Ca2(+)-activated K+ current involvement in neuronal function revealed by in situ single-channel analysis in Helix neurones

Characterization of Functional Effects of Two New Active Fractions Isolated From Scorpion Venom on Neuronal Ca2+ Spikes: A Possible Action on Ca2+-Dependent Dependent K+ Channels

INTRODUCTION

It is a long time that natural toxin research is conducted to unlock the medical potential of toxins. Although venoms-toxins cause pathophysiological conditions, they may be effective to treat several diseases. Since toxins including scorpion toxins target voltage-gated ion channels, they may have profound effects on excitable cells. Therefore, elucidating the cellular and electrophysiological impacts of toxins, particularly scorpion toxins would be helpful in future drug development opportunities.

METHODS

Intracellular recording was made from F1 cells of Helix aspersa in the presence of calcium Ringer solution in which Na⁺ and K⁺ channels were blocked. Then, the modulation of channel function in the presence of extracellular application of F4 and F6 toxins and kaliotoxin (KTX; 50 nM and 1 μM) was examined by assessing the electrophysiological characteristics of calcium spikes.

RESULTS

The two active toxin fractions, similar to KTX, a known Ca²⁺-activated K⁺ channel blocker, reduced the amplitude of AHP, enhanced the firing frequency of calcium spikes and broadened the duration of Ca²⁺ spikes. Therefore, it might be inferred that these two new fractions induce neuronal hyperexcitability possibly, in part, by blocking calcium-activated potassium channel current. However, this supposition requires further investigation using voltage clamping technique.

CONCLUSION

These toxin fractions may act as blocker of calcium-activated potassium channels.

Functional characterization of the cardiac ryanodine receptor pore-forming region

Ryanodine receptors are homotetrameric intracellular calcium release channels. The efficiency of these channels is underpinned by exceptional rates of cation translocation through the open channel and this is achieved at the expense of the high degree of selectivity characteristic of many other types of channel. Crystallization of prokaryotic potassium channels has provided insights into the structures and mechanisms responsible for ion selection and movement in these channels, however no equivalent structural detail is currently available for ryanodine receptors. Nevertheless both molecular modeling and cryo-electron microscopy have identified the probable pore-forming region (PFR) of the ryanodine receptor (RyR) and suggest that this region contains structural elements equivalent to those of the PFRs of potassium-selective channels. The aim of the current study was to establish if the isolated putative cardiac RyR (RyR2) PFR could form a functional ion channel. We have expressed and purified the RyR2 PFR and shown that function is retained following reconstitution into planar phospholipid bilayers. Our data provide the first direct experimental evidence to support the proposal that the conduction pathway of RyR2 is formed by structural elements equivalent to those of the potassium channel PFR.

The fruit essential oil of Pimpinella anisum L. (Umblliferae) induces neuronal hyperexcitability in snail partly through attenuation of after-hyperpolarization

AIM OF THE STUDY

Many biological actions of Pimpinella anisum L. (Ainse), including antiepileptic activity have been demonstrated; however, there is no data concerning its precise cellular mechanisms of action. We determined whether the fruit essential oil of anise affect the bioelectrical activity of snail neurons in control condition or after pentylenetetrazol (PTZ) induced epileptic activity.

MATERIALS AND METHODS

Intracellular recordings were made under the current clamp condition and the effects of anise oil (0.01% or 0.05%) alone or in combination with PTZ were assessed on the firing pattern, action potential configuration and postspike potentials.

RESULTS

Anise oil changed the firing pattern from regular tonic discharge to irregular and then to bursting in intact cells or resulted in the robustness of the burst firing and the steepness of the paroxysmal shift induced by PTZ treatment. It also significantly increased the firing frequency and decreased both the after-hyperpolarization potential (AHP) following single action potential and the post-pulse AHP.

CONCLUSIONS

Likely candidate cellular mechanisms underlying the hyperexcitability produced by anise oil include enhancement of Ca(2+) channels activity or inhibition of voltage and/or Ca(2+) dependent K(+) channels activity underlying AHPs. These finding indicates that a certain caution is needed when Pimpinella anisum is used for treating patients suffering from epilepsy.

The functional consequences of paraoxon exposure in central neurones of land snail, Caucasotachea atrolabiata, are partly mediated through modulation of Ca2+ and Ca2+-activated K+-channels

Toxicity of paraoxon has been attributed to inhibition of cholinesterase, but little is known about its direct action on ionic channels. The effects of paraoxon (0.3 microM-0.6 microM) were studied on the firing behaviour of snail neurones. Paraoxon significantly increased the frequency of spontaneously generated action potentials, shortened the afterhyperpolarization (AHP) and decreased the precision of firing. Short periods of high frequency-evoked trains of action potentials led to an accumulation in the depth and duration of post-train AHPs that was evidenced as an increase in time to resumption of autonomous activity. The delay time in autonomous activity initiation was linearly related to the frequency of spikes in the preceding train and the slope of the curve significantly decreased by paraoxon. The paraoxon induced hyperexcitability and its depressant effect on the AHP and the post-train AHP were not blocked by atropine and hexamethonium. Calcium spikes were elicited in a Na+ free Ringer containing voltage dependent potassium channel blockers. Paraoxon significantly decreased the duration of calcium spikes and following AHP and increased the frequency of spikes. These findings suggest that a reduction in calcium influx during action potential may decrease the activation of calcium dependent potassium channels that participate in AHP generation and act as a mechanism of paraoxon induced hyperexcitability.

