Inhibition of endothelium-derived hyperpolarizing factor by ascorbate in the bovine eye

The aim of this study was to characterise vasodilator responses in the perfused ciliary vascular bed of the bovine eye. When bovine eyes were perfused at a constant rate of 2.5 ml min-1, infusion of the powerful vasodilator, papaverine (150 muM), produced a very small reduction in perfusion pressure. Under the same conditions, the nitric oxide synthase inhibitor, L- NAME (100 muM), had no effect but the inhibitor of soluble guanylate cyclase, ODQ (10 muM), produced a small vasoconstrictor response. These results indicate that there is a small component of intrinsic (myogenic) tone that may be suppressed by a basal release of nitric oxide. In the bovine eye, vasodilatation to acetylcholine or bradykinin was unaffected by L- NAME (100 muM), or the cyclo-oxygenase inhibitor, flurbiprofen (30 muM), but was significantly attenuated following treatment with a high concentration of KC1 (30 muM), or by damaging the endothelium with the detergent, CHAPS (0.3%, 2 min). Thus agonist-induced vasodilatation is not mediated by nitric oxide or prostacyclin but involves a K+ conductance and is endothelium-dependent. Acetylcholine-induced vasodilatation in the bovine eye was unaffected by glibenclamide (10 muM), an inhibitor of ATP-sensitive K+ channels (KATP), but was significantly attenuated by TEA (10 mM), a non-selective inhibitor of K+ channels. The blockade of vasodilatation by TEA but not glibenclamide could indicate that a calcium- sensitive K+ channel is involved in the response. The small conductance calcium-sensitive K+ channel (SKCa) inhibitor, apamin (100 nM), and the large conductance calcium-sensitive K+ channel (BKCa) inhibitor, iberiotoxin (50 nM), had no significant effect on acetylcholine-induced vasodilatation. In contrast, the intermediate (IKCa)/large conductance calcium-sensitive K+ channel inhibitor, charybdotoxin (50 nM), powerfully blocked these vasodilator responses, and uncovered a vasoconstrictor response. Thus, vasodilator responses appear to involve the opening of IKCa channels. The combination of apamin (100 nM) with a sub-threshold concentration of charybdotoxin (10 nM) significantly attenuated acetylcholine-induced vasodilatation, but the combination of apamin (100 nM) with iberiotoxin (50 nM) had no effect. This profile of blockade is consistent with the vasodilator responses being mediated by endothelium-derived hyperpolarizing factor (EDHF). Ascorbate is known to protect nitric oxide dependent vasodilatation under conditions of oxidant stress, however, EDHF-mediated vasodilator responses induced by acetylcholine or bradykinin were powerfully blocked when ascorbate (50 muM, 120 min) was included in the perfusion medium; with acetylcholine a normally masked muscarinic vasoconstrictor response was also uncovered. These results indicate that, ascorbate at a physiologically relevant concentration, can inhibit EDHF-mediated vasodilatations. The blockade of EDHF-mediated vasodilatation by ascorbate was time-dependent (maximum blockade at 120 min) and concentration-dependent (10-150 muM). Thus, the blocking action of ascorbate has a slow onset and occurs concentrations across the normal plasma concentration range (10 -150 muM). The ability of ascorbate to block EDHF-mediated vasodilatation in the bovine eye is likely to result from its reducing properties, since this action was mimicked by two other reducing agents, namely, N-acetyl-L-cysteine (1 mM) and dithiothreitol (100 muM), but not by the redox-inactive analogue, dehydroascorbate (50 muM). In the bovine eye, vasodilatations induced by the KATP opener, levcromakalim (100 pmol-30 nmol), or the nitric oxide donor, glyceryl trinitrate (10 nmol), were completely unaffected by the infusion of ascorbate (50 muM). Furthermore, the L-NAME induced vasoconstrictor response in the presence of U46619 (~200 nM) was unaffected by infusion of ascorbate (50 muM). Thus, the blockade of EDHF-mediated vasodilatation by ascorbate is highly selective and does not result from non-selective damage of the endothelium as basal release of nitric oxide is unaffected. (Abstract shortened by ProQuest.)

