Vasorelaxant effects of the chronic treatment with melatonin on mesenteric artery and aorta of spontaneously hypertensive rats

OBJECTIVE: To investigate the effect of a chronic treatment with melatonin on arterial pressure and a possible improvement of the vascular muscarinic and NO synthase (NOS) pathways in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. DESIGN AND METHODS: Mean arterial pressure (MAP), systolic (SBP), diastolic blood pressure (DBP), and heart rate (HR) were evaluated in conscious rats treated with 30 mg/kg per day of melatonin during 4 weeks. Changes in MAP were evaluated following an intravenous injection of the NOS inhibitor N-omega-nitro-L-arginine methyl ester (L-NAME). Relaxant effects of acetylcholine (Ach), sodium nitroprusside (SNP), and the calcium ionophore A23187 were examined on mesenteric beds and aortic rings with or without treatment with melatonin. RESULTS: Melatonin produced a significant reduction of MAP, SBP, DBP and HR in SHR (P < 0.05). L-NAME increased the MAP of melatonin-treated SHR by the same magnitude as that of WKY rats which was significantly higher than that of non-treated SHR (P< 0.05). Melatonin treatment improved the maximal relaxation of mesenteric arteries to A23187 in SHR (P < 0.001) to the WKY level and caused a slight increment in Ach- and A23187-induced vasodilations in aorta from SHR and WKY rats (P < 0.05). CONCLUSION: The present study showed that melatonin exerted a bradycardic and an antihypertensive action in SHR. The enhancement by melatonin of the endothelium-dependent vasodilation (Ach and/or A23187) in mesenteric artery and aorta from SHR and WKY rats and the higher increase in MAP following L-NAME treatment in melatonin-treated SHR suggest the contribution of an improved vascular NOS pathway activity in the hypotensive effect of melatonin.     

Ferulic acid restores endothelium-dependent vasodilation in aortas of spontaneously hypertensive rats

BACKGROUND: Ferulic acid (FA), a phytochemical constituent, has antihypertensive effects, but a detailed understanding of its effects on vascular function remains unclear. The vasoreactivity of FA was assessed using aortic rings isolated from normotensive Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). METHODS: The effects of FA (10(-5) to 10(-3) mol/L) on vasodilatory responses were evaluated based on contractile responses induced by phenylephrine (10(-6) mol/L) in thoracic aortic rings from male WKY rats and SHR. Basal nitric oxide (NO) bioavailability in the aorta was determined from the contractile response induced by the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 10(-4) mol/L). The effects of FA on the production of NADPH-dependent superoxide anion were examined in SHR aortas. The impact of hydroxyhydroquinone, a generator of superoxide anions, on the FA-induced enhancement in acetylcholine-stimulated vasodilation was also investigated. RESULTS: The FA (10(-3) mol/L)-induced relaxation was partially blocked by removal of the endothelium or by pretreating SHR aortas with L-NAME. FA increased NO bioavailability, and decreased NADPH-dependent superoxide anion levels in SHR aortas. Ferulic acid improved acetylcholine-induced endothelium-dependent vasodilation in SHR, but not in WKY. Furthermore, the simultaneous addition of hydroxyhydroquinone significantly inhibited the increase in acetylcholine-induced vasodilation by FA. CONCLUSIONS: Ferulic acid restores endothelial function through enhancing the bioavailability of basal and stimulated NO in SHR aortas. The results explain, in part, the mechanisms underlying the effects of FA on blood pressure (BP) in SHR.     

Effect of 5-lipoxygenase blockade on blood pressure and acetylcholine-evoked endothelium-dependent contraction in aorta from spontaneously hypertensive rats

