Vasodilatation produced by adenosine in isolated rat perfused mesenteric artery: a role for endothelium

Adenosine and adenosine triphosphate (ATP) induced vasodilatation was studied in isolated rat perfused mesenteric artery at constant flow. Decrease in perfusion pressure was measured after induction of tone by continuous infusion with phenylephrine (5–7 M). Adenosine and ATP caused dose-dependent vasodilatation. Following infusion with selective A₂ adenosine receptor antagonist, 3,7-dimethyl-l-propargylxanthine (DMPX) (10 M), or non-selective adenosine receptor antagonist, theophylline (30 M), vasodilatation produced by adenosine were significantly reduced at lower doses. Responses to adenosine were not affected by pretreatment of tissues with either the P₂-purinoceptor desensitizing agent, , methylene ATP (30 M), or the P₂-purinoceptor antagonist, suramin (10 M). In contrast, both , methylene ATP and suramin significantly attenuate relaxation produced by ATP. Further, it was found that relaxation elicited by either adenosine or ATP was not significantly affected by the presence of glibenclamide (30 M). Vasodilatation induced by adenosine and ATP was greatly reduced in denuded arteries but more so for ATP than adenosine. It is concluded that adenosine-mediated vasodilatation may hardly be due to the stimulation of A₂ adenosine receptors and is strongly dependent on the presence of functional endothelium whereas ATP-mediated vasodilator responses were mediated via the activation of P_2y-purinoceptors and appeared to be entirely dependent upon the presence of functional endothelium. Further, vasodilator responses to neither adenosine nor ATP were sensitive to inhibition by the potassium channel blocker glibenclamide, in isolated mesenteric perfused bed. This would imply that ATP-sensitive potassium channels were not involved in adenosine and ATP mediated vasodilatation.     

Modelling the changes due to the endothelium and hypertension in the alpha-adrenoreceptor-mediated responses of rat aorta

The present study focuses on the role of endothelium on alpha1-adrenoceptor-mediated vasoconstriction in aorta from Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). To define and quantify changes in the parameters governing agonism at alpha1-adrenoceptor by hypertension and/or endothelium, the operational model of analysis was used. In either endothelium intact or denuded aorta, the sensitivity (P < 0.001) and the maximum contraction (P < 0.05) to phenylephrine were smaller in SHR than in WKY. However, in each strain of rats, removal of endothelium increased the sensitivity (P < 0.001) to phenylephrine but reduced (P < 0.05) KCl-evoked vasoconstriction, suggesting a modulation of these responses by the endothelium. The observed differences in sensitivity and maximum contraction are interpreted in terms of the operational parameters: Em, the maximum possible effect; pK(A), the agonist affinity; alpha, the agonist efficacy and n, the slope for the function relating receptor occupancy to pharmacological effect. The estimated parameters reflected differences, between strains, in the signal transduction processes linked to alpha1-adrenoceptor stimulation ascribed to the presence of the endothelium. N(G)-nitro-L-arginine methyl ester (L-NAME), enhanced to a similar extend in both rat strains the sensitivity (P < 0.001) and the maximum contraction to phenylephrine. Indomethacin reduced the maximum contraction to phenylephrine by 56.85% in SHR and by 11.80% in WKY suggesting that contracting prostanoids play a more major role in this response in SHR than in WKY. Nevertheless, these inhibitors were without effect on denuded vessels from both strains suggesting that NO and cyclooxygenase products from the media or the adventitia do not play a role on the phenylephrine-mediated responses. The studied endothelial factors partially explain the observed differences in modulation of alpha1-adrenoceptor responses by the endothelium but suggest the participation of other compounds released by the endothelium.     

