Endothelin-1 enhances calcium entry through T-type calcium channels in cultured neonatal rat ventricular myocytes

Voltage-Gated T-Type Calcium Channel Modulation by Kinases and Phosphatases: The Old Ones, the New Ones, and the Missing Ones

Calcium (Ca²⁺) can regulate a wide variety of cellular fates, such as proliferation, apoptosis, and autophagy. More importantly, changes in the intracellular Ca²⁺ level can modulate signaling pathways that control a broad range of physiological as well as pathological cellular events, including those important to cellular excitability, cell cycle, gene-transcription, contraction, cancer progression, etc. Not only intracellular Ca²⁺ level but the distribution of Ca²⁺ in the intracellular compartments is also a highly regulated process. For this Ca²⁺ homeostasis, numerous Ca²⁺ chelating, storage, and transport mechanisms are required. There are also specialized proteins that are responsible for buffering and transport of Ca²⁺. T-type Ca²⁺ channels (TTCCs) are one of those specialized proteins which play a key role in the signal transduction of many excitable and non-excitable cell types. TTCCs are low-voltage activated channels that belong to the family of voltage-gated Ca²⁺ channels. Over decades, multiple kinases and phosphatases have been shown to modulate the activity of TTCCs, thus playing an indirect role in maintaining cellular physiology. In this review, we provide information on the kinase and phosphatase modulation of TTCC isoforms Cav3.1, Cav3.2, and Cav3.3, which are mostly described for roles unrelated to cellular excitability. We also describe possible potential modulations that are yet to be explored. For example, both mitogen-activated protein kinase and citron kinase show affinity for different TTCC isoforms; however, the effect of such interaction on TTCC current/kinetics has not been studied yet.

Calcium signalling in developing cardiomyocytes: implications for model systems and disease

Adult cardiomyocytes exhibit complex Ca(2+) homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca(2+) sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca(2+) homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as cultured neonatal cells and stem cells.

Lipopolysaccharides upregulate calcium concentration in mouse uterine smooth muscle cells through the T-type calcium channels

Infection is a significant cause of preterm birth. Abnormal changes in intracellular calcium signals are the ultimate triggers of early uterine contractions that result in preterm birth. T‑type calcium channels play an important role in the pathogenesis of cancer, as well as endocrine and cardiovascular diseases. However, there are limited studies on their role in uterine contractions and parturition. In the present study, mouse uterine smooth muscle cells were isolated and treated with lipopolysaccharides (LPS) to mimic the microenvironment of uterine infection in vitro to investigate the role of T‑type calcium channels in the process of infection‑induced preterm birth. The results from quantitative polymerase chain reaction and western blot analysis showed that LPS significantly induced the expression of the Cav3.1 and Cav3.2 subtypes of T‑type calcium channels. Measurements of intracellular calcium concentration showed a significant increase in response to LPS. However, these effects can be reversed by T‑type calcium channel blockers. Western blot analysis further indicated that LPS induced the activation of the nuclear factor (NF)‑κB signaling pathway, and endothelin‑1 (ET‑1) was significantly upregulated, whereas NF‑κB inhibitors significantly inhibited the LPS‑induced upregulation of Cav3.1, Cav3.2 and ET‑1 expression. In addition, ET‑1 directly induced Cav3.1 and Cav3.2 expression, whereas ET‑1 antagonists inhibited the LPS‑induced upregulation of Cav3.1 and Cav3.2 expression. In conclusion, the present study demonstrates that infection triggers the upregulation of T‑type calcium channels and promotes calcium influx. This process relies on the activation of the NF‑κB/ET‑1 signaling pathway. The T‑type calcium channel is expected to become an effective target for the prevention of infection‑induced preterm birth.

The role of voltage-operated and non-voltage-operated calcium channels in endothelin-induced vasoconstriction of rat cerebral arteries

Endothelin-1 has been identified as a potential mediator in the pathogenesis of ischaemic stroke and cerebral vasospasm. The aim of this study was to analyse the role of voltage-operated calcium channels (VOCC) and non-VOCC in endothelin-1 induced vasoconstriction of rat cerebral arteries. Arterial segments were dissected from different regions of the cerebral circulation and responses assessed using wire myography. Endothelin-1 concentration-contraction curves were constructed in calcium-free medium or in the presence of nifedipine, NNC 55-0396 ((1S,2S)-2-(2-(N-[(3-benzimidazol-2-yl)propyl]-N-methylamino)ethyl)-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-naphtyl cyclopropanecarboxylate dihydrochloride) or SK&F 96365 (1-(2-(3-(4-methoxyphenyl)propoxy)-4-methoxyphenylethyl)-1H-imidazole) to inhibit the l-type VOCC, T-type VOCC and non-VOCC, respectively. Inhibition of the calcium channels or removal of calcium from the medium variably decreased the maximum effects (Emax) of endothelin-1, however its potency (pEC50) was unaltered. Endothelin-1 caused a small contraction (<22%) in calcium-free solution. Pre-treatment with nifedipine (1µM) did not affect responses to low concentrations of endothelin-1 but decreased Emax, while NNC 55-0396 (1µM) and SK&F 96365 (30-100µM) generally attenuated the endothelin-1-induced contraction. Combination of nifedipine with SK&F 96365 further decreased the Emax. The relaxant effect of the calcium channel antagonists was also assessed in pre-contracted arteries. Only nifedipine and SK&F 96365 relaxed the arteries pre-contracted with endothelin-1. In conclusion, VOCC and non-VOCC calcium channels are involved in different phases of the endothelin-1 contraction in rat cerebral vessels. T-type VOCC may be involved in contraction induced by low concentrations of endothelin-1, while l-type VOCC mediate the maintenance phase of contraction. VOCC and non-VOCC may work in concert in mediating contraction induced by endothelin-1.

Ethanol inhibition of a T-type Ca²+ channel through activity of protein kinase C

BACKGROUND

T-type calcium channels (T-channels) are widely distributed in the central and peripheral nervous system, where they mediate calcium entry and regulate the intrinsic excitability of neurons. T-channels are dysregulated in response to alcohol administration and withdrawal. We therefore investigated acute ethanol (EtOH) effects and the underlying mechanism of action in human embryonic kidney (HEK) 293 cell lines, as well as effects on native currents recorded from dorsal root ganglion (DRG) neurons cultured from Long-Evans rats.

METHODS

Whole-cell voltage-clamp recordings were performed at 32 to 34°C in both HEK cell lines and DRG neurons. The recordings were taken after a 10-minute application of EtOH or protein kinase C (PKC) activator (phorbol 12-myristate 13-acetate [PMA]).

RESULTS

We recorded T-type Ca²⁺ currents (T-currents) from 3 channel isoforms (CaV3.1, CaV3.2, and CaV3.3) before and during administration of EtOH. We found that only 1 isoform, CaV3.2, was significantly affected by EtOH. EtOH reduced current density as well as producing a hyperpolarizing shift in steady-state inactivation of both CaV3.2 currents from HEK 293 cell lines and in native T-currents from DRG neurons that are known to be enriched in CaV3.2. A myristoylated PKC peptide inhibitor (MPI) blocked the major EtOH effects, in both the cell lines and the DRG neurons. However, PMA effects were more complex. Lower concentration PMA (100 nM) replicated the major effects of EtOH, while higher concentration PMA (1 μM) did not, suggesting that the EtOH effects operate through activation of PKC and were mimicked by lower concentration of PMA.

CONCLUSIONS

EtOH primarily affects the CaV3.2 isoform of T-type Ca²⁺ channels acting through PKC, highlighting a novel target and mechanism for EtOH effects on excitable membranes.

