Inactivation properties of T-type calcium current in canine cardiac Purkinje cells

Deletion of TRPV3 and CaV3.2 T-type channels in mice undermines fertility and Ca2+ homeostasis in oocytes and eggs

Ca2+ influx during oocyte maturation and after sperm entry is necessary to fill the internal Ca2+ stores and for complete egg activation. We knocked out the transient receptor potential vanilloid member 3 (TRPV3) and the T-type channel, CaV3.2, to determine their necessity for maintaining these functions in mammalian oocytes/eggs. Double-knockout (dKO) females were subfertile, their oocytes and eggs showed reduced internal Ca2+ stores, and, following sperm entry or Plcz (also known as Plcz1) cRNA injection, fewer dKO eggs displayed Ca2+ responses compared to wild-type eggs, which were also of lower frequency. These parameters were rescued and/or enhanced by removing extracellular Mg2+, suggesting that the residual Ca2+ influx could be mediated by the TRPM7 channel, consistent with the termination of divalent-cation oscillations in dKO eggs by a TRPM7 inhibitor. In total, we demonstrated that TRPV3 and CaV3.2 mediate the complete filling of the Ca2+ stores in mouse oocytes and eggs. We also showed that they are required for initiating and maintaining regularly spaced-out oscillations, suggesting that Ca2+ influx through PM ion channels dictates the periodicity and persistence of Ca2+ oscillations during mammalian fertilization.

Modeling our understanding of the His-Purkinje system

The His-Purkinje System (HPS) is responsible for the rapid electric conduction in the ventricles. It relays electrical impulses from the atrioventricular node to the muscle cells and, thus, coordinates the contraction of ventricles in order to ensure proper cardiac pump function. The HPS has been implicated in the genesis of ventricular tachycardia and fibrillation as a source of ectopic beats, as well as forming distinct portions of reentry circuitry. Despite its importance, it remains much less well characterized, structurally and functionally, than the myocardium. Notably, important differences exist with regard to cell structure and electrophysiology, including ion channels, intracellular calcium handling, and gap junctions. Very few computational models address the HPS, and the majority of organ level modeling studies omit it. This review will provide an overview of our current knowledge of structure and function (including electrophysiology) of the HPS. We will review the most recent advances in modeling of the system from the single cell to the organ level, with considerations for relevant interspecies distinctions.

TRPV3 channels mediate strontium-induced mouse-egg activation

In mammals, calcium influx is required for oocyte maturation and egg activation. The molecular identities of the calcium-permeant channels that underlie the initiation of embryonic development are not established. Here, we describe a transient receptor potential (TRP) ion channel current activated by TRP agonists that is absent in TrpV3(-/-) eggs. TRPV3 current is differentially expressed during oocyte maturation, reaching a peak of maximum density and activity at metaphase of meiosis II (MII), the stage of fertilization. Selective activation of TRPV3 channels provokes egg activation by mediating massive calcium entry. Widely used to activate eggs, strontium application is known to yield normal offspring in combination with somatic cell nuclear transfer. We show that TRPV3 is required for strontium influx, because TrpV3(-/-) eggs failed to conduct Sr(2+) or undergo strontium-induced activation. We propose that TRPV3 is a major mediator of calcium influx in mouse eggs and is a putative target for artificial egg activation.

High purity human-induced pluripotent stem cell-derived cardiomyocytes: electrophysiological properties of action potentials and ionic currents

Human-induced pluripotent stem cells (hiPSCs) can differentiate into functional cardiomyocytes; however, the electrophysiological properties of hiPSC-derived cardiomyocytes have yet to be fully characterized. We performed detailed electrophysiological characterization of highly pure hiPSC-derived cardiomyocytes. Action potentials (APs) were recorded from spontaneously beating cardiomyocytes using a perforated patch method and had atrial-, nodal-, and ventricular-like properties. Ventricular-like APs were more common and had maximum diastolic potentials close to those of human cardiac myocytes, AP durations were within the range of the normal human electrocardiographic QT interval, and APs showed expected sensitivity to multiple drugs (tetrodotoxin, nifedipine, and E4031). Early afterdepolarizations (EADs) were induced with E4031 and were bradycardia dependent, and EAD peak voltage varied inversely with the EAD take-off potential. Gating properties of seven ionic currents were studied including sodium (I(Na)), L-type calcium (I(Ca)), hyperpolarization-activated pacemaker (I(f)), transient outward potassium (I(to)), inward rectifier potassium (I(K1)), and the rapidly and slowly activating components of delayed rectifier potassium (I(Kr) and I(Ks), respectively) current. The high purity and large cell numbers also enabled automated patch-clamp analysis. We conclude that these hiPSC-derived cardiomyocytes have ionic currents and channel gating properties underlying their APs and EADs that are quantitatively similar to those reported for human cardiac myocytes. These hiPSC-derived cardiomyocytes have the added advantage that they can be used in high-throughput assays, and they have the potential to impact multiple areas of cardiovascular research and therapeutic applications.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Creating a cardiac pacemaker by gene therapy

