Actions of epoxyeicosatrienoic acid on large-conductance Ca(2+)-activated K(+) channels in pituitary GH(3) cells

Coronary Large Conductance Ca2+-Activated K+ Channel Dysfunction in Diabetes Mellitus

Diabetes mellitus (DM) is an independent risk of macrovascular and microvascular complications, while cardiovascular diseases remain a leading cause of death in both men and women with diabetes. Large conductance Ca2+-activated K+ (BK) channels are abundantly expressed in arteries and are the key ionic determinant of vascular tone and organ perfusion. It is well established that the downregulation of vascular BK channel function with reduced BK channel protein expression and altered intrinsic BK channel biophysical properties is associated with diabetic vasculopathy. Recent efforts also showed that diabetes-associated changes in signaling pathways and transcriptional factors contribute to the downregulation of BK channel expression. This manuscript will review our current understandings on the molecular, physiological, and biophysical mechanisms that underlie coronary BK channelopathy in diabetes mellitus.

P450 Eicosanoids and Reactive Oxygen Species Interplay in Brain Injury and Neuroprotection

Significance: Eicosanoids are endogenous lipid mediators that play important roles in brain function and disease. Acute brain injury such as that which occurs in stroke and traumatic brain injury increases the formation of eicosanoids, which, in turn, exacerbate or diminish injury. In chronic neurodegenerative diseases such as Alzheimer's disease and vascular dementia (VD), eicosanoid synthetic and metabolizing enzymes are altered, disrupting the balance between neuroprotective and neurotoxic eicosanoids. Recent Advances: Human and experimental studies have established the opposing roles of hydroxy- and epoxyeicosanoids and their potential utility as diagnostic biomarkers and therapeutic targets in neural injury. Critical Issues: A gap in knowledge remains in understanding the cellular and molecular mechanisms underlying the neurovascular actions of specific eicosanoids, such as specific isomers of epoxyeicosatrienoic (EETs) and hydroxyeicosatetraenoic acids (HETEs). Future Directions: EETs and HETEs exert their actions on brain cells by targeting multiple mechanisms, which include surface G-protein coupled receptors. The identification of high-affinity receptors for EETs and HETEs and their cellular localization in the brain will be a breakthrough in our understanding of these eicosanoids as mediators of cell-cell communications and contributors to brain development, function, and disease. Antioxid. Redox Signal. 28, 987-1007.

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.

Physico-chemical properties and fatty acid composition of pomegranate, cherry and pumpkin seed oils

BACKGROUND

Nut and seed oils are often considered waste products but in recent years they have been receiving growing interest due to their high concentration of hydrophilic and lipophilic bioactive components, which have important pharmacological properties on human health. The aim of this work was to compare the physico-chemical and biochemical properties of pomegranate (Punicagranatum), sweet cherry (Prunusavium) and pumpkin (Cucurbita maxima) seed oils obtained by solvent extraction.

RESULTS

High amount of linoleic acid was found in the cherry and pumpkin seed oils, while pomegranate seed oil showed relevant content of polyunsaturated fatty acids (PUFAs) and monounsaturated fatty acids (MUFAs) along to eicosapentaenoic acid (EPA) and nervonic acid. Pumpkin seed oil had high concentration of carotenoids, while pomegranate oil was the best absorber in the UV-A and UV-B ranges.

CONCLUSION

Pomegranate, cherry and pumpkin seed oils can be an excellent source of bioactive molecules and antioxidant compounds such as polyphenols, carotenoids and unsaturated fatty acids. These seed oils can be included both as preservatives and functional ingredients in the food, pharmaceutical and cosmetic fields and can contribute to disease prevention and health promotion. Moreover, high absorbance of UV light indicates a potential use of these oils as filters from radiations in the food, pharmaceutical, and cosmetic fields.

