Molecular diversity of voltage-dependent Ca2+ channels

Functional analysis of calcium channel-mediated exocytosis in synaptic terminals by FM imaging technique

OBJECTIVE

Presynaptic voltage-gated Ca(2+) channels mediate rapid Ca(2+) influx into the synaptic terminal which triggers synaptic vesicle exocytosis and neurotransmitter release. The FM 1-43 dye was firstly introduced as a fluorescence probe by Betz and his colleagues in 1992, and has been used to monitor exocytosis, endocytosis and endosomal traffic in a variety of cell types. The present study aims to investigate the feasibility of applying the FM 1-43 dye in the functional analysis of calcium channel-mediated exocytosis in synaptic terminals.

METHODS

The hippocampi were isolated from embryos of pregnant rats, and hippocampal neurons were then transfected with Ds-Red conjugated plasmid. The neurons were then loaded with 8 micromol/L FM 1-43 and 47 mmol/L KCl for 90 s after transfection. After that, 90 mmol/L KCl was employed to induce FM dye destaining, which was recorded by FM imaging system.

RESULTS

The neuron synaptic terminals of rat hippocampus could be effectively stained by the FM 1-43 dye. Besides, the destaining of the labeled neuron terminals was in accordance with the transmitter release, which could be blocked by the application of nifedipine (inhibitor for L-type calcium channel).

CONCLUSION

The FM imaging technique is an advanced and effective method for analyzing synaptic vesicle exocytosis and neurotransmitter release, and can be applied in various synaptic functional studies.

Targeting voltage-gated calcium channels for neuropathic pain management

Voltage-gated calcium channels (VGCC) play obligatory roles in diverse physiological functions. Pathological conditions leading to changes in their biophysical properties and expression levels may cause malfunctions of VGCC-mediated activities, resulting in disease states. It is believed that changes in VGCC properties under pain-inducing conditions may play a causal role in the development of chronic pain, including nerve injury-induced pain or neuropathic pain. For the past several decades, preclinical and clinical research in developing VGCC blockers or modulators for chronic pain management has been fruitful, leading to some U.S. Food and Drug Administration-approved drugs currently available for chronic pain management. However, their efficacy in pain relief is limited in some patients, and their long-term use is limited by their side-effect profiles. Certainly, there is room for improvement in developing more subtype-specific VGCC blockers or modulators for chronic pain conditions. In this review, we summarized the most recent preclinical and clinical studies related to chronic pain medications acting on the VGCC. We also included clinical trials aiming to expand the application of approved VGCC drugs to different pain states derived from various pathological conditions, as well as drug combination therapies trying to improve the efficacies and side-effect profiles of current pain medications.

Respiratory circuits: function, mechanisms, topology, and pathology

Neuroscientists have long sought to understand how circuits in the nervous system are organized to generate the precise neural outputs that underlie particular behaviors. Recent studies deepened our understanding of the mechanisms responsible for the generation of the rhythmic output for breathing. Here, the author focuses on issues that are pertinent for the respiratory network and considers its organization and how it derives the functional output. The author discusses pacemaker and network mechanisms of rhythm generation, which are now combined into a novel concept of emergent network activity due to coherent excitation of pacemaker groups. He discusses subcellular basis of this hypothesis and possible mechanisms of synchronization within respiratory network. These new findings in respiratory neuroscience are further applied to explain modifications in breathing during hypoxia and possible origins of respiratory disorders that may be acquired during neural development and aging.

Three-dimensional structure of CaV3.1: comparison with the cardiac L-type voltage-gated calcium channel monomer architecture

Calcium entry through voltage-gated calcium channels has widespread cellular effects upon a host of physiological processes including neuronal excitability, muscle excitation-contraction coupling, and secretion. Using single particle analysis methods, we have determined the first three-dimensional structure, at 23 A resolution, for a member of the low voltage-activated voltage-gated calcium channel family, CaV3.1, a T-type channel. CaV3.1 has dimensions of approximately 115x85x95 A, composed of two distinct segments. The cytoplasmic densities form a vestibule below the transmembrane domain with the C terminus, unambiguously identified by the presence of a His tag being approximately 65 A long and curling around the base of the structure. The cytoplasmic assembly has a large exposed surface area that may serve as a signaling hub with the C terminus acting as a "fishing rod" to bind regulatory proteins. We have also determined a three-dimensional structure, at a resolution of 25 A, for the monomeric form of the cardiac L-type voltage-gated calcium (high voltage-activated) channel with accessory proteins beta and alpha2delta bound to the ion channel polypeptide CaV1.2. Comparison with the skeletal muscle isoform finds a good match particularly with respect to the conformation, size, and shape of the domain identified as that formed by alpha2. Furthermore, modeling of the CaV3.1 structure (analogous to CaV1.2 at these resolutions) into the heteromeric L-type voltage-gated calcium channel complex volume reveals multiple interaction sites for beta-CaV1.2 binding and for the first time identifies the size and organization of the alpha2delta polypeptides.

Characterization of voltage-gated Ca(2+) conductances in layer 5 neocortical pyramidal neurons from rats

Neuronal voltage-gated Ca(2+) channels are involved in electrical signalling and in converting these signals into cytoplasmic calcium changes. One important function of voltage-gated Ca(2+) channels is generating regenerative dendritic Ca(2+) spikes. However, the Ca(2+) dependent mechanisms used to create these spikes are only partially understood. To start investigating this mechanism, we set out to kinetically and pharmacologically identify the sub-types of somatic voltage-gated Ca(2+) channels in pyramidal neurons from layer 5 of rat somatosensory cortex, using the nucleated configuration of the patch-clamp technique. The activation kinetics of the total Ba(2+) current revealed conductance activation only at medium and high voltages suggesting that T-type calcium channels were not present in the patches. Steady-state inactivation protocols in combination with pharmacology revealed the expression of R-type channels. Furthermore, pharmacological experiments identified 5 voltage-gated Ca(2+) channel sub-types - L-, N-, R- and P/Q-type. Finally, the activation of the Ca(2+) conductances was examined using physiologically derived voltage-clamp protocols including a calcium spike protocol and a mock back-propagating action potential (mBPAP) protocol. These experiments enable us to suggest the possible contribution of the five Ca(2+) channel sub-types to Ca(2+) current flow during activation under physiological conditions.

Membrane signalling complexes: implications for development of functionally selective ligands modulating heptahelical receptor signalling

Technological development has considerably changed the way in which we evaluate drug efficacy and has led to a conceptual revolution in pharmacological theory. In particular, molecular resolution assays have revealed that heptahelical receptors may adopt multiple active conformations with unique signalling properties. It is therefore becoming widely accepted that ligand ability to stabilize receptor conformations with distinct signalling profiles may allow to direct the stimulus generated by an activated receptor towards a specific signalling pathway. This capacity to induce only a subset of the ensemble of responses regulated by a given receptor has been termed "functional selectivity" (or "stimulus trafficking"), and provides the bases for a highly specific regulation of receptor signalling. Concomitant with these observations, heptahelical receptors have been shown to associate with G proteins and effectors to form multimeric arrays. These complexes are constitutively formed during protein synthesis and are targeted to the cell surface as integral signalling units. Herein we summarize evidence supporting the existence of such constitutive signalling arrays and analyze the possibility that they may constitute viable targets for developing ligands with "functional selectivity".

