Omega-conotoxin binding and effects on calcium channel function in human neuroblastoma and rat pheochromocytoma cell lines

Bioactive Mimetics of Conotoxins and other Venom Peptides

Ziconotide (Prialt^®), a synthetic version of the peptide ω-conotoxin MVIIA found in the venom of a fish-hunting marine cone snail Conus magnus, is one of very few drugs effective in the treatment of intractable chronic pain. However, its intrathecal mode of delivery and narrow therapeutic window cause complications for patients. This review will summarize progress in the development of small molecule, non-peptidic mimics of Conotoxins and a small number of other venom peptides. This will include a description of how some of the initially designed mimics have been modified to improve their drug-like properties.

Beneficial effects of L- and N-type calcium channel blocker on glucose and lipid metabolism and renal function in patients with hypertension and type II diabetes mellitus

It has been proved that cilnidipine has N-type calcium channels inhibitory activity as well as L-type calcium channels and inhibits excessive release of norepinephrine from the sympathetic nerve ending. This study was undertaken to compare the efficacy of amlodipine (an inhibitor of L-type calcium channels) and cilnidipine (an inhibitor of both L-type and N-type calcium channels) in patients with hypertension and type II diabetes mellitus. Seventy-seven hypertensive patients were divided into two groups according to presence/absence of type II diabetes mellitus. In these two groups of patients, the effects of amlodipine and cilnidipine on glucose and lipid metabolism and renal function were compared. As for glucose and lipid metabolism, homeostasis model assessment insulin resistance (HOMA-R) level in the non-diabetic group and triglyceride in the diabetes group were significantly lower with cilnidipine than with amlodipine. As regards renal function in the diabetic group, estimated glomerular filtration rate (eGFR) was significantly higher and urinary albumin/creatinine ratio was significantly lower with cilnidipine than with amlodipine. Cilnidipine which inhibits N-type calcium channels is more useful for patients with hypertension and diabetes mellitus from its effects on glucose and lipid metabolism and renal function.

High-Throughput Screening for N-Type Calcium Channel Blockers Using a Scintillation Proximity Assay

N-type calcium channels located on presynaptic nerve terminals regulate neurotransmitter release, including that from the spinal terminations of primary afferent nociceptors. Accordingly, N-type calcium channel blockers may have clinical utility as analgesic drugs. A selective N-type calcium channel inhibitor, ziconotide (Prialt), is a neuroactive peptide recently marketed as a novel nonopioid treatment for severe chronic pain. To develop a small-molecule N-type calcium channel blocker, the authors developed a 96-well plate high-throughput screening scintillation proximity assay (SPA) for N-type calcium channel blockers using [125I]-labeled ω-conotoxin GVIA as a channel-specific ligand. Assay reagents were handled using Caliper’s Allegro automation system, and bound ligands were detected using a PerkinElmer TopCount. Using this assay, more than 150,000 compounds were screened at 10 μM and approximately 340 compounds were identified as hits, exhibiting at least 40% inhibition of [125I]GVIA binding. This is the 1st demonstration of the use of [125I]-labeled peptides with SPA beads to provide a binding assay for the evaluation of ligand binding to calcium channels. This assay could be a useful tool for drug discovery.

Modulation of N-type calcium channels translocation in RINm5F insulinoma cells

An intracellular pool of N-type voltage-operated calcium channels has recently been described in both IMR32 human neuroblastoma and PC12 rat pheochromocytoma cells. These channels were found to be accumulated in subcellular fractions where the chromogranin B-containing secretory granules were also enriched. Upon exocytosis N-type calcium channels were reversibly inserted in the plasma membrane. We have now extended this study to RINm5F rat insulinoma cells, and characterized the parallelism between the 'regulated' secretion of serotonin and the recruitment of surface calcium channels. Exocytosis was stimulated by different means, such as depolarization with high KCl, high Ba(2+)alone or protein kinase C activation; on the other hand exocytosis was inhibited with the non-selective calcium channel antagonist Cd(2+)or with noradrenaline. Stimulated release was always accompanied, with parallel kinetics, by calcium channel recruitment, while inhibition of secretion blocked calcium channel recruitment too. During repetitive depolarizations we revealed a potentiation of [Ca(2+)]()i transients in single Fura-2 loaded RINm5F cells, that was accompanied by an increase in surface VOCCs, suggesting a physiological role for the newly recruited channels. 2000 Academic Press@p$hr

Metabolism and trafficking of N-type voltage-operated calcium channels in neurosecretory cells

The N-type voltage-operated calcium channel has been characterized over the years as a high-threshold channel, with variable inactivation kinetics, and a unique ability to bind with high affinity and specificity omega-conotoxin GVIA and related toxins. This channel is particularly expressed in some neurons and endocrine cells, where it participates in several calcium-dependent processes, including secretion. Omega-conotoxin GVIA was instrumental not only for the biophysical and pharmacological characterization of N-type channels but also for the development of in vitro assays for studying N-type VOCC subcellular localization, biosynthesis, turnover, as well as short-and long-term regulation of its expression. We here summarize our studies on N-type VOCC expression in neurosecretory cells, with a major emphasis on recent data demonstrating the presence of N-type channels in intracellular secretory organelles and their recruitment to the cell surface during regulated exocytosis.

Altered [Ca2+] homeostasis in PC12 cells after nerve growth factor deprivation

[Ca2+]i homeostasis in individual PC12 cells after elevated [K+]o was studied by ratiometric microscopy, during nerve growth factor (NGF) deprivation. A significantly lower number of cells responded with an increased [Ca2+]i in the NGF deprived condition. Moreover, the responding cells were more deficient in regulating their [Ca2+]i back to control levels, after the transient peak. This suggests that differentiated neurons do not traverse the apoptotic program homogeneously with regard to their [Ca2+]i regulation and that NGF deprived PC12 cells have more difficulties to reduce their [Ca2+]i after influx of [Ca2+]o.