Pharmacological consequences of the coexpression of BK channel α and auxiliary β subunits

Coded by a single gene (Slo1, KCM) and activated by depolarizing potentials and by a rise in intracellular Ca(2+) concentration, the large conductance voltage- and Ca(2+)-activated K(+) channel (BK) is unique among the superfamily of K(+) channels. BK channels are tetramers characterized by a pore-forming α subunit containing seven transmembrane segments (instead of the six found in voltage-dependent K(+) channels) and a large C terminus composed of two regulators of K(+) conductance domains (RCK domains), where the Ca(2+)-binding sites reside. BK channels can be associated with accessory β subunits and, although different BK modulatory mechanisms have been described, greater interest has recently been placed on the role that the β subunits may play in the modulation of BK channel gating due to its physiological importance. Four β subunits have currently been identified (i.e., β1, β2, β3, and β4) and despite the fact that they all share the same topology, it has been shown that every β subunit has a specific tissue distribution and that they modify channel kinetics as well as their pharmacological properties and the apparent Ca(2+) sensitivity of the α subunit in different ways. Additionally, different studies have shown that natural, endogenous, and synthetic compounds can modulate BK channels through β subunits. Considering the importance of these channels in different pathological conditions, such as hypertension and neurological disorders, this review focuses on the mechanisms by which these compounds modulate the biophysical properties of BK channels through the regulation of β subunits, as well as their potential therapeutic uses for diseases such as those mentioned above.

Lipid regulation of BK channel function

This mini-review focuses on lipid modulation of BK (MaxiK, BK_Ca) current by a direct interaction between lipid and the BK subunits and/or their immediate lipid environment. Direct lipid-BK protein interactions have been proposed for fatty and epoxyeicosatrienoic acids, phosphoinositides and cholesterol, evidence for such action being less clear for other lipids. BK α (slo1) subunits are sufficient to support current perturbation by fatty and epoxyeicosatrienoic acids, glycerophospholipids and cholesterol, while distinct BK β subunits seem necessary for current modulation by most steroids. Subunit domains or amino acids that participate in lipid action have been identified in a few cases: hslo1 Y318, cerebral artery smooth muscle (cbv1) R334,K335,K336, cbv1 seven cytosolic CRAC domains, slo1 STREX and β1 T169,L172,L173 for docosahexaenoic acid, PIP₂, cholesterol, sulfatides, and cholane steroids, respectively. Whether these protein motifs directly bind lipids or rather transmit the energy of lipid binding to other areas and trigger protein conformation change remains unresolved. The impact of direct lipid-BK interaction on physiology is briefly discussed.

Temporal changes of cytochrome P450 (Cyp) and eicosanoid-related gene expression in the rat brain after traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) induces arachidonic acid (ArA) release from cell membranes. ArA metabolites form a class of over 50 bioactive eicosanoids that can induce both adaptive and/or maladaptive brain responses. The dynamic metabolism of ArA to eicosanoids, and how they affect the injured brain, is poorly understood due to their diverse activities, trace levels, and short half-lives. The eicosanoids produced in the brain postinjury depend upon the enzymes present locally at any given time. Eicosanoids are synthesized by heme-containing enzymes, including cyclooxygenases, lipoxygenases, and arachidonate monoxygenases. The latter comprise a subset of the cytochrome P450 "Cyp" gene family that metabolize fatty acids, steroids, as well as endogenous and exogenous toxicants. However, for many of these genes neither baseline neuroanatomical nor injury-related temporal expression have been studied in the brain.In a rat model of parietal cortex TBI, Cyp and eicosanoid-related mRNA levels were determined at 6 h, 24 h, 3d, and 7d postinjury in parietal cortex and hippocampus, where dynamic changes in eicosanoids have been observed. Quantitative real-time polymerase chain reaction with low density arrays were used to assay 62 rat Cyps, 37 of which metabolize ArA or other unsaturated fatty acids; 16 eicosanoid-related enzymes that metabolize ArA or its metabolites; 8 eicosanoid receptors; 5 other inflammatory- and recovery-related genes, plus 2 mouse Cyps as negative controls and 3 highly expressed "housekeeping" genes.

RESULTS

Sixteen arachidonate monoxygenases, 17 eicosanoid-related genes, and 12 other Cyps were regulated in the brain postinjury (p < 0.05, Tukey HSD). Discrete tissue levels and distinct postinjury temporal patterns of gene expression were observed in hippocampus and parietal cortex.

