scholarly journals Fast Inactivation of Delayed Rectifier K Conductance in Squid Giant Axon and Its Cell Bodies

1997 ◽  
Vol 109 (4) ◽  
pp. 435-448 ◽  
Author(s):  
Chris Mathes ◽  
Joshua J.C. Rosenthal ◽  
Clay M. Armstrong ◽  
William F. Gilly
Keyword(s):  
Large Fraction ◽  
Giant Axon ◽  
Delayed Rectifier ◽  
Inactivation Kinetics ◽  
Temperature Sensitive ◽  
Fast Inactivation ◽  
Giant Fiber ◽  
Experimental Conditions ◽  
Giant Axons ◽  
External K

Inactivation of delayed rectifier K conductance (gK) was studied in squid giant axons and in the somata of giant fiber lobe (GFL) neurons. Axon measurements were made with an axial wire voltage clamp by pulsing to VK (∼−10 mV in 50–70 mM external K) for a variable time and then assaying available gK with a strong, brief test pulse. GFL cells were studied with whole-cell patch clamp using the same prepulse procedure as well as with long depolarizations. Under our experimental conditions (12–18°C, 4 mM internal MgATP) a large fraction of gK inactivates within 250 ms at −10 mV in both cell bodies and axons, although inactivation tends to be more complete in cell bodies. Inactivation in both preparations shows two kinetic components. The faster component is more temperature-sensitive and becomes very prominent above 12°C. Contribution of the fast component to inactivation shows a similar voltage dependence to that of gK, suggesting a strong coupling of this inactivation path to the open state. Omission of internal MgATP or application of internal protease reduces the amount of fast inactivation. High external K decreases the amount of rapidly inactivating IK but does not greatly alter inactivation kinetics. Neither external nor internal tetraethylammonium has a marked effect on inactivation kinetics. Squid delayed rectifier K channels in GFL cell bodies and giant axons thus share complex fast inactivation properties that do not closely resemble those associated with either C-type or N-type inactivation of cloned Kv1 channels studied in heterologous expression systems.

scholarly journals Molecular identification of SqKv1A. A candidate for the delayed rectifier K channel in squid giant axon.

1996 ◽  
Vol 108 (3) ◽  
pp. 207-219 ◽  
Author(s):  
J J Rosenthal ◽  
R G Vickery ◽  
W F Gilly
Keyword(s):  
Time Course ◽  
Xenopus Oocytes ◽  
Giant Axon ◽  
K Channel ◽  
Delayed Rectifier ◽  
Giant Fiber ◽  
Western Blots ◽  
Internal Solution ◽  
Giant Axons ◽  
K Currents

We have cloned the cDNA for a squid Kvl potassium channel (SqKv1A). SqKv1A mRNA is selectively expressed in giant fiber lobe (GFL) neurons, the somata of the giant axons. Western blots detect two forms of SqKv1A in both GFL neuron and giant axon samples. Functional properties of SqKv1A currents expressed in Xenopus oocytes are very similar to macroscopic currents in GFL neurons and giant axons. Macroscopic K currents in GFL neuron cell bodies, giant axons, and in Xenopus oocytes expressing SqKv1A, activate rapidly and inactivate incompletely over a time course of several hundred ms. Oocytes injected with SqKv1A cRNA express channels of two conductance classes, estimated to be 13 and 20 pS in an internal solution containing 470 mM K. SqKv1A is thus a good candidate for the "20 pS" K channel that accounts for the majority of rapidly activating K conductance in both GFL neuron cell bodies and the giant axon.

scholarly journals The influence of external cations and membrane potential on Ca-activated Na efflux in Myxicola giant axons.

1978 ◽  
Vol 71 (4) ◽  
pp. 453-466 ◽  
Author(s):  
R A Sjodin ◽  
R F Abercrombie
Keyword(s):  
Membrane Potential ◽  
Direct Action ◽  
Monovalent Cation ◽  
Coupling Ratio ◽  
Large Component ◽  
Experimental Conditions ◽  
Giant Axons ◽  
Threefold Increase ◽  
Na Efflux ◽  
External K