Modulation of low-voltage-activated T-type Ca²⁺ channels

Low-voltage-activated T-type Ca²⁺ channels contribute to a wide variety of physiological functions, most predominantly in the nervous, cardiovascular and endocrine systems. Studies have documented the roles of T-type channels in sleep, neuropathic pain, absence epilepsy, cell proliferation and cardiovascular function. Importantly, novel aspects of the modulation of T-type channels have been identified over the last few years, providing new insights into their physiological and pathophysiological roles. Although there is substantial literature regarding modulation of native T-type channels, the underlying molecular mechanisms have only recently begun to be addressed. This review focuses on recent evidence that the Ca(v)3 subunits of T-type channels, Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3, are differentially modulated by a multitude of endogenous ligands including anandamide, monocyte chemoattractant protein-1, endostatin, and redox and oxidizing agents. The review also provides an overview of recent knowledge gained concerning downstream pathways involving G-protein-coupled receptors. This article is part of a Special Issue entitled: Calcium channels.

ZnT-1 enhances the activity and surface expression of T-type calcium channels through activation of Ras-ERK signaling

Zinc transporter-1 (ZnT-1) is a putative zinc transporter that confers cellular resistance from zinc toxicity. In addition, ZnT-1 has important regulatory functions, including inhibition of L-type calcium channels and activation of Raf-1 kinase. Here we studied the effects of ZnT-1 on the expression and function of T-type calcium channels. In Xenopus oocytes expressing voltage-gated calcium channel (CaV) 3.1 or CaV3.2, ZnT-1 enhanced the low-threshold calcium currents ( IcaT) to 182 ± 15 and 167.95 ± 9.27% of control, respectively ( P < 0.005 for both channels). As expected, ZnT-1 also enhanced ERK phosphorylation. Coexpression of ZnT-1 and nonactive Raf-1 blocked the ZnT-1-mediated ERK phosphorylation and abolished the ZnT-1-induced augmentation of IcaT. In mammalian cells (Chinese hamster ovary), coexpression of CaV3.1 and ZnT-1 increased the IcaT to 166.37 ± 6.37% compared with cells expressing CaV3.1 alone ( P < 0.01). Interestingly, surface expression measurements using biotinylation or total internal reflection fluorescence microscopy indicated marked ZnT-1-induced enhancement of CaV3.1 surface expression. The MEK inhibitor PD-98059 abolished the ZnT-1-induced augmentation of surface expression of CaV3.1. In cultured murine cardiomyocytes (HL-1 cells), transient exposure to zinc, leading to enhanced ZnT-1 expression, also enhanced the surface expression of endogenous CaV3.1 channels. Consistently, in these cells, endothelin-1, a potent activator of Ras-ERK signaling, enhanced the surface expression of CaV3.1 channels in a PD-98059-sensitive manner. Our findings indicate that ZnT-1 enhances the activity of CaV3.1 and CaV3.2 through activation of Ras-ERK signaling. The augmentation of CaV3.1 currents by Ras-ERK activation is associated with enhanced trafficking of the channel to the plasma membrane.

Pituitary adenylate cyclase-activating peptide (PACAP) recruits low voltage-activated T-type calcium influx under acute sympathetic stimulation in mouse adrenal chromaffin cells

Low voltage-activated T-type Ca(v)3.2 calcium channels are expressed in neurosecretory chromaffin cells of the adrenal medulla. Previous studies have shown that naïve adrenal chromaffin cells express a nominal Ca(v)3.2-dependent conductance. However, Ca(v)3.2 conductance is up-regulated following chronic hypoxia or long term exposure to cAMP analogs. Thus, although a link between chronic stressors and up-regulation of Ca(v)3.2 exists, there are no reports testing the specific role of Ca(v)3.2 channels in the acute sympathoadrenal stress response. In this study, we examined the effects of acute sympathetic stress on T-type Ca(v)3.2 calcium influx in mouse chromaffin cells in situ. Pituitary adenylate cyclase-activating peptide (PACAP) is an excitatory neuroactive peptide transmitter released by the splanchnic nerve under elevated sympathetic activity to stimulate the adrenal medulla. PACAP stimulation did not evoke action potential firing in chromaffin cells but did cause a persistent subthreshold membrane depolarization that resulted in an immediate and robust Ca(2+)-dependent catecholamine secretion. Moreover, PACAP-evoked secretion was sensitive to block by nickel chloride and was acutely inhibited by protein kinase C blockers. We utilized perforated patch electrophysiological recordings conducted in adrenal tissue slices to investigate the mechanism of PACAP-evoked calcium entry. We provide evidence that stimulation with exogenous PACAP and native neuronal stress stimulation both lead to a protein kinase C-mediated phosphodependent recruitment of a T-type Ca(v)3.2 Ca(2+) influx. This in turn evokes catecholamine release during the acute sympathetic stress response.

Alpha-cobratoxin inhibits T-type calcium currents through muscarinic M4 receptor and Gο-protein βγ subunits-dependent protein kinase A pathway in dorsal root ganglion neurons

The long-chain neurotoxic protein, alpha-cobratoxin (α-CTx), has been shown to have analgesic effects. However, the underlying mechanisms still remain unclear. In this study, we examined the effects of α-CTx on T-type calcium channel currents (T-currents) and elucidated the relevant mechanisms in mouse dorsal root ganglion (DRG) neurons. Our results showed that α-CTx reversibly inhibited T-currents in a dose-dependent manner. This inhibitory effect was blocked by the selective muscarinic M4 receptor antagonist tropicamide, while methyllycaconitine, a specific antagonist for the α7 subtype of nicotinic receptor had no effect. siRNA targeting the M4 receptor in small DRG neurons abolished α-CTx-induced T-current inhibition. Intracellular application of GDP-β-S or a selective antibody against the G(o)α-protein, as well as pretreatment of the cells with pertussis toxin, abolished the inhibitory effects of α-CTx. The M4 receptor-mediated response was blocked by dialyzing cells with QEHA peptide or anti-G(β) antibody. Pretreatment of the cells with protein kinase A (PKA) inhibitor H89 or intracellular application of PKI 6-22 abolished α-CTx-induced T-current inhibition in small DRG neurons, whereas inhibition of phosphatidylinositol 3-kinase or PKC elicited no such effects. In addition, α-CTx significantly increased PKA activity in DRG neurons, whereas pretreatment of the cells with tropicamide abolished this effect. In summary, our results suggest that activation of muscarinic M4 receptor by α-CTx inhibits T-currents via the G(βγ) of G(o)-protein and PKA-dependent pathway. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Pharmacological effects of carvedilol on T-type calcium current in murine HL-1 cells

Carvedilol is widely used in the treatment of cardiovascular diseases including atrial fibrillation. T-type Ca(2+) channels have been recognized recently in the mechanisms underlying atrial arrhythmias. However, it is unclear whether carvedilol may affect the T-type Ca(2+) channel. The present study evaluated the pharmacological effects of carvedilol on T-type calcium current (I(Ca,T)) in the murine HL-1 cell line. I(Ca)(,T) was recorded by the whole-cell patch-clamp technique. Calcium transient was monitored by the fluorescent dye Fluo-4/AM and confocal laser scanning microscopy. Carvedilol reversibly inhibited I(Ca)(,T) in a concentration-dependent manner, with an IC50 of 2.1 microM. 3 microM carvedilol was found to decrease the peak I(Ca)(,T) amplitude at -20 mV from 20.1+/-1.8pA/pF to 10.9+/-2.1pA/pF. Carvedilol significantly shifted the steady-state inactivation curve of I(Ca)(,T) towards more negative potential by 12.8 mV, while the activation curve was not significantly altered. Carvedilol delayed recovery from inactivation of I(Ca)(,T), time constant (tau) was 112.4+/-3.5 ms in control and 270.1+/-4.7 ms in carvedilol. Carvedilol-induced inhibition rate in I(Ca)(,T) was enhanced with the increase in stimuli frequency, the inhibitory rate was 23.2+/-4.1% at 0.2 Hz and 47.2+/-0.6% at 2 Hz. Carvedilol still produced the significant decrease in the amplitude of I(Ca)(,T) in the presence of H-89, PKA inhibitor. Carvedilol significantly inhibited the amplitude of the calcium transient in a concentration-dependent manner. These findings indicate that carvedilol inhibits I(Ca)(,T) in atrial cells by mechanisms involving preferential interaction with the inactivated state of T-type Ca(2+) channel.

