Endothelium-derived hyperpolarizing factor and endothelium-dependent relaxations

Biphasic relaxation caused by electrical field stimulation of the mesenteric arteries of rats

Electrical field stimulation (EFS) caused biphasic relaxation, first transient and then sustained, of rat mesenteric arteries precontracted by prostaglandin (PG) F2alpha. The transient relaxation was reduced about 45%, and the sustained relaxation was not observed after endothelium denudation of the arteries. N(omega)-Nitro-L-arginine (L-NOARG) inhibited the biphasic relaxation induced by EFS. At 1 - 100 microM, L-NOARG inhibited the transient relaxation more than the sustained relaxation. Methylene blue inhibited the biphasic relaxation and at 100 microM, L-NOARG abolished the transient relaxation. These results suggest that the transient relaxation mainly involves nitric oxide (NO), whereas the sustained relaxation involves both NO and some other factor(s).

Welsh onion (Allium fistulosum L.) extracts alter vascular responses in rat aortae

Welsh onion, a member of the genus Allium, has been consumed for prevention of cardiovascular disorders. However, its underlying mechanisms are still unclear. We investigated whether Welsh onion extracts (green or white portion, raw or boiled) can alter vascular responses in vitro in the thoracic aortae of Sprague-Dawley rats. The possible roles of endothelium-derived factors in the Welsh onion extract-induced vascular responses were examined by applying various inhibitors, such as Nomega-nitro-L-arginine (10(-4) M), tetraethylammonium (10(-3) M), and SQ29548 (10(-5) M). Our results showed that Welsh onion extracts caused vasodilation on precontracted vessel rings. These effects were most pronounced in vessel rings treated with raw green-leaf extract (RG). Low doses of RG induced vasorelaxation, which was mediated by endothelium-derived nitric oxide. High doses of RG induced endothelium-independent vasorelaxation. On the other hand, the boiled Welsh onion extract also stimulated the release of an endothelium-derived contracting factor, which might be thromboxane A2. We conclude that Welsh onion extract can modulate vascular tone in both endothelium-dependent and endothelium-independent manners.

Long-term treatment with eicosapentaenoic acid augments both nitric oxide-mediated and non-nitric oxide-mediated endothelium-dependent forearm vasodilatation in patients with coronary artery disease

Long-term treatment with eicosapentaenoic acid (EPA) is known to improve impaired endothelium-dependent relaxations of atherosclerotic blood vessels in animals and humans. However, it remains to be determined which mechanisms are involved in this beneficial effect of EPA. In this study, we investigated our hypothesis that EPA improves both nitric oxide (NO)-mediated and non-NO-mediated endothelium-dependent vasodilatation in patients with coronary artery disease. The study included eight patients with documented coronary artery disease. The forearm vascular responses to the endothelium-dependent vasodilator acetylcholine and substance P were examined before and after intraarterial infusion of NG-monomethyl-L-arginine (L-NMMA). Same measurements were repeated after the treatment with EPA (1,800 mg/day) for 6 weeks. The long-term treatment with EPA augmented forearm blood-flow response to both acetylcholine and substance P. Furthermore, acute administration of L-NMMA significantly inhibited the EPA-induced augmented response to acetylcholine but not that to substance P. The forearm vascular response to sodium nitroprusside was unchanged by the EPA treatment. These results indicate that long-term treatment with EPA augments both NO-dependent and non-NO-dependent endothelium-dependent forearm vasodilatation in patients with coronary artery disease. Thus the beneficial effects of EPA appear to extend to non-NO-dependent mechanism(s).

Pharmacologic study of muscarinic receptor subtypes and arteriolar dilations: a comparison of conducted and local responses

Arteriolar relaxation caused by the application of muscarinic agonists is mediated by multiple factors. One factor causes dilation only at the point of drug microapplication (local response), and a second factor causes responses remote (500 microm away) from the site of application (conducted response). This study was performed to determine if different muscarinic subtypes mediate the two responses. Arterioles of anesthetized hamster cheek pouch were studied with videomicroscopy. Muscarinic antagonists methscopolamine, scopolamine, pirenzepine, 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide), and AFDX-116 [(11-2[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5, 11-dihydro-6H-pyrido [2,3-b][1,4]benzodiazepin-6-one)] were cumulatively applied, and the K(B) for each antagonist was determined for the local and conducted responses caused by methacholine microapplication (10(-4) M, 5 s). The pK(B) (local, conducted) were not significantly different for the two responses when using scopolamine (10.5, 10.4). When the antagonist AFDX-116 (5.6, 6.3), selective for muscarinic receptor (m2) subtype was applied, the K(B) was greater for the conducted response. The pK(B) was greater, however, for the local response when the m1 subtype-selective pirenzepine (7.7, 6.9) or m3 subtype-selective 4-DAMP (10.1, 9.8) was applied. Thus the antagonist pK(B) ratio for on the local and conducted responses depends on the subtype selectivity of the antagonist. These data strongly suggest that different receptors are involved in the two responses.

Effects of antihypertensive therapy on factors mediating endothelium-dependent relaxation in rats treated chronically with L-NAME

OBJECTIVES

To evaluate the relative participation of endothelium-derived factors mediating relaxation in response to acetylcholine in isolated mesenteric vascular beds from rats treated chronically with N(G)-nitro-L-arginine methylester (L-NAME); and to compare the consequences of prolonged treatment with either an angiotensin converting enzyme inhibitor or a calcium channel blocker on the components of acetylcholine-induced relaxation in this vascular preparation.

MATERIALS AND METHODS

Male Sprague- Dawley rats were treated for 8 weeks with L-NAME (40 mg/kg per day), quinapril (10 mg/kg per day), diltiazem (100 mg/kg per day), L-NAME + quinapril and L-NAME + diltiazem. Systolic blood pressure was estimated by a tail-cuff plethysmograph. Relaxing responses to acetylcholine (10(-12) to 10(-8) mol) in mesenteric vascular beds precontracted with phenylephrine (10(-5) mol/l) were studied in the presence and absence of L-NAME (10(-5) mol/l), L-NAME + indomethacin (10(-5) mol/l) or L-NAME + indomethacin + potassium chloride (6 x 10(-5) mol/l). The area under the dose- response curve was used to calculate the approximate participation of nitric oxide, prostaglandins or endothelium-derived hyperpolarizing factor in the acetylcholine-induced relaxation.

RESULTS

Chronic administration of L-NAME increased blood pressure levels and vascular responsiveness to phenylephrine. Treatments with either quinapril or diltiazem reduced blood pressure levels and attenuated the increased response to phenylephrine. Relaxing responses to acetylcholine were similar in all groups, independently of the treatment received. The calculated participation of endothelium-derived hyperpolarizing factor in the acetylcholine-induced relaxation was higher than that of nitric oxide and prostaglandins in all groups, but was higher in L-NAME-treated than in untreated rats. In contrast, the participation of both nitric oxide and prostaglandins was higher in control than in L-NAME-treated rats. Quinapril increased the participation of prostaglandins in L-NAME-treated rats. Diltiazem increased the participation of nitric oxide in L-NAME-treated rats.

CONCLUSIONS

The administration of L-NAME in Sprague-Dawley rats increased the production of endothelium-derived hyperpolarizing factor as a compensatory mechanism to maintain acetylcholine-induced relaxation. Antihypertensive therapy with either quinapril or diltiazem produced a selective redistribution of the endothelial factors mediating acetylcholine-induced relaxation.

