Relaxation Of Rat Aorta Research Articles

Room E, 10/17/2000 9: 00 AM - 11: 00 AM (PS) Propofol-Induced Relaxation of Rat Aorta Is Altered by Aging

<b>Room E, 10/17/2000 9: 00 AM - 11: 00 AM (PS) Propofol-Induced Relaxation of Rat Aorta Is Altered by Aging</b>

Paradoxical Effects of Temperature on Vascular Tone

Temperature may have significant influence on vascular tone in such cases as organ preservation, coronary bypass surgery, and extracorporeal circulation. The aim of this research was to study the direct effect of temperature variation on vascular tone in an attempt to elucidate the mechanisms involved. In a first series of experiments, the isometric tension of two different vessels (rat thoracic aorta and pig renal branch artery) was studied at different temperatures. To study the role of calcium in this response, a second series of experiments was performed. In this the vessels were incubated with the intracellular chelator BAPTA/AM. Further experiments were performed to test the effect of cold storage. Our results show that changes in temperature lead to different results in pig renal artery and rat aorta. A decrease in temperature induced a highly reproducible relaxation in rat aorta, whereas pig renal artery presented cooling-induced contraction. Moreover, whereas calcium depletion failed to inhibit cooling-induced relaxation in rat aorta, it did not provoke cooling-induced contraction in pig renal artery. Similar responses were obtained with cold storage and calcium depletion. We intend to demonstrate that, just as the effect of temperature variation on pig renal artery is due to a metabolic mechanism, its effect on rat aorta may be due to structural factors. This hypothesis is supported by the result of histological studies which demonstrate a higher proportion of elastin fibres in rat aorta than in pig renal artery.

Erratum: Potentiation of carbon monoxide-induced relaxation of rat aorta by YC-1

The authors would like to revise a statement in their paper on page 348, first column, lines 12–16, “Thus, Furchgott and Jothianandan (1991) found that CO was much less potent than NO (by a factor of one thousand) in relaxing rabbit aorta, but both gaseous molecules had nearly the same potency in relaxing dog coronary artery.” The statement should be amended to read, “Thus, Furchgott and Jothianandan (1991) found that CO was much less potent than NO (by a factor of one thousand) in -relaxing rabbit aorta and dog coronary artery.” 679

Effect of Spironolactone and Its Metabolites on Contractile Property of Isolated Rat Aorta Rings

Spironolactone and its active metabolites canrenone and potassium canrenoate are normally used as antihypertensive drugs. Although they are classified as antagonists of aldosterone, their mechanism of action cannot be ascribed solely to the regulation of ion transport in the distal tubule of nephrons. Here we have evaluated the effects of spironolactone, canrenone, and potassium canrenoate on contractile properties of isolated rat aorta rings. Spironolactone (1-300 microM), canrenone (1-300 microM), and potassium canrenoate (0.01-10 mM), in a concentration-dependent manner, relaxed rat aorta rings precontracted with phenylephrine (1 microM) or KCl (40 mM). These relaxant effects were not affected by prior treatment with either aldosterone (100 microM), glibenclamide (10 microM), or tetraethylammonium (10 mM), excluding the possibility that these drugs can be involved in either the nongenomic effect of aldosterone or on activation of potassium channels. Spironolactone and canrenone at concentrations of 30 and 100 microM, but not at 10 microM, and potassium canrenoate at concentrations of 0.3 and 1 mM, but not at 0.1 mM, significantly inhibited the phenylephrine (0.001-3 microM) concentration-response curve. Conversely, all tested concentrations of spironolactone (10, 30, and 100 microM), canrenone (10, 30, and 100 microM), and potassium canrenoate (0.1, 0.3, and 1 mM) significantly inhibited the concentration-response curve induced by cumulative concentrations of KCI (10-80 mM). Because both phenylephrine- and KCl-induced contractions imply an intracellular Ca2+ influx, we suggest that these drugs could act through an inhibition of voltage-dependent Ca2+ channels.

Endothelium-dependent relaxation of rat aorta and main pulmonary artery by the phytoestrogens genistein and daidzein.

