Localization of functional endothelin receptor signaling complexes in cardiac transverse tubules

Disruption of transverse-tubular network reduces energy efficiency in cardiac muscle contraction

AIM

Altered organization of the transverse-tubular network is an early pathological event occurring even prior to the onset of heart failure. Such t-tubular remodelling disturbs the synchrony and signalling between membranous and intracellular ion channels, exchangers, receptors and ATPases essential in the dynamics of excitation-contraction coupling, leading to ionic abnormality and mechanical dysfunction in heart disease progression. In this study, we investigated whether a disrupted t-tubular network has a direct effect on cardiac mechano-energetics. Our aim was to understand the fundamental link between t-tubular remodelling and impaired energy metabolism, both of which are characteristics of heart failure. We thus studied healthy tissue preparations in which cellular processes are not altered by any disease event.

METHODS

We exploited the "formamide-detubulation" technique to acutely disrupt the t-tubular network in rat left-ventricular trabeculae. We assessed the energy utilization by cellular Ca²⁺ cycling and by crossbridge cycling, and quantified the change of energy efficiency following detubulation. For these measurements, trabeculae were mounted in a microcalorimeter where force and heat output were simultaneously measured.

RESULTS

Following structural disorganization from detubulation, muscle heat output associated with Ca²⁺ cycling was reduced, indicating impaired intracellular Ca²⁺ homeostasis. This led to reduced force production and heat output by crossbridge cycling. The reduction in force-length work was not paralleled by proportionate reduction in the heat output and, as such, energy efficiency was reduced.

CONCLUSIONS

These results reveal the direct energetic consequences of disrupted t-tubular network, linking the energy disturbance and the t-tubular remodelling typically observed in heart failure.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Subcellular compartmentalization of proximal Gαq-receptor signaling produces unique hypertrophic phenotypes in adult cardiac myocytes

G protein-coupled receptors that signal through Gα_q (G_q receptors), such as α₁-adrenergic receptors (α₁-ARs) or angiotensin receptors, share a common proximal signaling pathway that activates phospholipase Cβ1 (PLCβ1), which cleaves phosphatidylinositol 4,5-bisphosphate (PIP₂) to produce inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol. Despite these common proximal signaling mechanisms, G_q receptors produce distinct physiological responses, yet the mechanistic basis for this remains unclear. In the heart, G_q receptors are thought to induce myocyte hypertrophy through a mechanism termed excitation-transcription coupling, which provides a mechanistic basis for compartmentalization of calcium required for contraction versus IP₃-dependent intranuclear calcium required for hypertrophy. Here, we identified subcellular compartmentalization of G_q-receptor signaling as a mechanistic basis for unique G_q receptor-induced hypertrophic phenotypes in cardiac myocytes. We show that α₁-ARs co-localize with PLCβ1 and PIP₂ at the nuclear membrane. Further, nuclear α₁-ARs induced intranuclear PLCβ1 activity, leading to histone deacetylase 5 (HDAC5) export and a robust transcriptional response (i.e. significant up- or down-regulation of 806 genes). Conversely, we found that angiotensin receptors localize to the sarcolemma and induce sarcolemmal PLCβ1 activity, but fail to promote HDAC5 nuclear export, while producing a transcriptional response that is mostly a subset of α₁-AR-induced transcription. In summary, these results link G_q-receptor compartmentalization in cardiac myocytes to unique hypertrophic transcription. They suggest a new model of excitation-transcription coupling in adult cardiac myocytes that accounts for differential G_q-receptor localization and better explains distinct physiological functions of G_q receptors.

Caged lipids as tools for investigating cellular signaling

Lipid derivatives that can be activated by light, often referred to as 'caged' lipids, are useful tools to manipulate intact cells non-invasively. Here we focus on experimental approaches that have made use of caged lipids. Apart from summarizing the recent advances and available tools in the field, we strive to highlight the experimental challenges that arise from lipid-specific biophysical properties and the abundance of an enormous diversity of distinct molecular lipid species in cells. This article is part of a Special Issue entitled Tools to study lipid functions.

The role of the paracrine/autocrine mediator endothelin-1 in regulation of cardiac contractility and growth

Endothelin-1 (ET-1) is a critical autocrine and paracrine regulator of cardiac physiology and pathology. Produced locally within the myocardium in response to diverse mechanical and neurohormonal stimuli, ET-1 acutely modulates cardiac contractility. During pathological cardiovascular conditions such as ischaemia, left ventricular hypertrophy and heart failure, myocyte expression and activity of the entire ET-1 system is enhanced, allowing the peptide to both initiate and maintain maladaptive cellular responses. Both the acute and chronic effects of ET-1 are dependent on the activation of intracellular signalling pathways, regulated by the inositol-trisphosphate and diacylglycerol produced upon activation of the ET(A) receptor. Subsequent stimulation of protein kinases C and D, calmodulin-dependent kinase II, calcineurin and MAPKs modifies the systolic calcium transient, myofibril function and the activity of transcription factors that coordinate cellular remodelling. The precise nature of the cellular response to ET-1 is governed by the timing, localization and context of such signals, allowing the peptide to regulate both cardiomyocyte physiology and instigate disease.

LINKED ARTICLES

This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Modulation of distinct isoforms of L-type calcium channels by G(q)-coupled receptors in Xenopus oocytes: antagonistic effects of Gβγ and protein kinase C

L-type voltage dependent Ca(2+) channels (L-VDCCs; Ca(v)1.2) are crucial in cardiovascular physiology. In heart and smooth muscle, hormones and transmitters operating via G(q) enhance L-VDCC currents via essential protein kinase C (PKC) involvement. Heterologous reconstitution studies in Xenopus oocytes suggested that PKC and G(q)-coupled receptors increased L-VDCC currents only in cardiac long N-terminus (NT) isoforms of α(1C), whereas known smooth muscle short-NT isoforms were inhibited by PKC and G(q) activators. We report a novel regulation of the long-NT α(1C) isoform by Gβγ. Gβγ inhibited whereas a Gβγ scavenger protein augmented the G(q)--but not phorbol ester-mediated enhancement of channel activity, suggesting that Gβγ acts upstream from PKC. In vitro binding experiments reveal binding of both Gβγ and PKC to α(1C)-NT. However, PKC modulation was not altered by mutations of multiple potential phosphorylation sites in the NT, and was attenuated by a mutation of C-terminally located serine S1928. The insertion of exon 9a in intracellular loop 1 rendered the short-NT α(1C) sensitive to PKC stimulation and to Gβγ scavenging. Our results suggest a complex antagonistic interplay between G(q)-activated PKC and Gβγ in regulation of L-VDCC, in which multiple cytosolic segments of α(1C) are involved.

