Ascorbate blocks endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation in the bovine ciliary vascular bed and rat mesentery

Ex vivo ocular perfusion model to study vascular physiology in the mouse eye

Several hypotheses have been tested to understand whole organ regulation in other organs such as the brain and kidney, but no such hypothesis has yet been proposed for ocular circulations. To some extent resolve this deficit our ex vivo mouse eye perfusion model takes the first step in elucidating the mechanisms controlling the individual components of the ocular circulation. Various isolated ocular vascular preparations have been utilized in studies of ocular vascular biology, physiology, and pharmacology, including studies on both normal and pathological conditions. However, there is still significant potential for further studies to improve our understanding of ocular circulation and its regulation. The choroid specifically is inaccessible to direct visualization due to the retina's high metabolic requirement with a transparency that cannot be compromised by an overly rich vascular network on the inner retinal side hindering the visualization of the choroid. In this technical paper, we provide a detailed description of all the steps to be followed from the enucleation of mouse eyes to cannulation of the ophthalmic artery and perfusion and ex vivo confocal microscopy imaging of the dynamic nature of the choroid circulation.

Any Role of High-Dose Vitamin C for Septic Shock in 2021?

While the use of vitamin C as a therapeutic agent has been investigated since the 1950s, there has been substantial recent interest in the role of vitamin C supplementation in critical illness and particularly, sepsis and septic shock. Humans cannot synthesize vitamin C and rely on exogenous intake to maintain a plasma concentration of approximately 70 to 80 μmol/L. Vitamin C, in healthy humans, is involved with antioxidant function, wound healing, endothelial function, and catecholamine synthesis. Its function in the human body informs the theoretical basis for why vitamin C supplementation may be beneficial in sepsis/septic shock.Critically ill patients can be vitamin C deficient due to low dietary intake, increased metabolic demands, inefficient recycling of vitamin C metabolites, and loss due to renal replacement therapy. Intravenous supplementation is required to achieve supraphysiologic serum levels of vitamin C. While some clinical studies of intravenous vitamin C supplementation in sepsis have shown improvements in secondary outcome measures, none of the randomized clinical trials have shown differences between vitamin C supplementation and standard of care and/or placebo in the primary outcome measures of the trials. There are some ongoing studies of high-dose vitamin C administration in patients with sepsis and coronavirus disease 2019; the majority of evidence so far does not support the routine supplementation of vitamin C in patients with sepsis or septic shock.

Chronic Low-Level Lead Exposure Increases Mesenteric Vascular Reactivity: Role of Cyclooxygenase-2-Derived Prostanoids

Lead (Pb) exposure causes hazardous effects as hypertension and other cardiovascular diseases. We evaluated whether chronic Pb exposure alters the peripheral vascular resistance measuring the vascular reactivity of mesenteric resistance arteries in rats to identify the underlying mechanisms that are associated to the development of Pb-induced hypertension. Mesenteric resistance arteries from lead-treated and untreated Wistar rats (1st dose: 10 μg/100 g; subsequent doses: 0.125 μg/100 g, intramuscular, 30 days) were used. Contractile responses to phenylephrine increased, while acetylcholine and sodium nitroprusside-induced relaxation was not affected by lead treatment. Endothelium removal and inhibition of NO synthase by L-NAME similarly enhanced the response to phenylephrine in untreated and lead-treated rats. The antioxidants apocynin and superoxide dismutase (SOD) did not affect vasoconstriction in either group. The vascular expression of cyclooxygenase-2 (COX-2) protein increased after lead exposure. The respective non-specific or specific COX-2 inhibitors indomethacin and NS398 reduced more strongly the response to phenylephrine in treated rats. Antagonists of EP1 (SC19220), TP (SQ29548), IP (CAY10441) and angiotensin II type 1 (losartan) receptors reduced vasoconstriction only in treated rats. These conclusions present further evidence that lead, even in small concentration, produces cardiovascular hazards being an environmental contaminant that account for lead-induced hypertension.

