Hyperpolarizing shift by quinine in the steady-state inactivation curve of delayed rectifier-type potassium current in bullfrog sympathetic neurons

2001 ◽  
Vol 300 (2) ◽  
pp. 87-90 ◽  
Author(s):  
Susumu Kotani ◽  
Jun Hasegawa ◽  
Hongxu Meng ◽  
Tomotaro Suzuki ◽  
Kazunori Sato ◽  
...  
Keyword(s):  
Steady State ◽  
Potassium Current ◽  
Sympathetic Neurons ◽  
Delayed Rectifier ◽  
Inactivation Curve ◽  
Steady State Inactivation ◽  
Hyperpolarizing Shift

Eicosatetraynoic acid (ETYA), a non-metabolizable analogue of arachidonic acid, blocks the fast-inactivating potassium current of rat pituitary melanotrophs

2001 ◽  
Vol 79 (4) ◽  
pp. 338-345 ◽  
Author(s):  
S J Kehl
Keyword(s):  
Arachidonic Acid ◽  
Potassium Current ◽  
Eicosatetraenoic Acid ◽  
Delayed Rectifier ◽  
Patch Clamp Technique ◽  
Inactivation Curve ◽  
Current Decay ◽  
Bath Application ◽  
Rat Pituitary ◽  
Steady State Inactivation

The effects of arachidonic acid (5,8,11,14-eicosatetraenoic acid, AA) and 5,8,11,14-eicosatetraynoic acid (ETYA), a non-metabolizable analogue of AA, were examined on the transient [IK(f)] and the delayed rectifier-like [IK(s)] voltage-gated potassium currents in rat pituitary melanotrophs. The main questions addressed were whether AA and ETYA blocked IK(f) and if any blocking action was specific. Macroscopic currents were measured using the patch clamp technique. Bath application of 20 µM AA reduced IK(f), however, the degree of the block varied between cells. In contrast, ETYA consistently inhibited IK(f). Fitting of the charge transfer or the peak current amplitude yielded KD estimates for ETYA of 1.2 µM and 3.3 µM, respectively. The reduction by ETYA of peak IK(f) was always associated with an increased rate of current decay, but there was no detectable change of the kinetics of activation. ETYA caused a small left shift of the IK(f) steady-state inactivation curve and significantly slowed recovery from inactivation. At 20 µM, ETYA also reduced IK(s), indicating that it is not specific. The possibility that ETYA acts as an open-channel blocker is discussed.Key words: transient potassium current, melanotroph, eicosatetraynoic acid, ETYA, arachidonic acid.

Synaptic Current at the Rat Ganglionic Synapse and Its Interactions With the Neuronal Voltage-Dependent Currents

1998 ◽  
Vol 79 (2) ◽  
pp. 727-742 ◽  
Author(s):  
Oscar Sacchi ◽  
Maria Lisa Rossi ◽  
Rita Canella ◽  
Riccardo Fesce
Keyword(s):  
Steady State ◽  
Action Potential ◽  
Time Constant ◽  
Voltage Dependence ◽  
Sympathetic Neurons ◽  
Excitatory Postsynaptic Potential ◽  
Synaptic Current ◽  
Inactivation Curve ◽  
Voltage Dependent ◽  
Steady State Inactivation

Sacchi, Oscar, Maria Lisa Rossi, Rita Canella, and Riccardo Fesce. Synaptic current at the rat ganglionic synapse and its interactions with the neuronal voltage-dependent currents. J. Neurophysiol. 79: 727–742, 1998. The membrane current activated by fast nicotinic excitation of intact and mature rat sympathetic neurons was studied at 37°C, by using the two-microelectrode voltage-clamp technique. The excitatory postsynaptic current (EPSC) was modeled as the difference between two exponentials. A fast time constant (τ2; mean value 0.57 ms), which proves to be virtually voltage-independent, governs the current rise phase and a longer time constant (τ1; range 5.2–6.8 ms in 2 mM Ca2+) describes the current decay and shows a small negative voltage dependence. A mean peak synaptic conductance of 0.58 μS per neuron is measured after activation of the whole presynaptic input in 5 mM Ca2+ external solution (0.40 μS in 2 mM Ca2+). The miniature EPSCs also rise and decay with exponential time constants very similar to those of the compound EPSC recorded at the same voltage. A mean peak conductance of 4.04 nS is estimated for the unitary event. Deconvolution procedures were employed to decompose evoked macrocurrents. It is shown that under appropriate conditions the duration of the driving function describing quantal secretion can be reduced to <1 ms. The shape of the EPSC is accurately mimicked by a complete mathematical model of the sympathetic neuron incorporating the kinetic properties of five different voltage-dependent current types, which were characterized in a previous work. We show that I A channels are opened by depolarizing voltage steps or by synaptic potentials in the subthreshold voltage range, provided that the starting holding voltage is sufficiently negative to remove I A steady-state inactivation (less than −50 mV) and the voltage trajectories are sufficiently large to enter the I A activation range (greater than −65 mV). Under current-clamp conditions, this gives rise to an additional fast component in the early phase of membrane repolarization—in response to voltage pulses—and to a consistent distortion of the excitatory postsynaptic potential (EPSP) time course around its peak—in response to the synaptic signal. When the stimulation initiates an action potential, I A is shown to significantly increase the synaptic threshold conductance (up to a factor of 2 when I A is fully deinactivated), compared with that required when I A is omitted. The voltage dependence of this effect is consistent with the I A steady-state inactivation curve. It is concluded that I A, in addition to speeding up the spike repolarization process, also shunts the excitatory drive and delays or prevents the firing of the neuron action potential.