PKC-ε mediates multiple endothelin-1 actions on systolic Ca2+ and contractility in ventricular myocytes

Endothelin-1 (ET-1) induces positive inotropy (enhanced contractility) in cardiac muscle, but establishing underlying cellular mechanisms has been controversial in part because of a growing number of signaling pathways and end effectors targeted by ET-1. Here we present evidence that ET-1 induces positive inotropism in ventricular tissue by increasing both systolic Ca2+ and myofilament Ca2+ sensitivity. To examine the roles of PKC-δ and PKC-ε in these acute responses to ET-1, kinase inactive dominant negative PKC (dn-PKC) constructs were expressed in adult rat ventricular myocytes. Yellow fluorescent protein (YFP) was fused to dn-PKC constructs to visualize expression and localization of dn-PKC in living myocytes. Due to an alanine to glutamate mutation in the pseudosubstrate site, dn-PKCs constitutively translocated to anchoring sites and were unaffected by agonist or phorbol ester treatment. Dn-PKC-δ-YFP mainly distributed at Z-lines and at intercalated disks in adult myocytes, whereas dn-PKC-ε-YFP stained the surface sarcolemma, T-tubules/Z-lines and perinuclear region. Myocytes expressing dn-PKC-δ-YFP showed normal systolic Ca2+ and contractile responses to ET-1. In contrast, the entire ensemble of ET-1 responses was blocked in myocytes expressing dn-PKC-ε-YFP including increased Ca2+ transients, enhanced myofilament Ca2+ sensitivity, and positive inotropy. This report provides direct evidence that PKC-ε is activated early and robustly following ET-1 stimulation and thus mediates multiple intracellular changes underlying the acute actions of ET-1 on myocardium.

GATA4 expression is primarily regulated via a miR-26b-dependent post-transcriptional mechanism during cardiac hypertrophy

AIMS

GATA4 is a transcription factor that is up-regulated during cardiac hypertrophy and plays a fundamental role in myocyte growth and survival. In this study, we investigate the transcriptional vs. post-transcriptional mechanisms that are involved in regulating GATA4 in the heart during neonatal and pressure overload-induced hypertrophic growth.

METHODS AND RESULTS

GATA4 protein is significantly higher during pressure overload-induced (2.9 ± 0.4-fold) and neonatal (6.8 ± 1-fold) hypertrophic growth vs. the normal adult mouse heart. Using RNA polymerase II immunoprecipitation combined with deep sequencing, we confirmed that active transcription of the Gata4 gene remained unchanged during hypertrophy, whereas it was two-fold higher in the neonatal vs. adult heart, commensurate with the mRNA levels. These results suggested a post-transcriptional mode of regulation of its expression, which prompted the identification of a conserved sequence in its 3'-untranslated region that was responsible for reduced translation via miR-26b. Overexpression of miR-26b reduced GATA4-dependent transcription, endothelin-induced hypertrophy, and sensitized the cells to apoptotic insults. Additionally, miR-26b targeted phospholipase C-β1, which, in turn, inhibited miR-26b expression, creating a double-negative feedback loop. Accordingly, overexpression of miR-26b in the heart inhibited up-regulation of its targets and the development of hypertrophy. However, knockdown of miR-26b is not sufficient for inducing hypertrophy.

CONCLUSION

Down-regulation of miR-26b in the heart is required for the up-regulation of GATA4 and the induction of pressure-induced cardiac hypertrophy. The results also underscore the functional relevance of miRNAs in regulating gene expression during cardiac hypertrophy.

Myofibrillogenesis in live neonatal cardiomyocytes observed with hybrid two-photon excitation fluorescence-second harmonic generation microscopy

We developed a hybrid two-photon excitation fluorescence-second harmonic generation (TPEF-SHG) imaging system with an on-stage incubator for long-term live-cell imaging. Using the imaging system, we observed the addition of new sarcomeres during myofibrillogenesis while a cardiomyocyte was spreading on the substrate. The results suggest that the TPEF-SHG imaging system with an on-stage incubator is an effective tool for investigation of dynamic myofibrillogenesis.

Synthetic caged DAG-lactones for photochemically controlled activation of protein kinase C

Switching on kinases: Synthetic caged DAG-lactones have been developed and showed decreases of two orders of magnitude, relative to the corresponding parent compounds, in their binding affinities towards PKC. The caged compounds had no effect on the translocation of PKC until after photoactivation. This approach is a potentially powerful tool for probing the PKC signaling cascade.

Structural evidence for perinuclear calcium microdomains in cardiac myocytes

At each heartbeat, cardiac myocytes are activated by a cytoplasmic Ca(2+) transient in great part due to Ca(2+) release from the sarcoplasmic reticulum via ryanodine receptors (RyRs) clustered within calcium release units (peripheral couplings/dyads). A Ca(2+) transient also occurs in the nucleoplasm, following the cytoplasmic transient with some delay. Under conditions where the InsP3 production is stimulated, these Ca(2+) transients are regulated actively, presumably by an additional release of Ca(2+) via InsP3 receptors (InsP3Rs). This raises the question whether InsP3Rs are appropriately located for this effect and whether sources of InsP3 and Ca(2+) are available for their activation. We have defined the structural basis for InsP3R activity at the nucleus, using immunolabeling for confocal microscopy and freeze-drying/shadowing, T tubule "staining" and thin sectioning for electron microscopy. By these means we establish the presence of InsP3R at the outer nuclear envelope and show a close spatial relationship between the nuclear envelope, T tubules (a likely source of InsP3) and dyads (the known source of Ca(2+)). The frequency, distribution and distance from the nucleus of T tubules and dyads appropriately establish local perinuclear Ca(2+) microdomains in cardiac myocytes.

