- Kirk A Abraham
- Department of Physiology, College of Medicine, University of Missouri, Columbia, MO 65211, USA
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