1
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Denomme N, Lukowski AL, Hull JM, Jameson MB, Bouza AA, Narayan ARH, Isom LL. The voltage-gated sodium channel inhibitor, 4,9-anhydrotetrodotoxin, blocks human Na v1.1 in addition to Na v1.6. Neurosci Lett 2020; 724:134853. [PMID: 32114117 PMCID: PMC7096269 DOI: 10.1016/j.neulet.2020.134853] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2019] [Revised: 02/12/2020] [Accepted: 02/18/2020] [Indexed: 11/23/2022]
Abstract
Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of action potentials in neurons. The human genome includes ten human VGSC α-subunit genes, SCN(X)A, encoding Nav1.1-1.9 plus Nax. To understand the unique role that each VGSC plays in normal and pathophysiological function in neural networks, compounds with high affinity and selectivity for specific VGSC subtypes are required. Toward that goal, a structural analog of the VGSC pore blocker tetrodotoxin, 4,9-anhydrotetrodotoxin (4,9-ah-TTX), has been reported to be more selective in blocking Na+ current mediated by Nav1.6 than other TTX-sensitive VGSCs, including Nav1.2, Nav1.3, Nav1.4, and Nav1.7. While SCN1A, encoding Nav1.1, has been implicated in several neurological diseases, the effects of 4,9-ah-TTX on Nav1.1-mediated Na+ current have not been tested. Here, we compared the binding of 4,9-ah-TTX for human and mouse brain preparations, and the effects of 4,9-ah-TTX on human Nav1.1-, Nav1.3- and Nav1.6-mediated Na+ currents using the whole-cell patch clamp technique in heterologous cells. We show that, while 4,9-ah-TTX administration results in significant blockade of Nav1.6-mediated Na+ current in the nanomolar range, it also has significant effects on Nav1.1-mediated Na+ current. Thus, 4,9-ah-TTX is not a useful tool in identifying Nav1.6-specific effects in human brain networks.
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Affiliation(s)
- Nicholas Denomme
- Department of Pharmacology, University of Michigan, Ann Arbor, Michigan, 48109 United States; Center for Consciousness Science, University of Michigan, Ann Arbor, Michigan, 48109 United States
| | - April L Lukowski
- Program in Chemical Biology, University of Michigan, Ann Arbor, Michigan, 48109 United States; Life Sciences Institute, University of Michigan, Ann Arbor, Michigan, 48109 United States
| | - Jacob M Hull
- Neuroscience Graduate Program, University of Michigan, Ann Arbor, Michigan, 48109 United States
| | - Margaret B Jameson
- Department of Pharmacology, University of Michigan, Ann Arbor, Michigan, 48109 United States; Molecular and Cellular Pharmacology Training Program, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705 United States
| | - Alexandra A Bouza
- Department of Pharmacology, University of Michigan, Ann Arbor, Michigan, 48109 United States
| | - Alison R H Narayan
- Program in Chemical Biology, University of Michigan, Ann Arbor, Michigan, 48109 United States; Life Sciences Institute, University of Michigan, Ann Arbor, Michigan, 48109 United States; Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109 United States
| | - Lori L Isom
- Department of Pharmacology, University of Michigan, Ann Arbor, Michigan, 48109 United States; Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, Michigan, 48109 United States; Neuroscience Graduate Program, University of Michigan, Ann Arbor, Michigan, 48109 United States; Department of Neurology, University of Michigan, Ann Arbor, Michigan, 48109 United States.
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2
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Castrec J, Soudant P, Payton L, Tran D, Miner P, Lambert C, Le Goïc N, Huvet A, Quillien V, Boullot F, Amzil Z, Hégaret H, Fabioux C. Bioactive extracellular compounds produced by the dinoflagellate Alexandrium minutum are highly detrimental for oysters. AQUATIC TOXICOLOGY (AMSTERDAM, NETHERLANDS) 2018; 199:188-198. [PMID: 29653309 DOI: 10.1016/j.aquatox.2018.03.034] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/12/2018] [Revised: 03/27/2018] [Accepted: 03/28/2018] [Indexed: 06/08/2023]
Abstract
Blooms of the dinoflagellate Alexandrium spp., known as producers of paralytic shellfish toxins (PSTs), are regularly detected on the French coastline. PSTs accumulate into harvested shellfish species, such as the Pacific oyster Crassostrea gigas, and can cause strong disorders to consumers at high doses. The impacts of Alexandrium minutum on C. gigas have often been attributed to its production of PSTs without testing separately the effects of the bioactive extracellular compounds (BECs) with allelopathic, hemolytic, cytotoxic or ichthyotoxic properties, which can also be produced by these algae. The BECs, still uncharacterized, are excreted within the environment thereby impacting not only phytoplankton, zooplankton but also marine invertebrates and fishes, without implicating any PST. The aim of this work was to compare the effects of three strains of A. minutum producing either only PSTs, only BECs, or both PSTs and BECs, on the oyster C. gigas. Behavioral and physiological responses of oysters exposed during 4 days were monitored and showed contrasted behavioral and physiological responses in oysters supposedly depending on produced bioactive substances. The non-PST extracellular-compound-producing strain primarily strongly modified valve-activity behavior of C. gigas and induced hemocyte mobilization within the gills, whereas the PST-producing strain caused inflammatory responses within the digestive gland and disrupted the daily biological rhythm of valve activity behavior. BECs may therefore have a significant harmful effect on the gills, which is one of the first organ in contact with the extracellular substances released in the water by A. minutum. Conversely, the PSTs impact the digestive gland, where they are released and mainly accumulated, after degradation of algal cells during digestion process of bivalves. This study provides a better understanding of the toxicity of A. minutum on oyster and highlights the significant role of BECs in this toxicity calling for further chemical characterization of these substances.
