Effect of arachidonic acid on the L-type calcium current in frog cardiac myocytes

CYP2J2-mediated metabolism of arachidonic acid in heart: A review of its kinetics, inhibition and role in heart rhythm control

Cytochrome P450 2 J2 (CYP2J2) is primarily expressed extrahepatically and is the predominant epoxygenase in human cardiac tissues. This highlights its key role in the metabolism of endogenous substrates. Significant scientific interest lies in cardiac CYP2J2 metabolism of arachidonic acid (AA), an omega-6 polyunsaturated fatty acid, to regioisomeric bioactive epoxyeicosatrienoic acid (EET) metabolites that show cardioprotective effects including regulation of cardiac electrophysiology. From an in vitro perspective, the accurate characterization of the kinetics of CYP2J2 metabolism of AA including its inhibition and inactivation by drugs could be useful in facilitating in vitro-in vivo extrapolations to predict drug-AA interactions in drug discovery and development. In this review, background information on the structure, regulation and expression of CYP2J2 in human heart is presented alongside AA and EETs as its endogenous substrate and metabolites. The in vitro and in vivo implications of the kinetics of this endogenous metabolic pathway as well as its perturbation via inhibition and inactivation by drugs are elaborated. Additionally, the role of CYP2J2-mediated metabolism of AA to EETs in cardiac electrophysiology will be expounded.

Multi-Omics Approach Profiling Metabolic Remodeling in Early Systolic Dysfunction and in Overt Systolic Heart Failure

Metabolic remodeling plays an important role in the pathophysiology of heart failure (HF). We sought to characterize metabolic remodeling and implicated signaling pathways in two rat models of early systolic dysfunction (MOD), and overt systolic HF (SHF). Tandem mass tag-labeled shotgun proteomics, phospho-(p)-proteomics, and non-targeted metabolomics analyses were performed in left ventricular myocardium tissue from Sham, MOD, and SHF using liquid chromatography–mass spectrometry, n = 3 biological samples per group. Mitochondrial proteins were predominantly down-regulated in MOD (125) and SHF (328) vs. Sham. Of these, 82% (103/125) and 66% (218/328) were involved in metabolism and respiration. Oxidative phosphorylation, mitochondrial fatty acid β-oxidation, Krebs cycle, branched-chain amino acids, and amino acid (glutamine and tryptophan) degradation were highly enriched metabolic pathways that decreased in SHF > MOD. Glycogen and glucose degradation increased predominantly in MOD, whereas glycolysis and pyruvate metabolism decreased predominantly in SHF. PKA signaling at the endoplasmic reticulum–mt interface was attenuated in MOD, whereas overall PKA and AMPK cellular signaling were attenuated in SHF vs. Sham. In conclusion, metabolic remodeling plays an important role in myocardial remodeling. PKA and AMPK signaling crosstalk governs metabolic remodeling in progression to SHF.

Linolenic acid enhances contraction induced by phenylephrine in isolated rat aorta

This study examined the effect of linolenic acid on the contraction of isolated endothelium-intact and -denuded rat aorta induced by phenylephrine and its underlying mechanism. This was conducted in the presence or absence of N^W-nitro-L-arginine methyl ester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), methylene blue, and calmidazolium. The effects of linolenic acid on contraction induced by calcium chloride in calcium-free Krebs solution containing 60 mM potassium chloride were also examined. Moreover, the effect of linolenic acid on the association between intracellular calcium level ([Ca²⁺]_i) and tension induced by phenylephrine was examined. Finally, we examined the effects of linolenic acid on cGMP formation and endothelial nitric oxide synthase (eNOS) phosphorylation induced by phenylephrine. Linolenic acid (5 × 10^-5 M) increased phenylephrine-induced contraction in endothelium-intact aorta (standardized mean difference [SMD] of log ED₅₀: 2.23), whereas it decreased this contraction in endothelium-denuded aorta (SMD: 1.96). L-NAME, ODQ, methylene blue, and calmidazolium increased phenylephrine-induced contraction in endothelium-intact aorta. Linolenic acid decreased contraction induced by calcium chloride in calcium-free Krebs solution containing 60 mM potassium chloride in endothelium-denuded aorta. Linolenic acid caused an increase in [Ca²⁺]_i (SMD at 3 × 10^-7 M phenylephrine: 1.63) and calcium sensitivity induced by phenylephrine in endothelium-intact aorta. Conversely, linolenic acid decreased [Ca²⁺]_i (SMD: 0.99) induced by phenylephrine in endothelium-denuded aorta. Linolenic acid decreased cGMP formation and eNOS phosphorylation induced by phenylephrine. These results suggest that linolenic acid increases phenylephrine-induced contraction, which is attributed to linolenic acid inhibition of endothelial NO release rather than its decrease of [Ca²⁺]_i in vascular smooth muscle.

The Mechanisms Underlying Lipid Resuscitation Therapy

The experimental use of lipid emulsion for local anesthetic toxicity was originally identified in 1998. It was then translated to clinical practice in 2006 and expanded to drugs other than local anesthetics in 2008. Our understanding of lipid resuscitation therapy has progressed considerably since the previous update from the American Society of Regional Anesthesia and Pain Medicine, and the scientific evidence has coalesced around specific discrete mechanisms. Intravenous lipid emulsion therapy provides a multimodal resuscitation benefit that includes both scavenging (eg, the lipid shuttle) and nonscavenging components. The intravascular lipid compartment scavenges drug from organs susceptible to toxicity and accelerates redistribution to organs where drug (eg, bupivacaine) is stored, detoxified, and later excreted. In addition, lipid exerts nonscavenging effects that include postconditioning (via activation of prosurvival kinases) along with cardiotonic and vasoconstrictive benefits. These effects protect tissue from ischemic damage and increase tissue perfusion during recovery from toxicity. Other mechanisms have diminished in favor based on lack of evidence; these include direct effects on channel currents (eg, calcium) and mass-effect overpowering a block in mitochondrial metabolism. In this narrative review, we discuss these proposed mechanisms and address questions left to answer in the field. Further work is needed, but the field has made considerable strides towards understanding the mechanisms.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Medium-chain fatty acids modulate myocardial function via a cardiac odorant receptor

Several studies have demonstrated the expression of odorant receptors (OR) in various human tissues and their involvement in different physiological and pathophysiological processes. However, the functional role of ORs in the human heart is still unclear. Here, we firstly report the functional characterization of an OR in the human heart. Initial next-generation sequencing analysis revealed the OR expression pattern in the adult and fetal human heart and identified the fatty acid-sensing OR51E1 as the most highly expressed OR in both cardiac development stages. An extensive characterization of the OR51E1 ligand profile by luciferase reporter gene activation assay identified 2-ethylhexanoic acid as a receptor antagonist and various structurally related fatty acids as novel OR51E1 ligands, some of which were detected at receptor-activating concentrations in plasma and epicardial adipose tissue. Functional investigation of the endogenous receptor was carried out by Ca²⁺ imaging of human stem cell-derived cardiomyocytes. Application of OR51E1 ligands induced negative chronotropic effects that depended on activation of the OR. OR51E1 activation also provoked a negative inotropic action in cardiac trabeculae and slice preparations of human explanted ventricles. These findings indicate that OR51E1 may play a role as metabolic regulator of cardiac function.

Putative binding sites for arachidonic acid on the human cardiac Kv 1.5 channel

BACKGROUND AND PURPOSE

In human heart, the Kv 1.5 channel contributes to repolarization of atrial action potentials. This study examined the electrophysiological and molecular mechanisms underlying arachidonic acid (AA)-induced inhibition of the human Kv 1.5 (hKv 1.5) channel.

EXPERIMENTAL APPROACH

Site-directed mutagenesis was conducted to mutate amino acids that reside within the pore domain of the hKv 1.5 channel. Whole-cell patch-clamp method was used to record membrane currents through wild type and mutant hKv 1.5 channels heterologously expressed in CHO cells. Computer docking simulation was conducted to predict the putative binding site(s) of AA in an open-state model of the Kv 1.5 channel.

