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Structure of the squid axon membrane as derived from charge-pulse relaxation studies in the presence of absorbed lipophilic ions (1981)

Benz, Roland ; Conti, Franco

Springer

The journal of membrane biology 59 (1981), S. 91-104

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ISSN: 1432-1424

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Chemistry and Pharmacology

Notes: Summary Charge-pulse relaxation studies were performed on squid giant axons in the presence of membrane absorbed lipophilic anions, dipicrylamine (DPA) and tetraphenylborate (TPhB), and of specific blockers of sodium and potassium active currents. With the instrumentation used in this work a time resolution of 5 to 10 μsec was easily obtained without any averaging, although the voltage relaxations were always smaller than 5 mV in amplitude in order to keep the membrane voltage in a range where the used theory cyn be linearized. Two well distinguishable linear relaxations were invariably observed in the presence of the lipophilic anions. With DPA the fast relaxation (time constants between 8 and 70 μsec) was attributed to the redistribution of the lipophilic ions within the membrane following the change in membrane potential. The long relaxation process (time constant in the millisecond range) corresponds to the normal voltage relaxation of the passive squid axon membrane slightly modified by the process of redistribution of the extrinsic ions. The results support the same model for the translocation of lipophilic ions within the nerve membrane proposed earlier for artificial lipid bilayers. The fit of the data with a single barrier model yields the translocation rate constant,K, and the total concentration,N t , of membrane absorbed ions, from which the membrane-solution partition coefficient, β, can be derived. Both for DPA and TPhB,K had values close to those measured for solvent-free artificial lipid bilayers. The axon membrane appears as fluid mosaic membrane with a thickness of about 2.5 nm for the lipid bilayer part. In axons treated with DPA the dependence of relaxation data upon the holding membrane potential, $$\bar E_m$$ , provided information on the asymmetry of the membrane structure. The data were best fitted by assuming that nearly 100% of the membrane potential drops between the two free energy minima where the extrinsic ions are located, indicating that these minima lie very close to the membrane-solution interfaces, in the region of the phospholipid polar heads. The asymmetry voltage,E o, at which the extrinsic ions are expected to be equally distributed between the two sides of the membrane was found to range between −35 and −65 mV (inside negative), depending on the assumed shape of the free energy barrier describing the ion translocation process. This voltage is of the same sign and of the same order of magnitude as the equilibrium voltages for the open-close transitions of the gates of sodium and potassium channels, suggesting that all these voltages result from the same membrane asymmetry. A similar analogy was found between the asymmetry of the free energy barrier which best fitted DPA relaxation data and the asymmetrical voltage dependence of the gating of ionic channels. Our data were best fitted by assuming that about 70% of the potential drop occurs between the free energy minimum on the intracellular membrane face and the top of the barrier.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01875707

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Structural parts involved in activation and inactivation of the sodium channel (1989)

Stühmer, Walter ; Conti, Franco ; Suzuki, Harukazu ; [et al.]

[s.l.] : Nature Publishing Group

Nature 339 (1989), S. 597-603

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] Structure–function relationships of the sodium channel expressed inXenopus oocytes have been investigated by the combined use of site–directed mutagenesis and patch-clamp recording. This study provides evidence that the positive charges in segment S4 are involved in the ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/339597a0

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3

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Single channel recordings of K + currents in squid axons (1980)

Conti, Franco ; Neher, Erwin

[s.l.] : Nature Publishing Group

Nature 285 (1980), S. 140-143

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] Ionic currents from individual K+ channels in squid axon membrane have been recorded. At hyperpolarizing membrane voltages, unit events occur as widely spaced rectangular pulses with short interruptions. The frequency of occurrence of the units increases strongly when the ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/285140a0

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4

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Activation and deactivation properties of rat brain K+ channels of the Shaker-related subfamily (1994)

