Two distinct inactivation processes related to phosphorylation in cardiac L-type Ca(2+) channel currents

Bi-component HlgC/HlgB and HlgA/HlgB γ-hemolysins from S. aureus: Modulation of Ca2+ channels activity through a differential mechanism

Staphylococcal bi-component leukotoxins known as *pore-forming toxins* induce upon a specific binding to membrane receptors, two independent cellular events in human neutrophils. First, they provoke the opening of pre-existing specific ionic channels including Ca²⁺ channels. Then, they form membrane pores specific to monovalent cations leading to immune cells death. Among these leukotoxins, HlgC/HlgB and HlgA/HlgB γ-hemolysins do act in synergy to induce the opening of different types of Ca²⁺ channels in the absence as in the presence of extracellular Ca²⁺. Here, we investigate the mechanism underlying the modulation of Ca²⁺-independent Ca²⁺ channels in response to both active leukotoxins in human neutrophils. In the absence of extracellular Ca²⁺, the Mn²⁺ has been used as a Ca²⁺ surrogate to determine the activity of Ca²⁺-independent Ca²⁺ channels. Our findings provide new insights about different mechanisms involved in the staphylococcal γ-hemolysins activity to regulate three different types of Ca²⁺-independent Ca²⁺ channels. We conclude that (i) HlgC/HlgB stimulates the opening of La³⁺-sensitive Ca²⁺ channels, through a cholera toxin-sensitive G protein, (ii) HlgA/HlgB stimulates the opening of Ca²⁺ channels not sensitive to La³⁺, through a G protein-independent process, and (iii) unlike HlgA/HlgB, HlgC/HlgB toxins prevent the opening of a new type of Ca²⁺ channels by phosphorylation/de-phosphorylation-dependent mechanisms.

L-type channel inactivation balances the increased peak calcium current due to absence of Rad in cardiomyocytes

The L-type Ca²⁺ channel (LTCC) provides trigger calcium to initiate cardiac contraction in a graded fashion that is regulated by L-type calcium current (I_Ca,L) amplitude and kinetics. Inactivation of LTCC is controlled to fine-tune calcium flux and is governed by voltage-dependent inactivation (VDI) and calcium-dependent inactivation (CDI). Rad is a monomeric G protein that regulates I_Ca,L and has recently been shown to be critical to β-adrenergic receptor (β-AR) modulation of I_Ca,L. Our previous work showed that cardiomyocyte-specific Rad knockout (cRadKO) resulted in elevated systolic function, underpinned by an increase in peak I_Ca,L, but without pathological remodeling. Here, we sought to test whether Rad-depleted LTCC contributes to the fight-or-flight response independently of β-AR function, resulting in I_Ca,L kinetic modifications to homeostatically balance cardiomyocyte function. We recorded whole-cell I_Ca,L from ventricular cardiomyocytes from inducible cRadKO and control (CTRL) mice. The kinetics of I_Ca,L stimulated with isoproterenol in CTRL cardiomyocytes were indistinguishable from those of unstimulated cRadKO cardiomyocytes. CDI and VDI are both enhanced in cRadKO cardiomyocytes without differences in action potential duration or QT interval. To confirm that Rad loss modulates LTCC independently of β-AR stimulation, we crossed a β₁,β₂-AR double-knockout mouse with cRadKO, resulting in a Rad-inducible triple-knockout mouse. Deletion of Rad in cardiomyocytes that do not express β₁,β₂-AR still yielded modulated I_Ca,L and elevated basal heart function. Thus, in the absence of Rad, increased Ca²⁺ influx is homeostatically balanced by accelerated CDI and VDI. Our results indicate that the absence of Rad can modulate the LTCC without contribution of β₁,β₂-AR signaling and that Rad deletion supersedes β-AR signaling to the LTCC to enhance in vivo heart function.

