Epoxyeicosatrienoic acids inhibit Ca2+ entry into platelets stimulated by thapsigargin and thrombin

Epoxides and soluble epoxide hydrolase in cardiovascular physiology

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites that importantly contribute to vascular and cardiac physiology. The contribution of EETs to vascular and cardiac function is further influenced by soluble epoxide hydrolase (sEH) that degrades EETs to diols. Vascular actions of EETs include dilation and angiogenesis. EETs also decrease inflammation and platelet aggregation and in general act to maintain vascular homeostasis. Myocyte contraction and increased coronary blood flow are the two primary EET actions in the heart. EET cell signaling mechanisms are tissue and organ specific and provide significant evidence for the existence of EET receptors. Additionally, pharmacological and genetic manipulations of EETs and sEH have demonstrated a contribution for this metabolic pathway to cardiovascular diseases. Given the impact of EETs to cardiovascular physiology, there is emerging evidence that development of EET-based therapeutics will be beneficial for cardiovascular diseases.

Oleic acid versus linoleic and α-linolenic acid. different effects on Ca2+ signaling in rat thymocytes

BACKGROUND

Oleic acid is the principal fatty acid of olive oil composition and is reported to play a crucial role in its healthy aspects. However, the detailed mechanism of action is poorly understood.

AIMS

This study aims to elucidate the role of oleic acid in calcium signaling in rat thymocytes, in comparison to linoleic and linolenic acid.

METHODS

Fatty acids were applied to thymocytes isolated from wistar rats and loaded with Fura-2 to measure calcium signals.

RESULTS

The main results showed a concentration-dependent increase in [Ca(2+)](i) induced by the 3 fatty acids. Raising the number of unsaturations resulted in greater increases. Two different pathways contributed to the increase induced by the polyunsaturated fatty acids: an IP(3)- independent release from the thapsigargin-sensitive stores and an extracellular calcium entry by econazole and nifedipine-insensitive channels. However, the OA-induced increases in [Ca(2+)](i) seemed to be due mostly to the Ca(2+) recruited from the intracellular stores.

CONCLUSION

This study demonstrates that the fatty acids tested induce increases in [Ca(2+)](i) in rat thymocytes, with differences in close relation to the degree of unsaturation. Such differences could be responsible for their different physiological action.

Interaction of arachidonic acid with electrogenic properties of mouse chemosensory neurons

Chemosensory neurons respond to stimulation induced by gasses, volatile and non-volatile compounds. Neuronal excitation mediated via second messengers involves typically: cGMP, cAMP, or IP₃. Transduction pathways based on cyclic nucleotide have three-phosphate nucleotide as substrate, while IP₃ has a membrane lipid substrate. These derivatives of cholesterol are signaling molecules with modulator-like effects on many proteins, including membrane ion channels. In the present study, spontaneous and induced activities were recorded in a whole-cell configuration, in current and voltage clamp modes, in isolated chemosensory neurons obtained from the mouse. Chemosensory neurons responded with an inward depolarizing current to application of arachidonic acid, which suggests a role for it in putative mechanisms of signal transduction.

Epoxide hydrolase and epoxygenase metabolites as therapeutic targets for renal diseases

Renal epoxygenase metabolites are involved in blood flow regulation and long-term blood pressure control. One feature of renal and cardiovascular diseases is the inability of the kidney to properly increase epoxyeicosatrienoic acid (EET) levels. Others (Busse R, Edwards G, Félétou M, Fleming I, Vanhoutte PM, and Weston AH. Trends Phamacol Sci 23: 374–380, 2002; Campbell WB, Gebremedhin D, Pratt PF, and Harder DR. Circ Res 78: 415–423, 1996; Capdevila JH and Falck JR. Biochem Biophys Res Commun 285: 571–576, 2001; Roman RJ. Physiol Rev 82: 131–185, 2002; Zeldin DC. J Biol Chem 276: 36059–36062, 2001) and we (Imig JD, Falck JR, Wei S, and Capdevila JH. J Vasc Res 38: 247–255, 2001; Imig JD, Zhao X, Capdevila JH, Morisseau C, and Hammock BD. Hypertension 39: 690–694, 2002; Zhao X, Pollock DM, Inscho EW, Zeldin DC, and Imig JD. Hypertension 41: 709–714, 2003; Zhao X, Pollock DM, Zeldin DC, and Imig JD. Hypertension 42: 775–780, 2003) have provided compelling evidence that cytochrome P-450-derived EETs have antihypertensive properties and are endothelially derived hyperpolarizing factors (EDHFs) in the kidney. EETs also possess anti-inflammatory actions that could protect the kidney vasculature from injury during renal and cardiovascular diseases. A tactic that has been used to increase EET levels has been inhibition of the soluble epoxide hydrolase enzyme. Epoxide hydrolase inhibitors have been demonstrated to be antihypertensive and renal protective. Thus the renal and cardiovascular protective actions of increasing epoxygenase levels could be translated to therapies for preventing end-organ damage.