While electronic cardiac pacing in its various modalities represents standard of care for treatment of symptomatic bradyarrhythmias and heart failure, it has limitations ranging from absent or rudimentary autonomic modulation to severe complications. This has prompted experimental studies to design and validate a biological pacemaker that could supplement or replace electronic pacemakers. Advances in cardiac gene therapy have resulted in a number of strategies focused on beta-adrenergic receptors as well as specific ion currents that contribute to pacemaker function. This article reviews basic pacemaker physiology, as well as studies in which gene transfer approaches to develop a biological pacemaker have been designed and validated in vivo. Additional requirements and refinements necessary for successful biopacemaker function by gene transfer are discussed.

Evidence for common structural determinants of activation and inactivation in T-type Ca2+ channels

One of the most distinctive features of T-type Ca(2+) channels is their fast inactivation. Recent structure-function studies indicate that the rate of macroscopic inactivation of these channels is influenced by several structural components, including intracellular linkers, transmembrane segments, and pore loops. The macroscopic inactivation of T-type channels is partially coupled to activation. It is therefore possible that changes in the rate of macroscopic inactivation after alteration in the structure of these channels might actually result from changes in activation kinetics. In this study, we use kinetic simulations to illustrate how the alteration of the rate of channel activation may lead to changes in the rate of macroscopic inactivation. By examining data pooled from several structure-function studies we demonstrate that gating modifications induced by alteration in the channel structure unveils a correlation between the time constants of macroscopic inactivation and activation. This analysis underscores the relevance of considering the inactivation-activation coupling when analyzing the structural determinants of T-type channel inactivation. Furthermore, we demonstrate that slow-inactivating mutants, with modifications in the IIIS6 segment and the proximal C terminus, display significant alterations in the voltage dependencies of activation and deactivation with respect to the wild type channel Ca(V)3.1. Our results indicate that common structures, most likely the S6 transmembrane segments, are involved in the conformational changes occurring during both channel activation and inactivation.

Remodeled cardiac calcium channels

Cardiac calcium channels play a pivotal role in the proper functioning of cardiac cells. In response to various pathologic stimuli, they become remodeled, changing how they function, as they adapt to their new environment. Specific features of remodeled channels depend upon the particular disease state. This review will summarize what is known about remodeled cardiac calcium channels in three disease states: hypertrophy, heart failure and atrial fibrillation. In addition, it will review the recent advances made in our understanding of the function of the various molecular building blocks that contribute to the proper functioning of the cardiac calcium channel.

Augmentation of Cav3.2 T-type calcium channel activity by cAMP-dependent protein kinase A

Ca2+ influx through T-type Ca2+ channels is crucial for important physiological activities such as hormone secretion and neuronal excitability. However, it is not clear whether these channels are regulated by cAMP-dependent protein kinase A (PKA). In the present study, we examined whether PKA modulates Cav3.2 T-type channels reconstituted in Xenopus oocytes. Application of 10 microM forskolin, an adenylyl cyclase stimulant, increased Cav3.2 channel activity by 40+/-4% over 30 min and negatively shifted the steady-state inactivation curve (V50=-61.4+/-0.2 versus -65.5+/-0.1 mV). Forskolin did not affect other biophysical properties of Cav3.2 channels, including activation curve, current kinetics, and recovery from inactivation. Similar stimulation was achieved by applying 200 microM 8-bromo-cAMP, a membrane-permeable cAMP analog. The augmentation of Cav3.2 channel activity by forskolin was strongly inhibited by preincubation with 20 microM N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline (H89), and reversed by subsequent application of 500 nM protein kinase A inhibitor peptide. The stimulation of Cav3.2 channel activity by PKA was mimicked by serotonin when 5HT7 receptor was coexpressed with Cav3.2 in Xenopus oocytes. Finally, using chimeric channels constructed by replacing individual cytoplasmic loops of Cav3.2 with those of the Nav1.4 channel, which is insensitive to PKA, we localized a region required for the PKA-mediated augmentation to the II-III loop of the Cav3.2.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

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.

