I A in Kenyon Cells of the Mushroom Body of Honeybees Resembles Shaker Currents: Kinetics, Modulation by K+, and Simulation

1999 ◽  
Vol 81 (4) ◽  
pp. 1749-1759 ◽  
Author(s):  
Corinna Pelz ◽  
Johannes Jander ◽  
Hendrik Rosenboom ◽  
Martin Hammer ◽  
Randolf Menzel
Keyword(s):  
Action Potential ◽  
Time Constant ◽  
Mushroom Body ◽  
Delayed Rectifier ◽  
Patch Clamp Technique ◽  
Time Constants ◽  
Voltage Gated ◽  
Kenyon Cells ◽  
Recovery From Inactivation ◽  
A Current

I A in Kenyon cells of the mushroom body of honeybees resembles shaker currents: kinetics, modulation by K+, and simulation. Cultured Kenyon cells from the mushroom body of the honeybee, Apis mellifera, show a voltage-gated, fast transient K+ current that is sensitive to 4-aminopyridine, an A current. The kinetic properties of this A current and its modulation by extracellular K+ ions were investigated in vitro with the whole cell patch-clamp technique. The A current was isolated from other voltage-gated currents either pharmacologically or with suitable voltage-clamp protocols. Hodgkin- and Huxley-style mathematical equations were used for the description of this current and for the simulation of action potentials in a Kenyon cell model. Activation and inactivation of the A current are fast and voltage dependent with time constants of 0.4 ± 0.1 ms (means ± SE) at +45 mV and 3.0 ± 1.6 ms at +45 mV, respectively. The pronounced voltage dependence of the inactivation kinetics indicates that at least a part of this current of the honeybee Kenyon cells is a shaker-like current. Deactivation and recovery from inactivation also show voltage dependency. The time constant of deactivation has a value of 0.4 ± 0.1 ms at −75 mV. Recovery from inactivation needs a double-exponential function to be fitted adequately; the resulting time constants are 18 ± 3.1 ms for the fast and 745 ± 107 ms for the slow process at −75 mV. Half-maximal activation of the A current occurs at −0.7 ± 2.9 mV, and half-maximal inactivation occurs at −54.7 ± 2.4 mV. An increase in the extracellular K+concentration increases the conductance and accelerates the recovery from inactivation of the A current, affecting the slow but not the fast time constant. With respect to these modulations the current under investigation resembles some of the shaker-like currents. The data of the A current were incorporated into a reduced computational model of the voltage-gated currents of Kenyon cells. In addition, the model contained a delayed rectifier K+ current, a Na+current, and a leakage current. The model is able to generate an action potential on current injection. The model predicts that the A current causes repolarization of the action potential but not a delay in the initiation of the action potential. It further predicts that the activation of the delayed rectifier K+ current is too slow to contribute markedly to repolarization during a single action potential. Because of its fast activation, the A current reduces the amplitude of the net depolarizing current and thus reduces the peak amplitude and the duration of the action potential.

scholarly journals Voltage-gated transient currents in bovine adrenal fasciculata cells. I. T-type Ca2+ current.

1993 ◽  
Vol 102 (2) ◽  
pp. 217-237 ◽  
Author(s):  
B Mlinar ◽  
B A Biagi ◽  
J J Enyeart
Keyword(s):  
Time Constant ◽  
Low Voltage ◽  
Zona Fasciculata ◽  
Patch Clamp Technique ◽  
Time Constants ◽  
Voltage Gated ◽  
Wide Range ◽  
Boltzmann Function ◽  
Voltage Dependent ◽  
Ca2 Channels

