Ion-dependent inactivation of barium current through L-type calcium channels

Endogenous and exogenous Ca2+ buffers differentially modulate Ca2+-dependent inactivation of Ca(v)2.1 Ca2+ channels

Voltage-gated Ca2+ channels undergo a negative feedback regulation by Ca2+ ions, Ca2+-dependent inactivation, which is important for restricting Ca2+ signals in nerve and muscle. Although the molecular details underlying Ca2+-dependent inactivation have been characterized, little is known about how this process might be modulated in excitable cells. Based on previous findings that Ca2+-dependent inactivation of Ca(v)2.1 (P/Q-type) Ca2+ channels is suppressed by strong cytoplasmic Ca2+ buffering, we investigated how factors that regulate cellular Ca2+ levels affect inactivation of Ca(v)2.1 Ca2+ currents in transfected 293T cells. We found that inactivation of Ca(v)2.1 Ca2+ currents increased exponentially with current amplitude with low intracellular concentrations of the slow buffer EGTA (0.5 mm), but not with high concentrations of the fast Ca2+ buffer BAPTA (10 mm). However, when the concentration of BAPTA was reduced to 0.5 mm, inactivation of Ca2+ currents was significantly greater than with an equivalent concentration of EGTA, indicating the importance of buffer kinetics in modulating Ca2+-dependent inactivation of Ca(v)2.1. Cotransfection of Ca(v)2.1 with the EF-hand Ca2+-binding proteins, parvalbumin and calbindin, significantly altered the relationship between Ca2+ current amplitude and inactivation in ways that were unexpected from behavior as passive Ca2+ buffers. We conclude that Ca2+-dependent inactivation of Ca(v)2.1 depends on a subplasmalemmal Ca2+ microdomain that is affected by the amplitude of the Ca2+ current and differentially modulated by distinct Ca2+ buffers.

Modulation ofICa-Lby α1-adrenergic stimulation in rat ventricular myocytes

We found when L-type calcium current (ICa-L) was recorded with the perforated patch-clamp method in rat ventricular myocytes that bath application of phenylephrine (with propranolol) evoked a biphasic response characterized by an initial transient suppression followed by a sustained potentiation. The transient suppression occurred 30–60 s after phenylephrine perfusion and reached peak inhibition at approximately 2 min. The biphasic modulation of ICa-Lwas also elicited by methoxamine, and the effects of phenylephrine were blocked by prazosin, indicating that the responses were mediated through α1-adrenoceptors. Pretreatment of cells with H7 (100 µmol/L), a broad-spectrum protein kinase inhibitor that inhibits both protein kinase C and A, eliminated potentiation but did not affect transient suppression. The transient suppression occurred concurrently with the acceleration of the fast component of ICa-Linactivation. Depletion of intracellular Ca2+stores by ryanodine plus caffeine or thapsigargin eliminated the transient suppression. When ICa-Lwas recorded with whole-cell patch-clamp and with 0.05 mmol/L EGTA in the pipette solution to allow intracellular Ca2+to fluctuate, phenylephrine evoked a transient suppression as in the perforated patch recordings. Heparin, a specific blocker of IP3(inositol 1,4,5-trisphosphate) receptors, eliminated the phenylephrine-induced transient suppression of ICa-Lwhen added to the pipette solution. Intensive chelation of intracellular Ca2+by 5 mmol/L BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid) in the pipette solution also eliminated the phenylephrine-induced transient suppression of ICa-L. We conclude that transient increase in the concentration of intracellular calcium ([Ca2+]i) caused by Ca2+release from intracellular stores underlies the transient suppression of ICa-L, whereas the potentiation of ICa-Lis a result of activation of protein kinases.Key words: Ca2+mobilization, IP3, Ca2+-induced inactivation of Ca2+current, perforated patch-clamp.

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.

Ca2+- and voltage-dependent inactivation of the expressed L-type Ca(v)1.2 calcium channel

Ca2+-dependent regulation of the ion current through the alpha1Cbeta2aalpha2delta-1 (L-type) calcium channel transiently expressed in HEK 293 cells was investigated using whole cell patch clamp method. Ca2+ or Na+ ions were used as a charge carrier. Intracellular Ca2+ was either buffered by 10 mM EGTA or 200 microM Ca2+ was added into non-buffered intracellular solution. Free intracellular Ca2+ inactivated permanently about 80% of the L-type calcium current. The L-type calcium channel inactivated during a depolarizing pulse with two time constants, tau(fast) and tau(slow). Free intracellular calcium accelerated both time constants. Effect on the tau(slow) was more pronounced. About 80% of the channel inactivation during brief depolarizing pulse could be attributed to a Ca2+-dependent mechanism and 20% to a voltage-dependent mechanism. When Na+ ions were used as a charge carrier, the L-type current still inactivated with two time constants that were 10 times slower and were virtually voltage-independent. Ca2+ ions stabilized the inactivated state of the channel in a concentration-dependent manner.

Ionic currents involved in shock-induced nonlinear changes in transmembrane potential responses of single cardiac cells

An exhaustive characterization of how an isolated cardiac cell responds to applied electric fields could serve as an important groundwork for understanding responses of more complex higher order systems. Field stimulation of single cardiac cells during the early plateau of the action potential results in a nonuniform change in transmembrane potential (Vm) across the cell length that is more heavily weighted in the negative direction. These negatively shifted Vm responses are not replicated theoretically using present day membrane models. The goal of this study was to explore the membrane currents involved in the field responses during the plateau by selectively blocking various ion channels. Enzymatically isolated single guinea pig cells were stimulated with uniform field S1-S2 pulses, and the transmembrane potential responses were optically recorded from several sites along the cell length to assess the drug effect. We used nine different pharmacological agents to manipulate the conductance of major cardiac ion channels of which only barium (Ba2+) altered the transmembrane potential responses. At 50 microM Ba2+, which specifically blocks inwardly rectifying current I(K1), the negative shift in Vm responses was accentuated. At 1 mM Ba2+ , which blocks both I(K1) and sustained plateau current I(Kp), the negative shift diminished. However, 1 mM Ba2+ also depolarized the cells, and depressed or completely eliminated the action potential. Based on these results we conclude that I(K1) contributes to field-induced responses during the plateau stimulation by passing a net inward current, which when blocked accentuates the negative shift in the Vm responses. A conclusive role of I(Kp) could not be demonstrated because of confounding changes in membrane potential. However, from our results it remains as the most viable candidate for the elusive current that contributes a net outward current to produce negatively weighted Vm responses during plateau stimulation and warrants further investigation.

Voltage-gated rearrangements associated with differential beta-subunit modulation of the L-type Ca(2+) channel inactivation

Auxiliary beta-subunits bound to the cytoplasmic alpha(1)-interaction domain of the pore-forming alpha(1C)-subunit are important modulators of voltage-gated Ca(2+) channels. The underlying mechanisms are not yet well understood. We investigated correlations between differential modulation of inactivation by beta(1a)- and beta(2)- subunits and structural responses of the channel to transition into distinct functional states. The NH(2)-termini of the alpha(1C)- and beta-subunits were fused with cyan or yellow fluorescent proteins, and functionally coexpressed in COS1 cells. Fluorescence resonance energy transfer (FRET) between them or with membrane-trapped probes was measured in live cells under voltage clamp. It was found that in the resting state, the tagged NH(2)-termini of the alpha(1C)- and beta-subunit fluorophores are separated. Voltage-dependent inactivation generates strong FRET between alpha(1C) and beta(1a) suggesting mutual reorientation of the NH(2)-termini, but their distance vis-à-vis the plasma membrane is not appreciably changed. These voltage-gated rearrangements were substantially reduced when the beta(1a)-subunit was replaced by beta(2). Differential beta-subunit modulation of inactivation and of FRET between alpha(1C) and beta were eliminated by inhibition of the slow inactivation. Thus, differential beta-subunit modulation of inactivation correlates with the voltage-gated motion between the NH(2)-termini of alpha(1C)- and beta-subunits and targets the mechanism of slow voltage-dependent inactivation.