ATP-sensitive K+ channels in smooth muscle cells of guinea-pig mesenteric lymphatics: role in nitric oxide and beta-adrenoceptor agonist-induced hyperpolarizations

1. Intracellular microelectrode recordings were performed to investigate the membrane K+ conductances involved in smooth muscle hyperpolarization of lymphatic vessels in the guinea-pig mesentery. 2. Nitric oxide (NO), released either by the endothelium after acetylcholine (ACh; 10 microM) stimulation or by sodium nitroprusside (SNP; 50-100 microM), hyperpolarized lymphatic smooth muscle. These responses were inhibited with the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazole [4,3-a]quinoxalin-1-one (ODQ, 10 microM). 3. ACh and SNP-induced hyperpolarizations were inhibited (by about 90%) upon application of the ATP-sensitive K+(K(ATP)) channel blocker, glibenclamide (10 microM), or with 4-aminopyridine (2.5 mM), but were not affected by the Ca2+-activated K+ channels blocker, penitrem A (100 nM). 4. Hyperpolarization caused by the K+ channel opener, cromakalim (0.1-10 microM), isoprenaline (0.1 microM) or forskolin (0.5 microM) were all significantly blocked by glibenclamide. 5. Hyperpolarization evoked by ACh and SNP were inhibited with N-[2-(p-bromociannamylamino)-ethyl]-5-isoquinolinesulfonamide-dich loride (H89, 10 microM), suggesting the involvement of cyclic AMP dependent protein kinase (PKA). 6. These results suggest that K(ATP) channels play a central role in lymphatic smooth muscle hyperpolarization evoked by a NO-induced increase in cyclic GMP synthesis, as well as by beta-adrenoceptor-mediated production of cyclic AMP. Interestingly, both pathways lead to K(ATP) channels opening through the activation of PKA.

Role of cyclic guanosine monophosphate and K+ channels as mediators of the mesenteric vascular hyporesponsiveness in portal hypertensive rats

The mechanisms mediating the hyporesponsiveness to vasoconstrictors in portal hypertension are not completely established. In the present study, we evaluated the role of cyclic guanosine monophosphate (cGMP) and potassium channels as contributors to the pressor hyporesponsiveness to methoxamine (MTX) of the mesenteric vascular bed of portal vein-ligated (PVL) hypertensive rats. In basal conditions, and compared with sham-operated control rat (SHAM) vessels, PVL preparations showed a blunted pressor response (maximum: 39.3 +/- 6.1 vs. 94.5 +/- 8.9 mm Hg), which increased by pretreatment with methylene blue (MB), a guanylate cyclase inhibitor (118.7 +/- 8.9 vs. 152.0 +/- 10.0, respectively), and even more with the nitric oxide (NO) synthesis inhibitor N(omega)-nitro-L-arginine (NNA) (159.9 +/- 7.4 vs. 194.1 +/- 5.7, respectively), suggesting that NO acts through cGMP-dependent and independent mechanisms. In all cases, however, the pressor responses of PVL vessels were lower than those of SHAM. Pretreatment of the vessels with the potassium channel inhibitors, tetraethylammonium (TEA), glibenclamide (GLB), or charybdotoxin (CHX), did not improve the reduced pressor responses of the PVL rats. However, when the preparations were simultaneously pretreated with MB and TEA or with NNA and TEA, the pressor responses were potentiated with respect to groups treated with MB or NNA alone, and the differences between PVL and SHAM vessels were completely corrected. These data suggest that both NO and potassium channels mediate the vascular hyporesponsiveness to methoxamine of the PVL mesenteric vasculature. Our results also disclose that NO blunts the pressor response of the PVL vessels by a dual mechanism of action, through activation of potassium channels and through the formation of cGMP. Finally, the NO-independent component mediated by potassium channels can be only seen when the main cGMP-NO component is inactivated. In conclusion, both cGMP and potassium channels mediate the vascular hyporesponsiveness to MTX of the mesenteric bed of portal hypertensive rats.

Roles of calcium-activated and voltage-gated delayed rectifier potassium channels in endothelium-dependent vasorelaxation of the rabbit middle cerebral artery

1. The cellular mechanism(s) of action of endothelium-derived vasodilator substances in the rabbit middle cerebral artery (RMCA) were investigated. Specifically, the subtypes of potassium channels involved in the effects of endothelium-derived relaxing factors (EDRFs) in acetylcholine (ACh)-induced endothelium-dependent vasorelaxation in this vessel were systematically compared. 2. In the endothelium-intact RMCA precontracted with histamine (3 microM), ACh induced a concentration-dependent vasorelaxation, which was sensitive to indomethacin (10 microM) or N(G)-nitro-L-arginine (L-NOARG; 100 microM); pD2 values 8.36 vs 7.40 and 6.38, P < 0.01 for both, n = 6 and abolished by a combination of both agents. ACh caused relaxation in the presence of high K+ PSS (40 mM KCl), which was not affected by indomethacin, but abolished by L-NOARG and a combination of indomethacin and L-NOARG. 3. In the presence of indomethacin, relaxation to ACh in the endothelium-intact RMCA precontracted with histamine was unaffected by either glibenclamide (10 microM), an ATP-sensitive K+ channel (K[ATP]) blocker, 4-aminopyridine (4-AP, 1 mM) or dendrotoxin (DTX, 0.1 microM), delayed rectifier K channel (Kv) blockers. However, relaxation responses to ACh were significantly inhibited by either LY83583 (10 microM) and 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 10 microM), guanylyl cyclase inhibitors, or charybdotoxin (CTX; 0.1 microM), iberiotoxin (ITX, 0.1 microM) and apamin (APA, 0.1 microM), large conductance Ca2+-activated K+ channels (BK[Ca]) blocker and small conductance Ca2+-activated K+ channel (SK[Ca]) blocker, respectively. 4. In the presence of L-NOARG, relaxation to ACh was unaffected by glibenclamide or the cytochrome P450 mono-oxygenase inhibitor, clotrimazole (1 microM), but was significantly inhibited by either 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22,536, 10 microM) and 2',3'-dideoxyadenosine (2',3'-DDA, 30 microM), adenylyl cyclase inhibitors, or 4-AP, DTX, CTX, ITX and APA. 5. In the endothelium-denuded RMCA precontracted with histamine, authentic NO-induced relaxation was unaffected by glibenclamide, 4-AP and DTX, but significantly reduced by ODQ, ITX and APA. Authentic prostaglandin I2 (PGI2)-induced relaxation was unaffected by glibenclamide, but significantly reduced by 2',3'-DDA, 4-AP, DTX, ITX and APA. Forskolin-induced relaxation was significantly inhibited by high K+, CTX and 4-AP. 6. These results indicate that: (1) in the RMCA the EDRFs released by ACh are NO and a prostanoid (presumably PGI2), and there is no evidence for the release of a non-NO/PGI2 endothelium-derived hyperpolarizing factor (EDHF), (2) K(Ca) channels are involved in NO-mediated relaxation of the RMCA but both K(Ca) and Kv channels are involved in PGI2-mediated relaxation.

Microsphere-induced bronchial artery vasodilation: role of adenosine, prostacyclin, and nitric oxide

We previously found that injection of 15-micron microspheres into the bronchial artery of sheep decreased bronchial artery resistance. This effect was inhibited partially by indomethacin or 8-phenyltheophylline, suggesting that microspheres caused release of a dilating prostaglandin and adenosine. To identify the prostaglandin and confirm adenosine release, we perfused the bronchial artery in anesthetized sheep. In 12 sheep, bronchial artery blood samples were obtained before and after the infusion of 1 x 10(6) microspheres or microsphere diluent into the bronchial artery. Microspheres, but not diluent, decreased bronchial artery resistance by 40% and increased bronchial artery plasma 6-ketoprostaglandin F1 alpha (194.7 +/- 45.0 to 496.5 +/- 101.3 pg/ml), the stable metabolite of prostacyclin, and prostaglandin (PG) F2 alpha (28.1 +/- 4.4 to 46.2 +/- 9.7 pg/ml). There were no changes in PGD2, PGE2, thromboxane B2, adenosine, inosine, or hypoxanthine. Pretreatment with dipyridamole, an adenosine uptake inhibitor, did not affect bronchial artery nucleoside concentrations (n = 7). Microsphere-induced vasodilation was not enhanced by dipyridamole (n = 9) and was not inhibited by either the adenosine receptor antagonist xanthine amine congener (n = 4) or the nitric oxide (NO) synthase inhibitor NG-monomethyl-L-arginine (n = 8). These results do not support a role for either adenosine or NO and suggest that microspheres caused bronchial artery vasodilation through release of prostacylin and an unidentified vasodilator.

