(+)-Tubocurarine blocks the Ca2+-dependent K+-channel of the bullfrog sympathetic ganglion cell

Synthesis and pharmacological testing of polyaminoquinolines as blockers of the apamin-sensitive Ca2+-activated K+ channel (SK(Ca))

The synthesis and pharmacological testing of a series of non-peptidic blockers of the SK(Ca) (SK-3) channel is described. Target compounds were designed to mimic the spatial relationships of selected key residues in the energy-minimised structure of the octadecapeptide apamin, which are a highly potent blocker of this channel. Structures consist of a central unit, either a fumaric acid or an aromatic ring, to which are attached two alkylguanidine or two to four alkylaminoquinoline substituents. Potency was tested by the ability to inhibit the SK(Ca) channel-mediated after-hyperpolarization (AHP) in cultured rat sympathetic neurones. It was found that bis-aminoquinoline derivatives are significantly more potent as channel blockers than are the corresponding guanidines. This adds to the earlier evidence that delocalisation of positive charge through the more extensive aminoquinolinium ring system is important for effective channel binding. It was also found that an increase in activity can be gained by the addition of a third aminoquinoline residue to give non-quaternized amines which have submicromolar potencies (IC(50)=0.13-0.36 microM). Extension to four aminoquinoline residues increased the potency to IC(50)=93 nM.

Synthesis and biological evaluation of N-methyl-laudanosine iodide analogues as potential SK channel blockers

Neuronal action potentials are followed by an afterhyperpolarisation (AHP), which is mediated by small conductance Ca2+-activated K+ channels (SK channels or KCa2 channels). This AHP plays an important role in regulating neuronal activity and agents modulating AHP amplitude could have a potential therapeutic interest. It was previously shown that N-methyl-bicuculline iodide blocks SK channels but its GABA) activity represents a serious drawback. In view of the structural analogy between bicuculline and laudanosine 14, several N-quaternary analogues of the latter were developed. It was shown that N-methyl-laudanosine 15 (NML) and N-ethyl-laudanosine 16 induce a reversible and relatively specific blockade of the apamin sensitive AHP in dopaminergic neurones with mean IC50s of 15, and 47 microM, respectively. Laudanosine 14, N-butyl-17 and N-benzyl-18 derivatives were less potent. In order to find pharmacophore elements, modifications were performed at different positions such as C-1, C-6 and C-7. Intracellular recordings on rat midbrain dopaminergic neurones were made in order to evaluate the putative blockade of SK channels by these molecules. Simplified structures such as tetrahydroisoquinoline derivatives with H or Me at C-1 1-6 presented no significant activity at 300 microM. The presence of a 1-(3,4-dimethoxybenzyl) moiety seems an important feature. Indeed, compound 8 showed a blockade of the AHP of only 33% at 300 microM while compound 13 blocked it by 67%, respectively, at the same concentration. Binding experiments were also performed. Binding affinities for SK channels are in good agreement with electrophysiological data. These results indicate that the presence of a charged nitrogen group is an essential point for the affinity on SK channels. Finally, because of the similar activity of both enantiomers of NML 19 and 20, the interaction site may present a symmetrical configuration.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.

Compounds that block both intermediate-conductance (IK(Ca)) and small-conductance (SK(Ca)) calcium-activated potassium channels

1. Nine bis-quinolinyl and bis-quinolinium compounds related to dequalinium, and previously shown to block apamin-sensitive small conductance Ca(2+)-activated K(+) channels (SK(Ca)), have been tested for their inhibitory effects on actions mediated by intermediate conductance Ca(2+)-activated K(+) channels (IK(Ca)) in rabbit blood cells. 2. In most experiments, a K(+)-sensitive electrode was employed to monitor the IK(Ca)-mediated net loss of cell K(+) that followed the addition of the Ca(2+) ionophore A23187 (2 microM) to red cells suspended at an haematocrit of 1% in a low K(+) (0.12 - 0.17 mM) solution. The remainder used an optical method based on measuring the reduction in light transmission that occurred on applying A23187 (0.4 or 2 microM) to a very dilute suspension of red cells (haematocrit 0.02%). 3. Of the compounds tested, the most potent IK(Ca) blocker was 1,12 bis[(2-methylquinolin-4-yl)amino]dodecane (UCL 1407) which had an IC(50) of 0.85+/-0.06 microM (mean+/-s.d. mean). 4. The inhibitory action of UCL 1407 and its three most active congeners was characterized by (i) a Hill slope greater than unity, (ii) sensitivity to an increase in external [K(+)], and (iii) a time course of onset that suggested use-dependence. Also, the potency of the nonquaternary compounds tested increased with their predicted lipophilicity. These findings suggested that the IK(Ca) blocking action resembles that of cetiedil rather than of clotrimazole. 5. Some quaternized members of the series were also active. The most potent was the monoquaternary UCL 1440 ((1-[N-[1-(3, 5-dimethoxybenzyl)-2-methylquinolinium-4-yl]amino]-10-[N'-(2-me thylqu inolinium-4yl)amino] decane (trifluoroacetate) which had an IC(50) of 1.8+/-0.1 microM. The corresponding bisquaternary UCL 1438 (1, 10-bis[N-[1-(3,5-dimethoxybenzyl)-2-methylquinolinium-4-yl]amino] decane bis(trifluoroacetate) was almost as active (IC(50) 2.7+/-0.3 microM). 6. A bis-aminoquinolium cyclophane (UCL 1684) had little IK(Ca) blocking action despite its great potency at SK(Ca) channels (IC(50) 4.1+/-0.2 nM). 7. The main outcome is the identification of new intermediate-conductance Ca(2+)-activated K(+) channel blockers with a wide range of IK(Ca)/SK(Ca) selectivities.

Pharmacological characterization of small-conductance Ca(2+)-activated K(+) channels stably expressed in HEK 293 cells

Three genes encode the small-conductance Ca(2+)-activated K(+) channels (SK channels). We have stably expressed hSK1 and rSK2 in HEK 293 cells and addressed the pharmacology of these subtypes using whole-cell patch clamp recordings. The bee venom peptide apamin blocked hSK1 as well as rSK2 with IC(50) values of 3.3 nM and 83 pM, respectively. The pharmacological separation between the subtypes was even more prominent when applying the scorpion peptide blocker scyllatoxin, which blocked hSK1 with an IC(50) value of 80 nM and rSK2 at 287 pM. The potent small molecule blockers showed little differentiation between the channel subtypes. The bis-quinolinium cyclophane UCL 1684 blocked hSK1 with an IC(50) value of 762 pM and rSK2 at 364 pM. The antiseptic compound dequalinium chloride blocked hSK1 and rSK2 with IC(50) values of 444 nM and 162 nM, respectively. The nicotinic acetylcholine receptor antagonist d-tubocurarine was found to block hSK1 and rSK2 with IC(50) values of 27 microM and 17 microM when measured at +80 mV. The inhibition by d-tubocurarine was voltage-dependent with increasing affinities at more hyperpolarized potentials. The GABA(A) receptor antagonist bicuculline methiodide also blocked hSK1 and rSK2 in a voltage-dependent manner with IC(50) values of 15 and 25 microM when measured at +80 mV. In conclusion, the pharmacological separation between SK channel subtypes expressed in mammalian cells is too small to support the notion that the apamin-insensitive afterhyperpolarization of neurones is mediated by hSK1.