KEY RESULTS

The hKv 1.5 current was minimally affected at the onset of depolarization but was progressively reduced during depolarization by the presence of AA, suggesting that AA acts as an open-channel blocker. AA itself affected the channel at extracellular sites independently of its metabolites and signalling pathways. The blocking effect of AA was attenuated at pH 8.0 but not at pH 6.4. The blocking action of AA developed rather rapidly by co-expression of Kv β1.3. The AA-induced block was significantly attenuated in H463C, T480A, R487V, I502A, I508A, V512A and V516A, but not in T462C, A501V and L510A mutants of the hKv 1.5 channel. Docking simulation predicted that H463, T480, R487, I508, V512 and V516 are potentially accessible for interaction with AA.

CONCLUSIONS AND IMPLICATIONS

AA itself interacts with multiple amino acids located in the pore domain of the hKv 1.5 channel. These findings may provide useful information for future development of selective blockers of hKv 1.5 channels.

Role of phospholipase A2 pathway in regulating activation of Bufo arenarum oocytes

Transient increases in the concentration of cytosolic Ca2+ are essential for triggering egg activation events. Increased Ca2+ results from its rapid release from intracellular stores, mainly mediated by one or both intracellular calcium channels: the inositol trisphosphate receptor (IP3R) and the ryanodine receptor (RyR). Several regulatory pathways that tailor the response of these channels to the specific cell type have been proposed. Among its many modulatory actions, calcium can serve as an activator of a cytosolic phospholipase A2 (cPLA2), which releases arachidonic acid from phospholipids of the endoplasmic reticulum as well as from the nuclear envelope. Previous studies have suggested that arachidonic acid and/or its metabolites were able to modulate the activity of several ion channels. Based on these findings, we have studied the participation of the phospholipase A2 (PLA2) pathway in the process of Bufo arenarum oocyte activation and the interrelation between any of its metabolites and the ion channels involved in the calcium release from the intracellular reservoirs at fertilization. We found that addition of both melittin, a potent PLA2 activator, and arachidonic acid, the main PLA2 reaction metabolite, was able to induce activation events in a bell-shaped manner. Differential regulation of IP3Rs and RyRs by arachidonic acid and its products could explain melittin and arachidonic acid behaviour in Bufo arenarum egg activation. The concerted action of arachidonic acid and/or its metabolites could provide controlled mobilization of calcium from intracellular reservoirs and useful tools for understanding calcium homeostasis in eggs that express both types of receptors.

Fatty acid synthase modulates homeostatic responses to myocardial stress

Fatty acid synthase (FAS) promotes energy storage through de novo lipogenesis and participates in signaling by the nuclear receptor PPARα in noncardiac tissues. To determine if de novo lipogenesis is relevant to cardiac physiology, we generated and characterized FAS knockout in the myocardium (FASKard) mice. FASKard mice develop normally, manifest normal resting heart function, and have normal cardiac PPARα signaling as well as fatty acid oxidation. However, they decompensate with stress. Most die within 1 h of transverse aortic constriction, probably due to arrhythmia. Voltage clamp measurements of FASKard cardiomyocytes show hyperactivation of L-type calcium channel current that could not be reversed with palmitate supplementation. Of the classic regulators of this current, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) but not protein kinase A signaling is activated in FASKard hearts, and knockdown of FAS in cultured cells activates CaMKII. In addition to being intolerant of the stress of acute pressure, FASKard hearts were also intolerant of the stress of aging, reflected as persistent CaMKII hyperactivation, progression to dilatation, and premature death by ∼1 year of age. CaMKII signaling appears to be pathogenic in FASKard hearts because inhibition of its signaling in vivo rescues mice from early mortality after transverse aortic constriction. FAS was also increased in two mechanistically distinct mouse models of heart failure and in the hearts of humans with end stage cardiomyopathy. These data implicate a novel relationship between FAS and calcium signaling in the heart and suggest that FAS induction in stressed myocardium represents a compensatory response to protect cardiomyocytes from pathological calcium flux.

Arachidonic acid inhibition of L-type calcium (CaV1.3b) channels varies with accessory CaVbeta subunits

Arachidonic acid (AA) inhibits the activity of several different voltage-gated Ca²⁺ channels by an unknown mechanism at an unknown site. The Ca²⁺ channel pore-forming subunit (Ca_Vα₁) is a candidate for the site of AA inhibition because T-type Ca²⁺ channels, which do not require accessory subunits for expression, are inhibited by AA. Here, we report the unanticipated role of accessory Ca_Vβ subunits on the inhibition of Ca_V1.3b L-type (L-) current by AA. Whole cell Ba²⁺ currents were measured from recombinant channels expressed in human embryonic kidney 293 cells at a test potential of −10 mV from a holding potential of −90 mV. A one-minute exposure to 10 µM AA inhibited currents with β_1b, β₃, or β₄ 58, 51, or 44%, respectively, but with β_2a only 31%. At a more depolarized holding potential of −60 mV, currents were inhibited to a lesser degree. These data are best explained by a simple model where AA stabilizes Ca_V1.3b in a deep closed-channel conformation, resulting in current inhibition. Consistent with this hypothesis, inhibition by AA occurred in the absence of test pulses, indicating that channels do not need to open to become inhibited. AA had no effect on the voltage dependence of holding potential–dependent inactivation or on recovery from inactivation regardless of Ca_Vβ subunit. Unexpectedly, kinetic analysis revealed evidence for two populations of L-channels that exhibit willing and reluctant gating previously described for Ca_V2 channels. AA preferentially inhibited reluctant gating channels, revealing the accelerated kinetics of willing channels. Additionally, we discovered that the palmitoyl groups of β_2a interfere with inhibition by AA. Our novel findings that the Ca_Vβ subunit alters kinetic changes and magnitude of inhibition by AA suggest that Ca_Vβ expression may regulate how AA modulates Ca²⁺-dependent processes that rely on L-channels, such as gene expression, enzyme activation, secretion, and membrane excitability.

The Ca2+ channel beta subunit determines whether stimulation of Gq-coupled receptors enhances or inhibits N current

In superior cervical ganglion (SCG) neurons, stimulation of M₁ receptors (M₁Rs) produces a distinct pattern of modulation of N-type calcium (N-) channel activity, enhancing currents elicited with negative test potentials and inhibiting currents elicited with positive test potentials. Exogenously applied arachidonic acid (AA) reproduces this profile of modulation, suggesting AA functions as a downstream messenger of M₁Rs. In addition, techniques that diminish AA's concentration during M₁R stimulation minimize N-current modulation. However, other studies suggest depletion of phosphatidylinositol-4,5-bisphosphate during M₁R stimulation suffices to elicit modulation. In this study, we used an expression system to examine the physiological mechanisms regulating modulation. We found the β subunit (Ca_Vβ) acts as a molecular switch regulating whether modulation results in enhancement or inhibition. In human embryonic kidney 293 cells, stimulation of M₁Rs or neurokinin-1 receptors (NK-1Rs) inhibited activity of N channels formed by Ca_V2.2 and coexpressed with Ca_Vβ1b, Ca_Vβ3, or Ca_Vβ4 but enhanced activity of N channels containing Ca_Vβ2a. Exogenously applied AA produced the same pattern of modulation. Coexpression of Ca_Vβ2a, Ca_Vβ3, and Ca_Vβ4 recapitulated the modulatory response previously seen in SCG neurons, implying heterogeneous association of Ca_Vβ with Ca_V2.2. Further experiments with mutated, chimeric Ca_Vβ subunits and free palmitic acid revealed that palmitoylation of Ca_Vβ2a is essential for loss of inhibition. The data presented here fit a model in which Ca_Vβ2a blocks inhibition, thus unmasking enhancement. Our discovery that the presence or absence of palmitoylated Ca_Vβ2a toggles M₁R- or NK-1R–mediated modulation of N current between enhancement and inhibition identifies a novel role for palmitoylation. Moreover, these findings predict that at synapses, modulation of N-channel activity by M₁Rs or NK-1Rs will fluctuate between enhancement and inhibition based on the presence of palmitoylated Ca_Vβ2a.

