Diverse current and voltage responses to baclofen in an identified molluscan photoreceptor

1995 ◽  
Vol 74 (2) ◽  
pp. 506-518 ◽  
Author(s):  
L. D. Matzel ◽  
I. A. Muzzio ◽  
R. F. Rogers
Keyword(s):  
Voltage Clamp ◽  
Action Potentials ◽  
Gabab Receptor ◽  
Outward Current ◽  
Gabab Receptors ◽  
Equilibrium Potential ◽  
K Channel ◽  
Electrode Voltage ◽  
Voltage Dependent ◽  
K Current

1. gamma-Aminobuturic acid-B (GABAB) receptors play a role in the mediation of slow inhibitory postsynaptic potentials in mammalian as well as some nonmammalian species. In identified photoreceptors from the marine mollusc Hermissenda, recent evidence has suggested that GABA, as well as the GABAB receptor agonist baclofen, might simultaneously modulate multiple conductances on the postsynaptic membrane. Here, using intracellular current-clamp and single-electrode voltage-clamp techniques, we have characterized responses to baclofen in the B photoreceptors of the Hermissenda eye. 2. Microapplication of baclofen (12.5–62.5 microM) to the terminal branches of the B photoreceptors induced a slow, concentration-dependent hyperpolarization (approximately 3–8 mV) that was accompanied by a cessation of spontaneous action potentials and a positive shift in firing threshold. Both the hyperpolarization and the shift in spike threshold in response to baclofen were attenuated largely by the K+ channel blocker tetraethylammonium chloride (TEA; 50 mM). 3. Bath application of baclofen (100 microM) decreased the amplitude, duration, and the afterhyperpolarization (AHP) of evoked action potentials. Although baclofen's effect on spike duration and amplitude persisted in the absence of extracellular Ca2+, the reduction of the AHP by baclofen was eliminated, suggesting that multiple conductances mediated the baclofen-induced modification of the action potential. 4. Using a single-electrode voltage-clamp technique, microapplication of baclofen to the terminal branches of the B photoreceptor produced a slow, net outward current (< 0.5 nA) that reversed near the equilibrium potential for K+ and shifted to more positive potentials when extracellular K+ was increased, in approximate agreement with the Nernst equation for K+. 5. Baclofen induced an increase in amplitude of the nonvoltage dependent leak conductance (IL), and the increase was blocked by TEA. The baclofen-induced increase of IL was accompanied by an increase in amplitude and a negative shift in the voltage dependence of a slow, steeply voltage-dependent K+ current (IK), which displays selective sensitivity to TEA but does not normally contribute to leak conductance. The amplitude and steady-state inactivation of a fast, transient K+ current, as well as the amplitude of an inwardly rectifying K+ current were unaffected by baclofen. 6. Both the rate of activation as well as the amplitude of a voltage-dependent Ca2+ current (ICa) were reduced by baclofen. The reduction of ICa resulted in a concomitant suppression of a Ca(2+)-dependent K+ current (IK-Ca) that was sufficient to account for the reduction of the AHP after evoked action potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca-Induced K+-Outward Current in Paramecium Tetraurelia

1980 ◽  
Vol 88 (1) ◽  
pp. 293-304 ◽  
Author(s):  
YOUKO SATOW ◽  
CHING KUNG
Keyword(s):  
Steady State ◽  
Voltage Clamp ◽  
Outward Current ◽  
Paramecium Tetraurelia ◽  
Ca Channels ◽  
Outward Currents ◽  
Ciliary Reversal ◽  
Voltage Dependent ◽  
K Current ◽  
K Outward Current

Late K-outward currents upon membrane depolarization were recorded in Paramecium tetraurelia under a voltage clamp. A Ca-induced K-outward component is demonstrated by subtracting the value of the outward current in a pawn A mutant lacking functional Ca-channels (pwA500). The Ca-induced K-outward current activates slowly, reaching a peak after 100 to 1000 ms. The current then remains steady or reaches the steady state after a decline of several seconds. EGTA2- injection experiments show that the Ca-induced K-outward current is dependent on the internal Ca2+ concentration. The current is shown to depend on the voltage-dependent Ca conductance, by study of the leaky pawn A mutant (pwA132), which has a lowered Ca conductance as well as a lowered Ca-induced K-current. The Ca-induced GK is thus indirectly dependent on the voltage. The maximal GK is about 40 nmho/cell at + 7 mV in 4 mM-K+. The Ca-induced K current is sustained throughout the prolonged depolarization and the prolonged ciliary reversal.