Role of endothelin-1 receptor blockers on hemodynamic parameters and oxidative stress

Endothelin (ET) was first isolated and described by Yanagisawa et al. and has since been described as one of the most potent known vasoconstrictor compounds. ET-1 mediates its effects via two types of receptors, ETA and ETB, which are expressed in the vascular smooth muscle cells, endothelial cells, intestines and brain. Secretion of ET-1 results in long-lasting vasoconstriction, increased blood pressure and, in turn, overproduction of free radicals. As dysregulation of the endothelin system is an important factor in the pathogenesis of several diseases including atherosclerosis, hypertension and endotoxic shock, the ETA and ETB receptors are attractive therapeutic targets for treatment of these disorders. The biosynthesis and release of ET-1 are regulated at the transcriptional level. Studies have shown that p38MAP kinase, nuclear factor kappaB (NF-kappaB), PKC/ERK and JNK/c-Jun all take part in the ROS-activated production of ET-1. Furthermore, administration of ET(A) significantly reduces the generation of free radicals. However, treatment with ETB receptor blockers does not elicit the same effect. Therefore, the effects of endothelin receptor blockers on blood pressure and the generation of free radicals remain debatable. This review summarizes recent investigations into the role of endothelin receptor blockers with respect to the modulation of hemodynamic parameters and the generation of free radicals.

Role of Ca(2+)/calmodulin-dependent protein kinase II in the regulation of the cardiac L-type Ca(2+) current during endothelin-1 stimulation

Endothelin-1 (ET-1) shows a positive inotropic effect on cardiac muscle. Although the L-type Ca(2+) current (I(Ca)) is one of the important determinants of cardiac excitation-contraction coupling, the effect of ET-1 on the I(Ca) is not always clear. The controversial results appear to be due to different patch-clamp methods. The present study measured the effect of ET-1 on the I(Ca) of rat ventricular myocytes using the perforated patch-clamp technique. The holding potential was set to -40 mV, and depolarization was applied every 10 s. ET-1 (10 nM) increased the I(Ca) in a monophasic manner. The current reached a steady state 15 min after the application of ET-1, when the measurement was done. Endothelin receptor subtype expression was also investigated using Western immunoblotting. ET(A)-receptor protein was expressed, but ET(B)-receptor protein was not expressed, in the cell membranes of rat ventricular myocytes. The effect of ET-1 on the I(Ca) was inhibited by a selective ET(A)-receptor antagonist, BQ-123, but not by a selective ET(B)-receptor antagonist, BQ-788. The effect was inhibited by protein kinase C (PKC) inhibitor chelerythrine and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93, but not by its inactive analog KN-92. The effect of ET-1 was also blocked by another CaMKII inhibitor, autocamtide-2-related inhibitory peptide. These results suggest that ET-1 increases the I(Ca) via the ET(A)-receptor-PKC-CaMKII pathway.

The cardiovascular physiology and pharmacology of endothelin-1

One year after the discovery in 1980 that the endothelium was obligatory for acetylcholine to relax isolated arteries, it was clearly shown that the endothelium could also promote contraction. In 1988, Dr Yanagisawa's group identified endothelin-1 (ET-1) as the first endothelium-derived contracting factor. The circulating levels of this short (21 amino acids) peptide were quickly determined in humans and it was reported that in most cardiovascular diseases, circulating levels of ET-1 were increased and ET-1 was then recognized as a likely mediator of pathological vasoconstriction in human. The discovery of two receptor subtypes in 1990, ET(A) and ET(B), permitted optimization of bosentan, which entered clinical development in 1993, and was offered to patients with pulmonary arterial hypertension in 2001. In this report, we discuss the physiological and pathophysiological role of endothelium-derived ET-1, the pharmacology of its two receptors, focusing on the regulation of the vascular tone and as much as possible in humans. The coronary bed will be used as a running example, but references to the pulmonary, cerebral, and renal circulation will also be made. Many of the cardiovascular complications associated with aging and cardiovascular risk factors are initially attributable, at least in part, to endothelial dysfunction, particularly dysregulation of the vascular function associated with an imbalance in the close interdependence of NO and ET-1, in which the implication of the ET(B) receptor may be central.

Protein kinase C isoforms differentially phosphorylate Ca(v)1.2 alpha(1c)

The regulation of Ca(2+) influx through the phosphorylation of the L-type Ca(2+) channel, Ca(v)1.2, is important for the modulation of excitation-contraction (E-C) coupling in the heart. Ca(v)1.2 is thought to be the target of multiple kinases that mediate the signals of both the renin-angiotensin and sympathetic nervous systems. Detailed biochemical information regarding the protein phosphorylation reactions involved in the regulation of Ca(v)1.2 is limited. The protein kinase C (PKC) family of kinases can modulate cardiac contractility in a complex manner, such that contractility is either enhanced or depressed and relaxation is either accelerated or slowed. We have previously reported that Ser(1928) in the C-terminus of alpha(1c) was a target for PKCalpha, -zeta, and -epsilon phosphorylation. Here, we report the identification of seven PKC phosphorylation sites within the alpha(1c) subunit. Using phospho-epitope specific antibodies to Ser(1674) and Ser(1928), we demonstrate that both sites within the C-terminus are phosphorylated in HEK cells in response to PMA. Phosphorylation was inhibited with a PKC inhibitor, bisindolylmaleimide. In Langendorff-perfused rat hearts, both Ser(1674) and Ser(1928) were phosphorylated in response to PMA. Phosphorylation of Ser(1674), but not Ser(1928), is PKC isoform specific, as only PKCalpha, -betaI, -betaII, -gamma, -delta, and -theta, but not PKCepsilon, -zeta, and -eta, were able to phosphorylate this site. Our results identify a molecular mechanism by which PKC isoforms can have different effects on channel activity by phosphorylating different residues.

Nonlinear optical imaging of cellular processes in breast cancer

Nonlinear optical imaging techniques such as multiphoton and second harmonic generation (SHG) microscopy used in conjunction with novel signal analysis techniques such as spectroscopic and fluorescence excited state lifetime detection have begun to be used widely for biological studies. This is largely due to their promise to noninvasively monitor the intracellular processes of a cell together with the cell's interaction with its microenvironment. Compared to other optical methods these modalities provide superior depth penetration and viability and have the additional advantage in that they are compatible technologies that can be applied simultaneously. Therefore, application of these nonlinear optical approaches to the study of breast cancer holds particular promise as these techniques can be used to image exogeneous fluorophores such as green fluorescent protein as well as intrinsic signals such as SHG from collagen and endogenous fluorescence from nicotinamide adenine dinucleotide or flavin adenine dinucleotide. In this article the application of multiphoton excitation, SHG, and fluorescence lifetime imaging microscopy to relevant issues regarding the tumor-stromal interaction, cellular metabolism, and cell signaling in breast cancer is described. Furthermore, the ability to record and monitor the intrinsic fluorescence and SHG signals provides a unique tool for researchers to understand key events in cancer progression in its natural context.

