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F N Quandt

Publications and source records attributed to F N Quandt.

15 recordsLinked to original sources
PubMed1992

Action potential refractory period in axonal demyelination: a computer simulation.

Axonal demyelination leads to an increase in the refractory period for propagation of the action potential. Computer simulations were used to investigate the mechanism by which changes in the passive properties of the internodal membrane increase the refractory period. The properties of the voltage dependent ion channels can be altered to restore conduction in demyelinated nerve fibers. The ability of these alterations to decrease the refractory period of demyelinated model nerve fibers was compared. The model nerve fiber contained six nodes. The action potential was stimulated at node one and propagated to node six. The internode between nodes three and four was demyelinated in a graded manner. The absolute refractory period for propagation of the action potential through the demyelinated internode increased as the number of myelin wraps was reduced to less than 25% of the normal value. The increase in refractory period was found to be due to a reduction in the rate or repolarization of the action potential at node three. The delay in repolarization reduced the rate of recovery of inactivated Na channels and slowed the closing of K channels. The rate of repolarization of node three was reduced by the conduction delay for the depolarization of node four caused by demyelination of the preceding internode. In these simulations the increase in refractory period due to demyelination was eliminated by slowing the onset of Na channel inactivation. A small reduction of the K conductance also decreased the refractory period. However, larger reductions eliminated this effect.

Action Potentials↗
PubMed1992

Mechanism of asymmetric block of K channels by tetraalkylammonium ions in mouse neuroblastoma cells.

Experiments were performed to compare the mechanism of block of voltage-dependent K channels by various short and long alkyl chain tetraalkylammonium (TAA) ions at internal and external sites. Current through single channels was recorded from excised membrane patches of cultured neuroblastoma cells using the patch-clamp technique. All of the TAA derivatives tested blocked the open channel when applied to either side of the membrane. Tetraethylammonium (TEA) reduced the amplitude of current through the open channel. Tetrabutylammonium (TBA) and tetrapentylammonium (TPeA) reduced the open time as a function of the concentration. An additional nonconducting state was observed when TBA or TPeA was applied internally or externally, due to the presence of a drug-bound and blocked state of the channel. The closing rate under control conditions was similar to that in the presence of external tetramethylammonium (TMA), suggesting that channel closing is independent of external drug binding. The concentration for half maximal block of the channel by external TEA was 80 microM. The channel was less sensitive to internal TEA, which half blocked the channel at 27 mM. The dissociation rate of long alkyl chain TAA ions from the channel was slower when applied to the inside, compared to external application, suggesting the presence of distinct internal and external receptors. Long alkyl chain TAA derivatives, such as TBA had a faster association rate with the open channel when applied to the inside of the membrane than when applied to the outside.

Animals↗
PubMed1992

Tetraalkylammonium ion block of potassium currents in mouse neuroblastoma cells.

Experiments were performed to compare the mechanism of block of the delayed rectifier K+ channels in cultured mouse neuroblastoma cells by various derivatives of tetraethylammonium (TEA) which have symmetric alkyl chains of one to six carbons. Current from the whole cell was studied using the patch clamp technique. TEA blocked the whole cell K+ current with a Ki of 0.6 mM when applied to the external solution. The Ki for block by other derivatives were (mM): tetrapropylammonium, 9.2; tetrabutylammonium (TBA), 1.9; tetrapentylammonium (TPeA), 0.088; and tetrahexylammonium, (THA), 0.006. Block of the whole cell current by TEA or tetrapropylammonium did not increase with time after a step depolarization. However, block by TBA, TPeA or THA was time dependent. TEA did not compete with TPeA for the same receptor. Block by externally applied TEA was not appreciably voltage dependent, and the receptor for TPeA had an apparent electrical distance of 0.3. These observations suggest that TEA and TPeA block at separate receptors. THA could block the open channel in cell-attached patches when the compound was applied to the bath. This observation and the observation that externally applied TPeA and TEA do not occupy the same receptor suggest that derivatives having long alkyl chain lengths can reach the internal receptor from the external solution.

Animals↗
PubMed1991

Voltage-dependent gating and block of the cyclic-GMP-dependent current in bovine rod outer segments.