CONCLUSIONS

The results suggest complex regulation of ArA and other lipid metabolism after TBI. Due to the temporal nature of brain injury-induced Cyp gene induction, manipulation of each gene (or its products) at a given time after TBI will be required to assess their contributions to secondary injury and/or recovery. Moreover, a better understanding of brain region localization and cell type-specific expression may be necessary to deduce the role of these eicosanoid-related genes in the healthy and injured brain.

Homology of insect corpora allata and vertebrate adenohypophysis?

Animal species of various phyla possess neuroendocrine glands whose hormonal products regulate developmental and physiological mechanisms and directly impact behavior. Two examples, the corpora allata of insects and the vertebrate adenohypophysis have previously been regarded as analogous tissues that evolved independently from diffuse epidermal nerve nets of early metazoans. More recent developmental and functional studies accumulated evidence suggesting that the bilaterian nervous systems including its modern parts (e.g. pallium or cortex and mushroom bodies) and its neuroendocrine appendages (that are considered to be more ancient structures) possess a single evolutionary origin. The corpora allata of insects and the vertebrate adenohypophysis share a number of characteristics in respect of morphology, control of hormone release by RFamides, metabolites produced by closely related cytochrome P450 enzymes and gene expression during embryonic development. This review incorporates latest findings into an extensive description of similarities between insect corpora allata and vertebrate adenohypophysis that should encourage further studies about the onto- and phylogenetic origin of these neuroendocrine glands.

Modulation of BKCa channel gating by endogenous signaling molecules

Large-conductance Ca(2+)- and voltage-activated K(+) (BK(Ca), MaxiK, or Slo1) channels are expressed in almost every tissue in our body and participate in many critical functions such as neuronal excitability, vascular tone regulation, and neurotransmitter release. The functional versatility of BK(Ca) channels owes in part to the availability of a spectacularly wide array of biological modulators of the channel function. In this review, we focus on modulation of BK(Ca) channels by small endogenous molecules, emphasizing their molecular mechanisms. The mechanistic information available from studies on the small naturally occurring modulators is expected to contribute to our understanding of the physiological and pathophysiological roles of BK(Ca) channels.

Endothelial Ca+-activated K+ channels in normal and impaired EDHF-dilator responses--relevance to cardiovascular pathologies and drug discovery

The arterial endothelium critically contributes to blood pressure control by releasing vasodilating autacoids such as nitric oxide, prostacyclin and a third factor or pathway termed 'endothelium-derived hyperpolarizing factor' (EDHF). The nature of EDHF and EDHF-signalling pathways is not fully understood yet. However, endothelial hyperpolarization mediated by the Ca(2+)-activated K(+) channels (K(Ca)) has been suggested to play a critical role in initializing EDHF-dilator responses in conduit and resistance-sized arteries of many species including humans. Endothelial K(Ca) currents are mediated by the two K(Ca) subtypes, intermediate-conductance K(Ca) (KCa3.1) (also known as, a.k.a. IK(Ca)) and small-conductance K(Ca) type 3 (KCa2.3) (a.k.a. SK(Ca)). In this review, we summarize current knowledge about endothelial KCa3.1 and KCa2.3 channels, their molecular and pharmacological properties and their specific roles in endothelial function and, particularly, in the EDHF-dilator response. In addition we focus on recent experimental evidences derived from KCa3.1- and/or KCa2.3-deficient mice that exhibit severe defects in EDHF signalling and elevated blood pressures, thus highlighting the importance of the KCa3.1/KCa2.3-EDHF-dilator system for blood pressure control. Moreover, we outline differential and overlapping roles of KCa3.1 and KCa2.3 for EDHF signalling as well as for nitric oxide synthesis and discuss recent evidence for a heterogeneous (sub) cellular distribution of KCa3.1 (at endothelial projections towards the smooth muscle) and KCa2.3 (at inter-endothelial borders and caveolae), which may explain their distinct roles for endothelial function. Finally, we summarize the interrelations of altered KCa3.1/KCa2.3 and EDHF system impairments with cardiovascular disease states such as hypertension, diabetes, dyslipidemia and atherosclerosis and discuss the therapeutic potential of KCa3.1/KCa2.3 openers as novel types of blood pressure-lowering drugs.

