Properties and role of voltage-dependent calcium channels during mouse skeletal muscle differentiation

Skeletal muscle differentiation depends on calcium ions, but it is yet unclear whether calcium entry through voltage-dependent calcium channels (VDCCs) contributes to the myoblast fusion process. In this study, we investigate whether calcium influx through functional T-type VDCCs precedes and affects mouse satellite cell fusion. We report here on the properties and the role of the VDCCs expressed in differentiating mouse muscular cells using both the C2C12 cell line and primary cultures of satellite cells. We present electrophysiological and biochemical evidence demonstrating that T-type and L-type VDCCs are not present in C2C12 and primary cultures of mouse satellite cells prior to the fusion stage. Although mRNA for the T-type Ca_V3.2 subunit was detected in differentiated C2C12 cells, no T-type calcium currents could be recorded, while both T-type and L-type calcium currents were detected after the fusion process in primary cultures. In addition, chronic application of 30 μM nickel, known to inhibit T-type Ca_V3.2 channels, did not alter the fusion of C2C12 cells and mouse satellite cells in primary culture. Overall, the data indicate that, unlike in humans, Ca_V3.2 T-type calcium channels play no role in mouse satellite cell fusion.     

Short- and long-term roles of phosphatidylinositol 4,5-bisphosphate PIP2 on Cav3.1- and Cav3.2-T-type calcium channel current

T-type calcium (Ca²⁺) channels play important physiological functions in excitable cells including cardiomyocyte. Phosphatidylinositol-4,5-bisphosphate (PIP₂) has recently been reported to modulate various ion channels’ function. However the actions of PIP₂ on the T-type Ca²⁺ channel remain unclear. To elucidate possible effects of PIP₂ on the T-type Ca²⁺ channel, we applied patch clamp method to investigate recombinant Ca_V3.1- and Ca_V3.2-T-type Ca²⁺ channels expressed in mammalian cell lines with PIP₂ in acute- and long-term potentiation. Short- and long-term potentiation of PIP₂ shifted the activation and the steady-state inactivation curve toward the hyperpolarization direction of Ca_V3.1-I_Ca.T without affecting the maximum inward current density. Short- and long-term potentiation of PIP₂ also shifted the activation curve toward the hyperpolarization direction of Ca_V3.2-I_Ca.T without affecting the maximum inward current density. Conversely, long-term but not short-term potentiation of PIP₂ shifted the steady-state inactivation curve toward the hyperpolarization direction of Ca_V3.2-I_Ca.T. Long-term but not short-term potentiation of PIP₂ blunted the voltage-dependency of current decay Ca_V3.1-I_Ca.T. PIP₂ modulates Ca_V3.1- and Ca_V3.2-I_Ca.T not by their current density but by their channel gating properties possibly through its membrane-delimited actions.     

Surface expression and function of Cav3.2 T-type calcium channels are controlled by asparagine-linked glycosylation

Low-voltage-activated T-type calcium channels play important roles in neuronal physiology where they control cellular excitability and synaptic transmission. Alteration in T-type channel expression has been linked to various pathophysiological conditions such as pain arising from diabetic neuropathy. In the present study, we looked at the role of asparagine (N)-linked glycosylation on human Ca_v3.2 T-type channel expression and function. Manipulation of N-glycans on cells expressing a recombinant Ca_v3.2 channel revealed that N-linked glycosylation is critical for proper functional expression of the channel. Using site-directed mutagenesis to disrupt the canonical N-linked glycosylation sites of Ca_v3.2 channel, we show that glycosylation at asparagine N192 is critical for channel expression at the surface, whereas glycosylation at asparagine N1466 controls channel activity. Moreover, we demonstrate that N-linked glycosylation of Ca_v3.2 not only controls surface expression and activity of the channel but also underlies glucose-dependent potentiation of T-type Ca²⁺ current. Our data suggest that N-linked glycosylation of T-type channels may play an important role in aberrant upregulation of T-type channel activity in response to glucose elevations.     