Rectification of muscarinic K+ current by magnesium ion in guinea pig atrial cells

1987 ◽  
Vol 253 (1) ◽  
pp. H210-H214
Author(s):  
M. Horie ◽  
H. Irisawa
Keyword(s):  
Guinea Pig ◽  
Action Potentials ◽  
Single Channel ◽  
Outward Current ◽  
Atrial Cells ◽  
Voltage Dependent ◽  
K Current ◽  
Single Channel Currents ◽  
Channel Currents ◽  
Low K

Rectifying properties of the acetylcholine (ACh)-sensitive K+ channels were studied using a patch-clamp method in single atrial cells prepared enzymatically from adult guinea pig hearts. In the presence of micromolar concentration of nonhydrolyzable guanosine 5'-triphosphate (GTP) analogue 5'-guanylylimidodiphosphate (GppNHp) and the absence of Mg2+ at the inner surface of patch membrane [( Mg2+]i), the channel activity recovered in inside-out patch condition. The single channel conductance became ohmic between -80 and +80 mV (symmetrical 150 mM K+ solutions). The rapid relaxation of outward single channel currents was disclosed on a depolarization. [Mg2+]i blocked the outward current through the channel dose- and voltage-dependently and also induced a dose-dependent increase in the channel activation. The apparent paradoxical role of [Mg2+]i is important for the cholinergic control in the heart; voltage-dependent Mg block ensures a low K+ conductance of cell membrane at the plateau of action potentials during the exposure to ACh, thereby slowing the heart rate without unfavorable shortening of the action potentials.

Slowly inactivating outward currents in a cuticular mechanoreceptor neuron of the cockroach (Periplaneta americana)

1995 ◽  
Vol 74 (3) ◽  
pp. 1200-1211 ◽  
Author(s):  
P. H. Torkkeli ◽  
A. S. French
Keyword(s):  
Voltage Clamp ◽  
Time Constant ◽  
Action Potentials ◽  
Periplaneta Americana ◽  
Outward Current ◽  
Equilibrium Potential ◽  
Channel Blockers ◽  
Outward Currents ◽  
Frequency Of Action Potentials ◽  
K Outward Current

1. Although rapid adaptation is a widespread feature of sensory receptors, its ionic basis has not been clearly established in any touch receptor, because their small sizes have severely restricted the range of experiments tat can be performed. In the cockroach tactile spine, intracellular voltage-clamp recordings are now possible. 2. The basic electrophysiological properties of the cockroach femoral tactile spine neuron were studied using discontinuous (switching) single-electrode current- and voltage-clamp recordings. A slowly inactivating voltage-sensitive K+ outward current was detected after the major inward currents were blocked with tetrodotoxin. 3. The total outward current activated in < 1 ms at voltages above 0 mV. At moderate depolarizations it did not inactivate, but at higher depolarizations an inactivation time constant of approximately 260 ms was measured. Some recordings also revealed an additional, slower inactivation time constant of approximately 2.5 s. 4. More than half of the voltage-sensitive K+ outward current could be blocked with the Ca2+ channel blockers Co2+ and Cd2+. Tetraethylammonium chloride (TEA) also reduced the amplitude of the outward current to about half of its original amplitude. The actions of both blockers were reversible and probably reflect overlapping blockades of two separate outward currents. 5. The reversal potentials of the currents that remained after block with Co2+ (-91.7 mV) or TEA (-86.8 mV) were both near the K+ equilibrium potential expected for the tactile spine neuron. The voltage dependencies of activation of the Co(2+)- and TEA-resistant currents were both well fitted by Boltzmann distributions, giving values of half maximal activation (V50) equal to -34.5 mV for the Co(2+)-resistant current and -51.3 mV for the TEA-resistant current. 6. Current-clamp recordings revealed that the TEA-sensitive K+ current was the major component of action potential repolarization but that it did not effect the frequency of action potentials evoked by steady depolarization. On the other hand, blockers of Ca(2+)-sensitive K+ currents (Cd2+, Co2+, or charybdotoxin) reduced adaptation and increased the frequency of action potentials significantly but did not effect the duration or amplitude of individual action potentials.