The CYP2J2 G-50T polymorphism and myocardial infarction in patients with cardiovascular risk profile

Background

Cytochrome P450 (CYP) enzyme 2J2, an epoxygenase predominantly expressed in the heart, metabolises arachidonic acid to biologically active eicosanoids. One of the CYP2J2 products, 11, 12-epoxyeicosatrienoic acid, has several vasoprotective effects. The CYP2J2-G-50T-promotor polymorphism decreases gene expression and is associated with coronary artery disease. This association supports the vascular protective role of CYP-derived eicosanoids in cardiovascular disease. In the present study, we investigated the influence of this polymorphism on survived myocardial infarction in two study groups of patients with on average high cardiovascular risk profile.

Methods

The CYP2J2 polymorphism was genotyped in two groups of patients that were collected with the same method of clinical data collection. Data from 512 patients with sleep apnoea (group: OSA) and on average high cardiovascular risk profile and from another 488 patients who were admitted for coronary angiography (CAR-group) were evaluated for a potential correlation of the CYP2J2 polymorphism G-50T and a history of myocardial infarction. The G-50T polymorphism of the CYP2J2 gene was genotyped by allele specific restriction and light cycler analysis.

Results

The T-allele of the polymorphism was found in 111 (11.1%; CAR-group: N = 65, 13.3%; OSA: N = 46, 9.0%). 146 patients had a history of myocardial infarction (CAR: N = 120, 24.6%; OSA: N = 26, 5.1%). Cardiovascular risk factors were equally distributed between the different genotypes of the CYP2J2 G-50T polymorphism. In the total group of 1000 individuals, carriers of the T-allele had significantly more myocardial infarctions compared to carriers of the wild type (T/T or G/T: 21.6%; G/G: 13.7%; p = 0.026, odds ratio 1.73, 95%-CI [1.06–2.83]). In the multivariate logistic regression analysis the odds ratio for a history of myocardial infarction in carriers of the T-allele was 1.611, 95%-CI [0.957–2.731] but this trend was not significant (p = 0.073).

Conclusion

In presence of other risk factors, the CYP2J2 G-50T failed to show a significant role in the development of myocardial infarction. However, since our result is close to the border of significance, this question should be clarified in larger, prospective studies in the future.

Protein phosphatase 2A and Ca2+-activated K+ channels contribute to 11,12-epoxyeicosatrienoic acid analog mediated mesenteric arterial relaxation

Epoxyeicosatrienoic acids (EETs) are considered to be endothelium-derived hyperpolarizing factors, and are potent activators of the large-conductance, Ca(2+)-activated K(+) (BK(Ca)) channel in vascular smooth muscle. Here, we investigate the signal transduction pathway involved in the activation of BK(Ca) channels by 11,12-EET and 11,12-EET stable analogs in rat mesenteric vascular smooth muscle cells. 11,12-EET and the 11,12-EET analogs, 11-nonyloxy-undec-8(Z)-enoic acid (11,12-ether-EET-8-ZE), 11-(9-hydroxy-nonyloxy)-undec-8(Z)-enoic acid (11,12-ether-EET-8-ZE-OH) and 11,12-trans-oxidoeicosa-8(Z)-enoic acid (11,12-tetra-EET-8-ZE), caused vasorelaxation of mesenteric resistance arteries. Mesenteric myocyte whole-cell (perforated-patch) currents were substantially (approximately 150%) increased by 11,12-EET and 11,12-EET analogs. Single-channel recordings were conducted to identify the target for 11,12-EET. 11,12-EET and 11,12-EET analogs also increased mesenteric myocyte BK(Ca) channel activity in cell-attached patches. Similar results were obtained in cell-free patches. Baseline mesenteric myocyte BK(Ca) channel activity (NPo) in cell-free patches averaged less than 0.001 at +50 mV and 11,12-EET (1 micromol/L) increased NPo to 0.03+/-0.02 and 11,12-EET analogs (1 micromol/L) increased NPo to 0.09+/-0.006. Inhibition of protein phosphatase 2A (PP2A) activity with okadaic acid (10 nmol/L) completely reversed 11,12-EET stimulated BK(Ca) channel activity and greatly attenuated 11,12-ether-EET-8-ZE mesenteric resistance artery vasorelaxation. 11,12-EET and 11,12-EET analogs increased mesenteric myocyte PP2A activity by 3.5-fold. Okadaic acid and the EET inhibitor, 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE) inhibited the 11,12-EET mediated increase in PP2A activity. These findings provide initial evidence that PP2A activity contributes to 11,12-EET and 11,12-EET analog activation of mesenteric resistant artery BK(Ca) channels and vasorelaxation.