An apamin-sensitive Ca2+-activated K+ current in hippocampal pyramidal neurons

In hippocampal and other cortical neurons, action potentials are followed by afterhyperpolarizations (AHPs) generated by the activation of small-conductance Ca2+-activated K+ channels (SK channels). By shaping the neuronal firing pattern, these AHPs contribute to the regulation of excitability and to the encoding function of neurons. Here we report that CA1 pyramidal neurons express an AHP current that is suppressed by apamin and is involved in the control of repetitive firing. This current presents distinct kinetic and pharmacological features, and it is modulated differently than the apamin-insensitive slow AHP current. Furthermore, our in situ hybridizations show that the apamin-sensitive SK subunits are expressed in CA1 pyramidal neurons, providing a potential molecular correlate to the apamin-sensitive AHP current. Altogether, these results clarify the discrepancy between the reported high density of apamin-binding sites in the CA1 region and the apparent lack of an apamin-sensitive current in CA1 pyramidal neurons, and they may explain the effects of this toxin on hippocampal synaptic plasticity and learning.

Activation of Ca(2+)-dependent K+ channels by cyanide in guinea pig adrenal chromaffin cells

The effects of cyanide (CN) on whole cell current measured with the perforated-patch method were studied in adrenal medullary cells. Application of CN produced initially inward and then outward currents at -52 mV or more negative. As the membrane potential was hyperpolarized, amplitude and latency of the outward current (Io) by CN became small and long, respectively. A decrease in the external Na+ concentration did not affect the latency for CN-induced Io but enhanced the amplitude markedly. The CN Io reversed polarity at -85 mV, close to the Nernst potential for K+, and was suppressed by the K+ channel blockers curare and apamin but not by glibenclamide, suggesting that Io is due to the activation of Ca(2+)-dependent K+ channels. Consistent with this notion, the Ca(2+)-mobilizing agents, muscarine and caffeine, also produced Io. Exposure to CN in a Ca(2+)-deficient medium for 4 min abolished caffeine- or muscarine-induced Io without development of Io, and addition of Ca2+ to the CN-containing solution induced Io. We conclude that exposure to CN produces Ca(2+)-dependent K+ currents in an external Ca(2+)-dependent manner, probably via facilitation of Ca2+ influx.

On the concept of a bivalent pharmacophore for SKCa channel blockers: synthesis, pharmacological testing, and radioligand binding studies on mono-, bis-, and tris-quinolinium compounds

The dissociation equilibrium constants (Kd values) of dequalinium (2) and the monoquinolinium compounds 1a and 1b have been determined from competition equilibrium radioligand binding with [125I]apamin on rat brain synaptic plasma membranes (SPMs). Dequalinium binds to the channel with 2 orders of magnitude higher affinity than 1a or 1b, suggesting that both quinolinium groups are needed for potent and selective SKCa channel blockade. The trisquinolinium compound 3 (1,1'-[5-[4-(4- aminoquinolinium-1-yl)but-1-yl]non-4-en-1,9-diyl]-bis-(4- aminoquinolinium)) has been synthesized and tested for inhibition of the afterhyperpolarization of rat sympathetic neurones and on the binding assay. Compound 3 shows approximately one order of magnitude higher potency than 2, being the most potent non-peptidic SKCa channel blocker reported so far (Kd approximately 30 nM). The higher affinity of 3 compared with 2 may be due to direct binding of the third quinolinium group to the channel or may arise from a reduction of the unfavorable entropy of binding via an increase of the "local concentration" of quinolinium groups.

M-current suppression in PC12 cells by bradykinin is mediated by a pertussis toxin-insensitive G-protein and modulated by intracellular calcium

Inhibition of the M-current in PC12 cells by bradykinin has been studied under whole-cell recording conditions. In cells dialyzed with GTP-gamma-S, bradykinin produced a total and irreversible M-current suppression, while the inhibition was attenuated in cells dialyzed with GDP-beta-S. Inhibition occurred in cells pretreated with pertussis toxin, while this treatment prevented the modulation of Ca2+ currents by muscarine. The rate and extent of inhibition increased with the level of intracellular Ca2+ from 0 to 70 nM. These results indicate that a pertussis toxin insensitive G-protein mediates the action of bradykinin on the M-current, and some steps in the second messenger cascade are modulated by Ca2+.

Synthesis, molecular modeling, and K+ channel-blocking activity of dequalinium analogues having semirigid linkers

Dequalinium [1,1'-(decane-1, 10-diyl)bis(2-methyl-4-aminoquinolinium)] is an effective blocker of the small conductance Ca2(+)-activated K+ channel. It has been shown that the number of methylene groups in the alkyl chain linking the two quinolinium rings of this type of molecule is not critical for activity. To further investigate the role of the linker, analogues of dequalinium have been synthesized, in which the alkyl chain has been replaced by CH2XCH2 where X is a rigid or semirigid group containing aromatic rings. The compounds have been tested for blockade of the slow after-hyperpolarization on rat sympathetic neurons. The most potent compounds have X = phenanthryl, fluorenyl, cis-stilbene, and C6H4(CH2)nC6H4, where n = 0-4. The conformational preferences of the compounds were investigated using the XED/COSMIC molecular modeling system. Although there is some dependence of the potency of the analogue on the conformational properties of the linker (X), overall, X groups having substantial structural differences are tolerated. It seems that X provides a support for the two quinolinium groups and does not interact with the channel directly. The intramolecular separation between the quinolinium rings, which is provided by rigid groups X, is not critical for activity; this may be attributed to the residual conformational mobility of the heterocycles and to the extensive delocalization of the positive charge. These two factors may permit favorable contacts between the quinolinium groups and the channel over a range of intramolecular separations.

Synthesis and pharmacological testing of dequalinium analogues as blockers of the apamin-sensitive Ca(2+)-activated K+ channel: variation of the length of the alkylene chain

Dequalinium is a potent and selective blocker of the small conductance Ca(2+)-activated K+ (SKCa) channel in rat sympathetic neurones. Analogues of dequalinium possessing 3-6, 8, 10, and 12 methylene groups in the linking chain have been synthesized and tested for inhibition of the afterhyperpolarization in rat sympathetic neurones. The compounds having a 5-12-carbon chain showed very little variation in their activity as SKCa channel blockers. The analogues possessing four and three methylenes exhibited 3- and 8-fold lower potency, respectively, compared with dequalinium. These results are discussed in the context of possible modes of binding of the compounds to the SKCa channel.

Synthesis and quantitative structure-activity relationship of a novel series of small conductance Ca(2+)-activated K+ channel blockers related to dequalinium

The synthesis, pharmacological testing, and quantitative structure-activity relationship studies of a novel series of bisquinolinium small conductance Ca(2+)-activated K+ channel blockers (23) related to dequalinium are described. In this series, two quinolinium rings are linked via the 4-position to an alpha, omega-diamino alkylene chain and the ring N atom is quaternized with a methyl or benzyl group. The exocyclic N atom can be replaced by O, S, or CH2 but with some loss of potency. The quinoline groups do not have to be quaternized for blocking activity, as long as they are basic enough to be protonated at the site of action. For the quaternary compounds, there is considerable steric tolerance for the group R attached to the ring N atom of the quinoline; a benzyl group gave the optimum potency in this series. Moreover, and in contrast to previously reported results for dequalinium analogues, there is no correlation of activity to previously reported results for dequalinium analogues, there is no correlation of activity with N1 charge or EHOMO. On the other hand, a good correlation was obtained between the blocking potency of the compounds and ELUMO [pEMR = 1.16(+/-0.26)ELUMO + 5.33(+/-01.29)(n = 11, r= 0.83, s = 0.243)]. It has been possible to combine this equation with the previously reported ELUMO correlation for a series of dequalinium analogues to include all the compounds of both series [pEMR = 1.17(+/-0.15)ELUMO +5.33(+/-0.76)(n =24, r = 0.85, s = 0.249)]. A possible physical meaning for the ELUMO correlation based upon the principle of maximum hardness is discussed.