Modulation of the voltage sensor of L-type Ca2+ channels by intracellular Ca2+

Both intracellular calcium and transmembrane voltage cause inactivation, or spontaneous closure, of L-type (CaV1.2) calcium channels. Here we show that long-lasting elevations of intracellular calcium to the concentrations that are expected to be near an open channel (>/=100 microM) completely and reversibly blocked calcium current through L-type channels. Although charge movements associated with the opening (ON) motion of the channel's voltage sensor were not altered by high calcium, the closing (OFF) transition was impeded. In two-pulse experiments, the blockade of calcium current and the reduction of gating charge movements available for the second pulse developed in parallel during calcium load. The effect depended steeply on voltage and occurred only after a third of the total gating charge had moved. Based on that, we conclude that the calcium binding site is located either in the channel's central cavity behind the voltage-dependent gate, or it is formed de novo during depolarization through voltage-dependent rearrangements just preceding the opening of the gate. The reduction of the OFF charge was due to the negative shift in the voltage dependence of charge movement, as previously observed for voltage-dependent inactivation. Elevation of intracellular calcium concentration from approximately 0.1 to 100-300 microM sped up the conversion of the gating charge into the negatively distributed mode 10-100-fold. Since the "IQ-AA" mutant with disabled calcium/calmodulin regulation of inactivation was affected by intracellular calcium similarly to the wild-type, calcium/calmodulin binding to the "IQ" motif apparently is not involved in the observed changes of voltage-dependent gating. Although calcium influx through the wild-type open channels does not cause a detectable negative shift in the voltage dependence of their charge movement, the shift was readily observable in the Delta1733 carboxyl terminus deletion mutant, which produces fewer nonconducting channels. We propose that the opening movement of the voltage sensor exposes a novel calcium binding site that mediates inactivation.

Physiological modulation of inactivation in L-type Ca2+ channels: one switch

The relative contributions of voltage- and Ca(2+)-dependent mechanisms of inactivation to the decay of L-type Ca(2+) channel currents (I(CaL)) is an old story to which recent results have given an unexpected twist. In cardiac myocytes voltage-dependent inactivation (VDI) was thought to be slow and Ca(2+)-dependent inactivation (CDI) resulting from Ca(2+) influx and Ca(2+)-induced Ca(2+)-release (CICR) from the sarcoplasmic reticulum provided an automatic negative feedback mechanism to limit Ca(2+) entry and the contribution of I(CaL) to the cardiac action potential. Physiological modulation of I(CaL) by Beta-adrenergic and muscarinic agonists then involved essentially more or less of the same by enhancing or reducing Ca(2+) channel activity, Ca(2+) influx, sarcoplasmic reticulum load and thus CDI. Recent results on the other hand place VDI at the centre of the regulation of I(CaL). Under basal conditions it has been found that depolarization increases the probability that an ion channel will show rapid VDI. This is prevented by Beta-adrenergic stimulation. Evidence also suggests that a channel which shows rapid VDI inactivates before CDI can become effective. Therefore the contributions of VDI and CDI to the decay of I(CaL) are determined by the turning on, by depolarization, and the turning off, by phosphorylation, of the mechanism of rapid VDI. The physiological implications of these ideas are that under basal conditions the contribution of I(CaL) to the action potential will be determined largely by voltage and by Ca(2+) following Beta-adrenergic stimulation.

Molecular determinants of Ca(2+)/calmodulin-dependent regulation of Ca(v)2.1 channels

Ca2+-dependent facilitation and inactivation (CDF and CDI) of Cav2.1 channels modulate presynaptic P/Q-type Ca2+ currents and contribute to activity-dependent synaptic plasticity. This dual feedback regulation by Ca2+ involves calmodulin (CaM) binding to the alpha1 subunit (alpha12.1). The molecular determinants for Ca2+-dependent modulation of Cav2.1 channels reside in CaM and in two CaM-binding sites in the C-terminal domain of alpha12.1, the CaM-binding domain (CBD) and the IQ-like domain. In transfected tsA-201 cells, CDF and CDI were both reduced by deletion of CBD. In contrast, alanine substitution of the first two residues of the IQ-like domain (IM-AA) completely prevented CDF but had little effect on CDI, and glutamate substitutions (IM-EE) greatly accelerated voltage-dependent inactivation but did not prevent CDI. Mutational analyses of the Ca2+ binding sites of CaM showed that both the N- and C-terminal lobes of CaM were required for full development of facilitation, but only the N-terminal lobe was essential for CDI. In biochemical assays, CaM12 and CaM34 were unable to bind CBD, whereas CaM34 but not CaM12 retained Ca2+-dependent binding to the IQ-like domain. These findings support a model in which Ca2+ binding to the C-terminal EF-hands of preassociated CaM initiates CDF via interaction with the IQ-like domain. Further Ca2+ binding to the N-terminal EF-hands promotes secondary CaM interactions with CBD, which enhance facilitation and cause a conformational change that initiates CDI. This multifaceted mechanism allows positive regulation of Cav2.1 in response to local Ca2+ increases (CDF) and negative regulation during more global Ca2+ increases (CDI).

Critical residues of the Caenorhabditis elegans unc-2 voltage-gated calcium channel that affect behavioral and physiological properties

The Caenorhabditis elegans unc-2 gene encodes a voltage-gated calcium channel alpha1 subunit structurally related to mammalian dihydropyridine-insensitive high-threshold channels. In the present paper we describe the characterization of seven alleles of unc-2. Using an unc-2 promoter-tagged green fluorescent protein construct, we show that unc-2 is primarily expressed in motor neurons, several subsets of sensory neurons, and the HSN and VC neurons that control egg laying. Examination of behavioral phenotypes, including defecation, thrashing, and sensitivities to aldicarb and nicotine suggests that UNC-2 acts presynaptically to mediate both cholinergic and GABAergic neurotransmission. Sequence analysis of the unc-2 alleles shows that e55, ra605, ra606, ra609, and ra610 all are predicted to prematurely terminate and greatly reduce or eliminate unc-2 function. In contrast, the ra612 and ra614 alleles are missense mutations resulting in the substitution of highly conserved residues in the C terminus and the domain IVS4-IVS5 linker, respectively. Heterologous expression of a rat brain P/Q-type channel containing the ra612 mutation shows that the glycine to arginine substitution affects a variety of channel characteristics, including the voltage dependence of activation, steady-state inactivation, as well as channel kinetics. Overall, our findings suggest that UNC-2 plays a pivotal role in mediating a number of physiological processes in the nematode and also defines a number of critical residues important for calcium channel function in vivo.

Critical residues of the Caenorhabditis elegans unc-2 voltage-gated calcium channel that affect behavioral and physiological properties

The Caenorhabditis elegans unc-2 gene encodes a voltage-gated calcium channel alpha1 subunit structurally related to mammalian dihydropyridine-insensitive high-threshold channels. In the present paper we describe the characterization of seven alleles of unc-2. Using an unc-2 promoter-tagged green fluorescent protein construct, we show that unc-2 is primarily expressed in motor neurons, several subsets of sensory neurons, and the HSN and VC neurons that control egg laying. Examination of behavioral phenotypes, including defecation, thrashing, and sensitivities to aldicarb and nicotine suggests that UNC-2 acts presynaptically to mediate both cholinergic and GABAergic neurotransmission. Sequence analysis of the unc-2 alleles shows that e55, ra605, ra606, ra609, and ra610 all are predicted to prematurely terminate and greatly reduce or eliminate unc-2 function. In contrast, the ra612 and ra614 alleles are missense mutations resulting in the substitution of highly conserved residues in the C terminus and the domain IVS4-IVS5 linker, respectively. Heterologous expression of a rat brain P/Q-type channel containing the ra612 mutation shows that the glycine to arginine substitution affects a variety of channel characteristics, including the voltage dependence of activation, steady-state inactivation, as well as channel kinetics. Overall, our findings suggest that UNC-2 plays a pivotal role in mediating a number of physiological processes in the nematode and also defines a number of critical residues important for calcium channel function in vivo.

Biophysical and pharmacological characterization of voltage-gated calcium currents in turtle auditory hair cells

Hair cell calcium channels regulate membrane excitability and control synaptic transmission. The present investigations focused on determining whether calcium channels vary between hair cells of different characteristic frequencies or if multiple channel types exist within a hair cell, each serving a different function. To this end, turtle auditory hair cells from high- (317 +/- 27 Hz) and low-frequency (115 +/- 6 Hz) positions were voltage clamped using the whole-cell recording technique, and calcium currents were characterized based on activation, inactivation and pharmacological properties. Pharmacological sensitivity to dihydropyridines (nimodipine, Bay K 8644), benzothiazepines (diltiazem) and acetonitrile derivatives (verapamil, D600) and the insensitivity to non-L-type calcium channel antagonists support the conclusion that only L-type calcium channels were present. Fast activation rise times (< 0.5 ms), hyperpolarized half-activation potentials and a relative insensitivity to nimodipine suggest the channels were of the alpha1D (CaV1.3) variety. Although no pharmacological differences were found between calcium currents obtained from high- and low-frequency cells, low-frequency cells activated slightly faster and at hyperpolarized potentials, with half-activating voltages of -43 +/- 1 mV compared to -35 +/- 1 mV. Inactivation was observed in both high- and low-frequency cells. The time course of inactivation required three time constants for a fit. Long depolarizations could result in complete inactivation. The voltage of half-inactivation was -40 +/- 2 mV for high-frequency cells and -46 +/- 2 mV for low-frequency cells. Calcium channel inactivation did not significantly alter hair cell electrical resonant properties elicited from protocols where the membrane potential was hyperpolarized or depolarized prior to characterizing the resonance. A bell-shaped voltage dependence and modest sensitivities to intracellular calcium chelators and external barium ions suggest that inactivation was calcium dependent.