Substituted adamantyl-urea inhibitors of the soluble epoxide hydrolase dilate mesenteric resistance vessels

The epoxyeicosatrienoic acids (EETs) have been identified as endothelium-derived hyperpolarizing factors. Metabolism of the EETs to the dihydroxyeicosatrienoic acids is catalyzed by soluble epoxide hydrolase (sEH). Administration of urea-based sEH inhibitors provides protection from hypertension-induced renal injury at least in part by lowering blood pressure. Here, we investigated the hypothesis that a mechanism by which sEH inhibitors elicit their cardiovascular protective effects is via their action on the vasculature. Mesenteric resistance arteries were isolated from Sprague-Dawley rats, pressurized, and constricted with the thromboxane A2 agonist U46619 (9,11-dideoxy-11,9-epoxymethano-prostaglandin F2alpha). Mesenteric arteries were then incubated with increasing concentrations of the sEH inhibitor 12-(3-adamantan-1-yl-ureido)dodecanoic acid (AUDA). AUDA resulted in a concentration-dependent relaxation of mesenteric arteries, with 10 microM resulting in a 48 +/- 7% relaxation. Chain-shortened analogs of AUDA had an attenuated vasodilatory response. Interestingly, at 10 microM, the sEH inhibitors 1-cyclohexyl-3-dodecylurea, 12-(3-cyclohexylureido)dodecanoic acid, and 950 [adamantan-1-yl-3-{5-[2-(2-ethoxyethoxy)ethoxy]pentyl}urea] were significantly less active, resulting in a 25 +/- 8%, 10 +/- 9%, and -8 +/- 3% relaxation, respectively. Treatment of mesenteric arteries with tetraethylammonium, iberiotoxin, ouabain, or glibenclamide did not alter AUDA-induced relaxation. The AUDA-induced relaxation was completely inhibited when constricted with KCl. In separate experiments, denuding mesenteric resistance vessels did not alter AUDA-induced relaxation. Taken together, these data demonstrate that adamantyl-urea inhibitors have unique dilator actions on vascular smooth muscle compared with other sEH inhibitors and that these dilator actions depend on the adamantyl group and carbon chain length.

Large-conductance, calcium-activated potassium channels: structural and functional implications

The large-conductance, calcium-activated potassium channels (BK, also termed BK(Ca), Slo, or MaxiK) distributed in both excitable and non-excitable cells are involved in many cellular functions such as action potential repolarization; neuronal excitability; neurotransmitter release; hormone secretion; tuning of cochlear hair cells; innate immunity; and modulation of the tone of vascular, airway, uterine, gastrointestinal, and urinary bladder smooth muscle tissues. Because of their high conductance, activation of BK channels has a strong effect on membrane potential. BK channels differ from all other potassium (K(+)) channels due to their high sensitivity to both intracellular calcium (Ca(2+)) concentrations and voltage. These features make BK channels ideal negative feedback regulators in many cell types by decreasing voltage-dependent Ca(2+) entry through membrane potential hyperpolarization. The current review aims to give a comprehensive understanding of the structure and molecular biology of BK channels and their relevance to various pathophysiological conditions. The review will also focus on the therapeutic potential and pharmacology of the various BK channel activators and blockers.