Toxins affecting calcium channels in neurons

Calcium enters the cytoplasm mainly via voltage-activated calcium channels (VACC), and this represents a key step in the regulation of a variety of cellular processes. Advances in the fields of molecular biology, pharmacology and electrophysiology have led to the identification of several types of VACC (referred to as T-, N-, L-, P/Q- and R-types). In addition to possessing distinctive structural and functional characteristics, many of these types of calcium channels exhibit differential sensitivities to pharmacological agents. In recent years a large number of toxins, mainly small peptides, have been purified from the venom of predatory marine cone snails and spiders. Many of these toxins have specific actions on ion channels and neurotransmitter receptors, and the toxins have been used as powerful tools in neuroscience research. Some of them (omega-conotoxins, omega-agatoxins) specifically recognize and block certain types of VACC. They have common structural backbones and some been synthesized with identical potency as the natural ones. Natural, synthetic and labeled calcium channel toxins have contributed to the understanding of the diversity of the neuronal calcium channels and their function. In particular, the toxins have been useful in the study of the role of different types of calcium channels on the process of neurotransmitter release. Neuronal calcium channel toxins may develop into powerful tools for diagnosis and treatment of neurological diseases.

N-type Ca2+ channels are present in secretory granules and are transiently translocated to the plasma membrane during regulated exocytosis

An intracellular pool of N-type voltage-operated calcium channels has recently been described in different neuronal cell lines. We have now further characterized the intracellular pool of N-type calcium channels in both IMR32 human neuroblastoma and PC12 rat pheochromocytoma cells. Intracellular N-type calcium channels were found to be accumulated in subcellular fractions where the chromogranin B-containing secretory granules were also enriched. 125I-omega-Conotoxin GVIA binding assays on fixed and permeabilized cells revealed that intracellular N-type calcium channels translocate to the plasma membrane in cells exposed to secretagogues (KCl, ionomycin, and phorbol esters). The kinetics, Ca2+ and protein kinase C dependence, and brefeldin A insensitivity of N-type calcium channels translocation were similar to the regulated release of chromogranin B, while no correlation was found with the constitutive secretion of a heparan sulfate proteoglycan. A PC12 subclone deficient in the regulated but not in the constitutive pathway of secretion had a small intracellular pool of N-type calcium channels, and no secretagogue-induced translocation occurred in these cells. Calcium channel translocation was accompanied by a stronger response of Fura-2-loaded cells to depolarizing stimuli, suggesting that the newly inserted channels are functional.

AR4-2J cell line coexpresses dihydropyridine and omega-conotoxin sensitive Ca2+ channels

For the first time, we have demonstrated in AR4-2J cells, an experimental model of azaserine-induced carcinoma in the rat exocrine pancreas, the co-expression of alpha 1 subunit of dihydropyridine-sensitive Ca2+ channel and the alpha 1 sub-unit of omega-conotoxin-sensitive Ca2+ channel RNA messengers which share homologous sequences with, respectively, rbC II and rbB I sub-types described in the rat brain. These two types of voltage-dependent Ca2+ channels which are functionally expressed, emphasize the acquisition during carcinogenesis of neuroendocrine features of AR4-2J cells. Additionally, using antisense phosphorothioate oligodeoxynucleotide, we demonstrated clearly the involvement of dihydropyridine-sensitive Ca2+ channels in the control of AR4-2J cell proliferation.

omega-Conotoxin GVIA blocks nicotine-induced catecholamine secretion by blocking the nicotinic receptor-activated inward currents in bovine chromaffin cells

We have studied the contribution of N-type voltage-dependent Ca2+ channels to both norepinephrine and epinephrine secretion from bovine chromaffin cells induced by high K+ or nicotine using omega-conotoxin GVIA, a selective blocker of N-type voltage-dependent Ca2+ channels. We found that high K+ (75 mM) induced catecholamine secretion was not affected by exposure of bovine chromaffin cells to omega-conotoxin GVIA (1 microM). However, nicotine-induced both norepinephrine and epinephrine secretion were similarly blocked (about 25%) by omega-conotoxin GVIA (1 microM). This effect could be explained by a potent (about 80%) and reversible blockade of the inward current induced by nicotine receptor activation in bovine chromaffin cells. The results indicate that besides the blockade of N-type voltage-dependent channels, omega-conotoxin GVIA is a potent and reversible blocker of the nicotinic receptor-induced currents in chromaffin cells.

Multiple calcium channel subtypes in isolated rat chromaffin cells

By using the whole-cell configuration of the patch-clamp technique we have investigated the pharmacological properties of Ca2+ channels in short-term cultured rat chromaffin cells. In cells held at a membrane potential of --80 mV, using 10 mM Ba2+ as the charge carrier, only high-voltage-activated (HVA) Ca2+ channels were found. Ba2+ currents (IBa) showed variable sensitivity to dihydropyridine (DHP) Ca2+ channel agonists and antagonists. Furnidipine, a novel DHP antagonist, reversibly blocked the current amplitude by 22% and 48%, at 1 microM and 10 microM respectively, during short (15-50 ms) depolarizing pulses to 0 mV. The L-type Ca2+ channel agonist Bay K 8644 (1 microM) caused a variable potentiation of HVA currents that could be better appreciated at low rather than at high depolarizing steps. Increase of IBa was accompanied by a 20-mV shift in the activation curves for Ca2+ channels towards more hyperpolarizing potentials. Application of the conus toxin omega-conotoxin GVIA (GVIA; 1 microM) blocked 31% of IBa; blockade was irreversible upon removal of the toxin from the extracellular medium. omega-Agatoxin IVA (IVA; 100 nM) produced a 15% blockade of IBa. omega-Conotoxin MVIIC (MVIIC; 5 microM) produced a 36% blockade of IBa; such blockade seems to be related to both GVIA-sensitive (N-type) and GVIA-resistant Ca2+ channels. The sequential addition of supramaximal concentrations of furnidipine (10 microM), GVIA (1 microM), IVA (100 nM) and MVIIC (3 microM) produced partial inhibition of IBa, which were additive. Our data suggest that the whole cell IBa in rat chromaffin cells exhibits at least four components.(ABSTRACT TRUNCATED AT 250 WORDS)

Double effect of ethanol on intracellular Ca2+ levels in undifferentiated PC12 cells

In PC12, a cellular line derived from a rat pheochromocytoma, ethanol (EtOH) induces a different effect depending on the concentration used. When resting cells are incubated with an alcohol concentration less than or equal to 120 mM, the [Ca2+]i increased with a double phase pattern. If the alcohol concentration was increased over 120-160 mM, EtOH reversed its effect and the [Ca2+]i decreased. This decrease was strongly inhibited if KCl-depolarized cells were used and was completely abolished if the substrate constituted EtOH-chronically treated cells. The Ca2+ increase is the consequence of an activation of L-type voltage-activated channels, while the other voltage-dependent channels (N-type), the receptor-operated channels and the Ca2+ extrusion pump present in these cells are not involved in EtOH action. These findings indicate that EtOH can induce (by different mechanisms) both potentiating and inhibiting effects on [Ca2+]i in PC12 cells in relation to the alcohol dose effectively present in the suspension medium.