Modelling the changes due to the endothelium and hypertension in the α-adenoreceptor-mediated responses of rat aorta

1 The present study focuses on the role of endothelium on α₁-adrenoceptor-mediated vasoconstriction in aorta from Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). To define and quantify changes in the parameters governing agonism at α₁-adrenoceptor by hypertension and/or endothelium, the operational model of analysis was used. 2 In either endothelium intact or denuded aorta, the sensitivity (P < 0.001) and the maximum contraction (P < 0.05) to phenylephrine were smaller in SHR than in WKY. 3 However, in each strain of rats, removal of endothelium increased the sensitivity (P < 0.001) to phenylephrine but reduced (P < 0.05) KCl-evoked vasoconstriction, suggesting a modulation of these responses by the endothelium. The observed differences in sensitivity and maximum contraction are interpreted in terms of the operational parameters: E_m, the maximum possible effect; pK_A, the agonist affinity; α, the agonist efficacy and n, the slope for the function relating receptor occupancy to pharmacological effect. The estimated parameters reflected differences, between strains, in the signal transduction processes linked to α₁-adrenoceptor stimulation ascribed to the presence of the endothelium. 4 N^G-nitro- l -arginine methyl ester (L-NAME), enhanced to a similar extend in both rat strains the sensitivity (P < 0.001) and the maximum contraction to phenylephrine. Indomethacin reduced the maximum contraction to phenylephrine by 56.85% in SHR and by 11.80% in WKY suggesting that contracting prostanoids play a more major role in this response in SHR than in WKY. Nevertheless, these inhibitors were without effect on denuded vessels from both strains suggesting that NO and cyclooxygenase products from the media or the adventitia do not play a role on the phenylephrine-mediated responses. 5 The studied endothelial factors partially explain the observed differences in modulation of α₁-adrenoceptor responses by the endothelium but suggest the participation of other compounds released by the endothelium.     

Relaxation of rat arteries by urocortin: effects of gender and diabetes

Urocortin is a peptide recently identified, structurally related to corticotropin releasing factor (CRF). We have compared the effects of urocortin in different vascular beds, and have investigated whether there are gender differences in these effects or whether they are altered by diabetes. We have studied the response of isolated segments (2-mm long) from basilar, coronary and tail arteries to urocortin. The segments were obtained from male and female, normoglycaemic and streptozotocin-induced diabetic rats. In the arterial segments precontracted with endothelin-1, urocortin produced concentration-dependent relaxation, and the order of sensitivity was: tail > basilar > coronary. This relaxation was similar in arteries from male and female, diabetic and normoglycaemic rats. In tail arteries from normoglycaemic male rats, the cyclooxygenase inhibitor meclofenamate (10(-5) M) increased the relaxation to urocortin, and the inhibitor of nitric oxide synthesis N(omega)-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) or the potassium-channel-blocker charybdotoxin (10(-7) M) did not modify it. In tail arteries from normoglycaemic female rats meclofenamate, charybdotoxin or L-NAME did not modify the relaxation to urocortin. These results suggested that urocortin produced vasodilation which showed regional differences between basilar, coronary and tail arteries, but was not affected by diabetes. The mechanisms underlying this relaxation in tail arteries might differ between males and females.     

Increased influence of endothelium in obese Zucker rat aorta

The ability of endothelium to alter contractile events in phenylephrine (PE)-triggered contraction has been tested on ring segments of the thoracic aorta removed from obese Zucker rats (plasma cholesterol 3.63 mM; n = 8) and from age matched lean rats (plasma cholesterol 2.38 mM; n = 8). In normal medium, PE (1 microM) elicited similar contractions in endothelium-denuded arteries of both strains. However, the presence of endothelium reduced these contractile events and the endothelium-dependent relaxation induced by carbachol (10 microM) was higher in obese rats. In rings incubated in Ca2+ free medium containing EGTA (1 mM), PE (1 microM) induced a phasic contraction and a sustained contraction following addition of Ca2+ (2.5 mM) to the medium. The phasic contraction was due to intracellular Ca2+ release, whereas the sustained response was dependent on extracellular Ca2+ influx. In endothelium-free preparations, the size of both the phasic and sustained contraction was similar for the two strains. The Ca2+ antagonist gallopamil (1 microM) reduced the sustained contraction of lean (24%) and obese (34%) rats without affecting the phasic contraction. In preparations possessing endothelium, the sustained, but not the phasic contraction, of both strains was inhibited. This inhibitory effect of endothelium on the sustained contraction was significantly higher in obese than in lean rats. Thus, it can be concluded that phenylephrine elicited quantitatively and qualitatively similar contractions in obese and lean rats. In both strains, the endothelium diminished the contraction induced by PE, however, this effect was more pronounced in obese rats than in lean ones. These results may explain, in part, the described absence of atherosclerotic lesions in the obese strain.     

