Molecular physiology of low-voltage-activated t-type calcium channels

The 'window' T-type calcium current in brain dynamics of different behavioural states

All three forms of recombinant low voltage-activated T-type Ca(2)(+) channels (Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3) exhibit a small, though clearly evident, window T-type Ca(2)(+) current (I(Twindow)) which is also present in native channels from different neuronal types. In thalamocortical (TC) and nucleus reticularis thalami (NRT) neurones, and possibly in neocortical cells, an I(Twindow)-mediated bistability is the key cellular mechanism underlying the expression of the slow (< 1 Hz) sleep oscillation, one of the fundamental EEG rhythms of non-REM sleep. As the I(Twindow)-mediated bistability may also represent one of the cellular mechanisms underlying the expression of high frequency burst firing in awake conditions, I(Twindow) is of critical importance in neuronal population dynamics associated with different behavioural states.

Low voltage-activated calcium channels in vascular smooth muscle: T-type channels and AVP-stimulated calcium spiking

An important path of extracellular calcium influx in vascular smooth muscle (VSM) cells is through voltage-activated Ca2+ channels of the plasma membrane. Both high (HVA)- and low (LVA)-voltage-activated Ca2+ currents are present in VSM cells, yet little is known about the relevance of the LVA T-type channels. In this report, we provide molecular evidence for T-type Ca2+ channels in rat arterial VSM and characterize endogenous LVA Ca2+ currents in the aortic smooth muscle-derived cell line A7r5. AVP is a vasoconstrictor hormone that, at physiological concentrations, stimulates Ca2+ oscillations (spiking) in monolayer cultures of A7r5 cells. The present study investigated the role of T-type Ca2+ channels in this response with a combination of pharmacological and molecular approaches. We demonstrate that AVP-stimulated Ca2+ spiking can be abolished by mibefradil at low concentrations (<1 microM) that should not inhibit L-type currents. Infection of A7r5 cells with an adenovirus containing the Cav3.2 T-type channel resulted in robust LVA Ca2+ currents but did not alter the AVP-stimulated Ca2+ spiking response. Together these data suggest that T-type Ca2+ channels are necessary for the onset of AVP-stimulated calcium oscillations; however, LVA Ca2+ entry through these channels is not limiting for repetitive Ca2+ spiking observed in A7r5 cells.

Redox modulation of peripheral T-type Ca2+ channels in vivo: alteration of nerve injury-induced thermal hyperalgesia

We reported recently that redox agents, including the endogenous amino acid L-cysteine, modulate T-type Ca2+ currents in primary sensory neurons in vitro, and alter mechanical and thermal nociception in peripheral nociceptors in vivo in intact animals [Neuron 31 (2001) 75]. Here, we studied the effects of locally applied redox agents (L-cysteine and 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) on thermal hyperalgesia in animals with neuropathic pain due to chronic constrictive injury (CCI) of the sciatic nerve. We found that, following injection into the peripheral receptive fields, the endogenous reducing agent L-cysteine increased thermal hyperalgesia in a dose-dependent manner in rats with CCI of the sciatic nerve as well as in sham-operated rats. However, the magnitude of the increase was smaller and duration of effect was shorter in rats with CCI of the sciatic nerve compared to sham-operated animals. DTNB, an exogenous oxidizing agent, induced dose-dependent alleviation of thermal hyperalgesia in rats with CCI of the sciatic nerve and caused analgesia in sham-operated rats. DTNB completely blocked L-cysteine-induced thermal hyperalgesia in both animal groups. Mibefradil, a potent and preferential T-type Ca2+ channel blocker, abolished L-cysteine-induced increase in thermal hyperalgesia in both animal groups suggesting the involvement of T-type Ca2+ channels in peripheral nociception. These results indicate for the first time that redox modulation of T-type Ca2+ channels in rat peripheral nociceptors is operational in pain states caused by peripheral axonal injury. Since thermal hyperalgesia is a common symptom of axonal injury, locally applied oxidizing agents could be used as a novel treatment to ameliorate neuropathic pain.

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.

Mechanism of arachidonic acid modulation of the T-type Ca2+ channel alpha1G

Arachidonic acid (AA) modulates T-type Ca(2+) channels and is therefore a potential regulator of diverse cell functions, including neuronal and cardiac excitability. The underlying mechanism of modulation is unknown. Here we analyze the effects of AA on the T-type Ca(2+) channel alpha(1G) heterologously expressed in HEK-293 cells. AA inhibited alpha(1G) currents within a few minutes, regardless of preceding exposure to inhibitors of AA metabolism (ETYA and 17-ODYA). Current inhibition was also observed in cell-free inside-out patches, indicating a membrane-delimited interaction of AA with the channel. AA action was consistent with a decrease of the open probability without changes in the size of unitary currents. AA shifted the inactivation curve to more negative potentials, increased the speed of macroscopic inactivation, and decreased the extent of recovery from inactivation at -80 mV but not at -110 mV. AA induced a slight increase of activation near the threshold and did not significantly change the deactivation kinetics or the rectification pattern. We observed a tonic current inhibition, regardless of whether the channels were held in resting or inactivated states during AA perfusion, suggesting a state-independent interaction with the channel. Model simulations indicate that AA inhibits T-type currents by switching the channels into a nonavailable conformation and by affecting transitions between inactivated states, which results in the negative shift of the inactivation curve. Slow-inactivating alpha(1G) mutants showed an increased affinity for AA with respect to the wild type, indicating that the structural determinants of fast inactivation are involved in the AA-channel interaction.

[Genetic diversity of voltage-gated calcium channels]

Understanding of the properties of normal and diseased voltage-dependent calcium channels has greatly improved these last Years after the extensive development of the patch-clamp and molecular biology studies and the functional expression strategies. The calcium channel diversity is based on the expression of numerous genes that encode pore channel subunits (10 genes) and auxiliary/regulatory subunits (16 genes). In addition, most of these genes are subject to alternative splicing. The study of calcium channels has also benefited from the discovery of genetic diseases linked to calcium channel mutations: the calcium channelopathies. The review describes the recent data and working hypothesis that address the challenging question of how the calcium channel diversity occurs and how alterations in channel function lead to selective cellular dysfunction.

Functional impact of alternative splicing of human T-type Cav3.3 calcium channels

Low-voltage-activated T-type (Cav3) Ca2+ channels produce low-threshold spikes that trigger burst firing in many neurons. The CACNA1I gene encodes the Cav3.3 isoform, which activates and inactivates much more slowly than the other Cav3 channels. These distinctive kinetic features, along with its brain-region-specific expression, suggest that Cav3.3 channels endow neurons with the ability to generate long-lasting bursts of firing. The human CACNA1I gene contains two regions of alternative splicing: variable inclusion of exon 9 and an alternative acceptor site within exon 33, which leads to deletion of 13 amino acids (Delta33). The goal of this study is to determine the functional consequences of these variations in the full-length channel. The cDNA encoding these regions were cloned using RT-PCR from human brain, and currents were recorded by whole cell patch clamp. Introduction of the Delta33 deletion slowed the rate of channel opening. Addition of exon 9 had little effect on kinetics, whereas its addition to Delta33 channels unexpectedly slowed both activation and inactivation kinetics. Modeling of neuronal firing showed that exon 9 or Delta33 alone reduced burst firing, whereas the combination enhanced firing. The major conclusions of this study are that the intracellular regions after repeats I and IV play a role in channel gating, that their effects are interdependent, suggesting a direct interaction, and that splice variation of Cav3.3 channels provides a mechanism for fine-tuning the latency and duration of low-threshold spikes.

Ca(v)3.2 calcium channels control an autocrine mechanism that promotes neuroblastoma cell differentiation

Calcium influx via low-voltage activated alpha(1H) (Ca(v)3.2) T-currents participates in the morphological and electrical differentiation of neuroblastoma NG108-15 cells. We investigated whether an autocrine mechanism could contribute to this differentiation process. The presence of factors secreted by NG108-15 cells was identified through the use of conditioned media (CM) obtained from differentiated cells. These CM significantly increased neuritogenesis with no change in the HVA calcium channel expression. CM-induced neuritogenesis persists during alpha(1H) current block, whereas CM obtained from cells transfected with an alpha(1H) antisense did not induce neuritogenesis. These data indicate that morphological differentiation of NG108-15 cells depends on an autocrine mechanism, which is controlled by alpha(1H) currents. Such a mechanism is likely to play a role in the various differentiation processes that imply alpha(1H) T-type Ca(2+) channels.