The S-100: a protein family in search of a function

The S-100 is a group of low molecular weight (10-12 kD) calcium-binding proteins highly conserved among vertebrates. It is present in different tissues as dimers of homologous or different subunits (alpha, beta). In the nervous system, the S-100 exists as a mixture composed of beta beta and alpha beta dimers with the monomer beta represented more often. Its intracellular localisation is mainly restricted to the glial cytoplasmic compartment with a small fraction bound to membranes. In this compartment the S-100 acts as a potent inhibitor of phosphorylation on several substrates including the synaptosomal C-Kinase and Tau, a microtubule-associated protein. The S-100 in particular conditions, after binding with specific membrane sites (Kd = 0.2 microM; Bmax = 4.5 nM), is able to modify the activity of adenylate cyclase, probably via G-proteins. In addition, the Ca2+ homeostasis is also modulated by S-100 via an increase of specific membrane conductance and/or Ca2+ release from intracellular stores. "In vitro" and "in vivo" experiments showed that lower (nM) concentrations of extracellular S-100 beta act on glial and neuronal cells as a growth-differentiating factor. On the other hand, higher concentrations of the protein induce apoptosis of some cells such as the sympathetic-like PC12 line. Finally, data obtained from physiological (development, ageing) or pathological (dementia associated with Down's syndrome, Alzheimer's disease) conditions showed that a relationship could be established between the S-100 levels and some aspects of the statii.

Sarafotoxin-induced calcium mobilization in cultured dog tracheal smooth muscle cells

Sarafotoxin b (S6b)-induced changes in intracellular Ca2+ concentration ([Ca2+]i) were monitored in cultured canine tracheal smooth muscle cells (TSMCs) by a fluorescent Ca2+ indicator fura-2. S6b elicited an initial transient peak followed by a sustained elevation of [Ca2+]i. BQ-123, an endothelin-A (ETA) receptor antagonist, had a high affinity to block the rise in [Ca2+]i response to S6b. In the absence of external Ca2+, only an initial transient peak of [Ca2+]i was seen, the sustained elevation of [Ca2+]i could then be evoked by addition of 1.8 mM Ca2+. Ca2+ influx was required for the changes of [Ca2+]i, since the Ca(2+)-channel blockers, diltiazem, verapamil, and Ni2+, decreased both the initial and sustained elevation of [Ca2+]i in response to S6b. TSMCs pretreated with phorbol 12-myristate 13-acetate (PMA, 1 microM) for 30 min attenuated Ca2+ mobilization induced by S6b, which was reversed by staurosporine, a protein kinase C (PKC) inhibitor. The change of [Ca2+]i induced by S6b was attenuated by cholera toxin pretreatment, but not by pertussis toxin. These data demonstrate that the initial detectable increase in [Ca2+]i stimulated by S6b is due to the activation of ETA receptors and subsequent release of Ca2+ from internal stores, whereas the contribution of external Ca2+ follows and partially involves a diltiazem- and verapamil-sensitive process. The inhibition of PMA on S6b-induced Ca2+ mobilization was inversely correlated with membraneous PKC activity.

5-Hydroxytryptamine-stimulated calcium mobilization in cultured canine tracheal smooth muscle cells

5-Hydroxytryptamine (5-HT)-induced increase of intracellular Ca2+ concentration ([Ca2+]i) was monitored in cultured canine tracheal smooth muscle cells (TSMCs) using a fluorescent Ca2+ indicator Fura-2. Stimulation of TSMCs by 5-HT produced an initial transient peak followed by a sustained, concentration-dependent elevation of [Ca2+]i. The log (EC50) values of 5-HT for the peak and sustained plateau responses were -7.43 and -7.60 M, respectively. 5-HT1A and 5-HT3 receptor antagonists, NAN-190 and metoclopramide, inhibited the 5-HT-stimulated increase in [Ca2+]i with pKB values of 6.3 and 6.2, respectively, indicating that the 5-HT receptors mediating Ca2+ signal had low affinity for these receptor antagonists. In contrast, 5-HT2A receptor antagonists, ketanserin and mianserin, had high affinity in antagonizing the changes in [Ca2+]i response to 5-HT with pKB values of 8.3 and 8.3, respectively. The sustained elevation of [Ca2+]i was dependent on the presence of extracellular Ca2+. Removal of extracellular Ca2+ by addition of 2 mM EGTA during the sustained phase caused a rapid decline in [Ca2+]i to the resting level. In the absence of extracellular Ca2+, only an initial peak was observed which then declined to the resting level; the sustained elevation of [Ca2+]i could then be evoked by addition of 1.8 mM Ca2+ in the continued presence of 5-HT. Ca2+ influx was required for the changes of [Ca2+]i, since the Ca(2+)-channel blockers, diltiazem, verapamil, and Ni2+, decreased both the initial and sustained elevation of [Ca2+]i in response to 5-HT. These Ca(2+)-channel blockers also decreased the sustained elevation of [Ca2+]i when applied during the plateau phase. In conclusion, these findings indicate that the initial increase in [Ca2+]i stimulated by 5-HT acting on 5-HT2A receptors is due to the release of Ca2+ from internal stores, followed by the influx of external Ca2+ into the cells. The influx of extracellular Ca2+ partially involves a diltiazem and verapamil sensitive Ca2+ channel.

