Evidence against a role of cytochrome P450-derived arachidonic acid metabolites in endothelium-dependent hyperpolarization by acetylcholine in rat isolated mesenteric artery

Monodora myristica (African nutmeg) modulates redox homeostasis and alters functional chemistry in sickled erythrocytes

The antioxidative effect of Monodora myristica seed acetone extract and its effect on chemical functional groups were investigated in sickled erythrocytes as well as molecular modeling of the antisickling potentials of its secondary metabolites. The extract was subjected to gas chromatography–mass spectrometry to identify the compounds present, which were then docked into the allosteric-binding site of deoxy-hemoglobin. The extract was incubated with sickled erythrocytes at 37°C for 6, 12, and 24 h and were subjected to antioxidative analysis for reduced glutathione (GSH), superoxide dismutase (SOD), catalase, and lipid peroxidation (LPO). Chemical functional group of the treated cells was analyzed via Fourier transform infrared spectroscopy (FTIR). The predominant compounds identified were 17-octadecynoic acid; oleic acid, androstan-3-one, 17-hydroxy-2-methyl- (2.beta.,5.beta.,17.beta.)-; estran-3-one, 17-(acetyloxy)-2-methyl-, (2.alpha., 5.alpha., 17.beta.), and (+)-3-carene, 10-(acetylmethyl)-. They all fitted well within the active site of Hb with good binding affinity, as evidenced by the negative CDocker interaction energies of their complexes ranging between −54.4 and −26.7 kcal/mol. Treatment with the extract exacerbated SOD and catalase activities as well as GSH level, while LPO was suppressed. This antioxidative activity was time and/or dose dependent, with 6 and 12 h incubation showing the optimum activity. FTIR analysis of the treated cells showed the presence of hydrophobic functional groups. The synergetic molecular interaction of the major compounds of the extract with the α-dimer of Hb depicts an antisickling effect of M. myristica acetone extract. This is accompanied by exacerbation of endogenous antioxidant enzymes activity and modification of the functional chemistry of the cells.

The production of vasoconstriction-induced residual NO modulates perfusion pressure in rat mesenteric vascular bed

In the presence of nitric oxide synthase (NOS) inhibitors, the contribution of residual NO to endothelium-dependent relaxation induced by chemical agonists acetylcholine and bradykinin has been documented in resistance vessels. However, the contribution of residual NO to the vasodilatation in response to pressure and fluid shear stress is not well understood. In this study, to demonstrate the activity of residual NO, we applied a NO scavenger, hydroxocobalamin (HCX), on the phenylephrine-induced increase in perfusion pressure in the presence of NOS inhibitors, Nω-nitro-L-arginine (L-NA) or Nω-nitro-L-arginine methyl ester (L-NAME) in the rat perfused mesenteric bed. The perfusion pressure was increased by phenylephrine (1-2 µM), an α1-adrenoceptor agonist. This increase was augmented by the addition of L-NA or L-NAME. In the presence of any NOS inhibitors, the application of hydroxocobalamin (100 µM) further increased the perfusion pressure. The removal of endothelium by saponin (50 mg/L) and the use of a non-selective protein kinase inhibitor, staurosporine (5 nM), and a tyrosine kinase inhibitor, erbstatin A (30 µM), but not a calmodulin inhibitor, calmidazolium (0.5 µM), inhibited the additional pressor responses induced by L-NA or L-NAME and a combination of either of them with hydroxocobalamine. These findings show that there could be a NOS inhibitor-resistant residual NO production in response to pressure in the rat mesenteric vascular bed. This residual NO production may be associated with the activation of tyrosine kinase and protein kinases, but not calmodulin. Finally, this pressure-induced residual NO exerts a modulatory role against vasoconstriction induced by phenylephrine.

Characteristics of ACh-induced hyperpolarization and relaxation in rabbit jugular vein

BACKGROUND AND PURPOSE

The roles played by endothelium-derived NO and prostacyclin and by endothelial cell hyperpolarization in ACh-induced relaxation have been well characterized in arteries. However, the mechanisms underlying ACh-induced relaxation in veins remain to be fully clarified.

EXPERIMENTAL APPROACH

ACh-induced smooth muscle cell (SMC) hyperpolarization and relaxation were measured in endothelium-intact and -denuded preparations of rabbit jugular vein.

KEY RESULTS

In endothelium-intact preparations, ACh (≤ 10⁻⁸ M) marginally increased the intracellular concentration of Ca²⁺ ([Ca²⁺](i)) in endothelial cells but did not alter the SMC membrane potential. However, ACh (10⁻¹⁰ -10⁻⁸ M) induced a concentration-dependent relaxation during the contraction induced by PGF(2α) and this relaxation was blocked by the NO synthase inhibitor N(ω) -nitro-l-arginine. ACh (10⁻⁸ -10⁻⁶ M) concentration-dependently increased endothelial [Ca²⁺](i) and induced SMC hyperpolarization and relaxation. These SMC responses were blocked in the combined presence of apamin [blocker of small-conductance Ca²⁺-activated K⁺ (SK(Ca) , K(Ca) 2.3) channel], TRAM 34 [blocker of intermediate-conductance Ca²⁺ -activated K⁺ (IK(Ca) , K(Ca) 3.1) channel] and margatoxin [blocker of subfamily of voltage-gated K⁺ (K(V) ) channel, K(V) 1].

CONCLUSIONS AND IMPLICATIONS

In rabbit jugular vein, NO plays a primary role in endothelium-dependent relaxation at very low concentrations of ACh (10⁻¹⁰ -10⁻⁸ M). At higher concentrations, ACh (10⁻⁸ -3 × 10⁻⁶ M) induces SMC hyperpolarization through activation of endothelial IK(Ca) , K(V) 1 and (possibly) SK(Ca) channels and produces relaxation. These results imply that ACh regulates rabbit jugular vein tonus through activation of two endothelium-dependent regulatory mechanisms.

Epoxides and soluble epoxide hydrolase in cardiovascular physiology

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

Increased myoendothelial gap junctions mediate the enhanced response to epoxyeicosatrienoic acid and acetylcholine in mesenteric arterial vessels of cirrhotic rats

BACKGROUND

Cirrhotic portal hypertension is characterized by mesenteric arterial vasodilation and hyporeactivity to vasoconstrictors.

AIM

We evaluated the role of epoxyeicosatrienoic acid (EET) and of myoendothelial gap junctions (GJ) in the haemodynamic alterations of experimental cirrhosis.

METHODS

Thirty-five control rats and 35 rats with carbon tetrachloride (CCl(4))-induced cirrhosis were studied. Small resistance mesenteric arteries (diameter <350 μm) were connected to a pressure servo controller in a video-monitored perfusion system. Concentration-response curves to acetylcholine (ACh) were evaluated in mesenteric arteries pre-incubated with indomethacin, N(G)-nitro-L-arginine-methyl-ester and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one before and after the epoxygenase inhibitor miconazole or 18α-glycyrrhetinic acid (18α-GA) (GJ inhibitor). EC(50) was calculated. Concentration-response curves to 11,12-EET were also evaluated. mRNA and protein expression of connexins (Cxs) in the mesenteric arteries was evaluated by real-time PCR and immunohistochemistry.

RESULTS

The ACh response was increased in cirrhotic rats (EC(50): -6.55±0.10 vs. -6.01±0.10 log[M]; P<0.01) and was blunted by miconazole only in cirrhotic animals. 18α-GA blunted the response to ACh more in cirrhotic than that in control rats (P<0.05). Concentration-response curves to 11,12-EET showed an increased endothelium-dependent vasodilating response in cirrhotic rats (P<0.05); the BK(Ca) inhibitor Iberiotoxin (25 nM) blocked the response in normal rats but not in cirrhotic rats, while 18α-GA blunted the response in cirrhotic rats but not in control rats. An increased mRNA and protein expression of Cx40 and Cx43 in cirrhotic arteries was detected (P<0.05).

CONCLUSIONS

The increased nitric oxide/PGI(2)-independent vasodilation of mesenteric arterial circulation in cirrhosis is because of, at least in part, hyperreactivity to 11,12-EET through an increased expression of myoendothelial GJs.

Connexins and gap junctions in the EDHF phenomenon and conducted vasomotor responses

It is becoming increasingly evident that electrical signaling via gap junctions plays a central role in the physiological control of vascular tone via two related mechanisms (1) the endothelium-derived hyperpolarizing factor (EDHF) phenomenon, in which radial transmission of hyperpolarization from the endothelium to subjacent smooth muscle promotes relaxation, and (2) responses that propagate longitudinally, in which electrical signaling within the intimal and medial layers of the arteriolar wall orchestrates mechanical behavior over biologically large distances. In the EDHF phenomenon, the transmitted endothelial hyperpolarization is initiated by the activation of Ca(2+)-activated K(+) channels channels by InsP(3)-induced Ca(2+) release from the endoplasmic reticulum and/or store-operated Ca(2+) entry triggered by the depletion of such stores. Pharmacological inhibitors of direct cell-cell coupling may thus attenuate EDHF-type smooth muscle hyperpolarizations and relaxations, confirming the participation of electrotonic signaling via myoendothelial and homocellular smooth muscle gap junctions. In contrast to isolated vessels, surprisingly little experimental evidence argues in favor of myoendothelial coupling acting as the EDHF mechanism in arterioles in vivo. However, it now seems established that the endothelium plays the leading role in the spatial propagation of arteriolar responses and that these involve poorly understood regenerative mechanisms. The present review will focus on the complex interactions between the diverse cellular signaling mechanisms that contribute to these phenomena.

