Prolonged blockade of endothelin ET(A) receptors decreases vascular reactivity in the aorta of spontaneously hypertensive rats in vitro.

We investigated the effect of prolonged endothelin-1 type A (ET(A)) receptors blockade on the constrictor response to phenylephrine and the dilator response to acetylcholine (ACh) in isolated aortic rings from normotensive [Wistar-Kyoto (WKY)] rats and spontaneously hypertensive rats (SHRs). Animals were treated for 2 weeks with the ET(A)-receptor blocker LU135252 (50 mg/kg/day; n = 8). LU135252 treatment did not affect blood pressure in both strains. In isolated aortic segments, dilation to ACh and contractions to phenylephrine were decreased only in SHRs. Nitric oxide (NO) synthesis blockade (L-NAME, 0.1 mM) inhibited 90+/-11% (WKY rats) and 76+/-8% (SHRs) of ACh-induced dilation. Cyclooxygenases blockade (indomethacin, 10 microM) had no effect in both strains. Endothelium-derived hyperpolarizing factor(s) (EDHF) blockade (KCl, 20 mM) suppressed the remaining ACh-induced dilation in both strains. Treatment with LU135252 significantly decreased NO-dependent dilation, as compared with controls [70+/-8% vs. 90+/-11% (WKY rats) and 54+/-6% vs. 76+/-8% (SHRs) of total dilation; p<0.05]. On the other hand, EDHF-dependent dilation was significantly higher in the LU135252 groups [29+/-5% vs. 10+/-3% (WKY rats) and 44+/-7% vs. 19+/-4% (SHRs) of total dilation; p<0.05]. Thus prolonged ET(A)-receptor blockade decreased the responsiveness to phenylephrine and ACh in SHR aortas and changed the proportion of dilator agents in ACh-induced dilation.     

The abnormal renal vasodilator response to D1-like receptor stimulation in conscious SHR can be normalized by AT1 blockade

BACKGROUND: We previously showed that the renal vasodilator response to a D1-like receptor agonist is blunted in conscious SHR compared with WKY rats. The mechanism of this impaired dopaminergic responsiveness in SHR is unclear. An altered balance between the renin-angiotensin-aldosterone system (RAAS) and the dopaminergic system may be involved. To determine the interaction between the RAAS and the dopaminergic system in the blunted D1-like responsiveness in SHR, we studied the renal vasodilator response to the D1-like receptor agonist fenoldopam before and after 7 days of pretreatment with the AT1-receptor antagonist (AT1-A) L158,809 in conscious SHR and WKY rats. METHODS: Effective renal plasma flow (ERPF) was measured by the clearance of I-hippuran. Mean arterial pressure (MAP) was measured via an intraarterial catheter. RESULTS: Without pretreatment, MAP was reduced to comparable degrees by fenoldopam in WKY (-7 +/- 4%, ns) and SHR (-6 +/- 1%, P < 0.05). However, ERPF was significantly more increased (P < 0.006) by fenoldopam in WKY (+26 +/- 2%, P < 0.0001) than in SHR (+2 +/- 2%, ns). AT1-A treatment reduced MAP and increased ERPF and glomerular filtration rate significantly in both strains. Pretreatment with AT1-A significantly potentiated the fenoldopam-induced rise in ERPF in SHR, but not in WKY, without affecting the blood pressure responses in either strain. As a result, during pretreatment with an AT1-A, the rise in ERPF by fenoldopam was similar in both strains (SHR +25 +/- 2%, P < 0.0001; WKY +33 +/- 2%, P < 0.0001). CONCLUSIONS: These results suggest that the RAAS accounts for the blunted renal vasodilator response to a D1-like receptor agonist in SHR. A dysbalance between the dopaminergic system and the RAAS may be involved in the abnormal renal hemodynamic regulation in SHR.     

Nitric oxide inhibition accelerates hypertension and induces perivascular inflammation in rats

1. Inflammatory changes in peripheral arteries have been reported in animal models of hypertension. Whether they occur in cerebral arteries (CA) with hypertension induced by deprivation of endogenous nitric oxide (NO) remains unknown. 2. In the present study, we compared the arteriolar injury score (AIS) and perivascular inflammation in CA between hypertensive and normotensive rats following NO deprivation with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). Five-week-old male spontaneously hypertensive rats (SHR) and Wistar -Kyoto (WKY) rats were fed with L-NAME (1 mg/mL) for 4 weeks. 3. Nitric oxide deprivation resulted in time-dependent elevations in tail-cuff pressure (representing systolic blood pressure (SBP)) in both SHR and WKY rats. The magnitude of increase in SBP was larger in SHR (+81.0 +/- 3.2 vs+25.0 +/- 2.2 mmHg; P < 0.01). Arteriolar hyalinosis and AIS in various segments of the CA were assessed with periodic acid-Schiff staining and inflammatory cells were immunostained with the antibody against macrophage/monocyte marker (ED1). The ED1+ cells appeared in the middle CA of L-NAME-treated SHR as early as 2 weeks after treatment. These cells were not observed in L-NAME-treated WKY rats and untreated SHR. More ED1+ cells were found in L-NAME-treated SHR than L-NAME-treated WKY rats after 4 weeks treatment. 4. The AIS and number of ED1+ cells around the perivascular area of the internal carotid artery were significantly higher in L-NAME-treated compared with untreated rats (AIS: 137 +/- 28 vs 46 +/- 10 for WKY rats, respectively; 169 +/- 18 vs 53 +/- 6 for SHR, respectively (P < 0.01); ED1+ cells: 7.9 +/- 0.6 vs 1.3 +/- 0.9 for WKY rats, respectively; 13.6 +/- 2.7 vs 2.1 +/- 0.9 for SHR, respectively (P < 0.01)), although SBP was higher in untreated SHR than in L-NAME-treated WKY rats (170 +/- 4 vs 137 +/- 4 mmHg, respectively; P < 0.05). 5. These findings suggest that ED1+ cells appeared in the middle CA of L-NAME-SHR as early as 2 weeks after treatment. Chronic inhibition of NO accelerates hypertension and induces perivascular inflammation.     

