Contrasting Effects of Cd2+ and Co2+ on the Blocking/Unblocking of Human Cav3 Channels

Prosthetic Metals: Release, Metabolism and Toxicity

The development of metallic joint prostheses has been ongoing for more than a century alongside advancements in hip and knee arthroplasty. Among the materials utilized, the Cobalt-Chromium-Molybdenum (Co-Cr-Mo) and Titanium-Aluminum-Vanadium (Ti-Al-V) alloys are predominant in joint prosthesis construction, predominantly due to their commendable biocompatibility, mechanical strength, and corrosion resistance. Nonetheless, over time, the physical wear, electrochemical corrosion, and inflammation induced by these alloys that occur post-implantation can cause the release of various metallic components. The released metals can then flow and metabolize in vivo, subsequently causing potential local or systemic harm. This review first details joint prosthesis development and acknowledges the release of prosthetic metals. Second, we outline the metallic concentration, biodistribution, and elimination pathways of the released prosthetic metals. Lastly, we discuss the possible organ, cellular, critical biomolecules, and significant signaling pathway toxicities and adverse effects that arise from exposure to these metals.

Zn2+-induced changes in Cav2.3 channel function: An electrophysiological and modeling study

Loosely bound Zn²⁺ ions are increasingly recognized as potential modulators of synaptic plasticity and neuronal excitability under normal and pathophysiological conditions. Ca_v2.3 voltage-gated Ca²⁺ channels are among the most sensitive targets of Zn²⁺ and are therefore likely to be involved in the neuromodulatory actions of endogenous Zn²⁺. Although histidine residues on the external side of domain I have been implicated in the effects on Ca_v2.3 channel gating, the exact mechanisms involved in channel modulation remain incompletely understood. Here, we use a combination of electrophysiological recordings, modification of histidine residues, and computational modeling to analyze Zn²⁺-induced changes in Ca_v2.3 channel function. Our most important findings are that multiple high- and low-affinity mechanisms contribute to the net Zn²⁺ action, that Zn²⁺ can either inhibit or stimulate Ca²⁺ influx through Ca_v2.3 channels depending on resting membrane potential, and that Zn²⁺ effects may persist for some time even after cessation of the Zn²⁺ signal. Computer simulations show that (1) most salient features of Ca_v2.3 channel gating in the absence of trace metals can be reproduced by an obligatory model in which activation of two voltage sensors is necessary to open the pore; and (2) most, but not all, of the effects of Zn²⁺ can be accounted for by assuming that Zn²⁺ binding to a first site is associated with an electrostatic modification and mechanical slowing of one of the voltage sensors, whereas Zn²⁺ binding to a second, lower-affinity site blocks the channel and modifies the opening and closing transitions. While still far from complete, our model provides a first quantitative framework for understanding Zn²⁺ effects on Ca_v2.3 channel function and a step toward the application of computational approaches for predicting the complex actions of Zn²⁺ on neuronal excitability.

Cav2.3 channel function and Zn2+-induced modulation: potential mechanisms and (patho)physiological relevance

Voltage-gated calcium channels (VGCCs) are critical for Ca²⁺ influx into all types of excitable cells, but their exact function is still poorly understood. Recent reconstruction of homology models for all human VGCCs at atomic resolution provides the opportunity for a structure-based discussion of VGCC function and novel insights into the mechanisms underlying Ca²⁺ selective flux through these channels. In the present review, we use these data as a basis to examine the structure, function, and Zn²⁺-induced modulation of Ca_v2.3 VGCCs, which mediate native R-type currents and belong to the most enigmatic members of the family. Their unique sensitivity to Zn²⁺ and the existence of multiple mechanisms of Zn²⁺ action strongly argue for a role of these channels in the modulatory action of endogenous loosely bound Zn²⁺, pools of which have been detected in a number of neuronal, endocrine, and reproductive tissues. Following a description of the different mechanisms by which Zn²⁺ has been shown or is thought to alter the function of these channels, we discuss their potential (patho)physiological relevance, taking into account what is known about the magnitude and function of extracellular Zn²⁺ signals in different tissues. While still far from complete, the picture that emerges is one where Ca_v2.3 channel expression parallels the occurrence of loosely bound Zn²⁺ pools in different tissues and where these channels may serve to translate physiological Zn²⁺ signals into changes of electrical activity and/or intracellular Ca²⁺ levels.