Role of vitamin C in the function of the vascular endothelium

SIGNIFICANCE

Vitamin C, or ascorbic acid, has long been known to participate in several important functions in the vascular bed in support of endothelial cells. These functions include increasing the synthesis and deposition of type IV collagen in the basement membrane, stimulating endothelial proliferation, inhibiting apoptosis, scavenging radical species, and sparing endothelial cell-derived nitric oxide to help modulate blood flow. Although ascorbate may not be able to reverse inflammatory vascular diseases such as atherosclerosis, it may well play a role in preventing the endothelial dysfunction that is the earliest sign of many such diseases.

RECENT ADVANCES

Beyond simply preventing scurvy, evidence is mounting that ascorbate is required for optimal function of many dioxygenase enzymes in addition to those involved in collagen synthesis. Several of these enzymes regulate the transcription of proteins involved in endothelial function, proliferation, and survival, including hypoxia-inducible factor-1α and histone and DNA demethylases. More recently, ascorbate has been found to acutely tighten the endothelial permeability barrier and, thus, may modulate access of ascorbate and other molecules into tissues and organs.

CRITICAL ISSUES

The issue of the optimal cellular content of ascorbate remains unresolved, but it appears that low millimolar ascorbate concentrations are normal in most animal tissues, in human leukocytes, and probably in the endothelium. Although there may be little benefit of increasing near maximal cellular ascorbate concentrations in normal people, many diseases and conditions have either systemic or localized cellular ascorbate deficiency as a cause for endothelial dysfunction, including early atherosclerosis, sepsis, smoking, and diabetes.

FUTURE DIRECTIONS

A key focus for future studies of ascorbate and the vascular endothelium will likely be to determine the mechanisms and clinical relevance of ascorbate effects on endothelial function, permeability, and survival in diseases that cause endothelial dysfunction.

H₂S is an endothelium-derived hyperpolarizing factor

AIMS

Endothelium-dependent vasorelaxation is mediated by endothelium-derived relaxing factor and endothelium-derived hyperpolarizing factor (EDHF). However, the molecular entity of EDHF remains unclear. The present study examined whether hydrogen sulfide (H₂S) acts as EDHF and how H₂S mediates EDHF pathways from endothelial origination to downstream target of smooth muscle cells (SMCs).

RESULTS

We found that knocking-out the expression of cystathionine γ-lyase (CSE) in mice (CSE-knockout [KO]) elevated resting-membrane-potential of SMCs and eliminated methacholine-induced endothelium-dependent relaxation of mesenteric arteries, but not that of aorta. Methacholine, a cholinergic-muscarinic agonist, hyperpolarized SMC in endothelium-intact mesenteric arteries from wide-type mice. This effect was inhibited by muscarinic antagonist (atropine) or the co-application of charybdotoxin and apamin, which blocked intermediate- and small-conductance KCa (IKCa and SKCa) channels, or abolished in CSE-KO mice. Supplementation of exogenous H₂S hyperpolarized vascular SMCs and endothelial cells from wide-type and CSE-KO mice. Both methacholine and H₂S induced greater SMC hyperpolarization of female wide-type mesenteric arteries than that of male ones. H2S-induced hyperpolarization is blocked by -SH oxidants and -SSH inhibitor. The expression of SK2.3 but not IK3.1 channel in vascular tissues was increased by H₂S and decreased by CSE inhibitor or CSE gene KO.

INNOVATION AND CONCLUSIONS

Taken together, H₂S is an EDHF. The identification of H2S as an EDHF will not only solve one of the long-lasting perplexing puzzles for the mechanisms underlying endothelium-dependent vasorelaxation, but also shed light on potential therapeutic effects of H₂S on pathological abnormalities in peripheral resistance arteries.

Endosomal proteolysis regulates calcitonin gene-related peptide responses in mesenteric arteries

BACKGROUND AND PURPOSE

Calcitonin gene-related peptide (CGRP) is a potent vasodilator, implicated in the pathogenesis of migraine. CGRP activates a receptor complex comprising, calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1). In vitro studies indicate recycling of CLR●RAMP1 is regulated by degradation of CGRP in early endosomes by endothelin-converting enzyme-1 (ECE-1). However, it is not known if ECE-1 regulates the resensitization of CGRP-induced responses in functional arterial tissue.