The properties of the cyclic-GMP-activated conductance in the plasma membrane of bovine rod outer segments were studied in excised membranes. Multiple-channel and single-channel currents were recorded by the patch-clamp technique in symmetrical NaCl solutions which were free of divalent cations. The current-voltage relationship for the current, recorded when a large population of channels was activated, exhibited outward rectification. Rectification decreased as the concentration of cyclic-GMP was increased, and the concentration of cyclic-GMP required for half maximal activation of the channel decreased with depolarization. At a concentration of 1-3 microM cyclic-GMP, single-channel activity could be observed from these excised patches. The conductance of the open channel was 6 pS and was independent of the membrane potential. These results are consistent with the interpretation that under these conditions, the mechanism responsible for the outward rectification is due to an increase in the probability of an open channel as the membrane is depolarized. The cyclic-GMP-activated current could be blocked by L-cis-diltiazem. Block was voltage and time dependent. The time constant for the onset of block and its steady state level increased with depolarization. The extent of block by diltiazem was not enhanced as the cyclic-GMP concentration was increased, suggesting that the channel is not required to be open for block to occur. Complete block was never attained even for high concentrations of diltiazem. However, the diltiazem-resistant component of the cyclic-GMP-activated current could be blocked by tetracaine.

Animals↗
PubMed1990

Perfusing pipettes.

A simple modification of a patch-clamp set-up allows the fluid in a patch pipette to be changed. The time course of the solution exchange is estimated from the time course of change of the reversal potential of the current through an open Ca2+ activated K+ channel: solution exchange takes less than 1 min. Measurements of the power spectrum of noise show that the modified set-up introduces little excess noise below 1 kHz.

Electrophysiology↗
PubMed1988

Three kinetically distinct potassium channels in mouse neuroblastoma cells.

1. Mouse neuroblastoma cells were utilized to examine the electrical properties of single K+ channels which might underlie multiple components of outward current in vertebrate neurones. The conductance, kinetics of activation, inactivation, and pharmacology of three types of channels were compared. 2. Two types of voltage-dependent channels, primarily permeable to K+, were identified which did not require the presence of internal Ca2+. The first had gating kinetics best classified as a delayed rectifier. The conductance of the open channel was 35 pS (22 degrees C) in solutions having symmetrical 125 mM-K+ concentrations. 3. The second type of channel had a conductance of 14 pS under identical conditions. The gating kinetics of this type of channel were distinct from those of the delayed rectifier. The mean first latency, and lifetime of the open state at any voltage, were longer. The maximum probability of an open channel was smaller, so that this parameter appeared less sensitive to the membrane potential. The rate of inactivation of the channel was slower. Further, at the more negative membrane potentials tested, the level of steady-state inactivation was less for this type of channel. 4. The delayed rectifier channel was more sensitive to the blocking action of 4-aminopyridine than the channel with low conductance. 5. A Ca2+ -activated, voltage-dependent K+ channel, having a conductance of 140 pS, was also identified. The maximum probability of an open channel increased, and the voltage for half-maximal activation shifted to a more negative potential as the internal Ca2+ was increased. 6. The time course of inactivation of K+ currents recorded from the whole cell declined in two phases, probably due to the presence of the two types of voltage-dependent K+ channels.

4-Aminopyridine↗
PubMed1988

Modification of slow inactivation of single sodium channels by phenytoin in neuroblastoma cells.

Modifications of Na+ channels by phenytoin (PT), an anticonvulsant drug, were examined. Previous work using voltage-clamp methods indicated that PT could interact with inactivated states of the channel to reduce excitability. Single-channel analysis was used to test the idea that the fast inactivation process was not required for modification of the channel. The hypothesis that PT could interact with open or slow inactivated states to produce a drug-bound, long duration, nonconducting state was also tested. Currents due to the opening of single Na+ channels were measured in inside-out patches of membrane excised from N1E-115 mouse neuroblastoma cells grown in tissue culture. After the removal of the fast inactivation process enzymatically, the average Na+ current in response to a step depolarization decayed due to the slow inactivation process. The time constant of decay decreased as a function of the concentration of PT. The average current appeared to be caused by extensive reopening of Na+ channels. During maintained depolarization, the reopening of Na+ channels occurred in bursts interrupted by long silent periods, due to the slow inactivated state. PT decreased the burst duration and increased the interval between bursts. The average open time of Na+ channels was reduced in the presence of PT. All of the alterations were enhanced as the concentration of PT was increased. The amplitude of current through the open channel was not effected by PT. PT was able to modify the Na+ channel in the absence of fast inactivation. The results suggest that PT can bind to the Na+ channel and produce a nonconducting state from which the probability of a channel opening is small. These modifications could underly the selective block of action potentials during chronic depolarization of the membrane or during high frequency discharge.

Animals↗
PubMed1987

Burst kinetics of sodium channels which lack fast inactivation in mouse neuroblastoma cells.