Interaction of MDIMP with the Voltage-Gated Calcium Channels

Amino acid-derived isoindolines are synthetic compounds that were created with the idea of investigating their biological actions. The amino acid moiety was included on the grounds that it may help to avoid toxic effects. Recently, the isoindoline MDIMP was shown to inhibit both cardiac excitation-contraction coupling and voltage-dependent calcium channels. Here, we revealed that MDIMP binds preferentially to low-voltage-activated (LVA) channels. Using a holding potential of -90 mV, the following IC₅₀ values were found (in micromolars): >1000 (Ca_V2.3), 957 (Ca_V1.3), 656 (Ca_V1.2), 219 (Ca_V3.2), and 132 (Ca_V3.1). Moreover, the isoindoline also promoted both accelerated inactivation kinetics of high-voltage-activated Ca²⁺ channels and a modest upregulation of Ca_V1.3 and Ca_V2.3. Additional data indicate that although MDIMP binds to the closed state of the channels, it has more preference for the inactivated one. Concerning Ca_V3.1, the compound did not alter the shape of the instantaneous current-voltage curve, and substituting one or two residues in the selectivity filter drastically increased the IC₅₀ value, suggesting that MDIMP binds to the extracellular side of the pore. However, an outward current failed in removing the inhibition, which implies an alternative mechanism may be involved. The enantiomer (R)-MDIMP [methyl (R)-2-(1,3-dihydroisoindol-2-yl)-4-methylpentanoate], on the other hand, was synthesized and evaluated, but it did not improve the affinity to LVA channels. Implications of these findings are discussed in terms of the possible underlying mechanisms and pharmacological relevance. SIGNIFICANCE STATEMENT: We have studied the regulation of voltage-gated calcium channels by MDIMP, which disrupts excitation-contraction coupling in cardiac myocytes. The latter effect is more potent in atrial than ventricular myocytes, and this could be explained by our results showing that MDIMP preferentially blocks low-voltage-activated channels. Our data also provide mechanistic insights about the blockade and suggest that MDIMP is a promising member of the family of Ca²⁺ channel blockers, with possible application to the inhibition of subthreshold membrane depolarizations.

Selective blockage of Serratia marcescens ShlA by nickel inhibits the pore-forming toxin-mediated phenotypes in eukaryotic cells

Serratia marcescens is an opportunistic pathogen with increasing incidence in clinical settings. This is mainly attributed to the timely expression of a wide diversity of virulence factors and intrinsic and acquired resistance to antibiotics, including β-lactams, aminoglycosides, quinolones, and polypeptides. For these reasons, S. marcescens has been recently categorised by the World Health Organization as one priority to strengthen efforts directed to develop new antibacterial agents. Therefore, it becomes critical to understand the underlying mechanisms that allow Serratia to succeed within the host. S. marcescens ShlA pore-forming toxin mediates phenotypes that alter homeostatic and signal transduction pathways of host cells. It has been previously demonstrated that ShlA provokes cytotoxicity, haemolysis and autophagy and also directs Serratia egress and dissemination from invaded nonphagocytic cells. However, molecular details of ShlA mechanism of action are still not fully elucidated. In this work, we demonstrate that Ni²⁺ selectively and reversibly blocks ShlA action, turning wild-type S. marcescens into a shlA mutant strain phenocopy. Combined use of Ni²⁺ and calcium chelators allow to discern ShlA-triggered phenotypes that require intracellular calcium mobilisation and reveal ShlA function as a calcium channel, providing new insights into ShlA mode of action on target cells.

T-type Calcium Channels in Health and Disease

Low Voltage-Activated (LVA) T-type calcium channels are characterized by transient current and Low Threshold Spikes (LTS) that trigger neuronal firing and oscillatory behavior. Combined with their preferential localization in dendrites and their specific "window current", T-type calcium channels are considered to be key players in signal amplification and synaptic integration. Assisted by the emerging pharmacological tools, the structural determinants of channel gating and kinetics, as well as novel physiological and pathological functions of T-type calcium channels, are being uncovered. In this review, we provide an overview of structural determinants in T-type calcium channels, their involvement in disorders and diseases, the development of novel channel modulators, as well as Structure-Activity Relationship (SAR) studies that lead to rational drug design.