Hyperpolarizing factors

There is now overwhelming evidence for factors, other than nitric oxide (NO), that mediate endothelium-dependent vasodilation by hyperpolarizing the underlying smooth muscle via activation of Ca2+-activated K+ channels. Although the identity of endothelium-derived hyperpolarizing factor (EDHF) remains to be established, cytochrome P450 (CYP)-dependent metabolites of arachidonic acid (AA), namely, the epoxides, fulfill several of the criteria required for consideration as putative mediators of endothelium-dependent hyperpolarization. They are produced by the endothelium, released in response to vasoactive hormones, and elicit vasorelaxation via stimulation of Ca2+-activated K+ channels. Our studies in the rat indicate that, of the epoxides, 5,6-epoxyeicosatrienoic acid (5,6-EET) is the most likely mediator of NO-independent, but CYP-dependent coronary vasodilation in response to bradykinin. Studies in the rat kidney, however, support the existence of additional EDHFs as acetylcholine also exhibits NO-independent vasodilation that is unaffected by CYP inhibitors in concentrations that attenuate responses to bradykinin. In some blood vessels, NO may tonically suppress the expression of CYP-dependent EDHF. In the event of impaired NO synthesis, therefore, a CYP-dependent vasodilator mechanism may serve as a backup to a primary NO-dependent mechanism, although they may act in concert. In other vessels, particularly microvessels, an EDHF may constitute the major vasodilator mechanism for hormones and other physiological stimuli. EDHFs appear to be important regulators of vascular tone; alterations in this system can be demonstrated in hypertension and diabetes, conditions associated with altered endothelium-dependent vasodilator responsiveness.

Autacoids mediate coronary vasoconstriction induced by nitric oxide synthesis inhibition

Inhibition of nitric oxide (NO) synthesis results in coronary vasoconstriction. Using a Langendorff rat heart preparation, we tested the hypothesis that this vasoconstriction is caused by the unopposed effect of the autacoids prostaglandin H2 (PGH2) or thromboxane A2 (TxA2) or both through a mechanism that involves oxygen free radicals. The vasoconstriction induced by NO synthesis inhibition was studied with two different NO synthase inhibitors, N(omega)-nitro-L-arginine methyl ester (L-NAME) and N(omega)-monomethyl-L-arginine (L-NMMA). We found that the decrease in coronary flow (CF) induced by L-NAME (from 19.3 +/- 0.9 to 13.2 +/- 0.9 ml/min; p < 0.001) and L-NMMA (from 20.1 +/- 0.4 to 15.0 +/- 0.3 ml/min; p < 0.001) was completely blocked by the cyclooxygenase inhibitor indomethacin. A different cyclooxygenase inhibitor (ibuprofen), a PGH2/TxA2-receptor antagonist (SQ29548), and a TxA2 synthase inhibitor (CGS 13080) also completely abolished the vasoconstrictor effect of L-NAME, suggesting that this vasoconstriction is mediated by TxA2. Two different scavengers of superoxide radical anions (O2-), the enzyme superoxide dismutase (SOD) and a cell-permeable SOD mimic, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol), also blocked the vasoconstriction induced by NO synthesis inhibition. In contrast, catalase, which inactivates hydrogen peroxide (H2O2), failed to do so, indicating that O2- is needed for the vasoconstrictor effect of L-NAME, whereas H2O2 is not. To determine whether O2- acts on the conversion of PGH2 to TxA2 or at the receptor or postreceptor level, we studied whether the vasoconstriction induced by exogenous PGH2 or the TxA2 receptor agonist U46619 is blocked by scavengers of O2-. CF decreased by 50% with PGH2 (from 21 +/- 2.1 to 10.6 +/- 5.8 ml/min; p < 0.01), and this decrease was abolished by SOD and Tempol but not catalase. However, SOD had no effect on the vasoconstriction induced by U46619, which decreased CF by 45% (from 17.3 +/- 2.5 to 9.5 +/- 1.8 ml/min; p < 0.01). In addition, PGH2 increased the release of TxB2 (the stable metabolite of TxA2) in the coronary effluent (from 5.1 +/- 1.2 to 136.1 +/- 11.8 pg/ml/min). The release of TxB2 was significantly lower in hearts treated with SOD (76.8 +/- 14.2 pg/ml/min) and CGS (65.7 +/- 13.9 pg/ml/min). We conclude that the coronary vasoconstriction induced by inhibition of NO synthesis is the result of the unopposed effect of the autacoid TxA2 through activation of its receptor, and that O2- is necessary for conversion of PGH2 to TxA2.

Short-term estrogen augments both nitric oxide-mediated and non-nitric oxide-mediated endothelium-dependent forearm vasodilation in postmenopausal women

Estrogen is known to improve in the short term the impaired endothelium-dependent vasodilating responses in postmenopausal women, which may account in part for the beneficial cardiovascular effects of the female hormone. Endothelium-dependent vasodilation is achieved by combined effects of endothelium-derived prostacyclin, nitric oxide (NO), and hyperpolarizing factor. In this study, we investigated our hypothesis that short-term estrogen improves both NO-mediated and non-NO-mediated endothelium-dependent vasodilation in postmenopausal women. The study included 12 postmenopausal women (aged 64 +/- 3 years). The forearm blood flow was measured by strain-gauge plethysmography. The forearm vascular responses to the endothelium-dependent vasodilators, acetylcholine and substance P, were examined before and after intravenous administration of conjugated estrogen and subsequently after intraarterial infusion of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthesis. Short-term estrogen augmented the forearm vasodilating responses to both acetylcholine and substance P. The treatment with L-NMMA almost abolished the augmented response to acetylcholine but did not affect that to substance P. The forearm vascular response to sodium nitroprusside was unchanged by the estrogen administration. These results indicate that estrogen augments (in the short-term) both NO-mediated and non-NO-mediated endothelium-dependent forearm vasodilation in postmenopausal women. Thus the beneficial effect of estrogen on endothelial vasodilator function appears to extend to non-NO-dependent mechanism(s).

Hyperpolarization and relaxation of canine vascular smooth muscle to vasoactive intestinal polypeptide

This study was designed to determine the influence of the endothelium on the hyperpolarization induced by vasoactive intestinal polypeptide (VIP) in smooth muscle cells of canine blood vessels, and the potential contribution that these electrophysiologic changes may make to the relaxant effects of VIP. Membrane potential was measured in isolated canine coronary arteries and saphenous veins, using glass microelectrodes. Isometric force was recorded in a conventional organ chamber. All experiments were performed in the presence of indomethacin. VIP induced concentration-dependent and endothelium-independent hyperpolarization of the saphenous vein. This response was abolished by glibenclamide. VIP did not induce hyperpolarization of coronary arterial smooth muscle either in the presence or absence of the endothelium. VIP caused concentration-dependent and endothelium-independent relaxations of both arterial and venous rings. The relaxation of the saphenous vein to VIP was not influenced by glibenclamide. These data suggest that hyperpolarization of the cell membrane does not play a significant role in the relaxation of canine blood vessels to VIP.

Signalling pathways in bradykinin- and nitric oxide-induced hypotension in the normotensive rat; role of K+-channels

1. Bradykinin and nitric oxide (NO) are potent hypotensive agents. In the present study, the role of K+-channels in the signalling pathways responsible for their hypotensive action was investigated in normotensive, anaesthetized rats. The rats were treated with ion-channel inhibitors before administration of bradykinin (2.8, 5.6, 28 and 56 nmol kg(-1), i.v.) followed in some of the protocols by nitroprusside (1.1, 3.5, 7, 14, and 28 nmol kg(-1), i.v.). 2. No attenuation of the hypotensive response to bradykinin was detected for inhibitors of the Na-K-Cl-cotransporter (30 micromol kg(-1) furosemide), the ATP-sensitive K+-channel (40 micromol kg(-1) glibenclamide), high conductance Ca2+-activated K+-channel (180 micromol kg(-1) tetraethylammonium, 54 micromol kg(-1) tetrabutylammonium, 35 nmol kg(-1) iberiotoxin, 35 nmol kg(-1) charybdotoxin) or the low conductance Ca2+-activated K+-channel (74 nmol kg(-1) apamin). 3. However, the voltage-sensitive K+-channel (I(A)) inhibitor 4-aminopyridine (4.05-40.5 micromol kg(-1)) induced a concentration-dependent (P<0.0001) attenuation of the hypotensive response (P<0.0001). Bradykinin had no effect on heart rate in anaesthetized rats and this observation was not altered by pretreatment with 4-aminopyridine. 4. 4-Aminopyridine (53 micromol kg(-1)) also significantly attenuated the hypotensive response to nitroprusside (P<0.0003) without altering the heart rate concentration-response curve. Of the two Ca2+-activated K+-channel inhibitors tested on nitroprusside-induced hypotension, tetrabutylammonium induced a slight attenuation (P<0.0101), whereas iberiotoxin had no effect. 5. We therefore concluded that, although the acute hypotensive response to bradykinin in the normotensive rat is not mediated through nitric oxide synthesis, the hypotensive response to both agents was mediated through opening of voltage-sensitive K+-channels (I(A)), resulting in a decrease in peripheral vascular resistance.