EXPERIMENTAL APPROACH

CLR, ECE-1a-d and RAMP1 expression in rat mesenteric artery smooth muscle cells (RMA-SMCs) and mesenteric arteries was analysed by RT-PCR and by immunofluorescence and confocal microscopy. CGRP-induced signalling in cells was examined by measuring cAMP production and ERK activation. CGRP-induced relaxation of arteries was measured by isometric wire myography. ECE-1 was inhibited using the specific inhibitor, SM-19712.

KEY RESULTS

RMA-SMCs and arteries contained mRNA for CLR, ECE-1a-d and RAMP1. ECE-1 was present in early endosomes of RMA-SMCs and in the smooth muscle layer of arteries. CGRP induced endothelium-independent relaxation of arteries. ECE-1 inhibition had no effect on initial CGRP-induced responses but reduced cAMP generation in RMA-SMCs and vasodilation in mesenteric arteries responses to subsequent CGRP challenges.

CONCLUSIONS AND IMPLICATIONS

ECE-1 regulated the resensitization of responses to CGRP in RMA-SMCs and mesenteric arteries. CGRP-induced relaxation did not involve endothelium-derived pathways. This is the first report of ECE-1 regulating CGRP responses in SMCs and arteries. ECE-1 inhibitors may attenuate an important vasodilatory pathway, implicated in primary headaches and may represent a new therapeutic approach for the treatment of migraine.

Hydrogen sulfide as endothelium-derived hyperpolarizing factor sulfhydrates potassium channels

RATIONALE

Nitric oxide, the classic endothelium-derived relaxing factor (EDRF), acts through cyclic GMP and calcium without notably affecting membrane potential. A major component of EDRF activity derives from hyperpolarization and is termed endothelium-derived hyperpolarizing factor (EDHF). Hydrogen sulfide (H(2)S) is a prominent EDRF, since mice lacking its biosynthetic enzyme, cystathionine γ-lyase (CSE), display pronounced hypertension with deficient vasorelaxant responses to acetylcholine.

OBJECTIVE

The purpose of this study was to determine if H(2)S is a major physiological EDHF.

METHODS AND RESULTS

We now show that H(2)S is a major EDHF because in blood vessels of CSE-deleted mice, hyperpolarization is virtually abolished. H(2)S acts by covalently modifying (sulfhydrating) the ATP-sensitive potassium channel, as mutating the site of sulfhydration prevents H(2)S-elicited hyperpolarization. The endothelial intermediate conductance (IK(Ca)) and small conductance (SK(Ca)) potassium channels mediate in part the effects of H(2)S, as selective IK(Ca) and SK(Ca) channel inhibitors, charybdotoxin and apamin, inhibit glibenclamide-insensitive, H(2)S-induced vasorelaxation.

CONCLUSIONS

H(2)S is a major EDHF that causes vascular endothelial and smooth muscle cell hyperpolarization and vasorelaxation by activating the ATP-sensitive, intermediate conductance and small conductance potassium channels through cysteine S-sulfhydration. Because EDHF activity is a principal determinant of vasorelaxation in numerous vascular beds, drugs influencing H(2)S biosynthesis offer therapeutic potential.

Oxidation by trace Cu2+ ions underlies the ability of ascorbate to induce vascular dysfunction in the rat perfused mesentery

Ascorbate has both antioxidant and pro-oxidant activities. We have previously shown that plasma levels of ascorbate induce constriction and blockade of dilatation mediated by endothelium-derived hyperpolarizing factor (EDHF). In this study we sought to determine if these detrimental actions were mediated by a prooxidant action of ascorbate. Since trace levels of transition metal ions including, Cu2+ and Fe3+, promote oxidation of ascorbate, we examined the effects of the chelating agents, cuprizone and deferoxamine, and of CuSO4 and FeCl3 on ascorbate-induced constriction and blockade of EDHF in the perfused rat mesentery. Cuprizone abolished and Cu2+ but not Fe3+ ions enhanced both ascorbate (50 microM)-induced constriction and blockade of EDHF. The blockade of EDHF produced by ascorbate in the presence of CuSO4 (0.5 microM) was abolished by the hydrogen peroxide scavenger, catalase, but unaffected by the scavengers of hydroxyl radical or superoxide anion, mannitol and superoxide dismutase (SOD), respectively. Consistent with these observations, the oxidation of ascorbate by CuSO4 led to the rapid production of hydrogen peroxide. Catalase, mannitol and SOD had no effect on ascorbate-induced constriction. Thus, in the rat perfused mesentery, both the constrictor and EDHF-blocking actions of ascorbate arise from its oxidation by trace Cu2+ ions. The blockade of EDHF results from the consequent generation of hydrogen peroxide, but the factor producing constriction remains unidentified. These detrimental actions of ascorbate may help explain the disappointing outcome of clinical trials investigating dietary supplementation with the vitamin on cardiovascular health.

