Separation and identification of multiple potassium currents regulating the pacemaker activity of insect neurosecretory cells (DUM neurons)

1995 ◽  
Vol 73 (1) ◽  
pp. 160-171 ◽  
Author(s):  
F. Grolleau ◽  
B. Lapied
Keyword(s):  
Time Course ◽  
Voltage Dependence ◽  
Outward Current ◽  
Potassium Currents ◽  
Neurosecretory Cells ◽  
Transient Current ◽  
Pacemaker Activity ◽  
Dum Neurons ◽  
K Current ◽  
A Current

1. Whole cell voltage-clamp studies performed in isolated adult neurosecretory cells identified as dorsal unpaired median (DUM) neurons of the terminal abdominal ganglion of the cockroach Periplaneta americana have allowed us to reveal a complex voltage-dependent outward current regulating the pacemaker activity. 2. The global outward current remaining after tetrodotoxin treatment was activated by depolarization above -50 mV, showing steep voltage dependence and outward rectification. 3. We used tail current analysis to determine the ionic selectivity of this outward current. The reversal potentials for two extracellular potassium concentrations (-92.7 and -65.4 mV for 3.1 and 10 mM, respectively) is consistent with the expected equilibrium potential for potassium ions. 4. Both peak and sustained components of the global outward K+ current were reduced by external application of 20 mM tetraethylammonium chloride, 10 nM iberiotoxin, 1 nM charybdotoxin (CTX) and 1 mM cadmium chloride. Subtraction of current recorded in CTX solution from that in control solution revealed an unusual biphasic Ca(2+)-dependent K+ current. The fast transient current resistant to 5 mM 4-aminopyridine (4-AP) is distinguished by its dependence on holding potential and time course from the late sustained current. 5. In addition, two other components of CTX-resistant outward K+ current could be separated by sensitivity to 4-AP, time course, and voltage dependence. Beside a calcium-independent delayed outwardly rectifying current, a 4-AP-sensitive fast transient current resembling the A-current has been also identified. It activates at negative potential (about -65 mV) and unlike the A-current of other neurons, it inactivates rapidly with complex inactivation kinetics. A-like current is half-inactivated at -63.5 mV and half-activated at -35.6 mV. 6. Our findings demonstrate for the first time in DUM neuron cell bodies the existence of multiple potassium currents underlying the spontaneous electrical activity. Their identification and characterization represent a fundamental step in further understanding the pacemaker properties of these insect neurosecretory cells.

scholarly journals A time- and voltage-dependent K+ current in single cardiac cells from bullfrog atrium.

1986 ◽  
Vol 88 (6) ◽  
pp. 777-798 ◽  
Author(s):  
J R Hume ◽  
W Giles ◽  
K Robinson ◽  
E F Shibata ◽  
R D Nathan ◽  
...  
Keyword(s):  
Time Course ◽  
Voltage Dependence ◽  
Outward Current ◽  
Cardiac Cells ◽  
Delayed Rectifier ◽  
Mechanical Dispersion ◽  
Atrial Cells ◽  
Voltage Dependent ◽  
K Current ◽  
Kinetics Of

Individual myocytes were isolated from bullfrog atrium by enzymatic and mechanical dispersion, and a one-microelectrode voltage clamp was used to record the slow outward K+ currents. In normal [K+]o (2.5 mM), the slow outward current tails reverse between -95 and -100 mV. This finding, and the observed 51-mV shift of Erev/10-fold change in [K+]o, strongly suggest that the "delayed rectifier" in bullfrog atrial cells is a K+ current. This current, IK, plays an important role in initiating repolarization, and it is distinct from the quasi-instantaneous, inwardly rectifying background current, IK. In atrial cells, IK does not exhibit inactivation, and very long depolarizing clamp steps (20 s) can be applied without producing extracellular K+ accumulation. The possibility of [K+]o accumulation contributing to these slow outward current changes was assessed by (a) comparing reversal potentials measured after short (2 s) and very long (15 s) activating prepulses, and (b) studying the kinetics of IK at various holding potentials and after systematically altering [K+]o. In the absence of [K+]o accumulation, the steady state activation curve (n infinity) and fully activated current-voltage (I-V) relation can be obtained directly. The threshold of the n infinity curve is near -50 mV, and it approaches a maximum at +20 mV; the half-activation point is approximately -16 mV. The fully activated I-V curve of IK is approximately linear in the range -40 to +30 mV. Semilog plots of the current tails show that each tail is a single-exponential function, which suggests that only one Hodgkin-Huxley conductance underlies this slow outward current. Quantitative analysis of the time course of onset of IK and of the corresponding envelope of tails demonstrate that the activation variable, n, must be raised to the second power to fit the sigmoid onset accurately. The voltage dependence of the kinetics of IK was studied by recording and curve-fitting activating and deactivating (tail) currents. The resulting 1/tau n curve is U-shaped and somewhat asymmetric; IK exhibits strong voltage dependence in the diastolic range of potentials. Changes in the [Ca2+]o in the superfusing Ringer's, and/or addition of La3+ to block the transmembrane Ca2+ current, show that the time course and magnitude of IK are not significantly modulated by transmembrane Ca2+ movements, i.e., by ICa. These experimentally measured voltage- and time-dependent descriptors of IK strongly suggest an important functional role for IK in atrial tissue: it initiates repolarization and can be an important determinant of rate-induced changes in action potential duration.