The whole cell version of the patch clamp technique was used to identify and characterize voltage-gated Ca2+ channels in enzymatically dissociated bovine adrenal zona fasciculata (AZF) cells. The great majority of cells (84 of 86) expressed only low voltage-activated, rapidly inactivating Ca2+ current with properties of T-type Ca2+ current described in other cells. Voltage-dependent activation of this current was fit by a Boltzmann function raised to an integer power of 4 with a midpoint at -17 mV. Independent estimates of the single channel gating charge obtained from the activation curve and using the "limiting logarithmic potential sensitivity" were 8.1 and 6.8 elementary charges, respectively. Inactivation was a steep function of voltage with a v1/2 of -49.9 mV and a slope factor K of 3.73 mV. The expression of a single Ca2+ channel subtype by AZF cells allowed the voltage-dependent gating and kinetic properties of T current to be studied over a wide range of potentials. Analysis of the gating kinetics of this Ca2+ current indicate that T channel activation, inactivation, deactivation (closing), and reactivation (recovery from inactivation) each include voltage-independent transitions that become rate limiting at extreme voltages. Ca2+ current activated with voltage-dependent sigmoidal kinetics that were described by an m4 model. The activation time constant varied exponentially at test potentials between -30 and +10 mV, approaching a voltage-independent minimum of 1.6 ms. The inactivation time constant (tau i) also decreased exponentially to a minimum of 18.3 ms at potentials positive to 0 mV. T channel closing (deactivation) was faster at more negative voltages; the deactivation time constant (tau d) decreased from 8.14 +/- 0.7 to 0.48 +/- 0.1 ms at potentials between -40 and -150 mV. T channels inactivated by depolarization returned to the closed state along pathways that included two voltage-dependent time constants. tau rec-s ranged from 8.11 to 4.80 s when the recovery potential was varied from -50 to -90 mV, while tau rec-f decreased from 1.01 to 0.372 s. At potentials negative to -70 mV, both time constants approached minimum values. The low voltage-activated Ca2+ current in AZF cells was blocked by the T channel selective antagonist Ni2+ with an IC50 of 20 microM. At similar concentrations, Ni2+ also blocked cortisol secretion stimulated by adrenocorticotropic hormone. Our results indicate that bovine AZF cells are distinctive among secretory cells in expressing primarily or exclusively T-type Ca2+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Contribution of L-type Ca2+current to electrical activity in sinoatrial nodal myocytes of rabbits

1999 ◽  
Vol 276 (3) ◽  
pp. H1064-H1077 ◽  
Author(s):  
E. Etienne Verheijck ◽  
Antoni C. G. van Ginneken ◽  
Ronald Wilders ◽  
Lennart N. Bouman
Keyword(s):  
Action Potential ◽  
Calcium Current ◽  
Sodium Current ◽  
Delayed Rectifier ◽  
Current Clamp ◽  
Patch Clamp Technique ◽  
Inward Current ◽  
Delayed Rectifier Current ◽  
Diastolic Depolarization ◽  
Recovery From Inactivation

The role of L-type calcium current ( I Ca,L) in impulse generation was studied in single sinoatrial nodal myocytes of the rabbit, with the use of the amphotericin-perforated patch-clamp technique. Nifedipine, at a concentration of 5 μM, was used to block I Ca,L. At this concentration, nifedipine selectively blocked I Ca,L for 81% without affecting the T-type calcium current ( I Ca,T), the fast sodium current, the delayed rectifier current ( I K), and the hyperpolarization-activated inward current. Furthermore, we did not observe the sustained inward current. The selective action of nifedipine on I Ca,L enabled us to determine the activation threshold of I Ca,L, which was around −60 mV. As nifedipine (5 μM) abolished spontaneous activity, we used a combined voltage- and current-clamp protocol to study the effects of I Ca,L blockade on repolarization and diastolic depolarization. This protocol mimics the action potential such that the repolarization and subsequent diastolic depolarization are studied in current-clamp conditions. Nifedipine significantly decreased action potential duration at 50% repolarization and reduced diastolic depolarization rate over the entire diastole. Evidence was found that recovery from inactivation of I Ca,L occurs during repolarization, which makes I Ca,L available already early in diastole. We conclude that I Ca,L contributes significantly to the net inward current during diastole and can modulate the entire diastolic depolarization.

