The release of endothelium-derived relaxing factor via alpha 2-adrenergic receptor activation is specifically mediated by Gi alpha 2

The effect of inflammation on sympathetic nerve mediated contractions in rat isolated caudal artery

Chronic inflammatory process(es) contributes to changes in vascular function in a variety of diseases. Sympathetic nerve-mediated responses in blood vessels play a pivotal role in regular physiological functions. We tested the hypothesis that sympathetic neuro-effector function will be altered as consequence of inflammatory state. Sympathetic nerve-mediated contractions and alpha adrenergic receptor expressions were evaluated in isolated caudal arteries of rats treated with saline and Complete Freund's adjuvant (CFA). While CFA-treated animals had significantly higher plasma levels of tumor necrosis factor-alpha compared to saline, blood pressure remained unchanged. Immunofluorescence revealed increased expression of ionized calcium adapter binding molecule-1 in the adventitia of blood vessels from CFA-treated animals compared to saline. In isolated arteries, electrical field stimulations between 1.25 and 40Hz resulted in frequency-dependent contractions that wasabolished by tetrodotoxin. Neurogenic contractions from CFA groups were significantly greater than saline. While the presence of alpha₁-adrenoceptor antagonist (prazosin) significantly inhibited contractions at lower frequencies of stimulation (1.25-5Hz) in isolated arteries of CFA-treated rats compared to controls, alpha₂-adrenoceptor antagonist (rauwolscine) had modest effects. Inhibition of neuronal reuptake by cocaine comparably enhanced field-stimulated responses in vessels of experimental and control animals. Immunofluorescence revealed a difference in expression of alpha₁- and alpha₂-adrenoceptors in the endothelium of blood vessels of CFA compared to saline controls. Collectively, our observations lend support to enhanced neurogenic contractions in blood vessels of inflamed animals possibly attributing to alterations in responsiveness and/or distribution of post-junctional alpha₁-adrenoceptors.

A novel role for an endothelial adrenergic receptor system in mediating catecholestradiol-induced proliferation of uterine artery endothelial cells

Sequential conversion of estradiol-17β to its biologically active catecholestradiols, 2-hydroxyestradiol (OHE(2)) and 4-OHE(2), contributes importantly to its angiogenic effects on uterine artery endothelial cells (UAECs) derived from pregnant, but not nonpregnant ewes via an estrogen receptor-independent mechanism. Because catecholestradiols and catecholamines exhibit structural similarities and have high affinity for α- and β-adrenergic receptors (ARs), we investigated whether the endothelial α- or β-ARs mediate catecholestradiol-induced proliferation of P-UAECs and whether catecholamines alter these responses. Western analyses revealed expression of specific AR subtypes in nonpregnant UAECs and P-UAECs, including α(2)-, β(2)-, and β(3)-ARs but not α(1)- and β(1)-ARs. Levels of β(2)-ARs and β(3)-ARs were unaltered by pregnancy, whereas α(2)-ARs were decreased. Norepinephrine and epinephrine increased P-UAEC, but not nonpregnant UAEC proliferation, and these effects were suppressed by propranolol (β-AR blocker) but not phentolamine (α-AR blocker). Catecholamines combinations with 2-OHE(2) or 4-OHE(2) enhanced P-UAEC mitogenesis. Catecholestradiol-induced P-UAEC proliferation was also inhibited by propranolol but not phentolamine. β(2)-AR and β(3)-AR antagonists (ICI 118 551and SR 59230A, respectively) abrogated the mitogenic effects of both 2-OHE(2) and 4-OHE(2). Stimulation of β(2)-ARs and β(3)-ARs using formoterol and BRL 37344 dose-dependently stimulated P-UAEC proliferation, which was abrogated by ICI 118 551 and SR 59230A, respectively. Proliferation effects of both catecholamines and catecholestradiols were only observed in P-UAECs (not nonpregnant UAECs) and were mediated via β(2)-ARs and β(3)-ARs. We demonstrate for the first time convergence of the endothelial AR and estrogenic systems in regulating endothelial proliferation, thus providing a distinct evolutionary advantage for modulating uterine perfusion during stressful pregnancies.

Direct effect of dexmedetomidine on rat isolated aorta involves endothelial nitric oxide synthesis and activation of the lipoxygenase pathway

1. The aims of the present in vitro study were to examine the roles of pathways associated with arachidonic acid metabolism in dexmedetomidine-induced contraction and to determine which endothelium-derived vasodilators are involved in the endothelium-dependent attenuation of vasoconstriction elicited by dexmedetomidine. 2. Dexmedetomidine (10(-9)-10(-6) mol/L) concentration-response curves were constructed in: (i) aortic rings with no drug pretreatment; (ii) endothelium-denuded aortic rings pretreated with either 2 x 10(-5) mol/L quinacrine dihydrochloride, 10(-5) mol/L nordihydroguaiaretic acid (NDGA), 3 x 10(-5) mol/L indomethacin or 10(-5) mol/L fluconazole; and (iii) endothelium-intact aortic rings pretreated with either 5 x 10(-5) mol/L N(G)-nitro-l-arginine methyl ester (l-NAME), 10(-5) mol/L fluconazole, 10(-5) mol/L indomethacin, 10(-5) mol/L glibenclamide, 5 x 10(-3) mol/L tetraethylammonium or 5 x 10(-5) mol/L l-NAME plus rauwolscine (10(-5), 10(-6) mol/L). The production of nitric oxide (NO) metabolites was determined in human umbilical vein endothelial cells treated with dexmedetomidine. 3. Quinacrine dihydrochloride, NDGA and indomethacin attenuated the dexmedetomidine-induced contraction of endothelium-denuded rings. Dexmedetomidine (10(-7)-10(-6) mol/L)-induced contractions of endothelium-denuded rings were enhanced compared with those of endothelium-intact rings, as were dexmedetomidine-induced contractions of endothelium-intact rings pretreated with l-NAME or tetraethylammonium. Rauwolscine attenuated dexmedetomidine-induced contractions in endothelium-intact rings pretreated with l-NAME. Dexmedetomidine (10(-6) mol/L) was found to activate NO production. 4. Taken together, the results indicate that dexmedetomidine-induced contraction of aortic rings involves activation of the lipoxygenase and cyclo-oxygenase pathways and is attenuated by increased NO production following stimulation of endothelial alpha(2)-adrenoceptors by dexmedetomidine.