Endothelium-derived relaxing factor released by 5-HT: distinct from nitric oxide in basilar arteries of normotensive and hypertensive rats.

1. The role of the endothelium in cerebrovascular responses to 5-hydroxytryptamine (5-HT) was investigated in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) in vitro. 2. Cumulative addition of 5-HT caused concentration-dependent contractions in ring preparations of SHR basilar arteries; the contractile response was smaller in WKY basilar arteries. 3. Removal of the endothelium enhanced markedly the contractile responses to 5-HT in WKY arteries but had only a slight effect in SHR arteries. The responsiveness to 5-HT in WKY arteries after removal of endothelium was comparable to that in SHR arteries. 4. The endothelium-dependent relaxation induced by acetylcholine in WKY basilar arteries was almost abolished by treatment with 10 microM methylene blue or 10 microM NG-nitro-L-arginine (L-NOARG). However, the response to 5-HT was not affected by treatment with methylene blue, L-NOARG or indomethacin. 5. Application of 10-20 mM K+ or 3.2 mM tetraethylammonium (TEA) did not change significantly, or only increased slightly, the resting tension, but markedly enhanced the contractile response to 5-HT in WKY arteries with endothelium. In contrast, the submaximal response to 5-HT in SHR arteries with endothelium was significantly enhanced by 0.3 mM TEA. 6. In the presence of 1 mM TEA, the application of 10 microM L-NOARG further enhanced the responses of 5-HT in WKY arteries with endothelium. In SHR arteries with endothelium, 10 microM L-NOARG per se enhanced slightly but significantly the responses to 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of the flavonoid quercetin and its methylated metabolite isorhamnetin in isolated arteries from spontaneously hypertensive rats

Chronic oral quercetin exerts antihypertensive effects in spontaneously hypertensive rats (SHR). In the present study, the vasodilator effects of the flavonoid quercetin and its main metabolite isorhamnetin were analysed in isolated thoracic aorta, iliac artery and on the isolated perfused mesenteric resistance vascular bed from SHR and normotensive Wistar Kyoto rats (WKY). In noradrenaline-precontracted vessels from SHR there was an inverse correlation between the relaxant potency (pIC50) of quercetin (4.76 +/- 0.02, 5.08 +/- 0.12, 5.30 +/- 0.18, in aorta, iliac arteries and mesentery, respectively) and isorhamnetin (4.90 +/- 0.11, 5.38 +/- 0.15 and 5.80 +/- 0.10, respectively) and the diameter of the vessel studied. Both flavonoids were more potent in endothelium-denuded aortae and iliac arteries from SHR than from normotensive WKY rats. In addition, in aortae from SHR both flavonoids restored the endothelial-dependent vasodilation. Isorhamnetin, but not quercetin, also reduced the endothelium-dependent contractile responses induced by acetylcholine. These direct vasodilator effects, together with the improvement of endothelial function, are good candidates to explain the blood pressure reduction and vascular protective effects of quercetin in animal models of hypertension and possibly in human cardiovascular diseases.     

Aldosterone causes vasoconstriction in coronary arterioles of rats via angiotensin II type-1 receptor: influence of hypertension