Contribution of SK and BK channels in the control of catecholamine release by electrical stimulation of the cat adrenal gland

1. Transmural electrical stimulation (10 Hz, 1 ms, 40 V for 10 s) of cat adrenal glands perfused at room temperature with Krebs-Hepes solution produced catecholamine secretory responses which were reproducible when stimulations were applied at 5 min intervals. Such responses were inhibited about 20% by atropine (1 microM) and 80% by hexamethonium (30 microM). Apamin (100 nM) increased the secretory response 2.5-fold in the presence of atropine and 8-fold in the presence of hexamethonium. 2. Potentiation by apamin of secretory responses evoked by 100-pulse trains was similar at 5, 10 and 20 Hz (about 2-fold). When glands were continuously stimulated at 3 Hz, apamin increased 4-fold the initial secretion plateau. Continuous stimulation at a higher frequency (20 Hz) produced a sharp secretory peak followed by a small, sustained plateau; apamin did not alter this plateau. Apamin also enhanced the secretory responses obtained with sustained stimulation with acetylcholine (10 or 200 microM). 3. Secretion peaks induced by brief acetylcholine pulses (10 microM for 10 s) applied to isolated and superfused cat adrenal chromaffin cells were enhanced more than 3-fold by 100 nM apamin. Charybdotoxin (10 nM) did not enhance these secretory peaks. 4. In perfused cat adrenal glands, charybdotoxin (10 nM) affected neither the secretion evoked by trains of electrical stimulation applied at different frequencies nor the secretion evoked by acetylcholine pulses. 5. In 0.5 mM [Ca2+]o, apamin enhanced 3-fold the secretion evoked by electrical stimulation trains of 100 pulses (10 Hz, 10 s) and almost 6-fold the acetylcholine (10 microM for 10 s)-induced secretion. In 5 mM Ca2+, apamin enhanced the secretory responses to electrical stimulation and acetylcholine 2- and 10-fold, respectively. Charybdotoxin enhanced 2.5-fold the secretory response to electrical stimulation in 0.5 mM Ca2+, although this effect was not statistically significant. A synergistic interaction between the two toxins on catecholamine release induced by electrical stimulation was observed at low but not at high [Ca2+]o. 6. Simultaneous release of acetylcholine and catecholamines upon electrical stimulation was achieved in glands in which the endogenous acetylcholine stores in the splanchnic nerve terminals had been prelabelled by perfusion with [3H]choline. While apamin enhanced more than 2-fold the postsynaptic release of catecholamines, the presynaptic release of acetylcholine remained unaffected. 7. The results are compatible with the hypothesis that, under physiological conditions, Ca(2+)-activated SK channels present in chromaffin cells control the firing patterns of action potentials induced by the acetylcholine released from splanchnic nerves during stress.(ABSTRACT TRUNCATED AT 400 WORDS)

The inhibitory effects of nicotinic antagonists on currents elicited by GABA in rat hippocampal neurons

The nicotinic antagonists d-tubocurarine and trimethaphan camsylate competitively inhibit GABA-induced currents. Hexamethonium, mecamylamine and dihydro-beta-erythroidine, other nicotinic antagonists, do not affect GABA-elicited currents. The trimethaphan effect is completely reversed by a putative convulsant receptor antagonist, alpha-isopropyl-alpha-methyl-gamma-butyrolactone, which implies that the trimethaphan binding site may be closely associated with the convulsant site. However, nicotine was ineffective in competing for either the d-tubocurarine or trimethaphan effect at the GABAA receptor. From these observations, we propose that the nicotinic and GABAA receptor ionophore complexes share similar configurational patterns that accommodate some of the same molecules. Possible mechanisms for the trimethaphan and d-tubocurarine blockades are discussed.

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.

The slow Ca(2+)-activated K+ current, IAHP, in the rat sympathetic neurone

1. Adult and intact sympathetic neurones of the rat superior cervical ganglion maintained in vitro at 37 degrees C were analysed using the two-electrode voltage-clamp technique in order to investigate the slow component of the Ca(2+)-dependent K+ current, IAHP. 2. The relationship between the after-hyperpolarization (AHP) conductance, gAHP, and estimated Ca2+ influx resulting from short-duration calcium currents evoked at various voltages proved to be linear over a wide range of injected Ca2+ charge. An inflow of about 1.7 x 10(7) Ca2+ ions was required before significant activation of gAHP occurred. After priming, the gAHP sensitivity was about 0.3 nS pC-1 of Ca2+ inward charge. 3. IAHP was repeatedly measured at different membrane potentials; its amplitude decreased linearly with membrane hyperpolarization and was mostly abolished close to the K+ reversal potential, EK (-93 mV). The monoexponential decay rate of IAHP was a linear function of total Ca2+ entry and was not significantly altered by membrane potential in the -40 to -80 mV range. 4. Voltage-clamp tracings of IAHP could be modelled as a difference between two exponentials with tau on approximately 5 ms and tau off = 50-250 ms. 5. Sympathetic neurones discharged only once at the onset of a long-lasting depolarizing step. If IAHP was selectively blocked by apamin or D-tubocurarine treatments, accommodation was abolished and an unusual repetitive firing appeared. 6. Summation of IAHP was demonstrated under voltage-clamp conditions when the depolarizing steps were repeated sufficiently close to one another. Under current-clamp conditions the threshold depolarizing charge for action potential discharge significantly increased with progressive pulse numbers in the train, suggesting that an opposing conductance was accumulating with repetitive firing. This frequency-dependent spike firing ability was eliminated by pharmacological inhibition of the slow IAHP. 7. The IAHP was significantly activated by a single action potential; it was turned on cumulatively by Ca2+ load during successive action potential discharge and acted to further limit cell excitability.

The role of inositol trisphosphate on ACh-induced outward currents in bullfrog saccular hair cells

Acetylcholine (ACh) is considered as the most likely candidate for a neurotransmitter of the efferent synapse onto hair cell. In this paper, the nature of this cholinergic receptor mechanism on dissociated bullfrog saccular hair cell was examined by using whole cell recording and Ca2+ sensitive fluorophotometric technique. Both the ACh-induced current and the increase of [Ca2+]i were observed in an oscillatory manner, and were the largest around the basal part of the cell where the efferent synapse is thought to make a contact with the membrane. The reversal potential of ACh-induced current indicated that ACh activated a K+ conductance. The ACh-induced current was reversibly blocked by atropine, d-tubocurarine (dTC), apamin, tetraethylammonium (TEA) and quinine. Neither muscarine nor nicotine mimicked the ACh-induced current. When GTP gamma S was injected into a hair cell, the first ACh application induced an outward current of transient kinetics, but in subsequent trials ACh-induced current lost its decay phase. Intracellularly injected D-myo-inositol 1,4,5-trisphosphate (InsP3) generated outward currents. Intracellularly injected heparin suppressed ACh-induced currents, and lithium (Li+) increased ACh-induced currents. These results indicate that ACh activates a receptor coupled with a guanine nucleotide binding protein (G-protein) which triggers metabolic cascades of InsP3 and Ca2+ leading to the activation of the Ca(2+)-activated K+ channel.

Dequalinium, a selective blocker of the slow afterhyperpolarization in rat sympathetic neurones in culture

The actions of dequalinium have been investigated in cultured rat sympathetic neurones. It produced a rapid and reversible inhibition of the slow apamin-sensitive component of the afterhyperpolarization (AHP) which follows a single action potential in these cells. The IC50 for this effect was 1.1 microM and in voltage clamp experiments, 1 microM dequalinium produced 45% inhibition of the underlying current IAHP. When the small conductance Ca(2+)-activated K+ channels were blocked by 20 nM apamin the slow component of the AHP was abolished, and dequalinium (10 microM) produced no further change in the residual AHP. Dequalinium (10 microM) had no effect on the voltage-activated Ca2+ current in these cells, suggesting that the inhibition of the AHP was the result of a direct interaction with the K+ channels. The A-current as well as a composite current made up of IK and IC were all unaffected by 10 microM dequalinium. However, at this concentration it did produce 18% inhibition of the M-current. These results show dequalinium to be a potent and selective non-peptide blocker of the apamin-sensitive small conductance Ca(2+)-activated K+ channel in rat sympathetic neurones.