Ionic currents in hair cells dissociated from frog semicircular canals after preconditioning under microgravity conditions

2009 ◽  
Vol 296 (5) ◽  
pp. R1585-R1597 ◽  
Author(s):  
Marta Martini ◽  
Rita Canella ◽  
Alessandro Leparulo ◽  
Ivo Prigioni ◽  
Riccardo Fesce ◽  
...  
Keyword(s):  
Steady State ◽  
Hair Cells ◽  
Potassium Current ◽  
Time Course ◽  
Calcium Influx ◽  
Current System ◽  
Patch Clamp Technique ◽  
Inactivation Curve ◽  
Steady State Inactivation ◽  
Microgravity Conditions

The effects of microgravity on the biophysical properties of frog labyrinthine hair cells have been examined by analyzing calcium and potassium currents in isolated cells by the patch-clamp technique. The entire, anesthetized frog was exposed to vector-free gravity in a random positioning machine (RPM) and the functional modification induced on single hair cells, dissected from the crista ampullaris, were subsequently studied in vitro. The major targets of microgravity exposure were the calcium/potassium current system and the kinetic mechanism of the fast transient potassium current, IA. The amplitude of ICa was significantly reduced in microgravity-conditioned cells. The delayed current, IKD (a complex of IKV and IKCa), was drastically reduced, mostly in its IKCa component. Microgravity also affected IKD kinetics by shifting the steady-state inactivation curve toward negative potentials and increasing the sensitivity of inactivation removal to voltage. As concerns the IA, the I- V and steady-state inactivation curves were indistinguishable under normogravity or microgravity conditions; conversely, IA decay systematically displayed a two-exponential time course and longer time constants in microgravity, thus potentially providing a larger K+ charge; furthermore, IA inactivation removal at −70 mV was slowed down. Stimulation in the RPM machine under normogravity conditions resulted in minor effects on IKD and, occasionally, incomplete IA inactivation at −40 mV. Reduced calcium influx and increased K+ repolarizing charge, to variable extents depending on the history of membrane potential, constitute a likely cause for the failure in the afferent mEPSP discharge at the cytoneural junction observed in the intact labyrinth after microgravity conditioning.

scholarly journals Arachidonic acid modulation of α1H, a cloned human T-type calcium channel

2000 ◽  
Vol 278 (1) ◽  
pp. H184-H193 ◽  
Author(s):  
Yi Zhang ◽  
Leanne L. Cribbs ◽  
Jonathan Satin
Keyword(s):  
Arachidonic Acid ◽  
Steady State ◽  
Low Voltage ◽  
Specific Inhibitor ◽  
Nordihydroguaiaretic Acid ◽  
Ca Channel ◽  
Lipoxygenase Inhibitor ◽  
Inactivation Curve ◽  
Steady State Inactivation ◽  
Window Current