Paraoxon suppresses Ca2+ spike and afterhyperpolarization in snail neurons: Relevance to the hyperexcitability induction

The effects of organophosphate (OP) paraoxon, active metabolite of parathion, were studied on the Ca(2+) and Ba(2+) spikes and on the excitability of the neuronal soma membranes of land snail (Caucasotachea atrolabiata). Paraoxon (0.3 muM) reversibly decreased the duration and amplitude of Ca(2+) and Ba(2+) spikes. It also reduced the duration and the amplitude of the afterhyperpolarization (AHP) that follows spikes, leading to a significant increase in the frequency of Ca(2+) spikes. Pretreatment with atropine and hexamethonium, selective blockers of muscarinic and nicotinic receptors, respectively, did not prevent the effects of paraoxon on Ca(2+) spikes. Intracellular injection of the calcium chelator BAPTA dramatically decreased the duration and amplitude of AHP and increased the duration and frequency of Ca(2+) spikes. In the presence of BAPTA, paraoxon decreased the duration of the Ca(2+) spikes without affecting their frequency. Apamin, a neurotoxin from bee venom, known to selectively block small conductance of calcium-activated potassium channels (SK), significantly decreased the duration and amplitude of the AHP, an effect that was associated with an increase in spike frequency. In the presence of apamin, bath application of paraoxon reduced the duration of Ca(2+) spike and AHP and increased the firing frequency of nerve cells. In summary, these data suggest that exposure to submicromolar concentration of paraoxon may directly affect membrane excitability. Suppression of Ca(2+) entry during the action potential would down regulate Ca(2+)-activated K(+) channels leading to a reduction of the AHP and an increase in cell firing.

Development of Transient Outward Currents Coupled With Ca2+-Induced Ca2+Release Mediates Oscillatory Membrane Potential in Ascidian Muscle Cells

Isolated ascidian Halocynthia roretzi blastomeres of the muscle lineage exhibit muscle cell-like excitability on differentiation despite the arrest of cell cleavage early in development. This characteristic provides a unique opportunity to track changes in ion channel expression during muscle cell differentiation. Here, we show that the intrinsic membrane property of ascidian cleavage-arrested muscle-type cells becomes oscillatory by expressing transient outward currents ( Ito) activated by Ca2+-induced Ca2+release (CICR) in a maturation-dependent manner. In current-clamp mode, most day 4 (72 h after fertilization) cleavage-arrested muscle cells exhibited an oscillatory membrane potential of –20 mV at 15 Hz, whereas most day 3 (48 h after fertilization) cells exhibited a spiking pattern. In voltage-clamp mode, the day 4 cells exhibited prominent transient outward currents that were not present in day 3 cells. Itowas abolished by the application of 10 mM caffeine, implying that CICR was involved in Itoactivation. Itowas based on K+efflux and sensitive to tetraethylammonium and some Ca2+-activated K+channel inhibitors. We found a 60-pS single channel conductance that was activated by local Ca2+release in ascidian muscle cell. Voltage-clamp recording with an oscillatory waveform as a command pulse showed that CICR-activated K+currents were activated during the falling phase of the membrane potential oscillation. These results suggest that developmental expression of CICR-activated K+current plays a role in the maturation of larval locomotion by modifying the intrinsic membrane excitability of muscle cells.

The effect of mebudipine and dibudipine, two new Ca2+ channel blockers, in comparison with nifedipine on Ca2+ spikes of F1 neuronal soma membrane inHelix aspersa

Mebudipine and dibudipine are two new dihydropyridine (DHP) Ca2+ channel blockers that have been synthesized by Mahmoudian et al. (1997). In previous studies, they showed considerable relaxant effect on vascular and ileal smooth muscles. These two compounds also reduced the contraction force of rat left atrium (20, 22). In the present study, we attempted to compare the inhibitory actions of these new DHPs and nifedipine on the high threshold Ca2+ spikes of F1 neuronal soma membrane in the subesophageal ganglia of Helix aspersa, using current-clamp method. At a concentration of 1 microM, two new DHP compounds (mebudipine and dibudipine) were tested for their L-type Ca2+ channel blocker activity. Both compounds reversibly reduced the peak amplitude of action potential and after hyperpolarization potential and markedly decreased the duration of Ca2+ spikes. The most potent of these DHPs was mebudipine. Neither the two new DHPs nor nifedipine changed the resting membrane potential in a statistically significant way.

Modulatory effects of parallel fiber and molecular layer interneuron synaptic activity on purkinje cell responses to ascending segment input: a modeling study

Based on anatomical, physiological, and model-based studies, it has been proposed that synapses associated with the ascending segment of granule cell axons provide the principle excitatory drive on Purkinje cells which is then modulated by the more numerous parallel fiber synapses. In this study we have evaluated this idea using a detailed compartmental model of a cerebellar Purkinje cell by providing identical ascending segment synaptic inputs during different levels of random parallel fiber and molecular interneuron input. Results suggest that background inputs from parallel fibers and molecular layer interneurons can have a substantial effect on the response of Purkinje cells to ascending segment inputs. Interestingly, these effects are not reflected in the average firing rate of the Purkinje cell and are thus entirely dendritic in effect. These results are considered in the context of the known segregated spatial distribution of the parallel fibers and ascending segment synapses and a new hypothesis concerning the functional organization of cerebellar cortical circuitry.

