Influence of the vascular endothelium on angiotensin II-induced contractions in rabbit renal artery

Interaction of nitric oxide and renin angiotensin system in pulmonary arterial circulation of RHR

We investigated the interaction between nitric oxide and the renin angiotensin system in regulating isolated pulmonary arterial tension and pulmonary arterial pressure (PAP) in renal hypertensive rats (RHR) made by complete ligation of left renal artery. Losartan induced a depressor response that was smaller in RHR than in normotensive rats (NR) (3.3 and 7.0 mmHg, respectively, at 3.0 mg/kg, p<0.05), and the response was significantly reduced by N(G)-nitro-L-arginine methyl ester (L-NAME). Angiotensin II elevated the PAP (7.6 and 10.8 mmHg at 0.1 mug/kg; 20.3 and 23.6 mmHg at 1.0 mug/kg, respectively) and contracted the isolated pulmonary artery (pD(2): 8.79 and 8.71, respectively) from both NR and RHR with similar magnitude, and these effects were significantly enhanced by L-NAME in NR, but not in HRR. Acetylcholine lowered the PAP slightly less effectively in RHR than in NR (3.8 and 6.0 mmHg at 10 mug/kg, respectively) and relaxed the pulmonary artery precontracted with norepinephrine in both rats with similar magnitude (E(max): 60.8 and 63.6%, respectively), and the effect being completely abolished after pretreatment with L-NAME or removal of endothelial cells. These results suggest that nitric oxide interacts with renin angiotensin system to control the pulmonary vascular tension and pulmonary arterial circulation of RHR.

Mechanisms underlying enhanced contractile response to endothelin-1 in diabetic rat basilar artery

We investigated the influence of streptozotocin-induced diabetes on the responsiveness of the rat basilar artery to endothelin-1 (ET-1) and nitric oxide (NO), which is known to counteract ET-1. In basilar arteries isolated from diabetic rats: (a) the ET-1-induced contraction was enhanced, (b) the contraction induced by N(G)-nitro-l-arginine [a nitric oxide synthase (NOS) inhibitor] was weaker, and (c) the levels of the mRNAs for ET(A)/ET(B) receptors and prepro-ET-1, but not for NOS, were significantly elevated (all versus age-matched controls). These data indicate that ET-1-induced vasoconstriction may be increased in the diabetic rat basilar artery, and that this hyper-reactivity to ET-1 may be due to an overproduction of ET-1, an up-regulation of ET(A)/ET(B) receptors, and a defect in the bioavailability of NO.

Cross Talk between Angiotensin II and Alpha 1 Adrenergic Receptors in Rabbit Aorta: Role of Endothelium

Interaction between the renin-angiotensin system and the sympathetic nervous system has been proposed to be like a physiological regulation mechanism. The present work was designed to study the cross talk between angiotensin II and adrenergic receptors on the smooth muscle contractile response and the endothelium influence in this phenomenon. Homologous and endothelium independent desensitization of angiotensin II-contractile response was observed. Treatment with noradrenaline between two cumulative doses response curves (CDRC) to angiotensin II caused a rightward shift of the second CDRC in unrubbed arteries and increased the maximal response in rubbed arteries. Prazosin blocked these effects. No homologous desensitization of noradrenaline contractile response was found. Treatment with angiotensin II between two CDRC to noradrenaline caused a loss of affinity in the second CDRC in unrubbed arteries. Losartan was able to avoid this phenomenon. Maximal response was enhanced both in arteries with and without endothelium treated or not with angiotensin II. Results demonstrate homologous and endothelium-independent desensitization of the contractile response to angiotensin II but not to noradrenaline. In addition, heterologous and endothelium-dependent desensitization induced by noradrenaline and angiotensin II on the contractile response to each other was found. Furthermore, results provided the first evidence that there is an endothelium-dependent cross talk between alpha1-adrenergic and angiotensin II receptors in smooth muscle of rabbit aorta.