Two components of voltage-dependent inactivation in Ca(v)1.2 channels revealed by its gating currents

Voltage-dependent inactivation (VDI) was studied through its effects on the voltage sensor in Ca(v)1.2 channels expressed in tsA 201 cells. Two kinetically distinct phases of VDI in onset and recovery suggest the presence of dual VDI processes. Upon increasing duration of conditioning depolarizations, the half-distribution potential (V(1/2)) of intramembranous mobile charge was negatively shifted as a sum of two exponential terms, with time constants 0.5 s and 4 s, and relative amplitudes near 50% each. This kinetics behavior was consistent with that of increment of maximal charge related to inactivation (Qn). Recovery from inactivation was also accompanied by a reduction of Qn that varied with recovery time as a sum of two exponentials. The amplitudes of corresponding exponential terms were strongly correlated in onset and recovery, indicating that channels recover rapidly from fast VDI and slowly from slow VDI. Similar to charge "immobilization," the charge moved in the repolarization (OFF) transient became slower during onset of fast VDI. Slow VDI had, instead, hallmarks of interconversion of charge. Confirming the mechanistic duality, fast VDI virtually disappeared when Li(+) carried the current. A nine-state model with parallel fast and slow inactivation pathways from the open state reproduces most of the observations.

Role of individual ionic current systems in ventricular cells hypothesized by a model study

Individual ion channels or exchangers are described with a common set of equations for both the sinoatrial node pacemaker and ventricular cells. New experimental data are included, such as the new kinetics of the inward rectifier K+ channel, delayed rectifier K+ channel, and sustained inward current. The gating model of Shirokov et al. (J Gen Physiol 102: 1005-1030, 1993) is used for both the fast Na+ and L-type Ca2+ channels. When combined with a contraction model (Negroni and Lascano: J Mol Cell Cardiol 28: 915-929, 1996), the experimental staircase phenomenon of contraction is reconstructed. The modulation of the action potential by varying the external Ca2+ and K+ concentrations is well simulated. The conductance of I(CaL) dominates membrane conductance during the action potential so that an artificial increase of I(to), I(Kr), I(Ks), or I(KATP) magnifies I(CaL) amplitude. Repolarizing current is provided sequentially by I(Ks), I(Kr), and I(K1). Depression of ATP production results in the shortening of action potential through the activation of I(KATP). The ratio of Ca2+ released from SR over Ca2+ entering via I(CaL) (Ca2+ gain = approximately 15) in excitation-contraction coupling well agrees with the experimental data. The model serves as a predictive tool in generating testable hypotheses.

Is there an A-type K+ current in guinea pig ventricular myocytes?

A unique transient outward K(+) current (I(to)) has been described to result from the removal of extracellular Ca(2+) from ventricular myocytes of the guinea pig (15). This study addressed the question of whether this current represented K(+)-selective I(to) or the efflux of K(+) via L-type Ca(2+) channels. This outward current was inhibited by Cd(2+), Ni(2+), Co(2+), and La(3+) as well as by nifedipine. All of these compounds were equally effective inhibitors of the L-type Ca(2+) current. The current was not inhibited by 4-aminopyridine. Apparent inhibition of the outward current by extracellular Ca(2+) was shown to result from the displacement of the reversal potential of cation flux through L-type Ca(2+) channels. The current was found not to be K(+) selective but also permeant to Cs(+). The voltage dependence of inactivation of the outward current was identical to that of the L-type Ca(2+) current. It is concluded that extracellular Ca(2+) does not mask an A-type K(+) current in guinea pig ventricular myocytes.

Ca2+ entry-dependent inactivation of L-type Ca current: a novel formulation for cardiac action potential models

Cardiac L-type Ca current (I(Ca,L)) is controlled not only by voltage but also by Ca(2+)-dependent mechanisms. Precise implementation of I(Ca,L) in cardiac action potential models therefore requires thorough understanding of intracellular Ca(2+) dynamics, which is not yet available. Here, we present a novel formulation of I(Ca,L) for action potential models that does not explicitly require the knowledge of local intracellular Ca(2+) concentration ([Ca(2+)](i)). In this model, whereas I(Ca,L) is obtained as the product of voltage-dependent gating parameters (d and f), Ca(2+)-dependent inactivation parameters (f(Ca): f(Ca-entry) and f(Ca-SR)), and Goldman-Hodgkin-Katz current equation as in previous studies, f(Ca) is not a instantaneous function of [Ca(2+)](i) but is determined by two terms: onset of inactivation proportional to the influx of Ca(2+) and time-dependent recovery (dissociation). We evaluated the new I(Ca,L) subsystem in the framework of the standard cardiac action potential model. The new formulation produced a similar temporal profile of I(Ca,L) as the standard, but with different gating mechanisms. Ca(2+)-dependent inactivation gradually proceeded throughout the plateau phase, replacing the voltage-dependent inactivation parameter in the LRd model. In typical computations, f declined to approximately 0.7 and f(Ca-entry) to approximately 0.1, whereas deactivation caused fading of I(Ca,L) during final repolarization. These results support experimental findings that Ca(2+) entering through I(Ca,L) is essential for inactivation. After responses to standard voltage-clamp protocols were examined, the new model was applied to analyze the behavior of I(Ca,L) when action potential was prolonged by several maneuvers. Our study provides a basis for theoretical analysis of I(Ca,L) during action potentials, including the cases encountered in long QT syndromes.

Slow deactivation and U-shaped inactivation properties in cloned Cav1.2b channels in Chinese hamster ovary cells

Whole-cell patch-clamp techniques were applied to Chinese hamster ovary cells stably expressing cloned smooth muscle Ca(2+) channel alpha(1)-subunits. In the presence of Ba(2+) as a charge carrier, U-shaped inactivation was observed in the presence and absence of Ca(2+) agonists. Also, tail currents deactivated slowly when conditioning steps of positive potential were applied. The deactivation time constant was decreased by hyperpolarizing the repolarization step. Application of ATP-gamma-S or H-7 had little effect on the conditions necessary to induce slow tail, suggesting involvement of physical processes in the channel protein. In the presence of Bay K 8644, additional application of nifedipine decreased the amplitudes of the test and tail currents induced by a test step preceded by a conditioning step to +80 mV, but did not affect the decay time constant of the tail current. From these results and assumptions we have drawn up a kinetic scheme with one closed state, two open states (O(1), O(2)) and two inactivated states linked to the closed state and open state O(1), respectively, i.e., open state O(2) protected from inactivation. Computer calculation reconstructed slow deactivation and U-shaped inactivation properties. A similar kinetic scheme with Ca(2+)-agonist-binding states accounted for the results in the presence of Ca(2+) agonists.

Voltage-dependent inactivation of L-type Ca2+ currents in guinea-pig ventricular myocytes

The objective of this study was to describe the kinetics of voltage-dependent inactivation of native cardiac L-type Ca(2+) currents. Whole-cell currents were recorded from guinea-pig isolated ventricular myocytes. Voltage-dependent inactivation was separated from Ca(2+)-dependent inactivation by replacing extracellular Ca(2+) with Mg(2+) and recording outward currents through Ca(2+) channels. Voltage-dependent inactivation accelerated from slow monophasic decay at -30 mV to maximal rapid biphasic decay at +20 mV. Maximal voltage-dependent inactivation occurred with tau(f) approximately equal 30 ms and tau(s) approximately equal 300 ms, the fast component of decay accounted for 70 % of the current amplitude. In basal conditions Ca(2+) current availability was sigmoid. Isoproterenol (isoprenaline) evoked a large increase in a time-independent component of the Ca(2+) current which also increased with depolarisation. This was responsible for the apparent recovery of Ca(2+) channel current availability at positive membrane potentials and thus a U-shaped availability-voltage (A-V) relationship. It is concluded that beta-adrenergic stimulation altered the reaction of native cardiac L-type Ca(2+) channels to membrane voltage. In basal conditions, voltage accelerated inactivation. In isoproterenol, voltage could also reduce inactivation.