OBJECTIVE: Cysteinyl leukotrienes (cysLT) are pro-inflammatory and vasoactive products suspected to be involved in the regulation of vascular tone and blood pressure in hypertension. DESIGN: We investigated, in spontaneously hypertensive rats (SHR), the involvement of cysLT in the in-vivo regulation of blood pressure and the in-vitro endothelium-dependent contraction to acetylcholine in isolated aorta. METHODS: SHR and Wistar-Kyoto rats (WKY) were orally treated for 3 weeks with either the cysLT biosynthesis inhibitor MK-886 (0.1 mg/ml) or vehicle. After mean arterial blood pressure (MABP) measurement, aortic ring preparations were removed from all groups of animals, and contractions and relaxations were monitored subsequent to stimulation with acetylcholine. RESULTS: MABP was higher in SHR. Chronic treatment with MK-886 did not alter MABP in either SHR or WKY. In the presence of the N-nitro-L-arginine (L-NA, 100 micromol/l), and on prostaglandin F2alpha (PGF2alpha)-induced tone, acetylcholine evoked concentration-dependent contractions in intact aortic rings from SHR only. Pretreatment with either MK-886 (10 micromol/l), the 5-lipoxygenase (5-LO) inhibitor AA861 (10 micromol/l), or the cysLT1 receptor antagonist MK571 (1 micromol/l) reduced (P < 0.05) acetylcholine-induced contractions in intact aortic rings from SHR only. Acetylcholine-induced contractions were weaker (P < 0.01) in SHR chronically treated with MK-886 than in SHR. In the presence of L-NA, leukotriene (LT) D4 induced greater (P < 0.05) concentration-dependent contractions in aortic rings from SHR than from WKY. MK571 abolished LTD4-evoked contractions. CONCLUSION: These data suggested that 5-LO-derived products, through the activation of cysLT1 receptors, could be involved in the endothelium-dependent contraction to acetylcholine in aorta from SHR but not in the regulation of MABP in SHR.     

Effects of chronic treatment with simvastatin on endothelial dysfunction in spontaneously hypertensive rats.

OBJECTIVE: To investigate the effects of chronic treatment with simvastatin (SV) on endothelium-dependent relaxation and ouabain-induced contractions in aortic rings from spontaneously hypertensive rats (SHR), comparing with normotensive Wistar-Kyoto rats (WKY). METHODS: After a 12-week period of administration of 1 or 2 mg/kg SV to SHR and WKY, systolic blood pressure (SBP) and vascular reactivity in endothelium-intact aortic rings were assessed. RESULTS: Relaxation in response to acetylcholine (ACh) in WKY remained unaltered, but in SHR treated with 1 mg/kg SV, enhanced ACh-induced relaxation (P<0.05 versus untreated SHR) reached values observed in untreated WKY. The 2 mg/kg treatment also improved ACh relaxation (P<0.01 and P<0.05 versus untreated SHR and WKY respectively). Inhibiting cyclo-oxygenase (COX) with indomethacin (INDO) improved ACh relaxation in SHR (P<0.05) but not in WKY, independent of treatment with SV. Inhibition of nitric oxide synthase (NOS) with N(G)-nitro-L-arginine (L-NOARG) abolished ACh relaxations in all cases (P<0.001). The result was unaltered when combining INDO plus L-NOARG. SV treatment also decreased ouabain-induced contractions in endothelium-intact aortic rings from SHR, diminishing the percentage effect of contraction from 64.56+/-2.95 (untreated SHR) to 26.98+/-7.06 and 38.10+/-8.21 (1 and 2 mg/kg treated SHR respectively). Response to ouabain in WKY was not significantly affected by SV treatment CONCLUSIONS: Chronic treatment of SHR with SV improves endothelium-dependent ACh relaxation of the aortic rings, probably by an NO-involving mechanism more than by inhibiting contractile COX-derived factors. An improvement in endothelial modulation of ouabain-induced contractions was also observed after treatment with SV in SHR, which might be due to an inhibition of a calcium-sodium exchanger.     

Enhanced phospholipase C activity in the vascular wall of spontaneously hypertensive rats

To explore the roles of vascular phospholipase C activity in the development of hypertension, phospholipase C activity was examined in the aortic wall of spontaneously hypertensive rats (SHR). Phospholipase C activity was significantly enhanced (+87%, p less than 0.005) in 14-week-old SHR as compared with normotensive Wistar-Kyoto rats (WKY). The enzymatic activities were positively correlated with the levels of blood pressure in both of the rat strains (r = 0.62, p less than 0.003). Vascular phospholipase C was also significantly activated (+62%; p less than 0.006) in the aortic wall of 4-week-old prehypertensive SHR, as compared with age-matched WKY. In contrast, vascular phospholipase A2 activity was unaffected in the aortic wall of either adult or very young SHR. There was no difference in the cardiac phospholipase C activity between adult SHR and WKY. The vascular phospholipase C of SHR had a lower Michaelis constant (Km) value than that of WKY. Moreover, its pH profile and calcium requirement differed in part from those of WKY. These results indicate that the activation of vascular phospholipase C precedes the development of hypertension and that the enhancement may be induced by both quantitative and qualitative changes in phospholipase C in SHR.     