Capillary electrophoresis of cytochrome P-450 epoxygenase metabolites of arachidonic acid. 2. Resolution of stereoisomers

Each of the four regioisomers of epoxyeicosatrienoic acids (EETs) is a candidate for being an endothelial-dependent hyperpolarizing factor (EDHF). One regioisomer, 14,15-EET, stereospecifically blocks cyclooxygenases from converting arachidonic acid to prostaglandins and stereospecifically binds to cellular receptors. Both stereospecific actions emphasize the need to establish the tissue availability of the 14,15-EET enantiomers. The present work describes a method to quantitate picogram amounts of 14,15-EET enantiomers by capillary electrophoresis. The 14,15-EET enantiomers were baseline resolved (R = 1.3) using unsubstituted beta-cyclodextrin and 32% acetonitrile (v/v). When absorption at 194 nm was monitored using a photodiode array detector, 8 and 1 pg of underivatized 14,15-EET were readily quantitated and detected, respectively. Capillary electrophoresis accurately assessed chiral excesses up to 97:3 for either 14,15-EET enantiomer. Moreover, capillary electrophoresis with a photodiode array detector was sufficiently sensitive to detect and measure 14,15-EET enantiomers from murine liver. Thus, unlike chiral-phase high-performance liquid chromatography, capillary electrophoresis can be used to directly assess the chirality of trace amounts of underivatized eicosanoids.

Mechanisms of bradykinin-mediated Ca(2+) signalling in canine cultured corneal epithelial cells

Experiments were designed to differentiate the mechanisms of bradykinin receptors mediating the changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) in canine cultured corneal epithelial cells (CECs). Bradykinin and Lys-bradykinin caused an initial transient peak of [Ca(2+)](i) in a concentration-dependent manner, with half-maximal stimulation (pEC(50)) obtained at 6.9 and 7.1, respectively. Pretreatment of CECs with pertussis toxin (PTX) or cholera toxin (CTX) for 24 h did not affect the bradykinin-induced [Ca(2+)](i) changes. Application of Ca(2+) channel blockers, diltiazem and Ni(2+), inhibited the bradykinin-induced Ca(2+) mobilization, indicating that Ca(2+) influx was required for the bradykinin-induced responses. Addition of thapsigargin (TG), which is known to deplete intracellular Ca(2+) stores, transiently increased [Ca(2+)](i) in Ca(2+)-free buffer, and subsequently induced Ca(2+) influx when Ca(2+) was readded to this buffer. Pretreatment of CECs with TG completely abolished bradykinin-induced initial transient [Ca(2+)](i), but had slight effect on bradykinin-induced Ca(2+) influx. Pretreatment of CECs with 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole (SKF96365) and 1-(6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122) inhibited the bradykinin-induced Ca(2+) release and Ca(2+) influx, consistent with the inhibition of receptor-gated Ca(2+) channels and phospholipase C (PLC) in CECs, respectively. These results demonstrate that bradykinin directly stimulates B(2) receptors and subsequently Ca(2+) mobilization via a PTX-insensitive G protein in canine CECs. These results suggest that bradykinin-induced Ca(2+) influx into the cells is not due to depletion of these Ca(2+) stores, as prior depletion of these pools by TG has no effect on the bradykinin-induced Ca(2+) influx that is dependent on extracellular Ca(2+) in CECs.

Mechanism and signal transduction of 14 (R), 15 (S)-epoxyeicosatrienoic acid (14,15-EET) binding in guinea pig monocytes

14(R), 15(S)-epoxyeicosatrienoic acid (14,15-EET) is a cytochrome P-450 monooxygenase (epoxygenase) metabolite of arachidonic acid (AA). In this study, we have identified a population of specific high affinity binding sites for 14,15-EET in the guinea pig mononuclear (GPM) cells. The results of competition studies showed that 14(R), 15(S)-EET was an effective competing ligand with a Ki of 226.3 nM followed by 11(R), 12(S)-EET, 14(S), 15(R)-EET, 14,15 thia(S)-ET, and 14,15-aza(N)-ET. The binding was sensitive to various protease treatments suggesting that the binding site is protein in nature. Cholera toxin (CT) and dibutyryl cAMP attenuated 14,15-EET binding in GPM cells. Mean binding site density (Bmax), decreased 32.0% and 19.1% by the pretreatment with cholera toxin (200 micrograms/ml) and dibutyryl cAMP (100 nM), respectively, without changing the dissociation constant. A specific protein kinase A (PKA) inhibitor, H-89, but not the PKC inhibitor K252a reversed the down regulation of 14,15-EET receptor binding caused by dibutyryl cAMP in GPM cells. Thus, the results sug-gest that the specific binding site of 14,15-EET in GPM cells be associated with a receptor that could be down regulated through an increase in intracellular cAMP and activation of a PKA signal trans-duction. We propose that the signal transduction mechanism begins with the binding of 14,15-EET to its receptor that leads to increase intracellular cAMP levels and the activation of PKA, and finally, with the down regulation of 14,15-EET receptor binding.