Interaction of α 1 -Adrenoceptor Subtypes With Different G Proteins Induces Opposite Effects on Cardiac L-type Ca 2+ Channel

We examined the effect of α 1 -adrenoceptor subtype-specific stimulation on L-type Ca 2+ current ( I Ca ) and elucidated the subtype-specific intracellular mechanisms for the regulation of L-type Ca 2+ channels in isolated rat ventricular myocytes. We confirmed the protein expression of α 1A - and α 1B -adrenoceptor subtypes at the transverse tubules (T-tubules) and found that simultaneous stimulation of these 2 receptor subtypes by nonsubtype selective agonist, phenylephrine, showed 2 opposite effects on I Ca (transient decrease followed by sustained increase). However, selective α 1A -adrenoceptor stimulation (≥0.1 μmol/L A61603) only potentiated I Ca , and selective α 1B -adrenoceptor stimulation (10 μmol/L phenylephrine with 2 μ mol/L WB4101) only decreased I Ca . The positive effect by α 1A -adrenoceptor stimulation was blocked by the inhibition of phospholipase C (PLC), protein kinase C (PKC), or Ca 2+ /calmodulin-dependent protein kinase II (CaMKII). The negative effect by α 1B -adrenoceptor stimulation disappeared after the treatment of pertussis toxin or by the prepulse depolarization, but was not attriburable to the inhibition of cAMP-dependent pathway. The translocation of PKCδ and ε to the T-tubules was observed only after α 1A -adrenoceptor stimulation, but not after α 1B -adrenoceptor stimulation. Immunoprecipitaion analysis revealed that α 1A -adrenoceptor was associated with G q/11 , but α 1B -adrenoceptor interacted with one of the pertussis toxin-sensitive G proteins, G o . These findings demonstrated that the interactions of α 1 -adrenoceptor subtypes with different G proteins elicit the formation of separate signaling cascades, which produce the opposite effects on I Ca . The coupling of α 1A -adrenoceptor with G q/11 -PLC-PKC-CaMKII pathway potentiates I Ca . In contrast, α 1B -adrenoceptor interacts with G o , of which the βγ-complex might directly inhibit the channel activity at T-tubules.

Chronic ethanol intake modulates vascular levels of endothelin-1 receptor and enhances the pressor response to endothelin-1 in anaesthetized rats

BACKGROUND AND PURPOSE

The contribution of endothelin-1 (ET-1) to vascular hyper-reactivity associated with chronic ethanol intake, a major risk factor in several cardiovascular diseases, remains to be investigated.

EXPERIMENTAL APPROACH

The biphasic haemodynamic responses to ET-1 (0.01-0.1 nmol kg(-1), i.v.) or to the selective ETB agonist, IRL1620 (0.001-1.0 nmol kg(-1), i.v.), with or without ETA or ETB antagonists (BQ123 (c(DTrp-Dasp-Pro-Dval-Leu)) at 1 and 2.5 mg kg(-1) and BQ788 (N-cis-2,6-dimethyl-piperidinocarbonyl-L-gamma-methylleucyl1-D-1methoxycarbonyltryptophanyl-D-norleucine) at 0.25 mg kg(-1), respectively) were tested in anaesthetized rats, after 2 weeks' chronic ethanol treatment. Hepatic parameters and ET receptor protein levels were also determined.

KEY RESULTS

The initial hypotensive responses to ET-1 or IRL1620 were unaffected by chronic ethanol intake, whereas the subsequent pressor effects induced by ET-1, but not by IRL1620, were potentiated. BQ123 at 2.5 but not 1 mg kg(-1) reduced the pressor responses to ET-1 in ethanol-treated rats. Conversely, BQ788 (0.25 mg kg(-1)) potentiated ET-1-induced increases in mean arterial blood pressure in control as well as in ethanol-treated rats. Interestingly, in the latter group, increases in heart rate, induced by ET-1 at a dose of 0.025 mg kg(-1) were enhanced following ETB receptor blockade. Finally, we observed higher levels of ETA receptor in the heart and mesenteric artery and a reduction of ETB receptor protein levels in the aorta and kidney from rats chronically treated with ethanol.

CONCLUSIONS AND IMPLICATIONS

Increased vascular reactivity to ET-1 and altered protein levels of ETA and ETB receptors could play a role in the pathogenesis of cardiovascular complications associated with chronic ethanol consumption.

Contractile regulation by overexpressed ETArequires intact T tubules in adult rat ventricular myocytes

Endothelin (ET)-1 regulates the contractility and growth of the heart by binding G protein-coupled receptors of the ET type A receptor (ETA)/ET type B (ETB) receptor family. ETA, the predominant ET-1 receptor subtype in myocardium, is thought to localize preferentially within cardiac T tubules, but the consequences of mislocalization are not fully understood. Here we examined the effects of the overexpression of ETAin conjunction with T-tubule loss in cultured adult rat ventricular myocytes. In adult myocytes cultured for 3 to 4 days, the normally robust positive inotropic effect (PIE) of ET-1 was lost in parallel with T-tubule degeneration and a decline in ETAprotein levels. In these T tubule-compromised myocytes, an overexpression of ETAusing an adenoviral vector did not rescue the responsiveness to ET-1, despite the robust expression in the surface sarcolemma. The inclusion of the actin polymerization inhibitor cytochalasin D (CD) during culture prevented gross morphological changes including a loss of T tubules and a rounding of intercalated discs, but CD alone did not rescue the responsiveness to ET-1 or prevent ETAdownregulation. The rescue of a normal PIE in 3- to 4-day cultured myocytes required both an increased expression of ETAand intact T tubules (preserved with CD). Therefore, the activation of ETAlocalized in T tubules was associated with a strong PIE, whereas the activation of ETAin surface sarcolemma was not. The results provide insight into the pathological cardiac conditions in which ETAis upregulated and T-tubule morphology is altered.

