Molecular diversity of voltage-dependent Ca2+ channels

omega-Agatoxin-IVA-sensitive calcium channels in bovine chromaffin cells

A large component of the whole-cell currents through Ca2+ channels in bovine adrenomedullary chromaffin cells has been shown to be insensitive to both L-type and N-type Ca2+ channel blockers, suggesting the existence of a third type of Ca2+ channel. In the present paper, omega-agatoxin-IVA (AgTx), a selective blocker of P-type Ca2+ channels in mammalian neurons, has been used to investigate the presence of this subtype of Ca2+ channel in bovine chromaffin cells. Barium currents (IBa) through Ca2+ channels were recorded in whole-cell patch-clamped bovine chromaffin cells. IBa was blocked by AgTx in a dose-dependent and irreversible manner. At the maximal concentration used (1 microM), AgTx inhibited IBa by 49.5 +/- 3%. Such a blockade was also present when bovine chromaffin cells were pretreated with 10 microM furnidipine, a novel 1,4-dihydropyridine L-type channel blocker, and after treatment with 1 microM of the N-type channel blocker, omega-conotoxin GVIA (CgTx). A combination of these three types of Ca2+ channel blockers suppressed the macroscopic Ba2+ currents by 88%. We conclude that bovine chromaffin cells, in addition to N- and L-type Ca2+ channels, possess a P-like component in their whole-cell currents through the Ca2+ channels.

The mechanism of Ba(2+)-induced exocytosis from single chromaffin cells

Dialysis of Ba2+ into voltage-clamped single bovine chromaffin cells produced a concentration-dependent increase in cell capacitance, reflecting an enhanced rate of exocytotic events. Between 0.1 and 1 mM, Ba2+ linearly increased both the rate and the total amount of exocytosis. In unclamped cells also, extracellular Ba2+ induced the release of catecholamines, as assayed with a carbon-fibre electrode in the amperometric mode. Additionally, extracellular application of Ba2+ increased the apparent internal Ca2+ concentration ([Ca2+]app) in fura-2-loaded chromaffin cells. These observations were made both in the presence and absence of external Ca2+ (Cao2+), as well as after depletion of the intracellular Ca2+ stores with ionomycin. Under current-clamp conditions, Ba2+ induced pronounced depolarization of the cells. These results are compatible with the following conclusions: by blocking K+ channels, Ba2+ causes depolarization of chromaffin cells. This results in opening of voltage-gated Ca2+ channels and Ba2+ entry into the cytosol. Ba2+ then directly triggers exocytotic events, although it induces exocytosis only at concentrations more than a 100-fold higher than Ca2+. Various effects contribute to the generally observed greater secretory responses with Ba2+ as compared with Ca2+; these are the depolarizing effects of extracellular Ba2+, its greater entry through non-inactivating Ca2+ channels and its poor intracellular buffering largely arising from its weak affinity for plasmalemmal Ca2+ extrusion mechanisms. In some cases, Ba2+ additionally induces release of Ca2+ from internal stores, as evidenced by its effect on fura-2 fluorescence at different wavelengths.

Molecular localization of regions in the L-type calcium channel critical for dihydropyridine action

Sensitivity to dihydropyridines (DHPs) is a distinct characteristic that differentiates L-type Ca2+ channels from T-, N-, and P-type Ca2+ channels. To identify regions necessary for the functional effects of DHPs, chimeric Ca2+ channels were constructed in which portions of motif III or motif IV of a DHP-insensitive brain Ca2+ channel, BI-2, were introduced into the DHP-sensitive cardiac L-type Ca2+ channel. The resultant chimeric Ca2+ channels were expressed in Xenopus oocytes, and the effects of a DHP agonist and antagonist were studied. The results show that the linker region between S5 and S6 in motif IV of the L-type Ca2+ channel is a major site for DHP action. The DHP agonist and antagonist molecules interact with distinct sites on the alpha 1 subunit of the L-type Ca2+ channel. The data further show that the SS2-S6 region of motif III is not involved in DHP action but may be an important structural component of inactivation.

Identification and differential subcellular localization of the neuronal class C and class D L-type calcium channel alpha 1 subunits

To identify and localize the protein products of genes encoding distinct L-type calcium channels in central neurons, anti-peptide antibodies specific for the class C and class D alpha 1 subunits were produced. Anti-CNC1 directed against class C immunoprecipitated 75% of the L-type channels solubilized from rat cerebral cortex and hippocampus. Anti-CND1 directed against class D immunoprecipitated only 20% of the L-type calcium channels. Immunoblotting revealed two size forms of the class C L-type alpha 1 subunit, LC1 and LC2, and two size forms of the class D L-type alpha 1 subunit, LD1 and LD2. The larger isoforms had apparent molecular masses of approximately 200-210 kD while the smaller isoforms were 180-190 kD, as estimated from electrophoresis in gels polymerized from 5% acrylamide. Immunocytochemical studies using CNC1 and CND1 antibodies revealed that the alpha 1 subunits of both L-type calcium channel subtypes are localized mainly in neuronal cell bodies and proximal dendrites. Relatively dense labeling was observed at the base of major dendrites in many neurons. Staining in more distal dendritic regions was faint or undetectable with CND1, while a more significant level of staining of distal dendrites was observed with CNC1, particularly in the dentate gyrus and the CA2 and CA3 areas of the hippocampus. Class C calcium channels were concentrated in clusters, while class D calcium channels were generally distributed in the cell surface membrane of cell bodies and proximal dendrites. Our results demonstrate multiple size forms and differential localization of two subtypes of L-type calcium channels in the cell bodies and proximal dendrites of central neurons. The differential localization and multiple size forms may allow these two channel subtypes to participate in distinct aspects of electrical signal integration and intracellular calcium signaling in neuronal cell bodies. The preferential localization of these calcium channels in cell bodies and proximal dendrites implies their involvement in regulation of calcium-dependent functions occurring in those cellular compartments such as protein phosphorylation, enzyme activity, and gene expression.

Human neuronal voltage-dependent calcium channels: studies on subunit structure and role in channel assembly

Voltage-dependent calcium (Ca2+) channels, expressed in the CNS, appear to be multimeric complexes comprised of at least alpha 1, alpha 2 and beta subunits. Previously, we cloned and expressed human neuronal alpha 1, alpha 2 and beta subunits to study recombinant channel complexes that display properties of those expressed in vivo. The alpha 1B-mediated channel subtype binds omega-conotoxin (CgTx) GVIA with high affinity and exhibits properties of N-type voltage-dependent Ca2+ channels. Here we describe several alpha 2 and beta splice variants and report results on the expression of omega-CgTx GVIA binding sites, assembly of the subunit complex and biophysical function of alpha 1B-mediated channel complexes containing some of these splice variants. We optimized recombinant expression in human embryonic kidney (HEK) 293 cells of alpha 1B alpha 2b beta 1 subunit complexes by controlling the expression levels of subunit mRNAs and monitored cell surface expression by binding of omega-CgTx GVIA to the alpha 1B subunit. Co-expression of either alpha 2b or beta 1 subunits with an alpha 1B subunit increased expression of binding sites while the most efficient expression was achieved when both alpha 2b and beta 1 subunits were co-expressed with an alpha 1B subunit. The presence of alpha 2b affects the affinity of omega-CgTx GVIA binding and barium (Ba2+) current magnitudes, although it does not appear to alter kinetic properties of the Ba2+ current. This is the first evidence of an alpha 2 subunit modulating the binding affinity of a cell-surface Ca2+ channel ligand. Our results demonstrate that alpha 1, alpha 2 and beta subunits together contribute to the efficient assembly and functional expression of voltage-dependent Ca2+ channel complexes.