Modulation of endothelial cell KCa3.1 Channels during endothelium-derived hyperpolarizing factor signaling in mesenteric resistance arteries

Arterial hyperpolarization to acetylcholine (ACh) reflects coactivation of KCa3.1 (IKCa) channels and KCa2.3 (SKCa) channels in the endothelium that transfers through myoendothelial gap junctions and diffusible factor(s) to affect smooth muscle relaxation (endothelium-derived hyperpolarizing factor [EDHF] response). However, ACh can differentially activate KCa3.1 and KCa2.3 channels, and we investigated the mechanisms responsible in rat mesenteric arteries. KCa3.1 channel input to EDHF hyperpolarization was enhanced by reducing external [Ca2+]o but blocked either with forskolin to activate protein kinase A or by limiting smooth muscle [Ca2+]i increases stimulated by phenylephrine depolarization. Imaging [Ca2+]i within the endothelial cell projections forming myoendothelial gap junctions revealed increases in cytoplasmic [Ca2+]i during endothelial stimulation with ACh that were unaffected by simultaneous increases in muscle [Ca2+]i evoked by phenylephrine. If gap junctions were uncoupled, KCa3.1 channels became the predominant input to EDHF hyperpolarization, and relaxation was inhibited with ouabain, implicating a crucial link through Na+/K+-ATPase. There was no evidence for an equivalent link through KCa2.3 channels nor between these channels and the putative EDHF pathway involving natriuretic peptide receptor-C. Reconstruction of confocal z-stack images from pressurized arteries revealed KCa2.3 immunostain at endothelial cell borders, including endothelial cell projections, whereas KCa3.1 channels and Na+/K+-ATPase {alpha}2/{alpha}3 subunits were highly concentrated in endothelial cell projections and adjacent to myoendothelial gap junctions. Thus, extracellular [Ca2+]o appears to modify KCa3.1 channel activity through a protein kinase A-dependent mechanism independent of changes in endothelial [Ca2+]i. The resulting hyperpolarization links to arterial relaxation largely through Na+/K+-ATPase, possibly reflecting K+ acting as an EDHF. In contrast, KCa2.3 hyperpolarization appears mainly to affect relaxation through myoendothelial gap junctions. Overall, these data suggest that K+ and myoendothelial coupling evoke EDHF-mediated relaxation through distinct, definable pathways

Characterisation of the vasodilation effects of DHA and EPA, n-3 PUFAs (fish oils), in rat aorta and mesenteric resistance arteries

Background and Purpose Increasing evidence suggests that the omega-3 polyunsaturated acids (n-3 PUFA), docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), are beneficial to cardiovascular health, promoting relaxation of vascular smooth muscle cells and vasodilation. Numerous studies have attempted to study these responses, but to date there has not been a systematic characterisation of both DHA and EPA mediated vasodilation in conduit and resistance arteries. Therefore, we aimed to fully characterise the n-3 PUFA-induced vasodilation pathways in rat aorta and mesenteric artery. Methods Wire myography was used to measure the vasomotor responses of freshly dissected rat mesenteric artery and aorta. Arteries were pre-constricted with U46619 and cumulative concentrations of either DHA or EPA (10 nM-30 μM) were added. The mechanisms by which n-3 PUFA relaxed arteries were investigated using inhibitors of vasodilator pathways, which include: nitric oxide synthase (NOS; L-NAME), cycloxygenase (COX; indomethacin), cytochrome P450 epoxygenase (CYP450; clotrimazole); and calcium-activated potassium channels (KCa), SKCa (apamin), IKCa (TRAM-34) and BKCa (paxilline). Results Both DHA- and EPA-induced relaxations were partially inhibited following endothelium removal in rat mesenteric arteries. Similarly, in aorta EPA-induced relaxation was partially suppressed due to endothelium removal. CYP450 also contributed to EPA-induced relaxation in mesenteric artery. Inhibition of IKCa partially attenuated DHA-induced relaxation in aorta and mesenteric artery along with EPA-induced relaxation in mesenteric artery. Furthermore, this inhibition of DHA- and EPA-induced relaxation was increased following the additional blockade of BKCa in these arteries. Conclusions This study provides evidence of heterogeneity in the vasodilation mechanisms of DHA and EPA in different vascular beds. Our data also demonstrates that endothelium removal has little effect on relaxations produced by either PUFA. We demonstrate IKCa and BKCa are involved in DHA-induced relaxation in rat aorta and mesenteric artery; and EPA-induced relaxation in rat mesenteric artery only. CYP450 derived metabolites of EPA may also be involved in BKCa dependent relaxation. To our knowledge this is the first study indicating the involvement of IKCa in n-3 PUFA mediated relaxation

