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H M Fishman

Publications and source records attributed to H M Fishman.

At least 37 records · Page 2Linked to original sources
PubMed1992

Assessment of conduction properties and thermal noise in cell membranes by admittance spectroscopy.

Advances in the speed of signal processing enable application of a Fourier-synthesized function as a small perturbation (1 mV) superposed on voltage clamp steps to rapidly (< 1 sec) acquire cell membrane complex driving-point functions (impedance or admittance) in several frequency bands ranging from 1 Hz to 10 kHz. Curve fits of admittance models to these data yield a complete quantitative linear description of membrane conduction systems and their kinetics. Furthermore, the rate constants between microscopic states of an ion channel can be calculated from conductance parameters derived from model curve fits of membrane admittances. Additionally, the power spectrum of membrane thermal noise is obtainable from impedance determinations by use of the Nyquist relation. Consequently, rapid driving-point function determinations provide the most complete macroscopic assessment of membrane conduction properties presently available. Admittance determinations of the potassium conduction system in squid giant axon and the potassium conducting "inward rectifier" in snail neuron are used to illustrate the above points.

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PubMed1991

Determination of K(+)-channel relaxation times in squid axon membrane by Hodgkin-Huxley and by direct linear analysis.

An assumption in the use of the Hodgkin-Huxley (HH) formulation (A.L. Hodgkin and A.F. Huxley, J. Physiol. 117 (1952) 500) to extract kinetic parameters from ion conductance responses to step voltage changes across biological membranes is that estimates obtained from such an analysis are equivalent to those obtained by direct, small-perturbation analysis. Comparison of the estimates of the K(+)-conductance relaxation time, tau n, derived from HH vs rapid, complex admittance determinations in the same squid giant axons shows significant differences for the same step changes over a 60 mV range from holding (-65 mV). The admittance determinations (2.5-5000 Hz) are shown to satisfy criteria of linear analysis (i.e., estimates are equivalent to a small-step analysis and are time invariant). The discrepancies between the two methods arise from the fact that the HH power-law description for a constant power does not yield a best fit of data over the voltage range examined and thus best estimates of tau n are power dependent. Furthermore, the large step changes in membrane voltage may excite nonlinear modes unrelated to conductance gating that contaminate the data to which the nonlinear formulation is applied to estimate linear kinetic parameters. Thus, the long-standing assumption that application of the HH methodology and empiricism is equivalent to a direct linear analysis is not substantiated. This result suggests that in comparisons between microscopic and macroscopic conduction data, microkinetic parameters derived from analysis of single ion-channel data should not be compared to macrokinetic parameters from a large population of the channel derived by HH analysis.

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PubMed1990

Injury-induced vesiculation and membrane redistribution in squid giant axon.

Injury of isolated squid giant axons in sea water by cutting or stretching initiates the following unreported processes: (i) vesiculation in the subaxolemmal region extending along the axon several mm from the site of injury, followed by (ii) vesicular fusions that result in the formation of large vesicles (20-50 micron diameter), 'axosomes', and finally (iii) axosomal migration to and accumulation at the injury site. Some axosomes emerge from a cut end, attaining sizes up to 250 microns in diameter. Axosomes did not form after axonal injury unless divalent cations (Ca2+ or Mg2+) were present (10mM) in the external solution. The requirement for Ca2+ and the action of other ions are similar to that for cut-end cytoskeletal constriction in transected squid axons (Gallant, P.E. (1988) J. Neurosci. 8, 1479-1484) and for electrical sealing in transected axons of the cockroach (Yawo, H. and Kuno, M. (1985) J. Neurosci. 5, 1626-1632). Axosomes probably consist of membrane from different sources (e.g., axolemma, organelles and Schwann cells); however, localization of axosomal formation to the inner region of the axolemma and the formation dependence on divalent cations suggest principal involvement of cisternae of endoplasmic reticulum. Patch clamp of excised patches from axosomes liberated spontaneously from cut ends of transected axons showed a 12-pS K+ channel and gave indications of other channel types. Injury-induced vesiculation and membrane redistribution seem to be fundamental processes in the short-term (minutes to hours) that precede axonal degeneration or repair and regeneration. Axosomal formation provides a membrane preparation for the study of ion channels and other membrane processes from inaccessible organelles.