Flow-induced enhancement of vasoconstriction and blockade of endothelium-derived hyperpolarizing factor (EDHF) by ascorbate in the rat mesentery

BACKGROUND AND PURPOSE

We previously reported that ascorbate inhibits flow- and agonist-induced, EDHF-mediated vasodilatation in the bovine ciliary circulation. This study examined whether ascorbate had similar actions in the rat mesenteric vasculature.

EXPERIMENTAL APPROACH

The effects of ascorbate were examined both in rat second order mesenteric arterial rings suspended in a static wire myograph and the rat mesentery perfused at different rates of flow.

KEY RESULTS

Ascorbate (50 microM) had no effect on U46619-induced tone or acetylcholine-induced, EDHF-mediated vasodilatation in either rings of mesenteric artery or the perfused mesentery at rates of flow below 10 ml min(-1). At higher rates of flow, ascorbate produced two distinct effects in the rat mesentery: a rapid and maintained enhancement of vasoconstrictor tone and a slow (max at 3 h) inhibition of acetylcholine-induced, EDHF-mediated vasodilatation. The enhancement of vasoconstrictor tone appeared to be due to inhibition of flow-induced EDHF-like activity, since it was endothelium-dependent, but could be elicited during blockade of nitric oxide synthase and cyclooxygenase. Despite this, the classical inhibitors of EDHF, apamin and charybdotoxin, failed to affect the ascorbate-induced enhancement of tone, although they inhibited acetylcholine-induced vasodilatation.

CONCLUSIONS AND IMPLICATIONS

Ascorbate inhibits both flow- and agonist-induced EDHF in the rat mesentery. The strikingly different timecourses of these two effects, together with their differential sensitivity to apamin and charybdotoxin, suggest that the flow- and agonist-induced EDHFs in the rat mesenteric vasculature may either be different entities or operate by different mechanisms.

Modulation of gap-junction-dependent arterial relaxation by ascorbic acid

AIMS

To investigate whether ascorbic acid (AA) can influence endothelium-dependent relaxation by modulating the spread of endothelial hyperpolarization through the arterial wall via gap junctions.

METHODS

Force development and membrane potential were monitored by myography and sharp electrode techniques in isolated rabbit iliac arteries.

RESULTS

AA prevented the ability of the gap junction blocker 2-aminoethoxydiphenyl borate to inhibit endothelium-dependent relaxations and subintimal smooth muscle hyperpolarizations evoked by cyclopiazonic acid in the presence of nitric oxide (NO) synthase and cyclooxygenase blockade. AA also prevented the ability of a connexin-mimetic peptide targeted against Cx37 and Cx40 (37,40Gap 26) to attenuate the transmission of endothelial hyperpolarization to subintimal smooth muscle, and a peptide targeted against Cx43 (43Gap 26) to attenuate the spread of subintimal hyperpolarization to subadventitial smooth muscle and the associated mechanical relaxation. Parallel studies with endothelium-denuded preparations demonstrated that AA and cyclopiazonic acid both depressed relaxation evoked by the NO donor MAHMA NONOate.

CONCLUSIONS

The data suggest that AA can modulate arterial function through a previously unrecognized ability to preserve electrotonic signalling via myoendothelial and homocellular smooth muscle gap junctions under conditions where cell coupling is depressed. Underlying mechanisms do not involve amplification of 'residual' NO activity by AA.