A new Conus peptide ligand for Ca channel subtypes

A cDNA clone encoding a new omega-conotoxin was identified from Conus magus. The predicted peptide was chemically synthesized using a novel strategy that efficiently yielded the biologically active disulfide-bonded isomer. This peptide, omega-conotoxin MVIID, targets other voltage-gated calcium channels besides the N-subtype and exhibits greater discrimination against the N-channel subtype than any other omega-conotoxin variant to date. Consequently, omega-conotoxin MVIID may be a particularly useful ligand for calcium channel subtypes that are not of the L- or N-subclasses. Of the eight major sequence variants of omega-conotoxins that have been elucidated, four come from Conus magus venom. We suggest that sequence variants from the same venom may be designed to optimally interact with different molecular variants of calcium channels; such omega-conotoxin sets from a single venom may therefore be useful for helping to identify novel calcium channel subtypes.

Voltage-dependent L-type Ca channels and a novel type of non-selective cation channel activated by cAMP-dependent phosphorylation in mesoderm-like (MES-1) cells

Undifferentiated P19 embryonal carcinoma cells (ECC P19), the P19-derived clonal cell lines END-2 (visceral endoderm-like), EPI-7 (epithelioid ectoderm-like), MES-1 (mesoderm-like) and a parietal yolk sac cell line (PYS-2) were used as cellular models to examine the functional expression of voltage-dependent Ca channels and other Ca-permeable cation channels at various stages of early embryonic development. Whole-cell currents were recorded by means of the patch clamp technique. Whereas more than 75% of MES-1 cells possessed Ca channel currents, neither P19, END-2, EPI-7 nor PYS-2 cells had detectable voltage-dependent inward currents. Ca channel currents of MES-1 cells were highly sensitive towards 1,4-dihydropyridines and blocked by cadmium. Adrenaline (10 microM) caused Ca channel stimulation in only 14% of MES-1 cells examined. However, in 62% of the cells adrenaline activated a linear current component which under physiological conditions reversed close to 0 mV. Removal of extracellular Na+ suppressed the adrenaline-induced inward current, while reducing extracellular Cl- had no significant effect. These findings suggest that the adrenaline-induced current is carried through non-selective cation channels which were found to be permeable for Na+, K+, Cs+ >> Ca2+. Remarkably, the intracellular signalling pathway for activation of the non-selective cation current involved the cascade of reactions leading to cAMP-dependent phosphorylation, a regulatory pathway well known for cardiac Ca channels. A possible functional role of adrenaline-induced non-selective cation currents and Ca channels in embryonal development is discussed.

Functional properties of a neuronal class C L-type calcium channel

The rat brain class C calcium channel alpha 1 subunit cDNA, rbC-II, was subcloned into a vertebrate expression vector and transient expression was assayed following nuclear injection into Xenopus oocytes. Whole cell recordings showed that rbC-II currents (recorded with 40 mM Ba2+ as the charge carrier) had variable activation rates and minimal inactivation over an approximately 700 msec depolarizing step pulse. The pharmacological properties of the rbC-II current were consistent with those of an L-type calcium channel, being sensitive to dihydropyridines (10 microM nifedipine blocked approximately 85% of the current, 10 microM Bay K 8644 enhanced the current between 2- and 10-fold) and not affected by the N- and P-type calcium channel antagonists, omega-conotoxin GVIA and omega-agatoxin IVA, respectively. Coexpression of rbC-II with cloned rat neuronal calcium channel alpha 2 and beta subunits resulted in several changes to the electrophysiological properties of the rbC-II current including, an increased whole cell peak current, an increased rate of activation and a hyperpolarizing shift in the voltage dependence of activation. Taken together with results showing that the neuronal class D alpha 1 subunit also encodes an L-type calcium channel [Williams M. E., Feldman D. H., McCue A. F., Brenner R., Velicelebi G., Ellis S. B. and Harpold M. M. (1992a) Neuron 8: 71-84], these results indicate that the mammalian nervous system expresses two distinct genes encoding L-type calcium channels.

Characterization of the purified N-type Ca2+ channel and the cation sensitivity of omega-conotoxin GVIA binding

A functional N-type Ca2+ channel (omega-conotoxin GVIA receptor) has been purified from rabbit brain and shown to be composed of four subunits of molecular weights 230 K (alpha 1B), 160 K (alpha 2 delta), 95 K and 57 K (beta 3) [Witcher D. R., De Waard M., Sakamoto J., Franzini-Armstrong C., Pragnell M., Kahl S.D. and Campbell K. D. (1993) Science 261: 486-489]. These four subunits migrate on sucrose density gradients as a single complex and are identified by subunit specific polyclonal antibodies. Polyclonal antibodies against the purified receptor complex immunoprecipitate greater than 90% of the [125I]omega-conotoxin GVIA (omega-CgTx) binding sites in solubilized crude rabbit brain membranes. Furthermore, polyclonal antibodies affinity-purified against unique GST fusion proteins from two of the cloned subunits in the complex (alpha 1B and beta 3) specifically immunoprecipitated [125I]omega-CgTx binding sites and not [3H]PN200-110 binding sites. Analysis of [125I]omega-CgTx binding to the purified N-type Ca2+ channel demonstrated that the equilibrium binding was sensitive to increasing cation concentration. The IC50 for calcium and barium was 2.5 and 5 mM, respectively. [125I]omega-CgTx binding was not significantly reduced within 15 min after the addition of 50 mM barium. However, single channel analysis of the purified N-type Ca2+ channel preincubated with 10 microM omega-CgTx demonstrated that in the presence of 50 mM barium and 0.5 microM omega-CgTx, channel activity was detected but at a low open state probability (P < 0.10). These data suggest that the Ca2+ binding site(s) allosterically regulates the omega-CgTx binding site. Since the channel gating persisted in the presence of omega-CgTx, the omega-CgTx binding site may not be located within the pore of the channel and may be different from intra-pore Ca2+ binding sites.