Receptor-mediated activation of nitric oxide synthesis by arginine in endothelial cells

Arginine contains the guanidinium group and thus has structural similarity to ligands of imidazoline and alpha-2 adrenoceptors (alpha-2 AR). Therefore, we investigated the possibility that exogenous arginine may act as a ligand for these receptors in human umbilical vein endothelial cells and activate intracellular nitric oxide (NO) synthesis. Idazoxan, a mixed antagonist of imidazoline and alpha-2 adrenoceptors, partly inhibited L-arginine-initiated NO formation as measured by a Griess reaction. Rauwolscine, a highly specific antagonist of alpha-2 AR, at very low concentrations completely inhibited NO formation. Like L-arginine, agmatine (decarboxylated arginine) also activated NO synthesis, however, at much lower concentrations. We found that dexmedetomidine, a specific agonist of alpha-2 AR was very potent in activating cellular NO, thus indicating a possible role for alpha-2 AR in L-arginine-mediated NO synthesis. D-arginine also activated NO production and could be inhibited by imidazoline and alpha-2 AR antagonists, thus indicating nonsubstrate actions of arginine. Pertussis toxin, an inhibitor of G proteins, attenuated L-arginine-mediated NO synthesis, thus indicating mediation via G proteins. L-type Ca(2+) channel blocker nifedipine and phospholipase C inhibitor U73122 inhibited NO formation and thus implicated participation of a second messenger pathway. Finally, in isolated rat gracilis vessels, rauwolscine completely inhibited the L-arginine-initiated vessel relaxation. Taken together, these data provide evidence for binding of arginine to membrane receptor(s), leading to the activation of endothelial NO synthase (eNOS) NO production through a second messenger pathway. These findings provide a previously unrecognized mechanistic explanation for the beneficial effects of L-arginine in the cardiovascular system and thus provide new potential avenues for therapeutic development.

Clonidine induces nitric oxide- and prostaglandin-mediated vasodilation in healthy human skin

Sustained sympathetic activation not only leads to vasoconstriction but also might induce paradox vasodilation. This study was performed to explore whether and how alpha(2)-receptor stimulation mediates this vasodilation. We investigated 11 healthy subjects in 33 dermal microdialysis (MD) sessions. After nerve trunk blockade, MD fibers were inserted and perfused with physiological saline until skin trauma-related vasodilation subsided. Thereafter, fibers were perfused with either clonidine solutions (10(-3), 5 x 10(-4), 10(-4) mol/l), N(G)-monomethyl-l-arginine (L-NMMA; nitric oxide synthase blocker), acetylsalicylic acid (ASA; cyclooxygenase blocker), or combinations of these. Laser-Doppler scanning of the investigated skin revealed that clonidine not only induces vasoconstriction but subsequently also vasodilation with higher concentrations (P < 0.001). In contrast, both L-NMMA and ASA induced vasoconstriction (P < 0.001). By coapplication of 10(-3) mol/l clonidine with L-NMMA or ASA, vasodilation was partially prevented (P < 0.001). Our results demonstrate that sustained alpha(2)-receptor stimulation induces vasodilation in a dose-dependent way, which is mediated by nitric oxide and prostaglandin mechanisms in human skin.

Clonidine induces rat aorta relaxation by nitric oxide-dependent and -independent mechanisms

The alpha1- and alpha2-adrenoceptors coexist in vascular smooth muscle cells producing vascular contraction and relaxation. This study was designed to investigate which is the mechanism activated by clonidine in the rat aorta, and the endothelial factors possibly involved in the relaxation induced by clonidine. The alpha2-adrenoceptors agonist clonidine relaxed rat aortas pre-contracted with phenylephrine, with or without endothelium. In non-contracted denuded arteries, clonidine produced contractions instead of relaxation. In intact endothelium aortic rings, clonidine induced greater relaxation than in denuded aortic rings. In aortas with intact endothelium, the NO-synthase inhibitor L-NAME (10 micromol/L) and the NO-scavenger hemoglobin (10 micromol/L) reduced the relaxation to clonidine. On the other hand, indomethacin (10 micromol/L) failed to alter the relaxation induced by clonidine. These results suggest the participation of NO, but not prostacyclin in clonidine-induced relaxation. In aortic rings pre-contracted with KCl (60 mmol/L) the relaxation induced by clonidine was abolished; however, the K+ channel blockers glibenclamide (K(ATP)), tetraethylamonium (K(Ca)), and the combination of apamin and charybdotoxin (K(Ca)) did not change the relaxation induced by clonidine. The relaxation induced by clonidine on PGF2alpha-contracted arteries was not affected by prazosin. However, in the absence of prazosin, clonidine had an additional contractile effect in PGF2alpha-contracted arteries. In conclusion, our results show that in rat aorta clonidine can activate alpha2-adrenoceptors in the smooth muscle cells and alpha2-adrenoceptors in the endothelial cells that activates NO production, but not prostacyclin and/or EDHF. In the absence of phenylephrine and prazosin, clonidine can also activate alpha1-adrenoceptors and rat aorta contraction.

Role of the eNOS Glu298Asp variant on the GNB3825T allele dependent determination of alpha-adrenergic coronary constriction

The 825T allele of a polymorphism at GNB3, encoding the G protein beta(3) subunit, is associated with an enhanced coronary blood flow (CBF) reduction in response to alpha(2)-, but not to alpha(1)-adrenoceptor activation. Because the regulation of vascular tone by alpha(2)-adrenoceptors includes direct vasoconstriction as well as vasodilatation by endothelial release of nitric oxide, the eNOS Glu298Asp variant might further contribute to explain the variability of CBF reduction in response to alpha-adrenoceptor activation. Genotyping at the GNB3 and the eNOS gene was performed on 48 individuals receiving either the alpha(1)-adrenoceptor agonist methoxamine (5 mg i.c.) and/or the alpha(2)-adrenoceptor agonist BHT 933 (5 mg i.c.). CBF was calculated from quantitative coronary angiography and intracoronary Doppler flow velocity measurement. To analyse the impact of genotypes and coronary artery disease, a linear regression model was used, including cholesterol levels, heart rate, smoking and mean aortic blood pressure. An initial, univariate analysis suggested an impact of the eNOS Glu298Asp variant on alpha(2)-adrenoceptor-induced coronary constriction (CBF reduction 53.4 ch006.1 TT/TG versus 30.7 ch006.9% GG; P = 0.003). However, multifactorial analysis showed that the GNB3825T allele was associated exclusively with the CBF reduction on alpha(2)-adrenoceptor activation (58.2 +/- 4.4% TT/TC versus 27.9 +/- 4.3% CC; P < 0.0001). Contrary to the initial analysis, the Glu298Asp variant of the eNOS gene provides no additional information on the genetic basis of alpha(2)-adrenoceptor-induced coronary vasoconstriction, which appears exclusively associated to the 825T allele at GNB3. Analysis of modifying genes appears crucial for the understanding of genetic associations.

Beta(2)-adrenergic receptor gene delivery to the endothelium corrects impaired adrenergic vasorelaxation in hypertension

BACKGROUND

Impaired beta-adrenergic receptor (AR)-mediated vasorelaxation in hypertension plays a role in increased peripheral vascular resistance and blood pressure. Because the beta(2)AR is the most abundant vascular AR subtype, we sought to enhance betaAR vasorelaxation by overexpressing beta(2)ARs via adenoviral-mediated gene transfer (ADbeta(2)AR) to the vascular endothelium of the carotid artery.

METHODS AND RESULTS

In normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats, we exposed the right common carotid artery to ADbeta(2)AR in situ for 15 minutes by injection into the lumen while the blood flow was interrupted. Control carotids received an empty vector (ADempty). Three days later, transgene expression and selective endothelial localization were confirmed in infected vessels. Vasoregulation after beta(2)AR overexpression (2-fold) was studied in isolated organ baths. ADbeta(2)AR carotid responses to alpha(1)AR and alpha(2)AR agonists were not affected, whereas responses to epinephrine were altered and betaAR-mediated vasorelaxation was enhanced after beta(2)AR overexpression. As expected, betaAR-mediated vasodilatation in control carotids of SHR rats was significantly less than in similar control WKY carotid arteries. ADbeta(2)AR treatment enhanced betaAR vasorelaxation in SHR to levels similar to those seen in ADbeta(2)AR WKY carotids.