Involvement of voltage-dependent potassium channels in the EDHF-mediated relaxation of rat hepatic artery

1. In the rat hepatic artery, the acetylcholine-induced relaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) is abolished by a combination of apamin and charybdotoxin, inhibitors of small (SKCa) and large (BKCa) conductance calcium-sensitive potassium (K)-channels, respectively, but not by each toxin alone. The selective BKCa inhibitor iberiotoxin cannot replace charybdotoxin in this combination. Since delayed rectifier K-channels (KV) represent another target for charybdotoxin, we explored the possible involvement of KV in EDHF-mediated relaxation in this artery. 2. The KV inhibitors, agitoxin-2 (0.3 microM), kaliotoxin (0.3 microM), beta-dendrotoxin (0.3 microM), dofetilide (1 microM) and terikalant (10 microM), each in combination with apamin (0.3 microM) had no effect on the EDHF-mediated relaxation induced by acetylcholine in the presence of N omega-nitro-L-arginine (0.3 mM) and indomethacin (10 microM), inhibitors of nitric oxide (NO) synthase and cyclo-oxygenase, respectively (n = 2-3). Although the KV inhibitor margatoxin (0.3 microM) was also without effect (n = 5), the combination of margatoxin and apamin produced a small inhibition of the response (pEC50 and Emax values were 7.5 +/- 0.0 and 95 +/- 1% in the absence and 7.0 +/- 0.1 and 81 +/- 6% in the presence of margatoxin plus apamin, respectively; n = 6; P < 0.05). 3. Ciclazindol (10 microM) partially inhibited the EDHF-mediated relaxation by shifting the acetylcholine-concentration-response curve 12 fold to the right (n = 6; P < 0.05) and abolished the response when combined with apamin (0.3 microM; n = 6). This combination did not inhibit acetylcholine-induced relaxations mediated by endothelium-derived NO (n = 5). 4. A 4-aminopyridine-sensitive delayed rectifier current (IK(V)) was identified in freshly-isolated single smooth muscle cells from rat hepatic artery. None of the cells displayed a rapidly-activating and -inactivating A-type current. Neither charybdotoxin (0.3 microM; n = 3) nor ciclazindol (10 microM; n = 5), alone or in combination with apamin (0.3 microM; n = 4-5), had an effect on IK(V). A tenfold higher concentration of ciclazindol (0.1 mM, n = 4) markedly inhibited IK(V), but this effect was not increased in the additional presence of apamin (0.3 microM; n = 2). 5. By use of membranes prepared from rat brain cortex. [125I]-charybdotoxin binding was consistent with an interaction at a single site with a KD of approximately 25 pM. [125I]-charybdotoxin binding was unaffected by iberiotoxin (0.1 microM, n = 6), but was increased by apamin in a concentration-dependent manner (Emax 43 +/- 10%, P < 0.05 and pEC50 7.1 +/- 0.2; n = 7-8). Agitoxin-2 (10 nM) displaced [125I]-charybdotoxin binding by 91 +/- 3% (n = 6) and prevented the effect of apamin (1 microM; n = 6). 6. It is concluded that the EDHF-mediated relaxation in the rat hepatic artery is not mediated by the opening of either KV or BKCa. Instead, the target K-channels for EDHF seem to be structurally related to both KV and BKCa. The possibility that a subtype of SKCa may be the target for EDHF is discussed.

Surgical preparation abolishes endothelium-derived hyperpolarizing factor-mediated hyperpolarization in the human saphenous vein

BACKGROUND

The impairment of the synthesis and release of endothelium-derived relaxing factors may be related to the high incidence of atherosclerosis and occlusion in saphenous vein grafts. This study focused on the effect of surgical preparation on one of the endothelium-derived relaxing factors, endothelium-derived hyperpolarizing factor, in the human saphenous vein.

METHODS

Human saphenous vein segments taken from patients undergoing coronary bypass were placed in an organ bath. A glass microelectrode was inserted into a smooth muscle cell. The membrane potential in response to acetylcholine (-9 to -5 log M) was measured in normal or surgically prepared saphenous vein with presence or absence of NG-nitro-L-arginine (300 mumol/L) and indomethacin (7 mumol/L).

RESULTS

The resting membrane potential was -71.28 +/- 1.91 mV (n = 7) with intact endothelium and -65.5 +/- 2.92 mV (n = 6, p > 0.05) without endothelium. Acetylcholine hyperpolarized membrane potential with intact endothelium (-90.57 +/- 1.48 mV, n = 7, p < 0.001), but not without endothelium (-69.67 +/- 2.93 mV, n = 6, p > 0.05). In the surgically prepared saphenous vein, acetylcholine did not hyperpolarize membrane potential (-71.83 +/- 3.84 mV versus the resting membrane potential of -69.50 +/- 3.53 mV, n = 6, p > 0.05).

CONCLUSIONS

The endothelium-derived hyperpolarizing factor plays a role in the human saphenous vein. The surgical preparation abolishes the endothelium-derived hyperpolarizing factor-mediated hyperpolarization in the saphenous vein. This study provides evidence of functional changes of endothelium by traditional surgical preparation from another point of view, and it may be related to the high incidence of occlusion in saphenous vein grafts.

NO/PGI2-independent vasorelaxation and the cytochrome P450 pathway in rabbit carotid artery

1. The nature and cellular mechanisms that are responsible for endothelium-dependent relaxations resistant to indomethacin and NG-nitro-L-arginine methyl ester (L-NAME) were investigated in phenylephrine (PE) precontracted isolated carotid arteries from the rabbit. 2. In the presence of the cyclo-oxygenase inhibitor, indomethacin (10 microM), acetylcholine (ACh) induced a concentration- and endothelium-dependent relaxation of PE-induced tone which was more potent than the calcium ionophore A23187 with pD2 values of 7.03 +/- 0.12 (n = 8) and 6.37 +/- 0.12 (n = 6), respectively. The ACh-induced response was abolished by removal of the endothelium, but was not altered when indomethacin was omitted (pD2 value 7.00 +/- 0.10 and maximal relaxation 99 +/- 3%, n = 6). Bradykinin and histamine (0.01-100 microM) had no effect either upon resting or PE-induced tone (n = 5). 3. In the presence of indomethacin plus the NO synthase inhibitor, L-NAME (30 microM), the response to A23187 was abolished. However, the response to ACh was not abolished, although it was significantly inhibited with the pD2 value and the maximal relaxation decreasing to 6.48 +/- 0.10 and 67 +/- 3%, respectively (for both P < 0.01, n = 8). The L-NAME/indomethacin insensitive vasorelaxation to ACh was completely abolished by preconstriction of the tissues with potassium chloride (40 mM, n = 8). 4. The Ca(2+)-activated K+ (KCa) channel blockers, tetrabutylammonium (TBA, 1 mM, n = 5) and charybdotoxin (CTX, 0.1 microM, n = 5), completely inhibited the nitric oxide (NO) and prostacyclin (PGI2)-independent relaxation response to ACh. However, iberiotoxin (ITX, 0.1 microM, n = 8) or apamin (1-3 microM, n = 6) only partially inhibited the relaxation. 5. Inhibitors of the cytochrome P450 mono-oxygenase, SKF-525A (1-10 microM, n = 6), clotrimazole (1 microM, n = 5) and 17-octadecynoic acid (17-ODYA, 3 microM, n = 7) also reduced the NO/PGI2-independent relaxation response to ACh. 6. In endothelium-denuded rings of rabbit carotid arteries, the relaxation response to exogenous NO was not altered by either KCa channel blockade with apamin (1 microM, n = 5) or CTX (0.1 microM, n = 5), or by the cytochrome P450 mono-oxygenase blockers SKF-525A (10 microM, n = 4) and clotrimazole (10 microM, n = 5). However, the NO-induced response was shifted to the right by LY83583 (10 microM, n = 4), a guanylyl cyclase inhibitor, with the pD2 value decreasing from 6.95 +/- 0.14 to 6.04 +/- 0.09 (P < 0.01). 7. ACh (0.01-100 microM) induced a concentration-dependent relaxation of PE-induced tone in endothelium-denuded arterial segments sandwiched with endothelium-intact donor segments. This relaxation to ACh was largely unaffected by indomathacin (10 microM) plus L-NAME (30 microM), but abolished by the combination of indomethacin, L-NAME and TBA (1 mM, n = 5). 8. These data suggest that in the rabbit carotid artery: (a) ACh can induce the release of both NO and EDHF, whereas A23187 only evokes the release of NO from the endothelium, (b) the diffusible EDHF released by ACh may be a cytochrome P450-derived arachidonic acid metabolite, and (c) EDHF-induced relaxation involves the opening of at least two types of KCa channels, whereas NO mediates vasorelaxation via a guanosine 3': 5'-cyclic monophosphate (cyclic GMP)-mediated pathway, in which a cytochrome P450 pathway and KCa channels do not seem to be involved.

