Spontaneously Hypertensive Aorta Research Articles

Sodium nitrite causes relaxation of the isolated rat aorta: By stimulating both endothelial NO synthase and activating soluble guanylyl cyclase in vascular smooth muscle

Ingestion of dietary nitrites lowers arterial blood pressure in experimental animals and in humans. However, the exact mechanism underlying the hypotensive effect of nitrite remains unclear. The present study compared nitrite-induced responses in rings (with or without endothelium) of aortae of 18–20weeks old Wistar–Kyoto Rats (WKY) and spontaneously hypertensive (SHR) rats and investigated the underlying mechanism. Relaxations of aortae from WKY and SHR to increasing concentrations (1nM–100μM) of sodium nitrite (NaNO2) were determined during sustained contractions to phenylephrine, in the absence and presence of pharmacological agents. The nitrite-induced relaxations were concentration-dependent and larger in SHR than in WKY aortic rings. Inhibition of endothelial nitric oxide synthase (eNOS) and the absence of endothelium decreased nitrite-induced relaxations in both WKY and SHR aortae, indicating the role of endothelium-derived nitric oxide (NO) in the response. The involvement of eNOS was further confirmed by increases in phosphorylation of eNOS at ser1177 in HUVEC cells following treatment with sodium nitrite. The presence of NO scavengers decreased the relaxation to nitrite in both WKY and SHR preparations while inhibition of soluble guanylyl cyclase (sGC) abolished the response, indicating that besides producing NO, nitrite also induces relaxation by directly activating the enzyme. Thus, the present study demonstrates that the sensitivity to exogenous nitrite is increased in the aorta of the SHR compared to that of the WKY. The endothelium-dependent component of the relaxation to nitrite involves activation of eNOS with production of endothelium-derived NO, while the endothelium-independent component is due to stimulation of sGC.

α1 -Adrenoceptor activation of PKC-ε causes heterologous desensitization of thromboxane receptors in the aorta of spontaneously hypertensive rats.

In the aorta of adult spontaneously hypertensive (SHR), but not in that of normotensive Wistar-Kyoto (WKY), rats, previous exposure to phenylephrine inhibits subsequent contractions to PGE2 . The present experiments were designed to examine the mechanism(s) underlying this inhibition. Isometric tension was measured in isolated rings of SHR and WKY aortae. Gene expression and protein presence were measured by quantitative real-time PCR and Western blotting respectively. In aorta of 18 weeks SHR, but not age-matched WKY, pre-exposure to phenylephrine inhibited subsequent contractions to PGE2 that were mediated by thromboxane prostanoid (TP) receptors. This inhibition was not observed in preparations of pre-hypertensive 5-week-old SHR, and was significantly larger in those of 36- than 18-week-old SHR. Pre-exposure to the PKC activator, phorbol 12,13-dibutyrate, also inhibited subsequent contractions to PGE2 in SHR aortae. The selective inhibitor of PKC-ε, ε-V1-2, abolished the desensitization caused by pre-exposure to phenylephrine. Two molecular PKC bands were detected and their relative intensities differed in 36-week-old WKY and SHR vascular smooth muscle. The mRNA expressions of PKC-α, PKC-ε, PK-N2 and PKC-ζ and of G protein-coupled kinase (GRK)-2, GRK4 and β-arrestin2 were higher in SHR than WKY aortae. These experiments suggest that in the SHR but not the WKY aorta, α1 -adrenoceptor activation desensitizes TP receptors through activation of PKC-ε. This heterologous desensitization is a consequence of the chronic exposure to high arterial pressure.

Endothelial Dysfunction in Femoral Artery of the Hypertensive Rats Is Nitric Oxide Independent