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Affiliation(s)
- J Castrec
- LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, IUEM, rue Dumont d'Urville, 29280 Plouzané, France.
| | - P Soudant
- LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, IUEM, rue Dumont d'Urville, 29280 Plouzané, France
| | - L Payton
- CNRS, EPOC, UMR 5805, F-33120 Arcachon, France
| | - D Tran
- CNRS, EPOC, UMR 5805, F-33120 Arcachon, France
| | - P Miner
- Ifremer, LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, Centre de Bretagne, CS 10070, 29280 Plouzané, France
| | - C Lambert
- LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, IUEM, rue Dumont d'Urville, 29280 Plouzané, France
| | - N Le Goïc
- LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, IUEM, rue Dumont d'Urville, 29280 Plouzané, France
| | - A Huvet
- Ifremer, LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, Centre de Bretagne, CS 10070, 29280 Plouzané, France
| | - V Quillien
- Ifremer, LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, Centre de Bretagne, CS 10070, 29280 Plouzané, France
| | - F Boullot
- LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, IUEM, rue Dumont d'Urville, 29280 Plouzané, France
| | - Z Amzil
- Ifremer, Laboratoire Phycotoxines, BP 21105, F-44311 Nantes, France
| | - H Hégaret
- LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, IUEM, rue Dumont d'Urville, 29280 Plouzané, France
| | - C Fabioux
- LEMAR UMR 6539 CNRS/UBO/IRD/Ifremer, IUEM, rue Dumont d'Urville, 29280 Plouzané, France
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3
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Jones DK, Claydon TW, Ruben PC. Extracellular protons inhibit charge immobilization in the cardiac voltage-gated sodium channel. Biophys J 2014; 105:101-7. [PMID: 23823228 DOI: 10.1016/j.bpj.2013.04.022] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2013] [Revised: 04/10/2013] [Accepted: 04/12/2013] [Indexed: 11/25/2022] Open
Abstract
Low pH depolarizes the voltage-dependence of cardiac voltage-gated sodium (NaV1.5) channel activation and fast inactivation and destabilizes the fast-inactivated state. The molecular basis for these changes in protein behavior has not been reported. We hypothesized that changes in the kinetics of voltage sensor movement may destabilize the fast-inactivated state in NaV1.5. To test this idea, we recorded NaV1.5 gating currents in Xenopus oocytes using a cut-open voltage-clamp with extracellular solution titrated to either pH 7.4 or pH 6.0. Reducing extracellular pH significantly depolarized the voltage-dependence of both the QON/V and QOFF/V curves, and reduced the total charge immobilized during depolarization. We conclude that destabilized fast-inactivation and reduced charge immobilization in NaV1.5 at low pH are functionally related effects.
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Affiliation(s)
- D K Jones
- Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, British Columbia, Canada
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4
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Huang CJ, Schild L, Moczydlowski EG. Use-dependent block of the voltage-gated Na(+) channel by tetrodotoxin and saxitoxin: effect of pore mutations that change ionic selectivity. ACTA ACUST UNITED AC 2013; 140:435-54. [PMID: 23008436 PMCID: PMC3457692 DOI: 10.1085/jgp.201210853] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Voltage-gated Na(+) channels (NaV channels) are specifically blocked by guanidinium toxins such as tetrodotoxin (TTX) and saxitoxin (STX) with nanomolar to micromolar affinity depending on key amino acid substitutions in the outer vestibule of the channel that vary with NaV gene isoforms. All NaV channels that have been studied exhibit a use-dependent enhancement of TTX/STX affinity when the channel is stimulated with brief repetitive voltage depolarizations from a hyperpolarized starting voltage. Two models have been proposed to explain the mechanism of TTX/STX use dependence: a conformational mechanism and a trapped ion mechanism. In this study, we used selectivity filter mutations (K1237R, K1237A, and K1237H) of the rat muscle NaV1.4 channel that are known to alter ionic selectivity and Ca(2+) permeability to test the trapped ion mechanism, which attributes use-dependent enhancement of toxin affinity to electrostatic repulsion between the bound toxin and Ca(2+) or Na(+) ions trapped inside the channel vestibule in the closed state. Our results indicate that TTX/STX use dependence is not relieved by mutations that enhance Ca(2+) permeability, suggesting that ion-toxin repulsion is not the primary factor that determines use dependence. Evidence now favors the idea that TTX/STX use dependence arises from conformational coupling of the voltage sensor domain or domains with residues in the toxin-binding site that are also involved in slow inactivation.
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5
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Capes DL, Arcisio-Miranda M, Jarecki BW, French RJ, Chanda B. Gating transitions in the selectivity filter region of a sodium channel are coupled to the domain IV voltage sensor. Proc Natl Acad Sci U S A 2012; 109:2648-53. [PMID: 22308389 PMCID: PMC3289344 DOI: 10.1073/pnas.1210413109] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/03/2024] Open
Abstract
Voltage-dependent ion channels are crucial for generation and propagation of electrical activity in biological systems. The primary mechanism for voltage transduction in these proteins involves the movement of a voltage-sensing domain (D), which opens a gate located on the cytoplasmic side. A distinct conformational change in the selectivity filter near the extracellular side has been implicated in slow inactivation gating, which is important for spike frequency adaptation in neural circuits. However, it remains an open question whether gating transitions in the selectivity filter region are also actuated by voltage sensors. Here, we examine conformational coupling between each of the four voltage sensors and the outer pore of a eukaryotic voltage-dependent sodium channel. The voltage sensors of these sodium channels are not structurally symmetric and exhibit functional specialization. To track the conformational rearrangements of individual voltage-sensing domains, we recorded domain-specific gating pore currents. Our data show that, of the four voltage sensors, only the domain IV voltage sensor is coupled to the conformation of the selectivity filter region of the sodium channel. Trapping the outer pore in a particular conformation with a high-affinity toxin or disulphide crossbridge impedes the return of this voltage sensor to its resting conformation. Our findings directly establish that, in addition to the canonical electromechanical coupling between voltage sensor and inner pore gates of a sodium channel, gating transitions in the selectivity filter region are also coupled to the movement of a voltage sensor. Furthermore, our results also imply that the voltage sensor of domain IV is unique in this linkage and in the ability to initiate slow inactivation in sodium channels.