Regulation of voltage-gated Ca2+ channels by lipids

Great skepticism has surrounded the question of whether modulation of voltage-gated Ca(2+) channels (VGCCs) by the polyunsaturated free fatty acid arachidonic acid (AA) has any physiological basis. Here we synthesize findings from studies of both native and recombinant channels where micromolar concentrations of AA consistently inhibit both native and recombinant activity by stabilizing VGCCs in one or more closed states. Structural requirements for these inhibitory actions include a chain length of at least 18 carbons and multiple double bonds located near the fatty acid's carboxy terminus. Acting at a second site, AA increases the rate of VGCC activation kinetics, and in Ca(V)2.2 channels, increases current amplitude. We present evidence that phosphatidylinositol 4,5-bisphosphate (PIP(2)), a palmitoylated accessory subunit (beta(2a)) of VGCCs and AA appear to have overlapping sites of action giving rise to complex channel behavior. Their actions converge in a physiologically relevant manner during muscarinic modulation of VGCCs. We speculate that M(1) muscarinic receptors may stimulate multiple lipases to break down the PIP(2) associated with VGCCs and leave PIP(2)'s freed fatty acid tails bound to the channels to confer modulation. This unexpectedly simple scheme gives rise to unanticipated predictions and redirects thinking about lipid regulation of VGCCs.

Inhibition of L-type Ca2+channel current and negative inotropy induced by arachidonic acid in adult rat ventricular myocytes

We have previously shown an increase in arachidonic acid (AA) release in response to proinflammatory cytokines in adult rat ventricular myocytes (ARVM). AA is known to alter channel activities; however, its effects on cardiac L-type Ca2+channel current ( ICa,L) and excitation-contraction coupling remain unclear. The present study examined effects of AA on ICa,L, using the whole cell patch-clamp technique, and on cell shortening (CS) and the Ca2+transient of ARVM. ICa,Lwas monitored in myocytes held at −70 mV and internally equilibrated and externally perfused with Na+- and K+-free solutions. Exposure to AA caused a voltage-dependent block of ICa,Lconcentration dependently (IC508.5 μM). The AA-induced inhibition of ICa,Lis consistent with its hyperpolarizing shift in the voltage-dependent properties and reduction in maximum slope conductance. In the presence of AA, BSA completely blocked the AA-induced suppression of ICa,Land CS. Intracellular load with AA had no effect on the current density but caused a small depolarizing shift in the ICa,Lactivation curve, suggesting a site-specific action of AA. Moreover, intracellular AA had no effect on the extracellular AA-induced decrease in ICa,L. Pretreatment with indomethacin, an inhibitor of cyclooxygenase, or addition of nordihydroguaiaretic acid, an inhibitor of lipoxygenase, had no effect on AA-induced changes in ICa,L. Furthermore, AA suppressed CS and Ca2+transients of intact ARVM with no significant effect on SR function and myofilament Ca2+sensitivity. Therefore, these results suggest that AA inhibits contractile function of ARVM, primarily due to its direct inhibition of ICa,Lat an extracellular site.

Anticonvulsant efficacy of valproate-like carboxylic acids: a potential target for anti-bipolar therapy

BACKGROUND

Bipolar disorder (BPD) is a severe and chronic illness, with a lifetime prevalence of approximately 1.5%. Despite the availability of some mood stabilizing drugs including lithium, valproate (valproic acid), lamotrigine and carbamazepine, BPD is characterized by high rates of recurrence, as treatment with these and other drugs is ineffective for and not well-tolerated by a significant percentage of patients. Most drugs currently used for the maintenance treatment of BPD are anticonvulsants (e.g., valproate, carbamazepine and lamotrigine).

OBJECTIVES

The aim of this paper is to review the studies characterizing the anticonvulsant efficacy of valproate-like carboxylic acids and related compounds, some of which may have potential for the treatment of manic-depressive illness.

RESULTS

The data reviewed herein demonstrate clearly that some dietary fatty acids and other valproate-like carboxylic acids exhibit potent anticonvulsant activity, and may thus be candidates for mood stabilizing treatment options for BPD.

M1 muscarinic receptors inhibit L-type Ca2+ current and M-current by divergent signal transduction cascades

Ion channels reside in a sea of phospholipids. During normal fluctuations in membrane potential and periods of modulation, lipids that directly associate with channel proteins influence gating by incompletely understood mechanisms. In one model, M(1)-muscarinic receptors (M(1)Rs) may inhibit both Ca(2+) (L- and N-) and K(+) (M-) currents by losing a putative interaction between channels and phosphatidylinositol-4,5-bisphosphate (PIP(2)). However, we found previously that M(1)R inhibition of N-current in superior cervical ganglion (SCG) neurons requires loss of PIP(2) and generation of a free fatty acid, probably arachidonic acid (AA) by phospholipase A(2) (PLA(2)). It is not known whether PLA(2) activity and AA also participate in L- and M-current modulation in SCG neurons. To test whether PLA(2) plays a similar role in M(1)R inhibition of L- and M-currents, we used several experimental approaches and found unanticipated divergent signaling. First, blocking resynthesis of PIP(2) minimized M-current recovery from inhibition, whereas L-current recovered normally. Second, L-current inhibition required group IVa PLA(2) [cytoplasmic PLA(2) (cPLA(2))], whereas M-current did not. Western blot and imaging studies confirmed acute activation of cPLA(2) by muscarinic stimulation. Third, in type IIa PLA(2) [secreted (sPLA(2))](-/-)/cPLA(2)(-/-) double-knock-out SCG neurons, muscarinic inhibition of L-current decreased. In contrast, M-current inhibition remained unaffected but recovery was impaired. Our results indicate that L-current is inhibited by a pathway previously shown to control M-current over-recovery after washout of muscarinic agonist. Our findings support a model of M(1)R-meditated channel modulation that broadens rather than restricts the roles of phospholipids and fatty acids in regulating ion channel activity.

Fluid transport and ion fluxes in mammalian kidney proximal tubule: a model analysis of isotonic transport

AIM

By mathematical modelling, we analyse conditions for near-isotonic and isotonic transport by mammalian kidney proximal tubule.

METHODS

The model comprises compliant lateral intercellular space (lis) and cells, and infinitely large luminal and peritubular compartments with diffusible species: Na+, K+, Cl- and an intracellular non-diffusible anion. Unknown model variables are solute concentrations, electrical potentials, volumes and hydrostatic pressures in cell and lis, and transepithelial potential. We used data mainly from rat proximal tubule to model epithelial cells and interspace with luminal and peritubular baths of identical composition.

RESULTS

The model of the tubular epithelium with physiological water permeability and paracellular electrical resistance generates solute coupled water uptake with an approx. 3% hypertonic absorbate. This function remains unperturbed following 'blocking' of apical water channels and in 'aquaporin-null' simulation. Reduced rate of volume reabsorption in AQP(-/-) mice would also require decreased apical sodium permeability. Paracellular convection accounts for approx. 36% of the net Na+ absorption, and the model epithelium accomplishes uphill water transport similar to rat proximal tubule. Na+ recirculation is required for truly isotonic transport. The tonicity of the absorbate and the recirculation flux depend critically on ion permeabilities of interspace basement membrane.

CONCLUSION

Our model based on solute-solvent coupling in lateral space simulates major physiological features of proximal tubule, including significantly lower water permeability of the AQP1-null preparation, and a ratio of net sodium uptake and oxygen consumption exceeding that predicted from stoichiometry of the Na+/K+-pump. Physical properties of interspace basement membrane are critical for obtaining near-isotonic and truly isotonic transport.