Intracellular Ca(2+)- and PKC-dependent upregulation of T-type Ca(2+) channels in LPC-stimulated cardiomyocytes

Lysophosphatidylcholine (LPC) accumulation in intracellular and/or interstitial space in cardiomyocytes may underlie as a mechanism for tachycardia and various arrhythmias during cardiac ischemia, which is usually accompanied by elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)). The present study was therefore designed to investigate possible mechanisms responsible for [Ca(2+)](i) elevation by LPC focusing on T-type Ca(2+) channel current (I(Ca.T)). LPC as well as phorbol 12-myristate 13-acetate (PMA) significantly accelerated the beating rates of neonatal rat cardiomyocytes. Augmentation of I(Ca.T) by LPC was dependent on the intracellular Ca(2+) concentration: an increase of I(Ca.T) was significantly larger in high [Ca(2+)](i) condition (pCa=7) than those in low [Ca(2+)](i) condition (pCa=11). In heterologous expression system by use of human cardiac Ca(V)3.1 and Ca(V)3.2 channels expressed in HEK293 cells, LPC augmented Ca(V)3.2 channel current (I(Cav3.2)) in a concentration-dependent manner but not Ca(V)3.1 channel current (I(Cav3.1)). Augmentation of I(Cav3.2) by LPC was highly [Ca(2+)](i) dependent: I(Cav3.2) was unchanged when pCa was 11 but was markedly increased when [Ca(2+)](i) was higher than 10(-10) M (pCa<or=10) by LPC application (10-50 microM). A specific inhibitor of protein kinase Calpha (Ro-32-0432) attenuated the increase of I(Cav3.2) by LPC. LPC stimulates I(Ca.T) in a [Ca(2+)](i)-dependent manner via PKCalpha activation, which may play a role in triggering arrhythmias in pathophysiological conditions of the heart.

The design of the Mortality Assessment in Congestive Heart Failure Trial (MACH-1, mibefradil)

BACKGROUND

Despite the advances in therapy for heart failure with the use of angiotensin-converting enzyme inhibitors, heart failure remains a major medical problem, impacting on well-being and survival.

HYPOTHESIS

Mibefradil is a member of new class of agents that, due to its unique mechanism of action and pharmacologic and physiologic profile, may offer a significant advance in the treatment of heart failure. Mibefradil is a nonvoltage-regulated T-channel calcium blocker that differs from all the other currently available L-channel calcium blockers. The drug is a potent peripheral and coronary vasodilator, with no clinical negative inotropic activity. Furthermore, because of its long half-life, it can be given once a day and appears to be free of the common side effects seen with other calcium-channel blockers. The trial was designed to investigate whether the addition of mibefradil to standard therapy for heart failure will reduce mortality in patients with symptomatic heart failure.

METHODS

This manuscript describes the design, organization, and status of MACH-1 (Mortality Assessment in Congestive Heart Failure), a double-blind, placebo-controlled study. It is to be a 2,400-patient international trial assessing the ability of mibefradil to impact upon survival and symptoms of heart failure in patients who are in NYHA classes II-IV and who were already treated with standard therapy.

Molecular and electrical remodeling of L- and T-type Ca(2+) channels in rat right atrium with monocrotaline-induced pulmonary hypertension

BACKGROUND

Atrial arrhythmia is often encountered in chronic pulmonary disease with pulmonary hypertension (PH), but few studies have investigated the electrical remodeling of atrial Ca(2+) channels under PH.

METHODS AND RESULTS

Wistar rats were injected with monocrotaline (MCT), resulting in PH with right atrial and ventricular hypertrophy. The L-type Ca(2+) channel current density was significantly decreased in right atrial cells of MCT-treated rats, accompanied by a significant reduction in mRNA expression of the CaV1.2 (alpha(1C)) subunit and accessory beta(2) subunit. Conversely, the low voltage-activated Ca(2+) current was more marked in the right atrial cells of MCT-treated rats than in those of control rats. The current-voltage relationship and the time course of inactivation closely resembled those of T-type Ca(2+) channels, although the current was only slightly inhibited by 10-100 micromol/L Ni(2+). No significant differences were observed in the mRNA expression levels of CaV3.1 (alpha(1G)) and CaV3.2 (alpha(1H)) or the protein level of the CaV3.1 subunit. In left atrial cells, the electrophysiological molecular properties of Ca(2+) channels were unaffected by MCT treatment.

CONCLUSIONS

PH causes right atrial hypertrophy, associated with alteration of the electrophysiological molecular properties of Ca(2+) channels.

Mathematical model of mouse embryonic cardiomyocyte excitation-contraction coupling

Excitation-contraction (E-C) coupling is the mechanism that connects the electrical excitation with cardiomyocyte contraction. Embryonic cardiomyocytes are not only capable of generating action potential (AP)-induced Ca(2+) signals and contractions (E-C coupling), but they also can induce spontaneous pacemaking activity. The spontaneous activity originates from spontaneous Ca(2+) releases from the sarcoplasmic reticulum (SR), which trigger APs via the Na(+)/Ca(2+) exchanger (NCX). In the AP-driven mode, an external stimulus triggers an AP and activates voltage-activated Ca(2+) intrusion to the cell. These complex and unique features of the embryonic cardiomyocyte pacemaking and E-C coupling have never been assessed with mathematical modeling. Here, we suggest a novel mathematical model explaining how both of these mechanisms can coexist in the same embryonic cardiomyocytes. In addition to experimentally characterized ion currents, the model includes novel heterogeneous cytosolic Ca(2+) dynamics and oscillatory SR Ca(2+) handling. The model reproduces faithfully the experimentally observed fundamental features of both E-C coupling and pacemaking. We further validate our model by simulating the effect of genetic modifications on the hyperpolarization-activated current, NCX, and the SR Ca(2+) buffer protein calreticulin. In these simulations, the model produces a similar functional alteration to that observed previously in the genetically engineered mice, and thus provides mechanistic explanations for the cardiac phenotypes of these animals. In general, this study presents the first model explaining the underlying cellular mechanism for the origin and the regulation of the heartbeat in early embryonic cardiomyocytes.

Activation of protein kinase C augments T-type Ca2+ channel activity without changing channel surface density

T-type Ca2+ channels play essential roles in numerous cellular processes. Recently, we reported that phorbol-12-myristate-13-acetate (PMA) potently enhanced the current amplitude of Cav3.2 T-type channels reconstituted in Xenopus oocytes. Here, we have compared PMA modulation of the activities of Cav3.1, Cav3.2 and Cav3.3 channels, and have investigated the underlying mechanism. PMA augmented the current amplitudes of the three T-type channel isoforms, but the fold stimulations and time courses differed. The augmentation effects were not mimicked by 4alpha-PMA, an inactive stereoisomer of PMA, but were abolished by preincubation with protein kinase C (PKC) inhibitors, indicating that PMA augmented T-type channel currents via activation of oocyte PKC. The stimulation effect on Cav3.1 channel activity by PKC was mimicked by endothelin when endothelin receptor type A was coexpressed with Cav3.1 in the Xenopus oocyte system. Pharmacological studies combined with fluorescence imaging revealed that the surface density of Cav3.1 T-type channels was not significantly changed by activation of PKC. The PKC effect on Cav3.1 was localized to the cytoplasmic II-III loop using chimeric channels with individual cytoplasmic loops of Cav3.1 replaced by those of Cav2.1.