Aldosterone is involved in many cardiovascular diseases with increased oxidative stress. Aldosterone-induced cardiac fibrosis is abolished by blockade of angiotensin II Type-1 (AT1) receptor. Recently, non-genomic vasoconstrictor effects of aldosterone were reported in various vascular beds. We tested the hypothesis that aldosterone stimulates angiotensin AT1 receptor, and causes vasoconstriction by increasing oxidative stress in coronary microcirculation. Coronary arterioles (60-120 microm) were isolated from spontaneously hypertensive rats (SHR) and control Wistar Kyoto (WKY) rats, aged 23-26 weeks. They were cannulated, and pressurized at 60 cm H2O. Effect of aldosterone (10(-15) to 10(-6) M) on coronary arteriolar diameter was examined. Aldosterone rapidly and dose-dependently decreased coronary arteriolar diameter in WKY rats and SHR (diameter changes, 8.4+/-0.7% vs 13.9+/-0.8%, P<0.05). Aldosterone-induced vasoconstriction was enhanced by 1.6-folds in SHR compared to WKY rats (P<0.05). Mineralocorticoid receptor antagonist spironolactone (10(-6) M) did not influence aldosterone-induced vasoconstriction. Selective angiotensin AT1 receptor blocker valsartan (10(-4) M) or candesartan (10(-7) M) abolished aldosterone-induced vasoconstriction. Similarly, superoxide dismutase (SOD, 300 U/ml), and NADPH oxidase inhibitor apocynin (10(-4) M) abolished it. Moreover, the vasoconstrictor effect of aldosterone disappeared in denuded vessels. Real-time quantitative RT-PCR revealed that angiotensin AT1 receptor mRNA level in coronary arterioles of SHR was upregulated by 1.5-folds compared to that in WKY rats (P<0.05). Aldosterone causes vasoconstriction in coronary arterioles, and this vasoconstrictor effect is enhanced by genetically defined hypertension. Aldosterone-induced vasoconstriction is mediated by angiotensin AT1 receptor presumably via oxidative stress.     

Increased 45Ca influx in response to alpha 1-adrenoceptor stimulation in spontaneously hypertensive rat caudal artery

The isometric tension development and 45Ca influx in response to norepinephrine (NE) and methoxamine stimulation were investigated in caudal arteries of spontaneously hypertensive rats (SHR) and Wistar Kyoto normotensive rats (WKY). The maximum isometric tension developed as well as 45Ca influx in response to NE and methoxamine stimulation were significantly increased (p less than 0.05) in SHR caudal arteries as compared with WKY. On the other hand, neither the isometric tension developed nor the 45Ca influx in response to K+ depolarization were different between WKY and SHR caudal arteries. Estimation of [3H]prazosin binding to the membranes isolated from caudal artery of WKY and SHR showed a single class of high-affinity binding sites with Kd values for SHR 128 +/- 14 pM and for WKY 141 +/- 19 pM, and Bmax values for SHR 108 +/- 14 fmol/mg protein and for WKY 113 +/- 21 fmol/mg protein. From these results, we conclude: (a) Increased contractile response of SHR caudal artery rings to alpha 1-adrenoceptor stimulation appears at least in part to be due to an increased Ca2+ influx through receptor-operated Ca2+ channels; (b) the affinity or density of alpha 1-adrenoceptors estimated by [3H]prazosin binding is not altered in the SHR caudal artery.     

Tyrosine kinase inhibition attenuates vasopressin-induced contraction of mesenteric resistance arteries: alterations in spontaneously hypertensive rats

This study examined the role of tyrosine kinase-dependent signaling pathways in arginine vasopressin (AVP)-induced contractile responses in resistance arteries from spontaneously hypertensive rats (SHR). Systolic blood pressure was measured in conscious 6- and 21-week old SHR and Wistar Kyoto rats (WKY) by tail cuff measurements. Segments of third-order mesenteric arteries (about 200 microm in diameter, 2mm in length) were mounted in a pressurized chamber with the intraluminal pressure maintained at 45 mmHg. Contractile effects of AVP (10-12 to 10-7 mol/l) were determined in the absence and presence of the selective tyrosine kinase inhibitor tyrphostin A23 (10-5 mol/l) and the inactive analogue, tyrphostin A1 (10-5 mol/l). Systolic blood pressure was significantly higher in SHR compared with age-matched WKY (p < 0.01). AVP increased contraction in a dose-dependent manner with significantly greater responses in adult SHR (pD2 = 10.3 +/- 0.06) than age-matched WKY (pD2 = 9.4 +/- 0.04). Tyrphostin A23 shifted the AVP dose response curve to the right in 6- and 21-week WKY and 6-week SHR, but had little effect on AVP-induced responses in 21-week-old SHR. Tyrphostin A1 did not influence contraction in any groups. Protein tyrosine phosphorylation in VSMCs and mesenteric arteries was increased 2-3 fold in 21-week SHR compared with WKY counterparts. AVP significantly increased tyrosine phosphorylation in VSMCs, with enhanced effects in SHR compared with WKY (p < 0.05). These effects were inhibited by tyrphostin A23. Our findings demonstrate that protein tyrosine kinases contribute to AVP-induced contraction of resistance arteries from WKY and SHR during the phase of developing hypertension. These processes do not seem to play an important role in AVP-induced hypercontractility in SHR with established hypertension.     