The dietary phytoestrogens genistein and daidzein have been shown to relax agonist-preconstricted arteries in vitro; the mechanisms of relaxation remain incompletely understood. This study aimed to determine whether the relaxation of phenylephrine (PE)-constricted rat aorta and main pulmonary artery by genistein and daidzein was endothelium-dependent. Effects of endothelial-denudation, and pretreatment with with 100 microM L-N(G)-nitroarginine methyl ester (L-NAME) and/or 10 microM indomethacin on relaxation of PE (1 microM)-preconstricted contractures by genistein (1-100 microM) and daidzein (3-100 microM) were assessed by measuring isometric force development by rat arterial rings. The effect of L-NAME on relaxation to 17beta-estradiol (10 microM) was also measured in aorta. Genistein and daidzein caused concentration-dependent relaxation of aorta rings preconstricted with PE (1 microM). The IC50 values were 5.7 microM (n=8, 95% confidence limits 4.3-7.7 microM) and 36.7 microM (n=12, 95% confidence limits 25.7-44.1 microM), respectively. Removal of the endothelium and pretreatment with L-NAME (100 microM) significantly inhibited relaxation at 3, 10 and 30 microM genistein and 10 and 30 microM daidzein. The contracture evoked in rat aorta by depolarization with 75 mM K+ solution was similarly relaxed by genistein in a partially endothelium-dependent manner. 17Beta-estradiol (10 microM) caused a 48.7+/-5.0% (n=11) relaxation of the PE contracture, which was significantly reduced to 25.1+/-5.3% (n=7) by L-NAME. Relaxations brought about by 17beta-estradiol, genistein, and daidzein were not significantly affected by the genomic estrogen receptor antagonist ICI 182,780 (10 microM). Similar endothelium-dependent effects of genistein were observed in the main pulmonary artery. The results show that the relaxation of these rat arteries by concentrations of genistein and daidzein which overlap those present in human plasma after ingestion of soybean-containing meals is largely endothelium dependent.

Calcitonin gene-related peptide-dependent vascular relaxation of rat aorta: An additional mechanism for nitroglycerin

We investigated the involvement of calcitonin gene-related peptide (CGRP) in the vasodilatory mechanism of action of nitric oxide (NO) donors. The functional role of CGRP in NO donor-induced vasodilation of isolated rat aortic rings was determined by incubating these drugs with and without CGRP 8–37, a selective CGRP receptor antagonist. CGRP 8–37 (0.63 μM) induced rightward shifts in the vasodilatory concentration–response curves for nitroglycerin (NTG), Piloty’s acid (PA), and SIN-1 (linsidomine). The ec 50 values for NTG, PA, and SIN-1 were increased by 8.3-, 5.2-, and 2.3-fold, respectively ( P < 0.05). The release of CGRP from rat aorta in response to NTG and PA was measured specifically by radioimmunoassay. Thirty-minute incubations of NTG or PA with rat aorta induced 189.5 and 214.6% increases, respectively, in CGRP release when compared with the control ( P < 0.05). The concentration–response curves of sodium nitroprusside (SNP), S-nitroso-acetylpenicillamine (SNAP), tetranitromethane (TNM), diethylamine NO complex (DEA-NO), and diethylenetriamine/nitric oxide adduct (DETA NONOate) were not inhibited significantly by CGRP 8–37 co-incubation ( P > 0.05). NO donors also were incubated with aortic strips, and NTG and PA alone induced significant formation of hydroxylamine, a NO - metabolite (232.4 and 364.9%, respectively, P < 0.05). These results indicate that only NTG and PA, and to a lesser extent SIN-1, stimulate the release of CGRP from the rat aorta, which subsequently contributes to the vasodilatory activity of these agents. The hydroxylamine formation suggests a possible link between NO - generation and CGRP release from the vascular wall.

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

The signalling pathway involved in β-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.