Role of nitric oxide on the vasorelaxant effect of atrial natriuretic peptide on rabbit aorta basal tone

The role of nitric oxide (NO) on the vasorelaxant effect of atrial natriuretic peptide (ANP) on the basal tone of rabbit aortic rings conditioned to angiotensin II (Ang II) was studied. ANP aortic relaxation and nitrite release were measured in the presence and absence of endothelium and a NO-synthase inhibitor. Ang II at 10(-8) M triggered a contractile response, conditioning the vessel to a vasorelaxant effect of ANP (10(-8) M). This effect was significantly enhanced by endothelium removal, NG-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), and methylene blue (10(-5) M). ANP decrease of basal tone in Ang-II-sensitized aortic rings was improved when a higher concentration of Ang II was used (l0(-6) M). Basal and Ang-II-stimulated nitrite release were measured in stretched (S) and nonstretched (NS) aortic rings. Nitrite release was significantly increased in S rings (p < 0.001). L-NAME (10(-4) M) partially inhibited nitrite release in both basal and Ang-II-stimulated S aortic rings. In NS aortic rings, the NO inhibitor did not inhibit basal nitrite release but blunted the Ang-II-stimulated nitrite level. A significant negative correlation between nitrite release and the ANP vasorelaxant effect on basal tone was dependent on the Ang-II-sensitizing dose. The present results demonstrate that ANP relaxant effects on aortic basal tone are related to NO levels, which are regulated by S- and Ang-II-concentration-dependent NO generation and quenching.

Endothelium-dependent desensitization to angiotensin II in rabbit aorta: the mechanisms involved

The aim of this study was to characterize the role of the endothelium in angiotensin II-desensitization and its mechanisms of action. Rabbit aortic rings were exposed to increasing doses of angiotensin II (Ang II, 10–9 to 2.5 × 10–6) to generate two cumulative dose-response curves (CDRC I and II). A 50-min interval separated CDRC I and II. Desensitization was observed at all doses in unrubbed aortic tissue and at lower doses in rubbed aortic tissue. Tachyphylaxis was greater in arteries with endothelium. Treatment of intact rings with L-NG-nitroarginine methyl ester (L-NAME, 10–4 M) did not prevent this phenomenon. However, indomethacin (10–5 M) and miconazol (10–6 M) attenuated Ang II-desensitization. Treatment of unrubbed rings with nifedipine (10–6 M) and cromakalim (10–6 M) inhibited the effect of indomethacin. To confirm the involvement of K+ channels, unrubbed and rubbed aortic rings were treated with the KCa2+ blockers apamin (10–7 M), tetraethylammonium (TEA, 10–3 M), and iberiotoxin (10–8 M), and the KATP blocker glibenclamide (10–5 M). In both arteries apamin, TEA, and glibenclamide abolished the tachyphylaxis without changes in the maximal response. Iberiotoxin diminished Ang II-desensitization in rubbed but not unrubbed arteries. Results from this study suggest that Ang II-desensitization involves endothelium-dependent and -independent mechanisms. Endothelium-dependent desensitization could be mediated by a cyclooxygenase-cytochrome P450 product, which could act by increasing KCa2+ channel activity.Key words: angiotensin II, rabbit aorta, desensitization, endothelium, cyclooxygenase products.

Different types of antagonism by losartan and irbesartan on the effects of angiotensin II and its degradation products in rabbit arteries

A previous study by our group has demonstrated that the selective AT1-receptor antagonist losartan behaves as a noncompetitive antagonist in rabbit isolated renal artery (RA). In the present investigation, the influence of losartan and irbesartan on the contractile effects of angiotensin II (AII) and its degradation products angiotensin III (AIII) and angiotensin IV (AIV) was determined in the rabbit isolated RA and femoral artery (FA). The arteries were set up in organ chambers and changes in isometric force were recorded. In both rabbit isolated RA and FA preparations, AII, AIII and AIV elicited significant contractile responses with a similar efficacy. These effects were impaired by the presence of functional endothelium in RA preparations but not in FA preparations. In both preparations studied, the effects of AII, AIII and AIV were influenced neither by the aminopeptidase-A and -M inhibitor amastatin (10 microM), nor by the aminopeptidase-B and -M inhibitor bestatin (10 microM). In endothelium-denuded FA preparations, preincubation with losartan (3-300 nM) antagonized AII-, AIII- and AIV-induced contractions in a competitive manner. However, in endothelium-denuded RA preparations, losartan depressed the maximal contractile responses induced by AII but not those induced by AIII and AIV. In the same preparations, preincubation of another selective AT1-receptor antagonist irbesartan (3-30 nM) concentration-dependently shifted AII and AIII curves to the right in an insurmountable manner. The reduction of the maximal response of AII is more potent when compared to that of AIII (47.7 +/- 1.51% vs. 66.7 +/- 1.88%, percentage of the initial maximal response; P < 0.05; n=5). The selective AT2-receptor antagonist PD123177 (1 microM) did not influence the responses to all three peptides in both RA and FA preparations. These heterogeneous antagonistic effects of the two AT1-receptor antagonists studied with respect to the contractile actions of AII, AIII and AIV suggest the possible existence of multiple, functionally relevant AT1-receptor subtypes in rabbit RA preparations.