Membrane currents in a calcitonin-secreting human C cell line

1992 ◽  
Vol 263 (5) ◽  
pp. C986-C994 ◽  
Author(s):  
B. A. Biagi ◽  
B. Mlinar ◽  
J. J. Enyeart
Keyword(s):  
Cell Line ◽  
Time Constant ◽  
Membrane Currents ◽  
Human Thyroid ◽  
Delayed Rectifier ◽  
Patch Clamp Technique ◽  
Voltage Gated ◽  
Voltage Dependent ◽  
Tt Cells ◽  
K Currents

The whole cell version of the patch-clamp technique was used to identify and characterize voltage-gated Ca2+, Na+, and K+ currents in the calcitonin-secreting human thyroid TT cell line. Ca2+ current consisted of a single low-voltage-activated rapidly inactivating component. The current was one-half maximally activated at a potential of -27 mV, while steady-state voltage-dependent inactivation was one-half complete at -51 mV. The Ca2+ current inactivated with a voltage-dependent time constant that reached a minimum of 16 ms at potentials positive to -15 mV. Deactivation kinetics could also be fit with a single voltage-dependent time constant of approximately 2 ms at -80 mV. Replacing Ca2+ with Ba2+ reduced the maximum current by 18 +/- 5% (n = 6). The dihydropyridine Ca2+ agonist (-)BAY K 8644 did not affect the Ca2+ current, but 50 microM Ni2+ reduced it by 81 +/- 0.8% (n = 5). TT cells also possessed tetrodotoxin-sensitive voltage-gated Na+ channels and tetraethylammonium-sensitive delayed rectifier type K+ currents. These results indicate that TT cells possess membrane currents necessary for the generation of action potentials. T-type Ca2+ channels are the sole pathway for voltage-dependent Ca2+ entry into these cells and may couple electrical activity to calcitonin secretion.

Voltage-gated potassium currents in acutely dissociated rat cortical neurons

1993 ◽  
Vol 70 (1) ◽  
pp. 51-63 ◽  
Author(s):  
R. C. Foehring ◽  
D. J. Surmeier
Keyword(s):  
Time Constant ◽  
Cortical Neurons ◽  
Voltage Dependence ◽  
Inactivation Kinetics ◽  
Voltage Gated ◽  
Time To Peak ◽  
Recovery From Inactivation ◽  
Rat Cortical Neurons ◽  
K Current ◽  
A Current

1. We describe three outward K+ current components in acutely dissociated neurons from rat sensorimotor cortex on the basis of inactivation kinetics and voltage dependence. 2. The fast A current (IAf) was completely inactivated at -40 mV and half-inactivated at -52 mV. It activated [time to peak (TTP) 8 ms at -10 mV] and was inactivated (tau inact = 12 ms at -10 mV) rapidly. Recovery from inactivation had a time constant of approximately 80 ms at -100 mV. It was insensitive to tetraethyl ammonium (TEA) and dendrotoxin but was blocked by 4-aminopyridine (4-AP, IC50 = 1 mM). 3. The slowly inactivating current (IKS) was the largest current seen in acutely dissociated adult neurons. It was completely inactivated at -40 mV, half-inactivated at -98 mV, and was kinetically slower (TTP = 130 ms at -10 mV; tau inact = 293 ms at -10 mV) than the fast A current. Deactivation tails were fit with the sum of two exponentials with time constants of 2-10 and 15-40 ms. IKS recovered from inactivation with a time constant of approximately 1,200 ms at -100 mV. 4. There were two components that inactivated with even slower kinetics. The very slowly inactivating current (IKSS) was operationally defined as the current remaining after a 5-s hold at -40 mV. One component inactivated with a time constant of 1,927 ms at -10 mV. The other component showed no inactivation over a 5-s test command, but in 40- to 50-s steps to -10 mV, inactivated with a tau of approximately 20 s. The very slowly inactivating current activated with similar kinetics to IKS (TTP = 121 ms at -10 mV), and two deactivation tails, with kinetics similar to those after the -100 mV prepulse, were observed after holding at -40 mV. 5. Both IKS and IKSS were sensitive to TEA. Seventy-six percent (76%) of IKSS was blocked by 30 mM TEA. Two components to the TEA block were present for IKSS, with IC50s of 88 microM (67% of blockable current) and 7 mM (33%). Seventy percent (70%) of IKS was blocked by 30 mM TEA. For the IKS current, there were also two effective concentrations, with IC50s of 8 microM (21% of blockade current) and 3 mM (79%). 6. IKS and IKSS were also sensitive to 4-AP. Seventy-six percent (76%) of IKSS was blocked by 3-5 mM 4-AP. IKSS exhibited two components of 4-AP block.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization and functional consequences of delayed rectifier current transient in ventricular repolarization