Epoxyeicosatrienoic acids and endothelium-dependent responses

Epoxyeicosatrienoic acids (EETs) are cytochrome P450 metabolites of arachidonic acid that are produced by the vascular endothelium in response to agonists such as bradykinin and acetylcholine or physical stimuli such as shear stress or cyclic stretch. In the vasculature, the EETs have biological actions that are involved in the regulation of vascular tone, hemostasis, and inflammation. In preconstricted arteries in vitro, EETs activate calcium-activated potassium channels on vascular smooth muscle and the endothelium causing membrane hyperpolarization and relaxation. These effects are observed in a variety of arteries from experimental animals and humans; however, this is not a universal finding in all arteries. The mechanism of EET action may vary. In some arteries, EETs are released from the endothelium and are transferred to the smooth muscle where they cause potassium channel activation, hyperpolarization, and relaxation through a guanine nucleotide binding protein-coupled mechanism or transient receptor potential (TRP) channel activation. In other arteries, EETs activate TRP channels on the endothelium to cause endothelial hyperpolarization that is transferred to the smooth muscle by gap junctions or potassium ion. Some arteries use a combination of mechanisms. Acetylcholine and bradykinin increase blood flow in dogs and humans that is inhibited by potassium channel blockers and cytochrome P450 inhibitors. Thus, the EETs are endothelium-derived hyperpolarizing factors mediating a portion of the relaxations to acetylcholine, bradykinin, shear stress, and cyclic stretch and regulate vascular tone in vitro and in vivo.

Cytochrome P-450 metabolites of 2-arachidonoylglycerol play a role in Ca2+-induced relaxation of rat mesenteric arteries

The perivascular sensory nerve (PvN) Ca(2+)-sensing receptor (CaR) is implicated in Ca(2+)-induced relaxation of isolated, phenylephrine (PE)-contracted mesenteric arteries, which involves the vascular endogenous cannabinoid system. We determined the effect of inhibition of diacylglycerol (DAG) lipase (DAGL), phospholipase A(2) (PLA(2)), and cytochrome P-450 (CYP) on Ca(2+)-induced relaxation of PE-contracted rat mesenteric arteries. Our findings indicate that Ca(2+)-induced vasorelaxation is not dependent on the endothelium. The DAGL inhibitor RHC 802675 (1 microM) and the CYP and PLA(2) inhibitors quinacrine (5 microM) (EC(50): RHC 802675 2.8 +/- 0.4 mM vs. control 1.4 +/- 0.3 mM; quinacrine 4.8 +/- 0.4 mM vs. control 2.0 +/- 0.3 mM; n = 5) and arachidonyltrifluoromethyl ketone (AACOCF(3), 1 microM) reduced Ca(2+)-induced relaxation of mesenteric arteries. Synthetic 2-arachidonoylglycerol (2-AG) and glycerated epoxyeicosatrienoic acids (GEETs) induced concentration-dependent relaxation of isolated arteries. 2-AG relaxations were blocked by iberiotoxin (IBTX) (EC(50): control 0.96 +/- 0.14 nM, IBTX 1.3 +/- 0.5 microM) and miconazole (48 +/- 3%), and 11,12-GEET responses were blocked by IBTX (EC(50): control 55 +/- 9 nM, IBTX 690 +/- 96 nM) and SR-141716A. The data suggest that activation of the CaR in the PvN network by Ca(2+) leads to synthesis and/or release of metabolites of the CYP epoxygenase pathway and metabolism of DAG to 2-AG and subsequently to GEETs. The findings indicate a role for 2-AG and its metabolites in Ca(2+)-induced relaxation of resistance arteries; therefore this receptor may be a potential target for the development of new vasodilator compounds for antihypertensive therapy.

Effects of methyl beta-cyclodextrin on EDHF responses in pig and rat arteries; association between SK(Ca) channels and caveolin-rich domains

BACKGROUND AND PURPOSE

The small and intermediate conductance, Ca2+-sensitive K+ channels (SK(Ca) and IK(Ca), respectively) which are pivotal in the EDHF pathway may be differentially activated. The importance of caveolae in the functioning of IK(Ca) and SK(Ca) channels was investigated.

EXPERIMENTAL APPROACH

The effect of the caveolae-disrupting agent methyl-beta-cyclodextrin (MbetaCD) on IK(Ca) and SK(Ca) localization and function was determined.

KEY RESULTS

EDHF-mediated, SK(Ca)-dependent myocyte hyperpolarizations evoked by acetylcholine in rat mesenteric arteries (following blockade of IK(Ca) with TRAM-34) were inhibited by MbetaCD. Hyperpolarizations evoked by direct SK(Ca) channel activation (using NS309 in the presence of TRAM-34) were also inhibited by MbetaCD, an effect reversed by cholesterol. In contrast, IK(Ca)-dependent hyperpolarizations (in the presence of apamin) were unaffected by MbetaCD. Similarly, in porcine coronary arteries, EDHF-mediated, SK(Ca)-dependent (but not IK(Ca)-dependent) endothelial cell hyperpolarizations evoked by substance P were inhibited by MbetaCD. In mesenteric artery homogenates subjected to sucrose-density centrifugation, caveolin-1 and SK3 (SK(Ca)) proteins but not IK1 (IK(Ca)) protein migrated to the buoyant, caveolin-rich fraction. MbetaCD pretreatment redistributed caveolin-1 and SK3 proteins into more dense fractions. In immunofluorescence images of porcine coronary artery endothelium, SK3 (but not IK1) and caveolin-1 were co-localized. Furthermore, caveolin-1 immunoprecipitates prepared from native porcine coronary artery endothelium contained SK3 but not IK1 protein.

CONCLUSIONS AND IMPLICATIONS

These data provide strong evidence that endothelial cell SK(Ca) channels are located in caveolae while the IK(Ca) channels reside in a different membrane compartment. These studies reveal cellular organisation as a further complexity in the EDHF pathway signalling cascade.

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

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

A role for heterocellular coupling and EETs in dilation of rat cremaster arteries

OBJECTIVE

The authors probed endothelium-dependent dilation and endothelial cell Ca2+ handling in myogenically active resistance arteries.

METHODS

First-order arteries were removed from rat cremaster muscles, cannulated, and pressurized (75 mmHg). Vessel diameter and endothelial cell Ca2+ were monitored using confocal microscopy, and arterial ultrastructure was determined using electron microscopy.

RESULTS

Acetylcholine (ACh) stimulated elevations and oscillations in endothelial cell Ca2+, and concentration-dependently dilated arteries with myogenic tone. NO-independent dilation was blocked by 35 mM K+. Combined IK(Ca) (1 microM TRAM-34) and SK(Ca) (100 nM apamin) blockade partially inhibited NO-independent relaxations, with residual relaxations sensitive to BK(Ca) or cytochrome P-450 inhibition (100 nM iberiotoxin, and 20 microM 17-ODYA or 10 microM MS-PPOH). 11,12-EET stimulated iberiotoxin-sensitive dilation, but did not affect endothelial cell Ca2+. 15 mM K+ evoked dilation sensitive to inhibition of K(IR) (30 microM Ba2+) and Na+/K+-ATPase (10 microM ouabain), whereas these blockers did not affect ACh-mediated dilations. Homo- and heterocellular gap junctions were identified in radial sections through arteries.

CONCLUSION

These data suggest that rises in endothelial cell Ca2+ stimulate SK(Ca) and IK(Ca) channels, leading to hyperpolarization and dilation, likely due to electrical coupling. In addition, a component was unmasked following SK(Ca) and IK(Ca) blockade, attributable to activation of BK(Ca) channels by cytochrome P-450 metabolites.