The modulatory role of vascular endothelium in the interaction of agonists and antagonists with alpha-adrenoceptors in the rat aorta.

1. We have examined the effect of endothelium on the antagonistic action of prazosin, doxazosin, yohimbine and phentolamine against phenylephrine, clonidine and noradrenaline. 2. The action of prazosin against phenylephrine was similar to that earlier reported against noradrenaline, acting as a non-competitive antagonist in the presence of endothelium and as a competitive antagonist in the absence of endothelium. Prazosin also acted as a non-competitive antagonist against clonidine in the absence of endothelium. 3. Doxazosin behaved in a similar way to prazosin against noradrenaline, phenylephrine and clonidine acting as a non-competitive antagonist in the presence of endothelium and as competitive antagonist after removal of endothelium. In contrast, yohimbine and phentolamine acted as competitive antagonists both in the presence and in the absence of endothelium. 4. Analysis of the concentration-response curves for noradrenaline, phenylephrine and clonidine in the presence and in the absence of endothelium showed that the affinity for all three agonists was the same but not the efficacy and the receptor reserve, both of which were lower in the presence than in the absence of endothelium. 5. The rank order of agonist potency in the absence of endothelium was noradrenaline greater than phenylephrine greater than clonidine. The rank order of antagonist potency was prazosin greater than or equal to doxazosin greater than phentolamine greater than yohimbine. 6. The results show that vascular endothelium modulates the contractile response to alpha-adrenoceptor agonists and also modifies the action of the antagonists prazosin and doxazosin but not that of yohimbine and phentolamine. This effect of endothelium was related to a change in agonist efficacy and receptor reserve. These results also suggest that the alpha-adrenoceptors of the isolated aorta of the rat are predominantly, if not exclusively of the alpha 1-subtype.     

Role of endothelium in the contractions induced by norepinephrine and clonidine in rat aorta

The inhibitory effects of endothelium-derived relaxing factor (EDRF) on the contractions induced by norepinephrine and clonidine in rat aorta were examined. Carbachol induced a relaxation of norepinephrine-induced contraction in rat aorta with endothelium. Removal of endothelium inhibited the carbachol-induced relaxation and increased the magnitude of norepinephrine-induced contraction. Quinacrine, a phospholipase A2 inhibitor, methylene blue, a guanylate cyclase inhibitor and tetraethylammonium, a potassium permeability inhibitor, inhibited carbachol-induced relaxation and augmented the magnitude of norepinephrine-induced contraction only when endothelium was present. Clonidine induced a contraction when endothelium was removed or muscle was treated with methylene blue. The contractions induced by norepinephrine and clonidine were equally sensitive to prazosin and equally less sensitive to yohimbine. Clonidine inhibited the norepinephrine-induced contraction, whereas it potentiated the angiotensin 11- or 12 mM K-induced contractions in the aorta with endothelium. The inhibitory effect of clonidine on the norepinephrine-induced contraction was reduced by endothelium-removal and by methylene blue but not by yohimbine. These results suggest that norepinephrine has a strong direct stimulating action and clonidine has a weak one on vascular smooth muscle cells possibly mediated by alpha 1-adrenoceptors, and their contractile effects are inhibited by the spontaneously released EDRF.     

Relaxation of rabbit aorta by chlordimeform

Chlordimeform N'-(4-choro-O-tolyl)-N,N-dimethyl-formamidine; CDF), an acaracide-insecticide, relaxed helically cut strips of rabbit thoracic aorta and deadventitiated aorta strips. Relaxations by CDF of aorta strips contracted by 5 x 10(-7) M prostaglandin F2 alpha were not altered by the antimuscarinic agent atropine (10(-5) M), the beta-adrenergic antagonist propranolol (10(-5) M), the histamine H2 antagonist cimetidine (2 x 10(-4) M), the dopamine antagonist haloperidol (10(-5) M) or the Na+-K+-ATPase inhibitor ouabain (5 x 10(-6) M). CDF further relaxed deadventitiated strips contracted by 10(-6) M NE or 40 mM potassium and then partially relaxed by 10(-5) M verapamil. CDF (10(-5) M) potentiated contractions caused by the addition of calcium to potassium depolarized strips in zero calcium media, but higher concentrations reduced calcium-induced contractions. The highest CDF concentration (10(-2) M) not only prevented tension increase on calcium addition, but also progressively decreased tension to below baseline levels. The addition of 10(-2) M CDF to aorta strips not contracted by a vasoactive agent decreased resting tension. This relaxation was usually maximal at 10(-3) M and less at 10(-2) M. Thus, CDF relaxes vascular smooth muscle, not by antagonism at the usual vascular relaxant receptors, but by interference with calcium utilization.     