Proliferation of human lens epithelial cells (HLE-B3) is inhibited by blocking of voltage-gated calcium channels

Calcium, as an integral part of a large number of cellular regulatory pathways, is selective in the control of specific cell functions like the start of G1 phase in cell cycle. Cell proliferation has been suggested to depend on increasing intracellular calcium levels. A major regulatory pathway for intracellular calcium is the calcium influx into the cell via voltage-gated calcium channels. T-type and L-type calcium channels are substantially present in human lens epithelial cell (hLEC), and total calcium currents are inhibited by mibefradil. Here, the hypothesis was tested if calcium influx via Ca_v channels regulates proliferation in epithelial cells. Cell proliferation was determined by cell culture assays using the L- and T-type Ca_v channel blockers mibefradil and verapamil as modulators for calcium influx. Calcium influx was investigated using the Manganese quench technique. Western blot experiments were accomplished under standard conditions using antibodies against MAPK 3. Mibefradil as well as verapamil impaired cell proliferation, but in different concentration ranges. Furthermore, the activation of MAPK 3 was reduced by both antagonists. Calcium influx was also reduced in the presence of both blockers. We conclude that the transmembrane influx of Ca²⁺ through Ca_v channels contributes to the regulation of hLEC proliferation, identifying Ca_v channel blockers as potential therapeutic substances in ocular diseases.     

Role of voltage-gated calcium channels in epilepsy

It is well established that idiopathic generalized epilepsies (IGEs) show a polygenic origin and may arise from dysfunction of various types of voltage- and ligand-gated ion channels. There is an increasing body of literature implicating both high- and low-voltage-activated (HVA and LVA) calcium channels and their ancillary subunits in IGEs. Ca_v2.1 (P/Q-type) calcium channels control synaptic transmission at presynaptic nerve terminals, and mutations in the gene encoding the Ca_v2.1 α1 subunit (CACNA1A) have been linked to absence seizures in both humans and rodents. Similarly, mutations and loss of function mutations in ancillary HVA calcium channel subunits known to co-assemble with Ca_v2.1 result in IGE phenotypes in mice. It is important to note that in all these mouse models with mutations in HVA subunits, there is a compensatory increase in thalamic LVA currents which likely leads to the seizure phenotype. In fact, gain-of-function mutations have been identified in Ca_v3.2 (an LVA or T-type calcium channel encoded by the CACNA1H gene) in patients with congenital forms of IGEs, consistent with increased excitability of neurons as a result of enhanced T-type channel function. In this paper, we provide a broad overview of the roles of voltage-gated calcium channels, their mutations, and how they might contribute to the river that terminates in epilepsy.     

Modulation of T-type calcium channels by bioactive lipids

T-type calcium channels (T-channels/Ca_V3) have unique biophysical properties allowing a calcium influx at resting membrane potential of most cells. T-channels are ubiquitously expressed in many tissues and contribute to low-threshold spikes and burst firing in central neurons as well as to pacemaker activities in cardiac cells. They also emerged as potential targets to treat cancer and hypertension. Regulation of these channels appears complex, and several studies have indicated that Ca_V3.1, Ca_V3.2, and Ca_V3.3 currents are directly inhibited by multiple endogenous lipids independently of membrane receptors or intracellular pathways. These bioactive lipids include arachidonic acid and ω3 poly-unsaturated fatty acids; the endocannabinoid anandamide and other N-acylethanolamides; the lipoamino-acids and lipo-neurotransmitters; the P450 epoxygenase metabolite 5,6-epoxyeicosatrienoic acid; as well as similar molecules with 18–22 carbons in the alkyl chain. In this review, we summarize evidence for direct effects of these signaling molecules, the molecular mechanisms underlying the current inhibition, and the involved chemical features. The impact of this modulation in physiology and pathophysiology is discussed with a special emphasis on pain aspects and vasodilation. Overall, these data clearly indicate that T-current inhibition is an important mechanism by which bioactive lipids mediate their physiological functions.     

