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Skov M, Ruijs TQ, Grønnebæk TS, Skals M, Riisager A, Winther JB, Dybdahl KLT, Findsen A, Morgen JJ, Huus N, Broch-Lips M, Nielsen OB, de Cuba CMKE, Heuberger JAAC, de Kam ML, Tannemaat M, Verschuuren JJGM, Knutsen LJS, Kelly NM, Jensen KG, Arnold WD, Burghes AH, Olesen C, Bold J, Petersen TK, Quiroz JA, Hutchison J, Chin ER, Groeneveld GJ, Pedersen TH. The ClC-1 chloride channel inhibitor NMD670 improves skeletal muscle function in rat models and patients with myasthenia gravis. Sci Transl Med 2024; 16:eadk9109. [PMID: 38507469 DOI: 10.1126/scitranslmed.adk9109] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2023] [Accepted: 02/23/2024] [Indexed: 03/22/2024]
Abstract
Myasthenia gravis (MG) is a neuromuscular disease that results in compromised transmission of electrical signals at the neuromuscular junction (NMJ) from motor neurons to skeletal muscle fibers. As a result, patients with MG have reduced skeletal muscle function and present with symptoms of severe muscle weakness and fatigue. ClC-1 is a skeletal muscle specific chloride (Cl-) ion channel that plays important roles in regulating neuromuscular transmission and muscle fiber excitability during intense exercise. Here, we show that partial inhibition of ClC-1 with an orally bioavailable small molecule (NMD670) can restore muscle function in rat models of MG and in patients with MG. In severely affected MG rats, ClC-1 inhibition enhanced neuromuscular transmission, restored muscle function, and improved mobility after both single and prolonged administrations of NMD670. On this basis, NMD670 was progressed through nonclinical safety pharmacology and toxicology studies, leading to approval for testing in clinical studies. After successfully completing phase 1 single ascending dose in healthy volunteers, NMD670 was tested in patients with MG in a randomized, placebo-controlled, single-dose, three-way crossover clinical trial. The clinical trial evaluated safety, pharmacokinetics, and pharmacodynamics of NMD670 in 12 patients with mild MG. NMD670 had a favorable safety profile and led to clinically relevant improvements in the quantitative myasthenia gravis (QMG) total score. This translational study spanning from single muscle fiber recordings to patients provides proof of mechanism for ClC-1 inhibition as a potential therapeutic approach in MG and supports further development of NMD670.
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Affiliation(s)
- Martin Skov
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Titia Q Ruijs
- Centre for Human Drug Research, 2333 CL Leiden, Netherlands
- Leiden University Medical Centre, 2333 ZA Leiden, Netherlands
| | | | - Marianne Skals
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Anders Riisager
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | | | | | - Anders Findsen
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Jeanette J Morgen
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Nete Huus
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Martin Broch-Lips
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Ole B Nielsen
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
- Department of Biomedicine, Aarhus University, Ole Worms Alle 4, 8000 Aarhus C, Denmark
| | - Catherine M K E de Cuba
- Centre for Human Drug Research, 2333 CL Leiden, Netherlands
- Leiden University Medical Centre, 2333 ZA Leiden, Netherlands
| | | | | | | | | | - Lars J S Knutsen
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Nicholas M Kelly
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Klaus G Jensen
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - William D Arnold
- NextGen Precision Health, University of Missouri, 1030 Hitt St, Columbia, MO 65212, USA
| | - Arthur H Burghes
- Department of Biological Chemistry and Pharmacology, Ohio State University Wexner Medical Center, 1060 Carmack Road, Columbus, OH 43210, USA
- Department of Neurology, Neuromuscular Division, Ohio State University Wexner Medical Center, 395 W. 12(th) Ave, Columbus, OH 43210, USA
| | - Claus Olesen
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
- Department of Biomedicine, Aarhus University, Ole Worms Alle 4, 8000 Aarhus C, Denmark
| | - Jane Bold
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Thomas K Petersen
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Jorge A Quiroz
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - John Hutchison
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Eva R Chin
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
| | - Geert J Groeneveld
- Centre for Human Drug Research, 2333 CL Leiden, Netherlands
- Leiden University Medical Centre, 2333 ZA Leiden, Netherlands
| | - Thomas H Pedersen
- NMD Pharma A/S, Palle Juul-Jensens Boulevard 82, 8200 Aarhus N, Denmark
- Department of Biomedicine, Aarhus University, Ole Worms Alle 4, 8000 Aarhus C, Denmark
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Andersen OE, Kristensen AM, Nielsen OB, Overgaard K. Force potentiation during eccentric contractions in rat skeletal muscle. J Appl Physiol (1985) 2023; 134:777-785. [PMID: 36759160 DOI: 10.1152/japplphysiol.00676.2022] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2022] [Revised: 02/01/2023] [Accepted: 02/06/2023] [Indexed: 02/11/2023] Open
Abstract
Postactivation potentiation refers to an acute enhancement of contractile properties following muscle activity. Previously, the effects of prior muscle activation on eccentric force at tetanic activation frequencies have only been sparsely reported. This paper aimed to study acute activity-induced effects on eccentric force of slow and fast-twitch muscles and characterize them in relation to postactivation potentiation. We elicited eccentric contractions in isolated rat extensor digitorum longus and soleus muscles by actively lengthening muscles at a constant velocity. We assessed contractile properties by measuring force over shortly interspaced, identical eccentric, and isometric contractions. We then analyzed stretch force, isometric peak force, rate of force development, and relaxation times. Finally, we compared the time courses for the development and cessation of changes in stretch force to known features of postactivation potentiation. In extensor digitorum longus, muscles stretch force consistently increased in a contraction-to-contraction manner by up to 49% [95% confidence interval (CI): 35-64%] whereas isometric peak force simultaneously showed minor declines (8%, 95% CI: 5-10%). The development and cessation of eccentric force potentiation coincided with the development of twitch potentiation and increases in rate of force development. In soleus muscles we found no consistent eccentric potentiation. Characterization of the increase in eccentric force revealed that force only increased in the very beginning of an active stretch. Eccentric force at tetanic activation frequencies potentiates substantially in extensor digitorum longus muscles over consecutive contractions with a time course coinciding with postactivation potentiation. Such eccentric potentiation may be important in sport performance.NEW & NOTEWORTHY Force during eccentric contractions can increase to a magnitude that may have profound consequences for our understanding of skeletal muscle locomotion. This increase in eccentric force occurs over consecutive, shortly interspaced, tetanic contractions in rat extensor digitorum longus muscles-not in rat soleus muscles-and coincides with well-known traits of postactivation potentiation. Eccentric force potentiation may significantly enhance muscle performance in activities involving stretch-shortening cycles.