Potassium channel remodeling in cardiac hypertrophy

Cardiac hypertrophy is an adaptive process against increased work loads; however, hypertrophy also presents substrates for lethal ventricular arrhythmias, resulting in sudden arrhythmic deaths that account for about one third of deaths in cardiac hypertrophy. To maintain physiological cardiac function in the face of increased work loads, hypertrophied cardiomyocytes undergo K(+) channel remodeling that provides a prolongation in action potential duration and an increase in Ca(2+) entry. Increased Ca(2+) entry, in turn, activates signaling mechanisms including a calcineruin/NFAT pathway to permit remodeling of the K(+) channels. This results in a positive feedback loop between the K(+) channel remodeling and altered Ca(2+) handling; this loop may represent a potential therapeutic target against sudden arrhythmic deaths in cardiac hypertrophy. The purposes of this review are to: (1) discuss types of K(+) channels and their mRNA that undergo remodeling in cardiac hypertrophy; (2) report on recent research on molecular mechanisms of K(+) channel remodeling; and (3) address physiological events underlying new therapeutic modalities to ameliorate arrhythmias and sudden death in cardiac hypertrophy.

Lysophosphatidylcholine augments Ca(v)3.2 but not Ca(v)3.1 T-type Ca(2+) channel current expressed in HEK-293 cells

Lysophosphatidylcholine (LPC) has been shown to induce electrophysiological disturbances to arrhythmogenesis. However, the effects of LPC on the low-voltage-activated T-type Ca(2+) channels in the heart are not understood yet. We found that LPC increases the T-type Ca(2+) channel current (I(Ca.T)) in neonatal rat cardiomyocytes. To further investigate the underlying modulatory mechanism of LPC on T-type Ca(2+) channels, we utilized HEK-293 cells stably expressing alpha1G and alpha1H subunits (HEK-293/alpha1G and HEK-293/alpha1H), by use of patch-clamp techniques. A low concentration of LPC (10 micromol/l) significantly increased Ca(v)3.2 I(Ca.T) (alpha1H) that were similar to those observed in neonatal rat cardiomyocytes. Activation and steady-state inactivation curves were shifted in the hyperpolarized direction by 5.1 +/- 0.2 and 4.6 +/- 0.4 mV, respectively, by application of 10 micromol/l LPC. The pretreatment of cells with a protein kinase C inhibitor (chelerythrine) attenuated the effects of LPC on I(Ca.T) (alpha1H). However, the application of LPC failed to modify Ca(v)3.1 (alpha1G) I(Ca.T) at concentrations of 10-50 micromol/l. In conclusion, these data demonstrate that extracellularly applied LPC augments Ca(v)3.2 I(Ca.T) (alpha1H) but not Ca(v)3.1 I(Ca.T) (alpha1G) in a heterologous expression system, possibly by modulating protein kinase C signaling.

Towards selective antagonists of T-type calcium channels: design, characterization and potential applications of NNC 55-0396

NNC 55-0396 is a structural analog of mibefradil (Ro 40-5967) that inhibits both T-type and high-voltage-activated (HVA) Ca2+ channels with a higher selectivity for T-type Ca2+ channels. The inhibitory effect of mibefradil on HVA Ca2+ channels can be attributed to a hydrolyzed metabolite of the drug: the methoxy acetate side chain of mibefradil is removed by intracellular enzymes, thus it forms (1S,2S)-2-(2-(N-[(3-benzoimidazol-2-yl)propyl]-N-methylamino)ethyl)-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-naphtyl hydroxy dihydrochloride (dm-mibefradil), which causes potent inhibition of HVA Ca2+ currents. By replacing the methoxy acetate chain of mibefradil with cyclopropanecarboxylate, a more stable analog was developed (NNC 55-0396). The acute IC50 of NNC 55-0396 to block recombinant Cav3.1 T-type channels expressed in HEK293 cells is approximately 7 muM, whereas 100 microM NNC 55-0396 has no detectable effect on high voltage-activated currents in INS-1 cells. Block of T-type Ca2+ current was partially reduced by membrane hyperpolarization and was enhanced at high stimulus frequency. Washing NNC 55-0396 out of the recording chamber did not reverse the T-type Ca2+ current activity, suggesting that the compound dissolves in or passes through the plasma membrane to exert its effect; however, intracellular perfusion of the compound did not block T-type Ca2+ currents, arguing against a cytoplasmic route of action. We conclude that NNC 55-0396, by virtue of its modified structure, does not produce the metabolite that causes inhibition of L-type Ca2+ channel channels, thus rendering it more selective to T-type Ca2+ channels.

Pathophysiological significance of T-type Ca2+ channels: properties and functional roles of T-type Ca2+ channels in cardiac pacemaking

Calcium channels are essential for excitation-contraction coupling and pacemaker activity in cardiac myocytes. While L-type Ca(2+) channels (LCC) have been extensively studied, functional roles of T-type channels (TCC) in native cardiac myocytes are still debatable. TCC are activated at more negative membrane potentials than LCC and therefore facilitate slow diastolic depolarization in sinoatrial node cells. Recent studies showed that selective inhibition of TCC produced a marked slowing of the pacemaker rhythm, indicating that contribution of TCC to cardiac automaticity was relatively larger than what had been speculated in previous studies. To re-evaluate TCC, we measured current density and kinetics of TCC in sinoatrial node cells of various mammalian species. Current density of TCC was larger in mice and guinea pigs than in rabbit and porcine sinoatrial node cells. Interestingly, few or no obvious TCC were recorded in porcine sinoatrial node cells. Furthermore, it was demonstrated that TCC could be enhanced by several vasoactive substances, thereby increasing spontaneous firing rate of sinoatrial node cells. TCC may, at least in part, account for different heart rates among various mammalian species. In addition, TCC might be involved in physiological and/or pathophysiological modulations of the heart rate.

Role of high-voltage-activated calcium channels in glucose-regulated beta-cell calcium homeostasis and insulin release

High-voltage-activated (HVA) calcium channels are known to be the primary source of calcium for glucose-stimulated insulin secretion. However, few studies have investigated how these channels can be regulated by chronically elevated levels of glucose. In the present study, we determined the level of expression of the four major HVA calcium channels (N-type, P/Q-type, L(C)-type, and L(D)-type) in rat pancreatic beta-cells. Using quantitative real-time PCR (QRT-PCR), we found the expression of all four HVA genes in rat insulinoma cells (INS-1) and in primary isolated rat islet cells. We then determined the role of each channel in insulin secretion by using channel-selective antagonists. Insulin secretion analysis revealed that N- and L-type channels are both involved in immediate glucose-induced insulin secretion. However, L-type was preferentially coupled to secretion at later time points. P/Q-type channels were not found to play a role in insulin secretion at any stage. It was also found that long-term exposure to elevated glucose increases basal calcium in these cells. Interestingly, chronically elevated glucose decreased the mRNA expression of the channels involved with insulin secretion and diminished the level of stimulated calcium influx in these cells. Using whole cell patch clamp, we found that N- and L-type channel currents increase gradually subsequent to lower intracellular calcium perfusion, suggesting that these channels may be regulated by glucose-induced changes in calcium.

Angiotensin II-mediated constriction of afferent and efferent arterioles involves T-type Ca2+ channel activation

BACKGROUND/AIMS

Previous studies have shown that L-type Ca2+ channel (LCC) blockers prevent the afferent arteriolar (AA) vasoconstriction elicited by angiotensin II (Ang II), but do not influence its vasoconstrictor effect on efferent arterioles (EA). The present study tested the hypothesis that Ang II-mediated constriction of AA and EA involves T-type Ca2+ channel (TCC) activation, which may mediate Ca2+ entry responsible for Ang II-induced EA and possibly AA constriction.

METHODS

Video-microscopic measurements of vascular dimensions were performed on isolated blood-perfused juxtamedullary nephrons from Sprague-Dawley rats. Single AA or EA were visualized and superfused with solutions containing Ang II alone or with a TCC blocker, pimozide, or a LCC blocker, diltiazem.