Impaired activities of antioxidant enzymes elicit endothelial dysfunction in spontaneous hypertensive rats despite enhanced vascular nitric oxide generation

OBJECTIVE: Enhanced oxidative stress is involved in mediating the endothelial dysfunction associated with hypertension. The aim of this study was to investigate the relative contributions of pro-oxidant and anti-oxidant enzymes to the pathogenesis of endothelial dysfunction in genetic hypertension. METHODS: Dilator responses to endothelium-dependent and endothelium-independent agents such as acetylcholine (ACh) and sodium nitroprusside were measured in the thoracic aortas of 28-week-old spontaneously hypertensive rats (SHR) and their matched normotensive counterparts, Wistar Kyoto rats (WKY). The activity and expression (mRNA and protein levels) of endothelial nitric oxide synthase (eNOS), p22-phox, a membrane-bound component of NAD(P)H oxidase, and antioxidant enzymes, namely, superoxide dismutases (CuZn- and Mn-SOD), catalase and glutathione peroxidase (GPx), were also investigated in aortic rings. RESULTS: Relaxant responses to ACh were attenuated in phenylephrine-precontracted SHR aortic rings, despite a 2-fold increase in eNOS expression and activity. Although the activity and/or expression of SODs, NAD(P)H oxidase (p22-phox) and GPx were elevated in SHR aorta, catalase activity and expression remained unchanged compared to WKY. Pretreatment of SHR aortic rings with the inhibitor of xanthine oxidase, allopurinol, and the inhibitor of cyclooxygenase, indomethacin, significantly potentiated ACh-induced relaxation. Pretreatment of SHR rings with catalase and Tiron, a superoxide anion (O(2)(-)) scavenger, increased the relaxant responses to the levels observed in WKY rings whereas pyrogallol, a O(2)(-)-generator, abolished relaxant responses to ACh. CONCLUSION: These data demonstrate that dysregulation of several enzymes, resulting in oxidative stress, contributes to the pathogenesis of endothelial dysfunction in SHR and indicate that the antioxidant enzyme catalase is of particular importance in the reversal of this defect.     

Prostaglandin H2 may be the endothelium-derived contracting factor released by acetylcholine in the aorta of the rat

The present experiment was performed to identify endothelium-derived contracting factor produced by acetylcholine stimulation in the aorta of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. The rings of the thoracic aorta were obtained from age-matched SHR and WKY rats, and changes in isometric tension were recorded. The relaxant responses to acetylcholine in the aortic rings from SHR were significantly weaker than those from WKY rats. The relaxant responses to acetylcholine were significantly enhanced by pretreatment with a cyclooxygenase inhibitor (indomethacin) or thromboxane A2/prostaglandin H2 receptor antagonist (ONO-3708) in aortic rings from both SHR and WKY rats. A thromboxane A2 synthetase inhibitor (OKY-046) did not affect the acetylcholine-induced relaxation in the aortic rings from SHR or WKY rats. In the organ bath solution, after acetylcholine stimulation, prostaglandin E2 and 6-keto-prostaglandin F1 alpha concentrations increased but not prostaglandin F2 alpha and thromboxane B2 concentrations. Exogenous prostaglandin H2, a stable analogue of thromboxane A2, and prostaglandin F2 alpha induced contractions of the SHR rings at a lower concentration than prostaglandin E2, prostaglandin D2, and prostaglandin I2. These contractile responses to various prostaglandins were markedly inhibited by pretreatment with ONO-3708. A prostacyclin synthetase inhibitor did not affect the relaxant responses to acetylcholine in the SHR rings. These results show that endothelium-derived contracting factor is produced and released by acetylcholine stimulation not only in the aorta of SHR but also in those of WKY rats and suggest that prostaglandin H2, a precursor of the released prostaglandins, is a strong candidate for endothelium-derived contracting factor produced by acetylcholine stimulation.     

Reversal of endothelial nitric oxide synthase uncoupling and up-regulation of endothelial nitric oxide synthase expression lowers blood pressure in hypertensive rats.

