EDHF -- are there gaps in the pathway?

Vascular endothelial function in health and diseases

The vascular endothelium constitutes approximately 1% of body mass (1kg) and has a surface area of approximately 5000m(2). The endothelium is a multifunctional endocrine organ strategically placed between the vessel wall and the circulating blood, and has a key role in vascular homeostasis. The endothelium is both a target for and mediator of cardiovascular disease. The endothelium releases several relaxing and constricting factors, which can affect vascular homeostasis. Endothelial dysfunction, whether caused by physical injury or cellular damage, leads to compensatory responses that alter the normal homeostatic properties of the endothelium. In this review, we summarized some physiological aspects of endothelial function and then we discussed endothelial dysfunction during some pathological conditions.

Characterization of agonist-induced endothelium-dependent vasodilatory responses in the vascular bed of the equine digit

The role of endothelium-derived relaxing factors was studied in the regulation of vascular responses in the Krebs perfused equine isolated digit. Perfusion pressure was recorded in response to bolus doses of 5-hydroxytryptamine (6 nmol) alone or co-administered with carbachol (CCh; 0.2 micromol), bradykinin (BK; 0.2 nmol), substance P (SP; 0.2 nmol) or sodium nitroprusside (SNP; 0.2 micromol). N(omega)-Nitro-L-Arginine methyl ester hydrochloride (L-NAME; 300 microm) caused partial but significant inhibition of CCh-induced vasodilatory response, whereas BK and SP-induced responses were resistant to L-NAME. High potassium (K(+), 30 mm) and the cytochrome P-450 (CYP) epoxygenase inhibitor, clotrimazole (10 microm) plus L-NAME (100 microm), completely abolished the CCh, BK and SP-induced vasodilatory responses, whereas the response to SNP was unaffected. In contrast, the L-NAME-resistant proportion of CCh, BK and SP-induced vasodilatory response was not inhibited by the highly selective CYP2C9 inhibitor, sulphaphenazole (10 microm). The cyclo-oxygenase inhibitor, ibuprofen (10 microm) did not affect the CCh, BK and SP-induced responses. These data demonstrate that CCh, BK and SP-induced relaxation in the equine digit involve a combination of the NO and endothelium-derived hyperpolarizing factor (EDHF) pathways. These results do not support the evidence for the involvement of CYP-derived epoxyeicosatrienoic acids and the exact nature of EDHF in the equine digit remains to be established.

Endothelium-derived hyperpolarizing factor as an in vivo back-up mechanism in the cutaneous microcirculation in old mice

There is now strong evidence that an endothelium-derived hyperpolarizing factor (EDHF), other than nitric oxide (NO) or prostaglandin (PG), exists for dilating arteries and arterioles. In vitro studies on isolated vessels pointed out a role for EDHF as a back-up mechanism when the NO pathway is impaired, but there was a lack of in vivo studies showing a functional role for EDHF. Ageing has pronounced effects on vascular function and particularly on endothelium-dependent relaxation, providing a novel situation in which to assess the contributions of EDHF. The purpose of the present study was thus to determine if, in vivo, there was a functional role for EDHF as a back-up mechanism in the cutaneous microcirculation in the ageing process. We investigated in vivo the contribution of each endothelial factor (NO, PG and EDHF) in the cutaneous vasodilatation induced by iontophoretic delivery of acetylcholine and local pressure application in young adult (6-7 months) and old (22-25 months) mice, using pharmacological inhibitors. The cutaneous vasodilator responses induced by acetylcholine and local pressure application were dependent upon NO and PG pathways in young adult mice, whereas they were EDHF-dependent in old mice. EDHF appears to serve as a back-up mechanism when ageing reaches pathological states in terms of the ability for NO and PG to relax cutaneous microvessels, allowing for persistent cutaneous vasodilatator responses in old mice. However, as a back-up mechanism, EDHF did not completely restore cutaneous vasodilatation, since endothelial responses were reduced in old mice compared to young adult mice.

