Relaxation In Spontaneously Hypertensive Rats Research Articles

Effect of AT1 receptor blockade on hepatic redox status in SHR: possible relevance for endothelial function?

The study investigated whether the amelioration of endothelial dysfunction by candesartan (2 mg.kg-1.day-1; 10 wk) in spontaneously hypertensive rats (SHR) was associated with modification of hepatic redox system. Systolic arterial pressure (SAP) was higher (P < 0.05) in SHR than in Wistar-Kyoto rats (WKY) and was reduced (P < 0.05) by candesartan in both strains. Acetylcholine (ACh) relaxations were smaller (P < 0.05) and contractions induced by ACh + NG-nitro-l-arginine methyl ester (l-NAME) were greater (P < 0.05) in SHR than in WKY. Treatment with candesartan enhanced (P < 0.05) ACh relaxations in SHR and reduced (P < 0.05) ACh + l-NAME contractions in both strains. Expression of aortic endothelial nitric oxide synthase (eNOS) mRNA was similar in WKY and SHR, and candesartan increased (P < 0.05) it in both strains. Aortic mRNA expression of the subunit p22phox of NAD(P)H oxidase was higher (P < 0.05) in SHR than in WKY. Treatment with candesartan reduced (P < 0.05) p22phox expression only in SHR. Malonyl dialdehyde (MDA) levels were higher (P < 0.05), and the ratio reduced/oxidized glutathione (GSH/GSSG) as well as glutathione peroxidase activity (GPx) were lower (P < 0.05) in liver homogenates from SHR than from WKY. Candesartan reduced (P < 0.05) MDA and increased (P < 0.05) GSH/GSSG ratio without affecting GPx. Vessel, lumen, and media areas were bigger (P < 0.05) in SHR than in WKY. Candesartan treatment reduced (P < 0.05) media area in SHR without affecting vessel or lumen area. The results suggest that hypertension is not only associated with elevation of vascular superoxide anions but with alterations of the hepatic redox system, where ANG II is clearly involved. The results further support the key role of ANG II via AT1 receptors for the functional and structural vascular alterations produced by hypertension.

Selective cyclooxygenase-2 inhibition with etoricoxib elevates blood pressure and alters vascular reactivity

Selective inhibitors of cyclooxygenase-2 (COX-2) have been shown to be effective anti-inflammatory drugs with reduced gastrointestinal toxicity relative to conventional nonsteroidal anti-inflammatory drugs. However, given the ability of selective COX-2 inhibitors to suppress vascular prostacyclin synthesis, apart from their deleterious action on renal function, we decided to test the effects of the novel COX-2 inhibitor etoricoxib (ETO) on blood pressure (BP) and vascular reactivity. Normotensive male Wistar rats and spontaneously hypertensive rats (SHRs) received ETO (10 mg/kg, p.o.) once daily for 5 weeks. BP measurements were performed weekly by the tail-cuff method, and at the end of the treatment period the animals had their aortae removed in order to test the in vitro responses to both acetylcholine and sodium nitroprusside. ETO had no effect on the BP levels of normotensive Wistar rats. However, significant elevation of BP was observed in ETO-treated SHRs in comparison with the untreated SHRs (third week, 204.2 ± 5.8 vs 175.8 ± 5.5 mmHg, P < 0.001; fourth week, 206.4 ± 8.9 vs 185.0 ± 3.8 mmHg, P < 0.05; fifth week, 237.4 ± 4.9 vs 209.8 ± 4.0 mmHg, P < 0.001). Norepinephrine-precontracted aorta rings obtained from ETO-treated SHRs also showed a decreased relaxation response to both acetylcholine and sodium nitroprusside in vitro in comparison with the untreated SHR group. No ETO-related effects were observed in the in vitro reactivity of aorta rings obtained from the normotensive animals. These preliminary results suggest that the selective COX-2 inhibitor ETO significantly impairs vascular relaxation in SHRs, thus contributing, at least in part, to the significant potentiation of the hypertensive status of these animals.