Activation of a Large-conductance Ca2+-Dependent K+ Channel by Stimulation of Glutamate Phosphoinositide-coupled Receptors in Cultured Cerebellar Granule Cells

Trans-1-amino-cyclopentyl-1,3-dicarboxylic acid (trans-ACPD), a specific agonist of the glutamate phosphoinositide-coupled receptor (Qp receptor), increased the amplitude of the outward K+ current recorded in the whole-cell configuration of the patch-clamp technique in mouse cultured cerebellar granule cells. This effect was abolished by buffering internal Ca2+ with BAPTA [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid]. Activation of a large-conductance K+ channel was observed when trans-ACPD or quisqualic acid (QA), another Qp receptor agonist, was applied outside the cell-attached patch pipettes. No activation was observed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a specific agonist of ionotropic non-N-methyl-d-aspartate (non-NMDA) receptors. The effects of trans-ACPD or QA were potentiated in the presence of external Ca2+. The channel was also directly activated by both micromolar concentrations of internal Ca2+ and membrane depolarization. Its unitary conductance was 100 - 115 pS under asymmetrical K+ and 195 - 235 pS under high symmetrical K+ conditions. In the absence of agonist, the channel was blocked by 1 mM external tetraethylammonium. This is the first description of a large conductance Ca2+-activated K+ channel in cultured cerebellar granule cells. It possesses properties similar to those of the so-called 'big K+ channel' described in other preparations. Our cell-attached experiments demonstrated an indirect coupling between Qp receptors and this channel. The most likely hypothesis is that the second messenger system inositol 1,4,5-triphosphate (IP3)-Ca2+ was involved in the coupling process. This hypothesis was further strengthened by our whole-cell experiments. On the basis of the voltage- and Ca2+-sensitivities of the studied channel, we estimated an increase of 350 to 570 nM in internal Ca2+ concentration when Qp receptors were stimulated by 100 microM trans-ACPD. Under physiological conditions, stimulation of Qp receptors by the endogenous neurotransmitter should lead to similar K+ channel activation and therefore would tend to reduce the efficacy of ionotropic glutamate synaptic receptor stimulation responsible for cell excitation.

Nitric oxide increases excitability by depressing a calcium activated potassium current in snail neurons

In gastropods, the interneuronal messenger, nitric oxide (NO), modulates spike frequency and synaptic transmission. We have characterized the effect of NO on ion currents underlying neuronal excitability, using current-clamp and two-electrode voltage-clamp techniques. Identified neurons of the pulmonate snail, Helix pomatia, respond to the NO donor sodium nitroprusside (SNP) by increasing the firing frequency and decreasing the latency. Voltage-clamp experiments revealed that SNP or S-nitro-N-acetylpenicillamine (SNAP) depressed the macroscopic outward current, while the control compound N-acetylpenicillamine (NAP) had no effect. Current voltage curves generated from voltage steps to different membrane potentials ranging from -40 to +180 mV showed an N-shaped outward current. Superfusion of ganglia with Ca(2+) free Helix solution abolished the N-shape, indicating the contribution of a Ca(2+) activated K(+) current (I(K,Ca)). Exposure of neurons to SNP or SNAP diminished the N-shape, indicating that NO affects I(K,Ca). The depressing effect of SNP on the outward current was slow and reached steady state in about 5 min. In conclusion, our findings indicate that NO enhances excitability in Helix nervous system by decreasing I(K,Ca).

The soma and neurites of primary afferent neurons in the guinea-pig intestine respond differentially to deformation

1. Intrinsic primary afferent neurons in the small intestine are exposed to distortion of their processes and of their cell bodies. Recordings of mechanosensitivity have previously been made from these neurons using intracellular microelectrodes, but this form of recording has not permitted detection of generator potentials from the processes, or of responses to cell body distortion. 2. We have developed a technique to record from enteric neurons in situ using patch electrodes. The mechanical stability of the patch recordings has allowed recording in cell-attached and whole cell configuration during imposed movement of the neurons. 3. Pressing with a fine probe initiated generator potentials (14 +/- 9 mV) from circumscribed regions of the neuron processes within the same myenteric ganglion, at distances from 100 to 500 microm from the cell body that was patched. Generator potentials persisted when synaptic transmission was blocked with high Mg2+, low Ca2+ solution. 4. Soma distortion, by pressing down with the whole cell recording electrode, inhibited action potential firing. Consistent with this, moderate intra-electrode pressure (10 mbar; 1 kPa) increased the opening probability of large-conductance (BK) potassium channels, recorded in cell-attached mode, but suction was not effective. In outside-out patches, suction, but not pressure, increased channel opening probability. Mechanosensitive BK channels have not been identified on other neurons. 5. The BK channels had conductances of 195 +/- 25 pS. Open probability was increased by depolarization, with a half-maximum activation at a patch potential of 20 mV and a slope factor of 10 mV. Channel activity was blocked by charybdotoxin (20 nM). 6. Stretch that increased membrane area under the electrode by 15 % was sufficient to double open probability. Similar changes in membrane area occur when the intestine changes diameter and wall tension under physiological conditions. Thus, the intestinal intrinsic primary afferent neurons are detectors of neurite distortion and of compression of the soma, these stimuli having opposite effects on neuron excitability.