Angiotensin II-induced vasodilatation in cerebral arteries is mediated by endothelium-derived hyperpolarising factor

The angiotensin II-induced vasodilatation was evaluated in rat middle cerebral artery, especially regarding endothelium-derived hyperpolarising factor (EDHF), by use of a pressurised arteriograph. The angiotensin II dilatation was partly antagonised by inhibitors of nitric oxide synthase and cyclo-oxygenase. The remaining dilatation was inhibited by the potassium channel blockers, charybdotoxin and apamin, providing direct evidence that angiotensin II induces EDHF-mediated dilatation in cerebral arteries. The angiotensin II dilatation was blocked by the angiotensin AT1 and AT2 receptor blockers candesartan and PD 123319. Both angiotensin AT1 and AT2 receptors were detected on the endothelium by immunohistochemistry.

Responses of cerebral arterioles to ADP: eNOS-dependent and eNOS-independent mechanisms

ADP mediates platelet-induced relaxation of blood vessels and may function as an important intercellular signaling molecule in the brain. We used pharmacological and genetic approaches to examine mechanisms that mediate responses of cerebral arterioles to ADP, including the role of endothelial nitric oxide synthase (eNOS). We examined responses of cerebral arterioles (control diameter approximately 30 microm) in anesthetized wild-type (WT, eNOS+/+) and eNOS-deficient (eNOS-/-) mice using a cranial window. In WT mice, local application of ADP produced vasodilation that was not altered by indomethacin but was reduced by approximately 50% by NG-nitro-L-arginine (L-NNA) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (inhibitors of NOS and soluble guanylate cyclase, respectively). In eNOS-/- mice, responses to ADP were largely preserved, and a significant component of the response was resistant to L-NNA (a finding similar to that in WT mice treated with L-NNA). In the absence of L-NNA, responses to ADP were markedly reduced by charybdotoxin plus apamin [inhibitors of Ca2+-dependent K+ channels and responses mediated by endothelium-derived hyperpolarizing factor (EDHF)] in both WT and eNOS-/- mice. Thus pharmacological and genetic evidence suggests that a significant portion of the response to ADP in cerebral microvessels is mediated by a mechanism independent of eNOS. The eNOS-independent mechanism is functional in the absence of inhibited eNOS and most likely is mediated by an EDHF.

Receptor-activating peptides for PAR-1 and PAR-2 relax rat gastric artery via multiple mechanisms

Receptor-activating peptides for protease-activated receptors (PARs) 1 or 2 enhance gastric mucosal blood flow (GMBF) and protect against gastric mucosal injury in rats. We thus examined and characterized the effects of PAR-1 and PAR-2 agonists on the isometric tension in isolated rat gastric artery. The agonists for PAR-2 or PAR-1 produced vasodilation in the endothelium-intact arterial rings, which was abolished by removal of the endothelium. The mechanisms underlying the PAR-2- and PAR-1-mediated relaxation involved NO, endothelium-derived hyperpolarizing factor (EDHF) and prostanoids, to distinct extent, as evaluated by use of inhibitors of NO synthase, cyclo-oxygenase and Ca2+-activated K+ channels. The EDHF-dependent relaxation responses were significantly attenuated by gap junction inhibitors. These findings demonstrate that endothelial PAR-1 and PAR-2, upon activation, dilate the gastric artery via NO and prostanoid formation and also EDHF mechanisms including gap junctions, which would enhance GMBF.

A protective role of protease-activated receptor 1 in rat gastric mucosa

BACKGROUND AND AIMS

On activation, protease-activated receptor (PAR)-2 modulates multiple gastric functions and exerts mucosal protection via activation of sensory neurons. The role of PAR-1, a thrombin receptor, in the stomach remains unknown. We thus examined if the PAR-1 agonist could protect against gastric mucosal injury in rats.

METHODS

Gastric mucosal injury was created by oral administration of ethanol/HCl or absolute ethanol in conscious rats. Gastric mucosal blood flow and acid secretion were determined in anesthetized rats. Immunohistochemical analyses of PAR-1 and cyclooxygenase (COX)-1 were also performed in rat and human stomach.