Endothelium-mediated and N omega-nitro-L-arginine methyl ester-sensitive responses to cromakalim and diazoxide in the rat mesenteric bed

The effects of two 'K+ channel openers', (+/-)-6-cyano-3,4-dihydro-2,2-dimethyl-trans-4-(2-oxo-1-pyrrolidyl )-2 H-benzo[b]-pyran-3-ol (cromakalim) and 7-chloro-3-methyl-2 H-1,2,4-benzothiadiazine 1,1-dioxide (diazoxide), were studied on the rat isolated mesenteric bed. Differences in the perfusion pressure were measured as a parameter of vascular resistance. Cromakalim (0.1-700 microM) and diazoxide (1 microM-1 mM) reduced to 60% the contractions elicited by 10 microM noradrenaline and to 30% those evoked by 100 mM KCl. The relaxant effects of cromakalim and diazoxide on the noradrenaline-induced contractions were reduced by the K(+)-ATP channel blocker, 5-chloro-N-[2-[4-[[[(cyclohexylamino) carbonyl]amino]-sulfonyl]phenyl]ethyl]-2-methoxybenzamide (glibenclamide, 0.01-0.3 microM), endothelium removal with 0.1% saponin and pretreatment with the nitric oxide synthesis inhibitor, S(+/-)-N5-[imino(nitroamino)methyl]-L-ornithine methyl ester hydrochloride (L-NAME, 500 microM). Reductions in the relaxant responses after endothelium removal or L-NAME pretreatment were observed with 1-100 microM cromakalim and with 30 microM diazoxide but not with 100 and 300 microM diazoxide. Pretreatment with the inactive stereoisomer D-NAME as well as with the prostanoid synthesis inhibitor, 1-[p-chlorobenzoyl]-5-methoxy-2-methylindole-3-acetic acid (indomethacin, 10 microM), did not affect the reductions in contractile responses to noradrenaline caused by either cromakalim or diazoxide. It is concluded that the relaxant effects of cromakalim and diazoxide in the rat mesenteric bed are endothelium-mediated and L-NAME-sensitive and could at least partially involve the participation of nitric oxide.

Endothelium-derived hyperpolarizing factor

1. Not all endothelium-dependent relaxations can be fully explained by the release of either nitric oxide (NO) and/or prostacyclin. Another unidentified substance(s) that hyperpolarizes the underlying vascular smooth muscle cells (endothelium-derived hyperpolarizing factor; EDHF) contributes to endothelium-dependent relaxations. 2. In blood vessels from various species these hyperpolarizations are resistant to inhibitors of NO synthase (NOS) and cyclo-oxygenase. In canine, porcine and human blood vessels the hyperpolarization cannot be mimicked by nitrovasodilators or exogeneous NO. However, in other species (rat, guinea-pig, rabbit) endothelium-dependent hyperpolarizations resistant to inhibitors of NOS and cyclo-oxygenase and hyperpolarizations to endothelium-derived or exogeneous NO can be observed in the same vascular smooth muscle cells. 3. In blood vessels where NO causes hyperpolarization, the response is blocked by glibenclamide, suggesting the involvement of ATP-dependent potassium channels. Hyperpolarizations caused by EDHF are insensitive to glibenclamide but, depending on the tissue, are inhibited by relatively small concentrations of tetraethylammonium (TEA) or by apamin or the combination of charybdotoxin plus apamin, indicating that calcium-dependent potassium channels are likely to be involved. 4. Metabolites of arachidonic acid, through the cytochrome P450 mono-oxygenase pathway (epoxyeicosatrienoic acids), are produced by the endothelial cells, increase the open-state probability of calcium-activated potassium channels sensitive to TEA or charybdotoxin, and induce the hyperpolarization of arterial smooth muscle cells, indicating that epoxyeicosatrienoic acids could be EDHF. However, in blood vessels from various species, cytochrome P450 inhibitors do not affect endothelium-dependent hyperpolarizations, indicating that EDHF is not yet identified with certainty. 5. Endothelium-derived hyperpolarizing factor released from cultured endothelial cells reduces the intracellular calcium concentration in vascular smooth muscle cells and the EDHF component of the relaxation is proportionally more important in smaller than larger arteries. In aging animals and in various models of diseases, endothelium-dependent hyperpolarizations are diminished. 6. The identification of EDHF and/or the discovery of specific inhibitors of its synthesis and its action may allow a better understanding of its physiological and pathophysiological role(s).

The importance of the hyperpolarizing mechanism increases as the vessel size decreases in endothelium-dependent relaxations in rat mesenteric circulation

Endothelium-dependent relaxations are achieved by a combination of endothelium-derived prostacyclin (PGI2), nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). However, it remains to be fully clarified whether the relative contribution of these three mechanisms to endothelium-dependent relaxations varies as a function of the vessel size. This study was designed to clarify this point. Acetylcholine (ACh)-induced endothelium-dependent relaxations were examined in isolated blood vessels taken from the aorta and the proximal and distal mesenteric arteries of the rat. The contributions of PGI2, NO, and EDHF were evaluated by the inhibitory effects of indomethacin, N omega-nitro-L-arginine methyl ester (L-NAME) in the presence of indomethacin, and KCl in the presence of indomethacin and L-NAME, respectively. The membrane potentials were recorded with microelectrodes. The expression of endothelial No synthase (eNOS) was examined by both immunostaining and immunoblotting. The contribution of PGI2 was negligible in three different-sized blood vessels. The contribution of NO was most prominent in the aorta, whereas that of EDHF was most prominent in the distal mesenteric arteries. The resting membrane potential was significantly deeper and the ACh-induced hyperpolarization was greater in the distal mesenteric arteries than those in the aorta. The expression of eNOS was the highest in the aorta and the lowest in the distal mesenteric arteries. These results indicate that the importance of EDHF increases as the vessel size decreases in endothelium-dependent relaxations in the rat mesenteric circulation.

Developmental changes in endothelium-dependent relaxation of pulmonary arteries: role of EDNO and prostanoids

We hypothesized that maturational changes in both prostaglandin and endothelium-derived nitric oxide (EDNO) activity contribute to developmental changes in endothelium-dependent relaxation of newborn pulmonary arteries. Responses to endothelium-dependent vasodilators acetylcholine, bradykinin, and calcium ionophore A-23187 were determined in phenylephrine-constricted third- and fourth-generation (1- to 2-mm-diameter) pulmonary artery rings from 2-day (2d)- and 1-mo (1m)-old lambs under control conditions (Con), after inhibition of EDNO synthesis with N omega-nitro-L-arginine (L-NNA), after inhibition of prostanoid synthesis with meclofenamate (Mec), or both modulators with both inhibitors. Endothelium-independent responses to sodium nitroprusside (SNP) were also measured in Con rings. Endothelium-dependent relaxation was greater in 2d than 1m Con rings, particularly at high concentrations when an increase in tension occurred in 1m rings. L-NNA attenuated endothelium-dependent relaxation more in 2d rings, and SNP caused greater relaxation in 2d rings. However, Mec abolished all age-related differences by attenuating relaxation in 2d rings and constriction in 1m rings. These data suggest that developmental changes in endothelium-dependent responses of ovine pulmonary artery rings reflect both a decrease in EDNO activity and maturational differences in the relative influence of dilator and constrictor prostanoids.