Neural cell adhesion molecule induces intracellular signaling via multiple mechanisms of Ca2+ homeostasis

The neural cell adhesion molecule (NCAM) plays a pivotal role in the development of the nervous system, promoting neuronal differentiation via homophilic (NCAM-NCAM) as well as heterophilic (NCAM-fibroblast growth factor receptor [FGFR]) interactions. NCAM-induced intracellular signaling has been shown to affect and be dependent on the cytoplasmic Ca2+ concentration ([Ca2+]i). However, the molecular basis of this remains unclear. In this study, we determined [Ca2+]i regulating mechanisms involved in intracellular signaling induced by NCAM. To mimic the effect of homophilic NCAM interaction on [Ca2+]i in vitro, we used a peptide derived from a homophilic binding site of NCAM, termed P2, which triggers signaling cascades similar to those activated by NCAM-NCAM interaction. We found that P2 increased [Ca2+]i in primary hippocampal neurons. This effect depended on two signaling pathways. The first pathway was associated with activation of FGFR, phospholipase Cgamma, and production of diacylglycerol, and the second pathway involved Src-family kinases. Moreover, NCAM-mediated Ca2+ entry required activation of nonselective cation and T-type voltage-gated Ca2+ channels. These channels, together with the Src-family kinases, were also involved in neuritogenesis induced by physiological, homophilic NCAM interactions. Thus, unanticipated mechanisms of Ca2+ homeostasis are shown to be activated by NCAM and to contribute to neuronal differentiation.

Effects of unsaturated fatty acids on calcium-activated potassium current in gastric myocytes of guinea pigs

AIM: To investigate the effects of exogenous unsaturated fatty acids on calcium-activated potassium current [I_K(Ca)] in gastric antral circular myocytes of guinea pigs.

METHODS: Gastric myocytes were isolated by collagenase from the antral circular layer of guinea pig stomach. The whole-cell patch clamp technique was used to record I_K(Ca) in the isolated single smooth muscle cells with or without different concentrations of arachidonic acid (AA), linoleic acid (LA), and oleic acid (OA).

RESULTS: AA at concentrations of 2,5 and 10 μmol/L markedly increased I_K(Ca) in a dose-dependent manner. LA at concentrations of 5, 10 and 20 μmol/L also enhanced I_K(Ca) in a dose-dependent manner. The increasing potency of AA, LA, and oleic acid (OA) on I_K(Ca) at the same concentration (10 μmol/L) was in the order of AA>LA>OA. AA (10 μmol/L)-induced increase of I_K(Ca) was not blocked by H-7 (10 μmol/L), an inhibitor of protein kinase C (PKC), or indomethacin (10 μmol/L), an inhibitor of the cyclooxygenase pathway, and 17-octadecynoic acid (10 μmol/L), an inhibitor of the cytochrome P450 pathway, but weakened by nordihydroguaiaretic acid (10 μmol/L), an inhibitor of the lipoxygenase pathway.

CONCLUSION: Unsaturated fatty acids markedly increase I_K(Ca), and the enhancing potencies are related to the number of double bonds in the fatty acid chain. The lipoxygenase pathway of unsaturated fatty acid metabolism is involved in the unsaturated fatty acid-induced increase of I_K(Ca) in gastric antral circular myocytes of guinea pigs.

Mechanism of arachidonic acid modulation of the T-type Ca2+ channel alpha1G

Arachidonic acid (AA) modulates T-type Ca(2+) channels and is therefore a potential regulator of diverse cell functions, including neuronal and cardiac excitability. The underlying mechanism of modulation is unknown. Here we analyze the effects of AA on the T-type Ca(2+) channel alpha(1G) heterologously expressed in HEK-293 cells. AA inhibited alpha(1G) currents within a few minutes, regardless of preceding exposure to inhibitors of AA metabolism (ETYA and 17-ODYA). Current inhibition was also observed in cell-free inside-out patches, indicating a membrane-delimited interaction of AA with the channel. AA action was consistent with a decrease of the open probability without changes in the size of unitary currents. AA shifted the inactivation curve to more negative potentials, increased the speed of macroscopic inactivation, and decreased the extent of recovery from inactivation at -80 mV but not at -110 mV. AA induced a slight increase of activation near the threshold and did not significantly change the deactivation kinetics or the rectification pattern. We observed a tonic current inhibition, regardless of whether the channels were held in resting or inactivated states during AA perfusion, suggesting a state-independent interaction with the channel. Model simulations indicate that AA inhibits T-type currents by switching the channels into a nonavailable conformation and by affecting transitions between inactivated states, which results in the negative shift of the inactivation curve. Slow-inactivating alpha(1G) mutants showed an increased affinity for AA with respect to the wild type, indicating that the structural determinants of fast inactivation are involved in the AA-channel interaction.

Ca2+ selectivity and fatty acid specificity of the noncapacitative, arachidonate-regulated Ca2+ (ARC) channels

The arachidonate-regulated, Ca(2+)-selective ARC channels represent a novel receptor-activated pathway for the entry of Ca(2+) in nonexcitable cells that is entirely separate from the widely studied store-operated, Ca(2+) release-activated Ca(2+) channels. Activation of ARC channels occurs specifically at the low agonist concentrations typically associated with oscillatory Ca(2+) signals and appears to provide the predominant mode of Ca(2+) entry under these conditions (Mignen, O., Thompson, J. L., and Shuttleworth, T. J. (2001) J. Biol. Chem. 276, 35676-35683). In this study we demonstrate that ARC channels are present in a variety of different cell types including both cell lines and primary cells. Examination of their pharmacology revealed that currents through these channels are significantly inhibited by low concentrations (< 5 microm) of Gd(3+), are unaffected by 100 microm 2-aminoethyoxydiphenyl borane, and are not activated by the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (100 microm). Their selectivity for Ca(2+) was assessed by determining the EC(50) for external Ca(2+) block of the monovalent currents observed in the absence of external divalent cations. The value obtained (150 nm) indicates that the Ca(2+) selectivity of ARC channels is extremely high. Examination of the ability of various fatty acids, including arachidonic acid, to activate the ARC channels demonstrated that activation does not reflect any nonspecific membrane fluidity or detergent effects, shows a high degree of specificity for arachidonic acid over other fatty acids (especially monounsaturated and saturated fatty acids), and is independent of any arachidonic acid metabolite. Moreover, studies using the charged analogue arachidonyl coenzyme A demonstrate that activation of the ARC channels reflects an action of the fatty acid specifically at the internal face of the plasma membrane. Whether this involves a direct action of arachidonic acid on the channel protein itself or an action on some intermediary molecule is, at present, unclear.

Modulation of BK(Ca) channel activity by fatty acids: structural requirements and mechanism of action

To determine the mechanism of fatty acid modulation of rabbit pulmonary artery large-conductance Ca2+ -activated K+ (BK(Ca)) channel activity, we studied effects of fatty acids and other lipids on channel activity in excised patches with patch-clamp techniques. The structural features of the fatty acid required to increase BK(Ca) channel activity (or average number of open channels, NP(o)) were identified to be the negatively charged head group and a sufficiently long (C > 8) carbon chain. Positively charged lipids like sphingosine, which have a sufficiently long alkyl chain (C >or= 8), produced a decrease in NP(o). Neutral and short-chain lipids did not alter NP(o). Screening of membrane surface charge with high-ionic-strength bathing solutions (330 mM K+ or 130 mM K+, 300 mM Na+) did not alter the modulation of the BK(Ca) channel NP(o) by fatty acids and other charged lipids, indicating that channel modulation is unlikely to be due to an alteration of the membrane electric field or the attraction of local counterions to the channel. Fatty acids and other negatively charged lipids were able to modulate BK(Ca) channel activity in bathing solutions containing 0 mM Ca2+, 20 mM EGTA, suggesting that calcium is not required for this modulation. Together, these results indicate that modulation of BK(Ca) channels by fatty acids and other charged lipids most likely occurs by their direct interaction with the channel protein itself or with some other channel-associated component.