Enigma homolog 1 scaffolds protein kinase D1 to regulate the activity of the cardiac L-type voltage-gated calcium channel

AIMS

In cardiomyocytes, protein kinase D1 (PKD1) plays a central role in the response to stress signals. From a yeast two-hybrid assay, we have identified Enigma Homolog 1 (ENH1) as a new binding partner of PKD1. Since in neurons, ENH1, associated with protein kinase Cepsilon, was shown to modulate the activity of N-type calcium channels, and the pore-forming subunit of the cardiac L-type voltage-gated calcium channel, alpha1C, possesses a potential phosphorylation site for PKD1, we studied here a possible role of ENH1 and PKD1 in the regulation of the cardiac L-type voltage-gated calcium channel.

METHODS AND RESULTS

PKD1-interacting proteins were searched by yeast two-hybrid screening. In vivo protein interactions in cardiomyocytes isolated from heart ventricles of newborn rats were tested by co-immunoprecipitation. Small interfering RNA and a dominant negative mutant of PKD1 were delivered into cardiomyocytes by use of an adenovirus. Calcium currents were measured by the patch-clamp technique. Both ENH1 and PKD1 interact with alpha1C in cardiomyocytes. This interaction is increased upon stimulation. Silencing of ENH1 prevented the binding of PKD1 to alpha1C. Moreover, a dominant negative mutant of PKD1 or the silencing of ENH1 inhibited the alpha-adrenergic-induced increase of L-type calcium currents.

CONCLUSION

We found a new binding partner, ENH1, and a new target, alpha1C, for PKD1 in neonatal rat cardiomyocytes. We propose a model where ENH1 scaffolds PKD1 to alpha1C in order to form a signalling complex that regulates the activity of cardiac L-type voltage-gated Ca(2+) channels.

Overexpression of diacylglycerol kinase zeta inhibits endothelin-1-induced decreases in Ca2+ transients and cell shortening in mouse ventricular myocytes

Endothelin-1 (ET-1) is released in various cardiovascular disorders including congestive heart failure, and may modulate significantly the disease process by its potent action on vascular and cardiac muscle cell function and gene regulation. In adult mouse ventricular cardiomyocytes loaded with indo-1, ET-1 induced a sustained negative inotropic effect (NIE) in association with decreases in Ca(2+) transients. The ET-1-induced effects on Ca(2+) transients and cell shortening were abolished in diacylglycerol (DAG) kinase zeta-overexpressing mouse ventricular myocytes. A nonselective protein kinase C (PKC) inhibitor, GF109203X, inhibited the ET-1-induced decreases in Ca(2+) transients and cell shortening in concentration-dependent manners, whereas a selective Ca(2+)-dependent PKC inhibitor, Gö6976, did not affect the ET-1-induced effects. A phospholipase Cbeta inhibitor, U73122, and an inhibitor of phospholipase D, C(2)-ceramide, partially, but significantly, attenuated the ET-1-induced effects. Derivatives of the respective inhibitors with no specific effects, U73343 and dihydro-C(2)-ceramide, did not affect the ET-1-induced effects. Taken together, these results indicate that activation of a Ca(2+)-independent PKC isozyme by 1,2-DAG, which is generated by phospholipase Cbeta and phospholipase D activation and inactivated by phosphorylation via DAG kinase, is responsible for the ET-1-induced decreases in Ca(2+) transients and cell shortening in mouse ventricular cardiomyocytes.

Dynamic expression patterns of leucine-rich repeat containing protein 10 in the heart

Leucine-rich repeat containing protein 10 (LRRC10) is a heart-specific factor whose function remains unknown. Examination of the intracellular location of the gene products is a critical step in determining the biological functions of the protein. Our expression analyses in mice indicate that LRRC10 is exclusively expressed from the precardiac region in early embryos to the adult heart. LRRC10 expression is markedly elevated upon birth, suggesting its role in the embryonic as well as adult hearts. Of interest, LRRC10 exhibits dynamic intracellular expression patterns in cardiomyocytes. Cardiomyocytes from embryos and newborns show diffuse cytoplasmic and nuclear staining of LRRC10. In contrast, striking striations are observed in adult cardiomyocytes, which are colocalized with the markers for the Z-line, sarcoplasmic reticulum (SR), and transverse (T)-tubule by double immunostaining. Further investigation by electron micrographs places LRRC10 in a diad region where the SR interacts with the T-tubule that locates along the Z-line.

G-protein coupled receptor signaling in myocardium: not for the faint of heart

Catecholamines, endothelin-1 and angiotensin II are among a diverse group of diffusible extracellular signals that regulate pump function of the heart by binding to G-protein coupled receptors (GPCR). When the body demands a temporary boost of power output or if temporary budgeting of resources is required, these signals can adjust heart rate and contractile strength to maintain continuous perfusion of all vascular beds with nutrient- and oxygen-rich blood. Given adequate time in the face of prolonged challenges, activation of GPCRs can also promote "remodeling of the heart" by increasing cell size, organ size, and chamber dimensions, or by varying tissue composition and altering the expression of protein isoforms controlling excitability and contractility. A common feature of heart disease is the state of chronic activation of GPCR signaling systems. Paradoxically, whereas acute activation is beneficial, chronic activation often contributes to further deterioration of cardiac performance. A better understanding of how chronic GPCR activation contributes to the development of heart disease is needed so that it can be translated into better prevention and therapeutic strategies in the clinic.

Interaction and inhibitory cross-talk between endothelin and ErbB receptors in the adult heart