Bradykinin-stimulated calcium mobilization in cultured canine tracheal smooth muscle cells

Bradykinin (BDK)-induced increases in intracellular Ca2+ concentration ([Ca2+]i) were monitored in cultured canine tracheal smooth muscle cells (TSMCs) using a fluorescent Ca2+ indicator, Fura-2. BDK and kallidin caused an initial transient peak followed by a sustained elevation of [Ca2+]i in a concentration-dependent manner, with half-maximal stimulation (log EC50) obtained at -8.10 M and -8.04 M, respectively. The BDK-induced rise in [Ca2+]i was not affected by the BDK B1 receptor antagonist, des-Arg9[Leu8]-BDK (10 microM). However, the BDK B2 receptor antagonists des-Arg[Hyp3, Thi5,8, D-Phe7]-BDK and Hoe 140 had high affinity in antagonizing BDK with pKB values of 7.5 +/- 0.3 and 8.7 +/- 0.3, respectively. The sustained phase of the rise in [Ca2+]i was dependent on the presence of external Ca2+, as evidenced by a decline to the resting level on addition of EGTA. In the absence of external Ca2+, only an initial transient peak was seen which then declined to the resting level; a sustained elevation of [Ca2+]i could then be evoked by addition of 1.8 mM Ca2+ in the continued presence of BDK. Ca2+ influx was required for the changes in [Ca2+]i, since Ca(2+)-channel blockers, diltiazem, verapamil, and Ni2+, decreased both the initial and sustained elevation of [Ca2+]i in response to BDK. In conclusion, these findings indicate that the initial increase in [Ca2+]i stimulated by BDK acting on BDK B2 receptors is due to the release of Ca2+ from internal stores, followed by the influx of external Ca2+ into the cells. The influx of extracellular Ca2+ partially involves a diltiazem- and verapamil-sensitive Ca2+ channel.

Calcium mobilization induced by endothelins and sarafotoxin in cultured canine tracheal smooth muscle cells

Endothelins (ETs)- and sarafotoxin (S6b)-induced rises in intracellular Ca2+ concentration ([Ca2+]i) were monitored in cultured canine tracheal smooth muscle cells by using a fluorescent Ca2+ indicator fura-2. ET-1, ET-2, ET-3 and S6b elicited an initial transient peak and followed by a sustained elevation of [Ca2+]i, with half-maximal effect (EC50) of 18, 20, 38 and 21 nM, respectively. BQ-123, an ETA receptor antagonist, had a high affinity to block the rise in [Ca2+]i response to ET-1, ET-2, and S6b, as well as a low affinity for ET-3. Removal of external Ca2+ by addition of EGTA during the sustained phase, caused a rapid decline in [Ca2+]i to the resting level. In the absence of external Ca2+, only an initial transient peak of [Ca2+]i was seen, the sustained elevation of [Ca2+]i could then be evoked by addition of 1.8 mM Ca2+. Ca2+ influx was required for the changes of [Ca2+]i, since the Ca(2+)-channel blockers, diltiazem, verapamil, and Ni2+, decreased both the initial and sustained elevation of [Ca2+]i response to these peptides. ETs exhibited homologous desensitization of the Ca2+ response, but partial heterologous desensitization of the Ca2+ response mediated by carbachol to different extents. In contrast, ETs did not desensitize the Ca2+ response induced by ATP or vice versa. These data demonstrate that the initial detectable increase in [Ca2+]i stimulated by these peptides is due to the activation of ETA receptors and subsequently the release of Ca2+ from internal stores, whereas the contribution of external Ca2+ follows and partially involves a diltiazem- and verapamil-sensitive process.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel subtypes in cat chromaffin cells

1. Using the patch-clamp technique we have investigated the kinetic and pharmacological properties of high-voltage-activated (HVA) Ca2+ channels in short-term-cultured cat chromaffin cells. 2. In 10 mM Ba2+, HVA currents activated around -40 mV, reached maximal amplitude at 0 mV and reversed at about +60 mV. At 0 mV, HVA current activation was fast (mean tau act, 2.45 ms), and followed by either an incomplete inactivation or by a second slow phase of activation (mean tau slow, 36.8 ms) that was lost when Ba2+ was replaced by Ca2+. HVA Ba2+ currents deactivate quickly on repolarization to -50 mV (mean tau deact, 0.36 ms). 3. In most cells, HVA currents were sensitive to common dihydropyridine (DHP) derivatives. Nisoldipine blocked the currents maximally at low membrane potentials (mean block 76% at -30 mV, 3 microM) and gradually less at higher voltages. Nisoldipine block was clearly time dependent (33 and 56% after 30 and 600 ms, respectively, to 0 mV). 4. Bay K 8644 (3 microM) action was variable and caused (1) a 2- to 4-fold increase of Ba2+ currents at -40 to -20 mV, (2) a -15 mV shift of the current-voltage relationship and (3) a 10- to 20-fold prolongation of HVA channel deactivation at -50 mV. 5. Nisoldipine block and Bay K 8644 potentiation of HVA currents increased markedly in omega-conotoxin GVIA (omega-CgTX)-pretreated cells, suggesting an increased fraction of DHP-sensitive currents in these cells. Nisoldipine block of residual omega-CgTX-resistant currents was almost complete (mean block, 82%) during pulses of 1 s to 0 mV. 6. The degree of inhibition produced by omega-CgTX (2 microM for 1 min) varied from cell to cell (mean block, 46%) and was partly reversible. Residual omega-CgTX-resistant currents exhibited faster activation-deactivation kinetics than control currents. 7. The slow phase of HVA current activation was abolished if a conditioning prepulse of 40 ms to +70 mV preceded a test pulse to 0 mV. Double-pulse protocols caused an average current increase (facilitation) of 37% that was voltage dependent and which correlated with the slow phase of Ca2+ channel activation. Facilitation was lost in most omega-CgTX-treated cells and was little affected by nisoldipine (3 microM) and Bay K 8644 (1 microM). Facilitation was potentiated in cells dialysed with 100 microM guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and fully prevented by 1 mM guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S).(ABSTRACT TRUNCATED AT 400 WORDS)

omega-Conotoxin and Cd2+ stimulate the recruitment to the plasmamembrane of an intracellular pool of voltage-operated Ca2+ channels

125I-omega-conotoxin binding to neuroblastoma cells at 37 degrees C continuously increased, reaching a plateau after 6-8 hr; this was up to 6 times higher than that observed at lower temperatures. The same effect was induced by short pulses with omega-conotoxin followed by a chase period at 37 degrees C in control medium. Cd2+ also induced up-regulation of surface 125I-omega-conotoxin-binding sites. Fura-2 and patch-clamp experiments showed that the recruited binding sites corresponded to functional voltage-operated Ca2+ channels. Permeabilization experiments revealed a large intracellular pool of 125I-omega-conotoxin-binding sites, whose recruitment to the plasmamembrane was prevented by brefeldin A and nocodazole. These data suggest that specific stimuli might induce voltage-operated Ca2+ channel translocation to plasmamembrane and, in this way, modulate presynaptic events.

