The coexistence in rat muscle cells of two distinct classes of Ca2+-dependent K+ channels with different pharmacological properties and different physiological functions

Kalium 3.0 is a comprehensive depository of natural, artificial, and labeled polypeptides acting on potassium channels

Here, we introduce the third release of Kalium database (http://kaliumdb.org/), a manually curated comprehensive depository that accumulates data on polypeptide ligands of potassium channels. The major goal of this amplitudinous update is to summarize findings for natural polypeptide ligands of K+ channels, as well as data for the artificial derivatives of these substances obtained over the decades of exploration. We manually analyzed more than 700 original manuscripts and systematized the information on mutagenesis, production of radio- and fluorescently labeled derivatives, and the molecular pharmacology of K+ channel ligands. As a result, data on more than 1200 substances were processed and added enriching the database content fivefold. We also included the electrophysiological data obtained on the understudied and neglected K+ channels including the heteromeric and concatenated channels. We associated target channels in Kalium with corresponding entries in the official database of the International Union of Basic and Clinical Pharmacology. Kalium was supplemented with an adaptive Statistics page, where users are able to obtain actual data output. Several other improvements were introduced, such as a color code to distinguish the range of ligand activity concentrations and advanced tools for filtration and sorting. Kalium is a fully open-access database, crosslinked to other databases of interest. It can be utilized as a convenient resource containing ample up-to-date information about polypeptide ligands of K+ channels.

Apamin structure and pharmacology revisited

Apamin is often cited as one of the few substances selectively acting on small-conductance Ca²⁺-activated potassium channels (K_Ca2). However, published pharmacological and structural data remain controversial. Here, we investigated the molecular pharmacology of apamin by two-electrode voltage-clamp in Xenopus laevis oocytes and patch-clamp in HEK293, COS7, and CHO cells expressing the studied ion channels, as well as in isolated rat brain neurons. The microtitre broth dilution method was used for antimicrobial activity screening. The spatial structure of apamin in aqueous solution was determined by NMR spectroscopy. We tested apamin against 42 ion channels (K_Ca, K_V, Na_V, nAChR, ASIC, and others) and confirmed its unique selectivity to K_Ca2 channels. No antimicrobial activity was detected for apamin against Gram-positive or Gram-negative bacteria. The NMR solution structure of apamin was deposited in the Protein Data Bank. The results presented here demonstrate that apamin is a selective nanomolar or even subnanomolar-affinity K_Ca2 inhibitor with no significant effects on other molecular targets. The spatial structure as well as ample functional data provided here support the use of apamin as a K_Ca2-selective pharmacological tool and as a template for drug design.

Small and Intermediate Calcium Activated Potassium Channels in the Heart: Role and Strategies in the Treatment of Cardiovascular Diseases

Calcium-activated potassium channels are a heterogeneous family of channels that, despite their different biophysical characteristics, structures, and pharmacological signatures, play a role of transducer between the ubiquitous intracellular calcium signaling and the electric variations of the membrane. Although this family of channels was extensively described in various excitable and non-excitable tissues, an increasing amount of evidences shows their functional role in the heart. This review aims to focus on the physiological role and the contribution of the small and intermediate calcium-activated potassium channels in cardiac pathologies.

Identification of a pharmacophore of SKCa channel blockers

Small conductance calcium-activated potassium channels (SK) are widely expressed throughout the central nervous system (CNS) and the periphery. Three subtypes of SK channels have so far been identified in different parts of the brain. Activation of the SK channels by a rise in intracellular calcium leads to the hyperpolarisation of the membrane, reducing cell excitability. Blocking the SK channels might be beneficial in the treatment of depression, Parkinson's disease and cognitive disorders. However, few blockers of SK channels have been characterized. In this study, a pharmacophoric model of SK channels blockers is presented. It is based on a series of nonpeptidic compounds and apamin, a peptidic blocker. To create the pharmacophore model, the conformational space of nonpeptidic blockers was investigated to generate a series of distance constraints applied to a simulated annealing study of apamin. The resulting conformation was superimposed with the nonpeptidic blockers to give a pharmacophore.

Expression of small-conductance calcium-activated potassium channels (SK3) in skeletal muscle: regulation by muscle activity

The type 3 small conductance calcium-activated potassium channel (SK3) is expressed in embryonic and adult denervated skeletal muscles where it contributes to hyperexcitability. This study aimed at determining the role of muscle activity in regulating SK3 channels. Soleus muscles of adult rats were denervated by cutting the sciatic nerve. In reinnervation studies, the soleus nerve was crushed: in one group, muscles were reinnervated with electrically silent axons, by chronic sciatic nerve perfusion with tetrodotoxin. Several groups of denervated muscles were subjected to chronic direct electrical stimulation, using either fast (100 Hz) or slower patterns (20 or 30 Hz). The SK3 mRNA and protein levels in soleus muscle were determined by reverse transcriptional-PCR, Western blot and immunofluorescence. Both denervated and reinnervated-paralysed soleus muscles displayed similar up-regulation of SK3 mRNA and protein. Reinnervation with electrically active axons instead inhibited SK3 up-regulation. Chronic muscle direct stimulation in vivo, irrespective of the pattern used, reversed the denervation-induced up-regulation of SK3 expression or prevented it when initiated at the time of denervation. Chronic electrical stimulation of denervated muscles also completely prevented the development of the after-hyperpolarization (AHP) following the action potential, normally induced in the muscle fibres by denervation. We conclude that action potential activity evoked by motor neurones in muscle fibres is both necessary and sufficient to account for the physiological down-regulation of SK3 channels in the non-junctional membrane of skeletal muscle.

Nucleotide regulation of the voltage-dependent nonselective cation conductance in murine colonic myocytes

ATP is proposed to be a major inhibitory neurotransmitter in the gastrointestinal (GI) tract, causing hyperpolarization and smooth muscle relaxation. ATP activates small-conductance Ca(2+)-activated K(+) channels that are involved in setting the resting membrane potential and causing inhibitory junction potentials. No reports are available examining the effects of ATP on voltage-dependent inward currents in GI smooth muscle cells. We previously reported two types of voltage-dependent inward currents in murine proximal colonic myocytes: a low-threshold voltage-activated, nonselective cation current (I(VNSCC)) and a relatively high-threshold voltage-activated (L-type) Ca(2+) current (I(L)). Here we have investigated the effects of ATP on these currents. External application of ATP (1 mM) did not affect I(VNSCC) or I(L) in dialyzed cells. ATP (1 mM) increased I(VNSCC) and decreased I(L) in the perforated whole-cell configuration. UTP and UDP (1 mM) were more potent than ATP on I(VNSCC). ADP decreased I(L) but had no effect on I(VNSCC). The order of effectiveness was UTP = UDP > ATP > ADP. These effects were not blocked by pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS), but the phospholipase C inhibitor U-73122 reversed the effects of ATP on I(VNSCC). ATP stimulation of I(VNSCC) was also reversed by protein kinase C (PKC) inhibitors chelerythrine chloride or bisindolylmaleimide I. Phorbol 12,13-dibutyrate mimicked the effects of ATP. RT-PCR showed that P2Y(4) is expressed by murine colonic myocytes, and this receptor is relatively insensitive to PPADS. Our data suggest that ATP activates I(VNSCC) and depresses I(L) via binding of P2Y(4) receptors and stimulation of the phospholipase C/PKC pathway.