RESULTS

The PAR-1 agonist TFLLR-NH(2), administered intravenously in combination with amastatin, protected against the gastric mucosal injury induced by ethanol/HCl or absolute ethanol. The protective effect of TFLLR-NH(2) was abolished by indomethacin or a COX-1 inhibitor but not by ablation of sensory neurons with capsaicin. TFLLR-NH(2) produced an NO-independent increase in gastric mucosal blood flow that was partially inhibited by blockade of the endothelium-derived hyperpolarizing factor pathway. This inhibitory effect was promoted by indomethacin. TFLLR-NH(2) suppressed carbachol-evoked acid secretion in an indomethacin-reversible manner. Immunoreactive PAR-1 and COX-1 were expressed abundantly in rat gastric muscularis mucosae and smooth muscle, and the former protein was also detectable in blood vessels. Similar staining was observed in human gastric muscularis mucosae.

CONCLUSIONS

The PAR-1 agonist, given systemically, protects against gastric mucosal injury via COX-1-dependent formation of prostanoids, modulating multiple gastric functions. Our data identify a novel protective role for PAR-1 in gastric mucosa, and the underlying mechanism is entirely different from that for PAR-2.

Small- and intermediate-conductance calcium-activated K+ channels provide different facets of endothelium-dependent hyperpolarization in rat mesenteric artery

Activation of both small-conductance (SKCa) and intermediate-conductance (IKCa) Ca2+-activated K+ channels in endothelial cells leads to vascular smooth muscle hyperpolarization and relaxation in rat mesenteric arteries. The contribution that each endothelial K+ channel type makes to the smooth muscle hyperpolarization is unknown. In the presence of a nitric oxide (NO) synthase inhibitor, ACh evoked endothelium and concentration-dependent smooth muscle hyperpolarization, increasing the resting potential (approx. -53 mV) by around 20 mV at 3 microM. Similar hyperpolarization was evoked with cyclopiazonic acid (10 microM, an inhibitor of sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA)) while 1-EBIO (300 microM, an IKCa activator) only increased the potential by a few millivolts. Hyperpolarization in response to either ACh or CPA was abolished with apamin (50 nM, an SKCa blocker) but was unaltered by 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (1 microM TRAM-34, an IKCa blocker). During depolarization and contraction in response to phenylephrine (PE), ACh still increased the membrane potential to around -70 mV, but with apamin present the membrane potential only increased just beyond the original resting potential (circa -58 mV). TRAM-34 alone did not affect hyperpolarization to ACh but, in combination with apamin, ACh-evoked hyperpolarization was completely abolished. These data suggest that true endothelium-dependent hyperpolarization of smooth muscle cells in response to ACh is attributable to SKCa channels, whereas IKCa channels play an important role during the ACh-mediated repolarization phase only observed following depolarization.

Involvement of EDHF in the hypotension and increased gastric mucosal blood flow caused by PAR-2 activation in rats

1. Agonists for protease-activated receptor-2 (PAR-2) cause hypotension and an increase in gastric mucosal blood flow (GMBF) in vivo. We thus studied the mechanisms underlying the circulatory modulation by PAR-2 activation in vivo, especially with respect to involvement of endothelium-derived hyperpolarizing factor (EDHF). 2. Arterial blood pressure and GMBF were measured in anesthetized rats in vivo. Vascular relaxation was assessed in the precontracted rat gastric arterial rings in vitro. 3. The PAR-2-activating peptide SLIGRL-NH2 and/or trypsin, administered i.v., produced largely NO-independent hypotension and increase in GMBF accompanied by decreased gastric mucosal vascular resistance (GMVR) in rats. 4. Combined administration of apamin and charybdotoxin, but not each of them, specifically abolished the hypotension, increased GMBF and decreased GMVR caused by the PAR-2 agonists. 5. In the isolated rat gastric artery, SLIGRL-NH2 elicited endothelium-dependent relaxation even in the presence of an NO synthase inhibitor and indomethacin, which was abolished by apamin plus charybdotoxin. 6. Our data suggest involvement of apamin/charybdotoxin-sensitive K+ channels in the PAR-2-triggered hypotension and increased GMBF, predicting a role of EDHF-like factors.