Epoxyeicosatrienoic acids increase intracellular calcium concentration in vascular smooth muscle cells

Epoxyeicosatrienoic acids (EETs) are cytochrome P450-derived metabolites of arachidonic acid. They are potent endogenous vasodilator compounds produced by vascular cells, and EET-induced vasodilation has been attributed to activation of vascular smooth muscle cell (SMC) K(+) channels. However, in some cells, EETs activate Ca(2+) channels, resulting in Ca(2+) influx and increased intracellular Ca(2+) concentration ([Ca(2+)](i)). We investigated whether EETs also can activate Ca(2+) channels in vascular SMC and whether the resultant Ca(2+) influx can influence vascular tone. The 4 EET regioisomers (1 micromol/L) increased porcine aortic SMC [Ca(2+)](i) by 52% to 81%, whereas arachidonic acid, dihydroxyeicosatrienoic acids, and 15-hydroxyeicosatetraenoic acid (1 micromol/L) produced little effect. The increases in [Ca(2+)](i) produced by 14,15-EET were abolished by removal of extracellular Ca(2+) and by pretreatment with verapamil (10 micromol/L), an inhibitor of voltage-dependent (L-type) Ca(2+) channels. 14,15-EET did not alter Ca(2+) signaling induced by norepinephrine and thapsigargin. When administered to porcine coronary artery rings precontracted with a thromboxane mimetic, 14,15-EET produced relaxation. However, when administered to rings precontracted with acetylcholine or KCl, 14,15-EET produced additional contractions. In rings exposed to 10 mmol/L KCl, a concentration that did not affect resting ring tension, 14,15-EET produced small contractions that were abolished by EGTA (3 mmol/L) or verapamil (10 micromol/L). These observations indicate that 14,15-EET enhances [Ca(2+)](i) influx in vascular SMC through voltage-dependent Ca(2+) channels. This 14,15-EET-induced increase in [Ca(i)(2+)] can produce vasoconstriction and therefore may act to modulate EET-induced vasorelaxation.

Bradykinin-induced phosphoinositide hydrolysis and Ca2+ mobilization in canine cultured tracheal epithelial cells

1. Experiments were designed to differentiate the mechanisms and subtype of kinin receptors mediating the changes in intracellular Ca2+ concentration ([Ca2+]i) induced by bradykinin (BK) in canine cultured tracheal epithelial cells (TECs). 2. BK and Lys-BK caused an initial transient peak of [Ca2+]i in a concentration-dependent manner, with half-maximal stimulation (pEC50) obtained at 7.70 and 7.23, respectively. 3. Kinin B2 antagonists Hoe 140 (10 nM) and [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK (1 microM) had high affinity in antagonizing BK-induced Ca2+ response with pKB values of 8.90 and 6.99, respectively. 4. Pretreatment of TECs with pertussis toxin (100 ng ml(-1)) or cholera toxin (10 microg ml(-1)) for 24 h did not affect the BK-induced IP accumulation and [Ca2+]i changes in TECs. 5. Removal of Ca2+ by the addition of EGTA or application of Ca2+-channel blockers, verapamil, diltiazem, and Ni2+, inhibited the BK-induced IP accumulation and Ca2+ mobilization, indicating that Ca2+ influx was required for the BK-induced responses. 6. Addition of thapsigargin (TG), which is known to deplete intracellular Ca2+ stores, transiently increased [Ca2+]i in Ca2+-free buffer and subsequently induced Ca2+ influx when Ca2+ was re-added to this buffer. Pretreatment of TECs with TG completely abolished BK-induced initial transient [Ca2+]i, but had slight effect on BK-induced Ca2+ influx. 7. Pretreatment of TECs with SKF96365 and U73122 inhibited the BK-induced Ca2+ influx and Ca2+ release, consistent with the inhibition of receptor-gated Ca2+-channels and phospholipase C in TECs, respectively. 8. These results demonstrate that BK directly stimulates kinin B2 receptors and subsequently phospholipase C-mediated IP accumulation and Ca2+ mobilization via a pertussis toxin-insensitive G protein in canine TECs. These results also suggest that BK-induced Ca2+ influx into the cells is not due to depletion of these Ca2+ stores, as prior depletion of these pools by TG has no effect on the BK-induced Ca2+ influx that is dependent on extracellular Ca2+ in TECs.