Comparative effects of tramadol on vascular reactivity in normotensive and spontaneously hypertensive rats

The aim of the present study was to determine the effects of tramadol on vascular reactivity in aortic rings from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Aortic rings, with or without endothelium, were obtained from male WKY rats and SHR (15-20 weeks old) and prepared for isometric tension recording. Aortic rings were precontracted with phenylephrine (10 micromol/L) or 40 mmol/L KCl and then exposed to cumulative concentrations of tramadol (0.1-1 mmol/L). Tramadol produced a concentration-dependent relaxation of precontracted aortic rings from WKY rats and SHR, which was not dependent on functional endothelium. Vascular relaxation was significantly greater in rings from SHR than WKY rats. The concentration of tramadol necessary to produce a 50% reduction of the maximal contraction to phenylephrine (IC(50)) in rings with and without endothelium from SHR was 0.47 +/- 0.08 and 0.44 +/- 0.03 mmol/L, respectively (P = 0.76). Tramadol attenuated the contracture elicited by Ca2+ in depolarized tissue, suggesting that it may inhibit L-type Ca2+ channels. However, pretreatment with nicardipine (1 micromol/L) prevented the relaxation induced by tramadol in aortic rings from WKY rats and partially reduced its inhibitory effect in aortic rings from SHR. 6. Pretreatment of endothelium-denuded aorta with glybenclamide (3 micromol/L), 4-aminopyridine (3 mmol/L), tetraethylammonium (3 mmol/L) and naloxone (100 micromol/L) did not affect tramadol-induced vasodilation of aortic rings from either WKY rats or SHR. Intravenous administration of tramadol (10 mg/kg) to conscious SHR significantly reduced both systolic and diastolic blood pressure from 171.4 +/- 5.3 to 129.3 +/- 5.3 (P = 0.002) and from 125.0 +/- 6.5 to 57.8 +/- 8.9 mmHg (P = 0.003), respectively.     

Nitric oxide-releasing aspirin inhibits vasoconstriction in perfused tail artery of normotensive and spontaneously hypertensive rats

The aim of this study was to investigate the capacity of the 2-(acetyloxy)benzoic acid 3-(nitrooxymethyl)phenyl ester (NCX 4016), a nitric oxide (NO)-releaser derivative of aspirin, to decrease blood pressure in spontaneously hypertensive rats (SHR) and to counteract the adrenergic vasoconstriction in perfused tail artery of these animals. Oral treatment for 10 consecutive days with NCX 4016 (100 micromol/kg) in SHR and their genetic controls Wistar Kyoto (WKY) rats resulted in a reduction of blood pressure in SHR but not in WKY rats. In SHR, the NCX 4016 treatment increased the serum nitrite/nitrate and diminished the serum thromboxane B2, whereas aspirin did not change blood pressure but abolished the serum thromboxane B2. Perfused tail arteries excised from vehicle-treated SHR exhibited a significant impairment of endothelium-dependent vasorelaxant function. These vessels, prepared from SHR or WKY rats treated orally with NCX 4016 (10, 30 and 100 micromol/kg for 7 consecutive days), revealed a dose-dependent decrease in vasoconstriction in response to transmural nerve stimulation and norepinephrine, whereas aspirin was ineffective. Furthermore, in tail arteries of both SHR and WKY rats treated orally with NCX 4016 (100 micromol/kg for 7 consecutive days), the cGMP increased significantly. In conclusion, NCX 4016, by releasing NO and increasing cGMP in vascular tissue, reduces sympathetic-mediated vasoconstriction in resistance vessels and lowers blood pressure in SHR.     

Upregulated function of phosphatidylinositol-3-kinase in genetically hypertensive rats: a moderator of arterial hypercontractility

1. The growth enzyme phosphatidylinositol 3-kinase (PI3K) was recently implicated in the mediation of arterial spontaneous tone, an event observed in arteries from hypertensive, but not normotensive, subjects that contributes to changes in total peripheral resistance in the hypertensive state. We have shown this occurrence in experimentally induced models of hypertension. However, because the majority of hypertension is genetically based, it is important to demonstrate a similar change in genetically hypertensive animals. 2. Aorta from spontaneously hypertensive rats (SHR; systolic blood pressure = 183 +/- 4 mmHg) and Wistar Kyoto (WKY) rats (115 +/- 2 mmHg) were isolated for the measurement of isometric contractile force. Aorta from SHR displayed small increases (approximately 5% maximum phenylephrine (PE)-induced contraction) in spontaneous tone, whereas aorta from WKY rats displayed none. The non-selective PI3K inhibitor LY294002 (20 micromol/L) and the selective inhibitor of the p110delta catalytic subunit of PI3K IC87114 (20 micromol/L) caused a fall of basal tone in SHR aorta (20 +/- 7 and 24 +/- 6% of the initial PE contraction, respectively), but did not alter tone in arteries from WKY rats. LY294002, but not IC87114, normalized the increased potency of noradrenaline (NA) observed in aorta from SHR (-log EC50 values for NA in the presence of vehicle in WKY rats and SHR 7.5 +/- 0.1 and 7.8 +/- 0.1, respectively (P < 0.05); -log EC(50) values for NA in the presence of LY294002 in WKY rats and SHR 7.0 +/- 0.1 and 7.0 +/- 0.1, respectively). 3. Biochemical expression of the p110 catalytic and p85 regulator subunits of PI3K in western analyses revealed no difference in expression of the regulatory p85alpha or p110alpha protein subunits between WKY rats and SHR; p110gamma was not detected. In contrast, p110delta expression was increased greater than 30% in aorta from SHR compared with WKY rats (827.6 +/- 88.5 vs 576.8 +/- 53.4 arbitrary densitometry units, respectively). Immunohistochemical analyses revealed expression of the p110delta isoform in the smooth muscle of arteries. 4. These data underscore the relevance of an enzyme historically classified as one committed to growth/anti-apoptosis in modifying contractility and supports involvement of PI3K in genetically based hypertension.     