Endothelium-dependent relaxation and endothelial hyperpolarization by P2Y receptor agonists in rat-isolated mesenteric artery

(1) Vasorelaxation and hyperpolarization of endothelial cells by adenosine 5'-[beta-thio]diphosphate (ADPbetaS) and adenosine 5'-[gamma-thio]triphosphate (ATPgammaS) were studied in rat-isolated mesenteric artery. Effects from stimulation of P2X receptors were avoided by desensitization with alpha,beta-methylene adenosine triphosphate. (2) ADPbetaS caused concentration- and endothelium-dependent relaxations of methoxamine-precontracted small (third generation) and main mesenteric artery. These were inhibited by N(omega)-nitro-L-arginine methyl ester (L-NAME) or a combination of apamin plus charybdotoxin (inhibitors of Ca(2+)-activated K(+) channels); L-NAME, apamin and charybdotoxin applied together abolished the response. (3) ATPgammaS induced limited relaxation (35% of methoxamine-induced tone at 10 micro M) of small mesenteric artery, which was sensitive to L-NAME or endothelium denudation. However, it almost completely relaxed the main mesenteric artery over an extended concentration range (>6 orders of magnitude) in an endothelium-dependent manner. This relaxation was inhibited by either L-NAME or a combination of apamin with charybdotoxin, and abolished by a combination of all the three inhibitors. (4) The P2Y(1) receptor antagonist MRS 2179 (2'-deoxy-N(6)-methyladenosine 3',5'-bisphosphate; 0.3-3 micro M) caused parallel rightward shifts of the concentration/relaxation curve to ADPbetaS (pA(2)=7.1). However, MRS 2179 did not inhibit, but potentiated, relaxant responses to ATPgammaS. MRS 2179 did not affect the contractile responses ATPgammaS in small mesenteric artery; ATPgammaS did not contract the main mesenteric artery. (5) ADPbetaS hyperpolarized the endothelium of the main mesenteric artery in a concentration-dependent manner. This was unaffected by L-NAME but antagonized by MRS 2179. ATPgammaS also hyperpolarized the mesenteric artery endothelium in a concentration-dependent manner but, when ATPgammaS was applied at 10 micro M, its effect was potentiated by MRS 2179 (3 micro M). (6) It is concluded that both relaxation and hyperpolarization to ADPbetaS are mediated by P2Y(1) receptors and that the endothelial hyperpolarization is related to the L-NAME-resistant relaxation. Relaxation to the P2Y(2) agonist ATPgammaS shows regional variation along the mesenteric vasculature. The mechanisms for potentiation of relaxation and hyperpolarization by ATPgammaS are unknown, but may indicate interactions between P2Y receptor subtypes.

Structure, function, and endothelium-derived hyperpolarizing factor in the caudal artery of the SHR and WKY rat

OBJECTIVE

To quantify structural and functional characteristics of the caudal artery from spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats with particular reference to endothelium-derived hyperpolarizing factor (EDHF).

METHODS AND RESULTS

Ultrastructural studies showed that the number of myoendothelial gap junctions, smooth muscle cell (SMC) layers, and medial cross-sectional area were significantly greater in SHR than WKY. Intracellular dye labeling demonstrated hyperplasia of SMCs in SHR. Analysis of nerve-mediated excitatory junction potentials recorded in SMCs at the adventitial and luminal borders demonstrated decreased radial coupling of SMCs in SHR. In both SHR and WKY, in the presence of NG-nitro-L-arginine methyl ester and indomethacin, acetylcholine-elicited EDHF was abolished by charybdotoxin and apamin, while iberiotoxin had no effect, implicating the involvement of small and intermediate, but not large, calcium-activated potassium channels. EDHF was abolished by Gap-mimetic peptides, 18beta-glycyrrhetinic acid, and endothelial removal but not affected by the NO scavengers hydroxocobalamin and carboxy-PTIO.