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Affiliation(s)
- Ole Emil Andersen
- Department of Public Health, Aarhus University, Aarhus, Denmark
- Steno Diabetes Center Aarhus, Aarhus University, Aarhus, Denmark
| | | | - Ole B Nielsen
- Department of Public Health, Aarhus University, Aarhus, Denmark
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Leermakers PA, Skov M, Riisager A, Nielsen OB, Pedersen TH. Alterations in fast-twitch muscle membrane conductance regulation do not explain decreased muscle function of SOD1 G93A rats. Muscle Nerve 2021; 64:755-764. [PMID: 34486134 DOI: 10.1002/mus.27413] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2021] [Revised: 08/29/2021] [Accepted: 08/31/2021] [Indexed: 11/06/2022]
Abstract
INTRODUCTION/AIMS Both neuromuscular junction (NMJ) dysfunction and altered electrophysiological properties of muscle fibers have been reported in amyotrophic lateral sclerosis (ALS) patients. ALS-related preclinical studies typically use rodent SOD1G93A overexpression models, but translation to the human disease has been challenged. The present work explored NMJ function and cellular electrophysiological properties of muscles fibers in SOD1G93A overexpression rats. METHODS Longitudinal studies of compound muscle action potentials (CMAPs) were performed in SOD1G93A rats. Cellular studies were performed to evaluate electrophysiological properties of muscle fibers, including the resting membrane conductance (Gm ) and its regulation during prolonged action potential (AP) firing. RESULTS SOD1G93A rats showed a substantial loss of gastrocnemius CMAP amplitude (35.8 mV, P < .001) and a minor increase in CMAP decrement (8.5%, P = .002) at 25 weeks. In addition, SOD1G93A EDL muscle fibers showed a lower baseline Gm (wild-type, 1325 μS/cm2 ; SOD1G93A , 1137 μS/cm2 ; P < .001) and minor alterations in Gm regulation during repeated firing of APs as compared with wild-type rats. DISCUSSION The current data suggest that loss of CMAP amplitude is largely explained by defects in either lower motor neuron or skeletal muscle with only minor indications of a role for neuromuscular transmission defects in SOD1G93A rats. Electrophysiological properties of muscle fibers were not markedly affected, and an elevated Gm , as has been reported in motor neuron disease (MND) patients, was not replicated in SOD1G93A muscles. Collectively, the neuromuscular pathology of SOD1G93A rats appears to differ from that of ALS/MND patients with respect to neuromuscular transmission defects and electrophysiological properties of muscle fibers.
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Affiliation(s)
| | - Martin Skov
- Department of Biomedicine, Aarhus University, Aarhus, Denmark
| | - Anders Riisager
- Department of Biomedicine, Aarhus University, Aarhus, Denmark
| | - Ole B Nielsen
- Department of Biomedicine, Aarhus University, Aarhus, Denmark
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Kristensen AM, Nielsen OB, Pedersen TH, Overgaard K. Fatiguing stimulation increases curvature of the force-velocity relationship in isolated fast-twitch and slow-twitch rat muscles. ACTA ACUST UNITED AC 2019; 222:jeb.204545. [PMID: 31292165 DOI: 10.1242/jeb.204545] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/03/2019] [Accepted: 07/04/2019] [Indexed: 01/14/2023]
Abstract
In skeletal muscles, the ability to generate power is reduced during fatigue. In isolated muscles, maximal power can be calculated from the force-velocity relationship. This relationship is well described by the Hill equation, which contains three parameters: (1) maximal isometric force, (2) maximum contraction velocity and (3) curvature. Here, we investigated the hypothesis that a fatigue-induced loss of power is associated with changes in curvature of the force-velocity curve in slow-twitch muscles but not in fast-twitch muscles during the development of fatigue. Isolated rat soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscles were incubated in Krebs-Ringer solution at 30°C and stimulated electrically at 60 Hz (soleus) and 150 Hz (EDL) to perform a series of concentric contractions to fatigue. Force-velocity data were fitted to the Hill equation, and curvature was determined as the ratio of the curve parameters a/F 0 (inversely related to curvature). At the end of the fatiguing protocol, maximal power decreased by 58±5% in the soleus and 69±4% in the EDL compared with initial values in non-fatigued muscles. At the end of the fatiguing sequence, curvature increased as judged from the decrease in a/F 0 by 81±20% in the soleus and by 31±12% in the EDL. However, during the initial phases of fatiguing stimulation, we observed a small decrease in curvature in the EDL, but not in the soleus, which may be a result of post-activation potentiation. In conclusion, fatigue-induced loss of power is strongly associated with an increased curvature of the force-velocity relationship, particularly in slow-twitch muscles.
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Affiliation(s)
| | - Ole B Nielsen
- Department of Public Health, Aarhus University, DK-8000 Aarhus C, Denmark
| | - Thomas H Pedersen
- Department of Biomedicine, Aarhus University, DK-8000 Aarhus C, Denmark
| | - Kristian Overgaard
- Department of Public Health, Aarhus University, DK-8000 Aarhus C, Denmark
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Overgaard K, Pedersen KK, Nielsen OB. Doublets Initiating Low Frequency Stimulation Improve Dynamic Contractility Without Exacerbating Fatigue. Med Sci Sports Exerc 2019. [DOI: 10.1249/01.mss.0000562341.95016.12] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Pedersen KK, Nielsen OB, Overgaard K. Effects Of Doublet Stimulation On Dynamic Muscle Contractility In Muscles With K+-suppressed Excitability. Med Sci Sports Exerc 2019. [DOI: 10.1249/01.mss.0000561470.51524.47] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Skov M, de Paoli FV, Nielsen OB, Pedersen TH. The anti-convulsants lacosamide, lamotrigine, and rufinamide reduce myotonia in isolated human and rat skeletal muscle. Muscle Nerve 2017; 56:136-142. [PMID: 27783415 DOI: 10.1002/mus.25452] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/26/2015] [Revised: 10/14/2016] [Accepted: 10/19/2016] [Indexed: 11/11/2022]
Abstract
INTRODUCTION In myotonia congenita, loss of ClC-1 Cl- channel function results in skeletal muscle hyperexcitability and myotonia. Anti-myotonic treatment has typically targeted the voltage-gated sodium channel in skeletal muscle (Nav1.4). In this study we explored whether 3 sodium channel-modulating anti-epileptics can reduce myotonia in isolated rat and human muscle. METHODS Dissected muscles were rendered myotonic by ClC-1 channel inhibition. The ability of the drugs to suppress myotonia was then assessed from subclinical to maximal clinical concentrations. Drug synergy was determined using isobole plots. RESULTS All drugs were capable of abolishing myotonia in both rat and human muscles. Lamotrigine and rufinamide completely suppressed myotonia at submaximal clinical concentrations, whereas lacosamide had to be raised above the maximal clinical concentration to suppress myotonia completely. A synergistic effect of lamotrigine and rufinamide was observed. CONCLUSION These findings suggest that lamotrigine and rufinamide could be considered for anti-myotonic treatment in myotonia congenita. Muscle Nerve 56: 136-142, 2017.
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Affiliation(s)
- Martin Skov
- Department of Biomedicine, Aarhus University, Ole Worms Allé 4, 8000, Aarhus C, Denmark
| | - Frank V de Paoli
- Department of Biomedicine, Aarhus University, Ole Worms Allé 4, 8000, Aarhus C, Denmark.,Department of Cardiothoracic and Vascular Surgery, Aarhus University Hospital Skejby, Aarhus, Denmark
| | - Ole B Nielsen
- Department of Biomedicine, Aarhus University, Ole Worms Allé 4, 8000, Aarhus C, Denmark
| | - Thomas H Pedersen
- Department of Biomedicine, Aarhus University, Ole Worms Allé 4, 8000, Aarhus C, Denmark
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Pedersen KK, Nielsen OB, Overgaard K. Effects of high-frequency stimulation and doublets on dynamic contractions in rat soleus muscle exposed to normal and high extracellular [K(+)]. Physiol Rep 2013; 1:e00026. [PMID: 24303113 PMCID: PMC3831922 DOI: 10.1002/phy2.26] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2013] [Revised: 05/28/2013] [Accepted: 06/13/2013] [Indexed: 12/04/2022] Open
Abstract
The development of maximal velocity and power in muscle depends on the ability to transmit action potentials (AP) at very high frequencies up to about 400 Hz. However, for every AP there is a small loss of K+ to the interstitium, which during intense exercise, may build up to a point where excitability is reduced, thus limiting the intensity of further exercise. It is still unknown how the muscle responds to high-frequency stimulation when exposed to high [K+]. Contractile parameters of the muscles (force [F], velocity [V], power [P], rate of force development [RFD], and work) were examined during dynamic contractions, performed in vitro using rat soleus muscles incubated in buffers containing 4 or 8 mmol/L K+ and stimulated with constant trains of tetanic or supratetanic frequency or with trains initiated by a high-frequency doublet, followed by tetanic or subtetanic trains. At 4 mmol/L K+, an increase in frequency increased Pmax when using constant train stimulation. When stimulating with trains containing high-frequency doublets an increase in 120-msec work was seen, however, no increase in Pmax was observed. At 8 mmol/L K+, no differences were seen for either Pmax or 120-msec work when increasing frequency or introducing doublets. In all experiments where the frequency was increased or doublets applied, an increase in RFD was seen in both normal and high [K+]. The results indicate that stimulation with supratetanic frequencies can improve dynamic muscle contractility, but improvements are attenuated when muscles are exposed to high extracellular [K+].