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)

Comparison of lowest energy conformations of dimethylcurine and methoxyverapamil: evidence of ternary association of calcium channel, Ca2+, and calcium entry blockers

Verapamil and dimethylcurine are Ca2+ entry blockers of essentially different chemical structures which presumably bind to the same arylalkylamine receptor of the L-type Ca channel. A systematic conformational analysis of methoxyverapamil (D-600) and dimethylcurine has been carried out using a molecular mechanics method. The lowest minimum-energy conformations of D-600 are predisposed to chelate Ca2+ by four oxygen atoms of the stacked methoxyphenyl moieties. Comparison of the lowest energy conformations of D-600-Ca2+ and dimethylcurine revealed a similar spatial disposition of cationic groups and methoxyphenyl moieties in the two compounds. A three-dimensional model of arylalkylamine receptor was suggested which incorporates two nucleophilic areas of the Ca channel. Dimethylcurine binds to these areas by its quaternary amine functions, whereas D-600 does so by amine function and via coordinated Ca2+. The results support the hypotheses on ternary complex formation between the ligands of Ca channel, their receptors, and Ca2+.

Dequalinium: a potent inhibitor of apamin-sensitive K+ channels in hepatocytes and of nicotinic responses in skeletal muscle

The bisquaternary compound dequalinium has been tested for its ability to inhibit the loss of K+ which angiotensin II causes in guinea-pig hepatocytes and which occurs through apamin-sensitive Ca(2+)-activated K+ (SKCa) channels. Dequalinium blocked angiotensin II-evoked K+ loss with an IC50 of 1.5 +/- 0.1 microM and also inhibited 125I-monoiodoapamin binding with a KI of 1.1 +/- 0.1 microM. It is the most active non-peptide SKCa blocker so far described. The neuromuscular blocking agent vecuronium was also tested, and proved to be considerably less effective (IC50, 4.5 +/- 0.3 microM; KI, 3.6 +/- 0.5 microM). Dequalinium was also examined for its actions at nicotinic receptors in skeletal muscle and was found to be a potent, non-competitive antagonist of carbachol contractions of the frog rectus abdominis. In the frog cutaneous pectoris muscle, end-plate depolarizations induced by carbachol became smaller and more transient in the presence of dequalinium at 10 nM. However, contractions of the frog sartorius and rat diaphragm in response to nerve stimulation were inhibited only by concentrations > 1 microM. These apparently discrepant effects of dequalinium on nicotinic responses could be explained either by open channel block of slow onset or by 'stabilization' of the desensitized state of the receptor. The potency of dequalinium will make it a useful agent for the study of nicotinic receptors as well as of SKCa channels.

Synaptic transmission in rat cardiac neurones

Intracellular recordings of spontaneous synaptic activity and synaptic responses to fibre tract stimulation were taken from neurones of ganglia isolated from the left atrium and interatrial septum of the rat. In six out of 57 neurones studied, spontaneous fast excitatory postsynaptic potentials (EPSPs) were recorded. Single stimulation of fibre tracts approaching the ganglion resulted in an all-or-none response consisting of an EPSP, from which an action potential abruptly appeared. This response disappeared in Ca(2+)-free/high-Mg2+ solution, indicating that it was orthodromic in origin. EPSPs were markedly exaggerated and prolonged by neostigmine (1-5 microM). EPSPs produced by high-frequency (0.1-20 Hz) fibre tract stimulation were markedly attenuated when compared with responses to single fibre tract stimulation, although they usually remained suprathreshold for spike initiation. High concentrations of hexamethonium (1 mM) and d-tubocurarine (300 microM) failed to inhibit responses to single fibre tract stimulation, although they completely abolished responses to high-frequency stimulation. Responses to single fibre tract stimulation were abolished by trimetaphan (greater than or equal to 100 microM). No slow synaptic responses were detected during single or high-frequency fibre tract stimulation. All cardiac neurones that responded orthodromically were highly excitable: they had a short post-spike after-hyperpolarization (AHP) and responded with multiple firing to prolonged membrane depolarization. It is concluded that cardiac neurones, in the region of the heart studied here, receive single 'strong' cholinergic inputs from some fibre tracts approaching the ganglion that elicit EPSPs accompanied by spikes. EPSPs are rather resistant to ganglion-blocking agents and subject to frequency modulation.

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.

Acetylcholine receptors in rat cardiac neurones

Membrane potential changes produced by acetylcholine (ACh), and their underlying mechanisms, were studied in neurones of isolated cardiac ganglia of the rat by means of intracellular microelectrodes. Five components of membrane potential change could be detected in cardiac neurones following 1-5 s micro-application of ACh: (i) fast depolarization resulting from an activation of nonselective cationic conductance; (ii) slow depolarization associated with a decreased membrane conductance, presumably for potassium ions; slow hyperpolarization which consisted of (iii) early and (iv) late parts resulting from an activation of calcium-sensitive potassium current and from inhibition of steady-state inward current, respectively; and (v) delayed slow hyperpolarization associated with an increased conductance, most likely for potassium ions. Components (i), (iii) and (iv) persisted in the presence of atropine and were inhibited by nicotinic antagonists. Thus they were due to activation of nicotinic ACh receptors. However, the sensitivity of component (i) to ganglion-blocking agents appeared to be rather low: IC50s for inhibiting (i) were 226 +/- 34.2 microM, 31.2 +/- 4.31 microM and 15.3 +/- 3.27 microM for hexamethonium, d-tubocurarine, and trimetaphan, respectively. Components (ii) and (v) were abolished by atropine (1 microM) and mimicked by muscarine (component (ii) also persisted in d-tubocurarine), hence they resulted from activation of muscarinic ACh receptors. It is concluded that cardiac neurones are endowed with both nicotinic and muscarinic ACh receptors. Their activation leads to membrane depolarization and discharges followed by hyperpolarization and inhibition of discharges.

Ca(2+)-activated K+ channels modulate muscarinic secretion in cat chromaffin cells

1. This study was aimed at testing the hypothesis that Ca(2+)-dependent K+ channels regulate the release of catecholamines mediated by muscarinic stimulation of cat adrenal chromaffin cells. Two parameters were measured: the secretory response to brief pulses of methacholine (100 microM for 10 s) in intact cat adrenal glands perfused at a high rate with oxygenated Krebs solution; and the changes in cytosolic Ca2+ concentrations, [Ca2+]i, produced by puff applications of methacholine pulses (also 100 microM for 10 s) in isolated single cat adrenal chromaffin cells loaded with Fura-2. 2. A pulse of methacholine released 805 +/- 164 ng of catecholamines (mean of thirty-two pulses). d-Tubocurarine (DTC) increased the secretory response in a concentration-dependent manner. The maximum increase (around 1000 ng catecholamines over control values) was reached at 100 microM-DTC and the EC50 was around 10 microM. 3. The secretory responses to methacholine alone, or to the combination of methacholine plus DTC, were strongly dependent on the extracellular Ca2+ concentration, [Ca2+]o. Thus Ca2+o removal from the perfusing solution for 5-10 min abolished catecholamine release. 4. At 0.1 microM, isradipine (an L-type Ca2+ channel blocker) inhibited by 71% the secretory response to DTC plus methacholine. At 1 microM, Bay K 8644 (an L-type Ca2+ channel activator) increased 2-fold the secretory response to DTC plus methacholine (2746 ng of catecholamines). 5. Apamin (1 microM) increased 3.5-fold the secretory response to methacholine pulses (from 500 to 1800 ng of catecholamines). 6. Methacholine pulses enhanced [Ca2+]i from the resting level of 100 nM to a peak of 1000 nM which quickly declined to basal level. DTC (100 microM) enhanced by 20% the [Ca2+]i peak and substantially prolonged its duration. 7. Apamin (1 microM) increased by 60% the [Ca2+]i peak evoked by methacholine, and delayed the initiation of decline of the [Ca2+]i peak. 8. These results are compatible with the idea that muscarinic stimulation depolarizes the cat adrenal chromaffin cell through an unidentified mechanism. Depolarization is probably counteracted by activation of Ca2+i-dependent K+ channels. Therefore, inhibition of these channels enhances depolarization and firing of action potentials which activate voltage-dependent L-type Ca2+ channels to increase further the Ca2+i signal and the secretory response. Thus Ca2+i-dependent K+ channels, probably of the small-conductance type (SK), seem to be involved in the modulation of muscarinic-evoked catecholamine release responses in cat adrenal chromaffin cells.