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.

The Gating and Conductance Properties of CaV3.2 Low-Voltage-Activated T-Type Calcium Channels

Calcium channels are essential for excitation-contraction coupling and pacemaker potentials in cardiac muscle cells. Whereas L-type Ca(2+) channels have been extensively studied, T-type channels have been poorly characterized in cardiac myocytes. We describe here the functional properties of recombinant Ca(V)3.2 T-type Ca(2+) channels expressed in mammalian cell lines. The T-type Ca(2+) current showed a rapid activation and an inactivation phase in response to depolarization, and it displayed a window current over the voltage range from -60 to -40 mV in 1 to 10 mM external Ca(2+). Barium (Ba(2+)) and strontium (Sr(2+)) permeate the channel with similar activation kinetics. On the other hand, monovalent cations, Li(+) and Na(+), permeate the T-type Ca(2+) channel more easily than the L-type Ca(2+) channel. The permeability order of the Ca(V)3.2 T-type Ca(2+) channel among monovalent and divalent cations was determined as Ba(2+)>Mn(2+)>Ca(2+)>Sr(2+)>Li(+1)>Na(+) with the permeability order of 1.39:1.25:1.00:0.95:0.55:0.29. The ionic conductance sequence for cations relative to calcium was Sr(2+)>Ba(2+)>Ca(2+)>Li(+1)>Mn(2+)>Na(+) with the conductance ratio of 1.39:1.21:1.00:0.40:0.23:0.11. The permeation profile of manganese (Mn(2+)) is complex. Mn(2+) permeates the Ca(2+) channel with a permeability similar to Ca(2+) or Ba(2+), but with a much smaller current density, resulting in a much smaller conductance. The properties relating to progression and recovery from inactivation in the Ca(V)3.2 channel are substantially identical with either Ca(2+) or Ba(2+) as the charge carrier.

Effects of mibefradil, a T-type calcium current antagonist, on electrophysiology of Purkinje fibers that survived in the infarcted canine heart

INTRODUCTION

We studied the effects of mibefradil (MIB), a nondihydropyridine T-type Ca2+ channel antagonist, on T- and L-type Ca2+ (I(CaT), I(CaL)) currents in Purkinje myocytes dispersed from the subendocardium of the left ventricle of normal (NZPC) and 48-hour infarcted (IZPC) hearts.

METHODS AND RESULTS

Currents were recorded with Cs+- and EGTA-rich pipettes and in Na+-K+-free external solutions to eliminate overlapping currents. In all cells, I(Ca) was reduced by MIB (0.1 to 10 microM). No change in the time course of decay of peak I(Ca) was noted. Average peak T/L ratio decreased in NZPCs but not IZPCs with 1 microM MIB. Steady-state availability of I(CaL) was altered with 1 microM MIB in both cell types (mean +/- SEM) (V0.5 = -22 +/- 4 mV for NZPC and -25 +/- 5 mV for IZPC before drug; -63 +/- 9 mV for NZPC and -67 +/- 6 mV for IZPC after drug; P < 0.05). For I(CaT), V0.5 (-50 +/- 3 mV for NZPC and -52 +/- 1 mV for IZPC before drug) shifted to -60 +/- 2 mV (NZPC) and -62 +/- 3 mV (IZPC) (P < 0.05) after drug. We also determined the effects of MIB on spontaneously beating Purkinje normal fibers and on depolarized abnormally automatic fibers from the infarcted heart using standard microelectrode techniques. When NZPC and IZPC fibers were superfused with [K+]o = 2.7 mM, MIB 3 microM and 10 microM had no effect on rate or the maximum diastolic potential, but action potential plateau shifted to more negative values, the slope of repolarization phase 3 decreased, and action potential duration increased.