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.

Calcium influx factor, further evidence it is 5, 6-epoxyeicosatrienoic acid

We present evidence in astrocytes that 5,6-epoxyeicosatrienoic acid, a cytochrome P450 epoxygenase metabolite of arachidonic acid, may be a component of calcium influx factor, the elusive link between release of Ca2+ from intracellular stores and capacitative Ca2+ influx. Capacitative influx of extracellular Ca2+ was inhibited by blockade of the two critical steps in epoxyeicosatrienoic acid synthesis: release of arachidonic acid from phospholipid stores by cytosolic phospholipase A2 and cytochrome P450 metabolism of arachidonic acid. AAOCF3, which inhibits cytosolic phospholipase A2, blocked thapsigargin-stimulated release of arachidonic acid as well as thapsigargin-stimulated elevation of intracellular free calcium. Inhibition of P450 arachidonic acid metabolism with SKF525A, econazole, or N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide, a substrate inhibitor of P450 arachidonic acid metabolism, also blocked thapsigargin-stimulated Ca2+ influx. Nano- to picomolar 5, 6-epoxyeicosatrienoic acid induced [Ca2+]i elevation consistent with capacitative Ca2+ influx. We have previously shown that 5, 6-epoxyeicosatrienoic acid is synthesized and released by astrocytes. When 5,6-epoxyeicosatrienoic acid was applied to the rat brain surface, it induced vasodilation, suggesting that calcium influx factor may also serve a paracrine function. In summary, our results suggest that 5,6-epoxyeicosatrienoic acid may be a component of calcium influx factor and may participate in regulation of cerebral vascular tone.

Dissociation of intracellular Ca2+ release and Ca2+ entry response to 5-hydroxytryptamine in cultured canine tracheal smooth muscle cells

The relationship between the agonist-sensitive Ca2+ pool and those discharged by the Ca2+ -ATPase inhibitor thapsigargin (TG) were investigated in canine tracheal smooth muscle cells (TSMCs). In fura-2-loaded TSMCs, 5-hydroxytryptamine (5-HT) stimulated a rapid increase in intracellular Ca2+ ([Ca2+]i), followed by a sustained plateau phase that was dependent on extracellular Ca2+. In such cells, TG produced a concentration-dependent increase in [Ca2+]i, which remained elevated over basal level for several minutes and was substantially attenuated in the absence of extracellular Ca2+. Application of 5-HT after TG demonstrated that the TG-sensitive compartment partly overlapped the 5-HT-sensitive stores. Pre-treatment of TSMCs with TG significantly inhibited the increase in [Ca2+]i induced by 5-HT in a time-dependent manner. Similar results were obtained with two other Ca2+ -ATPase inhibitors, cyclopiazonic acid and 2,5-di-t-butylhydroquinone. Although these inhibitors had no effect on phosphoinositide hydrolysis, Ca2+ -influx was stimulated by these agents. These results suggest that depletion of the agonist-sensitive Ca2+ stores is sufficient for activation of Ca2+ influx. Some characteristics of the Ca2+ -influx activated by depletion of internal Ca2+ stores were compared with those of the agonist-activated pathway. 5-HT-stimulated Ca2+ influx was inhibited by La3+, membrane depolarisation, and the novel Ca2+ -influx blocker 1-¿beta-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl¿-1H-imidazole hydrochloride (SKF96365). Likewise, activation of Ca2+ influx by TG also was blocked by La3+, membrane depolarisation, and SKF96365. These results suggest that (1) in the absence of PI hydrolysis, depletion of the agonist-sensitive internal Ca2+ stores in TSMCs is sufficient for activation of Ca2+ influx, and (2) the agonist-activated Ca2+ influx pathway and the influx pathway activated by depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool are indistinguishable.