RESULTS

Pimozide at 10 micromol/l significantly dilated EA (19.7 +/- 1.4%) as well as AA (24.8 +/- 3.6%). In response to superfusion with Ang II at concentrations of 0.1, 1.0 and 10.0 nmol/l, AA diameter decreased significantly by 15.2 +/- 1.7, 23.3 +/- 3.2 and 36.1 +/- 3.4% and EA diameter also decreased significantly by 11.9 +/- 1.7, 19.6 +/- 2.8 and 31.0 +/- 2.6%, respectively. Pimozide (10 micromol/l) markedly blunted AA (4.6 +/- 1.2, 7.5 +/- 0.6 and 7.9 +/- 1.2%) and EA (2.2 +/- 0.6, 5.4 +/- 1.5 and 7.7 +/- 1.3%) diameter responses to Ang II. Diltiazem (10 micromol/l) significantly dilated AA (26.8 +/- 2.2%), and prevented Ang II-mediated constriction of AA. In contrast, diltiazem did not dilate EA (3.3 +/- 0.6%) and failed to inhibit the Ang II-induced EA vasoconstriction; however, the vasoconstriction was reversed by the subsequent addition of pimozide (5 micromol/l).

CONCLUSION

This study provides further functional evidence for TCC channels in the regulation of AA and EA indicating that Ang II-mediated arteriolar constriction may involve activation of TCC in both AA and EA. TCC may play an important role in mediating Ca2+ entry responsible for Ang-induced EA and AA constriction. The role of TCC in mediating Ang II-constrictor actions on EA may be of particular significance because LCC are not normally functional in these vessels.

CytochromeP-450 metabolites in endothelin-stimulated cardiac hormone secretion

We examined the role of cytochrome P-450-arachidonate (CYP450-AA) metabolites in endothelin-1 (ET-1)-stimulated atrial natriuretic peptide (ANP) and pro-ANP-(1-30) secretion from the heart. 17-Octadecynoic acid (17-ODYA, 10-5M) significantly inhibited ANP secretion stimulated by ET-1 (10-8M) in the isolated perfused rat atria and inhibited pro-ANP-(1-30) secretion stimulated by ET-1 (10-8M) or 20-hydroxyeicosatetraenoic acid in cultured neonatal rat ventricular myocytes (NRVM). In NRVM, 17-ODYA significantly ( P < 0.05) increased secretion of cAMP but had no significant effect on the secretion of cGMP from NRVM. Staurosporine, an inhibitor of protein kinase C, completely blocked the inhibitory action of 17-ODYA, whereas a protein kinase A inhibitor, H-89 (5 × 10-5M), did not significantly attenuate the effects of 17-ODYA. The results show that the inhibitory action of 17-ODYA on ET-1-augmented ANP secretion is mediated through cAMP and suggest that CYP450-AA may play an important role in ET-1-induced cardiac hormone secretion.

Dynamic remodeling of K+ and Ca2+ currents in cells that survived in the epicardial border zone of canine healed infarcted heart

Action potentials (APs) of the epicardial border zone (EBZ) cells from the day 5 infarcted heart continue to be altered by day 14 postocclusion, namely, they shortened. However, by 2 mo, EBZ APs appear "normal," yet conduction of wave fronts remains abnormal. We hypothesize that the changes in transmembrane APs are due to a change in the distribution of ion channels in either density or function. Thus we focused on the changes in Ca2+ and K+ currents in cells isolated from the 14-day (IZ14d) and 2-mo (IZ2m) EBZ and compared them with those occurring in cells from the same hearts but remote (Rem) from the EBZ. Whole cell voltage-clamp techniques were used to measure and compare Ca2+ and K+ currents in cells from the different groups. Ca2+ current densities remain reduced in cells of the 14-day and 2-mo infarcted heart and the kinetic changes previously identified in the 5-day heart begin to, but do not recover to, cells from noninfarcted epicardium (NZ) values. Importantly, I(Ca,L) in both the EBZ and Rem regions still show a slowed recovery from inactivation. Furthermore, during the remodeling process, there is an increased expression of T-type Ca2+ currents, but only regionally, and only within a specific time window postmyocardial infarction (MI). Regional heterogeneity in beta-adrenergic responsiveness of I(Ca,L) exists between EBZ and remote cells of the 14-day hearts, but this regional heterogeneity is gone in the healed infarcted heart. In IZ14d, the transient outward K+ current (Ito) begins to reemerge and is accompanied by an upregulated tetraethylammonium-sensitive outward current. By 2-mo postocclusion, Ito and sustained outward K+ current have completed the reverse remodeling process. During the healing process post-MI, canine epicardial cells downregulate the fast Ito but compensate by upregulating a K+ current that in normal cells is minimally functional. For recovering I(Ca,L) of the 14-day and 2-mo EBZ cells, voltage-dependent processes appear to be reset, such that I(Ca,L) "window" current occurs at hyperpolarized potentials. Thus dynamic changes in both Ca2+ and K+ currents contribute to the altered AP observed in 14-day fibers and may account for return of APs of 2 mo EBZ fibers.

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.

Modulation of Ca(v)3.2 T-type Ca2+ channels by protein kinase C

Although T-type Ca2+ channels have been implicated in numerous physiological functions, their regulations by protein kinases have been obscured by conflicting reports. We investigated the effects of protein kinase C (PKC) on Ca(v)3.2 T-type channels reconstituted in Xenopus oocytes. Phorbol-12-myristate-13-acetate (PMA) strongly enhanced the amplitude of Ca(v)3.2 channel currents (approximately 3-fold). The augmentation effects were not mimicked by 4alpha-PMA, an inactive stereoisomer of PMA, and abolished by preincubation with PKC inhibitors. Our findings suggest that PMA upregulates Ca(v)3.2 channel activity via activation of oocyte PKC.

Enhancement of low-voltage-activated calcium currents by group II metabotropic glutamate receptors in rat retinal ganglion cells

Current through voltage-gated calcium channels of rat retinal ganglion cells was recorded using the whole-cell patch-clamp technique. All cells displayed high-voltage-activated currents, and 75% of these also displayed low-voltage-activated (LVA) currents. Currents could be separated on the basis of their voltage/time dependence and sensitivity to nickel ions. The group II metabotropic glutamate receptor (mGluR) agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC; 100 microM) increased LVA current by 40% as did the nonselective mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (tACPD; 100 microM). Neither the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (100 microM) nor 5-hydroxytryptamine (100 microM) enhanced LVA current. In the presence of (S)-alpha-methyl-4-carboxyphenylglycine (100 microM), a group I/II mGluR antagonist, the tACPD-induced enhancement of LVA current was blocked. The voltage dependence of the activation or inactivation kinetics was unchanged in the presence of tACPD. Inclusion in the pipette solution of GDP-beta-S (1 mM) blocked the enhancement of the LVA current by APDC, whereas GTP-gamma-S (0.5 mM) prevented recovery of the enhancement. The tACPD-mediated enhancement of the LVA current was still present in cells pretreated with pertussis or cholera toxins (500 ng x ml(-1)). Genistein (10 microM) prevented the enhancement of the LVA current. These results suggest that LVA current can be enhanced by activation of mGluR2, by a mechanism that is G-protein dependent and may involve a protein tyrosine kinase step.

Neuroendocrine facets of human puberty

The unfolding of pubertal growth and maturation entails multisystem collaboration. Most notably, the outflow of gonadotropins and growth hormone (GH) proceeds both independently and jointly. The current update highlights this unique dependency in the human.