Skeletal muscle L-type Ca(2+) current modulation in gamma1-deficient and wildtype murine myotubes by the gamma1 subunit and cAMP

Modulation of the steady-state inactivation and current amplitude by the gamma1 subunit of the murine skeletal muscle L-type Ca(2+) channel were investigated using the whole-cell patch-clamp technique. Transient expression of the gamma1 subunit, but not of the gamma2 (stargazin) protein, in primary cultured myotubes from gamma1-deficient mice shifted the steady-state inactivation approximately -15 mV, thereby restoring wildtype (WT) steady-state inactivation and current amplitude. The increased Ca(2+) current amplitude in gamma1-deficient cells was abolished in myotubes from animals of 4 weeks and older whereas the positive shift in steady-state inactivation was independent of mouse age. Raising intracellular cAMP levels using the membrane-permeant analogue 8-Br-cAMP led to an increase in Ca(2+) current amplitude in WT cells to the level in gamma1-deficient myotubes. There was no effect on the current amplitude in gamma1-deficient cells or on the steady-state inactivation in either genotype. Rp-cAMPS, a competitive inhibitor of cAMP-dependent protein kinase, had no effect on the WT Ca(2+) current amplitude and steady-state inactivation, but diminished the current amplitude in gamma1-deficient myotubes without affecting the steady-state inactivation in these cells. These data show that the increased Ca(2+) influx in myotubes lacking the gamma1 subunit, due to right-shifted steady-state inactivation and increased L-type Ca(2+) current amplitude, is determined by the gamma1 subunit. The effect on current amplitude depends on the age of the mice and its cAMP-dependent modulation appears to be controlled by the gamma1 subunit.

The role of phospholipase A(2) and cyclooxygenase in renal toxicity induced by microcystin-LR

We have shown previously that exposure to microcystin-LR (MCLR) causes renal toxic effects in isolated perfused rat kidney. That study was extended further to approach the perspective of pharmacological blockade of renal toxic effects by MCLR through the use of experimental therapeutic agents. An isolated kidney perfusion system was utilized and samples of urine and perfusate were collected at 10min intervals to determine the levels of inulin, sodium, potassium and osmolality. Dexamethasone (20microg ml(-1)) and indomethacin (10microg ml(-1)) were administered in the beginning of the perfusion and MCLR was employed in a dose of 1microg ml(-1) after an internal control of 30min to evaluate the perfusion pressure (PP), renal vascular resistance (RVR), glomerular filtration rate (GFR) and urinary flow (UF). Dexamethasone and indomethacin antagonized the toxic effects of MCLR on PP, RVR, GFR and UF. Histologic analysis of dexamethasone and indomethacin treated groups did not show any vascular or interstitial alterations. MCLR potentially impairs the renal function, probably causing vascular and glomerular lesions and, promoting renal alterations through direct or indirect actions. These data seem to indicate that the renal alterations promoted by MCLR involves also phospholipase A(2) and arachidonic acid-derived mediators.

Arachidonic acid both inhibits and enhances whole cell calcium currents in rat sympathetic neurons

We recently reported that arachidonic acid (AA) inhibits L- and N-type Ca(2+) currents at positive test potentials in the presence of the dihydropyridine L-type Ca(2+) channel agonist (+)-202-791 in dissociated neonatal rat superior cervical ganglion neurons [Liu L and Rittenhouse AR. J Physiol (Lond) 525: 291-404, 2000]. In this first of two companion papers, we characterized the mechanism of inhibition by AA at the whole cell level. In the presence of either omega-conotoxin GVIA or nimodipine, AA decreased current amplitude, confirming that L- and N-type currents, respectively, were inhibited. AA-induced inhibition was concentration dependent and reversible with an albumin-containing wash solution, but appears independent of AA metabolism and G protein activity. In characterizing inhibition, an AA-induced enhancement of current amplitude was revealed that occurred primarily at negative test potentials. Cell dialysis with albumin minimized inhibition but had little effect on enhancement, suggesting that AA has distinct sites of action. We examined AA's actions on current kinetics and found that AA increased holding potential-dependent inactivation. AA also enhanced the rate of N-type current activation. These findings indicate that AA causes multiple changes in sympathetic Ca(2+) currents.

Modulation of the synaptic Ca2+ current in salamander photoreceptors by polyunsaturated fatty acids and retinoids

Synaptic transmission between retinal photoreceptors and second-order neurones is controlled by an L-type Ca2+ conductance (gCa) in the photoreceptor inner segment. Modulation of this conductance therefore influences the flow of visual information to higher centres. Possible modulation of gCa by retinal factors was investigated using patch clamp and Ca2+ imaging. No significant modulation of gCa by retinal neurotransmitters nor by intracellular signalling pathways was found. gCa was inhibited by retinoids (all-trans retinal) and by polyunsaturated fatty acids (PUFAs) such as arachidonic acid and docosahexaenoic acid, which are known to be released in the retina by exposure to light. Some PUFAs tested are physiological substrates for the cyclo-oxygenase, lipoxygenase and epoxygenase pathways, but specific inhibitors of these pathways had no effect on the inhibition of gCa. Treatments designed to activate or inhibit G-protein-coupled pathways or protein kinases A and C similarly had no effect on the inhibition by PUFAs nor on gCa itself. Inhibitors of phosphatases 1 and 2A were also largely ineffective. The inhibition by PUFAs is, however, dependent on membrane potential, suggesting that it arises from a direct interaction of fatty acids with the Ca2+ channel. The effect was not use or frequency dependent, suggesting that the effect does not depend on channel gating state. Control by retinoids and by PUFAs may be an important mechanism by which the Ca2+ conductance, and consequently the transmission of the visual signal, is modulated at the first retinal synapse.

Endogenous cannabinoid anandamide directly inhibits voltage-dependent Ca(2+) fluxes in rabbit T-tubule membranes

The effect of the endogenous cannabinoid, anandamide on Ca(2+) flux responses mediated by voltage-dependent Ca(2+) channels was studied in transverse tubule membrane vesicles from rabbit skeletal muscle. Vesicles were loaded with 45Ca(2+) and membrane potentials were generated by establishing K(+) gradients across the vesicle using the ionophore, valinomycin. Anandamide, in the range of 1-100 microM, inhibited depolarization-induced efflux responses. Anandamide also functionally modulated the effects of nifedipine (1-10 microM) and Bay K 8644 (1 microM) on Ca(2+) flux responses. Pretreatment with the specific cannabinoid receptor antagonist, SR141716A (1 microM), pertussis toxin (5 microg/ml), the amidohydrolase inhibitor, phenylmethylsulfonyl fluoride (0.2 mM) or the cyclooxygenase inhibitor, indomethacin (5 microM) did not alter the inhibition of efflux responses by anandamide. Arachidonic acid (10-100 microM) also effectively inhibited 45Ca(2+) efflux from membrane vesicles. In radioligand binding studies, it was found that both anandamide and arachidonic acid inhibited the specific binding of [3H]PN 200-110 to transverse tubule membranes with IC(50) values of 4.4+/-0. 7 and 13.4+/-3.5 microM, respectively. These results indicate that anandamide, independent of cannabinoid receptor activation, directly inhibits the function of voltage-dependent calcium channels and modulates the specific binding of calcium channel ligands of the dihydropyridine class.

Effects of arachidonic acid on unitary calcium currents in rat sympathetic neurons

We have characterized the actions of arachidonic acid (AA) on whole cell and unitary calcium (Ca2+) currents in rat neonatal superior cervical ganglion (SCG) neurons using barium (Ba2+) as the charge carrier. Whole cell currents were elicited by stepping the membrane potential from -90 mV to +10 mV. Arachidonic acid (5 microM) was introduced into the bath in the continued presence of 1 microM (+)-202-791, an L-type Ca2+ channel agonist. Under these conditions, the peak current, comprised mainly of N-type current, and a slow, (+)-202-791-induced component of the tail current were inhibited by 67 +/- 6 and 60 +/- 10 %, respectively, indicating that AA inhibits both N- and L-type currents. At a test potential of +30 mV, AA (5 microM) decreased unitary L- and N-type Ca2+ channel open probability (Po) in cell-attached patches that contained a single channel. For both channels, the underlying causes of the decrease in Po were similar. Arachidonic acid caused an increase in the percentage of null sweeps and in the number of null sweeps that clustered together. In sweeps with activity, the average number of openings per sweep decreased, while first latency and mean closed time increased. Arachidonic acid had no significant effect on unitary current amplitude or mean open time. Our findings are the first description of the inhibition of unitary L- and N-type Ca2+ channel activity by AA and are consistent with both channels spending more time in their null mode and with increased dwell time in one or more closed states.