OBJECTIVES: We sought to examine the hypothesis that a pharmacologic up-regulation of endothelial nitric oxide synthase (eNOS) combined with a reversal of eNOS uncoupling provides a protective effect against cardiovascular disease. BACKGROUND: Many cardiovascular diseases are associated with oxidant stress involving protein kinase C (PKC) and uncoupling of eNOS. METHODS: Messenger ribonucleic acid (mRNA) expression was analyzed with RNase protection assay or quantitative real-time polymerase chain reaction, vascular nitric oxide (NO) with spin trapping, and reactive oxygen species (ROS) with dihydroethidium fluorescence. RESULTS: Aortas of spontaneously hypertensive rats (SHR) showed an elevated production of ROS when compared with aortas of Wistar-Kyoto rats (WKY). The aortic expression of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunits (Nox1, Nox2, Nox4, and p22phox) was higher in SHR compared with WKY. In SHR, aortic production of ROS was reduced by the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), indicating eNOS "uncoupling" in hypertension. Oral treatment with the PKC inhibitor midostaurin reduced aortic Nox1 expression, diminished ROS production, and reversed eNOS uncoupling in SHR. Aortic levels of (6R)-5,6,7,8-tetrahydro-L-biopterin (BH4) were significantly reduced in SHR compared with WKY. Midostaurin normalized BH4 levels in SHR. In both WKY and SHR, midostaurin increased aortic expression of eNOS mRNA and protein, stimulated bioactive NO production, and enhanced relaxation of the aorta to acetylcholine. Midostaurin lowered blood pressure in SHR and, to a lesser extent, in WKY; the compound did not change blood pressure in WKY made hypertensive with L-NAME. CONCLUSIONS: Pharmacologic interventions that combine eNOS up-regulation and reversal of eNOS uncoupling can markedly increase bioactive NO in the vasculature and produce beneficial hemodynamic effects such as a reduction of blood pressure.     

Different mechanisms for testosterone-induced relaxation of aorta between normotensive and spontaneously hypertensive rats

The tension in isolated ring preparations of the thoracic aortae from Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) was measured isometrically to study the differences in testosterone-induced relaxation between WKY and SHR aortic rings. Testosterone (9 to 300 micromol/L) induced a concentration-dependent relaxation in both WKY and SHR aortic rings, and the relaxation induced by testosterone was greater in SHR than WKY. The relaxation induced by testosterone was significantly reduced by denudation of endothelium in SHR but not WKY. Indomethacin, an inhibitor of cyclooxygenase, and N(G)-nitro-L-arginine, an inhibitor of nitric oxide (NO) synthase, showed little influence on the relaxation induced by testosterone in both WKY and SHR aortic rings. Glibenclamide, a selective inhibitor of ATP-sensitive potassium channels, significantly reduced the relaxation induced by testosterone in both WKY and SHR aortic rings, although the extent of reduction was greater in WKY than SHR. On the other hand, 4-aminopyridine, a selective inhibitor of voltage-dependent potassium channels, and tetraethylammonium, an inhibitor of calcium-activated potassium channels, significantly reduced the relaxation induced by testosterone in SHR but not WKY. These results suggest that the mechanisms of testosterone-induced vasorelaxation in both WKY and SHR involve, in part, ATP-sensitive potassium channels in the thoracic aortae and that in SHR aortic rings, testosterone may release endothelium-derived substances that may cause hyperpolarization of the cells by a mechanism that involves potassium channels. Moreover, the data show differences between WKY and SHR in the function of ATP-sensitive, voltage-dependent, and calcium-activated potassium channels.     

Antihypertensive and vasodilator actions of antioxidants in spontaneously hypertensive rats

This study was designed to determine whether the antioxidants ascorbic acid, aminotriazole, and glutathione acutely reduce blood pressure (BP) by endothelium-independent or -dependent vasorelaxation in spontaneously hypertensive rats. Blood pressure of male Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) was measured before and 4 h after administration of antioxidants. Thoracic aortic rings with and without endothelium were suspended in organ chambers for isometric tension recordings. Each of the antioxidants, administered in vivo, significantly decreased blood pressure in SHR but had no significant effect on BP in WKY rats. The endothelium-dependent impaired relaxation of SHR aortic rings to acetylcholine (ACh) was improved by prior in vivo administration of each antioxidant. ACh-induced relaxations of aortic rings from WKY was not affected by prior antioxidant treatment. Addition of each antioxidant directly to the organ chamber containing SHR or WKY aortas produced dose- and endothelium-dependent relaxations. Moreover, antioxidant pretreatment of SHR aortic rings significantly potentiated ACh-induced relaxations in these aortas, suggesting that this effect was endothelium dependent. Relaxations induced by the antioxidants alone or by ACh in the presence of antioxidants were inhibited by addition of either xanthine plus xanthine oxidase or nitro-l-arginine. These findings suggest that either excess production of oxidants or a deficiency of antioxidant systems may contribute to the high blood pressure and the endothelium-dependent impairment of vascular relaxation in SHR.     