In microinjected Myxicola giant axons with elevated [Na]i, Na efflux was sensitive to Cao under some conditions. In Li seawater, sensitivity to Cao was high whereas in Na seawater, sensitivity to Cao was observed only upon elevation of [Ca]o above the normal value. In choline seawater, the sensitivity of Na efflux to Cao was less than that observed in Li seawater whereas Mg seawater failed to support any detectable Cao-sensitive Na efflux. Addition of Na to Li seawater was inhibitory to Cao-sensitive Na efflux, the extent of inhibition increasing with rising values of [Na]o. The presence of 20 mM K in Li seawater resulted in about a threefold increase in the Cao-activated Na efflux. Experiments in which the membrane potential, Vm, was varied or held constant when [K]o was changed showed that the augmentation of Ca-activated Na efflux by Ko was not due to changes in Vm but resulted from a direct action of K on activation by Ca. The same experimental conditions that favored a large component of Cao-activated Na efflux also caused a large increase in Ca influx. Measurements of Ca influx in the presence of 20 mM K and comparison with values of Ca-activated Na efflux suggest that the Na:Ca coupling ratio may be altered by increasing external [K]o. Overall, the results suggest that the Cao-activated Na efflux in Myxicola giant axons requires the presence of an external monovalent cation and that the order of effectiveness at a total monovalent cation concentration of 430 mM is K + Li greater than Li greater than Choline greater than Na.

scholarly journals A rapidly activating and slowly inactivating potassium channel cloned from human heart. Functional analysis after stable mammalian cell culture expression.

1993 ◽  
Vol 101 (4) ◽  
pp. 513-543 ◽  
Author(s):  
D J Snyders ◽  
M M Tamkun ◽  
P B Bennett
Keyword(s):  
Cell Lines ◽  
Time Course ◽  
Reversal Potential ◽  
Delayed Rectifier ◽  
Inactivation Kinetics ◽  
Constant Field ◽  
Slow Inactivation ◽  
Time Constants ◽  
K Concentration ◽  
External K

The electrophysiological properties of HK2 (Kv1.5), a K+ channel cloned from human ventricle, were investigated after stable expression in a mouse Ltk- cell line. Cell lines that expressed HK2 mRNA displayed a current with delayed rectifier properties at 23 degrees C, while sham transfected cell lines showed neither specific HK2 mRNA hybridization nor voltage-activated currents under whole cell conditions. The expression of the HK2 current has been stable for over two years. The dependence of the reversal potential of this current on the external K+ concentration (55 mV/decade) confirmed K+ selectivity, and the tail envelope test was satisfied, indicating expression of a single population of K+ channels. The activation time course was fast and sigmoidal (time constants declined from 10 ms to < 2 ms between 0 and +60 mV). The midpoint and slope factor of the activation curve were Eh = -14 +/- 5 mV and k = 5.9 +/- 0.9 (n = 31), respectively. Slow partial inactivation was observed especially at large depolarizations (20 +/- 2% after 250 ms at +60 mV, n = 32), and was incomplete in 5 s (69 +/- 3%, n = 14). This slow inactivation appeared to be a genuine gating process and not due to K+ accumulation, because it was present regardless of the size of the current and was observed even with 140 mM external K+ concentration. Slow inactivation had a biexponential time course with largely voltage-independent time constants of approximately 240 and 2,700 ms between -10 and +60 mV. The voltage dependence of slow inactivation overlapped with that of activation: Eh = -25 +/- 4 mV and k = 3.7 +/- 0.7 (n = 14). The fully activated current-voltage relationship displayed outward rectification in 4 mM external K+ concentration, but was more linear at higher external K+ concentrations, changes that could be explained in part on the basis of constant field (Goldman-Hodgkin-Katz) rectification. Activation and inactivation kinetics displayed a marked temperature dependence, resulting in faster activation and enhanced inactivation at higher temperature. The current was sensitive to low concentrations of 4-aminopyridine, but relatively insensitive to external TEA and to high concentrations of dendrotoxin. The expressed current did not resemble either the rapid or the slow components of delayed rectification described in guinea pig myocytes. However, this channel has many similarities to the rapidly activating delayed rectifying currents described in adult rat atrial and neonatal canine epicardial myocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Preliminary observations on escape swimming and giant neurons in Aglantha digitale (Hydromedusae: Trachylina)

1980 ◽  
Vol 58 (4) ◽  
pp. 549-552 ◽  
Author(s):  
S. Donaldson ◽  
G. O. Mackie ◽  
A. Roberts
Keyword(s):  
Action Potentials ◽  
Giant Axon ◽  
Synaptic Delay ◽  
Giant Axons ◽  
A Value ◽  
Giant Synapses ◽  
Run Up

Aglantha can swim in two ways, one of which, fast swimming, is evoked by contact with predators and serves for escape. The response consists of two or three violent contractions of which the first propels the animal a distance equivalent to five body lengths. Peak velocities in the range 0.3–0.4 m s−1 were measured. Drag is reduced by contraction of the tentacles.Coordination of escape swimming and tentacle contraction is achieved by a system of giant axons. A giant axon runs down each tentacle; action potentials in these elements show a one-for-one correspondence with potentials recorded from a ring-shaped axon lying in the margin near the tentacle bases. The ring giant synapses with eight motor giants which run up the subumbrella innervating the swimming muscles.Conduction velocities in the giant axons may be as high as 4.0 m s−1 in the case of the largest (40 μm diameter) axons. A value of 1.6 ms was obtained for minimum synaptic delay between the ring and motor giant axons.

