Antagonists of neuronal calcium channels: structure, function, and therapeutic implications

Electrophysiological properties of the human N-type Ca2+ channel: I. Channel gating in Ca2+, Ba2+ and Sr2+ containing solutions

We have characterized the properties of the human N-type Ca2+ channel produced by the stable co-expression of the alpha(1B-1), alpha(2b)delta and beta(1b) subunits. The channel displayed the expected pharmacology with respect to the toxins omega-CTx-GVIA and omega-CTx-MVIIC, which depressed currents in a voltage-independent fashion. We characterized a variety of biophysical properties of the channel under conditions in which either Ca2+, Ba2+ or Sr2+ was the sole extracellular divalent ion. In all three ions, current-voltage relationships revealed that the channel was clearly high-voltage activated. Current activation was significantly slower in Ca2+ than either Sr2+ or Ba2+. Construction of conductance-voltage relationships from tail current measurements indicated that the channel was more high-voltage activated in Ca2+ than in either Sr2+ or Ba2+. The rank order of current amplitude at +4 mV was Ba2+ > Sr2+ > or = Ca2+. Elevation of the extracellular concentration of Ba2+ increased maximal current amplitude and shifted the current-voltage relationship to the right. In all three ions channel inactivation was complex consisting of three distinct exponentials. Recovery from inactivation was slow taking several seconds to reach completion. Steady-state inactivation curves revealed that channel inactivation became detectable at holding potentials of between -101 and -91 mV depending on the permeating species. The rank order of mid-points of steady state inactivation was (most negative) Sr2+ > Ca2+ > Ba2+ (most positive). Deactivation of the N-type Ca2+ channel was voltage-dependent and very fast in all three ions. The deactivation rate in Ba2+ was significantly slower than that in both Ca2+ and Sr2+, however the voltage-dependence of deactivation rate was indistinguishable in all three ions.

Structure-function relationships of omega-conotoxin GVIA. Synthesis, structure, calcium channel binding, and functional assay of alanine-substituted analogues

The structure-function relationships of the N-type calcium channel blocker, omega-conotoxin GVIA (GVIA), have been elucidated by structural, binding and in vitro and in vivo functional studies of alanine-substituted analogues of the native molecule. Alanine was substituted at all non-bridging positions in the sequence. In most cases the structure of the analogues in aqueous solution was shown to be native-like by 1H NMR spectroscopy. Minor conformational changes observed in some cases were characterized by two-dimensional NMR. Replacement of Lys2 and Tyr13 with Ala caused reductions in potency of more than 2 orders of magnitude in three functional assays (sympathetic nerve stimulation of rat isolated vas deferens, right atrium and mesenteric artery) and a rat brain membrane binding assay. Replacement of several other residues with Ala (particularly Arg17, Tyr22 and Lys24) resulted in significant reductions in potency (<100-fold) in the functional assays, but not the binding assay. The potencies of the analogues were strongly correlated between the different functional assays but not between the functional assays and the binding assay. Thus, the physiologically relevant assays employed in this study have shown that the high affinity of GVIA for the N-type calcium channel is the result of interactions between the channel binding site and the toxin at more sites than the previously identified Lys2 and Tyr13.

L-type calcium channels: binding domains for dihydropyridines and benzothiazepines are located in close proximity to each other

We investigated the binding of a fluorescent diltiazem analogue (3R,4S)-cis-1-[2-[[3-[[3-[4,4-difluoro-3a,4-dihydro-5,7-dimethyl-4-bo ra-3a,4a-diaza-s-indacen-3-yl]propionyl]amino]propyl]amin o]ethy]-1,3,4,5-tetrahydro-3-hydroxy-4-(4-methoxyphenyl)-6-(triflu oromethyl)-2H-1-benzazepin-2-one (DMBODIPY-BAZ) to L-type Ca2+ channels in the presence of different 1,4-dihydropyridines (DHPs) by using fluorescence resonance energy transfer (FRET) [Brauns, T., Cai, Z.-W., Kimball, S. D., Kang, H.-C., Haugland, R. P., Berger, W., Berjukov, S., Hering, S., Glossmann, H., & Striessnig, J. (1995) Biochemistry 34, 3461]. When channels are occupied with DMBODIPY-BAZ, a rapid fluorescence change occurred upon addition of different DHPs. The direction of this intensity modulation was found to be only dependent on the chemical composition of the dihydropyridine employed. DHPs containing a nitro group decreased, whereas others (e.g., isradipine) enhanced the fluorescence signal. In addition, all DHPs markedly decreased the association rate constant for DMBODIPY-BAZ without affecting equilibrium binding. Both observations together are best explained by a steric model where the DHP binding site is located in close proximity to the accession pathway of DMBODIPY-BAZ.