CONCLUSIONS

Significant differences in SMC morphology and homocellular and heterocellular coupling exist between the caudal artery of SHR and WKY rats. In the caudal artery of SHR, significantly greater heterocellular coupling compensates for other structural changes in the media to maintain a functional role for EDHF.

Coronary myogenic constriction antagonizes EDHF-mediated dilation: role of KCa channels

In hypertension, pressure-induced myogenic constriction and impaired endothelium-derived hyperpolarizing factor (EDHF)-mediated dilation may contribute to increased vasomotor tone. Myogenic constriction as well as EDHF-mediated dilation may share common signaling mechanisms, and both may control KCa channel activity to set arterial tone. To investigate a potential relation between the 2 mechanisms, we studied coronary arteries of Sprague-Dawley rats for individual myogenic constriction compared with EDHF-mediated dilation of the same artery. EDHF-mediated dilation was measured as the maximal dilation to acetylcholine (100 micromol/L) after preconstriction, resistant to NO inhibition (NG-methyl-l-arginine acetate salt, L-NMMA, 100 micromol/L), and prostaglandin inhibition (indomethacin, 10 micromol/L) but abolished by charybdotoxin (100 nmol/L) plus apamin (500 nmol/L). Individual coronary myogenic constriction at an intraluminal pressure of 70 mm Hg (n=9) ranged from 6% to 44% (24+/-4%). EDHF-mediated dilation ranged from 18% to 84% (42+/-7%). Elevating pressure to 130 mm Hg (n=8) increased myogenic constriction by 2-fold (P<0.01) and decreased EDHF-mediated dilation by 2.6-fold (P<0.01). Interestingly, individual myogenic constriction inversely correlated to individual EDHF-mediated dilation (r=-0.75, P<0.001, n=17). Pretreatment with the KCa channel opener NS1619 (30 micromol/L) prevented coronary myogenic constriction and increased EDHF-mediated dilation by 2.2-fold (P<0.01), whereas the KATP channel opener cromakalim (3 micromol/L) had no effect on EDHF-mediated dilation. For comparison, in mesenteric arteries (at 70 mm Hg) low myogenic constriction (2+/-1%) was associated with high EDHF-mediated dilation (93+/-2%), and pretreatment with NS1619 had no effect. Our results demonstrate that myogenic constriction in coronary arteries antagonizes EDHF-mediated dilation. Activation of KCa channels with NS1619 reduces myogenic constriction and profoundly increases EDHF-mediated dilation, specifically in coronary arteries, suggesting a potential therapeutic impact to reduce coronary risk in hypertension.

Essential role of Gap junctions in NO- and prostanoid-independent relaxations evoked by acetylcholine in rabbit intracerebral arteries

BACKGROUND AND PURPOSE

Direct intercellular communication via gap junctions may play a central role in endothelium-dependent relaxations that are mediated by a conducted hyperpolarization and do not involve the synthesis of NO and prostanoids. In the present study, inhibitory peptides homologous to the Gap27 domain of the second extracellular loop of connexin37/connexin43 and connexin40, designated as 37,43Gap27 and 40Gap27, respectively, were used to evaluate the role of this mechanism in intracerebral arteries.

METHODS

Isolated rings of rabbit middle cerebral artery were constricted by histamine (10 micromol/L) in the presence of N(G)-nitro-L-arginine methyl ester (300 micromol/L) and indomethacin (10 micromol/L). Concentration-relaxation curves for acetylcholine were constructed in the presence and absence of 37,43Gap27 and 40Gap27. Specific antibodies were used to delineate the distribution of connexin37, connexin40, connexin43, and connexin45 within the arterial wall.

RESULTS

Individually, 37,43Gap27 and 40Gap27 minimally affected endothelium-dependent relaxations to acetylcholine at concentrations of 300 micro mol/L, whereas their combination (at 300 micromol/L each) inhibited the maximal response by approximately 70% and increased the EC50 value for relaxation by approximately 15-fold. In endothelium-denuded rings, this peptide combination did not attenuate responses to sodium nitroprusside, an exogenous source of NO. Gap junction plaques, whose incidence was highest in endothelium, were constructed from connexin40 and connexin43 in the media and connexin37, connexin40, and connexin43 in the endothelium.