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PubMed1988

Inward rectifier K+-channel kinetics from analysis of the complex conductance of Aplysia neuronal membrane.

Conduction in inward rectifier, K+-channels in Aplysia neuron and Ba++ blockade of these channels were studied by rapid measurement of the membrane complex admittance in the frequency range 0.05 to 200 Hz during voltage clamps to membrane potentials in the range -90 to -40 mV. Complex ionic conductances of K+ and Cl- rectifiers were extracted from complex admittances of other membrane conduction processes and capacitance by vector subtraction of the membrane complex admittance during suppressed inward K+ current (near zero-mean current and in zero [K+]0) from complex admittances determined at other [K+]0 and membrane potentials. The contribution of the K+ rectifier to the admittance is distinguishable in the frequency domain above 1 Hz from the contribution of the Cl- rectifier, which is only apparent at frequencies less than 0.1 Hz. The voltage dependence (-90 to -40 mV) of the chord conductance (0.2 to 0.05 microS) and the relaxation time (4-8 ms) of K+ rectifier channels at [K+]0 = 40 mM were determined by curve fits of admittance data by a membrane admittance model based on the linearized Hodgkin-Huxley equations. The conductance of inward rectifier, K+ channels at a membrane potential of -80 mV had a square-root dependence on external K+ concentration, and the relaxation time increased from 2 to 7.5 ms for [K+]0 = 20 and 100 mM, respectively. The complex conductance of the inward K+ rectifier, affected by Ba++, was obtained by complex vector subtraction of the membrane admittance during blockage of inward rectifier, K+ channels (at -35 mV and [Ba++]0 = 5 mM) from admittances determined at -80 mV and at other Ba++ concentrations. The relaxation time of the blockade process decreased with increases in Ba++ concentration. An open-closed channel state model produces the inductive-like kinetic behavior in the complex conductance of inward rectifier, K+ channels and the addition of a blocked channel state accounts for the capacitive-like kinetic behavior of the Ba++ blockade process.

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PubMed1983

Fluctuation and linear analysis of Na-current kinetics in squid axon.

The power spectrum of current fluctuations and the complex admittance of squid axon were determined in the frequency range 12.5 to 5,000 Hx during membrane voltage clamps to the same potentials in the same axon during internal perfusion with cesium. The complex admittance was determined rapidly and with high resolution by a fast Fourier transform computation of the current response, acquired after a steady state was attained, to a synthesized signal with predetermined spectral characteristics superposed as a continuous, repetitive, small perturbation on step voltage clamps. Linear conduction parameters were estimated directly from admittance data by fitting an admittance model, derived from the linearized Hodgkin-Huxley equations modified by replacing the membrane capacitance with a "constant-phase-angle" capacitance, to the data. The constant phase angle obtained was approximately 80 degrees. At depolarizations the phase of the admittance was 180 degrees, and the real part of the impedance locus was in the left-half complex plane for frequencies below 1 kHz, which indicates a steady-state negative Na conductance. The fits also yielded estimates of the natural frequencies of Na "activation" and "inactivation" processes. By fitting Na-current noise spectra with a double Lorentzian function, a lower and an upper corner frequency were obtained; these were compared with the two natural frequencies determined from admittance analysis at the corresponding potentials. The frequencies from fluctuation analyses ranged from 1.0 to 10.3 times higher than those from linear (admittance) analysis. This discrepancy is consistent with the concept that the fluctuations reflect a nonlinear rate process that cannot be fully characterized by linear perturbation analysis. Comparison of the real part of the admittance and the current noise spectrum shows that the Nyquist relation, which generally applies to equilibrium conductors, does not hold for the Na process in squid axon. The Na-channel conductance, gamma Na, was found to increase monotonically from 0.1 to 4.8 pS for depolarizations up to 50 mV from a holding potential of -60 mV, with no indication of a maximum value.