Ethosuximide reverses paclitaxel- and vincristine-induced painful peripheral neuropathy

Paclitaxel (Taxol) is one of the most effective and frequently used chemotherapeutics for the treatment of solid tumours. However, paclitaxel produces peripheral neurotoxicity with patients reporting sensory abnormalities and neuropathic pain during and often persisting after paclitaxel therapy. The mechanisms underlying this dose-limiting side effect are currently unknown and there are no validated drugs for its prevention or control. Male Sprague-Dawley rats received four intraperitoneal (i.p.) injections on alternate days of 2 mg/kg paclitaxel. Behavioural assessment using von Frey filaments and acetone showed that such paclitaxel treatment induced a pronounced mechanical and cold allodynia/hyperalgesia. Thus these studies aim to test potential analgesics on established paclitaxel-induced pain. Paclitaxel-induced pain appears to be relatively resistant to opioid therapy i.p. 4 mg/kg morphine was ineffective and i.p. 8 mg/kg morphine only elicited up to a 50% reversal of mechanical allodynia/hyperalgesia. Interestingly, a maximally tolerated dose (i.p. 0.2 mg/kg) of the potent NMDA receptor antagonist MK-801 produced no significant reversal of the mechanical allodynia/hyperalgesia suggesting that NMDA receptors have little role in paclitaxel-induced pain. Ethosuximide (i.p. 450 mg/kg) an anti-epileptic and relatively selective T-type calcium channel blocker elicited a near complete reversal of mechanical allodynia/hyperalgesia. Repetitive dosing with ethosuximide (i.p. 100 or 300 mg/kg daily for 3 days) showed a dose-related consistent reversal of mechanical allodynia/hyperalgesia with no evidence of tolerance. Ethosuximide (i.p. 300 mg/kg) also reversed paclitaxel-induced cold allodynia and vincristine-induced mechanical allodynia/hyperalgesia. These data suggest that T-type calcium channels may play a role in chemotherapy-induced neuropathy and moreover identify ethosuximide as a new potential treatment for chemotherapy-induced pain.

R-type calcium channels in myenteric neurons of guinea pig small intestine

Currents carried by L-, N-, and P/Q-type calcium channels do not account for the total calcium current in myenteric neurons. This study identified all calcium channels expressed by guinea pig small intestinal myenteric neurons maintained in primary culture. Calcium currents were recorded using whole cell techniques. Depolarizations (holding potential = -70 mV) elicited inward currents that were blocked by CdCl(2) (100 microM). Combined application of nifedipine (blocks L-type channels), Omega-conotoxin GVIA (blocks N-type channels), and Omega-agatoxin IVA (blocks P/Q-type channels) inhibited calcium currents by 56%. Subsequent addition of the R-type calcium channel antagonists, NiCl(2) (50 microM) or SNX-482 (0.1 microM), abolished the residual calcium current. NiCl(2) or SNX-482 alone inhibited calcium currents by 46%. The activation threshold for R-type calcium currents was -30 mV, the half-activation voltage was -5.2 +/- 5 mV, and the voltage sensitivity was 17 +/- 3 mV. R-type currents activated fully in 10 ms at 10 mV. R-type calcium currents inactivated in 1 s at 10 mV, and they inactivated (voltage sensitivity of 16 +/- 1 mV) with a half-inactivation voltage of -76 +/- 5 mV. These studies have accounted for all of the calcium channels in myenteric neurons. The data indicate that R-type calcium channels make the largest contribution to the total calcium current in myenteric neurons. The relatively positive half-activation voltage and rapid activation kinetics suggest that R-type channels could contribute to calcium entry during somal action potentials or during action potential-induced neurotransmitter release.

Conductance-Based Model of the Voltage-Dependent Generation of a Plateau Potential in Subthalamic Neurons

Because the subthalamic nucleus (STN) acts as a driving force of the basal ganglia, it is important to know how the activities of STN neurons are regulated. Previously, we have reported that a subset of STN neurons generates a plateau potential in a voltage-dependent manner. These plateau potentials can be evoked only when the cell is hyperpolarized. Here, to examine the mechanism of the voltage-dependent generation of the plateau potential in STN neurons, we constructed a conductance-based model of the plateau-generating STN neuron based on experimental observations and compared simulation results with recordings in slices. The model consists of a single compartment containing a Na+ current, a delayed-rectifier K+ current, an A-type K+ current, an L-like long-lasting Ca2+ current, a T-type Ca2+ current, a Ca2+-dependent K+ current, and a leak current. Our simulation results showed that a plateau potential in the model could be induced in a voltage-dependent manner that depended on the inactivation properties of L-like long-lasting Ca2+ current. The model could also reproduce the generation of a plateau potential as a rebound potential after termination of hyperpolarizing current injection. In addition, we tested the stability of simulated plateau potentials against inhibitory perturbation and found that the model showed similar properties observed for the plateau potentials of STN neurons in slices. The effects of K+ channel blockade by TEA and intracellular Ca2+ ion chelation by BAPTA on the plateau duration were also tested in the model and were found to match experimental observations. Thus our STN neuron model could qualitatively reproduce a number of experimental observations on plateau potentials. Our results suggest that the inactivation of L-type Ca2+ channels plays an important role in the voltage-dependent generation of the plateau potential.

Intracellular calcium changes induced by the endozepine triakontatetraneuropeptide in human polymorphonuclear leukocytes: role of protein kinase C and effect of calcium channel blockers

Background

The endozepine triakontatetraneuropeptide (TTN) induces intracellular calcium ([Ca⁺⁺]_i) changes followed by activation in human polymorphonuclear leukocytes (PMNs). The present study was undertaken to investigate the role of protein kinase (PK) C in the modulation of the response to TTN by human PMNs, and to examine the pharmacology of TTN-induced Ca⁺⁺ entry through the plasma membrane of these cells.

Results

The PKC activator 12-O-tetradecanoylphorbol-13-acetate (PMA) concentration-dependently inhibited TTN-induced [Ca⁺⁺]_i rise, and this effect was reverted by the PKC inhibitors rottlerin (partially) and Ro 32-0432 (completely). PMA also inhibited TTN-induced IL-8 mRNA expression. In the absence of PMA, however, rottlerin (but not Ro 32-0432) per se partially inhibited TTN-induced [Ca⁺⁺]_i rise. The response of [Ca⁺⁺]_i to TTN was also sensitive to mibefradil and flunarizine (T-type Ca⁺⁺-channel blockers), but not to nifedipine, verapamil (L-type) or ω-conotoxin GVIA (N-type). In agreement with this observation, PCR analysis showed the expression in human PMNs of the mRNA for all the α1 subunits of T-type Ca⁺⁺ channels (namely, α1G, α1H, and α1I).

Conclusions

In human PMNs TTN activates PKC-modulated pathways leading to Ca⁺⁺ entry possibly through T-type Ca⁺⁺ channels.

Ca2+ channel regulation by transforming growth factor-beta 1 and bone morphogenetic protein-2 in developing mice myotubes

In skeletal muscle myogenesis, precursor cells or myoblasts fuse to form multinucleated cells (myotubes), which then further develop into functional muscle. We investigated if the inhibition of myogenesis by transforming growth factor-beta1 (TGF-beta1) and bone morphogenetic protein-2 (BMP-2) involve regulation of voltage-dependent Ca(2+) channels. Primary cultured myoblasts were kept in fusion medium (0-6 days) in either the absence (control conditions) or the presence of 40 pm TGF-beta1 or 5 nm BMP-2. Subsequently, the developing myotubes were transferred to a growth factor-free recording solution, and subjected to whole cell patch-clamp experiments. At day 0, 14% of non-fusing myoblasts exhibited T-current, whereas the L-current was practically absent. Under control conditions, however, the percentage of T- and L-channel-expressing myotubes increased sharply, from 25% at day 1 to approximately 100% at days 2-6. In addition, parallel increases were determined for Ca(2+)-currents density and cell membrane capacitance (C(m)), which is proportional to the size of myotubes. Interestingly, at days 1-2 TGF-beta1 and BMP-2 eliminated the T-current on initial 14% of T-channel-expressing myoblasts. Moreover, at day 6 the growth factors significantly reduced the maximal values of both T-current density (80%) and C(m) (60%). The effect of BMP-2 was selective on T-channels, whereas TGF-beta1 decreased also the L-current density (90%). A similar reduction in maximal conductance of the Ca(2+) channels was determined, in the absence of significant alterations in other essential properties of the channels, including the time course and voltage dependence of activation and inactivation. The results suggest these growth factors markedly reduce the number of functional T- (both TGF-beta1 and BMP-2) and L-channels (only TGF-beta1) in the surface of the plasma membrane, and contribute to explaining the associated effects on myogenesis.