CONCLUSION

MIB blocks L- and T-type Ca2+ currents in Purkinje myocytes but lacks an effect on either normal or abnormal automaticity in Purkinje fibers.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Proarrhythmic effects of antidepressants and neuroleptic drugs on isolated, spontaneously beating guinea-pig Purkinje fibers

Antidepressants and neuroleptic drugs are sometimes the reason for the occurrence of the polymorphic ventricular arrhythmia torsades de pointes in patients. Therefore, it was of interest to study the actions of some of these drugs such as imipramine, amitriptyline, doxepin, chlorpromazine, trifluoperazine and thioridazine in isolated, spontaneously beating Purkinje fibers of guinea-pig hearts using the intracellular microelectrode technique because experimentally induced early afterdepolarizations (EADs) may be associated with this special type of arrhythmia. If the extracellular K+ concentration was 2.7mM none of these drugs could elicit EADs. For that reason the K+ concentration was lowered to 1. 35mM and EADs were evoked by imipramine (2 and 5 microM). Amitriptyline (2 and 5 microM) and doxepin (2 microM) did not induce EADs. Only a concentration of 5 microM doxepin elicited EADs. Among the neuroleptic drugs, chlorpromazine at a concentration of 2 and 5 microM was responsible for the occurrence of EADs as well as thioridazine in the same concentrations. When trifluoperazine (2 and 5 microM) was applied no EADs could be observed. Tetrodotoxin (0. 2 microMl-1) abolished thioridazine-induced EADs. Several membrane depolarizing currents may participate in the initiation of these EADs. Our results demonstrate that in guinea-pig Purkinje fibers some tricyclic antidepressants and some neuroleptic drugs are responsible for the rare occurrence of EADs under hypokalemic conditions.

Enantioselective blockade of T-type Ca2+ current in adult rat sensory neurons by a steroid that lacks gamma-aminobutyric acid-modulatory activity

A number of steroids seem to have anesthetic effects resulting primarily from their ability to potentiate currents gated by gamma-aminobutyric acidA (GABAA) receptor activation. One such compound is (3alpha,5alpha, 17beta)-3-hydroxyandrostane-17-carbonitrile [(+)-ACN]. We were interested in whether carbonitrile substitution at other ring positions might result in other pharmacological consequences. Here we examine effects of (3beta,5alpha, 17beta)-17-hydroxyestrane-3-carbonitrile [(+)-ECN] on GABAA receptors and Ca2+ channels. In contrast to (+)-ACN, (+)-ECN does not potentiate GABAA-receptor activated currents, nor does it directly gate GABAA-receptor mediated currents. However, both steroids produce an enantioselective reduction of T-type current. (+)-ECN blocked T current with an IC50 value of 0.3 microM with a maximal block of 41%. (+)-ACN produced a partial block of T current (44% maximal block) with an IC50 value of 0.4 microM. Block of T current showed mild use- and voltage-dependence. The (-)-ECN enantiomer was about 33 times less potent than (+)-ECN, with an IC50 value of 10 microM and an amount of maximal block comparable to (+)-ECN. (+)-ECN was less effective at blocking high-voltage-activated Ca2+ current in DRG neurons (IC50 value of 9. 3 microM with maximal block of about 27%) and hippocampal neurons. (+)-ECN (10 microM) had minimal effects on voltage-gated sodium and potassium currents in rat chromaffin cells. The results identify a steroid with no effects on GABAA receptors that produces a partial inhibition of T-type Ca2+ current with reasonably high affinity and selectivity. Further study of steroid actions on T currents may lead to even more selective and potent agents.

Both T- and L-type Ca2+ channels can contribute to excitation-contraction coupling in cardiac Purkinje cells

Although L-type Ca2+ channels have been shown to play a central role in cardiac excitation-contraction (E-C) coupling, little is known about the role of T-type Ca2+ channels in this process. We used the amphotericin B perforated patch method to study the possible role of T-type Ca2+ current in E-C coupling in isolated canine Purkinje myocytes where both Ca2+ currents are large. T-type Ca2+ current was separated from L-type Ca2+ current using protocols employing the different voltage dependencies of the channel types and their different sensitivities to pharmacological blockade. We showed that Ca2+ admitted through either T- or L-type Ca2+ channels is capable of initiating contraction and that the contractions depended on Ca2+-induced Ca2+ release from the sarcoplasmic reticulum (SR). The contractions, however, had different properties. Those initiated by Ca2+ entry through T-type Ca2+ channels had a longer delay to the onset of shortening, slower rates of shortening and relaxation, lower peak shortening, and longer time to peak shortening. These differences were present even when L-type Ca2+ current amplitude, or charge entry, was less than that of T-type Ca2+ current, suggesting that Ca2+ entry through the T-type Ca2+ channel is a less effective signal transduction mechanism to the SR than is Ca2+ entry through the L-type Ca2+ channel. We conclude that under our experimental conditions in cardiac Purkinje cells Ca2+ entry through the T-type Ca2+ channel can activate cell contraction. However, Ca2+ entry through the L-type Ca2+ channel is a more effective signal transduction mechanism. Our findings support the concept that different structural relationships exist between these channel types and the SR Ca2+ release mechanism.