Neuropathic pain: role for presynaptic T-type channels in nociceptive signaling

Pain is an important clinical problem and, in its chronic form, may be a disabling condition. Most currently available therapies are insufficient and/or accompanied by serious side effects. Recent studies have implicated the Ca_V3.2 isoform of T-type Ca channels in nociceptive signaling. Ca_V3.2 channels are located in the somas of dorsal root ganglion cells and in the central endings of these cells in the dorsal horn of the spinal cord. These channels can support the development and maintenance of both physiological (nociceptive) and pathological (neuropathic) pain. In this review, we summarize the most recent evidence linking the presynaptic Ca_V3.2 channels to the etiology of neuropathic pain disorders. In particular, we focus on data linking plasticity of Ca_V3.2 channels with neuropathic pain disorders associated with mechanical peripheral nerve injury and with diabetic peripheral neuropathy. We also discuss the development of potential pain therapies aimed at these channels.     

Mibefradil reduces blood glucose concentration in db/db mice

OBJECTIVE:

Numerous recent studies suggest that abnormal intracellular calcium concentration ([Ca²⁺]_i) is a common defect in diabetic animal models and patients. Abnormal calcium handling is an important mechanism in the defective pancreatic β-cell function in type 2 diabetes. T-type Ca²⁺ channel antagonists lower blood glucose in type 2 diabetes, but the mechanism remains unknown.

METHODS:

We examined the effect of the Ca²⁺ channel antagonist mibefradil on blood glucose in male db/db mice and phenotypically normal heterozygous mice by intraperitoneal injection.

RESULTS:

Mibefradil (15 mg/kg, i.p., b.i.d.) caused a profound reduction of fasting blood glucose from 430.92±20.46 mg/dl to 285.20±5.74 mg/dl in three days. The hypoglycemic effect of mibefradil was reproduced by NNC 55-0396, a compound structurally similar to mibefradil but more selective for T-type Ca²⁺ channels, but not by the specific L-type Ca²⁺ channel blocker nicardipine. Mibefradil did not show such hypoglycemic effects in heterozygous animals. In addition, triglycerides, basal insulin and food intake were significantly decreased by mibefradil treatment in the db/db mice but not in the controls. Western blot analysis, immunohistochemistry and immunofluorescence staining showed a significantly increased expression of T-type Ca²⁺ channel α-subunits Cav3.1 and Cav3.2 in liver and brain tissues from db/db mice compared to those from heterozygous animals.

CONCLUSIONS:

Collectively, these results suggest that T-type Ca²⁺ channels are potential therapeutic targets for antidiabetic drugs.     

Novel vistas of calcium-mediated signalling in the thalamus

Traditionally, the role of calcium ions (Ca²⁺) in thalamic neurons has been viewed as that of electrical charge carriers. Recent experimental findings in thalamic cells have only begun to unravel a highly complex Ca²⁺ signalling network that exploits extra- and intracellular Ca²⁺ sources. In thalamocortical relay neurons, interactions between T-type Ca²⁺ channel activation, Ca²⁺-dependent regulation of adenylyl cyclase activity and the hyperpolarization-activated cation current (I_h) regulate oscillatory burst firing during periods of sleep and generalized epilepsy, while a functional triad between Ca²⁺ influx through high-voltage-activated (most likely L-type) Ca²⁺ channels, Ca²⁺-induced Ca²⁺ release via ryanodine receptors (RyRs) and a repolarizing mechanism (possibly via K⁺ channels of the BK_Ca type) supports tonic spike firing as required during wakefulness. The mechanisms seem to be located mostly at dendritic and somatic sites, respectively. One functional compartment involving local GABAergic interneurons in certain thalamic relay nuclei is the glomerulus, in which the dendritic release of GABA is regulated by Ca²⁺ influx via canonical transient receptor potential channels (TRPC), thereby presumably enabling transmitters of extrathalamic input systems that are coupled to phospholipase C (PLC)-activating receptors to control feed-forward inhibition in the thalamus. Functional interplay between T-type Ca²⁺ channels in dendrites and the A-type K⁺ current controls burst firing, contributing to the range of oscillatory activity observed in these interneurons. GABAergic neurons in the reticular thalamic (RT) nucleus recruit a specific set of Ca²⁺-dependent mechanisms for the generation of rhythmic burst firing, of which a particular T-type Ca²⁺ channel in the dendritic membrane, the Ca²⁺-dependent activation of non-specific cation channels (I_CAN) and of K⁺ channels (SK_Ca type) are key players. Glial Ca²⁺ signalling in the thalamus appears to be a basic mechanism of the dynamic and integrated exchange of information between glial cells and neurons. The conclusion from these observations is that a localized calcium signalling network exists in all neuronal and probably also glial cell types in the thalamus and that this network is dedicated to the precise regulation of the functional mode of the thalamus during various behavioural states.     