Block of calcium channels in rat neurons by synthetic omega-Aga-IVA

We investigated block of voltage-dependent Ca channels in freshly dissociated rat central and peripheral neurons by the synthetic peptide omega-Aga-IVA. Synthetic omega-Aga-IVA blocked approximately 90% of the high-threshold Ca current in cerebellar Purkinje neurons with an estimated Kd of approximately 1.5 nM, slightly higher than that determined for block by toxin purified from Agelenopsis aperta venom. At 200 nM, the synthetic peptide blocked a small fraction of current in dorsal root ganglion neurons but had no effect on identified components of current carried by low-threshold T-type channels or by dihydropyridine-sensitive L-type Ca channels. Up to 800 nM synthetic peptide had no effect on the current in sympathetic neurons, carried mainly by omega-conotoxin GVIA-sensitive N-type channels. In spinal cord neurons, the same fraction of high-threshold current was blocked by synthetic peptide or purified toxin. We conclude that synthetic omega-Aga-IVA has the same high selectivity for blocking P-type Ca channels as does toxin purified from spider venom.

Distinctive pharmacology and kinetics of cloned neuronal Ca2+ channels and their possible counterparts in mammalian CNS neurons

This paper provides a brief overview of the diversity of voltage-gated Ca2+ channels and our recent work on neuronal Ca2+ channels with novel pharmacological and biophysical properties that distinguish them from L, N, P or T-type channels. The Ca2+ channel alpha 1 subunit known as alpha 1A or BI [Mori Y., Friedrich T., Kim M.-S., Mikami A., Nakai J., Ruth P., Bosse E., Hofmann F., Flockerzi V., Furuichi T., Mikoshiba K., Imoto K., Tanabe T. and Numa S. (1991) Nature 350, 398-402] is generally assumed to encode the P-type Ca2+ channel. However, we find that alpha 1A expressed in Xenopus oocytes differs from P-type channels in its kinetics of inactivation and its degree of sensitivity to block by the peptide toxins omega-Aga-IVA and omega-CTx-MVIIC [Sather W. A., Tanabe T., Zhang J.-F., Mori Y., Adams M. E. and Tsien R. W. (1993) Neuron 11, 291-303]. Thus, alpha 1A is capable of generating a Ca2+ channel with characteristics quite distinct from P-type channels. Doe-1, recently cloned from the forebrain of a marine ray, is another alpha 1 subunit which exemplifies a different branch of the Ca2+ channel family tree [Horne W. A., Ellinor P. T., Inman I., Zhou M., Tsien R. W. and Schwarz T. L. (1993) Proc. Natn. Acad. Sci. U.S.A. 90, 3787-3791]. When expressed in Xenopus oocytes, doe-1 forms a high voltage-activated (HVA) Ca2+ channel [Ellinor P. T., Zhang J.-F., Randall A. D., Zhou M., Schwarz T. L., Tsien R. W. and Horne W. (1993) Nature 363, 455-458]. It inactivates more rapidly than any previously expressed calcium channel and is not blocked by dihydropyridine antagonists or omega-Aga-IVA. Doe-1 current is reduced by omega-CTx-GVIA, but the inhibition is readily reversible and requires micromolar toxin, in contrast to this toxin's potent and irreversible block of N-type channels. Doe-1 shows considerable sensitivity to block by Ni2+ or Cd2+. We have identified components of Ca2+ channel current in rat cerebellar granule neurons with kinetic and pharmacological features similar to alpha 1A and doe-1 in oocytes [Randall A. D., Wendland B., Schweizer F., Miljanich G., Adams M. E. and Tsien R. W. (1993) Soc. Neurosci. Abstr. 19, 1478]. The doe-1-like component (R-type current) inactivates much more quickly than L, N or P-type channels, and also differs significantly in its pharmacology.(ABSTRACT TRUNCATED AT 400 WORDS)

R56865 inhibits catecholamine release from bovine chromaffin cells by blocking calcium channels

1. The effects of R56865 (a new class of cardioprotective agent which prevents Na+ and Ca2+ overload in cardiac myocytes) on catecholamine release, whole-cell current through Ca2+ channels (IBa) and cytosolic Ca2+ concentrations, [Ca2+]i, have been studied in bovine chromaffin cells. 2. R56865 caused a time- and concentration-dependent blockade of catecholamine release from superfused cells stimulated intermittently with 5 s pulses of 59 mM K+. After 5 min superfusion, a 3 microM concentration inhibited secretion by 20%; the blockade increased gradually with perfusion time, to reach 85% after 40 min. The IC50 to block secretion after 5 min periods of exposure to increasing concentrations of R56865 was around 3.1 microM. The blocking effects of R56865 were reversible after 5-15 min wash out. In high Ca2+ solution (10 mM Ca2+), the degree of blockade of secretion diminished by 20% in comparison with 1 mM Ca2+. 3. In electroporated cells, R56865 (10 microM) did not modify the secretory response induced by the application of 10 microM free Ca2+. 4. R56865 blocked the peak IBa current in a concentration- and time-dependent manner; its IC50 was very similar to that obtained for secretion (3 microM). The compound not only reduced the size of the peak current but also promoted its inactivation; when the effects of R56865 were measured at the most inactivated part of the current, its IC50 was 1 microM. Both the inactivation and the reduction of the peak currents were reversible upon washing out the drug. 5. In fura-2-loaded single chromaffin cells the basal [Ca2+]i of around 100 nM was elevated to a peak of1.5 microM by the application of a 5 s pulse of 59 mM K+. R56865 (10 microM) did not affect the basal [Ca2+]but blocked by 90% the K+-evoked increase. This effect was fully reversible after 5-10 min of wash out.6. The results are compatible with the idea that R56865 blocks Ca2+ entry into K+-depolarized chromaffin cells by promoting the inactivation of voltage-dependent Ca2+ channels; this would lead to the limitation of the rise in [Ca2+]i and of the release of catecholamines. The restriction of catecholamine release may favour indirectly the known direct beneficial cardioprotective actions of R56865.

Pharmacological characterisation of the calcium channels coupled to the plateau phase of KCl-induced intracellular free Ca2+ elevation in chicken and rat synaptosomes

The effect of various blockers of voltage operated calcium channels (VOCCs) was studied on the non-inactivating, plateau phase of KCl-induced intracellular free Ca2+ ([Ca2+]i) elevation in rat cortical and chicken forebrain synaptosomes. In chicken synaptosomes, omega-CgTx GVIA (0.1 nM to 1 microM) and omega-CgTx MVIIA (0.1 nM to 1 microM), both selective blockers of N-type Ca2+ channels, produced a concentration-dependent inhibition of the plateau phase of [Ca2+]i elevation. omega-CgTx GVIA (IC50 value 28 nM) was more potent than omega-CgTx MVIIA (IC50 value 78 nM), but at submaximal concentrations, took longer to reach its maximum effect (20 min for omega-CgTx GVIA; 10 min for omega-CgTx MVIIA). At 1 microM, the highest concentration tested, each toxin blocked > 85% of [Ca2+]i elevation. The effect of omega-CgTx GVIA on the extent and time-course of inhibition of [Ca2+]i elevation was maintained in a Na(+)-free, choline substituted, medium. omega-Aga IVA (300 nM), a selective blocker of P-type calcium channels, inhibited 28 +/- 5% of [Ca2+]i elevation. The effect of a combination of submaximal inhibitory concentrations of omega-CgTx GVIA (100 nM) and omega-Aga IVA (300 nM) was less than additive. In rat synaptosomes, omega-CgTx GVIA (1 microM) and omega-CgTx MVIIA (1 microM), blocked only 18 +/- 5% and 17 +/- 4% of the plateau phase of free Ca2+ elevation, respectively. omega-Aga IVA produced a concentration-dependent inhibition of [Ca2+]i elevation in this preparation. Threshold inhibition was observed at 1 nM, and maximum inhibition (64 +/- 8%) at 1 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