The alpha-adrenoceptor antagonist, zolertine, inhibits alpha1D- and alpha1A-adrenoceptor-mediated vasoconstriction in vitro

1. The antagonist effect of zolertine (4-phenyl-1-[2-(5-tetrazolyl)ethyl]piperazine trihydrochloride), on vascular contraction elicited by noradrenaline in aorta, carotid (alpha1D-adrenoceptors), mesenteric (alpha1A/D-adrenoceptors) and caudal arteries (alpha1A-adrenoceptors) from Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats and rabbit aorta (alpha1B-adrenoceptors), was investigated in endothelium-denuded arterial rings. 2. The selective alpha1D-adrenoceptor agonist, noradrenaline, elicited concentration-dependent contractions in all arterial rings from both species. Noradrenaline selectivity was: carotid = aorta > mesenteric = rabbit aorta > caudal arteries. 3. The contractile responses induced by noradrenaline were competitively antagonized by zolertine in rat carotid and aorta arteries, yielding pA2 values of WKY, 7.48 +/- 0.18; SHR, 7.43 +/- 0.13 and WKY, 7.57 +/- 0.24; SHR, 7.40 +/- 0.08, respectively. Zolertine was a non-competitive antagonist in some blood vessels as Schild plot slopes were lower than unity. The pKb estimates for zolertine were WKY, 6.98 +/- 0.16; SHR, 6.81 +/- 0.18 in the mesenteric artery, WKY, 5.73 +/- 0.11; SHR, 5.87 +/- 0.25 in the caudal artery and 6.65 +/- 0.09 in rabbit aorta. 4. Competition binding experiments using the alpha1-adrenoceptor antagonist [3H]prazosin showed a zolertine pKi of 6.81 +/- 0.02 in rat liver (alpha1B-adrenoceptors) and 6.35 +/- 0.04 in rabbit liver (alpha1A-adrenoceptors) membranes. 5. Zolertine showed higher affinity for alpha1D-adrenoceptors compared to alpha1A-adrenoceptors, while it had an intermediate affinity for alpha1B-adrenoceptors. The ability of the alpha1-adrenoceptor antagonist zolertine to block alpha1D-adrenoceptor-mediated constriction in different vessels of WKY and SHR rats may explain its antihypertensive efficacy despite its low order of potency.     

Effect of ouabain in pressurized middle cerebral arteries from normotensive and hypertensive rats.

The aim of the present study was to assess the ability of ouabain to induce vasomotor responses and interfere with the myogenic tone in isolated segments of middle cerebral arteries from male Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) subjected to different pressures and no-flow conditions using a pressure myograph. At 60 mmHg, ouabain (1 nM-1 mM) caused relaxations at concentrations of 10 microM and up in segments from WKY while 1 nM ouabain produced a relaxation that was unaltered by the remaining concentrations in SHR segments. The relaxations were higher in SHR than in WKY arteries. Endothelium removal practically abolished the relaxation in arteries from both strains, whereas 10 microM L-NAME (an inhibitor of nitric oxide synthase) had no effect. When arteries were pressurized from 20-120 mmHg, myogenic activity developed in 3 out of 10 WKY arteries while SHR arteries did not show myogenic tone. Endothelium removal did not alter the effects of pressure increase in both strains, and incubation of segments in a Ca(2+)-free medium to abolish myogenic tone, shifted the pressure-response curve of WKY segments to the left; pressure-response curves from SHR were not modified. Although ouabain (0.1 mM) did not alter the pressure-response curve from WKY segments, curves obtained from SHR were shifted to the left. These results suggest that: 1) ouabain produces vasodilation in pressurized middle cerebral arteries of WKY and SHR which is positively modulated by an endothelial factor distinct from nitric oxide; and 2) only WKY arteries develop myogenic activity while the diameter of SHR arteries is passively enhanced with increases in intraluminal pressure. This passive increase is facilitated by ouabain. Therefore, hypertension modifies the mechanical properties of cerebral arteries resulting in a loss in the capacity for autoregulation.     