Regulation of KCa2.3 andendothelium-dependent hyperpolarization (EDH) in the rat middle cerebral artery: the role of lipoxygenase metabolites and isoprostanes

Background and Purpose. In rat middle cerebral arteries, endothelium-dependent hyperpolarization (EDH) is mediated by activation of calcium-activated potassium(KCa) channels specifically KCa2.3 and KCa3.1. Lipoxygenase (LOX) products function as endothelium-derived hyperpolarizing factors (EDHFs) in rabbit arteries by stimulating KCa2.3. We investigated if LOX products contribute to EDH in rat cerebral arteries. Methods. Arachidonic acid (AA) metabolites produced in middle cerebral arteries were measured using HPLC and LC/MS. Vascular tension and membrane potential responses to SLIGRL were simultaneously recorded using wire myography and intracellular microelectrodes. Results. SLIGRL, an agonist at PAR2 receptors, caused EDH that was inhibited by a combination of KCa2.3 and KCa3.1 blockade. Non-selective LOX-inhibition reduced EDH, whereas inhibition of 12-LOX had no effect. Soluble epoxide hydrolase (sEH) inhibition enhanced the KCa2.3 component of EDH. Following NO synthase (NOS) inhibition, the KCa2.3 component of EDH was absent. Using HPLC, middle cerebral arteries metabolized 14C-AA to 15- and 12-LOX products under control conditions. With NOS inhibition, there was little change in LOX metabolites, but increased F-type isoprostanes. 8-iso-PGF2α inhibited the KCa2.3 component of EDH. Conclusions. LOX metabolites mediate EDH in rat middle cerebral arteries. Inhibition of sEH increases the KCa2.3 component of EDH. Following NOS inhibition,loss of KCa2.3 function is independent of changes in LOX production or sEH inhibition but due to increased isoprostane production and subsequent stimulation of TP receptors. These findings have important implications in diseases associated with loss of NO signaling such as stroke; where inhibition of sEH and/or isoprostane formation may of benefit

Statins and selective inhibition of Rho kinase protect small conductance calcium-activated potassium channel function (KCa2.3) in cerebral arteries

Background: In rat middle cerebral and mesenteric arteries the KCa2.3 component of endothelium-dependent hyperpolarization (EDH) is lost following stimulation of thromboxane (TP) receptors, an effect that may contribute to the endothelial dysfunction associated with cardiovascular disease. In cerebral arteries, KCa2.3 loss is associated with NO synthase inhibition, but is restored if TP receptors are blocked. The Rho/Rho kinase pathway is central for TP signalling and statins indirectly inhibit this pathway. The possibility that Rho kinase inhibition and statins sustain KCa2.3 hyperpolarization was investigated in rat middle cerebral arteries (MCA). Methods: MCAs were mounted in a wire myograph. The PAR2 agonist, SLIGRL was used to stimulate EDH responses, assessed by simultaneous measurement of smooth muscle membrane potential and tension. TP expression was assessed with rt-PCR and immunofluorescence. Results: Immunofluorescence detected TP in the endothelial cell layer of MCA. Vasoconstriction to the TP agonist, U46619 was reduced by Rho kinase inhibition. TP receptor stimulation lead to loss of KCa2.3 mediated hyperpolarization, an effect that was reversed by Rho kinase inhibitors or simvastatin. KCa2.3 activity was lost in L-NAME-treated arteries, but was restored by Rho kinase inhibition or statin treatment. The restorative effect of simvastatin was blocked after incubation with geranylgeranyl-pyrophosphate to circumvent loss of isoprenylation. Conclusions: Rho/Rho kinase signalling following TP stimulation and L-NAME regulates endothelial cell KCa2.3 function. The ability of statins to prevent isoprenylation and perhaps inhibit of Rho restores/protects the input of KCa2.3 to EDH in the MCA, and represents a beneficial pleiotropic effect of statin treatment

Statins and selective inhibition of Rho kinase protect small conductance calcium-activated potassium channel function (KCa2.3) in cerebral arteries