Apical and basal neurones isolated from the mouse vomeronasal organ differ for voltage-dependent currents

The mammalian vomeronasal organ (VNO) contains specialized neurones that transduce the chemical information related to pheromones into discharge of action potentials to the brain. Molecular and biochemical studies have shown that specific components of the pheromonal transduction systems are segregated into two distinct subsets of vomeronasal neurones: apical neurones and basal neurones. However, it is still unknown whether these neuronal subsets also differ in other functional characteristics, such as their membrane properties. We addressed this issue by studying the electrophysiological properties of vomeronasal neurones isolated from mouse VNO. We used the patch-clamp technique to examine both the passive membrane properties and the voltage-gated Na+, K+ and Ca2+ currents. Apical neurones were distinguished from basal ones by the length of their dendrites and by their distinct immunoreactivity for the putative pheromone receptor V2R2. The analysis of passive properties revealed that there were no significant differences between the two neuronal subsets. Also, apical neurones were similar to basal neurones in their biophysical and pharmacological properties of voltage-gated Na+ and K+ currents. However, we found that the density of Na+ currents was about 2-3 times greater in apical neurones than in basal neurones. Consistently, in situ hybridization analysis revealed a higher expression of the Na+ channel subtype III in apical neurones than in basal ones. In contrast, basal neurones were endowed with Ca2+ currents (T-type) of greater magnitude than apical neurones. Our findings indicate that apical and basal neurones in the VNO exhibit distinct electrical properties. This might have a profound effect on the sensory processes occurring in the VNO during pheromone detection.

Pre- and postsynaptic contributions of voltage-dependent Ca2+ channels to nociceptive transmission in rat spinal lamina I neurons

Activation of voltage-dependent Ca2+ channels (VDCCs) is critical for neurotransmitter release, neuronal excitability and postsynaptic Ca2+ signalling. Antagonists of VDCCs can be antinociceptive in different animal pain models. Neurons in lamina I of the spinal dorsal horn play a pivotal role in the processing of pain-related information, but the role of VDCCs to the activity-dependent Ca2+ increase in lamina I neurons and to the synaptic transmission between nociceptive afferents and second order neurons in lamina I is not known. This has now been investigated in a lumbar spinal cord slice preparation from young Sprague-Dawley rats. Microfluorometric Ca2+ measurements with fura-2 have been used to analyse the Ca2+ increase in lamina I neurons after depolarization of the cells, resulting in a distinct and transient increase of the cytosolic Ca2+ concentration. This Ca2+ peak was reduced by the T-type channel blocker, Ni2+, by the L-type channel blockers, nifedipine and verapamil, and by the N-type channel blocker, omega-conotoxin GVIA. The P/Q-type channel antagonist, omega-agatoxin TK, had no effect on postsynaptic [Ca2+]i. The NMDA receptor channel blocker D-AP5 reduced the Ca2+ peak, whereas the AMPA receptor channel blocker CNQX had no effect. Postsynaptic currents, monosynaptically evoked by electrical stimulation of the attached dorsal roots with C-fibre and Adelta-fibre intensity, respectively, were reduced by N-type channel blocker omega-conotoxin GVIA and to a much lesser extent, by P/Q-type channel antagonist omega-agatoxin TK, and the L-type channel blockers verapamil, respectively. No difference was found between unidentified neurons and neurons projecting to the periaqueductal grey matter. This is the first quantitative description of the relative contribution of voltage-dependent Ca2+ channels to the synaptic transmission in lamina I of the spinal dorsal horn, which is essential in the processing of pain-related information in the central nervous system.

CaV 3.1 and CaV 3.3 account for T-type Ca2+ current in GH3 cells

T-type Ca2+ channels are important for cell signaling by a variety of cells. We report here the electrophysiological and molecular characteristics of the whole-cell Ca2+ current in GH3 clonal pituitary cells. The current inactivation at 0 mV was described by a single exponential function with a time constant of 18.32 +/- 1.87 ms (N = 16). The I-V relationship measured with Ca2+ as a charge carrier was shifted to the left when we applied a conditioning pre-pulse of up to -120 mV, indicating that a low voltage-activated current may be present in GH3 cells. Transient currents were first activated at -50 mV and peaked around -20 mV. The half-maximal voltage activation and the slope factors for the two conditions are -35.02 +/- 2.4 and 6.7 +/- 0.3 mV (pre-pulse of -120 mV, N = 15), and -27.0 +/- 0.97 and 7.5 +/- 0.7 mV (pre-pulse of -40 mV, N = 9). The 8-mV shift in the activation mid-point was statistically significant (P < 0.05). The tail currents decayed bi-exponentially suggesting two different T-type Ca2+ channel populations. RT-PCR revealed the presence of alpha1G (CaV3.1) and alpha1I (CaV3.3) T-type Ca2+ channel mRNA transcripts.

Roles of Molecular Regions in Determining Differences between Voltage Dependence of Activation of CaV3.1 and CaV1.2 Calcium Channels

Voltage-dependent calcium channels are classified into low voltage-activated and high voltage-activated channels. We have investigated the molecular basis for this difference in voltage dependence of activation by constructing chimeras between a low voltage-activated channel (Ca(V)3.1) and a high voltage-activated channel (Ca(V)1.2), focusing on steady-state activation properties. Wild type and chimeras were expressed in oocytes, and two-electrode voltage clamp recordings were made of calcium channel currents. Replacement of domains I, III, or IV of the Ca 3.1 channel with the corresponding domain of Ca(V)1.2 led (V)to high voltage-activated channels; for these constructs the current/voltage (I/V) curves were similar to those for Ca(V)1.2 wild type. However, replacement of domain II gave only a small shift to the right of the I/V curve and modulation of the activation kinetics but did not lead to a high voltage-activating channel with an I/V curve like Ca 1.2. We also investigated the role of the voltage sensor (V)S4 by replacing the S4 segment of Ca(V)3.1 with that of Ca 1.2. For domain I, there was no shift in the I/V curve (V)as compared with Ca(V)3.1, and there were relatively small shifts to the right for domains III and IV. Taken together, these results suggest that domains I, III, and IV (rather than domain II) are apparently critical for channel opening and, therefore, contribute strongly to the difference in voltage dependence of activation between Ca 3.1 and Ca(V)1.2. However, the S4 segments in domains I, (V)III, and IV did not account for this difference in voltage dependence.

Depolarization-induced calcium influx in rat mesenteric small arterioles is mediated exclusively via mibefradil-sensitive calcium channels

1. In this study, intracellular Ca(2+) was measured as the Fura-2 ratio (R) of fluorescence excited at 340 and 380 nm (F(340)/F(380)) in nonpressurized rat mesenteric small arterioles ( (lumen diameter) 10-25 microm). 2. The response to depolarization using 75 mm KCl was an increase in R from a baseline of 0.96+/-0.01 ([Ca(2+)](i) approximately 74 nm) to 1.04+/-0.01 ( approximately 128 nm) (n=80). The response to 75 mm K(+) was reversibly abolished in Ca(2+)-free physiological saline solution, whereas phentolamine (10 microm) or tetrodotoxin (1 microm) had no effects. LaCl(3) (200 microm) inhibited 61+/-9% of the response. 3. A [K(+)]-response curve indicated that the Ca(2+) response was activated between 15 and 25 mm K(+). The data suggest that the Ca(2+) response was caused by the activation of voltage-dependent Ca(2+) channels. 4. Mibefradil use dependently inhibited the Ca(2+) response to 75 mm K(+) by 29+/-2% (100 nm), 73+/-7% (1 microm) or 89+/-7% (10 microm). Pimozide (500 nm) use dependently inhibited the Ca(2+) response by 85+/-1%. 5. Nifedipine (1 microm) inhibited the Ca(2+) response to 75 mm K(+) by 41+/-12%. The response was not inhibited by calciseptine (500 nm), omega-agatoxin IVA (100 nm), omega-conotoxin MVIIA (500 nm), or SNX-482 (100 nm). 6. Using reverse transcriptase-polymerase chain reaction, it was shown that neither Ca(V)2.1a (P-type) nor Ca(V)2.1b (Q-type) voltage-dependent Ca(2+) channels were expressed in mesenteric arterioles, whereas the Ca(V)3.1 (T-type) channel was expressed. Furthermore, no amplification products were detected when using specific primers for the beta(1b), beta(2), or beta(3) auxiliary subunits of high-voltage-activated Ca(2+) channels. 7. The results suggest that the voltage-dependent Ca(2+) channel activated by sustained depolarization in mesenteric arterioles does not classify as any of the high-voltage-activated channels (L-, P/Q-, N-, or R-type), but is likely to be a T-type channel. The possibility that the sustained Ca(2+) influx observed was the result of a T-type window current is discussed.