Development of nitric oxide and prostaglandin mediation of shear stress-induced arteriolar dilation with aging and hypertension

We hypothesized that during hypertension, the impairment of mediation of shear stress-induced dilation by nitric oxide (NO) is due to the prevailing hemodynamic forces, and that mediation of this response by NO should still be present in young spontaneously hypertensive rats (SHR). Thus, responses to increases in perfusate flow eliciting increases in wall shear stress were investigated in pressurized (80 mm Hg), isolated arterioles ( approximately 70 to 100 microm) of the left or right gracilis muscle obtained from the same WKY and SHR at 4 and 12 weeks of age. Flow-induced dilations were similar in WKY and SHR at 4 weeks (maximum, 26.5+/-1.8 and 24. 2+/-2.0 microm, respectively). Also, the middle of the upward portion of the shear stress-diameter curves was similar in arterioles of the 2 strains. Inhibition of NO synthase with N(omega)-nitro-L-arginine (L-NNA) or inhibition of synthesis of prostaglandins (PGs) with indomethacin elicited an approximately 50% reduction in flow-dependent dilation, whereas their combined administration eliminated the responses in both groups. In arterioles of 12-week-old WKY, flow-induced dilation became significantly greater (maximum, 46.1+/-2.3 microm) than responses of arterioles of 4-week-old WKY and 12-week-old SHR (maximum, 18.3+/-5. 9 microm), which shifted only the shear stress-diameter curve of the 12-week-old WKY significantly to the left. Also, at 12 weeks of age, flow-dependent dilation of arterioles from SHR is mediated solely by PGs. Thus, shear stress-induced arteriolar dilation is mediated by NO and PGs in 4-week-old WKY and SHR. With aging, the release of NO and PGs increases in normotensive rats, whereas the contribution of NO to the regulation of shear stress disappears in 12-week-old SHR, which suggests that this change is probably caused by the increase in intraluminal pressure as hypertension develops.     

Treatment with the arginase inhibitor N(omega)-hydroxy-nor-L-arginine improves vascular function and lowers blood pressure in adult spontaneously hypertensive rat

OBJECTIVE: High vascular arginase activity and subsequent reduction in vascular nitric oxide production were recently reported in animal models of hypertension. The present study investigated the effects of in-vivo arginase inhibition on blood pressure and vascular function in adult spontaneously hypertensive rats. METHODS: Ten-week-old spontaneously hypertensive rats and normotensive age-matched Wistar-Kyoto rats were treated with or without the selective arginase inhibitor N-hydroxy-nor-L-arginine for 3 weeks (10 or 40 mg/kg per day, intraperitoneally). Systolic blood pressure and cardiac rate were measured before and during treatment. Flow and pressure-dependent reactivity as well as remodeling of mesenteric arteries, acetylcholine-dependent vasodilation of aortic rings, cardiac hypertrophy, arginase activity and nitric oxide production were investigated in 13-week-old spontaneously hypertensive rats. RESULTS: In spontaneously hypertensive rats, N-hydroxy-nor-L-arginine treatment decreased arginase activity (30-40%), reduced blood pressure ( approximately 35 mmHg) and improved the reactivity of mesenteric vessels. However, vascular and cardiac remodeling was not different between treated and untreated spontaneously hypertensive rats. In Wistar-Kyoto rats, N-hydroxy-nor-L-arginine did not affect blood pressure. Finally, arginase inhibition was associated with increased nitric oxide production. Consistent with this, the response of aortic rings to acetylcholine was fully restored by N-hydroxy-nor-L-arginine, and the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester significantly reduced the effect of N-hydroxy-nor-L-arginine on flow-dependent vasodilation. CONCLUSION: Pharmacological inhibition of arginase in adult spontaneously hypertensive rats decreases blood pressure and improves the reactivity of resistance vessels. These data represent in-vivo argument in favor of selective arginase inhibition as a new therapeutic strategy against hypertension.     