Contribution of different Ca2+-activated K+ channels to the first phase of the response to TRH in clonal rat anterior pituitary cells

AIMS

Thyrotropin-releasing hormone (TRH) induces biphasic changes in electrical activity, cytosolic free Ca(2+) level ([Ca(2+)](i)), and prolactin secretion from both clonal GH cells and native lactotrophs. The first phase of the TRH response is characterized by hyperpolarization because of activation of Ca(2+)-activated K(+) channels (K(Ca)). In the present study, the relative contribution of BK, SK, and IK channels to the first phase of the TRH response in GH(4) cells was assessed.

METHODS

The expression of IK channels was confirmed by PCR with specific primers for SK4 (IK). The response to TRH was studied using the perforated patch technique and Ca(2+) microfluoromety (fura-2). The involvement of different K(Ca) channels was estimated by employing the specific channel blockers iberiotoxin (BK), apamin (SK) and clotrimazole (IK).

RESULTS

Application of 100 nM iberiotoxin, 1 microM apamin, and 10 microM clotrimazole reduced the peak value of the outward K(+) current during the first phase of the TRH response by 33, 26, and 33%, respectively. Clotrimazole also shortened the duration of the outward current response by 60%, causing a reduction of total charge movement by 73%. All these toxin-induced reductions were significant (P < 0.05). A combination of all three toxins abolished the current response almost completely.

CONCLUSION

All the three main types of K(Ca) channels are involved in the first phase of the TRH response, with IK as the major contributor. This is the first demonstration of a dominant role of IK compared with BK and SK channels in excitable cells.

Calcium-activated potassium channels: multiple contributions to neuronal function

Calcium-activated potassium channels are a large family of potassium channels that are found throughout the central nervous system and in many other cell types. These channels are activated by rises in cytosolic calcium largely in response to calcium influx via voltage-gated calcium channels that open during action potentials. Activation of these potassium channels is involved in the control of a number of physiological processes from the firing properties of neurons to the control of transmitter release. These channels form the target for modulation for a range of neurotransmitters and have been implicated in the pathogenesis of neurological and psychiatric disorders. Here the authors summarize the varieties of calcium-activated potassium channels present in central neurons and their defining molecular and biophysical properties.

Post-spike distance-to-threshold trajectories of neurones in monkey motor cortex

A recently developed method permits calculation of the post-spike distance-to-threshold trajectory from an extracellularly recorded spontaneous spike train, using a transform of the interspike interval histogram. We applied this method to 61 single neurones recorded from the primary motor cortex of an awake behaving monkey; 39 cells were antidromically identified as pyramidal tract neurones (PTNs). The cells fell into three categories. Fifty-three trajectories (37 from PTNs) had statistically significant peaks 10-60 ms after the preceding spike. Six neurones (2 PTNs) had non-peaked trajectories which rose exponentially towards threshold. Two cells (both unidentified) had trajectories which declined monotonically away from threshold with increasing post-spike latency. The peaked trajectories were unlikely simply to be an artefact of changing firing rate, which potentially can invalidate this method. Firstly, computer simulations confirmed that the method could accurately re-create both exponential and peaked trajectories, even in the presence of the same rate modulation as seen experimentally. Secondly, the responses of eight cells to weak single pulse intracortical microstimulation (20 microA) through a nearby electrode were measured. For each cell, including representatives of all three trajectory shapes, the modulation of response probability with post-spike latency was consistent with the trajectory computed from the spontaneous discharge. We also demonstrated that cells showed a peaked trajectory during periods with either high or low spontaneous network oscillations, so that the peaks were likely to be generated in part by single cell properties rather than exclusively by network activity. We conclude that many single neurones in motor cortex have an increased probability of firing a spike around 30 ms after the previous action potential. This could act to enhance synchronized oscillatory discharge among populations of cells at functionally relevant frequencies.

SK channels and the varieties of slow after-hyperpolarizations in neurons

Action potentials and associated Ca2+ influx can be followed by slow after-hyperpolarizations (sAHPs) caused by a voltage-insensitive, Ca2+-dependent K+ current. Slow AHPs are a widespread phenomenon in mammalian (including human) neurons and are present in both peripheral and central nervous systems. Although, the molecular identity of ion channels responsible for common membrane potential mechanisms has been largely determined, the nature of the channels that underlie the sAHPs in neurons, both in the brain and in the periphery, remains unresolved. This short review discusses why there is no clear molecular candidate for sAHPs.

Channels underlying neuronal calcium-activated potassium currents

In many cell types rises in cytosolic calcium, either due to influx from the extracellular space, or by release from an intracellular store activates calcium dependent potassium currents on the plasmalemma. In neurons, these currents are largely activated following calcium influx via voltage gated calcium channels active during the action potentials. Three types of these currents are known: I(c), I(AHP) and I(sAHP). These currents can be distinguished by clear differences in their pharmacology and kinetics. Activation of these potassium currents modulates action potential time course and the repetitive firing properties of neurons. Single channel studies have identified two types of calcium-activated potassium channel which can also be separated on biophysical and pharmacological grounds and have been named BK and SK channels. It is now clear that BK channels underlie I(c) whereas SK channels underlie I(AHP). The identity of the channels underlying I(sAHP) are not known. In this review, we discuss the properties of the different types of calcium-activated potassium channels and the relationship between these channels and the macroscopic currents present in neurons.

Diversity of K+ channels in circular smooth muscle of opossum lower esophageal sphincter

We previously demonstrated that a balance of K+ and Ca2+-activated Cl– channel activity maintained the basal tone of circular smooth muscle of opossum lower esophageal sphincter (LES). In the current studies, the contribution of major K+ channels to the LES basal tone was investigated in circular smooth muscle of opossum LES in vitro. K+ channel activity was recorded in dispersed single cells at room temperature using patch-clamp recordings. Whole-cell patch-clamp recordings displayed an outward current beginning to activate at –60 mV by step test pulses lasting 400 ms (–120 mV to +100 mV) with increments of 20 mV from holding potential of –80 mV ([K+]I = 150 mM, [K+]o = 2.5 mM). However, no inward rectification was observed. The outward current peaked within 50 ms and showed little or no inactivation. It was significantly decreased by bath application of nifedipine, tetraethylammonium (TEA), 4-aminopyridine (4-AP), and iberiotoxin (IBTN). Further combination of TEA with 4-AP, nifedipine with 4-AP, and IBTN with TEA, or vice versa, blocked more than 90% of the outward current. Ca2+-sensitive single channels were recorded at asymetrical K+ gradients in cell-attached patch-clamp configurations (100.8 ± 3.2 pS, n = 8). Open probability of the single channels recorded in inside-out patch-clamp configurations were greatly decreased by bath application of IBTN (100 nM) (Vh = –14.4 ± 4.8 mV in control vs. 27.3 ± 0.1 mV, n = 3, P < 0.05). These data suggest that large conductance Ca2+-activated K+ and delayed rectifier K+ channels contribute to the membrane potential, and thereby regulate the basal tone of opossum LES circular smooth muscle.Key words: large conductance Ca2+-activated K+ channels, delayed rectifier K+ channels, patch-clamp recording, visceral smooth muscle.