Gain-of-function mutations in the calcium channel CACNA1C (Cav1.2) cause non-syndromic long-QT but not Timothy syndrome

Gain-of-function mutations in CACNA1C, encoding the L-type Ca(2+) channel Cav1.2, cause Timothy syndrome (TS), a multi-systemic disorder with dysmorphic features, long-QT syndrome (LQTS) and autism spectrum disorders. TS patients have heterozygous mutations (G402S and G406R) located in the alternatively spliced exon 8, causing a gain-of-function by reduced voltage-dependence of inactivation. Screening 540 unrelated patients with non-syndromic forms of LQTS, we identified six functional relevant CACNA1C mutations in different regions of the channel. All these mutations caused a gain-of-function combining different mechanisms, including changes in current amplitude, rate of inactivation and voltage-dependence of activation or inactivation, similar as in TS. Computer simulations support the theory that the novel CACNA1C mutations prolong action potential duration. We conclude that genotype-negative LQTS patients should be investigated for mutations in CACNA1C, as a gain-of-function in Cav1.2 is likely to cause LQTS and only specific and rare mutations, i.e. in exon 8, cause the multi-systemic TS.

A new phosphorylation site in cardiac L-type Ca2+ channels (Cav1.2) responsible for its cAMP-mediated modulation

Cardiac L-type Ca(2+) channels are modulated by phosphorylation by protein kinase A (PKA). To explore the PKA-targeted phosphorylation site(s), five potential phosphorylation sites in the carboxyl (COOH) terminal region of the α1C-subunit of the guinea pig Cav1.2 Ca(2+) channel were mutated by replacing serine (S) or threonine (T) residues with alanine (A): S1574A (C1 site), S1626A (C2), S1699A (C3), T1908A, (C4), S1927A (C5), and their various combinations. The wild-type Ca(2+) channel activity was enhanced three- to fourfold by the adenylyl cyclase activator forskolin (Fsk, 5 μM), and that of mutants at C3, C4, C5, and combination of these sites was also significantly increased by Fsk. However, Fsk did not modulate the activity of the C1 and C2 mutants and mutants of combined sites involving the C1 site. Three peptides of the COOH-terminal tail of α1C, termed CT1 [corresponding to amino acids (aa) 1509-1789, containing sites C1-3], CT2 (aa 1778-2003, containing sites C4 and C5), and CT3 (aa 1942-2169), were constructed, and their phosphorylation by PKA was examined. CT1 and CT2, but not CT3, were phosphorylated in vitro by PKA. Three CT1 mutants at two sites of C1-C3 were also phosphorylated by PKA, but the mutant at all three sites was not. The CT2 mutant at the C4 site was phosphorylated by PKA, but the mutant at C5 sites was not. These results suggest that Ser(1574) (C1 site) may be a potential site for the channel modulation mediated by PKA.

H-89 decreases the gain of excitation-contraction coupling and attenuates calcium sparks in the absence of beta-adrenergic stimulation

This study used the selective protein kinase A (PKA) inhibitor H-89 (N-[2-(p-Bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide) to determine the role of basal PKA activity in modulating cardiac excitation-contraction coupling in the absence of β-adrenergic stimulation. Basal intracellular cyclic AMP (cAMP) levels measured in isolated murine ventricular myocytes with an enzyme immunoassay were increased upon adenylyl cyclase activation (forskolin; 1 and 10 μM) or phosphodiesterase inhibition (3-isobutyl-1-methylxanthine, IBMX; 300 μM). Forskolin and IBMX also caused concentration-dependent increases in peak Ca(2+) transients (fura-2) and cell shortening (edge-detector) measured simultaneously in field-stimulated myocytes (37 °C). Similar effects were seen upon application of dibutyryl cAMP. In voltage-clamped myocytes, H-89 (2 μM) decreased basal Ca(2+) transients, contractions and underlying Ca(2+) currents. H-89 also decreased diastolic Ca(2+) and the gain of excitation-contraction coupling (Ca(2+) release/Ca(2+) current), especially at negative membrane potentials. This was independent of alterations in sarcoplasmic reticulum (SR) Ca(2+) loading, as SR stores were unchanged by PKA inhibition. H-89 also decreased the frequency, amplitude and width of spontaneous Ca(2+) sparks measured in quiescent myocytes (loaded with fluo-4), but increased time-to-peak. Thus, H-89 suppressed SR Ca(2+) release by decreasing Ca(2+) current and by reducing the gain of excitation-contraction coupling, in part by decreasing the size of individual Ca(2+) release units. These data suggest that basal PKA activity enhances SR Ca(2+) release in the absence of ß-adrenergic stimulation. This may depress contractile function in models such as aging, where the cAMP/PKA pathway is altered due to low basal cAMP levels.