Relaxation Response of Histamine in the Pulmonary Artery of the Wistar‐Kyoto and Spontaneously Hypertensive Rats

Mast cells are thought of primarily in the context of allergic disorders and certain acute inflammatory responses. Recent studies suggest, however, that mast cells are also implicated in the expression of a wide variety of biologic responses such as pulmonary vascular disease. In this study, we evaluated the pulmonary arterial relaxation effect, in vitro, of histamine and the receptor subtype(s) involved in the normotensive Wistar-Kyoto (WKY) and age-matched Spontaneously hypertensive rats (SHR) (male, age: 14–22 weeks). Isometric tension change of phenylephrine (1 μM) pre-contracted pulmonary artery in response to histamine and histaminergic agonists challenge was recorded and compared. Histamine (with 10 μM SKF 91488, a histamine N-methyl-transferase inhibitor) caused a concentration-dependent relaxation (endothelium-dependent and -independent) of both strains of rats. However, the magnitude of relaxation response observed in SHR was smaller and it is corresponded to a diminished H1-receptor-mediated (competitively inhibited by diphenhydramine), endothelium-dependent (L-NAME (20 μM)-sensitive) relaxation. The endothelium-independent (H2- and H3-receptors, suppressed by cimetidine and clobenpropit respectively) component was indistinguishable between both strains of rats. Unlike histamine, dimaprit (a H2-receptor agonist) consistently produced a similar degree of relaxation in the WKY and SHR. Under the SKF 91488-free or clobenpropit (1 nM, a H3-receptor blocker)-containing conditions, histamine-evoked relaxation was significantly enhanced in both the WKY and SHR. No potentiation was observed with tetrodotoxin (100 nM) present. Imetit (a H3-receptor agonist) failed to produce relaxation and a further contraction was observed in both strains of rats. The magnitude of imetit-induced contraction was greater in the WKY than in SHR. Imetit-evoked contraction was reduced in he presence of tetrodotoxin (100 nM) and clobenpropit (3 nM). Application of SQ 22536 (100 μM), L-NAME (50 μM), ouabain (10 μM), iberiotoxin (100 nM), glibenclamide (3 μM) and apamin (500 nM) failed to modify the endothelium-independent relaxation of histamine. A supplementation of L-arginine (500 μM) significantly potentiated histamine-evoked relaxation in the WKY and SHR. In conclusion, multiple histaminergic (H1-, H2- and H3-) receptors are present in rat pulmonary artery. A reduced histamine-induced relaxation in SHR is due to the diminished H1-receptor-mediated, endothelium-dependent relaxation. The endothelium-independent (H2- and H3-receptors) component of histamine-induced relaxation, however, was not modified by hypertension.

Altered effect of cyclopiazonic acid on endothelium-dependent relaxation in femoral arteries from hypertensive rats.

The function of endoplasmic reticulum in hypertensive vascular endothelium has not been intensively studied. The current study was designed to investigate a role of intracellular Ca2+ stores in endothelium-dependent relaxations to acetylcholine using femoral arteries obtained from Wistar-Kyoto (WKYs) and spontaneously hypertensive rats (SHRs). Rings were prepared from the femoral arteries and changes in isometric tension were recorded. Endothelium-dependent relaxations induced by acetylcholine in rings contracted with serotonin were identical in WKYs and SHRs. Cyclopiazonic acid (CPA) inhibited the relaxation in SHRs but not in WKYs. In WKYs, acetylcholine evoked smaller relaxations in rings contracted with KCl than in those contracted with serotonin, whereas in SHRs the relaxation was not affected by the contractile agonists used. The relaxation in rings contracted with KCl was abolished by Nw-nitro-l-arginine methyl ester (l-NAME) and was reduced by CPA to a similar extent in both strains. In rings contracted with serotonin, l-NAME abolished the relaxation in SHRs, but the inhibitor only partially reduced the relaxation in WKYs. CPA did not alter the relaxation in the presence of l-NAME. Endothelium-independent relaxations to sodium nitroprusside were not affected by CPA. These results suggest that acetylcholine relaxes rat femoral arteries by releasing both nitric oxide and endothelium-derived hyperpolarizing factor (EDHF). In SHRs, the relaxation is preserved, but the release of EDHF is absent. CPA inhibits the relaxation mediated by nitric oxide, but not EDHF and, thus, inhibits the relaxation in SHRs but not in WKYs. Functional alteration of endoplasmic reticulum in the hypertensive endothelium cannot be detected.