CONCLUSIONS

The findings confirm that direct communication via gap junctions contributes to agonist-induced relaxations of intracerebral arteries. More than one connexin subtype appears to participate in such responses.

NO- and non-NO-, non-prostanoid-dependent vasodilatation in rat sciatic nerve during maturation and developing experimental diabetic neuropathy

This study examined NO- and non-NO-, non-prostanoid-dependent pathways of agonist-induced vasodilatation in streptozotocin (STZ)-induced diabetic rats and their age-matched controls at 1-2, 8-10 and 18-20 weeks after induction of diabetes. Using laser Doppler flowmetry, vasodilatory responses to acetylcholine (ACh; 0.1 mM) and morpholino-sydnonimine (SIN-1) were determined in the presence of Ringer solution, during inhibition of NO synthase (NOS) and cyclo-oxygenase (COX) with N(omega)-nitro-L-arginine (L-NNA; 1 mM) + indomethacin (10(-5) M), and during inhibition of K(+) channels, NOS and COX with tetraethylammonium (TEA; 10 mM) + L-NNA + indomethacin. Basal NOS activity and nerve conduction velocity were also determined. In age-matched controls, SIN-1-induced vasodilatation in the presence of TEA + L-NNA + indomethacin, basal NOS activity and the initial vasodilatory response to ACh during NOS and COX inhibition all decreased with maturation. In STZ-induced diabetics, SIN-1-induced vasodilatation in the presence of TEA + L-NNA + indomethacin was impaired immediately after induction of diabetes, but not at 18-20 weeks. NOS activity in STZ-induced diabetics displayed a transient 2-fold increase at 8-10 weeks, decreasing to age-matched control levels at 18-20 weeks. At 18-20 weeks of STZ-induced diabetes, ACh-induced vasodilatation during NOS and COX inhibition was prolonged due to increased K(+) channel activity and experimental diabetic sensory neuropathy (EDN) had developed. Thus, in sciatic nerve microcirculation of STZ-induced diabetic rats: (1) diabetic impairment of vasodilatation in response to exogenous NO was transient; (2) non-NO-, non-prostanoid-dependent vasodilatation and K(+) channel activity were augmented in STZ-induced diabetes; and (3) alterations in NO bioactivity were not related to the development of EDN.

Discrete store-operated calcium influx into an intracellular compartment in rabbit arteriolar smooth muscle

This study tested the hypothesis that store-operated channels (SOCs) exist as a discrete population of Ca2+ channels activated by depletion of intracellular Ca(2+) stores in cerebral arteriolar smooth muscle cells and explored their direct contractile function. Using the Ca2+ indicator fura-PE3 it was observed that depletion of sarcoplasmic reticulum (SR) Ca2+ by inhibition of SR Ca2+-ATPase (SERCA) led to sustained elevation of [Ca2+]i that depended on extracellular Ca2+ and slightly enhanced Mn2+ entry. Enhanced background Ca2+ influx did not explain the raised [Ca2+]i in response to SERCA inhibitors because it had marked gadolinium (Gd3+) sensitivity, which background pathways did not. Effects were not secondary to changes in membrane potential. Thus SR Ca2+ depletion activated SOCs. Strikingly, SOC-mediated Ca2+ influx did not evoke constriction of the arterioles, which were in a resting state. This was despite the fura-PE3-indicated [Ca2+]i rise being greater than that evoked by 20 mM [K+]o (which did cause constriction). Release of endothelial vasodilators did not explain the absence of SOC-mediated constriction, nor did a change in Ca2+ sensitivity of the contractile proteins. We suggest SOCs are a discrete subset of Ca2+ channels allowing Ca2+ influx into a 'non-contractile' compartment in cerebral arteriolar smooth muscle cells.

