Calmodulin kinase II activation is required for the maintenance of basal activity of L-type Ca2+ channels in guinea-pig ventricular myocytes

CaM-dependent modulation of human CaV1.3 whole-cell and single-channel currents by C-terminal CaMKII phosphorylation site S1475

Phosphorylation enables rapid modulation of voltage-gated calcium channels (VGCC) in physiological and pathophysiological conditions. How phosphorylation modulates human CaV1.3 VGCC, however, is largely unexplored. We characterized modulation of CaV1.3 gating via S1475, the human equivalent of a phosphorylation site identified in the rat. S1475 is highly conserved in CaV1.3 but absent from all other high-voltage activating calcium channel types co-expressed with CaV1.3 in similar tissues. Further, it is located in the C-terminal EF-hand motif, which binds calmodulin (CaM). This is involved in calcium-dependent channel inactivation (CDI). We used amino acid exchanges that mimic either sustained phosphorylation (S1475D) or phosphorylation resistance (S1475A). Whole-cell and single-channel recordings of phosphorylation state imitating CaV1.3 variants in transiently transfected HEK-293 cells revealed functional relevance of S1475 in human CaV1.3. We obtained three main findings: (1) CaV1.3_S1475D, imitating sustained phosphorylation, displayed decreased current density, reduced CDI and (in-) activation kinetics shifted to more depolarized voltages compared with both wildtype CaV1.3 and the phosphorylation-resistant CaV1.3_S1475A variant. Corresponding to the decreased current density, we find a reduced open probability of CaV1.3_S1475D at the single-channel level. (2) Using CaM overexpression or depletion, we find that CaM is necessary for modulating CaV1.3 through S1475. (3) CaMKII activation led to CaV1.3_WT-current properties similar to those of CaV1.3_S1475D, but did not affect CaV1.3_S1475A, confirming that CaMKII modulates human CaV1.3 via S1475. Given the physiological and pathophysiological importance of CaV1.3, our findings on the S1475-mediated interplay of phosphorylation, CaM interaction and CDI provide hints for approaches on specific CaV1.3 modulation under physiological and pathophysiological conditions. KEY POINTS: Phosphorylation modulates activity of voltage-gated L-type calcium channels for specific cellular needs but is largely unexplored for human CaV1.3 channels. Here we report that S1475, a CaMKII phosphorylation site identified in rats, is functionally relevant in human CaV1.3. Imitating phosphorylation states at S1475 alters current density and inactivation in a calmodulin-dependent manner. In wildtype CaV1.3 but not in the phosphorylation-resistant variant S1475A, CaMKII activation elicits effects similar to constitutively mimicking phosphorylation at S1475. Our findings provide novel insights on the interplay of modulatory mechanisms of human CaV1.3 channels, and present a possible target for CaV1.3-specific gating modulation in physiological and pathophysiological conditions.

Regulation of Cardiac Cav1.2 Channels by Calmodulin

Cav1.2 Ca2+ channels, a type of voltage-gated L-type Ca2+ channel, are ubiquitously expressed, and the predominant Ca2+ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca2+-binding protein that causes Ca2+-dependent facilitation (CDF) and inactivation (CDI). Ca2+-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca2+/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

The L-type Ca(2+) channel facilitates abnormal metabolic activity in the cTnI-G203S mouse model of hypertrophic cardiomyopathy

KEY POINTS

Genetic mutations in cardiac troponin I (cTnI) are associated with development of hypertrophic cardiomyopathy characterized by myocyte remodelling, disorganization of cytoskeletal proteins and altered energy metabolism. The L-type Ca(2+) channel is the main route for calcium influx and is crucial to cardiac excitation and contraction. The channel also regulates mitochondrial function in the heart by a functional communication between the channel and mitochondria via the cytoskeletal network. We find that L-type Ca(2+) channel kinetics are altered in cTnI-G203S cardiac myocytes and that activation of the channel causes a significantly greater increase in mitochondrial membrane potential and metabolic activity in cTnI-G203S cardiac myocytes. These responses occur as a result of impaired communication between the L-type Ca(2+) channel and cytoskeletal protein F-actin, involving decreased movement of actin-myosin and block of the mitochondrial voltage-dependent anion channel, resulting in a 'hypermetabolic' mitochondrial state. We propose that L-type Ca(2+) channel antagonists, such as diltiazem, might be effective in reducing the cardiomyopathy by normalizing mitochondrial metabolic activity.