Absence of regulation of the T-type calcium current by Cav1.1, β1a and γ1 dihydropyridine receptor subunits in skeletal muscle cells

The subunit structure of low voltage activated T-type Ca²⁺ channels is still unknown. Co-expression of dihydropyridine receptor (DHPR) auxiliary subunits with T-type α₁ subunits in heterologous systems has produced conflicting results. In developing foetal skeletal muscle fibres which abundantly express DHPR subunits, Ca_v3.2 (α_1H) subunits are believed to underlie T-type calcium currents which disappear 2 to 3 weeks after birth. Therefore, a possible regulation of foetal skeletal muscle T-type Ca²⁺ channels by DHPR subunits was investigated in freshly isolated foetal skeletal muscle using knockout mice, which provide a powerful tool to address this question. The possible involvement of α_1S (Ca_v1.1), β₁ and γ₁ DHPR subunits was tested using dysgenic (α_1S-null), β_1a and γ₁ knockout mice. The results show that the absence of α_1S, β₁ or γ₁ DHPR subunits does not significantly affect the electrophysiological properties of T-type Ca²⁺ currents in skeletal muscle, suggesting that (1) native Ca_v3.2 is not regulated by β₁ or γ₁ DHPR subunits; (2) T-type and L-type currents have distinct and not interchangeable roles.     

Regional distribution of calcium elevation during sensory transduction in spider mechanoreceptor neurons

Spider mechanosensory VS-3 neurons receive peripheral efferent synaptic modulation, with regional variations in the types of efferent synapses and transmitter receptors. VS-3 somata possess a voltage-activated calcium current, but the levels and time courses of calcium changes in other regions are unknown. The roles of calcium in these neurons are not completely understood, but could include modulation of both mechanosensitivity and response dynamics. Here, we measured calcium concentration rises caused by single, mechanically induced action potentials in VS-3 sensory dendrites, somata and axons, using Oregon Green BAPTA-1 fluorescence. Calcium concentration rose by ∼1 nM following each action potential. Time courses of calcium rise and fall were similar in the three regions but the rise in amplitude was about 50% higher in the sensory dendrite than in the soma. Antibody to the Ca_V3.1(α_1g) isotype of T-type calcium channel labeled all three neuronal regions. Some Ca_V3.1 labeling colocalized with synapsin labeling, suggesting that calcium channels play some part in efferent modulation. We conclude that mechanically stimulated action potentials start near sensory dendrite tips and pass rapidly through the neurons to the axons, activating low voltage activated calcium channels in all three regions and causing calcium concentration to rise rapidly in each region. These results suggest important roles for calcium in several stages of mechanosensation.     

Ca2+ signaling by T-type Ca2+ channels in neurons

Among the major families of voltage-gated Ca²⁺ channels, the low-voltage-activated channels formed by the Ca_v3 subunits, referred to as T-type Ca²⁺ channels, have recently gained increased interest in terms of the intracellular Ca²⁺ signals generated upon their activation. Here, we provide an overview of recent reports documenting that T-type Ca²⁺ channels act as an important Ca²⁺ source in a wide range of neuronal cell types. The work is focused on T-type Ca²⁺ channels in neurons, but refers to non-neuronal cells in cases where exemplary functions for Ca²⁺ entering through T-type Ca²⁺ channels have been described. Notably, Ca²⁺ influx through T-type Ca²⁺ channels is the predominant Ca²⁺ source in several neuronal cell types and carries out specific signaling roles. We also emphasize that Ca²⁺ signaling through T-type Ca²⁺ channels occurs often in select subcellular compartments, is mediated through strategically co-localized targets, and is exploited for unique physiological functions. Lucius Cueni and Marco Canepari contributed equally to this review.     