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

Activation of ionotropic γ-aminobutyric acid type A (GABAA) receptors depolarizes neurons that have high intracellular [Cl−], causing inhibition or excitation in different cell types. The depolarization often leads to inactivation of voltage-gated Na channels, but additional ionic mechanisms may also be affected. Previously, a simulated model of spider VS-3 mechanosensory neurons suggested that although voltage-activated Na+current is partially inactivated during GABA-induced depolarization, a slowly activating and inactivating component remains and may contribute to the depolarization. Here, we confirmed experimentally, by blocking Na channels prior to GABA application, that Na+current contributes to GABA-induced depolarization in VS-3 neurons. Ratiometric Ca2+imaging experiments combined with intracellular recordings revealed a significant increase in intracellular [Ca2+] when GABAAreceptors were activated, synchronous with the depolarization and probably due to Ca2+influx via low-voltage–activated (LVA) Ca channels. In contrast, GABAB-receptor activation in these neurons was previously shown to inhibit LVA current. Blockade of voltage-gated K channels delayed membrane repolarization, extending GABA-induced depolarization. However, inhibition of Ca channels significantly increased the amplitude of GABA-induced depolarization, indicating that Ca2+-activated K+current has an even stronger repolarizing effect. Regulation of intracellular [Ca2+] is important for many cellular processes and Ca2+control of K+currents may be particularly important for some functions of mechanosensory neurons, such as frequency tuning. These data show that GABAA-receptor activation participates in this regulation.

Multiple conductances cooperatively regulate spontaneous bursting in mouse olfactory bulb external tufted cells

External tufted (ET) cells are juxtaglomerular neurons that spontaneously generate bursts of action potentials, which persist when fast synaptic transmission is blocked. The intrinsic mechanism of this autonomous bursting is unknown. We identified a set of voltage-dependent conductances that cooperatively regulate spontaneous bursting: hyperpolarization-activated inward current (I(h)), persistent Na+ current (I(NaP)), low-voltage-activated calcium current (I(L/T)) mediated by T- and/or L-type Ca2+ channels, and large-conductance Ca2+-dependent K+ current (I(BK)). I(h) is important in setting membrane potential and depolarizes the cell toward the threshold of I(NaP) and I(T/L), which are essential to generate the depolarizing envelope that is crowned by a burst of action potentials. Action potentials depolarize the membrane and induce Ca2+ influx via high-voltage-activated Ca2+ channels (I(HVA)). The combined depolarization and increased intracellular Ca2+ activates I(BK), which terminates the burst by hyperpolarizing the membrane. Hyperpolarization activates I(h) and the cycle is regenerated. A novel finding is the role of L-type Ca2+ channels in autonomous ET cells bursting. A second novel feature is the role of BK channels, which regulate burst duration. I(L) and I(BK) may go hand-in-hand, the slow inactivation of I(L) requiring I(BK)-dependent hyperpolarization to deactivate inward conductances and terminate the burst. ET cells receive monosynaptic olfactory nerve input and drive the major inhibitory interneurons of the glomerular circuit. Modulation of the conductances identified here can regulate burst frequency, duration, and spikes per burst in ET cells and thus significantly shape the impact of glomerular circuits on mitral and tufted cells, the output channels of the olfactory bulb.

Microtubule-associated protein 2-positive cells derived from microglia possess properties of functional neurons

Microglia are believed to play an important role in the regulation of phagocytosis, neuronal survival, neuronal cell death, and inflammation. Recent studies have demonstrated that microglia are multipotential stem cells that give rise to neurons, astrocytes, and oligodendrocytes. However, the functional properties of neurons derived from microglia are poorly understood. In this study, we investigated the possibility that microglia differentiate into functional neurons. Immunocytochemical study demonstrated that microtubule-associated protein 2 (MAP2)-positive cells were derived from microglia under differentiation conditions. Intracellular Ca(2+) imaging study demonstrated that KCl caused no significant changes in [Ca(2+)](i) in microglia, whereas it caused a remarkable increase in [Ca(2+)](i) in microglia-derived cells. Furthermore, electrophysiological study demonstrated that the spike waveform, firing rate, and tetrodotoxin sensitivity of extracellular action potentials evoked by 4-aminopyridine from microglia-derived MAP2-positive cells were nearly identical to those from cultured cortical neurons. These results suggest that microglia-derived MAP2-positive cells possess properties of functional neurons.

The leaner P/Q-type calcium channel mutation renders cerebellar Purkinje neurons hyper-excitable and eliminates Ca2+-Na+ spike bursts

The leaner mouse mutation of the Cacna1a gene leads to a reduction in P-type Ca2+ current, the dominant Ca2+ current in Purkinje cells (PCs). Here, we compare the electro-responsiveness and structure of PCs from age-matched leaner and wild-type (WT) mice in pharmacological isolation from synaptic inputs in cerebellar slices. We report that compared with WT, leaner PCs exhibit lower current threshold for Na+ spike firing, larger subthreshold membrane depolarization, rapid adaptation followed by complete block of Na+ spikes upon strong depolarization, and fail to generate Ca2+-Na+ spike bursts. The Na+ spike waveforms in leaner PCs have slower kinetics, reduced spike amplitude and afterhyperpolarization. We show that a deficit in the P-type Ca2+ current caused by the leaner mutation accounts for most but not all of the changes in mutant PC electro-responsiveness. The selective P-type Ca2+ channel blocker, omega-agatoxin-IVA, eliminated differences in subthreshold membrane depolarization, adaptation of Na+ spikes upon strong current-pulse stimuli, Na+ spike waveforms and Ca2+-Na+ burst activity. In contrast, a lower current threshold for eliciting repetitive Na+ spikes in leaner PCs was still observed after blockade of the P-type Ca2+ current, suggesting secondary effects of the mutation that render PCs hyper-excitable. Higher input resistance, reduced whole-cell capacitance and smaller dendritic size accompanied the enhanced excitability in leaner PCs, indicative of developmental retardation in these cells caused by P/Q-type Ca2+ channel malfunction. Our data indicate that a deficit in P-type Ca2+ current leads to complex functional and structural changes in PCs, impairing their intrinsic and integrative properties.

Loss of skeletal muscle strength by ablation of the sarcoplasmic reticulum protein JP45

Skeletal muscle constitutes approximately 40% of the human body mass, and alterations in muscle mass and strength may result in physical disability. Therefore, the elucidation of the factors responsible for muscle force development is of paramount importance. Excitation-contraction coupling (ECC) is a process during which the skeletal muscle surface membrane is depolarized, causing a transient release of calcium from the sarcoplasmic reticulum that activates the contractile proteins. The ECC machinery is complex, and the functional role of many of its protein components remains elusive. This study demonstrates that deletion of the gene encoding the sarcoplasmic reticulum protein JP45 results in decreased muscle strength in young mice. Specifically, this loss of muscle strength in JP45 knockout mice is caused by decreased functional expression of the voltage-dependent Ca(2+) channel Ca(v)1.1, which is the molecule that couples membrane depolarization and calcium release from the sarcoplasmic reticulum. These results point to JP45 as one of the molecules involved in the development or maintenance of skeletal muscle strength.