ABSTRACT

Genetic mutations in cardiac troponin I (cTnI) account for 5% of families with hypertrophic cardiomyopathy. Hypertrophic cardiomyopathy is associated with disorganization of cytoskeletal proteins and altered energy metabolism. The L-type Ca(2+) channel (ICa-L ) plays an important role in regulating mitochondrial function. This involves a functional communication between the channel and mitochondria via the cytoskeletal network. We investigate the role of ICa-L in regulating mitochondrial function in 25- to 30-week-old cardiomyopathic mice expressing the human disease-causing mutation Gly203Ser in cTnI (cTnI-G203S). The inactivation rate of ICa-L is significantly faster in cTnI-G203S myocytes [cTnI-G203S: τ1  = 40.68 ± 3.22, n = 10 vs. wild-type (wt): τ1  = 59.05 ± 6.40, n = 6, P < 0.05]. Activation of ICa-L caused a greater increase in mitochondrial membrane potential (Ψm , 29.19 ± 1.85%, n = 15 vs. wt: 18.84 ± 2.01%, n = 10, P < 0.05) and metabolic activity (24.40 ± 6.46%, n = 8 vs. wt: 9.98 ± 1.57%, n = 9, P < 0.05). The responses occurred because of impaired communication between ICa-L and F-actin, involving lack of dynamic movement of actin-myosin and block of the mitochondrial voltage-dependent anion channel. Similar responses were observed in precardiomyopathic mice. ICa-L antagonists nisoldipine and diltiazem decreased Ψm to basal levels. We conclude that the Gly203Ser mutation leads to impaired functional communication between ICa-L and mitochondria, resulting in a 'hypermetabolic' state. This might contribute to development of cTnI-G203S cardiomyopathy because the response is present in young precardiomyopathic mice. ICa-L antagonists might be effective in reducing the cardiomyopathy by altering mitochondrial function.

Role of protein phosphatases in the run down of guinea pig cardiac Cav1.2 Ca2+ channels

This study aimed to investigate protein phosphatases involved in the run down of Cav1.2 Ca2+ channels. Single ventricular myocytes obtained from adult guinea pig hearts were used to record Ca2+ channel currents with the patch-clamp technique. Calmodulin (CaM) and ATP were used to restore channel activity in inside-out patches. Inhibitors of protein phosphatases were applied to investigate the role of phosphatases. The specific protein phosphatase type 1 (PP1) inhibitor (PP1 inhibitor-2) and protein phosphatase type 2A (PP2A) inhibitor (fostriecin) abolished the slow run down of Cav1.2 Ca2+ channels, which was evident as the time-dependent attenuation of the reversing effect of CaM/ATP on the run down. However, protein phosphatase type 2B (PP2B, calcineurin) inhibitor cyclosporine A together with cyclophilin A had no effect on the channel run down. Furthermore, PP1 inhibitor-2 mainly prolonged the open time constants of the channel, specifically, the slow open time. Fostriecin primarily shortened the slow close time constants. Our data suggest that PP1 and PP2A were involved in the slow phase of Cav1.2 Ca2+ channel run down. In addition, they exerted different effects on the open-close kinetics of the channel. All above support the view that PP1 and PP2A may dephosphorylate distinct phosphorylation sites on the Cav1.2 Ca2+ channel.