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Affiliation(s)
- Katja K Pedersen
- Department of Public Health, Section of Sport Science, Aarhus University Aarhus, Denmark
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Skov M, Riisager A, Fraser JA, Nielsen OB, Pedersen TH. Extracellular magnesium and calcium reduce myotonia in ClC-1 inhibited rat muscle. Neuromuscul Disord 2013; 23:489-502. [PMID: 23623567 DOI: 10.1016/j.nmd.2013.03.009] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2012] [Revised: 03/08/2013] [Accepted: 03/13/2013] [Indexed: 11/16/2022]
Abstract
Loss-of-function mutations in the ClC-1 Cl(-) channel trigger skeletal muscle hyperexcitability in myotonia congenita. For reasons that remain unclear, the severity of the myotonic symptoms can vary markedly even among patients with identical ClC-1 mutations, and may become exacerbated during pregnancy and with diuretic treatment. Since both these conditions are associated with hypomagnesemia and hypocalcemia, we explored whether extracellular Mg(2+) and Ca(2+) ([Mg(2+)]o and [Ca(2+)]o) can affect myotonia. Experimental myotonia was induced in isolated rat muscles by ClC-1 inhibition and effects of [Mg(2+)]o or [Ca(2+)]o on myotonic contractions were determined. Both cations dampened myotonia within their physiological concentration ranges. Thus, myotonic contractile activity was 6-fold larger at 0.3 than at 1.2 mM [Mg(2+)]o and 82-fold larger at 0.3 than at 1.27 mM [Ca(2+)]o. In intracellular recordings of action potentials, the threshold for action potential excitation was raised by 4-6 mV when [Mg(2+)]o was elevated from 0.6 to 3 mM, compatible with an increase in the depolarization of the membrane potential necessary to activate the Na(+) channels. Supporting this notion, mathematical simulations showed that myotonia went from appearing with normal Cl(-) channel function to disappearing in the absence of Cl(-) channel function when Na(+) channel activation was depolarized by 6 mV. In conclusion, variation in serum Mg(2+) and Ca(2+) may contribute to phenotypic variation in myotonia congenita patients.
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Affiliation(s)
- Martin Skov
- Department of Biomedicine, Aarhus University, Ole Worms Allé 4, 8000 Aarhus C, Denmark
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Halldorsdottir O, Nielsen OB, Pedersen TH. Effect of Chloride Channel Inhibition in Muscle with Reduced Neuromuscular Transmission. Med Sci Sports Exerc 2010. [DOI: 10.1249/01.mss.0000389465.70771.d5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Abstract
High-frequency stimulation of skeletal muscle has long been associated with ionic perturbations, resulting in the loss of membrane excitability, which may prevent action potential propagation and result in skeletal muscle fatigue. Associated with intense skeletal muscle contractions are large changes in muscle metabolites. However, the role of metabolites in the loss of muscle excitability is not clear. The metabolic state of isolated rat extensor digitorum longus muscles at 30 degrees C was manipulated by decreasing energy expenditure and thereby allowed investigation of the effects of energy conservation on skeletal muscle excitability. Muscle ATP utilization was reduced using a combination of the cross-bridge cycling blocker N-benzyl-p-toluene sulfonamide (BTS) and the SR Ca2+ release channel blocker Na-dantrolene, which reduce activity of the myosin ATPase and SR Ca2+-ATPase. Compared with control muscles, the resting metabolites ATP, phosphocreatine, creatine, and lactate, as well as the resting muscle excitability as measured by M-waves, were unaffected by treatment with BTS plus dantrolene. Following 20 or 30 s of continuous 60-Hz stimulation, BTS-plus-dantrolene-treated muscles showed a 25% lower ATP utilization compared with control muscles. Furthermore, the ability of muscles to maintain excitability during high-frequency stimulation was significantly improved in BTS-plus-dantrolene-treated muscles, indicating a strong link between metabolites, energetic state, and the excitability of the muscle.
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Affiliation(s)
- W A Macdonald
- Institute of Physiology and Biophysics, University of Aarhus, DK-8000, Aarhus C, Denmark.
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Macdonald WA, Pedersen TH, Clausen T, Nielsen OB. N-Benzyl-p-toluene sulphonamide allows the recording of trains of intracellular action potentials from nerve-stimulated intact fast-twitch skeletal muscle of the rat. Exp Physiol 2005; 90:815-25. [PMID: 16049056 DOI: 10.1113/expphysiol.2005.031435] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
In skeletal muscle, the intracellular recording of trains of action potentials is difficult owing to the movement of the muscle upon stimulation. A potential tool for the removal of muscle movement is the cross-bridge cycle blocker, N-benzyl-p-toluene sulphonamide (BTS), although the effects of BTS on the passive and active membrane properties of intact muscle fibres are not known. Rat extensor digitorum longus (EDL) muscle was used to show that 50 mum BTS reduced tetanic force to approximately 10% of control force, without markedly altering muscle excitability. Incubation with BTS did not alter intracellular K+ content or Na+-K+ pump activity, but produced minor decreases in intracellular Na+ content (7%), resting 22Na+ influx (14%) and excitation-induced 22Na+ influx (29%). Despite these alterations to Na+ fluxes, BTS did not impair muscle excitability, since membrane conductance, resting membrane potential (RMP), rheobase current and the amplitude, overshoot and maximum rate of depolarization of the action potential were all unaltered. However, BTS did induce a small (8%) decrease in the maximum rate of repolarization of the action potential and an increase in the refractory period. The minor effects of BTS on muscle membrane properties did not compromise the ability of the muscle to propagate action potentials, even during tetanic stimulation. In conclusion, BTS can be used successfully to reduce contractility, allowing the intracellular recording of action potentials during both twitch and tetanic contraction of nerve-stimulated muscle, thus making it an excellent tool for the study of electrophysiology in fast-twitch skeletal muscle.
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Affiliation(s)
- W A Macdonald
- Institute of Physiology and Biophysics, University of Aarhus, Arhus, Denmark.
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Macdonald WA, Nielsen OB, Clausen T. Na+-K+ pump stimulation restores carbacholine-induced loss of excitability and contractility in rat skeletal muscle. J Physiol 2005; 563:459-69. [PMID: 15649983 PMCID: PMC1665601 DOI: 10.1113/jphysiol.2004.080390] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Revised: 12/02/2004] [Accepted: 01/06/2005] [Indexed: 11/08/2022] Open
Abstract
Intense exercise results in increases in intracellular Na+ and extracellular K+ concentrations, leading to depolarization and a loss of muscle excitability and contractility. Here, we use carbacholine to chronically activate the nicotinic acetylcholine (nACh) receptors to mimic the changes in membrane permeability, chemical Na+ and K+ gradients and membrane potential observed during intense exercise. Intact rat soleus muscles were mounted on force transducers and stimulated electrically to evoke short tetani at regular intervals. Carbacholine produced a 2.6-fold increase in Na+ influx that was tetrodotoxin (TTX) insensitive, but abolished by tubocurarine, resulting in a significant 36% increase in intracellular Na+, and 8% decrease in intracellular K+ content. The mid region, near the motor end plate, had much larger alterations than the more distal regions of the muscle, and showed a larger membrane depolarization from -73 +/- 1 to -60 +/- 1 mV compared with -64 +/- 1 mV. Carbacholine (10(-4) M) significantly reduced tetanic force to 31 +/- 3% of controls, which underwent significant recovery upon application of Na+-K+ pump stimulators: salbutamol (10(-5) M), adrenaline (10(-5) M) and calcitonin gene-related peptide (CGRP; 10(-7) M). The force recovery with salbutamol was accompanied by a recovery of intracellular Na+ and K+ contents, and a small but significant 4-5 mV recovery of membrane potential. Similar results were obtained using succinylcholine (10(-4) M), indicating that Na+-K+ pump stimulation may prevent or restore succinylcholine-induced hyperkalaemia. The stimulation of the Na+-K+ pump allows muscle to partially recover contractility by regaining excitability through electrogenically driven repolarization of the muscle membrane.