Arachidonic acid (AA) and the products of its metabolism are central mediators of changes in cellular excitability. We show that the recently cloned and expressed T-type or low-voltage-activated Ca channel, α1H, is modulated by external AA. AA (10 μM) causes a slow, time-dependent attenuation of α1H current. At a holding potential of −80 mV, 10 μM AA reduces peak inward α1H current by 15% in 15 min and 70% in 30 min and shifts the steady-state inactivation curve −25 mV. AA inhibition was not affected by applying the cyclooxygenase inhibitor indomethacin or the lipoxygenase inhibitor nordihydroguaiaretic acid. The epoxygenase inhibitor octadecynoic acid partially antagonized AA attenuation of α1H. The epoxygenase metabolite epoxyeicosatrienoic acid (8,9-EET) mimicked the inhibitory effect of AA on α1H peak current. A protein kinase C (PKC)-specific inhibitor (peptide fragment 19–36) only partially antagonized the AA-induced reduction of peak α1H current and the shift of the steady-state inactivation curve but had no effect on 8,9-EET-induced attenuation of current. In contrast, PKA has no role in the modulation of α1H. These results suggest that AA attenuation and shift of α1H may be mediated directly by AA. The heterologous expression of T-type Ca channels allows us to study for the first time properties of this important class of ion channel in isolation. There is a significant overlap of the steady-state activation and inactivation curves, which implies a substantial window current. The selective shift of the steady-state inactivation curve by AA reduces peak Ca current and eliminates the window current. We conclude that AA may partly mediate physiological effects such as vasodilatation via the attenuation of T-type Ca channel current and the elimination of a T-type channel steady window current.

scholarly journals Effect of acidosis on transient outward potassium current in isolated rat ventricular myocytes

2000 ◽  
Vol 278 (1) ◽  
pp. H50-H59 ◽  
Author(s):  
J. T. Hulme ◽  
C. H. Orchard
Keyword(s):  
Potassium Current ◽  
Voltage Dependence ◽  
Ventricular Myocytes ◽  
Solution Ph ◽  
Patch Clamp Technique ◽  
Inactivation Curve ◽  
Rat Ventricular Myocytes ◽  
Steady State Inactivation ◽  
Perforated Patch Clamp ◽  
Outward Potassium Current

The effect of acidosis on the transient outward K+ current ( Ito ) of rat ventricular myocytes has been investigated using the perforated patch-clamp technique. When the holding potential was −80 mV, depolarizing pulses to potentials positive to −20 mV activated Ito in subepicardial cells but activated little Ito in subendocardial cells. Exposure to an acid solution (pH 6.5) had no significant effect on Ito activated from this holding potential in either subepicardial or subendocardial cells. When the holding potential was −40 mV, acidosis significantly increased Ito at potentials positive to −20 mV in subepicardial cells but had little effect on Ito in subendocardial cells. The increase in Ito in subepicardial cells was inhibited by 10 mM 4-aminopyridine. In subepicardial cells, acidosis caused a +8.57-mV shift in the steady-state inactivation curve. It is concluded that in subepicardial rat ventricular myocytes acidosis increases the amplitude of Ito as a consequence of a depolarizing shift in the voltage dependence of inactivation.

Effects of estrogens on Ca channels in myometrial cells isolated from pregnant rats

1995 ◽  
Vol 268 (1) ◽  
pp. C64-C69 ◽  
Author(s):  
T. Yamamoto
Keyword(s):  
Steady State ◽  
Ca Channel ◽  
Ca Channels ◽  
Inhibitory Effects ◽  
Inactivation Curve ◽  
Pregnant Rat ◽  
Negative Direction ◽  
Myometrial Cells ◽  
Steady State Inactivation ◽  
Channel Currents

Whole cell patch-clamp techniques were applied to cultured smooth muscle cells isolated from the longitudinal layer of the late pregnant rat myometrium. Effects of estrogens on Ca channels were examined. Inhibitory effects of beta-estradiol (1 microM) on Ca channel currents were recognized. The inhibitory effects of beta-estradiol depended on holding potentials. beta-Estradiol shifted the steady-state inactivation curve in the negative direction by 7 mV at mid potential (n = 9). Diethylstilbestrol, a synthetic estrogen, gave similar effects on Ca channel currents at lower concentration (2 microM) to those of beta-estradiol. Strong inhibitory effects on Ca channel currents were obtained by higher concentration (20 microM). Diethylstilbestrol shifted the steady-state inactivation curve in the negative direction by 7 mV at mid potential (n = 5). The results indicate that estrogens influence the voltage dependency and the whole cell conductance of Ca channels of pregnant rat myometrial cells. The acute effect of estrogens may cause both electrical and mechanical depression of myometrium.

scholarly journals Voltage-dependent inactivation of slow calcium channels in intact twitch muscle fibers of the frog.