Influence of hypertension on nitric oxide synthase expression and vascular effects of lipopolysaccharide in rat mesenteric arteries

1. Experiments were designed to investigate the effects of the inducible nitric oxide synthase (iNOS) stimulator, lipopolysaccharide (LPS), on noradrenaline (NA) responses and on NOS activity and its expression in intact mesenteric resistance arteries (MRAs) from Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. 2. In MRAs from WKY, LPS (10 microg ml(-1); 1-5 h) reduced the vasoconstrictor responses to NA (0.1 - 30 microM) in the presence, but not in the absence of L-arginine (L-Arg, 10 microM). However, in SHR arteries, LPS induced an incubation-time dependent reduction of NA responses in the absence, as well as the presence, of L-Arg. The LPS inhibitory effect was reduced by the non-specific NOS inhibitor L-N(G)-nitroarginine methyl ester (L-NAME, 100 microM) and the selective iNOS inhibitor, aminoguanidine (100 microM). 3. L-NAME alone similarly shifted the concentration-response curve to NA leftward in arteries from both strains, while aminoguanidine had no effect. L-Arg shifted the curve to NA rightward only in SHR MRAs. 4. Basal activity of both iNOS and constitutive NOS (conversion of [(3)H]-L-Arg to [(3)H]-L-citrulline) was similar in arteries from both strains. After 5 h incubation with LPS, only iNOS activity in arteries from SHR was increased. 5. Basal iNOS protein expression was undetectable; basal endothelial (eNOS) protein expression was similar in arteries from both strains, while neuronal (nNOS) was greater in arteries from SHR. LPS induced iNOS protein expression, that was higher in arteries from SHR than in those from WKY. 6. These results indicate that NO production, via iNOS induction, is greater than in those from MRAs from SHR to WKY.     

Angiotensin II receptors involved in the enhancement of noradrenergic transmission in the caudal artery of the spontaneously hypertensive rat.

1. The effects of the AT1 receptor antagonist losartan and the AT2 receptor antagonist PD 123319, on actions of angiotensin II in isolated caudal arteries of spontaneously hypertensive (SH) and age-matched normotensive (Wistar-Kyoto) rats were compared. 2. Angiotensin II (0.1-3 microM) produced concentration-dependent increases in perfusion pressure in artery preparations from both SH and Wistar-Kyoto (WKY) rats, the maximal increase in the SH rat being significantly greater than the increase in WKY rats. The increase in perfusion pressure in preparations from both strains of rats was prevented by losartan (0.1 microM) and unaffected by PD 123319 (0.1 microM), indicating that the vasoconstrictor action of angiotensin II is subserved by AT1 receptors. 3. Angiotensin II (0.1-3 microM) produced concentration-dependent enhancement of both stimulation-induced (S-I) efflux of [3H]-noradrenaline and stimulation-evoked vasoconstrictor responses in isolated preparations of caudal artery from both SH and WKY rats, in which the noradrenergic transmitter stores had been labelled with [3H]-noradrenaline. The maximum enhancement of S-I efflux produced by angiotensin II (1 microM) was significantly greater in artery preparations from WKY rats than in preparations from SH rats, whereas the maximum enhancement of stimulation-evoked vasoconstrictor responses was greater in preparations from SH rats than in those from WKY rats. 4. In artery preparations from both WKY and SH rats, the AT1 angiotensin II receptor antagonist, losartan (0.01 and 0.1 microM), reduced or abolished the enhancement of both S-I efflux and vasoconstrictor responses by 1 microM angiotensin II. 5. The combination of 0.01 microM losartan and 0.1 microM angiotensin II enhanced both the S-I efflux and stimulation-evoked vasoconstrictor response in caudal artery preparations from WKY rats, whereas 0.1 microM angiotensin alone was ineffective. The AT2 receptor antagonist PD 123319 (0.01 and 0.1 microM) prevented the enhancement of both S-I efflux and stimulation-evoked vasoconstrictor responses by the combination of angiotensin II and losartan. 6. In contrast to findings in WKY preparations and those previously obtained for arteries from another normotensive strain (Sprague-Dawley), in artery preparations from SH rats there was no synergistic interaction between losartan and angiotensin II. Rather, combinations of 0.1 microM angiotensin II and PD 123319 (both 0.01 and 0.1 microM) enhanced S-I [3H]-noradrenaline efflux, whereas 0.1 microM angiotensin II alone was without effect. Moreover, losartan (0.1 microM) prevented the enhancement of S-I efflux by the combination of angiotensin II and PD 123319. 7. The present findings indicate that in the caudal artery of WKY and SH rats, and as previously found in Sprague-Dawley preparations, angiotensin II receptors similar to the AT1B subtype subserve enhancement of transmitter noradrenaline release. 8. As previously suggested for Sprague-Dawley caudal artery preparations, the synergistic prejunctional interaction of losartan and 0.1 microM angiotensin II in caudal artery preparations from WKY rats may be due to either the unmasking by losartan of a latent population of angiotensin II receptors subserving facilitation of transmitter noradrenaline release, or blockade by losartan of an inhibitory action of angiotensin II on transmitter release. 9. The synergistic interaction of PD 123319 and 0.1 microM angiotensin II in caudal arteries of SH rats may also be explained by either of the mechanisms proposed for the normotensive strains, but the involvement of different receptor subtypes would need to be postulated for each of the proposed mechanisms.     