cAMP facilitates EDHF-type relaxations in conduit arteries by enhancing electrotonic conduction via gap junctions

We have investigated the role of cAMP in NO- and prostanoid-independent relaxations that are widely attributed to an endothelium-derived hyperpolarizing factor (EDHF). Under control conditions EDHF-type relaxations evoked by acetylcholine (ACh) in rabbit iliac arteries were transient, but in the presence of the cAMP phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) or the cell permeant cAMP analog 8-bromo-cAMP, relaxations became sustained with their maxima potentiated approximately 2-fold. Relaxation was associated with transient approximately 1.5-fold elevations in smooth muscle cAMP levels with both mechanical and nucleotide responses being abolished by interrupting gap junctional communication with the connexin-mimetic peptide Gap 27 and by endothelial denudation. However, IBMX induced a sustained endothelium-independent approximately 2-fold rise in cAMP levels, which was not further amplified by ACh, suggesting that the contribution of cAMP to the EDHF phenomenon is permissive. After selective loading of the endothelium with calcein AM, direct transfer of dye from the endothelium to the media was enhanced by IBMX or 8-bromo-cAMP, but not by 8-bromo-cGMP, whereas Gap 27 promoted sequestration within the intima. ACh-induced hyperpolarizations of subintimal smooth muscle in arterial strips with intact endothelium were abolished by Gap 27 and the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine but were unaffected by IBMX. By contrast, in strips partially denuded of endothelium, IBMX enhanced the transmission of hyperpolarization from the endothelium to remote smooth muscle cells. These findings support the hypothesis that endothelial hyperpolarization underpins the EDHF phenomenon, with cAMP governing subsequent electrotonic signaling via both myoendothelial and homocellular smooth muscle gap junctions.

Gap junction-dependent and -independent EDHF-type relaxations may involve smooth muscle cAMP accumulation

We have compared the mechanisms that contribute to endothelium-derived hyperpolarizing factor (EDHF)-type responses induced by ACh and the Ca(2+) ionophore A-23187 in the rabbit iliac artery. Relaxations to both agents were associated with ~1.5-fold elevations in smooth muscle cAMP levels and were attenuated by the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine (DDA) and potentiated by the cAMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). Mechanical responses were inhibited by coadministration of the Ca(2+)-activated K(+) channel blockers apamin and charybdotoxin, both in the absence and presence of IBMX, but were unaffected by blockade of ATP-sensitive K(+) channels with the sulphonylurea glibenclamide. Relaxations and elevations in cAMP evoked by ACh were abolished by 18alpha-glycyrrhetinic acid, which disrupts gap junction plaques, whereas the corresponding responses to A-23187 were unaffected by this agent. Consistently, in "sandwich" bioassay experiments, A-23187, but not ACh, elicited extracellular release of a factor that evoked relaxations that were inhibited by DDA and potentiated by IBMX. These findings provide evidence that EDHF-type relaxations of rabbit iliac arteries evoked by ACh and A-23187 depend on cAMP accumulation in smooth muscle, but involve signaling via myoendothelial gap junctions and the extracellular space, respectively.

Characterization of an apamin-sensitive small-conductance Ca(2+)-activated K(+) channel in porcine coronary artery endothelium: relevance to EDHF