Oxytocin produces thermal analgesia via vasopressin-1a receptor by modulating TRPV1 and potassium conductance in the dorsal root ganglion neurons

Recent studies have provided several lines of evidence that peripheral administration of oxytocin induces analgesia in human and rodents. However, the exact underlying mechanism of analgesia still remains elusive. In the present study, we aimed to identify which receptor could mediate the analgesic effect of intraperitoneal injection of oxytocin and its cellular mechanisms in thermal pain behavior. We found that oxytocin-induced analgesia could be reversed by d(CH₂)₅[Tyr(Me)²,Dab⁵] AVP, a vasopressin-1a (V1a) receptor antagonist, but not by desGly-NH₂-d(CH₂)₅[DTyr², Thr⁴]OVT, an oxytocin receptor antagonist. Single cell RT-PCR analysis revealed that V1a receptor, compared to oxytocin, vasopressin-1b and vasopressin-2 receptors, was more profoundly expressed in dorsal root ganglion (DRG) neurons and the expression of V1a receptor was predominant in transient receptor potential vanilloid 1 (TRPV1)-expressing DRG neurons. Fura-2 based calcium imaging experiments showed that capsaicin-induced calcium transient was significantly inhibited by oxytocin and that such inhibition was reversed by V1a receptor antagonist. Additionally, whole cell patch clamp recording demonstrated that oxytocin significantly increased potassium conductance via V1a receptor in DRG neurons. Taken together, our findings suggest that analgesic effects produced by peripheral administration of oxytocin were attributable to the activation of V1a receptor, resulting in reduction of TRPV1 activity and enhancement of potassium conductance in DRG neurons.

Increase in Ca2+ current by sustained cAMP levels enhances proliferation rate in GH3 cells

AIMS

Ca²⁺ and cAMP are important intracellular modulators. In order to generate intracellular signals with various amplitudes, as well as different temporal and spatial properties, a tightly and precise control of these modulators in intracellular compartments is necessary. The aim of this study was to evaluate the effects of elevated and sustained cAMP levels on voltage-dependent Ca²⁺ currents and proliferation in pituitary tumor GH3 cells.

MAIN METHODS

Effect of long-term exposure to forskolin and dibutyryl-cyclic AMP (dbcAMP) on Ca²⁺ current density and cell proliferation rate were determined by using the whole-cell patch-clamp technique and real time cell monitoring system. The cAMP levels were assayed, after exposing transfected GH3 cells with the EPAC-1 cAMP sensor to forskolin and dbcAMP, by FRET analysis.

KEY FINDINGS

Sustained forskolin treatment (24 and 48h) induced a significant increase in total Ca²⁺ current density in GH3 cells. Accordingly, dibutyryl-cAMP incubation (dbcAMP) also elicited increase in Ca²⁺ current density. However, the maximum effect of dbcAMP occurred only after 72h incubation, whereas forskolin showed maximal effect at 48h. FRET-experiments confirmed that the time-course to elevate intracellular cAMP was distinct between forskolin and dbcAMP. Mibefradil inhibited the fast inactivating current component selectively, indicating the recruitment of T-type Ca²⁺ channels. A significant increase on cell proliferation rate, which could be related to the elevated and sustained intracellular levels of cAMP was observed.

SIGNIFICANCE

We conclude that maintaining high levels of intracellular cAMP will cause an increase in Ca²⁺ current density and this phenomenon impacts proliferation rate in GH3 cells.

Ecotoxicological assessment of cobalt using Hydra model: ROS, oxidative stress, DNA damage, cell cycle arrest, and apoptosis as mechanisms of toxicity

The mechanisms underlying cobalt toxicity in aquatic species in general and cnidarians in particular remain poorly understood. Herein we investigated cobalt toxicity in a Hydra model from morphological, histological, developmental, and molecular biological perspectives. Hydra, exposed to cobalt (0-60 mg/L), were altered in morphology, histology, and regeneration. Exposure to standardized sublethal doses of cobalt impaired feeding by affecting nematocytes, which in turn affected reproduction. At the cellular level, excessive ROS generation, as the principal mechanism of action, primarily occurred in the lysosomes, which was accompanied by the upregulation of expression of the antioxidant genes SOD, GST, GPx, and G6PD. The number of Hsp70 and FoxO transcripts also increased. Interestingly, the upregulations were higher in the 24-h than in the 48-h time-point group, indicating that ROS overwhelmed the cellular defense mechanisms at the latter time-point. Comet assay revealed DNA damage. Cell cycle analysis indicated the induction of apoptosis accompanied or not by cell cycle arrest. Immunoblot analyses revealed that cobalt treatment triggered mitochondria-mediated apoptosis as inferred from the modulation of the key proteins Bax, Bcl-2, and caspase-3. From this data, we suggest the use of Hydra as a model organism for the risk assessment of heavy metal pollution in aquatic ecosystems.