Interplay of voltage and Ca-dependent inactivation of L-type Ca current

Inactivation of L-type Ca channels (LTCC) is regulated by both Ca and voltage-dependent processes (CDI and VDI). To differentiate VDI and CDI, several experimental and theoretical studies have considered the inactivation of Ba current through LTCC (I(Ba)) as a measure of VDI. However, there is evidence that Ba can weakly mimic Ca, such that I(Ba) inactivation is still a mixture of CDI and VDI. To avoid this complication, some have used the monovalent cation current through LTCC (I(NS)), which can be measured when divalent cation concentrations are very low. Notably, I(NS) inactivation rate does not depend on current amplitude, and hence may reflect purely VDI. However, based on analysis of existent and new data, and modeling, we find that I(NS) can inactivate more rapidly and completely than I(Ba), especially at physiological temperature. Thus VDI that occurs during I(Ba) (or I(Ca)) must differ intrinsically from VDI during I(NS). To account for this, we have extended a previously published LTCC mathematical model of VDI and CDI into an excitation-contraction coupling model, and assessed whether and how experimental I(Ba) inactivation results (traditionally used in VDI experiments and models) could be recapitulated by modifying CDI to account for Ba-dependent inactivation. Thus, the view of a slow and incomplete I(NS) inactivation should be revised, and I(NS) inactivation is a poor measure of VDI during I(Ca) or I(Ba). This complicates VDI analysis experimentally, but raises intriguing new questions about how the molecular mechanisms of VDI differ for divalent and monovalent currents through LTCCs.

Physiological modulation of voltage-dependent inactivation in the cardiac muscle L-type calcium channel: A modelling study

The inactivation of the L-type Ca2+ current is composed of voltage-dependent and calcium-dependent mechanisms. The relative contribution of these processes is still under dispute and the idea that the voltage-dependent inactivation could be subject to further modulation by other physiological processes had been ignored. This study sought to model physiological modulation of inactivation of the current in cardiac ventricular myocytes, based upon the recent detailed experimental data that separated total and voltage-dependent inactivation (VDI) by replacing extracellular Ca2+ with Mg2+ and monitoring L-type Ca2+ channel behaviour by outward K+ current flowing through the channel in the absence of inward current flow. Calcium-dependent inactivation (CDI) was based upon Ca2+ influx and formulated from data that was recorded during beta-adrenergic stimulation of the myocytes. Ca2+ influx and its competition with non-selective monovalent cation permeation were also incorporated into channel permeation in the model. The constructed model could closely reproduce the experimental Ba2+ and Ca2+ current results under basal condition where no beta-stimulation was added after a slight reduction of the development of fast voltage-dependent inactivation with depolarization. The model also predicted that under beta-adrenergic stimulation voltage-dependent inactivation is lost and calcium-dependent inactivation largely compensates it. The developed model thus will be useful to estimate the respective roles of VDI and CDI of L-type Ca2+ channels in various physiological and pathological conditions of the heart which would otherwise be difficult to show experimentally.