Bertoli, Alessandro ; Moran, Oscar ; Conti, Franco

Springer

European biophysics journal 23 (1994), S. 379-384

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ISSN: 1432-1017

Keywords: Potassium channels ; Rat brain ; Shaker ; Oocyte expression

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Physics

Notes: Abstract We studied the activation properties of members of the Shaker-related subfamily of voltage-gated K+ channels cloned from rat brain and expressed in Xenopus oocytes. We find that Kv1.1, Kv1.4, Kv1.5, and Kv1.6 have similar activation and deactivation kinetics. The K+ currents produced by step depolarisations increase with a sigmoidal time course that can be described by a delay and by the derivative of the current at the inflection point. The delay tends to zero and the logarithmic derivative seems to approach a finite value at large positive voltages, but these asymptotic values are not yet reached at +80 mV. Deactivation of the currents upon stepping to negative membrane potentials below -60 mV is fairly well described by a single exponential. The decrease of the deactivation time constant at increasingly negative voltages tends to become less steep, indicating that this parameter also has a finite limiting value, which is not yet reached, however, at −160 mV The various clones studied have very similar voltage dependencies of activation with half-activation voltages ranging between −50 and −11 mV and maximum steepness yielding an e-fold change for voltage increments between 3.8 and 7.0 mV The shallower activation curve of Kv1.4 is likely to be due to coupling with the fast inactivation process present in this clone.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00188662

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Fast- and Slow-Gating Modes of the Sodium Channel Are Altered by a Paramyotonia Congenita-Linked Mutation (1998)

Moran, Oscar ; Melani, Raffaella ; Nizzari, Mario ; [et al.]

Springer

Journal of bioenergetics and biomembranes 30 (1998), S. 579-588

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ISSN: 1573-6881

Keywords: Sodium channel ; paramyotonia congenita ; inactivation ; modal gating ; oocytes ; hereditary disease

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Chemistry and Pharmacology , Physics

Notes: Abstract We have studied the expression in frog oocytes of the α subunit of the rat skeletal muscle sodium channel mutation T1306M, homologous to the mutation T1313M of the human isoform that causes the muscular hereditary disease paramyotonia congenita. Wild-type (WT) channels show a bimodal behavior, with two gating modes characterized by inactivation time constants that differ at least by one order of magnitude and with voltage dependencies shifted by +27 mV in the slow mode (M2) relative to the fast (M1) mode. In the myopathy-linked mutant the propensity of the channel for the mode M2 is increased fourfold and the kinetics and voltage dependence of inactivation in both modes are altered. In mode M1, the onset of inactivation is faster and the recovery from inactivation is slower whereas both processes are slowed in mode M2. The half-inactivation potential of both modes is shifted by the mutation to positive potentials. Coexpression of β subunit causes a threefold reduction of the M2 propensity of both WT and T1306M channels, with small changes in the voltage dependency and kinetic properties of inactivation. All the changes are consistent with the hyperexcitability of the muscle fibers observed in patients affected by potassium-aggrevated myotonia (PAM).

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1020536601658

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6

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Myopathic Mutations Affect Differently the Inactivation of the Two Gating Modes of Sodium Channels (1999)

Moran, Oscar ; Nizzari, Mario ; Conti, Franco

Springer

Journal of bioenergetics and biomembranes 31 (1999), S. 591-608

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ISSN: 1573-6881

Keywords: Heterologous expression ; oocytes ; modal gate ; skeletal muscle ; paramyotonia congenita ; hyperkalemic periodic paralysis ; potassium aggravated myotonia

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Chemistry and Pharmacology , Physics

Notes: Abstract Three groups of mutations of the α subunit of the rat skeletal muscle sodium channel (rSkM1),homologous to mutations linked to human muscle hereditary diseases, have been studied byheterologous expression in frog oocytes: S798F, G1299E, G1299V, and G1299A, linked withpotassium-aggravated myotonia (PAM); T1306M, R1441C and R1441P, linked withparamyotonia congenita (PC); T698M and M1353V, linked with the hyperkalemic periodic paralysis(HyPP). Wild-type rSkM1 channels (WT) show two gating modes, M1 and M2, which differmainly in the process of inactivation. The naturally most representative mode M1 is tenfoldfaster and develops at ∼30 mV less depolarized potentials. A common feature ofmyopathy-linked mutants is an increase in the mode M2 probability, PM2, but phenotype-specific alterationsof voltage-dependence and kinetics of inactivation of both modes are also observed. Thecoexpression of the sodium channel β1 subunit, which has been studied for WT and for thefive best expressing mutants, generally caused a threefold reduction of PM2 without changingthe properties of the individual modes. This indicates that the mutations do not affect theα − β1 interaction and that the phenotypic changes in PM2 observed for the enhanced mode M2behavior of the sole α subunits, although largely depressed in the native tissue, are likely to bethe most important functional modification that causes the muscle hyperexcitability observed inall patients carrying the myotonic mutations. The interpretation of the more phenotype-specificchanges revealed by our study is not obvious, but it may offer clues for understanding the differentclinical manifestations of the diseases associated with the various mutations.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1005473129183

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