Voltage-gated potassium channels activated during action potentials in layer V neocortical pyramidal neurons

To investigate voltage-gated potassium channels underlying action potentials (APs), we simultaneously recorded neuronal APs and single K(+) channel activities, using dual patch-clamp recordings (1 whole cell and 1 cell-attached patch) in single-layer V neocortical pyramidal neurons of rat brain slices. A fast voltage-gated K(+) channel with a conductance of 37 pS (K(f)) opened briefly during AP repolarization. Activation of K(f) channels also was triggered by patch depolarization and did not require Ca(2+) influx. Activation threshold was about -20 mV and inactivation was voltage dependent. Mean duration of channel activities after single APs was 6.1 +/- 0.6 ms (mean +/- SD) at resting membrane potential (-64 mV), 6.7 +/- 0.7 ms at -54 mV, and 62 +/- 15 ms at -24 mV. The activation and inactivation properties suggest that K(f) channels function mainly in AP repolarization but not in regulation of firing. K(f) channels were sensitive to a low concentration of tetraethylammonium (TEA, 1 mM) but not to charybdotoxin (ChTX, 100 nM). Activities of A-type channels (K(A)) also were observed during AP repolarization. K(A) channels were activated by depolarization with a threshold near -45 mV, suggesting that K(A) channels function in both repolarization and timing of APs. Inactivation was voltage dependent with decay time constants of 32 +/- 6 ms at -64 mV (rest), 112 +/- 28 ms at -54 mV, and 367 +/- 34 ms at -24 mV. K(A) channels were localized in clusters and were characterized by steady-state inactivation, multiple subconductance states (36 and 19 pS), and inhibition by 5 mM 4-aminopyridine (4-AP) but not by 1 mM TEA. A delayed rectifier K(+) channel (K(dr)) with a unique conductance of 17 pS was recorded from cell-attached patches with TEA/4-AP-filled pipettes. K(dr) channels were activated by depolarization with a threshold near -25 mV and showed delayed long-lasting activation. K(dr) channels were not activated by single action potentials. Large conductance Ca(2+)-activated K(+) (BK) channels were not triggered by neuronal action potentials in normal slices and only opened as neuronal responses deteriorated (e.g., smaller or absent spikes) and in a spike-independent manner. This study provides direct evidence for different roles of various K(+) channels during action potentials in layer V neocortical pyramidal neurons. K(f) and K(A) channels contribute to AP repolarization, while K(A) channels also regulate repetitive firing. K(dr) channels also may function in regulating repetitive firing, whereas BK channels appear to be activated only in pathological conditions.

Fast BK-type channel mediates the Ca(2+)-activated K(+) current in crayfish muscle

The role of the Ca(2+)-activated K(+) current (I(K(Ca))) in crayfish opener muscle fibers is functionally important because it regulates the graded electrical activity that is characteristic of these fibers. Using the cell-attached and inside-out configurations of the patch-clamp technique, we found three different classes of channels with properties that matched those expected of the three different ionic channels mediating the depolarization-activated macroscopic currents previously described (Ca(2+), K(+), and Ca(2+)-dependent K(+) currents). We investigated the properties of the ionic channels mediating the extremely fast activating and persistent I(K(Ca)). These voltage- and Ca(2+)-activated channels had a mean single-channel conductance of approximately 70 pS and showed a very fast activation. Both the single-channel open probability and the speed of activation increased with depolarization. Both parameters also increased in inside-out patches, i.e., in high Ca(2+) concentration. Intracellular loading with the Ca(2+) chelator bis(2-aminophenoxy) ethane-N, N,N',N'-tetraacetic acid gradually reduced and eventually prevented channel openings. The channels opened at very brief delays after the pulse depolarization onset (<5 ms), and the time-dependent open probability was constant during sustained depolarization (< or =560 ms), matching both the extremely fast activation kinetics and the persistent nature of the macroscopic I(K(Ca)). However, the intrinsic properties of these single channels do not account for the partial apparent inactivation of the macroscopic I(K(Ca)), which probably reflects temporal Ca(2+) variations in the whole muscle fiber. We conclude that the channels mediating I(K(Ca)) in crayfish muscle are voltage- and Ca(2+)-gated BK channels with relatively small conductance. The intrinsic properties of these channels allow them to act as precise Ca(2+) sensors that supply the exact feedback current needed to control the graded electrical activity and therefore the contraction of opener muscle fibers.

Diversity of single potassium channels in isolated snail neurons

Comparison of K+ channels in mollusk and mammalian neurons has been made to elucidate their fundamental properties. Using patch clamp cell-attached configuration, K+ channels in isolated snail neurons were separated into three subtypes: with big (BKC), medium (MKC) and small (SKC) unitary conductances. BKC and MKC were activated at -30 mV and SKC at more negative potentials. BKC and MKC proved sensitive to TEA, whereas SKC were sensitive to 4-AP. Cd2+ in the pipet decreased unitary conductance of BKC by 55% and of MKC by about 31%. Bath application of 5-HT selectively suppressed MKC. It is suggested that BKC can be referred to large conductance Ca(2+)-dependent K+ currents (KCa), MKC to intermediate conductance KCa and SKC channels comply with the characteristics of A current of mammals. These data show that KCa and A currents may be the most general types of currents generated by K+ channels.