Effects of chronic N-acetylcysteine treatment on the actions of peroxynitrite on aortic vascular reactivity in hypertensive rats.

Peroxynitrite (ONOO-), the product of superoxide and nitric oxide, seems to be involved in vascular alterations in hypertension. To evaluate the effects of ONOO- on endothelium-dependent and independent aortic vascular responsiveness, oxidized/reduced glutathione balance (GSSG/GSH), malondialdehyde aortic content, and the formation of 3-nitrotyrosine (3-NT), a stable marker of ONOO-, in N-acetylcysteine (NAC)-treated normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). In SHR only, NAC significantly reduced heart rate and systolic, but not diastolic, blood pressure. It also improved endothelium-dependent aortic relaxation in SHR, but not after exposure to ONOO-. Endothelium-dependent and independent aortic relaxations were markedly impaired by ONOO- in both strains of rat. NAC partially protected SHR against the ONOO- -induced reduction in endothelium-independent relaxation. Aortic GSSG/GSH ratio and malondialdehyde, which were higher in SHR than in WKY rats, showed a greater increase in SHR after exposure to ONOO-. NAC decreased GSSG/GSH and malondialdehyde in both strains of rat before and after exposure to ONOO-. The 3-NT concentration, which was similar in both strains of rat under basal conditions, was greater in SHR than in WKY rats after the addition of ONOO-, with a reduction only in NAC-treated SHR. These findings suggest an increased vulnerability of SHR aortas to the effects of ONOO- as compared with those of WKY rats. The selective improvements produced by NAC, in systolic arterial pressure, heart rate, aortic endothelial function, ONOO- -induced impairment of endothelium-independent relaxation, aortic GSSG/GSH balance, malondialdehyde content and 3-NT formation in SHR suggest that chronic administration of NAC may have a protective effect against aortic vascular dysfunction in the SHR model of hypertension.

Diminished β-adrenoceptor-mediated relaxation of femoral arteries from young spontaneously hypertensive rats

A β-adrenoceptor agonist, norepinephrine (NE)-induced relaxation in the presence of an α-adrenoceptor antagonist and indomethacin was investigated in isolated femoral arteries from 5-week-old Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). NE elicited endothelium-dependent and -independent relaxation in WKY. In endothelium-intact WKY artery, the NE-induced relaxation was reduced by nitro l-arginine ( l-NA) and methylene blue. The residual response to NE in the presence of l-NA was further reduced by tetraethylammonium (TEA). Glibenclamide attenuated the NE-induced, endothelium-independent relaxation in WKY. In SHR, on the other hand, the relaxation to NE was solely endothelium-independent, unaffected by a combination of l-NA and TEA and inhibited by glibenclamide. The relaxation in response to NE in SHR was less than that in WKY, regardless of the presence and absence of endothelial cells. When WKY and SHR were treated for 10 days with captopril, the response to NE was increased not only in WKY but also in SHR. The relaxation in captopril-treated SHR consisted of endothelium-dependent and -independent components. The former was attenuated by l-NA and to a greater extent by TEA with l-NA. Sodium nitroprusside- and forskolin-induced, endothelium-independent relaxations in SHR were not significantly different from those in WKY. Captopril did not affect the response to these drugs. The present results indicate that the relaxation to NE is in part mediated by NO and a vasorelaxing factor distinct from NO in WKY but not in SHR. It is suggested that NE-induced, endothelium-independent relaxation in both groups is in part mediated by ATP-sensitive K + channels. It is also suggested that in SHR, captopril increases the response to NE through increases in endothelial production of NO and the non-NO vasorelaxing factor.

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.