Effects of pyrethroids on voltage-sensitive calcium channels: a critical evaluation of strengths, weaknesses, data needs, and relationship to assessment of cumulative neurotoxicity

The Food Quality Protection Act of 1996 requires that the U.S. Environmental Protection Agency conduct cumulative risk assessments for classes of pesticides that have a common mode or mechanism of action. For the pyrethroid insecticides, disruption of voltage-sensitive sodium channel function is generally accepted as the mechanism underlying acute neurotoxicity. However, data exist which suggest that voltage-sensitive calcium (Ca(2+)) channels (VSCC) may also be important targets of pyrethroid action. VSCC are important to neuronal function during development and for neurotransmitter release, gene expression, and electrical excitability in the nervous system. Disruption of these and other processes mediated by VSCC can result in neurotoxicity. If effects on VSCC are demonstrated to contribute to pyrethroid neurotoxicity, then such effects will have to be considered when making decisions regarding cumulative risk of exposure to this class of compounds. This document provides a critical review of the data related to the hypothesis that VSCC are important targets of pyrethroid effects. Data supporting effects of pyrethroids on VSCC have been generated by several different laboratories using different techniques and biological preparations. Thus, the many reports of effects on VSCC provide evidence that pyrethroids may interact with VSCC. However, evidence to support a role of VSCC in pyrethroid neurotoxicity is based entirely on in vitro observations, and numerous limitations exist in these data, including: (1) lack of defined concentration-response relationships, with some effects observed only at relatively high concentrations, (2) the use of indirect measures of VSCC function, (3) data from nonmammalian species, (4) data from studies that have not been peer-reviewed, (5) the need for replication of some effects, and (6) inconsistent or contradictory results from different laboratories/preparations. Thus, at the present time, it is premature to conclude that effects on VSCC play an important role in the acute neurotoxicity of pyrethroid insecticides in mammals. To demonstrate that VSCC are important targets of pyrethroid neurotoxicity in mammals, in vivo studies supporting a role for pyrethroid effects on VSCC are needed. Additional support could be provided by demonstration of direct effects of pyrethroid compounds on mammalian neuronal VSCC in vitro, including demonstration that concentration-response relationships are similar, or greater, in sensitivity to effects of pyrethroids on voltage-sensitive sodium channels. If such effects were to be demonstrated, the rationale for considering VSCC as targets of pyrethroid compounds when assessing cumulative risk would be strengthened. However, at the present time, the data available neither support nor refute conclusively the hypothesis that effects on VSCC are important to the acute neurotoxicity of pyrethroids.

Exposure to cAMP and beta-adrenergic stimulation recruits Ca(V)3 T-type channels in rat chromaffin cells through Epac cAMP-receptor proteins

T-type channels are expressed weakly or not at all in adult rat chromaffin cells (RCCs) and there is contrasting evidence as to whether they play a functional role in catecholamine secretion. Here we show that 3-5 days after application of pCPT-cAMP, most RCCs grown in serum-free medium expressed a high density of low-voltage-activated T-type channels without altering the expression and characteristics of high-voltage-activated channels. The density of cAMP-recruited T-type channels increased with time and displayed the typical biophysical and pharmacological properties of low-voltage-activated Ca(2+) channels: (1) steep voltage-dependent activation from -50 mV in 10 mm Ca(2+), (2) slow deactivation but fast and complete inactivation, (3) full inactivation following short conditioning prepulses to -30 mV, (4) effective block of Ca(2+) influx with 50 microM Ni(2+), (5) comparable permeability to Ca(2+) and Ba(2+), and (6) insensitivity to common Ca(2+) channel antagonists. The action of exogenous pCPT-cAMP (200 microM) was prevented by the protein synthesis inhibitor anisomycin and mimicked in most cells by exposure to forskolin and 1-methyl-3-isobutylxanthine (IBMX) or isoprenaline. The protein kinase A (PKA) inhibitor H89 (0.3 microM) and the competitive antagonist of cAMP binding to PKA, Rp-cAMPS, had weak or no effect on the action of pCPT-cAMP. In line with this, the selective Epac agonist 8CPT-2Me-cAMP nicely mimicked the action of pCPT-cAMP and isoprenaline, suggesting the existence of a dominant Epac-dependent recruitment of T-type channels in RCCs that may originate from the activation of beta-adrenoceptors. Stimulation of beta-adrenoceptors occurs autocrinally in RCCs and thus, the neosynthesis of low-voltage-activated channels may represent a new form of 'chromaffin cell plasticity', which contributes, by lowering the threshold of action potential firing, to increasing cell excitability and secretory activity during sustained sympathetic stimulation and/or increased catecholamine circulation.

Multiple Structural Elements Contribute to the Slow Kinetics of the Cav3.3 T-type Channel

Molecular cloning and expression studies established the existence of three T-type Ca(2+) channel (Ca(v)3) alpha(1) subunits: Ca(v)3.1 (alpha(1G)), Ca(v)3.2 (alpha(1H)), and Ca(v)3.3 (alpha(1I)). Although all three channels are low voltage-activated, they display considerable differences in their kinetics, with Ca(v)3.1 and Ca(v)3.2 channels activating and inactivating much faster than Ca(v)3.3 channels. The goal of the present study was to determine the structural elements that confer the distinctively slow kinetics of Ca(v)3.3 channels. To address this question, a series of chimeric channels between Ca(v)3.1 and Ca(v)3.3 channels were constructed and expressed in Xenopus oocytes. Kinetic analysis showed that the slow activation and inactivation kinetics of the Ca(v)3.3 channel were not completely abolished by substitution with any one portion of the Ca(v)3.1 channel. Likewise, the Ca(v)3.1 channel failed to acquire the slow kinetics by simply adopting one portion of the Ca(v)3.3 channel. These findings suggest that multiple structural elements contribute to the slow kinetics of Ca(v)3.3 channels.

The cytosolic II-III loop of Cav2.3 provides an essential determinant for the phorbol ester-mediated stimulation of E-type Ca2+ channel activity

There is growing evidence that E-type voltage dependent Ca(2+) channels (Ca(v)2.3) are involved in triggering and controlling pivotal cellular processes like neurosecretion and long-term potentiation. The mechanism underlying a novel Ca(2+) dependent stimulation of E-type Ca(2+) channels was investigated in the context of the recent finding that influx of Ca(2+) through other voltage dependent Ca(2+) channels is necessary and sufficient to directly activate protein kinase C (PKC). With Ba(2+) as charge carrier through Ca(v)2.3 channel alpha(1) subunits expressed in HEK-293 cells, activation of PKC by low concentrations of phorbol ester augmented peak I(Ba) by approximately 60%. In addition, the non-inactivating fraction of I(Ba) was increased by more than three-fold and recovery from short-term inactivation was accelerated. The effect of phorbol ester on I(Ba) was inhibited by application of the specific PKC inhibitor bisindolylmaleimide I. With Ca(2+) as charge carrier, application of phorbol ester did not change the activity of Ca(v)2.3 currents but they were modified by the PKC inhibitor bisindolylmaleimide I. These results suggest that with Ca(2+) as charge carrier the incoming Ca(2+) can activate PKC, thereby augmenting Ca(2+) influx into the cytosol. No modulation of Ca(v)2.3 channels by PKC was observed when an arginine rich region in the II-III loop of Ca(v)2.3 was eliminated. Receptor independent stimulation of PKC and its interaction with Ca(v)2.3 channels therefore represents an important positive feedback mechanism to decode electrical signals into a variety of cellular functions.

Testosterone is a potent inhibitor of L-type Ca2+ channels

Testosterone administration is beneficial in alleviating myocardial ischaemia in men with significant coronary artery disease (CAD), a condition which is associated with hypotestosteronaemia. Infusion of physiological concentrations of testosterone into coronary arteries at angiography results in rapid vasodilatation in patients with CAD. Whilst the cardiovascular benefits of testosterone have long been documented, the underlying mechanism(s) have not yet been revealed. Here, we have investigated whether testosterone might act like widely prescribed antihypertensive dihydropyridines, as an endogenous Ca(2+) channel antagonist. To do this, we used the whole-cell patch-clamp technique to record Ca(2+) currents from the A7r5 smooth muscle cell line and HEK 293 cells stably expressing either L- or T-type Ca(2+) channels. We demonstrate that testosterone directly inhibited both native and human recombinant vascular L-type Ca(2+) channels in a manner that was voltage-independent and, crucially, displayed an IC(50) value of 38 nM, a value within the physiological range. At higher (supraphysiological) concentrations both native and human recombinant T-type channels were also inhibited by testosterone. Our data indicate that testosterone acts like widely prescribed antihypertensive dihydropyridines to reduce Ca(2+) influx into vascular smooth muscle and so promote vasodilation. This effect is likely to account for its beneficial cardiovascular actions.