Mechanism of rat mesenteric arterial KATP channel activation by 14,15-epoxyeicosatrienoic acid

Recently, we reported that 11,12-epoxyeicosatrienoic acid (11,12-EET) potently activates rat mesenteric arterial ATP-sensitive K+ (KATP) channels and produces significant vasodilation through protein kinase A-dependent mechanisms. In this study, we tried to further delineate the signaling steps involved in the activation of vascular KATP channels by EETs. Whole cell patch-clamp recordings [0.1 mM ATP in the pipette, holding potential (HP) = 0 mV and testing potential (TP) = −100 mV] in freshly isolated rat mesenteric smooth muscle cells showed small glibenclamide-sensitive KATP currents (19.0 ± 7.9 pA, n = 5) that increased 6.9-fold on exposure to 5 μM 14,15-EET (132.0 ± 29.0 pA, n = 7, P < 0.05 vs. control). With 1 mM ATP in the pipette solution, KATP currents (HP = 0 mV and TP = −100 mV) were increased 3.5-fold on exposure to 1 μM 14,15-EET (57.5 ± 14.3 pA, n = 9, P < 0.05 vs. baseline). In the presence of 100 nM iberiotoxin, 1 μM 14,15-EET hyperpolarized the membrane potential from −20.5 ± 0.9 mV at baseline to −27.1 ± 3.0 mV ( n = 6 for both, P < 0.05 vs. baseline), and the EET effects were significantly reversed by 10 μM glibenclamide (−21.8 ± 1.4 mV, n = 6, P < 0.05 vs. EET). Incubation with 5 μM 14,15-epoxyeicosa-5( Z)-enoic acid (14,15-EEZE), a 14,15-EET antagonist, abolished the 14,15-EET effects (31.0 ± 11.8 pA, n = 5, P < 0.05 vs. 14,15-EET, P = not significant vs. control). The 14,15-EET effects were inhibited by inclusion of anti-Gsα antibody (1:500 dilution) but not by control IgG in the pipette solution. The effects of 14,15-EET were mimicked by cholera toxin (100 ng/ml), an exogenous ADP-ribosyltransferase. Treatment with the ADP-ribosyltransferase inhibitors 3-aminobenzamide (1 mM) or m-iodobenzylguanidine (100 μM) abrogated the effects of 14,15-EET on KATP currents. These results were corroborated by vasodilation studies. 14,15-EET dose-dependently dilated isolated small mesenteric arteries, and this was significantly attenuated by treatment with 14,15-EEZE or 3-aminobenzamide. These results suggest that 14,15-EET activates vascular KATP channels through ADP-ribosylation of Gsα.

Endothelial mediators of the acetylcholine-induced relaxation of the rat femoral artery

This study examined endothelium-derived mediators of acetylcholine-induced relaxation in male rat femoral arteries. Arterial rings were suspended in a myograph for the measurement of isometric force. The generation of hydrogen peroxide (H2O2) in endothelial cells was detected using the fluorescent probe, 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate acetyl ester. N(G)-nitro-L-arginine methyl ester (L-NAME, NOS inhibitor) and 1H-[1,2,4]oxadiazolo[4,2-alpha]quinoxalin-1-one (ODQ, guanylate cyclase inhibitor) alone or in combination with indomethacin (cycloxygenase inhibitor) diminished acetylcholine-induced endothelium-dependent relaxation to a similar extent. A small relaxation to acetylcholine in 60 mM KCl-constricted rings was abolished by L-NAME. Acetylcholine-induced relaxation was reduced by charybdotoxin plus apamin (intermediate- and small-conductance Ca2+-activated K+ channel blockers, respectively) or by 30 mM KCl. Both ouabain (Na+/K+ ATPase inhibitor) and BaCl2 (K(IR) channel blocker) also inhibited the relaxation albeit to a lesser degree. In the presence of L-NAME, ODQ plus indomethacin, charybdotoxin plus apamin or ouabain plus BaCl2 produced further inhibition. Catalase attenuated acetylcholine-induced relaxations and this attenuation was prevented by 3-amino-1,2,4-triazole (catalase inhibitor). Catalase did not affect acetylcholine-induced relaxations in rings treated with L-NAME or ODQ. Acetylcholine increased the dichlorofluorescein fluorescence intensity in native endothelial cells and this effect was abolished by catalase and by L-NAME. Exogenous H2O2 caused endothelium-independent relaxation that was slightly inhibited by iberiotoxin, ODQ or significantly reduced by elevated KCl, and abolished by catalase. The present results indicate that in addition to nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF, sensitive to charybdotoxin plus apamin, ouabain, and BaCl2), the endothelium of rat femoral artery can release H2O2 in response to acetylcholine, which was sensitive to L-NAME. Thus, the eNOS-dependent H2O2 is likely to be the third mediator of acetylcholine-mediated relaxations in rat femoral arteries.

ENHANCED RELEASE OF ENDOTHELIUM-DERIVED HYPERPOLARIZING FACTOR IN SMALL CORONARY ARTERIES FROM RATS WITH CONGESTIVE HEART FAILURE

1. Previous studies have suggested that the production of nitric oxide (NO) is reduced in coronary vessels of animals with congestive heart failure (CHF). However, the response to endothelium-derived hyperpolarizing factor (EDHF) in small coronary resistance arteries from CHF rats has not been investigated. The aim of the present study was to determine whether flow-induced dilation (FID) is altered in small coronary arteries from CHF rats and to characterize the role of EDHF in this process. 2. Small coronary arteries (97 +/- 6 microm) were isolated from control rats and from rats in which CHF was induced by left coronary artery ligation. The arteries were cannulated at 60 mmHg with flow. Changes in internal diameter were examined using videomicroscopy. 3. There was no significant difference in FID in small coronary arteries between control and CHF rats (68 +/- 6 vs 61 +/- 4% (expressed as a percentage of maximal dilation induced by nitroprusside (%MaxD(NP))), respectively). Flow-induced dilation in control rat vessels showed greater attenuation by N(G)-monomethyl-L-arginine (L-NMMA) than vessels from CHF rats (%NO-mediated FID 32 +/- 5 vs 16 +/- 3% (%MaxD(NP)), respectively). Pretreatment with indomethacin had no significant effect on FID in vessels from either rat group. Flow-induced dilation was attenuated by KCl (40 mmol/L) to a greater degree in vessels from CHF rats in the presence of L-NMMA and indomethacin compared with vessels from control rats (%EDHF-mediated FID: 36 +/- 4 vs 25 +/- 5% (%MaxD(NP)), respectively). Flow-induced dilation was abolished by removal of the endothelium and was significantly decreased in vessels from CHF rats in response to charybdotoxin plus apamin or tetrabutylammonium compared with control rat vessels. 17-Octadecynoic acid had no significant effect on FID in vessels from either control or CHF rats. 4. In conclusion, the FID of small coronary arteries is mediated by K+ channels, including the K(Ca) channels. Endothelium-derived hyperpolarizing factor-mediated dilation may compensate for the loss of NO-mediated dilation in CHF.

Reciprocal changes in endothelium-derived hyperpolarizing factor- and nitric oxide-system in the mesenteric artery of adult female rats following ovariectomy

1. To explore the effects of estrogen on arterial functions, we examined endothelium-derived hyperpolarizing factor (EDHF)- and NO-mediated responses in isolated mesenteric arteries of female rats, 4 weeks after sham-operation (CON), ovariectomy (OVX) and OVX plus chronic estrogen treatment (OVX+E(2)). Tissue levels of connexins-40, 43 (major components of gap junction), inducible NOS (iNOS), endothelial NOS (eNOS) and eNOS regulator proteins such as calmodulin, heat shock protein 90 (hsp90) and caveolin-1 were also examined using Western blot. 2. In OVX, acetylcholine (ACh)-induced EDHF-mediated relaxation and membrane hyperpolarization of arterial smooth muscles were reduced, whereas ACh-induced NO-mediated relaxation was enhanced, leading to no change in ACh-induced relaxation. 3. In OVX, connexin-40 and 43 were decreased. Tissue levels of eNOS and its positive regulators (calmodulin and hsp90) were unchanged, but that of its negative regulator, caveolin-1, was decreased. The levels of iNOS in mesenteric artery and aorta and plasma levels of NO metabolites and cholesterol were elevated. 4. In OVX, contraction of the artery by phenylephrine was reduced, but augmented by nonspecific inhibitor of NOS to the comparable level as that in CON group. The contraction in OVX group unlike that in CON group was augmented by specific iNOS inhibitor, and the difference between contractions in the presence of nonspecific and specific inhibitor as an index of eNOS activity was increased. 5. In OVX+E(2), all these changes were recovered. 6. In all groups, EDHF-mediated relaxation was suppressed by 18beta-glycyrrhetinic acid, an inhibitor of gap junction. 7. These results indicate that estrogen deficiency does not change the diameter of mesenteric artery: it reduces EDHF-mediated relaxation by decreasing gap junction, whereas it augments NO-mediated relaxation via an increase in NO release. Increased NO result from increased activity of eNOS subsequent to a decrease in caveolin-1 and from induction of iNOS. However, excessive NO generation with elevated plasma cholesterol would raise a risk for atherosclerosis.

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.