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Affiliation(s)
- Deborah L. Capes
- Department of Neuroscience, University of Wisconsin, Madison, WI 53706
- Molecular and Cellular Pharmacology Graduate Program, and
| | - Manoel Arcisio-Miranda
- Department of Neuroscience, University of Wisconsin, Madison, WI 53706
- Department of Biophysics, Federal University of Sao Paulo, 04023-060, Sao Paulo, Brazil; and
| | - Brian W. Jarecki
- Department of Neuroscience, University of Wisconsin, Madison, WI 53706
| | - Robert J. French
- Department of Physiology and Pharmacology, University of Calgary, Calgary, AB, Canada T2N 4N1
| | - Baron Chanda
- Department of Neuroscience, University of Wisconsin, Madison, WI 53706
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6
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Gating transitions in the selectivity filter region of a sodium channel are coupled to the domain IV voltage sensor. Proc Natl Acad Sci U S A 2012. [PMID: 22308389 DOI: 10.1073/pnas.1115575109] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Voltage-dependent ion channels are crucial for generation and propagation of electrical activity in biological systems. The primary mechanism for voltage transduction in these proteins involves the movement of a voltage-sensing domain (D), which opens a gate located on the cytoplasmic side. A distinct conformational change in the selectivity filter near the extracellular side has been implicated in slow inactivation gating, which is important for spike frequency adaptation in neural circuits. However, it remains an open question whether gating transitions in the selectivity filter region are also actuated by voltage sensors. Here, we examine conformational coupling between each of the four voltage sensors and the outer pore of a eukaryotic voltage-dependent sodium channel. The voltage sensors of these sodium channels are not structurally symmetric and exhibit functional specialization. To track the conformational rearrangements of individual voltage-sensing domains, we recorded domain-specific gating pore currents. Our data show that, of the four voltage sensors, only the domain IV voltage sensor is coupled to the conformation of the selectivity filter region of the sodium channel. Trapping the outer pore in a particular conformation with a high-affinity toxin or disulphide crossbridge impedes the return of this voltage sensor to its resting conformation. Our findings directly establish that, in addition to the canonical electromechanical coupling between voltage sensor and inner pore gates of a sodium channel, gating transitions in the selectivity filter region are also coupled to the movement of a voltage sensor. Furthermore, our results also imply that the voltage sensor of domain IV is unique in this linkage and in the ability to initiate slow inactivation in sodium channels.
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7
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Cervenka R, Zarrabi T, Lukacs P, Todt H. The outer vestibule of the Na+ channel-toxin receptor and modulator of permeation as well as gating. Mar Drugs 2010; 8:1373-93. [PMID: 20479982 PMCID: PMC2866490 DOI: 10.3390/md8041373] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2010] [Revised: 03/31/2010] [Accepted: 04/19/2010] [Indexed: 12/19/2022] Open
Abstract
The outer vestibule of voltage-gated Na(+) channels is formed by extracellular loops connecting the S5 and S6 segments of all four domains ("P-loops"), which fold back into the membrane. Classically, this structure has been implicated in the control of ion permeation and in toxin blockage. However, conformational changes of the outer vestibule may also result in alterations in gating, as suggested by several P-loop mutations that gave rise to gating changes. Moreover, partial pore block by mutated toxins may reverse gating changes induced by mutations. Therefore, toxins that bind to the outer vestibule can be used to modulate channel gating.
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Affiliation(s)
| | | | - Peter Lukacs
- Institute of Pharmacology, Centre of Physiology and Pharmacology, Medical University of Vienna, 1090 Vienna, Austria; E-Mails:
(R.C.);
(T.Z.);
(P.L.)
| | - Hannes Todt
- Institute of Pharmacology, Centre of Physiology and Pharmacology, Medical University of Vienna, 1090 Vienna, Austria; E-Mails:
(R.C.);
(T.Z.);
(P.L.)
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8
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The tetrodotoxin binding site is within the outer vestibule of the sodium channel. Mar Drugs 2010; 8:219-34. [PMID: 20390102 PMCID: PMC2852835 DOI: 10.3390/md8020219] [Citation(s) in RCA: 91] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2009] [Revised: 01/10/2010] [Accepted: 01/28/2010] [Indexed: 12/21/2022] Open
Abstract
Tetrodotoxin and saxitoxin are small, compact asymmetrical marine toxins that block voltage-gated Na channels with high affinity and specificity. They enter the channel pore’s outer vestibule and bind to multiple residues that control permeation. Radiolabeled toxins were key contributors to channel protein purification and subsequent cloning. They also helped identify critical structural elements called P loops. Spacial organization of their mutation-identified interaction sites in molecular models has generated a molecular image of the TTX binding site in the outer vestibule and the critical permeation and selectivity features of this region. One site in the channel’s domain I P loop determines affinity differences in mammalian isoforms.
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9
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The Structural Basis and Functional Consequences of Interactions Between Tetrodotoxin and Voltage-Gated Sodium Channels. Mar Drugs 2006. [PMCID: PMC3663409] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022] Open
Abstract
Tetrodotoxin (TTX) is a highly specific blocker of voltage-gated sodium channels. The dissociation constant of block varies with different channel isoforms. Until recently, channel resistance was thought to be primarily imparted by amino acid substitutions at a single position in domain I. Recent work reveals a novel site for tetrodotoxin resistance in the P-region of domain IV.