Involvement of calcium/calmodulin-dependent protein kinase II in methamphetamine-induced neural damage

Methamphetamine (METH), an illicit drug, is widely abused in many parts of the world. Mounting evidence shows that METH exposure contributes to neurotoxicity, particularly for the monoaminergic neurons. However, to date, only a few studies have tried to unravel the mechanisms involved in METH-induced non-monoaminergic neural damage. Therefore, in the present study, we tried to explore the mechanisms for METH-induced neural damage in cortical neurons. Our results showed that METH significantly increased intracellular [Ca(2) (+) ]i in Ca(2) (+) -containing solution rather than Ca(2) (+) -free solution. Moreover, METH also upregulated calmodulin (CaM) expression and activated CaM-dependent protein kinase II (CaMKII). Significantly, METH-induced neural damage can be partially retarded by CaM antagonist W7 as well as CaMKII blocker KN93. In addition, L-type Ca(2) (+) channel was also proved to be involved in METH-induced cell damage, as nifedipine, the L-type Ca(2) (+) channel-specific inhibitor, markedly attenuated METH-induced neural damage. Collectively, our results suggest that Ca(2) (+) -CaM-CaMKII is involved in METH-mediated neurotoxicity, and it might suggest a potential target for the development of therapeutic strategies for METH abuse. Copyright © 2016 John Wiley & Sons, Ltd.

PKA and phosphatases attached to the Ca(V)1.2 channel regulate channel activity in cell-free patches

Calmodulin (CaM) + ATP can reprime voltage-gated L-type Ca(2+) channels (Ca(V)1.2) in inside-out patches for activation, but this effect decreases time dependently. This suggests that the Ca(V)1.2 channel activity is regulated by additional cytoplasmic factors. To test this hypothesis, we examined the role of cAMP-dependent protein kinase A (PKA) and protein phosphatases in the regulation of Ca(V)1.2 channel activity in the inside-out mode in guinea pig ventricular myocytes. Ca(V)1.2 channel activity quickly disappeared after the patch was excised from the cell and recovered to only 9% of that in the cell-attached mode on application of CaM + ATP at 10 min after the inside out. However, immediate exposure of the excised patch to the catalytic subunit of PKA + ATP or the nonspecific phosphatase inhibitor okadaic acid significantly increased the Ca(V)1.2 channel activity recovery by CaM + ATP (114 and 96%, respectively) at 10 min. Interestingly, incubation of the excised patches with cAMP + ATP also increased CaM/ATP-induced Ca(V)1.2 channel activity recovery (108%), and this effect was blocked by the nonspecific protein kinase inhibitor K252a. The channel activity in the inside-out mode was not maintained by either catalytic subunit of PKA or cAMP + ATP in the absence of CaM, but was stably maintained in the presence of CaM for more than 40 min. These results suggest that PKA and phosphatase(s) attached on or near the Ca(V)1.2 channel regulate the basal channel activity, presumably through modulation of the dynamic CaM interaction with the channel.

Nucleotides maintain the activity of Cav1.2 channels in guinea-pig ventricular myocytes

The activity of Cav1.2 Ca(2+) channels is maintained in the presence of calmodulin and ATP, even in cell-free patches, and thus a channel ATP-binding site has been suggested. In this study, we examined whether other nucleotides, such as GTP, UTP, CTP, ADP and AMP, could be substituted for ATP in guinea-pig ventricular myocytes. We found that all the nucleotides tested could re-prime the Ca(2+) channels in the presence of 1 μM calmodulin in the inside-out mode. The order of efficacy was ATP > GTP > UTP > ADP > CTP ≈ AMP. Thus, the presumed nucleotide-binding site in the channel seemed to favor a purine rather than pyrimidine base and a triphosphate rather than a di- or mono-phosphate group. Furthermore, a high concentration (10 mM) of GTP, UTP, CTP, ADP and AMP had inhibitory effects on the channel activity. These results provide information on the putative nucleotide-binding site(s) in Cav1.2 Ca(2+) channels.