Effect of Potassium Channel and Cytochrome P450 Inhibition on Transient Hypotension and Survival during Lipopolysaccharide-Induced Endotoxic Shock in the Rat

The purpose of this study was to determine whether inhibition of potassium channels or cytochrome P450 attenuates the transient phase of hypotension during endotoxic shock in vivo, and to determine whether these interventions improve the rate of survival. Male Sprague-Dawley rats were pretreated with saline (0.2 ml, i.v.), tetraethylammonium chloride (TEA 30 mg/kg; 0.2 ml, i.v.), proadifen (SKF-525 A; 50 mg/kg, i.p.) or ketoconazole (50 mg/kg, i.p.) and challenged with lipopolysaccharide (LPS; 20 mg/kg, i.p.). Changes in heart rate, mean (MAP), systolic (SP) and diastolic (DP) arterial pressures as well as survival rate were then monitored for 45 min. Potassium channel inhibition with TEA had no effect on LPS-induced hypotension at any time point compared with saline (maximal fall in MAP of 79 +/- 18 and 80 +/- 13 mm Hg, respectively). Pretreatment with proadifen or ketoconazole, inhibitors of cytochrome P450, significantly attenuated LPS-induced hypotension compared with saline (maximal fall in MAP of 34, 26 and 63% below baseline, respectively). This effect was evident in all arterial pressures measured, MAP, SP and DP. At 45 min, the survival rate in the saline group was 66%. Pretreatment with TEA significantly reduced survival rate to 50% and pretreatment with proadifen or ketoconazole improved survival to 100% (p < 0.05). These results suggest that an arachidonic acid metabolite produced by a cytochrome P450-catalyzed reaction may contribute to the transient phase of LPS-induced hypotension. However, these effects do not appear to be mediated through potassium channel activation.

Cytochrome P450-2C11 mRNA is not expressed in endothelial cells dissected from rat renal arterioles

BACKGROUND

Cytochrome P450 (CYP) isoenzymes (CYP2C and CYP2J) are involved in the production of epoxyeicosatrienoic acids, which are postulated as endothelium-derived hyperpolarizing factors (EDHFs). We hypothesized that if CYP2C11 is involved in the EDHF-mediated responses, its mRNA should be expressed in endothelial cells. We, therefore, examined the mRNA expression of CYP2C11 in endothelial cells of renal arterioles.

METHODS

Laser microdissection was applied to isolate endothelial cells from the renal arterioles of 4 male and 4 female Wistar rats. As a positive control of CYP2C11 expression, hepatocytes were also dissected from these rats. RNA was isolated and real-time quantitative polymerase chain reaction (Q-PCR) analysis was applied.

RESULTS

Q-PCR analysis showed that CYP2C11 mRNA was not expressed in laser microdissected endothelial cells of renal arterioles of male and female rats. CYP2C11 mRNA expression was highly abundant in hepatocytes dissected from male livers, but in female livers hardly any CYP2C11 mRNA was detected.

CONCLUSION

We have shown that endothelial cells can be dissected from small renal arterioles by laser microdissection to study the mRNA expression of specific genes by Q-PCR. Using this novel tool, we demonstrated that the CYP2C11 mRNA was not expressed in the endothelial cells of renal arterioles. Therefore, we speculate that CYP2C11 does not contribute to the EDHF-mediated responses in renal arterioles.

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.

Activation of rat mesenteric arterial KATP channels by 11,12-epoxyeicosatrienoic acid

Epoxyeicosatrienoic acids (EETs), the cytochrome P-450 epoxygenase metabolites of arachidonic acid, are candidates of endothelium-derived hyperpolarizing factors. We have previously reported that EETs are potent activators of cardiac ATP-sensitive K(+) (K(ATP)) channels, but their effects on the vascular K(ATP) channels are unknown. With the use of whole cell patch-clamp techniques with 0.1 mM ATP in the pipette and holding at -60 mV, freshly isolated smooth muscle cells from rat mesenteric arteries had small glibenclamide-sensitive currents at baseline (13.1 +/- 3.9 pA, n = 5) that showed a 7.2-fold activation by 10 microM pinacidil (94.1 +/- 21.9 pA, n = 7, P < 0.05). 11,12-EET dose dependently activated the K(ATP) current with an apparent EC(50) of 87 nM. Activation of the K(ATP) channels by 500 nM 11,12-EET was inhibited by inclusion of the PKA inhibitor peptide (5 microM) but not by the inclusion of the PKC inhibitor peptide (100 microM) in the pipette solution. These results were corroborated by vasoreactivity studies. 11,12-EET produced dose-dependent vasorelaxation in isolated small mesenteric arteries, and this effect was reduced by 50% with glibenclamide (1 microM) preincubation. The 11,12-EET effects on vasorelaxation were also significantly attenuated by preincubation with cell-permeant PKA inhibitor myristoylated PKI(14-22), and, in the presence of PKA inhibitor, glibenclamide had no additional effects. These results suggest that 11,12-EET is a potent activator of the vascular K(ATP) channels, and its effects are dependent on PKA activities.

NOS II Inhibition Restores Attenuation of Endothelium-Dependent Hyperpolarization in Rat Mesenteric Artery Exposed to Lipopolysaccharide

The aim of this study was to assess the effects of lipopolysaccharide (LPS) exposure on the endothelium-dependent hyperpolarization in the rat mesenteric artery using isometric tension recordings and electrophysiological studies. Mesenteric arterial rings of male Sprague-Dawley rats were incubated with LPS for 6 hours. All experiments were performed in the presence of indomethacin to inhibit the formation of vasoactive prostanoids. Contraction to phenylephrine was significantly reduced in rings incubated with LPS, which was restored in the presence of N(omega)-nitro-L-arginine methyl ester (L-NAME). L-NAME resistant relaxation to acetylcholine was attenuated in LPS-treated rings. LPS exposure hyperpolarized resting membrane potentials of arterial smooth muscle cells, which was repolarized by incubation with either L-NAME or 1400W, a selective inhibitor of nitric oxide synthase II (NOS II). Endothelium-dependent hyperpolarization to acetylcholine was attenuated in arteries incubated with LPS, while incubation with LPS and 1400W restored EDHF-mediated hyperpolarization. LPS-induced membrane potential change was mimicked by incubation with either SIN-1 or diethylamine NONOate, a donor of nitric oxide. These data suggest that LPS exposure attenuates EDHF-mediated both relaxation and hyperpolarization in the rat mesenteric artery. The possible mechanisms underlying decreased EDHF-mediated responses might be due to, at least in some part, massive nitric oxide induced by NOS II.

EDHF-mediated vasodilation involves different mechanisms in normotensive and hypertensive rat lungs

The role of endothelium-derived hyperpolarizing factor (EDHF) in regulating the pulmonary circulation and the participation of cytochrome P-450 (CYP450) activity and gap junction intercellular communication in EDHF-mediated pulmonary vasodilation are unclear. We tested whether tonic EDHF activity regulated pulmonary vascular tone and examined the mechanism of EDHF-mediated pulmonary vasodilation induced by thapsigargin in salt solution-perfused normotensive and hypoxia-induced hypertensive rat lungs. After blockade of both cyclooxygenase and nitric oxide synthase, inhibition of EDHF with charybdotoxin plus apamin did not affect either normotensive or hypertensive vascular tone or acute hypoxic vasoconstriction but abolished thapsigargin vasodilation in both groups of lungs. The CYP450 inhibitors 7-ethoxyresorufin and sulfaphenazole and the gap junction inhibitor palmitoleic acid, but not 18alpha-glycyrrhetinic acid, inhibited thapsigargin vasodilation in normotensive lungs. None of these agents inhibited the vasodilation in hypertensive lungs. Thus tonic EDHF activity does not regulate either normotensive or hypertensive pulmonary vascular tone or acute hypoxic vasoconstriction. Whereas thapsigargin-induced EDHF-mediated vasodilation in normotensive rat lungs involves CYP450 activity and might act through gap junctions, the mechanism of vasodilation is apparently different in hypertensive lungs.

[Recent advances in the study of endothelium-dependent hyperpolarizing factor (EDHF)]

Recent advances in the properties and physiological functions of endothelium-dependent hyperpolarizing factor (EDHF) in vascular tissues were reviewed briefly. The EDHF-induced hyperpolarization is inhibited by charybdotoxin, indicating that the potential is produced mainly by activation of intermediate conductance Ca-sensitive K-channels. During generation of EDHF responses, endothelial Ca2+ concentration was elevated, suggesting that the activated K-channels were distributed on the endothelial membrane. This was confirmed by direct recording of membrane potentials from endothelial and smooth muscle cells using double patch electrodes. Measurement of the propagation of potentials applied to endothelial or smooth muscle cells to surrounding cells revealed that there were tight electrical connections between endothelial cells much more than between endothelial and smooth muscle cells or between smooth muscle cells, and these observations yielded a possible spread of electrical signal along the endothelial layer first, and then the signals would be conducted to smooth muscle cell layers. These properties of vascular tissues allow speculating that EDHF is an electrical signal propagated from endothelial cells electrotonically through myoendothelial gap junctions. Several candidates have been proposed as EDHF, and possibilities of individual substances for EDHF were discussed. The cellular mechanism of the hyperpolarization-induced vasodilatation remains unclear, and this should be clarified in the future for further understanding of the EDHF-induced vasodilatation.