This study examined nitric oxide (NO) production, oxidative load and endothelium-dependent relaxation (NO-dependent and NO-independent) in adult male borderline hypertensive (BHR) and spontaneously hypertensive (SHR) rats as compared to normotensive Wistar-Kyoto (WKY) rats. Systolic blood pressure (BP) was determined by tail-cuff. NO production was determined by conversion of [(3)H]-L-arginine. Conjugated dienes (CD) and concentrations of thiobarbituric acid-reactive substances (TBARS) were measured for assessment of oxidative load. Vascular function was investigated in rings of the femoral artery (FA) using a wire myograph. BP of WKY, BHR and SHR was 106+/-2, 143+/-3 and 191+/-3 mm Hg, respectively (p<0.01 for each). Significant left ventricle (LV) hypertrophy and elevated levels of CD and TBARS in the LV were present in BHR and SHR as compared to WKY. NO production was elevated significantly in the aorta of BHR and SHR vs. WKY as well as in the LV of SHR vs. WKY. Acetylcholine (ACh)-induced relaxation of the FA was reduced significantly in both BHR and SHR vs. WKY. The NO-dependent component of ACh-induced relaxation had increasing tendency in hypertensive groups and it correlated positively with BP. The NO-independent component of vasorelaxation was reduced significantly in BHR and SHR vs. WKY and it correlated negatively with BP. In conclusion, the results showed that endothelial dysfunction in the experimental model of borderline hypertensive and hypertensive rats is NO-independent. The results suggest that borderline hypertension represents a risk of other cardiovascular disorders which is qualitatively similar to that of fully developed hypertension.

Endogenous acetylcholine contributes to endothelium‐dependent relaxations induced by mild hypothermia in the SHR aorta.

The present study was designed to investigate whether or not endothelial endogenous acetylcholine contributes to endothelium‐dependent relaxations induced by mild hypothermia. Aortic rings of spontaneously hypertensive (SHR) and Wistar‐Kyoto normotensive (WKY) rats were incubated with antagonists of m‐ and n‐acetylcholine receptors (atropine and tubocurarine, respectively) or enzyme and inhibitors that can interfere with the synthesis, degradation and transport of acetylcholine (acetylcholinesterase, bromoacetylcholine, hemicholinium‐3, and vesamicol) and exposed to progressive mild hypothermia (from 37°C to 31°C) to trigger relaxations. Mild hypothermia induced endothelium‐dependent relaxations which were inhibited significantly by all the inhibitors used in SHR but not in WKY aortae. The endothelium of both SHR and WKY aortae were able to take up extracellular choline and synthesize endogenous acetylcholine. Thus, the present study demonstrates that endothelium of both normotensive and hypertensive rats can produce acetylcholine. However, mild hypothermia causes endothelium‐dependent relaxations which can be reduced by inhibiting acetylcholine receptors or interfering with the metabolism of acetylcholine only in SHR rat aortae. Thus, in the hypertensive rat, endothelial endogenous acetylcholine can in an autocrine manner activate acetylcholine receptors and elicit endothelium‐dependent relaxations.

Endothelial NOS‐independent release of nitric oxide in the aorta of the spontaneously hypertensive rat

Preliminary experiments showed that in the aorta of male spontaneously hypertensive (SHR), but not in that of normotensive Wistar‐Kyoto (WKY) rats, contractions to phenylephrine (PE) are inhibited in preparations with endothelium despite the presence of indomethacin and L‐NAME. The present study aimed to identify the mechanism underlying this endothelium‐dependent inhibition in the SHR aorta. Aortic rings, with and without endothelium, of male SHR and WKY were suspended in organ chambers for the measurement of isometric tension. Contractions to PE were smaller in SHR aorta with than in those without endothelium. The inhibitory effect of the endothelium was larger in preparations from adult than in those from young SHR. The endothelium‐dependent difference in contraction to PE was abolished by the NO scavengers oxyhemoglobin and carboxy‐PTIO, by the inhibitor of soluble guanylyl cyclase ODQ, and the by inhibitor of cytochrome P450 reductase (CPR) diphenyleneiodonium. PE‐induced contractions in adult WKY aortic rings with endothelium were not affected by carboxy‐PTIO. Thus, in the SHR, there is an endothelium‐dependent release of NO which is not produced by NOS but by CPR. Nitrate is the likely source of this NOS‐independent endothelial NO. The NOS‐independent NO release is observed only in preparations from hypertensive animals and increases with age (supported by the Hong Kong Research Grant Council).