Bradykinin induced a positive chronotropic effect via stimulation of T- and L-type calcium currents in heart cells

Using Fluo-3 calcium dye confocal microscopy and spontaneously contracting embryonic chick heart cells, bradykinin (10(-10) M) was found to induce positive chronotropic effects by increasing the frequency of the transient increase of cytosolic and nuclear free Ca2+. Pretreatment of the cells with either B1 or B2 receptor antagonists (R126 and R817, respectively) completely prevented bradykinin (BK) induced positive chronotropic effects on spontaneously contracting single heart cells. Using the whole-cell voltage clamp technique and ionic substitution to separate the different ionic current species, our results showed that BK (10(-6) M) had no effect on fast Na+ inward current and delayed outward potassium current. However, both L- and T-type Ca2+ currents were found to be increased by BK in a dose-dependent manner (10(-10)-10(-7) M). The effects of BK on T- and L-type Ca2+ currents were partially blocked by the B1 receptor antagonist [Leu8]des-Arg9-BK (R592) (10(-7) M) and completely reversed by the B2 receptor antagonist D-Arg[Hyp3,D-Phe7,Leu8]BK (R-588) (10(-7) M) or pretreatment with pertussis toxin (PTX). These results demonstrate that BK induced a positive chronotropic effect via stimulation of T- and L-type Ca2+ currents in heart cells mainly via stimulation of B2 receptor coupled to PTX-sensitive G-proteins. The increase of both types of Ca2+ current by BK in heart cells may explain the positive inotropic and chronotropic effects of this hormone.

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

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Identification of PKCalpha isoform-specific effects in cardiac myocytes using antisense phosphorothioate oligonucleotides

Members of the mammalian protein kinase C (PKC) superfamily play key regulatory roles in multiple cellular processes. In the heart, PKC signaling is involved in hypertrophic agonist-induced gene expression and hypertrophic growth. To investigate the specific function of PKC signaling in regulating cardiomyocyte growth, we used antisense oligonucleotides to inhibit PKC alpha, the major isozyme present in the neonatal heart. Transfection of cultured neonatal cardiomyocytes with antisense PKCalpha oligonucleotides resulted in a marked reduction in both PKCalpha mRNA and protein levels. PKCalpha antisense treatment also reduced phenylephrine (PE)-induced PKC activity and perinuclear translocation of PKCalpha. Antisense inhibition of PKCalpha led to reduction of PE-induced increase in skeletal alpha-actin mRNA levels and atrial natriuretic peptide (ANP) secretion but had no significant effects on PE-induced beta-myosin heavy chain, ANP, or B-type natriuretic peptide (BNP) gene expression. On the other hand, antisense PKCalpha treatment attenuated endothelin-1-induced increase in ANP and BNP peptide secretion, whereas endothelin-1-induced gene expression of ANP and BNP remained unchanged. The hypertrophic agonist-induced growth of cardiomyocytes, characterized by increased [(3)H]leucine incorporation, was not affected with antisense PKCalpha treatment. Furthermore, we found that PE-induced increase in extracellular signal-regulated kinase (ERK) activity was partially inhibited by antisense PKCalpha treatment, implicating ERK as a downstream mediator for PKCalpha signaling. These results indicate that PKCalpha isozyme is involved in hypertrophic signaling in cardiomyocytes and provide novel strategies for future studies to identify other cellular targets controlled selectively by PKCalpha or other PKC isozymes.

Endothelin antagonism with bosentan: current status and future perspectives

Endothelin receptor antagonists have been proposed for the treatment of a variety of disorders in which the endothelins may act as pathogenic mediators, such as hypertension, congestive heart failure, and cerebral vasospasm. Bosentan is a nonpeptide competitive antagonist, which can be a good tool for studying the endothelin system. It is specific for the endothelin system and blocks the actions of endothelin at both mammalian receptors (A and B). Bosentan has recently moved into Phase III clinical trial. This review will attempt to overview the experimental and clinical effects of bosentan.

Mibefradil improves beta-adrenergic responsiveness and intracellular Ca(2+) handling in hypertrophied rat myocardium

The present study investigated the effects of mibefradil, a novel T-type channel blocker, on ventricular function and intracellular Ca(2+) handling in normal and hypertrophied rat myocardium. Ca(2+) transient was measured with the bioluminescent protein, aequorin. Mibefradil (2 microM) produced nonsignificant changes in isometric contraction and peak systolic intracellular Ca(2+) concentration ([Ca(2+)](i)) in normal rat myocardium. Hypertrophied papillary muscles isolated from aortic-banded rats 10 weeks after operation demonstrated a prolonged duration of isometric contraction, as well as decreased amplitudes of developed tension and peak Ca(2+) transient compared with the sham-operated group. Additionally, diastolic [Ca(2+)](i) increased in hypertrophied rat myocardium. The positive inotropic effect of isoproterenol stimulation was blunted in hypertrophied muscles despite a large increase in Ca(2+) transient amplitude. Afterglimmers and corresponding aftercontractions were provoked with isoproterenol (10(-5) and 10(-4) M) stimulation in 4 out of 16 hypertrophied muscles, but were eliminated in the presence of mibefradil (2 microM). In addition, hypertrophied muscles in the presence of mibefradil had a significant improvement of contractile response to isoproterenol stimulation and a reduced diastolic [Ca(2+)](I), although a mild decrease of peak Ca(2+)-transient was also shown. However, verapamil (2 microM) did not restore the inotropic and Ca(2+) modulating effects of isoproterenol in hypertrophied myocardium. Mibefradil partly restores the positive inotropic response to beta-adrenergic stimulation in hypertrophied myocardium from aortic-banded rats, an effect that might be useful in hypertrophied myocardium with impaired [Ca(2+)](i) homeostasis.

Stimulation of recombinant Ca(v)3.2, T-type, Ca(2+) channel currents by CaMKIIgamma(C)

Molecular cloning of low-voltage activated (LVA) T-type calcium channels has enabled the study of their regulation in heterologous expression systems. Here we investigate the regulation of Ca(v)3.2 alpha(1)-subunits (alpha1H) by calcium- and/or calmodulin-dependent protein kinase II (CaMKII). 293 cells stably expressing alpha1H were transiently transfected with CaMKIIgamma(C). Using the whole-cell recording configuration, we observed that elevation of pipette free Ca(2+) (1 microM) in the presence of CaM (2 microM) increases T-type channel activity selectively at negative potentials, evoking an 11 mV hyperpolarizing shift in the half-maximal potential (V(1/2)) for activation. The V(1/2) of channel inactivation is not altered by Ca(2+)/CaM. These effects reproduced modulation observed in adrenal zona glomerulosa cells. The potentiation by Ca(2+)/CaM was dependent on the co-expression of CaMKIIgamma(C) and required Ca(2+)/CaM-dependent kinase activity. Peptide (AIP) and lipophilic (KN-62) protein kinase inhibitors prevented the Ca(2+)/CaM-induced changes in channel gating without altering basal Ca(v)3.2 channel activity (27 nM free Ca(2+)) as did replacing pipette ATP with adenylyl imidodiphosphate (AMP-PNP), a non-hydrolysable analogue. CaMKII-dependent potentiation of channel opening resulted in significant increases in apparent steady-state open probability (P(o)) and sustained channel current at negative voltages. Under identical conditions, CaMKII activation did not regulate the activity of Ca(v)3.1 channels, the first cloned member (alpha1G) of the T-type Ca(2+) channel family. Our results provide the first evidence for the differential regulation of two members of the Ca(v)3 family by protein kinase activation and the first report reconstituting CaMKII-dependent regulation of any cloned Ca(2+) channel.