Arachidonic acid modulation of alpha1H, a cloned human T-type calcium channel

Arachidonic acid (AA) and the products of its metabolism are central mediators of changes in cellular excitability. We show that the recently cloned and expressed T-type or low-voltage-activated Ca channel, alpha1H, is modulated by external AA. AA (10 microM) causes a slow, time-dependent attenuation of alpha1H current. At a holding potential of -80 mV, 10 microM AA reduces peak inward alpha1H current by 15% in 15 min and 70% in 30 min and shifts the steady-state inactivation curve -25 mV. AA inhibition was not affected by applying the cyclooxygenase inhibitor indomethacin or the lipoxygenase inhibitor nordihydroguaiaretic acid. The epoxygenase inhibitor octadecynoic acid partially antagonized AA attenuation of alpha1H. The epoxygenase metabolite epoxyeicosatrienoic acid (8,9-EET) mimicked the inhibitory effect of AA on alpha1H peak current. A protein kinase C (PKC)-specific inhibitor (peptide fragment 19-36) only partially antagonized the AA-induced reduction of peak alpha1H current and the shift of the steady-state inactivation curve but had no effect on 8,9-EET-induced attenuation of current. In contrast, PKA has no role in the modulation of alpha1H. These results suggest that AA attenuation and shift of alpha1H may be mediated directly by AA. The heterologous expression of T-type Ca channels allows us to study for the first time properties of this important class of ion channel in isolation. There is a significant overlap of the steady-state activation and inactivation curves, which implies a substantial window current. The selective shift of the steady-state inactivation curve by AA reduces peak Ca current and eliminates the window current. We conclude that AA may partly mediate physiological effects such as vasodilatation via the attenuation of T-type Ca channel current and the elimination of a T-type channel steady window current.

CAMs and FGF cause a local submembrane calcium signal promoting axon outgrowth without a rise in bulk calcium concentration

Binding of basic fibroblast growth factor (bFGF) and cell adhesion molecules to the nerve cell membrane promotes axon outgrowth. This response can be blocked by antagonists of voltage-gated calcium channels, yet no change of cytosolic calcium concentration in the growth cone can be detected upon binding of the growth factor bFGF or the cell adhesion molecule L1. Using barium as a charge carrier, we show that bFGF and L1 open a calcium influx pathway in growth cones of rat sensory neurons without changing the membrane voltage. L1 does not activate influx in cells expressing a dominant negative mutant of the fibroblast growth factor receptor (FGFR) tyrosine kinase. FGFR-activated influx is blocked by specific antagonists of L- and N-type voltage-gated calcium channels and by an inhibitor of diacylglycerol lipase. We propose that both L1 and bFGF act via the FGFR to generate polyunsaturated fatty acids which in turn cause calcium channels to flicker open and shut. Short-lived domains of raised calcium at the cytosolic mouth of open channels activate axon outgrowth without raising bulk cytosolic calcium concentration. In confirmation of this model, the rapidly-acting calcium buffer BAPTA is significantly more effective at blocking FGF-induced axon outgrowth when compared with the slower buffer EGTA. Generation of short-lived calcium domains may provide a crucial mechanism for axon guidance during development and for promoting regeneration of damaged axons.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

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

Inhibition of cardiac L-type calcium channels by epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs), products of the cytochrome P-450 monooxygenase metabolism of arachidonic acid, can regulate the activity of ion channels. We examined the effects of EETs on cardiac L-type Ca2+ channels that play important roles in regulating cardiac contractility, controlling heart rate, and mediating slow conduction in normal nodal cells and ischemic myocardium. Our experimental approach was to reconstitute porcine L-type Ca2+ channels into planar lipid bilayers where we could control the aqueous and lipid environments of the channels and the regulatory pathways that change channel properties. We found that 20 to 125 nM EETs inhibited the open probability of reconstituted L-type Ca2+ channels, accelerated the inactivation of the channels, and reduced the unitary current amplitude of open channels. There was no selectivity among different EET regioisomers or stereoisomers. When 11,12-EET was esterified to the sn-2 position of phosphatidylcholine, restricting it to the hydrophobic phase of the planar lipid bilayer, the reconstituted channels were similarly inhibited, suggesting that the EET interacts directly with Ca2+ channels through the lipid phase. The inhibitory effects of EET persisted in the presence of microcystin, an inhibitor of protein phosphatases 1 and 2A, suggesting that dephosphorylation was not the mechanism through which these eicosanoids down-regulate channel activity. This inhibition may be an important protective mechanism in the setting of cardiac ischemia where arachidonic acid levels are dramatically increased and EETs have been shown to manifest preconditioning-like effects.

Calcineurin involvement in the regulation of high-threshold Ca2+ channels in NG108-15 (rodent neuroblastoma x glioma hybrid) cells

1. We examined the relationship between calcineurin (protein phosphatase 2B (PP2B) and voltage-operated Ca2+ channels (VOCCs) in NG108-15 cells. PP2B expression in NG108-15 cells was altered by transfection with plasmid constructs containing a full length cDNA of human PP2B beta(3) in sense (CN-15) and antisense (CN-21) orientation. 2. Confocal immunocytochemical localization showed that in wild-type cells, PP2B immunoreactivity is uniformly distributed in undifferentiated cells and located at the inner surface of soma membrane and neurites in differentiated cells. 3. To test the Ca2+ dependence of the VOCC, we used high-frequency stimulation (HFS). The L- and N-type VOCCs decreased by 37 and 52%, respectively, whereas the T-type current was only marginally sensitive to this procedure. FK-506 (2 microM), a specific blocker of PP2B, reduced the inhibition of L- and N-type VOCCs induced by HFS by 30 and 33%, respectively. 4. In CN-15-transfected cells overexpressing PP2B, total high-voltage-activated (HVA) VOCCs were suppressed by about 60% at a test potential of +20 mV. Intracellular addition of EGTA or FK-506 into CN-15-transfected cells induced an up to 5-fold increase of HVA VOCCs. 5. These findings indicate that PP2B activity does not influence the expression of HVA Ca2+ channels, but modulates their function by Ca(2+)-dependent dephosphorylation. Thus HVA VOCCs, in a phosphorylated state under control conditions, are downregulated by PP2B upon stimulation, with the major effect on N-type VOCCs.