Voltage-gated and Ca2+-activated conductances mediating and controlling graded electrical activity in crayfish muscle

Crayfish opener muscle fibers provide a unique preparation to quantitatively evaluate the relationships between the voltage-gated Ca2+ (ICa) and Ca2+-activated K+ (IK(Ca)) currents underlying the graded action potentials (GAPs) that typify these fibers. ICa, IK(Ca), and the voltage-gated K+ current (IK) were studied using two-electrode voltage-clamp applying voltage commands that simulated the GAPs evoked in current-clamp conditions by 60-ms current pulses. This methodology, unlike traditional voltage-clamp step commands, provides a description of the dynamic aspects of the interaction between different conductances participating in the generation of the natural GAP. The initial depolarizing phase of the GAP was due to activation of the ICa on depolarization above approximately -40 mV. The resulting Ca2+ inflow induced the activation of the fast IK(Ca) (<3 ms), which rapidly repolarized the fiber (<6 ms). Because of its relatively slow activation, the contribution of IK to the GAP repolarization was delayed. During the final steady GAP depolarization ICa and IK(Ca) were simultaneously activated with similar magnitudes, whereas IK aided in the control of the delayed sustained response. The larger GAPs evoked by higher intensity stimulations were due to the increase in ICa. The resulting larger Ca2+ inflow increased IK(Ca), which acted as a negative feedback that precisely controlled the fiber's depolarization. Hence IK(Ca) regulated the Ca2+-inflow needed for the contraction and controlled the depolarization that this Ca2+ inflow would otherwise elicit.

Activity-dependent [Ca2+]i changes in guinea pig vagal motoneurons: relationship to the slow afterhyperpolarization

Vagal motoneurons in slices from the guinea-pig brain stem were injected with the fluorescent [Ca2+]i indicators fura-2, furaptra, or Calcium Green-1. Spike-induced fluorescence changes were measured in the soma and dendrites and simultaneously the long-lasting afterhyperpolarization was recorded with a sharp microelectrode in the soma. Na+ spikes or Ca2+ spikes increased [Ca2+]i (measured as a change in indicator fluorescence) in all locations in the soma and dendrites. Each spike in a train of action potentials caused a step increase in fluorescence of about equal amplitude when nonsaturating indicators were used. Peak changes at all locations occurred at the time of the last action potential. Transients measured with low concentrations of Calcium Green-1 or furaptra had a recovery time constant of approximately 500-1,500 ms in the cell body. The recovery time course was faster in the dendrites than in the soma. The norepinephrine-sensitive, slow afterhyperpolarization (sAHP) had a time to peak of approximately 800 ms and a recovery time constant of 2-5 s, much longer than the recovery time course of the fluorescence changes. Some of these experiments were repeated on pyramidal neurons from the CA1 region of the rat hippocampus with similar results. In both cell types, the data suggest that the time course of neither the rising phase nor the falling phase of the sAHP, nor the underlying conductance, directly reflects the time course of the [Ca2+]i change. The mechanism connecting the parameters remains unclear. One possibility is that an additional second messenger system is involved.

Channels underlying the slow afterhyperpolarization in hippocampal pyramidal neurons: neurotransmitters modulate the open probability

The slow afterhyperpolarization in hippocampal pyramidal neurons is mediated by a calcium-activated potassium current (IAHP) and is a target for variety of different neurotransmitters. The characteristics of the channels underlying IAHP and how they are modulated by neurotransmitters are, however, unknown. In this study, we have examined the properties of the channels underlying IAHP using fluctuation analysis of the macroscopic current. Our results indicate that this channel has a unitary conductance of 2-5 pS and a mean open time of about 2 ms. When the peak amplitude of IAHP was maximal, these channels have an open probability of 0.4. Noradrenaline and carbachol reduced IAHP amplitude by lowering open channel probability. These result indicate that a novel calcium-activated potassium channel underlies IAHP. This channel is modulated in a similar fashion by two different transmitter systems that utilize distinct protein kinases.

Fast, persistent, Ca(2+)-dependent K+ current controls graded electrical activity in crayfish muscle

The early outward current in opener muscle fibres of crayfish (Procambarus clarkii) was studied using the two-electrode voltage-clamp technique. This current was abolished in Ca(2+)-free and 5 mM Cd2+ solutions, and was blocked by extra- or intracellular tetraethylammonium, indicating that it was a Ca(2+)-dependent K+ current [IK(Ca)]. IK(Ca) was voltage dependent, apamin insensitive and sensitive to charybdotoxin (CTX), which, in addition to its tetraethylammonium sensitivity, suggests that the channels mediating IK(Ca) behave in a BK type manner. IK(Ca) activation was extremely fast, reaching a maximum within 5 ms, and the inactivation was incomplete, stabilizing at a persistent steady-state. IK(Ca) was insensitive to intracellular ethylenebis(oxonitrilo)tetraacetate (EGTA), but was abolished by injection of the faster Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), suggesting that voltage-dependent Ca2+ channels and those mediating IK(Ca) should be clustered closely on the membrane. Under two-electrode current-clamp recording mode, low amplitude, graded responses were evoked under control conditions, whereas repetitive all-or-none spikes were elicited by application of CTX or after loading the cells with BAPTA. We conclude that IK(Ca) activates extremely quickly, is persistent and is responsible for the generation and control of the low amplitude, graded, active responses of opener muscle fibres.