Nitric oxide-mediated endothelium-dependent relaxation of rat thoracic aorta induced by aqueous extract of red rice fermented with Monascus ruber

Vasodilatory effects of aqueous extract of red rice fermented with Monascus ruber IFO32318 were examined on the isolated rat aorta. The water phase of fermented rice with Monascus (WP/FRM, 0.1–10 mg/ml) caused a transient relaxation of the endothelium-intact rat aorta precontracted with norepinephrine (NE, 300 nM). The WP/FRM-induced relaxation was abolished by removal of endothelium or in the presence of N G-nitro- l-arginine ( l-NNA, 10 μM), a nitric oxide (NO) synthase inhibitor. Neither atropine, a muscarinic receptor antagonist (10 μM), nor indomethacin, a cyclooxygenase inhibitor (10 μM), altered the WP/FRM-induced endothelium-dependent relaxation. γ-Aminobutyric acid (GABA), one of the principle components of the extract, did not affect the muscle tension of the aorta with intact endothelium. In addition, WP/FRM increased the production of NO in primary cultured endothelial cells from human umbilical vein. The enhanced production of NO by WP/FRM was diminished by pretreatment with l-NNA (10 μM). In conclusion, WP/FRM induces relaxation of rat aorta by releasing NO from endothelium. There seem to be some unknown factor(s) other than acetylcholine (Ach) and GABA, in the aqueous extract of red rice, which stimulate vascular endothelial cells to produce and/or release NO leading to endothelium-dependent relaxation by WP/FRM.

Mg(2+)-induced endothelium-dependent relaxation of blood vessels and blood pressure lowering: role of NO.

In vitro extracellular Mg(2+) concentration ([Mg(2+)](0)) produces endothelium-dependent and endothelium-independent relaxations in rat aorta in a concentration-dependent manner. These relaxant effects of Mg(2+) on intact rat aortic rings, but not denuded rat aortic rings, were suppressed by either N(G)-monomethyl-L-arginine (L-NMMA), N(omega)-nitro-L-arginine methyl ester (L-NAME), or methylene blue. The inhibitory effects of L-NMMA and L-NAME could be reversed partly by L-arginine. [Mg(2+)](0)-induced dilatation in vivo in rat mesenteric arterioles and venules was almost completely inhibited by N(G)-nitro-L-arginine and L-NMMA. Removal of extracellular Ca(2+) concentration ([Ca(2+)](0)) or buffering intracellular Ca(2+) concentration in endothelial cells, with 10 microM 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, markedly attenuated the relaxant effects of Mg(2+). Mg(2+) produced nitric oxide (NO) release from the intact aortic rings in a concentration-dependent manner. Removal of [Ca(2+)](0) diminished the increased NO release induced by elevated levels of [Mg(2+)](0). In vivo infusion of increasing doses (1-30 microM/min) of MgSO(4), directly into the femoral veins of anesthetized rats, elicited significant concentration-dependent sustained increases in serum total Mg and concomitant decreases in arterial blood pressure. Before and after employment of various doses of MgSO(4), intravenous administration of either L-NMMA (10 mg/kg) or L-NAME (10 mg/kg) increased (i.e., reversed) the MgSO(4)-lowered blood pressure markedly, and intravenous injection of L-arginine restored partially the increased blood pressure effects of both L-NMMA and L-NAME. Our results suggest that 1) small blood vessels are very dependent on NO release for Mg(2+) dilatations and 2) the endothelium-dependent relaxation induced by extracellular Mg(2+) is mediated by release of endothelium-derived relaxing factor-NO from the endothelium, and requires Ca(2+) and formation of guanosine 3',5'-cyclic monophosphate.

Potentiation of carbon monoxide-induced relaxation of rat aorta by YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole

The hypothesis that endogenous carbon monoxide (CO), produced during the oxidation of heme catalyzed by heme oxygenase (HO), plays a role similar to that of nitric oxide (NO) in the regulation of cardiovascular tone has been criticized because of the low potency of CO compared with NO in relaxing blood vessels and stimulating soluble guanylyl cyclase (sGC). This criticism has been muted by the demonstration that, in the presence of YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole], CO has similar potency to NO in stimulating sGC activity. In this study, we determined that YC-1 potentiated CO-induced relaxation of rat aortic strips (RtAS) by approximately ten-fold. Furthermore, CO-induced relaxation of RtAS was shown to be mediated through stimulation of sGC because vasorelaxation was inhibited by ODQ (1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one), a selective sGC inhibitor, in the absence and presence of YC-1. A gas chromatographic-headspace method was used to measure CO concentration in Krebs' solution following the addition of CO-saturated saline solution to the tissue bath, in order to provide an accurate determination of RtAS exposure to CO. The tissue bath concentration of CO was shown to be approximately one-half of that calculated to be present. We conclude that should an endogenous compound exist with properties similar to that of YC-1, then the potency of CO as a vasorelaxant in the presence of this factor would be increased. As a consequence, CO could play a role in the regulation of cardiovascular tone, comparable to that of NO.