Nitroglycerin-induced aortic relaxation mediated by calcium-activated potassium channel is markedly diminished in hypertensive rats

Nitroglycerin (NTG), a nitric oxide (NO) donor, is considered to relax vascular smooth muscle by stimulating soluble guanylate cyclase, which in turn increases cy clic GMP (cGMP) level. Recently it became evident that NO-induced vasodilatation is also mediated by stimulating Ca-activated K (K Ca) channels directly and/or indirectly through cGMP. We, therefore, tried to investigate the possible involvement or the alteration of K Ca, channels in the mechanism of vasodilation induced by NTG in physiological and pathological conditions. Using rings prepared from thoracic aortas of spontaneously hypertensive rats (SHR) and those of age-matched Wistar-Kyoto rats (WKY), we studied changes in isometric tension of the rings in response to NTG to evaluate effects of a soluble guanylate cyclase inhibitor methylene blue (MB), and a specific blocker of K Ca channel charybdotoxin (CTX). Rings from WKY and SHR precontracted with norepinephrine showed similar aortic relaxation to NTG. MB markedly suppressed the NTG-induced relaxation in both strains, leaving about 30% of MB-resistant relaxation. CTX nearly completely eliminated this MB-resistant relaxation in WHY but did not affect this relaxation in SHR. These results suggest that NTG-induced vasorelaxation is mediated through i) cGMP-dependent and ii) cGMP-independent K Ca channel involving mechanisms, the latter may be diminished or virtually eliminated in hypertensive state.

Benidipine Improves Endothelial Function in Renal Resistance Arteries of Hypertensive Rats

We studied the effects of long-term antihypertensive treatment on endothelial function in renal resistance arteries from spontaneously hypertensive rats (SHR). Wistar-Kyoto rats (WKY) were used as a normotensive reference. Adult SHR were treated with benidipine (a calcium antagonist) or ecarazine (a vasodilator) for 10 weeks; the drugs caused similar reductions in blood pressure. Changes in isometric tension of rings prepared from the third-order branches of the renal arteries were recorded. Endothelium-dependent relaxations induced by acetylcholine in rings contracted with norepinephrine were smaller in SHR than in WKY. The impaired relaxation was improved by benidipine treatment, but ecarazine had no significant effect. In vitro treatment with meclofenamic acid, a cyclooxygenase inhibitor, did not alter the differences in the relaxations. In the presence of meclofenamic acid, N omega-nitro-L-arginine methyl ester slightly reduced the relaxations; the relaxation was smaller in SHR than in WKY and was not affected by benidipine treatment. In rings contracted with 40 mmol/L. KCI, the relaxations induced by acetylcholine in the presence of meclofenamic acid were smaller than those in rings contracted with norepinephrine. The relaxation was smaller in SHR than in WKY but was normalized by benidipine treatment. Thus, acetylcholine relaxes rat renal resistance arteries by releasing nitric oxide and endothelium-derived hyperpolarizing factor from the endothelium, which is impaired in SHR. Long-term benidipine treatment improves the impaired relaxation in SHR by enhancing nitric oxide-mediated relaxation.

Regulation of vascular contraction by ionic matrix surfaces of the smooth muscle cell: changes in SHR?

1. KCl-induced contractions in aortic rings of spontaneously hypertensive rats (SHR) and two normotensive strains, Sprague-Dawley (SD) and Wistar-Kyoto (WKY) rats, were studied. 2. The contraction elicited by isotonic Na-free, KCl solution containing 2.5 mmol/L Ca and 1 mmol/L Mg was concentration-dependently inhibited by Na and K. Maximal inhibition was induced by 20 mmol/L NaCl in WKY and SHR (65 and 85%, respectively) and by 60 mmol/L NaCl in SD (50%). KCl (10-40 mmol/L) had no effect on SD but produced almost full relaxation in WKY and about 50% relaxation in SHR. 3. Isotonic Ca-free, Na-free high Mg (2 mmol/L), K-solution caused strong phasic contractions in WKY and SHR, but weak responses in SD. 4. Aortic rings of SD, WKY and SHR developed strong, sustained and reproducible contractions in isotonic Ca-free, Na-free, K-solution containing EDTA, which were fully relaxed by 1.2, 1.6 and 8 mmol/L Na, respectively. 5. We suggest that the primary mechanism of KCl-contraction is potential-independent competition between monovalent and divalent cations for binding sites on both surfaces of the plasma membrane (e.g. K with Mg and Na with Ca) and that differences in aortic contractility in SHR, WKY and SD merely reflect strain differences in the composition of the ionic matrix, independent of hypertension.