Choline blocks large conductance KCa channels in mammalian sympathetic neurones

Using the outside-out configuration of the patch clamp technique, we examined the effects of externally applied choline chloride (ChCl) on the large-conductance calcium-dependent K+ (KCa) channel in sympathetic neurones of the rabbit coeliac ganglion. Isotonically replacing the bath sodium chloride by ChCl significantly decreased the unitary current amplitude of the KCa channels but did not affect their gating properties. The blocking effect was dose-dependent and required 24 mM ChCl to reach half-reduction of the KCa channel conductance (from 134 to 64 pS). In whole-cell voltage-clamped neurones, ChCl activated both nicotinic and muscarinic receptors, which induced various membrane current changes including Ca2+ current decrease. We therefore tested the effects of the cholinergic agonists acetylcholine and carbachol on excised KCa channels. Both agents failed to mimic the blocking effects of ChCl. We therefore conclude that KCa channels in sympathetic neurones are occluded by external choline chloride.

Differential effects of heavy metal ions on Ca(2+)-dependent K+ channels

1. The ability of various divalent metal ions to substitute for Ca2+ in activating distinct types of Ca(2+)-dependent K+ [K+(Ca2+)] channels has been investigated in excised, inside-out membrane patches of human erthrocytes and of clonal N1E-115 mouse neuroblastoma cells using the patch clamp technique. The effects of the various metal ions have been compared and related to the effects of Ca2+. 2. At concentrations between 1 and 100 microM Pb2+, Cd2+ and Co2+ activate intermediate conductance K+(Ca2+) channels in erythrocytes and large conductance K+(Ca2+) channels in neuroblastoma cells. Pb2+ and Co2+, but not Cd2+, activate small conductance K+(Ca2+) channels in neuroblastoma cells. Mg2+ and Fe2+ do not activate any of the K+(Ca2+) channels. 3. Rank orders of the potencies for K+(Ca2+) activation are Pb2+, Cd2+ > Ca2+, Co2+ >> Mg2+, Fe2+ for the intermediate erythrocyte K+(Ca2+) channel, and Pb2+, Cd2+ > Ca2+ > Co2+ >> Mg2+, Fe2+ for the small, and Pb2+ > Ca2+ > Co2+ >> Cd2+, Mg2+, Fe2+ for the large K+(Ca2+) channel in neuroblastoma cells. 4. At high concentrations Pb2+, Cd2+, and Co2+ block K+(Ca2+) channels in erythrocytes by reducing the opening frequency of the channels and by reducing the single channel amplitude. The potency orders of the two blocking effects are Pb2+ > Cd2+, Co2+ >> Ca2+, and Cd2+ > Pb2+, Co2+ >> Ca2+, respectively, and are distinct from the potency orders for activation. 5. It is concluded that the different subtypes of K+(Ca2+) channels contain distinct regulatory sites involved in metal ion binding and channel opening. The K+(Ca2+) channel in erythrocytes appears to contain additional metal ion interaction sites involved in channel block.

Polyamines block Ca(2+)-activated K+ channels in pituitary tumor cells (GH3)

The effects of the natural polyamines, putrescine, spermidine and spermine on single calcium-activated potassium channels from clonal rat pituitary tumor cells (GH3) were studied. Applied to inside-out patches, polyamines were found to reduce the current amplitude and open probability of the channels in a dose- and voltage-dependent manner, indicating that polyamines act as fast blockers which sense a fraction of the electrical field in the channel pore. The Kd for spermine was 11.2 mM for the reduction of unitary current amplitude and 0.7 mM for the reduction of the open probability. The order of effectiveness was spermine > spermidine > putrescine. From fitting beta-functions to current amplitude histograms, blocking and unblocking rates were determined as 11.4 x 10(4) sec-1 and 21.9 x 10(4) sec-1, respectively. The reduction of the channel open probability was relieved by an increase of the Ca2+ concentration of the internal solution, indicating that polyamines compete with Ca2+ at the Ca2+ sensor of the channel. Putrescine antagonized the effect of spermine on the channel current amplitude. The results suggest that polyamines at intracellular millimolar concentrations suppress ion channel activity and therefore may effect electrical discharge behavior of excitable cells.

Single-channel and whole-cell recordings from on-neurone glial cells in Helix pomatia ganglia

A procedure is described for performing patch-clamp recordings on satellite glial cells kept in place within the nervous ganglia in the mollusc Helix. Glial cell properties were deduced from whole-cell and cell-attached recordings. The glial membrane was found to contain densely packed inwardly rectifying K+ channels. Activation of the neurones, under either current-clamp or voltage-clamp conditions, depolarized the glial cell layer wrapped around the neurones and induced a delayed persistent increase in the K+ channel opening probability. These results suggest that the glial channels opened in response to a signal emanating from the active neurones. This preparation provides a useful means of detecting and analysing neurone-glial interactions at the cell and unitary channel levels.