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Affiliation(s)
- W A Macdonald
- Institute of Physiology and Biophysics, University of Aarhus, DK-8000, Arhus C, Denmark.
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Pedersen TH, de Paoli F, Nielsen OB. Increased excitability of acidified skeletal muscle: role of chloride conductance. J Gen Physiol 2005; 125:237-46. [PMID: 15684096 PMCID: PMC2217490 DOI: 10.1085/jgp.200409173] [Citation(s) in RCA: 110] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2004] [Accepted: 01/10/2005] [Indexed: 11/20/2022] Open
Abstract
Generation of the action potentials (AP) necessary to activate skeletal muscle fibers requires that inward membrane currents exceed outward currents and thereby depolarize the fibers to the voltage threshold for AP generation. Excitability therefore depends on both excitatory Na+ currents and inhibitory K+ and Cl- currents. During intensive exercise, active muscle loses K+ and extracellular K+ ([K+]o) increases. Since high [K+]o leads to depolarization and ensuing inactivation of voltage-gated Na+ channels and loss of excitability in isolated muscles, exercise-induced loss of K+ is likely to reduce muscle excitability and thereby contribute to muscle fatigue in vivo. Intensive exercise, however, also leads to muscle acidification, which recently was shown to recover excitability in isolated K(+)-depressed muscles of the rat. Here we show that in rat soleus muscles at 11 mM K+, the almost complete recovery of compound action potentials and force with muscle acidification (CO2 changed from 5 to 24%) was associated with reduced chloride conductance (1731 +/- 151 to 938 +/- 64 microS/cm2, P < 0.01) but not with changes in potassium conductance (405 +/- 20 to 455 +/- 30 microS/cm2, P < 0.16). Furthermore, acidification reduced the rheobase current by 26% at 4 mM K+ and increased the number of excitable fibers at elevated [K+]o. At 11 mM K+ and normal pH, a recovery of excitability and force similar to the observations with muscle acidification could be induced by reducing extracellular Cl- or by blocking the major muscle Cl- channel, ClC-1, with 30 microM 9-AC. It is concluded that recovery of excitability in K(+)-depressed muscles induced by muscle acidification is related to reduction in the inhibitory Cl- currents, possibly through inhibition of ClC-1 channels, and acidosis thereby reduces the Na+ current needed to generate and propagate an AP. Thus short term regulation of Cl- channels is important for maintenance of excitability in working muscle.
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Affiliation(s)
- Thomas H Pedersen
- Institute of Physiology and Biophysics, University of Aarhus, Aarhus C, Denmark.
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15
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Abstract
Intracellular acidification of skeletal muscles is commonly thought to contribute to muscle fatigue. However, intracellular acidosis also acts to preserve muscle excitability when muscles become depolarized, which occurs with working muscles. Here, we show that this process may be mediated by decreased chloride permeability, which enables action potentials to still be propagated along the internal network of tubules in a muscle fiber (the T system) despite muscle depolarization. These results implicate chloride ion channels in muscle function and emphasize that intracellular acidosis of muscle has protective effects during muscle fatigue.
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16
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Nielsen OB, Ørtenblad N, Lamb GD, Stephenson DG. Excitability of the T-tubular system in rat skeletal muscle: roles of K+ and Na+ gradients and Na+-K+ pump activity. J Physiol 2004; 557:133-46. [PMID: 15034125 PMCID: PMC1665049 DOI: 10.1113/jphysiol.2003.059014] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2003] [Accepted: 03/15/2004] [Indexed: 11/08/2022] Open
Abstract
Strenuous exercise causes an increase in extracellular [K(+)] and intracellular Na(+) ([Na(+)](i)) of working muscles, which may reduce sarcolemma excitability. The excitability of the sarcolemma is, however, to some extent protected by a concomitant increase in the activity of muscle Na(+)-K(+) pumps. The exercise-induced build-up of extracellular K(+) is most likely larger in the T-tubules than in the interstitium but the significance of the cation shifts and Na(+)-K(+) pump for the excitability of the T-tubular membrane and the voltage sensors is largely unknown. Using mechanically skinned fibres, we here study the role of the Na(+)-K(+) pump in maintaining T-tubular function in fibres with reduced chemical K(+) gradient. The Na(+)-K(+) pump activity was manipulated by changing [Na(+)](i). The responsiveness of the T-tubules was evaluated from the excitation-induced force production of the fibres. Compared to control twitch force in fibres with a close to normal intracellular [K(+)] ([K(+)](i)), a reduction in [K(+)](i) to below 60 mM significantly reduced twitch force. Between 10 and 50 mM Na(+), the reduction in force depended on [Na(+)](i), the twitch force at 40 mM K(+) being 22 +/- 4 and 54 +/- 9% (of control force) at a [Na(+)](i) of 10 and 20 mM, respectively (n= 4). Double pulse stimulation of fibres at low [K(+)](i) showed that although elevated [Na(+)](i) increased the responsiveness to single action potentials, it reduced the capacity of the T-tubules to respond to high frequency stimulation. It is concluded that a reduction in the chemical gradient for K(+), as takes place during intensive exercise, may depress T-tubular function, but that a concomitant exercise-induced increase in [Na(+)](i) protects T-tubular function by stimulating the Na(+)-K(+) pump.
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Affiliation(s)
- O B Nielsen
- Department of Physiology, University of Aarhus, DK-8000 Arhus C, Denmark.
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17
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Clausen T, Overgaard K, Nielsen OB. Evidence that the Na+-K+ leak/pump ratio contributes to the difference in endurance between fast- and slow-twitch muscles. ACTA ACUST UNITED AC 2004; 180:209-16. [PMID: 14738479 DOI: 10.1111/j.0001-6772.2003.01251.x] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
AIM Muscles containing predominantly fast-twitch (type II) fibres [ext. dig. longus (EDL)] show considerably lower contractile endurance than muscles containing mainly slow-twitch (type I) fibres (soleus). To assess whether differences in Na+-K+ fluxes and excitability might contribute to this phenomenon, we compared excitation-induced Na+-K+ leaks, Na+ channels, Na+-K+ pump capacity, force and compound action potentials (M-waves) in rat EDL and soleus muscles. METHODS Isolated muscles were mounted for isometric contractions in Krebs-Ringer bicarbonate buffer and exposed to direct or indirect continuous or intermittent electrical stimulation. The time-course of force decline and concomitant changes in Na+-K+ exchange and M-waves were recorded. RESULTS During continuous stimulation at 60-120 Hz, EDL showed around fivefold faster rate of force decline than soleus. This was associated with a faster loss of excitability as estimated from the area and amplitude of the M-waves. The net uptake of Na+ and the release of K+ per action potential were respectively 6.5- and 6.6-fold larger in EDL than in soleus, which may in part be due to the larger content of Na+ channels in EDL. During intermittent stimulation with 1 s 60 Hz pulse trains, EDL showed eightfold faster rate of force decline than soleus. CONCLUSION The considerably lower contractile endurance of fast-twitch compared with slow-twitch muscles reflects differences in the rate of excitation-induced loss of excitability. This is attributed to the much larger excitation-induced Na+ influx and K+ efflux, leading to a faster rise in [K+]o in fast-twitch muscles. This may only be partly compensated by the concomitant activation of the Na+-K+ pumps, in particular in fibres showing large passive Na+-K+ leaks or reduced content of Na+-K+ pumps. Thus, endurance depends on the leak/pump ratio for Na+ and K+.