β2-Adrenergic receptor agonists stimulate L-type calcium current independent of PKA in newborn rabbit ventricular myocytes

Selective stimulation of β2-adrenergic receptors (ARs) in newborn rabbit ventricular myocardium invokes a positive inotropic effect that is lost during postnatal maturation. The underlying mechanisms for this age-related stimulatory response remain unresolved. We examined the effects of β2-AR stimulation on L-type Ca2+ current ( ICa,L) during postnatal development. ICa,L was measured (37°C; either Ca2+ or Ba2+ as the charge carrier) using the whole-cell patch-clamp technique in newborn (1 to 5 days old) and adult rabbit ventricular myocytes. Ca2+ transients were measured concomitantly by dialyzing the cell with indo-1. Activation of β2-ARs (with either 100 nM zinterol or 1 μM isoproterenol in the presence of the β1-AR antagonist, CGP20712A) stimulated ICa,L twofold in newborns but not in adults. The β2-AR-mediated increase in Ca2+ transient amplitude in newborns was due exclusively to the augmentation of ICa,L. Zinterol increased the rate of inactivation of ICa,L and increased the Ca2+ flux integral. The β2-AR inverse agonist, ICI-118551 (500 nM), but not the β1-AR antagonist, CGP20712A (500 nM), blocked the response to zinterol. Unexpectedly, the PKA blockers, H-89 (10 μM), PKI 6-22 amide (10 μM), and Rp-cAMP (100 μM), all failed to prevent the response to zinterol but completely blocked responses to selective β1-AR stimulation of ICa,L in newborns. Our results demonstrate that in addition to the conventional β1-AR/cAMP/PKA pathway, newborn rabbit myocardium exhibits a novel β2-AR-mediated, PKA-insensitive pathway that stimulates ICa,L. This striking developmental difference plays a major role in the age-related differences in inotropic responses to β2-AR agonists.

The role of constitutive PKA-mediated phosphorylation in the regulation of basal I(Ca) in isolated rat cardiac myocytes

1. Pharmacological inhibitors of protein kinase A (PKA) and protein phosphatases 1/2A were used to determine whether basal L-type Ca(2+) current (I(Ca)) observed in the absence of exogenous beta-adrenergic receptor stimulation is sustained by PKA-mediated phosphorylation. Amphotericin B was used to record whole-cell I(Ca) in the perforated patch-clamp configuration. 2. Calyculin A and isoprenaline (both 1 micromol l(-1)) increased basal I(Ca) (P<0.05), whereas H-89 inhibited I(Ca) in a concentration-dependent manner with an IC(50) approximately 5 micromol l(-1). H-89 also inhibited the response to 1.0 micromol l(-1) isoprenaline, although relatively high concentrations (30 micromol l(-1)) were required to achieve complete suppression of the response. 3. Double-pulse protocols were used to study the effects of 10 micromol l(-1) H-89 on time-dependent recovery of I(Ca) from voltage-dependent inactivation as well as the steady-state gating of I(Ca). T(0.5) (time for I(Ca) to recover to 50% of the preinactivation amplitude) increased in the presence of H-89 (P<0.05) but was unaffected by calyculin A or isoprenaline. 4. Steady-state activation/inactivation properties of I(Ca) were unaffected by 10 micromol l(-1) H-89 or 1 micromol l(-1) calyculin A, whereas isoprenaline caused a leftward shift in both curves so that V(0.5) for activation and inactivation became more negative. 5. Data show that basal I(Ca) is regulated by cAMP-PKA-mediated phosphorylation in the absence of externally applied beta-receptor agonists and that relatively high concentrations of H-89 are required to fully suppress the response to beta-adrenergic receptor stimulation, thereby limiting the value of H-89 as a useful tool in dissecting signalling pathways in intact myocytes.

Tyrosine kinase inhibitors and ATP modulate the conversion of smooth muscle L-type Ca2+ channels toward a second open state

Properties of smooth and cardiac L-type Ca2+ channels differ prominently in several physiological aspects, including sympathetic modulation. To assess the possible underlying mechanisms, we applied the whole cell patch-clamp technique to guinea pig detrusor smooth muscle cells, in which only L-type Ca2+ channel currents are observed in practice. During depolarization to large positive potentials, the conformation of the majority of L-type Ca2+ channels is converted from the normal (O1) to a second open state (O2), which undergoes little inactivation during depolarization. Extracellular application of genistein, a known tyrosine kinase inhibitor, significantly attenuated the voltage-dependent conversion of Ca2+ channels to O2, accompanied by reduction of availability, whereas genistin, an inactive analog, had little effect. In the absence of ATP in the patch pipette, intracellular application of either genistein or tyrphostin-47 suppressed the conversion to O2. Computer calculation revealed that the acceleration of the O1 to an inactivated state qualitatively reconstructs the unique effects of PTK inhibitors antagonized by ATP. We concluded that under normal conditions smooth muscle L-type Ca2+ channels are already modulated by tyrosine-kinase and ATP-related mechanism(s) and thereby easily achieve the second conversion, which yields voltage-dependent modulation of L-type Ca2+ current analogous to that in cardiac myocytes during beta-adrenoceptor stimulation.