Stimulation of α1-adrenoceptors reduces glutamatergic synaptic input from primary afferents through GABAA receptors and T-type Ca channels

Activation of the descending noradrenergic system inhibits nociceptive transmission in the spinal cord. Although both α₁- and α₂-adrenoceptors in the spinal cord are involved in the modulation of nociceptive transmission, it is not clear how α₁-adrenoceptors regulate excitatory and inhibitory synaptic transmission at the spinal level. In this study, inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs, respectively) were recorded from lamina II neurons in rat spinal cord slices. The specific α₁-adrenoceptor agonist phenylephrine significantly increased the frequency of GABAergic spontaneous IPSCs in a concentration dependent manner, and this effect was abolished by the α₁-adrenoceptor antagonist 2-(2,6-dimethoxyphenoxy)ethylaminomethyl-1,4-benzodioxane (WB4101). Phenylephrine also significantly reduced the amplitude of monosynaptic and polysynaptic EPSCs evoked from primary afferents. The inhibitory effect of phenylephrine on evoked monosynaptic glutamatergic EPSCs was largely blocked by the GABA_A receptor antagonist picrotoxin and, to a lesser extent, by the GABA_B receptor antagonist CGP55845. Furthermore, blocking T-type Ca²⁺ channels with amiloride or mibefradil diminished the inhibitory effect produced by phenylephrine or the GABA_A receptor agonist muscimol on monosynaptic EPSCs evoked from primary afferents. Collectively, these findings suggest that activation of α₁-adrenoceptors in the spinal cord increases synaptic GABA release, which attenuates glutamatergic input from primary afferents mainly through GABA_A receptors and T-type Ca²⁺ channels. This mechanism of presynaptic inhibition in the spinal cord may be involved in the regulation of nociception by the descending noradrenergic system.     

Calcium-dependent inhibition of T-type calcium channels by TRPV1 activation in rat sensory neurons

We studied the inhibitory effects of transient receptor potential vanilloid-1 (TRPV1) activation by capsaicin on low-voltage-activated (LVA, T-type) Ca²⁺ channel and high-voltage-activated (HVA; L, N, P/Q, R) currents in rat DRG sensory neurons, as a potential mechanism underlying capsaicin-induced analgesia. T-type and HVA currents were elicited in whole-cell clamped DRG neurons using ramp commands applied before and after 30-s exposures to 1 μM capsaicin. T-type currents were estimated at the first peak of the I–V characteristics and HVA at the second peak, occurring at more positive potentials. Small and medium-sized DRG neurons responded to capsaicin producing transient inward currents of variable amplitudes, mainly carried by Ca²⁺. In those cells responding to capsaicin with a large Ca²⁺ influx (59% of the total), a marked inhibition of both T-type and HVA Ca²⁺ currents was observed. The percentage of T-type and HVA channel inhibition was prevented by replacing Ca²⁺ with Ba²⁺ during capsaicin application or applying high doses of intracellular BAPTA (20 mM), suggesting that TRPV1-mediated inhibition of T-type and HVA channels is Ca²⁺-dependent and likely confined to membrane nano-microdomains. Our data are consistent with the idea that TRPV1-induced analgesia may derive from indirect inhibition of both T-type and HVA channels which, in turn, would reduce the threshold of nociceptive signals generation (T-type channel inhibition) and nociceptive synaptic transmission (HVA-channels inhibition).     