Abnormalities in intracellular Ca2+ regulation in fetal vascular smooth muscle in pre-eclampsia: enhanced sensitivity to arachidonic acid

Pre-eclampsia (PE) is a leading cause of maternal and fetal mortality and morbidity. As free fatty acid metabolism is abnormally regulated in PE, we investigated the intracellular Ca2+([Ca2+]i) response to arachidonic acid (AA) in primary cultures of human umbilical artery smooth muscle cells (HUASMC). AA (50 microM) caused a significantly greater [Ca2+]i elevation in PE than in normal HUASMC, with many cells displaying a delayed secondary increase. The nonmetabolizable AA analog ETYA did not induce a response, suggesting that the augmented PE response depends on an AA metabolite. Inhibition of the AA metabolizing cyclooxygenase or lipoxygenase pathways did not affect the AA response of PE HUASMC but induced in normal cells the secondary rise of [Ca2+]i observed in PE cells. This potentiated response and the response in PE cells were blocked by inhibitors of the monooxygenase pathway, a third AA metabolizing pathway. We conclude that the [Ca2+]i response of HUASMC is elevated in PE because of an increased level of a monooxygenase metabolite that stimulates Ca2+ influx and that this can be mimicked in normal cells by blocking cyclooxygenase or lipoxygenase to divert AA to the monooxygenase. This and our work with fetal endothelial cells (FASEB J. 10.1096/fj.01-0916fje) demonstrate phenotypic changes in the fetal vasculature in PE.

20-Hydroxyeicosatetraenoic acid potentiates stretch-induced contraction of canine basilar artery via PKC alpha-mediated inhibition of KCa channel

1. The present study was undertaken to elucidate whether PKCalpha plays a role in the mechanism of the stretch-induced contraction potentiated by 20-hydroxyeicosatetraenoic acid (20-HETE). The effects of 20-HETE on the canine basilar artery were compared with those of iberiotoxin, a blocker of large conductance Ca(2+)-activated K(+) channels (K(Ca) channels), as this blocker was shown earlier to sensitize these arteries to mechanical stretch. 2. Slow stretch at rates of 0.1 to 3 mm s(-1) did not produce any contraction in normal physiological solution. 3. In the presence of 20-HETE, the slow stretch could produce contraction, which was inhibited by nicardipine, a 1,4-dihydropyridine Ca(2+) channel blocker, and gadolinium, a blocker of stretch-activated cation channels. 4. 20-HETE inhibited whole-cell K(+) current and depolarized the membrane by approximately 10 mV. These effects of 20-HETE were similar to those of iberiotoxin. 5. Calphostin C, an inhibitor of protein kinase C (PKC), inhibited the action of 20-HETE, but not that of iberiotoxin. 6. In response to 20-HETE PKCalpha isoform was translocated from the cytosol to the membrane fraction, which translocation was inhibited by calphostin C. 7. These results suggest that 20-HETE induced sensitization of the canine basilar artery to stretch was caused by PKCalpha-mediated inhibition of K(Ca) channel activity.

Role of cytoplasmic phospholipase A2 in endothelium-derived hyperpolarizing factor dilations of rat middle cerebral arteries

Very little is known regarding the mechanism of action for the endothelium-derived hyperpolarizing factor (EDHF) response in cerebral vessels. The authors tested two hypotheses: (1) activation of the cytoplasmic form of phospholipase A (cPLA ) is involved with EDHF-mediated dilations in rat middle cerebral arteries; and (2) activation of the cPLA involves an increase in endothelial Ca through activation of phospholipase C. Middle cerebral arteries were isolated from the rat, pressurized to 85 mm Hg, and luminally perfused. The EDHF response was elicited by luminal application of uridine triphosphate (UTP) after NO synthase and cyclooxygenase inhibition (10 mol/L -nitro-l-arginine methyl ester and 10 mol/L indomethacin, respectively). AACOCF and PACOCF, inhibitors of cPLA (Ca -sensitive) and Ca -insensitive PLA (iPLA ), dose dependently attenuated the EDHF response. A selective inhibitor for iPLA2, haloenol lactone suicide substrate, had no effect on the EDHF response. The EDHF response elicited by UTP was accompanied by an increase in endothelial Ca (144 to 468 nmol/L), and the EDHF dilation was attenuated with U73122, a phospholipase C inhibitor. The authors conclude that the EDHF response elicited by luminal UTP in rat middle cerebral arteries involved activation of phospholipase C, an increase in endothelial Ca, and activation of cPLA.

Myoendothelial electrical coupling in arteries and arterioles and its implications for endothelium-derived hyperpolarizing factor

1. Considerable progress has been made over the past decade in evaluating the presence of electrical coupling between the endothelial and smooth muscle layers of blood vessels, prompted, in part, by ultrastructural evidence for the presence of myoendothelial junctions. 2. In a variety of vessels ranging in size from conduit arteries down to small arterioles, action potentials have been recorded from endothelial cells that were associated with constriction of the vessels and/or occurred in synchrony with and were indistinguishable from action potentials recorded from the smooth muscle. From these results, it is now firmly established that myoendothelial electrical coupling occurs in at least some blood vessels. 3. Spread of hyperpolarizing current from the endothelium to the smooth muscle is the most likely explanation of the smooth muscle hyperpolarization attributed to endothelium-derived hyperpolarizing factor. Because this hyperpolarization can evoke considerable relaxation of the smooth muscle, myoendothelial electrical coupling has important implications for endothelial regulation of the contractile activity of blood vessels.

Critical role of gap junctions in endothelium-dependent hyperpolarization in rat mesenteric arteries

1. Acetylcholine (ACh) evokes endothelium-dependent hyperpolarization in arterial cells, presumably through endothelium-derived hyperpolarizing factor (EDHF). The identity of EDHF is still elusive; however, several recent studies suggest the possible involvement of myoendothelial gap junctions in the EDHF response. 2. To elucidate the role of gap junctions in endothelium-dependent hyperpolarization, we examined the effects of the gap junction inhibitors 18 alpha-glycyrrhetinic acid (18 alpha-GA; 10(-4) mol/L) and carbenoxolone (3 x 10(-4) mol/L), a water-soluble form of 18 beta-GA, on hyperpolarization and relaxation to ACh in rat proximal and distal mesenteric arteries. Experiments were performed in the presence of indomethacin (10(-5) mol/L) and N(G)-nitro-L-arginine (10(-4) mol/L). 3. In both proximal and distal mesenteric arteries, ACh-induced hyperpolarization and relaxation were partially inhibited by 18 alpha-GA and abolished by carbenoxolone. 4. Endothelium-independent hyperpolarization to levcromakalim, an ATP-sensitive K+ channel opener, were unaffected by 18 alpha-GA or carbenoxolone in both arteries. 5. Relaxations to levcromakalim were unaffected by 18 alpha-GA, but were inhibited somewhat by carbenoxolone in proximal mesenteric arteries. 6. These findings suggest that myoendothelial gap junctions play a critical role in EDHF-mediated responses in both proximal and distal mesenteric arteries of the rat.

Involvement of nitric oxide and potassium channels in the bradykinin-induced vasodilatation in the rat kidney perfused ex situ

The role of nitric oxide (NO), K(+) channels, and arachidonic acid metabolism, via cytochrome P450 and cyclooxygenase pathways, in the renal vasodilatory effect of bradykinin was examined in the isolated rat kidney perfused ex situ with a blood-free solution. Bradykinin (BK, 0.25-1.0 microM) induced a dose-dependent reduction of 10-35% in the relative renal vascular resistance (rRVR) of isolated kidneys preconstricted with phenylephrine (PHE, 0.17-0.35 microM). The vasodilating effect of 0.5 microM bradykinin was significantly inhibited by the nitric oxide synthase inhibitors, N(G)-nitro-L-arginine (95% inhibition) and N(G)-nitro-L-arginine methyl ester (45-75% inhibition). Clotrimazole, an inhibitor of cytochrome P450 pathway but not indomethacin, a cyclooxygenase inhibitor, reduced the renal vasodilator response to bradykinin by 84%. The nonspecific K(+) channel inhibitor, tetraethylammonium ion (TEA) and the selective inhibitor of Ca(2+)-activated K(+) channels, charybdotoxin (ChTX) greatly attenuated the vasodilator response to bradykinin by approximately 84% and 79%, respectively. These two K(+) channel inhibitors showed similar effects on vasodilatation induced by S-nitroso-acetyl-D,L-penicillamine (1 microM), a nitric oxide donor. The results suggest that bradykinin releases nitric oxide which, by opening potassium channels specifically the Ca(+)-dependent type, mediates the renal vasodilator response to bradykinin in the isolated kidney perfused ex situ.