Plasma membrane expression of T-type calcium channel alpha(1) subunits is modulated by high voltage-activated auxiliary subunits

It has been suggested that the auxiliary subunits of high voltage-activated (HVA) calcium channels modulate T-type, low voltage-activated (LVA) calcium channels. Such a regulation has yet to be documented, especially because there has been no biochemical characterization of T-channels. To monitor total protein levels and plasma membrane expression of T-channels in living cells, external epitopes (hemagglutinin, FLAG) were introduced into human recombinant Ca(V)3 channels that were also N-terminally fused to green fluorescent protein. Utilizing Western blot techniques, fluorescence flow cytometry, immunofluorescence, luminometry, and electrophysiology, we describe here that beta(1b) and alpha(2)-delta(1) subunits enhance the level of Ca(V)3 proteins as well as their plasma membrane expression in various expression systems. We also report that, in both Xenopus oocytes and mammalian cells, the alpha(2)-delta(1) subunits increase by at least and beta(1b) 2-fold the current density of Ca(V)3 channels with no change in the electrophysiological properties. Altogether, these data indicate that HVA auxiliary subunits modulate Ca(V)3 channel surface expression, suggesting that the membrane targeting of HVA and LVA alpha(1) subunits is regulated dynamically through the expression of a common set of regulatory subunits.

Identification of Channels Promoting Calcium Spikes and Waves in HT1080 Tumor Cells

Intracellular Ca(2+) signals have been associated with cell polarization and locomotion. As cell motility underlies metastasis, we have sought to better characterize the Ca(2+) signaling events in HT1080 fibrosarcoma cells. We have tested the hypothesis that low voltage-activated (LVA) and nonvoltage-gated (NVG) channels of HT1080 cells participate in dynamic Ca(2+)-signaling events leading to cell migration and invasion. Immunofluorescence microscopy has shown that HT1080 cells express LVA T-type Ca(2+) channels uniformly about the cell periphery, whereas the transient receptor potential-1 (a NVG cation channel) protein appears as punctate spots about a cell's periphery. HT1080 cells exhibit periodic intracellular Ca(2+) spikes. High-speed imaging revealed that the Ca(2+) spikes were composed of a single Ca(2+) wave traveling unidirectionally about the periphery of the cytoplasm in a clockwise fashion (as viewed from basal to apical surfaces). The T-type Ca(2+) channel blocker mibefradil inhibited Ca(2+) spikes and waves on cells and, in parallel, inhibited cell motility and invasion in a dose-dependent manner. Similar changes were noted with the NVG cation channel blockers Gd(3+) and carboxyamido-triazole. The combination of LVA and NVG blockers further reduced Matrigel invasiveness. However, the Ca(2+) channel blockers nicardipine, SKF96365, diltiazem, and verapamil had no effect at appropriate doses. These results indicate that certain LVA and NVG channels regulate HT1080 cell motility. In addition to providing novel information regarding cancer cell motility, we suggest that it may be possible to design drugs that inhibit a key Ca(2+) wave, thereby enhancing the efficacy of emerging therapeutic protocols.

Insulin-like growth factor-induced transcriptional activity of the skeletal alpha-actin gene is regulated by signaling mechanisms linked to voltage-gated calcium channels during myoblast differentiation

IGF-I activates signaling pathways that increase the expression of muscle-specific genes in differentiating myoblasts. Induction of skeletal alpha-actin expression occurs during differentiation through unknown mechanisms. The purpose of this investigation was to examine the mechanisms that IGF-I uses to induce skeletal alpha-actin gene expression in C2C12 myoblasts. IGF-I increased skeletal alpha-actin promoter activity by 107% compared with the control condition. Ni(+) [T-type voltage-gated Ca(2+) channel (VGCC) inhibitor] reduced basal-induced activation of the skeletal alpha-actin promoter by approximately 84%, and nifedipine (L-type VGCC inhibitor) inhibited IGF-I-induced activation of the skeletal alpha-actin promoter by 29-48%. IGF-I failed to increase skeletal alpha-actin promoter activity in differentiating dysgenic (lack functional L-type VGCC) myoblasts; 30 mm K(+) and 30 mm K(+)+IGF-I increased skeletal alpha-actin promoter activity by 162% and 76% compared with non-IGF-I or IGF-I-only conditions, respectively. IGF-I increased calcineurin activity, which was inhibited by cyclosporine A. Further, cyclosporine A inhibited K(+)+IGF-I-induced activation of the skeletal alpha-actin promoter. Constitutively active calcineurin increased skeletal alpha-actin promoter activity by 154% and rescued the nifedipine-induced inhibition of L-type VGCC but failed to rescue the Ni(+)-inhibition of T-type VGCC. IGF-I-induced nuclear factor of activated T-cells transcriptional activity was not inhibited by nifedipine or Ni(+). IGF-I failed to increase serum response factor transcriptional activity; however, serum response factor activity was reduced in the presence of Ni(+). These data suggest that IGF-I-induced activation of the skeletal alpha-actin promoter is regulated by the L-type VGCC and calcineurin but independent of nuclear factor of activated T-cell transcriptional activity as C2C12 myoblasts differentiate into myotubes.

Design, synthesis and evaluation of novel hydroxyamides as orally available anticonvulsants

Themisone, also known as Atrolactamide, was found, in the 1950s, to be a very potent anticonvulsant. It was hypothesized that the -CF(3) substitution would maintain the anticonvulsant activity. Anticonvulsant testing of our novel compounds by the National Institute of Health's Anticonvulsant Screening Project of the Antiepileptic Drug Discovery Program identified analogue 1, 3,3,3-trifluoro-2-hydroxy-2-phenyl-propionamide, to have potent anticonvulsant activity (MES ED(50) of 9.9 mg/kg, ScMET ED(50) of 34 mg/kg and TD(50) of 100 mg/kg). Therefore, a diverse range of analogues were synthesized utilizing multiple synthetic pathways to explore the structure-activity relationship. Patch clamp electrophysiology experiments demonstrate that compound 1 is an effective T-type calcium channel blocker. Altogether, these results suggest these compounds as a class of orally available anticonvulsants.

Differential involvement of N-type calcium channels in transmitter release from vasoconstrictor and vasodilator neurons

1. The effects of calcium channel blockers on co-transmission from different populations of autonomic vasomotor neurons were studied on isolated segments of uterine artery and vena cava from guinea-pigs. 2. Sympathetic, noradrenergic contractions of the uterine artery (produced by 200 pulses at 1 or 10 Hz; 600 pulses at 20 Hz) were abolished by the N-type calcium channel blocker omega-conotoxin (CTX) GVIA at 1-10 nm. 3. Biphasic sympathetic contractions of the vena cava (600 pulses at 20 Hz) mediated by noradrenaline and neuropeptide Y were abolished by 10 nm CTX GVIA. 4. Neurogenic relaxations of the uterine artery (200 pulses at 10 Hz) mediated by neuronal nitric oxide and neuropeptides were reduced <50% by CTX GVIA 10-100 nm. 5. Capsaicin (3 microm) did not affect the CTX GVIA-sensitive or CTX GVIA-resistant neurogenic relaxations of the uterine artery. 6. The novel N-type blocker CTX CVID (100-300 nm), P/Q-type blockers agatoxin IVA (10-100 nm) or CTX CVIB (100 nm), the L-type blocker nifedipine (10 microm) or the 'R-type' blocker SNX-482 (100 nm), all failed to reduce CTX GVIA-resistant relaxations. The T-type channel blocker NiCl(2) (100-300 microm) reduced but did not abolish the remaining neurogenic dilations. 7. Release of different neurotransmitters from the same autonomic vasomotor axon depends on similar subtypes of calcium channels. N-type channels are responsible for transmitter release from vasoconstrictor neurons innervating a muscular artery and capacitance vein, but only partly mediate release of nitric oxide and neuropeptides from pelvic vasodilator neurons.