Transient voltage and calcium-dependent outward currents in hippocampal CA3 pyramidal neurons

1985 ◽  
Vol 53 (4) ◽  
pp. 1038-1058 ◽  
Author(s):  
K. L. Zbicz ◽  
F. F. Weight
Keyword(s):  
Time Course ◽  
Pyramidal Neurons ◽  
Sympathetic Neurons ◽  
Outward Current ◽  
Transient Current ◽  
Transient Outward Current ◽  
Molluscan Neurons ◽  
Voltage Sensitivity ◽  
Transient Currents ◽  
Fast Transient

Membrane currents activated by step changes in membrane potential were studied in hippocampal pyramidal neurons of region CA3 using the single microelectrode voltage-clamp technique. The transient outward current activated by depolarizing steps appeared to be composed of two transient currents that could be distinguished by differences in voltage sensitivity, time course, and pharmacological sensitivity. The more slowly decaying current was activated by voltage steps positive to -60 mV and declined exponentially with a time constant between 200 and 400 ms. This current inactivated as the holding potential was made more positive over the range of -75 to -45 mV and was 50% inactivated near -60 mV. The more slowly decaying transient current was selectively blocked by 0.5 mM 4-aminopyridine (4-AP) but not by 5-10 mM tetraethylammonium (TEA) or 2-5 mM Mn2+. The second transient current had a much faster time course than the 4-AP-sensitive current, having a duration of 5-20 ms. This very fast transient current was observed during potential steps positive to -45 mV. The fast transient current was inactivated when the holding potential was made positive to -45 mV. The amplitude of the fast transient current was greatly reduced by the application of 4 mM Mn2+ or Ca2+-free artificial cerebrospinal fluid (CSF). The fast transient current appeared to be unaffected by 0.5 mM 4-AP but was greatly reduced by 10 mM TEA. These results suggest that the transient outward current observed during depolarizing steps is composed of at least two distinct transient currents. The more slowly decaying current resembles the A-current originally described in molluscan neurons (9, 32, 42) in voltage sensitivity, time course, and pharmacological sensitivity. The faster transient current resembles a fast, Ca2+-dependent transient current previously observed in bull-frog sympathetic neurons (5, 27).

scholarly journals Voltage-dependent conductances in Limulus ventral photoreceptors.

1982 ◽  
Vol 79 (2) ◽  
pp. 187-209 ◽  
Author(s):  
J E Lisman ◽  
G L Fain ◽  
P M O'Day
Keyword(s):  
Outward Current ◽  
Delayed Rectifier ◽  
Molluscan Neurons ◽  
Inward Current ◽  
Transient Component ◽  
Outward Currents ◽  
Voltage Dependent ◽  
Inward Currents ◽  
K Current ◽  
A Current

The voltage-dependent conductances of Limulus ventral photoreceptors have been investigated using a voltage-clamp technique. Depolarization in the dark induces inward and outward currents. The inward current is reduced by removing Na+ or Ca2+ and is abolished by removing both ions. These results suggest that both Na+ and Ca2+ carry voltage-dependent inward current. Inward current is insensitive to tetrodotoxin but is blocked by external Ni2+. The outward current has a large transient component that is followed by a smaller maintained component. Intracellular tetraethylammonium preferentially reduces the maintained component, and extracellular 4-amino pyridine preferentially reduces the transient component. Neither component is strongly affected by removal of extracellular Ca2+ or by intracellular injection of EGTA. It is concluded that the photoreceptors contain at least three separate voltage-dependent conductances: 1) a conductance giving rise to inward currents; 2) a delayed rectifier giving rise to maintained outward K+ current; and 3) a rapidly inactivating K+ conductance similar to the A current of molluscan neurons.