Pharmacological vascular reactivity in isolated diabetic rabbit ciliary artery

Impairment of the ocular circulation induced by diabetes mellitus has not been fully defined, but is thought to be related to hemodynamic changes in the ocular circulation. The purpose of the present study is to investigate the functional and morphological changes occurring in the ciliary artery wall of rabbits with alloxan-induced diabetes mellitus. A single intravenous bolus injection of alloxan (100 mg/kg) was given to each of 26 10-week-old rabbits and 16 sham-injected control rabbits. Twenty weeks later, control rabbits and diabetic rabbits were sacrificed, and their ciliary arteries were mounted in a myograph system. The responses of these arteries to high K+ solution (K-Krebs solution), phenylephrine and carbachol were investigated using isometric tension recording. L-NAME (NG-nitro-l-arginine methyl ester; 100 microM) and indomethacin (1 microM) were also used to test the mechanism causing the carbachol induced relaxation. The arteries were also examined morphologically. The maximum tensions induced by K-Krebs solution in this tissue were not significantly different: 17.2+/-0.8 mN (n=16) in the control rabbits and 17.6+/-0.8 mN (n=23) in the diabetic rabbits (P=0.36). Phenylephrine caused dose-dependent contraction with EC50 values of 1.3+/-0.4 microM (n=6) in the control and 5.1+/-2.3 microM (n=6) in the diabetic rabbits, but there was no significant difference between the two (P=0.36). Carbachol induced dose-dependent relaxations in segments precontracted with K-Krebs solution. These relaxations were significantly reduced in the diabetic rabbits. The maximum relaxation induced by carbachol was 77.0+/-2.4% (10 microM) and 66.4+/-2.5% (100 microM) in the control and diabetic rabbits, respectively. These values were significantly different (P=0.0076). The IC(50) value for carbachol was 396.3+/-58.4 nM (n=16) in the control, and 443.6+/-141.1 nM (n=23) in the diabetic rabbit (P=0.87). Application of a 100 microM nitric oxide synthase inhibitor, L-NAME, significantly inhibited the amplitude of relaxations evoked by carbachol (P=0.0066). However, these relaxations were not inhibited by pretreatment with 1 microM indomethacin (P=0.60). Histologically, the frequency of invaginations was less in the diabetic arterioles with a flattening of the lamina in the diabetic rabbits than in the controls. The cytoplasm of endothelial cells contained large vacuoles, indicating weak adhesion to the lamina. Some endothelial cells even showed vacuolar degeneration due to breakdown of the cell membranes. However, the smooth muscle cells were well preserved in the diabetic rabbit. These results suggest that the mechanism of impairment of ocular circulation induced by diabetes mellitus is mainly the reduction of NO synthase due to endothelial cell dysfunction. Furthermore, the characteristics of rabbits with alloxan-induced diabetes mellitus probably make them a useful model for investigating ocular complications induced by diabetic mellitus.

Ascorbate elevates perfusion pressure in the bovine extraocular long posterior ciliary artery: role of endothelium-derived hyperpolarizing factor (EDHF)

Ascorbate blocks agonist-induced, endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation in the bovine perfused ciliary artery and this is associated with a rise in perfusion pressure. We now report the origins of this ascorbate-induced rise in perfusion pressure. In segments of ciliary artery perfused at 2.5 ml/min, the addition of ascorbate (10-150 microM) enhanced U46619-induced perfusion pressure. Ascorbate produced no enhancement in the absence of U46619, suggesting that its effects resulted not from a constrictor action but through removal of a tonic vasodilator influence. Experiments revealed the endothelial source of this vasodilator influence, and EDHF, but not nitric oxide or prostanoids, appeared to be involved. The ascorbate-induced enhancement of vasoconstrictor tone was not seen in a static myograph or in segments perfused at low rates of flow, but was seen at flow rates of 2.5 ml(-1) and above. We conclude that ascorbate augments vasoconstrictor tone through inhibition of flow-induced EDHF activity.