Beta-adrenergic and muscarinic agonists modulate inactivation of L-type Ca2+ channel currents in guinea-pig ventricular myocytes

The objective of this study was to examine the effects of isoproterenol (isoprenaline) and carbachol upon voltage-dependent inactivation of L-type Ca(2+) current (I(Ca,L)). I(Ca,L) was recorded in guinea-pig isolated ventricular myocytes in the presence and absence of extracellular Ca(2+) to separate total inactivation and voltage-dependent inactivation. In the presence of Ca(2+), isoproterenol and carbachol had 'competitive' effects upon the relationships between membrane voltage and I(Ca,L) amplitude and inactivation. Neither agonist had a marked effect upon the decay of inward I(Ca,L) carried by Ca(2+). In the absence of Ca(2+), isoproterenol severely reduced and slowed I(Ca,L) inactivation; this effect was reversed by carbachol. Under control conditions decay was dominated by fast inactivation. Isoproterenol reduced fast-inactivating and increased time-independent currents in a dose-dependent manner. These effects were counteracted by carbachol. There was a reciprocal relationship between the amplitude of fast-inactivating and time-independent currents with agonist stimulation. It is concluded that agonist modulation of rapid voltage-dependent inactivation of L-type Ca(2+) channels involves an 'on-off' switch.

Calcium-dependent inactivation and depletion of synaptic cleft calcium ions combine to regulate rod calcium currents under physiological conditions

L-type Ca2+ currents (I(Ca)) in rod photoreceptors exhibit Ca2+-dependent inactivation. Perforated-patch whole-cell recordings were obtained from isolated rods of the tiger salamander using 1.8 mm Ca2+ in the bathing medium to determine the extent of Ca2+-dependent inactivation of I(Ca) with physiological [Ca2+] and endogenous buffering. I(Ca) was measured with voltage ramps applied before and after 5-s steps to -40, -30, -20, or -10 mV. Long depolarizing steps in isolated rods produced inactivation of I(Ca) ranging from 15% at -40 mV to > 80% at -10 mV. Because, in addition to Ca2+-dependent inactivation, depletion of synaptic cleft Ca2+ accompanying activation of I(Ca) can reduce presynaptic I(Ca) at calycal synapses, we investigated whether a similar mechanism worked at the invaginating rod synapse. Rods from retinal slices with intact synapses were compared with isolated rods in which synaptic cleft depletion is absent. I(Ca) was more strongly depressed by depolarization of rods in retinal slices, with ICa reduced by 47% following voltage steps to -40 mV. The depression of currents by depolarization was also greater for rods from retinal slices than isolated rods when Ca2+ was replaced with Ba2+ to reduce Ca2+-dependent inactivation. The stronger depolarization-evoked inhibition of I(Ca) in retinal slices compared to isolated rods probably reflects depletion of synaptic cleft Ca2+ arising from sustained Ca2+ influx. Inactivation of I(Ca) exhibited slow onset and recovery. These findings suggest that Ca2+-dependent inactivation and depletion of synaptic cleft Ca2+ may combine to regulate I(Ca) in response to light-evoked changes in rod membrane potential.

Modulation of the gating of unitary cardiac L-type Ca(2+) channels by conditioning voltage and divalent ions

Although a considerable number of studies have characterized inactivation and facilitation of macroscopic L-type Ca(2+) channel currents, the single channel properties underlying these important regulatory processes have only rarely been examined using Ca(2+) ions. We have compared unitary L-type Ca(2+) channel currents recorded with a low concentration of Ca(2+) ions with those recorded with Ba(2+) ions to elucidate the ionic dependence of the mechanisms responsible for the prepulse-dependent modulation of Ca(2+) channel gating kinetics. Conditioning prepulses were applied across a wide range of voltages to examine their effects on the subsequent Ca(2+) channel activity, recorded at a constant test potential. All recordings were made in the absence of any Ca(2+) channel agonists. Moderate-depolarizing prepulses resulted in a decrease in the probability of opening of the Ca(2+) channels during subsequent test voltage steps (inactivation), the extent of which was more dramatic with Ca(2+) ions than Ba(2+) ions. Facilitation, or increase of the average probability of opening with strong predepolarization, was due to long-duration mode 2 openings with Ca(2+) ions and Ba(2+) ions, despite a decrease in Ca(2+) channel availability (inactivation) under these conditions. The degree of both prepulse-induced inactivation and facilitation decreased with increasing Ba(2+) ion concentration. The time constants (and their proportions) describing the distributions of Ca(2+) channel open times (which reflect mode switching) were also prepulse-, and ion-dependent. These results support the hypothesis that both prior depolarization and the nature and concentration of permeant ions modulate the gating properties of cardiac L-type Ca(2+) channels.

A carbamate-type cholinesterase inhibitor 2-sec-butylphenyl N-methylcarbamate insecticide blocks L-type Ca2+ channel in guinea pig ventricular myocytes

2-sec-Butylphenyl N-methylcarbamate (BPMC) is a carbamate-type cholinesterase (ChE) inhibitor with unique toxicological properties such as noncholinergic cardiovascular collapse. Effects of BPMC on L-type Ca2+ channel currents (ICa(L)) were studied in isolated guinea pig ventricular myocytes using the whole-cell patch-clamp technique, since the examination of cardiovascular responses indicated its Ca2+ antagonistic action. BPMC induced bradycardic and hypotensive responses in vivo and inhibited contraction of isolated papillary muscles (IC50 = 1.3 x 10(-4) M) in guinea pigs. BPMC produced reversible block of ICa(L) in the concentration range of 10(-4) - 10(-3) M. At test potentials between -30 mV and +20 mV, BPMC at 3 x 10(-4) M caused marked acceleration of decay rate of ICa(L) with moderate reduction of peak ICa(L) amplitude. BPMC (3 x 10(-4) M) shifted the steady-state inactivation curve to the hyperpolarizing direction by 12.7 mV. Decay rate of Ba2+ currents (IBa(L)) was also accelerated by BPMC. Fitting analysis of inactivation kinetics of IBa(L) with a two-exponential equation revealed that BPMC accelerates the slow inactivation component. At concentrations for blocking peak IBa(L) by ca. 30%, the inactivation kinetics of IBa(L) were significantly accelerated by BPMC, but merely slightly accelerated by Ca2+ channel antagonists such as diltiazem, nifedipine, or verapamil. These results indicate that BPMC, in addition to the inhibition of ChE, blocks L-type Ca2+ channels by accelerating voltage-dependent inactivation.

beta-Adrenergic stimulation modulates Ca2+- and voltage-dependent inactivation of L-type Ca2+ channel currents in guinea-pig ventricular myocytes

The objective of this study was to examine the effect of beta-adrenergic stimulation upon voltage- and Ca2+-induced inactivation of native cardiac L-type Ca2+ channels. Whole-cell currents were recorded from guinea-pig isolated ventricular myocytes. Total and voltage-dependent inactivation was separated by replacing extracellular Ca2+ with Mg2+. L-type Ca2+ channel behaviour was monitored with outward Ca2+ channel currents. First, the voltage dependence of inactivation was studied at fixed times (50 and 1000 ms) after activation. This showed that under control conditions Ca2+ contributed little to inactivation. In isoproterenol (isoprenaline), voltage-dependent inactivation was markedly reduced and Ca2+ contributed largely to total inactivation. Second, the time dependence of inactivation was studied at a fixed voltage (+10 mV). In control conditions the fast phase of inactivation (tau(f) approximately 15 ms) was reduced to the same extent by ryanodine (tau(f) approximately 30 ms) and the absence of Ca2+ (tau(f) approximately 30 ms) while the slow phase of inactivation (tau(s) approximately 70 ms) was reduced by ryanodine (tau(s) approximately 160 ms) and further reduced in the absence of Ca2+ (tau(s) approximately 300 ms). In isoproterenol, biphasic inactivation of Ca2+ currents (tau(f) approximately 4 ms, tau(s) approximately 60 ms) was replaced by a single slow (tau approximately 450 ms) phase of inactivation in the absence of Ca2+. It is concluded that, under control conditions Ca2+ channel current decay is largely dominated by rapid voltage-dependent inactivation, while in isoproterenol this is replaced by Ca2+-induced inactivation.