Effect of chronic lithium administration on endothelium-dependent relaxation in rat aorta.

1. The effects of chronic lithium administration on the relaxant responses of rat thoracic aortic rings in the presence of indomethacin (a cyclo-oxygenase inhibitor) and/or NG-nitro-L-arginine (L-NOARG; a nitric oxide synthase inhibitor) to acetylcholine (ACh) or sodium nitroprusside were investigated in the present study. 2. Acetylcholine produced a concentration-dependent relaxation in vessels precontracted by phenylephrine (PE), while in lithium-treated rats the maximal relaxation was significantly increased. 3. Indomethacin (20 mumol/L) significantly potentiated the ACh-induced relaxation in lithium-treated and control rats. 4. NG-Nitro-L-arginine (1 mumol/L) decreased the ACh-induced relaxation in both control and lithium-treated rats. In contrast, indomethacin (20 mumol/L) reversed the inhibitory effect of L-NOARG. 5. Sodium nitroprusside produced similar concentration-dependent relaxations of vessels from both control and lithium-treated rats, which was not affected by indomethacin. In endothelium-denuded rings, indomethacin (20 mumol/L) caused a rightward shift in the concentration-contraction curve to PE. 6. These data support evidence for a possible increase in endothelium-dependent relaxation induced by ACh during long-term administration of lithium in rat aortic rings.

Pharmacological effects of novel quinone compounds, 6-(fluorinated-phenyl)amino-5,8-quinolinediones, on inhibition of drug-induced relaxation of rat aorta and their putative action mechanism

Two 6-(fluorinated-phenyl)amino-5,8-quinolinedione derivatives, OQ21 and OQ1, were newly synthesized as potent inhibitors of endothelial-dependent vasorelaxation. The purpose of the present study was to investigate the effects of OQ21 and OQ1 on different types of vasorelaxation and to pursue their action mechanisms. For acetylcholine both compounds, at a low concentration (0.1 μM), reduced the maximal response with increase of EC 50 values. OQ21 is a novel quinone compound and showed a more potent and efficacious inhibitory effect on acetylcholine-induced relaxation of rat aorta than that of LY83583 (6-anilino-5,8-quinolinedione). Relatively high concentrations (1 μM) of OQ21 and OQ1 inhibited the sodium nitroprusside–induced relaxation of endothelium-denuded ring, producing rightward shifts of the curve for sodium nitroprusside without altering the maximal response. They also prevented acetylcholine and sodium nitroprusside-induced elevations of cyclic GMP. In addition, OQ21 and OQ1 (1 μM) significantly decreased (52–72%) the sensitivity of L-arginine–induced relaxation of precontracted endothelium-denuded aortic rings from lipopolysaccaride-treated (20 mg/kg, i.p.) rats. The inhibitory effect of OQ21 on endothelium-dependent vasodilation was enhanced by N G-nitro-L-arginine, which inhibits nitric oxide synthase (NOS) by binding the oxygenase domain of the enzyme, but not by diphenylendiodonium, which inhibits NOS by binding to the reductase domain of the enzyme. Treatment of blood vessels with OQ21 or OQ1 showed a significant increase in chemiluminescence output, which was prevented by adding superoxide dismutase, suggesting that superoxide generation is involved in the action mechanism for OQ21. Present results indicate that a novel naphthoquinone compound, OQ21, potently inhibits endothelial NOS, possibly by interacting with the reductase domain of the enzyme, which leads to induce superoxide formation. The new benzoquinone compounds, OQ21 and OQ1, inhibit not only endothelium-dependent vasorelaxation but also endothelium-independent relaxation induced by exogenous NO generated from a nitrovasodilator via the reduction of cyclic GMP. They also reduced L-arginine–induced vasorelaxation in endotoxin-treated rats, indicating their possession of inhibitory effect on inducible NOS.