A beta-subunit normalizes the electrophysiological properties of a cloned N-type Ca2+ channel alpha 1-subunit

The electrophysiological and pharmacological properties of a cloned rat brain N-type Ca2+ channel were determined by transient expression in Xenopus oocytes. Expression of the class B Ca2+ channel alpha 1 subunit, rbB-I, resulted in a high voltage-threshold current that activated slowly and showed little inactivation over 800 msec. Characteristic of N-type currents, the rbB-I current was completely blocked by omega-conotoxin GVIA and was insensitive to nifedipine and Bay K8644. The modulatory effects on the rbB-I current by cloned rat brain Ca2+ channel alpha 2 and beta 1b subunits were also examined. Coexpression of rbB-I with the beta 1b subunit caused significant changes in the properties of the rbB-I current making it more similar to N-type currents in neurons. These included: (1) an increase in the whole-cell current, (2) an increased rate of activation, (3) a shift of the voltage-dependence of inactivation to hyperpolarized potentials and (4) a pronounced inactivation of the current over 800 msec. Coexpression with the rat brain alpha 2 subunit had no significant effect on the rbB-I current alone but appeared to potentiate the rbB-I+beta 1b whole cell current. The results show that coexpression with the brain beta 1b subunit normalizes the rbB-I N-type current, and suggests the possibility that differences in subunit composition may contribute to the heterogeneous properties described for N-type channels in neurons.

Subunit-dependent modulation of recombinant L-type calcium channels. Molecular basis for dihydropyridine tissue selectivity

At least four calcium channel subtypes (P, T, N, and L) have now been classified on the basis of their biophysical and/or pharmacological properties. L-type channels, a channel family particularly important to physiological function of the cardiovascular system, are identified by their slow voltage- and calcium-dependent inactivation as well as their sensitivity to dihydropyridine (DHP) calcium channel antagonists. In this study, we report the results of experiments in which we have measured the DHP modulation of recombinant calcium channel activity in cells transfected with alpha 1 subunits of cardiac and smooth muscle L-type calcium channels. We find subunit-dependent differences in the voltage and concentration dependence of channel modulation. Our results provide evidence for a molecular basis for DHP sensitivity of heart and smooth muscle calcium channels and, additionally, indicate that, even within one family of calcium channels, slight differences in channel structure can cause marked differences in channel pharmacology.

Biotinylated derivatives of omega-conotoxins GVIA and MVIID: probes for neuronal calcium channels

The omega-conotoxins are small, disulfide-rich peptides which inhibit voltage-sensitive calcium channels. Biotinylated omega-conotoxins are potentially useful reagents for characterizing distinct subsets of calcium channels. We describe the preparation and characterization of biotinylated derivatives of two specific omega-conotoxins, GVIA and MVIID, which bind different calcium channel subtypes. Eight biotinylated derivatives were tested; all specifically displaced binding of the radiolabeled unbiotinylated omega-conotoxin. In general, the addition of one biotin moiety decreased the apparent affinity for the receptor target site by only approximately 10-fold. However, derivatization of omega-conotoxin MVIID at the Lys10 residue caused a much more marked effect, a ca 500-fold decrease in affinity. These results indicate that the vicinity of the Lys10 residue of omega-conotoxin MVIID may be more critical for binding to the receptor target site than regions around other amino groups in omega-conotoxins GVIA and MVIID. Thus, high affinity biotinylated omega-conotoxin GVIA and MVIID derivatives have been chemically defined; the biotin groups have been shown to be accessible to streptavidin. Given the commercial availability of streptavidin coupled to various reporter groups, the biotinylated omega-conotoxin derivatives described here should be widely useful for fluorescence, electron microscopic or immunological applications.

The nicotinic acetylcholine receptor of the bovine chromaffin cell, a new target for dihydropyridines

The effects of 1,4-dihydropyridine derivatives on divalent cation transients and catecholamine release stimulated by either high K+ or the nicotinic receptor agonist dimethyl-phenyl-piperazinium (DMPP) have been compared in bovine adrenal chromaffin cells. The activation of Ca2+ entry pathways was followed by measuring 45Ca2+ or Mn2+ uptake, or by the changes of [Ca2+]i in fura-2-loaded chromaffin cells. Various dihydropyridine Ca2+ channel blockers (nimodipine, PCA50938, nifedipine, nitrendipine, furnidipine) abolished the DMPP-mediated effects, but prevented only partially the activation by high [K+]0 of 45Ca2+ uptake. The IC50 for DMPP-induced activation was around 1 microM. The L-type Ca2+ channel activator Bay K 8644 potentiated the uptake of 45Ca2+ induced by K+ depolarization at concentrations between 10 nM and 1 microM, but completely inhibited the uptake of 45Ca2+ by DMPP (IC50, 0.9 microM). Both high [K+]0 and DMPP produced membrane depolarization as measured using bis-oxonol. The DMPP-evoked, but not the K(+)-evoked membrane depolarization was prevented by Na+ removal, suggesting that the depolarization was due to Na+ entry through the acetylcholine receptor ionophore. Nimodipine at 10 microM abolished the depolarization induced by DMPP, leaving the K(+)-evoked depolarization unaffected. Tetrodotoxin (2 microM) did not affect the DMPP- or high K(+)-mediated cell depolarization. Whole-cell inward current evoked by 100 microM DMPP (IDMPP) was measured in cells voltage-clamped at -80 mV. Nimodipine (10 microM) reduced IDMPP by 36%; Bay K 8644 (10 microM) inhibited IDMPP by 67%. DMPP-evoked catecholamine release from superfused chromaffin cells was reduced by over 90% with 10 microM nimodipine; in contrast, K(+)-evoked release was decreased by 20%.(ABSTRACT TRUNCATED AT 250 WORDS)

A possible role of sarcoplasmic Ca2+ release in modulating the slow Ca2+ current of skeletal muscle

Ca2+ channels are regulated in a variety of different ways, one of which is modulation by the Ca2+ ion itself. In skeletal muscle, Ca2+ release sites are presumably located in the vicinity of the dihydropyridine-sensitive Ca2+ channel. In this study, we have tried to investigate the effects of Ca2+ release from the sarcoplasmic reticulum on the L-type Ca2+ channel in frog skeletal muscle, using the double Vaseline gap technique. We found an increase in Ca2+ current amplitude on application of caffeine, a well-known potentiator of Ca2+ release. Addition of the fast Ca2+ buffer BAPTA to the intracellular solution led to a gradual decline in Ca2+ current amplitude and eventually caused complete inhibition. Similar observations were made when the muscle fibre was perfused internally with the Ca2+ release channel blocker ruthenium red. The time course of Ca2+ current decline followed closely the increase in ruthenium red concentration. This suggests that Ca2+ release from the sarcoplasmic reticulum is involved in the regulation of L-type Ca2+ channels in frog skeletal muscle.