Compound 48/80 blocks transmission and increases the excitability of ganglion neurons

Compound 48/80 (5.0-50 micrograms/ml) significantly and reversibly decreased (1) the amplitude, but not the shape of the compound action potential, (2) the amplitude and duration of the acetylcholine potential and (3) the residual fast excitatory postsynaptic potential recorded from neurons of the 9th and 10th paravertebral ganglia of the bullfrog Rana catesbeiana. The excitability of B-type ganglion neurons in the presence of nicotinic and muscarinic receptor antagonists was increased by compound 48/80 without altering the input resistance or membrane potential. In addition, compound 48/80 (10-50 micrograms/ml) significantly decreased the duration of the spike afterhyperpolarization (AHP). The amplitude but not the decay rate of the current underlying the slow component of the spike AHP was decreased by compound 48/80. Compound 48/80 did not, however, alter either the amplitude or the duration of calcium-dependent spikes. Intracellular recordings from dissociated sympathetic neurons also demonstrated a compound 48/80-induced increase in neuronal excitability. These results suggest that compound 48/80 interacts with the nicotinic receptor/channel complex to decrease ganglionic transmission, and also has a direct action to increase neuronal excitability by blocking potassium channels mediating the duration of the spike AHP.

Effects of muscarine on single rat adrenal chromaffin cells

1. The action of muscarine on membrane currents and cytosolic calcium (Ca2+) of dissociated rat adrenal chromaffin cells was investigated using standard whole-cell voltage-clamp techniques and microfluorimetry of unclamped single cells. 2. In cells held at a constant holding potential negative to -40 mV, brief (5-10 s) applications of muscarine produced a transient activation of outward current. The activation of this current by muscarine also occurs in the presence of 5 mM-Co2+. 3. The outward current activated by muscarine at holding potentials negative to about -40 mV is blocked over 90% by either 200 microM-curare or 200 nM-apamin. One millimolar TEA produces variable blocking effects at such potentials. 4. The outward current activated by muscarine is transient even in the continuing presence of muscarine. Complete recovery between pairs of muscarine applications occurs over a 1-2 min period. If sufficient time was allowed for recovery between muscarine applications, the muscarine-activated outward current could be reliably elicited in dialysed cells for periods of 20-30 min. 5. Voltage ramps were used to examine effects of muscarine on currents over a range of membrane potentials. Over all potentials, muscarine activates a relatively voltage-independent component which is blocked almost completely by 200 nM-apamin and by 200 microM-curare. At potentials negative to about -40 mV, the apamin- and curare-sensitive current accounts for virtually all muscarine-activated current. This current appears to correspond to a Ca(2+)-activated, voltage-independent current found in these cells. Effects of muscarine on currents activated at potentials positive to 0 mV are complex. At potentials above 0 mV, muscarine can produce either an activation or an inhibition of outward current. The outward current activated at positive potentials was primarily voltage dependent and blocked by 1 mM-TEA. However, in some cells activation of voltage-dependent current was not observed and, in such cases, muscarine produced an inactivation of the voltage-dependent component of current. The inactivation of outward current could also be observed in the presence of 5 mM-Co2+ indicating that the inactivation does not occur secondarily to an effect of muscarine on Ca2+ current. The possibility is discussed that the inactivation of outward current occurs as a result of intrinsic inactivation properties of the voltage-dependent Ca(2+)-dependent K+ current. According to this hypothesis, the extent to which inactivation of voltage-dependent outward current is observed depends on the magnitude of the muscarine-induced cytosolic Ca2+ elevation and the level of depolarization of the cell.(ABSTRACT TRUNCATED AT 400 WORDS)

Two components of calcium-activated potassium current in rat adrenal chromaffin cells

1. The activation of calcium (Ca2+)-dependent potassium (K+) currents in dissociated rat adrenal chromaffin cells was investigated using the dialysed cell recording technique. 2. Ca(2+)-dependent K+ current was the major component of outward current at command potentials from -30 mV to about +50 mV. 3. Two components of Ca(2+)-dependent outward current could be distinguished based on the voltage dependence of activation, the properties of tail currents following repolarization, and pharmacological properties. 4. One Ca(2+)-dependent current was similar to an after-hyperpolarization current (often termed IAHP) observed in other cell types. This current was largely blocked by 200 nM-apamin or 200 microM-curare, was associated with slow Ca(2+)-dependent tail current, and exhibited little dependence on voltage. In cells with cytosolic Ca2+ buffered to 500 nM-1 microM, curare-sensitive current accounted for most of the membrane current at potentials negative to about -40 mV. 5. A second component of Ca(2+)-activated K+ current exhibited voltage-dependent activation, was completely blocked by 1 mM-TEA, and turned off rapidly following repolarization. An unusual aspect of the TEA-sensitive currents was that they appeared to inactivate under conditions of constant cytosolic Ca2+. 6. A novel observation during these experiments was a slow hump of outward current which appears to result from a non-monotonic elevation in cytosolic Ca2+ during prolonged voltage jumps.

Characteristics of multiple Ca(2+)-activated K+ channels in acutely dissociated chick ciliary-ganglion neurones

1. Whole-cell and single-channel recordings were used to characterize Ca(2+)-activated K+ channels (IK(Ca)) in acutely dissociated chick-ganglion neurones. 2. Application of depolarizing voltage steps resulted in outward currents that could be separated according to their dependence on external Ca2+ and/or holding potential. IK(Ca) was the only outward current that could be evoked from holding potentials of -50 mV or less. IK(Ca) was eliminated by bath application of Ca(2+)-free salines. A voltage-dependent outward current (IK(V)) could be evoked from more negative holding potentials in Ca(2+)-free salines. IK(V) was only partially blocked by as much as 30 mM-tetraethylammonium (TEA). 3. Tail currents associated with IK(Ca) reversed close to the K+ equilibrium potential (EK). IK(Ca) tail currents appeared sigmoidal, but the falling phase of the tail currents could be fitted with exponential curves that decayed faster at more negative membrane potentials. 4. IK(Ca) was blocked completely and reversibly by 10 mM-TEA. IK(Ca) was substantially reduced (80-90%) by as little as 1 mM-TEA. 5. Total IK(Ca) was reduced but not eliminated by saturating concentrations of apamin (200 nM). This blockade was not reversible with up to 30 min of washing. Application of 100 microM-d-tubocurare (dTC) also produced a partial blockade of total IK(Ca). 6. Whole-cell current-clamp recordings showed that IK(Ca) contributed to the late phases of spike repolarization and was the dominant current flowing during the spike after-hyperpolarization (AHP). Application of 200 nM-apamin caused a reduction in the duration of the AHP. This reduction was best seen when multiple spikes were evoked by prolonged (20-50 ms) injections of depolarizing current. 7. Three distinct types of IK(Ca) channels could be observed in inside-out patches in the presence of free Ca2+ concentrations of 2 x 10(-7) M, but not in the presence of free Ca2+ at concentrations of less than 10(-9) M. These had unitary chord conductances of 190 pS (i1), 110 pS (i2), and 45 pS (i3) with [K+]o = 150 mM and [K+]i = 75 mM. Each of these three channels had distinct kinetic properties. The 45 pS channel was most sensitive to activation by Ca2+ and could be detected at free Ca2+ concentrations as low as 10(-8) M. 8. All three IK(Ca) channels could be observed in inside-out patches held at membrane potentials where IK(V) was fully inactivated. Application of 10 mM-TEA caused a complete block of IK(Ca) channels in outside-out patches.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of soman on the electrical properties and excitability of bullfrog sympathetic ganglion neurones