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10
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The Structural Basis and Functional Consequences of Interactions Between Tetrodotoxin and Voltage-Gated Sodium Channels. Mar Drugs 2006. [DOI: 10.3390/md403143] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
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11
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Abstract
Saxitoxin (STX) and tetrodotoxin (TTX) are frequently used to selectively block sodium channels. In this study, we provide evidence that commercial STX also inhibits L-type Ca2+ currents (I(Ca,L)) in adult mouse ventricular myocytes (VMs) and tsA-201 cells that were transiently cotransfected with three calcium channel subunits. We measured inhibition of sodium currents (INa) in mouse VMs, of I(Ca,L) in mouse VM and tsA-201 cells, and intracellular calcium concentration ([Ca2+]i) transients in single mouse VMs. STX or TTX was abruptly applied before the test voltage pulse using a rapid solution switcher device. STX (10 microM; Calbiochem) and TTX (60 microM; Sigma-Aldrich) completely blocked INa in mouse VMs. However, STX at 10 microM also reduced I(Ca,L) in mouse VM by 39% (P < 0.0001; n = 14), whereas TTX at 60 microM had no effect on I(Ca,L). STX (10 microM; Calbiochem) reduced the amplitude of the [Ca2+]i transients in mouse VMs by 36% (P < 0.0001; n = 10). In contrast, TTX (60 microM; Sigma-Aldrich) only reduced the amplitude of the [Ca2+]i transients by 9% (P = 0.003; n = 5). STX (10 microM) obtained from Sigma-Aldrich showed a similar inhibitory effect on I(Ca,L) (33%) (P < 0.0001; n = 5) in mouse VMs. STX (Calbiochem) inhibited the calcium currents of tsA-201 cells in a dose-dependent manner. This inhibition was voltage-independent. The current-voltage relationship of calcium currents in tsA-201 cells was not altered by STX. These results indicate that STX partially blocks L-type Ca2+ channels and thus provide further evidence that its effects are not specific for Na+ channels.
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Affiliation(s)
- Zhi Su
- Cardiology Division, University of Utah Health Sciences Center, Salt Lake City, Utah 84132, USA
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12
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Keynes RD, Greeff NG, Forster IC. Activation, inactivation and recovery in the sodium channels of the squid giant axon dialysed with different solutions. Philos Trans R Soc Lond B Biol Sci 1997; 337:471-84. [PMID: 1359591 DOI: 10.1098/rstb.1992.0122] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Comparisons were made between families of ion currents recorded in voltage-clamped squid axons dialysed with 20 mM NaF and 330 mM CsF or TMAF, and bathed in a solution in which four fifths of the Na was replaced by Tris. The permeability coefficient
P
Na,fast
for the fast-inactivating current in the initial open state was calculated as a function of test potential from the size of the initial peak of
I
Na
. The permeability coefficient
P
Na,non
for the non-inactivating open state was calculated from the steady-state
I
Na
that persisted until the end of the test pulse. Dialysis with TMA had no direct effect on the
Qv
curve for gating charge. The reversal potential for
I
Na,non
was always lower than that for
I
Na,fast
, the mean difference being about — 9 mV when dialysing with Cs, but only about — 1 mV with TMA. Except close to threshold,
P
Na,fast
was roughly halved by dialysis with TMA as compared with Cs, but
P
Na,non
was substantially increased. The time constant τ
h
for inactivation of the sodium system was slightly increased during dialysis with TMA in place of Cs, and there were small shifts in the steady-state inactivation curve, but the rate of recovery from inactivation was not measurably altered. The flattening off of the τ
h
curve at increasingly positive test potentials corresponded to a steady reduction of the apparent inactivation charge until a value of about 0.2
e
was reached for pulses to 100 mV. The instantaneous
I-V
relationship in the steady state was also investigated. The results have a useful bearing on the effects of dialysis with TMA, on the differences between the initial and steady open states of the sodium channel, and on the relative voltage-dependences of the transitions in each direction between the resting and inactivated states.
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13
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Conti F, Gheri A, Pusch M, Moran O. Use dependence of tetrodotoxin block of sodium channels: a revival of the trapped-ion mechanism. Biophys J 1996; 71:1295-312. [PMID: 8874004 PMCID: PMC1233597 DOI: 10.1016/s0006-3495(96)79330-x] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
The use-dependent block of sodium channels by tetrodotoxin (TTX) has been studied in cRNA-injected Xenopus oocytes expressing the alpha-subunit of rat brain IIA channels. The kinetics of stimulus-induced extra block are consistent with an underlying relaxation process involving only three states. Cumulative extra block induced by repetitive stimulations increases with hyperpolarization, with TTX concentration, and with extracellular Ca2+ concentration. We have developed a theoretical model based on the suggestion by Salgado et al. that TTX blocks the extracellular mouth of the ion pore less tightly when the latter has its external side occupied by a cation, and that channel opening favors a tighter binding by allowing the escape of the trapped ion. The model provides an excellent fit of the data, which are consistent with Ca2+ being more efficient than Na+ in weakening TTX binding and with bound Ca2+ stabilizing the closed state of the channel, as suggested by Armstrong and Cota. Reports arguing against the trapped-ion mechanism are critically discussed.
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Affiliation(s)
- F Conti
- Istituto di Cibernetica e Biofisica, CNR, Genoa, Italy.
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14
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Abstract
When Hodgkin & Huxley (1952) first embarked on the analysis of their voltageclamp data on the ionic currents in the squid giant axon, they hoped to be able to deduce a mechanism from it, but it soon became clear that the electrical data would by themselves yield only very general information about the class of system likely to be involved.
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Affiliation(s)
- R D Keynes
- Physiological Laboratory, Cambridge, England
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15
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Fleig A, Fitch JM, Goldin AL, Rayner MD, Starkus JG, Ruben PC. Point mutations in IIS4 alter activation and inactivation of rat brain IIA Na channels in Xenopus oocyte macropatches. Pflugers Arch 1994; 427:406-13. [PMID: 7971139 DOI: 10.1007/bf00374254] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Macroscopic currents of wild-type rat brain IIA (RBIIA) and mutant Na channels were recorded in excised patches from Xenopus oocytes. A charge deletion (K859Q) and an adjacent conservative mutation (L860F) in the second domain S4 membrane-spanning region differentially altered voltage sensitivity and kinetics. Analysis of voltage dependence was confined to Na currents with fast inactivation kinetics, although RBIIA and K859Q (but not L860F) also showed proportional shifts between at least two gating modes, rendering currents with fast or slow inactivation kinetics, respectively. Compared to RBIIA, the midpoint of the activation curve was shifted in both K859Q and L860F by 22 mV to more positive potentials, yet this shift was not associated with a corresponding change in the voltage dependence of time constants for activation (tau a) or inactivation (tau h1, tau h2). L860F showed faster activation time constants tau a than RBIIA, while K859Q was slower for both the activation (tau a) and the inactivation components (tau h1). Similarly, the steady-state inactivation curve of L860F but not K859Q shifted by 9 mV in the hyperpolarizing direction. Thus, the fourth charge in the IIS4 transmembrane segment exerts control over voltage sensitivity and kinetics of activation and may interact with structure that influence other aspects of channel gating.