Excitatory versus inhibitory modulation by ATP of neurohypophysial terminal activity in the rat

Much is now known about the electrophysiological properties of the magnocellular neurones of the hypothalamus. Oxytocin neurones are characterized by an intermittent high frequency discharge during suckling that leads to the pulsatile release of oxytocin into the blood and to subsequent milk ejection. Vasopressin neurones are characterized by their asynchronous phasic activity (bursting) during maintained vasopressin release and the subsequent regulation of water balance. In both cases, it is the clustering of spikes, albeit with different time courses for each peptide, that facilitates hormone release. The mechanism underlying this differential facilitation is one of the major unanswered questions in neuroendocrinology. This paper considers recent evidence that indicates that ATP, co-secreted with vasopressin and oxytocin, may play a key role in the regulation of stimulus-secretion coupling in the neurohypophysis. The activity of the type (II) Ca2+-activated K+ (K(Ca)) channel found in the nerve terminals was significantly increased in the presence of ATP on the cytoplasmic side of the channel. Extracellular ATP, in contrast, inhibited the type II K(Ca) current in a dose-dependent manner. Thus, intracellular and extracellular ATP exert opposite effects on the type II K(Ca) channel of neurohypophysial terminals. Furthermore, ATP opens P2X2 channels to increase intracellular [Ca2+] in the nerve terminals and subsequent arginine vasopressin (AVP) release. In contrast, adenosine, acting via A1 receptors, specifically inhibits only the N-type Ca2+ channel, thus decreasing neuropeptide release. These multiple, conflicting effects of ATP and its metabolite adenosine could explain the patterns of AVP release observed during physiological stimulation in vivo.

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.

Discoordinate regulation of different K channels in cultured rat skeletal muscle by nerve growth factor

We investigated the effects of nerve growth factor (NGF) on expression of K+ channels in cultured skeletal muscle. The channels studied were (1) charybdotoxin (ChTx)-sensitive channels by using a polyclonal antibody raised in rabbits against ChTx, (2) Kv1.5 voltage-sensitive channels, and (3) apamin-sensitive (afterhyperpolarization) channels. Crude homogenates were prepared from cultures made from limb muscles of 1-2-day-old rat pups for identification of ChTx-sensitive and Kv1.5 channels by Western blotting techniques. Apamin-sensitive K+ channels were studied by measurement of specific [125I]-apamin binding by whole cell preparations. ChTx-sensitive channels display a fusion-related increase in expression, and NGF downregulates these channels in both myoblasts and myotubes. Voltage-dependent Kv1.5 channel expression is low in myoblasts and increases dramatically with fusion; NGF induces early expression of these channels and causes expression after fusion to increase even further. NGF downregulates apamin-sensitive channels. NGF increases the rate of fall of the action potential recorded intracellularly from single myotubes with intracellular microelectrodes. The results confirm and extend those of previous studies in showing a functional role for NGF in the regulation of membrane properties of skeletal muscle. Moreover, the findings demonstrate that the different K+ channels in this preparation are regulated in a discoordinate manner. The divergent effects of NGF on expression of different K+ channels, however, do not appear sufficient to explain the NGF-induced increase in the rate of fall of the action potential. The changes during the falling phase may rather be due to increases in channel properties or may result from an increased driving force on the membrane potential secondary to the NGF-induced hyperpolarization.

Properties of inhibitory junctional transmission in smooth muscle of the guinea pig lower esophageal sphincter

Inhibitory neurotransmission in guinea pig lower esophageal sphincter (LES) muscles was investigated by using electrophysiological methods. Transmural nerve stimulation (TNS) initiated an inhibitory junction potential (i.j.p.); the amplitude increased 35% by atropine (10(-6) M) and converted to a muscarinic excitatory junction potential (e.j.p.) by apamin (10(-7) M) plus Nomega-nitro-L-arginine (L-NNA, 10(-5) M). In atropinized tissue, the i.j.p. amplitude was reduced 58% by guanethidine (5 x 10(-6) M), 41% by L-NNA (10(-5) M), 57% by suramin (10(-4) M), and it was abolished by apamin (10(-7) M), suggesting that this potential was produced by ATP and nitric oxide (NO) released from adrenergic and nitrergic nerves, respectively, through the activation of Ca2+-sensitive K+ channels. Hyperpolarizations produced by ATP and NO were inhibited by apamin. The i.j.p. amplitude was reduced after desensitizing the membrane with ATP. In atropinized tissue, TNS produced a relaxation that was reduced 15% by guanethidine (5 x 10(-6) M), 50% by L-NNA (10(-5) M), and 30% by apamin (10(-7) M). Thus the LES receives cholinergic excitatory and adrenergic and nitrergic inhibitory innervations; the latter two components contribute evenly to the i.j.p. generation. The relaxation is mainly produced by NO in a membrane potential-independent way.

Tubocurarine blocks a calcium-dependent potassium current in rat tumoral pituitary cells

We investigated the effects of potassium channel inhibitors on electrical activity, membrane ionic currents, intracellular calcium concentration ([Ca2+]i) and hormone release in GH3/B6 cells (a line of pituitary origin). Patch-clamp recordings show a two-component after hyperpolarization (AHP) following each action potential (current clamp) or a two-component tail current (voltage-clamp). Both components can be blocked by inhibiting Ca2+ influx. Application of D-tubocurarine (dTc) (20-500 microM) reversibly suppressed the slowly decaying Ca2+-activated K+ tail current (I AHPs) in a concentration-dependent manner. On the other hand, low doses of tetraethylammonium ions (TEA+) only blocked the rapidly decaying voltage- and Ca2+-activated K+ tail current (I AHPf). Therefore, GH3/B6 cells exhibit at least two quite distinct Ca2+-dependent K+ currents, which differ in size, voltage- and Ca2+-sensitivity, kinetics and pharmacology. These two currents also play quite separate roles in shaping the action potential. d-tubocurarine increased spontaneous Ca2+ action potential firing, whereas TEA increased action potential duration. Thus, both agents stimulated Ca2+ entry. I AHPs is activated by a transient increase in [Ca2+]i such as a thyrotrophin releasing hormone-induced Ca2+ mobilization. All the K+ channel inhibitors we tested: TEA, apamin, dTC and charybdotoxin, stimulated prolactin and growth hormone release in GH3/B6 cells. Our results show that I AHPs is a good sensor for subplasmalemmal Ca2+ and that dTc is a good pharmacological tool for studying this current.

A model for binaural response properties of inferior colliculus neurons. II. A model with interaural time difference-sensitive excitatory and inhibitory inputs and an adaptation mechanism

The inferior colliculus (IC) model of Cai et al. [J. Acoust. Soc. Am. 103, 475-493 (1998)] simulated the binaural response properties of low-frequency IC neurons in response to various acoustic stimuli. This model, however, failed to simulate the sensitivities of IC neurons to dynamically changing temporal features, such as the sharpened dynamic interaural phase difference (IPD) functions. In this paper, the Cai et al. (1998) model is modified such that an adaptation mechanism, viz., an additional channel simulating a calcium-activated, voltage-independent potassium channel which is responsible for afterhyperpolarization, is incorporated in the IC membrane model. Simulations were repeated with this modified model, including the responses to pure tones, binaural beat stimuli, interaural phase-modulated stimuli, binaural clicks, and pairs of binaural clicks. The discharge patterns of the model in response to current injection were also studied and compared with physiological data. It was demonstrated that this model showed all the properties that were simulated by the Cai et al. (1998) model. In addition, it showed some properties that were not simulated by that model, such as the sharpened dynamic IPD functions and adapting discharge patterns in response to current injection.