NO hyperpolarizes pulmonary artery smooth muscle cells and decreases the intracellular Ca2+ concentration by activating voltage-gated K+ channels

NO causes pulmonary vasodilation in patients with pulmonary hypertension. In pulmonary arterial smooth muscle cells, the activity of voltage-gated K+ (Kv) channels controls resting membrane potential. In turn, membrane potential is an important regulator of the intracellular free calcium concentration ([Ca2+]i) and pulmonary vascular tone. We used patch clamp methods to determine whether the NO-induced pulmonary vasodilation is mediated by activation of Kv channels. Quantitative fluorescence microscopy was employed to test the effect of NO on the depolarization-induced rise in [Ca2+]i. Blockade of Kv channels by 4-aminopyridine (5 mM) depolarized pulmonary artery myocytes to threshold for initiation of Ca2+ action potentials, and thereby increased [Ca2+]i. NO (approximately 3 microM) and the NO-generating compound sodium nitroprusside (5-10 microM) opened Kv channels in rat pulmonary artery smooth muscle cells. The enhanced K+ currents then hyperpolarized the cells, and blocked Ca(2+)-dependent action potentials, thereby preventing the evoked increases in [Ca2+]i. Nitroprusside also increased the probability of Kv channel opening in excised, outside-out membrane patches. This raises the possibility that NO may act either directly on the channel protein or on a closely associated molecule rather than via soluble guanylate cyclase. In isolated pulmonary arteries, 4-aminopyridine significantly inhibited NO-induced relaxation. We conclude that NO promotes the opening of Kv channels in pulmonary arterial smooth muscle cells. The resulting membrane hyperpolarization, which lowers [Ca2+]i, is apparently one of the mechanisms by which NO induces pulmonary vasodilation.

The arterial wall: a new pharmacological and therapeutic target

In recent years, two key concepts having numerous interrelationships were advanced for the understanding of various cardiovascular diseases: the "endothelial dysfunction" and the "arterial remodelling". Both endothelial dysfunction and arterial remodelling occur in various pathologies including essential hypertension, heart failure, atherosclerosis, restenosis after angioplasty, and pulmonary hypertension, and have modified the therapeutic approach by offering new pharmacological targets: specific receptors not only at the site of the vascular smooth muscle cells but also on the endothelial cells, growth factors that stimulate proliferation of smooth muscle, and receptors and enzymes of the extra-cellular matrix. Among the various substances under research, the present review will discuss angiotensin II receptor antagonists, endothelin receptor antagonists, nitrates-NO donors, potassium channel activators, and substances interfering with proteoglycans and other components of the extra-cellular matrix.

Role of potassium channels in endothelium-dependent relaxation resistant to nitroarginine in the rat hepatic artery

1. In the presence of indomethacin (IM, 10 microM) and N omega-nitro-L- arginine (L-NOARG, 0.3 mM), acetylcholine (ACh) induces an endothelium-dependent smooth muscle hyperpolarization and relaxation in the rat isolated hepatic artery. The potassium (K) channel inhibitors, tetrabutylammonium (TBA, 1 mM) and to a lesser extent 4-aminopyridine (4-AP, 1 mM) inhibited the L-NOARG/IM-resistant relaxation induced by ACh, whereas apamin (0.1-0.3 microM), charybdotoxin (0.1-0.3 microM), iberiotoxin (0.1 microM) and dendrotoxin (0.1 microM) each had no effect. TBA also inhibited the relaxation induced by the receptor-independent endothelial cell activator, A23187. 2. When combined, apamin (0.1 microM) + charybdotoxin (0.1 microM), but not apamin (0.1 microM) + iberiotoxin (0.1 microM) or a triple combination of 4-AP (1 mM) + apamin (0.1 microM) + iberiotoxin (0.1 microM), inhibited the L-NOARG/IM-resistant relaxation induced by ACh. At a concentration of 0.3 microM, apamin + charybdotoxin completely inhibited the relaxation. This toxin combination also abolished the L-NOARG/ IM-resistant relaxation induced by A23187. 3. In the absence of L-NOARG, TBA (1 mM) inhibited the ACh-induced relaxation, whereas charybdotoxin (0.3 microM) + apamin (0.3 microM) had no effect, indicating that the toxin combination did not interfere with the L-arginine/NO pathway. 4. The gap junction inhibitors halothane (2 mM) and 1-heptanol (2 mM), or replacement of NaCl with sodium propionate did not affect the L-NOARG/IM-resistant relaxation induced by ACh. 5. Inhibition of Na+/K(+)-ATPase by ouabain (1 mM) had no effect on the L-NOARG/IM-resistant relaxation induced by ACh. Exposure to a K(+)-free Krebs solution, however, reduced the maximal relaxation by 13% without affecting the sensitivity to ACh. 6. The results suggest that the L-NOARG/IM-resistant relaxation induced by ACh in the rat hepatic artery is mediated by activation of K-channels sensitive to TBA and a combination of apamin + charybdotoxin. Chloride channels, Na+/K(+)-ATPase and gap junctions are probably not involved in the response. It is proposed that endothelial cell activation induces secretion of an endothelium-derived hyperpolarizing factor(s) (EDHF), distinct from NO and cyclo-oxygenase products, which activates more than one type of K-channel on the smooth muscle cells. Alternatively, a single type of K-channel, to which both apamin and charybdotoxin must bind for inhibition to occur, may be the target for EDHF.

Pathophysiology, Diagnosis, and Management of Pulmonary Hypertension in Infants and Children

Pulmonary hypertension (PH) may occur as a primary process or as a complication of several diseases. In the pediatric population, PH secondary to congenital heart disease, chronic hypoxemia, or acute respiratory failure is more common than primary PH. Regardless of etiology, PH may lead to significant morbidity or mortality as a consequence of right-to-left shunting across cardiovascular channels or right heart failure. In this review, PH is defined in terms of the determinants of pulmonary blood flow: pulmonary artery pressure, downstream pressure, and pulmonary vascular resistance. Research addressing both normal developmental changes in pulmonary vascular resistance and abnormal pulmonary vascular reactivity is then reviewed, followed by a discussion of the etiologies of PH in children. Some of the more common clinical presentations of PH are presented focussing on the differences seen between patients with and without intracardiac communications. Assessment of the severity of PH using both noninvasive (electrocardiogram, echocardiogram, magnetic resonance imaging) and invasive (cardiac catheterization, lung biopsy) techniques is then discussed. Treatment of PH is presented, focussing on restoration of adequate pulmonary blood flow through use of both conventional and newer vasodilator therapies. The review concludes by noting the limits to our understanding of the pathogenesis and therapy of PH.

Vasoactive effects of fibrinogen on saphenous vein

Normal plasma fibrinogen concentrations are critical to haemostasis. Higher fibrinogen concentrations are associated with increasing risk of atherosclerotic disease and with graft stenosis and occlusion after saphenous vein bypass surgery. Vein graft stenosis is characterized by the localized proliferation of intimal smooth muscle cells, causing narrowing of the graft with increased risk of thrombotic occlusion. In rabbit arteries, fibrinopeptide B is reported to have both vasoconstrictor and mitogenic properties. We report here that fibrinopeptides had no vasoactive effects on saphenous vein rings; however, fibrinogen (0-2 microM) affected an endothelium-dependent relaxation, followed by recontraction at higher concentrations. The fibrinogen-mediated relaxation was inhibited by K+-channel blockers and antibodies to ICAM-1. Coupled signalling pathways for the synthesis of vasoactive mediators and mitogens could underlie the association between fibrinogen and the development of vein graft pathology.