Blockade by funnel web toxin of a calcium current in the intermediate pituitary of the rat

The pharmacological sensitivities of the low threshold (LT) and high threshold (HT) calcium currents were studied using single electrode voltage clamp techniques in melanotrophs of the intact rat intermediate pituitary. The T-type LT current was selectively abolished by 200 microM nickel whereas the HT current was preferentially abolished by 25 microM cadmium. The HT current consisted of both sustained and inactivating components. The sustained portion of the HT current was increased by BAY K-8644 indicating the presence of an L-type current. The inactivating component of the HT current was not affected by omega-conotoxin, which blocks the N-type calcium current in many other cell types, but was rapidly and reversibly reduced by funnel-web toxin, a blocker of the P-type calcium channel. These data suggest the presence of a P-type channel in a neuroendocrine cell.

Expression of synaptotagmin and syntaxin associated with N-type calcium channels in small cell lung cancer

The presence of synaptic proteins involved in excitation/secretion coupling was examined in ten small cell lung cancer lines. N-Type calcium channels (omega-conotoxin receptors), synaptotagmin (p65) and syntaxin (HPC-1) were detected in eight. Co-immunoprecipitation experiments indicated that syntaxin can form a complex with synaptotagmin and calcium channels. The expression of synaptotagmin in small cell lung cancer may elicit an autoimmune response that reduces transmitter release at the nerve terminal.

Sensory neuronopathy and small cell lung cancer: antineuronal antibody reacting with neuroblastoma cells

The anti-Hu antibody is associated with a paraneoplastic subacute sensory neuronopathy (SSN) described in cases of small cell lung cancer (SCLC). The Hu antigen is a pan-neuronal nuclear antigen with a molecular weight of 35-40 kDa. In this study we demonstrated the presence of the paraneoplastic Hu antigen in different neuroblastoma cell lines. We showed that by indirect immunocytochemistry the serum of patients with SSN and SCLC reacts with the nuclei of neuroblastoma cell lines SKN-SH and LAN-1. Western blot analysis of nuclear extracts from neuroblastoma cell lines SKN-SH, IMR-32 and LAN-1 confirmed the presence of the Hu antigen in these neuroblastoma cell lines. By comparing the immunocytochemical method and the Western blot analysis we were able to determine that the Western blot analysis was a more sensitive test. Screening of the sera of a large population (a total of 122 patients with SCLC, 17 with paraneoplastic disorders as well as 121 controls with other neurological disorders) was performed and showed all 5 of the patients with SSN and SCLC to be positive for the anti-Hu antibody, whereas only 11 of the 122 SCLC patients and none of the controls were positive, thereby suggesting that this test has a very high degree of sensitivity.

G protein modulation of omega-conotoxin binding sites in neuroblastoma x glioma NG 108-15 hybrid cells

Electrophysiological evidence shows that voltage-dependent calcium channel (VDCC) activity can be regulated by a large number of neurotransmitters. In particular, guanine nucleotide binding regulatory protein (G protein)-mediated inhibitory modulation of the channel activity has been deduced from evidence that GTP analogues and purified G proteins are able to mimic this effect. The G proteins involved are pertussis toxin (PTx) sensitive. The purpose of the present study was to investigate, using biochemical techniques, whether G protein activation modulates the recognition site for omega-conotoxin GVIA (CgTx), a peptide neurotoxin that selectively labels a population of high-threshold VDCC. Undifferentiated and differentiated (1 mM dibutyryl cyclic AMP, 4 days) NG 108-15 cells were used. In both crude cellular extracts specific binding of 125I-CgTx was characterized. Differentiation induced a sixfold increase in the number of binding sites and doubled the KD value. The in vitro addition of guanylylimidodiphosphate (GMP-PNP; a nonhydrolyzable analogue of GTP) to extracts prepared from differentiated cells reduced the 125I-CgTx binding by 48%. This effect, observed in undifferentiated cells as well, was also caused by other triphosphate guanine nucleotides, such as GTP, but not by guanosine 5'-O-(2-thiodiphosphate) or adenine nucleotides. Treatment of the cells with PTx prevented the GMP-PNP effect. Moreover, the results obtained after preincubation with specific antisera raised against the alpha subunits of Gi1-2 and Go suggest that Go is the G protein responsible for the observed effect.

Structure and functional expression of an omega-conotoxin-sensitive human N-type calcium channel

N-type calcium channels are omega-conotoxin (omega-CgTx)-sensitive, voltage-dependent ion channels involved in the control of neurotransmitter release from neurons. Multiple subtypes of voltage-dependent calcium channel complexes exist, and it is the alpha 1 subunit of the complex that forms the pore through which calcium enters the cell. The primary structures of human neuronal calcium channel alpha 1B subunits were deduced by the characterization of overlapping complementary DNAs. Two forms (alpha 1B-1 and alpha 1B-2) were identified in human neuroblastoma (IMR32) cells and in the central nervous system, but not in skeletal muscle or aorta tissues. The alpha 1B-1 subunit directs the recombinant expression of N-type calcium channel activity when it is transiently co-expressed with human neuronal beta 2 and alpha 2b subunits in mammalian HEK293 cells. The recombinant channel was irreversibly blocked by omega-CgTx but was insensitive to dihydropyridines. The alpha 1B-1 alpha 2b beta 2-transfected cells displayed a single class of saturable, high-affinity (dissociation constant = 55 pM) omega-CgTx binding sites. Co-expression of the beta 2 subunit was necessary for N-type channel activity, whereas the alpha 2b subunit appeared to modulate the expression of the channel. The heterogeneity of alpha 1B subunits, along with the heterogeneity of alpha 2 and beta subunits, is consistent with multiple, biophysically distinct N-type calcium channels.