Apamin, a blocker of the calcium-activated potassium channel, induces neurodegeneration of Purkinje cells exclusively

Following acute intracerebroventricular injections of 1 ng of apamin and chronic apamin infusion (0.4 ng/microl, 0.5 microl/h, 14 days), the rat brains exhibited bilateral damage only in the cerebellum. The argyrophilic cells were Purkinje cells in copula pyramis, flocculus, paraflocculus, and paramedian lobules. These data demonstrate that the inactivation of small conductance Ca2+-activated K+ channels by apamin induces a non-limbic neurodegeneration.

hSK4, a member of a novel subfamily of calcium-activated potassium channels

The gene for hSK4, a novel human small conductance calcium-activated potassium channel, or SK channel, has been identified and expressed in Chinese hamster ovary cells. In physiological saline hSK4 generates a conductance of approximately 12 pS, a value in close agreement with that of other cloned SK channels. Like other members of this family, the polypeptide encoded by hSK4 contains a previously unnoted leucine zipper-like domain in its C terminus of unknown function. hSK4 appears unique, however, in its very high affinity for Ca2+ (EC50 of 95 nM) and its predominant expression in nonexcitable tissues of adult animals. Together with the relatively low homology of hSK4 to other SK channel polypeptides (approximately 40% identical), these data suggest that hSK4 belongs to a novel subfamily of SK channels.

Age-dependent expression of the apamin-sensitive calcium-activated K+ channel in fast and slow rat skeletal muscle

An altered expression of the apamin-sensitive K+ channel from skeletal muscle is apparently implicated in human myotonic dystrophy (MD). We found, in rats, that the expression of this channel depends on age and the type of muscle. This result may be one of the bases of the different susceptibilities of fast and slow muscles to drug-induced myotonia.

Analysis of current fluctuations during after-hyperpolarization current in dentate granule neurones of the rat hippocampus

1. We have studied macroscopic current fluctuations associated with the after-hyperpolarization current (IAHP) that follows a 200 ms voltage-clamp step to 0 mV in dentate granule (DG) neurones of the rat hippocampus. This maximally effective stimulus produced a peak IAHP of 205 +/- 20 pA. Background noise was minimized by using the whole-cell single-electrode voltage-clamp configuration. 2. Conventional current-variance analysis was performed on IAHP to obtain estimates of the unitary AHP channel current (i) and the maximal attainable AHP current (Imax). A second approach, utilizing changes in the power spectrum of IAHP 'noise' during the decay of IAHP, was employed to yield an independent estimate of Imax as well as an estimate of the mean open-state duration of AHP channels. 3. Changes in the power spectrum during IAHP decay revealed that the mean channel open time is fixed at 6.9 +/- 0.5 ms and that the decay is due to changes in channel closed-state duration. The same analysis gave a value for Imax of 320 +/- 20 pA (n = 7). 4. Current-variance analysis suggests that channels responsible for generation of IAHP have a unitary current of 0.29 +/- 0.08 pA at -45 mV in 5 mM extracellular potassium and an Imax of 400 +/- 180 (n = 7). Thus, both methods indicate that about 1200 channels are available to generate IAHP in DG neurones and that about 60% are open at the peak of a maximal IAHP. 5. Computer simulations of IAHP currents in a model neurone show that dendritic current sources will result in an underestimation of i while Imax is underestimated to a lesser extent. Estimates of Imax obtained from power-spectrum analysis are more accurate and less affected by neuronal electrotonic structure than estimates of Imax based on current-variance analysis.

Potential-dependent block of human delayed rectifier K+ channels by internal Na+

The delayed rectifier K+ currents in differentiated human SH-SY5Y neuroblastoma cells were characterized with tight-seal recording techniques. Activation and inactivation parameters were measured. At high positive potentials, the current showed a marked rectification, causing a region of negative slope conductance in the current vs. potential curve. The rectification depended markedly on the pipette Na+ concentration. Without Na+, no rectification was observed, whereas with high Na+ (20-60 mM), a marked rectification was always observed. Tail current measurements showed a fast ( < 400 microseconds) block of K+ currents in the presence of internal Na+. With 60 mM Na+ in the pipette 8% of the K+ current was blocked at 0 mV, 27% at +20 mV, and 82% at +100 mV. Similar degrees of block were often seen with 30 mM Na+ in the pipette. The submembrane Na+ concentration in intact cells was estimated, on the basis of the reversal of Na+ current, to be approximately 15 mM. Single-channel K+ currents, in the cell-attached configuration, showed a conductance of approximately 20 pS at 40-60 mV above rest but showed rectification at high potentials.

Modulation of Ca(2+)-activated K+ current by isoprenaline, carbachol, and phorbol ester in cultured (and fresh) rat aortic vascular smooth muscle cells

1. Effects of isoprenaline (ISO), carbachol, and phorbol ester on the outward K+ currents in single cultured (or fresh) rat aortic vascular smooth muscle (A7r5 and A-10) cells were examined using a whole-cell voltage-clamp (at room temperature 22 degrees C). 2. With 10 mM EGTA in the pipette solution, the delayed rectifier K+ current (IK) was activated by Ca2+ at pCa 7 more than at pCa 10, and was TEA (10 mM) and apamin (200 nM) sensitive, which represents a Ca(2+)-activated K+ current (IKCa). 3. In cultured A7r5 cells, isoprenaline (1 and 5 microM) and carbachol (0.1 and 1 microM) inhibited IKCa. Phorbol ester, 4-beta-phorbol-12, 13-dibutyrate (PDB), at 0.1 and 1 microM also inhibited IKCa, and increased the inhibitory effects induced by isoprenaline (1 microM). 4. In fresh aortic cells, these drugs, at the same concentrations, also produced the similar effects. 5. In A-10 cells, PDB (1 microM) enhanced the transient outward current (4-AP-sensitive), but ISO (1 microM) inhibited the current. 6. These results suggest that the IKCa current would be inhibited by cyclic nucleotides (cAMP and cGMP) and also by PK-C stimulation, and thereby be directly contributed to excitation-contraction coupling of the vascular smooth muscle cells.

Characterization and regulation of apamin-binding K+ channels in skeletal muscle

The Pattern of development and regulation of the apamin receptor (afterhyperpolarization channel) was studied in cultures of skeletal muscle prepared from 1-2-day-old rat pups. Expression was measured by the specific binding of (125)I-apamin. Apamin binding was virtually undetectable until the time of fusion (3-4 days in culture) of single myoblasts into myotubes. Mature myotubes (5-7 days in vitro) displayed a Bmax of 7.4 fmol/mg protein and a Kd of 376 pmol/L. When studied in mature muscle cells apamin binding was found to increase twofold in response to tetrodotoxin (TTX) and elevated Ko, which resulted in decreased Na(i). In contrast, treatments causing an increase in Na(i), such as monensin and veratridine, caused a decrease in apamin binding. The increase in apamin binding following TTX treatment was due mainly to synthesis of new channels, as the effect was blocked by cycloheximide. Alterations in cytosolic Ca2+ by calcium ionophore or Ca-channel blockers were without effect on apamin-sensitive channel expression. We conclude that afterhyperpolarization channel expression is regulated by the level of intracellular Na+ ions.

Properties of the inhibitory junction potential in smooth muscle of the guinea-pig gastric fundus

1. In circular smooth muscle of the guinea-pig gastric fundus, transmural nerve stimulation evoked a cholinergic excitatory junction potential (e.j.p.), and blockade of the e.j.p. by atropine revealed a non-adrenergic non-cholinergic (NANC) inhibitory junction potential (i.j.p.). 2. The amplitude of the e.j.p. was increased by apamin, suramin or NGnitro-L-arginine (L-NOARG), with no significant change in the membrane potential. 3. The i.j.p. consisted of two components (fast and slow); apamin inhibited the former, nitroarginine inhibited the latter, and suramin inhibited both components. 4. Apamin inhibited the hyperpolarization produced by adenosine 5'-triphosphate (ATP) but not by vasoactive intestinal polypeptide (VIP). Suramin inhibited the hyperpolarization produced by VIP but not by ATP. The sodium nitroprusside (SNP)-induced hyperpolarization was not blocked by apamin or suramin. L-NOARG or tetrodotoxin did not inhibit the hyperpolarization produced by ATP, VIP or SNP. 5. The data did not support the hypothesis that ATP, VIP or nitric oxide (NO) is the main transmitter responsible for generation of the NANC i.j.p. in the fundus. 6. Actions of L-NOARG suggest that endogenous NO may be involved in junctional transmission, mainly as an inhibitory modulator of cholinergic transmission.