Analysis of adenosine vascular effect in isolated rat aorta: possible role of Na+/K+-ATPase

The present experiments were undertaken in order to examine the effect of adenosine in isolated rat aorta, to investigate the possible role of intact endothelium and endothelial relaxing factors in this action and to determine which population of adenosine receptors is involved in rat aorta response to adenosine. Adenosine (0.1-300 microM) produced concentration-dependent (intact rings: pD2=4.39+/-0.09) and endothelium-independent (denuded rings: pD2=4.52+/-0.12) relaxation of isolated rat aorta. In the presence of high concentration of K+ (100 mM) adenosine-evoked relaxation was significantly reduced (maximal relaxation in denuded rings: control - 92.1+/-9.8 versus K+- 54.4+/-5.0). Similar results were obtained after incubation of ouabain (100 microM) or glibenclamide (1 microM). In K+-free solution, K+ (1-10 mM)-induced rat aorta relaxant response was significantly inhibited by ouabain (100 microM). Application of indomethacin (10 microM), NG-nitro-L-arginine (10 microM) or tetraethylammonium (500 microM) did not alter the adenosine-elicited effect in rat aorta. 8-(3-Chlorostyril)-caffeine (0.3-3 microM), a selective A2A-receptor antagonist, significantly reduced adenosine-induced relaxation of rat aorta in a concentration-dependent manner (pKB=6.57). Conversely, 1,3-dipropyl-8-cyclopentylxanthine (10 nM), an A1-receptor antagonist, did not affect adenosine-evoked dilatation. These results indicate that in isolated rat aorta, adenosine produces endothelium-independent relaxation, which is most probably dependent upon activation of smooth muscle Na+/K+-ATPase, and opening of ATP-sensitive K+ channels, to a smaller extent. According to receptor analysis, vasorelaxant action of adenosine in rat aorta is partly induced by activation of smooth muscle adenosine A2A receptors.     

Effects of liposome-entrapped adenosine in the isolated rat aorta

This study examined the effects of adenosine- and adenosine deaminase-loaded liposomes upon the contractile activity of the vascular smooth muscle, using the isolated, de-endothelised rat aorta ring as in vitro model. While control liposomes had no effect, intraliposomal adenosine (5 × 10⁻³ M) induced contraction of the preparation. Intraliposomal adenosine deaminase induced partial relaxation of high K⁺-precontracted rings. The adenosine-induced contraction seems to involve Ca²⁺ influx through L-type channels as an essential component, but protein kinase C may also have a modulatory role.     

Characterization of adenosine receptors in the rat isolated aorta

• 1.1. Adenosine and its analogues relaxed the isolated rat aorta by an endothelium-dependent mechanism with an order of potency of 5′-N-ethylcarboxamidoadenosine (NECA) > 2-(p-(2-carboxyethyl)phenethylamino)-5′-N-ethylcarboxamidoadenosine (CGS 21680) > adenosine = N⁶-(2-(4-aminophenyl)ethyl)adenosine (APNEA = N⁶-cyclopentyladenosine (CPA) > 5′ - methylthioadenosine (MTA), although the maximal response achieved by CGS 21680 was less than that achieved by NECA.
• 2.2. Both 8-sulphophenyltheophylline (8-SPT) and MTA antagonized responses to the adenosine analogues, but there were some anomolous features of this antagonism and NECA was inhibited more powerfully than the other agonists. This suggests that as well as A_2a receptors mediating relaxation, the rat aorta may relax to adenosine analogues by other mechanisms.