Inositol 1,4,5-trisphosphate receptor immunoreactivity in SH-SY5Y human neuroblastoma cells is reduced by chronic muscarinic receptor activation

Inositol 1,4,5-trisphosphate (InsP3) receptor immunoreactivity in SH-SY5Y human neuroblastoma cells was monitored with a monoclonal antibody raised against the mouse cerebellar InsP3 receptor. Recognition of a protein corresponding to the InsP3 receptor (molecular mass, approximately 275 kDa) was inhibited markedly following exposure of cells to 0.1 mM carbachol. This effect was half-maximal and maximal at approximately 2 and approximately 6 h, respectively; was blocked by atropine; but was not mimicked by thapsigargin, K+, or phorbol 12-myristate 13-acetate. However, the decrease in immunoreactivity following exposure of cells to carbachol for 5 h was blocked if the extracellular Ca2+ concentration was reduced from 1.3 mM to 200 nM. This manipulation also reduced markedly carbachol-induced increases in InsP3 concentration at 5 h. These data indicate that chronic muscarinic stimulation of phosphoinositide hydrolysis reduces InsP3 receptor concentration in SH-SY5Y cells, perhaps via a mechanism that involves prolonged elevation of InsP3 levels.

omega-Conotoxin-sensitive, voltage-operated Ca2+ channels in insulin-secreting cells

The properties of voltage-operated Ca2+ channel subtypes were investigated in insulin-secreting RINm5F cells. Two types of channels were identified: a dihydropyridine-sensitive (L-type) channel, and an omega-conotoxin-sensitive (omega-type) channel. 125I-omega-Conotoxin bound with high affinity (Kd 46.7 pM) to a saturable number of binding sites (10.3 fmol/mg of protein). Toxin binding was not antagonized by L-type channel ligands, but was sensitive to Ca2+ and neomycin. 125I-omega-Conotoxin-labeled Ca2+ channels were recognized by autoantibodies of Lambert-Eaton myasthenic patients. These antibodies are known to be specific for the neuronal omega-type channel. High-voltage-activated Ca2+ currents, investigated with the patch-clamp technique, consisted of a major dihydropyridine-sensitive (L-type) component, and a minor fraction irreversibly blocked by omega-conotoxin. Depolarizing secretagogues, such as D-glyceraldehyde and alanine, induced Ca(2+)-dependent insulin secretion, which was attenuated by omega-conotoxin. Taken together, these results show that voltage-operated Ca2+ channels in insulin-secreting RINm5F cells are heterogeneous and, in particular, that an omega-type channel, pharmacologically, immunologically and electrophysiologically similar to the neuronal omega-type channel, is also expressed in endocrine cells where it might have a role in the control of hormone secretion.

The synaptic vesicle protein synaptotagmin associates with calcium channels and is a putative Lambert-Eaton myasthenic syndrome antigen

Immunoglobulin G fractions from patients with Lambert-Eaton myasthenic syndrome (LEMS), an autoimmune disease of neuromuscular transmission, immunoprecipitate 125I-labeled omega-conotoxin GVIA-labeled calcium channels solubilized from rat brain. A 58-kDa antigen was detected by probing Western blots of partially purified calcium channels with LEMS plasma and IgG and was shown to be the relevant antigen in omega-conotoxin receptor immunoprecipitation. Monoclonal antibody 1D12, produced by immunizing mice with synaptic membranes, has properties similar to these autoimmune IgGs in both immunoprecipitation and Western blotting assays. 1D12 antigen was purified by immunoaffinity chromatography and shown to bind LEMS IgG. The antigen was identified by screening a rat brain cDNA library with 1D12 and was found to have strong homology to the synaptic vesicle membrane protein synaptotagmin. Our results indicate therefore that these antibodies immunoprecipitate omega-conotoxin receptors by binding to synaptotagmin that is associated with calcium channels. We suggest that the interaction between synaptotagmin and the voltage-gated calcium channel plays a role in docking synaptic vesicles at the plasma membrane prior to rapid neurotransmitter release and that autoantibody binding to a synaptotagmin-calcium-channel complex may be involved in the etiology of LEMS.

Voltage-sensitive calcium channels in a human small-cell lung cancer cell line

Utilizing the whole-cell patch-clamp method we assessed the Ca2+ current (ICa) in well-established cell lines from human small-cell carcinoma (SCC) of the lung, NCI-H209 and NCI-H187. The Ca2+ current was readily observed in H209 tumour cells (90% of the cells tested), whereas H187 tumour cells only occasionally expressed Ca2+ channels (26% of the cells tested). H209 Ca2+ current was evoked by potentials greater than -30 mV and exhibited partial inactivation over the duration of a 40 ms command potential. This inward current was unchanged by alteration of the holding potential from -80 to -40 mV and the activation phase of the Ca2+ current was best fitted by Hodgkin-Huxley m(t)2 kinetics. H209 Ca2+ current was reduced over 80% by verapamil (100 microM), whereas w-conotoxin (5 microM) appeared to be without effect. In contrast, H209 Ca2+ current was rapidly abolished by nifedipine (10 microM), strongly suggesting the presence of L-type Ca2+ channels. Voltage-gated Ca2+ channels may be important to the secretion of ectopic hormones and the etiology and pathogenesis of Lambert-Eaton syndrome, an autoimmune disorder of the motor nerve terminal in which autoantibodies directed against voltage-gated Ca2+ channels are produced.

Two types of high-threshold calcium currents inhibited by omega-conotoxin in nerve terminals of rat neurohypophysis

1. The neurohypophysis comprises the nerve terminals of hypothalamic neurosecretory cells, which contain arginine vasopressin (AVP) and oxytocin. The secretory terminals of rat neurohypophyses were acutely dissociated. The macroscopic calcium currents (ICa) of these isolated peptidergic terminals were studied using 'whole-cell' patch-clamp recording techniques. 2. There are two types ('Nt' (where the subscript 't' denotes terminal) and 'L') of high-threshold voltage-activated ICa in the terminals, which can be distinguished by holding at different potentials i.e. -90 and -50 mV. Replacement of Ca2+ in the bathing solution by Ba2+ increased the amplitude of ICa, primarily due to an increase in the L-type component. Both inward currents were eliminated by adding 50 microM-Cd2+ or when in a Ca(2+)-free bathing solution. 3. omega-Conotoxin GVIA (omega-CgTx) has been widely used as a Ca2+ channel blocker. However, whether this toxin can discriminate between different types of Ca2+ channels is still a subject of controversy. We applied omega-CgTx over a wide range of concentrations (0.01-2 microM) to examine its effects on both Nt- and L-type ICa in these terminals. At a concentration of 30 nM, omega-CgTx selectively reduced, by 48%, the amplitude of Nt-type ICa. In contrast, a higher concentration (300 nM) of omega-CgTx was necessary to inhibit the L-type ICa. 4. omega-CgTx inhibited both Nt- and L-type ICa in a dose-dependent manner, and the half-maximum inhibition (IC50) of the ICa by the toxin was 50 and 513 nM, respectively, which was approximately a tenfold difference. The reduction in both types of currents did not result from any shift in their current-voltage or steady-state inactivation relationships. 5. In contrast, omega-CgTx, at a concentration of 300 nM, had no effect on the tetrodotoxin-sensitive sodium current (INa) of the isolated peptidergic nerve terminals. Furthermore, omega-CgTx did not reduce the long-lasting, non-inactivating ICa in the isolated non-neuronal secretory cells of the pars intermedia (PI) (intermediate lobe of the pituitary). 6. Our studies suggest that omega-CgTx might exert specific blocking effects on both Nt- and L-type Ca2+ channels, but that in the isolated peptidergic nerve terminals, the Nt-type component is more susceptible to this toxin.