Forward genetic screen of mouse reveals dominant missense mutation in the P/Q-type voltage-dependent calcium channel, CACNA1A

Dominant mutations of the P/Q-type Ca(2+) channel (CACNA1A) underlie several human neurological disorders, including episodic ataxia type 2, familial hemiplegic migraine 1 (FHM1) and spinocerebellar ataxia 6, but have not been found previously in the mouse. Here we report the first dominant ataxic mouse model of Cacna1a mutation. This Wobbly mutant allele of Cacna1a was identified in an ethylnitrosourea (ENU) mutagenesis dominant behavioral screen. Heterozygotes exhibit ataxia from 3 weeks of age and have a normal life span. Homozygotes have a righting reflex defect from postnatal day 8 and later develop severe ataxia and die prematurely. Both heterozygotes and homozygotes exhibit cerebellar atrophy with focal reduction of the molecular layer. No obvious loss of Purkinje cells or decrease in size of the granule cell layer was observed. Real-time polymerase chain reaction revealed altered expression levels of Cacna1g, Calb2 and Th in Wobbly cerebella, but Cacna1a messenger RNA and protein levels were unchanged. Positional cloning revealed that Wobbly mice have a missense mutation leading to an arginine to leucine (R1255L) substitution, resulting in neutralization of a positively charged amino acid in repeat III of voltage sensor segment S4. The dominance of the Wobbly mutation more closely resembles patterns of CACNA1A mutation in humans than previously described mouse recessive mutants (tottering, leaner, rolling Nagoya and rocker). Positive-charge neutralization in S4 has also been shown to underlie several cases of human dominant FHM1 with ataxia. The Wobbly mutant thus highlights the importance of the voltage sensor and provides a starting point to unravel the neuropathological mechanisms of this disease.

Functional interaction of neuronal Cav1.3 L-type calcium channel with ryanodine receptor type 2 in the rat hippocampus

Neuronal L-type Ca(2+) channels do not support synaptic transmission, but they play an essential role in synaptic activity-dependent gene expression. Ca(v)1.2 and Ca(v)1.3 are the two most widely expressed L-type Ca(2+) channels in neurons and have different biophysical and subcellular distributions. The function of the Ca(v) 1.3 L-type Ca(2+) channel and its cellular mechanisms in the central nervous system are poorly understood. In this study, using a yeast two-hybrid assay, we found that the N terminus of the rat Ca(v)1.3 alpha(1) subunit interacts with a partial N-terminal amino acid sequence of ryanodine receptor type 2 (RyR2). Reverse transcription-PCR and Western blot assays revealed high expression of both Ca(v)1.3 and RyR2 in the rat hippocampus. We also demonstrate a physical association of Ca(v)1.3 with RyR2 using co-immunoprecipitation assays. Moreover, immunocytochemistry revealed prominent co-localization between Ca(v)1.3 and RyR2 in hippocampal neurons. Depolarizing cells by an acute treatment of a high concentration of KCl (high-K, 60 mm) showed that the activation of L-type Ca(2+) channels induced RyR opening and led to RyR-dependent Ca(2+) release, even in the absence of extracellular Ca(2+). Furthermore, we found that RyR2 mRNA itself is increased by long term treatment of high-K via activation of L-type Ca(2+) channels. These acute and long term effects of high-K on RyRs were selectively blocked by small interfering RNA-mediated silencing of Ca(v)1.3. These results suggest a physical and functional interaction between Ca(v)1.3 and RyR2 and important implications of Ca(v)1.3/RyR2 clusters in translating synaptic activity into alterations in gene expression.

The sequence of events that underlie quantal transmission at central glutamatergic synapses

The properties of synaptic transmission were first elucidated at the neuromuscular junction. More recent work has examined transmission at synapses within the brain. Here we review the remarkable progress in understanding the biophysical and molecular basis of the sequential steps in this process. These steps include the elevation of Ca2+ in microdomains of the presynaptic terminal, the diffusion of transmitter through the fusion pore into the synaptic cleft and the activation of postsynaptic receptors. The results give insight into the factors that control the precision of quantal transmission and provide a framework for understanding synaptic plasticity.

Mutation associated with an autosomal dominant cone-rod dystrophy CORD7 modifies RIM1-mediated modulation of voltage-dependent Ca2+ channels

Genetic analyses have revealed an association between the gene encoding the Rab3A-interacting molecule (RIM1) and the autosomal dominant cone-rod dystrophy CORD7. However, the pathogenesis of CORD7 remains unclear. We recently revealed that RIM1 regulates voltage-dependent Ca(2+) channel (VDCC) currents and anchors neurotransmitter-containing vesicles to VDCCs, thereby controlling neurotransmitter release. We demonstrate here that the mouse RIM1 arginine-to-histidine substitution (R655H), which corresponds to the human CORD7 mutation, modifies RIM1 function in regulating VDCC currents elicited by the P/Q-type Ca(v)2.1 and L-type Ca(v)1.4 channels. Thus, our data can raise an interesting possibility that CORD7 phenotypes including retinal deficits and enhanced cognition are at least partly due to altered regulation of presynaptic VDCC currents.

RIM1 confers sustained activity and neurotransmitter vesicle anchoring to presynaptic Ca2+ channels

The molecular organization of presynaptic active zones is important for the neurotransmitter release that is triggered by depolarization-induced Ca2+ influx. Here, we demonstrate a previously unknown interaction between two components of the presynaptic active zone, RIM1 and voltage-dependent Ca2+ channels (VDCCs), that controls neurotransmitter release in mammalian neurons. RIM1 associated with VDCC beta-subunits via its C terminus to markedly suppress voltage-dependent inactivation among different neuronal VDCCs. Consistently, in pheochromocytoma neuroendocrine PC12 cells, acetylcholine release was significantly potentiated by the full-length and C-terminal RIM1 constructs, but membrane docking of vesicles was enhanced only by the full-length RIM1. The beta construct beta-AID dominant negative, which disrupts the RIM1-beta association, accelerated the inactivation of native VDCC currents, suppressed vesicle docking and acetylcholine release in PC12 cells, and inhibited glutamate release in cultured cerebellar neurons. Thus, RIM1 association with beta in the presynaptic active zone supports release via two distinct mechanisms: sustaining Ca2+ influx through inhibition of channel inactivation, and anchoring neurotransmitter-containing vesicles in the vicinity of VDCCs.

Selectivity signatures of three isoforms of recombinant T-type Ca2+ channels

Voltage-gated Ca(2+) channels (VGCCs) are recognized for their superb ability for the preferred passage of Ca(2+) over any other more abundant cation present in the physiological saline. Most of our knowledge about the mechanisms of selective Ca(2+) permeation through VGCCs was derived from the studies on native and recombinant L-type representatives. However, the specifics of the selectivity and permeation of known recombinant T-type Ca(2+)-channel alpha1 subunits, Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3, are still poorly defined. In the present study we provide comparative analysis of the selectivity and permeation Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3 functionally expressed in Xenopus oocytes. Our data show that all Ca(v)3 channels select Ca(2+) over Na(+) by affinity. Ca(v)3.1 and Ca(v)3.2 discriminate Ca(2+), Sr(2+) and Ba(2+) based on the ion's effects on the open channel probability, whilst Ca(v)3.3 discriminates based on the ion's intrapore binding affinity. All Ca(v)3s were characterized by much smaller difference in the K(D) values for Na(+) current blockade by Ca(2+) (K(D1) approximately 6 microM) and for Ca(2+) current saturation (K(D2) approximately 2 mM) as compared to L-type channels. This enabled them to carry notable mixed Na(+)/Ca(2+) current at close to physiological Ca(2+) concentrations, which was the strongest for Ca(v)3.3, smaller for Ca(v)3.2 and the smallest for Ca(v)3.1. In addition to intrapore Ca(2+) binding site(s) Ca(v)3.2, but not Ca(v)3.1 and Ca(v)3.3, is likely to possess an extracellular Ca(2+) binding site that controls channel permeation. Our results provide novel functional tests for identifying subunits responsible for T-type Ca(2+) current in native cells.