Biphasic neurogenic vasodilatation in the bovine intraocular long posterior ciliary artery: involvement of nitric oxide and an additional unidentified neurotransmitter

We have investigated the neurogenic factors inducing relaxation in the intraocular segment of the bovine long posterior ciliary artery. In precontracted vessels, electrical field stimulation (EFS, 0.5-128 Hz, 10 s trains) in the presence of guanethidine (30 microM) evoked biphasic relaxation: optimal relaxation for the first and second components occurred at 10 and 50 s, respectively. The first component, but not the second, was abolished by L-NAME (100 microM) or ODQ (3 microM). Relaxation to exogenous CGRP (0.1-300 nM) was inhibited by the CGRP antagonist, CGRP(8-37) (1-5 microM), but neither component of neurogenic relaxation was affected. Preincubation with the sensory nerve excitotoxin, capsaicin (1 microM), had no effect on either the first or second components of neurogenic relaxation. Substance P (0.1 nM-0.1 microM) induced relaxation, but rapid and complete desensitisation occurred within minutes. Neither desensitisation to substance P (0.1 microM) nor incubation with the NK(1) antagonist, L-733,060 (0.3 microM), had any effect on the first or second components of neurogenic relaxation.VIP (0.1 nM-0.3 microM) induced relaxation and this was followed by substantial desensitisation. Neither desensitisation to VIP (0.6 microM) nor treatment with the protease, alpha-chymotrypsin (10 U ml(-1)), had any effect on the first or second components of neurogenic relaxation. The results indicate that nitric oxide mediates the first component of neurogenic relaxation in the bovine intraocular ciliary artery. The neurotransmitter mediating the second component remains to be determined but is unlikely to be CGRP, substance P or VIP.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

Hydrogen peroxide and endothelium-dependent hyperpolarization in the guinea-pig carotid artery

This study was designed to determine whether or not endothelium-dependent hyperpolarizations evoked by acetylcholine in the isolated guinea-pig carotid artery involve hydrogen peroxide. Membrane potential was recorded in the vascular smooth muscle cells of that artery. Under control conditions, acetylcholine induced endothelium-dependent hyperpolarization of the vascular smooth muscle cells which was not affected by the presence of catalase, superoxide dismutase or their combination. Neither the superoxide dismutase mimetic, tiron nor the thiol-reducing agent N-acetyl-L-cysteine modified the hyperpolarization evoked by 0.1 microM acetylcholine but each produced a partial and significant inhibition of the hyperpolarization induced by 1 microM acetylcholine. Neither 10 nor 100 microM hydrogen peroxide influenced the resting membrane potential of the smooth muscle cells and the higher concentration did not significantly influence the hyperpolarization elicited by acetylcholine. These data indicate that, in the guinea-pig isolated carotid artery, hydrogen peroxide is unlikely to contribute to the endothelium-dependent hyperpolarization evoked by acetylcholine.

Enhanced endothelium derived hyperpolarising factor activity in resistance arteries from normal pressure glaucoma patients: implications for vascular function in the eye

BACKGROUND/AIMS

Endothelial cell dysfunction in the ocular circulation may contribute to normal pressure glaucoma (NPG). This study aimed to investigate the contributions made by endothelium derived relaxing factors to relaxation of (1) subcutaneous resistance arteries from patients with NPG, and (2) porcine ciliary arteries.

METHODS

Human gluteal resistance arteries were isolated from seven patients with NPG and matched controls. Human and porcine arteries produced endothelium dependent relaxation when exposed to acetylcholine (ACh) (10(-9)-3 x 10(-5)M) or bradykinin (10(-10)-3 x 10(-6)M). Pharmacological agents were used to inhibit the nitric oxide pathway (l-arginine analogues, soluble guanylate cyclase inhibitor), endothelium derived hyperpolarising factor (EDHF) activity (potassium channel antagonists), and prostaglandin synthesis (cyclo-oxygenase inhibitors).

RESULTS

In all arteries, endothelium dependent relaxation was attenuated by nitric oxide (NO) inhibition or potassium channel blockade, but not by cyclo-oxygenase inhibition. Inhibition of ACh mediated relaxation by potassium channel antagonists was greater (p<0.05) in patients with NPG (Emax, 55.4% (SD 8.16%) relaxation, n = 4) than controls (Emax, 81.8% (6.0%), n = 5). In contrast, combined inhibition of NO synthase (NOS) and cyclo-oxygenase produced similar inhibition of ACh mediated relaxation in both groups.

CONCLUSIONS

The enhanced contribution of EDHF to ACh mediated relaxation in systemic resistance arteries from NPG patients may contribute to the maintained endothelium mediated relaxation in these vessels. EDHF also contributes significantly to bradykinin mediated relaxation in porcine ocular ciliary arteries. Therefore, similar changes in the balance of relaxing factors in the ocular circulation could influence the response of the eye to vascular endothelial dysfunction in NPG.