Endothelium-dependent relaxations mediated by inducible B1 and constitutive B2 kinin receptors in the bovine isolated coronary artery

1. Rings of bovine left anterior descending coronary artery (LAD) were contracted with the thromboxane A2-mimetic, U46619 (1-30 nM), to approximately 40% of their maximum contraction to 125 mM KCl Krebs solution (KPSSmax) for comparison of responses to the B1 and B2 kinin receptor agonists, des-Arg9-bradykinin (des-Arg9-BK) and bradykinin (BK), respectively. Relaxation responses were normalized as percentages of the initial U46619-induced contraction level, while contractile responses were expressed as percentages of KPSSmax. 2. After 6 h of in vitro incubation in Krebs solution at 37 degrees C, des-Arg9-BK (pEC50, 8.00 +/- 0.08; maximum response (Rmax), 93.9 +/- 1.9%) and BK (pEC50, 9.75 +/- 0.07; Rmax, 100.1 +/- 0.7%) caused endothelium-dependent relaxations in precontracted rings of bovine LAD which were competitively and selectively antagonized by the B1 receptor antagonist, des-Arg9-[Leu8]-BK (pA2, 6.27 +/- 0.11) and the B2 receptor antagonist Hoc-140 (pA2, 9.63 +/- 0.14), respectively. 3. At 3 h of in vitro incubation, the sensitivity (pEC50, 7.45 +/- 0.10) and Rmax (84.6 +/- 3.3%) to des-Arg9-BK were significantly less than those obtained in the same tissues at 6 h (pEC50, 7.94 +/- 0.06; Rmax, 91.4 +/- 2.5%), whereas endothelium-dependent relaxations to BK and ACh were unaffected by incubation time. 4. Relaxation responses to des-ARg9-BK, but not BK, at both 3 h and 6 h were significantly attenuated by the protein synthesis inhibitors, cycloheximide (30 and 100 microM) and actinomycin D (2 microM). 5. At 6 h, the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine (L-NOARG, 100 microM), caused a significant 2 fold decrease in pEC50 (9.58 +/- 0.03) but had no effect on Rmax for BK. For des-Arg9-BK, L-NOARG (100 microM) caused a marked and significant decrease in both the pEC50 and Rmax and revealed contractions to low concentrations of des-Arg9-BK. In both cases, L-NOARG inhibition was reversed in the presence of L-arginine (10 mM). 6. At 6 h removal of the endothelium abolished relaxation responses to des-Arg9-BK and BK, and for des-Arg9-BK, but not BK, unmasked concentration-dependent contractions (pEC50, 7.57 +/- 0.09; Rmax, 83.4 +/- 9.1%). The sensitivity of contractions to des-Arg9-BK increased slightly from 3 h (pEC50, 7.37 +/- 0.08) to 6 h (pEC50, 7.62 +/- 0.12) of in vitro incubation; however, there was a small but significant depression in the maximum response over this time (Rmax, 126.8 +/- 8.5% and 103.3 +/- 8.6% for 3 h and 6 h of incubation respectively). 7. In conclusion, the bovine LAD contains inducible B1 and constitutive B2 endothelial cell kinin receptors, both of which mediate endothelium-dependent relaxation partly via the release of NO. B1 receptors were also present on the smooth muscle layer of the bovine LAD.

Regional differences in endothelium-dependent relaxation in the rat: contribution of nitric oxide and nitric oxide-independent mechanisms

Relaxant effects of acetylcholine (ACh), histamine, calcitonin gene-related peptide (CGRP) and the calcium ionophore A23187 were examined in rat femoral (phi approximately 0.2 mm), mesenteric (0.2 mm), intrarenal (0.2 mm) and hepatic (0.3 mm) arteries, and aorta (2 mm). Acetylcholine elicited an endothelium-dependent relaxation in all arteries. Histamine induced an endothelium-dependent relaxation in aorta, and mesenteric and intrarenal arteries, whereas a partly endothelium-dependent and mainly endothelium-independent relaxation was observed in hepatic and femoral arteries, respectively. In hepatic, mesenteric and intrarenal arteries, CGRP induced an endothelium-independent relaxation, whereas either small or no relaxation was obtained in aorta and femoral arteries respectively. A23187 induced an endothelium-dependent relaxation in the aorta and hepatic artery, whereas A23187 had no relaxant effect in femoral, mesenteric and intrarenal arteries. N omega-nitro-L-arginine (L-NOARG, 0.3 mM) reduced the maximum ACh-induced relaxation (in the presence of 10 microM indomethacin) by 66% in the aorta, and abolished the relaxation in femoral and intrarenal arteries. A marked L-NOARG/indomethacin-resistant relaxation was obtained in mesenteric and hepatic arteries. Levcromakalim induced a concentration-dependent and almost complete relaxation in all arteries. When contracted by a 60 mM K+ solution, all arteries responded to ACh with a relaxation that was abolished by L-NOARG. These results demonstrate marked regional differences with regard to the vascular effects of ACh, histamine, CGRP and A23187. Whereas nitric oxide appears to mediate endothelium-dependent relaxation regardless of the vascular region, an L-NOARG/indomethacin-resistant relaxation, presumably mediated by an endothelium-deprived hyperpolarizing factor, was observed only in mesenteric and hepatic arteries, and aorta.

Persistent Pulmonary Hypertension of the Newborn: Role of Nitric Oxide

Persistent pulmonary hypertension of the newborn (PPHN) is a common cause of respiratory failure in the full-term neonate. Molecular and cellular studies in vascular biology have revealed that endothelial-derived mediators play a critical role in the pathogenesis and treatment of PPHN. Endothelial-derived vasoconstrictors, like endothelin, may increase smooth muscle cell contractility and growth, leading to the physiologic and structural changes observed in the pulmonary arterioles of infants with this disease. On the other hand, decreased production of the endothelial-derived relaxing factor, nitric oxide, may exacerbate pulmonary vasoreactivity and lead to more severe pulmonary hypertension. Exogenous (inhaled) nitric oxide therapy reduces pulmonary vascular resistance and improves oxygenation. The safety and efficacy of this therapy in reducing the need for extracorporeal membrane oxygenation and decreasing long-term morbidity is being tested in several trials nationally and abroad. Understanding the basic mechanisms that regulate the gene expression and production of these vasoactive mediators will lead to improved preventive and therapeutic strategies for PPHN.

The role of nitric oxide and other endothelium-derived vasoactive substances in vascular disease

Alteration in the release and action of endothelium-derived vasoactive factors is responsible for changes in vascular reactivity early in the course of vascular disease. These factors include nitric oxide, eicosanoids, endothelium-derived hyperpolarizing factor, endothelin, and angiotensin II. Because endothelial dysfunction occurs at early stages of disease, it may reflect physiological changes that, if allowed to become chronic, are responsible for changes in vascular structure and growth and adhesivity to platelets and leukocytes, ultimately leading to atherosclerosis and thrombosis. Each of the major risk factors predisposing to vascular disease are associated with endothelial cell dysfunction, suggesting a direct etiologic link between the effects of the risk factors on the endothelium and their propensity to accelerate vascular disease. Restoration or replacement of endothelium-derived factors such as nitric oxide and prostacyclin, which impede the progression of vascular disease, or preventing the action of mediators such as vasoconstrictor eicosanoids, angiotensin II, or endothelin, which accelerate the progression of vascular disease, has become a useful paradigm in the treatment and prevention of vascular disease. Thus, understanding the physiology of endothelium-derived vasoactive factors is a necessary part of every physician's education.

Substance P-induced relaxation and hyperpolarization in human cerebral arteries

1. Vascular effects of substance P were studied in human isolated pial arteries removed from 14 patients undergoing cerebral cortical resection. 2. Substance P induced a concentration-dependent relaxation in the presence of indomethacin. No relaxation was seen in arteries where the endothelium had been removed. 3. N omega-nitro-L-arginine (L-NOARG, 0.3 mM) abolished the relaxation in arteries from six patients. The relaxation was only partially inhibited in the remaining eight patients, the reduction of the maximum relaxation being less than 50% in each patient. 4. The L-NOARG-resistant relaxation was abolished when the external K+ concentration was raised above 30 mM. 5. Substance P caused a smooth muscle hyperpolarization (in the presence of L-NOARG and indomethacin), but only when the artery showed an L-NOARG-resistant relaxation. 6. The results indicate that nitric oxide is an important mediator of endothelium-dependent relaxation in human cerebral arteries. Furthermore, another endothelium-dependent pathway, causing hyperpolarization and vasodilatation, was identified in arteries from more than half the population of patients.