Acetylcholine release at neuromuscular junctions of adult tottering mice is controlled by N-(cav2.2) and R-type (cav2.3) but not L-type (cav1.2) Ca2+ channels

The mutation in the alpha(1A) subunit gene of the P/Q-type (Ca(v)2.1) Ca(2+) channel present in tottering (tg) mice causes ataxia and motor seizures that resemble absence epilepsy in humans. P/Q-type Ca(2+)channels are primarily involved in acetylcholine (ACh) release at mammalian neuromuscular junctions. Unmasking of L-type (Ca(v)1.1-1.2) Ca(2+) channels occurs in cerebellar Purkinje cells of tg mice. However, whether L-type Ca(2+) channels are also up-regulated at neuromuscular junctions of tg mice is unknown. We characterized thoroughly the pharmacological sensitivity of the Ca(2+) channels, which control ACh release at adult tg neuromuscular junctions. Block of N- and R-type (Ca(v)2.2-2.3), but not L-type Ca(2+) channels, significantly reduced quantal content of end-plate potentials in tg preparations. Neither resting nor KCl-evoked miniature end-plate potential frequency differed significantly between tg and wild type (WT). Immunolabeling of Ca(2+) channel subunits alpha(1A), alpha(1B), alpha(1C), and alpha(1E) revealed an apparent increase of alpha(1B), and alpha(1E) staining, at tg but not WT neuromuscular junctions. This presumably compensates for the deficit of P/Q-type Ca(2+)channels, which localized presynaptically at WT neuromuscular junctions. No alpha(1C) subunits juxtaposed with pre- or postsynaptic markers at either WT or tg neuromuscular junctions. Thus, in adult tg mice, immunocytochemical and electrophysiological data indicate that N- and R-type channels both assume control of ACh release at motor nerve terminals. Recruitment of alternate subtypes of Ca(2+) channels to control transmitter release seems to represent a commonly occurring method of neuronal plasticity. However, it is unclear which conditions underlie recruitment of Ca(v)2 as opposed to Ca(v)1-type Ca(2+) channels.

Direct G protein modulation of Cav2 calcium channels

The regulation of presynaptic, voltage-gated calcium channels by activation of heptahelical G protein-coupled receptors exerts a crucial influence on presynaptic calcium entry and hence on neurotransmitter release. Receptor activation subjects presynaptic N- and P/Q-type calcium channels to a rapid, membrane-delimited inhibition-mediated by direct, voltage-dependent interactions between G protein betagamma subunits and the channels-and to a slower, voltage-independent modulation involving soluble second messenger molecules. In turn, the direct inhibition of the channels is regulated as a function of many factors, including channel subtype, ancillary calcium channel subunits, and the types of G proteins and G protein regulatory factors involved. Twenty-five years after this mode of physiological regulation was first described, we review the investigations that have led to our current understanding of its molecular mechanisms.

Calcium-channel modulators for cardiovascular disease

It is generally accepted that hypertension doubles the risk of cardiovascular disease, of which coronary heart disease is the most common and lethal. Hypertension is a predisposing factor for the development of stroke, peripheral arterial disease, heart failure and end-state renal disease. Atherosclerosis-causing coronary heart disease is related to the severity of hypertension. Inhibition of calcium entry reduces the active tone of vascular smooth muscle and produces vasodilatation. This pharmacological action has been the basis for the use of calcium-channel blockers (CCBs) for the management of hypertension. Other drug families may achieve this: diuretics, beta-blockers, angiotensin-converting enzyme inhibitors, angiotensin-receptor antagonists. Cardiovascular hypertrophy and atherosclerosis are major complications related to high blood pressure. Cardiac hypertrophy is considered as an independent risk factor associated with abnormalities of diastolic function and can result in heart failure. Atherosclerosis is associated with activation of innate immunity. Atherosclerosis is expressing itself not only as coronary heart disease, but as a cerebrovascular and peripheral arterial disease. By impairing physiological vasomotor function, atherosclerosis includes ultimately necrosis of myocardium. CCBs reduce blood pressure. Do they prevent the progress of the main complications of hypertension? This major question is the matter of the present paper.

Expression pattern of voltage-dependent calcium channel subunits in hippocampal inhibitory neurons in mice

Different subtypes of voltage-dependent calcium channels (VDCCs) generate various types of calcium currents that play important role in neurotransmitter release, membrane excitability, calcium transients and gene expression. Well-established differences in the physiological properties and variable sensitivity of hippocampal GABAergic inhibitory neurons to excitotoxic insults suggest that the calcium homeostasis, thus VDCC subunits expression pattern is likely different in subclasses of inhibitory cells. Using double-immunohistochemistry, here we report that in mice: 1) Cav2.1 and Cav3.1 subunits are expressed in almost all inhibitory neurons; 2) subunits responsible for the L-type calcium current (Cav1.2 and Cav1.3) are infrequently co-localized with calretinin inhibitory cell marker while Cav1.3 subunit, at least in part, tends to compensate for the low expression of Cav1.2 subunit in parvalbumin-, metabotropic glutamate receptor 1alpha- and somatostatin-immunopositive inhibitory neurons; 3) Cav2.2 subunit is expressed in the majority of inhibitory neurons except in calbindin-reactive inhibitory cells; 4) Cav2.3 subunit is expressed in the vast majority of the inhibitory cells except in parvalbumin- and calretinin-immunoreactive neurons where the proportion of expression of this subunit is considerably lower. These data indicate that VDCC subunits are differentially expressed in hippocampal GABAergic interneurons, which could explain the diversity in their electrophysiological properties, the existence of synaptic plasticity in certain inhibitory neurons and their vulnerability to stressful stimuli.

The junctional SR protein JP-45 affects the functional expression of the voltage-dependent Ca2+ channel Cav1.1

JP-45, an integral protein of the junctional face membrane of the skeletal muscle sarcoplasmic reticulum (SR), colocalizes with its Ca2+ -release channel (the ryanodine receptor), and interacts with calsequestrin and the skeletal-muscle dihydropyridine receptor Cav1. We have identified the domains of JP-45 and the Cav1.1 involved in this interaction, and investigated the functional effect of JP-45. The cytoplasmic domain of JP-45, comprising residues 1-80, interacts with Cav1.1. JP-45 interacts with two distinct and functionally relevant domains of Cav1.1, the I-II loop and the C-terminal region. Interaction between JP-45 and the I-II loop occurs through the alpha-interacting domain in the I-II loop. beta1a, a Cav1 subunit, also interacts with the cytosolic domain of JP-45, and its presence drastically reduces the interaction between JP-45 and the I-II loop. The functional effect of JP-45 on Cav1.1 activity was assessed by investigating charge movement in differentiated C2C12 myotubes after overexpression or depletion of JP-45. Overexpression of JP-45 decreased peak charge-movement and shifted VQ1/2 to a more negative potential (-10 mV). JP-45 depletion decreased both the content of Cav1.1 and peak charge-movements. Our data demonstrate that JP-45 is an important protein for functional expression of voltage-dependent Ca2+ channels.