Action of AT1 receptor antagonists on angiotensin II-induced tone in human isolated subcutaneous resistance arteries

1. Human isolated subcutaneous arteries were studied under isometric conditions in a myograph. 2. Addition of angiotensin II (AII) induced a concentration-dependent increase in tone in isolated arteries. The active metabolite of candesartan (CV 11974), losartan and the active metabolite of losartan, E-3174 antagonized AII-induced tone in a non-competitive manner, but the AT2 selective antagonist, PD123319, was without effect on responses to AII. The effects of candesartan, losartan and E-3174 were analysed using a classical model of non-competitive antagonism and a two-state receptor model. 3. Mechanical removal of the endothelium; pre-incubation with Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME); pre-incubation with indomethacin, a cyclo-oxygenase inhibitor; or pre-incubation with BQ 485, an endothelin antagonist; had no significant effect on contractions induced by AII. 4. Our results suggest AII contracts human isolated resistance arteries by an action on AT1 receptors and does not involve release of endothelial factors. Use of a two-state receptor model successfully described the action of the AT1 antagonists without sacrificing assumptions regarding the competitive nature of binding of these antagonists.

Acute infusion of nicotine potentiates norepinephrine-induced vasoconstriction in the hamster cheek pouch

Although cigarette smoking and the components of cigarette smoke appear to alter nitric oxide synthase-dependent dilation of blood vessels, the effect of these substances on constrictor responses of resistance arterioles has not been examined. Thus the goal of this study was to examine the effect of a major component of cigarette smoke-that is, nicotine-on constrictor responses of cheek pouch arterioles. The diameter of cheek pouch arterioles (approximately 50 microm in diameter) was measured by using intravital microscopy. We examined the responses of arterioles to angiotensin II, arginine vasopressin, norepinephrine, and the thromboxane analog U-46619 before and after treatment with vehicle (saline solution), N(G)-monomethyl-L-arginine (L-NMMA; 1.0 micromol/L), or nicotine (2.0 microg/kg/min i.v. for 30 minutes followed by a maintenance dose of 0.35 microg/kg/min for 30 minutes). Topical application of angiotensin II (0.01 and 0.1 nmol/L), arginine vasopressin (1.0 and 10 pmol/L), norepinephrine (1.0 and 10 nmol/L), and U-46619 (0.01 and 0.1 nmol/L) produced marked reproducible constriction of cheek pouch arterioles in hamsters treated with vehicle. Topical application of L-NMMA potentiated constrictor responses of arterioles to the high dose of arginine vasopressin (28%+/-4% versus 36%+/-4%; P<.05) and to both doses of norepinephrine (14%+/-1% and 24%+/-2% versus 19%+/-1% and 31%+/-3%; P<.05). The infusion of nicotine did not alter responses to angiotensin II, arginine vasopressin, or U-46619 but modestly potentiated vasoconstriction in response to norepinephrine (12%+/-2% and 22%+/-2% versus 14%+/-2% and 26%+/-2%; P<.05). These findings suggest that the synthesis/release of nitric oxide may modulate constrictor responses of cheek pouch resistance arterioles to selected agonists. In addition, nicotine, at levels observed in smokers, may potentiate norepinephrine-induced vasoconstriction. We suggest that preservation/potentiation of vasoconstrictor responses may contribute to the pathogenesis of vascular abnormalities associated with cigarette smoking.