Effects of different tetra-n-alkylammonium ions on acetylcholine-induced endothelium-dependent hyperpolarization in rat mesenteric artery

The effects of a series of symmetric tetra-n-alkylammonium (TAA) compounds with alkyl side chains of one to six carbons in length on acetylcholine-induced endothelium-dependent hyperpolarization were examined in rat mesenteric artery. All TAA compounds caused a concentration-dependent inhibition of the hyperpolarizing response to acetylcholine. The potency of TAAs showed a general trend to increase with the lengths of the alkyl side chains. The inhibitory effects of TAAs, excepting the smallest compound, on the acetylcholine response were reversible. However, TAAs with long alkyl side chains may act as antagonists at muscarinic receptors, because the suppressive effect on A23187-induced endothelium-dependent hyperpolarization was more marked with TAAs having smaller alkyl side chains. Conversely, the hyperpolarizing response to pinacidil, an ATP-sensitive K+ channel opener, was significantly prevented only by TAA compounds with long alkyl side chains. TAA compounds with one-to three-carbon alkyl side chains caused a modest and reversible depolarization of the membrane, whereas the depolarizing effects of the compounds with four-to six-carbon alkyl side chains were marked and irreversible. These results suggest that TAAs could gain access to the target K+ channels for endothelium-derived hyperpolarizing factor, the ATP-sensitive K+ channels, and the K+ channels responsible for the regulation of the resting membrane potential in different ways.

P-450 metabolites of arachidonic acid in the control of cardiovascular function

Recent studies have indicated that arachidonic acid is primarily metabolized by cytochrome P-450 (CYP) enzymes in the brain, lung, kidney, and peripheral vasculature to 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) and that these compounds play critical roles in the regulation of renal, pulmonary, and cardiac function and vascular tone. EETs are endothelium-derived vasodilators that hyperpolarize vascular smooth muscle (VSM) cells by activating K(+) channels. 20-HETE is a vasoconstrictor produced in VSM cells that reduces the open-state probability of Ca(2+)-activated K(+) channels. Inhibitors of the formation of 20-HETE block the myogenic response of renal, cerebral, and skeletal muscle arterioles in vitro and autoregulation of renal and cerebral blood flow in vivo. They also block tubuloglomerular feedback responses in vivo and the vasoconstrictor response to elevations in tissue PO(2) both in vivo and in vitro. The formation of 20-HETE in VSM is stimulated by angiotensin II and endothelin and is inhibited by nitric oxide (NO) and carbon monoxide (CO). Blockade of the formation of 20-HETE attenuates the vascular responses to angiotensin II, endothelin, norepinephrine, NO, and CO. In the kidney, EETs and 20-HETE are produced in the proximal tubule and the thick ascending loop of Henle. They regulate Na(+) transport in these nephron segments. 20-HETE also contributes to the mitogenic effects of a variety of growth factors in VSM, renal epithelial, and mesangial cells. The production of EETs and 20-HETE is altered in experimental and genetic models of hypertension, diabetes, uremia, toxemia of pregnancy, and hepatorenal syndrome. Given the importance of this pathway in the control of cardiovascular function, it is likely that CYP metabolites of arachidonic acid contribute to the changes in renal function and vascular tone associated with some of these conditions and that drugs that modify the formation and/or actions of EETs and 20-HETE may have therapeutic benefits.

Factors involved in the time course of response to acetylcholine in mesenteric arteries from spontaneously hypertensive rats

The time course of the response to prolonged application of acetylcholine in mesenteric arteries from stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY) was compared. Only a relaxing response, which was blocked by N(omega)-nitro-L-arginine (L-NOARG), was observed after the prolonged application of a low concentration of acetylcholine (10(-8) M) in both preparations; the response was impaired in SHRSP preparations. Prolonged application of a high concentration of acetylcholine (10(-5) M) induced a second contractile response after a first relaxing response in SHRSP preparations under basal conditions and in WKY preparations in the presence of L-NOARG. This contractile response was attenuated by indomethacin. In the presence of a combination of apamin and charybdotoxin, the relaxing response to the high concentration of acetylcholine was reduced and a contractile response, which was abolished by indomethacin, appeared. In the presence of all of these blockers, a contractile response, which was blocked by cyclo(D-alpha-aspartyl-L-propyl-D-valyl-L-leucyl-D-tryptophyl) (BQ-123), was observed in preparations from WKY but not in preparations from SHRSP. Results indicate that prolonged application of acetylcholine in rat mesenteric arteries induces the release of endothelium-derived relaxing, contracting, hyperpolarizing factors and endothelin-1, and that the mode of action differs between preparations from WKY and SHRSP.

Cytochrome P-450 metabolite of arachidonic acid mediates bradykinin-induced negative inotropic effect

This study focused on the mechanisms of the negative inotropic response to bradykinin (BK) in isolated rat hearts perfused at constant flow. BK (100 nM) significantly reduced developed left ventricular pressure (LVP) and the maximal derivative of systolic LVP by 20-22%. The cytochrome P-450 (CYP) inhibitors 1-aminobenzotriazole (1 mM and 100 microM) or proadifen (5 microM) abolished the cardiodepression by BK, which was not affected by nitric oxide and cyclooxygenase inhibitors (35 microM NG-nitro-L-arginine methyl ester and 10 microM indomethacin, respectively). The CYP metabolite 14,15-epoxyeicosatrienoic acid (14,15-EET; 50 ng/ml) produced effects similar to those of BK in terms of the reduction in contractility. After the coronary endothelium was made dysfunctional by Triton X-100 (0.5 microl), the BK-induced negative inotropic effect was completely abolished, whereas the 14,15-EET-induced cardiodepression was not affected. In hearts with normal endothelium, after recovery from 14,15-EET effects, BK reduced developed LVP to a 35% greater extent than BK in the control. In conclusion, CYP inhibition or endothelial dysfunction prevents BK from causing cardiodepression, suggesting that, in the rat heart, endothelial CYP products mediate the negative inotropic effect of BK. One of these mediators appears to be 14,15-EET.

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.

Endothelium-independent, ouabain-sensitive relaxation of bovine coronary arteries by EETs

Endothelium-derived hyperpolarizing factor (EDHF) is released in response to agonists such as ACh and bradykinin and regulates vascular smooth muscle tone. Several studies have indicated that ouabain blocks agonist-induced, endothelium-dependent hyperpolarization of smooth muscle. We have demonstrated that epoxyeicosatrienoic acids (EETs), cytochrome P-450 metabolites of arachidonic acid, function as EDHFs. To further test the hypothesis that EETs represent EDHFs, we have examined the effects of ouabain on the electrical and mechanical effects of 14,15- and 11,12-EET in bovine coronary arteries. These arteries are relaxed in a concentration-dependent manner to 14,15- and 11,12-EET (EC(50) = 6 x 10(-7) M), bradykinin (EC(50) = 1 x 10(-9) M), sodium nitroprusside (SNP; EC(50) = 2 x 10(-7) M), and bimakalim (BMK; EC(50) = 1 x 10(-7) M). 11,12-EET-induced relaxations were identical in vessels with and without an endothelium. Potassium chloride (1-15 x 10(-3) M) inhibited [(3)H]ouabain binding to smooth muscle cells but failed to relax the arteries. Ouabain (10(-5) to 10(-4) M) increased basal tone and inhibited the relaxations to bradykinin, 11,12-EET, and 14,15-EET, but not to SNP or BMK. Barium (3 x 10(-5) M) did not alter EET-induced relaxations and ouabain plus barium was similar to ouabain alone. Resting membrane potential (E(m)) of isolated smooth muscle cells was -50.2 +/- 0.5 mV. Ouabain (3 x 10(-5) and 1 x 10(-4) M) decreased E(m) (-48.4 +/- 0.2 mV), whereas 11,12-EET (10(-7) M) increased E(m) (-59.2 +/- 2.2 mV). Ouabain inhibited the 11,12-EET-induced increase in E(m). In cell-attached patch clamp studies, 11,12-EET significantly increased the open-state probability (NP(o)) of a calcium-activated potassium channel compared with control cells (0.26 +/- 0.06 vs. 0.02 +/- 0.01). Ouabain did not change NP(o) but blocked the 14,15-EET-induced increase in NP(o). These results indicate that: 1) EETs relax coronary arteries in an endothelium-independent manner, 2) unlike EETs, potassium chloride does not relax the coronary artery, and 3) ouabain inhibits bradykinin- and EET-induced relaxations as has been reported for EDHF. These findings provide further evidence that EETs are EDHFs.