     

Modulation by endothelium of contractile responses in rat aorta in absence and presence of flunarizine.

The possible modulation by endothelium of phenylephrine- and prostaglandin F2 alpha-induced mobilization of calcium for contraction in the rat aorta has been investigated. Contractions elicited by these and other agonists are inhibited in the presence of endothelium. For any single concentration of phenylephrine in the presence of endothelium, the initial phasic components of contractions were significantly greater, the maximal contractions were achieved sooner and were less well maintained as compared to contractions elicited in the absence of endothelium. The kinetic characteristics of contractions stimulated by single concentrations of PGF2 alpha were similar in the presence and absence of endothelium and did not exhibit initial phasic components of contraction. Sub-maximal contractions-elicited by both PGF2 alpha and phenylephrine in the absence of endothelium were inhibited to a greater extent by flunarizine 3 microM than equieffective contractions elicited in the presence of endothelium. Maximal contractions elicited by phenylephrine (1 microM) were inhibited to a similar extent by flunarizine in the presence and absence of endothelium, but maximal contractions elicited by PGF2 alpha (30 microM) were inhibited by flunarizine to a greater extent in the presence than in the absence of endothelium. It is concluded that an endothelium-derived factor, perhaps distinct from endothelium-derived relaxing factor, can modulate the ability of both phenylephrine and PGF2 alpha to mobilize calcium for contraction. This modulatory effect is associated with an enhanced mobilization of intracellular calcium. Thus, submaximal concentrations of both agonists were less dependent on extracellular calcium than on intracellular calcium to elicit contractions in the presence of endothelium, as compared to contractions elicited in the absence of endothelium.     

山茱萸环烯醚萜总苷对糖尿病大鼠胸主动脉血管内皮的保护作用

在发达国家，糖尿病大血管病变如糖尿病心脏病变导致50％的糖尿病人死亡，美国糖尿病心脏病居致死原因的第5位。糖尿病血管病变已严重地威胁着人类的健康,寻找能阻止或控制糖尿病血管病变发生、发展的有效药物已成为世界医药学研究的重点之一。现代研究认为，血管内皮损伤在糖尿病血管病变中是一早期的关键过程，故本文从形态学的角度观察其对糖尿病大鼠胸主动脉血管内皮的保护作用。     

Age-related involvement of the endothelium in beta-adrenoceptor-mediated relaxation of rat aorta

The signalling pathway involved in beta-adrenoceptor relaxation was studied in aortas from rats either 8 or 54 weeks of age. The vasorelaxation produced by isoprenaline was almost completely abolished by endothelium removal in 54-week aortas, whereas in 8-week aortas, the effect was much smaller. The nitric oxide synthase inhibitor N-methyl-1-arginine acetate (L-NMMA) partially attenuated the isoprenaline induced relaxation to a similar extent in both age groups when the endothelium was intact, suggesting that although nitric oxide was involved, it could not explain the age-related difference. The K(+) channel inhibitor, tetraethylammonium inhibited isoprenaline vasorelaxation to a larger degree in 54-week compared to 8-week aortas indicating that K(+) channels were responsible for the age-related differences. We suggest that as the animals age, the smooth muscle cyclic AMP signalling system declines, and that this is compensated for by an alternate pathway involving the opening of K(+) channels.     

Uridine adenosine tetraphosphate induces contraction and relaxation in rat aorta

Uridine adenosine tetraphosphate (Up(4)A) has been recently reported as an endothelium-derived vasoconstrictor and plasma levels of this dinucleotide are increased in juvenile hypertensive subjects. This study aimed to evaluate the vascular actions of Up(4)A, typify the putative purinergic receptors that might mediate these effects and characterize the intracellular signaling pathways that may govern Up(4)A responses. Up(4)A induced a modest endothelium-dependent relaxation of rat aortic rings contracted with phenylephrine. From baseline, Up(4)A induced concentration-dependent contractions that were significantly potentiated by endothelium removal or nitric oxide synthase inhibition. The contractile response induced by Up(4)A was not tachyphylactic and was significantly reduced in the presence of P1 or P2X receptor antagonists, L-type Ca(2+) channel blocker and Rho-kinase inhibitor. Up(4)A-induced contraction apparently involves superoxide anion formation since it was significantly reduced by treatment with apocynin or tempol. This study presents the unique findings that the endogenous compound Up(4)A is able to induce relaxation in addition to contraction of rat aorta. Up(4)A-induced contraction is modulated by nitric oxide production, mediated by P1 and P2X receptor activation, and involves L-type Ca(2+) channels, Rho-kinase pathway and superoxide formation.     