scholarly journals Separation of the action potential into a Na-channel spike and a K-channel spike by tetrodotoxin and by tetraethylammonium ion in squid giant axons internally perfused with dilute Na-salt solutions.

1980 ◽  
Vol 76 (3) ◽  
pp. 337-354 ◽  
Author(s):  
I Inoue
Keyword(s):  
Action Potential ◽  
Voltage Clamp ◽  
Action Potentials ◽  
Outward Current ◽  
K Channel ◽  
Bathing Medium ◽  
Maximum Slope ◽  
Giant Axons ◽  
Tetraethylammonium Ion ◽  
Cacl2 Solution

Squid giant axons internally perfused with a 30 mM NaF solution and bathed in a 100 mM CaCl2 solution, which are known to produce long lasting action potentials in response to pulses of outward current, were investigated. The effects of tetrodotoxin (TTX) and of tetraethylammonium ion (TEA+) on such action potentials were studied. The results are summarized as follows: (a) An addition of 1--3 microM TTX to the external solution altered but did not block the action potentials; it increased the height of the action potential by approximately 15 mV, and it decreased the membrane conductance as the peak of excitation by about two-thirds. (b) Voltage-clamp experiments performed with both NaCl and TTX in the external CaCl2 solution revealed that the TTX-insensitive action potential does not involve a rise in gNa, whereas the experiments performed without TTX showed that the action potential is accompanied by a large rise in gNa. (c) Internally applied TEA+ was shown to selectively block the TTX-insensitive action potential, but it did not block the other component of the action potential, which is accompanied by a rise in gNa, and which is selectively suppressed by TTX. (d) The addition of a small amount of KCl to the external CaCl2 solution containing TTX greatly increased both the maximum peak inward current under voltage clamp and the maximum slope conductance. Furthermore, it was shown that K+ applied on both sides of the axon plays a dominant role in producing the membrane potential in the active state in the presence of TTX, even though a large amount of Ca2+ is presented in the bathing medium. These observations have led me to conclude that the sodium channel is responsible for the production of the TTX-sensitive component of the action potential under the ionic conditions of these experiments, and the potassium channel for the TTX-insensitive component of the action potential.

Activation of GABAB receptors increases a potassium conductance in rat bulbospinal neurons of the C1 area

1996 ◽  
Vol 271 (5) ◽  
pp. R1304-R1310 ◽  
Author(s):  
Y. W. Li ◽  
P. G. Guyenet
Keyword(s):  
Gabab Receptor ◽  
Outward Current ◽  
Gabab Receptors ◽  
Neonatal Rat ◽  
Ventrolateral Medulla ◽  
Equilibrium Potential ◽  
Inward Rectification ◽  
Dependent Manner ◽  
Gamma Aminobutyric Acid