CONCLUSIONS

Our results demonstrate a critical role for the endothelium in betaAR-mediated vasorelaxation and suggest that impaired betaAR signaling may account for dysfunctional betaAR vasorelaxation in hypertension rather than impaired endothelium-dependent nitric oxide metabolism.

Endothelial G protein beta-subunits trigger nitric oxide-but not endothelium-derived hyperpolarizing factor-dependent dilation in rabbit resistance arteries

A single subtype of heterotrimeric G protein-coupled receptor controls both nitric oxide (NO) (sensitive to L-arginine analogues) and endothelium-derived hyperpolarizing factor (EDHF) (sensitive to high-external K(+) and apamine) production by the vascular endothelium leading to dilation. We hypothesized that alpha- and betagamma-subunits of the G protein serve as distinct intermediates to produce NO and EDHF. In pressurized resistance arteries, selective pinocytotic endothelial incorporation of specific antibodies (Abs) directed against alpha(q/11)-subunits abolished acetylcholine (Ach)-mediated dilation but failed to influence oxymetazoline (Oxy, alpha(2)-adrenergic receptor agonist)-induced dilation. In contrast, alpha(i1-2)-subunit Abs prevented Oxy- but not Ach-induced dilation. Thus, as expected, endothelial muscarinic and alpha(2)-adrenoceptors couple to G(q) protein and G(i) proteins, respectively. beta-subunit Abs reduced both Ach- and Oxy-induced dilation. The beta-subunit Abs abolished the nitro-L-arginine (L-NNA)-sensitive component but did not impair the high-external K(+)-sensitive component of the dilation induced by Ach and Oxy. Thus, G protein beta-subunits primarily accounted for NO production. Neutralization of Hsp90 and inhibition of the phospholipase C by U73122 (1 micromol/L) or intracellular Ca(2+) buffering with BAPTA-AM (10 micromol/L) sharply reduced NO-dependent but not K(+)-sensitive dilation. In conclusion, mobilization of the G protein beta-subunit is pivotal to NO-dependent dilation triggered through muscarinic and alpha(2)-adrenergic receptors. In contrast, receptor-operated EDHF-dependent dilation was insensitive to beta-subunit Abs. Although not directly activating the NO pathway, alpha-subunit activation is an absolute prerequisite for receptor-operated endothelium-dependent dilation of resistance arteries.

Impaired insulin-like growth factor I vasorelaxant effects in hypertension

Insulin-like growth factor I (IGF-I) can be considered a factor potentially involved in arterial hypertension not only for its growth-promoting features but also for its effects on vascular tone. Nevertheless, the actions of the hormone on vascular reactivity are still unexplored in hypertension. Therefore, the vasodilation induced by increasing doses of IGF-I and the modulation of norepinephrine vasoconstriction induced by low levels of the hormone were tested on aortic rings of spontaneously hypertensive and normotensive rats. The results indicate that the vasodilation evoked by IGF-I is impaired in hypertensive rats (Delta% of maximal vasorelaxation, 30+/-1 versus 41+/-1; P<0.01), and after the removal of endothelium or the inhibition of endothelial NO synthase, the vasodilation evoked by the hormone was blunted in both rat strains and became similar between hypertensive and normotensive rats (Delta% of maximal vasorelaxation, 21+/-1 versus 20+/-1; P=NS). Moreover, IGF-I does not show any effect on norepinephrine vasoconstriction in hypertensive rats, and this alteration may depend on the lack of sensitizing effect exerted by IGF-I on alpha(2)-adrenergic-evoked NO vasorelaxation. The defect in IGF-I vascular action is also present in young spontaneously hypertensive rats (age 5 weeks). In conclusion, our data demonstrate that IGF-I vasorelaxant properties are impaired in spontaneously hypertensive rats, suggesting that such defect may play a causative or permissive role in the development of hypertensive conditions.

Activation of Galpha s mediates induction of tissue-type plasminogen activator gene transcription by epoxyeicosatrienoic acids

The epoxyeicosatrienoic acids (EETs) are products of cytochrome P450 (CYP) epoxygenases that have vasodilatory and anti-inflammatory properties. Here we report that EETs have additional fibrinolytic properties. In vascular endothelial cells, physiological concentrations of EETs, particularly 11,12-EET, or overexpression of the endothelial epoxygenase, CYP2J2, increased tissue plasminogen activator (t-PA) expression by 2.5-fold without affecting plasminogen activator inhibitor-1 expression. This increase in t-PA expression correlated with a 4-fold induction in t-PA gene transcription and a 3-fold increase in t-PA fibrinolytic activity and was blocked by the CYP inhibitor, SKF525A, but not by the calcium-activated potassium channel blocker, charybdotoxin, indicating a mechanism that does not involve endothelial cell hyperpolarization. The t-PA promoter is cAMP-responsive, and induction of t-PA gene transcription by EETs correlated with increases in intracellular cAMP levels and, functionally, with cAMP-driven promoter activity. To determine whether increases in intracellular cAMP levels were due to modulation of guanine nucleotide-binding proteins, we assessed the effects of EETs on Galpha(s) and Galpha(i2). Treatment with EETs increased Galpha(s), but not Galpha(i2), GTP-binding activity by 3.5-fold. These findings indicate that EETs possess fibrinolytic properties through the induction of t-PA and suggest that endothelial CYP2J2 may play an important role in regulating vascular hemostasis.

Increase in renal medullary nitric oxide synthase activity protects from norepinephrine-induced hypertension

Studies were performed in conscious Sprague-Dawley rats to determine the role of the alpha(2)-adrenergic receptor-mediated increase in the renal medullary nitric oxide synthase (NOS) activity as a counterregulatory mechanism of blood pressure control in response to increased renal adrenergic stimulation. A subpressor dose of norepinephrine (NE, 8 microg. kg(-1). h(-1)) was infused intravenously, and NOS activity was determined with arginine-citrulline conversion by high-performance liquid chromatography in renal cortical and outer and inner medullary tissues. It was found that after 7 days of intravenous NE infusion, NOS activity was significantly higher in both the outer and inner medullary tissues (158+/-45 versus 30+/-24 pmol. mg(-1). h(-1) [outer medulla] and 5.1+/-0.7 versus 2.0+/-0.5 nmol. mg(-1). h(-1) [inner medulla] for NE-treated versus control rats, respectively). To determine whether the increase of NOS activity was mediated through renal medullary alpha(2)-receptors, the receptor antagonist rauwolscine (RAU, 1 microg. kg(-1). min(-1)) was infused via an implanted renal medullary interstitial catheter, and the consequences of intravenous NE administration were evaluated. NOS activity was significantly lower in the RAU-infused animals and did not increase with infusion of NE. To determine the systemic effects of the renal medullary alpha(2)-receptors, studies were performed to determine the consequences of chronic intravenous infusion of subpressor amounts of NE in the presence and absence of renal medullary alpha(2)-receptor inhibition. Under conditions in which RAU was continuously infused into the renal medulla, the same subpressor dose of NE caused sustained and reversible hypertension (mean arterial pressure increased from 120+/-3 to 131+/-3 mm Hg). Chronic blunting of the renal medullary NOS activity with N(G)-nitro-L-arginine methyl ester (75 microg. kg(-1). h(-1)) also enabled NE to produce a significant rise in mean arterial pressure (from 117+/-2 to 134+/-4 mm Hg). We conclude that the hypertensive effects of moderate elevations of renal adrenergic activity were chronically buffered by the alpha(2)-receptor-mediated increase in NOS activity within the renal medulla.