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Affiliation(s)
- T Clausen
- Department of Physiology, University of Aarhus, Arhus, Denmark
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18
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Abstract
1. During strenuous exercise lactic acid accumulates producing a reduction in muscle pH. In addition, exercise causes a loss of muscle K(+) leading to an increased concentration of extracellular K(+) ([K(+)](o)). Individually, reduced pH and increased [K(+)](o) have both been suggested to contribute to muscle fatigue. 2. To study the combined effect of these changes on muscle function, isolated rat soleus muscles were incubated at a [K(+)](o) of 11 mM, which reduced tetanic force by 75 %. Subsequent addition of 20 mM lactic acid led, however, to an almost complete force recovery. A similar recovery was observed if pH was reduced by adding propionic acid or increasing the CO(2) tension. 3. The recovery of force was associated with a recovery of muscle excitability as assessed from compound action potentials. In contrast, acidification had no effect on the membrane potential or the Ca(2+) handling of the muscles. 4. It is concluded that acidification counteracts the depressing effects of elevated [K(+)](o) on muscle excitability and force. Since intense exercise is associated with increased [K(+)](o), this indicates that, in contrast to the often suggested role for acidosis as a cause of muscle fatigue, acidosis may protect against fatigue. Moreover, it suggests that elevated [K(+)](o) is of less importance for fatigue than indicated by previous studies on isolated muscles.
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Affiliation(s)
- O B Nielsen
- Department of Physiology, University of Aarhus, DK-8000 Arhus C, Denmark.
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19
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Abstract
Increased extracellular K(+) concentration ([K(+)](o)) can reduce excitability and force in skeletal muscle. Here we examine the effects of muscle activation on compound muscle action potentials (M waves), resting membrane potential, and contractility in isolated rat soleus muscles. In muscles incubated for 60 min at 10 mM K(+), tetanic force and M wave area decreased to 23 and 24%, respectively, of the control value. Subsequently, short (1.5 s) tetanic stimulations given at 1-min intervals induced recovery of force and M wave area to 81 and 90% of control levels, respectively, within 15 min (P < 0.001). The recovery of force and M wave was associated with a partial repolarization of the muscle fibers. Experiments with tubocurarine suggest that the force recovery was related to activation of muscle Na(+)-K(+) pumps caused by the release of some compound from sensory nerves in response to muscle activity. In conclusion, activity produces marked recovery of excitability in K(+)-depressed muscle, and this may protect muscles against fatigue caused by increased [K(+)](o) during exercise.
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Affiliation(s)
- K Overgaard
- Department of Physiology, University of Aarhus, DK-8000 Arhus C, Denmark.
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20
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Nielsen OB, Clausen T. The Na+/K(+)-pump protects muscle excitability and contractility during exercise. Exerc Sport Sci Rev 2000; 28:159-64. [PMID: 11064849] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/18/2023]
Abstract
In skeletal muscle, the concentration of Na+/K(+)-pumps is high and increases through training. In isolated muscles, contractile endurance depends in part on Na+/K(+)-pump concentration. Exercise leads to rundown of Na+/K(+)-gradients, compound action potentials, and force. Early and efficient activation of the Na+/K(+)-pump, however, protects excitability and contractility.
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Affiliation(s)
- O B Nielsen
- Department of Physiology, University of Aarhus, Denmark.
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Overgaard K, Nielsen OB, Flatman JA, Clausen T. Relations between excitability and contractility in rat soleus muscle: role of the Na+-K+ pump and Na+/K+ gradients. J Physiol 1999; 518:215-25. [PMID: 10373703 PMCID: PMC2269417 DOI: 10.1111/j.1469-7793.1999.0215r.x] [Citation(s) in RCA: 97] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
1. The effects of reduced Na+/K+ gradients and Na+-K+ pump stimulation on compound action potentials (M waves) and contractile force were examined in isolated rat soleus muscles stimulated through the nerve. 2. Exposure of muscles to buffer containing 85 mM Na+ and 9 mM K+ (85 Na+/9 K+ buffer) produced a 54% decrease in M wave area and a 50 % decrease in tetanic force compared with control levels in standard buffer containing 147 mM Na+ and 4 mM K+. Subsequent stimulation of active Na+-K+ transport, using the beta2-adrenoceptor agonist salbutamol, induced a marked recovery of M wave area and tetanic force (to 98 and 87% of the control level, respectively). Similarly, stimulation of active Na+-K+ transport with insulin induced a significant recovery of M wave area and tetanic force. 3. During equilibration with 85 Na+/9 K+ buffer and after addition of salbutamol there was a close linear correlation between M wave area and tetanic force (r = 0.92, P < 0.001). Similar correlations were found in muscles where tetrodotoxin was used to reduce excitability and in muscles fatigued by 120 s of continuous stimulation at a frequency of 30 Hz. 4. These results show a close correlation between excitability and tetanic force. Furthermore, in muscles depressed by a reduction in the Na+/K+ gradients, beta-adrenergic stimulation of the Na+-K+ pump induces a recovery of excitability which can fully explain the previously demonstrated recovery of tetanic force following Na+-K+ pump stimulation. Moreover, the data indicate that loss of excitability is an important factor in fatigue induced by high-frequency (30 Hz) stimulation.
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Affiliation(s)
- K Overgaard
- Department of Physiology, University of Aarhus, DK-8000 Arhus C, Denmark.
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22
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Nielsen OB, Hilsted L, Clausen T. Excitation-induced force recovery in potassium-inhibited rat soleus muscle. J Physiol 1998; 512 ( Pt 3):819-29. [PMID: 9769424 PMCID: PMC2231245 DOI: 10.1111/j.1469-7793.1998.819bd.x] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/1998] [Accepted: 07/20/1998] [Indexed: 11/28/2022] Open
Abstract
1. Excitation markedly stimulates the Na+-K+ pump in skeletal muscle. The effect of this stimulation on contractility was examined in rat soleus muscles exposed to high extracellular K+ concentration ([K+]o). 2. At a [K+]o of 10 mM, tetanic force declined to 58 % of the force in standard buffer with 5.9 mM K+. Subsequent direct stimulation of the muscle at 1 min intervals with 30 Hz pulse trains of 2 s duration induced a 97 % recovery of force within 14 min. Force recovery could also be elicited by stimulation via the nerve. In muscles exposed to 12.5 mM K+, 30 Hz pulse trains of 2 s duration at 1 min intervals induced a recovery of force from 16 +/- 2 to 62 +/- 4% of the initial control force at a [K+]o of 5.9 mM. 3. The recovery of force was associated with a decrease in intracellular Na+ and was blocked by ouabain. This indicates that the force recovery was secondary to activation of the Na+-K+ pump. 4. Excitation stimulates the release of calcitonin gene-related peptide (CGRP) from nerves in the muscle. Since CGRP stimulates the Na+-K+ pump, this may contribute to the excitation-induced force recovery. Indeed, reducing CGRP content by capsaicin pre-treatment or prior denervation prevented both the excitation-induced force recovery and the drop in intracellular Na+. 5. The data suggest that activation of the Na+-K+ pump in contracting muscles counterbalances the depressing effect of reductions in the chemical gradients for Na+ and K+ on excitability.