Endothelin-1 (ET-1) regulates contractility and growth of the mammalian heart by binding endothelin receptor type A (ET(A)) and endothelin receptor type B (ET(B)) G-protein-coupled receptors. To identify growth signaling pathways associated with ET-1 receptors in adult myocardium, a combined immunoprecipitation/proteomic analysis was performed. Signaling proteins believed to function downstream of ET(A) such as Galpha(q), phospholipase C-beta1, protein kinase C (PKC) epsilon, and PKCdelta were identified in immunoprecipitates of ET(A) by matrix-assisted laser desorption ionization/time of flight mass spectrometry. Also prominent were the growth factor receptor tyrosine kinases erbB2 and erbB4 and their downstream growth signaling effectors phosphoinositide-3 kinase (PI3 kinase), Akt, Raf-1, mitogen-activated protein kinase kinase (MEK), and extracellular signal-regulated kinase (Erk). Western blot analysis confirmed coimmunoprecipitation of erbB2/4, PI3 kinase, and Akt with ET(A), and confocal microscopy revealed their colocalization in cardiac transverse tubules (T-tubules). The erbB4 receptor ligand neuregulin-1beta (NRG1beta) promoted erbB2/4 tryosine phosphorylation and Akt serine phosphorylation in ventricular myocytes, whereas treatment with ET-1 did not. This observation argues against ET-1 growth signaling occurring via erbB2/4 transactivation in adult myocardium. ET-1 did, however, stimulate Erk1/2 phosphorylation and substantially blunted several NRG1beta-mediated actions, including erbB2/4 phosphorylation, serine phosphorylation of Akt, and negative inotropy. This inhibitory cross-talk between ET(A) and erbB2/4-Akt pathways was mimicked by a phorbol ester and blocked by pharmacological inhibition of PKC or MEK/Erk. The proteomic analysis and subsequent investigation of receptor cross-talk indicate that growth signaling between ET(A) and erbB pathways is fundamentally different in adult versus neonatal cardiac myocytes. The results may be relevant to cardiomyopathies associated with 1) prolonged exposure to ET-1; 2) degeneration of T-tubules; and 3) therapies targeted at erbB2 inhibition.

Protective role of protein kinase C epsilon activation in ischemia-reperfusion arrhythmia

PURPOSE

Ischemic heart disease carries an increased risk of malignant ventricular tachycardia (VT), fibrillation (VF), and sudden cardiac death. Protein kinase C (PKC) epsilon activation has been shown to improve the hemodynamics in hearts subjected to ischemia/reperfusion. However, very little is known about the role of epsilon PKC in reperfusion arrhythmias. Here we show that epsilon PKC activation is anti-arrhythmic and its inhibition is pro-arrhythmic.

METHOD

Langendorff-perfused isolated hearts from epsilonPKC agonist (epsilonPKC activation), antagonist (epsilonPKC inhibition) transgenic (TG), and wild-type control mice were subjected to 30 min stabilization period, 10 min global ischemia, and 30 min reperfusion. Action potentials (APs) and calcium transients (CaiT) were recorded simultaneously at 37 degrees C using optical mapping techniques. The incidence of VT and VF was assessed during reperfusion.

RESULTS

No VT/VF was seen in any group during the stabilization period in which hearts were perfused with Tyrode's solution. Upon reperfusion, 3 out of the 16 (19%) wild-type mice developed VT but no VF. In epsilonPKC antagonist group, in which epsilonPKC activity was downregulated, 10 out of 13 (76.9%) TG mice developed VT, of which six (46.2%) degenerated into sustained VF upon reperfusion. Interestingly, in epsilonPKC agonist mice, in which the activity of epsilonPKC was upregulated, no VF was observed and only 1 out of 12 mice showed only transient VT during reperfusion. During ischemia and reperfusion, CaiT decay was exceedingly slower in the antagonist mice compared to the other two groups.

CONCLUSION

Moderate in vivo activation of epsilonPKC exerts beneficial antiarrhythmic effect vis-a-vis the lethal reperfusion arrhythmias. Abnormal CaiT decay may, in part, contribute to the high incidence of reperfusion arrhythmias in the antagonist mice. These findings have important implications for the development of PKC isozyme targeted therapeutics and subsequently for the treatment of ischemic heart diseases.

Inositol 1,4,5-trisphosphate supports the arrhythmogenic action of endothelin-1 on ventricular cardiac myocytes

Although ventricular cardiomyocytes express inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptors, it is unclear how these Ca2+ channels contribute to the effects of Gq-coupled agonists. Endothelin-1 augmented the amplitude of pacing-evoked Ca2+ signals (positive inotropy), and caused an increasing frequency of spontaneous diastolic Ca2+-release transients. Both effects of endothelin-1 were blocked by an antagonist of phospholipase C, suggesting that Ins(1,4,5)P3 and/or diacylglycerol production was necessary. The endothelin-1-mediated spontaneous Ca2+ transients were abolished by application of 2-aminoethoxydiphenyl borate (2-APB), an antagonist of Ins(1,4,5)P3 receptors. Incubation of electrically-paced ventricular myocytes with a membrane-permeant Ins(1,4,5)P3 ester provoked the occurrence of spontaneous diastolic Ca2+ transients with the same characteristics and sensitivity to 2-APB as the events stimulated by endothelin-1. In addition to evoking spontaneous Ca2+ transients, stimulation of ventricular myocytes with the Ins(1,4,5)P3 ester caused a positive inotropic effect. The effects of endothelin-1 were compared with two other stimuli, isoproterenol and digoxin, which are known to induce inotropy and spontaneous Ca2+ transients by overloading intracellular Ca2+ stores. The events evoked by isoproterenol and digoxin were dissimilar from those triggered by endothelin-1 in several ways. We propose that Ins(1,4,5)P3 receptors support the development of both inotropy and spontaneous pro-arrhythmic Ca2+ signals in ventricular myocytes stimulated with a Gq-coupled agonist.

Endothelin-1-dependent leptin induction in gastric mucosal inflammatory responses to Helicobacter pylori lipopolysaccharide

Leptin, a multifunctional hormone that regulates food intake and energy expenditure, has emerged recently as an important modulator of gastric mucosal responses to Helicobacter pylori infection. We applied the animal model of H. pylori LPS-induced gastritis to investigate the role of endothelin-1 (ET-1) in the mucosal leptin production. We show that the histologic pattern of inflammation reached a maximum on the fourth day following the LPS and was reflected in a marked increase in the mucosal level of ET-1 and leptin. Therapeutic administration of phosphoramidon, an inhibitor of ECE-1 activity, led to a 61.2% decline in the mucosal ET-1 level and a 64.1% reduction in leptin, while the severity of mucosal inflammatory involvement increased by 28.6%. A drop in the level of leptin and the increase in severity of the inflammatory involvement elicited by the LPS was also attained in the presence of ET(A) receptor antagonist BQ610, but not the ET(B) receptor antagonist BQ788. Moreover, administration of ERK inhibitor, PD98059, in the presence of ET(B) receptor antagonist, but not the ET(A) receptor antagonist, caused reduction in the mucosal leptin level. Our findings are the first to implicate ET-1 as a key factor in up-regulation of gastric mucosal leptin-associated H. pylori infection. We also show that the effect of ET-1 on leptin production is a consequence of ET(A) receptor activation.