Modulation of vascular tone by nitric oxide and endothelin 1 in myometrial resistance arteries from pregnant women at term

OBJECTIVE

We evaluated the role of endothelium-derived nitric oxide and endothelin 1 in the modulation of myogenic tone, norepinephrine-induced tone, and flow-mediated responses in resistance arteries from pregnant women at term.

STUDY DESIGN

Arteries (approximately 200 microm at 50 mm Hg; n = 27) were dissected from myometrial biopsies obtained from women undergoing elective cesarean delivery at term and mounted in a pressure arteriograph. Responses to intraluminal flow, pressure, and norepinephrine were studied in the absence and presence of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine and the endothelin-converting enzyme inhibitor phosphoramidon.

RESULTS

Pressure-induced (80 mm Hg) myogenic tone was significantly enhanced after incubation with Nomega-nitro-L-arginine (33% +/- 8% vs 24% +/- 4%; P <.05), whereas phosphoramidon significantly reduced myogenic tone (24% +/- 5% vs 33% +/- 5%; P <.05). A combination of Nomega-nitro-L -arginine and phosphoramidon did not affect myogenic tone. Norepinephrine-induced tone was significantly enhanced after nitric oxide synthase inhibition (49% +/- 6% vs 41% +/- 5%; P <.05) but was not affected by phosphoramidon. Flow-mediated dilatation was increased in the presence of phosphoramidon compared with flow-induced dilatation in physiologic salt solution (maximum dilatation, 57% +/- 12% vs 30% +/- 5%; analysis of variance, P <.05), and all flow-induced dilatation was abolished by Nomega-nitro-L -arginine.

CONCLUSIONS

Nitric oxide and endothelin 1 may play a significant role in modulation of myogenic tone and flow-mediated responses in the resistance vasculature of the uterine circulation in normal pregnancy.

G protein beta3 subunit 825T allele and enhanced coronary vasoconstriction on alpha(2)-adrenoceptor activation

Recently, alpha(2)-adrenoceptor activation was shown to play an important role in the vasoconstriction of normal coronary arteries, whereas in the presence of atherosclerosis, the activation of both alpha(1)- and alpha(2)-adrenoceptors reduces coronary blood flow in humans. alpha(2)-Adrenoceptors activate pertussis toxin (PTX)-sensitive G proteins, whereas alpha(1)-adrenoceptors couple to PTX-insensitive G proteins. Thus, the 825T allele of the beta3 subunit of heterotrimeric G proteins, associated with enhanced PTX-sensitive G protein signaling, was expected to determine the alpha(2)-adrenoceptor-, but not the alpha(1)-adrenoceptor-, mediated reduction in coronary blood flow (CBF). Genotyping was performed on 48 individuals. Twelve of the 48 received the alpha(1)-adrenoceptor agonist methoxamine (MTX; 5 mg IC), and 12 received the alpha(2)-adrenoceptor agonist BHT 933 (BHT; 5 mg IC). Twenty-four additional individuals received both MTX and BHT during the same investigational procedure. CBF was calculated on the basis of coronary angiography and intracoronary Doppler flow velocity measurement. Drug-related ischemia was assessed on the basis of ST-segment changes and angina pectoris. In response to BHT, but not to MTX, CBF was reduced to a significantly greater extent in 825T allele carriers (58+/-4%, n=16) than in individuals homozygous for the C825 allele (28+/-4%, n=19, P=0.001). This finding was independent of cholesterol levels, mean arterial blood pressure, and the presence or absence of coronary artery disease. Ischemic events in response to BHT occurred more frequently in 825T allele carriers than in homozygous 825C allele carriers (P=0.01). alpha(2)-Adrenoceptor coronary vasoconstriction is genetically determined and significantly enhanced in GNB3 825T allele carriers.

Cholesterol-independent endothelial dysfunction in virgin and pregnant rats fed a diet high in saturated fat

1. Western diets high in saturated fat are associated with an increased incidence of cardiovascular diseases. In this study we have evaluated vascular endothelial function and oxidative stress in virgin rats fed a normal (VC) or high in saturated fat diet (VHF) (20 % lard and corn oil w/w) from weaning until adulthood, and throughout subsequent pregnancy (PC and PHF, respectively). 2. The saturated fat diet was associated with enhanced noradrenaline sensitivity in small mesenteric arteries from VHF rats (VHF vs. VC, P < 0.05) and blunted endothelium-dependent relaxation in VHF and PHF rats (VHF vs. VC, P < 0.001; PHF vs. PC, P < 0.05). Endothelial dysfunction was attributable to a reduced nitric oxide component of relaxation in VHF rats, and blunted prostacyclin and endothelium-derived hyperpolarizing factor components in PHF rats. 3. Other than plasma cholesterol, which was reduced in VHF and PHF rats, plasma lipids were normal. Fasting plasma insulin and glucose concentrations were raised in VHF rats (P < 0.05) and the plasma marker of oxidative stress, 8-iso PGF2alpha, was increased in PHF animals (P < 0.01). 4. These findings suggest that endothelial dysfunction induced by a saturated fat diet is cholesterol independent and likely to be of different mechanistic origin in virgin and pregnant rats.

Activation of guanine nucleotide-binding proteins and induction of endothelial tissue-type plasminogen activator gene transcription by alcohol

The mechanism by which moderate alcohol ingestion lowers the risk of cardiovascular disease is unknown but may be due, in part, to the ability of alcohol to increase the level of tissue-type plasminogen activator (t-PA). Human endothelial cells were treated with low concentrations of ethanol (0.25-25 mM, 0-24 h), which are associated with moderate alcohol consumption. Although treatment with ethanol alone did not affect t-PA gene transcription or mRNA expression, it augmented isoproterenol (ISO)-stimulated t-PA gene transcription and mRNA levels by 3.4- and 2.8-fold, respectively, and decreased plasminogen activator inhibitor-1 mRNA levels by 65%. These effects of ethanol correlated with 2.5- and 6.9-fold increases in ISO-stimulated cyclic AMP levels and 4x-cyclic AMP response element heterologous promoter activity, respectively. To determine whether alcohol-induced changes in agonist-stimulated cyclic AMP levels were because of modulation of guanine nucleotide-binding proteins (G proteins), we assessed the effects of ethanol on Galphas and Galphai2. Although ethanol did not affect the expression of Galphas or Galphai2, it increased ISO-stimulated Galphas GTPase and GTP binding activity by 2.2- and 2.9-fold and decreased UK14304-stimulated Galphai2 GTPase and GTP binding activity by 38 and 80%. These results indicate that treatment with relatively low concentrations of ethanol enhances agonist-stimulated cyclic AMP-dependent t-PA gene transcription in vascular endothelial cells through differential modulation of G protein.