In anesthetized rats, iontophoresis of the gamma-aminobutyric acid (GABAB)-receptor agonist and antispastic drug baclofen inhibits the bulbospinal vasomotor neurons of the rostral ventrolateral medulla (RVLM). The present study was carried out to determine whether C1 adrenergic and other bulbospinal neurons of the RVLM have postsynaptic GABAB receptors. Retrogradely labeled RVLM bulbospinal neurons (n = 52) were recorded in 120-micron-thick slices from neonatal rat brain (3-10 days old). Most neurons (48/52) were tonically active (3 +/- 0.6 spikes/s). Twenty-six neurons were recovered histologically, and 18 of them were immunoreactive for tyrosine hydroxylase (TH). In current clamp, baclofen (0.3-10 microM) hyperpolarized RVLM bulbospinal cells in a dose-dependent manner (16 +/- 0.5 mV hyperpolarization by 3 microM baclofen; n = 19) and decreased input resistance by 40% (n = 10). In voltage clamp (1 microM tetrodotoxin present; holding potential: -40 to -60 mV), 3 microM baclofen induced an outward current of 21 +/- 2 pA (n = 29). This current exhibited inward rectification and reversed polarity close to the K+ equilibrium potential (external K+ from 2.5 to 10 mM). The current induced by baclofen was reduced 90% by 0.1-0.2 mM BaCl2 (n = 6) and was blocked reversibly by the selective GABAB-receptor antagonist CGP-55845A (0.5-1 microM; n = 6). All histologically verified TH-immunoreactive cells (n = 18) were sensitive to baclofen. In summary, RVLM bulbospinal neurons including C1 adrenergic cells possess GABAB receptors. Activation of these receptors increases an inwardly rectifying K+ conductance. This effect reduces the intrinsic firing frequency of RVLM vasomotor neurons "in vitro" and may contribute to the sympatholytic action of baclofen "in vivo."

Activators of protein kinase C mimic serotonin-induced modulation of a voltage-dependent potassium current in pleural sensory neurons of Aplysia

1994 ◽  
Vol 72 (3) ◽  
pp. 1240-1249 ◽  
Author(s):  
S. Sugita ◽  
D. A. Baxter ◽  
J. H. Byrne
Keyword(s):  
Protein Kinase ◽  
Voltage Clamp ◽  
Sensory Neurons ◽  
Phorbol Esters ◽  
Outward Current ◽  
Membrane Currents ◽  
Kinase C ◽  
Voltage Dependent ◽  
K Current ◽  
Kinetics Of

1. In the pleural mechanoafferent sensory neurons of Aplysia, serotonin (5-HT)-induced spike broadening consists of at least two components: a cAMP and protein kinase A (PKA)-dependent, rapidly developing component and a protein kinase C (PKC)-dependent, slowly developing component. Voltage-clamp experiments were conducted to identify currents that are modulated by PKC and thus may contribute to the slowly developing component of 5-HT-induced spike broadening. 2. We compared the effects of phorbol esters, activators of PKC, on membrane currents with those of 5-HT. Bath application of 5-HT had complex modulatory effects on currents elicited by voltage-clamp pulses to potentials > 0 mV. The kinetics of both activation and inactivation of the membrane currents were slowed by 5-HT. This led to a decrease in an outward current at the beginning of the voltage-clamp pulse and an increase at the end of the pulse. Previous work has shown that these effects represent, in part, the modulation of a large, voltage-dependent K+ current (IK,V) by 5-HT. 3. Active phorbol esters mimicked some of the actions of 5-HT on membrane currents in that they slowed activation and inactivation kinetics of current responses to voltage-clamp pulses more positive than 0 mV. This led to a decrease in an outward current at the beginning of the pulse and an increase at the end of the pulse. Because inactive phorbols did not mimic the actions of 5-HT, the effects of active phorbol esters appeared to be PKC specific. In addition, preexposure of the sensory neurons to active phorbol esters appeared to occlude the modulatory actions of 5-HT on IK,V. Thus it is likely that modulation of IK,V by 5-HT is mediated, at lease in part, by PKC. 4. To further characterize which currents were modulated by PKC, low concentrations of tetraethylammonium (TEA, 2 mM) were used to block Ca(2+)-activated K+ current (IK,Ca). Low TEA partially blocked the phorbol ester-induced increase of the outward current at the end of voltage-clamp pulses. These results agreed with previous reports that activation of PKC enhanced a fast component of IK,Ca in these sensory neurons. Such an enhancement would lead to an increase in outward current that should be blocked by low TEA. Low TEA, however, did not affect phorbol ester-induced decrease of the outward current at the beginning of pulse, where the predominant current is IK,V, which is less sensitive to TEA.(ABSTRACT TRUNCATED AT 400 WORDS)

Choline chloride activates time-dependent and time-independent K+ currents in dog atrial myocytes