Calcium-channel blockers and gastrointestinal motility: basic and clinical aspects

Several calcium-channel blockers currently in use for the treatment of cardiovascular disorders have recently been tested for their effects on gastrointestinal motility. The rationale for this approach centers on the concept that calcium-channel blockers are at least as potent in inhibiting intestinal smooth muscle as in relaxing vascular smooth muscle. This review will give an outline of the most recent findings on the role of calcium and calcium channels in smooth muscle and neuronal function in the digestive system. It will also consider the mechanisms by which calcium-channel blockers may affect gastrointestinal motility and assess potential clinical applications in gastroenterology. The main goal for researchers in this field will be the development of gut-selective agents, with no cardiovascular side effects.

Analysis of rapid calcium signals in synaptosomes

A combination of the stopped-flow technology with dual channel spectrofluorometry of Ca(2+)-indicators was utilized for the measurement of rapid Ca(2+)-signals in rat cerebral cortical synaptosomes evoked by K(+)-depolarization. There was no observable contribution of Ca(2+)-ions from intracellular stores to the rise in [Ca2+]i. The kinetics of the fast increase in intracellular Ca2+ concentration was analysed in relation to the depolarization strength. The maximal increase in [Ca2+]i and the time course of Ca(2+)-channel inactivation were determined for depolarizations obtained by different extracellular K(+)-concentrations ([K+]o). An apparent threshold was observed at about 18 mM [K+]o; a maximal Ca(2+)-signal amplitude was estimated at about 40 mM [K+]o. Pharmacological properties of the involved Ca(2+)-channels were determined using selective Ca(2+)-channel blockers (Dihydropyridines, omega-Conotoxin, omega-Agatoxins); the results suggest that a P-type voltage-dependent Ca(2+)-channel is the relevant channel type, generating the evoked Ca(2+)-signals in rat cerebral cortical synaptosomes.

Changes in calcium channel current densities in rat colonic smooth muscle cells during development and aging

The age-related changes of Ca2+ channel currents were investigated in freshly isolated single smooth muscle cells from the circular layer of the distal colon from the rat using the whole cell voltage clamp technique. Under physiological conditions (Ca2+ concentration of 2.0 mM), the averaged total Ca2+ current density increased markedly from 1.25 pA/pF in the newborn rat to 6.46 pA/pF in the 60-day-old rat; it then gradually declined with aging. Two types of Ca2+ channel currents seemed to be present; one type possessed more negative threshold potentials (-70 to -60 mV) when the cells were held at -80 or -100 mV and inactivated quickly. The voltage for peak current was -20 to -10 mV, and the reversal potential was +60 to +70 mV. This current was highly sensitive to low concentrations of Ni2+ (30 microM) but was resistant to nifedipine, diltiazem, cadmium, and tetrodotoxin. In contrast, the other type of Ca2+ channel current possessed more positive threshold potential (-40 mV) and inactivated more slowly. The voltage for peak current was 0 mV, and the reversal potential was +60 to +70 mV. This current was insensitive to low concentrations of Ni2+ but highly sensitive to nifedipine, diltiazem, and cadmium. These results suggest that the fast inactivating (transient) current might be T-type Ca2+ current [ICa(T)], and such cells were ICa(T) positive cells; whereas the sustained Ca2+ current was L-type Ca2+ current [ICa(L)], and such cells were ICa(L) positive cells. Our results showed that the fraction of ICa(T) positive cells increased with development; the current densities of both ICa(L) and ICa(T) also increased with development.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca channel kinetics during the spontaneous heart beat in embryonic chick ventricle cells

The ability of Ca ions to inhibit Ca channels presents one of the most intriguing problems in membrane biophysics. Because of this negative feedback, Ca channels can regulate the current that flows through them. The kinetics of the channels depend on voltage, and, because the voltage controls the current, a strong interaction exists between voltage dependence and Ca dependence. In addition to this interaction, the proximity of pores and the local concentration of ions also determine how effectively the Ca ions influence channel kinetics. The present article proposes a model that incorporates voltage-dependent kinetics, current-dependent kinetics, and channel clustering. We have based the model on previous voltage-clamp data and on Ca and Ba action currents measured during the action potential in beating heart cells. In general we observe that great variability exists in channel kinetics from patch to patch: Ba or Ca currents have low or high amplitudes and slow or fast kinetics during essentially the same voltage regime, either applied step-protocols or spontaneous cell action potentials. To explain this variability, we have postulated that Ca channels interact through shared ions. The model we propose expands on our previous model for Ba currents. We use the same voltage-dependent rate constants for the Ca currents that we did for the Ba currents. However, we vary the current-dependent rate constants according to the species of the conducting ion. The model reproduces the main features of our data, and we use it to predict Ca channel kinetics under physiological conditions. Preliminary reports of this work have appeared (DeFelice et al., 1991, Biophys. J. 59:551a; Risso et al., 1992, Biophys. J. 61:248a).

Distinctive biophysical and pharmacological properties of class A (BI) calcium channel alpha 1 subunits

Transcripts for the class A Ca2+ channel alpha 1 subunit (also known as BI) are present at high levels in many parts of the mammalian CNS and are widely assumed to encode the P-type Ca2+ channel. To characterize the biophysical and pharmacological properties of alpha 1A channels, macroscopic and single-channel recordings were made in Xenopus oocytes injected with alpha 1A cRNA. alpha 1-specific properties were identified by making systematic comparisons with the more familiar class C alpha 1 subunit under the condition of a standard ancillary subunit (alpha 2/delta + beta) makeup. alpha 1A currents activate and inactivate more rapidly and display steeper voltage dependence of gating than alpha 1C currents. Unlike alpha 1C, alpha 1A channels are largely insensitive to dihydropyridines and FPL 64176, but respond to the cone snail peptide omega-CTx-MVIIC(SNX-230), a potent and fairly selective inhibitor. In comparison with P-type Ca2+ channels in rat cerebellar Purkinje cells, alpha 1A channels in oocytes are approximately 10(2)-fold less sensitive to omega-Aga-IVA and approximately 10-fold more sensitive to omega-CTx-MVIIC. alpha 1A channels are not inhibited by Bay K 8644 and inactivate much more rapidly than P-type Ca2+ channels. Thus, alpha 1A is capable of generating a Ca2+ channel phenotype quite different from P-type current.

Transporters, channels and human disease

The past year has seen significant advances in our understanding of the molecular biology of ion channels and transporters and their role in human disease. The star of the show has to be the cystic fibrosis chloride channel about which an enormous amount of information has been accumulated, and the functional effects of some of the mutations found in cystic fibrosis patients have been characterized.