Cilazapril reverses endothelium-dependent vasodilator response to acetylcholine in mesenteric artery from spontaneously hypertensive rats

This study was designed to evaluate the effect of chronic treatment with cilazapril on vascular reactivity of aorta and mesenteric artery from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Cilazapril (5 mg/kg), an angiotensin converting enzyme inhibitor, was injected intraperitoneally twice a day for 4 weeks. Results demonstrated that acetylcholine (ACh)-induced relaxation in aorta and mesenteric artery from SHR was significantly less than that from WKY, cilazapril-treated WKY, and SHR. The impairment of ACh-induced relaxation in SHR was significantly reversed after cilazapril treatment and there were no significant differences among WKY, cilazapril-treated WKY, and SHR. Meanwhile, both N omega-nitro-L-arginine (LNNA; 10(-4) mol/L) and methylene blue (MB; 10(-5) mol/L) completely blocked the vasodilator response to ACh in aorta but only partly inhibited in mesenteric artery from WKY, cilazapril-treated WKY, and SHR. These LNNA- and MB-resistant vasodilator responses to ACh in mesenteric artery were only slightly inhibited by TEA (10(-3) mol/L) but not by indomethacin (5 x 10(-6) mol/L). These findings suggest that there may be an unidentified endothelium-dependent relaxing factor(s) (EDRF), which exists in the endothelium and may participate in the modulation of blood pressure in SHR. Results further demonstrate that the antihypertensive effect of cilazapril may be partly mediated by the reversing function of endothelium to release EDRF and LNNA-resistant, unidentified relaxing factor(s).

Vasorelaxant Properties of n-3 Polyunsaturated Fatty Acids in Aortas from Spontaneously Hypertensive and Normotensive Rats

Dietary consumption of fish, rich in n-3 polyunsaturated fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), has been shown to reduce blood pressure in both animal studies and clinical trials. Although the antihypertensive mechanisms are not known, the blood-pressure-lowering effect of n-3 polyunsaturated fatty acids may be partially attributed to their vasorelaxant properties. Aortic rings with and without endothelium, from Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR), 16-17 weeks old, were suspended in tissue baths and isometric tension was measured. Concentration-response curves were generated for DHA and EPA (1-100 mu mol/l) in norepinephrine-contracted rings. Blood pressure was measured using the tail-cuff method and aortic media thickness was determined. Blood pressure was significantly increased in SHR (n=10; 194 +/- 4.4 mmHg) compared with WKY (n=10; 124 +/- 1.2 mmHg, P < or = 0.0001). DHA (1-100 mu mol/l) relaxed aortic rings f rom WKY (-3.3 +/- 0.7 to -13 +/- 2.3%, P < or = 0.001) and from SHR (-6.5 +/- 1.8 to -22.9 +/- 4%, P < or = 0.01) in a concentration-dependent manner. EPA (1-100 mu mol/l) evoked greater relaxation in SHR (-10.1 +/- 2.0 to -33 +/- 3.9%, P < 0.01) than in WKY (-2.9 +/- 1.1 to -18.3 +/- 2.1%, P < 0.01) aortic rings. The relaxant effect of DHA in both WKY and SHR and of EPA in WKY were not dependent on an intact endothelium. However, EPA (1-10 mu mol/l) induced greater responses in intact SHR rings (-10.1 +/- 2.0 to -14.5 +/- 3.1%) than in de-endothelialized SHR rings (0 to -2.1 +/- 1.7%, P = 0.001). The direct relaxant effects of n-3 fatty acids as seen in WKY and SHR may contribute, in part, toward the blood-pressure-lowering effect of dietary fish and fish-oil supplementation.

Purinergic endothelium-dependent and -independent contractions in rat aorta.