Electrophysiological analysis of the negative chronotropic effect of endothelin-1 in rabbit sinoatrial node cells

1. Electrophysiological effects of endothelin-1 (ET-1) were studied in rabbit sinoatrial node (SAN) using conventional microelectrode and whole-cell voltage and current recordings. 2. In rabbit SAN, RT-PCR detected ET(A) endothelin receptor mRNA. ET-1 (100 nM) increased the cycle length of action potentials (APs) from 305 +/- 15 to 388 +/- 25 ms; this effect was antagonised by the ET(A) receptor-selective antagonist BQ-123 (1 microM). ET-1 increased AP duration (APD50) by 22%, depolarised the maximum diastolic potential (MDP) from -59 +/- 1 to -53 +/- 2 mV, shifted the take-off potential by +5 mV and decreased the pacemaker potential (PMP) slope by 15%. Under exactly the same experimental conditions, ET-1 caused a positive chronotropic effect in guinea-pig SAN with a decrease of 13% in APD50, a shift of -4 mV in the take-off potential and an increase of 8% in the PMP slope. 3. Rabbit SAN exhibited two major cell types, distinguished both by their appearances and by their electrophysiological responses to ET-1. Whereas the spontaneous pacing rate and the PMP slope were similarly decreased by ET-1 (10 nM) in both cell types, ET-1 depolarised MDP from -67 +/- 1 to -62 +/- 4 mV in spindle-shaped cells but hyperpolarised it from -73 +/- 1 to -81 +/- 3 mV in rod-shaped cells. ET-1 decreased APD50 by 8 and 52% and shifted the take-off potential by +5 and -9 mV in spindle- and rod-shaped cells, respectively. 4. ET-1 decreased the high-threshold calcium current (I(CaL)) by about 50% in both cell types, without affecting its voltage dependence, and decreased the delayed rectifier K+ current (I(K)) with significant shifts (of +4.7 and +14.0 mV in spindle- and rod-shaped cells, respectively) in its voltage dependence. It was exclusively in rod-shaped cells that ET-1 activated a sizeable amount of time-independent inward-rectifying current. 5. The hyperpolarisation-activated current (I(f)), observed exclusively in spindle-shaped cells, was significantly increased by ET-1 at membrane potentials between -74.7 and -84.7 mV whereas it was significantly decreased at more negative potentials. ET-1 significantly decreased the slope of the current-voltage (I-V) relation of the I(f) tail without changing its half-maximum voltage. 6. The overall negative chronotropic influence of ET-1 on the whole rabbit SAN is interpreted as resulting from the integration of its different actions on spindle- and rod-shaped SAN cells through electrotonic interaction.

Comparison of L-type and mixed L- and T-type calcium channel blockers on kidney injury caused by deoxycorticosterone-salt hypertension in rats

The efficiency of calcium channel blockers (CCBs) in the treatment of chronic renal disease (CRD) is controversial. In this study, we investigated whether combined T- and L-type CCBs, using mibefradil (30 mg/kg/d), provided superior protection versus traditional L-type voltage-gated CCBs, using amlodipine (10 mg/kg/d), in the deoxycorticosterone acetate (DOCA)-salt model of high glomerular blood pressure (P(GC)) and rapidly developing kidney damage. After 4 to 5 weeks of DOCA-salt, amlodipine did not reduce proteinuria (protein, 341 +/- 90 versus 482 +/- 54 mg/24 h; P = not significant) or degree of glomerular damage (20% +/- 4% versus 28% +/- 6% damaged glomeruli; P = not significant) compared with untreated rats. Conversely, mibefradil reduced proteinuria and glomerular damage versus untreated DOCA-salt rats (protein, 244 +/- 75 mg/24 h; P < 0.02; damaged glomeruli, 11% +/- 3%; P < 0.05). Both CCBs had similar antihypertensive actions, returning blood pressure to the untreated sham value. Of note, P(GC) also was reduced by a similar extent (and to the sham value) with both mibefradil (58 +/- 2 mm Hg; P < 0.001) and amlodipine (61 +/- 2 mm Hg; P < 0.005) versus untreated DOCA-salt rats (70 +/- 1 mm Hg). This study shows that combined T- and L-type CCBs provide superior protection against CRD in the DOCA-salt model compared with L-type CCBs alone. However, this protection was not hemodynamic because similar systemic and glomerular antihypertensive responses occurred with both mibefradil and amlodipine. Although mibefradil was withdrawn from the market because of adverse drug interactions not associated with CCBs, other mixed channel blockers may provide an alternative or adjunctive therapy to angiotensin-converting enzyme inhibition in CRD.

L-type but not T-type calcium current changes during postnatal development in rabbit sinoatrial node

Although the neonatal sinus node beats at a faster rate than the adult, when a sodium current (I(Na)) present in the newborn is blocked, the spontaneous rate is slower in neonatal myocytes than in adult myocytes. This suggests a possible functional substitution of I(Na) by another current during development. We used ruptured [T-type calcium current (I(Ca,T))] and perforated [L-type calcium current (I(Ca,L))] patch clamps to study developmental changes in calcium currents in sinus node cells from adult and newborn rabbits. I(Ca,T) density did not differ with age, and no significant differences were found in the voltage dependence of activation or inactivation. I(Ca,L) density was lower in the adult than newborn (12.1 +/- 1.4 vs. 17.6 +/- 2.5 pA/pF, P = 0.049). However, activation and inactivation midpoints were shifted in opposite directions, reducing the potential contribution during late diastolic depolarization in the newborn (activation midpoints -17.3 +/- 0.8 and -22.3 +/- 1.4 mV in the newborn and adult, respectively, P = 0.001; inactivation midpoints -33.4 +/- 1.4 and -28.3 +/- 1.7 mV for the newborn and adult, respectively, P = 0.038). Recovery of I(Ca,L) from inactivation was also slower in the newborn. The results suggest that a smaller but more negatively activating and rapidly recovering I(Ca,L) in the adult sinus node may contribute to the enhanced impulse initiation at this age in the absence of I(Na).

Abnormal calcium and protein kinase C-epsilon signaling in hypertrophied atrial tumor myocytes (AT-1 cells)

Cardiac hypertrophy leads to contractile dysfunction and altered hormone responsiveness through incompletely understood mechanisms. Atrial tumor (AT-1) myocytes (AT-1 cells) are a cardiomyocyte lineage that proliferates but hypertrophies when proliferation is prevented with mitomycin C. Because both states maintain a highly differentiated phenotype, AT-1 cells were used to explore the signaling pathways that accompany and/or contribute to hypertrophic cardiomyocyte growth. Mitomycin C-induced AT-1 cell enlargement is associated with a pronounced increase in the amplitude and the duration of both electrically stimulated calcium transients and endothelin receptor-dependent calcium responses. Studies with caffeine indicate that the intracellular pool of releasable calcium is similar in control and hypertrophied AT-1 cells. This agrees with the results of Northern analyses that show similar steady-state levels of transcripts encoding the sarcoplasmic reticulum Ca-ATPase (and higher levels of transcripts encoding the Na+/Ca2+ exchanger) in hypertrophied AT-1 cells, relative to proliferating control cultures. However, immunoblot analyses reveal a marked increase in the expression of protein kinase C (PKC)-epsilon (a critical intermediate in the signaling pathway for endothelin receptor-dependent modulation of intracellular calcium) during AT-1 cell hypertrophy; the abundance of other PKC isoforms is not changed. Collectively, these results identify reciprocal regulation between calcium/PKC signaling and hypertrophic growth. The evidence that AT-1 cell hypertrophy leads to abnormalities in calcium regulation and specific changes in PKC-epsilon expression that alter endothelin receptor responsiveness supports the notion that pathophysiological changes in PKC-epsilon abundance lead to functionally important changes in hormonal modulation of cardiomyocyte function.