Availability of low-threshold Ca2+ current in retinal ganglion cells

Spiking in central neurons depends on the availability of inward and outward currents activated by depolarization and on the activation and priming of currents by hyperpolarization. Of these processes, priming by hyperpolarization is the least described. In the case of T-type Ca2+ current availability, the interplay of hyperpolarization and depolarization has been studied most completely in expression systems, in part because of the difficulty of pharmacologically separating the Ca2+ currents of native neurons. To facilitate understanding of this current under physiological conditions, we measured T-type current of isolated goldfish retinal ganglion cells with perforated-patch voltage-clamp methods in solutions containing a normal extracellular Ca2+ concentration. The voltage sensitivities and rates of current activation, inactivation, deactivation, and recovery from inactivation were similar to those of expressed alpha1G (CaV3.1) Ca2+ channel clones, except that the rate of deactivation was significantly faster. We reproduced the amplitude and kinetics of measured T currents with a numerical simulation based on a kinetic model developed for an alpha1G Ca2+ channel. Finally, we show that this model predicts the increase of T-type current made available between resting potential and spike threshold by repetitive hyperpolarizations presented at rates that are within the bandwidth of signals processed in situ by these neurons.

NRSF regulates the fetal cardiac gene program and maintains normal cardiac structure and function

Reactivation of the fetal cardiac gene program is a characteristic feature of hypertrophied and failing hearts that correlates with impaired cardiac function and poor prognosis. However, the mechanism governing the reversible expression of fetal cardiac genes remains unresolved. Here we show that neuron-restrictive silencer factor (NRSF), a transcriptional repressor, selectively regulates expression of multiple fetal cardiac genes, including those for atrial natriuretic peptide, brain natriuretic peptide and alpha-skeletal actin, and plays a role in molecular pathways leading to the re-expression of those genes in ventricular myocytes. Moreover, transgenic mice expressing a dominant-negative mutant of NRSF in their hearts exhibit dilated cardiomyopathy, high susceptibility to arrhythmias and sudden death. We demonstrate that genes encoding two ion channels that carry the fetal cardiac currents I(f) and I(Ca,T), which are induced in these mice and are potentially responsible for both the cardiac dysfunction and the arrhythmogenesis, are regulated by NRSF. Our results indicate NRSF to be a key transcriptional regulator of the fetal cardiac gene program and suggest an important role for NRSF in maintaining normal cardiac structure and function.

Imatinib-mesylate blocks recombinant T-type calcium channels expressed in human embryonic kidney-293 cells by a protein tyrosine kinase-independent mechanism

The 2-phenylaminopyrimidine derivative imatinib-mesylate, a powerful protein tyrosine kinase (PTK) inhibitor that targets abl, c-kit, and the platelet-derived growth factor receptors, is rapidly gaining a relevant role in the treatment of several types of neoplasms. Because first generation PTK inhibitors affect the activity of a large number of voltage-dependent ion channels, the present study explored the possibility that imatinib-mesylate could interfere with the activity of T-type channels, a class of voltage-dependent Ca2+ channels that take part in the chain of events elicited by PTK activation. The effect of the drug on T-type channel activity was examined using the whole-cell patch-clamp technique with Ba2+ (10 mM) as the permeant ion in human embryonic kidney-293 cells, stably expressing the rat Ca(V)3.3 channels. Imatinib-mesylate concentrations, ranging from 30 to 300 microM, reversibly decreased Ca(V)3.3 current amplitude with an IC(50) value of 56.9 microM. By contrast, when imatinib-mesylate (500 microM) was intracellularly dialyzed with the pipette solution, no reduction in Ba2+ current density was observed. The 2-phenylaminopyrimidine derivative modified neither the voltage dependence of activation nor the steady-state inactivation of Ca(V)3.3 channels. The decrease in extracellular Ba2+ concentration from 10 to 2 mM and the substitution of Ca2+ for Ba2+ increased the extent of 30 microM imatinib-mesylate-induced percentage of channel blockade from 25.9 +/- 2.4 to 36.3 +/- 0.9% in 2 mM Ba2+ and 44.2 +/- 2.3% in 2 mM Ca2+. In conclusion, imatinib-mesylate blocked the cloned Ca(V)3.3 channels by a PTK-independent mechanism. Specifically, the drug did not affect the activation or the inactivation of the channel but interfered with the ion permeation process.

Calcium channel blockers in the treatment of disease

Over the last 20 years the combination of patch clamp and molecular biology techniques have resulted in an explosion in our knowledge of the different calcium channel types and their roles in physiology. A crucial component to this advance has been the discovery of specific blockers for different calcium channel types. These blockers have not only permitted researchers to assign specific functions to different channel types, but their specificity allowed them to be used to treat diseases that resulted from enhanced calcium channel function. In some cases, the enhanced calcium channel function is secondary to other problems leading to increased cellular excitability. The specificity of calcium channel blockers, however, has provided a means to treat symptoms of the pathophysiology until more effective treatments become available to address the underlying problems. The goal of this review is to introduce the various types of calcium channels, their primary physiologic roles, drugs that block these channels, and diseases that are currently being treated with these drugs.

The R(−)-Enantiomer of Efonidipine Blocks T-type but Not L-type Calcium Current in Guinea Pig Ventricular Myocardium

In guinea pig ventricular cardiomyocytes, the R(-)-enantiomer of efonidipine concentration-dependently blocked T-type Ca2+ current with 85% inhibition at 1 microM. In contrast, R(-)-efonidipine (1 microM) had no effect on the L-type Ca2+ current and Ca2+ transient in cardiomyocytes and contractile force in papillary muscles. Thus, R(-)-efonidipine is a highly selective blocker of the T-type Ca2+ current in native myocardia.

Effect of Cilnidipine on L- and T-type Calcium Currents in Guinea Pig Ventricle and Action Potential in Rabbit Sinoatrial Node

Cilnidipine, a dihydropyridine Ca(2+) channel antagonist, is known to have inhibitory effects on both L- and N-type Ca(2+) currents. In the present study, we examined the effect of cilnidipine on myocardial L- and T-type Ca(2+) currents and sinoatrial node action potential configuration. In voltage clamped guinea pig ventricular myocytes, cilnidipine concentration-dependently decreased L- and T-type Ca(2+) currents. In rabbit sinoatrial node tissue, cilnidipine increased cycle length through reduction of phase 4 depolarization slope. In conclusion, cilnidipine has inhibitory effects on T-type Ca(2+) current, which may contribute to its negative chronotropic potency.

Differential expression of calcium channels in sympathetic and parasympathetic preganglionic inputs to neurons in paracervical ganglia of guinea-pigs

Neurons in pelvic ganglia receive nicotinic excitatory post-synaptic potentials (EPSPs) from sacral preganglionic neurons via the pelvic nerve, lumbar preganglionic neurons via the hypogastric nerve or both. We tested the effect of a range of calcium channel antagonists on EPSPs evoked in paracervical ganglia of female guinea-pigs after pelvic or hypogastric nerve stimulation. omega-Conotoxin GVIA (CTX GVIA, 100 nM) or the novel N-type calcium channel antagonist, CTX CVID (100 nM) reduced the amplitude of EPSPs evoked after pelvic nerve stimulation by 50-75% but had no effect on EPSPs evoked by hypogastric nerve stimulation. Combined addition of CTX GVIA and CTX CVID was no more effective than either antagonist alone. EPSPs evoked by stimulating either nerve trunk were not inhibited by the P/Q calcium channel antagonist, omega-agatoxin IVA (100 nM), nor the L-type calcium channel antagonist, nifedipine (30 microM). SNX 482 (300 nM), an antagonist at some R-type calcium channels, inhibited EPSPs after hypogastric nerve stimulation by 20% but had little effect on EPSPs after pelvic nerve stimulation. Amiloride (100 microM) inhibited EPSPs after stimulation of either trunk by 40%, while nickel (100 microM) was ineffective. CTX GVIA or CTX CVID (100 nM) also slowed the rate of action potential repolarization and reduced afterhyperpolarization amplitude in paracervical neurons. Thus, release of transmitter from the terminals of sacral preganglionic neurons is largely dependent on calcium influx through N-type calcium channels, although an unknown calcium channel which is resistant to selective antagonists also contributes to release. Release of transmitter from lumbar preganglionic neurons does not require calcium entry through either conventional N-type calcium channels or the variant CTX CVID-sensitive N-type calcium channel and seems to be mediated largely by a novel calcium channel.