Voltage- and cation-dependent inactivation of L-type Ca2+ channel currents in guinea-pig ventricular myocytes

L-type Ca2+ channel currents in native ventricular myocytes inactivate according to voltage- and Ca2+-dependent processes. This study sought to examine the effect of beta-adrenergic stimulation on the contributions of voltage and Ca2+ to Ca2+ current decay. Ventricular myocytes were enzymatically isolated from guinea-pig hearts. Inward whole-cell Cd2+-sensitive L-type Ca2+ channel currents were recorded with the patch clamp technique and comparison was made between inward currents carried by Ca2+ and either Ba2+, Sr2+ or Na+. In control conditions the decay of Ca2+ currents was faster than Ba2+, Sr2+ or Na+ currents at negative voltages while at positive voltages there was no difference. The relationship between voltage and inactivation for Ca2+ currents was bell-shaped, while that for Ba2+, Sr2+, and Na+ currents was sigmoid. Thus depolarisation progressively replaced Ca2+-dependent inactivation in the fast phase of decay of Ca2+ channel currents with rapid voltage-dependent inactivation. In the presence of isoproterenol (isoprenaline) the decay of Ca2+ currents was faster than Ba2+, Sr2+ or Na+ currents at all measured voltages (-40 to +30 mV). The relationship between voltage and inactivation for Ca2+, Ba2+ and Sr2+ currents was bell-shaped, while that for Na+ currents was sigmoid with less inactivation than under control conditions. Therefore the fast phase of decay of Ca2+ channel currents was now almost entirely due to Ca2+. It is concluded that the relative contributions of Ca2+- and voltage-dependent mechanisms of inactivation of L-type Ca2+ channels in native cardiac myocytes are modulated by beta-adrenergic stimulation influencing the amount of rapid voltage-dependent inactivation.

Modulation of Ca2+ currents in rat thalamocortical relay neurons by activity and phosphorylation

Rhythmic low and high frequency activity in thalamocortical networks depend critically on activation of low- and high-voltage-activated (LVA, HVA) Ca2+ currents. In order to test whether Ca2+ currents are modified during repetitive activation, acutely isolated thalamocortical relay neurons of rats, at postnatal days 12 (P12) to P20, were investigated using patch-clamp, Ca2+ imaging and Western blot techniques. High-voltage-activated, but not LVA Ca2+ currents were reduced significantly during 2 Hz stimulation. Ca2+ imaging experiments demonstrated a close correlation between the increase in intracellular Ca2+ levels and the decrease in HVA Ca2+ current amplitudes. Further examination of HVA Ca2+ currents revealed a 'U-shaped' inactivation curve and a time-dependent inactivation process that could be described by a two-exponential function. The 'U-shape' was significantly reduced, current amplitude was increased significantly and time-dependent inactivation revealed a one-exponential decline with Ba2+ as the charge carrier, following activation of the cAMP/PKA pathway, and following application of phosphatase inhibitors (ascomycin, calyculin A). Western blot analysis and the effect of ascomycin indicated an involvement of calcineurin in the inactivation process. Isolation of HVA Ca2+ current components by subtype-specific blockers revealed that changes in time-dependent inactivation, inactivation curve and current amplitude were carried mainly by L-type and N-type Ca2+ currents. Furthermore, Ca2+-dependent inactivation was operative during stimulation protocols mimicking tonic action potential firing. These data indicate a modulation of L- and N-type Ca2+ channels by phosphorylation, resulting jointly in an increased intracellular Ca2+ influx during activity of the ascending brainstem system, the latter occurring during states of wakefulness.

Role of Repeat I in the fast inactivation kinetics of the Ca(V)2.3 channel

The molecular basis for inactivation in Ca(V)2.3 (alpha 1E) channels was studied after expression of alpha 1E/alpha 1C (Ca(V)2.3/Ca(V)1.2) chimeras in Xenopus oocytes. In the presence of 10 mM Ba(2+), the CEEE chimera (Repeat I+part of the I-II linker from Ca(V)1.2) displayed inactivation properties similar to Ca(V)1.2 despite being more than 90% homologous to Ca(V)2.3. The transmembrane segments of Repeat I did not appear to be crucial as inactivation of EC(IS1-6)EEE was not significantly different than Ca(V)2.3. In contrast, EC(AID)EEE, with the beta-subunit binding domain from Ca(V)1.2, tended to behave like Ca(V)1.2 in terms of inactivation kinetics and voltage dependence. A detailed kinetic analysis revealed nonetheless that CEEE and EC(AID)EEE retained the fast inactivation time constant (tau(fast) approximately equal to 20-30 ms) that is a distinctive feature of Ca(V)2.3. Altogether, these data suggest that the region surrounding the AID binding site plays a pivotal albeit not exclusive role in determining the inactivation properties of Ca(V)2.3.

Calcium-dependent inhibition of L, N, and P/Q Ca2+ channels in chromaffin cells: role of mitochondria

The hypothesis that the buffering of Ca(2+) by mitochondria could affect the Ca(2+)-dependent inhibition of voltage-activated Ca(2+) channels, (I(Ca)), was tested in voltage-clamped bovine adrenal chromaffin cells. The protonophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), the blocker of the Ca(2+) uniporter ruthenium red (RR), and a combination of oligomycin plus rotenone were used to interfere with mitochondrial Ca(2+) buffering. In cells dialyzed with an EGTA-free solution, peak I(Ca) generated by 20 msec pulses to 0 or +10 mV, applied at 15 sec intervals, from a holding potential of -80 mV, decayed rapidly after superfusion of cells with 2 microm CCCP (tau = 16.7 +/- 3 sec; n = 8). In cells dialyzed with 14 mm EGTA, CCCP did not provoke I(Ca) loss. Cell dialysis with 4 microm ruthenium red or cell superfusion with oligomycin (3 microm) plus rotenone (4 microm) also accelerated the decay of I(Ca). After treatment with CCCP, decay of N- and P/Q-type Ca(2+) channel currents occurred faster than that of L-type Ca(2+) channel currents. These data are compatible with the idea that the elevation of the bulk cytosolic Ca(2+) concentration, [Ca(2+)](c), causes the inhibition of L- and N- as well as P/Q-type Ca(2+) channels expressed by bovine chromaffin cells. This [Ca(2+)](c) signal appears to be tightly regulated by rapid Ca(2+) uptake into mitochondria. Thus, it is plausible that mitochondria might efficiently regulate the activity of L, N, and P/Q Ca(2+) channels under physiological stimulation conditions of the cell.

Calcium-dependent inhibition of L, N, and P/Q Ca2+ channels in chromaffin cells: role of mitochondria

The hypothesis that the buffering of Ca(2+) by mitochondria could affect the Ca(2+)-dependent inhibition of voltage-activated Ca(2+) channels, (I(Ca)), was tested in voltage-clamped bovine adrenal chromaffin cells. The protonophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), the blocker of the Ca(2+) uniporter ruthenium red (RR), and a combination of oligomycin plus rotenone were used to interfere with mitochondrial Ca(2+) buffering. In cells dialyzed with an EGTA-free solution, peak I(Ca) generated by 20 msec pulses to 0 or +10 mV, applied at 15 sec intervals, from a holding potential of -80 mV, decayed rapidly after superfusion of cells with 2 microm CCCP (tau = 16.7 +/- 3 sec; n = 8). In cells dialyzed with 14 mm EGTA, CCCP did not provoke I(Ca) loss. Cell dialysis with 4 microm ruthenium red or cell superfusion with oligomycin (3 microm) plus rotenone (4 microm) also accelerated the decay of I(Ca). After treatment with CCCP, decay of N- and P/Q-type Ca(2+) channel currents occurred faster than that of L-type Ca(2+) channel currents. These data are compatible with the idea that the elevation of the bulk cytosolic Ca(2+) concentration, [Ca(2+)](c), causes the inhibition of L- and N- as well as P/Q-type Ca(2+) channels expressed by bovine chromaffin cells. This [Ca(2+)](c) signal appears to be tightly regulated by rapid Ca(2+) uptake into mitochondria. Thus, it is plausible that mitochondria might efficiently regulate the activity of L, N, and P/Q Ca(2+) channels under physiological stimulation conditions of the cell.

Ca2+-dependent regulation of cardiac L-type Ca2+ channels: is a unifying mechanism at hand?

Ca2+ entry (I(Ca)) through cardiac L-type Ca2+ channels (LTCC) drives critical cellular processes ranging from contraction to gene expression, and, when disordered, is implicated in arrhythmias and hypertrophy. LTCC activation occurs by cell membrane depolarization, but LTCCs are also regulated by auxiliary proteins, phosphorylation, and intracellular CA2+([Ca2+]i). LTCC regulation by [Ca2+]i is especially intriguing because increased [Ca2+]i signals dual and conflicting commands for I(Ca)inactivation and facilitation. A recent explosion of work has shed new light on the mechanisms and molecular identity of domains necessary for [Ca2+]i-dependent regulation of LTCC.