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

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

Ca2+ Dysregulation Induces Mitochondrial Depolarization and Apoptosis

We previously reported that constitutively activated Galpha(q) (Q209L) expression in cardiomyocytes induces apoptosis through opening of the mitochondrial permeability transition pore. We assessed the hypothesis that disturbances in Ca(2+) handling linked Galpha(q) activity to apoptosis because resting Ca(2+) levels were significantly increased prior to development of apoptosis. Treating cells with EGTA lowered Ca(2+) and blocked both loss of mitochondrial membrane potential (an indicator of permeability transition pore opening) and apoptosis (assessed by DNA fragmentation). When cytosolic Ca(2+) and mitochondrial membrane potential were simultaneously measured by confocal microscopy, sarcoplasmic reticulum (SR)-driven slow Ca(2+) oscillations (time-to-peak approximately 4 s) were observed in Q209L-expressing cells. These oscillations were seen to transition into sustained increases in cytosolic Ca(2+), directly paralleled by loss of mitochondrial membrane potential. Ca(2+) transients generated by caffeine-induced release of SR Ca(2+) were greatly prolonged in Q209L-expressing cells, suggesting a decreased ability to extrude Ca(2+). Indeed, the Na(+)/Ca(2+) exchanger (NCX), which removes Ca(2+) from the cell, was markedly down-regulated at the mRNA and protein levels. Adenoviral NCX expression normalized cytosolic Ca(2+) levels and prevented DNA fragmentation in cells expressing Q209L. Interestingly, constitutively activated Akt, which rescues cells from Q209L-induced apoptosis, prevented the decrease in NCX expression, normalized cytosolic Ca(2+) levels and spontaneous Ca(2+) oscillations, shortened caffeine-induced Ca(2+) transients, and prevented loss of the mitochondrial membrane potential. Our findings demonstrate that NCX down-regulation and consequent increases in cytosolic and SR Ca(2+) can lead to Ca(2+) overloading-induced loss of mitochondrial membrane potential and suggest that recovery of Ca(2+) dysregulation is a target of Akt-mediated protection.

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.

FPL 64176 modification of Ca(V)1.2 L-type calcium channels: dissociation of effects on ionic current and gating current

FPL 64176 (FPL) is a nondihydropyridine compound that dramatically increases macroscopic inward current through L-type calcium channels and slows activation and deactivation. To understand the mechanism by which channel behavior is altered, we compared the effects of the drug on the kinetics and voltage dependence of ionic currents and gating currents. Currents from a homogeneous population of channels were obtained using cloned rabbit Ca(V)1.2 (alpha1C, cardiac L-type) channels stably expressed in baby hamster kidney cells together with beta1a and alpha2delta1 subunits. We found a striking dissociation between effects of FPL on ionic currents, which were modified strongly, and on gating currents, which were not detectably altered. Inward ionic currents were enhanced approximately 5-fold for a voltage step from -90 mV to +10 mV. Kinetics of activation and deactivation were slowed dramatically at most voltages. Curiously, however, at very hyperpolarized voltages (< -250 mV), deactivation was actually faster in FPL than in control. Gating currents were measured using a variety of inorganic ions to block ionic current and also without blockers, by recording gating current at the reversal potential for ionic current (+50 mV). Despite the slowed kinetics of ionic currents, FPL had no discernible effect on the fundamental movements of gating charge that drive channel gating. Instead, FPL somehow affects the coupling of charge movement to opening and closing of the pore. An intriguing possibility is that the drug causes an inactivated state to become conducting without otherwise affecting gating transitions.