Impaired functional communication between the L-type calcium channel and mitochondria contributes to metabolic inhibition in the mdx heart

Duchenne muscular dystrophy is a fatal X-linked disease characterized by the absence of dystrophin. Approximately 20% of boys will die of dilated cardiomyopathy that is associated with cytoskeletal protein disarray, contractile dysfunction, and reduced energy production. However, the mechanisms for altered energy metabolism are not yet fully clarified. Calcium influx through the L-type Ca(2+) channel is critical for maintaining cardiac excitation and contraction. The L-type Ca(2+) channel also regulates mitochondrial function and metabolic activity via transmission of movement of the auxiliary beta subunit through intermediate filament proteins. Here, we find that activation of the L-type Ca(2+) channel is unable to induce increases in mitochondrial membrane potential and metabolic activity in intact cardiac myocytes from the murine model of Duchenne muscular dystrophy (mdx) despite robust increases recorded in wt myocytes. Treatment of mdx mice with morpholino oligomers to induce exon skipping of dystrophin exon 23 (that results in functional dystrophin accumulation) or application of a peptide that resulted in block of voltage-dependent anion channel (VDAC) "rescued" mitochondrial membrane potential and metabolic activity in mdx myocytes. The mitochondrial VDAC coimmunoprecipitated with the L-type Ca(2+) channel. We conclude that the absence of dystrophin in the mdx ventricular myocyte leads to impaired functional communication between the L-type Ca(2+) channel and mitochondrial VDAC. This appears to contribute to metabolic inhibition. These findings provide new mechanistic and functional insight into cardiomyopathy associated with Duchenne muscular dystrophy.

Adenosine triphosphate regulates the activity of guinea pig Cav1.2 channel by direct binding to the channel in a dose-dependent manner

The present study is to investigate the mechanism by which ATP regulates Cav1.2 channel activity. Ventricular tissue was obtained from adult guinea pig hearts using collagenase. Ca(2+) channel activity was monitored using the patch-clamp technique. Proteins were purified using wheat germ agglutinin-Sepharose, and the concentration was determined using the Coomassie brilliant blue technique. ATP binding to the Cav1.2 channel was examined using the photoaffinity method. EDA-ATP-biotin maintains Ca(2+) channel activity in inside-out membrane patches. ATP directly bound to the Cav1.2 channel in a dose-dependent manner, and at least two molecules of ATP bound to one molecule of the Cav1.2 channel. Low levels of calmodulin (CaM) increased ATP binding to the Cav1.2 channel, but higher levels of CaM decreased ATP binding to the Cav1.2 channel. In addition, Ca(2+) was another regulator for ATP binding to the Cav1.2 channel. Furthermore, ATP bound to GST-fusion peptides of NH2-terminal region (amino acids 6-140) and proximal COOH-terminal region (amino acids 1,509-1,789) of the main subunit (α1C) of the Cav1.2 channel. Our data suggest that ATP might regulate Cav1.2 channel activity by directly binding to the Cav1.2 channel in a dose-dependent manner. In addition, the ATP-binding effect to the Cav1.2 channel was both CaM- and Ca(2+) dependent.

Dynamic alterations in the CaV1.2/CaM/CaMKII signaling pathway in the left ventricular myocardium of ischemic rat hearts

Cardiac L-type calcium channel (CaV1.2), calmodulin (CaM), and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) form the CaV1.2/CaM/CaMKII signaling pathway, which plays an important role in maintaining intracellular Ca(2+) homeostasis. The roles of CaM and CaMKII in the regulation of CaV1.2 in Ca(2+)-dependent inactivation and facilitation have been reported; however, alterations in this signaling pathway in the heart after myocardial ischemia (MI) had not been well characterized. In this study, we investigated the dynamic changes in CaV1.2, CaM, and CaMKII mRNA and protein expression levels in the left ventricles of the heart following MI in rats. The MI model was induced by ligating the left anterior descending coronary artery; the rats were divided into the following five groups: the 6 h post-MI group (MI-6h), 24 h post-MI group (MI-24h), 1 week post-MI group (MI-1w), 2 weeks post-MI group (MI-2w), and the sham group. The mRNA levels were measured by quantitative real-time polymerase chain reaction and the protein expression was determined by western blotting and immunohistochemistry. There were no observable differences in the CaV1.2 mRNA and protein levels at the early stages of MI, but these levels decreased at MI-2w. Both the mRNA and protein levels of CaM increased at MI-6h, peaked at MI-24h, and then reduced to normal levels at MI-2w. CaMKII mRNA and protein levels decreased at MI-6h and reached their lowest level at MI-24h. Taken together, these data demonstrate that there are dynamic changes in the CaV1.2/CaM/CaMKII signaling pathway following MI injuries, which suggests that different therapeutic regimens should be used at different time points after MI injuries.

