Enhanced release of endothelium-derived relaxing factor in mineralocorticoid hypertension

The effects of LXR agonist GW3965 on vascular reactivity and inflammation in hypertensive rat aorta

AIMS

Liver X receptors (LXRs) play an important role in the regulation of cholesterol, fatty acid and glucose metabolisms together with inflammatory processes. In the present study, the effects of LXR agonist GW3965 on vascular reactivity and expression of functional proteins in DOCA-Salt induced hypertension were examined.

MAIN METHODS

Hypertension was induced through unilateral nephrectomy and deoxycorticosterone-acetate (DOCA) injection (20 mg/kg, twice a week) for 6 weeks in male Wistar albino rats (8 weeks old). An LXR agonist GW3965 (10 mg/kg/day, i.p.) was administered to animals for last seven days.

KEY FINDINGS

GW3965 treatment reduced systolic blood pressures in hypertensive rats. Acetylcholine-induced endothelium-dependent and sodium nitroprusside-induced endothelium-independent vasorelaxations were decreased in hypertensive rats but not affected by GW3965. GW3965 treatment enhanced plasma nitrite levels in normotensive rats. KCl and phenylephrine (Phe)-induced vasocontractions were reduced in hypertensive groups and increased with GW3965 treatment. Decreased sarco/endoplasmic reticulum Ca²⁺-ATPase2 (SERCA2) expression in the hypertensive aorta was not changed by GW3965 treatment. Expression of inositoltrisphosphate receptor1 (IP3R1) was increased by GW3965 in normotensive animals. The nuclear factor kappaB (NF-κB) and tumor necrosis factor alpha (TNF-α) expressions were increased in hypertensive rats and reduced by GW3965 treatment.

SIGNIFICANCE

The results of study indicate that the LXR agonist, GW3965, exhibited a beneficial effect on increased blood pressure and improved hypertension-induced impairment in contractile activity of vessel and inflammatory markers in vascular tissue. Therefore, these effects of LXR agonists on vessel should be taken into account in experimental or therapeutic approaches to hypertension.

Evaluation of the antihypertensive activity and alpha adrenergic receptor interaction of cleistanthins A and B

Hypertension was induced in male Sprague Dawley rats with twice weekly administration of deoxycorticosterone acetate (DOCA) salt (20 mg/kg s.c) for 4 weeks. They were divided into eight groups of six animals each viz., hypertensive control, standard (prazosin 1 mg/kg), cleistanthin A 12.5, 25, 50 mg/kg and cleistanthin B 12.5, 25 mg/kg, and 50 mg/kg. One more group served as normal control. The hypertension was induced in 4 weeks, and the animals were given assigned treatment in 5^th week. The alteration in blood pressure (BP) was recorded weekly using a rodent noninvasive blood pressure system. At the end of the experiment alpha-adrenergic receptor response of drugs like adrenaline, nor adrenaline, dopamine (doses 1 μg and 2 μg) was recorded invasively. Two-way repeated measures ANOVA followed by Bonferroni post-hoc test was used to analyze the data. The systolic BP and diastolic BP of test groups rose to a higher level after DOCA administration and fell to the normal range (P < 0.05) following the administration of cleistanthins A and B. There were no differences in the weekly heart rate among the groups. In the test group animals pretreated with prazosin and cleistanthins, adrenaline, noradrenaline and dopamine failed to raise the mean arterial pressure and the end-diastolic pressure from baseline (P > 0.05) cleistanthins A and B exert a significant antihypertensive effect through alpha-adrenergic receptor blockade similar to prazosin.