A model of the L-type Ca2+ channel in rat ventricular myocytes: ion selectivity and inactivation mechanisms

1. We have developed a mathematical model of the L-type Ca2+ current, which is based on data from whole-cell voltage clamp experiments on rat ventricular myocytes. Ion substitution methods were employed to investigate the ionic selectivity of the channel. Experiments were configured with Na+, Ca2+ or Ba2+ as the majority current carrier. 2. The amplitude of current through the channel is attenuated in the presence of extracellular Ca2+ or Ba2+. Our model accounts for channel selectivity by using a modified Goldman-Hodgkin-Katz (GHK) configuration that employs voltage-dependent channel binding functions for external divalent ions. Stronger binding functions were used for Ca2+ than for Ba2+. 3. Decay of the ionic current during maintained depolarization was characterized by means of voltage- and Ca2+-dependent inactivation pathways embedded in a five-state dynamic channel model. Particularly, Ca2+ first binds to calmodulin and the Ca2+-calmodulin complex is the mediator of Ca2+ inactivation. Ba2+-dependent inactivation was characterized using the ttau same scheme, but with a decreased binding to calmodulin. 4. A reduced amount of steady-state inactivation, as evidenced by a U-shaped curve at higher depolarization levels (>40 mV) in the presence of [Ca2+]o, was observed in double-pulse protocols used to study channel inactivation. To characterize this phenomenon, a mechanism was incorporated into the model whereby Ca2+ or Ba2+ also inhibits the voltage-dependent inactivation pathway. 5. The five-state dynamic channel model was also used to simulate single channel activity. Calculations of the open probability of the channel model are generally consistent with experimental data. A sixth state can be used to simulate modal activity by way of introducing long silent intervals. 6. Our model has been tested extensively using experimental data from a wide variety of voltage clamp protocols and bathing solution manipulations. It provides: (a) biophysically based explanations of putative mechanisms underlying Ca2+- and voltage-dependent channel inactivation, and (b) close fits to voltage clamp data. We conclude that the model can serve as a predictive tool in generating testable hypotheses for further investigation of this complex ion channel.

The alpha 1-subunit of smooth muscle Ca(2+) channel preserves multiple open states induced by depolarization

The cloned alpha 1-subunits of the smooth muscle Ca(2+) channel (alpha (1C-b)) from rabbit lung were expressed in Chinese hamster ovary cells. The effect of large depolarizations was examined using cell-attached patch clamp techniques. After large, long-duration depolarizations (to +80 mV, 4 s), the cloned smooth muscle Ca(2+) channels were still open, and also showed slow channel closure upon repolarization. The sum of unitary channel currents revealed that the tail current seen after large conditioning depolarizations had a slower deactivation time constant compared to that seen when the cell membrane was depolarized briefly with a test step (to +40 mV), suggesting that large depolarizations transform the conformation of the Ca(2+) channels to a second open state. The decay time course of the tail current induced by large conditioning depolarizations was prolonged by reducing the negativity of the repolarization step, and vice versa. Using the slow deactivating characteristic, the current-voltage relationship was directly measured by applying a ramp pulse after a large depolarization. Its slope conductance was approximately 26 pS. Since the patch pipettes contained Ca(2+) agonists, the transition of the Ca(2+) channel conformation to the second, long open state during a large depolarization was distinct from that caused by Ca(2+) agonists, suggesting that the cloned alpha 1-subunits of smooth muscle Ca(2+) channels preserve the characteristic features seen in native smooth muscle Ca(2+) channels. In addition, when skeletal muscle beta-subunits were coexpressed with the alpha 1-subunits, the long channel openings after large, long-duration depolarizations were frequently suppressed. This phenomenon could be explained if the skeletal muscle beta-subunits increased the inactivation rate during the preconditioning depolarization.

A blocker-resistant, fast-decaying, intermediate-threshold calcium current in palaeocortical pyramidal neurons

The whole-cell patch-clamp technique was used to record Ca2+ currents in acutely dissociated neurons from layer II of guinea-pig piriform cortex (PC). Ba2+ (5 mM) was used as charge carrier. In a subpopulation of layer II cells ( approximately 22%) total Ba2+ currents (IBas) displayed a high degree (> 70%) of inactivation after 300 ms of steady depolarization. The application of L-, N- and P/Q-type Ca2+-channel blockers to these high-decay IBas left their fast inactivating component largely unaffected. The inactivation phase of the blocker-resistant, fast-decaying IBa thus isolated had a bi-exponential time course, with a fast time constant of approximately 20 ms and a slower time constant of approximately 100 ms at voltage levels positive to -10 mV. The voltage dependence of activation of the blocker-resistant, fast-decaying IBa was shifted by approximately 7-9 mV in the negative direction in comparison with those of other pharmacologically and/or kinetically different high-voltage-activated Ca2+ currents. We named this blocker-resistant, fast-decaying, intermediate-threshold current IRfi. The amplitude of IRfi decreased only slightly (by approximately 9%) when extracellular Ca2+ was substituted for Ba2+, in contrast with that of slowly decaying, high-voltage-activated currents, which was reduced by approximately 41% on average. Moreover, IRfi was substantially inhibited by low concentrations of Ni2+ (50 microM). We conclude that IRfi, because of its fast inactivation kinetics, intermediate threshold of activation and resistance to organic blockers, represents a definite, identifiable Ca2+ current different from classical high-voltage-activated currents and clearly distinguishable from classical IT. The striking similarity found between IRfi and Ca2+ currents resulting from heterologous expression of alpha1E-type channel subunits is discussed.

Critical determinants of Ca(2+)-dependent inactivation within an EF-hand motif of L-type Ca(2+) channels

L-type (alpha(1C)) calcium channels inactivate rapidly in response to localized elevation of intracellular Ca(2+), providing negative Ca(2+) feedback in a diverse array of biological contexts. The dominant Ca(2+) sensor for such Ca(2+)-dependent inactivation has recently been identified as calmodulin, which appears to be constitutively tethered to the channel complex. This Ca(2+) sensor induces channel inactivation by Ca(2+)-dependent CaM binding to an IQ-like motif situated on the carboxyl tail of alpha(1C). Apart from the IQ region, another crucial site for Ca(2+) inactivation appears to be a consensus Ca(2+)-binding, EF-hand motif, located approximately 100 amino acids upstream on the carboxyl terminus. However, the importance of this EF-hand motif for channel inactivation has become controversial since the original report from our lab implicating a critical role for this domain. Here, we demonstrate not only that the consensus EF hand is essential for Ca(2+) inactivation, but that a four-amino acid cluster (VVTL) within the F helix of the EF-hand motif is itself essential for Ca(2+) inactivation. Mutating these amino acids to their counterparts in non-inactivating alpha(1E) calcium channels (MYEM) almost completely ablates Ca(2+) inactivation. In fact, only a single amino acid change of the second valine within this cluster to tyrosine (V1548Y) supports much of the functional knockout. However, mutations of presumed Ca(2+)-coordinating residues in the consensus EF hand reduce Ca(2+) inactivation by only approximately 2-fold, fitting poorly with the EF hand serving as a contributory inactivation Ca(2+) sensor, in which Ca(2+) binds according to a classic mechanism. We therefore suggest that while CaM serves as Ca(2+) sensor for inactivation, the EF-hand motif of alpha(1C) may support the transduction of Ca(2+)-CaM binding into channel inactivation. The proposed transduction role for the consensus EF hand is compatible with the detailed Ca(2+)-inactivation properties of wild-type and mutant V1548Y channels, as gauged by a novel inactivation model incorporating multivalent Ca(2+) binding of CaM.