Pharmacological properties of T-type Ca2+ current in adult rat sensory neurons: effects of anticonvulsant and anesthetic agents

We have used the whole cell patch-clamp method to study pharmacological properties of low-voltage-activated (LVA) Ca2+ current in freshly dissociated neurons from dorsal root ganglia of adult rats. Inward barium current [in the presence of internal fluoride to reduce L-type high-voltage-activated (HVA) and external 1 microM omega-conotoxin GVIA to block N-type HVA current- was evoked from negative holding potentials of -90 mV to test potentials of -25 mV and showed complete inactivation during 200-ms test pulses. Amiloride blocked approximately 90% of current with half-maximal block (EC50) of 75 microM and a Hill coefficient (n) of 0.99. LVA current was blocked completely by inorganic Ca2+ channel blockers: lanthanum (EC50 = 0. 53 microM) > zinc (EC50 = 11.3 microM) > cadmium (EC50 = 20 microM)> nickel (EC50 = 51 microM). The antiepileptics, ethosuximide (EC50 = 23.7 mM, n = 1.4), phenytoin (EC50 = 7.3 microM, n = 1.3), alpha-methyl-alpha-phenylsuccinimide (EC50 = 170 microM, n = 2.1), and valproic acid (EC50 = 330 microM, n = 1.9) maximally blocked approximately 100, 60, 26, and 17% of T current, respectively. Another antiepileptic, carbamazepine (</=100 microM), and convulsants such as pentylenetetrazole (1 mM) and tert-butyl-bicyclo [2.2.2] phosphorothionate (50 microM) had no effect on T current. Barbiturates completely blocked T current: thiopental (EC50 = 153 microM, n =1.2) > pentobarbital (EC50 = 334 microM, n = 1.2) > methohexital (EC50 = 502 microM, n = 1.3) > phenobarbital (EC50 = 1. 7 mM, n = 1.2). Blockade by thiopental and pentobarbital did not show voltage or use dependence. General anesthetics blocked T current completely and reversibly: propofol (EC50 = 12.9 microM, n = 1.3) > octanol(EC50 = 122 microM, n = 1.2) > etomidate (EC50 = 205 microM, n =1.3) > isoflurane (EC50 = 303 microM, n = 2.3) > halothane (EC50 = 655 microM, n = 2.0) > ketamine (EC50 = 2.5 mM, n = 1.1). Mibefradil, a novel Ca2+ channel blocker, blocked dorsal root ganglion T current in a voltage- and use-dependent fashion with an EC50 of approximately 3 microM (n = 1.3). When compared with results on other T currents, these data indicate that significant differences exist among different T currents in terms of pharmacological sensitivities. Furthermore, differences in pharmacological sensitivity of T currents among peripheral neurons, CNS, and neuroendocrine cells may contribute to the spectrum of effects of particular analgesic, anticonvulsant, and anesthetic drugs.

Role of Na+:Ca2+ exchange current in Cs(+)-induced early afterdepolarizations in Purkinje fibers

INTRODUCTION

The ionic mechanisms for early afterdepolarizations (EADs) have not been fully clarified. It has been suggested that L-type Ca2+ current (ICaL) is the primary current generating EADs that occur near the plateau level (E-EADs) of the membrane potential (Vm) when ICaL is enhanced. The purpose of these studies was to determine accurately the range of Vm at which EADs occur in Purkinje fibers with K+ currents blocked by Cs+ and to investigate the importance of Na+:Ca2+ exchange current (INa:Ca) as opposed to ICaL and other currents in the generation of EADs occurring later during repolarization (L-EADs).

METHODS AND RESULTS

Shortened Purkinje strands from dogs and guinea pigs were superfused with physiologic solution containing Cs+ (3.6 mM) and a low [K+]o (3.0 or 2.0 mM) to induce EADs. The Vm of origin of EADs and their evolution were measured with the aid of phase plane plots of the rate of repolarization against Vm. L-EADs occurred over a wide range of Vm (-35 to -90 mV), generally more negative in guinea pig than in dog. Elevation of [Ca2+]o from 1.8 to 3.6 mM suppressed L-EADs within a few cycles, and they returned with continued exposure. After repeated exposures to high [Ca2+]o, L-EADs migrated toward less negative Vm when [Ca2+]o was reestablished to 1.8 mM in the presence of Cs+. Reduction of [Na+]o from 147.5 to 112.5 mM by substitution with Li+ or sucrose also rapidly depressed L-EADs.