Role of endothelial Ni(2+)-sensitive Ca(2+) entry pathway in regulation of EDHF in porcine coronary artery

Elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in endothelial cells is proposed to be required for generation of vascular actions of endothelium-derived hyperpolarizing factor (EDHF). This study was designed to determine the endothelial Ca(2+) source that is important in development of EDHF-mediated vascular actions. In porcine coronary artery precontracted with U-46619, bradykinin (BK) and cyclopiazonic acid (CPA) caused endothelium-dependent relaxations in the presence of N(G)-nitro-L-arginine (L-NNA). The L-NNA-resistant relaxant responses were inhibited by high K(+), indicating an involvement of EDHF. In the presence of Ni(2+), which inhibits Ca(2+) influx through nonselective cation channels, the BK-induced EDHF relaxant response was greatly diminished and the CPA-induced response was abolished. BK and CPA elicited membrane hyperpolarization of smooth muscle cells of porcine coronary artery. Ni(2+) suppressed the hyperpolarizing responses in a manner analogous to removal of extracellular Ca(2+). EDHF-mediated relaxations and hyperpolarizations evoked by BK and CPA in porcine coronary artery showed a temporal correlation with the increases in [Ca(2+)](i) in porcine aortic endothelial cells. The extracellular Ca(2+)-dependent rises in [Ca(2+)](i) in endothelial cells stimulated with BK and CPA were completely blocked by Ni(2+). These results suggest that Ca(2+) influx into endothelial cells through nonselective cation channels plays a crucial role in the regulation of EDHF.

Bradykinin attenuates the [Ca(2+)](i) response to angiotensin II of renal juxtamedullary efferent arterioles via an EDHF

1. Bradykinin (BK) effect on the [Ca(2+)](i) response to 1 nM angiotensin II was examined in muscular juxtamedullary efferent arterioles (EA) of rat kidney. 2. BK (10 nM) applied during the angiotensin II-stimulated [Ca(2+)](i) increase, induced a [Ca(2+)](i) drop (73+/-2%). This drop was prevented by de-endothelialization and suppressed by HOE 140, a B2 receptor antagonist. It was neither affected by L-NAME or indomethacin, nor mimicked by sodium nitroprusside, 8-bromo-cyclic GMP or PGI(2). The BK effect did not occur when the [Ca(2+)](i) increase was caused by 100 mM KCl-induced membrane depolarization and was abolished by 0.1 microM charybdotoxin, a K(+) channel blocker. 3. Although proadifen prevented the BK-caused [Ca(2+)](i) fall, more selective cytochrome P450 inhibitors, 17-octadecynoic acid (50 microM) and 7-ethoxyresorufin (10 microM) were without effect. 4. Increasing extracellular potassium from 5 to 15 mM during angiotensin II stimulation caused a [Ca(2+)](i) decrease (26+/-4%) smaller than BK which was charybdotoxin-insensitive. Inhibition of inward rectifying K(+) channels by 30 microM BaCl(2) and/or of Na(+)/K(+) ATPase by 1 mM ouabain abolished the [Ca(2+)](i) decrease elicited by potassium but not by BK. 5. A voltage-operated calcium channel blocker, nifedipine (1 microM) did not prevent the BK effect but reduced the [Ca(2+)](i) drop. 6. These results indicate that the BK-induced [Ca(2+)](i) decrease in angiotensin II-stimulated muscular EA is mediated by an EDHF which activates charybdotoxin-sensitive K(+) channels. In these vessels, EDHF seems to be neither a cytochrome P450-derived arachidonic acid metabolite nor K(+) itself. The closure of voltage-operated calcium channels is not the only cellular mechanism involved in this EDHF-mediated [Ca(2+)](i) decrease.

Epoxygenase metabolites. Epithelial and vascular actions

Cytochrome P450 (CYP450) epoxygenase enzymes demonstrate organ and cell specific expression and each CYP450 enzyme isoform produces a distinct pattern of epoxyeicosatrienoic acids (EETs). Investigations are beginning to describe the regulation of specific tissue CYP450 epoxygenase isoforms that may be associated with alterations in organ function that occur during various physiological and pathophysiological states. The main biological actions of EETs are their ability to affect epithelial ion transport and vascular smooth muscle cell function. This chapter focuses on the organ localization and production of EETs and the action of EETs on epithelial and vascular smooth muscle cells.

Comparison of the pharmacological properties of EDHF-mediated vasorelaxation in guinea-pig cerebral and mesenteric resistance vessels

In the presence of L-NNA (100 microM), indomethacin (10 microM) and ODQ (10 microM), acetylcholine induced a concentration-dependent vasorelaxation of guinea-pig mesenteric and middle cerebral arteries precontracted with cirazoline or histamine, but not with high K(+), indicating the contribution of an endothelium-derived hyperpolarizing factor (EDHF). In cerebral arteries, charybdotoxin (ChTX; 0.1 microM) completely inhibited the indomethacin, L-NNA and ODQ-insensitive relaxation; iberiotoxin (IbTX, 0.1 microM), 4-aminopyridine (4-AP, 1 mM), or barium (30 microM) significantly reduced the response; in the mesenteric artery, ChTX and IbTX also reduced this relaxation. Glibenclamide (10 microM) had no affect in either the mesenteric or cerebral artery. Neither clotrimazole (1 microM) nor 7-ethoxyresorufin (3 microM) affected EDHF-mediated relaxation in the mesenteric artery, but abolished or attenuated EDHF-mediated relaxations in the cerebral artery. AM404 (30 microM), a selective anandamide transport inhibitor, did not affect the vasorelaxation response to acetylcholine in the cerebral artery, but in the mesenteric artery potentiated the vasorelaxation response to acetylcholine in an IbTX, and apamin-sensitive, but SR 141816A-insensitive manner. Ouabain (100 microM) almost abolished EDHF-mediated relaxation in the mesenteric artery, but enhanced the relaxation in the cerebral artery whereas the addition of K(+) (5 - 20 mM) to precontracted guinea-pig cerebral or mesenteric artery induced further vasoconstriction. These data suggest that in the guinea-pig mesenteric and cerebral arteries different EDHFs mediate acetylcholine-induced relaxation, however, EDHF is unlikely to be mediated by K(+).

Possible role of P-450-derived metabolites in endothelium-dependent relaxation of rat small mesenteric arteries

We reported previously that acetylcholine (ACh)-induced endothelium-dependent relaxation of rat mesenteric microvessels depended both on nitric oxide (NO) and on a charybdotoxin (CTX)-sensitive endothelium-derived hyperpolarizing vasodilator. Cytochrome P450 (CYP)-dependent arachidonic acid metabolites act in some systems as hyperpolarizing vasodilators. We sought to quantitate contributions of such metabolites to the CTX-sensitive component of ACh-induced vasodilation in isolated rat mesenteric resistance arteries. ACh relaxed these vessels nearly completely (93.3+/-1.2%, n = 71); cyclooxygenase inhibition with indomethacin did not diminish this response (94.3+/-11.4%, n = 9). NO synthase inhibition with Nitro-L-arginine (NNLA) reduced relaxation by 30% (n = 54, p<0.05). Pretreatment of vessels with CYP inhibitors, either clotrimazole (CTM) or 17-octadecynoic acid (17-ODYA), or with selective K+ channel inhibitors, either tetraethyammonium acetate (TEA) or CTX, each led to similar small reductions in maximal relaxation (17%, 22%, 16%, and 9% respectively, n = 3-6). Combined pretreatment with NNLA + either (CTM or 17-ODYA) or (TEA or CTX) each led to similar maximal relaxations (52.2+/-4.8%, 50.6+/-9.2, 37.6+/-8.6%, and 44.1+/-11.5%, respectively, n = 6-35; p<0.05 for NNLA+[CTM or TEA or CTX] vs NNLA alone). Combined pretreatment with NNLA+CTM+(CTX or TEA) led to similar maximal relaxations (43.0+/-7.3%, 43.7+/-15%, n = 6-11) that did not differ from values in vessels pretreated with either NNLA+CTM or NNLA+(CTX or TEA). We conclude that the ACh-induced vasodilation was insensitive to cyclooxygenase inhibition, partially sensitive to NO synthase inhibition, and that the K+ channel blockers TEA and CTX identified the same minor component of ACh relaxation as did the CYP inhibitor CTM.