Regulation of cardiac L-type Ca²⁺ channel CaV1.2 via the β-adrenergic-cAMP-protein kinase A pathway: old dogmas, advances, and new uncertainties

In the heart, adrenergic stimulation activates the β-adrenergic receptors coupled to the heterotrimeric stimulatory Gs protein, followed by subsequent activation of adenylyl cyclase, elevation of cyclic AMP levels, and protein kinase A (PKA) activation. One of the main targets for PKA modulation is the cardiac L-type Ca²⁺ channel (CaV1.2) located in the plasma membrane and along the T-tubules, which mediates Ca²⁺ entry into cardiomyocytes. β-Adrenergic receptor activation increases the Ca²⁺ current via CaV1.2 channels and is responsible for the positive ionotropic effect of adrenergic stimulation. Despite decades of research, the molecular mechanism underlying this modulation has not been fully resolved. On the contrary, initial reports of identification of key components in this modulation were later refuted using advanced model systems, especially transgenic animals. Some of the cardinal debated issues include details of specific subunits and residues in CaV1.2 phosphorylated by PKA, the nature, extent, and role of post-translational processing of CaV1.2, and the role of auxiliary proteins (such as A kinase anchoring proteins) involved in PKA regulation. In addition, the previously proposed crucial role of PKA in modulation of unstimulated Ca²⁺ current in the absence of β-adrenergic receptor stimulation and in voltage-dependent facilitation of CaV1.2 remains uncertain. Full reconstitution of the β-adrenergic receptor signaling pathway in heterologous expression systems remains an unmet challenge. This review summarizes the past and new findings, the mechanisms proposed and later proven, rejected or disputed, and emphasizes the essential issues that remain unresolved.

Mechanisms underlying the modulation of L-type Ca2+ channel by hydrogen peroxide in guinea pig ventricular myocytes

Although Cav1.2 Ca(2+) channels are modulated by reactive oxygen species (ROS), the underlying mechanisms are not fully understood. In this study, we investigated effects of hydrogen peroxide (H2O2) on the Ca(2+) channel using a patch-clamp technique in guinea pig ventricular myocytes. Externally applied H2O2 (1 mM) increased Ca(2+) channel activity in the cell-attached mode. A specific inhibitor of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) KN-93 (10 μM) partially attenuated the H2O2-mediated facilitation of the channel, suggesting both CaMKII-dependent and -independent pathways. However, in the inside-out mode, 1 mM H2O2 increased channel activity in a KN-93-resistant manner. Since H2O2-pretreated calmodulin did not reproduce the H2O2 effect, the target of H2O2 was presumably assigned to the Ca(2+) channel itself. A thiol-specific oxidizing agent mimicked and occluded the H2O2 effect. These results suggest that H2O2 facilitates the Ca(2+) channel through oxidation of cysteine residue(s) in the channel as well as the CaMKII-dependent pathway.

Rutin potentiates calcium uptake via voltage-dependent calcium channel associated with stimulation of glucose uptake in skeletal muscle

Rutin is a flavonoid with several pharmacological properties and it has been demonstrated that rutin can modulate glucose homeostasis. In skeletal muscle, an increase in intracellular calcium concentration may induce glucose transporter-4 (GLUT-4) translocation with consequent glucose uptake. The aim of this study was to investigate the effect of rutin and intracellular pathways on calcium uptake as well as the involvement of calcium in glucose uptake in skeletal muscle. The results show that rutin significantly stimulated calcium uptake through voltage-dependent calcium channels as well as mitogen-activated kinase (MEK) and protein kinase A (PKA) signaling pathways. Also, rutin stimulated glucose uptake in the soleus muscle and this effect was mediated by extracellular calcium and calcium-calmodulin-dependent protein kinase II (CaMKII) activation. In conclusion, rutin significantly stimulates calcium uptake in rat soleus muscles. Furthermore, the increase in intracellular calcium concentration is involved in DNA activation by rutin. Also, rutin-induced glucose uptake via CaMKII may result in GLUT-4 translocation to the plasma membrane, characterizing an insulin-independent pathway. These findings indicate that rutin is a potential drug candidate for diabetes therapy.