Two distinct low-voltage-activated Ca2+ currents contribute to the pacemaker mechanism in cockroach dorsal unpaired median neurons

1996 ◽  
Vol 76 (2) ◽  
pp. 963-976 ◽  
Author(s):  
F. Grolleau ◽  
B. Lapied
Keyword(s):  
Voltage Dependence ◽  
Low Voltage ◽  
Neurosecretory Cells ◽  
Pacemaker Activity ◽  
Tetraacetic Acid ◽  
Current Clamp ◽  
Patch Clamp Technique ◽  
Time To Peak ◽  
Whole Cell Patch Clamp ◽  
Dorsal Unpaired Median

1. The contribution of Ca2+ currents to the endogenous firing properties of cockroach isolated adult dorsal unpaired median neurons was investigated using whole cell patch-clamp technique with 5 mM Ca2+ as the charge carrier. At least three types of Ca2+ currents, a high-voltage-activated Ca2+ current and two low-voltage-activated (LVA) Ca2+ currents, have been found in these neurons. This study focused on the LVA Ca2+ currents, which are suitable candidates in the generation of the slow predepolarization because of their low threshold of activation. 2. The global LVA Ca2+ current could be dissociated by means of nickel sensitivity, deactivation time constant and voltage dependence of time to peak, tail current amplitude and inactivation, as transient and maintained LVA Ca2+ currents. 3. The transient LVA Ca2+ current, sensitive to 100 microM Ni2+, was isolated by using a subtraction procedure. It was activated at -70 mV and half-inactivated at -59.5 mV. The inactivation was purely voltage dependent. Current-clamp experiments performed with 150 microM Ni2+ indicated that this current was involved in the initial part of the predepolarization. 4. The maintained LVA Ca2+ current, resistant to 100 microM Ni2+, was activated in a range of potential 10 mV more positive than the transient LVA Ca2+ current, and its voltage dependence of inactivation displayed a U-shaped-curve. 5. Replacing Ca2+ with Ba2+ in equimolar amount or low internal Ca2+ concentration [5 mM bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) in the pipette] induced a monotonic voltage dependence of inactivation and increased the rate of relaxation of this current. These effects were mimicked by high internal Ca2+ concentration [0.1 mM Ca2+ and no ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid in the pipette]. This demonstrated an unusual Ca2+-sensitive inactivation process that varied over a narrow range of Ca2+ concentrations. 6. Current-clamp experiments performed under 150 microM Ni2+, with 15 mM external Ca2+ concentration (which potentiated the maintained LVA current within 30 s of superfusion) or with 5 mM BAPTA in the pipette demonstrated the participation of this current in the last two-thirds of the slow predepolarizing phase. 7. Our findings demonstrated, for the first time in neurosecretory cells, the coexistence of two distinct LVA Ca2+ currents, which have specialized function in the generation of the pacemaker activity.

scholarly journals A quantitative study of potassium channel kinetics in rat skeletal muscle from 1 to 37 degrees C.

1983 ◽  
Vol 81 (4) ◽  
pp. 485-512 ◽  
Author(s):  
K G Beam ◽  
P L Donaldson
Keyword(s):  
Skeletal Muscle ◽  
Low Temperatures ◽  
Time Course ◽  
Voltage Dependence ◽  
Potassium Currents ◽  
Effect Of Temperature ◽  
Time Axis ◽  
Rat Skeletal Muscle ◽  
Rapid Phase ◽  
Qualitative Effect