Regulation of L-type calcium current by intracellular magnesium in rat cardiac myocytes

The effects of changing cytosolic [Mg(2+)] ([Mg(2+)](i)) on L-type Ca(2+) currents were investigated in rat cardiac ventricular myocytes voltage-clamped with patch pipettes containing salt solutions with defined [Mg(2+)] and [Ca(2+)]. To control [Mg(2+)](i) and cytosolic [Ca(2+)] ([Ca(2+)](i)), the pipette solution included 30 mM citrate and 10 mM ATP along with 5 mM EGTA (slow Ca(2+) buffer) or 15 mM EGTA plus 5 mM BAPTA (fast Ca(2+) buffer). With pipette [Ca(2+)] ([Ca(2+)](p)) set at 100 nM using a slow Ca(2+) buffer and pipette [Mg(2+)] ([Mg(2+)](p)) set at 0.2 mM, peak l-type Ca(2+) current density (I(Ca)) was 17.0 +/- 2.2 pA pF(-1). Under the same conditions, but with [Mg(2+)](p) set to 1.8 mM, I(Ca) was 5.6 +/- 1.0 pA pF(-1), a 64 +/- 2.8% decrease in amplitude. This decrease in I(Ca) was accompanied by an acceleration and a -8 mV shift in the voltage dependence of current inactivation. The [Mg(2+)](p)-dependent decrease in I(Ca) was not significantly different when myocytes were preincubated with 10 microM forskolin and 300 microM 3-isobutyl-L-methylxanthine and voltage-clamped with pipettes containing 50 microM okadaic acid, to maximize Ca(2+) channel phosphorylation. However, when myocytes were voltage-clamped with pipettes containing protein phosphatase 2A, to promote channel dephosphorylation, I(Ca) decreased only 25 +/- 3.4% on changing [Mg(2+)](p) from 0.2 to 1.8 mM. In the presence of 0.2 mM[Mg(2+)](p), changing channel phosphorylation conditions altered I(Ca) over a 4-fold range; however, with 1.8 mM[Mg(2+)](p), these same manoeuvres had a much smaller effect on I(Ca). These data suggest that [Mg(2+)](i) can antagonize the effects of phosphorylation on channel gating kinetics. Setting [Ca(2+)](p) to 1, 100 or 300 nM also showed that the [Mg(2+)](p)-induced reduction of I(Ca) was smaller at the lowest [Ca(2+)](p), irrespective of channel phosphorylation conditions. This interaction between [Ca(2+)](i) and [Mg(2+)](i) to modulate I(Ca) was not significantly affected by ryanodine, fast Ca(2+) buffers or inhibitors of calmodulin, calmodulin-dependent kinase and calcineurin. Thus, physiologically relevant [Mg(2+)](i) modulates I(Ca) by counteracting the effects of Ca(2+) channel phosphorylation and by an unknown [Ca(2+)](i)-dependent mechanism. The magnitude of these effects suggests that changes in [Mg(2+)](i) could be critical in regulating L-type channel gating.

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.

L-type Ca2+ channels serve as a sensor of the SR Ca2+ for tuning the efficacy of Ca2+-induced Ca2+ release in rat ventricular myocytes

In cardiac excitation-contraction coupling, Ca2+-induced Ca2+ release (CICR) from ryanodine receptors (RyRs), triggered by Ca2+ entry through the nearby L-type Ca2+ channel, induces Ca2+-dependent inactivation (CDI) of the Ca2+ channel. Aiming at elucidating the physiological role of CDI produced by CICR (CICR-dependent CDI), we investigated the contribution of the CICR-dependent CDI to action potential (AP) waveform and the amount of Ca2+-influx through Ca2+ channels during AP in rat ventricular myocytes. The elimination of the CICR-dependent CDI, by depletion of the SR Ca2+ with thapsigargin, significantly prolonged AP duration (APD). APD changed in parallel with the magnitude of CICR during the recovery of the SR Ca2+ content after transient depletion by caffeine. Such CICR-dependent change of APD persisted under the highly Ca2+ buffered condition where the Ca2+ signalling was restricted to nanoscale domains. Blockers of the Ca2+-dependent Cl- channel or the BK channel did not affect AP waveform. The amount of Ca2+-influx through Ca2+ channels during the SR-depleted type AP waveform, measured in the SR-depleted myocyte, was increased by 40 % over that during the SR-intact type AP waveform measured in the SR-intact myocyte. The protein kinase A stimulation further enhanced the Ca2+-influx during AP under the SR-depleted condition to 70 % of that under the SR-intact condition. These results indicate that the CICR-dependent CDI of L-type Ca2+ channels, under control of the privileged cross-signalling between L-type Ca2+ channels and RyRs, play important roles for monitoring and tuning the SR Ca2+ content via changes of AP waveform and the amount of Ca2+-influx during AP in ventricular myocytes.