Activation of multiple sites by adenosine analogues in the rat isolated aorta.

1. The presence of A2 receptors mediating relaxation in the rat isolated aorta has been previously demonstrated. However, agonist dependency of the degree of rightward shift elicited by 8-sulphophenyltheophylline (8-SPT) led to the suggestion that the population of receptors in this tissue is not a homogeneous one. In this study we have re-examined the effects of 8-SPT in the absence and presence of the NO synthase inhibitor L-NAME (NG-nitro-L-arginine methyl ester) and investigated antagonism of responses by the potent A2a receptor ligands PD 115,199 (N-[2-dimethylamino)ethyl]-N-methyl-4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3 dipropyl-1H-purin-8-yl)) benzene sulphonamidexanthine), ZM 241385 (4-(2-[7-amino-2-(2-furyl) [1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-yl amino]ethyl)phenol), and CGS 21680 (2-[p-(2-carboxyethyl)phenylamino]-5'-N-ethylcarboxamidoadenosine). We have also investigated the antagonist effects of BWA1433 (1,3-dipropyl-8-(4-acrylate)phenylxanthine) which has been shown to have affinity at rat A3 receptors. 2. Adenosine, R-PIA (N6-R-phenylisopropyl adenosine), CPA (N6-cyclopentyladenosine) and NECA (5'-N-ethylcarboxamidoadenosine) all elicited relaxant responses in the phenylephrine pre-contracted rat isolated aorta with the following potency order (p[A50] values in parentheses): NECA (7.07 +/- 0.11) > R-PIA (5.65 +/- 0.10) > CPA (5.05 +/- 0.12) > adenosine (4.44 +/- 0.12). 3. 8-SPT (10-100 microM) caused parallel rightward shifts of the E/[A] curves to NECA (pKB = 5.23 +/- 0.16). A smaller rightward shift of E/[A] curves to CPA was observed (pA2 = 4.85 +/- 0.17). However, no significant shifts of E/[A] curves to either adenosine or R-PIA were observed. 4. In the absence of endothelium E/[A] curves to NECA and CPA were right-shifted compared to controls. However, removal of the endothelium did not produce a substantial shift of adenosine E/[A] curves, and E/[A] curves to R-PIA were unaffected by removal of the endothelium. 5. In the presence of L-NAME (100 microM) E/[A] curves to NECA and CPA were right-shifted. However, no further shift of the CPA E/[A] curve was obtained when 8-SPT (50 microM) was administered concomitantly. The locations of curves to R-PIA and adenosine were unaffected by L-NAME (100 microM). 6. In the presence of PD 115,199 (0.1 microM) a parallel rightward shift of NECA E/[A] curves was observed (pA2 = 7.50 +/- 0.19). PD 115,199 (0.1 and 1 microM) gave smaller rightward shifts of E/[A] curves to R-PIA and CPA, but E/[A] curves to adenosine were not significantly shifted in the presence of PD 115,199 (0.1 or 1 microM). 7. The presence of ZM 241385 (3 nM-0.3 microM) caused parallel rightwad shifts of NECA E/[A] curves (pKB = 8.73 +/- 0.11). No significant shifts of E/[A] curves to adenosine, CPA or R-PIA were observed in the presence of 0.1 microM ZM 241385. 8. CGS 21680 (1 microM) elicited a relaxant response equivalent to approximately 40% of the NECA maximum response. In the presence of this concentration of CGS 21680, E/[A] curves to NECA were right-shifted in excess of 2-log units, whereas E/[A] curves to R-PIA were not significantly shifted. 9. BWA1433 (100 microM) caused a small but significant right-shift of the E/[A] curve to R-PIA yielding a pA2 estimate of 4.1 IB-MECA (N6-(3-iodo-benzyl)adenosine-5(1)-N-methyl uronamide) elicited relaxant responses which were resistant to blockade by 8-SPT (p[A]50 = 5.26 +/- 0.13). 10. The results suggest that whereas relaxations to NECA (10 nM-1 microM) are mediated via adenosine A2a receptors, which are located at least in part on the endothelium, R-PIA and CPA may activate A2b receptors on the endothelium and an additional, as yet undefined site, which is likely to be located on the smooth muscle and which is not susceptible to blockade by 8-SPT, PD 115,199 or ZM 241385. This site is unlikely to be an A3 receptor since the very small shift obtained in the presence of BWA1433 (100 microM), and the low potency of IB-MECA is not consistent with the affin     