The molecular mode of action of the Ca agonist (-) BAY K 8644 on the cardiac Ca channel

The primary drug action of (-) BAY K 8644 on whole-cell Ca current in atrial myocytes was measured under conditions where secondary Ca-mediated changes of Ca channel activity were minimized. The most direct action of (-) BAY K 8644 is the change of gating kinetics which results in a strictly voltage-dependent increase of the peak current in the voltage range between -40 and 0 mV. Peak currents were increased dose dependently in the concentration range from 1 to 30 nM. Analysis of peak current/voltage relations revealed a linear shift of the current activation by approximately 23 mV to more negative membrane potentials, without any change in its voltage dependence and in the current reversal potential or the maximum whole-cell conductance. Measurement of Ca current activation and deactivation time constants suggests that (-) BAY K 8644 prolongs the single-channel open time without affecting the closed time. From the shift of the open time function to more negative voltages by about 50 mV the energy transferred to the gating process is calculated to be 5.4 kJ/mol (1.3 kcal/mol). The drug-induced slow component of tail current has been used to estimate the true dose/response relation for (-) BAY K 8644. A KD value of 4.3 nM and a Hill coefficient of 1.25 were determined. Flash-induced competition experiments with the Ca antagonist nifedipine allowed the measurement of binding kinetics of (-) BAY K 8644. The association rate constant is estimated to about 5 x 10(6) mol-1.s-1 and dissociation time constant is approximately 50-70 s; both are in close agreement with receptor binding studies. Results are discussed in relation to models for drug action of dihydropyridine-type compounds and to implications for the structure of the Ca channel protein.

Stimulatory effects of HSR-803 on ileal motor activity

Stimulatory effects of HSR-803 on intestinal motor activity in vitro were studied in guinea pig ileum. HSR-803 (1 x 10(-6)-1 x 10(-4) M) increased the amplitude of longitudinal muscle contractions and increased the frequency of peristalsis in isolated segments of guinea pig ileum. The stimulatory effect in amplitude and not frequency was abolished by 1 x 10(-6) M atropine. In the Magnus method with ileal segments, HSR-803 (1 x 10(-7) - 1 x 10(-4) M) produced contractions concentration-dependently, which were inhibited by atropine (1 x 10(-8) and 3 x 10(-8) M) and 3 x 10(-7) M tetrodotoxin (TTX). In the [3H]-quinuclidinyl benzilate (QNB) binding experiment with ileal smooth muscle, HSR-803 had low affinity for acetylcholine (ACh) receptors (pKi = 4.47 +/- 0.04). In addition, HSR-803 failed to increase the spontaneous release and the electrical stimulation-induced [3H]ACh release in ileal smooth muscle. On the other hand, HSR-803 (1 x 10(-5) M) enhanced contractions induced by ACh, but had no effect on contractions induced by carbachol, which is not hydrolyzed by acetylcholinesterase (AChE). In conclusion, HSR-803 stimulated ileal motor activity. However, HSR-803 had low affinity for ACh receptors and had no influence on ACh release. It is likely that HSR-803 stimulated motor activity mainly due to prevention of ACh hydrolysis.

Effect of elevated extracellular glucose concentrations on transmembrane calcium ion fluxes in cultured rat VSMC

Blood flow autoregulation is impaired in early diabetes mellitus, predisposing the renal microcirculation to injury. These hemodynamic changes have been strongly implicated in the development and progression of diabetic glomerulopathy. Blood flow autoregulation is predominantly a myogenic reflex which is strongly dependent on Ca2+ uptake by vascular smooth muscle cells (VSMC). Because impaired blood flow autoregulation may be responsive to glycemic control, the present study examined the effects of elevated extracellular glucose concentrations on basal, voltage sensitive and receptor operated Ca2+ uptake by VSMC. Confluent cultured rat VSMC were exposed to: (1) control medium (CM; 5 mM glucose); (2) high glucose medium (HGM; 10 to 30 mM glucose); or (3) osmotic control medium (OCM; glucose 5 mM + L-glucose 25 mM or mannitol 25 mM). A threshold glucose concentration of 15 mM markedly and maximally depressed basal Ca2+ uptake by VSMC (HGM 52% vs. CM). In addition, HGM significantly depressed voltage sensitive Ca2+ uptake by VSMC as determined by responses to BAY K 8644 (10(-7) M) or high extracellular [K+] (65 mM, HGM 50% vs. CM). HGM similarly depressed pressor hormone-stimulated Ca2+ uptake (AVP or Ang II 10(-7) M) by VSMC. The effects of HGM on Ca2+ uptake were time exposure dependent and reversible. Ca2+ uptake by VSMC in the presence of OCM did not differ from CM. Elevated extracellular glucose concentrations thus exert a direct and profound effect on basal, voltage sensitive and receptor operated Ca2+ uptake by VSMC. These observations may provide a biochemical basis for glucose-induced dysregulation of regional blood flow autoregulation in early diabetes mellitus.

Solution structure of omega-conotoxin GVIA using 2-D NMR spectroscopy and relaxation matrix analysis

We report here the solution structure of omega-conotoxin GVIA, a peptide antagonist of the N-type neuronal voltage-sensitive calcium channel. The structure was determined using two-dimensional NMR in combination with distance geometry and restrained molecular dynamics. The full relaxation matrix analysis program MARDIGRAS was used to generate maximum and minimum distance restraints from the crosspeak intensities in NOESY spectra. The 187 restraints obtained were used in conjunction with 23 angle restraints from vicinal coupling constants as input for the structure calculations. The backbones of the best 21 structures match with an average pairwise RMSD of 0.58 A. The structures contain a short segment of triple-stranded beta-sheet involving residues 6-8, 18-21, and 24-27, making this the smallest published peptide structure to contain a triple-stranded beta-sheet. Conotoxins have been shown to be effective neuroprotective agents in animal models of brain ischemia. Our results should aid in the design of novel nonpeptide compounds with potential therapeutic utility.

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.

Subunit identification and reconstitution of the N-type Ca2+ channel complex purified from brain

Calcium channels play an important role in regulating various neuronal processes, including synaptic transmission and cellular plasticity. The N-type calcium channels, which are sensitive to omega-conotoxin, are involved in the control of transmitter release from neurons. A functional N-type calcium channel complex was purified from rabbit brain. The channel consists of a 230-kilodalton subunit (alpha 1B) that is tightly associated with a 160-kilodalton subunit (alpha 2 delta), a 57-kilodalton subunit (beta 3), and a 95-kilodalton glycoprotein subunit. The complex formed a functional calcium channel with the same pharmacological properties and conductance as those of the native omega-conotoxin-sensitive calcium channel in neurons.