Inactivation characteristics reveal two calcium currents in adult bovine chromaffin cells

1. Two calcium currents were identified by differences in their inactivation characteristics in adult chromaffin cells maintained in short-term primary culture (3-5 days). Calcium currents were recorded by means of the whole-cell configuration using an intracellular medium highly buffered for pH and pCa. 2. Calcium current evoked from a holding potential of -90 mV inactivated along two components: an initial transient with a time constant of 250 ms followed by a plateau. 3. Steady-state inactivation followed two processes which developed at two distinct membrane potentials. One process was half-inactivated at low voltages around -55 mV and affected mainly the initial transient component. The other process, which affected mainly the sustained component of the calcium current, was half-inactivated at voltages around -10 mV. The proportions of these two processes varied greatly from cell to cell. 4. The dihydropyridine antagonists (nicardipine and nifedipine applied at 10(-5) M) and the phenylalkylamine D600 (5 x 10(-6) M) shifted the half-inactivation value towards -55 mV, indicating the suppression of the sustained component. The snail toxin, omega-conotoxin, had the opposite effect; it shifted the half-activation value towards -10 mV. 5. The calcium channel agonist Bay K 8644 (10(-5) M) either had no effect or induced only a slight increase of the response, as did its (-)-enantiomer (10(-6) M). To interpret the present results, we suggest that the L-component was maximally activated in our recording conditions. 6. In chromaffin cells, the calcium current recorded in whole-cell conditions is composed of two components with properties close to those of N- and L-type currents described in sympathetic neurons.

Characterization of a Voltage-dependent Calcium Current in the Human Neuroblastoma Cell Line SH-SY5Y During Differentiation

In the presence of retinoic acid, cultured human neuroblastoma SH-SY5Y cells grow processes indicative of neuronal differentiation. A voltage-gated Ca current is already present in undifferentiated cells. A gradual increase of the Ca current density occurs during cell differentiation. According to kinetic and pharmacological properties, Ca currents in differentiated cells are indistinguishable from those elicitable in undifferentiated cells and resemble features of the high-voltage activated currents present in mammalian neuronal cells. omega-conotoxin strongly depresses high-voltage activated currents, both in undifferentiated and in differentiated SH-SY5Y cells. Interestingly, the Ca agonist Bay K 8644 is effective, albeit with great variability from cell to cell, only in differentiated cells and only when barium is the current carrier through the Ca channels. A diversity of high-voltage activated Ca channels of distinct pharmacology has been recently observed in other kinds of neurons. This requires a redefinition of the role that voltage-dependent Ca channel subtypes can play in mammalian neurons.

Noradrenergic inhibition and voltage-dependent facilitation of omega-conotoxin-sensitive Ca channels in insulin-secreting RINm5F cells

We found that, besides dihydropyridine-sensitive Ca channels, insulin-secreting RINm5F cells also contain a minority (15-25%) of omega-conotoxin (omega-CgTx)-sensitive channels that show a high-affinity binding to [125I] omega-CgTx (Kd 51 pM). Noradrenaline (NA, 10 microM) slows down Ca-channel activation in these cells and produces a sizeable reduction of Ca currents that is relieved by strong pre-conditioning depolarizations (facilitation). The action of NA is mimicked by intracellular application of GTP-gamma-S and is prevented by pertussis toxin (PTX) or by cell pre-incubation with omega-CgTx. This suggests specific noradrenergic inhibition of omega-CgTx-sensitive Ca channels that is modulated by membrane potentials and PTX-sensitive G-protein activation.

The bradykinin receptor--a putative receptor-operated channel in PC12 cells: studies of neurotransmitter release and inositol phosphate accumulation

Bradykinin (BK) induced [3H]norepinephrine [( 3H]NE) release and phosphatidylinositol turnover were investigated in PC12 cells. Induction of [3H]NE release by BK is mediated by activation of BK-B2-receptors, as determined using type specific BK receptor antagonists. BK induces [3H]NE release with a half maximal effective concentration of 30 +/- 0.5 nM, and reaches maximal net fractional release of 9.0 +/- 1% with 200 nM BK. The BK-induced release is Ca2+ dependent, reaching maximal release at 1.0 mM Ca2+, is pertussis toxin insensitive (1 microgram/ml), slightly increased by a dibutyryl cAMP (1 mM) and not affected by inhibitors of the cyclooxygenase or lipoxygenase pathways. Voltage-sensitive Ca2+ channel blockers, verapamil (10 microM), nifedipine (10 microM), and omega-conotoxin (CgTx 10 nM), do not block the BK-induced release. However, a considerable inhibitory effect was obtained by divalent cations Co2+ (ED50 = 0.2 mM) and Ni2+ (ED50(2)+ = 1 mM). These results indicate the involvement of a Ca2+ channel in the BK-mediated release which is different from the L- or N-type voltage sensitive calcium channels. Whereas [Ca2+]ex is essential for the BK-induction of catecholamine release, the rise in level of InsP's induced by BK in the presence or in the absence of [Ca2+]ex is similar up to concentration of 1 microM. This indicates that the rise in InsP's induced by BK is not sufficient to cause neurotransmitter release. Moreover, subsequent addition of Ca2+ to BK-stimulated cells in Ca(2+)-free medium yields no release. Hence, no activity triggered by BK alone could be further stimulated by Ca2+ for induction of release.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel autoantibodies in the Lambert-Eaton myasthenic syndrome

We have tested 36 patients with the Lambert-Eaton myasthenic syndrome for serum antibodies to voltage-gated calcium channels by using an immunoprecipitation assay with [125I] omega-conotoxin-labeled voltage-gated calcium channels extracted from a human neuroblastoma cell line, SKN-SH. Forty-four percent of these patients had significant levels of antibody (30-1,466 pM) compared with healthy control individuals (less than 15 pM). The incidence of positive sera in patients without associated small cell lung carcinoma (61%) was greater than in those patients with small cell lung carcinoma (28%). Results correlated strongly with results obtained using voltage-gated calcium channels extracted from the small cell lung carcinoma line, MAR5. Anti-voltage-gated calcium channel antibody titers did not correlate with disease severity across individuals, but longitudinal studies in 2 patients receiving immunosuppressive therapy showed a clear inverse relation between antibody titer and an electromyographic index of disease severity. The incidence of positive sera among patients with other neurological disorders was not significant, but 8 of 12 patients with rheumatoid arthritis or systemic lupus erythematosus had raised titers (30-82 pM). We conclude that the antibodies detected in this assay are heterogeneous and that some of them are likely to be implicated in this disorder of neuromuscular transmission. The assay should prove useful as an additional diagnostic aid in patients with Lambert-Eaton myasthenic syndrome.