Stimulatory effects of chlorzoxazone, a centrally acting muscle relaxant, on large conductance calcium-activated potassium channels in pituitary GH3 cells

Chlorzoxazone, a centrally acting muscle relaxant, has been used as a marker for hepatic CYP2E1 activity. However, little is known about the mechanism of chlorzoxazone actions on ion currents in neurons or neuroendocrine cells. We thus investigated its effects on ion currents in GH(3) lactotrophs. Chlorzoxazone reversibly increased Ca(2+)-activated K(+) current (I(K(Ca))) in a concentration-dependent manner with an EC(50) value of 30 microM. The chlorzoxazone-stimulated I(K(Ca)) was inhibited by iberitoxin (200 nM) or clotrimazole (10 microM), but not by glibenclamide (10 microM) or apamin (200 nM). Chlorzoxazone (30 microM) suppressed voltage-dependent L-type Ca(2+) current. In the inside-out configuration, chlorzoxazone applied to the intracellular side of the patch did not modify single-channel conductance of large conductance Ca(2+)-activated K(+) (BK(Ca)) channels, but did increase channel activity by increasing mean open time and decreasing mean closed time. Chlorzoxazone also caused a left shift in the activation curve of BK(Ca) channels. However, Ca(2+)-sensitivity of these channels was unaffected by chlorzoxazone. 1-Ethyl-2-benzimidazolinone (30 microM), 2-amino-5-chlorobenzoxazole (30 microM) or chlormezanone (30 microM) enhanced BK(Ca) channel activity, while 6-hydroxychlorzoxazone (30 microM) slightly increased it; however, chlorphenesin carbamate (30 microM) had no effect on it. Under the current-clamp condition, chlorzoxazone (10 microM) reduced the firing rate of action potentials. In neuroblastoma IMR-32 cells, chlorzoxazone (30 microM) also stimulated BK(Ca) channel activity. The stimulatory effects of chlorzoxazone on these channels may be responsible for the underlying mechanism of chlorzoxazone actions on neurons and neuroendocrine cells.

Block of erg current by linoleoylamide, a sleep-inducing agent, in pituitary GH3 cells

Linoleoylamide is physiological constituent of neurons. The effects of this agent, also a sleep-inducing agent, on ion currents in pituitary GH(3) cells were investigated. Hyperpolarization-elicited K(+) currents in GH(3) cells bathed in a high-K(+), Ca(2+)-free solution were studied to determine the effects of linoleoylamide and other related compounds on the I(K(IR)) that was sensitive to inhibition by E-4031 and identified as an erg (ether-à-go-go-related-gene) current. Linoleoylamide suppressed the amplitude of I(K(IR)) in a concentration-dependent manner with an IC(50) value of 5 microM. Oleamide (20 microM) inhibited the amplitude of I(K(IR)), while neither arachidonic acid (20 microM) nor 14,15-epoxyeicosatrienoic acid (20 microM) had an effect on it. In GH(3) cells incubated with anandamide (20 microM) or arachidonic acid (20 microM), the linoleoylamide-induced inhibition of I(K(IR)) remained unaltered. In inside-out patches, arachidonic acid (20 microM) and 14,15-epoxyeicosatrienoic acid (20 microM) stimulated large-conductance Ca(2+)-activated K(+) channels; however, linoleoylamide (20 microM) had little or no effect on them. Under current-clamp mode, linoleoylamide (20 microM) increased the firing rate. In IMR-32 neuroblastoma cells, linoleoylamide also suppressed I(K(IR)). This study provides the evidence that linoleoylamide has a depressant effect on the erg current, and suggests that this effect may affect hormonal secretion.

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.