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Affiliation(s)
- O B Nielsen
- Department of Physiology, University of Aarhus, DK-8000 Arhus C, Denmark.
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23
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Abstract
Increased passive Na+,K+ fluxes necessitate an efficient activation of the Na+,K+ pump in working muscles to limit the rundown of the Na+,K+ chemical gradients and ensuing loss of excitability. Several studies have demonstrated an increase in Na+,K+-pump rate in working muscles, and in electrically stimulated muscles up to a 22-fold increase in active Na+,K+ transport has been observed. Excitation-induced increase in intracellular Na+ is believed to be the primary stimulus for Na+,K+ pumping in a contracting muscle. In muscles recovering from electrical stimulation, however, the activity of the pump may stay elevated even after intracellular Na+ has been reduced to below the resting level. Moreover, in rat soleus muscles 10-s stimulation at 60 Hz induced a 5-fold increase in the activity of the Na+,K+ pump although mean intracellular [Na+] was unchanged. These findings strongly suggest that a substantial part of the excitation-induced increase in Na+,K+-pump activity is caused by mechanisms other than increased intracellular [Na+]. The mechanism behind this activation is not clear, but may involve a change in the affinity of the Na+,K+ pump for intracellular Na+. In addition to intracellular [Na+], the Na+,K+ pump may be stimulated in contracting muscles by other factors such as catecholamines, calcitonin gene-related peptide (CGRP), free fatty acids and cytoskeletal links. Together, this activation may form a feed forward mechanism protecting muscles from loss of excitability during periods of contraction by increasing Na+,K+-pump activity prior to erosion of the Na+,K+ chemical gradients. During exercise of high intensity, however, intracellular [Na+] increases substantially constituting an additional stimulus for the pump.
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Affiliation(s)
- O B Nielsen
- Department of Physiology, University of Aarhus, Arhus, Denmark
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24
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Abstract
In most types of mammalian skeletal muscles the total concentration of Na+,K+ pumps is 0.2-0.8 nmol g wet wt(-1). At rest, only around 5% of these Na+,K+ pumps are active, but during high-frequency stimulation, virtually all Na+,K+ pumps may be called into action within a few seconds. Despite this large capacity for active Na+,K+ transport, excitation often induces a net loss of K+, a net gain of Na+, depolarization and ensuing loss of excitability. In muscles exposed to high [K+]o or low [Na+]o, alone or combined, excitability is reduced. Under these conditions, hormonal or excitation-induced stimulation of the Na+,K+ pump leads to considerable force recovery. This recovery can be blocked by ouabain and seems to be the result of Na+,K+ pump induced hyperpolarization and restoration of Na+,K+ gradients. In muscles where the capacity of the Na+,K+ pump is reduced, the decline in the force developing during continuous electrical stimulation (30-90 Hz) is accelerated and the subsequent force recovery considerably delayed. The loss of endurance is significant within a few seconds after the onset of stimulation. Increased concentration of Na+ channels or open-time of Na+ channels is also associated with reduced endurance and impairment of force recovery. This indicates that during contractile activity, excitability is acutely dependent on the ratio between Na+ entry and Na+,K+ pump capacity. Contrary to previous assumptions, the Na+,K+ pump, due to rapid activation of its large transport capacity seems to play a dynamic role in the from second to second ongoing restoration and maintenance of excitability in working skeletal muscle.
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Affiliation(s)
- T Clausen
- Department of Physiology, University of Aarhus, Arhus, Denmark
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25
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Abstract
1. In rat soleus muscle, high frequency electrical stimulation produced a rapid increase in intracellular Na+ (Na+i) content. This was considerably larger in muscles contracting without developing tension than in muscles contracting isometrically. During subsequent rest a net extrusion of Na+ took place at rates which, depending on the frequency and duration of stimulation, approached the maximum transport capacity of the Na(+)-K+ pumps present in the muscle. 2. In isometrically contracting muscles, the net extrusion of Na+ continued for up to 10 min after stimulation, reducing Na+i to values 30% below the resting level (P < 0.001). This undershoot in Na+i, seen in both soleus and extensor digitorum longus muscles, could be maintained for up to 30 min and was blocked by ouabain or cooling to 0 degree C. 3. The undershoot in Na+i could be elicited by direct stimulation as well as by tubocurarine-suppressible stimulation via the motor endplate. It could not be attributed to a decrease in Na+ influx, to effects of noradrenaline or calcitonin gene-related peptide released from nerve endings, to an increase in extracellular K+ or the formation of nitric oxide. 4. The results indicate that excitation rapidly activates the Na(+)-K+ pump, partly via a change in its transport characteristics and partly via an increase in intracellular Na+ concentration. This activation allows an approximately 20-fold increase in the rate of Na+ efflux to take place within 10 s. 5. The excitation-induced activation of the Na(+)-K+ pump may represent a feed-forward mechanism that protects the Na(+)-K+ gradients and the membrane potential in working muscle. Contrary to previous assumptions, the Na(+)-K+ pump seems to play a dynamic role in maintenance of excitability during contractile activity.
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Affiliation(s)
- O B Nielsen
- Department of Physiology, University of Aarhus, Denmark.
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26
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Overgaard K, Nielsen OB, Clausen T. Effects of reduced electrochemical Na+ gradient on contractility in skeletal muscle: role of the Na+-K+ pump. Pflugers Arch 1997; 434:457-65. [PMID: 9211813 DOI: 10.1007/s004240050421] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Continued excitation of skeletal muscle may induce a combination of a low extracellular Na+ concentration ([Na+]o) and a high extracellular K+ concentration ([K+]o) in the T-tubular lumen, which may contribute to fatigue. Here, we examine the role of the Na+-K+ pump in the maintenance of contractility in isolated rat soleus muscles when the Na+, K+ gradients have been altered. When [Na+]o is lowered to 25 mM by substituting Na+ with choline, tetanic force is decreased to 30% of the control level after 60 min. Subsequent stimulation of the Na+-K+ pump with insulin or catecholamines induces a decrease in [Na+]i and hyperpolarization. This is associated with a force recovery to 80-90% of the control level which can be abolished by ouabain. This force recovery depends on hyperpolarization and is correlated to the decrease in -Na+-i (r = 0. 93; P<0.001). The inhibitory effect of a low -Na+-o on force development is considerably potentiated by increasing [K+]o. Again, stimulation of the Na+-K+ pump leads to rapid force recovery. The Na+-K+ pump has a large potential for rapid compensation of the excitation-induced rundown of Na+, K+ gradients and contributes, via its electrogenic effect, to the membrane potential. We conclude that these actions of the Na+-K+ pump are essential for the maintenance of excitability and contractile force.
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Affiliation(s)
- K Overgaard
- Department of Physiology, University of Aarhus, Ole Worms Alle 160, 8000 Arhus C, Denmark
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27
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Abstract
The dependence of contractile performance on the leak-to-pump ratio for Na+ has been examined. In isolated rat soleus muscle the concentration of Na(+)-K+ pumps was shown to decrease with age (-57%) or K+ deficiency (-69%), whereas Na+ channel concentration remained constant. This relative increase in the ratio between Na+ channels and Na(+)-K+ pumps was associated with a markedly faster rate of force decline (58 and 97%, respectively; both P < 0.001) during stimulation at 90 Hz and reduced subsequent force recovery (-34 and -38%, respectively; both P < 0.001). Similar effects were elicited by acute inhibition of Na(+)-K+ pump activity with ouabain. Preincubation with aconitine and veratridine, resulting in a 91 and 118% increase in Na+ influx per contraction, respectively (both P < 0.05), significantly hastened the initial rate of force decline (19%; P < 0.05 for aconitine and 69%; P < 0.001 for veratridine) and slowed recovery (-59 and -86%, respectively, both P < 0.001). It is concluded that the ratio between excitation-induced Na+ influx and Na(+)-K+ pump capacity is an important determinant for endurance and rate of recovery of force in skeletal muscle.