Constrictor responses of the rat basilar artery during diabetes mellitus

Diabetes mellitus produces abnormalities of the endothelium and impairs endothelium-dependent dilatation of large and small cerebral blood vessels. However, the effect of diabetes mellitus on cerebral vasoconstriction and the modulatory influence of nitric oxide on cerebral vasoconstriction is unclear. Thus, the first goal of this study was to examine the effect of diabetes mellitus on constrictor responses of the basilar artery in vivo. Our second goal was to examine a potential role for nitric oxide in modulating constrictor responses of the basilar artery. A craniotomy was performed over the ventral medulla to expose the basilar artery. The diameter of the basilar artery was measured using intravital microscopy in nondiabetic and diabetic (3-4 months after injection of streptozotocin; 50-60 mg/kg i.p.) rats in response to angiotensin II, arginine vasopressin, endothelin-1, and the thromboxane analogue, U-46619. Topical application of angiotensin II (10 and 100 nM) produced only minimal changes in diameter of the basilar artery which were similar in nondiabetic and diabetic rats (p>0.05). Arginine vasopressin (0.1 and 1.0 nM), endothelin-1 (10 and 50 nM), and U-46619 (10 and 100 nM) produced marked dose-related constriction of the basilar artery which also was similar in nondiabetic and diabetic rats (p>0.05). Next, we examine whether the synthesis/release of nitric oxide played a role in constriction of the basilar artery in response to the agonists. We found that L-NMMA (1.0 microM) did not alter constrictor responses of the basilar artery in nondiabetic and diabetic rats. Thus, responses of the basilar artery to important vasoactive agonists are not altered by diabetes mellitus. In addition, it does not appear that the synthesis/release of nitric oxide modulates constrictor responses of the basilar artery to angiotensin II, arginine vasopressin, endothelin-1 and U-46619. We suggest that preservation of vasoconstrictor responses, coupled with impaired vasodilator responses, may contribute to the pathogenesis of cerebrovascular abnormalities associated with diabetes mellitus.

Influence of indomethacin and L-NMMA on vascular tone and angiotensin II-induced vasoconstriction in the human forearm

Stimulated release of vasodilator prostaglandins and nitric oxide by angiotensin II may counteract the vasoconstrictor effects of this octapeptide. We investigated the effects of inhibition of prostaglandin synthesis by indomethacin and of nitric oxide formation by NG-monomethyl-L-arginine (L-NMMA) on baseline forearm blood flow (FBF) and on angiotensin II-induced vasoconstriction in healthy subjects. For comparison, the effects of the AT1-receptor antagonist losartan on these parameters were determined. FBF was measured by venous occlusion plethysmography. Angiotensin II (0.01-10 ng/kg/min) was infused into the brachial artery, in the absence and presence of indomethacin (0.65 micrograms/kg/min; n = 8), L-NMMA (30 micrograms/kg/min; n = 5), and losartan (3 micrograms/kg/min; n = 12), respectively. Sodium nitroprusside was used to submaximally predilate the forearm vascular system. Baseline FBF remained unchanged with indomethacin and losartan, but was significantly decreased by -42 +/- 6% (mean +/- SEM) by L-NMMA. The dose-dependent angiotensin II-induced vasoconstriction was unaffected by indomethacin and L-NMMA, but was inhibited by losartan. Emax was -78 +/- 2% during control conditions, -84 +/- 3% during indomethacin (n.s.), -74 +/- 4% during L-NMMA (n.s.), and -17 +/- 6% during losartan infusion (p < 0.05). None of the interventions significantly changed the EC50 value of angiotensin II of -9.4 +/- 0.14 log M. In conclusion, in the human forearm of healthy subjects, neither endogenous angiotensin II nor cyclooxygenase-dependent prostaglandin synthesis plays a role in the genesis of vascular tone, whereas endogenous nitric oxide production does. The constrictor effects of angiotensin II are counteracted by neither stimulated release of prostaglandins nor by that of nitric oxide.