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Affiliation(s)
- A Fleig
- Department of Physiology, John A. Burns School of Medicine, University of Hawaii, Honolulu 96822
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16
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Hanck DA, Makielski JC, Sheets MF. Kinetic effects of quaternary lidocaine block of cardiac sodium channels: a gating current study. J Gen Physiol 1994; 103:19-43. [PMID: 8169596 PMCID: PMC2216854 DOI: 10.1085/jgp.103.1.19] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
The interaction of antiarrhythmic drugs with ion channels is often described within the context of the modulated receptor hypothesis, which explains the action of drugs by proposing that the binding site has a variable affinity for drugs, depending upon whether the channel is closed, open, or inactivated. Lack of direct evidence for altered gating of cardiac Na channels allowed for the suggestion of an alternative model for drug interaction with cardiac channels, which postulated a fixed affinity receptor with access limited by the conformation of the channel (guarded receptor hypothesis). We report measurement of the gating currents of Na channels in canine cardiac Purkinje cells in the absence and presence of QX-222, a quaternary derivative of lidocaine, applied intracellularly, and benzocaine, a neutral local anesthetic. These data demonstrate that the cardiac Na channel behaves as a modulated rather than a guarded receptor in that drug-bound channels gate with altered kinetics. In addition, the results suggest a new interpretation of the modulated receptor hypothesis whereby drug occupancy reduces the overall voltage-dependence of gating, preventing full movement of the voltage sensor.
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Affiliation(s)
- D A Hanck
- Department of Medicine, University of Chicago, Illinois 60637
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Starkus JG, Rayner MD, Fleig A, Ruben PC. Fast and slow inactivation of sodium channels: effects of photodynamic modification by methylene blue. Biophys J 1993; 65:715-26. [PMID: 8218899 PMCID: PMC1225774 DOI: 10.1016/s0006-3495(93)81098-1] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
Illumination of crayfish giant axons, during internal perfusion with 0.5 mM methylene blue (MB), produces photodynamic effects that include (i) reduction in total sodium conductance, (ii) shifting of the steady-state inactivation curve to the right along the voltage axis, (iii) reduction in the effective valence of steady-state inactivation and, (iv) potentially complete removal of fast inactivation. Additionally, the two kinetic components of fast inactivation in crayfish axons are differentially affected by MB+light. The intercept of the faster component (tau h1) is selectively reduced at shorter MB+light exposure times. Neither tau h1 nor the slower (tau h2) process was protected from MB+light by prior steady-state inactivation of sodium channels. However, carotenoids provide differing degrees of protection against each of the photodynamic actions listed above, suggesting that the four major effects of MB+light are mediated by changes occurring within different regions of the sodium channel molecule.
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Affiliation(s)
- J G Starkus
- Bekesy Laboratory of Neurobiology, John A. Burns School of Medicine, University of Hawaii, Honolulu 96822
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18
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Keynes RD, Meves H, Hof D. The dual effect of internal tetramethylammonium ions on the open states of the sodium channel in the squid giant axon. Proc Biol Sci 1992; 249:101-6. [PMID: 1359545 DOI: 10.1098/rspb.1992.0090] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Voltage-clamp recordings of INa in squid axons dialysed with Cs or TMA, and bathed in low Na choline seawater, showed that, except close to threshold, the initial peak of fast-inactivating current was invariably decreased by TMA, whereas the non-inactivating current in the steady state was simultaneously increased. The results suggest that although TMA does not act directly on the movements of the voltage sensors that activate the sodium system, it blocks single-channel conductance in a voltage-dependent fashion in both the open states of the Na channel, while it has an entirely different type of action by increasing the probability of late openings in the steady state. Another difference between the two open states was that the sodium permeability coefficient had a Q10 of 1.8 in the initial open state, whereas in the steady state the effect of temperature was much smaller or even negative.
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Affiliation(s)
- R D Keynes
- Physiological Laboratory, University of Cambridge, U.K
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19
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Ruben PC, Starkus JG, Rayner MD. Steady-state availability of sodium channels. Interactions between activation and slow inactivation. Biophys J 1992; 61:941-55. [PMID: 1316183 PMCID: PMC1260353 DOI: 10.1016/s0006-3495(92)81901-x] [Citation(s) in RCA: 54] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Changes in holding potential (Vh), affect both gating charge (the Q(Vh) curve) and peak ionic current (the F(Vh) curve) seen at positive test potentials. Careful comparison of the Q(Vh) and F(Vh) distributions indicates that these curves are similar, having two slopes (approximately 2.5e for Vh from -115 to -90 mV and approximately 4e for Vh from -90 to -65 mV) and very negative midpoints (approximately -86 mV). Thus, gating charge movement and channel availability appear closely coupled under fully-equilibrated conditions. The time course by which channels approach equilibration was explored using depolarizing prepulses of increasing duration. The high slope component seen in the F(Vh) and Q(Vh) curves is not evident following short depolarizing prepulses in which the prepulse duration approximately corresponds to the settling time for fast inactivation. Increasing the prepulse duration to 10 ms or longer reveals the high slope, and left-shifts the midpoint to more negative voltages, towards the F(Vh) and Q(Vh) distributions. These results indicate that a separate slow-moving voltage sensor affects the channels at prepulse durations greater than 10 ms. Charge movement and channel availability remain closely coupled as equilibrium is approached using depolarizing pulses of increasing durations. Both measures are 50% complete by 50 ms at a prepulse potential of -70 mV, with proportionately faster onset rates when the prepulse potential is more depolarized. By contrast, charge movement and channel availability dissociate during recovery from prolonged depolarizations. Recovery of gating charge is considerably faster than recovery of sodium ionic current after equilibration at depolarized potentials. Recovery of gating charge at -140 mV, is 65% complete within approximately 100 ms, whereas less than 30% of ionic current has recovered by this time. Thus, charge movement and channel availability appear to be uncoupled during recovery, although both rates remain voltage sensitive. These data suggest that channels remain inactivated due to a separate process operating in parallel with the fast gating charge. We demonstrate that this behavior can be simulated by a model in which the fast charge movement associated with channel activation is electrostatically-coupled to a separate slow voltage sensor responsible for the slow inactivation of channel conductance.