Cloning and characterization of an electrogenic Na/HCO3− cotransporter from the squid giant fiber lobe

2007 ◽  
Vol 292 (6) ◽  
pp. C2032-C2045 ◽  
Author(s):  
Peter M. Piermarini ◽  
Inyeong Choi ◽  
Walter F. Boron
Keyword(s):  
Giant Axon ◽  
Predicted Amino Acid Sequence ◽  
Giant Fiber ◽  
Reading Frame ◽  
Excitable Membranes ◽  
Current Voltage ◽  
Partial Cdna ◽  
Partial Cdna Clone ◽  
Current Voltage Relationship

The squid giant axon is a classic model system for understanding both excitable membranes and ion transport. To date, a Na+-driven Cl-HCO3− exchanger, sqNDCBE—related to the SLC4 superfamily and cloned from giant fiber lobe cDNA—is the only HCO3−-transporting protein cloned and characterized from a squid. The goal of our study was to clone and characterize another SLC4-like cDNA. We used degenerate PCR to obtain a partial cDNA clone (squid fiber clone 3, SF3), which we extended in both the 5′ and 3′ directions to obtain the full-length open-reading frame. The predicted amino-acid sequence of SF3 is similar to sqNDCBE, and a phylogenetic analysis of the membrane domains indicates that SF3 clusters with electroneutral Na+-coupled SLC4 transporters. However, when we measure pHi and membrane potential—or use two-electrode voltage clamping to measure currents—on Xenopus oocytes expressing SF3, the oocytes exhibit the characteristics of an electrogenic Na/HCO3− cotransporter, NBCe. That is, exposure to extracellular CO2/HCO3− not only causes a fall in pHi, followed by a robust recovery, but also causes a rapid hyperpolarization. The current-voltage relationship is also characteristic of an electrogenic NBC. The pHi recovery and current require HCO3− and Na+, and are blocked by DIDS. Furthermore, neither K+ nor Li+ can fully replace Na+ in supporting the pHi recovery. Extracellular Cl− is not necessary for the transporter to operate. Therefore, SF3 is an NBCe, representing the first NBCe characterized from an invertebrate.

Na+, K+ and Ca2+ currents in identified leech neurones in culture

1989 ◽  
Vol 141 (1) ◽  
pp. 1-20
Author(s):  
R. R. Stewart ◽  
J. G. Nicholls ◽  
W. B. Adams
Keyword(s):  
Minor Component ◽  
Outward Current ◽  
Membrane Properties ◽  
Delayed Rectifier ◽  
Inactivation Kinetics ◽  
Channel Blockers ◽  
Sensory Neurones ◽  
K Current ◽  
The Individual ◽  
K Currents

1. Na+, K+ and Ca2+ currents have been measured by voltage-clamp in Retzius (R), anterior pagoda (AP) and sensory (pressure, touch and nociceptive) cells dissected from the central nervous system (CNS) of the leech. These cells maintain their distinctive membrane properties and action potential configurations in culture. Currents carried by the individual ions were analysed by the use of channel blockers and by their kinetics. Since the cells are isopotential they can be voltage-clamped effectively. 2. Depolarization, as expected, gave rise to an early inward Na+ current followed by a delayed outward K+ current. In Na+-free medium containing tetraethylammonium (TEA+), and in the presence of 4-aminopyridine (4-AP), inward Ca2+ currents were revealed that inactivated slowly and were blocked by Cd2+ and Mn2+. 3. Na+ and Ca2+ currents were similar in their characteristics in R. AP and sensory neurones. In contrast, K+ currents showed marked differences. Three principal K+ currents were identified. These differed in their time courses of activation and inactivation and in their responses to Ca2+ channel blockers. 4. K+ currents of the A-type (IA) activated and inactivated rapidly, were not affected by Ca2+ channel blockers and were eliminated by steady-state inactivation at holding potentials of −30 mV. A-type K+ currents were found in AP cells and as a minor component of the outward current in R cells. A Ca2+-activated K+ current (IC), that inactivated more slowly and was reduced by Ca2+ channel blockers, constituted the major outward current in R cells. The third K+ current resembled the delayed rectifier currents (IK1 and IK2) of squid axons with slow activation and inactivation kinetics. Such currents were found in R cells and in the sensory neurones (T, P and N). 5. The principal differences in membrane properties of identified leech neurones can be explained in terms of the numbers of Na+ channels and the distinctive kinetics of K+ channels in each type of cell.