Comparative vasoconstrictor effects of angiotensin II, III, and IV in human isolated saphenous vein

Elevated levels of angiotensin (Ang II) and its degradation products angiotensin III (Ang III) and angiotensin IV (Ang IV) may contribute to the regulation of vascular tone under various clinical circumstances. We investigated the contractile effects of Ang III and Ang IV in endothelium-denuded human saphenous vein (SV) preparations and compared them with those of Ang II. The veins were suspended in organ chambers, and changes in isometric force were recorded. Ang II (0.1-100 nM), Ang III (1 nM-3 microM), and Ang IV (0.3 microM-0.1 mM) caused concentration-dependent contractions with comparable maximal responses (Emax). Ang III was 16 times less active than Ang II, whereas Ang IV was approximately 2,700-fold less potent than Ang II. In the presence of the aminopeptidase-A and -M inhibitor amastatin (10 microM), the potencies of Ang III and Ang IV were increased by approximately 16 and 12 times, respectively, although no changes of Ang II potency were observed. The AT1-selective Ang II receptor antagonist losartan (10 and 100 nM) but not the AT2-selective antagonist PD123177 (1 microM), shifted the concentration-response curves (CRC) for the angiotensin peptides to the right in a parallel manner. Preincubation with indomethacin (10 microM), a cyclooxygenase inhibitor, did not influence the CRCs for any of the angiotensin peptides studied. Tachyphylaxis was investigated by constructing a second series of CRCs for the angiotensin peptides after an interval of 60 min. Ang II showed strong tachyphylaxis (the Emax value of the second Ang II CRC was approximately 50% of the first), whereas Ang III and Ang IV did not. Our results indicate that in endothelium-denuded human SV, both Ang III and Ang IV are less potent but similarly efficacious vasoconstrictor agents compared with Ang II. Endogenous aminopeptidase activity may counteract the effects of the angiotensin peptides. The contractile responses to all three peptides are mediated via AT1-receptors but not AT2-receptors.

Age-related differences and roles of endothelial nitric oxide and prostanoids in angiotensin II responses of isolated, perfused mesenteric arteries and veins of rats

We examined whether or not cyclo-oxygenase products of arachidonic acid and endothelium-derived relaxing factor (nitric oxide, NO) regulate the vascular response to angiotensin II differently with aging or development. For this purpose angiotensin II responses of isolated, perfused rat mesenteric vascular beds were compared between rats aged 4 weeks and 32 weeks. Angiotensin II increased perfusion pressure in arteries and veins of both rats aged 4 weeks and 32 weeks. In the arteries of rats aged 32 weeks the increase was slight, and less than that in rats aged 4 weeks. In contrast, the veins showed similar increases in perfusion pressure in rats aged 4 weeks and 32 weeks. Indomethacin, an inhibitor of cyclo-oxygenase, at 5 x 10(-6) M depressed the increase in perfusion pressure only in the arteries of rats aged 32 weeks. NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthase, applied at 5 x 10(-6) M in the presence of indomethacin enlarged the perfusion pressure increase in the arteries of both rats aged 4 weeks and 32 weeks, while it failed to modify that in the veins. After removal of the endothelium from the blood vessels, the perfusion pressure responses in arteries were increased in both rats aged 4 weeks and 32 weeks, whereas those in veins were not affected. Regardless of the endothelium being intact or removed, the increase in arterial perfusion pressure of rats aged 32 weeks all but disappeared with 5 x 10(-6) M furegrelate, an inhibitor of thromboxane A2 synthase, and with a combined application of furegrelate and 10(-6) M SQ29,548, a blocker of thromboxane A2/prostaglandin H2 receptors. These results indicate the following: in rat mesenteric vascular beds the angiotensin II response in the arteries appears to diminish with aging or development, whereas that in the veins does not change. The NO released from the endothelium regulates the arterial response but vasodilating prostanoids have no role in the response. Moreover, in the arteries of rats aged 32 weeks, vasoconstricting prostanoids, such as prostaglandin H2 and thromboxane A2, seem to play a role in angiotensin II-induced vasoconstriction. With aging or development, and depending on the type of blood vessel, NO and prostanoids appear to modify the angiotensin II response differently.