1. The kinetics of the slow inactivation process of Na+ channels were examined by recording single-channel currents from cultured neuroblastoma cells. 2. In order to directly examine slow inactivation, fast inactivation was first removed irreversibly by briefly exposing the internal surface of excised membranes to papain. Following treatment, the time constant for the inactivation of averaged membrane Na+ current increased by over two orders of magnitude, while the open time of individual channels increased by a factor of three. The two effects are consistent with the idea that papain can selectively remove fast inactivation of Na+ channels. 3. In the absence of fast inactivation, Na+ channels continued to open during maintained depolarization of the membrane to potentials less negative than -60 mV. Under these conditions, the opening occurred in bursts 50 ms to hundreds of milliseconds long, followed by silent periods lasting many seconds. The average burst length was found to be equal to the time constant of the decline in average evoked current measured at the same potential, indicating that a burst was terminated by entry of the channel into the slow inactivated state. 4. Histograms of open times revealed two populations of open states at any potential. Bursts could also be classified as either short or long bursts. Bursts appeared to be due to the gating of a single channel, and long bursts contained both types of open states, suggesting that a Na+ channel could have more than one open state. 5. The kinetics of bursts of Na+ channels were voltage dependent. As the membrane was depolarized, the burst length, interval between bursts, and open time all increased. Although the probability of an open channel during a burst increased to almost 1.0 with depolarization, any channel was open less than 0.5% of the time when measured throughout the depolarization. The increase in burst duration with depolarization would occur if the rate of slow inactivation is faster from closed states of the channel than from open states. 6. Records of membrane current evoked by a series of step depolarizations were clustered into those with openings of Na+ channels and those without openings. Records in which a channel did not inactivate during the depolarization were less likely to lead to hibernation, suggesting that this phenomenon is caused by the slow inactivation process.

Action Potentials↗
PubMed1986

Single inward rectifier channels in horizontal cells.

The ion channels responsible for inward rectification in horizontal cells were studied using the patch clamp technique applied to isolated cells from goldfish retina. Inward currents recorded from these cells were identified as due to the opening of inward rectifier channels based on their ion selectivity, channel gating behavior, and the effects of external blocking ions. The single channel conductance was 20 pS in 125 mM external K+. The null current potential shifted with changes in the K+ concentration as expected for a channel permeable to K+, and the channel appeared to have little permeability to Na+. The probability of a channel being in an open state increased as the membrane was hyperpolarized from the K+ equilibrium potential (0 to -10 mV) over potentials ranging to -80 mV, in the presence of external Na+. The closing rate was insensitive to membrane potential in the presence of external Na+. The opening rate of the channel increased as the membrane was hyperpolarized. The increase in the probability of a channel being open at negative potentials was therefore caused by the voltage sensitivity of the rate of channel opening.

Animals↗
PubMed1986

Calcium activated potassium channels in cultured astrocytes.

The patch clamp technique was used to analyze single channel currents in intact and excised patches of glial cell membranes grown in primary cultures from newborn rat brain. Glial cells were morphologically identified by immunohistochemical staining for glial fibrillary acidic protein. Outward currents due to single channels were observed in recordings from both intact and excised patches obtained from the cell body region. The channel responsible for these currents was preferentially permeable to K+ because the reversal potential for this current was correlated with changes in the potassium equilibrium potential, when experimentally altered. The single channel conductance was 25 pS when measured between -20 and +20 mV in solutions with physiological K+ concentrations (10 degrees C). Channel gating was dependent on both the internal Ca2+ concentration and the membrane potential. Either depolarization of the membrane patch, or the addition of increasing Ca2+ concentrations to the internal surface, increased the probability of channel opening. Tetraethylammonium reversibly blocked the channel whereas 4-aminopyridine had no effect. The characteristics exhibited by this channel indicate that a Ca2+-activated K+ channel is present in the membrane of astrocytes grown in culture. These results, combined with previous evidence for a voltage dependent Ca2+ channel, suggest a dynamic role for glial cells in controlling excitability in the central nervous system. Influx of Ca2+ upon depolarization would increase the membrane permeability to K+ and could increase the "buffering" capacity of glial cells for extracellular K+.

Animals↗
PubMed1985

All or none block of single Na+ channels by tetrodotoxin.

The effects of tetrodotoxin on single Na+-channel currents recorded from excised patches of neuroblastoma cells were examined. Tetrodotoxin was found to cause a dose-dependent reduction in the frequency at which Na+ channels conduct during a series of depolarizations. Surviving conducting states had normal open times and current amplitudes. These effects could be explained by a model which includes initial binding of tetrodotoxin to a closed state of the channel with stable, complete block during the time the channel would normally be gated open.

Cell Line↗