Involvement of cytochrome P-450 enzyme activity in the control of microvascular permeability in canine lung

Products of cytochrome P-450 enzymes may play a role in capacitative Ca2+ entry in endothelial cells, which can promote a rise in vascular permeability. Thapsigargin (150 nM) stimulated capacitative Ca2+ entry and increased the capillary filtration coefficient (Kf,c) in isolated normal canine lung lobes. Pretreatment of the lobes with cytochrome P-450 inhibitors clotrimazole (10 microM) or 17-octadecynoic acid (5 microM) abolished the thapsigargin-induced increases in Kf,c. Because clotrimazole also blocks Ca2+-activated K+ channels, the K+-channel blocker tetraethylammonium (10 mM) was used to ensure that permeability was not influenced by this mechanism. Tetraethylammonium did not affect thapsigargin-induced permeability. The effects of the cytochrome P-450 arachidonic acid metabolite 5,6-epoxyeicosatrienoic acid (EET) were also investigated in lobes taken from control dogs and dogs with pacing-induced heart failure (paced at 245 beats/min for 4 wk). 5,6-EET (10 microM) significantly increased Kf,c in lobes from the control but not from the paced animals. We conclude that cytochrome P-450 metabolites are involved in mediating microvascular permeability in normal canine lungs, but an absence of 5,6-EET after heart failure does not explain the resistance of lungs from these animals to permeability changes.

Arachidonic acid inhibits capacitative calcium entry in rat thymocytes and human neutrophils

Emptying the intracellular Ca2+ stores by treatment with the endomembrane Ca2+-ATPase inhibitor thapsigargin activates capacitative Ca2+ entry (CCE). This can be evidenced in fura-2-loaded cells by an increase of [Ca2+]i or by an acceleration of Mn2+ entry. Micromolar concentrations of arachidonic acid inhibited CCE induced by treatment with thapsigargin in rat thymocytes and in human neutrophils. This inhibitory action was shared by other unsaturated fatty acids, but not by the saturated arachidic acid nor by arachidonic acid methyl ester. The effect was not due to metabolites derived from arachidonic acid since several non-metabolizable analogs were able to reproduce it. Phorbol dibutyrate (PDB) acted similarly, suggesting that the inhibitory effect could be mediated by activation of protein kinase C (PKC). However, whereas the inhibition of CCE by PDB was reversed by treatment with the PKC inhibitor staurosporin, the inhibition by arachidonic acid was not. We find that unsaturated fatty acids antagonized microsomal dealkylation of benzyl-resorufin, a cytochrome P450-mediated activity, with the same specificity profile as for inhibition of CCE. These results are consistent with previous proposals suggesting that a microsomal cytochrome P450 may be involved in the regulation of CCE.

Post-receptor signal transduction and regulation of 14(R),15(S)-epoxyeicosatrienoic acid (14,15-EET) binding in U-937 cells

14(R),15(S)-epoxyeicosatrienoic acid (14,15-EET), a cytochrome P-450 monooxygenase (epoxygenase) metabolite of arachidonic acid has been reported to induce adhesion of a monocyte cell line (U-937) to cultured endothelial cells. In this study, we identified a population of specific, high affinity binding sites for 14(R),15(S)-EET in U-937 cell surface with Kd of 13.84 +/- 2.58 nM and Bmax of 3.54 +/- 0.28 pmol/10(6) cells. The specific binding of [3H]-14,15-EET on U-937 cells is more effectively displaced by 14(R),15(S)-EET than the 14(S),15(R)-isomer thus indicating stereospecificity. The binding was sensitive to various protease treatments suggesting the binding site is protein in nature. 14,15-EET binding in U937 cells is attenuated by cholera toxin (CT) and dibutyryl cAMP. Mean binding site density (Bmax) decreased 31.61% and 34.8% by the pretreatment with cholera toxin (200 micrograms/ml) and dibutyryl cAMP (300 nM), respectively, without affecting the dissociation constant. Under similar conditions, pertussis toxin (20-200 ng/ml) was less effective as compared to CT and dibutyryl cAMP. The down regulation of 14,15-EET binding caused by dibutyryl cAMP in U-937 cell was reversed by a specific protein kinase A (PKA) inhibitor, H-89, but not by the PKC inhibitor K252a. Thus, the results suggest that the specific binding site of 14,15-EET in U-937 cells is associated with a receptor that could be down regulated through an increase in intracellular cAMP and activation of a PKA signal transduction mechanism. We propose that the signal transduction mechanism of 14,15-EET begins with the binding of the receptor, which leads to the increase of intracellular cAMP levels and the activation of PKA, and finally with the down regulation of 14,15-EET receptor binding.