CONCLUSIONS

The observation of Cs(+)-induced L-EADs over a wide range of Vm indicates that there is not a single inward gated current as a common ionic mechanism for L-EADs but does not exclude an important role for INa:Ca, which can operate over a wide range of Vm. The rapid suppression of L-EADs with elevated [Ca2+]o and reduced [Na+]o and the migration of EADs to more positive Vm after exposures to high [Ca2+]o are compatible with INa:Ca as the major charge carrier for L-EADs.

Developmental changes in Ca2+ currents from newborn rat cardiomyocytes in primary culture

Electrophysiological characteristics of neonatal rat ventricular cardiomyocytes in primary culture were studied using the whole-cell patch-clamp recording technique. Cell size, estimated by measurement of membrane capacitance, was significantly increased throughout the culture from 22.4 +/- 5.4 pF at day 2 to 55.0 +/- 16.1 pF at day 7, reflecting the hypertrophic process which characterises postnatal cell development. The Ca2+ current was investigated at day 2 and 7 of the culture which constituted the early postnatal and maximally developed stages, respectively, of isolated cells in our experimental conditions. At 2 days of culture, two types of Ca2+ current could be distinguished, as also observed in freshly dissociated newborn ventricular cells. From their potential dependence and pharmacological characteristics, they could be attributed to the T- (ICa-T) and L-type (ICa-L) Ca2+ current components. After 7 days of culture, only the latter ICa-L was present and its density was significantly increased when compared to the density in 2-day-old cells, but lower than that obtained in freshly dissociated adult cells. As the age of the culture progressed, the steady-state inactivation curve was shifted toward negative potentials, in the direction of the inactivation curve obtained for adult cells. Compared to the serum-free control conditions, the density of ICa-L was significantly increased in the presence of fetal calf serum throughout the culture. Consequently, the density of ICa-L obtained in 7-day-old cells was similar to the density of ICa-L obtained in freshly dissociated adult cardiac cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced calcium currents in subendocardial Purkinje myocytes that survive in the 24- and 48-hour infarcted heart

BACKGROUND

The abnormal transmembrane action potentials of subendocardial Purkinje fibers that survive 24 to 48 hours after coronary artery occlusion can be a source of the multiform ventricular tachycardias that occur during this time. A change in the density or function of either or both the T-type and L-type cardiac Ca2+ channels may contribute to the altered electrical activity of these Purkinje myocytes.

METHODS AND RESULTS

The purpose of this study was to determine the function of the T- and L-type Ca2+ currents (iCat and iCaL, respectively) in Purkinje myocytes dispersed from the subendocardium of the left ventricle 24 and 48 hours after coronary artery occlusion (IZPC24 and IZPC48, respectively). To do this we compared whole-cell Ca2+ currents from Purkinje myocytes enzymatically dispersed from free-running fiber bundles (SPCs), from the subendocardium of the noninfarcted canine heart (NZPCs), and from IZPC24 and IZPC48. ICaL and iCat were recorded with Cs(+)- and EGTA-rich pipettes and in Na(+)-K(+)-free external solutions to eliminate overlapping currents. ICaL density was significantly reduced in IZPC48 compared with NZPC or IZPC24. This was not accompanied by a shift in the current-voltage relation or by a change in the time course of decay of iCaL. Replacement of Ca2+ with equimolar Ba2+ increased iCaL density in all cell types, but peak iBaL of IZPC48 remained reduced compared with control iBaL values. T-type Ca2+ currents were recorded in all SPCs and NZPCs. In IZPC24 and IZPC48 there was a reduction in peak iCat amplitudes and densities. This was not accompanied by a shift in the current-voltage relation or by a change in the time course of decay of peak iCat. However, there was a hyperpolarizing shift in the steady-state availability relations in both IZPC24 and IZPC48. In addition, the maximally available iCat in IZPC24 was not different from control, whereas it was significantly reduced in IZPC48.