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Affiliation(s)
- P C Ruben
- Békésy Laboratory of Neurobiology, Pacific Biomedical Research Center, Honolulu, Hawaii
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20
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Rayner MD, Starkus JG, Ruben PC, Alicata DA. Voltage-sensitive and solvent-sensitive processes in ion channel gating. Kinetic effects of hyperosmolar media on activation and deactivation of sodium channels. Biophys J 1992; 61:96-108. [PMID: 1311613 PMCID: PMC1260226 DOI: 10.1016/s0006-3495(92)81819-2] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Kinetic effects of osmotic stress on sodium ionic and gating currents have been studied in crayfish giant axons after removal of fast inactivation with chloramine-T. Internal perfusion with media made hyperosmolar by addition of formamide or sucrose, reduces peak sodium current (before and after removal of fast inactivation with chloramine-T), increases the half-time for activation, but has no effect on tail current deactivation rate(s). Kinetics of ON and OFF gating currents are not affected by osmotic stress. These results confirm (and extend to sodium channels) the separation of channel gating mechanisms into voltage-sensitive and solvent-sensitive processes recently proposed by Zimmerberg J., F. Bezanilla, and V. A. Parsegian. (1990. Biophys. J. 57:1049-1064) for potassium delayed rectifier channels. Additionally, the kinetic effects produced by hyperosmolar media seem qualitatively similar to the kinetic effects of heavy water substitution in crayfish axons (Alicata, D. A., M. D. Rayner, and J. G. Starkus. 1990. Biophys. J. 57:745-758). However, our observations are incompatible with models in which voltage-sensitive and solvent-sensitive gating processes are presumed to be either (a) strictly sequential or, (b) parallel and independent. We introduce a variant of the parallel model which includes explicit coupling between voltage-sensitive and solvent-sensitive processes. Simulations of this model, in which the total coupling energy is as small as 1/10th of kT, demonstrate the characteristic kinetic changes noted in our data.
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Affiliation(s)
- M D Rayner
- Pacific Biomedical Research Center, Békésy Laboratory of Neurobiology, Honolulu, Hawaii 96822
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21
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Keynes RD, Greeff NG, Forster IC, Bekkers JM. The effect of tetrodotoxin on the sodium gating current in the squid giant axon. Proc Biol Sci 1991; 246:135-40. [PMID: 1685237 DOI: 10.1098/rspb.1991.0135] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
The effect of tetrodotoxin (TTX) on the sodium gating current in the squid giant axon was examined by recording the current that flowed at the pulse potential at which the ionic current fell to zero, first in the absence and then in the presence of TTX. The addition of 1 microM TTX to the bathing solution had no consistent effect on the size of the initial peak of the gating current, but resulted in small changes in the timecourse of its subsequent relaxation which were mainly caused by a reduction of about one quarter in the component that has a delayed onset and may possibly arise from changes in the state of ionization of groups in the channel wall when the lumen fills with water. Our findings suggest that the binding of TTX at the outer face of the sodium channel does not interfere with the mechanisms of activation and inactivation by the voltage sensors, but has an allosteric effect on the access of internal cations to the inside of the channel.
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22
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Abstract
Effects of changes in initial conditions on the magnitude and kinetics of gating current and sodium current were studied in voltage-clamped, internally-perfused, crayfish giant axons. We examined the effects of changes in holding potential, inactivating prepulses, and recovery from inactivation in axons with intact fast inactivation. We also studied the effects of brief interpulse intervals in axons pretreated with chloramine-T for removal of fast inactivation. We find marked effects of gating current kinetics induced by both prepulse inactivation and brief interpulse intervals. The apparent changes in gating current relaxation rates cannot be explained simply by changes in gating charge magnitude (charge immobilization) combined with "Cole-Moore-type" time shifts. Rather they appear to indicate selective suppression of kinetically-identifiable components within the control gating currents. Our results provide additional support for a model involving parallel, nonidentical, gating particles.
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Affiliation(s)
- J G Starkus
- Pacific Biomedical Research Center, Békésy Laboratory of Neurobiology, Honolulu, Hawaii
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23
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Patton DE, Goldin AL. A voltage-dependent gating transition induces use-dependent block by tetrodotoxin of rat IIA sodium channels expressed in Xenopus oocytes. Neuron 1991; 7:637-47. [PMID: 1657057 DOI: 10.1016/0896-6273(91)90376-b] [Citation(s) in RCA: 64] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
We have utilized molecular biological techniques to demonstrate that rat IIA sodium channels expressed in Xenopus oocytes were blocked by tetrodotoxin (TTX) in a use-dependent manner. This use dependence was the result of an increased affinity of the channels for TTX upon depolarization, most likely due to a conformational change in the channel. Using a mutant with a slower macroscopic rate of inactivation, we have demonstrated that this conformational change is not the transition into the fast-inactivated state. The transition is probably one occurring during activation of the channel, as suggested by the fact that one sodium channel mutant demonstrated comparable depolarizing shifts in the voltage dependence of both activation and use-dependent block by TTX. The transition occurred at potentials more negative than those resulting in channel conductance, suggesting that the conformational change that causes use-dependent block by TTX is a closed-state voltage-dependent gating transition.