Dextromethorphan blocks N-methyl-D-aspartate-induced currents and voltage-operated inward currents in cultured cortical neurons

The effect of dextromethorphan on several types of cation currents in cultured rat cortical neurons and PC12 cells was studied by using the whole-cell configuration of the patch-clamp technique. The Ba2+ current through L- and N-type Ca2+ channels was blocked with similar potencies (52-71 microM) in both types of cells. The effect was not voltage-dependent, in contrast to that of amlodipine (a dihydropyridine). Dextromethorphan was able to block the Ba2+ current completely unlike amlodipine and omega-conotoxin (an N-type channel blocker) which produced only partial inhibition. The voltage-activated Na+ and Ca2+ channels in cortical neurons were inhibited by similar concentrations of dextromethorphan (IC50 approximately 80 microM). The morphinan was at least 100 times more potent (IC50 = 0.55 microM) as a blocker of the current induced by N-methyl-D-aspartate (NMDA) in cortical neurons. Currents induced by (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ((RS)-AMPA) or kainic acid were not significantly affected even at 1 mM. The results suggest that the neuroprotective effect of dextromethorphan, previously found to occur in a concentration range of 10-100 microM, may be due to a complete blockade of the NMDA receptor channel and a partial inhibition of voltage-dependent Ca2+ and Na+ channels.

Functional expression of a rapidly inactivating neuronal calcium channel

Diverse types of calcium channels in vertebrate neurons are important in linking electrical activity to transmitter release, gene expression and modulation of membrane excitability. Four classes of Ca2+ channels (T, N, L and P-type) have been distinguished on the basis of their electrophysiological and pharmacological properties. Most of the recently cloned Ca2+ channels fit within this functional classification. But one major branch of the Ca2+ channel gene family, including BII (ref. 15) and doe-1 (ref. 16), has not been functionally characterized. We report here the expression of doe-1 and show that it is a high-voltage-activated (HVA) Ca2+ channel that inactivates more rapidly than previously expressed calcium channels. Unlike L-type or P-type channels, doe-1 is not blocked by dihydropyridine antagonists or the peptide toxin omega-Aga-IVA, respectively. In contrast to a previously cloned N-type channel, doe-1 block by omega-CTx-GVIA requires micromolar toxin and is readily reversible. Unlike most HVA channels, doe-1 also shows unusual sensitivity to block by Ni2+. Thus, doe-1 is an HVA Ca2+ channel with novel functional properties. We have identified a Ca2+ channel current in rat cerebellar granule neurons that resembles doe-1 in many kinetic and pharmacological features.

Agelenopsis aperta venom and FTX, a purified toxin, inhibit acetylcholine release in Torpedo synaptosomes

The presence of P-type calcium channels in synaptosomes prepared from electric organ of Torpedo marmorata was investigated by using the venom of Agelenopsis aperta, a toxin purified from it, FTX, and its synthetic analog. We analysed the action of these agents on acetylcholine release which was continuously followed using a chemiluminescent assay. Agelenopsis aperta venom, FTX and synthetic FTX inhibit acetylcholine release from synaptosomes induced by a presynaptic membrane depolarization with 60 mM KCl. A stronger inhibition of acetylcholine release was observed with the venom than with FTX (70 and 50%, respectively). Another way of triggering acetylcholine release from Torpedo synaptosomes is to insert in the presynaptic membrane a calcium ionophore A23187 which allows the bypass of the natural calcium channels. The venom of Agelenopsis aperta inhibits A23187-evoked acetylcholine release. Purified and synthetic FTX does not possess this property, suggesting that this inhibition of acetylcholine release was due to other toxins of the venom. Another type of pharmacological sensitivity of Torpedo calcium channels was also demonstrated using omega-conotoxin GVIA. At a concentration of 20 microM, this toxin was able to inhibit about 35% of KCl-evoked acetylcholine release. When FTX + omega-conotoxin GVIA were applied together, the inhibitory effect on KCl-evoked acetylcholine release was not significantly increased in comparison with the one observed with FTX alone. In conclusion, we examined the effect of different agents on acetylcholine release from Torpedo marmorata electric organ synaptosomes; acetylcholine release was elicited with KCl depolarization and followed continuously with a chemiluminescent assay.(ABSTRACT TRUNCATED AT 250 WORDS)

Action and binding of omega-conotoxin on the putative calcium channel of synaptosomal plasma membrane from electric organ of Japanese electric ray, Narke japonica

Actions of omega-conotoxin GVIA on synaptosomes isolated from a Japanese electric ray, Narke japonica, were investigated. omega-Conotoxin inhibited, in a dose-dependent manner, both increases in free calcium concentration in, and acetylcholine release from synaptosomes depolarized with a high concentration of potassium ions. The concentrations of omega-conotoxin required for half-maximal inhibition (IC50) of increase in intrasynaptosomal Ca and acetylcholine release were 8 and 7 microM, respectively. Assay using radioiodinated toxin derivative revealed a specific binding site with a dissociation constant (KD) of 2.8 microM and a density (Bmax) of 45 pmol/mg protein of synaptosome. Binding assay with synaptosomal plasma membrane showed a KD = 7 microM and a Bmax = 200 pmol/mg protein. Autoradiography with sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis after covalent cross-linking of the toxin, using disuccinimidyl suberate, revealed the 170,000 mol. wt peptide to be an omega-conotoxin receptor. The present study has directly and clearly shown that omega-conotoxin inhibits acetylcholine release by blocking Ca influx into nerve terminals. The 170,000 mol. wt peptide identified as a receptor of the toxin exists in high density in the plasma membrane of the presynaptic nerve terminal and is likely to be a component of a voltage-dependent Ca channel responsible for the neurotransmitter release.

Two high-voltage-activated, dihydropyridine-sensitive Ca2+ channel currents with distinct electrophysiological and pharmacological properties in cultured rat aortic myocytes

In smooth muscle cells, essentially two distinct types of voltage-gated Ca2+ channels have been shown, on the basis of their distinct electrophysiological and pharmacological properties, to coexist. Here we report that, in addition to a dihydropyridine (DHP)-sensitive, low-voltage-activated Ba2+ current (IBa,LVA), two types of high-voltage-activated Ba2+ currents with distinct waveforms were recorded in whole-cell clamped aortic myocytes; these were referred to as IBa,HVA1 and IBa,HVA2. They were investigated in cells where no IBa,LVA was detectable. IBa,HVA1 had a slow, monoexponential decay. In contrast, the decay of IBa,HVA2 was much faster and biexponential. In addition, IBa,HVA2 had more negative ranges of activation and steady-state inactivation than IBa,HVA1 and was more sensitive to the DHP antagonist nicardipine (concentrations for half maximum inhibition 0.2 microM and 2 microM, respectively). When using the physiological ion Ca2+ as the charge carrier, the decay of HVA1 currents was not altered, whereas both time constants of HVA2 current decay were accelerated five-fold. Moreover, permeability ratios (ICa/IBa) were also significantly different (0.2 and 0.6 for HVA1 and HVA2 respectively). IBa,HVA1 and IBa,HVA2 are consistent either with the existence and activation of two functionally distinct subtypes of the so-called "DHP-sensitive L-type" Ca2+ channel or with different gating behaviours of a single type of channel. Potentially, they may serve distinct biological functions and constitute distinct targets for neurotransmitters and drugs.