Cytochrome P450: a novel system modulating Ca2+ channels and contraction in mammalian heart cells

1. Cytochrome P450 (P450) is a ubiquitous enzyme system that catalyses oxidative reactions of numerous endogenous and exogenous compounds. The modulatory effects of P450 on the L-type Ca2+ current (ICa), intracellular free Ca2+ signals and cell shortening were assessed in adult rat single ventricular myocytes. 2. Bath administration of the imidazole antimycotics, clotrimazole, econazole and miconazole, which are potent P450 inhibitors, significantly suppressed cardiac ICa. While the Ca2+ channel antagonist nifedipine blocked ICa within 30 s, clotrimazole-induced suppression of ICa required 5.1 +/- 0.4 min (n = 14) to reach a steady low level. The suppression of ICa was dose dependent and recovered after washout of clotrimazole. Intracellular dialysis with the P450 antibody anti-rat CYP1A2 also significantly reduced cardiac ICa. 3. Additional administration of the beta-adrenergic agonist isoprenaline (1 microM) or the membrane-permeable 8-bromo-cAMP (2 mM) completely reversed the suppressant effects of clotrimazole and NaCN on ICa. In addition, intracellular dialysis with 2 mM cAMP abolished the P450 inhibitor-induced suppression of ICa. Phosphorylation of the channel with hydrolysis-resistant ATPgammaS prevented the suppressant effect of clotrimazole on ICa. Furthermore, dephosphorylation of the Ca2+ channel with intracellular dialysis with phosphatase types I and II reduced ICa by 85 +/- 3 % and abolished clotrimazole-induced suppression of ICa. 4. Extracellular administration of the phospholipase A2 inhibitors mepacrine and 4-bromophenacyl bromide significantly suppressed ICa. 5. Clotrimazole, econazole, miconazole and CN- also significantly inhibited intracellular free Ca2+ signals and cell shortening in rat single ventricular myocytes. 6. Intracellular cAMP content was significantly reduced in isolated ventricular myocytes incubated with clotrimazole or CN-. Extracellular administration of 11, 12-epoxyeicosatrienoic acid, one of the P450-mediated metabolites of arachidonic acid, enhanced ICa and intracellular cAMP content. The epoxyeicosatrienoic acid also restored the amplitude of the reduced ICa in P450 antibody-dialysed myocytes. 7. The present data suggest that cytochrome P450 modulates cardiac ICa and cell contraction, and the modulation may result from changes in intracellular levels of cAMP by P450- mediated metabolites of arachidonic acid.

Arachidonic acid inhibits both K+ and Ca2+ currents in isolated type I cells of the rat carotid body

Whole-cell patch-clamp recordings were used to investigate the effects of arachidonic acid (AA) on K+ and Ca2+ channels in isolated rat type I carotid body cells. AA (2-20 microM) produced a concentration-dependent inhibition of both K+ currents and Ca2+ channel currents. The effects of AA on K+ currents were unaffected by indomethacin (5 microM), phenidone (5 microM) or 1-aminobenzotriazole (3 mM), suggesting that AA did not exert its effects via cyclo-oxygenase, lipoxygenase or cytochrome P-450 (cP-450) metabolism. Our results suggest that AA directly and non-selectively inhibits ionic currents in rat type I carotid body cells.

Anticonvulsant effect of polyunsaturated fatty acids in rats, using the cortical stimulation model

Recent studies have shown that long-chain polyunsaturated fatty acids can prevent cardiac arrhythmias, attributed to the reduction in excitability of cardiomyocytes, owing mainly to a shift in hyperpolarizing direction of the inactivation curves of both Na+ and Ca2+ currents and to a slowed recovery from inactivation. Qualitatively similar effects of polyunsaturated fatty acids on inactivation parameters have been observed in freshly isolated hippocampal neurons. Since the same effects are presumed to underlie the action of some established anticonvulsant drugs, polyunsaturated fatty acids might have an anticonvulsant action as well. We have investigated this for eicosapentaenoic acid, docosahexaenoic acid, linoleic acid and oleic acid, employing cortical stimulation in rats, a seizure model allowing the determination of the full anticonvulsant effect-time profile in freely moving, individual animals. I.v. infusion of 40 micromol of eicosapentaenoic acid or docosahexaenoic acid over a period of 30 min, modestly increased the threshold for localized seizure activity after 6 h by 73 +/- 13 microA (mean +/- S.E.M.; n = 7) and 77 +/- 17 microA (n = 7), respectively, and the threshold for generalized seizure activity by 125 +/- 20 and 130 +/- 19 microA, respectively (P < 0.001). The thresholds remained elevated for 6 h after infusion, but returned to baseline the next day. Free plasma concentrations in rats treated with eicosapentaenoic acid or docosahexaenoic acid, averaged 5.7 +/- 1.6 microM (n = 4) for eicosapentaenoic acid and 12.9 +/- 1.8 microM (n = 5) for docosahexaenoic acid at the end of infusion, but declined to undetectable levels within 3 h. Linoleic acid and oleic acid were less effective. Possible mechanisms for the modest anticonvulsant effect but of long duration with the polyunsaturated fatty acids are discussed.

Multiple effects of nordihydroguaiaretic acid on ionic currents in rat isolated type I carotid body cells

1. The effects of the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA) on the ionic currents of rat carotid body type I cells were investigated by use of whole-cell and outside-out patch clamp techniques. 2. NDGA (5-50 microM) produced a concentration-dependent inhibition of whole-cell K+ currents at all activating test potentials (holding potential -70 mV). The time-course of the inhibition was also concentration-dependent and the effects of NDGA were only reversible following brief periods of exposure (<2 min). Another lipoxygenase inhibitor, phenidone (5 microM), was without effect on whole-cell K+ currents in carotid body type I cells. 3. NDGA (5-50 microM) also inhibited whole-cell Ca2+ channel currents (recorded with Ba2+ as charge carrier) in a concentration-dependent manner. 4. Isolation of voltage-gated K+ channels by use of high [Mg2+] (6 mM), low [Ca2+] (0.1 mM) solutions revealed a direct inhibition of the voltage-sensitive component of the whole-cell K+ current by NDGA (50 microM). 5. In excised, outside-out patches NDGA (20-50 microM) increased large conductance, Ca2+ activated K+ channel activity approximately 10 fold, an effect which could be reversed by either tetraethylammonium (10 mM) or charybdotoxin (30 nM). 6. It is concluded that NDGA activates maxi-K+ channels in carotid body type I cells and over the same concentration range inhibits voltage-sensitive K+ and Ca2+ channels. The inhibition of whole cell K+ currents seen is most likely due to a combination of direct inhibition of the voltage-sensitive K+ current and indirect inhibition of maxi-K+ channel activity through blockade of Ca2+ channels.

Purification of a fatty acid-stimulated protein-serine/threonine phosphatase from bovine brain and its identification as a homolog of protein phosphatase 5

An arachidonic acid-stimulated Ser/Thr phosphatase activity was detected in soluble extracts prepared from rat pituitary clonal GH4C1 cells, rat or bovine brain, and bovine heart. The enzyme activity was purified to homogeneity from bovine brain as a monomer with a Mr of 63,000 and a specific activity of 32 nmol of Pi released per min/mg of protein when assayed in the presence of 10 microM phosphocasein in the absence of lipid. Arachidonic acid stimulated activity 4-14-fold, with half-maximal stimulation at 50-100 microM, when assayed in the presence of a variety of phosphosubstrates including casein, reduced carboxamidomethylated and maleylated lysozyme, myelin basic protein, and histone. Oleic acid, linoleic acid, and palmitoleic acid also stimulated activity; however, saturated fatty acids and alcohol or methyl ester derivatives of fatty acids did not significantly affect activity. The lipid-stimulated phosphatase was identified as the bovine equivalent of protein phosphatase 5 or a closely related homolog by sequence analysis of proteolytic fragments generated from the purified enzyme. When recombinant rat protein phosphatase 5 was expressed as a cleavable glutathione S-transferase fusion protein, the affinity-purified thrombin-cleaved enzyme exhibited a specific activity and sensitivity to arachidonic acid similar to those of the purified bovine brain enzyme. These results suggest that protein phosphatase 5 may be regulated in vivo by a lipid second messenger or another endogenous activator.

Regulation of intracellular calcium release channel function by arachidonic acid and leukotriene B4

Arachidonic acid has been shown to affect the intracellular calcium concentration in many cell types (1-5), but the target of this regulation was unclear. Here we show that two types of intracellular calcium release channel, the inositol 1,4,5-trisphosphate-gated channel (IP3R) and the ryanodine receptor (RyR) are modulated in an opposing manner by arachidonic acid and its product leukotriene B4 (LTB4). The IP3R was inhibited by arachidonic acid (Ki = 27 nM), whereas the RyR was unaffected by this compound. In contrast, 100 nM LTB4 fully activated the RyR but did not influence the IP3R. The concerted action of arachidonic acid and LTB4 could provide specific mobilization of stored calcium by terminating IP3-induced release and activating the RyR/calcium release channel by its newly identified agonist.