A possible correlation between the correction of endothelial dysfunction and normalization of high blood pressure levels by 1,3,4-oxadiazole derivative, an L-type Ca2+ channel blocker in deoxycorticosterone acetate and N(G)-nitro-l-arginine hypertensive rats

We have previously demonstrated the vasorelaxant activity of 1,3,4-oxadiazole derivative (NOX-1) through L-type Ca2+ channel blockage. In the present study, we investigated whether the correction of endothelial dysfunction is dependent on the normalization of high blood pressure levels by 1,3,4-oxadiazole derivative (NOX-1) in deoxycorticosterone acetate (DOCA-salt) and N(G)-nitro-l-arginine (L-NNA) hypertensive rats. In DOCA-salt and L-NNA hypertensive rats, the mean systolic blood pressure (MSBB) was 185.3+/-4.7 and 170.2+/-4.1 mmHg, whereas after administration of NOX-1 to hypertensive rats, MSBB was 127.8+/-4.5 and 120.2+/-5.1 mmHg, respectively. To study the endothelial dysfunction, concentration-response curves of norepinephrine (NE) and acetylcholine (Ach) were constructed in rat aortic rings isolated from normotensive, hypertensive (DOCA and L-NNA) and NOX-1 treated rats. NE-induced contractions and Ach-induced relaxations were significantly (p<0.05) decreased and increased, respectively in the aorta of NOX-1 treated rats. Vasorelaxant activity of NOX-1 was not abolished by pretreatment of aortic rings with L-NNA, 1H-[1,2,4] oxadiazolo [4,3-A] quinoxalin-1-one (ODQ), indomethacin or glibenclamide. The results suggest that the endothelial dysfunction can be corrected by the L-type Ca2+ channel blocker with endothelium-independent action and that is dependent on the normalization of high blood pressure levels. The antihypertensive and vasorelaxant effects of NOX-1 are mainly endothelial-independent and it can be used to treat hypertension, a state associated with endothelial dysfunction.

Effect of Synthetic Corticosteroids on Vascular Reactivity in the Human Forearm

Exogenous cortisol raises blood pressure (BP) and suppresses acetylcholine (ACh)-induced vasodilatation in healthy male volunteers. This study tests the hypothesis that the activation of either classical type I or II corticosteroid receptors by synthetic corticosteroids induces endothelial dysfunction. In two separate studies, dexamethasone or fludrocortisone was administered to healthy male subjects over five days. BP, metabolic parameters, and forearm blood flow (FBF) responses to intra-arterial ACh and nitroprusside (SNP) were measured on day 5 of treatment. Fludrocortisone (800 microg/day) and dexamethasone (3 mg/day) increased BP from control measurements, but not when compared with placebo. Metabolic effects of the steroids were consistent with their known actions. Endothelium-dependent vasodilatation was enhanced by fludrocortisone, most obviously in the presence of nitric oxide (NO) synthase inhibition with NG-mono-methyl-L-arginine (LNMMA). Dexamethasone did not suppress endothelium dependent or independent vasodilatation. Non-NO-mediated endothelium-dependent vasodilatation was increased by systemic mineralocorticoid excess but unaffected by glucocorticoid excess. These results do not support the notion that cortisol-induced vascular effects are mediated through classical corticosteroid receptors.

Impaired effect of salt loading on nitric oxide-mediated relaxation in aortas from stroke-prone spontaneously hypertensive rats

1. The present study was designed to characterize the effects of salt on vasorelaxation via the nitric oxide (NO)/cGMP pathway in stroke-prone spontaneously hypertensive rats (SHRSP), which are highly salt sensitive. 2. Male 8-week-old SHRSP were given 1% NaCl solution as drinking water for 4 weeks, whereas control animals were given water only. 3. In aortic rings from salt-loaded SHRSP, relaxations in response to acetylcholine and sodium nitroprusside were significantly impaired compared with those in the control. In the presence of zaprinast, a cGMP-specific cyclic nucleotide phosphodiesterase (PDE)-5 inhibitor, the cGMP levels induced by these drugs were significantly reduced by salt loading, but remained unchanged in the absence of zaprinast. The protein levels of endothelial NO synthase, soluble guanylate cyclase (sGC) and cGMP-dependent protein kinase (PKG) remained unchanged with salt loading, but those of PDE-5 decreased significantly and those of phosphorylated PKG tended to decrease, although the change was not statistically significant. Salt loading significantly impaired the relaxation in response to 8-bromo-cGMP. 4. These results indicate that, in aortas from SHRSP, salt loading causes impairment of relaxation in response to NO, which may be due to a decrease in cGMP production by sGC and impairment of the relaxation pathway downstream of cGMP, which, in turn, probably causes a decrease in PKG activity. Reduced PDE-5 protein expression may act, in part, as a compensatory response to impairment of cGMP-mediated relaxation.