Involvement of N- and P/Q- but not L- or T-type voltage-gated calcium channels in ischaemia-induced striatal dopamine release in vitro

Calcium influx and transmitter efflux are central events in the neuropathological cascade that occurs during and following cerebral ischaemia. This study explored the role of voltage-gated calcium channels (VGCCs) in ischaemia-induced striatal dopamine (DA) release in vitro. Slices (350 microm thickness) of rat neostriatum were superfused (400 ml/h) with an artificial cerebrospinal fluid (aCSF) at 34 degrees C and subjected to episodes of 'ischaemia' by reduction of the glucose concentration from 4 to 2 mM and gassing with 95% N2/5% CO2. DA release was monitored with fast cyclic voltammetry at implanted carbon fibre microelectrodes. The time to onset, time to peak, rate and magnitude of DA release were measured. Non-selective blockade of VGCCs with a high concentration of Ni2+ (2.5 mM), markedly delayed (P < 0.01) and slowed (P < 0.05) DA release but preferential blockade of T-type VGCCs with a lower concentration (200 microM) had no effect. DA release was also unaffected by selective antagonism of L-type VGCCs with nimodipine and nicardipine (10 microM each). Selective blockade of N-type VGCCs with omega-conotoxin GVIA (100 nM) delayed DA release (P < 0.05) but did not affect its rate or magnitude. Blockade of P- and possibly Q-type VGCCs with omega-agatoxin IVA (up to 200 nM) both delayed (P < 0.05) and slowed (P < 0.05) DA release. Preferential blockade of P- type VGCCs with neomycin (500 microM) also delayed (P < 0.05) and slowed (P < 0.05) DA release. These findings suggest that N-, P- and possibly Q- but not L- or T-type VGCCs mediate ischaemia-induced DA release. Although it is not possible to say, on the basis of these results, that the effects are directly upon the dopamine terminals, these calcium channels nevertheless constitute promising targets for therapeutic intervention.

Innovations in pharmacologic therapies

OBJECTIVES

To describe the mechanism of action of four new neurotransmitters that may play a role in pain modulation and to review the clinical implications of these new classes of analgesics.

DATA SOURCES

Research studies, proceedings, abstracts, and book chapters pertaining to new pharmacologic therapies for pain relief.

CONCLUSIONS

Recent advances in our understanding of neurotransmitter and receptor physiology have provided new directions for the development and testing of analgesic compounds. NMDA antagonists and calcium channel blockers warrant investigations in humans. Additional human studies are needed of alpha 2 adrenergic agonists and GABA agonists.

IMPLICATIONS FOR NURSING PRACTICE

Knowledge of new novel classes of analgesics is important as further investigative studies will take place to determine which types of pain problems are most effectively treated by these classes of drugs.

Ca2+ currents in central insect neurons: electrophysiological and pharmacological properties

Ca2+ currents in dorsal unpaired median (DUM) neurons isolated from the fifth abdominal ganglion of the cockroach Periplaneta americana were investigated with the whole cell patch-clamp technique. On the basis of kinetic and pharmacological properties, two different Ca2+ currents were separated in these cells: mid/low-voltage-activated (M-LVA) currents and high-voltage-activated (HVA) currents. M-LVA currents had an activation threshold of -50 mV and reached maximal peak values at -10 mV. They were sensitive to depolarized holding potentials and decayed very rapidly. The decay was largely Ca2+ dependent. M-LVA currents were effectively blocked by Cd2+ median inhibiting concentration (IC50 = 9 microM), but they also had a remarkable sensitivity to Ni2+ (IC50 = 19 microM). M-LVA currents were insensitive to vertebrate LVA channel blockers like flunarizine and amiloride. The currents were, however, potently blocked by omega-conotoxin MVIIC (1 microM) and omega-agatoxin IVA (50 nM). The blocking effects of omega-toxins developed fast (time constant tau = 15 s) and were fully reversible after wash. HVA currents activated positive to -30 mV and showed maximal peak currents at + 10 mV. They were resistant to depolarized holding potentials up to -50 mV and decayed in a less pronounced manner than M-LVA currents. HVA currents were potently blocked by Cd2+ (IC50 = 5 microM) but less affected by Ni2+ (IC50 = 40 microM). These currents were reduced by phenylalkylamines like verapamil (10 microM) and benzothiazepines like diltiazem (10 microM), but they were insensitive to dihydropyridines like nifedipine (10 microM) and BAY K 8644 (10 microM). Furthermore, HVA currents were sensitive to omega-conotoxin GVIA (1 microM). The toxin-induced reduction of currents appeared slowly (tau approximately 120 s) and the recovery after wash was incomplete in most cases. The dihydropyridine insensitivity of the phenylalkylamine-sensitive HVA currents is a property the cockroach DUM cells share with other invertebrate neurons. Compared with Ca2+ currents in vertebrates, the DUM neuron current differ considerably from the presently known types. Although there are some similarities concerning kinetics, the pharmacological profile of the cockroach Ca2+ currents especially is very different from profiles already described for vertebrate currents.