Activation of Nicotinic Receptors Can Contribute to Endothelium-Dependent Relaxations to Acetylcholine in the Rat Aorta

Acetylcholine causes endothelium-dependent relaxations in the rat aorta. Both muscarinic acetylcholine receptors (mAChRs) and nicotinic acetylcholine receptors (nAChRs) are expressed in endothelial cells. It is generally accepted that mAChRs are responsible for the endothelium-dependent relaxations evoked by acetylcholine. The present study was designed to investigate whether nAChRs can also be involved in such responses evoked by the cholinergic transmitter. Rings with or without endothelium of aortae of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rats were suspended in organ chambers for the measurement of isometric tension. In WKY aortae the muscarinic antagonist atropine abolished the relaxations to increasing concentrations of acetylcholine, confirming that mAChRs are responsible mainly for the response under control conditions. In SHR aortae, atropine caused only partial inhibition of the endothelium-dependent relaxations to acetylcholine; the remaining decreases in tension were inhibited by the nicotinic antagonist mecamylamine, which did not significantly affect the response in the absence of atropine in either SHR or WKY preparations. Thus, when mAChRs are inhibited, nAChRs mediate relaxation to the cholinergic transmitter in the SHR but not the WKY aorta. Nicotine, a direct agonist of the nicotinic receptor, induced endothelium-dependent relaxations in both SHR and WKY rats via the activation of α7-nAChRs, but not by mecamylamine-sensitive nicotinic receptors (α3 subtype). The acetylcholine-induced, atropine-insensitive relaxations and those to nicotine both involve the phosphatidylinositol 3-kinase/AKT pathway. The present study demonstrates that the activation of nAChRs can contribute to acetylcholine-induced, endothelium-dependent relaxations in the aortae of hypertensive animals and suggests that these receptors may contribute to the endothelium-dependent regulation of vascular tone.

Structural properties of lipid reconstructs and lipid composition of normotensive and hypertensive rat vascular smooth muscle cell membranes

Multiple cell membrane alterations have been reported to be the cause of various forms of hypertension. The present study focuses on the lipid portion of the membranes, characterizing the microviscosity of membranes reconstituted with lipids extracted from the aorta and mesenteric arteries of spontaneously hypertensive (SHR) and normotensive control rat strains (WKY and NWR). Membrane-incorporated phospholipid spin labels were used to monitor the bilayer structure at different depths. The packing of lipids extracted from both aorta and mesenteric arteries of normotensive and hypertensive rats was similar. Lipid extract analysis showed similar phospholipid composition for all membranes. However, cholesterol content was lower in SHR arteries than in normotensive animal arteries. These findings contrast with the fact that the SHR aorta is hyporeactive while the SHR mesenteric artery is hyperreactive to vasopressor agents when compared to the vessels of normotensive animal strains. Hence, factors other than microviscosity of bulk lipids contribute to the vascular smooth muscle reactivity and hypertension of SHR. The excess cholesterol in the arteries of normotensive animal strains apparently is not dissolved in bulk lipids and is not directly related to vascular reactivity since it is present in both the aorta and mesenteric arteries. The lower cholesterol concentrations in SHR arteries may in fact result from metabolic differences due to the hypertensive state or to genes that co-segregate with those that determine hypertension during the process of strain selection.

Mechanisms underlying ATP-induced endothelium-dependent contractions in the SHR aorta

In mature spontaneously hypertensive (SHR) and Wistar–Kyoto (WKY) rats, acetylcholine, the calcium ionophore A 23187 and ATP release endothelium-derived contracting factor (EDCF), cyclooxygenase (COX) derivatives that activate thromboxane–endoperoxide (TP) receptors on vascular smooth muscle. The EDCFs released by acetylcholine have been identified as prostacyclin and prostaglandin (PG) H 2 while in response to A 23187 thromboxane A 2, along with the two other prostaglandins, contributes to the endothelium-dependent contractions. The purpose of the present study was to identify the EDCFs produced by ATP. Isometric tension and the release of prostaglandins were measured in isolated aortic rings of WKY rats and SHR. ATP produced the endothelium-dependent release of prostacyclin, thromboxane A 2 and PGE 2 (PGI 2 ≫ TXA 2 ≥ PGE 2 > PGF 2α) in a similar manner in aorta from WKY rats and SHR. In SHR aortas, the release of thromboxane A 2 was significantly larger in response to ATP than to acetylcholine while that to prostacyclin was significantly smaller. The inhibition of cyclooxygenase with indomethacin prevented the release of prostaglandins and the occurrence of endothelium-dependent contractions. The thromboxane synthase inhibitor dazoxiben selectively abolished the ATP-dependent production of thromboxane A 2 and partially inhibited the corresponding endothelium-dependent contractions. U 51605, a non-selective inhibitor of PGI-synthase, reduced the release of prostacyclin elicited by ATP but induced a parallel increase in the production of PGE 2 and PGF 2α, suggestive of a PGH 2-spillover, which was associated with the enhancement of the endothelium-dependent contractions. Thus, in the aorta of SHR, endothelium-dependent contractions elicited by ATP involve the release of thromboxane A 2 and prostacyclin with a possible contribution of PGH 2.