The role of endothelium-derived contracting factor or factors in modulating relaxations and contractions to adenine nucleotides was examined in aortas from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) and Wistar rats. During contractions to phenylephrine, the relaxations to ATP were impaired significantly in SHR compared with WKY aortas with endothelium. In rings treated with NG-nitro-L-arginine (to inhibit nitric oxide synthase), the endothelium significantly augmented contractions evoked by ATP; this enhancement was greater in SHR compared with WKY aortas. Indomethacin (inhibitor of cyclooxygenase) and SQ 29,458 (antagonist of thromboxane/prostaglandin endoperoxide receptors) but not dazoxiben (inhibitor of thromboxane synthase) significantly augmented the maximal relaxation in WKY rats, abolished the impairment of the relaxation in SHR, and prevented the potentiation by the endothelium of the contractions evoked by ATP. In older animals (10 to 12 months old), the endothelium-dependent concentration-relaxation curves to ATP in SHR and WKY aortas treated with indomethacin were superimposable, as were the concentration-contraction curves (with NG-nitro-L-arginine present). Endothelium-dependent concentration-relaxation and -contraction curves to ADP obtained in these preparations overlapped also. In Wistar rats, the magnitude of the endothelium-dependent relaxations to either ATP or ADP were significantly smaller compared with the other strains, and the endothelium-dependent contractions were even smaller. Results show that adenine nucleotides stimulate the production of both endothelium-derived relaxing and contracting factors. Although there is no obvious age-related alteration in the capacity of aortas to release endothelium-derived relaxing factor, aging enhances endothelium-derived contracting factor activity in WKY rats.

Endothelium-derived contracting factors in resistance arteries of young spontaneously hypertensive rats before development of overt hypertension.

Vascular relaxations are impaired in adult spontaneously hypertensive rats (SHRs) because of increased production of an endothelium-derived, cyclooxygenase-dependent contractile factor or factors. To test the hypothesis that alterations in endothelial function precede and contribute to the development of overt hypertension in SHRs, we compared in myographs endothelium-mediated relaxations of mesenteric resistance arteries from 4-week-old SHRs and Wistar-Kyoto (WKY) rats. Acetylcholine (10(-9) to 10(-4) M) induced comparable relaxations in SHR and WKY arteries precontracted (ED50) with norepinephrine. In arteries obtained from SHRs but not from WKY rats, relaxations were replaced by contractile responses with higher concentrations of acetylcholine (10(-6) to 10(-5) M). The contractile responses were endothelium dependent, were augmented by nitro L-arginine (10(-4) M), and were prevented by pretreatment with indomethacin (10(-5) M) or 3-amino-1,2,4-triazole (10(-3) M), an inhibitor of superoxide anion production via the cyclooxygenase pathway. Inhibition of thromboxane synthetase (CGS-13080, 5 x 10(-5) M) and antagonism of prostaglandin H2/thromboxane A2 receptors (SQ-29,548, 5 x 10(-5) M) failed to block the contractile response to acetylcholine in SHR arteries. Acetylcholine-mediated relaxations were significantly impaired in mesenteric arteries from 16-week-old SHRs but not from WKY rats. Endothelium-independent relaxations produced by sodium nitroprusside and contractile responses to norepinephrine and endothelin were comparable in arteries from SHRs and WKY rats of all ages. In summary, endothelium-dependent relaxations of mesenteric arteries from "prehypertensive" SHR rats were impaired by the production of a contractile factor (or factors) that appears to be superoxide anions.

Prostaglandin H2 and thromboxane A2 are contractile factors in intrarenal arteries of spontaneously hypertensive rats.

Vascular resistance is increased in the kidneys of spontaneously hypertensive rats (SHR). Previous studies have demonstrated impaired vascular relaxations of mesenteric resistance arteries of SHR because of increased production of a cyclooxygenase-dependent endothelium-derived contracting factor. To test the hypothesis that altered endothelial function contributes to the enhanced constriction in kidneys of SHR, endothelium-mediated relaxations of renal resistance arteries from 5-6-week-old prehypertensive SHR and Wistar-Kyoto rats were compared in arteriographs. Acetylcholine induced endothelium-dependent contractions in SHR arteries, while potent endothelium-dependent relaxations were noted in renal arteries from Wistar-Kyoto rats. Inhibition of cyclooxygenase (indomethacin) or blockade of prostaglandin H2-thromboxane A2 receptors (SQ 29,548) blocked acetylcholine-induced contractions in SHR renal arteries; relaxations in SHR renal arteries after either treatment were similar to those observed in renal arteries from Wistar-Kyoto rats. NG-Nitro-L-arginine inhibited acetylcholine-mediated relaxations in both SHR and Wistar-Kyoto arteries. Endothelium-independent relaxations induced by verapamil were comparable in SHR and Wistar-Kyoto arteries. Thus, the impaired response to acetylcholine in SHR renal resistance arteries may result from the release of endothelium-derived cyclooxygenase products (prostaglandin H2 or thromboxane A2), which oppose endothelium-derived nitric oxide-mediated relaxation.