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Affiliation(s)
- D E Patton
- Department of Microbiology and Molecular Genetics, University of California, Irvine 92717
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24
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Ruben PC, Starkus JG, Rayner MD. Holding potential affects the apparent voltage-sensitivity of sodium channel activation in crayfish giant axons. Biophys J 1990; 58:1169-81. [PMID: 1963328 PMCID: PMC1281062 DOI: 10.1016/s0006-3495(90)82458-9] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
Sodium channel activations, measured as the fraction of channels open to peak conductance for different test potentials (F[V]), shows two statistically different slopes from holding potential more positive than -90 mV. A high valence of 4-6e is indicated a test potentials within 35 mV of the apparent threshold potential (circa -65 mV at -85 mV holding potential). However, for test potentials positive to -30 mV, the F(V) curve shows a 2e valence. The F(V) curve for crayfish axon sodium channels at these "depolarized" holding potentials thus closely resembles classic data obtained from other preparations at holding potentials between -80 and -60 mV. In contrast, at holding potentials more negative than -100 mV, the high slope essentially disappears and the F(V) curve follows a single Boltzmann distribution with a valence of approximately 2e at all potentials. Neither the slope of this simple distribution nor its midpoint (-20 mV) was significantly affected by removal of fast inactivation with pronase. The change in F(V) slope, when holding potential is increased from -85 to -120 mV, does not appear to be caused by the contribution of a second channel type. The simple voltage dependence of sodium current found at Vh -120 mV be used by to discriminate between models of sodium channel activation, and rules out models with three particles of equal valence.
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Affiliation(s)
- P C Ruben
- Békésy Laboratory of Neurobiology, Pacific Biomedical Research Center, University of Hawaii, Honolulu 96822
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25
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Hanck DA, Sheets MF, Fozzard HA. Gating currents associated with Na channels in canine cardiac Purkinje cells. J Gen Physiol 1990; 95:439-57. [PMID: 2157792 PMCID: PMC2216329 DOI: 10.1085/jgp.95.3.439] [Citation(s) in RCA: 32] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
Gating currents (Ig) were recorded in single canine cardiac Purkinje cells at 10-12 degrees C. Ig characteristics corresponded closely to macroscopic INa characteristics and appeared to exhibit little contamination from other voltage-gated channels. Charge density predicted by peak INa was 0.14-0.22 fC micron -2 and this compared well with the measured value of 0.19 +/- 0.10 fC micron -2 (SD; n = 28). The charge-voltage relationship rose over a voltage similar to the peak INa conductance curve. The midpoints of the two relationships were not significantly different although the conductance curve was 1.5 +/- 0.3 (SD; n = 9) times steeper. Consistent with this observation, which predicted that a large amount of the gating charge would be associated with transitions close to the open state, an analysis of activation from Hodgkin-Huxley fits to the macroscopic currents showed that tau m corresponded well with a prominent component of Ig. Ig relaxations fitted two exponentials better than one over the range of voltages in which Na channels were activated. When the holding potential was hyperpolarized, relaxation of Ig during step depolarizations to 0 mV was prolonged but there was no substantial increase in charge, further suggesting that early closed-state transitions are less in charge, further suggesting that early closed-state transitions are less voltage dependent. The single cardiac Purkinje cell appears to be a good candidate for combining Ig and single-channel measurements to obtain a kinetic description of the cardiac Na channel.
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Affiliation(s)
- D A Hanck
- Department of Medicine, University of Chicago, Illinois 60637
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26
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Lönnendonker U. Use-dependent block of sodium channels in frog myelinated nerve by tetrodotoxin and saxitoxin at negative holding potentials. BIOCHIMICA ET BIOPHYSICA ACTA 1989; 985:153-60. [PMID: 2553115 DOI: 10.1016/0005-2736(89)90360-x] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Na+ currents were measured in myelinated frog nerve fibres in the presence of nanomolar concentrations of tetrodotoxin (TTX) or saxitoxin (STX) in the extracellular solution. The Na+ currents declined during a train of depolarizing pulses if the fibre was held at hyperpolarizing potentials between the pulses. At a pulse frequency of 0.8 Hz, the peak Na+ currents were reduced to 70 or 60% of the initial value in 9.3 nM TTX and 3.5 nM STX solutions, respectively. A decline of Na+ currents was also observed in two-pulse experiments. The peak Na+ current during a second test pulse did not depend on the duration (0.2 to 12 ms) of the first pulse. It decreased with increasing interval between the pulses, reached a minimum and increased again. The results are interpreted with a use-dependent blockage of Na+ channels by TTX or STX at negative holding potentials. The effects were described quantitatively, assuming a fast affinity increase of toxin receptors at Na+ channels triggered by Na+ activation followed by slow toxin binding to channels and relaxation of the receptor affinity.