Distinctive membrane and discharge properties of rat spinal lamina I projection neurones in vitro

Most lamina I neurones with a projection to the brainstem express the neurokinin 1 receptor and thus belong to a small subgroup of lamina I neurones that are necessary for the development of hyperalgesia in rat models of persisting pain. These neurones are prone to synaptic plasticity following primary afferent stimulation in the noxious range while other nociceptive lamina I neurones are not. Here, we used whole-cell patch-clamp recordings from lamina I neurones in young rat spinal cord transverse slices to test if projection neurones possess membrane properties that set them apart from other lamina I neurones. Neurones with a projection to the parabrachial area or the periaqueductal grey (PAG) were identified by retrograde labelling with the fluorescent tracer DiI. The properties of lamina I projection neurones were found to be fundamentally different from those of unidentified, presumably propriospinal lamina I neurones. Two firing patterns, the gap and the bursting firing pattern, occurred almost exclusively in projection neurones. Most spino-parabrachial neurones showed the gap firing pattern while the bursting firing pattern was characteristic of spino-PAG neurones. The underlying membrane currents had the properties of an A-type K(+) current and a Ca(2+) current with a low activation threshold, respectively. Projection neurones, especially those of the burst firing type, were more easily excitable than unidentified neurones and received a larger proportion of monosynaptic input from primary afferent C-fibres. Intracellular labelling with Lucifer yellow showed that projection neurones had larger somata than unidentified neurones and many had a considerable extension in the mediolateral plane.

Slow inactivation of the Ca(V)3.1 isotype of T-type calcium channels

T-type calcium channels (the Ca(V)3 channel family) are involved in defining the resting membrane potential and in neuronal activities such as oscillations and rebound depolarization. Their physiological roles depend upon the channel activation and inactivation kinetics. A fast inactivation that stops the ionic flux of calcium in tens of milliseconds has already been described in both native and heterologously expressed channels. Here, using HEK 293 cells expressing the rat Ca(V)3.1 channel and whole-cell voltage clamp, we investigate an additional inactivation process, which can be distinguished from the previously described fast inactivation by its slow time course of recovery from inactivation (tau= 1 s) and by its sensitivity to external calcium. Steady-state slow inactivation is voltage dependent around the resting membrane potential (the potential of half-inactivation (V(0.5)) =-70 mV, slope factor = 7.4 mV) and can reduce the calcium current by up to 50%. Near resting potential, the slow inactivation displays a half-time of induction of tens of seconds. The slow inactivation therefore modulates the availability of T-type calcium channels depending upon recent cell history, providing a mechanism to store information in a time scale of seconds.

Sustained Ca2+ entry elicits transient postsynaptic currents at a retinal ribbon synapse

Night (scotopic) vision is mediated by a distinct retinal circuit in which the light responses of rod-driven neurons are faster than those of the rods themselves. To investigate the dynamics of synaptic transmission at the second synapse in the rod pathway, we made paired voltage-clamp recordings from rod bipolar cells (RBCs) and postsynaptic AII and A17 amacrine cells in rat retinal slices. Depolarization of RBCs from -60 mV elicited sustained Ca2+ currents and evoked AMPA receptor (AMPAR)-mediated EPSCs in synaptically coupled amacrine cells that exhibited large, rapidly rising initial peaks that decayed rapidly to smaller, steady-state levels. The transient component persisted in the absence of feedback inhibition to the RBC terminal and when postsynaptic AMPA receptor desensitization was blocked with cyclothiazide, indicating that it reflects a time-dependent decrease in the rate of exocytosis from the presynaptic terminal. The EPSC waveform was similar when RBCs were recorded in perforated-patch or whole-cell configurations, but asynchronous release from RBCs was enhanced when the intraterminal Ca2+ buffer capacity was reduced. When RBCs were depolarized from -100 mV, inactivating, low voltage-activated (T-type channel-mediated) Ca2+ currents were evident. Although Ca2+ influx through T-type channels boosted vesicle release, as reflected by larger EPSCs, it did not make the EPSCs faster, indicating that activation of T-type channels is not necessary to generate a transient phase of exocytosis. We conclude that the time course of vesicle release from RBCs is inherently transient and, together with the fast kinetics of postsynaptic AMPARs, speeds transmission at this synapse.

Angiotensin II Signaling Pathways Mediate Expression of Cardiac T-Type Calcium Channels

Recent studies indicate that cardiac T-type Ca 2+ current ( I CaT ) reappears in hypertrophied ventricular cells. The aim of this study was to investigate the role of angiotensin II (Ang II), a major inducer of cardiac hypertrophy, in the reexpression of T-type channel in left ventricular hypertrophied myocytes. We induced cardiac hypertrophy in rats by abdominal aorta stenosis for 12 weeks and thereafter animals were treated for 2 weeks with losartan (12 mg/kg per day), an antagonist of type 1 Ang II receptors (AT 1 ). In hypertrophied myocytes, we showed that the reexpressed I CaT is generated by the Ca V 3.1 and Ca V 3.2 subunits. After losartan treatment, I CaT density decreased from 0.40±0.05 pA/pF (n=26) to 0.20±0.03 pA/pF (n=27, P <0.01), affecting Ca V 3.1- and Ca V 3.2-related currents. The amount of Ca V 3.1 mRNA increased during hypertrophy and retrieved its nonhypertrophic level after losartan treatment, whereas the amount of Ca V 3.2 mRNA was unaffected by stenosis. In cultured newborn ventricular cells, chronic Ang II application (0.1 μmol/L) also increased I CaT density and Ca V 3.1 mRNA amount. UO126, a mitogen-activated protein kinase kinase-1/2 (MEK1/2) inhibitor, reduced Ang II–increased I CaT density and Ca V 3.1 mRNA amount. Bosentan, an endothelin (ET) receptor antagonist, reduced Ang II–increased I CaT density without affecting the amount of Ca V 3.1 mRNA. Finally, cotreatment with bosentan and UO126 abolished the Ang II–increased I CaT density. Our results show that AT 1 -activated MEK pathway and autocrine ET-activated independent MEK pathway upregulate T-type channel expression. Ang II–increased of I CaT density observed in hypertrophied myocytes may play a role in the pathogenesis of Ca 2+ overload and arrhythmias seen in cardiac pathology.

Characterization of T-type calcium current and its contribution to electrical activity in rabbit urethra

Rabbit urethral smooth muscle cells were studied at 37 degrees C by using the amphotericin B perforated-patch configuration of the patch-clamp technique, using Cs(+)-rich pipette solutions. Two components of current, with electrophysiological and pharmacological properties typical of T- and L-type Ca(2+) currents, were recorded. Fitting steady-state inactivation curves for the L current with a Boltzmann equation yielded a V(1/2) of -41 +/- 3 mV. In contrast, the T current inactivated with a V(1/2) of -76 +/- 2 mV. The L currents were reduced by nifedipine (IC(50) = 225 +/- 84 nM), Ni(2+) (IC(50) = 324 +/- 74 microM), and mibefradil (IC(50) = 2.6 +/- 1.1 microM) but were enhanced when external Ca(2+) was substituted with Ba(2+). The T current was little affected by nifedipine at concentrations <300 nM but was increased in amplitude when external Ca(2+) was substituted with Ba(2+). Both Ni(2+) and mibefradil reduced the T current with an IC(50) = 7 +/- 1 microM and approximately 40 nM, respectively. Spontaneous electrical activity recorded with intracellular electrodes from strips of rabbit urethra consisted of complexes comprising a series of spikes superimposed on a slow spontaneous depolarization (SD). Inhibition of T current reduced the frequency of these SDs but had no effect on either the number of spikes per complex or the amplitude of the spikes. In contrast, application of nifedipine failed to significantly alter the frequency of the SD but reduced the number and amplitude of the spikes in each complex.

A mechanism for the direct regulation of T-type calcium channels by Ca2+/calmodulin-dependent kinase II

Low-voltage-activated (LVA) Ca2+ channels are widely distributed throughout the CNS and are important determinants of neuronal excitability, initiating dendritic and somatic Ca2+ spikes that trigger and shape the pattern of action potential firing. Here, we define a molecular mechanism underlying the dynamic regulation of alpha1H channels (Cav3.2), by Ca2+/CaM-dependent protein kinase II (CaMKII). We show that channel regulation is selective for the LVA alpha1H Ca2+ channel subtype, depends on determinants in the alpha1H II-III intracellular loop, and requires the phosphorylation of a serine residue absent from unregulated alpha1G (Cav3.1) channels. These studies identify the alpha1H channel as a new substrate for CaMKII and provide the first molecular mechanism for the direct regulation of T-type Ca2+ channels by a protein kinase. Our data suggest a novel mechanism for modulating the integrative properties of neurons.