Bisallylic hydroxylation and epoxidation of polyunsaturated fatty acids by cytochrome P450

Polyunsaturated fatty acids can be oxygenated by cytochrome P450 to hydroxy and epoxy fatty acids. Two major classes of hydroxy fatty acids are formed by hydroxylation of the omega-side chain and by hydroxylation of bisallylic methylene carbons. Bisallylic cytochrome P450-hydroxylases transform linoleic acid to 11-hydroxylinoleic acid, arachidonic acid to 13-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid, 10-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and 7-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and eicosapentaenoic acid to 16-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid, 13-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid and 10-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid as major metabolites. The bisallylic hydroxy fatty acids are chemically unstable and decompose rapidly to cis-trans conjugated hydroxy fatty acids during acidic extractive isolation. Bisallylic hydroxylase activity appears to be augmented in microsomes induced by the synthetic glucocorticoid dexamethasone and by some other agents, but the P450 gene families of these hydroxylases have yet to be determined. The fatty acid epoxides, which are formed by cytochrome P450, are chemically stable, but are hydrolyzed to diols by soluble epoxide hydrolases. Epoxidation of polyunsaturated fatty acids is a prominent pathway of metabolism in the liver and the renal cortex and epoxy-genase activity appears to be under homeostatic control in the kidney. Many arachidonate epoxygenases have been identified belonging to the CYP2C gene subfamily. Epoxygenases have also been found in the central nervous system, endocrine organs, the heart and endothelial cells. Epoxides of arachidonic acid have been found to exert pharmacological effects on many cells.

Ca2+ increase and Ca(2+)-influx in human tracheal smooth muscle cells: role of Ca2+ pools controlled by sarco-endoplasmic reticulum Ca(2+)-ATPase 2 isoform

1. The contribution of sarco-endoplasmic reticulum Ca(2+)-ATPases (SERCA)-regulated Ca2+ stores to the increase in intracellular free calcium ([Ca2+]i) induced by bradykinin (BK) was investigated in fura-2 loaded human tracheal smooth muscle cells (TSMC). For this purpose, we used thapsigargin, a selective inhibitor of Ca(2+)-ATPases of intracellular organelles. 2. Thapsigargin (10(-9) to 10(-6) M) induced a dose-dependent increase in [Ca2+]i in the presence of external Ca2+ with an EC50 value of 7.33 +/- 1.26 nM. In Ca(2+)-free conditions, the addition of Ca2+ (1.25 mM) caused an increase in [Ca2+]i which was directly proportional to the pre-incubation time of the cells with thapsigargin. Net increases of 60 +/- 9, 150 +/- 22 and 210 +/- 27 nM were obtained after 1, 3 and 5 min, respectively. 3. In the presence of extracellular Ca2+, BK induced a typical biphasic increase in [Ca2+]i with a fast transient phase and a sustained phase. The sustained component was reversed by addition of a bradykinin B2-receptor antagonist (Hoe 140, 10(-6) M) to the buffer as well as by deprivation of Ca2+. The transient phase induced by BK, histamine and carbachol was inhibited in a time-dependent way by preincubation of the cells with thapsigargin. 4. Comparative western blotting of human TSMC membranes using anti-SERCA2 isoform-specific antibodies clearly showed the greater expression of the 100-kDa SERCA2-b isoform compared with the SERCA2-a isoform. 5. Our data show that thapsigargin-sensitive Ca2+ stores contribute significantly to the activation of human TSMC which suggests a role for these stores in the subsequent induction of Ca2+ influx. These stores appear to be controlled by the Ca2+-ATPases (SERCA2-b isoform) which could also participate in the regulation of Ca2+ influx through the plasma membrane.

Free radicals irreversibly decrease Ca2+ currents in isolated guinea-pig ventricular myocytes

The effects of free radicals on voltage-gated Ca2+ currents (ICa) were investigated in single guinea-pig ventricular myocytes using the whole-cell clamp technique. ICa was measured in the baseline state and after the application of free radicals from cumene hydroperoxide or generated from the addition of purine to xanthine oxidase. ICa decreased from 846 +/- 533 (S.D.) pA to 688 +/- 444 pA (n = 7, P < 0.05) in the presence of 100 microM cumene hydroperoxide and from 708 +/- 157 pA to 457 +/- 163 pA (n = 5, P < 0.0001) in the presence of 500 microM cumene hydroperoxide. ICa also decreased from 1303 +/- 560 pA to 965 +/- 360 pA in the presence of the free radical generating system (2.3 mM purine plus 20 U/l xanthine oxidase). The reduced ICa could not be restored by washing for up to 5 min using normal recording solution. We conclude that ICa is decreased in the presence of cumene hydroperoxide and an oxygen-derived free radical generating system in single guinea-pig ventricular myocytes. The cellular Ca2+ overload observed in free radical mediated reperfusion injury is therefore unlikely to result from an increase in sarcolemmal Ca2+ entry via voltage-gated Ca2+ channels.