Cytochrome P450 pathways of arachidonic acid metabolism

Cytochrome P450s metabolize arachidonic acid to hydroxyeicosatetraenoic acids and epoxyeicosatrienoic acids. These eicosanoids are formed in a tissue and cell-specific manner and have numerous biological functions. Of major interest are the opposing actions of hydroxyeicosatetraenoic and epoxyeicosatrienoic acids within the vasculature. Regio- and stereoisomeric epoxyeicosatrienoic acids have potent vasodilatory properties while 20-hydroxyeicosatetraenoic acid is a potent vasoconstrictor. Both effects are mediated through actions on large-conductance Ca2+-activated K+ channels. Cytochrome P450-derived eicosanoids are also important in the regulation of ion transport, and have recently been shown to influence a number of fundamental biological processes including cellular proliferation, apoptosis, inflammation, and hemostasis. The formation of these functionally relevant eicosanoids is tightly controlled by the expression and activity of the cytochrome P450 epoxygenases and hydroxylases. In addition, soluble epoxide hydrolase catalyzes the hydrolysis of epoxyeicosatrienoic acids to dihydroxyeicosatrienoic acids, and the activity of this enzyme is a critical determinant of tissue epoxyeicosatrienoic and dihydroxyeicosatrienoic acid levels. The intracellular balance between epoxyeicosatrienoic, dihydroxyeicosatrienoic and hydroxyeicosatetraenoic acids influences the biological response to these eicosanoids and alterations in their levels have recently been associated with certain pathological conditions. The involvement of the cytochrome P450-derived eicosanoids in a wide array of biological functions and the observation that levels are altered in pathological conditions suggest that the enzymes involved in the formation and degradation of these fatty acids may be novel therapeutic targets.

Fenamates stimulate BKCa channel osteoblast-like MG-63 cells activity in the human

BACKGROUND

The fenamates, a family of nonsteroidal anti-inflammatory drugs that are derivatives of N-phenylanthranilic acid, are the inhibitors of cyclo-oxygenase. The ionic mechanism of actions of these compounds in osteoblasts is not well understood.

METHODS

The effects of the fenamates on ionic currents were investigated in a human osteoblast-like cell line (MG-63) with the aid of the whole-cell and inside-out configurations of the patch-clamp technique.

RESULTS

In MG-63 cells, niflumic acid and meclofenamic acid increased K+ outward currents (IK). The niflumic acid-stimulated IK was reversed by subsequent application of iberiotoxin or paxilline, yet not by that of glibenclamide or apamin. In the inside-out configuration, niflumic acid (30 micromol/L) added to the bath did not modify single-channel conductance but increased the activity of large-conductance Ca2+-activated K+ (BKCa) channels. The EC50 values for niflumic acid- and meclofenamic acid-induced channel activity were 22 and 24 micromol/L, respectively. Niflumic acid (30 micromol/L) and meclofenamic acid (30 micromol/L) shifted the activation curve of BKCa channels to less positive membrane potentials. Membrane stretch potentiated niflumic acid-stimulated channel activity. The rank order of potency for the activation of BKCa channels in these cells was niflumic acid = meclofenamic acid > tolfenamic acid > flufenamic acid > nimesulide. Evans blue and nordihydroguaiaretic acid increased channel activity; however, indomethacin, piroxicam, and NS-398 had no effect on it.

CONCLUSIONS

The fenamates can stimulate BKCa channel activity in a manner that seems to be independent of the action of these drugs on the prostaglandin pathway. The activation of the BKCa channel may hyperpolarize the osteoblast, thereby modulating osteoblastic function.

Epoxyeicosatrienoic acid-induced relaxation is impaired in insulin resistance

We assessed the effect of epoxyeicosatrienoic acids (EETs) in intact mesenteric arteries and Ca(2+)-activated K(+) (BK(Ca)) channels of isolated vascular smooth muscle cells from control and insulin-resistant (IR) rats. The response to 11,12-EET and 14,15-EET was assessed in small mesenteric arteries from control and IR rats in vitro. Mechanistic studies were performed in endothelium intact or denuded arteries and in the presence of pharmacological inhibitors. Moreover, EET-induced activation of the BK(Ca) channel was assessed in myocytes in both the cell-attached and the inside-out (I/O) patch-clamp configurations. In control arteries, both EET isomers induced relaxation. Relaxation was impaired by endothelium denudation, N(omega)-nitro-L-arginine, or iberiotoxin (IBTX), whereas it was abolished by IBTX + apamin or charybdotoxin + apamin. In contrast, the EETs did not relax IR arteries. In control myocytes, the EETs increased BK(Ca) activity in both configurations. Conversely, in the cell-attached mode, EETs had no effect on BK(Ca) channel activity in IR myocytes, whereas in the I/O configuration, BK(Ca) channel activity was enhanced. EETs induce relaxation in small mesenteric arteries from control rats through K(Ca) channels. In contrast, arteries from IR rats do not relax to the EETs. Patch-clamp studies suggest impaired relaxation is due to altered regulatory mechanisms of the BK(Ca) channel.