Effects of angiotensin I and angiotensin II in blood vessels: greater influence of converting enzyme activity in the rabbit basilar artery

We investigated the constrictor effects of Angiotensin I (Ang I) and Angiotensin II (Ang II) on rabbit peripheral (aorta, carotid artery, mesenteric artery, saphenous artery) and cerebral (basilar artery) vessels and in rat aorta in functional organ bath studies. The effect of angiotensin converting enzyme (ACE) inhibition by captopril was also assessed in these preparations. Ang II elicited concentration-dependent contractions with comparable potency in rabbit and rat endothelium-free vascular rings (pD2 about 8.5) which indicates a lack of species and regional variation in the contractile responses to Ang II. The responses to Ang II were reduced by the presence of a functional endothelium in rabbit mesenteric artery and in rat aorta. Since ACE determines the plasma and tissue conversion of Ang I to active Ang II, we calculated the ratio R (EC50 Ang I-induced contraction: EC50 Ang II-induced contraction) as an indicator of the tissue ACE effectiveness. In the aorta without endothelium, Ang I was found to be much less potent than Ang II in the rabbit (R = 44) compared with the rat (R = 3.5). This species difference in the aortic conversion of Ang I to Ang II was confirmed by the use of captopril. Captopril (10(-6) M) shifted the Ang I concentration/ response curve by 2- and 14-fold to the right in rabbit and rat respectively. In other rabbit blood vessels, the rank order of potency to Ang I in endothelium denuded rings was basilar artery > > carotid artery > or = aorta > or = saphenous artery. In addition, the R value was the lowest for the basilar artery (R = 2.5). This is in agreement with the highest rightward shift (78-fold) of the Ang I concentration/response curve by captopril for basilar artery in comparison with only 3-, 8- and 3-fold shifts observed in carotid artery, saphenous artery and aorta respectively. In conclusion, our data provide evidence for a greater influence of ACE in rabbit basilar artery than in peripheral vessels.

Endothelium-dependent responses and inhibition of angiotensin-converting enzyme

1. Experimental and clinical studies have demonstrated the efficacy of inhibitors of angiotensin-converting enzyme (ACE) in a variety of cardiovascular diseases. Both structural and functional improvements have been reported. 2. Hypertension, atherosclerosis, congestive heart failure or ageing are accompanied by endothelial dysfunctions. The vasoactive endothelium-derived relaxing factors, nitric oxide, endothelium-derived hyperpolarizing factor and prostacyclin, could be involved, depending on the pathology. 3. Some of the beneficial effects of ACE inhibitors may be due to the augmented release of these endothelial factors resulting from the protection of locally produced bradykinin, particularly at the endothelial level.

Comparative effects of angiotensin II and its degradation products angiotensin III and angiotensin IV in rat aorta

1. In the present study, the contractile effects of angiotensin III (AIII) and angiotensin IV (AIV) compared with those of angiotensin II (AII) were determined in rat aortic ring preparations. 2. All three peptides caused concentration-dependent contractions with similar maximal responses. AIII proved approximately 4 times less potent than AII, whereas AIV was about 1000 times less active than AII. 3. The selective AT1-receptor antagonist, losartan (10-300 nM) caused parallel rightward shifts of the concentration-response curves (CRC) for all three peptides. The Schild plot slopes for the effect of losartan on AIII curves were significantly lower than unity (P < 0.05). The selective AT2-receptor antagonist, PD123177 did not influence the CRCs for AII and AIV. However, the AIII curves were moderately shifted leftward in the presence of PD123177 (0.1 and 1 microM). 4. Destruction of the endothelium or incubation with the NO-synthesis inhibitor NG-monomethyl-L-arginine acetate (L-NMMA) (0.1 mM) significantly enhanced the contractile responses to all three peptides. 5. Tachyphylaxis was investigated by constructing a second CRC for all three peptides, after an interval of 1 h. The presence of endothelium significantly enhanced the development of tachyphylaxis to all three peptides. However, in endothelium-denuded preparations, the Emax value of the second curve elicited by AII was about 50%, compared with the first one, whereas for AIII and AIV Emax values were as high as 90% and 100%, respectively. 6. Our results indicate that both AIII and AIV are less potent but similarly efficacious vasoconstrictor agents compared with AII. Their contractile effects are also mediated by AT1-receptors and probably modulated by endothelium. Tachyphylaxis induced by AIII and AIV proved weaker than that for AII. Tachyphylaxis appears to be enhanced by the presence of an intact endothelium.