Nifedipine enhances cGMP production through the activation of soluble guanylyl cyclase in rat ventricular papillary muscle

It is known that nifedipine, an L-type calcium channel blocker, increases cGMP production, which partially contributes to the relaxation of vascular smooth muscle. The aim of our investigation was to clarify whether or not nifedipine regulates cGMP production, which has a physiological role in cardiac muscle. To measure contractile responses and tissue cGMP levels, left ventricular papillary muscles prepared from male Wistar rats (350-400 g) were mounted in the isolated organ chamber under isometric conditions and electrically paced by means of platinum punctate electrodes (1 Hz, 1 ms duration). In papillary muscle preparation, the negative inotropic effect induced by nifedipine (30 to 300 nM) was significantly inhibited in the presence of ODQ(1H-[1,2,4]oxidazolo[4,3-a]quinoxaline1-one; 10 microM), a soluble guanylyl cyclase inhibitor. Furthermore, nifedipine (100 nM) strongly increased the tissue cGMP level, which was significantly decreased in the presence of ODQ. On the other hand,N(G)-monomethyl-(L)-arginine (100 microM), a nitric oxide synthase inhibitor, did not inhibit either the negative inotropic effect or cGMP production induced by nifedipine. These results indicate that in rat left ventricular papillary muscle, nifedipine augments its negative inotropic effect at least partly through direct activation of cardiac soluble guanylyl cyclase but not nitric oxide synthase.

Frequency-dependent depression of exocytosis and the role of voltage-gated calcium channels

Synaptic vesicle exocytosis in primary cultures of baroreceptor neurons is reduced during high-frequency stimulation. Calcium influx through voltage-gated calcium channels (VGCC) is a key step in neurotransmitter release. With the help of FM2-10, a marker of synaptic vesicle recycling, the present study investigates the differential contribution of several VGCC subtypes to exocytosis in neuronal processes and how this contribution is altered at high frequencies. In control experiments, field stimulation at 0.5 Hz evoked about 66 +/- 5% destaining. Combined blockade of N- and P/Q-subtypes with Ctx-MVIIC was far more effective in reducing exocytosis (11 +/- 8%) than blocking N-type (49 +/- 5%, Ctx-GVIA) or P-type (46 +/- 1%, Agatoxin) alone. The effectiveness of the blockers also varied with the duration of stimulation: Ctx-GVIA attenuating exocytosis significantly in the first 60 s and Agatoxin affecting exocytosis only towards the end of 180 s stimulation period. Field stimulation at 10 Hz evoked exocytosis (36 +/- 18%) comparable to that evoked by 0.5 Hz in the presence of Ctx-GVIA. While blockade with Agatoxin had no effects, Ctx-GVIA, Ctx-MVIIC and L-type blocker Nifedepine had small but similar inhibitory effects on exocytosis at 10 Hz. The data suggest that N-type is the major contributor to exocytosis at 0.5 Hz, and this contribution is reduced during prolonged stimulation periods and at high frequencies.

R-type calcium channels contribute to afterdepolarization and bursting in hippocampal CA1 pyramidal neurons

Action potentials in pyramidal neurons are typically followed by an afterdepolarization (ADP), which in many cells contributes to intrinsic burst firing. Despite the ubiquity of this common excitable property, the responsible ion channels have not been identified. Using current-clamp recordings in hippocampal slices, we find that the ADP in CA1 pyramidal neurons is mediated by an Ni2+-sensitive calcium tail current. Voltage-clamp experiments indicate that the Ni2+-sensitive current has a pharmacological and biophysical profile consistent with R-type calcium channels. These channels are available at the resting potential, are activated by the action potential, and remain open long enough to drive the ADP. Because the ADP correlates directly with burst firing in CA1 neurons, R-type calcium channels are crucial to this important cellular behavior, which is known to encode hippocampal place fields and enhance synaptic plasticity.

A multifunctional cytoprotective agent that reduces neurodegeneration after ischemia

Cellular and molecular pathways underlying ischemic neurotoxicity are multifaceted and complex. Although many potentially neuroprotective agents have been investigated, the simplicity of their protective mechanisms has often resulted in insufficient clinical utility. We describe a previously uncharacterized class of potent neuroprotective compounds, represented by PAN-811, that effectively block both ischemic and hypoxic neurotoxicity. PAN-811 disrupts neurotoxic pathways by at least two modes of action. It causes a reduction of intracellular-free calcium as well as free radical scavenging resulting in a significant decrease in necrotic and apoptotic cell death. In a rat model of ischemic stroke, administration of PAN-811 i.c.v. 1 h after middle cerebral artery occlusion resulted in a 59% reduction in the volume of infarction. Human trials of PAN-811 for an unrelated indication have established a favorable safety and pharmacodynamic profile within the dose range required for neuroprotection warranting its clinical trial as a neuroprotective drug.

Functional compensation by other voltage-gated Ca2+ channels in mouse basal forebrain neurons with Ca(V)2.1 mutations

Tottering (tg/tg) and leaner (tg(la)/tg(la)) mutant mice exhibit distinct mutations in the gene encoding the voltage-activated Ca(2+) channel alpha(1A) subunit (CACNA1A), the pore-forming subunit of the Ca(V)2.1 (P/Q type) Ca(2+) channels. These mice exhibit absence seizures and deficiencies in motor control and other functions. Previous work in cerebellar Purkinje neurons has shown that these mutations cause dramatic reductions in calcium channel function. Because Purkinje cell somata primarily express the Ca(V)2.1 channels, the general decrease in Ca(V)2.1 channel function is observed as a profound decrease in whole-cell current. In contrast to Purkinje cells, basal forebrain (BF) neurons express all of the Ca(2+) channel alpha(1) subunits, with Ca(V)2.1 contributing approximately 30% to the whole-cell current in wild-type (+/+) mice. Here, we show that whole-cell Ba(2+) current densities in BF neurons are not reduced in the mutant genotypes despite a reduction in the Ca(V)2.1 contribution. By blocking the different Ca(2+) channel subtypes with specific pharmacological agents, we found a significant increase in the proportion of Ca(V)1 Ca(2+) current in mutant phenotypes. There was no change in tissue mRNA expression of calcium channel subtypes Ca(V)2.1, Ca(V)2.2, Ca(V)1.2, Ca(V)1.3, and Ca(V)2.3 in the tottering and leaner mutant mice. These results suggest that Ca(V)1 channels may functionally upregulate to compensate for reduced Ca(V)2.1 function in the mutants without an increase in Ca(v)1 message. Single-cell reverse transcription polymerase chain reaction (RT-PCR) experiments in a subset of sampled neurons revealed that approximately 90% of the cells could be considered cholinergic based on choline acetyltransferase (ChAT) mRNA expression.

Presenilin 1 deficiency alters the activity of voltage-gated Ca2+ channels in cultured cortical neurons

Presenilins 1 and 2 (PS1 and PS2, respectively) play a critical role in mediating gamma-secretase cleavage of the amyloid precursor protein (APP). Numerous mutations in the presenilins are known to cause early-onset familial Alzheimer's disease (FAD). In addition, it is well established that PS1 deficiency leads to altered intracellular Ca(2+) homeostasis involving endoplasmic reticulum Ca(2+) stores. However, there has been little evidence suggesting Ca(2+) signals from extracellular sources are influenced by PS1. Here we report that the Ca(2+) currents carried by voltage-dependent Ca(2+) channels are increased in PS1-deficient cortical neurons. This increase is mediated by a significant increase in the contributions of L- and P-type Ca(2+) channels to the total voltage-mediated Ca(2+) conductance in PS1 (-/-) neurons. In addition, chelating intracellular Ca(2+) with 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) produced an increase in Ca(2+) current amplitude that was comparable to the increase caused by PS1 deficiency. In contrast to this, BAPTA had no effect on voltage-dependent Ca(2+) conductances in PS1-deficient neurons. These data suggest that PS1 deficiency may influence voltage-gated Ca(2+) channel function by means that involve intracellular Ca(2+) signaling. These findings reveal that PS1 functions at multiple levels to regulate and stabilize intracellular Ca(2+) levels that ultimately control neuronal firing behavior and influence synaptic transmission.