CONCLUSIONS

The L-type ICa density in subendocardial Purkinje myocytes that survive in the infarcted heart is significantly decreased by 48 hours after the time of coronary artery occlusion. The peak T-type ICa density is decreased in subendocardial Purkinje myocytes that survive in the infarcted heart at 24 hours, but further reduction occurs in these myocytes by 48 hours. This loss in Ca2+ channel function could contribute to the abnormal transmembrane potentials of these myocytes surviving in the infarcted heart.

Cardiac T-type calcium current: pharmacology and roles in cardiac tissues

A low threshold, voltage-gated calcium current is reported in most cardiac tissues but rarely in ventricular cells. This article reports some recently described characteristics and discusses their possible pathophysiologic implications. It also reviews the alterations induced in this current by a variety of chemical agents including several neuromediators in cardiac and other tissues.

Direct measurement of L-type Ca2+ window current in heart cells

The activation and inactivation relations of several ion channel currents overlap, suggesting the existence of a steady-state or "window" current. We studied L-type Ca2+ channel window current in single cardiac Purkinje cells using a voltage-clamp protocol by which channels were first inactivated nearly completely during a long-duration depolarizing step, and then the recovery of Ca2+ current was observed during repolarizing steps into the L-type Ca2+ window voltage range. With these conditions, a small-amplitude inward Ca2+ current gradually developed after repolarization to voltages within the window but not after steps to voltages positive or negative to it. Window current was suppressed by Cd2+ (50 microM), nifedipine (1 microM), and nicardipine (1 microM), and it was augmented by isoproterenol (5 microM) and Bay K 8644 (1 microM). At voltages at which window current developed, L-type Ca2+ channels also recovered to a closed state from which they could be reopened by an additional depolarizing step. At voltages positive to the window range, channel recovery to a closed state(s) was absent, whereas at voltages negative to the window range, channel recovery to a closed state(s) increased, as expected from the "steady-state" inactivation relation. Our results provide direct measurement of L-type Ca2+ window current and distinguish it from other processes, such as slow inactivation. Our findings support the postulate that within a window there occur channel transitions from inactivated to closed states, and these channels (re)open, and this process may occur repetitively. Some physiological and pathophysiological roles for L-type Ca2+ window current are discussed.

L- and T-type Ca2+ channels in canine cardiac Purkinje cells. Single-channel demonstration of L-type Ca2+ window current

Canine cardiac Purkinje cells contain both L- and T-type calcium currents, yet the single Ca2+ channels have not been characterized from these cells. Additionally, previous studies have shown an overlap between the steady-state inactivation and activations curves for L-type Ca2+ currents, suggesting the presence of L-type Ca2+ "window" current. We used the on-cell, patch-clamp technique to study Ca2+ channels from isolated cardiac Purkinje cells. Patches contained one or more Ca2+ channels 75% of the time. L-type channels were seen in 69% and T-type channels in 73% of these patches. With 110 mM Ba2+ as the charge carrier, the conductances of the L- and T-type Ca2+ channels were 24.2 +/- 0.8 pS (n = 9) and 9.0 +/- 0.5 pS (n = 8), respectively (mean +/- SEM). With 110 mM Ca2+ as the charge carrier, the conductance of the L-type Ca2+ channel decreased to 9.7 +/- 1.2 pS (n = 4), whereas the T-type Ca2+ channel conductance was unchanged. Voltage-dependent inactivation was shown for both L- and T-type Ca2+ channels, although for L-type Ca2+ channel with Ba2+ as the charge carrier, inactivation took at least 30 seconds at a potential of +40 mV. After channel inactivation was complete, L-type Ca2+ channel reopenings were observed following repolarizing steps into the window voltage range. Thus, our data identify both L- and T-type Ca2+ channels in cardiac Purkinje cells and demonstrate, at the single-channel level, L-type channel transitions expected for a window current. Window current may play an important role in shaping the action potential and in arrhythmogenesis.