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Affiliation(s)
- A P Harrison
- Department of Physiology, University of Aarhus, Denmark
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28
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Nielsen OB. Effects of haemoglobin O2 saturation on volume regulation in adrenergically stimulated red blood cells from the trout, Oncorhynchus mykiss. J Comp Physiol B 1997; 167:159-68. [PMID: 9151427 DOI: 10.1007/s003600050060] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Adrenergic stimulation of trout red blood cells activates a Na+/H(+)-exchange. If unopposed, the ensuing increase in cell Na+ leads to an isosmotic cell swelling. In this study the effect of the level of haemoglobin O2 saturation on volume regulation has been investigated in adrenergically stimulated red blood cells from trout: at full haemoglobin O2 saturation, net influx of Na+ through the Na+/H(+)-exchanger was balanced by net efflux of K+ and no increases in cell volume took place. In contrast, at low O2 saturation (8-14%) adrenergic stimulation led to a substantial increase in cell Na+, K+ and volume. Moreover, cell volume recovery after adrenergic swelling was incomplete at low O2 saturation, whereas cells at high O2 saturation exhibited a fast and complete cell volume recovery. In cells exposed to alternating high and low O2 saturation, volume regulation was similar to the regulation found in cells maintained at high O2 saturation. In cells at high O2 saturation, extrusion of cellular Na+ by the Na+/K(+)-pump significantly contributed to the volume decrease. It is concluded that trout red blood cells at high or alternating O2 saturations possess a powerful regulatory volume decrease response that is shut off at low O2 saturation. The physiological implications of this regulation is discussed.
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Affiliation(s)
- O B Nielsen
- Department of Zoophysiology, University of Aarhus, Denmark.
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29
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Abstract
The effects of reduced Na+,K+ pump capacity on contractile endurance and excitation-induced changes in intracellular Na+ content were investigated in isolated rat soleus and extensor digitorum longus muscles. Pre-incubation with 10(-5) M ouabain increased the rate of force decline measured over the first 5-20 s of tetanic contraction from 0.32 to 0.94% s-1 and 1.4 to 4.6% s-1 in soleus and extensor digitorum longus muscles, respectively. Soleus muscles from K(+)- deficient rats exhibited 54% reduction in the concentration of Na+,K+ pumps and the force decline during 30 s of 60 Hz stimulation was increased from 0.53 to 1.15% s-1. A similar change was induced in control muscles when a comparable reduction in the concentration of functional Na+,K+ pumps was elicited by pre-incubation with ouabain (10(-6)-2 x 10(-6) M). In soleus, the force decline during 60 s of 60 Hz stimulation showed linear correlation to the increase in intracellular Na+ content. In extensor digitorum longus, force decline and increase in Na+ content during 60 Hz stimulation were both four times faster than in soleus as measured over 15 s of excitation. These results indicate that during maximal contractions the Na+,K+ pump capacity is one of the determinants for the contractile endurance in skeletal muscle. Furthermore, the maintenance of contractile force seems to be a function of the rate of Na(+)-influx and this relationship may account for the difference in endurance between slow-twitch and fast-twitch muscles.
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Affiliation(s)
- O B Nielsen
- Department of Physiology, University of Aarbus, Denmark
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30
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Abstract
Intensive contractile activity is associated with a significant net loss of K+ and a comparable gain of Na+ in the working muscle fibres. This leads to an increase in the interstitial and T-tubular K+ concentration and to a decrease in the T-tubular Na+ concentration. It is well established that the exposure of muscles to high extracellular K+ or low extracellular Na+ inhibits contractile performance. More importantly, the combination of high extracellular K+ and low extracellular Na+ has a much more pronounced inhibitory effect on force than the sum of the individual effects of the two ions. The inhibitory effects of high extracellular K+ or low extracellular Na+ can be alleviated within 5-10 min by acute hormonal stimulation of the Na+, K+ pump. In contrast, reductions in the capacity for active Na+, K+ transport by pre-incubation of isolated muscles with ouabain or by prior K+ depletion of the animals significantly decreases contractile endurance during high-frequency electrical stimulation. Thus, muscles from K(+)-depleted rats exhibiting a 54% reduction in Na+, K+ pump concentration showed a 110% increase in force decline during 30 s of 60 Hz stimulation. Reducing the Na+, K+ pump capacity to a similar extent by pre-incubation with ouabain led to a comparable decrease in endurance. Moreover, reductions in the Na+, K+ pump capacity were associated with an increased intracellular accumulation of Na+ during electrical stimulation. These observations support the notion that excitation-induced decreases in Na+, K+ gradients contribute to fatigue during intensive exercise and suggest that the capacity for active Na+, K+ transport is a determining factor for contractile endurance.
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Affiliation(s)
- O B Nielsen
- Department of Physiology, University of Aarbus, Denmark
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31
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Abstract
The role of protein phosphatases in the regulation of K flux pathways of the trout red blood cell has been investigated using the phosphatase inhibitors calyculin A and okadaic acid. Both inhibitors completely blocked an oxygenation-activated Cl-dependent K flux with a 50% inhibitory concentration of 17 and 675 nmol/l, respectively, but not the hypotonically activated Cl-independent K uptake. N-ethylmaleimide (NEM) and staurosporine caused an increase in the Cl-dependent flux. In both cases preincubation with calyculin A blocked activation but, when added during activation, it prevented any further increase with NEM but abolished the staurosporine-induced uptake. K uptake that was activated by NEM and "clamped" by calyculin A was volume sensitive, indicating a dual influence on this pathway. Chelerythrine, a protein kinase inhibitor, activated a Cl-independent K uptake that was unaffected by calyculin A. It is concluded that activation and deactivation of both Cl-dependent and Cl-independent pathways require changes in the phosphorylation of an as yet unidentified target protein(s), although with different sets of protein kinase and/or phosphatases. These observations also suggest a complex model of kinase-phosphatase regulation and provide drugs for the pharmacological definition and manipulation of Cl-dependent and Cl-independent K flux pathways.
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Affiliation(s)
- A R Cossins
- Department of Environmental and Evolutionary Biology, University of Liverpool, United Kingdom
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32
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Abstract
In skeletal muscle, the excitation induced influx of Na+ and efflux of K+ may be sufficient to exceed the activity or even the capacity of the available Na+,K(+)-pumps. This leads to a rise in intracellular Na+ and extracellular K+. Both events interfere with excitability and may present important limitations for the continuation of contractile activity. Furthermore, inhibition of the Na+,K(+)-pump or reduction of the concentration of functional Na+,K(+)-pumps decrease excitability and the maintenance of force during continued stimulation. Conversely, in muscles where contractile force is inhibited by exposure to high extracellular K+, acute stimulation of the Na+,K(+)-pump with catecholamines, CGRP or insulin leads to a rapid recovery of force. The large passive fluxes of Na+ and K+ associated with excitation constitute the major drive on the activity of the Na+,K(+)-pump, giving rise to up to 20-fold stimulation of the transport rate. In keeping with this, training induces an upregulation of the total concentration of Na+,K(+)-pumps in skeletal muscle. The activity and the capacity of the Na+,K(+)-pump are important limiting factors determining the maintenance of excitability and contractile performance.
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Affiliation(s)
- T Clausen
- Institute of Physiology, University of Aarhus, Denmark
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Vejlgaard TB, Nielsen OB. [Subacute thyroiditis in Parvovirus B19 infection]. Ugeskr Laeger 1994; 156:6039-40. [PMID: 7992447] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
In a 32-year-old women with clinical and scintigraphic signs of thyroiditis, serological screening showed positive IgG and IgM titres against Parvovirus B19 (PB19). Subacute thyroiditis has not previously been reported following PB19 infection. The present case suggests that subacute thyroiditis may be caused by PB19 infection. Further screening is necessary to establish whether this infection is more frequently associated with subacute thyroiditis than previously thought.