Extracellular acidosis results in higher intracellular acidosis and greater contraction in spontaneously hypertensive rat aorta

Acidic pH induces a contraction in aorta from spontaneously hypertensive (SHR) and Wistar–Kyoto (WKY) rats. The contractile response to acidic pH in SHR aorta is greater than that in WKY aorta. The purpose of this study was to investigate the correlation among extracellular pH (pH o), intracellular pH (pH i) and contraction in order to understand the exaggerated contractile response to acidic pH in SHR aorta. pH i measurement showed that at pH o 6.5, intracellular acidification was greater in SHR aorta than in WKY aorta. Decreasing pH o further to 6.2 in WKY aorta produced intracellular acidification close to that achieved at pH o 6.5 in SHR aorta, and at this level, the difference in contractile response between the two strains was also abolished. These results suggest that acidic pH i, but not pH o, is closely correlated with the contractile response and that the exaggerated contractile response in SHR aorta is due to a greater fall in pH i.

Functional role of Cl− channels in acidic pH-induced contraction of the aorta of spontaneously hypertensive and Wistar Kyoto rats

pH regulates various cellular functions. Previously, we have described that acidic pH produces depolarization and contraction in isolated aorta from spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats [Br. J. Pharmacol. 118 (1996) 485]. The aim of the present study was to investigate the involvement of Cl− channels in acidic pH-induced contraction. Changing the pH of the bathing solution from 7.4 to 6.5 induced a contraction in both SHR and WKY aorta, which was 127.50±13.32% and 79.27±0.94% of the 64.8 mM KCl-induced contraction, respectively. The acidic pH-induced contraction was partially inhibited by the voltage-dependent Ca2+ channel (VDCC) blockers, verapamil (1 μM) and nifedipine (0.1 μM). The Cl− channel inhibitors, diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) (0.5 mM), 9-anthracene chloride (0.5 mM), indanyloxyacetic acid (30 μM) and niflumic acid (3 μM) also inhibited the acidic pH-induced contraction and the degree of attenuation was comparable to that of VDCC blockers. DIDS, 9-anthracene chloride and niflumic acid at concentrations used to inhibit the acidic pH-induced contraction also inhibited the 10 μM phenylephrine-induced contraction partially, without affecting the 64.8 mM KCl-induced contraction, whereas both the contractions were inhibited by indanyloxyacetic acid with equal efficacy. Indanyloxyacetic acid but not DIDS, 9-anthracene chloride or niflumic acid inhibited the 24.8 mM KCl-induced contraction. Simultaneous measurement of cytosolic Ca2+ and tension showed that niflumic acid reversed the increase in intracellular Ca2+ level and inhibited the contraction caused by acidic pH. Similarly, acidic pH depolarized the cultured vascular smooth muscle cells from SHR and the depolarization was completely reversible after the administration of niflumic acid. All these results suggest that the activation of Cl− channels is an important mechanism underlying the depolarization and contraction induced by acidic pH in SHR and WKY aortas.

Differential response to chloroethylclonidine in blood vessels of normotensive and spontaneously hypertensive rats: role of alpha 1D- and alpha 1A-adrenoceptors in contraction.