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PubMed1981

Material from the internal surface of squid axon exhibits excess noise. Implications in modeling membrane noise.

A fluid material from a squid (Loligo pealei) axon was isolated by mechanical application of two types of microcapillary (1-3-mum Diam) to the internal surface of intact and cut-axon preparations. Current noise in the isolated material exceeded thermal levels and power spectra were 1/f in form in the frequency range 1.25-500 Hz with voltage-dependent intensities that were unrelated to specific ion channels. Whether conduction in this material is a significant source of excess noise during axon conduction remains to be determined. Nevertheless, a source of excess noise external to or within an ion channel may not be properly represented solely as an additive term to the spectrum of ion channel noise; a deconvolution of these spectral components may be required for modeling purposes.

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PubMed1980

Small-signal analysis of K+ conduction in squid axons.

The maximum potential displacement that gives a linear K conductance response was determined to be 1 mV (rms) from a voltage-clamp analysis of TTX treated axons. For perturbations below this amplitude the K conductance kinetics are indistinguishable from a first-order rate process. Linearity and order of kinetics were assessed by four types of measurements: (i) the shape of the onset of the potassium current (sigmoidal vs. exponential); (ii) the symmetry of small hyperpolarizing and depolarizing pulses, (iii) wide band admittance, and (iv) harmonic analysis. The simplest interpretation of the results is that the small-signal linear response arises from a first-order gating mechanism, whereas the large-signal conventional voltage-clamp pulse of tens of millivolts evokes nonlinear phenomena. The small-signal results are consistent with the Hodgkin-Huxley description or any other nonlinear model which fits the large signal data and produces a linear first-order response for small perturbations.

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PubMed1979

Complex admittance of Na+ conduction in squid axon.

The complex admittance, Y(p), of squid axon was measured (4--1000 Hz) during step voltage clamp to obtain linear data on Na+ conduction. Y(p) is used as a spectroscopic tool to identify Na+ and K+ conduction, which dominate Y(p) at low frequencies and can be separated from each other and from the static capacitance. Na+ conduction is readily distinguishable from K+ conduction in that it produces a steady-state negative conductance. The admittance of the Na+ system can show an anomalous resonance or an antiresonance depending on whether the net shunt conductance is negative or positive. Use of the Na+ negative conductance to neutralize leakage yields a measurement of dielectric capacitance at low frequency. A 90 degrees phase angle suggests that the capacitance is ideal.

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PubMed1979

Chemically induced K+ conduction noise in squid axon.

Internal perfusion of tetraethylammonium ions (TEA) in squid axons produces a significant high frequency noise component. Although internal TEA suppresses the potassium conductance (GK) noise at relatively low frequencies, it induces high frequency noise which exceeds the intensity of the normal potassium and sodium noise. In addition, the induced noise is dependent on the presence of internal potassium ions (K+) suggesting that this source of noise arises from a modulation of the K+ conductance due to the blocking and unblocking of the K+ channel. The simplest model describing the TEA data is a two-step sequential, pseudo-unimolecular reaction where TEA binds during an open conductance state. A unit channel conductance of 2 pS is estimated from the TEA data as well as noise induced by triethyldecylammonium (TEDA) ions. Thus, these data are consistent with the hypothesis that the channel is blocked whenever the quaternary ammonium ion binding site, located near or within the K+ channel, is occupied.

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PubMed1979

K+ conduction phenomena applicable to the low frequency impedance of squid axon.

The observation of peaking in power spectra of K current noise in squid axon (Fishman, H.M, Moore, L.E., Poussart, D.J.M. 1975, J. Membrane Biol. 24:305) led to the calculation of low frequency K conduction feature in the impedance (admittance) which was confirmed (Fishman, H.M., Poussart, D.J.M., Moore, L.E. & Siebenga, E., 1977, J. Membrane Biol. 32:255). This paper analyzes two physical phenomena, one within and the other outside of the excitable membrane, that might account for the low frequency impedance (admittance) feature. The accumulation of potassium ions in a space outside the axon in conjunction with diffusion through the Schwann cell layer produces a low-frequency mode that is similar in some respects to that observed experimentally. Alternatively, a hypothetical inactivation process, with a voltage-dependent time constant, associated with conduction in potassium channels gives a better account of the data. Either or both of these phenomena could be involved in producing the low-frequency impedance behavior in the squid axon.