Dihydroxyeicosatrienoic acids are potent activators of Ca(2+)-activated K(+) channels in isolated rat coronary arterial myocytes

1. Dihydroxyeicosatrienoic acids (DHETs), which are metabolites of arachidonic acid (AA) and epoxyeicosatrienoic acids (EETs), have been identified as highly potent endogenous vasodilators, but the mechanisms by which DHETs induce relaxation of vascular smooth muscle are unknown. Using inside-out patch clamp techniques, we examined the effects of DHETs on the large conductance Ca(2+)-activated K(+) (BK) channels in smooth muscle cells from rat small coronary arteries (150-300 microM diameter). 2. 11,12-DHET potently activated BK channels with an EC(50) of 1.87 +/- 0.57 nM (n = 5). Moreover, the three other regioisomers 5,6-, 8,9- and 14,15-DHET were equipotent with 11,12-DHET in activating BK channels. The efficacy of 11,12-DHET in opening BK channels was much greater than that of its immediate precursor 11,12-EET. In contrast, AA did not significantly affect BK channel activity. 3. The voltage dependence of BK channels was dramatically modulated by 11,12-DHET. With physiological concentrations of cytoplasmic Ca(2+) (200 nM), the voltage at which the channel open probability was half-maximal (V(1/2)) was shifted from a baseline of 115.6 +/- 6.5 mV to 95.0 +/- 10.1 mV with 5 nM 11,12-DHET, and to 60.0 +/- 8.4 mV with 50 nM 11,12-DHET. 4. 11,12-DHET also enhanced the sensitivity of BK channels to Ca(2+) but did not activate the channels in the absence of Ca(2+). 11,12-DHET (50 nM) reduced the Ca(2+) EC(50) of BK channels from a baseline of 1.02 +/- 0.07 microM to 0.42 +/- 0.11 microM. 5. Single channel kinetic analysis indicated that 11,12-DHET did not alter BK channel conductance but did reduce the first latency of BK channel openings in response to a voltage step. 11,12-DHET dose-dependently increased the open dwell times, abbreviated the closed dwell times, and decreased the transition rates from open to closed states. 6. We conclude that DHETs hyperpolarize vascular smooth muscle cells through modulation of the BK channel gating behaviour, and by enhancing the channel sensitivities to Ca(2+) and voltage. Hence, like EETs, DHETs may function as endothelium-derived hyperpolarizing factors.

Cytochrome P450 metabolites of arachidonic acid in the control of renal function

Recent studies indicate that arachidonic acid is primarily metabolized by cytochrome P450 enzymes of the 4A and 2C families in the kidney to 20-hydroxyeicosatetraenoic acid (HETE), epoxyeicosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids. These compounds play central roles in the regulation of renal tubular and vascular function. 20-HETE is produced by renal vascular smooth muscle (VSM) cells and is a potent constrictor that depolarizes VSM cells by blocking the calcium-activated potassium channel. Inhibition of the formation of 20-HETE blocks the myogenic response of isolated renal arterioles in vitro, and autoregulation of renal blood flow and tubuloglomerular feedback responses in vivo. EETs are products formed in the endothelium and are potent dilators that activate the calcium-activated potassium channel in renal VSM. Endothelial-dependent vasodilators stimulate the release of EETs, and these compounds appear to serve as an endothelial-derived hyperpolarizing factor. EETs and 20-HETE are produced in the proximal tubule. There, they regulate sodium/potassium-ATPase activity and serve as second messengers for the natriuretic effects of dopamine, parathyroid hormone and angiotensin II. 20-HETE is also produced in the thick ascending loop of Henle. It regulates sodium-potassium-chloride transport in this nephron segment. The renal production of cytochrome P450 metabolites of arachidonic acid is altered in hypertension, diabetes, toxemia of pregnancy, and hepatorenal syndrome. Given the importance of cytochrome P450 metabolites of arachidonic acid in the control of renal function, it is likely that changes in this system contribute to the abnormalities in renal function that are associated with many of these conditions.