Potassium currents were measured using the three-microelectrode voltage-clamp technique in rat omohyoid muscle at temperatures from 1 to 37 degrees C. The currents were fitted according to the Hodgkin-Huxley equations as modified for K currents in frog skeletal muscle (Adrian et al., 1970a). The equations provided an approximate description of the time course of activation, the voltage dependence of the time constant of activation (tau n), and the voltage dependence of gK infinity. At higher temperatures the relationship between gK infinity and voltage was shifted in the hyperpolarizing direction. The effect of temperature on tau n was much greater in the cold than in the warm: tau n had a Q10 of nearly 6 at temperatures below 10 degrees C, but a Q10 of only approximately 2 over the range of 30-38 degrees C. The decreasing dependence of tau n on temperature was gradual and the Arrhenius plot of tau n revealed no obvious break-points. In addition to its quantitative effect on activation kinetics, temperature also had a qualitative effect. Near physiological temperatures (above approximately 25 degrees C), the current was well described by n4 kinetics. At intermediate temperatures (approximately 15-25 degrees C), the current was well described by n4 kinetics, but only if the n4 curve was translated rightward along the time axis (i.e., the current had a greater delay than could be accounted for by simple n4 kinetics). At low temperatures (below approximately 15 degrees C), n4 kinetics provided only an approximate fit whether or not the theoretical curve was translated along the time axis. In particular, currents in the cold displayed an initial rapid phase of activation followed by a much slower one. Thus, low temperatures appear to reveal steps in the gating process which are kinetically "hidden" at higher temperatures. Taken together, the effects of temperature on potassium currents in rat skeletal muscle demonstrate that the behavior of potassium channels at physiological temperatures cannot be extrapolated, either quantitatively or qualitatively, from experiments carried out in the cold.

Is there an A-type K+ current in guinea pig ventricular myocytes?

2003 ◽  
Vol 284 (2) ◽  
pp. H598-H604 ◽  
Author(s):  
Ian Findlay
Keyword(s):  
Guinea Pig ◽  
Voltage Dependence ◽  
Ventricular Myocytes ◽  
Reversal Potential ◽  
Outward Current ◽  
Type K ◽  
K Current

A unique transient outward K+ current ( I to) has been described to result from the removal of extracellular Ca2+ from ventricular myocytes of the guinea pig (15). This study addressed the question of whether this current represented K+-selective I to or the efflux of K+ via L-type Ca2+ channels. This outward current was inhibited by Cd2+, Ni2+, Co2+, and La3+ as well as by nifedipine. All of these compounds were equally effective inhibitors of the L-type Ca2+ current. The current was not inhibited by 4-aminopyridine. Apparent inhibition of the outward current by extracellular Ca2+ was shown to result from the displacement of the reversal potential of cation flux through L-type Ca2+ channels. The current was found not to be K+ selective but also permeant to Cs+. The voltage dependence of inactivation of the outward current was identical to that of the L-type Ca2+ current. It is concluded that extracellular Ca2+ does not mask an A-type K+current in guinea pig ventricular myocytes.

Alpha 1-adrenergic receptor activation depolarizes rat supraoptic neurosecretory neurons in vitro

1986 ◽  
Vol 251 (3) ◽  
pp. R569-R574 ◽  
Author(s):  
J. C. Randle ◽  
C. W. Bourque ◽  
L. P. Renaud
Keyword(s):  
Time Course ◽  
Membrane Resistance ◽  
Receptor Activation ◽  
Membrane Hyperpolarization ◽  
Neurosecretory Neurons ◽  
Consistent Change ◽  
K Current ◽  
Supraoptic Neurons ◽  
A Current

Intracellular data were obtained from 35 supraoptic nucleus neurosecretory neurons maintained in vitro in intra-arterially perfused explants of rat hypothalamus. Addition of norepinephrine, phenylephrine, or methoxamine, but not isoproterenol (30-200 microM), consistently induced membrane depolarization, bursting activity, and an associated prolongation in action potential duration, effects that were reversibly antagonized by the alpha 1-antagonist prazosin. Norepinephrine-evoked depolarizations demonstrated no consistent change in membrane resistance and were reduced both by membrane hyperpolarization and by raising extracellular K+. Norepinephrine shortened the time course of spike hyperpolarizing afterpotentials and increased the magnitude of late depolarizing afterpotentials. It is proposed that one of norepinephrine's actions on supraoptic neurons involves K+ channels, perhaps by modulation of a transient K+ current known as A current.