Characterization and regulation of T-type Ca2+ channels in embryonic stem cell-derived cardiomyocytes

T-type Ca2+ channels may play a role in cardiac development. We studied the developmental regulation of the T-type currents (ICa,T) in cardiomyocytes (CMs) derived from mouse embryonic stem cells (ESCs). ICa,T was studied in isolated CMs by whole cell patch clamp. Subsequently, CMs were identified by the myosin light chain 2v-driven green fluorescent protein expression, and laser capture microdissection was used to isolate total RNA from groups of cells at various developmental time points. ICa,T showed characteristics of Cav3.1, such as resistance to Ni2+ block, and a transient increase during development, correlating with measures of spontaneous electrical activity. Real-time RT-PCR showed that Cav3.1 mRNA abundance correlated (r2 = 0.81) with ICa,T. The mRNA copy number was low at 7+4 days (2 copies/cell), increased significantly by 7+10 days (27/cell; P < 0.01), peaked at 7+16 days (174/cell), and declined significantly at 7+27 days (25/cell). These data suggest that ICa,T is developmentally regulated at the level of mRNA abundance and that this regulation parallels measures of pacemaker activity, suggesting that ICa,T might play a role in the spontaneous contractions during CM development.

Calcium current subtypes in GnRH neurons

Calcium plays roles in excitability, rhythm generation, and neurosecretion. Identifying channel subtypes that regulate calcium influx is thus important to understanding rhythmic GnRH secretion, which is a prerequisite for reproduction. Whole-cell voltage-clamp recordings were made from short-term dissociated GnRH adult ovariectomized (OVX) mice (n = 21) to identify channel subtypes that carry calcium current using selective channel blockers and voltage characteristics. Low-voltage activated (LVA) currents were not observed in 42 GnRH neurons tested, although most non-GnRH neurons (4/6) displayed LVA current. The L-type component of the high-voltage activated (HVA) calcium current was 25% +/- 2%. The remaining HVA calcium current passed through N-type (27% +/- 3%), P-type (15% +/- 1%), Q-type (18% +/- 3%), and R-type (15% +/- 1%) channels. Because these data differ substantially from reports on cultured GnRH neurons, which may represent reproductively immature models, we also examined GnRH neurons from gonadal-intact young (Postnatal Days 4-10, n = 8 mice) mice. LVA currents were still rare (2/28) in young mice. Although the same HVA components were observed, the proportions were shifted toward significantly more L-type and less N-type current, suggesting a possible developmental shift in calcium currents in GnRH neurons. These data suggest that calcium channel subtypes in GnRH neurons prepared in the short term from brain slices differ substantially from those in long-term cultured GnRH models. These findings provide a vital foundation to examine the role of calcium channels in the secretory and rhythmic machinery of GnRH neurons.

The impact of splice isoforms on voltage-gated calcium channel alpha1 subunits

Semi-conserved exon boundaries in members of the CACNA1 gene family result in recurring pre-mRNA splicing patterns. The resulting variations in the encoded pore-forming subunit of the voltage-gated calcium channel affect functionally significant regions, such as the vicinity of the voltage-sensing S4 segments or the intracellular loops that are important for protein interaction. In addition to generating functional diversity, RNA splicing regulates the quantitative expression of other splice isoforms of the same gene by producing transcripts with premature stop codons which encode two-domain or three-domain channels. An overview of some of the known splice isoforms of the alpha(1) calcium channel subunits and their significance is given.

Mammalian Voltage-Gated Calcium Channels Are Potently Blocked by the Pyrethroid Insecticide Allethrin

Pyrethroids are commonly used insecticides for both household and agricultural applications. It is generally reported that voltage-gated sodium channels are the primary target for toxicity of these chemicals to humans. The phylogenetic and structural relatedness between sodium channels and voltage-gated calcium (Ca) channels prompted us to examine the effects of the type 1 pyrethroid allethrin on the three major classes of mammalian calcium channels exogenously expressed in human embryonic kidney 293 cells. We report that all classes of mammalian calcium channels are targets for allethrin at concentrations very similar to those reported for interaction with sodium channels. Allethrin caused blockade with IC(50) values of 7.0 microM for T-type alpha(1G) (Ca(v)3.1), 6.8 microM for L-type alpha(1C) (Ca(v)1.2), and 6.7 microM for P/Q-type alpha(1A) (Ca(v)2.1) channels. Mechanistically, the blockade of calcium channels was found to be significantly different than the prolonged opening of mammalian sodium channels caused by pyrethroids. In all calcium channel subtypes tested, allethrin caused a significant acceleration of the inactivation kinetics and a hyperpolarizing shift in the voltage dependence of inactivation. The high-voltage-activated P/Q- and L-type channels showed a frequency of stimulation-dependent increase in block by allethrin, whereas the low-voltage-activated alpha(1G) subtype did not. Allethrin did not significantly modify the deactivation kinetics or current-voltage relationships of any of the calcium channel types. Our study indicates that calcium channels are another primary target for allethrin and suggests that blockade of different types of calcium channels may underlie some of the chronic effects of low-level pyrethroid poisoning.

Role of cell volume in K+-induced Ca2+ signaling by rat adrenal glomerulosa cells

The involvement of cell volume in the K+-evoked Ca2+ signaling was studied in cultured rat glomerulosa cells. Previously we reported that hyposmosis (250 mOsm) increased the amplitude of T-type Ca2+ current and, accordingly, enhanced the Ca2+ response of cultured rat glomerulosa cells to K+. In the present study we found that this enhancement is not influenced by the cytoskeleton-disrupting drugs cytochalasin-D (20 microM) and colchicine (100 microM). Elevation of extracellular potassium concentration ([K+]e) from 3.6 to 4.6-8.6 mM induced cell swelling, which had slower kinetics than the Ca2+ signal. Cytoplasmic Ca2+ signal measured in single glomerulosa cells in response to stimulation with 5 mm K+ for 2 min showed two phases: after a rapid rise reaching a plateau within 20-30 sec, [Ca2+]c increased further slowly by approximately one third. When 5 mM K+ was coapplied with elevation of extracellular osmolarity from 290 to 320 mOsm, the second phase was prevented. These results indicate that cell swelling evoked by physiological elevation of [K+]e may contribute to the generation of sustained Ca2+ signals by enhancing voltage-activated Ca2+ influx.

ZD7288 inhibits low-threshold Ca2+ channel activity and regulates sperm function

In this study, ZD7288, a blocker of hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels, has been found to inhibit the mouse sperm acrosome reaction (AR). HCN channels have not yet been either recorded or implicated in mouse sperm AR, but low-threshold (T-type) Ca(2+) channels have. Interestingly, ZD7288 blocked native T-type Ca(2+) currents in mouse spermatogenic cells with an IC(50) of about 100 microM. This blockade was more effective at voltages producing low levels of inactivation, suggesting a differential affinity of ZD7288 for different channel conformations. Furthermore, ZD7288 inhibited all cloned T-type but not high-threshold N-type channels heterologously expressed in HEK-293 cells. Our results further support the role of T-type Ca(2+) channels in the mouse sperm AR.

T-type calcium channels in the regulation of afferent and efferent arterioles in rats

L-type Ca2+ channels predominantly influence preglomerular arterioles, but there is less information regarding the role of T-type Ca2+ channels in regulating the renal microvasculature. We compared the effects of T- and L-type channel blockade on afferent and efferent arterioles using the in vitro blood-perfused juxtamedullary nephron preparation. Single afferent or efferent arterioles of Sprague-Dawley rats were visualized and superfused with solutions containing Ca2+ channel blockers. We confirmed that L-type channel blockade with diltiazem dilates afferent arterioles but has no significant effects on efferent arterioles. In contrast, T-type channel blockade with pimozide (10 micromol/l) or mibefradil (1 micromol/l) dilated both afferent (26.8 +/- 3.4 and 24.6 +/- 1.9%) and efferent (19.2 +/- 2.9 and 19.1 +/- 4.8%) arterioles. Adding diltiazem did not significantly augment the dilation of afferent arterioles elicited by pimozide and mibefradil, and adding pimozide after diltiazem likewise did not elicit further vasodilation. Diltiazem blocked the depolarization-induced afferent arteriolar constriction elicited by 55 mM KCl; however, the constrictor response to KCl remained intact during treatment with 10 microM pimozide. Pimozide also prevented the afferent arterioles from exhibiting autoregulatory-mediated constrictor responses to increases in perfusion pressure. We conclude that T-type channel blockers dilate efferent arterioles as well as afferent arterioles and diminish afferent arteriolar autoregulatory responses to changes in perfusion pressure. To the extent that these agents exert their effects primarily on T-type Ca2+ channels in our experimental setting, these results indicate that T-type channels are functionally expressed in juxtamedullary afferent and efferent arterioles and may act cooperatively with L-type channels to regulate afferent arteriolar resistance. Because L-type channels are not functionally expressed in efferent arterioles, T-type channels may be particularly significant in the regulation of efferent arteriolar function.