Gestational changes in endothelium-dependent relaxation (EDR) of rat aorta

Gestational changes in endothelium-dependent relaxation (EDR) of rat aorta

Hydroxamic acids are nitric oxide donors. Facile formation of ruthenium(II)-nitrosyls and NO-mediated activation of guanylate cyclase by hydroxamic acids

Hydroxamic acids are shown for the first time to be effective NO donors by their ability to readily form ruthenium(II)-nitrosyls, and to cause vascular relaxation of rat aorta by NO-mediated activation of the iron-containing guanylate cyclase.

Potentiation of carbon monoxide-induced relaxation of rat aorta by YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole

Potentiation of carbon monoxide-induced relaxation of rat aorta by YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole

Erratum: Potentiation of carbon monoxide-induced relaxation of rat aorta by YC-1

Erratum: Potentiation of carbon monoxide-induced relaxation of rat aorta by YC-1

Calcium-mediated mechanisms of eicosapentaenoic acid-induced relaxation in hypertensive rat aorta

We have previously demonstrated the vasorelaxant properties of the omega-3 fatty acid, eicosapentaenoic acid (EPA), in normotensive and spontaneously hypertensive rat (SHR) aorta, although the mechanism(s) of action are not fully understood. Because endothelial dysfunction and increased intracellular free calcium concentration ([Ca 2+] i) are seen in hypertensive rat aorta, we investigated the potential role of Ca 2+ signaling, endothelium and derived factors, and the opening of potassium (K +) channels in EPA-induced relaxation. In the presence of extracellular Ca 2+, EPA induced significant relaxations at > 10 μmol/L ( P < .01) in norepinephrine (NE) (10 −6 mol/L)-contracted aortic rings and at 30 μmol/L ( P <. 001) in high K + (80 mmol/L)-contracted aortic rings. In the absence of extracellular Ca 2+, EPA (10 to 30 μmol/L) inhibits the tonic component of NE-induced contraction ( P < .0001). The relaxant properties of EPA in SHR aorta appear specific to Ca 2+ release from an internal storage site associated with NE-induced tonic contraction. Further studies with the use of fura-2 to measure [Ca 2+] i in cultured vascular smooth muscle (VSM) cells from SHR aorta indicated that EPA (30 μmol/L)-pretreatment attenuated angiotensin II (50 nmol/L)-induced Ca 2+ transient by 95%, suggesting that an inhibitory effect on the Ca 2+ signaling may underlie EPA-induced relaxation of the vessel preparation. In addition, EPA per se induced an increase in [Ca 2+] i with a duration of approximately 20 min in VSM cells, and the effect was not altered by removal of extracellular Ca 2+. There was no increase in the level of inositol-1,4,5-trisphosphate in response to EPA (30 μmol/L). The actions of EPA are independent of endothelium-derived factors, cyclooxygenase metabolites, and activation of K + channels since endothelium removal, N ω-nitro- l-arginine methyl ester hydrochloride, (L-NAME, 100 μmol/L), indomethacin (10 μmol/L), tetraethylammonium (1 mmol/L), and glibenclamide (10 μmol/L) did not affect EPA-induced vasodilation in NE-precontracted aortic rings. These results suggest that EPA directly modulates intracellular Ca 2+ signaling in VSM cells, and that this may contribute to the vasorelaxant effect and, at least in part, the blood pressure-lowering effect of fish oil.