Inactivation properties of human recombinant class E calcium channels

The electrophysiological and pharmacological properties of alpha(1E)-containing Ca(2+) channels were investigated by using the patch-clamp technique in the whole cell configuration, in HEK 293 cells stably expressing the human alpha(1E) together with alpha(2b) and beta(1b) accessory subunits. These channels had current-voltage (I-V) characteristics resembling those of high-voltage-activated (HVA) Ca(2+) channels (threshold at -30 mV and peak amplitude at +10 mV in 5 mM Ca(2+)). The currents activated and deactivated with a fast rate, in a time- and voltage-dependent manner. No difference was found in their relative permeability to Ca(2+) and Ba(2+). Inorganic Ca(2+) channel blockers (Cd(2+), Ni(2+)) blocked completely and potently the alpha(1E,)/alpha(2b)delta/beta(1b) mediated currents (IC(50) = 4 and 24.6 microM, respectively). alpha(1E)-mediated currents inactivated rapidly and mainly in a non-Ca(2+)-dependent manner, as evidenced by the fact that 1) decreasing extracellular Ca(2+) from 10 to 2 mM and 2) changing the intracellular concentration of the Ca(2+) chelator 1. 2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA), did not affect the inactivation characteristics; 3) there was no clear-cut bell-shaped relationship between test potential and inactivation, as would be expected from a Ca(2+)-dependent event. Although Ba(2+) substitution did not affect the inactivation of alpha(1E) channels, Na(+) substitution revealed a small but significant reduction in the extent and rate of inactivation, suggesting that besides the presence of dominant voltage-dependent inactivation, alpha(1E) channels are also affected by a divalent cation-dependent inactivation process. We have analyzed the Ca(2+) currents produced by a range of imposed action potential-like voltage protocols (APVPs). The amplitude and area of the current were dependent on the duration of the waveform employed and were relatively similar to those described for HVA calcium channels. However, the peak latency resembled that obtained for low-voltage-activated (LVA) calcium channels. Short bursts of APVPs applied at 100 Hz produced a depression of the Ca(2+) current amplitude, suggesting an accumulation of inactivation likely to be calcium dependent. The human alpha(1E) gene seems to participate to a Ca(2+) channel type with biophysical and pharmacological properties partly resembling those of LVA and those of HVA channels, with inactivation characteristics more complex than previously believed.

Extracellular Ba(2+) blocks the cardiac transient outward K(+) current

Ba(2+) is widely used as a tool in patch-clamp studies because of its ability to block a variety of K(+) channels and to pass Ca(2+) channels. Its potential ability to block the cardiac transient outward K(+) current (I(to)) has not been clearly documented. We performed whole cell patch-clamp studies in canine ventricular and atrial myocytes. Extracellular application of Ba(2+) produced potent inhibition of I(to) with an IC(50) of approximately 40 microM. The effects were voltage independent, and the inactivation kinetics were not altered by Ba(2+). The potency of Ba(2+) was approximately 10 times higher than that of 4-aminopyridine (a selective I(to) blocker with an IC(50) of 430 microM) under identical conditions. By comparison, Ba(2+) blockade of the inward rectifier K(+) current was voltage dependent; the IC(50) was approximately 20 times lower (2.5 microM) than that for I(to) when determined at -100 mV and was comparable to I(to) as determined at -60 mV (IC(50) = 26 microM). Ba(2+) concentrations of </=1 mM or higher failed to block ultrarapid delayed rectifier K(+) current. Our data suggest that Ba(2+) can be considered a potent blocker of I(to).

Interaction between permeant ions and voltage sensor during inactivation of N-type Ca2+ channels

1. Inactivation of neuronal N-type Ca2+ channels transiently expressed in human kidney tSA-201 cells was studied at the level of whole-cell Ca2+ current and intramembrane charge movement. 2. Prolonged (5 s) depolarization to 40 mV shifted the voltage distribution of intramembrane charge movement from a transition potential (mid-point voltage) of 9.5 +/- 3.8 mV to -55.4 +/- 8.2 mV. Because of the large negative shift, it was possible to record intramembrane charge movement from unblocked inactivated channels and determine the effect of Ca2+ influx on inactivation of intramembrane charge movement. 3. In unblocked channels, the rate of inactivation of charge movement (21 +/- 3 s-1 at 0 mV) was close to that of Ca2+ current decay during the conditioning pulse. However, in blocked channels inactivation was significantly slower (4 +/- 1 s-1 at 0 mV). In unblocked channels, the availability of Ca2+ current was minimal and charge movement from inactivated channels was maximal after conditioning to about 10 mV. After the block of ionic current, inactivation of charge movement gradually increased with voltage. 4. Although the rate of Ca2+ current run-down was not affected by 10-15 microM free Ca2+ in the pipette solution, inactivation of Ca2+ currents during depolarization was about two times faster in high intracellular Ca2+. 5. The present results favour the current-dependent mechanism of inactivation of N-type channels. They also suggest that Ca2+ acting in the permeation pathway and transmembrane voltage are the proximate causes of the same inactivation transitions of voltage sensing moieties in these channels.

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.

CaM kinase augments cardiac L-type Ca2+ current: a cellular mechanism for long Q-T arrhythmias

Early afterdepolarizations (EAD) caused by L-type Ca2+ current (ICa, L) are thought to initiate long Q-T arrhythmias, but the role of intracellular Ca2+ in these arrhythmias is controversial. Rabbit ventricular myocytes were stimulated with a prolonged EAD-containing action potential-clamp waveform to investigate the role of Ca2+/calmodulin-dependent protein kinase II (CaM kinase) in ICa,L during repolarization. ICa,L was initially augmented, and augmentation was dependent on Ca2+ from the sarcoplasmic reticulum because the augmentation was prevented by ryanodine or thapsigargin. ICa,L augmentation was also dependent on CaM kinase, because it was prevented by dialysis with the inhibitor peptide AC3-I and reconstituted by exogenous constitutively active CaM kinase when Ba2+ was substituted for bath Ca2+. Ultrastructural studies confirmed that endogenous CaM kinase, L-type Ca2+ channels, and ryanodine receptors colocalized near T tubules. EAD induction was significantly reduced in current-clamped cells dialyzed with AC3-I (4/15) compared with cells dialyzed with an inactive control peptide (11/15, P = 0.013). These findings support the hypothesis that EADs are facilitated by CaM kinase.

N-type calcium channel inactivation probed by gating-current analysis

N-type calcium channels inactivate most rapidly in response to moderate, not extreme depolarization. This behavior reflects an inactivation rate that bears a U-shaped dependence on voltage. Despite this apparent similarity to calcium-dependent inactivation, N-type channel inactivation is insensitive to the identity of divalent charge carrier and, in some reports, to the level of internal buffering of divalent cations. Hence, the inactivation of N-type channels fits poorly with the "classic" profile for either voltage-dependent or calcium-dependent inactivation. To investigate this unusual inactivation behavior, we expressed recombinant N-type calcium channels in mammalian HEK 293 cells, permitting in-depth correlation of ionic current inactivation with potential alterations of gating current properties. Such correlative measurements have been particularly useful in distinguishing among various inactivation mechanisms in other voltage-gated channels. Our main results are the following: 1) The degree of gating charge immobilization was unchanged by the block of ionic current and precisely matched by the extent of ionic current inactivation. These results argue for a purely voltage-dependent mechanism of inactivation. 2) The inactivation rate was fastest at a voltage where only approximately (1)/(3) of the total gating charge had moved. This unusual experimental finding implies that inactivation occurs most rapidly from intermediate closed conformations along the activation pathway, as we demonstrate with novel analytic arguments applied to coupled-inactivation schemes. These results provide strong, complementary support for a "preferential closed-state" inactivation mechanism, recently proposed on the basis of ionic current measurements of recombinant N-type channels (Patil et al., . Neuron. 20:1027-1038).

Control of L-type calcium current during the action potential of guinea-pig ventricular myocytes

1. During an action potential the L-type Ca2+ current (ICa,L) activates rapidly, then partially declines leading to a sustained inward current during the plateau phase. The reason for the sustained part of ICa,L has been investigated here. 2. In the present study the mechanisms controlling the ICa,L during an action potential were investigated quantitatively in isolated guinea-pig ventricular myocytes by whole-cell patch clamp. To measure the actual time courses of ICa,L and the corresponding L-type channel inactivation (fAP) during an action potential, action potential-clamp protocols combined with square pulses were applied. 3. Within the first 10 ms of the action potential the ICa,L rapidly inactivated by about 50 %; during the plateau phase inactivation proceeded to 95 %. Later, during repolarization, the L-type channels recovered up to 25 %. 4. The voltage-dependent component of inactivation during an action potential was determined from measurements of L-type current carried by monovalent cations. This component of inactivation proceeded rather slowly and contributed only a little to fAP. ICa,L during an action potential is thus mainly controlled by Ca2+-dependent inactivation. 5. In order to investigate the source of the Ca2+ controlling fAP, internal Ca2+ homeostasis was manipulated by the use of Ca2+ buffers (EGTA, BAPTA), by blocking Na+-Ca2+ exchange, or by blocking Ca2+ release from the sarcoplasmic reticulum (SR). Internal BAPTA markedly reduced the L-type channel inactivation during the entire action potential, whereas EGTA affected fAP only during the middle and late plateau phases. Inhibition of Na+-Ca2+ exchange markedly increased inactivation of L-type channels. Although blocking SR Ca2+ release decreased the fura-2-measured cytoplasmic Ca2+ concentration ([Ca2+]i) transient by about 90 %, it reduced L-type channel inactivation only during the initial 50 ms of the action potential. Thus, it is Ca2+ entering the cell through the L-type channels that controls the inactivation process for the majority of the action potential. Nevertheless, SR Ca2+-release contributes 40-50 % to L-type channel inactivation during the initial period of the action potential. However, the maximum extent of inactivation reached during the plateau is independent of Ca2+ released from the SR. 6. For the first time, the actual time course of L-type channel inactivation has been directly determined during an action potential under various defined [Ca2+]i conditions. Thereby, the relative contribution to ICa,L inactivation of voltage, Ca2+ entering through L-type channels, and Ca2+ being released from the SR could be directly demonstrated.