Biomechanical properties and chemical composition of the aorta in genetic hypertensive rats.

OBJECTIVE: To study the alteration in the biomechanical properties of the thoracic aorta and its composition in young normotensive Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHR), and stroke-prone SHR (SHRSP). METHODS: The in-vitro biomechanical properties of the aorta in 4- and 12-week-old SHRSP were determined by means of a tensile testing machine and compared with those of the SHR and WKY rats; in addition, a biochemical analysis of collagen, elastin and advanced glycation endproducts was performed. RESULTS: The aortic biomechanical properties were altered in the 4- and 12-week-old SHRSP, compared with age-matched WKY rats and SHR. The maximum stress in the 12-week-old SHRSP was reduced by 27% compared with the normotensive WKY rats, and by 26% compared with the SHR. The maximum strain values in the 4- and 12-week-old SHRSP were lower than those in the age-matched WKY rats, by 12 and 9% respectively, whereas this value in the 12-week-old SHR was significantly increased (by 26%) compared with the age-matched WKY rats. No differences were observed in the aortic contents of collagen and elastin between the SHRSP and SHR. However, the extractability of collagen by pepsin digestion in the 12-week-old SHRSP was lower than that in the age-matched SHR and WKY rats, and a significantly larger accumulation of advanced glycation endproducts was observed in the 12-week-old SHRSP than in the age-matched SHR and WKY rats, suggesting a greater formation of collagen-derived cross-links in SHRSP. CONCLUSIONS: From these results, we conclude that decreased aortic distensibility and mechanical strength values are partly related to the greater formation of collagen-derived cross-links in 12-week-old SHRSP, and that the mechanical properties in SHRSP may be the result not only of the larger formation of collagen-derived cross-links but also of primary defects, since the aortic mechanical strength value was decreased even in 4-week-old SHRSP.     

Dietary gamma-linolenic acid lowers blood pressure and alters aortic reactivity and cholesterol metabolism in hypertension.

OBJECTIVE: To determine the effects of dietary gamma-linolenic acid upon blood pressure, aortic reactivity and cholesterol metabolism in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. DESIGN: Randomized parallel-group study. METHODS: SHR and WKY rats were fed a purified diet containing either sesame or borage oil rich in gamma-linolenic acid for 7 weeks. Blood pressure measured by the tail-cuff method and weight were monitored weekly. At the end of the study, intra-arterial pressor responses to norepinephrine and angiotensin II, and reactivity of isolated aortic rings to norepinephrine, angiotensin II, KCl and acetylcholine were determined. Serum cholesterol and triglycerides were measured. Hepatic and intestinal enzymes and receptors of cholesterol metabolism were also measured. RESULTS: Dietary borage oil significantly decreased blood pressure in SHR and WKY rats compared with sesame oil-fed rats. Pressor responses to norepinephrine and angiotensin II, and aortic reactivity to norepinephrine, angiotensin II, KCl and acetylcholine were not significantly different. The borage oil diet increased serum cholesterol levels in WKY rats and hepatic B-hydroxy-3-methylglutaryl coenzyme A reductase in SHR. CONCLUSION: These data indicate that dietary borage oil has a blood pressure lowering effect in hypertensive and normotensive rats. However, the effect cannot be explained by altered sensitivity to humoral and neural vasoconstrictors or changes in cholesterol metabolism. Other mechanisms should be investigated.     

Effect of Red Blood Cells and Hemoglobin on Spontaneously Hypertensive and Normotensive Rat Aortas

Experiments were designed to compare the contractile effect of red blood cells (RBC) on aortic rings with and without endothelium from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Red blood cells of 4 week old WKY and SHR rats induced a negligible increase in tension of aortic rings, either with or without endothelium, being slightly more effective in SHR rats. However, red blood cells of 16 week old rats increased tension of WKY and SHR aortic rings, with endothelium at this age being more pronounced then red blood cells in 4 week old animals. The contractions induced by WKY and SHR red blood cells both in WKY and SHR aortic rings without endothelium at this age are significantly greater compared to the effect on aortic rings with endothelium. Red blood cell ghosts of rats of both strains increased the tension of the rings without endothelium of SHR aorta to near 50% of those induced by red blood cells, whereas they were ineffective in aortic rings without endothelium of WKY rats. Oxyhemoglobin increased the tension of 16 week SHR aortic rings both with and without endothelium, whereas the effect on the rings of WKY rats was negligible. This increase in tension was inhibited by BM 13505, nordihydroguaiaretic acid, and indomethacin in SHR rings both with and without endothelium, demonstrating an eicosanoid involvement in oxyhemoglobin-induced contractions. Hemoglobin or its metabolites may be involved in development or in maintenance of spontaneous hypertensin.     