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PubMed1977

K+ conduction description from the low frequency impedance and admittance of squid axon.

The form of power spectra of K+ conduction fluctuations in patches of squid axon suggested that K+ conduction kinetics are higher than first order (Fishman, Moore & Poussart, 1975, J. Membrane Biol. 24:305). To obtain an alternative description of ion conduction kinetics consistent with spontaneous fluctuations, the complex impedance and admittance of squid (Loligo pealei) axon were measured at low frequencies (1-1000 Hz) with a four electrode system using white Gaussian noise as a stochastic perturbation. As predicted from the spontaneous noise measurements, a low frequency impedance feature is observed between 1 and 30 Hz which is voltage and temperature dependent, disappears after substantial reduction in [Ki+], and is unaffected by the state of Na+ conduction or active transport. These measurements confirm and constitute strong support for the patch noise measurements and interpretations. The linearized Hodgkin-Huxley (HH) equations do not produce the low frequency feature since first order ion conduction kinetics are assumed. Computation of diffusion polarization effects associated with the axon sheath gives a qualitative account of the low frequency feature, but the potential dependence is opposite to that of the data. Thus, K+ conduction kinetics in the axon are not adequately described by a single first order process. In addition, significant changes in HH parameter values were required to describe the usual impedance (resonance) feature in Loligo pealei axon data.

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PubMed1977

Asymmetry currents and admittance in squid axons.

The complex admittance of squid (Loligo pealei) axon was measured rapidly (within 1 s) with pseudo-random small signals and discrete Fourier transform techniques under guarded, "space-clamp" conditions and during suppression of ion conduction. Asymmetry currents were measured by paired step clam pulses of +/-70 mV from a holding potential of -97 mV and gave an apparent capacitance of 0.36 muF/cm2. However, the admittance data showed no change in capacitance at holding potentials from -97 to -67 mV and gave a decrease of 0.07 of 0.15 muF/cm2 at -37 mV. The failure to observe a capacitance increase at low membrane potentials suggests the following possibilities: (a) the asymmetry current is a displacement current that inactivates completely with time, and (b) the asymmetry current is not a displacement current and arises from large signal effects (i.e., delayed nonlinearity in ionic current) on the membrane.

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PubMed1975

Patch voltage clamp of squid axon membrane.

A small area (patch) of the external surface of a squid axon can be "isolated" electrically from the surrounding bath by means of a pair of concentric glass pipettes. The seawater-filled inner pipette makes contact with the axon and constitutes the external access to the patch. The outer pipette is used to direct flowing sucrose solution over the area surrounding the patch of membrane underlying the inner pipette. Typically, sucrose isolated patches remain in good condition (spike amplitude greater than 90 mV) for periods of approximately one half hour. Patches of axon membrane which had previously been exposed to sucrose solution were often excitable. Membrane survival of sucrose treatment apparently arises from an outflow of ions from the axon and perhaps satellite cells into the interstitial cell space surrounding the exolemma. Estimate of the total access resistance (electrode plus series resistance) to the patch is about 100 komega (7 omega cm2). Patch capacitance ranges from 10-100 pF, which suggests areas of 10(-4) to 10(-5) cm2 and resting patch resistances of 10-100 Momega. Shunt resistance through the interstitial space exposed to sucrose solution, which isolates the patch, is typically 1-2 Momega. These parameters indicate that good potential control and response times can be achieved on a patch. Furthermore, spatial uniformity is demonstrated by measurement of an exoplasmic isopotential during voltage clamp of an axon patch. The method may be useful for other preparations in which limited membrane area is available or in special instances such as in the measurement of membrane conduction noise.

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