T-type channels-secretion coupling: evidence for a fast low-threshold exocytosis

T-type channels are transient low-voltage-activated (LVA) Ca²⁺ channels that control Ca²⁺ entry in excitable cells during small depolarizations around resting potential. Studies in the past 20 years focused on the biophysical, physiological, and molecular characterization of T-type channels in most tissues. This led to a well-defined picture of the functional role of LVA channels in controlling low-threshold spikes, oscillatory cell activity, muscle contraction, hormone release, cell growth and differentiation. So far, little attention has been devoted to the role of T-type channels in transmitter release, which mainly involves channel types belonging to the high-voltage-activated (HVA) Ca²⁺ channel family. However, evidence is accumulating in favor of a unique participation of T-type channels in fast transmitter release. Clear data are now reported in reciprocal synapses of the retina and olfactory bulb, synaptic contacts between primary afferent and second order nociceptive neurons, rhythmic inhibitory interneurons of invertebrates and clonal cell lines transfected with recombinant α₁ channel subunits. T-type channels also regulate the large dense-core vesicle release of neuroendocrine cells where Ca²⁺ dependence, rate of vesicle release, and size of readily releasable pool appear comparable to those associated to HVA channels. This suggests that when sufficiently expressed and properly located near the release zones, T-type channels can trigger fast low-threshold secretion. In this study, we will review the main findings that assign a specific task to T-type channels in fast exocytosis, discussing their possible involvement in the control of the Ca²⁺-dependent processes regulating exocytosis like vesicle depletion and vesicle recycling.     

Voltage-gated calcium channels in chronic pain: emerging role of alternative splicing

N- and T-type voltage-gated calcium channels are key established players in chronic pain. Current work suggests that alternative splicing of these channels constitutes an important aspect in the investigation of their roles in the pathogenesis of chronic pain. Recent N-type channel studies describe a nociceptor-enriched alternatively spliced module responsible for voltage-independent G protein modulation and internalization, which is implicated in the control of distinct nociceptive pathways. On the contrary, although a large body of work has demonstrated that peripheral Ca_v3.2-encoded T-type currents are involved in several types of chronic pain, little is known with respect to the expression of numerous newly discovered splice variants in specific pain pathways. The elucidation of the new layers of molecular complexity uncovered in N- and T-type channel splice variants and their respective locations and roles in different pain pathways will allow for the development of better therapeutic strategies for the treatment of chronic pain.     

Ca2+-dependent large conductance K+ currents in thalamocortical relay neurons of different rat strains

Mutations in genes coding for Ca²⁺ channels were found in patients with childhood absence epilepsy (CAE) indicating a contribution of Ca²⁺-dependent mechanisms to the generation of spike-wave discharges (SWD) in humans. Since the involvement of Ca²⁺ signals remains unclear, the aim of the present study was to elucidate the function of a Ca²⁺-dependent K⁺ channel (BK_Ca) under physiological conditions and in the pathophysiological state of CAE. The activation of BK_Ca channels is dependent on both voltage and intracellular Ca²⁺ concentrations. Moreover, these channels exhibit an outstandingly high level of regulatory heterogeneity that builds the basis for the influence of BK_Ca channels on different aspects of neuronal activity. Here, we analyse the contribution of BK_Ca channels to firing of thalamocortical relay neurons, and we test the hypothesis that BK_Ca channel activity affects the phenotype of a genetic rat model of CAE. We found that the activation of the β₂-adrenergic receptor/protein kinase A pathway resulted in BK_Ca channel inhibition. Furthermore, BK_Ca channels affect the number of action potentials fired in a burst and produced spike frequency adaptation during tonic activity. The latter result was confirmed by a computer modelling approach. We demonstrate that the β₂-adrenergic inhibition of BK_Ca channels prevents spike frequency adaptation and, thus, might significantly support the tonic firing mode of thalamocortical relay neurons. In addition, we show that BK_Ca channel functioning differs in epileptic WAG/Rij and thereby likely contributes to highly synchronised, epileptic network activity.     