Apamin-sensitive, non-nitric oxide (NO) endothelium-dependent relaxations to bradykinin in the bovine isolated coronary artery: no role for cytochrome P450 and K+

Since cytochrome P(450)-derived metabolites of arachidonic acid and K(+) have been implicated in endothelium-derived hyperpolarizing factor (EDHF)-dependent responses, the aim of this study was to determine whether such factors contribute to non-nitric oxide (NO), endothelium-dependent relaxation to bradykinin (BK) in bovine isolated coronary artery. In rings of artery contracted with U46619 and treated with indomethacin (3 microM) and N(G)-nitro-L-arginine (L-NOARG; 100 microM), relaxation to BK (0.01 nM-0.3 microM) was blocked by approximately 60% after inhibition of K(+) channels with either high extracellular K(+) (high [K(+)](o); 15 - 67 mM) or apamin (0.3 microM). Ouabain (1 microM), an inhibitor of Na(+)/K(+)-ATPase, decreased the sensitivity to BK without affecting the maximum response. In L-NOARG-treated rings, ouabain had no further effect on the relaxation to BK. An inhibitor of inward-rectifying K(+) channels, Ba(2+) (30 microM), had no effect on relaxations to BK in the absence or presence of either L-NOARG or ouabain. KCl (2.5 - 10 mM) elicited small relaxations ( approximately 20%) that were abolished by nifedipine (0.3 microM) and ouabain. Both the high [K(+)](o)/apamin-sensitive relaxation to BK, and the relaxation to the K(ATP) channel-opener, levcromakalim (0.6 microM), were unaffected by the cytochrome P(450) inhibitor, 7-ethoxyresorufin (10 microM), or by co-treatment with a phospholipase A(2) inhibitor, arachidonyl trifluoromethyl ketone (AACOCF(3); 3 microM) and a diacylglycerol (DAG)-lipase inhibitor, 1, 6-bis-(cyclohexyloximinocarbonylamino)-hexane (RHC 80267; 30 microM). The non-NO/high [K(+)](o)-insensitive, approximately 40% relaxation to BK was, however, abolished by these treatments. Therefore, neither cytochrome P(450)-derived metabolites of arachidonic acid nor K(+) appear to mediate the EDHF-like relaxation to BK (i.e the non-NO, high [K(+)](o)/apamin-sensitive component) in bovine coronary arteries. Cytochrome P(450)-derived metabolites may be released at higher BK concentrations to act in parallel with NO and the high [K(+)](o)/apamin-sensitive mechanism.

K(+) channel blockers and cytochrome P450 inhibitors on acetylcholine-induced, endothelium-dependent relaxation in rabbit mesenteric artery

Acetylcholine caused an endothelium-dependent relaxation in isolated rabbit mesenteric small artery in the presence of nitro L-arginine and indomethacin. The acetylcholine-induced relaxation was attenuated by high K(+) solution, suggesting that the response is mediated by a membrane potential-sensitive mechanism, presumably an endothelium-derived hyperpolarizing factor. The acetylcholine-induced relaxation was also inhibited with tetraethylammonium, 4-aminopyridine and charybdotoxin, but not with Ba(2+), apamin, iberiotoxin nor glibenclamide. The relaxation was abolished by a combination of apamin and charybdotoxin, but iberiotoxin could not replace charybdotoxin in this combination. The responses to charybdotoxin and 4-aminopyridine were synergistic but neither apamin nor iberiotoxin increased the effect of 4-aminopyridine. Clotrimazole and proadifen inhibited the acetylcholine-induced relaxation, but these drugs also inhibited the cromakalim-induced relaxation, while protoporphyrin IX inhibited the acetylcholine- but not cromakalim-induced relaxation. 17-Octadecynoic acid and 1-aminobenzotriazole did not affect the response to acetylcholine. Four regioisomers of epoxyeicosatrienoic acids did not relax endothelium-denuded artery. A gap junction inhibitor 18alpha-glycyrrhetinic acid attenuated the relaxation to acetylcholine. It is suggested that in rabbit mesenteric artery, the acetylcholine-induced, nitric oxide- and prostacyclin-independent relaxation is mainly mediated by 4-aminopyridine- and charybdotoxin-sensitive K(+) channels and that the relaxation is not mediated through cytochrome P450 enzyme metabolites. The contribution of heterocellular gap junctional communication to the relaxation is discussed.

Characterization of nitric oxide- and prostaglandin-independent relaxation in response to acetylcholine in rabbit renal artery

1. We investigated the characterization of acetylcholine (ACh)-induced NG-nitro-L-arginine methyl ester (L-NAME)- and indomethacin (IND)-resistant relaxations, which can be mediated by endothelium-derived hyperpolarizing factor (EDHF), in rabbit renal arterial rings. 2. The relaxations were inhibited by SKF 525A, a cytochrome P450 inhibitor, but were not affected by other inhibitors, namely clotrimazole, 17-octadecynoic acid and alpha-naphthoflavone. Furthermore, 11,12-epoxyeicosatrienoic acid, a cytochrome P450 metabolite, did not relax arterial rings. 3. Arterial relaxations were significantly attenuated by charybdotoxin and iberiotoxin, but not by apamin, all K+ channel blockers. 4. In a sandwich bioassay experiment, ACh-induced L-NAME- and IND-resistant relaxations were not transferred to the detector site. 5. Relaxations were also significantly attenuated by 1-heptanol and 18 alpha-glycyrrhetinic acid, gap junctional coupling inhibitors. 6. These results indicate that, in the rabbit renal artery, L-NAME- and IND-resistant relaxations are mediated by factors other than cytochrome P450-derived arachidonic acid metabolites, which may be able to diffuse into the lumen but be partly transferred via myoendothelial gap junctions to adjacent vascular smooth muscle cells and relax muscles by opening high-conductance Ca(2+)-activated K+ channels.

Miconazole represses CO(2)-induced pial arteriolar dilation only under selected circumstances

Previous experimental findings have led to the suggestion that guanosine 3',5'-cyclic monophosphate (cGMP) plays a permissive role in hypercapnic cerebral vasodilation. However, we recently reported that the technique used to reveal a permissive role for cGMP [cGMP repletion in the presence of nitric oxide synthase (NOS) inhibition] created a situation where CO(2) reactivity was normalized but where different mechanisms (i.e., K(+) channels) participated in the response. In the present study, we examined whether that nascent K(+)-channel dependence is related in any way to an increase in the influence of the miconazole-inhibitable cytochrome P-450 epoxygenase pathway. Using intravital microscopy and a closed cranial window system in adult rats, we measured pial arteriolar diameters during normo- and hypercapnia, first in the absence and then in the presence of a neuronal NOS (nNOS) inhibitor [7-nitroindazole (7-NI)]. This was followed by suffusion of a cGMP analog and then cGMP plus miconazole. Separate groups of rats were used to evaluate whether miconazole either alone or in the presence of 8-bromoguanosine 3', 5'-cyclic monophosphate (8-BrcGMP) or its vehicle (0.1% ethanol) had any effect on CO(2) reactivity and whether miconazole affected K(+)-channel opener-induced dilations. Hypercapnic (arterial PCO(2), congruent with65 mmHg) pial arteriolar dilations, as expected, were reduced by 70-80% with 7-NI and restored with cGMP repletion. CO(2) reactivity was again attenuated after miconazole introduction. Miconazole, with and without 8-BrcGMP, and its vehicle had no influence on pial arteriolar CO(2) reactivity in the absence of nNOS inhibition combined with cGMP repletion. Miconazole alone also did not affect vasodilatory responses to K(+)-channel openers. Thus present results suggest that the nascent K(+)-channel dependence of the hypercapnic response found in our earlier study may be related to increased epoxygenase activity. The specific reasons why the pial arteriolar CO(2) reactivity gains a K(+)-channel and epoxygenase dependence only under conditions of nNOS inhibition and cGMP restoration remain to be identified. These findings again call into question the interpretations applied to data collected in studies evaluating potential permissive actions of cGMP or NO.

Important role of endothelium-derived hyperpolarizing factor in shear stress--induced endothelium-dependent relaxations in the rat mesenteric artery

Shear stress is one of the most important stimulators for the release of endothelium-derived relaxing factors. Although shear stress-induced release of nitric oxide (NO) has been extensively investigated, it remains to be elucidated whether endothelium-derived hyperpolarizing factor (EDHF) contributes to the endothelium-dependent relaxations to shear stress. This study was designed to address this point in the isolated rat mesenteric artery. Large mesenteric arteries (400-500 microm) and resistance mesenteric arteries (150-250 microm) of the rat were precontracted with phenylephrine (at 80 mm Hg of perfusion pressure), and the changes in vessel diameter in response to variable flow (0-300 microl/min) were continuously examined. The relative contributions of vasodilator prostaglandins, NO, and EDHF were analyzed by the inhibitory effects of indomethacin (10(-5) M), N(G)-nitro-L-arginine (L-NNA, 10(-4) M), and KCl (40 mM), respectively. The shear stress-induced relaxations were totally endothelium dependent in both-sized blood vessels, and the contribution of NO was more prominent in large arteries than in resistance arteries, whereas that of EDHF was noted in both-sized blood vessels. Tetrabutylammonium (a nonselective inhibitor of K channels) almost abolished, whereas the combination of charybdotoxin (an inhibitor of both large- and intermediate-conductance Ca2+ -activated K channels) and apamin (an inhibitor of small-conductance Ca2+ -activated K channels) significantly inhibited the EDHF-mediated component of the shear stress-induced relaxations. These results indicate that EDHF plays an important role in shear stress-induced endothelium-dependent relaxations, where K channels, especially calcium-activated K channels, appear to be involved.