Background: In rat middle cerebral and mesenteric arteries the KCa2.3 component of endothelium-dependent hyperpolarization (EDH) is lost following stimulation of thromboxane (TP) receptors, an effect that may contribute to the endothelial dysfunction associated with cardiovascular disease. In cerebral arteries, KCa2.3 loss is associated with NO synthase inhibition, but is restored if TP receptors are blocked. The Rho/Rho kinase pathway is central for TP signalling and statins indirectly inhibit this pathway. The possibility that Rho kinase inhibition and statins sustain KCa2.3 hyperpolarization was investigated in rat middle cerebral arteries (MCA). Methods: MCAs were mounted in a wire myograph. The PAR2 agonist, SLIGRL was used to stimulate EDH responses, assessed by simultaneous measurement of smooth muscle membrane potential and tension. TP expression was assessed with rt-PCR and immunofluorescence. Results: Immunofluorescence detected TP in the endothelial cell layer of MCA. Vasoconstriction to the TP agonist, U46619 was reduced by Rho kinase inhibition. TP receptor stimulation lead to loss of KCa2.3 mediated hyperpolarization, an effect that was reversed by Rho kinase inhibitors or simvastatin. KCa2.3 activity was lost in L-NAME-treated arteries, but was restored by Rho kinase inhibition or statin treatment. The restorative effect of simvastatin was blocked after incubation with geranylgeranyl-pyrophosphate to circumvent loss of isoprenylation. Conclusions: Rho/Rho kinase signalling following TP stimulation and L-NAME regulates endothelial cell KCa2.3 function. The ability of statins to prevent isoprenylation and perhaps inhibit of Rho restores/protects the input of KCa2.3 to EDH in the MCA, and represents a beneficial pleiotropic effect of statin treatment

Nitric oxide suppresses cerebral vasomotion by sGC-independent effects on ryanodine receptors and voltage-gated calcium channels

Background/Aims: In cerebral arteries, nitric oxide (NO) release plays a key role in suppressing vasomotion. Our aim was to establish the pathways affected by NO in rat middle cerebral arteries. Methods: In isolated segments of artery, isometric tension and simultaneous measurements of either smooth muscle membrane potential or intracellular [Ca 2+ ] ([Ca 2+ ] SMC ) changes were recorded. Results: In the absence of L -NAME, asynchronous propagating Ca 2+ waves were recorded that were sensitive to block with ryanodine, but not nifedipine. L -NAME stimulated pronounced vasomotion and synchronous Ca 2+ oscillations with close temporal coupling between membrane potential, tone and [Ca 2+ ] SMC . If nifedipine was applied together with L -NAME, [Ca 2+ ] SMC decreased and synchronous Ca 2+ oscillations were lost, but asynchronous propagating Ca 2+ waves persisted. Vasomotion was similarly evoked by either iberiotoxin, or by ryanodine, and to a lesser extent by ODQ. Exogenous application of NONOate stimulated endothelium-independent hyperpolarization and relaxation of either L -NAME-induced or spontaneous arterial tone. NO-evoked hyperpolarization involved activation of BK Ca channels via ryanodine receptors (RYRs), with little involvement of sGC. Further, in whole cell mode, NO inhibited current through L-type voltage-gated Ca 2+ channels (VGCC), which was independent of both voltage and sGC. Conclusion: NO exerts sGC-independent actions at RYRs and at VGCC, both of which normally suppress cerebral artery myogenic tone

Cannabidiol (CBD) improves survival and behavioural comorbidities of Dravet syndrome in mice

Background and Purpose Dravet syndrome is a severe, genetic form of paediatric epilepsy associated with premature mortality and comorbidities such as anxiety, depression, autism, motor dysfunction, and memory deficits. Cannabidiol is an approved anticonvulsive drug in USA and Europe for seizures associated with Dravet syndrome therapy in patients 2 years of age and older; we investigated its potential to prevent premature mortality and improve associated comorbidities. Experimental Approach The efficacy of sub-chronic cannabidiol administration in two mouse models which reproduce characteristics of Dravet syndrome was investigated. The effect of cannabidiol on neonatal welfare and survival was studied using Scn1a-/- mice. We then used a hybrid, heterozygote Scn1a+/- mouse model to study the effect of cannabidiol on survival and behavioural comorbidities; motor deficits (rotarod and static-beam test), gait abnormality (gait test), social anxiety (social interaction test), anxiety-like (elevated plus maze) and depressive-like behaviours (sucrose preference test) and cognitive impairment (radial arm maze test). Key Results In Scn1a-/- mice, cannabidiol increased survival and delayed worsening of neonatal welfare. In Scn1a+/- mice chronic cannabidiol administration did not show any adverse effect on motor function and gait, reduced premature mortality, improved social behaviour and memory function, and reduced anxiety-like and depressive-like behaviours. Conclusion and Implications We are the first to demonstrate a potential disease-modifying effect of cannabidiol in animal models of Dravet syndrome. cannabidiol treatment reduced premature mortality and improved several behavioural comorbidities in Dravet syndrome mice. These crucial findings may be translated into human therapy to address behavioural comorbidities associated with Dravet syndrome