Ginsenosides of the protopanaxatriol group cause endothelium-dependent relaxation in the rat aorta

The vasoactive effects of several ginsenosides, purified from Panax ginseng, were tested in the rat aorta. Ginsenosides from the protopanaxatriol group and its purified ginsenosides Rg1 and Re cause endothelium dependent relaxation which is associated with the formation of cyclic GMP. Both responses to these protopanaxatriol-containing ginsenosides are inhibited by methylene blue, an inhibitor of soluble guanylate cyclase and of nitric oxide synthase. In contrast, ginsenosides from the protopanaxadiol group and its purified ginsenosides Rb1 and Re do not affect vascular tone or production of cyclic GMP in the rat aorta. These findings demonstrate that ginsenosides from the protopanaxatriol group but not from the protopanaxadiol group enhance the release of nitric oxide from endothelial cells and may contribute to the beneficial effect of ginseng on the cardiovascular system.

The endothelium mediates a nitric oxide-independent hyperpolarization and relaxation in the rat hepatic artery

The rat hepatic artery responds to acetylcholine (ACh) with an endothelium-dependent relaxation, which is unaffected by nitric oxide (NO) synthase and cyclooxygenase inhibition. The purpose of this study was to investigate whether the NO-independent relaxation is caused by hyperpolarization of the smooth muscle cells. In vessels with endothelium ACh induced a hyperpolarization in the presence of 0.3 mM N omega-nitro-L-arginine (L-NOARG) and 10 microM indomethacin. The hyperpolarization, which slowly decayed after an initial maximum, generally lasted for at least 20 min. ACh in contrast to levcromakalim failed to hyperpolarize the smooth muscle cells in endothelium-denuded vessels. In vessels contracted by phenylephrine (PhE) ACh caused a concentration-dependent hyperpolarization and relaxation, and both events occurred over the same concentration interval. Curve fitting using the Hill equation showed a close correlation between the hyperpolarization and the relaxation. Exposure to a 30 mM K+ solution abolished the hyperpolarization and suppressed the relaxation induced by ACh. Nimodipine did not affect the ACh-induced hyperpolarization, whereas the relaxation induced by ACh and levcromakalim, but not that evoked by the NO donor 3-morpholino-sydnonimin, were attenuated. Glibenclamide had no effect on the ACh-induced hyperpolarization and relaxation, but abolished the corresponding responses to levcromakalim. The results demonstrate a NO-independent hyperpolarization and relaxation in the rat hepatic artery. The hyperpolarization and relaxation were endothelium-dependent, and apparently causally related to each other, since interference with the hyperpolarization or the subsequent effector pathway inhibited the relaxation.

Relaxation of porcine coronary artery to bradykinin. Role of arachidonic acid

Bradykinin-induced relaxation of precontracted, porcine coronary artery (PCA) rings is mediated by distinctly different endothelium-derived relaxing factors depending on the contractile agent used. Thus when contracted with KCl, bradykinin-induced relaxation of PCA rings is mediated solely by nitric oxide (NO), whereas when contracted with the thromboxane mimetic U46619, a small component of the relaxation is attributable to NO and a large component is attributable to a non-NO mechanism that is independent of cyclooxygenase activity. We hypothesized that the non-NO component was mediated by arachidonic acid (AA) or by a non-cyclooxygenase product of AA metabolism. Bradykinin-induced relaxations of PCA rings precontracted with U46619 in the presence of indomethacin (10 mumol/L) were moderately attenuated by the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 100 mumol/L), whereas when precontracted with KCl, L-NAME abolished the relaxations. AA produced endothelium-dependent relaxations of rings precontracted with U46619 that were unaffected by L-NAME, whereas AA did not relax rings precontracted with KCl. In rings precontracted with U46619, in the presence of L-NAME and indomethacin the phospholipase inhibitors quinacrine (50 mumol/L) and 4-bromophenacyl bromide (10 mumol/L) attenuated bradykinin- but not AA-induced relaxations. Inhibitors of both lipoxygenase (BW 755c [100 mumol/L] and nafazatrom [20 mumol/L]) and cytochrome P-450 (proadifen [10 mumol/L] and clotrimazole [10 mumol/L]) pathways did not eliminate bradykinin- or AA-induced relaxations, although clotrimazole partially attenuated AA-induced relaxations. These findings suggest that bradykinin-induced relaxation of PCA rings is mediated by AA through a mechanism that is not dependent on cyclooxygenase, lipoxygenase, or cytochrome P-450 pathways.

Nutrition, endothelial cell metabolism, and atherosclerosis

The vascular endothelium that forms an interface between the blood and the surrounding tissues is continuously exposed to both physiologic and pathophysiologic stimuli. These stimuli are often mediated by nutrients that can contribute to the overall function of the endothelial cell in the regulation of vascular tone, coagulation and fibrinolysis, cellular growth and differentiation, and immune and inflammatory responses. Therefore, nutrient-mediated functional changes of the endothelium and the underlying tissues may be significantly involved in the atherosclerotic disease process. There is evidence that individual nutrients or nutrient derivatives may either provoke or prevent metabolic and physiologic perturbations of the vascular endothelium. Preservation of nutrients that exhibit antiatherogenic properties may, therefore, be a critical issue in the preparation and processing of foods. This review focuses on selected nutrients as they affect endothelial cell metabolism and their possible implications in atherosclerosis.

NG-nitro-L-arginine-resistant endothelium-dependent relaxation induced by acetylcholine in the rabbit renal artery

Studies were designed to determine the extent of the involvement of endothelium-derived relaxing factor(s) other than nitric oxide (NO) in vascular relaxation in response to acetylcholine (ACh) in the rabbit renal artery. ACh (10(-9)-10(-6) M) induced concentration-dependent relaxation of isolated endothelium-intact arterial rings preconstricted with noradrenaline. NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthase, partly inhibited the ACh-induced endothelium-dependent relaxation, whereas it almost completely abolished the production of cyclic-3', 5'-guanosine monophosphate (cGMP) in these rings in response to ACh. Methylene blue, an inhibitor of guanylate cyclase, had an essentially similar effect to L-NAME on the relaxation. Indomethacin, an inhibitor of cyclooxygenase, had no effect. High concentrations of potassium chloride (to inhibit endothelium-dependent hyperpolarization), tetraethylammonium (TEA) or 4-aminopyridine (4-AP), a voltage-dependent or Ca(2+)-dependent K+ channel blocker, partly inhibited the relaxation while, in contrast, glibenclamide, an ATP-sensitive K+ channel blocker, had no effect. Ouabain, an inhibitor of Na+, K(+)-ATPase, also partly inhibited the ACh-induced relaxation, especially the higher concentration effect. Application of L-NAME together with ouabain, TEA, or a high concentration of potassium chloride completely abolished the relaxation. These results suggest that ACh-induced endothelium-dependent relaxation in the rabbit renal artery is mediated by NO, and by an other factor(s), which relaxes the vascular smooth muscle through opening K+ channels other than ATP-sensitive ones, and/or through the activation of a Na+, K(+)-pump.

Response to the endothelium-dependent vasodilator acetylcholine in perfused kidneys of normotensive and spontaneously hypertensive rats

The possible role of endothelium-derived relaxing factor (EDRF) was investigated in resistance vessels of the kidney obtained from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. Kidneys were studied in parallel and perfused with Tyrode's solution at constant optimal flow rates. In the presence of indomethacin, during vasoconstriction (increase in perfusion pressure) produced by prostaglandin F2 alpha, acetylcholine caused a graded dilatation (pressure fall) that was greater in kidneys of WKY than of SHR. Methylene blue and hydroquinone, but not oxyhemoglobin, inhibited the decreases in perfusion pressure induced by acetylcholine, but not those by papaverine. The results suggest that part of the renal vasodilatation induced by acetylcholine is mediated by endothelium-derived relaxing factor, and that the response is impaired in the resistance vessels of the hypertensive rat kidney.