1989 ◽  
Vol 94 (5) ◽  
pp. 937-951 ◽  
Author(s):  
G Cota ◽  
E Stefani
Keyword(s):  
Steady State ◽  
Rate Constant ◽  
Voltage Dependence ◽  
Muscle Fibers ◽  
Dependent Manner ◽  
Inactivation Curve ◽  
Voltage Dependent ◽  
Steady State Inactivation ◽  
Intact Muscle ◽  
Ca2 Channels

Inactivation of slow Ca2+ channels was studied in intact twitch skeletal muscle fibers of the frog by using the three-microelectrode voltage-clamp technique. Hypertonic sucrose solutions were used to abolish contraction. The rate constant of decay of the slow Ca2+ current (ICa) remained practically unchanged when the recording solution containing 10 mM Ca2+ was replaced by a Ca2+-buffered solution (126 mM Ca-maleate). The rate constant of decay of ICa monotonically increased with depolarization although the corresponding time integral of ICa followed a bell-shaped function. The replacement of Ca2+ by Ba2+ did not result in a slowing of the rate of decay of the inward current nor did it reduce the degree of steady-state inactivation. The voltage dependence of the steady-state inactivation curve was steeper in the presence of Ba2+. In two-pulse experiments with large conditioning depolarizations ICa inactivation remained unchanged although Ca2+ influx during the prepulse greatly decreased. Dantrolene (12 microM) increased mechanical threshold at all pulse durations tested, the effect being more prominent for short pulses. Dantrolene did not significantly modify ICa decay and the voltage dependence of inactivation. These results indicate that in intact muscle fibers Ca2+ channels inactivate in a voltage-dependent manner through a mechanism that does not require Ca2+ entry into the cell.

Stimulatory effect of ouabain on T- and L-type calcium currents in guinea pig cardiac myocytes

1990 ◽  
Vol 258 (5) ◽  
pp. H1620-H1623 ◽  
Author(s):  
B. Le Grand ◽  
E. Deroubaix ◽  
A. Coulombe ◽  
E. Coraboeuf
Keyword(s):  
Steady State ◽  
Guinea Pig ◽  
Ventricular Myocytes ◽  
Calcium Currents ◽  
High Threshold ◽  
Inactivation Curve ◽  
Cell Recording ◽  
Steady State Inactivation ◽  
Low Threshold ◽  
Free Media

The effect of 10(-6) M ouabain on macroscopic low-threshold T-type Ca2+ and high-threshold L-type Ca2+ currents was studied by whole cell recording in isolated guinea pig ventricular myocytes superfused with K-free, Na-free media, i.e., after suppression of Na-K-ATPase activity and Na influx through the Na-Ca exchanger. Under such conditions, the amplitudes of the two currents were significantly increased by ouabain. In particular, the current occurring in the -50 to -20 mV range (T-type Ca2+) was increased two- to threefold by ouabain and suppressed by 40 microM Ni2+. Ouabain shifted by approximately 10 mV toward negative potentials the steady-state inactivation curve of the T-type Ca2+ current but not that of the L-type Ca2+ current. It is concluded that ouabain enhances not only L-type Ca2+ current but also T-type Ca2+ current possibly through different mechanisms.

Delayed rectifier K+ current of dog bronchial myocytes: effect of pollen sensitization and PKC activation

1998 ◽  
Vol 275 (2) ◽  
pp. L336-L347 ◽  
Author(s):  
Gareth J. Waldron ◽  
Stefan B. Sigurdsson ◽  
Ernesto A. Aiello ◽  
Andrew J. Halayko ◽  
Newman L. Stephens ◽  
...  
Keyword(s):  
Current Density ◽  
Voltage Dependence ◽  
Ragweed Pollen ◽  
Delayed Rectifier ◽  
Airway Smooth Muscle Cells ◽  
Average Current Density ◽  
Whole Cell Patch Clamp ◽  
Steady State Inactivation ◽  
Pkc Activation ◽  
Hyperpolarizing Shift

The properties of delayed rectifier K+ current [ I K(dr)] of canine airway smooth muscle cells isolated from small bronchi and its modulation by protein kinase C (PKC) were studied by whole cell patch clamp. I K(dr) activated positive to −40 mV, with half-maximal activation at −16 ± 1.2 mV ( n = 15) and average current density of 31 ± 2.6 pA/pF ( n = 15) at +30 mV. The capacitive surface area, current density, and voltage dependence of activation of I K(dr) of myocytes of ragweed pollen-sensitized dogs were not different from age-matched control dogs. However, the sensitization reduced the availability of I K(dr) between −40 and −20 mV due to a hyperpolarizing shift in the voltage dependence of steady-state inactivation (−29.9 ± 1.2 in sensitized versus −26.0 ± 0.7 mV in control dogs, n = 9 and 11, respectively; P < 0.05). PKC activation with diacylglycerol analog or phorbol ester depressed I K(dr) amplitude, whereas an inactive diacylglycerol analog had no effect. The hyperpolarizing shift in voltage dependence of inactivation and/or modulation of I K(dr) by PKC may be two mechanisms that contribute to the enhanced reactivity of bronchial tissues from ragweed pollen-sensitized dogs.

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