Chronic At1 receptor blockade alters the mechanisms mediating hypoxic dilation in middle cerebral arteries

The purpose of this study was to determine whether chronic blockade of the angiotensin II (ANG II) AT1 receptor under normal physiological conditions impairs vascular relaxation mechanisms in isolated middle cerebral arteries (MCA). Male Sprague-Dawley rats on a standard diet were given losartan (1 mg/mL) in the drinking water or normal water ad libitum for 7 days. Vessel diameters were measured by television microscopy before and during exposure to various vasodilator agonists and reductions in PO2 from 140 mm Hg to 35-45 mm Hg. Dilations to acetylcholine (1 microM), the stable prostacyclin analogue iloprost (10 pg/mL), and the Gs protein activator cholera toxin (1 ng/mL) were completely eliminated in vessels from losartan-treated animals. However, middle cerebral arteries from control and losartan-treated rats still demonstrated significant dilations in response to reduced PO2. Hypoxic dilation of middle cerebral arteries from control rats was eliminated by indomethacin (1 microM) and unaffected by the NOS inhibitor L-NAME (100 microM) whereas dilation in response to reduced PO2 in middle cerebral arteries from losartan-treated rats was eliminated by L-NAME and unaffected by indomethacin. Middle cerebral arteries from control and losartan-treated animals exhibited similar dilations in response to the NO-donor sodium nitroprusside (1 microM). These data suggest that AT1 receptor activation is important in maintaining normal vascular relaxation mechanisms in cerebral resistance arteries during normal physiological conditions, and that AT1 receptor blockade causes a shift in the mechanisms of hypoxic dilation of middle cerebral arteries from cyclooxygenase metabolites to NO.     

Neuropeptide Y2 receptors are involved in enhanced neurogenic vasoconstriction in spontaneously hypertensive rats

1. The present study addressed the role of neuropeptide (NPY) Y2 receptors in neurogenic contraction of mesenteric resistance arteries from female spontaneously hypertensive rats (SHR). Arteries were suspended in microvascular myographs, electrical field stimulation (EFS) was performed, and protein evaluated by Western blotting and immunohistochemistry. 2. In vasopressin-activated endothelium-intact arteries, NPY and fragments with selectivity for Y1 receptors, [Leu31,Pro34]NPY, Y2 receptors, NPY(13-36), and rat pancreatic polypeptide evoked more pronounced contractions in segments from SHR than in Wistar Kyoto (WKY) arteries, even in the presence of the Y1 receptor antagonist, BIBP3226 (0.3 microM, (R)-N(2)-(diphenacetyl)-N-[(4-hydroxyphenyl)methyl]D-arginineamide). 3. In the presence of prazosin and during vasopressin activation, EFS-evoked contractions were larger in arteries from SHR compared to WKY. EFS contractions were enhanced by the Y2 receptor selective antagonist BIIE0246TF (0.5 microM, (S)-N2-[[1-[2-[4-[(R,S)-5,11-dihydro-6(6h)-oxodibenz[b,e]azepin-11-y1]-1-piperazinyl]-2-oxoethyl]cyclo-pentyl-N-[2-[1,2-dihydro-3,5 (4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]-argininamide), reduced by BIBP3226, and abolished by the combination of BIBP3226 and BIIE0246TF. 4. Immunoblotting showed NPY Y1 and Y2 receptor expression to be similar in arteries from WKY and SHR, although a specific Y2 receptor band at 80 kDa was detected only in arteries from WKY. 5. Immunoreaction for NPY was enhanced in arteries from SHR. In contrast to arteries from WKY, BIIE0246TF increased NPY immunoreactivity in EFS-stimulated arteries from SHR. 6. The present results suggest that postjunctional neuropeptide Y1 and Y2 receptors contribute to neurogenic contraction of mesenteric small arteries. Moreover, both enhanced NPY content and altered neuropeptide Y1 and Y2 receptor activation apparently contribute to the enhanced neurogenic contraction of arteries from SHR.     

Effects of intravenous nicorandil on coronary circulation in humans: plasma concentration and action mechanism

We investigated the cardiovascular profile of nicorandil, an antianginal agent, in humans. Pharmacologically, nicorandil acts as both an adenosine triphosphate (ATP)-sensitive K+ (K(ATP)) channel opener and a nitrate. We examined which of these mechanistic components has a predominant vasodilatory effect at clinical doses. Fourteen patients underwent cardiac catheterization. The effects of the continuous intravenous infusion of nicorandil (12 mg/45 min) were examined in angiographically normal coronary arteries. Coronary vascular resistance was calculated from coronary artery diameter and coronary blood flow velocity measured using an intravascular Doppler catheter. We compared the hemodynamic responses to nicorandil with those to the intracoronary injection of nitroglycerin (250 microg) and papaverine (12 mg). The epicardial coronary arteries responded to nicorandil at the lowest plasma concentration examined (dilation of +14.0 +/- 3.3% at approximately 170 ng/ml), whereas dilation of the coronary resistance arteries (i.e., a decrease in coronary vascular resistance) took place only at higher concentrations (>200 ng/ml). Nitroglycerin caused no further changes in coronary artery diameter or coronary vascular resistance. Papaverine caused no further increase in coronary artery diameter, but markedly decreased coronary vascular resistance (1.6 +/- 0.3 to 0.4 +/- 0.1 mm Hg/ml/min; p < 0.05). Nicorandil significantly decreased pulmonary capillary wedge pressure (i.e., reduced cardiac preload) at a plasma level of >200 ng/ml, but did not change either systemic or pulmonary vascular resistance. Thus nicorandil preferentially dilated epicardial coronary arteries rather than coronary resistance arteries, and had a stronger effect on preload than on afterload. These changes in human coronary hemodynamics suggest that the nitrate actions of nicorandil as a coronary vasodilator predominate over those as a K(ATP) opener.     