Minor contribution of cytosolic Ca2+ transients to the pacemaker rhythm in guinea pig sinoatrial node cells

The question of the extent to which cytosolic Ca(2+) affects sinoatrial node pacemaker activity has been discussed for decades. We examined this issue by analyzing two mathematical pacemaker models, based on the "Ca(2+) clock" (C) and "membrane clock" (M) hypotheses, together with patch-clamp experiments in isolated guinea pig sinoatrial node cells. By applying lead potential analysis to the models, the C mechanism, which is dependent on potentiation of Na(+)/Ca(2+) exchange current via spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) during diastole, was found to overlap M mechanisms in the C model. Rapid suppression of pacemaker rhythm was observed in the C model by chelating intracellular Ca(2+), whereas the M model was unaffected. Experimental rupturing of the perforated-patch membrane to allow rapid equilibration of the cytosol with 10 mM BAPTA pipette solution, however, failed to decrease the rate of spontaneous action potential within ∼30 s, whereas contraction ceased within ∼3 s. The spontaneous rhythm also remained intact within a few minutes when SR Ca(2+) dynamics were acutely disrupted using high doses of SR blockers. These experimental results suggested that rapid disruption of normal Ca(2+) dynamics would not markedly affect spontaneous activity. Experimental prolongation of the action potentials, as well as slowing of the Ca(2+)-mediated inactivation of the L-type Ca(2+) currents induced by BAPTA, were well explained by assuming Ca(2+) chelation, even in the proximity of the channel pore in addition to the bulk cytosol in the M model. Taken together, the experimental and model findings strongly suggest that the C mechanism explicitly described by the C model can hardly be applied to guinea pig sinoatrial node cells. The possible involvement of L-type Ca(2+) current rundown induced secondarily through inhibition of Ca(2+)/calmodulin kinase II and/or Ca(2+)-stimulated adenylyl cyclase was discussed as underlying the disruption of spontaneous activity after prolonged intracellular Ca(2+) concentration reduction for >5 min.

Calmodulin- and Ca2+-dependent facilitation and inactivation of the Cav1.2 Ca2+ channels in guinea-pig ventricular myocytes

The L-type Ca(2+) channel (Ca(V)1.2) shows clear Ca(2+)-dependent facilitation and inactivation. Here we have examined the effects of calmodulin (CaM) and Ca(2+) on Ca(2+) channel in guinea-pig ventricular myocytes in the inside-out patch mode, where rundown of the channels was controlled. At a free [Ca(2+)] of 0.1 microM, CaM (0.15, 0.7, 1.4, 2.1, 3.5, and 7.0 microM) + ATP (2.4 mM) induced channel activities of 27%, 98%, 142%, 222%, 65%, and 20% relative to the control activity, respectively, showing a bell-shaped relationship. Similar results were observed at a free [Ca(2+)] <0.01 microM or with a Ca(2+)-insensitive mutant, CaM(1234), suggesting that apoCaM may induce facilitation and inactivation of the channel activity. The bell-shaped curve of CaM was shifted to the lower concentration side with increasing [Ca(2+)]. A simple model for CaM- and Ca(2+)-dependent modulations of the channel activity, which involves two CaM-binding sites, was proposed. We suggest that both apoCaM and Ca(2+)/CaM can induce facilitation and inactivation of Ca(V)1.2 Ca(2+) channels and that the basic role of Ca(2+) is to accelerate CaM-dependent facilitation and inactivation.