Regulation of cardiac inwardly rectifying potassium current IK1 and Kir2.x channels by endothelin-1

To elucidate the ionic mechanism of endothelin-1 (ET-1)-induced focal ventricular tachyarrhythmias, the regulation of I(K1) and its main molecular correlates, Kir2.1, Kir2.2 and Kir2.3 channels, by ET-1 was investigated. Native I(K1) in human atrial cardiomyocytes was studied with whole-cell patch clamp. Human endothelin receptors were coexpressed with human Kir2.1, Kir2.2 and Kir2.3 channels in Xenopus oocytes. Currents were measured with a two-microelectrode voltage clamp. In human cardiomyocytes, ET-1 induced a marked inhibition of I(K1) that could be suppressed by the protein kinase C (PKC) inhibitor staurosporine. To investigate the molecular mechanisms underlying this regulation, we studied the coupling of ET(A) receptors to homomeric and heteromeric Kir2.1, Kir2.2 and Kir2.3 channels in the Xenopus oocyte expression system. ET(A) receptors coupled functionally to Kir2.2 and Kir2.3 channels but not to Kir2.1 channels. In Kir2.2 channels lacking functional PKC phosphorylation sites, the inhibitory effect was abolished. The inhibition of Kir2.3 currents could be suppressed by the PKC inhibitors staurosporine and chelerythrine. The coupling of ET(A) receptors to heteromeric Kir2.1/Kir2.2 and Kir2.2/Kir2.3 channels resulted in a strong inhibition of currents comparable with the effect observed in Kir2.2 homomers. Surprisingly, in heteromeric Kir2.1/Kir2.3 channels, no effect was observed. ET-1 inhibits human cardiac I(K1) current via a PKC-mediated phosphorylation of Kir2.2 channel subunits and additional regulatory effects on Kir2.3 channels. This mechanism may contribute to the intrinsic arrhythmogenic potential of ET-1.

Endothelin-1-dependent up-regulation of leptin production in gastric mucosal injury by indomethacin

Leptin, a multifunctional hormone that regulates food intake and metabolic and endocrine responses, has emerged recently as an important modulatory factor in gastric mucosal resistance to injury. In this study, we applied the animal model of gastric mucosal injury caused by indomethacin to investigate the role of endothelin-1 (ET-1) in the mucosal leptin production. Using groups of rats subjected to intragastric administration of indomethacin (at 0-60 mg/kg), we show that gastric mucosal damage reached a maximum 4 h following the drug, and was accompanied by a marked elevation (up to 3.5-fold) in the mucosal leptin level, up to 4-fold enhancement in the expression of endothelin-converting enzyme-1 (ECE-1) activity and up to 4.5-fold increase in ET-1 generation. Pretreatment with phosphoramidon, an inhibitor of ECE-1 activity, not only led to a decline in ECE-1 and ET-1 generation, but also produced a dose-dependent reduction in the mucosal level of leptin and the extent of mucosal damage caused by indomethacin. This effect of phosphoramidon, however, was subject to suppression by the exogenous ET-1 administration. Moreover, a marked drop in the mucosal level of leptin and the reduction in the severity of mucosal damage was attained following pretreatment with ET(A) receptor antagonist BQ610, but not by ET(B) receptor antagonist BQ788. The results implicate ET-1 as a key factor in the regulation of leptin production associated with gastric mucosal response to injury, and show that the stimulatory effect of ET-1 on leptin production occurs via ET(A) receptor activation.

Up-regulation in endothelin-1 by Helicobacter pylori lipopolysaccharide interferes with gastric mucin synthesis via epidermal growth factor receptor transactivation

OBJECTIVE

Endothelin-1 (ET-1), a key mediator of inflammatory processes associated with bacterial infection, is a 21-amino acid peptide produced from a biologically inactive big ET-1 by the action of endothelin-converting enzyme-1 (ECE-1) that acts through G protein-coupled ET(A) and ET(B) receptors. Here we report on the role of ET-1 in the mediation of the detrimental influence of Helicobacter pylori on the synthesis of gastric mucin.

MATERIAL AND METHODS

Rat gastric mucosal cells were exposed to H. pylori key virulence factor, lipopolysaccharide (LPS).

RESULTS

The LPS inhibitory effect on gastric mucin synthesis was accompanied by a marked increase in ET-1 generation and enhancement in ECE-1 activity. Inhibition of ECE-1 with phosphoramidon not only led to the impedance of LPS-induced ET-1 generation, but also countered the detrimental effect of LPS on mucin synthesis. Moreover, the LPS inhibitory effect on mucin synthesis was blocked by ET(A) receptor antagonist BQ610, but not by ET(B) receptor antagonist BQ788. Furthermore, the LPS-induced suppression in gastric mucin synthesis was countered in a concentration-dependent fashion by PD153035 (81.7%), a specific inhibitor of epidermal growth factor receptor (EGFR) kinase as well as PP2 (69.8%), a selective inhibitor of tyrosine kinase Src responsible for ligand-independent EGFR transactivation.

CONCLUSIONS

Our findings are the first to show that the detrimental effect of H. pylori on gastric mucin synthesis is intimately linked to the events associated with ECE-1 up-regulation, enhancement in ET-1 production, and G protein-coupled ET(A) receptor activation that triggers the EGFR transactivation.

alpha1-adrenoceptor stimulation potentiates L-type Ca2+ current through Ca2+/calmodulin-dependent PK II (CaMKII) activation in rat ventricular myocytes

alpha1-Adrenoceptor stimulation (alpha1ARS) modulates cardiac muscle contraction under physiological conditions by means of changes in Ca2+ current through L-type channels (ICa,L) and Ca2+ sensitivity of the myofilaments. However, the cellular mechanisms of alpha1ARS are not fully clarified. In this study, we investigated the role of Ca2+/calmodulin-dependent PK II (CaMKII) in the regulation of ICa,L during alpha1ARS in isolated adult rat ventricular myocytes by using the perforated patch-clamp technique. CaMKII inhibition with 0.5 microM KN-93 abolished the potentiation in ICa,L observed during alpha1ARS by 10 microM phenylephrine. In the presence of PKC inhibitor (10 microM chelerythrine), the potentiation of ICa,L by phenylephrine also disappeared. In Western immunoblotting analysis, phenylephrine (> or =1 microM) increased the amount of autophosphorylated CaMKII (active CaMKII) significantly, and this increase was abolished by CaMKII inhibition or PKC inhibition. Also, we investigated changes in the subcellular localization of active CaMKII by using immunofluorescence microscopy and immunoelectron microscopy. Before alpha1ARS, active CaMKII was exclusively located just beneath the plasmalemma. However, after alpha1ARS, active CaMKII was localized close to transverse tubules, where most of L-type Ca2+ channels are located. From these results, we propose that CaMKII, which exists near transverse tubules, is activated and phosphorylated by alpha1ARS and that CaMKII activation directly potentiates ICa,L in rat ventricular myocytes.