IN VITRO AND IN VIVO EXPERIMENTAL EFFECT OF KOREAN RED GINSENG ON ERECTION

To elucidate the efficacy of Korean red ginseng (KRG) on penile erectile tissue and erectile response, the effect of ginseng was evaluated in both in vivo and in vitro experiments in laboratory animals. Fifty milligrams of KRG per kilogram in weight was mixed in physiologic saline and given by mouth for 3 months to both rabbits and rats. In vitro experiments were performed by observing the responses to acetylcholine (ACh) and KRG of strips of rabbit corpus cavernosum. In vivo experiments were performed by measuring the cavernosal pressures after the stimulation of pelvic nerves innervating rat corpus cavernosum. On rabbit cavernosal muscle strips precontracted with phenylephrine (5x10(-6) M), increasing concentrations of ACh (10(-7), 10(-6), 10(-5), 10(-4) M) showed dose dependent relaxation in the control group (10(-7) M: 15.32%, 10(-6) M: 35.44%, 10(-5) M: 59.45%, 10(-4) M: 76.54%)(p<0.01). After 3 months of KRG administration, the relaxation responses to ACh were increased significantly (10(-7) M: 34.18%, 10(-6) M: 56.35%, 10(-5) M: 75.33%, 10(-4) M: 89.86%)(p<0.01). Relaxation effects of KRG were significantly increased after administering KRG for 3 months, as evident by the intracavernousal pressure to electrostimulation being 107.52 cm. water in the control group and significantly increased to 138.34 cm. water after 3 months administration of KRG (p<0.01). We confirmed that the long-term administration of KRG enhances erectile capacity and that its action is mediated by endothelium-derived relaxing factor and peripheral neurophysiologic enhancement.

Effect of pre-exposure to vasoconstrictors on isoprenaline-induced relaxation in rat aorta: involvement of inducible nitric oxide synthase.

1. The aim of this study was to determine whether a brief (30 min) episode of contractile receptor stimulation could affect the degree of a subsequent vasorelaxation. Therefore, concentration - relaxation curves of the rat aorta to isoprenaline were compared before and after exposure of the tissue to noradrenaline (100 microM) or prostaglandin F2alpha (PGF2alpha, 100 microM). 2. Exposure to noradrenaline enhanced the second maximal relaxant effect of isoprenaline (from 20 - 95% relaxation). This effect was not due to significant differences in precontraction levels and was not modified by the presence of the endothelium. Treatment with PGF2alpha mimicked the actions of noradrenaline on subsequent vasorelaxation to isoprenaline. 3. Before exposure to noradrenaline (100 microM), forskolin (10 microM) did not produce any significant relaxation of the rat aorta. After exposure to noradrenaline, forskolin caused a concentration-dependent relaxation with a maximal effect of more than 90% in rings with and without endothelium suggesting that the change in vasorelaxation to isoprenaline occurred downstream from the beta-adrenoceptor. 4. The increase in relaxation due to exposure to noradrenaline was markedly attenuated by treatment with a protein synthase inhibitor (cycloheximide), a nitric oxide (NO) synthase inhibitor (L-NG-nitroarginine methyl ester, L-NAME) and an inhibitor of the activation of soluble guanylyl cyclase (methylene blue). 5. Western blot analysis showed an increase of inducible NO synthase (iNOS) in aortic rings exposed to noradrenaline or PGF2alpha. 6. Together, these findings suggest that pretreatment of rat aorta with noradrenaline or PGF2alpha could induce vascular NOS which would in turn result in an increase in isoprenaline-induced vasorelaxation, this increase occurring downstream from receptor activation. Such a mechanism might participate in cardioprotection during preconditioning induced by noradrenaline.

Erigeron breviscapus prevents defective endothelium-dependent relaxation in diabetic rat aorta

We examined the endothelium-dependent relaxation response to acetylcholine (Ach) in streptozotocin-induced diabetic rat aorta at the stages of 2- and 6-wks' duration in vitro, and compared with another two groups which were treated with dietary supplement of 0.1% Aminoquanidine (AG) and 0.5% Erigeron breviscapus(EB) from 1-week of diabetes induction. At the stage of 2-wks' duration of diabetes, relaxation responses to lower concentrations of Ach in 0.3uM phenylepherine-precontracted aortas were diminished significantly (P < 0.05) compared with age-matched control, but the maximal relaxation of Ach remained unchanged. At the stage of 6-wks' duration, diabetes caused an approximately 60% (P < 0.001) deficit in maximum relaxation, and this was significantly (P < 0.001) prevented in AG and EB treated groups. There was an approximately 40% enhancement in the maximum contractile response to phenylepherine with diabetes (P < 0.05), which was unaffected significantly by AG and EB treatments. The data suggest that the defective endothelium-dependent relaxation in diabetic rat aorta occurred as early as 2-wks' duration of diabetes, and the treatments of AG and EB could protect vascular endothelium although the deficits in vascular smooth muscle contractile responses were not protected.