Iron transport in the kidney: implications for physiology and cadmium nephrotoxicity

The kidney has recently emerged as an organ with a significant role in systemic iron (Fe) homeostasis. Substantial amounts of Fe are filtered by the kidney, which have to be reabsorbed to prevent Fe deficiency. Accordingly Fe transporters and receptors for protein-bound Fe are expressed in the nephron that may also function as entry pathways for toxic metals, such as cadmium (Cd), by way of "ionic and molecular mimicry". Similarities, but also differences in handling of Cd by these transport routes offer rationales for the propensity of the kidney to develop Cd toxicity. This critical review provides a comprehensive update on Fe transport by the kidney and its relevance for physiology and Cd nephrotoxicity. Based on quantitative considerations, we have also estimated the in vivo relevance of the described transport pathways for physiology and toxicology. Under physiological conditions all segments of the kidney tubules are likely to utilize Fe for cellular Fe requiring processes for metabolic purposes and also to contribute to reabsorption of free and bound forms of Fe into the circulation. But Cd entering tubule cells disrupts metabolic pathways and is unable to exit. Furthermore, our quantitative analyses contest established models linking chronic Cd nephrotoxicity to proximal tubular uptake of metallothionein-bound Cd. Hence, Fe transport by the kidney may be beneficial by preventing losses from the body. But increased uptake of Fe or Cd that cannot exit tubule cells may lead to kidney injury, and Fe deficiency may facilitate renal Cd uptake.

Cd(2+) sensitivity and permeability of a low voltage-activated Ca(2+) channel with CatSper-like selectivity filter

CatSper is a sperm-specific Ca(2+) channel that plays an essential role in the male fertility. However, its biophysical properties have been poorly characterized mainly due to its deficient heterologous expression. As other voltage-gated Ca(2+) channels (CaVs), CatSper possesses a conserved Ca(2+)-selective filter motif ([T/S]x[D/E]xW) in the pore region. Interestingly, CatSper conserves four aspartic acids (DDDD) as the negatively charged residues in this motif while high voltage-activated CaVs have four glutamic acids (EEEE) and low voltage-activated CaVs possess two glutamic acids and two aspartic acids (EEDD). Previous studies based on site-directed mutagenesis of L- and T-type channels showed that the number of D seems to have a negative correlation with their cadmium (Cd(2+)) sensitivity. These results suggest that CatSper (DDDD) would have low sensitivity to Cd(2+). To explore Cd(2+)-sensitivity and -permeability of CatSper, we performed two types of experiments: 1) Electrophysiological analysis of heterologously expressed human CaV3.1 channel and three pore mutants (DEDD, EDDD and DDDD), 2) Cd(2+) imaging of human spermatozoa with FluoZin-1. Electrophysiological studies showed a significant increase in Cd(2+) and manganese (Mn(2+)) currents through the CaV3.1 mutants as well as a reduction in the inhibitory effect of Cd(2+) on the Ca(2+) current. In fluorescence imaging with human sperm, we observed an increase in Cd(2+) influx potentiated by progesterone, a potent activator of CatSper. These results support our hypothesis, namely that Cd(2+)-sensitivity and -permeability are related to the absolute number of D in the Ca(2+)-selective filter independently to the type of the Cav channels.

Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents

Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (NaV) channels in this diversity has not been investigated because NaV currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal NaV channels. RT-PCR experiments, however, indicated expression of diverse NaV channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed NaV currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other NaV currents. Some VGNs had a small current with properties consistent with NaV1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed NaV1.5-like immunoreactivity. Other VGNs had a current with the properties of NaV1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two NaV1.8 reporter lines, subsets of VGNs were labeled. VGNs with NaV1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of NaV channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

Distinct roles of L- and T-type voltage-dependent Ca2+ channels in regulation of lymphatic vessel contractile activity