Effects of endogenous and exogenous catecholamines on LPS-induced neutrophil trafficking and activation

Endotoxemia produces elevations in catecholamine levels in the pulmonary and systemic circulation as well as rapid increases in neutrophil number and proinflammatory cytokine expression in the lungs. In the present experiments, we examined the effects of endogenous and exogenous adrenergic stimulation on endotoxin-induced lung neutrophil accumulation and activation. Levels of interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha, and macrophage inflammatory protein (MIP)-2 mRNAs were increased in lung neutrophils from endotoxemic mice compared with those present in lung neutrophils from control mice or in peripheral blood neutrophils from endotoxemic or control mice. Treatment with the beta-adrenergic antagonist propranolol before endotoxin administration did not affect trafficking of neutrophils to the lungs or the expression of IL-1beta, TNF-alpha, or MIP-2 by lung neutrophils. Administration of the alpha-adrenergic antagonist phentolamine before endotoxemia did not alter lung neutrophil accumulation as measured by myeloperoxidase (MPO) levels but did result in significant increases in IL-1beta, TNF-alpha, and MIP-2 mRNA expression by lung neutrophils compared with endotoxemia alone. Administration of the alpha1-adrenergic agonist phenylephrine before endotoxin did not affect trafficking of neutrophils to the lungs but was associated with significantly increased expression of TNF-alpha and MIP-2 mRNAs by lung neutrophils compared with that found after endotoxin alone. In contrast, treatment with the alpha2-adrenergic agonist UK-14304 prevented endotoxin-induced increases in lung MPO and lung neutrophil cytokine mRNA levels. The suppressive effects of UK-14304 on endotoxin-induced increases in lung MPO were not affected by administration of the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester. These data demonstrate that the initial accumulation and activation of neutrophils in the lungs after endotoxemia can be significantly diminished by alpha2-adrenergic stimulation. Therapy with alpha2-adrenergic agents may have a role in modulating inflammatory pulmonary processes associated with sepsis-induced acute lung injury.

Impaired function of alpha-2 adrenoceptors in smooth muscle of mesenteric arteries from spontaneously hypertensive rats

The alpha2-adrenoceptor function in mesenteric arteries of spontaneously hypertensive rats (SHR) was investigated by comparing membrane potential changes in response to adrenergic agonists in preparations from female SHR, Wistar-Kyoto (WKY) and normotensive Wistar rats (NWR). Resting membrane potential was found to be less negative in mesenteric arteries from SHR than in those from NWR and WKY. Apamin induced a decrease in the membrane potential of mesenteric artery rings without endothelium from NWR and WKY, but had no effects in those from SHR. Both UK 14,304 and adrenaline, in the presence of prazosin, induced a hyperpolarization that was significantly lower in de-endothelialized mesenteric rings from SHR than in those from NWR and WKY. In mesenteric rings with endothelium, however, similar hyperpolarization was observed in the three strains. In NWR mesenteric rings with endothelium the hyperpolarization induced by activation of alpha2-adrenoceptors was abolished by apamin, whereas in intact SHR mesenteric rings this hyperpolarization was slightly reduced by apamin and more efficiently reduced by Nomega-nitro-L-arginine. It is concluded that the activity of potassium channels coupled to alpha2-adrenoceptors is altered in the smooth muscle cells of SHR mesenteric arteries, contributing to their less negative membrane potential. On the other hand, the endothelial alpha2-receptors are functioning in mesenteric vessels from SHR and their stimulation induces a hyperpolarization mainly through the release of nitric oxide.

Functional cross-talk between endothelial muscarinic and alpha2-adrenergic receptors in rabbit cerebral arteries

Interactions between two classes of receptors have been observed in several cell lines and preparations. The aim of this work was to assess the impact of simultaneous stimulation of endothelial muscarinic and alpha2-adrenergic receptors (alpha2-AR) on vascular reactivity. Rabbit middle cerebral arteries were isolated and changes in isometric tension were recorded in the presence of indomethacin. Inhibition of nitric oxide (NO) synthase with Nomega-nitro-L-arginine (L-NOARG, 100 micromol l(-1)) revealed alpha-AR-dependent contractions. Pre-addition of acetylcholine (ACH, 1 micromol l(-1)) augmented oxymetazoline (OXY, 10 micromol l(-1), alpha2-AR agonist)-, but decreased phenylephrine (PE, 10 micromol(-1), alpha1-AR agonist)-induced contraction (P<0.05). The effects of ACH were endothelium-dependent. Vessels were precontracted with 40 mmol l(-1) KCl-physiological salt solution (PSS) in the absence of L-NOARG, or PE or OXY in the presence of L-NOARG. In the presence of high external K+ or PE, ACH induced a potent relaxation (P<0.05). In the presence of OXY, however, ACH mediated contraction (P<0.05). After pertussis toxin (PTX, inactivator of Galpha(i/o) proteins) pre-treatment, alpha2-AR-dependent contractions were abolished. Forty mmol l(-1) KCl-PSS induced contraction was not altered by PTX whereas ACH-induced relaxation was augmented (P<0.05). To investigate if endothelin-1 (ET-1) intervened in the endothelium-dependent contractile response to ACH in the presence of OXY-dependent tone, vessels were incubated in the presence of BQ123 (1 micromol l(-1)), an ETA receptor antagonist. OXY-mediated tone was not affected by BQ123; however, ACH-induced contraction was reversed to a relaxation (P<0.05). These data indicate that activation of endothelial alpha2-AR triggers an endothelium-dependent, ET-1 mediated, contraction to ACH. This suggests that activation of alpha2-AR affects muscarinic receptor/G protein coupling leading to an opposite biological effect.

Endothelial modulation of neural sympathetic vascular tone in canine skeletal muscle

The effect of nitric oxide synthase (NOS) inhibition and endothelin-A (ETA)-receptor blockade on neural sympathetic control of vascular tone in the gastrocnemius muscle was examined in anesthetized dogs under conditions of constant flow. Muscle perfusion pressure (MPP) was measured before and after NOS inhibition (Nomega-nitro-L-arginine methyl ester; L-NAME) and ETA-receptor blockade [cyclo-(D-Trp-d-Asp-Pro-D-Val-Leu); BQ-123]. Zero and maximum sympathetic nerve activities were achieved by sciatic nerve cold block and stimulation, respectively. In group 1 (n = 6), MPP was measured 1) before nerve cold block, 2) during nerve cold block, and 3) during nerve stimulation. Measurements under these conditions were repeated after L-NAME and then BQ-123. The same protocol was followed in group 2 (n = 6) except that the order of L-NAME and BQ-123 was reversed. MPP and muscle vascular resistance (MVR) increased after L-NAME and then decreased to control values after BQ-123. MVR decreased after BQ-123 alone and, with the addition of L-NAME, increased to a level not different from that observed during the control period. MVR fell during nerve cold block. This response was not affected by administration of L-NAME followed by BQ-123, but it was attenuated by administration of BQ-123 before L-NAME. The constrictor response during sympathetic nerve stimulation was enhanced by L-NAME; no further effect was observed with BQ-123, nor was the response affected when BQ-123 was given first. These findings indicate that endothelin contributes to 1) basal vascular tone in skeletal muscle and 2) the increase in skeletal muscle vascular resistance after NOS inhibition. Finally, nitric oxide "buffers" the degree of constriction in skeletal muscle vasculature during maximal sympathetic stimulation.