1994 ◽  
Vol 266 (1) ◽  
pp. C42-C51 ◽  
Author(s):  
B. Fermini ◽  
S. Nattel
Keyword(s):  
Choline Chloride ◽  
Outward Current ◽  
Time Dependent ◽  
Equilibrium Potential ◽  
Delayed Rectifier ◽  
Atrial Myocytes ◽  
Patch Clamp Technique ◽  
Voltage Dependent ◽  
K Current ◽  
K Currents

Using the whole cell configuration of the patch-clamp technique, we studied the effect of isotonic replacement of bath sodium chloride (NaCl) by choline chloride (ChCl) in dog atrial myocytes. Our results show that ChCl triggered 1) activation of a time-independent background current, characterized by a shift of the holding current in the outward direction at potentials positive to the K+ equilibrium potential (EK), and 2) activation of a time- and voltage-dependent outward current, following depolarizing voltage steps positive to EK. Because the choline-induced current obtained by depolarizing steps exhibited properties similar to the delayed rectifier K+ current (IK), we named it IKCh. The amplitude of IKCh was determined by extracellular ChCl concentration, and this current was generally undetectable in the absence of ChCl. IKCh was not activated by acetylcholine (0.001-1.0 mM) or carbachol (10 microM) and could not be recorded in the absence of ChCl or when external NaCl was replaced by sucrose or tetramethylammonium chloride. IKCh was inhibited by atropine (0.01-1.0 microM) but not by the M1 antagonist pirenzepine (up to 10 microM). This current was carried mainly by K+ and was inhibited by CsCl (120 mM, in the pipette) or barium (1 mM, in the bath). We conclude that in dog atrial myocytes, ChCl activates a background conductance comparable to ACh-dependent K+ current, together with a time-dependent K+ current showing properties similar to IK.

scholarly journals Delayed rectifying and calcium-activated K+ channels and their significance for action potential repolarization in mouse pancreatic beta-cells.

1990 ◽  
Vol 95 (6) ◽  
pp. 1041-1059 ◽  
Author(s):  
P A Smith ◽  
K Bokvist ◽  
P Arkhammar ◽  
P O Berggren ◽  
P Rorsman
Keyword(s):  
Beta Cells ◽  
Action Potentials ◽  
Pancreatic Beta Cells ◽  
Single Channel ◽  
Outward Current ◽  
K Channel ◽  
K Channels ◽  
Dependent Component ◽  
Voltage Dependent ◽  
Action Potential Repolarization

The contribution of Ca2(+)-activated and delayed rectifying K+ channels to the voltage-dependent outward current involved in spike repolarization in mouse pancreatic beta-cells (Rorsman, P., and G. Trube. 1986. J. Physiol. 374:531-550) was assessed using patch-clamp techniques. A Ca2(+)-dependent component could be identified by its rapid inactivation and sensitivity to the Ca2+ channel blocker Cd2+. This current showed the same voltage dependence as the voltage-activated (Cd2(+)-sensitive) Ca2+ current and contributed 10-20% to the total beta-cell delayed outward current. The single-channel events underlying the Ca2(+)-activated component were investigated in cell-attached patches. Increase of [Ca2+]i invariably induced a dramatic increase in the open state probability of a Ca2(+)-activated K+ channel. This channel had a single-channel conductance of 70 pS [( K+]o = 5.6 mM). The Ca2(+)-independent outward current (constituting greater than 80% of the total) reflected the activation of an 8 pS [( K+]o = 5.6 mM; [K+]i = 155 mM) K+ channel. This channel was the only type observed to be associated with action potentials in cell-attached patches. It is suggested that in mouse beta-cells spike repolarization results mainly from the opening of the 8-pS delayed rectifying K+ channel.

scholarly journals Activation-inactivation of potassium channels and development of the potassium-channel spike in internally perfused squid giant axons.