Defective phosphatidylinositol 3-kinase signaling in central control of cardiovascular effects in the nucleus tractus solitarii of spontaneously hypertensive rats

Recently we have shown functional involvement of the phosphatidylinositol 3-kinase (PI3K)-Akt-nitric oxide synthase (NOS) signaling pathway in central control of cardiovascular effects in the nucleus tractus solitarii (NTS) of normotensive Wistar-Kyoto (WKY) rats. In this study we determined whether PI3K/Akt signaling was defective in spontaneously hypertensive rats (SHR). WKY rats and SHR were anesthetized with urethane. Mean blood pressure (MBP) and heart rate (HR) were monitored intra-arterially. Unilateral microinjection (60 nL) of insulin (100 IU/mL) into the NTS produced prominent depressor and bradycardic effects in 8- and 16-week-old normotensive WKY and 8-week-old SHR. However, no significant cardiovascular effects were found in 16-week-old SHR after insulin injection. Furthermore, pretreatment with PI3K inhibitor LY294002 and NOS inhibitor L-NAME into the NTS attenuated the cardiovascular response evoked by insulin in WKY and 8-week-old SHR but not in 16-week-old SHR. Unilateral microinjection of 1 mmol/L of PI(3,4,5)P(3) (phosphatidylinositol 3,4,5-triphosphate), a phospholipids second messenger produced by PI3K, into the NTS produced prominent depressor and bradycardic effects in 8- or 16-week-old WKY rats as well as 8-week-old SHR but not in 16-week-old SHR. Western blot analysis showed no significant increase in Akt phosphorylation in 8-week-old pre-hypertensive SHR after insulin injection. Similar results were also found in hypertensive 16-week-old SHR. Our results indicate that the Akt-independent signaling pathway is involved in NOS activation to regulate cardiovascular effects in the NTS of 8-week-old pre-hypertensive SHR. Both Akt-dependent and Akt-independent signaling pathways are defective in hypertensive 16-week-old SHR.     

Nitric oxide synthase upregulation and the predelivery blood pressure decrease in spontaneously hypertensive rats

OBJECTIVE : The pregnant spontaneously hypertensive rat (SHR) exhibits a decrease in arterial blood pressure shortly before delivery; however, the mechanisms are unknown. Nitric oxide may be involved. DESIGN : Blood pressure in stroke-prone SHR (SHRSP) and Wistar-Kyoto control rats (WKY) was telemetrically measured. Four groups were studied: pregnant and non-pregnant WKY and SHRSP rats, respectively. Mean blood pressure in pregnant SHRSP rats decreased from 148 +/- 2 mmHg at conception to 120 +/- 4 mmHg at day 15, compared to 112 +/- 1 mmHg in pregnant WKY rats. At delivery, we determined the vasodilatory responses of isolated preconstricted aortic strips. RESULTS : Vasodilatory responses from late-term SHRSP rats were significantly greater following acetylcholine than either those from non-pregnant SHRSP or pregnant and non-pregnant WKY rats (acetylcholine IC50: 5, 22.8, 398, 1000 nmol/l, respectively), while contractile responses to increasing doses of norepinephrine were not different. Similar results were obtained with substance P. Indomethacin had no effect on the relaxation responses. Relaxation in response to sodium nitroprusside was not different in the groups. Western blot analysis showed that endothelial nitric oxide synthase (eNOS) levels were significantly increased in the pregnant SHRSP vessels compared to non-pregnant SHRSP, pregnant WKY, and non-pregnant WKY vessels. CONCLUSION : Increased NOS may explain the blood pressure decrease during late pregnancy in genetically hypertensive rats.     

Higher level of plasma nitric oxide in spontaneously hypertensive rats.