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Abstract
The effect of oxygenation on the dissipative fluxes of K in trout red blood cells has been determined. Unidirectional influx under low oxygen tension (PO2 = 1 kPa) was 0.56 +/- 0.07 mmol.l-1 packed cells.h-1. Within a few minutes of equilibration with high oxygen tension (PO2 = 120 kPa), influx was increased 14-fold, and this was associated with a progressive loss of KCl and a cell shrinkage. K influx progressively declined over the following 3 h to levels close to those characteristic of cells at low oxygen tension. Replacement of medium Cl by NO3- or methane sulfonate inhibited the stimulation due to high oxygen as did furosemide and low extracellular pH. The oxygenation-stimulated influx was highly volume sensitive, being increased by up to 100% by osmotic swelling and decreased by osmotic shrinkage. By contrast, the small influx under low oxygen tension was unaffected by either Cl replacement or by shrinkage and increased only with extreme swelling. Thus high oxygen tension activated a Cl-dependent and furosemide-sensitive K flux. Once activated, the mechanism was rapidly deactivated on transfer back to low oxygen tension but slowly deactivated when maintained at high PO2. The oxygenation-stimulated flux mechanism promotes a rapid and more complete volume regulatory decrease than in cells at low oxygen tension.
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Affiliation(s)
- O B Nielsen
- Department of Zoophysiology, University of Aarhus, Denmark
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Abstract
Trout were exposed to hypercapnia, two levels of aerobic exercise, or three successive periods of supramaximal exercise to evaluate the effects on erythrocyte and plasma K+. During aerobic exercise, plasma K+ increased slightly with the intensity of work, while no change was found in the erythrocyte K+ content. In contrast, both hypercapnia and supramaximal exercise induced a net erythrocyte K+ uptake. This uptake changed to a net loss of K+ as arterial pH and hemoglobin-bound oxygen saturation returned to control values during recovery. The maximal rates of net K+ uptake found during hypercapnia and supramaximal exercise corresponded to 195 and 350 mumol.kg fish-1.h-1, respectively, and the maximal rates of net K+ loss found during recovery corresponded in both cases to approximately 130 mumol.kg fish-1.h-1. Hypercapnia had only a minor effect on plasma K+, but return to normocapnic conditions induced a 0.8 mM rise in plasma K+. Of this increase, approximately 70% could be accounted for by the simultaneous net release of erythrocyte K+. Each period of supramaximal exercise induced an elevated plasma K+ level, resulting in accumulation of plasma K+ despite slight decreases in plasma K+ in between the exercise periods. At the same time the net erythrocyte K+ uptake caused an estimated reduction in plasma K+ of 1.5 mM. It is concluded that both hypercapnia and supramaximal exercise cause profound net changes in the erythrocyte K+ content with significant effects on plasma K+.
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Affiliation(s)
- O B Nielsen
- Department of Zoophysiology, University of Aarhus, Denmark
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Abstract
Plasma and erythrocyte K+ were monitored during storage and tonometry of blood samples taken from resting rainbow trout, Oncorhynchus mykiss. During storage of arterial blood samples, plasma K+ concentration increased by 38% in 12 min. During extended tonometry of blood with a pH near 7.9 and full hemoglobin-bound oxygen (HbO2) saturation the erythrocytes showed a net loss of K+. Plasma K+ concentration increased from 2.9 mM to a near steady-state value of 5.6 mM. When tonometered at a pH near 7.2 and a HbO2 saturation at approximately 4% the erythrocytes took up K+, leading to a dramatic reduction in plasma K+ concentration to 0.2 mM. This net uptake was stimulated by isoprenaline and was inhibited by ouabain. It is concluded that net erythrocyte K+ uptake and loss can be induced in trout by changes in blood pH or HbO2 saturation in vitro.
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Affiliation(s)
- O B Nielsen
- Department of Zoophysiology, University of Aarhus, Denmark
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Abstract
Various 2, 4- and 3, 4-disubstituted 5-methylsulfonylbenzoic acids were synthesized as methylsulfonyl analogues of previously described 5-sulfamoylbenzoic acid diuretics. The results of the diuretic screening in dogs reveal that substitution of the sulfamoyl group by the spatially and sterically similar methylsulfonyl group does not affect the diuretic pattern but leads generally to somewhat decreased potency. For the highly potent 3-benzylamino-4-phenoxy-5-methylsulfonylbenzoic acid the corresponding 5-methylthio and 5-methylsulfinyl analogs were prepared and found still to exhibit diuretic activity. Internal aldol condensation and subsequent dehydration of 3-benzylamino-and 3-n-butylamino-4-benzoyl-5-methylsulfonylbenzoic acid provided the corresponding inactive 4-alkylamino-6-carboxy-2, 3-dihydro-3-hydroxy-3-phenylbenzo[b]thiophene 1, 1-dioxides and 4-alkylamino-6-carboxy-3-phenyl-benzo[b]thiophene 1,1-dioxides.
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Nielsen OB, Bruun H, Bretting C, Feit PW. Aminobenzoic acid diuretics. 7. 3-Substituted 4-phenyl-, 4-arylcarbonyl-, and 4-arylmethyl-5-sulfamoylbenzoic acids and related compounds. J Med Chem 1975; 18:41-50. [PMID: 1109574 DOI: 10.1021/jm00235a010] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
Various 4-substituted 3-alkylamino-, 3-alkoxy-, 3-alkylthio-, and 3-alkyl-5-sulfamoylbenzoic acids related to known aminobenzoic acid diuretics were synthesized and screened for their diuretic properties in dogs. The tabulated results from a 3-hr test period revealed that generally the diuretic profile and potency could be retained when 3-alkoxy, 3-alkylthio, and 3-phenethyl were substituted for the 3-alkylamino moiety. The high potency of several 3-alkoxy-, 3-alkylthio-, and 3-phenethyl-4-benzoyl-5-sulfamoylbenzoic acids confirmed previous suggestions that the apparent diuretic effect of 4- and 5-alkylamino-6-carboxy-3-phenyl-1,2-benzisothiazole 1,1-dioxides originates from the corresponding 4-benzoyl-5-sulfamoylbenzoic acid derivatives due to an existing equilibrium in plasma. 4-Benzoyl-5-sulfamoyl-3-(3-thenyloxy) benzoic acid (118) is among the most potent benzoic acid diuretics hitherto synthesized and shows significant diuretic activity in dogs at 1 mug/kg. The results obtained with different 3-substituted 4-phenyl-5-sulfamoylbenzoic acids supported the earlier concept regarding the steric influence of the 4-substituent on the diuretic potency of sulfamoylbenzoic acid diuretics.
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Feit PW, Nielsen OB, Brunn H. Aminobenzoic acid diuretics. 6. 4-Substituted 3-alkylthio-5-sulfamoylbenzoic and 5-sulfamoylthiosalicylic acids. J Med Chem 1974; 17:572-8. [PMID: 4829937 DOI: 10.1021/jm00252a002] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
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Nielsen OB, Nielsen CK, Feit PW. Aminobenzoic acid diuretics. 5. 3-Amino-4-arylmethyl-5-sulfamylbenzoic acid derivatives and related compounds. J Med Chem 1973; 16:1170-7. [PMID: 4749473 DOI: 10.1021/jm00268a022] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
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Feit PW, Nielsen OB, Rastrup-Andersen N. Aminobenzoic acid diuretics. 4. 4-Benzyl-5-sulfamylanthranilic acid derivatives and related 1,2-benzisothiazole 1,1-dioxides. J Med Chem 1973; 16:127-30. [PMID: 4683107 DOI: 10.1021/jm00260a010] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
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