The VGL-Chanome: A Protein Superfamily Specialized for Electrical Signaling and Ionic Homeostasis

Complex multicellular organisms require rapid and accurate transmission of information among cells and tissues and tight coordination of distant functions. Electrical signals and resulting intracellular calcium transients, in vertebrates, control contraction of muscle, secretion of hormones, sensation of the environment, processing of information in the brain, and output from the brain to peripheral tissues. In nonexcitable cells, calcium transients signal many key cellular events, including secretion, gene expression, and cell division. In epithelial cells, huge ion fluxes are conducted across tissue boundaries. All of these physiological processes are mediated in part by members of the voltage-gated ion channel protein superfamily. This protein superfamily of 143 members is one of the largest groups of signal transduction proteins, ranking third after the G protein-coupled receptors and the protein kinases in number. Each member of this superfamily contains a similar pore structure, usually covalently attached to regulatory domains that respond to changes in membrane voltage, intracellular signaling molecules, or both. Eight families are included in this protein superfamily-voltage-gated sodium, calcium, and potassium channels; calcium-activated potassium channels; cyclic nucleotide-modulated ion channels; transient receptor potential (TRP) channels; inwardly rectifying potassium channels; and two-pore potassium channels. This article identifies all of the members of this protein superfamily in the human genome, reviews the molecular and evolutionary relations among these ion channels, and describes their functional roles in cell physiology.

Molecular pharmacology of high voltage-activated calcium channels

Voltage-gated calcium channels are key sources of calcium entry into the cytosol of many excitable tissues. A number of different types of calcium channels have been identified and shown to mediate specialized cellular functions. Because of their fundamental nature, they are important targets for therapeutic intervention in disorders such as hypertension, pain, stroke, and epilepsy. Calcium channel antagonists fall into one of the following three groups: small inorganic ions, large peptide blockers, and small organic molecules. Inorganic ions nonselectively inhibit calcium entry by physical pore occlusion and are of little therapeutic value. Calcium-channel-blocking peptides isolated from various predatory animals such as spiders and cone snails are often highly selective blockers of individual types of calcium channels, either by preventing calcium flux through the pore or by antagonizing channel activation. There are many structure-activity-relation classes of small organic molecules that interact with various sites on the calcium channel protein, with actions ranging from selective high affinity block to relatively nondiscriminatory action on multiple calcium channel isoforms. Detailed interactions with the calcium channel protein are well understood for the dihydropyridine and phenylalkylamine drug classes, whereas we are only beginning to understand the molecular actions of some of the more recently discovered calcium channel blockers. Here, we provide a comprehensive review of pharmacology of high voltage-activated calcium channels.

The α2δ Auxiliary Subunit Reduces Affinity of ω-Conotoxins for Recombinant N-type (Cav2.2) Calcium Channels

The omega-conotoxins from fish-hunting cone snails are potent inhibitors of voltage-gated calcium channels. The omega-conotoxins MVIIA and CVID are selective N-type calcium channel inhibitors with potential in the treatment of chronic pain. The beta and alpha(2)delta-1 auxiliary subunits influence the expression and characteristics of the alpha(1B) subunit of N-type channels and are differentially regulated in disease states, including pain. In this study, we examined the influence of these auxiliary subunits on the ability of the omega-conotoxins GVIA, MVIIA, CVID and analogues to inhibit peripheral and central forms of the rat N-type channels. Although the beta3 subunit had little influence on the on- and off-rates of omega-conotoxins, coexpression of alpha(2)delta with alpha(1B) significantly reduced on-rates and equilibrium inhibition at both the central and peripheral isoforms of the N-type channels. The alpha(2)delta also enhanced the selectivity of MVIIA, but not CVID, for the central isoform. Similar but less pronounced trends were also observed for N-type channels expressed in human embryonic kidney cells. The influence of alpha(2)delta was not affected by oocyte deglycosylation. The extent of recovery from the omega-conotoxin block was least for GVIA, intermediate for MVIIA, and almost complete for CVID. Application of a hyperpolarizing holding potential (-120 mV) did not significantly enhance the extent of CVID recovery. Interestingly, [R10K]MVIIA and [O10K]GVIA had greater recovery from the block, whereas [K10R]CVID had reduced recovery from the block, indicating that position 10 had an important influence on the extent of omega-conotoxin reversibility. Recovery from CVID block was reduced in the presence of alpha(2)delta in human embryonic kidney cells and in oocytes expressing alpha(1B-b). These results may have implications for the antinociceptive properties of omega-conotoxins, given that the alpha(2)delta subunit is up-regulated in certain pain states.

Expression analysis of P/Q-type Ca2+ channel α1A subunit mRNA in olfactory mitral cell in N-type Ca2+ channel α1B subunit gene-deficient mice

N-type and P/Q-type Ca2+ channels play an important role in the processing of olfactory information. However, N-type Ca2+ channel alpha1B-deficient mice show normal behavior, presumably owing to compensation by other Ca2+ channels. P/Q-type Ca2+ channel alpha1A mRNA was expressed at a higher level in olfactory bulb of homozygous alpha1B-deficient mice than wild-type or heterozygous mice. LacZ expression in olfactory mitral cells of homozygous alpha1B-deficient x alpha1A1.5-lacZ mice, carrying a 1.5-kb 5'-upstream fragment of the alpha1A gene fused to the lacZ reporter gene, was increased compared to that in wild-type or heterozygous mice. Therefore, a possible explanation for the normal behavior of alpha1B-deficient mice is compensation by the alpha1A gene and that the 1.5-kb 5'-upstream region of this gene contains an enhancer cis-element for compensation in olfactory mitral cells.

Increased expression of P/Q-type Ca2+ channel α1A subunit mRNA in cerebellum of N-type Ca2+ channel α1B subunit gene-deficient mice

The Ca(2+) channel alpha(1B) subunit is a pore-forming component capable of generating N-type Ca(2+) channel activity. Although the N-type Ca(2+) channel plays a role in a variety of neuronal functions, alpha(1B)-deficient mice show normal behavior, presumably owing to compensation by the other Ca(2+) channels. In this study, we examined the mRNA expression of the P/Q-type Ca(2+) channel alpha(1A) subunit in cerebellum of alpha(1B)-deficient mice. The alpha(1A) subunit mRNA in homozygous alpha(1B)-deficient mice was expressed at a significantly higher level than in wild or heterozygous mice. To examine whether the increased expression is induced by a cis-regulatory element within the 5'-upstream region of the alpha(1A) subunit gene, we examined lacZ expression in alpha(1B)-deficient x alpha(1A)3.0-lacZ mice (carrying a 3.0-kb 5'-upstream fragment of the alpha(1A) subunit gene fused to Escherichia coli lacZ reporter gene), which express lacZ in granule but not in Purkinje cells, and in alpha(1B)-deficient x alpha(1A)6.3-lacZ mice (carrying a 6.3-kb 5'-upstream region fused to lacZ gene), which express lacZ in Purkinje but not in granule cells. The levels of lacZ expression in homozygous alpha(1B)-deficient x alpha(1A)3.0-lacZ mice were significantly higher than in wild or heterozygous mice, but no difference in lacZ expression level was found among wild, heterozygous and homozygous alpha(1B)-deficient x alpha(1A)6.3-lacZ mice. Therefore, a possible explanation of the normal behavior of alpha(1B)-deficient mice is that compensation by alpha(1A) subunit gene occurs and that the 3.0-kb 5'-upstream region of alpha(1A) subunit gene contains an enhancer cis-element(s) for compensation in cerebellar granule cells.