The effects of chloroethylclonidine on alpha(1)-adrenoceptor-mediated contraction in endothelium-denuded caudal arteries and aorta from normotensive Wistar and Wistar Kyoto (WKY), and from spontaneously hypertensive (SHR) rats were evaluated. Chloroethylclonidine elicited concentration-dependent contractions. Maximal contraction was similar in caudal arteries among strains ( approximately 40% of noradrenaline effect). However, chloroethylclonidine elicited a higher contraction in aorta from SHR than from normotensive rats. In Wistar aorta chloroethylclonidine produced the smallest contractile response. In SHR aorta, BMY 7378 and 5-methylurapidil blocked chloroethylclonidine-elicited contraction, while (+)-cyclazocine did not inhibit it; while in caudal arteries, 5-methylurapidil blocked chloroethylclonidine action; the other antagonists had no effect. In chloroethylclonidine-treated aorta noradrenaline elicited biphasic contraction-response curves, indicating a high affinity (pD(2), 8.5 - 7.5) chloroethylclonidine-sensitive component and a low affinity (pD(2), 6.3 - 5.2) chloroethylclonidine-insensitive component. The high affinity component was blocked by chloroethylclonidine; while in caudal arteries noradrenaline elicited monophasic contraction-response curves with pD(2) values (6.5 - 5.7) similar to the low affinity component in aorta. Chloroethylclonidine inhibition of noradrenaline response was greater in aorta than in caudal arteries. Chloroethylclonidine increased the EC(50) values of noradrenaline approximately 1000 fold in aorta and approximately 10 fold in caudal arteries. In SHR aorta BMY 7378 protected alpha(1D)-adrenoceptors and in caudal arteries 5-methylurapidil protected alpha(1A)-adrenoceptors from chloroethylclonidine alkylation, allowing noradrenaline to elicit contraction. These results show marked strain-dependent differences in the ability of chloroethylclonidine to contract aorta; moreover, chloroethylclonidine stimulates alpha(1D)-adrenoceptors in aorta and alpha(1A)-adrenoceptors in caudal arteries. The higher contraction observed in aorta from SHR and WKY suggests an augmented number of alpha(1D)-adrenoceptors in these strains.

Downregulation of soluble guanylyl cyclase in young and aging spontaneously hypertensive rats.

Endothelial dysfunction, as observed in hypertension and atherosclerosis, is associated with a reduction in the bioavailability of endothelium-derived nitric oxide (NO). We tested the hypothesis that alterations in the soluble guanylyl cyclase (sGC) pathway may also contribute to the pathogenesis of hypertension. Therefore, we investigated the expression and activity of sGC in young (6 weeks) and aging (17 months) spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY). Endothelium-independent relaxation of aortic rings in response to the sGC activator YC-1 was attenuated in SHR, and expression of both alpha(1) and beta(1) subunits of heterodimeric sGC and the basal contents of cGMP were reduced specifically in SHR aorta. Moreover, mRNA expression of the cGMP receptor and effector protein cGMP-dependent protein kinase type Ialpha (cGKIalpha) was also reduced. Interestingly, downregulation of both sGC and cGKIalpha expression was observed in young, ie, normotensive SHR, whereas impairment of the endothelium-independent relaxation was found only in aging SHR. Accordingly, similar cGMP levels were reached in response to YC-1 in young SHR and young WKY, suggesting a compensatory increased sensitivity or effectiveness of the sGC pathway in young SHR. In aging SHR, however, increased sensitivity to YC-1 no longer compensated for the impairment of endothelium-independent relaxation, suggesting that other mechanisms were involved. In fact, endothelium-independent relaxations were partially restored by superoxide dismutase, suggesting a pathophysiological role of superoxide production, particularly at later disease stages. Thus, tissue-specific downregulation of components of the sGC/cGMP pathway is an early event in the pathogenesis of hypertension.

Differential effects of tyrosine kinase inhibitors on contraction and relaxation of the aortas of normotensive and hypertensive rats

The contribution of tyrosine kinase activity to vasoreactivity in normotensive Wistar–Kyoto (WKY) and spontaneously hypertensive (SHR) rats was investigated on isolated aortic preparations by the use of two tyrosine kinase inhibitors: methyl-2,5-dihydroxycinnamate (30 μM) and genistein (30 μM). The pretreatment of endothelium denuded aorta with methyl-2,5-dihydroxycinnamate reduced the sensitivity of the rings to noradrenaline to a larger extent in SHR than in WKY. The relaxing effects evoked by methyl-2,5-dihydroxycinnamate and genistein on the sustained contraction induced by endothelin-1 were also more pronounced in SHR denuded rings. Furthermore, in presence of methyl-2,5-dihydroxycinnamate, the endothelium-independent contractile responses to equipotent doses of cyclopiazonic acid were more depressed in SHR than in WKY. In WKY and SHR endothelium-intact aortas contracted with either phenylephrine or endothelin-1, carbachol and cyclopiazonic acid evoked endothelium derived relaxing factor (EDRF)/nitric oxide (NO)-dependent relaxations which were reduced by pretreatment of the rings with methyl-2,5-dihydroxycinnamate or genistein. These inhibitory effects were larger in WKY rings and more important on the cyclopiazonic acid response. In addition, sodium orthovanadate (30 μM) potentiated the noradrenaline-mediated contractions of endothelium-denuded SHR rings and reduced the cyclopiazonic acid-induced relaxation of endothelium-intact WKY rings. The present study suggests a regulatory role for tyrosine kinase in the smooth muscle contraction and the endothelium-dependent relaxation in WKY and SHR aortas and demonstrates the existence of a different relationship in the effect of tyrosine kinase inhibitors on vasoreactivity between SHR and WKY. We propose that an increase in the tyrosine kinase activity in SHR could lead to an enhanced reactivity of Ca 2+-linked contractile mechanisms. In addition, our results suggest a link between the loss of tyrosine kinase activity and the altered endothelium-dependent relaxation associated with hypertension.