Expression of G-protein subtypes in cultured cerebral endothelial cells

This paper describes Western-blotting evidence for the presence of various guanine nucleotide binding proteins, G-proteins in cultured rat cerebral endothelial cells (CECs) and two immortalized cerebral endothelial cell lines, RBE4 and GP8. By using specific antibodies raised against known sequences of appropriate G-protein types that were previously characterized, we demonstrated the presence of Gsalpha, Gi2alpha, Gi3alpha, Gq/11alpha, Goalpha and Gbeta in cell lysates of primary cultures of CECs, and plasma membranes of RBE4 and GP8 cells. The appearance of Goalpha proteins in CECs might be of special importance, since they were not detected in peripheral endothelial cells in previous studies. Isoproterenol and bradykinin displayed significant, dose-dependent stimulation of [35S]GTPgammaS binding above basal values. This assay, reflecting the GDP-GTP exchange reaction on Galpha-subunits by receptor agonists, suggested that there were functional, G-protein coupled beta-adrenergic and bradykinin receptors in these systems. No significant stimulation of [35S]GTP7gammaS binding was noted with serotonin under our experimental conditions. Since stimulation of [35S]GTPgammaS binding by isoproterenol and bradykinin was additive, it was concluded that different Galpha proteins were activated by these two ligands. In analogy to other systems, activation of Gs is most likely by isoproterenol, while Gi and/or Gq/11 proteins might be activated by bradykinin receptors. The possible significance of the receptors and G-proteins detected is being discussed in the functioning of cerebral endothelium, and thus the blood-brain barrier.

Nitric oxide inhibits alpha2-adrenoceptor-mediated endothelium-dependent vasodilation

This study was designed to investigate the interaction between the NO/L-arginine pathway and the alpha2-adrenoceptor-mediated endothelium-dependent vasorelaxation. Reactivity of isolated resistance mesenteric arterial segments from mice lacking the gene for constitutive endothelial NO synthase (eNOS- mice, n=14) and from their wild-type controls (WT mice, n=46) was studied in isometric conditions in the presence of indomethacin (blocker of cyclooxygenase). Oxymetazoline (OXY, 0.01 to 30 micromol/L; a selective alpha2-adrenoceptor agonist) induced an endothelium-dependent relaxation of eNOS- but not WT arteries preconstricted either with phenylephrine or serotonin. In the presence of Nomega-nitro-L-arginine (l-NNA, 100 micromol/L), an inhibitor of NOS, OXY induced an endothelium-dependent relaxation of WT mesenteric arteries. l-NNA had no effect on the relaxation caused by OXY in eNOS- arterial rings. Therefore, the relaxation caused by OXY was independent of NO formation. To demonstrate the inhibitory role of NO on the alpha2-adrenoceptor-mediated relaxation, subthreshold (0.1 nmol/L) to threshold (1 nmol/L) concentrations of sodium nitroprusside (donor of NO) were added to l-NNA-treated arteries before OXY challenges: in these conditions, the alpha2-adrenoceptor-mediated relaxation of eNOS- and WT arteries was inhibited. OXY-induced relaxation was restored on readdition of methylene blue (1 micromol/L, inhibitor of guanylate cyclase), suggesting that cGMP may be the mechanism of inhibition of the alpha2-adrenergic pathway in the presence of NO. Finally, OXY-mediated relaxation was blocked by tetraethylammonium (1 mmol/L) but not glibenclamide (1 micromol/L), suggesting the involvement of an endothelium-derived hyperpolarizing factor that activates Ca2+-activated K+ channels. In conclusion, alpha2-adrenoceptor activation caused relaxation of isolated murine mesenteric arteries that was functionally blocked by NO through a mechanism that may involve activation of the soluble guanylate cyclase and cGMP formation. The endothelium-dependent alpha2-adrenoceptor-mediated relaxation is likely to be due to an endothelium-derived hyperpolarizing factor, whose release and/or production is reduced by concurrent NO formation.

Endothelin-1 regulates tone of isolated small arteries in the rat: effect of hyperendothelinemia

Chronic elevation of plasma endothelin-1 (ET-1) levels has been reported in several pathological conditions. To investigate the consequences of increased circulating ET-1 on vascular responsiveness, Sprague-Dawley rats (n=16) were chronically instrumented with a minipump delivering ET-1 at a constant dose for 7 days. Plasma ET-1 levels were more than doubled in treated (0.98+/-0.09 pmol/L; P<.05) versus untreated sham-operated rats (0.43+/-0.04 pmol/L), whereas systolic arterial blood pressure increased (139+/-5 versus 128+/-4 mm Hg in untreated rats; P<.05). After rats were killed, segments of middle cerebral (MCA) and mesenteric (MES) arteries were mounted on an isometric myograph. ET-induced contraction was shifted to the right in ET-1-treated animals and not modified by BQ123 (an ET(A) receptor antagonist); bosentan (ET(A/B) receptor antagonist) prevented ET-1-induced contraction in both groups. After inhibition of nitric oxide synthase with N(omega)-nitro-L-arginine (L-NNA), both phenylephrine and oxymetazoline (an alpha2-adrenoceptor agonist) induced MCA contraction. The sensitivity to phenylephrine was decreased in ET-1-treated compared with control rats (P<.05). Sensitivity to phenylephrine-induced contraction was decreased by BQ123 in control rats only. In contrast, L-NNA revealed greater oxymetazoline-induced contractions in treated compared with control MCA rings (P<.05); this potentiation was blunted by bosentan but unaffected by BQ123. Removal of the endothelium revealed a direct constrictor effect of oxymetazoline that was insensitive to L-NNA alone or combined with bosentan; however, oxymetazoline induced significantly lower constriction in treated rat MCA segments. Responses to oxymetazoline were also blunted in treated compared with untreated denuded MES arteries. In conclusion, chronic elevated plasmatic ET-1 decreases smooth muscle cell sensitivity to contractile agonists both in MCA and MES rings. In cerebral vessels, endothelial alpha2-adrenoceptor-dependent stimulation induced greater contractile responses in treated rats which were sensitive to bosentan, suggesting that oxymetazoline stimulates ET-1 release from the endothelium. This may represent a compensatory mechanism for the loss of smooth muscle sensitivity.