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.

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.

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.

Mechanisms of L-type Ca(2+) current downregulation in rat atrial myocytes during heart failure

Downregulation of the L-type Ca(2+) current (I(Ca)) is an important determinant of the electrical remodeling of diseased atria. Using a rat model of heart failure (HF) due to ischemic cardiopathy, we studied I(Ca) in isolated left atrial myocytes with the whole-cell patch-clamp technique and biochemical assays. I(Ca) density was markedly reduced (1.7+/-0.1 pA/pF) compared with sham-operated rats (S) (4.1+/-0.2 pA/pF), but its gating properties were unchanged. Calcium channel alpha(1C)-subunit quantities were not significantly different between S and HF. The beta-adrenergic agonist isoproterenol (1 micromol/L) had far greater stimulatory effects on I(Ca) in HF than in S (2.5- versus 1-fold), thereby suppressing the difference in current density. Dialyzing cells with 100 micromol/L cAMP or pretreating them with the phosphatase inhibitor okadaic acid also increased I(Ca) and suppressed the difference in density between S and HF. Intracellular cAMP content was reduced more in HF than in S. The phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine had a greater effect on I(Ca) in HF than in S (76.0+/-11.2% versus 15.8+/-21.2%), whereas the inhibitory effect of atrial natriuretic peptide on I(Ca) was more important in S than in HF (54.1+/-4.8% versus 24.3+/-8.8%). Cyclic GMP extruded from HF myocytes was enhanced compared with S (55.8+/-8.0 versus 6.2+/-4.0 pmol. mL(-1)). Thus, I(Ca) downregulation in atrial myocytes from rats with heart failure is caused by changes in basal cAMP-dependent regulation of the current and is associated with increased response to catecholamines.

Rescue of contractile parameters and myocyte hypertrophy in calsequestrin overexpressing myocardium by phospholamban ablation

Cardiac-specific overexpression of murine cardiac calsequestrin results in depressed cardiac contractile parameters, low Ca(2+)-induced Ca(2+) release from sarcoplasmic reticulum (SR) and cardiac hypertrophy in transgenic mice. To test the hypothesis that inhibition of phospholamban activity may rescue some of these phenotypic alterations, the calsequestrin overexpressing mice were cross-bred with phospholamban-knockout mice. Phospholamban ablation in calsequestrin overexpressing mice led to reversal of the depressed cardiac contractile parameters in Langendorff-perfused hearts or in vivo. This was associated with increases of SR Ca(2+) storage, assessed by caffeine-induced Na(+)-Ca(2+) exchanger currents. The inactivation time of the L-type Ca(2+) current (I(Ca)), which has an inverse correlation with Ca(2+)-induced SR Ca(2+) release, and the relation between the peak current density and half-inactivation time were also normalized, indicating a restoration in the ability of I(Ca) to trigger SR Ca(2+) release. The prolonged action potentials in calsequestrin overexpressing cardiomyocytes also reversed to normal upon phospholamban ablation. Furthermore, ablation of phospholamban restored the expression levels of atrial natriuretic factor and alpha-skeletal actin mRNA as well as ventricular myocyte size. These results indicate that attenuation of phospholamban function may prevent or overcome functional and remodeling defects in hypertrophied hearts.