Nitric oxide modulation of calcium-activated potassium channels in postganglionic neurones of avian cultured ciliary ganglia

1. A study has been made of the modulation of calcium-activated potassium channels in cultured neurones of avian ciliary ganglia by sodium nitroprusside and L-arginine. 2. Sodium nitroprusside (100 microM) reduced the net outward current by 22 +/- 1% at 4.8 ms (mean +/- s.e. mean) and 25 +/- 1% at 350 ms during a test depolarization to +40 mV from a holding potential of -40 mV. The outward current remained reduced for the duration of the recording following a single application of sodium nitroprusside. These effects did not occur if the influx of calcium ions was first blocked with Cd2+ (500 microM). Application of ferrocyanide (100 microM) reduced the net outward current by only 6 +/- 3% at 350 ms during a test depolarization to +40 mV. 3. L-Arginine (270 microM) reduced the net outward current on average by 19 +/- 2% at 4.8 ms and 22 +/- 2% at 350 ms during a test depolarization to +40 mV. The current remained in this reduced state for the duration of the recording following a single application of L-arginine. These effects were reduced to 11 +/- 1% at 4.8 ms and 11 +/- 2% at 350 ms in the presence of N omega-nitro-L-arginine methyl ester (L-NAME, 100 microM). 4. In order to alleviate the dependence of calcium-activated potassium channels (Ik(Ca)) on the inward flux of calcium ions, the patch-clamp pipettes were filled with a solution containing 100 microM CaCl2, and the Ca2+ in the bathing solution was replaced with EGTA. Under these conditions sodium nitroprusside reduced the total outward current during a depolarizing pulse of + 40 mV by 9 +/_ 1% at 4.8 ms and by 36 +/- 3% at 350 ms. L-Arginine (270 microM) reduced this current under the same conditions by 9 +/- 1% at 4.8 ms and by 35 +/- 2% at 350 ms.5. Calcium-activated potassium currents were sensitive to apamin (50 nM), as this reduced the outward current by 23 +/- 3% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to + 40 mV. L-Arginine still decreased the outward current in the presence of apamin(50 nM), by 5 +/- 1% at 4.8 ms and by 19 +/- 2% at 350 ms, indicating that L-arginine could reduce an apamin-insensitive Ik(Ca)6. Calcium-activated potassium currents were also sensitive to charybdotoxin (10 nM), as this reduced the outward current by 34 +/- 4% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to + 40 mV. L-Arginine still decreased the outward current in the presence of charybdotoxin, by 6 +/- 1% at 4.8 ms and 12 +/- 4% at 350 ms, showing that L-arginine could reduce a charybdotoxin-insensitive Ik(Ca).7. The present results indicate that NO-synthase in ciliary ganglia can modulate Ik(Ca) by a method which is independent of the action of NO on the calcium channels. The Ik(ca) is decreased significantly at 4.8 ms into a depolarizing pulse, at a time that would decrease the rate of repolarization of the action potential. Ik(Ca) is also reduced at longer times (350 ms), indicating an affect on the inactivating process.

Large conductance Ca(2+)-activated K+ channels are involved in both spike shaping and firing regulation in Helix neurones

1. The role of BK-type calcium-dependent K+ channels (K+Ca) in cell firing regulation was evaluated by performing whole-cell voltage clamp and patch clamp experiments on the U cell neurones in the snail Helix pomatia. These cells were selected because most of the repolarizing K+ current flowed through K+Ca channels. 2. U cells generated overshooting Ca(2+)-dependent spikes in Na(+)-free saline. In response to prolonged depolarizing current, they fired a limited number of spikes of decreasing amplitude, and behaved like fast-adapting or phasic neurones. 3. Under voltage clamp conditions, the K+Ca current had a slow onset at voltages that induced small Ca2+ entries. By manipulating the Ca2+ entry (either with appropriate voltage programmes or by changing the Ca2+ content of the bath), the K+Ca channel opening was found to be rate limited by the Ca2+ binding step and not by the voltage-dependent conformational change to the open state. 4. Despite the slow activation rate observed in voltage-clamped cells, 25-30% of the available K+Ca current was found to be active during isolated spikes. These data were based on patch clamp, spike-like voltage clamp and hybrid current clamp-voltage clamp experiments. 5. The fact that spikes led the slowly rising K+Ca current to shift into a fast activating mode was accounted for by the large surge of Ca2+ current concomitant with spike upstroke. The early calcium surge resulted in local increases in cytosolic calcium, which speeded up the binding of calcium ions to the closed K+Ca channels. From changes in the null Ca2+ current voltage, it was calculated that the submembrane [Ca2+]i increase to 50-80 microM during the spike. 6. Due to their fast voltage dependence, K+Ca channels appeared to play no role in shaping the interspike trajectory. 7. Even in the fast activating mode, the K+Ca current had a finite rate of rise and was not involved in repolarizing short duration Na(+-dependent action potentials. The current became more and more active, however, when voltage-gated K+ channels were progressively inactivated during firing. 8. The fast adaptation exhibited by U cells upon sustained depolarization was not paralleled by a recruitment of K+Ca channels because of the cumulative Ca2+ entries. During a spike burst, the K+Ca current progressively overlapped the depolarizing Ca2+ current, which ultimately stopped the firing. The early opening of K+Ca channels was ascribed to residual Ca2+ accumulation that kept part of the channels in the Ca(2+)-bound state ready to be opened quickly by cell depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Patch-clamp recording of cAMP-induced membrane current noise in Helix pomatia neurons

Membrane current noise evoked by intracellular cAMP injection was studied in isolated Helix pomatia neurons with the patch-clamp technique. Fluctuation analysis was used to estimate the elementary current amplitude (i) and the single channel conductance. It was found that i decreased linearly with cell depolarization and the extrapolated reversal potential was approximately -12 mV. The calculated single-channel conductance was 0.9 +/- 0.14 pS, a value quite different from those obtained for cAMP-activated channels in Pleurobranchaea neurons.