Contractions induced by potassium-free solution and potassium relaxation in vascular smooth muscle of hypertensive and normotensive rats.

1. Vascular contractions induced by K(+)-free solution and relaxation responses following the return of K+ to the organ bath were studied in mesenteric arterial rings from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) with particular focus on the role of vascular adrenergic nerve-endings and endothelium. 2. In endothelium-denuded rings the omission of K+ from the incubation medium resulted in gradual contractions, the rate of which was slower in SHR than WKY. Nifedipine (1 microM) inhibited the contractions more effectively in SHR than WKY. 3. Adrenergic denervation in vitro with 6-hydroxydopamine reduced the contractions induced by the K(+)-free medium in endothelium-denuded rings. The remaining contractions after denervation were markedly greater in SHR than WKY. 4. The presence of intact vascular endothelium attenuated the K(+)-free contractions in both strains, the attenuation being smaller in SHR than WKY. NG-nitro-L-arginine methyl ester (L-NAME, 0.1 mM) and methylene blue (10 microM), but not indomethacin (10 microM), abolished the attenuating effect of endothelium on the K(+)-free contractions. L-Arginine (1 mM) reversed the effect of L-NAME in WKY but not in SHR. 5. The re-addition of K+ after full K(+)-free contractions dose-dependently relaxed the rings. The rate of this K(+)-induced relaxation was significantly slower in SHR than WKY at all K+ concentrations (0.1-5.9 mM) studied, whether the endothelium or functioning adrenergic nerve-endings were present or not. Ouabain (1 mM) totally inhibited the K+ relaxation in SHR but only partially in WKY.6. Vascular smooth muscle contractions induced by high concentrations of potassium were comparable between the strains. The EC50 for noradrenaline-induced contractions was lower in SHR than WKY, but the maximal forces did not differ significantly.7. In conclusion, the contractile response in K+-free solution more clearly differentiates vascular rings from SHR and WKY than the responses induced by the classical contractile agents noradrenaline and high concentrations of potassium. The depressant effect of the presence of intact endothelium on the K+-free contractions, which was smaller in SHR than WKY, is mediated via the endothelium-derived relaxing factor. Neurotransmitter release from vascular adrenergic nerve-endings participates less in the K+-free contractile response in SHR than WKY. Moreover, the contractile response is more dependent on calcium entry through nifedipine-sensitive calcium channels in SHR than WKY. The greater K+-free contractions of denervated endothelium-denuded rings and the reduced K+ relaxation rate in SHR when compared to WKY suggest increased cell membrane permeability and decreased activity of vascular Na+, K+-ATPase, respectively, in this type of genetic hypertension.

Sodium pump activity and contraction of renal artery from spontaneously hypertensive rats

We investigated the hypertensive changes in renal arteries isolated from 21-week-old spontaneously hypertensive rats (SHR), and from age-matched normotensive Wistar-Kyoto rats (WKY). The maximam contraction of renal arteries from SHR in response to norepinephrine (NE), serotonin (5-HT) and KCl was greater than that of arteries from WKY. The threshold and EC 50 concentrations of NE, 5-HT and KCl were not significantly different between SHR and WKY. Contraction induced by removal of K + was inhibited by 10 −8 M prazosin. Less than 10 −7 M NE in K +-free solution did not cause contraction. Addition of 5.9 mM KCl to K +-free solution in the presence of 10 −5 M NE induced relaxation, which was followed by contraction to about the same level as that before KCl addition. The duration of the K +-induced relaxation in SHR (22.4 ± 0.9 min) was slightly, but significantly shorter than that in WKY (26.6 ± 0.8 min) arteries. In K +-free solution with reduced Na +, the duration of the relaxation induced by KC1 was shorter than that in the normal solution, for both SHR (13.8 ± 0.3 min) and WKY (14.1 ± 0.5 min). Such differences could be caused by increased influx and decreased efflux of Ca 2+, which depend on the Na + concentration and are related to the Na +-Ca 2+ exchange. The results suggest that enhanced renal vascular reactivity in hypertension may depend on structural changes and increased Na + pump activity.

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.