Potassium currents of neurons isolated from medical nucleus tractus solitarius

1993 ◽  
Vol 265 (5) ◽  
pp. H1596-H1602 ◽  
Author(s):  
J. P. Moak ◽  
D. L. Kunze
Keyword(s):  
Potassium Current ◽  
Voltage Dependence ◽  
Nucleus Tractus Solitarius ◽  
Outward Current ◽  
Potassium Currents ◽  
Afferent Input ◽  
Calcium Currents ◽  
Voltage Dependent ◽  
Current Relaxation ◽  
Outward Potassium Current

Voltage-dependent potassium currents of neurons enzymatically isolated from the medial and dorsal subnuclei of the solitary tract (mNTS) of adult guinea pig have been characterized with respect to their voltage dependence, time dependence, and sensitivity to specific blocking agents. This region of the medulla receives baroreceptor afferent input and is involved in cardiovascular regulation. Our results showed the presence of three types of potassium currents. First, in all neurons studied (n = 58) a slowly developing outward current was present at potentials more positive than -30 mV. The time to half-peak current decreased with depolarization [24.8 ms at 0 mV; 19.2 ms at +10 mV; 12.5 ms at +20 mV; 9.9 ms at +30 mV (n = 4)]. This current required 20 mM tetraethylammonium (TEA) for full block and failed to show significant inactivation for voltage commands up to 300 ms. Second, a rapidly activating, 4-aminopyridine (4-AP)-sensitive transient outward potassium current was present in 83% of the cells examined (n = 39/47). Threshold for activation was -30 mV. The current relaxation consisted of three components: tau 1 = 14-49 ms; tau 2 = 174-362; tau 3 = 1.1-2.4 s. Finally, in all cells tested calcium activated a large nontransient outward potassium current that was inhibited by charybdotoxin. The studies reported here will be used in conjunction with studies describing sodium and calcium currents to understand the basis for generation of activity in the mNTS in response to baroreceptor input.

Choline acts as agonist and blocker for Aplysia cholinergic synapses

1984 ◽  
Vol 51 (1) ◽  
pp. 1-15 ◽  
Author(s):  
D. Gardner ◽  
R. L. Ruff ◽  
R. L. White
Keyword(s):  
Time Constant ◽  
Decay Time ◽  
Time Course ◽  
Voltage Dependence ◽  
Partial Agonist ◽  
Outward Current ◽  
Decay Time Constant ◽  
Inhibitory Synapses ◽  
Cholinesterase Inhibition ◽  
Current Component

Several identified neurons of the Aplysia buccal ganglia respond to choline. Iontophoretic applications of either choline or acetylcholine (ACh) to voltage-clamped inhibitory follower neurons produce similar currents. Peak amplitudes of choline responses were 10-100% of ACh responses on the same cell. Choline currents were curare blockable and reversed at -69 +/- 2 mV, within 1 mV of postsynaptic current (IPSC) reversal. Application of 1 mM choline to the bath produces more prolonged effects than an initial conductance change. Choline depressed IPSC amplitude by 42 +/- 5% and prolonged IPSC decay time constant by 25 +/- 7%. The slowing was reversible but the depression was not. Use of choline as a Na substitute may therefore involve unexpected partial agonist action; even where conductance changes are transient or inapparent, choline may alter synaptic responses. Bath choline had variable effects on cholinergic self-inhibitory synapses, blocking in six trials but not in three others. Voltage clamping cells BL and BR7, in which monosynaptic cholinergic PSPs are diphasic, reveals underlying early inward and late outward currents. Choline activates only the late outward current component. Correspondingly, bath choline blocks only the late outward component, as does eserine and ACh. This block is not seen with neostigmine, and so is unlikely to be related to cholinesterase inhibition. The early inward current component, revealed by block of the late component by choline or ACh, decays exponentially. Decay time constant is exponentially dependent on membrane potential over the range -20 to -100 mV, with 63-mV depolarization speeding decay e-fold. Eserine prolongs decay and steepens voltage dependence. The late outward component decays with voltage-independent time constant of 48 +/- 5 ms. Both the time integral of synaptic conductance and the ratio of synaptic charge transfer to peak synaptic current of the early inward component of the cell 7 response are reduced by depolarization. Voltage-dependent duration thus combines with reduced driving force in diminishing the excitatory effect of this component at depolarized levels, allowing the inhibitory component to predominate. In this diphasic synapse, voltage dependence of the time course of one component thus serves an easily identified function.

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