Calcium-activated potassium channels in adrenal chromaffin cells

Rat chromaffin cells express an interesting diversity of Ca(2+)-dependent K+ channels, including a voltage-independent, small-conductance, apamin-sensitive SK channel and two variants of voltage-dependent, large-conductance BK channels. The two BK channel variants are differentially segregated among chromaffin cells, such that BK current is completely inactivating in about 75-80% of rat chromaffin cells, while the remainder express a mix of inactivating and non-inactivating current or mostly non-inactivating BKs current. The single-channel conductance of BKi channels is identical to that of BKs channels. Although rates of current activation are similar in the two variants, the deactivation kinetics of the two channels also differ. Furthermore, BKi channels are somewhat less sensitive to scorpion toxins than BKs channels. The slow component of BKi channel deactivation may be an important determinant of the functional role of these channels. During blockade of SK current, cells with BKi current fire tonically during sustained depolarizing current injection, whereas cells with BKs current tend to fire only a few action potentials before becoming quiescent. The ability to repetitively fire requires functional BKi channels, since partial blockade of BKi channels by CTX makes a BKi cell behave much like a BKs cell. In contrast, the physiological significance of BKi inactivation may arise from the ability of secretagogue-induced [Ca2+]i elevations to regulate the availability of BKi channels during subsequent action potentials (Herrington et al., 1995). By reducing the number of BK channels available for repolarization, the time course of action potentials may be prolonged. This possibility remains to be tested directly. These results raise a number of interesting questions pertinent to the control of secretion in rat adrenal chromaffin cells. An interesting hypothesis is that cells with a particular kind of BK current may reflect particular subpopulations of chromaffin cells. These subpopulations might differ either in the nature of the material secreted from the cell (e.g., Douglass and Poisner, 1965) or in the responsiveness to particular secretagogues. The differences in electrical behavior between cells with BKi and BKs current suggest that the pattern of secretion that might be elicited by a single type of stimulus could differ. For BKi cells, secretion may occur in a tonic fashion during sustained depolarization, while secretion from cells with BKs current may be more phasic. In the absence of specific structural information about the domains responsible for inactivation of BKi channels, our understanding of the mechanism of inactivation remains indirect. BKi inactivation shares many features with N-terminal inactivation of voltage-dependent K+ channels. However, there are provocative differences between the two types of inactivation which require us to propose that the native inactivation domain of BKi channels may occlude access of permeant ions to the BK channel permeation pathway in a position at some distance from the actual mouth of the channel. Further understanding of the structural and mechanistic basis of inactivation of BKi channels promises to provide new insights into both the cytoplasmic topology of BK channels and the Ca(2+)- and voltage-dependent steps involved in channel activation.

Selective block of Ca(2+)-dependent K+ current in crayfish neuromuscular system and chromaffin cells by sea anemone Bunodosoma cangicum venom

The effects of the nematocyst venom of the sea anemone Bunodosoma cangicum on depolarization-activated currents were studies in opener crayfish muscle fibers and in cultured bovine chromaffin cells. The venom selectively and reversibly blocked the Ca(2+)-dependent K+ current (IK(Ca)) present in crayfish muscle in a dose-dependent manner without affecting voltage-gated Ca2+ or K+ currents. Furthermore, the venom also reduced IK(Ca) in chromaffin cells, without modifying voltage-gated Na+, Ca2+, or K+ currents. Synaptic transmission in crayfish muscle was also affected by the venom. Repetitive excitatory and inhibitory postsynaptic currents (each associated with a presynaptic action potential) were evoked by each nerve stimulus, suggesting that presynaptic IK(Ca) may control the electrical activity of excitatory and inhibitory presynaptic fibers. We conclude that B. cangicum venom includes a toxin that selectively and reversibly blocks Ca(2+)-dependent K+ currents in crayfish muscle and in bovine chromaffin cells, and modifies excitatory and inhibitory synaptic transmission, probably abolishing a similar conductance at the presynaptic fibers.

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.

Density of apamin-sensitive Ca(2+)-dependent K+ channels in bovine chromaffin cells: relevance to secretion

Three objectives were defined when planning this study: (i) to identify binding sites for [125I]-apamin in intact bovine adrenal medulla chromaffin cells and to estimate their density and selectivity; (ii) to determine whether apamin modified the release of catecholamines evoked by brief pulses of dimethylphenylpiperazinium (DMPP, 1 or 5 microM for 10 sec), histamine (10 microM for 10 sec) or high K+ (20, 35 or 70 mM for 10 sec) applied to superfused cells; and (iii) to test whether apamin affected the profiles of the changes in cytosolic Ca2+ concentrations [Ca2+]i obtained in suspensions of cells loaded with fura-2 and stimulated with DMPP or histamine. At equilibrium, increasing concentrations of [125I]-apamin gave a saturation curve whose Scatchard transformation produced a Kd of 132 pM and a Bmax of 0.72 fmol/10(6) cells. Quinine, tetraethylammonium, charybdotoxin or glibenclamide (blockers of various subtypes of K+ channels) did not inhibit [125I]apamin binding. Binding was blocked by apamin and by d-tubocurarine, two blockers of small-conductance Ca(2+)-activated K+ channels (SK channels). The number of binding sites for [125I]apamin amounted to approx. 900 per single chromaffin cell, 0.72 sites per micron 2 surface area. Apamin (1 microM) enhanced the secretory response to histamine (10 microM), DMPP (1 or 5 microM) and high K+ (20 or 35 mM) by 2-3-fold. The response to 70 mM K+, however, was unaffected. Apamin also enhanced the peak [Ca2+]i increase produced by DMPP or histamine by approx. 30%. Overall, these results strongly support the hypothesis that under physiological conditions, SK channels control some of the electrical activity of chromaffin cells and indirectly, the opening of voltage-dependent Ca2+ channels, the access of Ca2+ to the secretory machinery and the rate of catecholamine release to the circulation from the intact adrenal gland.

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.

Role of Ca(+)-dependent K-channels in the membrane potential and contractility of aorta from spontaneously hypertensive rats

1. Contractile responses to KCl and membrane potentials were determined in aortic rings from spontaneously hypertensive rats (SHR), normotensive Wistar rats (NWR) and Wistar Kyoto rats (WKY) both in the absence and in the presence of the Ca(2+)-dependent K-channel blockers, apamin and tetraethylammonium (TEA). 2. Compared to NWR, aortic rings from WKY and SHR were less reactive and their Ca2+ uptake after stimulation with K+ was decreased. 3. Smooth muscle cell membrane potentials were higher in aortae from SHR and WKY than in NWR aortae, whereas SHR had higher K+ and lower Na+ intracellular activities than WKY and NWR, suggesting overactivity of the Na+/K+ pump in the hypertensive animals. 4. Treatment with apamin caused depolarization of WKY and SHR aortae, and increased their contractile responses to the same level as those of the NWR. Treatment with TEA also caused depolarization of aortae from WKY and SHR, but in the SHR the depolarization induced by TEA was smaller than that produced by apamin and the contractile responses to KCl did not reach the level of those of aortae from NWR. 5. It is concluded that overactivity of Ca(2+)-dependent K-channels in aortae of WKY and SHR contributes to their higher membrane potentials and lower responsiveness to vasoconstrictor stimuli. In SHR, an overactive Na+/K+ pump is also present, and the contribution of apamin-sensitive Ca(2+)-dependent K-channels to the membrane potential and reactivity appears to be more relevant than that of TEA-sensitive channels.