Altered calcium signaling and neuronal injury: stroke and Alzheimer's disease as examples

Several cellular signaling systems have been implicated in the neuronal death that occurs both in development ("natural" cell death) or in pathological conditions such as stroke and Alzheimer's disease (AD). Here we consider the possibility that neuronal degeneration in an array of disorders including stroke and AD arises from one or more alterations in calcium-regulating systems that result in a loss of cellular calcium homeostasis. A long-standing hypothesis of neuronal injury, the excitatory amino acid (EAA) hypothesis, is revisited in light of new supportive data concerning the roles of EAAs in stroke and the neurofibrillary degeneration in AD. Two quite new concepts concerning mechanisms of neuronal injury and death are presented, namely: 1) growth factors normally "stabilize" intracellular free calcium levels ([Ca2+]i) and protect neurons against ischemic/excitotoxic injury, and 2) aberrant processing of beta-amyloid precursor protein (APP) can cause neurodegeneration by impairing a neuroprotective function of secreted forms of APP (APPs) which normally regulate [Ca2+]i. Altered APP processing also results in the accumulation of beta-amyloid peptide which contributes to neuronal damage by destabilizing calcium homeostasis; in AD beta-amyloid peptide may render neurons vulnerable to excitotoxic conditions that accrue with increasing age (e.g., altered glucose metabolism, ischemia). Growth factors may normally protect neurons against the potentially damaging effects of calcium influx resulting from energy deprivation and overexcitation. For example, bFGF, NGF and IGFs can protect neurons from several brain regions against excitotoxic/ischemic insults. Growth factors apparently stabilize [Ca2+]i by several means including: a reduction in calcium influx; enhanced calcium extrusion or buffering; and maintenance or improvement of mitochondrial function. For example, bFGF can suppress the expression of a N-methyl-D-aspartate (NMDA) receptor protein that mediates excitotoxic damage in hippocampal neurons. Growth factors may also prevent the loss of neuronal calcium homeostasis and the increased vulnerability to neuronal injury caused by beta-amyloid peptide. Since elevated [Ca2+]i can elicit cytoskeletal alterations similar to those seen in AD neurofibrillary tangles, we propose that neuronal damage in AD results from a loss of calcium homeostasis. The data indicate that a variety of alterations in [Ca2+]i regulation may contribute to the neuronal damage in stroke and AD, and suggest possible means of preventing neuronal damage in these disorders.

Structure and functional expression of a member of the low voltage-activated calcium channel family

Oscillatory firing patterns are an intrinsic property of some neurons and have an important function in information processing. In some cells, low voltage-activated calcium channels have been proposed to underlie a depolarizing potential that regulates bursting. The sequence of a rat brain calcium channel alpha 1 subunit (rbE-II) was deduced. Although it is structurally related to high voltage-activated calcium channels, the rbE-II channel transiently activated at negative membrane potentials, required a strong hyperpolarization to deinactivate, and was highly sensitive to block by nickel. In situ hybridization showed that rbE-II messenger RNA is expressed in regions throughout the central nervous system. The electrophysiological properties of the rbE-II current are consistent with a type of low voltage-activated calcium channel that requires membrane hyperpolarization for maximal activity, which suggests that rbE-II may be involved in the modulation of firing patterns.

Molecular diversity of Ca2+ channel alpha 1 subunits from the marine ray Discopyge ommata

In many neurons, transmitter release from presynaptic terminals is triggered by Ca2+ entry via dihydropyridine-insensitive Ca2+ channels. We have looked for cDNAs for such channels in the nervous system of the marine ray Discopyge ommata. One cDNA (doe-2) is similar to dihydropyridine-sensitive L-type channels, and two cDNAs (doe-1 and doe-4) are similar to the subfamily of dihydropyridine-insensitive non-L-type channels. doe-4, which encodes a protein of 2326 aa, most closely resembles a previously cloned N-type channel. doe-1, which encodes a protein of 2223 aa, is a member of a separate branch of the non-L-type channels. Northern blot analysis reveals that doe-1 is abundant in the forebrain. doe-4 is more plentiful in the electric lobe and, therefore, may control neurotransmitter release in motor nerve terminals. These results show that the familial pattern of Ca(2+)-channel genes has been preserved from a stage in evolution before the divergence of higher and lower vertebrates > 400 million years ago. The cloning of these channels may be a useful starting point for elucidating the role of the Ca2+ channels in excitation-secretion coupling in nerve terminals.

GABAB receptor inhibition of P-type Ca2+ channels in central neurons

P-type Ca2+ channels in cerebellar Purkinje neurons were inhibited by GABA and the GABAB receptor agonist baclofen. Inhibition of P-type Ca2+ channel current involved changes in voltage dependence and kinetics. Baclofen induced a slow phase of activation and altered tail current kinetics, and inhibition could be partly overcome by large depolarizations. These effects were mimicked by internal application of GTP gamma S, which also made the action of baclofen irreversible. In spinal cord neurons, use of selective channel blockers showed that baclofen inhibited both P-type and N-type Ca2+ channels, but not L-type Ca2+ channels; a high threshold current resistant to blockers of P-type, N-type, and L-type channels was also modulated by baclofen. These results show that stimulation of GABAB receptors in central neurons can modulate P-type Ca2+ channels through a G protein-mediated mechanism similar to the one linked to N-type Ca2+ channels.

Cyclic AMP-dependent regulation of P-type calcium channels expressed in Xenopus oocytes

Xenopus oocytes injected with rat cerebellum mRNA, express voltage-dependent calcium channels (VDCC). These were identified as P-type Ca2+ channels by their insensitivity to dihydropyridines and omega-conotoxin and by their blockade by Agelenopsis aperta venom (containing the funnel-web spider toxins: FTX and omega-Aga-IV-A). Coinjection of cerebellar mRNA and antisense oligonucleotide complementary to the dihydropyridine-resistant brain Ca2+ channel, named BI [Mori Y. et al. (1991) Nature 350:398-402] or rbA [Starr T. V. B. et al. (1991) Proc Natl Acad Sci USA 88:5621-5625], strongly reduced the expressed Ba2+ current suggesting that these clones encode a P-type VDCC. The macroscopic Ca2+ channel activity was increased by direct intraoocyte injection of cAMP. This increase in current amplitude was concomitant with a slowing of current inactivation, and was attributed to activation of protein kinase A, since it could be antagonized by a peptidic inhibitor of this enzyme. Positive regulation of P-type VDCC could be of importance in Purkinje neurons and motor nerve terminals where this channel is predominant.

Primary structure and functional expression of the omega-conotoxin-sensitive N-type calcium channel from rabbit brain

The complete amino acid sequence of a rabbit brain calcium channel (BIII) has been deduced by cloning and sequencing the cDNA. The open reading frame encodes 2339 amino acids, which corresponds to an M(r) of 261,167. A phylogenetic tree representing evolutionary relationships indicates that BIII is grouped together with the other rabbit brain calcium channels, BI and BII, into a subfamily that is distinct from the dihydropyridine-sensitive L-type subfamily. Transient expression in cultured skeletal muscle myotubes derived from muscular dysgenic mice demonstrates that the BIII channel mediates an omega-conotoxin-sensitive calcium current with kinetics and voltage dependence like those previously reported for whole-cell N-type current. Cell-attached patch recordings, with isotonic barium as the charge carrier, revealed distinct single channels with an average slope conductance of 14.3 pS.