scholarly journals Trafficking-deficient hERG K+ channels linked to long QT syndrome are regulated by a microtubule-dependent quality control compartment in the ER

2011 ◽  
Vol 301 (1) ◽  
pp. C75-C85 ◽  
Author(s):  
Jennifer L. Smith ◽  
Christie M. McBride ◽  
Parvathi S. Nataraj ◽  
Daniel C. Bartos ◽  
Craig T. January ◽  
...  
Keyword(s):  
Golgi Apparatus ◽  
Long Qt Syndrome ◽  
Functional Expression ◽  
Delayed Rectifier ◽  
Temperature Sensitive ◽  
Missense Mutations ◽  
Long Qt ◽  
Delayed Rectifier Current ◽  
Golgi Compartment ◽  
Qt Syndrome

The human ether-a-go-go related gene ( hERG) encodes the voltage-gated K+ channel that underlies the rapidly activating delayed-rectifier current in cardiac myocytes. hERG is synthesized in the endoplasmic reticulum (ER) as an “immature” N-linked glycoprotein and is terminally glycosylated in the Golgi apparatus. Most hERG missense mutations linked to long QT syndrome type 2 (LQT2) reduce the terminal glycosylation and functional expression. We tested the hypothesis that a distinct pre-Golgi compartment negatively regulates the trafficking of some LQT2 mutations to the Golgi apparatus. We found that treating cells in nocodazole, a microtubule depolymerizing agent, altered the subcellular localization, functional expression, and glycosylation of the LQT2 mutation G601S-hERG differently from wild-type hERG (WT-hERG). G601S-hERG quickly redistributed to peripheral compartments that partially colocalized with KDEL (Lys-Asp-Glu-Leu) chaperones but not calnexin, Sec31, or the ER golgi intermediate compartment (ERGIC). Treating cells in E-4031, a drug that increases the functional expression of G601S-hERG, prevented the accumulation of G601S-hERG to the peripheral compartments and increased G601S-hERG colocalization with the ERGIC. Coexpressing the temperature-sensitive mutant G protein from vesicular stomatitis virus, a mutant N-linked glycoprotein that is retained in the ER, showed it was not restricted to the same peripheral compartments as G601S-hERG at nonpermissive temperatures. We conclude that the trafficking of G601S-hERG is negatively regulated by a microtubule-dependent compartment within the ER. Identifying mechanisms that prevent the sorting or promote the release of LQT2 channels from this compartment may represent a novel therapeutic strategy for LQT2.

Molecular identification of a TTX-sensitive Ca2+current

2001 ◽  
Vol 280 (5) ◽  
pp. C1327-C1339 ◽  
Author(s):  
Silvia Guatimosim ◽  
Eric A. Sobie ◽  
Jader dos Santos Cruz ◽  
Laura A. Martin ◽  
W. J. Lederer
Keyword(s):  
Cardiac Myocytes ◽  
Ion Selectivity ◽  
Inactivation Kinetics ◽  
Experimental Conditions ◽  
Guinea Pig Heart ◽  
Hek 293 ◽  
Hek 293 Cells ◽  
293 Cells ◽  
Intracellular Ca ◽  
Rat Myocytes

The TTX-sensitive Ca2+ current [ I Ca(TTX)] observed in cardiac myocytes under Na+-free conditions was investigated using patch-clamp and Ca2+-imaging methods. Cs+ and Ca2+were found to contribute to I Ca(TTX), but TEA+ and N-methyl-d-glucamine (NMDG+) did not. HEK-293 cells transfected with cardiac Na+ channels exhibited a current that resembled I Ca(TTX) in cardiac myocytes with regard to voltage dependence, inactivation kinetics, and ion selectivity, suggesting that the cardiac Na+ channel itself gives rise to I Ca(TTX). Furthermore, repeated activation of I Ca(TTX) led to a 60% increase in intracellular Ca2+ concentration, confirming Ca2+ entry through this current. Ba2+ permeation of I Ca(TTX), reported by others, did not occur in rat myocytes or in HEK-293 cells expressing cardiac Na+channels under our experimental conditions. The report of block of I Ca(TTX) in guinea pig heart by mibefradil (10 μM) was supported in transfected HEK-293 cells, but Na+current was also blocked (half-block at 0.45 μM). We conclude that I Ca(TTX) reflects current through cardiac Na+ channels in Na+-free (or “null”) conditions. We suggest that the current be renamed I Na(null) to more accurately reflect the molecular identity of the channel and the conditions needed for its activation. The relationship between I Na(null)and Ca2+ flux through slip-mode conductance of cardiac Na+ channels is discussed in the context of ion channel biophysics and “permeation plasticity.”