Postjunctional alpha 2-adrenoceptors are not present in proximal arterioles of rat intestine

The purpose of this study was to evaluate two potential stimuli for nitric oxide (NO) release in rat intestinal arterioles during sympathetic nerve activation. To determine whether these vessels contain endothelial alpha 2-adrenoceptors linked to the L-arginine-NO pathway, intravital microscopy was used to study the response of first-order arterioles (1As, 20-40 microns ID) to direct application of 1) the selective alpha 2-agonist BHT-933 and 2) norepinephrine (NE) or sympathetic nerve stimulation before and after alpha 1- or alpha 2-receptor blockade. The effect of sympathetic nerve stimulation on 1A wall shear rate (WSR) was also determined to evaluate the possibility of hemodynamic shear stress as a stimulus for NO release. BHT-933 had no effect on 1A diameter, whereas NE produced dose-dependent constrictions of 5 +/- 3 to 15 +/- 3 microns, which were usually abolished by the alpha 1-antagonist prazosin but unaffected by the alpha 2-antagonist idazoxan. Sympathetic nerve stimulation at 3, 8, and 16 Hz induced constrictions of 4 +/- 1, 8 +/- 2, and 17 +/- 4 microns, respectively, and these constrictions were also usually abolished by prazosin but unaffected by idazoxan. Resting WSR averaged 1,997 +/- 163 s-1 and decreased to 1,587 +/- 209, 1,087 +/- 195, and 537 +/- 99 s-1 during 3-, 8-, and 16-Hz nerve stimulation. These results suggest that alpha 2-adrenoceptor-dependent pathways do not influence either resting tone or sympathetic constriction of proximal arterioles in the intestinal submucosa and that luminal shear stress in these vessels significantly decreases with sympathetic constriction. It therefore appears unlikely that either alpha 2-receptor activation or changes in hemodynamic shear serve as stimuli for arteriolar NO release during periods of increased sympathetic nerve activity.

17Beta-estradiol stimulates flow-induced vasodilatation in isolated small mesenteric arteries from prepubertal femal rats

OBJECTIVE

Flow-induced vasodilatation may contribute to lowering of peripheral resistance to pregnancy. This study investigated modulation of flow responses by 17beta-estradiol.

STUDY DESIGN

Small mesenteric arteries from prepubertal female Wistar rats were pretreated for 3 hours with 17beta-estradiol (10(-7) mol/L) with 17alpha-estradiol (10(-7) mol/L) or vehicle (0.1% dimethylsulfoxide). Responses to intraluminal flow were evaluated with use of a pressure arteriograph.

RESULTS

After pretreatment with 17beta-estradiol arteries relaxed to flow, whereas those treated with vehicle or 17alpha-estradiol did not (percent change in diameter at maximum flow rate after 17beta-estradiol 38.7% +/- 5.7%, n = 10, vs 1.1% +/- 4.3%, n = 10 after vehicle; p < 0.01). Endothelium removal or pretreatment with either a nitric oxide synthase inhibitor or a novel soluble guanylyl cyclase inhibitor diminished the response to 17beta-estradiol.

CONCLUSION

17Beta-estradiol stimulated nitric oxide-mediated flow-induced relaxation. Through this pathway 17beta-estradiol could play an important role in the control of vascular tone.

Insulin enhances endothelial alpha2-adrenergic vasorelaxation by a pertussis toxin mechanism

To investigate whether insulin effect on endothelium is related to a specific signal transduction pathway or reflects a more generalized action of the hormone, we studied in aortic rings of Wistar-Kyoto (WKY) rats the effects of the hormone on endothelium-dependent relaxations generated by acetylcholine, adenosine diphosphate, the selective alpha2-adrenergic agonist UK 14,304, and the calcium ionophore ionomycin. The responses were evaluated both in control conditions and after 30 minutes of exposure to three different levels of insulin (30, 100, and 500 microU/mL). Insulin failed to modify the phenylephrine aortic contractions and the relaxations induced by acetylcholine, adenosine diphosphate, and ionomycin. In contrast, both 100 and 500 microU/mL insulin were able to potentiate the UK 14,304-induced vasorelaxation (+96+/-19% and +91+/-12%, respectively). Pertussis toxin, which causes alpha2-adrenergic receptor Gi uncoupling, reduced the alpha2-adrenergic vasorelaxation and prevented the insulin potentiation of the response to UK 14,304. Furthermore, in primary cultured aortic endothelial cells from WKY, we evaluated the conversion of [3H]arginine to [3H]citrulline in response to acetylcholine, ionomycin, and UK 14,304, both in control conditions and during insulin exposure. Again, insulin did not affect basal citrulline production or the increase induced by acetylcholine and ionomycin, whereas it potentiated the response to UK 14,304. Finally, in aortic rings of spontaneously hypertensive rats, insulin treatment (100 and 500 microU/mL) was unable to enhance the alpha2-adrenergic vasodilator response; in vascular endothelial cells from spontaneously hypertensive rats, insulin did not potentiate the increase in citrulline production evoked by UK 14,304. In conclusion, insulin selectively enhances alpha2-adrenergic endothelial vasorelaxation through a pertussis toxin-sensitive mechanism, by potentiating endothelial nitric oxide production. This vasorelaxant mechanism is altered in spontaneously hypertensive rats.

Role of nitric oxide in the regulation of vascular tone in pressurized and perfused resistance myometrial arteries from term pregnant women

OBJECTIVE

Our purpose was to evaluate flow-induced responses, myogenic tone, and norepinephrine-induced constriction in myometrial resistance arteries from normal term pregnant women and the role that nitric oxide and prostanoids may play in these responses.

STUDY DESIGN

Arteries (approximately 200 microns, n = 14, at 40 mm Hg) were dissected from myometrial biopsy specimens from women undergoing cesarean section and then were mounted in a pressure arteriograph. Responses to intraluminal flow, pressure, and a constrictor agonist (norepinephrine 10(-6) mol/L) were studied in the absence and presence of N omega-nitro-L-arginine methyl ester (n = 7) or indomethacin (n = 5). Myogenic and norepinephrine-induced tone were calculated after the determination of artery diameter in the absence of extracellular calcium.

RESULTS

Arteries developed myogenic tone (80 mm Hg) that was not modulated by nitric oxide or prostanoid release, whereas norepinephrine-induced tone was significantly enhanced by the nitric oxide inhibitor. An increase in intraluminal flow led to dilatation in physiologic salt solution and indomethacin, but to constriction in the presence of N omega-nitro-L-arginine methyl ester (percent increase in diameter at flow rate of 184.6 microliters/min, 24% +/- 8% in physiologic salt solution and 20% +/- 4% in the presence of indomethacin versus -27% +/- 12% in N omega-nitro-L-arginine methyl ester alone and -21% +/- 10% in indomethacin and N omega-nitro-L-arginine methyl ester, respectively, analysis of variance, p < 0.05).

CONCLUSIONS

Flow-induced shear stress is a physiologic modulator of vascular tone in myometrial arteries from pregnant women. Nitric oxide, but not prostanoids, mediates this response and also blunts norepinephrine constriction. Nitric oxide may play a fundamental role in the maintenance of adequate blood supply to the fetus during human pregnancy.

Reversal of endothelin-1 release by stimulation of endothelial alpha2-adrenoceptor contributes to cerebral vasorelaxation

Agonists acting on the vascular endothelium can modulate the release of a number of factors that interact with the surrounding smooth muscle cells and influence their tone. One such factor is the vasoconstricting agent endothelin-1 (ET-1), which has been implicated in several disease states, including stroke. However, very little is known about the physiological role of ET-1 in the cerebral circulation. We demonstrate that activation of alpha2-adrenoceptors in human pial artery endothelial cells reduces both constitutive and agonist-stimulated release of immunoreactive ET-1. That this has physiological relevance is supported by our demonstration that in segments of rabbit middle cerebral arteries, alpha2-adrenoceptor activation reduces the release of endothelium-derived ET-1 and causes an endothelium-dependent relaxation. The adrenoceptor-dependent relaxation was not blocked by combined addition of indomethacin and N omega-nitro-L-arginine in 25 mmol/L KCl-depolarizing physiological solution but was selectively antagonized by a subthreshold concentration of exogenous ET-1. Our data suggest that activation of endothelial alpha2-adrenoceptor would favor a decrease in ET-1 production and possibly promote vascular relaxation.