Mutations in the EF-hand motif impair the inactivation of barium currents of the cardiac alpha1C channel

Calcium-dependent inactivation has been described as a negative feedback mechanism for regulating voltage-dependent calcium influx in cardiac cells. Most recent evidence points to the C-terminus of the alpha1C subunit, with its EF-hand binding motif, as being critical in this process. The EF-hand binding motif is mostly conserved between the C-termini of six of the seven alpha1 subunit Ca2+ channel genes. The role of E1537 in the C-terminus of the alpha1C calcium channel inactivation was investigated here after expression in Xenopus laevis oocytes. Whole-cell currents were measured in the presence of 10 mM Ba2+ or 10 mM Ca2+ after intracellular injection of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Against all expectations, our results showed a significant reduction in the rate of voltage-dependent inactivation as measured in Ba2+ solutions for all E1537 mutants, whereas calcium-dependent inactivation appeared unscathed. Replacing the negatively charged glutamate residue by neutral glutamine, glycine, serine, or alanine significantly reduced the rate of Ba2+-dependent inactivation by 1.5-fold (glutamine) to 3.5-fold (alanine). The overall rate of macroscopic inactivation measured in Ca2+ solutions was also reduced, although a careful examination of the distribution of the fast and slow time constants suggests that only the slow time constant was significantly reduced in the mutant channels. The fast time constant, the hallmark of Ca2+-dependent inactivation, remained remarkably constant among wild-type and mutant channels. Moreover, inactivation of E1537A channels, in both Ca2+ and Ba2+ solutions, appeared to decrease with membrane depolarization, whereas inactivation of wild-type channels became faster with positive voltages. All together, our results showed that E1537 mutations impaired voltage-dependent inactivation and suggest that the proximal part of the C-terminus may play a role in voltage-dependent inactivation in L-type alpha1C channels.

Molecular cloning and functional expression of a skeletal muscle dihydropyridine receptor from Rana catesbeiana

In skeletal muscle the dihydropyridine receptor is the voltage sensor for excitation-contraction coupling and an L-type Ca2+ channel. We cloned a dihydropyridine receptor (named Fgalpha1S) from frog skeletal muscle, where excitation-contraction coupling has been studied most extensively. Fgalpha1S contains 5600 base pairs coding for 1688 amino acids. It is highly homologous with, and of the same length as, the C-truncated form predominant in rabbit muscle. The primary sequence has every feature needed to be an L-type Ca2+ channel and a skeletal-type voltage sensor. Currents expressed in tsA201 cells had rapid activation (5-10 ms half-time) and Ca2+-dependent inactivation. Although functional expression of the full Fgalpha1S was difficult, the chimera consisting of Fgalpha1S domain I in the rabbit cardiac Ca channel had high expression and a rapidly activating current. The slow native activation is therefore not determined solely by the alpha1 subunit sequence. Its Ca2+-dependent inactivation strengthens the notion that in rabbit skeletal muscle this capability is inhibited by a C-terminal stretch (Adams, B., and Tanabe, T. (1997) J. Gen. Physiol. 110, 379-389). This molecule constitutes a new tool for studies of excitation-contraction coupling, gating, modulation, and gene expression.

Inactivation of gating currents of L-type calcium channels. Specific role of the alpha 2 delta subunit

In studies of gating currents of rabbit cardiac Ca channels expressed as alpha 1C/beta 2a or alpha 1C/beta 2a/alpha 2 delta subunit combinations in tsA201 cells, we found that long-lasting depolarization shifted the distribution of mobile charge to very negative potentials. The phenomenon has been termed charge interconversion in native skeletal muscle (Brum, G., and E. Ríos. 1987. J. Physiol. (Camb.). 387:489-517) and cardiac Ca channels (Shirokov, R., R. Levis, N. Shirokova, and E. Ríos. 1992. J. Gen. Physiol. 99:863-895). Charge 1 (voltage of half-maximal transfer, V1/2 approximately 0 mV) gates noninactivated channels, while charge 2 (V1/2 approximately -90 mV) is generated in inactivated channels. In alpha 1C/beta 2a cells, the available charge 1 decreased upon inactivating depolarization with a time constant tau approximately 8, while the available charge 2 decreased upon recovery from inactivation (at -200 mV) with tau approximately 0.3 s. These processes therefore are much slower than charge movement, which takes <50 ms. This separation between the time scale of measurable charge movement and that of changes in their availability, which was even wider in the presence of alpha 2 delta, implies that charges 1 and 2 originate from separate channel modes. Because clear modal separation characterizes slow (C-type) inactivation of Na and K channels, this observation establishes the nature of voltage-dependent inactivation of L-type Ca channels as slow or C-type. The presence of the alpha 2 delta subunit did not change the V1/2 of charge 2, but sped up the reduction of charge 1 upon inactivation at 40 mV (to tau approximately 2 s), while slowing the reduction of charge 2 upon recovery (tau approximately 2 s). The observations were well simulated with a model that describes activation as continuous electrodiffusion (Levitt, D. 1989. Biophys. J. 55:489-498) and inactivation as discrete modal change. The effects of alpha 2 delta are reproduced assuming that the subunit lowers the free energy of the inactivated mode.

Inactivation of presynaptic calcium current contributes to synaptic depression at a fast central synapse

Voltage-gated calcium channels are well characterized at neuronal somata but less thoroughly understood at the presynaptic terminal where they trigger transmitter release. In order to elucidate how the intrinsic properties of presynaptic calcium channels influence synaptic function, we have made direct recordings of the presynaptic calcium current (I(pCa)) in a brainstem giant synapse called the calyx of Held. The current was pharmacologically classified as P-type and exhibited marked inactivation. The inactivation was largely dependent upon the inward calcium current magnitude rather than the membrane potential, displayed little selectivity between divalent charge carriers (Ca2+, Ba2+ and Sr+), and exhibited slow recovery. Simultaneous pre- and postsynaptic whole-cell recording revealed that I(pCa) inactivation predominantly contributes to posttetanic depression of EPSCs. Thus, because of its slow recovery, I(pCa) inactivation underlies this short-term synaptic plasticity.

Structural regions of the cardiac Ca channel alpha subunit involved in Ca-dependent inactivation

We investigated the molecular basis for Ca-dependent inactivation of the cardiac L-type Ca channel. Transfection of HEK293 cells with the wild-type alpha or its 3' deletion mutant (alpha) produced channels that exhibited prominent Ca-dependent inactivation. To identify structural regions of alpha involved in this process, we analyzed chimeric alpha subunits in which one of the major intracellular domains of alpha was replaced by the corresponding region from the skeletal muscle alpha subunit (which lacks Ca-dependent inactivation). Replacing the NH terminus or the III-IV loop of alpha with its counterpart from alpha had no appreciable effect on Ca channel inactivation. In contrast, replacing the I-II loop of alpha with the corresponding region from alpha dramatically slowed the inactivation of Ba currents while preserving Ca-dependent inactivation. A similar but less pronounced result was obtained with a II-III loop chimera. These results suggest that the I-II and II-III loops of alpha may participate in the mechanism of Ca-dependent inactivation. Replacing the final 80% of the COOH terminus of alpha with the corresponding region from alpha completely eliminated Ca-dependent inactivation without affecting inactivation of Ba currents. Significantly, Ca-dependent inactivation was restored to this chimera by deleting a nonconserved, 211-amino acid segment from the end of the COOH terminus. These results suggest that the distal COOH terminus of alpha can block Ca-dependent inactivation, possibly by interacting with other proteins or other regions of the Ca channel. Our findings suggest that structural determinants of Ca-dependent inactivation are distributed among several major cytoplasmic domains of alpha.