Factors, fiction and endothelium-derived hyperpolarizing factor

1. The principal mediators of vascular tone are neural, endothelial and physical stimuli that result in the initiation of dilator and constrictor responses to facilitate the control of blood pressure. Two primary vasodilatory stimuli produced by the endothelium are nitric oxide (NO) and prostaglandins. An additional endothelium-dependent vasodilatory mechanism is characterized as the hyperpolarization-mediated relaxation that remains after the inhibition of the synthesis of NO and prostaglandins. This mechanism is due to the action of a so-called endothelium-derived hyperpolarizing factor (EDHF) and is dependent on either the release of diffusible factor(s) and/or to a direct contact-mediated mechanism. 2. Most evidence supports the concept that 'EDHF' activity is dependent on contact-mediated mechanisms. This involves the transfer of an endothelium-derived electrical current, as an endothelium-derived hyperpolarization (EDH), through direct heterocellular coupling of endothelial cells and smooth muscle cells via myoendothelial gap junctions (MEGJ). However, there is a lack of consensus with regard to the nature and mechanism of action of EDHF/EDH (EDH(F)), which has been shown to vary within and between vascular beds, as well as among species, strains, sex and during development, ageing and disease. 3. In addition to actual heterogeneity in EDH(F), further heterogeneity has resulted from the less-than-optimal design, analysis and interpretation of data in some key papers in the EDHF literature; with such views being perpetuated in the subsequent literature. 4. The focus of the present brief review is to examine what factors are proposed as EDH(F) and highlight the correlative structural and functional studies from our laboratory that demonstrate an integral role for MEGJ in the conduction of EDH, which account for the heterogeneity in EDH(F), while incorporating the reported diffusible mechanisms in the regulation of this activity. Furthermore, in addition to the reported heterogeneity in the nature and mechanism of action of EDH(F), the contribution of experimental design and technique to this heterogeneity will be examined.

Requirement for flow in the blockade of endothelium-derived hyperpolarizing factor (EDHF) by ascorbate in the bovine ciliary artery

We previously reported that ascorbate inhibits endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation in the bovine perfused ciliary circulation and rat perfused mesentery, but not in rings of bovine or porcine coronary artery. In this study, we have compared the ability of ascorbate to inhibit EDHF-mediated vasodilatation in a single vessel, the bovine long posterior ciliary artery, when perfused and when mounted as rings in a myograph. Both in segments perfused at a flow rate of 2.5 ml min(-1) and in rings mounted in a myograph, bradykinin and acetylcholine each induced vasodilator responses that were mediated jointly by EDHF and nitric oxide, as revealed by their respective blocking agents, apamin/charybdotoxin, and L-NAME. Ascorbate (50 and 150 microm) induced a time (max at 2-3 h)-dependent inhibition of the EDHF-mediated component of vasodilatation to bradykinin or acetylcholine in perfused segments, but not in rings. Ascorbate (50 microm) failed to inhibit bradykinin-induced vasodilatation at a flow rate of 1.25 ml min(-1) or below, but produced graded blockade at the higher flow rates of 2.5 and 5 ml min(-1). Furthermore, using a pressure myograph where pressure and flow were independently controlled, it was confirmed that the inhibitory action of ascorbate (150 microm) was directly related to flow per se and not any associated changes in pressure. Thus, we have shown in the bovine ciliary artery that ascorbate inhibits EDHF-mediated vasodilatation under conditions of flow but not in a static myograph. The mechanism by which flow renders EDHF susceptible to inhibition by ascorbate remains to be determined.

Ascorbate decreases Fabry cerebral hyperperfusion suggesting a reactive oxygen species abnormality: An arterial spin tagging study

PURPOSE

To test the hypothesis that reactive oxygen species contribute to the cerebral hyperperfusion in Fabry disease.

MATERIAL AND METHODS

We examined the effect of intravenous injection of ascorbate on cerebral blood flow (CBF) using magnetic resonance arterial spin tagging. Nineteen patients with Fabry disease and 15 control subjects were studied as part of a randomized double-blind placebo-controlled trial of enzyme replacement therapy (ERT).