Modulation of bradykinin receptor ligand binding affinity and its coupled G-proteins by nitric oxide

To determine whether nitric oxide (NO) can modulate bradykinin (BK) signaling pathways, we treated endothelial cells with an NO donor, S-nitrosoglutathione (GSNO), to determine its effect(s) on G-proteins (Gi and Gq) that are coupled to the type II kinin (BK2) receptor. Radioligand binding assays and Western analyses showed that GSNO (10-500 microM, 0-72 h) did not alter the expression of BK2 receptor, Gi, or Gq. However, GSNO caused a 6-fold increase in basal cGMP production and decreased high affinity BK bindings sites and GTPase activity by 74 and 85%, respectively. The cGMP analogue, dibutyryl-cGMP, also inhibited BK-stimulated GTPase activity by 74% suggesting that some of the effects of NO may be mediated through activation of guanylyl cyclase. The NO synthase inhibitor, Nomega-monomethyl-L-arginine, inhibited endogenous NO synthase activity and cGMP production by 91 and 76%, respectively, but increased BK-stimulated GTPase activity by 61%. To determine which G-proteins are affected by NO, we performed GTP binding assays with [35S]GTPgammaS followed by immunoprecipitation with specific G-protein antisera. Both GSNO and dibutyryl-cGMP increased basal G-protein GTP binding activities by 18-26%. However, GSNO decreased BK-stimulated Galphai2, Galphai3, and Galphaq/11 GTP binding activity by 93, 61, and 90%, respectively, whereas epinephrine-stimulated Galphas GTP binding activity was unaffected. These results suggest that NO can modulate BK signaling pathways by selectively inhibiting G-proteins of the Gi and Gq family.

Contribution of arachidonic acid metabolites to reduced norepinephrine-induced contractions in hypercholesterolemic rabbit aortas

Because alterations in the aortic metabolism of arachidonic acid and in vascular responsiveness occur in hypercholesterolemic rabbits, we hypothesized that an arachidonic acid metabolite may contribute to the regulation of vascular tone. Aortic contractions to norepinephrine were investigated in rabbits fed either standard chow or chow containing 2% cholesterol. In normal rabbits, norepinephrine (10(-6) M) elicited a 126 +/- 2% contraction compared with a 95 +/- 2% contraction in cholesterol-fed rabbits. The factor mediating the depressed response was endothelium-dependent because removal of the endothelium blocked the decrease in norepinephrine-induced contractions observed in the cholesterol-fed rabbits. The endothelium-derived factor was not nitric oxide, because blockade of nitric oxide synthase with nitro-L-arginine did not abolish the decreased response in the cholesterol-fed rabbits. Pretreatment of aortas with a cyclooxygenase inhibitor, indomethacin (10(-5) M) caused a slight decrease in the norepinephrine-induced contractions, suggesting that the factor could be a vasoconstrictor cyclooxygenase metabolite or a vasodilatory lipoxygenase or cytochrome P450 epoxygenase metabolite. Pretreatment with the thromboxane A2/prostaglandin H2-receptor antagonist, SQ 29458, had no effect on norepinephrine-induced contractions. Whereas the lipoxygenase inhibitor, nordihydroguaiaretic acid (5 x 10(-5) M), caused a slight increase in the contractions to norepinephrine in cholesterol-fed rabbits compared with normal rabbits, the cytochrome P450 epoxygenase inhibitor, metyrapone (10(-4) M), produced a greater enhancement of norepinephrine-induced contractions in cholesterol-fed rabbits but had no effect on responses in the normal rabbits. Characterization of [3H]arachidonic acid metabolism in cholesterol-fed aortic tissue indicated that norepinephrine stimulated the synthesis of both lipoxygenase and epoxygenase metabolites in an endothelium-dependent manner. This study demonstrated that (a) an endothelium-derived metabolite of arachidonic acid regulates vascular tone, (b) this metabolite appears to be a lipoxygenase or cytochrome P450 product or both, and (c) the activity or synthesis of the factor is enhanced by hypercholesterolemia.

Endothelium-dependent relaxation by alpha 2-adrenoceptor agonists in spontaneously hypertensive rat aorta

Differences in alpha(2)-adrenoceptor-induced relaxation of the aorta between stroke-prone spontaneously hypertensive rats (SHRSP) and control normotensive Wistar Kyoto rats (WKY) were studied. Changes in the tension of ring preparations of the aortas were measured isometrically. Relaxation was observed in the preparations precontracted in the presence of ONO-11113, a thromboxane A(2) analogue. The alpha(2)-agonist clonidine and UK-14304 induced dose-dependent relaxation in both the WKY and SHRSP preparations. The relaxation was impaired in the SHRSP preparation. A modified sandwich experiment showed that the relaxing substance from the SHRSP endothelium was decreased. Acetylcholine (ACh) also induced dose-dependent relaxation, and the relaxation was impaired in the SHRSP preparations. alpha(2)-Agonists induced a greater degree of impairment in the relaxation than did ACh. The relaxation induced by alpha(2)-agonists and by ACh was blocked by N G-nitro-L-arginine (L-NNA). Indomethacin improved the relaxation induced by ACh but not that induced by alpha(2)-agonists in the SHRSP aortas. These results suggest that the impairment of relaxation by alpha(2)-agonists in SHRSP is not caused by the increase in the release of endothelium-derived contracting factor (EDCF) but by the reduction in the release of nitric oxide (NO). Alteration of the alpha(2)-adrenoceptors and/or the intracellular mechanism through which NO is synthesized by stimulation of the alpha(2)-adrenoceptors may be the cause of the reduction in relaxation.

Oxidized low-density lipoprotein decreases the expression of endothelial nitric oxide synthase

The atherogenic effects of low-density lipoprotein (LDL) may be mediated, in part, by its effect(s) on endothelial-derived nitric oxide (NO). To determine whether LDL can modulate NO production by changing NO synthase expression, we treated human saphenous vein endothelial cells with increasing concentrations of native or oxidized LDL (0-100 micrograms/ml) for various durations (0-72 h). Oxidized, but not native LDL caused a time-dependent decrease in steady-state NO synthase mRNA levels. This coincided with a maximal 56% decrease in NOS activity was determined by [3H]arginine to [3H]citrulline conversion. In the presence of actinomycin D, treatment with oxidized LDL reduced the half-life of NO synthase mRNA from 36 to 10 h. This decrease in NO synthase mRNA correlated with the degree of LDL oxidation and was attenuated by pretreatment with cycloheximide. Nuclear run-off studies showed a biphasic transcriptional pattern of NO synthase gene with an initial 25% decrease during the first 6 h followed by a maximal 2.2-fold increase over baseline during the subsequent 18 h. These results indicate that oxidized LDL regulates endothelial NOS expression through a combination of early transcriptional inhibition and post-transcriptional mRNA destabilization.