Do calcium channel classifications account for neuronal calcium channel diversity?

Calcium (Ca2+) ions are involved in the development and control of a variety of neuronal properties and functions such as channel expression, synaptic transmission and neurosecretion. The main pathway by which Ca2+ enters the intracellular space is through voltage-activated Ca2+ channels that can be classified according to their different biophysical and pharmacological properties. Identification and characterization of these channel types are prerequisites for understanding the mechanisms that underlie Ca2(+)-controlled processes. In this article we summarize the efforts made to identify neuronal Ca2+ channel types, and we attempt to evaluate how useful existing classifications are in assigning specific properties and functions to distinct channel types in neurons.

Omega-conotoxin inhibits the acute activation of tyrosine hydroxylase and the stimulation of norepinephrine release by potassium depolarization of sympathetic nerve endings

Increased Ca2+ influx serves as a signal that initiates multiple biochemical and physiological events in neurons following depolarization. The most widely studied of these phenomena is the release of neurotransmitters. In sympathetic neurons, depolarization also increases the rate of synthesis of the transmitter norepinephrine (NE), via an activation of the enzyme tyrosine hydroxylase (TH), and this effect also seems to involve Ca2+ entry. We have examined whether the mechanism of Ca2+ entry relevant to TH activation is via voltage-sensitive Ca2+ channels and, if so, whether the type of Ca2+ channel involved is the same as that involved in the stimulation of NE release. We have investigated the isolated rat iris, allowing us to examine transmitter biosynthesis and release in sympathetic nerve terminals in the absence of sympathetic cell bodies and dendrites. Potassium depolarization produced a three- to fivefold increase in TH activity and an approximately 100-fold increase in NE release. Both effects were dependent on Ca2+ being present in the extracellular medium, and both were inhibited by omega-conotoxin (1 microM), which inhibits N-type voltage-sensitive Ca2+ channels. In contrast, the dihydropyridine nimodipine (1-3 microM), which blocks L-type Ca2+ channels, had no effect on either measure. These data support the hypothesis that increases in NE biosynthesis and release in sympathetic nerve terminals during periods of depolarization are both initiated by an influx of Ca2+ through voltage-sensitive Ca2+ channels and that a similar type of Ca2+ channel is involved in both processes.

Intracellular calcium in NCB-20 cells: elevation by depolarization and ethanol but not by glutamate

Intracellular Ca2+, measured in NCB-20 cells with the fluorescent indicator fura-2, was elevated by K(+)-depolarization and by ethanol, but not by the excitatory amino acids L-glutamate and N-methyl-D-aspartate. These amino acids failed to raise intracellular Ca2+ levels even in the presence of elevated extracellular Ca2+, in the absence of extracellular Mg2+ or Na+, under depolarizing conditions, or in differentiated cells, and induced neither 45Ca2+ uptake nor excitotoxic cell death. NCB-20 cells thus appear to express functional voltage-gated, but not N-methyl-D-aspartate receptor-gated, Ca2+ channels.

Two states of the L-type Ca2+ channel in PC12 cells: different sensitivity to 1,4-dihydropyridines

A omega-conotoxin-resistant component of high K(+)-induced [Ca2+]i increase in PC12 cells was further divided into two components by the difference in sensitivity to 1,4-dihydropyridines. The initial phase (up to 30s) was several times less sensitive to 1,4-dihydropyridines than the following plateau phase (several minutes). On the other hand, diltiazem blocked both phases with the same potency. Verapamil also was only a little more sensitive to the initial phase. All four 1,4-dihydropyridines tested showed 5-10 times more potent inhibition of (+)-[3H]PN200-110 binding in PC12 cells under the depolarizing (70 mM K+) condition than under the non-depolarizing (5 mM K+) condition. The biphasic blockade of the high K(+)-induced [Ca2+]i rise by 1,4-dihydropyridines appears to reflect their different affinities under depolarizing and non-depolarizing conditions.

Effects of glucose on calcium channels in neural cells

Hyperglycemia has been reported to alter outcome following experimental and clinical cerebral ischemia, but the mechanisms involved are incompletely understood. Since glucose influences the function of dihydropyridine-sensitive, voltage-gated Ca2+ channels in some non-neural cells, and since cellular Ca2+ overload has been implicated in the pathogenesis of ischemic neuronal injury, we examined whether glucose regulates Ca2+ channel function in a cultured neural cell line. Physiologic concentrations of glucose had no effect on free intracellular Ca2+ levels in PC12 cells, but 4-fold elevation of glucose above physiologic levels reduced the dihydropyridine-sensitive, depolarization-induced increase in Ca2+. This effect would not account for exacerbation of ischemic brain injury by hyperglycemia, but may contribute to attenuation of ischemic injury by glucose in certain settings.

Differential sensitivities of avian and mammalian neuromuscular junctions to inhibition of cholinergic transmission by omega-conotoxin GVIA

Nerve stimulation-induced contractions of the chick biventer cervicis muscle were slowly reduced by omega-conotoxin. However, omega-conotoxin had no effect on skeletal muscle function after i.v. injection in mice or on nerve stimulation-induced contractions of focally innervated muscle of the rat diaphragm or the rabbit proximal oesophagus, or the multiply innervated extra-ocular rectus muscle from rabbit. The lack of effect of omega-conotoxin on mammalian neuromuscular junctions was not due to the high safety factor in transmission or to a high local concentration of Ca2+ originating from the muscle, and could not be accounted for in terms of the operation of facilitatory or inhibitory feedback modulation of transmitter release from motoneurone terminals. It is concluded that the Ca2+ channels of mammalian motoneurone terminals differ from those of avian motoneurone terminals and other omega-conotoxin-sensitive nerve terminals.