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Affiliation(s)
- U Lönnendonker
- Physiologisches Institut der Universität des Saarlandes, Homburg/Saar, F.R.G
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27
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Pohl JA. Recovery from charge immobilization in sodium channels of the frog node of Ranvier. Pflugers Arch 1989; 414:516-22. [PMID: 2550889 DOI: 10.1007/bf00580986] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
The gating current off-response of the frog node of Ranvier shows a fast and a slow phase, reflecting presumably charges moving back into the resting position and charges returning from immobilization. The paper describes measurements of the time constant of the slow component, tau off2, at different potentials at 20 or 17 degrees C. The time constant tau off2 decreased markedly when the potential to which the fibre was repolarized at the end of the test pulses was decreased from -100 to -145 mV. tau off2 was compared with tau re gat and tau re Na, the time constants of the recovery of gating current and peak sodium current from the effect of a depolarizing conditioning pulse. The three time constants were equal at -145 mV, but somewhat different at -100 to -130 mV, the sequence being tau re gat greater than tau off2 greater than tau re Na. The inequality tau re gat greater than tau off2 was small and statistically not significant. It does not seem to be due to contamination of the charge movement with ionic currents because a) the Qoff/Qon ratio was near 1.0 at -100 to -120 mV, b) partial replacement of internal CsCl by KCl did not significantly alter tau off2 or Qoff/Qon, c) tau off2 was independent of pulse size. The small inequality tau re gat greater than tau off2 suggests that charges which have returned from immobilization are not immediately available for displacement. The inequality tau re gat greater than tau re Na was larger and statistically significant at -100 and -110 mV.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- J A Pohl
- I. Physiologisches Institut, Universität des Saarlandes, Homburg/Saar, Federal Republic of Germany
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28
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Alicata DA, Rayner MD, Starkus JG. Osmotic and pharmacological effects of formamide on capacity current, gating current, and sodium current in crayfish giant axons. Biophys J 1989; 55:347-53. [PMID: 2540848 PMCID: PMC1330477 DOI: 10.1016/s0006-3495(89)82811-5] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Internal perfusion with solutions made hyperosmolar by 10% formamide selectively reduces the initial fast component of ON gating current (fast Ig) in crayfish axons. This result parallels the effects of formamide perfusion seen in Myxicola giant axons (Schauf, C. L., and M. A. Chuman. 1986. Neural Membranes. Alan R. Liss, Inc., New York. 3-23). However, our findings do not confirm their conclusion that internal formamide has a specific pharmacological effect on fast Ig. Formamide-induced suppression of fast Ig is always associated with changes in linear capacity current, indicating a reduction in the rate of rise of the voltage clamp. Furthermore, this suppression of fast Ig can be reversed when clamp rise time is returned to its control rate by increasing compensation for series resistance (Rs) during formamide perfusion. Increases in Rs during 10% formamide perfusion of up to 5 omega.cm2 were measured by evaluating the increase in Rs compensation required to return the following parameters to their control levels: (a) peak capacity current, (b) peak gating current, (c) the voltage maximum of the /Na-V curve, and (d) "tau h". We conclude that hyperosmolar internal formamide increases Rs, reduces clamp speed, and thus selectively suppresses fast Ig. On the other hand, the reversible block of sodium ionic current by internal formamide, reported by Schauf and Chuman, is not eliminated by correcting for series resistance changes during formamide perfusion.
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Affiliation(s)
- D A Alicata
- Department of Physiology, John A. Burns School of Medicine, University of Hawaii, Honolulu 96822
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29
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Vassilev PM, Tien HT. Reconstitution of membrane molecular mechanisms in bilayer lipid membranes and patch-clamp bilayers. Subcell Biochem 1989; 14:97-143. [PMID: 2655201 DOI: 10.1007/978-1-4613-9362-7_3] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
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30
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Abstract
Progressive shifts of holding potential (Vh) in crayfish giant axons, from -140 to -70 mV, reduce gating currents seen in depolarizing steps (to 0 mV test potential) while proportionately increasing gating currents in hyperpolarizing steps (to -240 mV). The resulting sigmoid equilibrium charge distribution (Q-Vh curve) shows an effective valence of 1.9e and a midpoint of -100 mV. By contrast, Q-V curves obtained using hyperpolarizing and/or depolarizing steps from a single holding potential, change their "shape" depending on the chosen holding potential. For holding potentials at the negative end of the Q-Vh distribution (e.g., -140 mV), negligible charge moves in hyperpolarizing pulses and the Q-V curve can be characterized entirely from depolarizing voltage steps. The slope of the resulting simple sigmoid Q-V curve also indicates an effective valence of 1.9e. When the axon is held at less negative potentials significant charge moves in hyperpolarizing voltage steps. The component of the Q-V curve collected using hyperpolarizing pulses shows a significantly reduced slope (approximately 0.75e) by comparison with the 1.9e slope found using depolarizing pulses or from the Q-Vh curve. As holding potential is shifted in the depolarizing direction along the Q-Vh curve, an increasing fraction of total charge movement must be assessed in hyperpolarizing voltage steps. Thus charge moving in the low slope component of the Q-V curve increases as holding potential is depolarized, while charge moving with high apparent valence decreases proportionately. Additional results, together with simulations based on a simple kinetic model, suggest that the reduced apparent valence of the low slope component of the Q-V curve results from gating charge immobilization occurring at holding potential. Immobilization selectively retards that fraction of total charge moving in hyperpolarizing pulses. Misleading conclusions, as to the number and valence of the gating particles, may therefore be derived from Q-V curves obtained by other than depolarizing pulses from negative saturated holding potentials.
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Affiliation(s)
- M D Rayner
- Department of Physiology, John A. Burns School of Medicine, University of Hawaii, Honolulu 96822
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31
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Garritsen A, Ijzerman AP, Soudijn W. [3H]batrachotoxinin-A 20-alpha-benzoate binding to sodium channels in rat brain: sensitivity to tetrodotoxin and divalent cations. Eur J Pharmacol 1988; 145:261-6. [PMID: 2450762 DOI: 10.1016/0014-2999(88)90428-1] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
The sodium channel blocker tetrodotoxin partially inhibited [3H]batrachotoxinin-A 20-alpha-benzoate binding to rat brain synaptosomes. This inhibition results from a decrease in the apparent affinity of the radioligand, which indicates that sites 1 and 2 of the sodium channel are not independent as thought previously. Computer-assisted data analysis allowed two binding sites for BTX-B to be distinguished. These sites could be differentiated by means of the divalent cations Mg2+ and Ca2+, that inhibit BTX-B binding completely. Tetrodotoxin diminished the inhibition by Mg2+ and vice versa, suggesting a common mechanism of action for the inhibition.
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Affiliation(s)
- A Garritsen
- Division of Medicinal Chemistry, State University Leiden, The Netherlands
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32
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Sorokina ZA, Chizhmakov IV. Mechanism underlying the action of blocking toxins on voltage-operated sodium channels: Recent advances. NEUROPHYSIOLOGY+ 1988. [DOI: 10.1007/bf01058052] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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