2000 ◽  
Vol 278 (3) ◽  
pp. H806-H817 ◽  
Author(s):  
Gary A. Gintant
Keyword(s):  
Action Potential ◽  
Ventricular Myocytes ◽  
Outward Current ◽  
Inward Rectifier ◽  
Ventricular Repolarization ◽  
Delayed Rectifier ◽  
Voltage Range ◽  
Delayed Rectifier Current ◽  
Recovery From Inactivation ◽  
Voltage Dependent

Although inactivation of the rapidly activating delayed rectifier current ( I Kr) limits outward current on depolarization, the role of I Kr (and recovery from inactivation) during repolarization is uncertain. To characterize I Krduring ventricular repolarization (and compare with the inward rectifier current, I K1), voltage-clamp waveforms simulating the action potential were applied to canine ventricular, atrial, and Purkinje myocytes. In ventricular myocytes, I Kr was minimal at plateau potentials but transiently increased during repolarizing ramps. The I Kr transient was unaffected by repolarization rate and maximal after 150-ms depolarizations (+25 mV). Action potential clamps revealed the I Kr transient terminating the plateau. Although peak I Kr transient density was relatively uniform among myocytes, potentials characterizing the peak transients were widely dispersed. In contrast, peak inward rectifier current ( I K1) density during repolarization was dispersed, whereas potentials characterizing I K1 defined a narrower (more negative) voltage range. In summary, rapidly activating I Kr provides a delayed voltage-dependent (and functionally time-independent) outward transient during ventricular repolarization, consistent with rapid recovery from inactivation. The heterogeneous voltage dependence of I Kr provides a novel means for modulating the contribution of this current during repolarization.

scholarly journals Ginsenoside Rb1 exerts antiarrhythmic effects by inhibiting INa and ICaL in rabbit ventricular myocytes

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Zhipei Liu ◽  
Lv Song ◽  
Peipei Zhang ◽  
Zhenzhen Cao ◽  
Jie Hao ◽  
...  
Keyword(s):  
Ion Channels ◽  
Action Potential ◽  
Potassium Current ◽  
Ischemia Reperfusion ◽  
Calcium Currents ◽  
Delayed Rectifier ◽  
Dependent Manner ◽  
Ginsenoside Rb1 ◽  
Patch Clamp Technique ◽  
High Calcium

AbstractGinsenoside Rb1 exerts its pharmacological action by regulating sodium, potassium and calcium ion channels in the membranes of nerve cells. These ion channels are also present in cardiomyocytes, but no studies have been reported to date regarding the effects of Rb1 on cardiac sodium currents (INa), L-type calcium currents (ICaL) and action potentials (APs). Additionally, the antiarrhythmic potential of Rb1 has not been assessed. In this study, we used a whole-cell patch clamp technique to assess the effect of Rb1 on these ion channels. The results showed that Rb1 inhibited INa and ICaL, reduced the action potential amplitude (APA) and maximum upstroke velocity (Vmax), and shortened the action potential duration (APD) in a concentration-dependent manner but had no effect on the inward rectifier potassium current (IK1), delayed rectifier potassium current (IK) or resting membrane potential (RMP). We also designed a pathological model at the cellular and organ level to verify the role of Rb1. The results showed that Rb1 abolished high calcium-induced delayed afterdepolarizations (DADs), depressed the increase in intracellular calcium ([Ca2+]i), relieved calcium overload and protected cardiomyocytes. Rb1 can also reduce the occurrence of ventricular premature beats (VPBs) and ventricular tachycardia (VT) in ischemia-reperfusion (I-R) injury.