1. The effects of soman (0.1-10 microM), an irreversible inhibitor of acetylcholinesterase (AChE), were examined on the electrical properties of ganglion neurones of the paravertebral sympathetic chain of the bullfrog, Rana catesbeiana. 2. Soman (10 microM) depolarized 29 of 35 (83%) ganglion neurones studied by 6.4 +/- 0.65 mV within 10 min of application and reduced the cell input resistance in 9 of 11 neurones examined (82%) to 55 +/- 5.3% of control. 3. Soman (10 microM) significantly reduced the maximum amplitude and the maximum rate of rise of the action potential and the duration, but not the amplitude, of the after-hyperpolarization (AHP) following the action potential elicited by either direct or antidromic stimulation. The maximum rate of fall and the duration of the action potential were not significantly affected by soman. These actions of soman were independent of the agent-induced depolarization. When examined by a single microelectrode voltage clamp, soman reduced the amplitude and the time constant of the current underlying the slow AHP, IAHs. 4. Soman (1-10 microM) produced an increase in neuronal excitability which was evidenced as either an increase in the number of action potentials or a decrease in the interspike interval in response to constant-current depolarizing pulses. The soman-induced increase in excitability occurred independently of both the agent-induced depolarization and the decrease in input resistance, was reversible with washing, was not caused by an inhibition of the M-current and was also recorded in dissociated sympathetic ganglion neurones.5. The effects of soman on the membrane potential, input resistance and the duration of the AHP but not cell excitability were blocked by pretreatment with atropine (10 microM). Pretreatment with dihydro-/J-erythroidine (DHbetalE) (10 microM) was ineffective in blocking or reversing the effects of soman. These results suggest that the direct actions of soman on the electrical properties of these neurones are mediated by activation of muscarinic receptors.

A slow calcium-dependent chloride current in rhythmic hyperpolarization in neurones of the rabbit vesical pelvic ganglia

1. Voltage-clamp recordings were made from neurones of vesical pelvic ganglia isolated from the rabbit urinary bladder. A rhythmic outward current, ISH, which corresponds to the spontaneous hyperpolarization, occurred at fairly constant intervals in fifty-eight of eighty-four neurones superfused with Krebs solution. The peak amplitude of the ISH was 0.5 +/- 0.2 nA (n = 48; mean +/- S.E.M.). 2. The ISH was eliminated in a Krebs solution containing nominally zero calcium and 12 mM-magnesium. Lowering the temperature of the superfusing solution from 36 to 22 degrees C also inhibited the occurrence of the ISH. 3. Bath application of caffeine increased the frequency of ISH. In contrast, ryanodine and procaine reversibly blocked ISH. 4. In thirty-four of fifty-eight neurones, the ISH was composed of two current components, an initial fast ISH with duration of 1-10 s and a slow ISH lasting 15-60 s. In the remaining twenty-four neurones, ISH showed only the fast component. 5. The fast ISH was associated with an increased membrane conductance and the slow ISH was associated with a decreased membrane conductance. The reversal potentials of the fast and the slow ISH were -88 +/- 7 mV (n = 4) and -30 +/- 6 mV (n = 4), respectively. 6. Tetraethylammonium (5 mM) and barium (1 mM) blocked the fast ISH but not the slow ISH. Intracellular caesium injected by ionophoresis through a Cs(+)-filled microelectrode blocked the fast ISH, without affecting the slow ISH. Apamin and (+)-tubocurarine selectively suppressed the fast component of the ISH. 7. Substitution of isethionate (67 mM) for chloride increased the amplitude of the slow ISH and shifted the reversal potential of the slow ISH to +1 +/- 8 mV (n = 5). A slow ISH with amplitude of 0.1-1 nA and was still observed in a low-sodium (26.2 mM) solution. The stilbene derivative, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS), a chloride channel blocker, suppressed the slow ISH. 8. These results suggest that ISH is composed of two distinct calcium-dependent currents, a fast ISH produced by activation of potassium conductance and a slow ISH produced by inactivation of chloride conductance. 9. The after-hyperpolarization (AHP) following the action potential was also composed of apamin-sensitive and insensitive spontaneous hyperpolarizing oscillations. The apamin-insensitive component of IAHP was increased by lowering external chloride activity, while it was depressed by SITS.

Effects of Corticotrophin Releasing Factor, Muscarine and Somatostatin on Rubidium and Potassium Efflux from Mouse AtT-20 Pituitary Cells

Effects of secretagogues and anti-secretagogues of ACTH secretion on K+ permeability in the clonal pituitary cell line AtT-20 were measured by recording 86Rb or 42K efflux. Efflux was accelerated by the secretagogues K+, corticotrophin, forskolin, isoprenaline, and the Ca-ionophore A23187. Efflux was reduced by the inhibitors somatostatin, muscarine, and oxotremorine, or by removing external Ca. Efflux was also reduced by the K+-channel blocking compound d-tubocurarine but not by tetraethylammonium. Muscarine, oxotremorine, somatostatin, and 0 Ca2+ also reduced intracellular Ca2+ measured by quin-2 fluorescence. It is suggested that most of the resting 86Rb or 42K efflux measured under these conditions occurs via tubocurarine-sensitive Ca2+-dependent K+-channels, and that changes in efflux rate produced by secretagogues or anti-secretagogues are secondary to changes in intracellular Ca2+.

Muscarinic suppression of the M-current in the rat sympathetic ganglion is mediated by receptors of the M1-subtype

1. Under voltage-clamp dissociated adult and foetal rat superior cervical ganglion (s.c.g.) cells exhibited a non-inactivating voltage- and time-dependent component of K+ current termed the M-current (IM). IM was detected and measured from the current decay during hyperpolarizing voltage steps applied from potentials where IM was pre-activated. 2. Neither the resting membrane current nor the amplitude of these current decay relaxations were reduced by omitting Ca from the bathing fluid, showing that the M-current was not a 'Ca-activated' K-current dependent on a primary Ca-influx. Concentrations of (+)-tubocurarine sufficient to block the slow Ca-activated K-current IAHP did not inhibit IM or antagonize the effect of muscarinic agonists on IM, showing that IM was not contaminated by IAHP. Tetraethylammonium (1 mM), which blocks the fast Ca-activated K-current IC, produced a small inhibition of IM. This was not due to contamination of IM by IC since muscarinic agonists did not consistently block IC. 3. The muscarinic agonists muscarine, oxotremorine, McN-A-343 and methacholine reversibly suppressed IM, resulting in an inward (depolarizing) current. The rank order of potency was: oxotremorine greater than or equal to muscarine greater than McN-A-343 greater than methacholine. 4. The suppression of IM by muscarine was similar in cultured cells derived from adult and foetal tissue to that seen in the intact ganglia. 5. IM-suppression by muscarine was inhibited by pirenzepine (Pz) and AF-DX 116 with mean pKB values of 7.53 +/- 0.13 (n = 3) and 6.02 +/- 0.13 (n = 4) respectively. 6. The suppression of IM by muscarinic agonists was not affected by gallamine (10-30 microM). 4-Diphenylacetoxy-N-methylpiperidine methiodide inhibited the response at 300 nM. 7. Pirenzepine inhibited the contractions of the guinea-pig isolated ileum produced by muscarine with a mean pKB of 6.37 +/- 0.03 (n = 8). 8. These results suggest that the receptors mediating suppression of the M-current accord with those designated pharmacologically as M1 and that these receptors reach maturity at a very early stage in the development of the rat s.c.g.