Ca2+ release by inositol 1,4,5-trisphosphate is blocked by the K(+)-channel blockers apamin and tetrapentylammonium ion, and a monoclonal antibody to a 63 kDa membrane protein: reversal of blockade by K+ ionophores nigericin and valinomycin and purification of the 63 kDa antibody-binding protein

Ins(1,4,5)P3-induced Ca2+ release from platelet membrane vesicles was blocked by apamin, a selective inhibitor of low-conductance Ca(2+)-activated K+ channels, and by tetrapentylammonium ion, and was weakly inhibited by tetraethylammonium ion. Other K(+)-channel blockers, i.e. charybdotoxin, 4-aminopyridine and glybenclamide were ineffective. A monoclonal antibody (mAb 213-21) obtained by immunizing mice with the InsP3-sensitive membrane fraction from platelets also blocked Ca2+ release by InsP3 from membrane vesicles obtained from platelets, cerebellum, aortic smooth muscle, HEL cells and sea-urchin eggs. ATP-dependent Ca2+ uptake and binding of [3H]InsP3 to platelet membranes was unaffected by either K(+)-channel blockers or mAb 213-21. Blockade of Ca2+ release by apamin, tetrapentylammonium and mAb 213-21 was not affected by the Na+/H+ carrier monensin or the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), but could be completely reversed by the K+/H+ ionophore nigericin and partially reversed by the K+ carrier valinomycin. The antibody-binding protein (ABP) solubilized from platelets, cerebellum, and smooth muscle chromatographed identically on gel filtration, anion-exchange and heparin-TSK h.p.l.c. ABP was purified to apparent homogeneity from platelets and aortic smooth muscle as a 63 kDa protein by immunoaffinity chromatography on mAb 213-21-agarose. These results suggest that optimal Ca2+ release by InsP3 from platelet membrane vesicles may require the tandem function of a K+ channel. A counterflow of K+ ions could prevent the build-up of a membrane potential (inside negative) that would tend to oppose Ca2+ release. The 63 kDa protein may function to regulate K+ permeability that is coupled to the Ca2+ efflux via the InsP3 receptor.

Potassium currents operated by thyrotrophin-releasing hormone in dissociated CA1 pyramidal neurones of rat hippocampus

1. Membrane currents activated by thyrotrophin-releasing hormone (TRH) were investigated in the dissociated rat hippocampal CA1 pyramidal neurone using the nystatin perforated patch recording configuration. 2. Under current-clamp condition, TRH caused a transient hyperpolarization accompanied by a decrease of firing activity and a successive long-lasting depolarization. The latter induced a blockade of firing. 3. When neurones were held at a holding potential (VH) of -40 mV under voltage clamp, TRH elicited a transient outward current with an increase in the membrane conductance, which was followed by a sustained inward current with a decrease in membrane conductance. The inactive TRH metabolite, TRH free acid, did not induce any currents. 4. The reversal potential of TRH-induced outward current (ETRH) was close to the K+ equilibrium potential (EK). The change in ETRH for a 10-fold change in extracellular K+ concentration was 56.4 mV, indicating that the membrane behaves like a K+ electrode in the presence of TRH. On the other hand, the TRH-induced inward current was due to suppression of a slow inward current relaxation during hyperpolarizing voltage commands to -50 mV from a VH of -40 mV, indicating the suppression of the voltage- and time-dependent component of the K+ current (M-current). 5. The TRH-induced outward current (ITRH) increased in a concentration-dependent manner over the concentration range 10(-8)-10(-6) M. The half-maximum concentration was 7.4 x 10(-8) M and the Hill coefficient was 1.5. 6. The TRH-induced outward current (ITRH) was antagonized by K+ channel blockers such as tetraethylammonium (TEA), 4-aminopyridine (4-AP) and Ba2+ in a concentration-dependent manner. ITRH was insensitive to both apamin and iberiotoxin. 7. The first application of TRH to neurones perfused with Ca(2+)-free external solution containing 2 mM EGTA could induce ITRH but the TRH response diminished dramatically with successive applications. Intracellular perfusion with a Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), also diminished the TRH response. 8. The depletion of Ca2+ from the intracellular Ca2+ store by thapsigargin blocked the TRH response without affecting the caffeine response. Pretreatment with Li+ significantly enhanced ITRH, suggesting that ITRH is involved in the elevation of intracellular free Ca2+ released from the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store site but not from the caffeine-sensitive one. 9. Staurosporine, a protein kinase C (PKC) inhibitor, suppressed ITRH in a concentration-dependent manner (the half-maximum inhibitory concentration (IC50), was 2.45 x 10(-8) M).(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-activated hyperpolarizations in rat locus coeruleus neurons in vitro

1. Intracellular recordings were made from rat locus coeruleus (LC) neurons in completely submerged brain slices. Trains of action potentials in LC neurons were followed by a prolonged post-stimulus hyperpolarization (PSH). If trains were elicited with depolarizing current pulses of sufficient intensity, PSH was composed of a fast, early component (PSHE) and a slow, late component (PSHL). PSH which followed trains elicited with lower intensity depolarizing current pulses consisted only of PSHL. 2. Both PSHE and PSHL were augmented by increasing the number of action potentials in the train and both were associated with an increase in membrane conductance. The reversal potential for PSHE was -108 mV and for PSHL it was -114 mV. 3. When a hybrid voltage clamp protocol was used, the current underlying PSH (IPSH) was observed to consist of an early, rapidly decaying component, IE, followed by a late, slower decaying component, IL. The time course of decay of IPSH was biexponential with the time constant of decay of IL more than one order of magnitude larger than the time constant of decay of IE. An increase in the concentration of external K+ shifted the reversal potentials for IE and IL in the depolarizing direction; the mean value of shift per tenfold increase in external K+ concentration was 57.1 mV for IE and 57.6 mV for IL. 4. Both PSHE and PSHL were inhibited by lowering the external Ca2+ concentration or by application of the Ca2+ channel blockers Cd2+ (200-500 microM) or nifedipine (100 microM). Intracellular injection of EGTA abolished both components of PSH. Increasing the external Ca2+ concentration augmented both PSH components. 5. Superfusion of dantrolene (25 microM) or ryanodine (20 microM) decreased the amplitude and duration of PSHL with much less effect on PSHE. 6. d-Tubocurarine (20-200 microM) selectively blocked PSHE with no effect on PSHL; this effect is the same as that of apamin which we have previously described. Superfusion with charybdotoxin (40 nM) or TEA (400 microM-1 mM) did not reduce PSHE or PSHL. 7. Inhibition of IA by 4-aminopyridine or 2,4-diaminopyridine also did not reduce either component of PSH. In fact, these agents slightly augmented both components of PSH; this effect was probably secondary to the prolongation of action potential duration. Superfusion of TEA in concentrations of 2-10 mM increased the size and duration of PSHL and increased the duration but decreased the size of PSHE.(ABSTRACT TRUNCATED AT 400 WORDS)

S-nitrosocysteine, but not sodium nitroprusside, produces apamin-sensitive hyperpolarization in rat gastric fundus