Current- and Voltage-Clamp Recordings and Computer Simulations of Kenyon Cells in the Honeybee

2004 ◽  
Vol 92 (4) ◽  
pp. 2589-2603 ◽  
Author(s):  
Daniel G. Wüstenberg ◽  
Milena Boytcheva ◽  
Bernd Grünewald ◽  
John H. Byrne ◽  
Randolf Menzel ◽  
...  
Keyword(s):  
Voltage Clamp ◽  
Mushroom Body ◽  
Outward Current ◽  
Delayed Rectifier ◽  
Transient Outward Current ◽  
Insect Brain ◽  
Type Model ◽  
Kenyon Cells ◽  
Fast Transient

The mushroom body of the insect brain is an important locus for olfactory information processing and associative learning. The present study investigated the biophysical properties of Kenyon cells, which form the mushroom body. Current- and voltage-clamp analyses were performed on cultured Kenyon cells from honeybees. Current-clamp analyses indicated that Kenyon cells did not spike spontaneously in vitro. However, spikes could be elicited by current injection in approximately 85% of the cells. Of the cells that produced spikes during a 1-s depolarizing current pulse, approximately 60% exhibited repetitive spiking, whereas the remaining approximately 40% fired a single spike. Cells that spiked repetitively showed little frequency adaptation. However, spikes consistently became broader and smaller during repetitive activity. Voltage-clamp analyses characterized a fast transient Na+ current ( INa), a delayed rectifier K+ current ( IK,V), and a fast transient K+ current ( IK,A). Using the neurosimulator SNNAP, a Hodgkin–Huxley-type model was developed and used to investigate the roles of the different currents during spiking. The model led to the prediction of a slow transient outward current ( IK,ST) that was subsequently identified by reevaluating the voltage-clamp data. Simulations indicated that the primary currents that underlie spiking are INa and IK,V, whereas IK,A and IK,ST primarily determined the responsiveness of the model to stimuli such as constant or oscillatory injections of current.

Discrepant effects of heart failure on electrophysiological property in right ventricular outflow tract and left ventricular outflow tract cardiomyocytes

2017 ◽  
Vol 131 (12) ◽  
pp. 1317-1327 ◽  
Author(s):  
Yen-Yu Lu ◽  
Chen-Chuan Cheng ◽  
Chin-Feng Tsai ◽  
Yung-Kuo Lin ◽  
Ting-I Lee ◽  
...  
Keyword(s):  
Heart Failure ◽  
Action Potential ◽  
Ventricular Arrhythmias ◽  
Ionic Currents ◽  
Ventricular Outflow Tract ◽  
Left Ventricular ◽  
Outflow Tract ◽  
Delayed Rectifier ◽  
Patch Clamp Technique ◽  
Left Ventricular Outflow

Ventricular arrhythmias commonly arise from the right (RVOT) and left ventricular outflow tracts (LVOT) in patients without structural heart disease. Heart failure (HF) significantly increases the risk of ventricular arrhythmias. The regional differences and how HF affects the electrophysiological characteristics of RVOT and LVOT cardiomyocytes remain unclear. The whole-cell patch-clamp technique was used to investigate the action potentials and ionic currents in isolated single RVOT and LVOT cardiomyocytes from control rabbits and rabbits with HF induced by rapid ventricular pacing. Comparison with control LVOT cardiomyocytes showed that control RVOT cardiomyocytes have a shorter action potential duration (APD), smaller late Na+ currents (INa-late), larger transient outward (Ito) and larger delayed rectifier K+ currents (IKr-tail), but had similar L-type Ca2+ currents (ICa-L) and Na+/Ca2+ exchanger (NCX) current. HF increased APD, INa-late and NCX, but decreased ICa-L and Ito in RVOT cardiomyocytes. In contrast with this, HF decreased APD and ICa-L, but increased Ito and IKr-tail in LVOT cardiomyocytes. In conclusion, RVOT and LVOT cardiomyocytes had distinctive electrophysiological characteristics. HF differentially modulates action potential morphology and ionic currents in RVOT and LVOT cardiomyocytes.

scholarly journals Fast inactivation causes rectification of the IKr channel.