On the transduction mechanism for muscarine-induced inhibition of M-current in cultured rat sympathetic neurones

1. Dissociated adult or fetal rat superior cervical ganglion cells were voltage-clamped through a single patch pipette. The voltage-dependent K+ current, IM (M-current), was maintained by including MgATP in the pipette solution and by buffering the solution pH to 6.7. 2. Bath-applied muscarine (0.4 microM) produced a reversible inhibition of IM. 3. Addition of Gpp(NH)p (200 microM) or GTP-gamma-S (500 microM) to the pipette solution induced a slowly developing inhibition of IM and prevented recovery from subsequent muscarine-induced inhibition. 4. Addition of GDP-beta-S (500 microM) to the pipette solution reduced the amount of IM inhibition produced by 0.4 microM-muscarine by 42% and reduced the associated inward shift of the holding current by 56%. 5. Cells responded normally to muscarine after pre-treatment for 4-27 h with 500 ng ml-1 pertussis toxin (PTx). 6. IM was not diminished by extracellular addition of 1 mM-dibutyryl cyclic AMP, 8-bromo-cyclic AMP or dibutyryl cyclic GMP, or of 10 microM-forskolin. 7. IM was not reduced by inclusion of Li+ (2 mM) or inositol 1,4,5-trisphosphate (IP3, 100 microM) in the patch pipette, nor by ionophoretic injection of IP3 from an inserted micropipette. 8. Addition of 4-beta-phorbol 12,13-dibutyrate (PDBu, 0.5-2 microM) to the extracellular medium partly inhibited IM and reduced an additional component of resting membrane current. This effect was not replicated by 4-alpha-phorbol 12,13-didecanoate. 9. It is concluded that the inhibition of IM by muscarine is mediated through activation of a PTx-insensitive GTP-binding protein. The effect of muscarine appears not to be mediated by cyclic nucleotides or IP3 but may possibly involve the generation of diacylglycerols and activation of protein kinase C.

Calcium-dependent slow outward current in visceral primary afferent neurones of the rabbit

Slow outward currents were recorded from voltage-clamped neurones in nodose ganglia excised from rabbits. In the majority of Type C neurones, a short depolarizing command pulse evoked a slow outward tail current (ISAH) with a decay time constant ranging from 0.5 to 2 s. The ISAH was due to an increase in membrane conductance to K+ because its reversal potential was approximately equal to the Nernst potential for K+. The ISAH was reversibly blocked by removal of external Ca2+ or by Ca2+ antagonists. A Ca2+ ionophore, A23187, produced an outward current which was similar to the ISAH. The ISAH was resistant to tetraethylammonium and depressed by Ba2+, whereas it was not affected by Cs+ and 4-aminopyridine. The ISAH was initially augmented and subsequently depressed by apamin (1-10 nM) and (+)-tubocurarine (100-600 microM). It is concluded that the ISAH in visceral primary neurones may be due to a long-lasting increase in K+ conductance caused by an increase in the concentration of intracellular Ca2+, resulting from Ca2+ entry during the depolarizing command pulse.

Selective antagonism of muscarinic potentials on the superior cervical ganglion of the rat

Selective antagonists have been used to classify the muscarinic receptors involved in the slow excitatory postsynaptic potential and slow inhibitory postsynaptic potential of the superior cervical ganglia of the rat, as recorded in 1 microM neostigmine, using a grease-gap method. Cardioselective M2 antagonists, e.g. AF-DX 116, depressed the slow inhibitory postsynaptic potential and enhanced the slow excitatory postsynaptic potential. The M1 selective antagonist pirenzepine depressed both potentials equally. The high potency of pirenzepine against the slow excitatory postsynaptic potential, however, indicates that it is mediated by M1 receptors. The slow excitatory and inhibitory postsynaptic potentials were found to be pharmacologically similar to the muscarinic agonist-induced depolarisation and hyperpolarisation of this preparation, respectively. The actions of two muscarinic agonists on the postsynaptic potentials were also studied. It is concluded that the slow excitatory postsynaptic potential is mediated by M1 receptors and the slow inhibitory postsynaptic potential by cardiac-like M2 receptors.

Calcium-dependent potassium conductance in neurons of rabbit vesical pelvic ganglia

Intracellular recordings were made from neurons of vesical pelvic (parasympathetic) ganglia (VPG) isolated from the rabbit urinary bladder. Spontaneous hyperpolarizations (SH), occurring at intervals of 30 s to 5 min, could be recorded from 53% of VPG neurons in Krebs solution. The action potential was associated with inward sodium and calcium currents and was followed by fast and slow afterhyperpolarizations (AHPs). The action potential also evoked an additional hyperpolarization which was identical to the SH. The SH and the AHPs were associated with a decrease in the input resistance and reversed their polarity close to the potassium equilibrium potential. Intracellular cesium ions blocked the AHPs and the SH. Superfusing the preparation with a calcium-free solution produced a depolarization associated with an increased input resistance. The outward rectification activated at the resting membrane potential was depressed in the calcium-free solution. The removal of extracellular calcium ions also depressed both the SH and the spike AHPs. Bath-application of caffeine (1-3 mM) increased the frequency of the appearance of the SH. Injection of EGTA into VPG neurons caused a depolarization due to a blockade of the outward rectification. EGTA also depressed the slow AHP and the SH. These results suggest that the neuronal membrane of the rabbit VPG is endowed with a calcium-dependent potassium conductance (gKCa). Apamin (0.3-5 nM) and (+)-tubocurarine (30-300 microM) blocked the slow AHP and the SH without affecting the fast AHP and the resting membrane potential. Tetraethylammonium (TEA, 0.3-5 mM) suppressed the fast AHP and the SH without affecting the outward rectification. TEA augmented the slow AHP. Barium ions (0.1-1 mM) depressed the AHPs, the SH and the outward rectification. These pharmacological properties imply that at least 3 kinds of gKCa systems underlie the generation of the outward rectification, the spike AHPs and the SH.

Clindamycin-induced alteration of ganglionic function. I. Direct effects on ganglion cell properties

The influence of the lincosamide antibiotic, clindamycin, on the properties of bullfrog sympathetic ganglion B cells has been determined in vitro using conventional voltage recording methods or single microelectrode voltage-clamp recording techniques. Individual neurons were depolarized with both bath application or local perfusion of clindamycin. The amplitude of the depolarization was not altered by pretreatment with 50 microM (+)-tubocurarine, 10-microM atropine, or 1.5 microM tetrodotoxin (TTX), indicating that the clindamycin-induced depolarization does not result from either the activation of (1) nicotinic receptors, (2) muscarinic receptors, or (3) voltage-gated sodium channels. Clindamycin partially inhibited IM, an action which accounts for part of the clindamycin-induced depolarization. The duration of the hyperpolarizing afterpotential (HAP) following the action potential was decreased in the presence of clindamycin. Clindamycin decreased the amplitude and maximum rate of rise (MRR) of TTX-insensitive action potentials. As calcium influx is thought to contribute to the depolarizing phase of the TTX-insensitive spikes, we suggest that the decrease in HAP duration by clindamycin results from a decrease in the somal calcium current. Further, it is suggested that a decrease in IM and HAP duration may be responsible for the increased excitability exhibited during exposure to clindamycin.