RESULTS

Vertebro-basilar hyperperfusion was observed in patients with Fabry disease. A decrease in systemic ascorbate levels relative to healthy controls was found in the patients. CBF decreased after ascorbate infusion in both control subjects and patients treated with ERT. The placebo group had a significantly delayed decrease in the CBF response after ascorbate infusion. Myeloperoxidase levels were elevated in Fabry patients, consistent with ongoing inflammatory processes in these patients.

CONCLUSION

Increased CBF in Fabry disease may be related to increased production of reactive oxygen species, while low plasma ascorbate levels suggests a global redox imbalance. These abnormalities were improved by ERT. These observations have implications regarding oxidative processes contributing to accelerated atherosclerosis in Fabry disease.

Distinct hyperpolarizing and relaxant roles for gap junctions and endothelium-derived H2O2 in NO-independent relaxations of rabbit arteries

We have compared the contributions of gap junctional communication and chemical signaling via H2O2 to NO-independent relaxations evoked by the Ca2+ ionophore A23187 and acetylcholine (ACh) in rabbit ilio-femoral arteries. Immunostaining confirmed the presence of connexins (Cxs) 37 and 40 in the endothelium and Cxs 40 and 43 in smooth muscle. Maximal endothelium-dependent subintimal smooth muscle hyperpolarizations evoked by A23187 and ACh were equivalent (approximately 20 mV) and almost abolished by an inhibitory peptide combination targeted against Cxs 37, 40, and 43. However, maximal NO-independent relaxations evoked by A23187 were unaffected by such peptides, whereas those evoked by ACh were depressed by approximately 70%. By contrast, the enzyme catalase, which destroys H2O2, attenuated A23187-induced relaxations over a broad range of concentrations, but only minimally depressed the maximum response to ACh. Catalase did not affect A23187- or ACh-evoked hyperpolarizations. After loading with an H2O2-sensitive probe, A23187 caused a marked increase in endothelial fluorescence that correlated temporally with relaxation, whereas only a weak delayed increase was observed with ACh. In arteries without endothelium, the H2O2-generating system xanthine/xanthine oxidase induced a catalase-sensitive relaxation that mimicked the gap junction-independent response to A23187 as it was maximally equivalent to approximately 80% of induced tone, but associated with a smooth muscle hyperpolarization <5 mV. We conclude that myoendothelial gap junctions underpin smooth muscle hyperpolarizations evoked by A23187 and ACh, but that A23187-induced relaxation is dominated by extracellular release of H2O2. Endothelium-derived H2O2 may thus be regarded as a relaxing factor, but not a hyperpolarizing factor, in rabbit arteries.

Differential effects of ascorbate on endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation in the bovine ciliary vascular bed and coronary artery

1. The ability of ascorbate to inhibit endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation was compared in the bovine perfused ciliary vascular bed and isolated rings of coronary artery. 2. Acetylcholine-induced, EDHF-mediated vasodilatation of the ciliary circulation was blocked following inclusion of ascorbate (50 micro M, 120 min) in the perfusion fluid. The blockade was highly selective since ascorbate had no effect on the vasodilator actions of the K(ATP) channel opener, levcromakalim, nor on the tonic vasodepressor action of basally released nitric oxide. 3. The possibility that concentration of ascorbate by the ciliary body was a prerequisite for blockade to occur was ruled out, since EDHF was still blocked when the anterior and posterior chambers were continuously flushed with Krebs solution or when both the aqueous and vitreous humour were drained. 4. Ascorbate at 50 micro M failed to affect bradykinin- or acetylcholine-induced, EDHF-mediated vasodilatation in rings of bovine coronary artery. Raising the concentration to 3 mM did produce blockade of EDHF, but this was nonselective, since vasodilator responses to endothelium-derived nitric oxide were also inhibited. 5. Thus, ascorbate (50 micro M) is not a universal blocker of EDHF. Whether its ability to block in the bovine ciliary circulation, but not in the coronary artery, is due to differences in the nature of EDHF at the two sites, differences in vessel size (resistance arterioles versus conduit artery), the presence or absence of flow, or to some other factor remains to be determined.