The rationale for new therapies in acute ischaemic stroke

Although stroke is a major cause of morbidity and mortality, it is only relatively recently that a concerted effort has been made to develop acute treatments. Thrombolytics, such as recombinant tissue plasminogen activator (rt-PA), may benefit selected patients within 3 h of cerebral infarction. CUrrently, rt-PA is only licensed for use in the United States. Many potential strategies for neuroprotection exist and are currently under investigation. Because the mechanisms of neurotoxicity involve numerous interdependent processes, it may be that the interpretation of a single site in the cascade of events is insufficient to provide effective neuroprotection. Drugs acting at several sites in the neurotoxic cascade may be more effective, and the results of Phase III studies with the novel neoroprotectant lubeluzole are anticipated.

Behavioural and anticonvulsant effects of Ca2+ channel toxins in DBA/2 mice

This study investigated the behavioural and anticonvulsant effects of voltage-sensitive calcium channel blockers in DBA/2 mice. Omega-Conotoxin MVIIC (0.1, 0.3 micrograms ICV/mouse) and omega-agatoxin IVA (0.1, 0.3, 1 micrograms ICV), which act predominantly at P- and/or Q-type calcium channels, prevented clonic and tonic sound-induced seizures in this animal model of reflex epilepsy (ED50 values with 95% confidence limits for protection against clonic sound-induced seizures were 0.09 (0.04-0.36) micrograms ICV and 0.09 (0.05-0.15) micrograms ICV respectively and against tonic seizures 0.07 (0.03-0.16) micrograms ICV and 0.08 (0.04-0.13) micrograms ICV, respectively). The N-type calcium channel antagonists omega-conotoxin GVIA and omega-conotoxin MVIIA were also tested in this model. Omega-Conotoxin GVIA was anticonvulsant in DBA/2 mice, but only at high doses (3 micrograms ICV prevented tonic seizures in 60% of the animals; 10 micrograms ICV prevented clonic seizures in 60% and tonic seizures in 90% of the animals), whereas omega-conotoxin MVIIA did not inhibit sound-induced seizures in doses up to 10 micrograms ICV. Both omega-conotoxin GVIA and omega-conotoxin MVIIA induced an intense shaking syndrome in doses as low as 0.1 microgram ICV, whereas omega-conotoxin MVIIC and omega-agatoxin IVA did not produce shaking at any of the doses examined. Finally, omega-conotoxin GI (0.01-1 microgram ICV) and alpha-conotoxin SI (0.3-30 micrograms ICV), which both act at acetylcholine nicotinic receptors, were not anticonvulsant and did not induce shaking in DBA/2 mice. These results confirm that blockers of N- and P-/Q-type calcium channels produce different behavioural responses in animals. The anticonvulsant effects of omega-conotoxin MVIIC and omega-agatoxin IVA in DBA/2 mice are consistent with reports that P- and/or Q-type calcium channel blockers inhibit the release of excitatory amino acids and are worthy of further exploration.

Immunohistochemical and electrophysiological evidence for omega-conotoxin-sensitive calcium channels in unmyelinated C-fibres of biopsied human sural nerve

In vitro electrophysiological measurements of Ca2+ potentials in human sural nerve fascicles revealed that Ca2+ conductances might be present on unmyelinated C-fibres. Furthermore, these Ca2+ potentials were partially blocked by omega-conotoxin, a calcium antagonist for the N-type Ca2+ channels. Therefore, immunohistochemical staining with indirect immunofluorescent omega-conotoxin GVIA was used to localize N-type Ca2+ channels in intact and in enzymatically dissociated human sural nerve fascicles. Densities of toxin binding sites were highly heterogeneous throughout the different nerve fascicles investigated and putative N-type Ca2+ channels were localized in about 20% of the unmyelinated C-fibres. Myelinating Schwann cells as well as enzymatically demyelinated axons displayed no specific binding indicating the absence of N-type Ca2+ channels.