Membrane properties of single muscle cells of the rhabdosphincter of the male urethra

BACKGROUND

The electrophysiological properties of myoblast cultures established from the human and porcine rhabdosphincter (RS) and porcine lower limb muscle (LLSKM) were studied to elucidate their potential for tissue engineering applications in the lower urinary tract.

METHODS

Muscle biopsies were collected from the prostatic part of the RS, the RS of male pigs, and the porcine LLSKM. Ion channels were studied by means of the patch-clamp technique.

RESULTS

Only one subtype each of voltage gated Na+ and Ca2+ channels was observed in porcine RS and LLSKM. Two types of voltage gated Ca2+ channels were identified in human RS cells. The porcine RS and LLSKM myoblasts displayed similar fusion competence.

CONCLUSIONS

Porcine RS and LLSKM myoblasts and human RS and human skeletal muscle cells show a high degree of similarity. Injection of autologous skeletal muscle myoboblasts in the lower urinary tract might, therefore, represent a promising approach to treat stress incontinence after radical prostatectomy.

Effects of pH on vascular tone in rabbit basilar arteries

Effects of pH on vascular tone and L-type Ca2+ channels were investigated using Mulvany myograph and voltage-clamp technique in rabbit basilar arteries. In rabbit basilar arteries, high K+ produced tonic contractions by 11+/-0.6 mN (mean+/-S.E.,n=19). When extracellular pH (pHo) was changed from control 7.4 to 7.9 ([alkalosis]o), K+-induced contraction was increased to 128+/-2.1% of the control (n=13). However, K+-induced contraction was decreased to 73+/-1.3% of the control at pHo 6.8 ([acidosis] o, n=4). Histamine (10 microM) also produced tonic contraction by 11+/-0.6 mN (n=17), which was blocked by post-application of nicardipine (1 microM). [alkalosis]o and [acidosis]o increased or decreased histamine-induced contraction to 134+/-5.7% and 27+/-7.6% of the control (n=4, 6). Since high K+- and histamine-induced tonic contractions were affected by nicardipine and pHo, the effect of pHo on voltage-dependent L-type Ca2+ channel (VDCCL) was studied. VDCCL was modulated by pHo: the peak value of Ca2+ channel current (IBa) at a holding of 0 mV decreased in [acidosis]o by 41+/-8.8%, whereas that increased in [alkalosis]o by 35+/-2.1% (n=3). These results suggested that the external pH regulates vascular tone partly via the modulation of VDCC in rabbit basilar arteries.

Human group IIA secretory phospholipase A2 potentiates Ca2+ influx through L-type voltage-sensitive Ca2+ channels in cultured rat cortical neurons

Mammalian group IIA secretory phospholipase A2 (sPLA2-IIA) generates prostaglandin D2 (PGD2) and triggers apoptosis in cortical neurons. However, mechanisms of PGD2 generation and apoptosis have not yet been established. Therefore, we examined how second messengers are involved in the sPLA2-IIA-induced neuronal apoptosis in primary cultures of rat cortical neurons. sPLA2-IIA potentiated a marked influx of Ca2+ into neurons before apoptosis. A calcium chelator and a blocker of the L-type voltage-sensitive Ca2+ channel (L-VSCC) prevented neurons from sPLA2-IIA-induced neuronal cell death in a concentration-dependent manner. Furthermore, the L-VSCC blocker ameliorated sPLA2-IIA-induced morphologic alterations and apoptotic features such as condensed chromatin and fragmented DNA. Other blockers of VSCCs such as N type and P/Q types did not affect the neurotoxicity of sPLA2-IIA. Blockers of L-VSCC significantly suppressed sPLA2-IIA-enhanced Ca2+ influx into neurons. Moreover, reactive oxygen species (ROS) were generated prior to apoptosis. Radical scavengers reduced not only ROS generation, but also the sPLA2-IIA-induced Ca2+ influx and apoptosis. In conclusion, we demonstrated that sPLA2-IIA potentiates the influx of Ca2+ into neurons via L-VSCC. Furthermore, the present study suggested that eicosanoids and ROS generated during arachidonic acid oxidative metabolism are involved in sPLA2-IIA-induced apoptosis in cooperation with Ca2+.

A confirmation of 125I-omega-conotoxin labeled sites in a crude membrane fraction from chick brain as the alpha1 subunit of N-type calcium channels

Omega-conotoxin GVIA (omega-CTX), as a selective blocker for an N-type Ca2+ channel, has been conveniently used in many molecular biochemical and pharmacological experiments. There has been little elucidation of 125I-omega-CTX binding sites (mainly the 135-kDa band) in the crude membranes from chick brain, although the characteristics of specific 125I-omega-CTX binding and labeling sites in chick brain membranes have been investigated in our previous research. In this work, our goal is to further identify 125I-omega-CTX labeling sites in chick brain membranes by using anti-B1Nt antibodies (against the N-terminal segment B1Nt of N- or P-type Ca2+ channel alpha1-subunits). The 25I-omega-CTX-labeled sites in chick brain membranes could be solubilized and immunoprecipitated by using an anti B1Nt antibody. The molecular weight of the immunoprecipitated protein was determined as 135 kDa, which is inconsistent with that of the specific 125I-omega-CTX binding protein reported previously. Moreover, the 125-omega-CTX-labeled protein could be purified by the method of preparative SDS-PAGE and recognized by anti-B1Nt antibodies in Western blotting analysis. These results indicated that anti-B1Nt antibodies could truly recognize 125I-omega-CTX-labeled sites as the main band of 135 kDa from chick brain membranes, and the omega-CTX-labeled site (mainly the 135-kDa band) should be N-type Ca2+ channel alpha1-subunits.

Altered properties of quantal neurotransmitter release at endplates of mice lacking P/Q-type Ca2+ channels

Transmission at the mouse neuromuscular junction normally relies on P/Q-type channels, but became jointly dependent on both N- and R-type Ca(2+) channels when the PQ-type channel alpha(1A) subunit was deleted. R-type channels lay close to Ca(2+) sensors for exocytosis and I(K(Ca)) channel activation, like the P/Q-type channels they replaced. In contrast, N-type channels were less well localized, but abundant enough to influence secretion strongly, particularly when action potentials were prolonged. Our data suggested that active zone structures may select among multiple Ca(2+) channels in the hierarchy P/Q >R >N. The alpha(1A)-/- neuromuscular junction displayed several other differences from wild-type: lowered quantal content but greater ability to withstand reductions in the Ca(2+)/Mg(2+) ratio, and little or no paired-pulse facilitation, the latter findings possibly reflecting compensatory mechanisms at individual release sites. Changes in presynaptic function were also associated with a significant reduction in the size of postsynaptic acetylcholine receptor clusters.