Effects of 5,8,11,14-eicosatetraynoic acid on thapsigargin-induced calcium entry, and intracellular pH in thyroid FRTL-5 cells

The effect of 5,8,11,14-eicosatetraynoic acid (ETYA), an inhibitor of lipoxygenase and cytochrome P-450 epoxygenase enzymes, on calcium fluxes was investigated in Fura 2 loaded rat thyroid FRTL-5 cells. ETYA per se released sequestered calcium. ETYA also inhibited calcium influx in thapsigargin-stimulated cells in dose-dependent manner. Addition of calcium to cells treated with ETYA and stimulated with thapsigargin in a calcium-free buffer resulted in a blunted increase in intracellular free calcium compared with the response in control cells. In addition, ETYA per se acidified the cytosol in a dose-dependent manner. Acidification of the cytosol with the K+/H+ ionophore nigericin also decreased thapsigargin-induced calcium entry, but not to the same extent as that seen in cells treated with ETYA. The results suggest that ETYA is a potent modulator of calcium entry, and that part of the inhibitory effect of ETYA may be due to the ETYA-induced acidification of the cytosol.

n-3 fatty acids and renal diseases

There are numerous biologic rationales for the use of n-3 fatty acids in renal diseases, including a possible increase in the renal vasodilatory capacity by a rearrangement of renal prostanoid production, a reduction in the production of proinflammatory leukotrienes, a reduction in the transcapillary escape rate of albumin, and actions limiting cyclosporine-related nephrotoxicity. Studies of animal models of renal disease, mostly of immune-renal disease, support the idea of the possible usefulness of these compounds. The most promising areas of clinical investigation include the reduction of proteinuria in some chronic glomerular diseases, the treatment of immunoglobulin A nephropathy, and the prevention of cyclosporine-induced nephrotoxicity. However, the results of larger clinical studies, some of which are ongoing, are necessary to support the use of n-3 fatty acids in human renal diseases.

Thapsigargin-induced calcium entry in FRTL-5 cells: possible dependence on phospholipase A2 activation

Stimulating rat thyroid FRTL-5 cells with agonists that activate the inositol phosphate cascade results in the release of sequestered calcium and influx of extracellular calcium. In addition, phospholipase A2 (PLA2) is activated. Since PLA2 is a calcium-dependent enzyme we wanted to investigate the interrelationships between PLA2 activity and the entry of calcium. Stimulating 3H-arachidonic acid (3H-AA)-labelled cells with thapsigargin resulted in a substantial release of 3H-AA. This release was totally abolished in a calcium-free buffer. Pretreatment of Fura 2 loaded cells with 4-bromophenacyl bromide, an inhibitor of PLA2 activity, decreased the thapsigargin-induced entry of calcium, suggesting a role for PLA2 in the regulation of calcium entry. In cells treated with nordihydroguaiaretic acid (NDGA), clotramizole, or econazole, compounds with lipoxygenase and cytochrome P-450 inhibitory actions, the thapsigargin-induced entry of calcium was decreased in a dose-dependent manner. However, treatment of the cells with indomethacin, a cyclooxygenase inhibitor, had no effect on the thapsigargin-induced calcium entry. We also showed that stimulation of the cells with arachidonic acid released sequestered calcium, apparently from the same intracellular pool as did thapsigargin. The results suggested that the calcium-induced PLA2 activation and the metabolism of the produced arachidonic acid by a noncyclooxygenase pathway may be of importance in maintaining calcium entry after releasing sequestered Ca2+ in FRTL-5 cells.

TRH-evoked entry of extracellular calcium in GH4C1 cells: possible importance of arachidonic acid metabolites

Previous studies have shown that stimulating pituitary GH4C1 cells with thyrotropin-releasing hormone (TRH) evoked a biphasic change in cytosolic free Ca2+ concentration ([Ca2+]i): a rapid release of sequestered Ca2+ due to the production of inositol-1,4,5-trisphosphate, and Ca2+ entry via both voltage-operated Ca2+ channels and a presently unknown voltage-independent influx pathway. The aim of the present study was to further evaluate to which extent the TRH-evoked changes in [Ca2+]i were dependent on entry of extracellular Ca2+, and which mechanisms participated in regulating this Ca2+ entry. Pretreatment of the cells with 4-bromophenylacylbromide (an inhibitor of phospholipase A2), nordihydroguaiaretic acid (an inhibitor of lipoxygenase), and econazole (an inhibitor of both lipoxygenase and cytochrome P-450 enzymes), attenuated the TRH-evoked increase in [Ca2+]i, suggesting that noncyclooxygenase metabolites of arachidonic acid or cytochrome P-450 metabolites may participate in regulating the TRH-evoked entry of extracellular Ca2+. Both nordihydroguaiaretic acid and econazole showed a similar inhibition of the Ca2+ entry, as did SKF 96365, a compound previously shown to inhibit receptor-activated Ca2+ entry. We also showed that arachidonic acid per se increased [Ca2+]i, and acidified the cytosol in GH4C1 cells in a dose-dependent manner. The effects of arachidonic acid was reversed by addition of BSA to the cell suspension. The calcium entry and the activation of the metabolism of arachidonic acid may thus be important components of the TRH-evoked signal-transduction pathway in GH4C1 cells.