Inhibition of neuromuscular transmission in the myenteric plexus of guinea-pig ileum by omega-conotoxins GVIA, MVIIA, MVIIC and SVIB

1. The effects of a number of Ca2+ channel blockers on the transmural electrical stimulation or receptor agonist-elicited contractile responses of guinea-pig ileum were compared. 2. omega-Conotoxins (MVIIA, GVIA, SVIB and MVIIC), but not omega-agatoxin IVA, completely blocked the twitch responses evoked by low frequency (0.1 Hz) transmural stimulation without inhibition of the contractures evoked by exogenous acetylcholine. The concentration-inhibition curves were shifted by changes of external Ca2+. 3. The tetanic contractures produced by a high frequency (30 Hz) train of stimulation were inhibited by omega-conotoxins by only 25-30%, except for omega-conotoxin MVIIC, which produced about 55% inhibition, all significantly less than that produced by atropine (about 70%) or tetrodotoxin (about 85%). Combinations of omega-conotoxins did not produce additive inhibitory effects. 4. The four omega-conotoxins as well as atropine produced similar partial inhibition (53-62%) of the contractures evoked by dimethylphenylpiperazinium, while tetrodotoxin inhibited the contracture completely. 5. Nifedipine and Ni2+ depressed the nerve stimulation-evoked twitch response and tetanic contracture as well as acetylcholine contracture. 6. These observations suggest that, in the myenteric plexus, a subset of N-type Ca2+ channel dominates under low frequency stimulation, while high frequency stimulation may recruit additional channels and non-cholinergic pathways.

Calcium potentials and tetrodotoxin-resistant sodium potentials in unmyelinated C fibres of biopsied human sural nerve

Compound action potentials and electrotonic responses to 150 ms current pulses were recorded from isolated nerve fascicles of human sural nerve biopsies. Compound action potentials in normal bathing solution were characterized by previously described A beta, A delta and C fibre components. In addition, tetrodotoxin-resistant sodium- or calcium-dependent potential components were found when a mixture of tetrodotoxin and the potassium channel blockers 4-aminopyridine and tetraethylammonium was added to the bathing solution. In contrast to tetrodotoxin-sensitive action potentials, tetrodotoxin-resistant sodium- or calcium-dependent potentials could be recorded in the presence of high extracellular potassium concentrations (10-20 mM). Calcium action potentials were found to be sensitive to specific pharmacological antagonists or agonists of L-, N- and P-type calcium channels. Lidocaine, cadmium, verapamil and capsaicin showed unspecific blocking effects on calcium and tetrodotoxin-resistant potentials. Tetrodotoxin-resistant action potentials seem to originate from unmyelinated C fibres since a clear correlation was found between the number of C fibres and the amplitude of tetrodotoxin-resistant calcium and sodium spikes in preparations with different axon type composition. The evidence for tetrodotoxin-resistant Na+ and Ca2+ spikes in peripheral human axons offers new possibilities for a better understanding and/or treatment of abnormalities in the excitability of damaged or diseased peripheral nerves.

Solution structure of omega-conotoxin MVIIA using 2D NMR spectroscopy

The solution structure of omega-conotoxin MVIIA (SNX-111), a peptide toxin from the fish hunting cone snail Conus magus and a high-affinity blocker of N-type calcium channels, was determined by 2D NMR spectroscopy. The backbones of the best 44 structures match with an average pairwise RMSD of 0.59 angstroms. The structures contain a short segment of triple-stranded beta-sheet involving residues 6-8, 20-21, and 24-25. The structure of this toxin is very similar to that of omega-conotoxin GVIA with which is has only 40% sequence homology, but very similar calcium channel binding affinity and selectivity.

SNX-325, a novel calcium antagonist from the spider Segestria florentina

A novel selective calcium channel antagonist peptide, SNX-325, has been isolated from the venom of the spider Segestria florentina. The peptide was isolated using as bioassays the displacement of radioiodinated omega-conopeptide SNX-230 (MVIIC) from rat brain synaptosomal membranes, as well as the inhibition of the barium current through cloned expressed calcium channels in oocytes. The primary sequence of SNX-325 is GSCIESGKSCTHSRSMKNGLCCPKSRCNCRQIQHRHDYLGKRKYSCRCS, which is a novel amino acid sequence. Solid-phase synthesis resulted in a peptide that is chromatographically identical with the native peptide and which has the same configuration of cysteine residues as the spider venom peptide omega-Aga-IVa [Mintz, I. M., et al., (1992) Nature 355, 827-829]. At micromolar concentrations, SNX-325 is an inhibitor of most calcium, but not sodium or potassium, currents. At nanomolar concentrations, SNX-325 is a selective blocker of the cloned expressed class B (N-type), but not class C (cardiac L), A, or E, calcium channels. SNX-325 is approximately equipotent with the N-channel selective omega-conopeptides (GVIA and MVIIA as well as closely related synthetic derivatives) in blocking the potassium induced release of tritiated norepinephrine from hippocampal slices (IC50s, 0.1-0.5 nM) and in blocking the barium current through cloned expressed N-channels in oocytes (IC50s 3-30 nM). By contrast, SNX-325 is 4-5 orders of magnitude less potent than is SNX-111 (synthetic MVIIA) at displacing radioiodinated SNX-111 from rat brain synaptosomal membranes. SNX-325 will be a useful comparative tool in further defining the function and pharmacology of the N- and possibly other types of high-voltage activated calcium channels.