Ca2+ dependence of small Ca(2+)-activated K+ channels in cultured N1E-115 mouse neuroblastoma cells

Single-channel properties of Ca(2+)-activated K+ channels have been investigated in excised membrane patches of N1E-115 mouse neuroblastoma cells under asymmetric K+ concentrations at 0 mV. The SK channels are blocked by 3 nM external apamin, are unaffected by 20 mM external tetraethylammonium (TEA) and have a single-channel conductance of 5.4 pS. The half-maximum open probability and opening frequency of SK channels are observed at 1 microM internal Ca2+. Concentration/effect curves of these parameters are very steep with exponential slope factors between 7 and 13. Open-time distributions demonstrate the existence of at least two open states. The mean short open time increases with [Ca2+]i, whereas the mean long open time is independent of [Ca2+]i. At low [Ca2+]i the short-lived open state predominates. At saturating [Ca2+]i the number of long-lived openings is more enhanced than the number of short-lived openings and both open states occur equally frequently. The opening frequency as well as the open times of SK channels are independent of the membrane potential in the range of -16 to +40 mV. The results indicate that activation of K+ current through SK channels is mainly determined by the Ca(2+)-dependent single-channel opening frequency. BK channels in N1E-115 cells are insensitive to 100 nM external apamin, are sensitive to external TEA in the millimolar range and have a single-channel conductance of 98 pS. Half-maximum open probability and opening frequency of the BK channel are observed at 7.5-21 microM internal Ca2+. The slope factors of concentration/effect curves range between 1.7 and 2.9. As the BK channel open time is markedly enhanced at raised [Ca2+]i, the Ca2+ dependence of the current through BK channels is determined by the single-channel opening frequency as well as the open time. SK as well as BK channels appear to be clustered and interact in a negative cooperative manner in multiple channel patches. The differences in Ca2+ dependence suggest that BK channels are activated by a local high [Ca2+]i associated with Ca2+ influx, whereas SK channels may be activated by Ca2+ released from internal stores as well.

The effect of phloretin on single potassium channels in myelinated nerve

The effect of phloretin, a dipolar organic compound, on single potassium channel currents of myelinated nerve fibres of Xenopus laevis has been investigated, using inside-out patches prepared by the method of Jonas et al. (1989). The I channel, a potential dependent K channel with intermediate deactivation kinetics, was reversibly blocked by 20 microM phloretin applied on the inside; the block was strongest at negative membrane potentials and less pronounced at positive potentials. Phloretin shifted the curve relating open probability to membrane potential towards more positive potentials and reduced its slope and maximum. This confirms previous findings on the effect of phloretin on the voltage dependence of the fast macroscopic K conductance. Single channel conductance and deactivation kinetics were not altered by phloretin.

Calcium-activated potassium channels: regulation by calcium

A wide variety of calcium-activated K channels has been described and can be conveniently separated into three classes based on differences in single-channel conductance, voltage dependence of channel opening, and sensitivity to blockers. Large-conductance calcium-activated K channels typically require micromolar concentrations of calcium to open, and their sensitivity to calcium increases with membrane depolarization, suggesting that they may be involved in repolarization events. Small-conductance calcium-activated K channels are generally more sensitive to calcium at negative membrane potentials, but their sensitivity to calcium is independent of membrane potential, suggesting that they may be involved in regulating membrane properties near the resting potential. Intermediate-conductance calcium-activated K channels are a loosely defined group, where membership is determined because a channel does not fit in either of the other two groups. Within each broad group, variations in calcium sensitivity and single-channel conductance have been observed, suggesting that there may be families of closely related calcium-activated K channels. Kinetic studies of the gating of calcium-activated potassium channels have revealed some basic features of the mechanisms involved in activation of these channels by calcium, including the number of calcium ions participating in channel opening, the number of major conformations of the channels involved in the gating process, and the number of transition pathways between open and closed states. Methods of analysis have been developed that may allow identification of models that give accurate descriptions of the gating of these channels. Although such kinetic models are likely to be oversimplifications of the behavior of a large macromolecule, these models may provide some insight into the mechanisms that control the gating of the channel, and are subject to falsification by new data.

Ca(2+)-activated K+ currents underlying the afterhyperpolarization in guinea pig vagal neurons: a role for Ca(2+)-activated Ca2+ release

We examined the possibility that Ca2+ released from intracellular stores could activate K+ currents underlying the afterhyperpolarization (AHP) in neurons. In neurons of the dorsal motor nucleus of the vagus, the current underlying the AHP had two components: a rapidly decaying component that was maximal following the action potential (GkCa,1) and a slower component that had a distinct rising phase (GkCa,2). Both components required influx of extracellular Ca2+ for their activation, and neither was blocked by extracellular TEA (10 mM). GkCa,1 was selectively blocked by apamin, whereas GkCa,2 was selectively reduced by noradrenaline. The time course of GkCa,2 was markedly temperature sensitive. GkCa,2 was selectively blocked by application of ryanodine or sodium dantrolene, or by loading cells with ruthenium red. These results suggest that influx of Ca2+ directly gates one class of K+ channels and leads to release of Ca2+ from intracellular stores, which activates a different class of K+ channel.

Two subtypes of C current in identified Helix neurons

Ca-activated K currents (C currents) were identified in various Helix pomatia neurons under voltage-clamp conditions. This comparative study revealed the existence of two C currents: a fast-inactivating current and a non-inactivating current; both had identical gating and pharmacological properties. Inactivation of the C currents was induced by either cell depolarization or short Ca entry.