Nucleoplasmic calcium regulation in rabbit aortic vascular smooth muscle cells

In this study, we investigated whether nucleoplasmic free Ca2+ in aortic vascular smooth muscle cells (VSMCs) might be independently regulated from cytosolic free Ca2+. Understanding mechanisms and pathways responsible for this regulation is especially relevant given the role of a numerous intranuclear Ca2+-sensitive proteins in transcriptional regulation, apoptosis and cell division. The question of an independent regulatory mechanism remains largely unsettled because the previous use of intensitometric fluorophores (e.g., Fluo-3) has been criticized on technical grounds. To circumvent the potential problem of fluorescence artifact, we utilized confocal laser scanning microscopy to image intracellular Ca2+ movements with the ratiometric fluorophore Indo-1. In cultured rabbit VSMCs, we found sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) pumps and ryanodine receptor (RyR) Ca2+ channel proteins to be discretely arranged within a perinuclear locus, as determined by fluorescent staining patterns of BODIPY FL thapsigargin and BODIPY FL-X Ry. When intracellular Ca2+ stores were mobilized by addition of thapsigargin (5 microM) and activatory concentrations of ryanodine (1 microM), Indo-1 ratiometric signals were largely restricted to the nucleoplasm. Cytosolic signals, by comparison, were relatively small and even then its spatial distribution was largely perinuclear rather homogeneous. These observations indicate perinuclear RyR and SERCA proteins are intimately involved in regulating VSMC nucleoplasmic Ca2+ concentrations. We also observed a similar pattern of largely nucleoplasmic Ca2+ mobilization upon exposure of cells to the immunosuppressant drug FK506 (tacrolimus), which binds to the RyR-associated immunophillin-binding proteins FKBP12 and FKBP12.6. However, initial FK506-induced nucleoplasmic Ca2+ mobilization was followed by marked reduction of Indo-1 signal intensity close to pretreatment levels. This suggested FK506 exerts both activatory and inhibitory effects upon RyR channels. The latter was reinforced by observed effects of FK506 to only reduce nucleoplasmic Indo-1 signal intensity when added following pretreatment with both activatory and inhibitory concentrations of ryanodine. These latter observations raise the possibility that VSMC nuclei represent an important sink of intracellular Ca2+ and may help explain vasodilatory actions of FK506 observed by others.

Porcine pancreatic group IB secretory phospholipase A2 potentiates Ca2+ influx through L-type voltage-sensitive Ca2+ channels

Secretory phospholipase A(2) (sPLA(2)) exhibits neurotoxicity in the central nervous system. There are high-affinity binding sites of the porcine pancreatic group IB sPLA(2) (sPLA(2)-IB) in the brain. sPLA(2)-IB causes neuronal cell death via apoptosis in the rat cerebral cortex. Although apoptosis is triggered by an influx of Ca(2+) into neurons, it has not yet been ascertained whether the Ca(2+) influx is associated with the neurotoxicity of sPLA(2)-IB. We thus examined the possible involvement of Ca(2+) in the neurotoxicity of sPLA(2)-IB in the primary culture of rat cortical neurons. sPLA(2)-IB induced neuronal cell death in a concentration- and time-dependent manner. This death was accompanied by condensed chromatin and fragmented DNA, exhibiting apoptotic features. Before apoptosis, sPLA(2)-IB markedly enhanced the influx of Ca(2+) into neurons. A calcium chelator suppressed neurons from sPLA(2)-IB-induced neuronal cell death in a concentration-dependent manner. An L-type voltage-sensitive Ca(2+) channel (L-VSCC) blocker significantly protected the sPLA(2)-IB-potentiated influx of Ca(2+). On the other hand, blockers of N-VSCC and P/Q-VSCC did not. An L-VSCC blocker protected neurons from sPLA(2)-IB-induced neuronal cell death. In addition, the L-VSCC blocker ameliorated the apoptotic features of sPLA(2)-IB-treated neurons. Neither an N-VSCC blocker nor P/Q-VSCC blockers affected the neurotoxicity of the enzyme. In conclusion, these findings demonstrate that the influx of Ca(2+) into neurons play an important role in the neurotoxicity of sPLA(2)-IB. Furthermore, the present study suggests that L-VSCC contribute to the sPLA(2)-IB-potentiated influx of Ca(2+) into neurons.

The alpha(1A) subunits of rat brain calcium channels are developmentally regulated by alternative RNA splicing

Calcium influx through voltage-gated calcium channels governs important aspects of CNS development. Multiple alternative splicings of the pore-forming alpha(1) subunits have been evidenced in adult brain but little information about their expression during ontogenesis is presently available. The aim of this study was to focus on the expression of three rat voltage-gated calcium channel alpha(1A) splice variants (alpha(1A-a), alpha(1A-b) and alpha(1A-EFe)) during brain ontogenesis in vivo. Using a reverse transcription-polymerase chain reaction strategy, we found that the three isoforms have different timings of development throughout the brain: alpha(1A-b) is expressed from embryonic to the adult stage, alpha(1A--EFe) is restricted to the embryonic period whereas alpha(1A-a) is expressed only postnatally. In situ hybridization indicated that alpha(1A-a) and alpha(1A-b) isoforms develop with different regional and cellular patterns. In hippocampus and cerebellum, alpha(1A-b) represented the predominant isoform at all developmental stages. Taken together, these data reveal that alternative RNA splicing may modulate the alpha(1A) calcium channel properties during development.

Effects of the calcium antagonist nifedipine on the afferent impulse activity of isolated cat muscle spindles

The impulse activity of muscle spindles isolated from the cat tenuissimus muscle was investigated under varying concentrations of the L-type calcium channel blocker nifedipine. At a concentration of 25 microM nifedipine impulse activity was clearly diminished in both primary and secondary endings. However, low concentrations of the drug (5-10 microM) exerted unexpected excitatory effects. The dynamic properties of primary endings in particular were augmented; those of secondary endings were also increased, although only slightly. A detailed analysis of the afferent discharge patterns obtained under ramp-and-hold stretches yielded the following effects of 10 microM nifedipine. (1) The initial burst at the beginning of the ramp phase of a stretch was increased in primary endings; (2) the peak dynamic discharge frequency at the end of the ramp phase was considerably increased in most primary endings; (3) the sensitivity of the peak dynamic discharge value to varying amplitudes and velocities of stretch was significantly enhanced in primary endings, and also increased, although only slightly, in secondary endings; (4) the rise in the discharge frequency during the ramp phase of a stretch was augmented in both types of ending, the effect being again stronger in primary endings; (5) the fast adaptive decay of the impulse frequency following the ramp phase of a ramp-and-hold stretch was significantly increased in primary endings, but remained unaffected in secondary endings. The enhanced dynamic properties of primary endings were also observed under small sinusoidal stretch stimuli (10 microm, 40 Hz), where nifedipine induced a significant shift in the position of the 1:1 driven action potentials toward smaller phase values. In view of an increase in tension in the isolated muscle spindle and an increased initial burst in primary endings in the presence of nifedipine, it is suggested that the drug facilitates the attachment of cross-bridges in the poles of the intrafusal muscle fibers. The increase in the dynamic properties of primary endings points to the possibility that the drug preferentially affects the nuclear bag(1) fiber. The inhibitory effect on the afferent discharge rate at high doses of the drug is interpreted as the consequence of a calcium channel block in the membranes of the sensory endings. The membrane potential of sensory endings appears to be highly dependent on sustained Ca(2+) conductance.