Novel determinant of PKC-epsilon anchoring at cardiac Z-lines

The Z-line represents a critical link between the transverse tubule network and cytoskeleton of cardiac cells with a role in anchoring structural proteins, ion channels, and signaling molecules. Protein kinase C-epsilon (PKC-epsilon) regulates cardiac excitability, cardioprotection, and growth, possibly as a consequence of translocation to the Z-line/T tubule region. To investigate the mechanism of PKC-epsilon translocation, fragments of its NH2-terminal 144-amino acid variable domain, epsilonV1, were fused with green fluorescent protein and evaluated by quantitative Fourier image analysis of decorated myocytes. Deletion of 23 amino acids from the NH2-terminus of epsilonV1, including an EAVSLKPT motif important for binding to a receptor for activated C kinase (RACK2), reduced but did not abolish Z-line binding. Further deletions of up to 84 amino acids from the NH2-terminus of epsilonV1 also did not prevent Z-line decoration. However, deletions of residues 85-144 from the COOH-terminus strongly reduced Z-line binding. COOH-terminal deletions caused 2.5-fold greater loss of binding energy (deltadeltaG) than did NH2-terminal deletions. Synthetic peptides derived from these regions modulated epsilonV1 binding and cardiac myocyte function, but also revealed considerable heterogeneity within populations of adult cardiac myocytes. The COOH-terminal subdomain important for Z-line anchoring maps to a surface in the epsilonV1 crystal structure that complements the eight-amino acid RACK2 binding site and two previously identified membrane docking motifs. PKC-epsilon anchoring at the cardiac Z-line/T tubule appears to rely on multiple points of contact probably involving protein-lipid and protein-protein interactions.

Bhc-cNMPs as either water-soluble or membrane-permeant photoreleasable cyclic nucleotides for both one- and two-photon excitation

Cyclic nucleoside monophosphates (cNMPs) play key roles in many cellular regulatory processes, such as growth, differentiation, motility, and gene expression. Caged derivatives that can be activated by irradiation could be powerful tools for studying such diverse functions of intracellular second messengers, since the spatiotemporal dynamics of these molecules can be controlled by irradiation with appropriately focused light. Here we report the synthesis, photochemistry, and biological testing of 6-bromo-7-hydroxycoumarin-4-ylmethyl esters of cNMP (Bhc-cNMP) and their acetyl derivatives (Bhc-cNMP/Ac) as new caged second messengers. Irradiation of Bhc-cNMPs quantitatively produced the parent cNMPs with one-photon uncaging efficiencies (Phiepsilon) of up to one order of magnitude better than those of 2-nitrophenethyl (NPE) cNMPs. In addition, two-photon induced photochemical release of cNMP from Bhc-cNMPs (7 and 8) can be observed with the two-photon uncaging action cross-sections (delta(u)) of up to 2.28 GM (1 GM=10(-50) cm(4) s photon(-1)), which is the largest value among those of the reported Bhc-caged compounds. The wavelength dependence of the delta(u) values of 7 revealed that the peak wavelength was twice that of the one-photon absorption maximum. Bhc-cNMPs showed practically useful water solubility (nearly 500 microM), whereas 7-acetylated derivatives (Bhc-cNMPs/Ac) were expected to have a certain membrane permeability. Their advantages were demonstrated in two types of biological systems: the opening of cAMP-mediated transduction channels in newt olfactory receptor cells and cAMP-mediated motility responses in epidermal melanophores in scales from medaka fish. Both examples showed that Bhc and Bhc/Ac caged compounds have great potential for use in many cell biological applications.

Flash photolytic release of alcohols from photolabile carbamates or carbonates is rate-limited by decarboxylation of the photoproduct

Flash photolysis of a 7-nitroindolinyl carbamate derivative in neutral aqueous solution rapidly generated a monoalkyl carbonate salt. The rate constant for subsequent decarboxylation of this salt [mono(2-phosphoryloxyethyl) carbonate], determined by rapid scan IR difference spectroscopy, was 0.4 s(-1) at pH 7.0, 20 degrees C. This rate reflects release of the product alcohol upon photolysis of the parent compound. In general, alcohols protected as photolabile carbamate (or carbonate) derivatives will therefore be released too slowly for studies of the kinetics of millisecond time scale biological processes.

Ser1928 Is a Common Site for Cav1.2 Phosphorylation by Protein Kinase C Isoforms

Voltage-dependent Ca(2+) channel (Ca(v)1.2, L-type Ca(2+) channel) function is highly regulated by hormones and neurotransmitters in large part through the activation of kinases and phosphatases. Regulation of Ca(v)1.2 by protein kinase C (PKC) is of significant physiologic importance, mediating, in part, the cardiac response to hormonal regulation. Although PKC has been reported to mediate activation and/or inhibition of Ca(v)1.2 function, the molecular mechanisms mediating the response have not been definitively elucidated. We show that PKC forms a macromolecular complex with the alpha(1c) subunit of Ca(v)1.2 through direct interaction with the C terminus. This interaction leads to phosphorylation of the channel in response to activators of PKC. We identify Ser(1928) as the residue that is phosphorylated by PKC in vitro and in vivo. Ser(1928) has been identified previously as the site mediating, in part, the protein kinase A up-regulation of channel activity. Thus, the protein kinase A and PKC signaling pathways converge on the Ca(v)1.2 complex at Ser(1928) to increase channel activity. Our results identify two mechanisms leading to regulation of Ca(v)1.2 activity by PKC: pre-association of the channel with PKC isoforms and phosphorylation of specific sites within the alpha(1c) subunit.