Angiotensin II regulates endothelial cell migration through calcium influx via T-type calcium channel in human umbilical vein endothelial cells

Aim: The T-type calcium channel is expressed in vascular endothelial cells, but its role in endothelial cell function is yet to be elucidated. We analysed the endothelial functional role of T-type calcium channel-dependent calcium under angiotensin II (Ang II) stimulation. Methods: Human umbilical vein endothelial cells were co-incubated with hormone at 10⁻⁷ m and either Efonidipine 10⁻⁵ m or Verapamil 10⁻⁵ m or Mibefradil 10⁻⁵ m or Wortmannin 10⁻⁶ m . The contribution of Ang II receptors was evaluated using PD123319 10⁻⁷ m and ZD 7155 10⁻⁷ m . The calcium ion concentration was observed using Fluo-3 acetossimetil ester. The cells were observed after 3, 6, 9 and 12 h. Results: The microfluorescence method points out that Ang II induces intracellular calcium modulation in time by distinct mechanisms. AT2 receptor blockade is necessary to observe significant increase in [Ca²⁺]_i levels. Pre-treatment with Mibefradil abolishes Ang II -induced cell migration. Conclusions: Our data show that Ang II, via AT1 receptor, modulates calcium concentration involving T-type calcium channel and L-type calcium channel but only the calcium influx via T-type calcium channels regulates endothelial cell migration which is essential for angiogenesis.     

T-type channel-mediated neurotransmitter release

Besides controlling a wide variety of cell functions, T-type channels have been shown to regulate neurotransmitter release in peripheral and central synapses and neuroendocrine cells. Growing evidence over the last 10 years suggests a key role of Cav3.2 and Cav3.1 channels in controlling basal neurosecretion near resting conditions and sustained release during mild stimulations. In some cases, the contribution of low-voltage-activated (LVA) channels is not directly evident but requires either the activation of coupled presynaptic receptors, block of ion channels, or chelation of metal ions. Concerning the coupling to the secretory machinery, T-type channels appear loosely coupled to neurotransmitter and hormone release. In neurons, Cav3.2 and Cav3.1 channels mainly control the asynchronous appearance of “minis” [miniature inhibitory postsynaptic currents (mIPSCs) and miniature excitatory postsynaptic currents (mEPSCs)]. The same loose coupling is evident from membrane capacity and amperometric recordings in chromaffin cells and melanotropes where the low-threshold-driven exocytosis possesses the same linear Ca²⁺ dependence of the other voltage-gated Ca²⁺ channels (Cav1 and Cav2) that is strongly attenuated by slow calcium buffers. The intriguing issue is that, despite not expressing a consensus “synprint” site, Cav3.2 channels do interact with syntaxin 1A and SNAP-25 and, thus, may form nanodomains with secretory vesicles that can be regulated at low voltages. In this review, we discuss all the past and recent issues related to T-type channel-secretion coupling in neurons and neuroendocrine cells.     

Role of T-type channels in vasomotor function: team player or chameleon?

Low-voltage-activated T-type calcium channels play an important role in regulating cellular excitability and are implicated in conditions, such as epilepsy and neuropathic pain. T-type channels, especially Cav3.1 and Cav3.2, are also expressed in the vasculature, although patch clamp studies of isolated vascular smooth muscle cells have in general failed to demonstrate these low-voltage-activated calcium currents. By contrast, the channels which are blocked by T-type channel antagonists are high-voltage activated but distinguishable from their L-type counterparts by their T-type biophysical properties and small negative shifts in activation and inactivation voltages. These changes in T-channel properties may result from vascular-specific expression of splice variants of Cav3 genes, particularly in exon 25/26 of the III–IV linker region. Recent physiological studies suggest that T-type channels make a small contribution to vascular tone at low intraluminal pressures, although the relevance of this contribution is unclear. By contrast, these channels play a larger role in vascular tone of small arterioles, which would be expected to function at lower intra-vascular pressures. Upregulation of T-type channel function following decrease in nitric oxide bioavailability and increase in oxidative stress, which occurs during cardiovascular disease, suggests that a more important role could be played by these channels in pathophysiological situations. The ability of T-type channels to be rapidly recruited to the plasma membrane, coupled with their subtype-specific localisation in signalling microdomains where they could modulate the function of calcium-dependent ion channels and pathways, provides a mechanism for rapid up- and downregulation of vasoconstriction. Future investigation into the molecules which govern these changes may illuminate novel targets for the treatment of conditions such as therapy-resistant hypertension and vasospasm.