1996 ◽  
Vol 107 (5) ◽  
pp. 611-619 ◽  
Author(s):  
P S Spector ◽  
M E Curran ◽  
A Zou ◽  
M T Keating ◽  
M C Sanguinetti
Keyword(s):  
Outward Current ◽  
K Channel ◽  
Delayed Rectifier ◽  
Fast Inactivation ◽  
Time Constants ◽  
Recovery From Inactivation ◽  
Current Voltage ◽  
Voltage Dependent ◽  
Cardiac Delayed Rectifier ◽  
Cytoplasmic Side

The mechanism of rectification of HERG, the human cardiac delayed rectifier K+ channel, was studied after heterologous expression in Xenopus oocytes. Currents were measured using two-microelectrode and macropatch voltage clamp techniques. The fully activated current-voltage (I-V) relationship for HERG inwardly rectified. Rectification was not altered by exposing the cytoplasmic side of a macropatch to a divalent-free solution, indicating this property was not caused by voltage-dependent block of outward current by Mg2+ or other soluble cytosolic molecules. The instantaneous I-V relationship for HERG was linear after removal of fast inactivation by a brief hyperpolarization. The time constants for the onset of and recovery from inactivation were a bell-shaped function of membrane potential. The time constants of inactivation varied from 1.8 ms at +50 mV to 16 ms at -20 mV; recovery from inactivation varied from 4.7 ms at -120 mV to 15 ms at -50 mV. Truncation of the NH2-terminal region of HERG shifted the voltage dependence of activation and inactivation by +20 to +30 mV. In addition, the rate of deactivation of the truncated channel was much faster than wild-type HERG. The mechanism of HERG rectification is voltage-gated fast inactivation. Inactivation of channels proceeds at a much faster rate than activation, such that no outward current is observed upon depolarization to very high membrane potentials. Fast inactivation of HERG and the resulting rectification are partly responsible for the prolonged plateau phase typical of ventricular action potentials.

Ionic currents during action potentials in mammalian skeletal muscle fibers analyzed with loose patch clamp

1994 ◽  
Vol 267 (6) ◽  
pp. C1699-C1706 ◽  
Author(s):  
H. Wolters ◽  
W. Wallinga ◽  
D. L. Ypey ◽  
H. B. Boom
Keyword(s):  
Patch Clamp ◽  
Action Potential ◽  
Action Potentials ◽  
Potassium Current ◽  
Sodium Current ◽  
Delayed Rectifier ◽  
Mammalian Skeletal Muscle ◽  
Action Potential Generation ◽  
Potential Generation ◽  
Patch Clamp Technique

The loose patch-clamp technique was applied to analyze transmembrane currents during propagating action potentials in superficial fibers of musculi extensor digitorum longus of the mouse in vitro. Experimentally three components were identified in the transmembrane current: 1) a capacitive, 2) an inward sodium, and 3) an outward potassium current. Other components were negligible. The capacitive current was similar in shape to the first derivative of the intracellularly measured action potential. Tetrodotoxin, tetraethylammonium, and 4-aminopyridine, applied in the pipette, were used to identify the contribution in the current by sodium and potassium ions. With extracellularly applied depolarization steps only a sodium current was observed, not a potassium current. Occasionally found outward currents were artifactual. The behaviour of delayed rectifier potassium channels in muscle fiber membranes is discussed in the light of these unexpected findings. We conclude that potassium channel activity contributing to and measured during action potential generation is in some way inaccessible to loose patch extracellular voltage-clamp stimulation and that loose patch action current recording is a useful noninvasive method to analyze membrane conductances involved in action potential generation.

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