Localised Ca channel phosphorylation modulates the distribution of L-type Ca current in cardiac myocytes

The t-tubule network is central to excitation-contraction coupling in mammalian cardiac ventricular myocytes, with recent studies showing that the majority of Ca influx via the L-type Ca current (I(Ca)) occurs across the t-tubule membrane. The present study investigated whether tonic phosphorylation of the L-type Ca channel is different at the t-tubule and surface membranes, and if this could account for the high density of I(Ca) at the t-tubules. Ventricular myocytes were isolated from male Wistar rats and detubulated using formamide. I(Ca) was recorded using the whole cell patch clamp technique, and Ca transients were recorded using fluo-3 in conjunction with confocal microscopy. The protein kinase A (PKA) inhibitor H-89 (10micromol/L) and the CaMKII inhibitor KN-93 (5micromol/L) decreased the amplitude of I(Ca) in intact cells but had no effect on I(Ca) amplitude in detubulated cells. These inhibitors also decreased the amplitude of the Ca transient in intact cells but not in detubulated cells. Antibody staining for phosphorylated L-type Ca channel showed significantly higher phosphorylation at the t-tubules than at the surface membrane in intact cells. Thus it appears that tonic phosphorylation of the L-type Ca channel maintains the amplitude of I(Ca) and occurs predominantly at the t-tubules. This may have important implications in heart disease, in which changes of phosphorylation and t-tubule density have been reported.

The distinct roles of calmodulin and calmodulin kinase II in the reversal of run-down of L-type Ca(2+) channels in guinea-pig ventricular myocytes

In this study, we investigated the roles of calmodulin kinase II (CaMKII) and calmodulin (CaM) in the reversal of run-down of L-type Ca(2+) channels. Single Ca(2+)-channel activities in guinea-pig ventricular myocytes were recorded using the patch-clamp technique, and run-down of the channel activities was induced by inside-out patch formation in the basic internal solution. At 1 min after patch excision, 1 - 30 muM CaMKII mutant T286D (CaMKIIT286D), a constitutively active type of CaMKII, induced the Ca(2+)-channel activities to only 2% - 10% of that recorded in the cell-attached mode. However, in the presence of CaMKIIT286D, the time-dependent attenuation of CaM's effects in the reversal of run-down was abolished. A GST-fusion protein containing amino acids 1509 - 1789 of the C-terminal region of guinea-pig Cav1.2 (CT1) was prepared. In pull-down assays, CT1 treated with CaMKIIT286D showed a higher affinity for CaM compared with CT1 treated with phosphatase. We propose a model in which CaMKII-mediated phosphorylation of the channels regulates the binding of CaM to the channels in the reversal of run-down of L-type Ca(2+) channels.

Depression of Ca(2+)/calmodulin-dependent protein kinase II in dorsal root ganglion neurons after spinal nerve ligation

The enzyme calcium/calmodulin-dependent protein kinase II (CaMKII) is associated with memory and its alpha isoform is critical for development of activity-induced synaptic changes. Therefore, we hypothesized that CaMKII is involved in altered function of dorsal root ganglion (DRG) neurons after neuronal injury. To test this hypothesis, Sprague-Dawley rats were made hyperalgesic by L5 and L6 spinal nerve ligation (SNL), and changes in total phosphorylated and unphosphorylated CaMKII (tCaMKII) and phosphorylated form of its alpha isoform (pCaMKIIalpha) were analyzed using immunochemistry in different subpopulations of DRG. SNL did not induce any changes in tCaMKII between experimental groups, while the overall percentage of pCaMKIIalpha-positive neurons in injured L5 DRG SNL (24.8%) decreased significantly when compared to control (41.7%). SNL did not change the percentage of pCaMKIIalpha/N52 colabeled neurons but decreased the percentage of N52-negative nonmyelinated neurons that expressed pCaMKIIalpha from 27% in control animals to 11% after axotomy. We also observed a significant decrease in the percentage of small nonpeptidergic neurons labeled with IB4 (37.6% in control vs. 4.0% in L5 SNL DRG), as well as a decrease in the percentage of pCaMKIIalpha/IB4 colabeled neurons in injured L5 DRGs (27% in control vs. 1% in L5 DRG of SNL group). Our results show that reduction in pCaMKIIalpha levels following peripheral injury is due to the loss of IB4-positive neurons. These results indicate that diminished afferent activity after axotomy may lead to decreased phosphorylation of CaMKIIalpha.