We had detected a slightly, but significantly, higher level of plasma nitrite/nitrate in the spontaneously hypertensive rat (SHR) by using the nitric oxide (NO) analyzer (Sievers 280 NOA), which converts nitrate (including nitrate converted from nitrite) to NO. Here, we examined whether the release of NO from protein-bound dinitrosyl nonheme iron complexes (DNIC) contributes to the elevated plasma nitrate level in the SHR. The SHR and their genetic normotensive controls, Wistar-Kyoto rats (WKY), were anesthestized and cannulized for monitoring blood pressure, collecting a blood sample, and the administration of endotoxin (lipopolysaccharide [LPS]). The nitrate levels (an indicator of NO formation) in the plasma and the aorta were measured by an NO analyzer. In addition, the relaxation of acetylcholine (ACh) in the presence or absence of N(omega)-nitro-L-arginine methyl ester (L-NAME) was also examined in thoracic aortae obtained from both strains. The slight, but significant, increase of basal nitrate levels in the plasma and aorta were observed, and the former was further enhanced in SHR treated with LPS for 3 h. In vitro, the ACh-induced relaxation was attenuated in the aortae obtained from SHR. However, this difference between SHR and WKY (without LPS treatment) was abolished by treatment of rings with L-NAME (30 micromol/L), suggesting that an impairment of NO formation was observed in the SHR. After rats were treated with LPS for 3 h, the ACh-induced relaxation was reduced in the WKY, but not in the SHR. In addition, a 10-fold increase of L-NAME was needed to abolish the difference in ACh-induced relaxation between SHR and WKY, indicating an expression of inducible NO synthase in both strains treated with LPS. We suggest that the elevated plasma NO level in SHR may be due to the release of NO from DNIC in the vascular bed to combat the hypertensive state.     

Endothelium-dependent relaxation in resistance vessels from the spontaneously hypertensive rats

The endothelium-dependent vasodilator acetylcholine was used to observe relaxation responses of noradrenaline-contracted mesenteric resistance vessels from 3-, 6-, 12- and 18-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Relaxation responses were greater than normal in the 3-week-old SHR but the pattern of response was different in the 6-18-week-old SHR compared with the WKY. In these older animals, low concentrations of acetylcholine relaxed SHR and WKY vessels to a similar extent, but high concentrations (greater than 10(-7) mol/l) caused the partially relaxed vessels to contract again. Indomethacin enhanced relaxation in the 12-week-old SHR and reduced the difference between the SHR and WKY. The reduction in acetylcholine-induced, endothelium-dependent relaxation in SHR suggested that a functional change occurred, causing the vessels to release a vasoconstrictor factor that opposes the action of endothelium-derived relaxing factor.     

应用RNA阵列技术检测自发性高血压大鼠肌浆网钙释放通道基因表达变化

目的 探讨肌浆网Ca2 释放通道在原发性高血压发病机制中的变化特点。方法 提取2、4、6、8、10、12周龄各组雄性自发性高血压大鼠(spontaneously hypertensive rats,SHR)和正常血压大鼠(Wistar-kyoto rats,WKY)心室肌、血管平滑肌、肝脏和肾脏组织的总RNA,共294个样品,利用高通量RNA阵列技术检测肌浆网兰尼碱受体2(ryanodine receptor,RyR2)和1,4,5-三磷酸肌醇受体1(inositol 1,4,5-triphosphate receptors,IP3R1)在不同周龄SHR中mRNA的表达谱改变。结果 与同周龄WKY相比较,SHR在6、8、10、12周龄血压出现显著性升高(P均<0.01),10、12周龄心室肌重量/体重比出现显著增加(P均<0.01),心肌中RyR2基因表达在4、6、8、10、12周龄出现显著性升高(P<0.05或P<0.01),IP3R1基因表达在6、8、10、12周龄出现显著性升高(P<0.05或P<0.01)。血管平滑肌组织中RyR2基因表达在4、6、8、10、12周龄出现显著性升高(P<0.05),IP3R1基因表达在4、6、8、10、12周龄出现显著性升高(P<0.05或P<0.01)。肝脏和肾脏组织中未见上述基因的明显表达。结论 肌浆网Ca2 释放通道受体蛋白RyR2和IP3R1 mRNA表达变化是实验性高血压发生和发展过程中重要的分子生物学机制。