Changes in electrophysiological properties in the prostatic portion of vas deferens from spontaneously hypertensive rats

The main objective of this study has been to analyse the electrophysiological differences in the prostatic portion of vas deferens between spontaneously hypertensive (SHR) and Wistar Kyoto rats (WKY). Resting membrane potentials (RMP) recorded in SHR (-63.8+/-0.3 mV) and WKY (-68.1+/-0.3 mV) were significantly different. Bath applications of suramin (30 microM), alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP; 30 microM) or prazosin (0.1 microM) did not modify the control values of RMP. In control conditions, spontaneous excitatory junction potentials (SEJPs) were recorded in preparations from both groups of animals. SEJPs registered in SHR were greater than those in WKY in amplitude (7.0+/-0.4 mV vs. 2.6+/-0.1 mV) and frequency (0.25+/-0.02 Hz vs. 0.14+/-0.01 Hz). SEJP amplitude was abolished by bath applications of suramin (30 microM) or alpha,beta-meATP (30 microM). However, tetrodotoxin (TTX; 1 microM) and prazosin (0.1 microM) had no effect on this spontaneous activity. Electrical-field stimulation (EFS; 0.1 ms, 20 V, 0.2 Hz) induced an enhanced excitatory junction potential (EJP) in SHR but not in WKY (16.0+/-0.6 mV vs. 12.2+/-0.5 mV) which was abolished by TTX (1 microM), suramin (30 microM) and alpha,beta-meATP (30 microM). The degree of inhibition of both SEJP and EJP produced by alpha,beta-meATP (0.3-30 microM) was greater in SHR than in WKY. This study demonstrates an altered purinergic contribution to the co-transmission process in the prostatic portion of vas deferens from SHR.     

Alterations of cyclo-oxygenase products and NO in responses to angiotensin II of resistance arteries from the spontaneously hypertensive rat.

1. The involvement of cyclo-oxygenase (COX) products and nitric oxide (NO) in contractile responses of resistance arteries to angiotensin II (AII) were investigated in small mesenteric arteries from spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) rats. 2. In endothelium intact vessels, AII induced concentration-dependent responses without any significant difference between the two strains. However, removal of functional endothelium resulted in enhanced sensitivity to AII, the pD2 value increasing from 8.4 +/- 0.2 to 8.9 +/- 0.2 (P < 0.05) in WKY and from 8.2 +/- 0.1 to 8.6 +/- 0.1 (P < 0.05) in SHR (not significantly different between strains, n = 9 - 12). In addition, endothelium removal enhanced maximal contractions elicited by AII in SHR (1.4 +/- 0.1 to 2.1 +/- 0.2 mN mm-1, n = 5; P < 0.05) but not in WKY (1.0 +/- 0.1 to 1.2 +/- 0.1 mN mm-1, n = 5) vessels. 3. In the absence of functional endothelium, the COX inhibitor indomethacin (10(-5) M) reduced contractile responses elicited by AII in SHR arteries, resulting in 33 +/- 5% (n = 5) decrease in maximal contraction. However, it produced minimal if any, effect on responses of WKY vessels. In both strains, the TP receptor antagonist GR32191 B (3 x 10(-6) M) did not modify contractions elicited by AII in these conditions. 4. In the presence of functional endothelium, indomethacin (10(-5) M) almost abolished the responses to AII in both strains. GR32191 B (3 x 10(-6) M) reduced the sensitivity of WKY arteries to AII (pD2 = 8.1 +/- 0.1, P < 0.01) without any effect on maximal contraction. In SHR arteries, it markedly reduced maximal contraction (47 +/- 3.5%). 5. In both strains, the NO synthase inhibitor NG-nitro-L-arginine methy lester (L-NAME; 10(-4) M) had no effect in the absence of functional endothelium but it markedly reduced the inhibitory influence of endothelium on contractile responses to AII. Furthermore, in arteries with endothelium, it reduced the effect of both indomethacin and GR32191 B to the same level as observed in vessels without functional endothelium. 6. The results suggest that enhanced contraction caused by COX products was counteracted by enhanced relaxation caused by endothelium-derived NO in resistance mesenteric arteries of the SHR exposed to AII, compared to WKY arteries. The COX products involved in alterations of SHR responses comprised an endothelium-derived prostaglandin activating TP receptors and another nonendothelial unidentified vasoconstrictor compound which did not activate these receptors.