Alternatively spliced alpha(1G) (Ca(V)3.1) intracellular loops promote specific T-type Ca(2+) channel gating properties

At least three genes encode T-type calcium channel alpha(1) subunits, and identification of cDNA transcripts provided evidence that molecular diversity of these channels can be further enhanced by alternative splicing mechanisms, especially for the alpha(1G) subunit (Ca(V)3.1). Using whole-cell patch-clamp procedures, we have investigated the electrophysiological properties of five isoforms of the human alpha(1G) subunit that display a distinct III-IV linker, namely, alpha(1G-a), alpha(1G-b), and alpha(1G-bc), as well as a distinct II-III linker, namely, alpha(1G-ae), alpha(1G-be), as expressed in HEK-293 cells. We report that insertion e within the II-III linker specifically modulates inactivation, steady-state kinetics, and modestly recovery from inactivation, whereas alternative splicing within the III-IV linker affects preferentially kinetics and voltage dependence of activation, as well as deactivation and inactivation. By using voltage-clamp protocols mimicking neuronal activities, such as cerebellar train of action potentials and thalamic low-threshold spike, we describe that inactivation properties of alpha(1G-a) and alpha(1G-ae) isoforms can support channel behaviors reminiscent to those described in native neurons. Altogether, these data demonstrate that expression of distinct variants for the T-type alpha(1G) subunit can account for specific low-voltage-activated currents observed in neuronal tissues.

Calcium currents and arrhythmias: insights from molecular biology

Calcium channels are critical to normal cardiac function. They are involved in the generation and conduction of the action potential and in contraction. Three surface membrane channels have been identified. The L-type Ca channel is most abundant and is responsible for Ca entry into the cell that triggers contraction. T-type Ca channels are most prevalent in the conduction system and are probably involved in automaticity. A newly described TTX-sensitive calcium current may be important in "boosting" or enhancing conduction and contraction. The main intracellular Ca channel resides in the sarcoplasmic reticulum and is responsible for the release of the Ca that activates contraction. Oscillatory behavior of this channel influences the sarcolemmal membrane, causing delayed aftercontractions and arrhythmias such as those seen in digoxin toxicity. The on-going molecular characterization of these channels will enhance our knowledge of their normal function and dysfunction in disease states, leading to the development of new therapeutic agents to treat arrhythmias and contractile dysfunction.

Antiadrenergic effects of endothelin-1 on the L-type Ca2+ current in dog ventricular myocytes

The effect of endothelin-1 (ET-1) on L-type Ca2+ current (I(Ca)) and the interaction of ET-1 with beta-adrenergic stimulation were studied in dog ventricular myocytes by means of a whole-cell patch-clamp technique. ET-1 (10(-8) M) had no effect on the baseline I(Ca), but at 10(-9)-10(-7) M, it inhibited the isoproterenol (ISO)-induced increase in I(Ca). The maximal inhibition induced by ET-1 at 3 x 10(-8) M was approximately 30%, and the median inhibitory (IC50) value of ET-1 was 1.1 x 10(-9) M. The inhibitory action of ET-1 (10(-8) M) on the ISO-induced increase in I(Ca) was markedly attenuated by the ET(A) antagonist FR139317 (10(-6) M) and was partially inhibited by the ET(B) antagonist BQ-788 (10(-6) M). The inhibitory action of ET-1 was totally inhibited by the nonselective ET-receptor antagonist, TAK-044 (10(-6) M). These results indicate that ET-1 exerts an antiadrenergic effect on the ISO-induced increase in I(Ca), which is mediated mainly by ET(A), but activation of ET(B) receptors might contribute to the effect of ET-1 to a lesser extent.

Mibefradil prevents L-NAME-exacerbated nephrosclerosis in spontaneously hypertensive rats

OBJECTIVE

To determine the potential renal protective effects of a novel calcium channel blocker mibefradil in chronic renal failure.

METHOD

We compared the long-term effects of mibefradil with an angiotensin-converting enzyme inhibitor cilazapril on blood pressure, proteinuria, renal function and histological alterations in N-nitro-L-arginine methylester (L-NAME)-treated spontaneously hypertensive rats (SHR). Three groups of SHR were studied for 45 days: group 1 (n = 14), treated with L-NAME only (50 mg/l in the drinking water); group 2 (n = 15) L-NAME plus co-treatment with mibefradil (30 mg/kg per day); group 3 (n = 15), L-NAME plus co-treatment with cilazapril (10 mg/kg per day).

RESULTS

Both mibefradil and cilazapril attenuated the increased systolic blood pressure, and prevented the development of proteinuria and the decreased creatinine clearance (Ccr) seen at day 42 in the group treated with L-NAME alone. Notably, mibefradil had similar effects to cilazapril on proteinuria and Ccr, despite a reduced antihypertensive effect All animals receiving mibefradil co-treatment remained alive throughout the experiment, whereas the mortality rate was 43% in SHR treated with L-NAME alone. Both mibefradil and cilazapril completely prevented renal structural damage as assessed by scoring glomerular, tubulo-interstitial and vascular lesions.

CONCLUSIONS

Our data show that mibefradil prevented the development of hypertension and proteinuria, renal functional impairment and nephrosclerosis, and also improved animal survival. The renal protective effects of mibefradil were at least equivalent to those of an ACE inhibitor in this animal model of chronic renal failure.

Expression of T-type Ca(2+) channels in ventricular cells from hypertrophied rat hearts

In this study we examined the existence of T-type Ca(2+) current in ventricular myocytes isolated from rats with pressure-overload hypertrophy. The whole-cell clamp technique was used to record Ca(2+) currents in enzymatically dissociated ventricular cells. T- and L-type Ca(2+) currents were separated by applying voltage steps to different test potentials from a holding potential of -80 mV and -50 mV. T-type Ca(2+) current was defined as the difference between the currents from the two holding potentials. Ventricular myocytes from sham-operated rats showed only L-type Ca(2+) current (maximal density -13.9+/-1.3 pA/pF n=17), whereas ventricular myocytes isolated from rats with aortic stenosis showed both L- and T-type Ca(2+) currents. The average values of T- and L-type Ca(2+) current density were -4.8+/-0.4 pA/pF and -12.4+/-0.9 pA/pF (n=32), respectively. T-type Ca(2+) current was distinguished from L-type Ca(2+) current by its voltage dependence, its kinetics and by its strong blockade by nickel 50 microM. In conclusion, we have demonstrated that hypertrophied ventricular rat cells express T-type Ca(2+) channels and this finding strongly supports a role for this channel in regulating growth processes in cardiac tissue.

Role of Na+/Ca2+ exchange in endothelin-1-induced increases in Ca2+ transient and contractility in rabbit ventricular myocytes: pharmacological analysis with KB-R7943

1. The effects of endothelin-1 (ET-1) on intracellular Ca2+ ion level and cell contraction were simultaneously investigated in rabbit ventricular cardiac myocytes loaded with indo-1/A1. The role of Na+/Ca2+ exchange in ET-1-induced positive inotropic effect (PIE) was examined by using KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulphonate), a selective inhibitor of reverse mode Na+/Ca2+ exchange. 2. ET-1 at 0.3 pM - 1 nM increased cell contraction and Ca2+ transient (CaT) with EC50 values of 2.9 pM and 1.2 pM, respectively, and the increase in amplitude of CaT was much smaller relative to the PIE: ET-1 at 1 nM increased peak cell shortening by 237%, while it increased peak CaT by 167%. For a given level of PIE, ET-1-induced increase in CaT was much smaller than that induced by elevation of [Ca2+]o and by isoprenaline. Therefore, ET-1 shifted the relationship between peak CaT and cell shortening to the left relative to the relationship for increase in [Ca2+]o, an indication that ET-1 increased myofibrillar Ca2+ sensitivity. 3. KB-R7943 at 0.1 microM and higher inhibited contraction and CaT induced by 0.1 nM ET-1 and at 0.3 microM it abolished the increase in CaT while inhibiting the PIE by 48.1%. Over concentration range of 0.1-0.3 microM, KB-R7943 neither inhibited baseline contraction and CaT nor the isoprenaline-induced response, although at 1 microM and higher it had a significant inhibitory action on these responses. 4. These results indicate that in rabbit ventricular myocytes both increases in CaT and myofibrillar Ca2+ sensitivity contribute to the ET-induced PIE, and the activation of reverse mode Na+/Ca2+ exchange may play a crucial role in increase in CaT induced by ET-1 in rabbit ventricular cardiac myocytes.