1981 ◽  
Vol 78 (1) ◽  
pp. 43-61 ◽  
Author(s):  
I Inoue
Keyword(s):  
Potassium Channels ◽  
Potassium Channel ◽  
Voltage Clamp ◽  
Time Constant ◽  
Equilibrium Potential ◽  
Giant Axons ◽  
Dependent Inactivation ◽  
Voltage Dependent ◽  
Time Courses ◽  
Calcium Permeability

A spike that is the result of calcium permeability through potassium channels was separated from the action potential is squid giant axons internally perfused with a 30 mM NaF solution and bathed in a 100 mM CaCl2 solution by blocking sodium channels with tetrodotoxin. Currents through potassium channels were studied under voltage clamp. The records showed a clear voltage-dependent inactivation of the currents. The inactivation was composed of at least two components; one relatively fast, having a time constant of 20--30 ms, and the other very slow, having a time constant of 5--10 s. Voltage clamp was carried out with a variety of salt compositions in both the internal and external solutions. A similar voltage-dependent inactivation, also composed of the two components, was recognized in all the current through potassium channels. Although the direction and intensity of current strongly depended on the salt composition of the solutions, the time-courses of these currents at corresponding voltages were very similar. These results strongly suggest that the inactivation of the currents in attributable to an essential, dynamic property of potassium channels themselves. Thus, the generation of a potassium-channel spike can be understood as an event that occurs when the equilibrium potential across the potassium channel becomes positive.

scholarly journals Extracellular Hco3− Dependence of Electrogenic Na/Hco3 Cotransporters Cloned from Salamander and Rat Kidney

2000 ◽  
Vol 115 (5) ◽  
pp. 533-546 ◽  
Author(s):  
Irina I. Grichtchenko ◽  
Michael F. Romero ◽  
Walter F. Boron
Keyword(s):  
Membrane Potential ◽  
Voltage Clamp ◽  
Xenopus Oocytes ◽  
Rat Kidney ◽  
Outward Current ◽  
Membrane Vesicles ◽  
Disulfonic Acid ◽  
Ambystoma Tigrinum ◽  
Ph Titration ◽  
Electrode Voltage

We studied the extracellular [HCOabstract 3 −] dependence of two renal clones of the electrogenic Na/HCO3 cotransporter (NBC) heterologously expressed in Xenopus oocytes. We used microelectrodes to measure the change in membrane potential (ΔVm) elicited by the NBC cloned from the kidney of the salamander Ambystoma tigrinum (akNBC) and by the NBC cloned from the kidney of rat (rkNBC). We used a two-electrode voltage clamp to measure the change in current (ΔI) elicited by rkNBC. Briefly exposing an NBC-expressing oocyte to HCOabstract 3 −/CO2 (0.33–99 mM HCOabstract 3−, pHo 7.5) elicited an immediate, DIDS (4,4-diisothiocyanatostilbene-2,2-disulfonic acid)-sensitive and Na+-dependent hyperpolarization (or outward current). In ΔVm experiments, the apparent Km for HCOabstract 3− of akNBC (10.6 mM) and rkNBC (10.8 mM) were similar. However, under voltage-clamp conditions, the apparent Km for HCOabstract 3− of rkNBC was less (6.5 mM). Because it has been reported that SOabstract 3=/HSO abstract 3− stimulates Na/HCO3 cotransport in renal membrane vesicles (a result that supports the existence of a COabstract 3= binding site with which SOabstract 3= interacts), we examined the effect of SOabstract 3=/HSO abstract 3− on rkNBC. In voltage-clamp studies, we found that neither 33 mM SOabstract 4= nor 33 mM SOabstract 3 =/HSOabstract 3− substantially affects the apparent Km for HCO abstract 3−. We also used microelectrodes to monitor intracellular pH (pHi) while exposing rkNBC-expressing oocytes to 3.3 mM HCOabstract 3 −/0.5% CO2. We found that SO abstract 3=/HSOabstract 3 − did not significantly affect the DIDS-sensitive component of the pHi recovery from the initial CO2 -induced acidification. We also monitored the rkNBC current while simultaneously varying [CO2]o, pHo, and [COabstract 3=]o at a fixed [HCOabstract 3−]o of 33 mM. A Michaelis-Menten equation poorly fitted the data expressed as current versus [COabstract 3=]o . However, a pH titration curve nicely fitted the data expressed as current versus pHo. Thus, rkNBC expressed in Xenopus oocytes does not appear to interact with SOabstract 3 =, HSOabstract 3−, or COabstract 3=.

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