1. The apamin-sensitive small-conductance Ca(2+)-activated K(+) channel (SK(Ca)) was characterized in porcine coronary arteries. 2. In intact arteries, 100 nM substance P and 600 microM 1-ethyl-2-benzimidazolinone (1-EBIO) produced endothelial cell hyperpolarizations (27.8 +/- 0.8 mV and 24.1 +/- 1.0 mV, respectively). Charybdotoxin (100 nM) abolished the 1-EBIO response but substance P continued to induce a hyperpolarization (25.8 +/- 0.3 mV). 3. In freshly-isolated endothelial cells, outside-out patch recordings revealed a unitary K(+) conductance of 6.8 +/- 0.04 pS. The open-probability was increased by Ca(2+) and reduced by apamin (100 nM). Substance P activated an outward current under whole-cell perforated-patch conditions and a component of this current (38%) was inhibited by apamin. A second conductance of 2.7 +/- 0.03 pS inhibited by d-tubocurarine was observed infrequently. 4. Messenger RNA encoding the SK2 and SK3, but not the SK1, subunits of SK(Ca) was detected by RT - PCR in samples of endothelium. Western blotting indicated that SK3 protein was abundant in samples of endothelium compared to whole arteries. SK2 protein was present in whole artery nuclear fractions. 5. Immunofluorescent labelling confirmed that SK3 was highly expressed at the plasmalemma of endothelial cells and was not expressed in smooth muscle. SK2 was restricted to the peri-nuclear regions of both endothelial and smooth muscle cells. 6. In conclusion, the porcine coronary artery endothelium expresses an apamin-sensitive SK(Ca) containing the SK3 subunit. These channels are likely to confer all or part of the apamin-sensitive component of the endothelium-derived hyperpolarizing factor (EDHF) response.

Activation of endothelial cell IK(Ca) with 1-ethyl-2-benzimidazolinone evokes smooth muscle hyperpolarization in rat isolated mesenteric artery

1. In rat small mesenteric arteries contracted with phenylephrine, 1-ethyl-2-benzimidazolinone (1-EBIO; 3-300 microM) evoked concentration-dependent relaxation that, above 100 microM, was associated with smooth muscle hyperpolarization. 2. 1-EBIO-evoked hyperpolarization (maximum 22.1+/-3.6 mV with 300 microM, n=4) was endothelium-dependent and inhibited by charybdotoxin (ChTX 100 nM; n=4) but not iberiotoxin (IbTX 100 nM; n=4). 3. In endothelium-intact arteries, smooth muscle relaxation to 1-EBIO was not altered by either of the potassium channel blockers ChTX (100 nM; n=7), or IbTX (100 nM; n=4), or raised extracellular K(+) (25 mM). Removal of the endothelium shifted the relaxation curve to the right but did not reduce the maximum relaxation. 4. In freshly isolated mesenteric endothelial cells, 1-EBIO (600 microM) evoked a ChTX-sensitive outward K-current. In contrast, 1-EBIO had no effect on smooth muscle cell conductance whereas NS 1619 (33 microM) stimulated an outward current while having no effect on the endothelial cells. 5. These data show that with concentrations greater than 100 microM, 1-EBIO selectively activates outward current in endothelial cells, which presumably underlies the smooth muscle hyperpolarization and a component of the relaxation. Sensitivity to block with charybdotoxin but not iberiotoxin indicates this current is due to activation of IK(Ca). However, 1-EBIO can also relax the smooth muscle by an undefined mechanism, independent of any change in membrane potential.

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.

Gap junction-dependent increases in smooth muscle cAMP underpin the EDHF phenomenon in rabbit arteries

We have investigated the role of cAMP in nitric oxide (NO)- and prostanoid-independent vascular relaxations evoked by acetylcholine (ACh) in isolated arteries and perfused ear preparations from the rabbit. These EDHF-type responses are shown to be associated with elevated cAMP levels specifically in smooth muscle and are attenuated by blocking adenylyl cyclase or protein kinase A (PKA). Relaxations are amplified by 3-isobutyl-1-methylxanthine, which prevents cAMP hydrolysis, while remaining susceptible to inhibition by the combination of two K(Ca) channel blockers, apamin and charybdotoxin. Analogous endothelium- and cAMP-dependent relaxations were evoked by cyclopiazonic acid (CPA) which stimulates Ca(2+) influx via channels linked to the depletion of Ca(2+) stores. Responses to ACh and CPA were both inhibited by interrupting cell-to-cell coupling via gap junctions with 18alpha-glycyrrhetinic acid and a connexin-specific Gap 27 peptide. The findings suggest that EDHF-type responses are initiated by capacitative Ca(2+) influx into the endothelium and propagated by direct intercellular communication to effect relaxation via cAMP/PKA-dependent phosphorylation events in smooth muscle.