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.

Changes in the concentration of endothelin-1 during development of hypertensive rats.

The concentration of endothelin-1 (ET-1) in the brain regions, heart, and throacic aorta of 1-, 4-, 6- and 8-week-old spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats was determined using radioimmunoassay. ET-1-like immunoreactivity in the brain regions of 1-week-old WKY and SHR rats was lower compared to older (6 and 8 weeks) rats. ET-1 levels in the central nervous system gradually increased with age in both SHR and WKY rats. However, the concentration of ET-1 in 8-week-old rats was lower in the brain regions of SHR compared to WKY rats. The concentration of ET-1 in the thoracic aorta of SHR (224 +/- 43 pg/g tissue) rats was lower than that of WKY (452 +/- 11 pg/g tissue) rats at 1 week of age. However, ET-1 levels gradually increased with age in SHR rats. By 8 weeks of age, levels of ET-1 in SHR (623 +/- 33 pg/g tissue) rats were higher compared to WKY (439 +/- 62 pg/g tissue) rats. In the heart, ET-1 levels were similar in WKY and SHR rats at 4 weeks of age, but at 8 weeks of age ET-1 levels were higher in SHR rats (364 +/- 33 pg/g tissue) compared to WKY rats (260 +/- 31 pg/g tissue). It appears that at 8 weeks of age when hypertension is fully expressed in rats, ET-1 levels are lower in the central nervous system and are higher in the thoracic aorta and heart of SHR compared to WKY rats.

The endothelin ET A receptor antagonist, BQ-123, normalizes the response of SHR aorta to Ca 2+ channel activator

Hypertension is associated with a hypersensitive response to the Ca 2+ channel activator, Bay K 8644. We investigated the effect of the endothelin ET A receptor antagonist, BQ-123, on the contractions induced by Bay K 8644 in aorta from spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats. BQ-123 (1 μM) decreased the sensitivity to Bay K 8644 of aorta of SHR down to that of WKY. This observation is consistent with a role for endothelin in hypertension.

Increases in Flow Reduce the Release of Endothelium-Derived Relaxing Factor in the Aorta of Normotensive and Spontaneously Hypertensive Rats

Experiments were designed to compare basal and acetylcholine-induced release of endothelium-derived relaxing factor at different flow rates in the aorta of the normotensive (Wistar-Kyoto; WKY) and spontaneously hypertensive (SHR) rat. Aortic segments (with endothelium) of either WKY or SHR were perfused at different flow rates (1 or 4 mL/min) with modified Krebs-Ringer solution; the relaxing activity of the perfusate from the two types of segments was bioassayed by measuring the isometric force in rings (without endothelium, and contracted with norepinephrine) of aortas of both WKY and SHR. All studies were performed in the presence of indomethacin to prevent endothelial production of prostacyclin and other vasodilator prostanoids. The basal release of endothelium-derived relaxing factor was not significantly affected by the flow rate in either the WKY or the SHR aortas. At the two flow rates, and with both types of bioassay rings, the basal release of endothelium-derived relaxing factor was smaller with SHR than with WKY aortas, but this reached significance only at 4 mL/min using the SHR-aorta as bioassay tissue. Both with WKY and SHR aortas the release of endothelium-derived relaxing factor evoked by acetylcholine was significantly larger at 1 than at 4 mL/min; no significant differences in responsiveness to acetylcholine were noted between WKY and SHR segments. There was also no difference in responsiveness of WKY and SHR bioassay rings to acetylcholine and acetylcholine-induced release of endothelium-derived relaxing factor. These experiments suggest that prolonged exposure to increases in shear stress reduces the ability of the endothelium to release endothelium-derived relaxing factor in responses to muscarinic activation.