Abstract 17272: Ionic Current Remodeling Delays the Electrical Recovery of the Senescent Heart

Circulation ◽  
2014 ◽  
Vol 130 (suppl_2) ◽  
Author(s):  
Sergio Signore ◽  
Giulia Borghetti ◽  
Ramaswamy Kannappan ◽  
Andrea Sorrentino ◽  
Antonio Cannata ◽  
...  
Keyword(s):  
Qt Interval ◽  
Ventricular Arrhythmias ◽  
Ionic Current ◽  
Monophasic Action Potential ◽  
K Channel ◽  
Delayed Rectifier ◽  
Inactivation Kinetics ◽  
Na Channel ◽  
Surface Ecg ◽  
Gene And Protein Expression

Cardiac aging is associated with lengthening of the QT interval, a condition that enhances malignant ventricular arrhythmias and sudden death. The aim of this study was to establish whether ionic currents are altered in old myocytes contributing to the protracted electrical recovery of the senescent heart. Thus, mice at 3-30 months of age were studied by ECG and patch-clamp; these physiological determinations were complemented with molecular assays for the analysis of ion channel proteins. By surface ECG and telemetry system, PR, QRS and QT intervals were prolonged in mice at 25 months or older. These delays were maintained in ex-vivo Langendorff preparations. In comparison to young, epicardial monophasic action potential (AP) duration at 50% and 90% repolarization were 1.6- and 1.2-fold larger in old LV, respectively. Moreover, senescent hearts presented a 60% higher incidence of arrhythmias. In isolated myocytes, prolongation of the early (+47%), intermediate (+117%) and late (+75%) repolarization phases of the AP were identified in cells from old animals, whereas resting membrane potential, upstroke amplitude and +dV/dt were preserved. Voltage-clamp experiments were then performed to measure ionic current properties. The rapidly activating K+ current, which consists of the transient outward and ultrarapid delayed rectifier (Ito+Kur), is responsible for the early repolarization of the AP, and was significantly reduced in old myocytes. Molecular studies revealed low levels of transcripts and proteins for K+ channel subunits Kv1.4, Kv1.5 and KChiP2 in senescent cells. Also, the late Na+ current INaL, which presents slow inactivation kinetics and is operative during AP repolarization, was 1.5-fold larger in old cells. These changes were associated with alterations in gene and protein expression of Na+ channel subunits. Inhibition of INaL with mexiletine significantly shortened the intermediate and late repolarization phases of the AP in both myocytes and perfused myocardium from old mice. Importantly, INaL inhibition in vivo shortened the QT interval of senescent mice by 12%. Thus, defects in ionic current occur with aging resulting in prolongation of the AP and delays in electrical recovery which may lead to malignant ventricular arrhythmias.

Coordinated Excitation of Flexor Inhibitors in the Crayfish

1980 ◽  
Vol 86 (1) ◽  
pp. 187-195
Author(s):  
CHIKAO UYAMA ◽  
TAKASHI MATSUYAMA
Keyword(s):  
Giant Axon ◽  
Experimental Results ◽  
Abdominal Ganglion ◽  
Giant Axons ◽  
The Third ◽  
Nerve Cords

In isolated abdominal nerve cords of crayfish, the medial or lateral giant axons were stimulated at a position just rostral to the first abdominal ganglion. Recordings of the impulse sequences of the flexor inhibitor (FI) were made from the anterior five ganglia, three ganglia at a time. In 20% of our preparations, one giant axon impulse caused one to four FI impulses in every abdominal third root. An equal number of FI impulses were usually produced by each abdominal ganglion for any given stimulation. The earliest FI impulse was observed at the third root of the fourth ganglion. FI impulses occurred with increasing latencies rostrally and caudally from the fourth ganglion. The FI responses to medial and lateral giant axons stimulation were essentially equivalent. FI impulses were recorded from the rostral three abdominal ganglia, while the caudal ganglia were cut off one after another from the sixth to the third ganglion. Little change was noted until after the removal of the fourth ganglion, which usually caused all FI impulses to disappear. From these experimental results, we propose a model of central mechanisms for FI excitation.

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