Importance of arachidonic acid metabolites in regulating ATP-induced calcium fluxes in thyroid FRTL-5 cells

Stimulating rat thyroid FRTL-5 cells with the purinergic agonist ATP activates both the inositol phosphate signal-transduction pathway and the phospholipase A2 pathway. In the present study we wanted to investigate the possible inter-relationships between these two systems during ATP-induced changes in intracellular free calcium ([Ca2+]i). Pretreatment of Fura-2 loaded cells with 4-bromophenylacyl, an inhibitor of phospholipase A2, had no effect on the ATP-induced entry of Ca2+ but inhibited the release of sequestered Ca2+. Nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, and 5,8,11,14-eicosatetraynoic acid (ETYA), an inhibitor of cytochrome P-450 enzymes, attenuated the ATP-evoked transient increase in [Ca2+]i. Furthermore, the capacitative entry of Ca2+ was also attenuated in NDGA- and ETYA-treated cells stimulated with ATP. Similar results were obtained using econazole, an inhibitor of cytochrome P-450 enzymes. However, treatment of the cells with indomethacin, a cyclooxygenase inhibitor, had no effect on the ATP-evoked response in [Ca2+]i. We also showed that stimulation of intact or permeabilized FRTL-5 cells with arachidonic acid released sequestered calcium. This calcium originated, at least in part, from an IP3 sensitive calcium pool. In addition, arachidonic acid rapidly acidified the cytosol. The results suggest that metabolism of arachidonic acid by a non-cyclooxygenase pathway is of importance in supporting agonist-induced calcium fluxes evoked via stimulation of the inositol phosphate pathway in FRTL-5 cells. Furthermore, arachidonic acid per se may modify agonist-induced calcium fluxes in these cells.

Oxygenation of polyunsaturated fatty acids by cytochrome P450 monooxygenases

Polyunsaturated fatty acids can be oxygenated by P450 in different ways--by epoxidation, by hydroxylation of the omega-side chain, by allylic and bis-allylic hydroxylation and by hydroxylation with double bond migration. Major organs for these oxygenations are the liver and the kidney. P450 is an ubiquitous enzyme. It is therefore not surprising that some of these reactions have been found in other organs and tissues. Many observations indicate that P450 oxygenates arachidonic acid in vivo in man and in experimental animals. This is hardly surprising. omega-Oxidation was discovered in vivo 60 years ago. It was more unexpected that biological activities have been associated with many of the P450 metabolites of arachidonic acid, at least in pharmacological doses. Epoxygenase metabolites of arachidonic acid have attracted the largest interest. In their critical review on epoxygenase metabolism of arachidonic acid in 1989, Fitzpatrick and Murphy pointed out some major differences between the PGH synthase, the lipoxygenase and the P450 pathways of arachidonic acid metabolism. Their main points are still valid and have only to be modified slightly in the light of recent results. First, lipoxygenases show a marked regiospecificity and stereospecificity, while many P450 seem to lack this specificity. There are, however, P450 isozymes which catalyse stereospecific epoxidations or hydroxylations. Many hydroxylases and at least some epoxygenases also show regiospecificity, i.e. oxygenate only one double bond or one specific carbon of the fatty acid substrate. In addition, preference for arachidonic acid and eicosapentaenoic acid may occur in the sense that other fatty acids are oxygenated with less regiospecificity. A more important difference is that prostaglandins and leukotrienes affect specific and well characterised receptors in cell membranes, while receptors for epoxides of arachidonic acid or other P450 metabolites have not been characterised. Nevertheless, epoxides of arachidonic acid have been found to induce a large number of different pharmacological effects. In some systems, effects have been noted at pm concentrations which might conceivably be in the physiological concentration range of these epoxides, e.g. after release from phospholipids by phospholipase A2. An intriguing possibility is that the effects of [Ca]i on different ion channels might possibly explain their biological actions. In situations when pharmacological doses are used, metabolism to epoxyprostanoids or other interactions with PGH synthase could also be of importance. Finally, one report on a specific receptor for 14R,15S-EpETrE in mononuclear cell membranes has just been published.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.