Lymph drainage maintains tissue fluid homeostasis and facilitates immune response. It is promoted by phasic contractions of collecting lymphatic vessels through which lymph is propelled back into the blood circulation. This rhythmic contractile activity (i.e. lymphatic pumping) increases in rate with increase in luminal pressure and relies on activation of nifedipine-sensitive voltage-dependent Ca(2+) channels (VDCCs). Despite their importance, these channels have not been characterized in lymphatic vessels. We used pressure- and wire-myography as well as intracellular microelectrode electrophysiology to characterize the pharmacological and electrophysiological properties of L-type and T-type VDCCs in rat mesenteric lymphatic vessels and evaluated their particular role in the regulation of lymphatic pumping by stretch. We complemented our study with PCR and confocal immunofluorescence imaging to investigate the expression and localization of these channels in lymphatic vessels. Our data suggest a delineating role of VDCCs in stretch-induced lymphatic vessel contractions, as the stretch-induced increase in force of lymphatic vessel contractions was significantly attenuated in the presence of L-type VDCC blockers nifedipine and diltiazem, while the stretch-induced increase in contraction frequency was significantly decreased by the T-type VDCC blockers mibefradil and nickel. The latter effect was correlated with a hyperpolarization. We propose that activation of T-type VDCCs depolarizes membrane potential, regulating the frequency of lymphatic contractions via opening of L-type VDCCs, which drive the strength of contractions.

R-type calcium channels are crucial for semaphorin 3A-induced DRG axon growth cone collapse

Semaphorin 3A (Sema3A) is a secreted protein involved in axon path-finding during nervous system development. Calcium signaling plays an important role during axonal growth in response to different guidance cues; however it remains unclear whether this is also the case for Sema3A. In this study we used intracellular calcium imaging to figure out whether Sema3A-induced growth cone collapse is a Ca²⁺ dependent process. Intracellular Ca²⁺ imaging results using Fura-2 AM showed Ca²⁺ increase in E15 mice dorsal root ganglia neurons upon Sema3A treatment. Consequently we analyzed Sema3A effect on growth cones after blocking or modifying intracellular and extracellular Ca²⁺ channels that are expressed in E15 mouse embryos. Our results demonstrate that Sema3A increased growth cone collapse rate is blocked by the non-selective R- and T- type Ca²⁺ channel blocker NiCl₂ and by the selective R-type Ca²⁺ channel blocker SNX482_. These Ca²⁺ channel blockers consistently decreased the Sema3A-induced intracellular Ca²⁺ concentration elevation. Overall, our results demonstrate that Sema3A-induced growth cone collapses are intimately related with increase in intracellular calcium concentration mediated by R-type calcium channels.

Contribution of different Ca2+ channels to the acrosome reaction-mediated initiation of sperm motility in the newt Cynops pyrrhogaster

Initiation of sperm motility in urodeles, which is induced by a sperm motility-initiating substance (SMIS) in the sequestered granules on the surface of egg jelly, is mediated by the acrosome reaction (AR), which is triggered by an AR-inducing substance (ARIS) on a sheet-like structure. Details of the unique process of the interaction between egg jelly and sperm in these species is still unclear. The current study showed the fine structure of egg jelly in the newt Cynops pyrrhogaster, a urodele species, revealing that its outer surface was covered by a sheet-like structure of approximately 0.29 μm in thickness. Granules of approximately 2 μm in diameter with small particles of approximately 54 nm were attached to its surface and distributed inhomogeneously just beneath the sheet-like structure. Emission spectrometry revealed that the Ca2+ concentration was maintained at a high level compared with that of the blood plasma and the vas deferens fluid, suggesting that egg jelly is a reliable source of Ca2+ for the sperm–egg interaction. Blockers of the T-type voltage-dependent Ca2+ channel (VDCC), but not the L-type VDCC, inhibited both AR and initiation of sperm motility. Conversely, Ni+, which affects the α1 H subunit of T-type VDCC, only inhibited the initiation of sperm motility. These data suggest that, in response to ARIS and SMIS, sequential gating of distinct Ca2+ channels occurs in the AR, followed by the initiation of sperm motility on the surface of the egg jelly in C. pyrrhogaster at fertilization.