Cannabidiol reduces seizures and associated behavioral comorbidities in a range of animal seizure and epilepsy models

Objective Epilepsy is a progressive neurological disease characterized by recurrent seizures and behavioral comorbidities. We investigated the antiseizure effect of cannabidiol (CBD), in a battery of acute seizure models. Additionally, we defined the disease-modifying potential of chronic oral administration of CBD on associated comorbidities in the reduced intensity status epilepticus-spontaneous recurrent seizure (RISE-SRS) model of temporal lobe epilepsy (TLE). Methods We evaluated the acute antiseizure effect of CBD in the maximal electroshock seizure (MES), 6 Hz psychomotor seizure, and pentylenetetrazol (PTZ) acute seizure tests, as well as the corneal kindling model of chronic seizures in mice following intraperitoneal administration. Median effective (ED50) or behavioral toxic dose (TD50) was determined in both mice and rats. Next, we tested an intravenous preparation of CBD (10 mg/kg, single dose) in a rat model of pilocarpine-induced status epilepticus. We defined the effect of chronic CBD administration (200mg/kg, orally) on spontaneous seizures, motor control, gait, and memory function in the rat RISE-SRS model of TLE. Results CBD was effective in a battery of acute seizure models in both mice and rats following intraperitoneal administration. In the pilocarpine induced status epilepticus rat model, CBD attenuated maximum seizure severity following intravenous administration, further demonstrating CBD’s acute antiseizure efficacy in this rat model. We established that oral CBD attenuated the time-dependent increase in seizure burden and improved TLE-associated motor comorbidities of epileptic rats in the RISE-SRS model without affecting gait. Chronic administration of CBD after the onset of SRS ameliorated reference memory and working memory errors of epileptic animals in a spatial learning and memory task. Significance The present study illustrates that CBD is a well-tolerated and effective antiseizure agent and illustrates a potential disease-modifying effect of CBD on both reducing seizure burden and associated comorbidities well-after the onset of symptomatic seizures in a model of TLE

Omega-3 polyunsaturated fatty acid-induced vasodilation in mouse aorta and mesenteric arteries is not mediated by ATP-sensitive potassium channels

There is strong evidence that the omega-3 polyunsaturated fatty acids (n-3 PUFAs) docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) have cardioprotective effects. n-3 PUFAs cause vasodilation in hypertensive patients, in part controlled by increased membrane conductance to potassium. As KATP channels play a major role in vascular tone regulation and are involved in hypertension, we aimed to verify whether n-3 PUFA-mediated vasodilation involved the opening of KATP channels. We used a murine model in which the KATP channel pore subunit, Kir6.1, is deleted in vascular smooth muscle. The vasomotor response of preconstricted arteries to physiologically relevant concentrations of DHA and EPA was measured using wire myography, using the channel blocker PNU-37883A. The effect of n-3 PUFAs on potassium currents in wild-type native smooth muscle cells was investigated using whole-cell patch clamping. DHA and EPA induced vasodilation in mouse aorta and mesenteric arteries; relaxations in the aorta were sensitive to KATP blockade with PNU-37883A. Endothelium removal didn’t affect relaxation to EPA and caused a small but significant inhibition of relaxation to DHA. In the knock-out model, relaxations to DHA and EPA were unaffected by channel knockdown but were still inhibited by PNU-37883A, indicating that the action of PNU-37883A on relaxation may not reflect inhibition of KATP. In native aortic smooth muscle cells DHA failed to activate KATP currents. We conclude that DHA and EPA cause vasodilation in mouse aorta and mesenteric arteries. Relaxations in blocker-treated arteries from knock-out mice demonstrate that KATP channels are not involved in the n-3 PUFA-induced relaxation