Calcium and contractile responses to ouabain and potassium-free solution in aortae from spontaneously hypertensive rats

This study examines the role of calcium in contractions induced by Na+, K+ ATPase inhibition in blood vessels from spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY). Helical strips of aortae from SHR contract more rapidly in response to ouabain or potassium-free conditions than those from WKY rats. Dose-response curves to calcium in SHR aortae treated with 10(-3) mol/l ouabain were shifted to the left of those in WKY. Treatment with 10(-3) mol/l EGTA shifted dose-response curves to calcium in ouabain-treated strips to the left in both strains. The magnitude of the leftward shift induced by EGTA was greater in WKY aortic strips than in those from SHR. Similarly, treatment with EGTA increased the rate of contractile responses to potassium-free solution in WKY aortae to a greater extent than in SHR aortae. Verapamil (10(-6) mol/l) depressed contractions induced by ouabain and potassium-free solution and abolished the differences between SHR and WKY aortae in terms of contraction rate. Calcium-free conditions completely blocked contractions caused by sodium pump inhibition. These results suggest that the difference in responsiveness to sodium pump inhibition in SHR and WKY rats results from an alteration in calcium entry through verapamil-sensitive calcium channels.

Contractile responses to ouabain and K+-free solution in aorta from hypertensive rats.

This study characterizes isometric force development in response to ouabain and K+-free solution in isolated aortic strips from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. SHR aortas were more sensitive to ouabain than those from WKY (threshold: SHR, 3.1 X 10(-5) M; WKY, 25.6 X 10(-5) M), and force development in response to 10(-3) M ouabain was greater in SHR (SHR, 586 +/- 51 mg; WKY, 245 +/- 24 mg). Monensin, a Na+ ionophore, potentiated contractile responses to ouabain, whereas amiloride, a Na+ channel blocker, and low Na+ solutions depressed contractile responses to ouabain. Contractile responses of SHR aortic strips to K+-free solution were faster than those of WKY aortic strips [time to half-maximal response (t1/2): SHR, 24 +/- 5 min; WKY, 47 +/- 4 min]. Maximal force development by aortic strips from SHR in response to K+-free solution was not different from that of WKY aortic strips (SHR, 808 +/- 34 mg; WKY, 750 +/- 37 mg). Monensin (10(-5) M) increased the rate of force development to K+-free solution to a greater extent in WKY aortic strips than in those from SHR (t1/2: SHR, 3 +/- 1 min; WKY, 4 +/- 2 min). Amiloride and low Na+ solution depressed contractile responses to K+-free solution in both SHR and WKY aortic strips. These observations demonstrate that SHR aortas are more responsive to ouabain and K+-free solution compared with WKY aortas. Contractile responses to ouabain and K+-free solution were sensitive to experimental interventions that alter transmembrane Na+ movements.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased Vascular Sensitivity to Serotonin and Methysergide in Hypertension in Rats

1. The effect of the hypertensive process on vascular sensitivity to serotonin and methysergide was studied. 2. Isometric contractions were recorded from arteries isolated from spontaneously hypertensive (SHR), Kyoto-Wistar normotensive (WKY), deoxycorticosterone acetate (DOCA)-hypertensive and Sprague-Dawley normotensive rats. 3. Arteries from hypertensive rats (except SHR aorta) were more sensitive (lower ED30) to noradrenaline, serotonin and methysergide relative to those from controls. 4. The magnitude of the leftward shift in ED30 was similar for the three agonists in mesenteric and tail arteries from DOCA rats, whereas the shift in ED30 for serotonin and methylsergide in aortae exceeded that to noradrenaline. In SHR, the shift in ED30 for serotonin and methylsergide in tail and mesenteric arteries was greater than that to noradrenaline; SHR aortae did not differ from WKY aortae. 5. Arteries from hypertensive rats (except SHR aorta) developed a greater maximal contraction to methysergide than those from controls; maximal contractions to noradrenaline and serotonin were either less than or similar to those of controls. 6. The agonistic action of methysergide in DOCA rats paralleled the development of high arterial pressure. Low sodium diet abated the high pressure in DOCA rats and reduced vascular sensitivity to all agonists. 7. These observations support the view that in hypertension there is a generalized increase in vasoconstrictor sensitivity. The major finding is that, in two hypertensive models, there is a uniquely greater increase in vascular responsiveness to the agonistic action of methysergide.