Accumulation of arachidonic acid in ischemic/reperfused cardiac tissue: possible causes and consequences

Under physiological conditions, the content of unesterified arachidonic acid in cardiac tissue is very low. The bulk of arachidonic acid is present in the membrane phospholipid pool. Incorporation of arachidonic acid into phospholipids (reacylation) and liberation of this fatty acid from the phospholipid pool (deacylation) are controlled by a set of finely tuned enzymes, including lysophospholipid acyltransferase and phospholipase A2. At present, at least three subtypes of phospholipase A2 have been identified in cardiac structures, i.e., a low molecular mass group II phospholipase A2, a cytoplasmic high molecular mass phospholipase A2 and a plasmalogen-specific phospholipase A2. Cessation of flow to the heart (ischemia) gives rise to net degradation of membrane phospholipids accompanied by accumulation of fatty acids, including (unesterified) arachidonic acid. Restoration of flow to the previously ischemic cells results in a continued accumulation of fatty acids. The mechanism(s) underlying net phospholipid degradation in ischemic/reperfused myocardial tissue is (are) incompletely understood. Impaired reacylation, enhanced hydrolysis of phospholipids, or a combination of both may be responsible for the phenomena observed. Elevated tissue levels of arachidonic acid may exert both direct and indirect effects on the affected myocardium and healthy cardiac cells adjacent to the injured cardiomyocytes. Indirect effects might be evoked by arachidonic acid metabolites, i.e., eicosanoids. Arachidonic acid may directly influence ion channel activity, substrate metabolism and signal transduction, thereby affecting the functional characteristics of the ischemic/reperfused myocardium.

Role of G proteins in alpha1-adrenergic inhibition of the beta-adrenergically activated chloride current in cardiac myocytes

alpha1-Adrenergic receptor stimulation can inhibit the Cl- current activated by beta-adrenergic receptor agonists in guinea-pig ventricular myocytes. We investigated the role of G proteins in mediating this type of alpha-adrenergic response. The combined alpha- and beta-adrenergic agonist norepinephrine (NE) activated the Cl- current with an EC50 value of 53 nM. Preincubation of myocytes with PTX decreased the EC50 value for NE activation of the Cl- current to 5.9 nM, and addition of the alpha1-adrenergic receptor antagonist prazosin did not cause any further change in sensitivity to NE. These results suggest that the alpha1-adrenergic inhibition of beta-adrenergic responses is mediated through a PTX-sensitive G protein. However, PTX pretreatment also increased the sensitivity of the Cl- current to the selective beta-adrenergic agonist isoproterenol (Iso), which indicates that the PTX treatment increases the sensitivity to beta-adrenergic stimulation alone and that this could account for the PTX-induced change in sensitivity to NE. Consistent with this idea, the selective alpha1-adrenergic receptor agonist methoxamine was still able to inhibit the Cl- current activated by Iso in PTX-treated myocytes. However, the sensitivity to methoxamine was significantly decreased. In control cells, the Cl- current activated by 30 nM Iso was inhibited by methoxamine with an EC50 value of 8.3 microM, but in PTX-treated cells, the EC50 value was 284 microM. The EC50 for methoxamine inhibition was similarly increased when the Cl- current was activated by 300 nM Iso. These data suggest that the effects of PTX on alpha1-adrenergic responses can actually be explained by changes in the sensitivity to beta-adrenergic stimulation. To verify the role for a G protein in mediating the inhibitory alpha1-adrenergic response, we examined the effect of methoxamine on the Cl- current activated in cells dialyzed with the nonhydrolyzable GTP analogue guanosine-5'-O-(3-thio)triphosphate. Pre-exposure to methoxamine resulted in an attenuated response upon subsequent exposure to Iso alone. We conclude that alpha1-adrenergic inhibition of beta-adrenergic responses is mediated by a G protein-dependent mechanism that appears to be PTX-insensitive.

Suppression of voltage-gated L-type Ca2+ currents by polyunsaturated fatty acids in adult and neonatal rat ventricular myocytes

Our recent data show that in cardiac myocytes polyunsaturated fatty acids (PUFAs) are antiarrhythmic. They reduce I(Na), shorten the action potential, shift the threshold for excitation to more positive potentials, and prolong the relative refractory period. In this study we use patch-clamp techniques in whole-cell mode and confocal Ca2+ imaging to examine the effects of PUFAs on the voltage-gated L-type Ca2+ current (I(Ca,L)), elementary sarcoplasmic reticulum Ca2+-release events (Ca2+-sparks), and [Ca2+]i transients in isolated rat ventricular myocytes. Extracellular application of eicosapentaenoic acid (EPA; C20:5 n - 3) produced a prompt and reversible concentration-dependent suppression of I(Ca,L). The concentration of EPA to produce 50% inhibition of I(Ca) was 0.8 microM in neonatal rat heart cells and 2.1 microM in adult ventricular myocytes. While the EPA induced suppression of I(Ca,L), it did not significantly alter the shape of the current-voltage relation but did produce a small, but significant, negative shift of the steady-state inactivation curve. The inhibition of I(Ca,L) was voltage- and time-dependent, but not use- or frequency-dependent. Other PUFAs, such as docosahexaenoic acid, arachidonic acid, linolenic acid, linoleic acid, conjugated linoleic acid, and eicosatetraynoic acid had similar effects on I(Ca,L) as EPA. All-trans-retinoic acid, which had been shown to suppress induced arrhythmogenic activity in rat heart cells, also produced a significant inhibition of I(Ca,L). The saturated stearic acid and the monounsaturated oleic acid had no effect on I(Ca,L). Because both I(Ca,L) and sarcoplasmic reticulum Ca2+-release underlie many cardiac arrhythmias, we examined the effects of EPA on I(Ca,L) and Ca2+-sparks. While EPA suppressed both, it did not change the temporal or spatial character of the Ca2+-sparks, nor did it alter the ability of I(Ca,L) to trigger Ca2+-sparks. We conclude that PUFAs may act as antiarrhythmic agents in vivo in normal and Ca2+-overloaded cells principally because they reduce Ca2+ entry by blocking I(Ca,L). Furthermore, PUFAs act directly to decrease I(Na) and I(Ca,L), but indirectly to reduce the [Ca2+]i transients and [Ca2+]i-activated membrane current. Although a negative inotropic action is associated with application of PUFAs, it is clear that by reducing I(Ca,L), I(Na) and Ca2+-sparks, PUFAs can reduce spontaneous extrasystoles in the heart. The mechanisms by which PUFAs act are discussed.

Polyunsaturated fatty acids modulate sodium and calcium currents in CA1 neurons

Recent evidence indicates that long-chain polyunsaturated fatty acids (PUFAs) can prevent cardiac arrhythmias by a reduction of cardiomyocyte excitability. This was shown to be due to a modulation of the voltage-dependent inactivation of both sodium (INa) and calcium (ICa) currents. To establish whether PUFAs also regulate neuronal excitability, the effects of PUFAs on INa and ICa were assessed in CA1 neurons freshly isolated from the rat hippocampus. Extracellular application of PUFAs produced a concentration-dependent shift of the voltage dependence of inactivation of both INa and ICa to more hyperpolarized potentials. Consequently, they accelerated the inactivation and retarded the recovery from inactivation. The EC50 for the shift of the INa steady-state inactivation curve was 2.1 +/- 0.4 microM for docosahexaenoic acid (DHA) and 4 +/- 0.4 microM for eicosapentaenoic acid (EPA). The EC50 for the shift on the ICa inactivation curve was 2.1 +/- 0.4 for DHA and > 15 microM for EPA. Additionally, DHA and EPA suppressed both INa and ICa amplitude at concentrations > 10 microM. PUFAs did not affect the voltage dependence of activation. The monounsaturated oleic acid and the saturated palmitic acid were virtually ineffective. The combined effects of the PUFAs on INa and ICa may reduce neuronal excitability and may exert anticonvulsive effects in vivo.