1. To investigate the pharmacological properties of the membrane hyperpolarization induced by electrical field stimulation (EFS), sodium nitroprusside (SNP) and S-nitrosocysteine (NO-Cys) in circular smooth muscle cells of the rat gastric fundus (forestomach), the effects of various potassium channel blockers on these hyperpolarizations were investigated. 2. EFS (50 microseconds, 20 Hz, 3 pulses, 10-50 V) produced inhibitory junction potentials (i.j.ps), in the presence of atropine (1 microM) and guanethidine (1 microM). NO-Cys and SNP produced hyperpolarization of the membrane in the rat gastric fundus. L-NG-nitroarginine (L-NNA) inhibited the i.j.ps, but not the hyperpolarization induced by NO-Cys and SNP. This inhibitory action of L-NNA on the i.j.ps was partly reversed by subsequent application of L-arginine (1 mM) but not by D-arginine. 3. Oxyhaemoglobin (Oxy-Hb; 5 microM) inhibited these hyperpolarizations, although a higher concentration of Oxy-Hb was required to inhibit the SNP-induced hyperpolarization. Hydroquinone (50 microM) inhibited only the hyperpolarization induced by NO-Cys. 4. Apamin (1 microM) partly inhibited i.j.ps and NO-Cys-induced hyperpolarization, but not the SNP-induced hyperpolarization. Tetraethylammonium (TEA; 1 mM), 4-aminopyridine (4-AP; 1 mM) or glibenclamide (1 microM) did not affect hyperpolarization induced by NO-Cys and SNP. 5. 8-Bromo cyclic guanosine 3':5'-monophosphate (1 mM) also produced hyperpolarization. Apamin (1 microM), TEA (1 mM) and glibenclamide (5 microM) all failed to inhibit this hyperpolarization. 6. These results indicate that NO-Cys and EFS hyperpolarize the membrane by activating apaminsensitive and TEA-resistant K+ channels and favour the hypothesis that a NO-liberating substance may act as a neurotransmitter in non-adrenergic, non-cholinergic (NANC) neurones in the rat forestomach.Our results also suggest that increase in cyclic GMP may cause apamin-resistant hyperpolarization but the apamin-sensitive hyperpolarization is mediated by another mechanism.

Activation of a slow outward current by the calcium released during contraction of cultured rat skeletal muscle cells

A slow outward current, activated during depolarization, which induced contraction in whole-cell patch-clamped rat skeletal muscle cells in primary culture [10], was extensively characterized in the present study. This current, Io, was simultaneously recorded with the contraction as a slow outward current during the test pulse, and a slow outward bell-shaped tail after repolarization. Io never appeared below the threshold potential for contraction, and the tail amplitude displayed a similar evolution with peak contraction amplitude as a function of membrane potential. This feature is consistent with the fact that Io was suppressed when contraction was blocked by 5 microM nifedipine [10], and it suggests that Io was dependent on calcium released during contraction. This was confirmed by the fact that the presence of 10 mM EGTA in the patch pipette prevented the development of both contraction and Io, and that Io could be activated during caffeine-induced contractures without applying depolarizations. Io could be carried by K+ or Cs+ ions, but not by Na+. The pharmacology of Io was different from that of Ca(2+)-dependent BK and SK channels, since it was resistant to tetraethylammonium (135 mM), charybdotoxin (25 nM) and apamin (50 nM). Io was also insensitive to 4-aminopyridine (1 mM) but blocked by 5 mM Ba2+ without change to contraction. It was concluded that rat cultured myoballs exhibit a Cs+ permeation through an atypical K+ channel type, which is activated by the calcium released during contraction.

Small-conductance Ca(2+)-activated K+ channels in bovine chromaffin cells

Simultaneous whole-cell patch-clamp and fura-2 fluorescence [Ca2+]i measurements were used to characterize Ca(2+)-activated K+ currents in cultured bovine chromaffin cells. Extracellular application of histamine (10 microM) induced a rise of [Ca2+]i concomitantly with an outward current at holding potentials positive to -80 mV. The activation of the current reflected an increase in conductance, which did not depend on membrane potential in the range -80 mV to -40 mV. Increasing the extracellular K+ concentration to 20 mM at the holding potential of -78 mV was associated with inwardly directed currents during the [Ca2+]i elevations induced either by histamine (10 microM) or short voltage-clamp depolarizations. The current reversal potential was close to the K+ equilibrium potential, being a function of external K+ concentration. Current fluctuation analysis suggested a unit conductance of 3-5 pS for the channel that underlies this K+ current. The current could be blocked by apamin (1 microM). Whole-cell current-clamp recordings showed that histamine (10 microM) application caused a transient hyperpolarization, which evolved in parallel with the [Ca2+]i changes. It is proposed that a small-conductance Ca(2+)-activated K+ channel is present in the membrane of bovine chromaffin cells and may be involved in regulating catecholamine secretion by the adrenal glands of various species.

Comparison of the distribution of binding sites for the potassium channel ligands [125I]apamin, [125I]charybdotoxin and [125I]iodoglyburide in the rat brain

Potassium channels represent a diverse and promising target for drug development. Pharmacological subtypes of K channels have begun to emerge based on the development of both organic molecules and peptide toxins which possess subtype selectivity. In order to evaluate the neuroanatomical distribution of these subtypes we have utilized the ligands [125I]apamin, [125I]charybdotoxin and [125I]iodoglyburide in an autoradiographic study of rat brain. In the rat brain, these ligands have selectivity for the low conductance Ca(2+)-activated, voltage-gated K channels and ATP-sensitive K channels respectively. The distribution of binding sites for these three ligands were distinctly different. [125I]Apamin binding was highest in various thalamic and hippocampal structures, while only low to moderate levels of [125I]charybdotoxin binding were seen in these regions. In contrast, very high levels of [125I]charbydotoxin were seen in white matter regions such as the lateral olfactory tract and fasciculus retroflexus. High levels of [125I]charybdotoxin binding were also seen in gray matter-containing regions such as the zona incerta, medial geniculate and superior colliculus, where low to moderate [125I]apamin binding was found. [125I]Iodoglyburide presented a more uniform binding with the highest levels in the globus pallidus, islands of Calleja, anteroventral nucleus of the thalamus and zonas reticulata of the substantia nigra. These results indicate that subtypes of K channels have very different distributions in the brain. As such, the results imply differing CNS actions for potential modulators of K channel subtypes.

Characterization and partial purification from pheochromocytoma cells of an endogenous equivalent of scyllatoxin, a scorpion toxin which blocks small conductance Ca(2+)-activated K+ channels

This work describes the partial purification of a heat-stable peptide which has the same properties as the scorpion toxin, scyllatoxin, a specific blocker of one class of Ca(2+)-activated K+ channels: (i) it competes with [125I]apamin for binding to the same site, (ii) like apamin and scyllatoxin, it blocks the after-potential hyperpolarization in skeletal muscle cells in culture, (iii) like apamin and scyllatoxin, it contracts guinea-pig taenia coli relaxed by epinephrine, (iv) it cross-reacts with antibodies raised against scyllatoxin but not with antibodies raised against apamin.

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.

Inhibitory effects of psychotomimetic sigma ligands on nicotine-induced K+ flux from differentiated PC12 cells

In NGF-treated PC12 cells, nicotine-induced K+ release was measured with a K(+)-sensitive microelectrode. The K+ outflow via nicotinic ACh receptor cation channels was inhibited by various psychotomimetic sigma ligands in the sequence of PCP, dextromethorphan >> DTG, MK 801, (+)SKF10047 >> (+)3-PPP. The K+ release was not affected by the neuroleptic sigma ligand haloperidol nor by the calcium antagonist nifedipine. The results suggest that psychotomimetic sigma ligands inhibit nicotine-stimulated K+ flux by interacting with nicotinic, rather than via sigma 2 receptors.