Local formation of angiotensin II in the rat aorta: effect of endothelium.

1. The local formation of angiotensin II (AII) from its precursors, angiotensin I (AI) and tetradecapeptide (TDP) renin substrate, was studied in intact (with endothelium) and rubbed (without endothelium) aortic rings of the rat. 2. AI and TDP renin substrate maximally contracted intact tissues in a similar way to AII. The same observations were made in rubbed tissues. 3. The maximal response and the sensitivity of the aorta to these agonists were greater in rubbed than in intact tissues. 4. In intact preparations, methylene blue increased the contractile response to AII and TDP to the same extent as endothelium removal. 5. In intact preparations, AII receptor blockade completely suppressed all contractile responses, converting enzyme inhibition completely blocked the responses to AI and TDP, and renin inhibition partially blocked the responses to TDP. 6. In rubbed preparations, AII receptor blockade completely inhibited all contractile responses, converting enzyme inhibition completely suppressed the responses to AI but only partially inhibited those to TDP, and renin inhibition partially blocked the responses to TDP. 7. In conclusion, the formation of AII from TDP and its blockade by a converting enzyme inhibitor and a renin inhibitor shows that converting enzyme and a renin-like aspartic proteinase are present in the aortic wall. Furthermore, the results show that the endothelium is not essential for the conversion of the TDP to AII, but modulates the responses to locally formed AII through the release of endothelium-derived relaxing factor (EDRF).     

腺苷受体激动剂对大鼠离体主动脉的作用及其机制

目的　研究不同腺苷受体 (A R)激动剂对大鼠离体主动脉环的作用及其机制。方法　分别给予A2 R激动剂CPCA和A1 R激动剂CPA ,观察离体主动脉环对 0 6 2 μmol·L-1去甲肾上腺素的反应 ,并观察它们的作用是否依赖于血管内皮、细胞外钙和ATP敏感性钾通道 (KATP)。结果　5 0 μmol·L-1CPCA可引起血管扩张 ,去除血管内皮或换用无钙营养液后此作用消失 ,KATP阻滞剂格列苯脲并不能阻断CPCA的血管扩张作用。CPA在 10 μmol·L-1浓度时不引起血管扩张 ,10 0 μmol·L-1浓度时有血管扩张作用 ,此作用在去除内皮后或换用无钙营养液后仍然存在 ,格列苯脲不能阻断此作用。结论　CPCA引起主动脉的扩张依赖于血管内皮和细胞外钙的存在 ,高浓度CPA则通过直接作用于平滑肌细胞而引起血管扩张 ,KATP均不参与A R激动剂的扩血管作用     

Omeprazole-induced relaxation in rat aorta is partly dependent on endothelium.

We investigated the effect of omeprazole (1 x 10(-5)-3 x 10(-4)M), an inhibitor of H(+),K(+)-ATPase, on rat aortic rings pre-contracted with phenylephrine (10(-6)M). Omeprazole relaxed the tissue in a concentration-dependent manner. Either removal of the endothelium or incubation with nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 3 x 10(-5)M) significantly attenuated the relaxations. Pre-treatment with L-arginine (10(-3)M), but not with D-arginine, reversed the inhibitory action of L-NAME. Indomethacin (10(-6)M) and tetraethylammonium (TEA, 10(-2)M) did not affect the relaxant responses to omeprazole indicating the lack of involvement of cyclooxygenase products and K(+) channels, respectively. These results suggest a role of NO in the mechanism of action of omeprazole.