Examination of the role of calcium in the adrenaline-induced hyperpolarization of bullfrog sympathetic neurons

The adrenaline-induced hyperpolarization, which was recorded in neurons of bullfrog paravertebral sympathetic ganglia by means of the sucrose gap technique, was antagonized by 1 mM 4-aminopyridine. The response was unaffected by drugs which influence intracellular Ca2+ movements or Ca2+-sensitive K+ conductances, i.e. 100 or 200 microM Cd2+, 60 microM dantrolene Na+, 10 mM tetraethylammonium bromide, 0.5-2.0 microM apamin or 70 microM (+)-tubocurarine chloride. The spontaneous, rhythmic hyperpolarizations which occur in ganglionic neurons in the presence of 5 mM caffeine and reflect activation of Ca2+-sensitive K+ conductances following mobilization of intracellular Ca2+, were examined by means of intracellular recording. These responses were often biphasic, comprising a transient rapid early phase and a slow late phase. Tetraethylammonium (10 mM) and 0.5-2.0 microM apamin antagonized the rapid early phase and 70 microM (+)-tubocurarine chloride antagonized both phases of the response. Neither phase of these spontaneous, rhythmic, caffeine-induced hyperpolarizations were affected by 1 mM 4-aminopyridine. Although the adrenaline-induced hyperpolarization was antagonized by 50 microM 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate and by 50 microM quinidine, the majority of the results argue against the hypothesis that mobilization of intracellular Ca2+ is required for activation of the K+ conductance thought to underlie the adrenaline-induced hyperpolarization.

Voltage- and calcium-activated potassium currents in mouse neuroblastoma x rat glioma hybrid cells

1. Membrane currents were recorded from voltage-clamped, microelectrode-impaled cells of the NG108-15 mouse neuroblastoma x rat glioma clonal cell line, differentiated with prostaglandin E1. 2. A slow outward tail current reversing at post-pulse potentials between -80 and -90 mV was evoked by depolarizing pre-pulses to near 0 mV. The tail current was inhibited by Cd2+ ions (0.2-1 mM) and hence attributed to activation of a Ca2+-dependent K+ current by a priming voltage-activated Ca2+ current. 3. Two components to this tail current could be distinguished pharmacologically: an early (less than or equal to 50 ms) component inhibited by 1-5 mM-tetraethylammonium (TEA), and a late component lasting several hundred milliseconds inhibited by apamin (0.1-0.4 microM) or d-tubocurarine (0.1-0.5 mM). 4. Ionophoretic injection of Ca2+ ions evoked a transient outward current with an apparent reversal potential (from ramped current-voltage curves) of -70 mV. This current was succeeded or sometimes replaced by an inward current with an apparent reversal potential between -20 and -10 mV. 5. The outward current induced by Ca2+ injections was unaffected or partly inhibited by TEA (1-5 mM), but was strongly inhibited by apamin or d-tubocurarine. 6. Hyperpolarizing voltage steps from between -30 and -40 mV induced inward current relaxations reversing at between -80 and -90 mV. These were considered to result from deactivation of the voltage-dependent sustained K+ current, IM. 7. Application of methacholine, muscarine or Ba2+ ions produced an inward current, reduced input conductance and reduced IM deactivation relaxations. 8. It is concluded that differentiated NG108-15 cells possess several of the K+ currents present in sympathetic neurones, including a delayed rectifier current, two species of Ca2+-activated K+ current and the M-current.

Pharmacological and physiological properties of the after-hyperpolarization current of bullfrog ganglion neurones

1. The slowly decaying, calcium-dependent after-hyperpolarization (a.h.p.) that follows action potentials in bullfrog ganglion B cells has previously been shown to be generated by a potassium current called IAHP. We have recorded IAHP using a switched, single-electrode hybrid clamp where current-clamp mode was changed to voltage-clamp mode immediately after repolarization of a spike or the last spike of a train. 2. Reduction of extracellular calcium reduced the decay time of IAHP following a single spike. At all levels of extracellular calcium tested (0.5-4 mM), the decay time of IAHP was longer following a train of action potentials than following a single action potential. Thus, the time course of IAHP evoked by action potentials is a function of the calcium load induced by the action potentials. Conversely, agents that reduce the amount of IAHP activated without affecting its rate of decay, probably do not affect calcium influx. 3. Muscarine (2 or 10 microM) inhibits IAHP following an action potential by at most 30% and has no effect on decay rate of IAHP. These results suggest that muscarine has little or no effect on either calcium influx or sequestration. Decay of the a.h.p. is accelerated by muscarine but this effect is due to an increased leak conductance. 4. Charybdotoxin (CTX) between 4 and 20 nM, prolongs action potential duration in a manner consistent with blockade of the voltage- and calcium-dependent potassium current (Ic) involved in spike repolarization in these cells. This action is consistent with its reported action on analogous channels in other systems. However, CTX also reduces IAHP. Thus, in bullfrog ganglion neurones, two distinct calcium-dependent potassium currents exhibit a comparable sensitivity to CTX. This cannot be due to a decreased influx of calcium because the decay rate of IAHP following an action potential is unchanged. The action of CTX was observed with both crude and purified preparations of CTX. 5. Apamin (25 nM) and (+)-tubocurarine (concentration giving 50% of maximal inhibition = 20 microM) block IAHP without affecting action potential duration. The action of (+)-tubocurarine is more readily reversible than apamin. Approximately 20% of IAHP is resistant to blockade by either apamin or (+)-tubocurarine. 6. Muscarine was used to block the M-current (IM) selectively and (+)-tubocurarine was used to inhibit IAHP selectively. Both currents were shown to contribute to spike frequency adaptation. Inhibition of both IM and IAHP has a synergistic action to increase repetitive firing.

Apamin and d-tubocurarine block the afterhyperpolarization of rat supraoptic neurosecretory neurons

Magnocellular neurosecretory cells (MNCs) were impaled in perfused explants of rat hypothalamus. Evoked bursts of spikes were followed by an afterhyperpolarization (AHP) lasting 1-2 s. In each of 22 cells this AHP was selectively suppressed by low nanomolar concentrations of apamin (IC50 = 1.3 nM) or micromolar concentrations of d-tubocurarine (IC50 approximately 40 microM). Blockade of the AHP was accompanied by a decrease in spike accommodation and an unmasking of the late depolarizing afterpotential which induces burst firing in MNCs. Modulation of the Ca2+-dependent K+ (AHP) conductance by an endogenous apamin-like ligand could play an important role in the control of firing rate and pattern in MNCs.

The Ca2+-sensitive K+-currents underlying the slow afterhyperpolarization of bullfrog sympathetic neurones

Ca2+-sensitive K+ currents involved in the slow afterhyperpolarization (a.h.p.) of an action potential of bullfrog sympathetic neurones were studied with a single-electrode voltage clamp method. The outward tail current (IAH) generated after the end of a depolarizing command pulse (from the holding potential of -60 mV to 0 mV, 5-20 ms in duration), mimicking an action potential, was separated into at least two exponential components (IAHf and IAHs). They were identified as K+ currents, since their reversal potentials were close to the K+ equilibrium potential and they were sensitive to external K+. The time constant of IAHf (tf; 44 ms at -60 mV) was decreased by membrane hyperpolarization from -40 to -80 mV, while that of IAHs (ts; 213 ms) remained constant. Removal of external Ca2+ or addition of Cd2+ significantly decreased the IAHs amplitude (As) and tf without a change in ts and the IAHf amplitude (Af). On the other hand, increasing Ca2+ influx by applying repetitive command pulses enhanced both Af and As with negligible effects on tf and ts, and produced a much slower component. Intracellular injection of EGTA reduced Af with no effect on tf, and increased As with a decreased ts. Both muscarine and (+/-)-tubocurarine, which reduced IAHs, hardly affected IAHf. These results indicate that a.h.p. is induced by the activation of two distinct Ca2+-dependent K+ channels, which differ in voltage sensitivity, Ca2+-dependence and pharmacology.