Cd²⁺ block and permeation of CaV3.1 (α1G) T-type calcium channels: candidate mechanism for Cd²⁺ influx

Cd²⁺ is an industrial pollutant that can cause cytotoxicity in multiple organs. We examined the effects of extracellular Cd²⁺ on permeation and gating of Ca(v)3.1 (α1G) channels stably transfected in HEK293 cells, by using whole-cell recording. With the use of instantaneous I-V currents (measured after strong depolarization) to isolate the effects on permeation, Cd²⁺ rapidly blocked currents with 2 mM Ca²⁺ in a voltage-dependent manner. The block caused by Cd²⁺ was relieved at more-hyperpolarized potentials, which suggests that Cd²⁺ can permeate through the selectivity filter of the channel into the cytosol. In the absence of other permeant ions (Ca²⁺ and Na⁺ replaced by N-methyl-d-glucamine), Cd²⁺ carried sizable inward currents through Ca(v)3.1 channels (210 ± 20 pA at -60 mV with 2 mM Cd²⁺). Ca(v)3.1 channels have a significant "window current" at that voltage (open probability, ∼1%), which makes them a candidate pathway for Cd²⁺ entry into cells during Cd²⁺ exposure. Incubation with radiolabeled ¹⁰⁹Cd²⁺ confirmed uptake of Cd²⁺ into cells with Ca(v)3.1 channels.

Atypical Ca2+ currents in chromaffin cells from SHR and WKY rat strains result from the deficient expression of a splice variant of the α1D Ca2+ channel

Ca2+ currents ( ICa) recorded from adrenal chromaffin cells (CCs) of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats are similar to one another, but different from those recorded in other rodent species. ICa in WKY/SHR CCs comprises an early, transient ( ICae) and a late, sustained component ( ICas). In Wistar CCs, ICae is absent, and ICas is of greater amplitude. Activation and steady-state inactivation of ICae and ICas in WKY/SHR CCs suggest the recruitment of at least two populations of Ca2+ channels with different voltage dependence and kinetics. In WKY/SHR CCs, ICae is inhibited by nifedipine, enhanced by BAY K 8644, is not blocked by the mibefradil analog NNC 55–0396, and displays Ca2+-dependent inactivation and fast deactivation kinetics, suggesting that it results from the opening of L-type rather than T-type Ca2+ channels. ICae properties suggest that it originates from the opening of Ca2+ channels formed with the short splice variant (CaV1.342A). RT-PCR showed that expression of CaV1.342A mRNA is similar in both Wistar and WKY/SHR, but that the long variant (CaV1.342) is virtually absent in WKY/SHR. Thus ICae corresponds to the recruitment of CaV1.342A channels, unmasked by the absence of CaV1.342 channels. Studies in WKY CCs do not report major functional alterations, despite the unusual expression pattern of CaV1.3 splice variants. It remains to be established if more subtle functional alterations exist, and if the atypical splicing pattern of CaV1.3 could be related to the functional and behavioral alterations reported in WKY/SHR rats, including their susceptibility to develop hypertension.

Ni2+ block of CaV3.1 (alpha1G) T-type calcium channels

Ni²⁺ inhibits current through calcium channels, in part by blocking the pore, but Ni²⁺ may also allosterically affect channel activity via sites outside the permeation pathway. As a test for pore blockade, we examined whether the effect of Ni²⁺ on Ca_V3.1 is affected by permeant ions. We find two components to block by Ni²⁺, a rapid block with little voltage dependence, and a slow block most visible as accelerated tail currents. Rapid block is weaker for outward vs. inward currents (apparent K_d = 3 vs. 1 mM Ni²⁺, with 2 mM Ca²⁺ or Ba²⁺) and is reduced at high permeant ion concentration (110 vs. 2 mM Ca²⁺ or Ba²⁺). Slow block depends both on the concentration and on the identity of the permeant ion (Ca²⁺ vs. Ba²⁺ vs. Na⁺). Slow block is 2–3× faster in Ba²⁺ than in Ca²⁺ (2 or 110 mM), and is ∼10× faster with 2 vs. 110 mM Ca²⁺ or Ba²⁺. Slow block is orders of magnitude slower than the diffusion limit, except in the nominal absence of divalent cations (∼3 μM Ca²⁺). We conclude that both fast and slow block of Ca_V3.1 by Ni²⁺ are most consistent with occlusion of the pore. The exit rate of Ni²⁺ for slow block is reduced at high Ni²⁺ concentrations, suggesting that the site responsible for fast block can “lock in” slow block by Ni²⁺, at a site located deeper within the pore. In contrast to the complex pore block observed for Ca_V3.1, inhibition of Ca_V3.2 by Ni²⁺ was essentially independent of voltage, and was similar in 2 mM Ca²⁺ vs. Ba²⁺, consistent with inhibition by a different mechanism, at a site outside the pore.