Stimulation-induced potentiation of T-type Ca2+ channel currents in myocytes from guinea-pig coronary artery

1. Whole-cell Ca2+ channel currents were studied in myocytes isolated from guinea-pig circumflex coronary artery at 36 degrees C and with 10 mM-Ba2+ (or Ca2+) as charge carrier. With 180 ms clamp steps from the holding potential of -100 mV, currents at -30 mV were carried mostly through the T-type calcium channels while at positive potentials currents were mostly of the L-type. 2. The increase in frequency of pulsing from 0.1 to 2.5 Hz resulted in a reduction of peak inward current ('negative staircase') with the 180 ms pulses to + 10 mV, but in a 2-fold potentiation ('positive staircase') with pulses to -30 mV. T-type currents and their frequency-mediated potentiation did not change significantly when Ba2+ was substituted by Ca2+ or Sr2+. 3. Potentiation of T-type currents was further analysed with a paired-pulse protocol: at a basal frequency of 0.1 Hz, a pre-pulse (inducing current I1) was followed by a 200 ms repolarization to -100 mV and a test pulse (inducing current I2). The potentiation could only be recorded using test pulses depolarizing the membrane to potentials between -40 and -10 mV; at more positive test potentials it was masked by the depressant effect of pre-pulses on the L-type current. 4. Potentiation of I2 by 200 ms pre-pulses started at pre-pulse potentials more positive than -60 mV and saturated at -20 mV (I2 potentiated by a factor 2.4). Between -20 and +130 mV the potentiation was not dependent on the pre-pulse potential suggesting that the influx of Ba2+ or Ca2+ is not required for this effect. Potentiation of I2 by a 10 s pre-pulse followed the voltage dependence of the steady-state inactivation curve of the T-type Ca2+ channel; potentiation became visible at potentials more positive than -80 mV and saturated at about -50 mV. 5. When changing the interval between two identical 200 ms pulses, the T-type current was found to recover completely from inactivation within 40 ms at -100 mV; at intervals of 160-320 ms maximal potentiation of I2 occurred. 6. With pre-pulses shorter than 200 ms, potentiation became attenuated when inactivation became less complete. When the potential during the interval between the pulses was -80 instead of -100 mV, maximal potentiation was reduced (I2 potentiated by a factor of 1.3 instead of 2.2) and occurred later (1.28 s). 7. Potentiated T-type currents inactivated faster.(ABSTRACT TRUNCATED AT 400 WORDS)

L-type Ca2+ currents in ascidian eggs

We have studied Ca2+ currents in ascidian eggs using the whole-cell clamp technique. T and L components, as observed in somatic cells, are present and the L-type current predominates. Since the IV relationship for these inward currents overlap at -30 mV, separation of the two components using different voltage regimes is not feasible. Increasing external Ca2+ results in larger currents. The L-type current decreases in a dose-dependent fashion in the presence of Mn2+ and Nifedipine, while the T-type current is inhibited in Ni2+. When Ba2+ was used as the carrier ion, channel kinetics and conductance were completely altered. Considering the density and kinetics of L-type channels in unfertilized eggs it is probable they play an important role in regulating cytosolic Ca2+ during early developmental processes.

Ca channel gating during cardiac action potentials

How do Ca channels conduct Ca ions during the cardiac action potential? We attempt to answer this question by applying a two-microelectrode technique, previously used for Na and K currents, in which we record the patch current and the action potential at the same time (Mazzanti, M., and L. J. DeFelice. 1987. Biophys. J. 12:95-100, and 1988. Biophys. J. 54:1139-1148; Wellis, D., L. J. DeFelice, and M. Mazzanti. 1990. Biophys. J. 57:41-48). In this paper, we also compare the action currents obtained by the technique with the step-protocol currents obtained during standard voltage-clamp experiments. Individual Ca channels were measured in 10 mM Ca/1 Ba and 10 mM Ba. To describe part of our results, we use the nomenclature introduced by Hess, P., J. B. Lansman, and R. W. Tsien (1984. Nature (Lond.). 311:538-544). With Ba as the charge carrier, Ca channel kinetics convert rapidly from long to short open times as the patch voltage changes from 20 to -20 mV. This voltage-dependent conversion occurs during action potentials and in step-protocol experiments. With Ca as the charge carrier, the currents are brief at all voltages, and it is difficult to define either the number of channels in the patch or the conductance of the individual channels. Occasionally, however, Ca-conducting channels spontaneously convert to long-open-time kinetics (in Hess et al., 1984, notation, mode 2). When this happens, which is about once in every 100beats, there usually appears to be only one channel in the patch. In this rare configuration, the channel is open long enough to measure its conductance in 10 Ca/ 1 Ba. The value is 8-10 pS, which is about half the conductance in Ba. Because the long openings occur so infrequently with Ca as the charge carrier, they contribute negligibly to the average Ca current at any particular time during an action potential. However, the total number of Ca ions entering during these long openings may be significant when compared to the number entering by the more usual kinetics.