A novel large-conductance Ca(2+)-activated potassium channel and current in nerve terminals of the rat neurohypophysis

1. Nerve terminals of the rat posterior pituitary were acutely dissociated and identified using a combination of morphological and immunohistochemical techniques. Terminal membrane currents were studied using the 'whole-cell' patch clamp technique and channels were studied using inside-out and outside-out patches. 2. In physiological solutions, but with 7 mM 4-aminopyridine (4-AP), depolarizing voltage clamp steps from different holding potentials (-90 or -50 mV) elicited a fast, inward current followed by a slow, sustained, outward current. This outward current did not appear to show any steady-state inactivation. 3. The threshold for activation of the outward current was -30 mV and the current-voltage relation was 'bell-shaped'. The amplitude increased with increasingly depolarized potential steps. The outward current reversal potential was measured using tail current analysis and was consistent with that of a potassium current. 4. The sustained potassium current was determined to be dependent on the concentration of intracellular calcium. Extracellular Cd2+ (80 microM), a calcium channel blocker, also reversibly abolished the outward current. 5. The current was delayed in onset and was sustained over the length of a 150 ms-duration depolarizing pulse. The outward current reached a peak plateau and then decayed slowly. The decay was fitted by a single exponential with a time constant of 9.0 +/- 2.2 s. The decay constants did not show a dependence on voltage but rather on intracellular Ca2+. The time course of recovery from this decay was complex with full recovery taking > 190 s. 6. 4-AP (7 mM), dendrotoxin (100 nM), apamin (40-80 nM), and charybdotoxin (10-100 nM) had no effect on the sustained outward current. In contrast Ba2+ (200 microM) and tetraethylammonium inhibited the current, the latter in a dose-dependent manner (apparent concentration giving 50% of maximal inhibition (IC50) = 0.51 mM). 7. The neurohypophysial terminal outward current recorded here corresponds most closely to a Ca(2+)-activated K+ current (IK(Ca)) and not to a delayed rectifier or IA-like current. It also has properties different from that of the Ca(2+)-dependent outward current described in the magnocellular neuronal cell bodies of the hypothalamus. 8. A large conductance channel is often observed in isolated rat neurohypophysial nerve terminals. The channel had a unit conductance of 231 pS in symmetrical 150 mM K+.(ABSTRACT TRUNCATED AT 400 WORDS)

Increase of apamin receptors in skeletal muscle induced by colchicine: possible role in myotonia

We have shown an increase of apamin receptors in rat skeletal muscle membranes following the application of colchicine to the sciatic nerve. 125I-apamin binding to partially purified membrane fractions was observed since day 4, reached a maximum around days 6-15, and was negligible at day 35 after the application of colchicine. Control muscles (nerves treated with buffer solution) showed low binding values (11 fmol/mg protein). Maximal 125I-apamin binding values to partially purified muscle membranes of colchicine-treated rats (42 fmol/mg protein) were lower than those obtained in denervated muscle (95 fmol/mg protein). The affinity binding constant values were 37 (colchicine) and 95 pM (denervation). No signs of muscle denervation were observed on histological examination of the nerve submitted to colchicine treatment nor in the muscles innervated by it. Muscle tension developed by indirect stimulation was the same as in controls. We here show also that partially purified membranes of normal untreated muscles have measurable amounts of 125I-apamin binding (13 fmol/mg protein), similar to those obtained in control muscles. Electromyographic recordings of the muscles after colchicine treatment of the nerve showed abnormal repetitive electrical discharges, similar to myotonic discharges, that were present with a similar temporal course as the increase in apamin receptors. The myotonic-like discharges were suppressed by the topical application of apamin to the muscle, whereas the toxin had no effect on anthracene-9-carbolytic acid-induced myotonia. Our results suggest that a neurotrophic factor that travels by axonal flow is involved in the regulation of the expression of apamin receptors in skeletal muscle membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Trypsin-sensitive, rapid inactivation of a calcium-activated potassium channel

Most calcium-activated potassium channels couple changes in intracellular calcium to membrane excitability by conducting a current with a probability that depends directly on submembrane calcium concentration. In rat adrenal chromaffin cells, however, a large conductance, voltage- and calcium-activated potassium channel (BK) undergoes rapid inactivation, suggesting that this channel has a physiological role different than that of other BK channels. The inactivation of the BK channel, like that of the voltage-gated Shaker B potassium channel, is removed by trypsin digestion and channels are blocked by the Shaker B amino-terminal inactivating domain. Thus, this BK channel shares functional and possibly structural homologies with other inactivating voltage-gated potassium channels.

An after-depolarization following action potentials and its modulation by substance P in rat sympathetic neurons

Sympathetic neurons in rat coeliac-superior mesenteric ganglia (C-SMG) displayed an after-depolarization (ADP) following action potentials and after-hyperpolarization. The ADP had amplitudes of 3-10 mV and lasted for 2-6 s, during which membrane resistance and excitability of C-SMG neurons increased. The reversal potential of ADP was dependent on external K+ concentrations. The ADP was suppressed in a solution containing Cd2+ or zero Ca2+. The ADP thus appears to be produced by inhibition of certain K+ channels via a Ca(2+)-dependent process. Substances P (SP) depolarized ganglion cells and increased the ADP, leading to a long-lasting increase in membrane excitability of rat C-SMG neurons.

Apamin-sensitive potassium channels mediate agonist-induced oscillations of membrane potential in pituitary gonadotrophs

In cultured rat pituitary gonadotrophs, gonadotropin-releasing hormone (GnRH) induces rapid hyperpolarization of the cell membrane and causes cessation of the spontaneous electrical activity present in non-stimulated cells. This initial response to GnRH is followed by slow oscillations of membrane potential (Vm) which often exhibit brief bursts of action potentials (AP) fired from the peak of the oscillations. The hyperpolarization waves are synchronous with GnRH-induced elevations of cytoplasmic Ca2+ concentration ([Ca2+]i), such that Vm maxima alternate with the peak values of [Ca2+]i. The Vm oscillations result from repetitive activation of apamin-sensitive K+ channels by cytoplasmic Ca2+. Thus, GnRH activation of Ca2+ mobilization can generate a bursting pattern of membrane potential through the activation of K+ channels against a background of spontaneous electrical activity.

A small-conductance charybdotoxin-sensitive, apamin-resistant Ca(2+)-activated K+ channel in aortic smooth muscle cells (A7r5 line and primary culture)

A small conductance K+ channel was identified in smooth muscle cells of the rat aortic cell line A7r5 and also in rat aortic smooth muscle cells in primary culture, using conventional single-channel recording techniques. The single-channel conductance shows no rectification, either in the range -70 to +40 mV under asymmetrical conditions (9.1 pS), or in the range -100 to +50 mV in symmetrical 150 mM K+ (37 pS). Channel activity is reversibly inhibited by extracellular application of charybdotoxin, with a concentration of 8 nM producing half-maximal inhibition. It is unaffected by apamin or scyllatoxin. Channel activity depends on the presence of free Ca2+ on the cytosolic face of the membrane, with an activation zone between 0.1 and 1 microM. This small-conductance, charybdotoxin-sensitive, Ca(2+)-regulated K+ channel is activated by vasoconstrictors such as vasopressin and endothelin.