The endocrine system in chronic nitric oxide deficiency

The experimental model of chronic inhibition of nitric oxide (NO) production has proven to be a useful tool to study cardiovascular and renal lesions produced by this type of hypertension, which are similar to those found in human hypertension. It also offers a unique opportunity to study the interaction of NO with the humoral systems, known to have a role in the normal physiology of vascular tone and renal function. This review provides a thorough and updated analysis of the interactions of NO with the endocrine system. There is special focus on the main vasoactive factors, including the renin-angiotensin-aldosterone system, catecholamines, vasopressin, and endothelin among others. Recent discoveries of crosstalk between the endocrine system and NO are also reported. Study of these humoral interactions indicates that NO is a molecule with ubiquitous function and that its inhibition alters virtually to all other known regulatory systems. Thus, hypothyroidism attenuates the pressor effect of NO inhibitor N-nitro-L-arginine methyl ester, whereas hyperthyroidism aggravates the effects of NO synthesis inhibition; the sex hormone environment determines the blood pressure response to NO blockade; NO may play a homeostatic role against the prohypertensive effects of mineralocorticoids, thyroid hormones and insulin; and finally, NO deficiency affects not only blood pressure but also glucose and lipid homeostasis, mimicking the human metabolic syndrome X, suggesting that NO deficiency may be a link between metabolic and cardiovascular disease.

Differential expression of α2D-adrenoceptor and eNOS in aortas from early and later stages of diabetes in Goto-Kakizaki rats

The aim of the present study was to compare vascular dysfunction between the early (12 wk old) and later (36 wk old) stages of spontaneous diabetes in Goto-Kakizaki (GK) rats. We also evaluated the aortic expression of the α2D-adrenoceptor and endothelial nitric oxide synthase (eNOS). Vascular reactivity was assessed in thoracic aortas from age-matched control rats and 12- and 36-wk GK rats. Using RT-PCR and immunoblots, we also examined the changes in expression of the α2D-adrenoceptor and eNOS. In aortas from GK rats (vs. those from age-matched control rats): 1) the relaxation response to ACh was enhanced at 12 wk but decreased at 36 wk; 2) the relaxation response to sodium nitroprusside was decreased at both 12 and 36 wk, 3) norepinephrine (NE)-induced contractility was decreased at 12 wk but not at 36 wk, 4) the expressions of α1B- and α1D-adrenoceptors were unaffected, whereas those of α2D-adrenoceptor and eNOS mRNAs were increased at both 12 and 36 wk; and 5) NE- and ACh-stimulated NOx (nitrite and nitrate) levels were increased at 12 wk, although at 36 wk ACh-stimulated NOx was lower, whereas NE-stimulated NOx showed no change. These results clearly demonstrate that enhanced ACh-induced relaxation and impaired NE-induced contraction, due to NO overproduction via eNOS and increased α2D-adrenoceptor expression, occur in early-stage GK rats and that the impaired ACh-induced relaxation in later-stage GK rats is due to reductions in both NO production and NO responsiveness (but not in eNOS expression).

[Endothelium-derived factors in hypertensive blood vessels, especially nitric oxide and hypertension]

Endothelium-dependent relaxation (EDR) in the blood vessels of spontaneously hypertensive rats (SHR) and the role of nitric oxide (NO) in the initiation of hypertension are reviewed. EDR was impaired in blood vessels of SHR depending on age and degree of hypertension when compared with those of normotensive rats. The cause of the impairment varied among the type of blood vessels: a decrease in the production of NO and endothelium-derived relaxing factor (EDRF) and an increase in the production of endothelium-derived contracting factor (EDCF) are the main causes of the impairment in large arteries, while a decrease in endothelium-dependent hyperpolarization and increased release of EDCF are the main causes of the impairment in small arteries. Interactions among these endothelium-derived factors and changes in the interactions are also causes of impairment. Superoxide may be involved in the impairment of EDR by destroying NO. The endothelium depresses smooth muscle contraction, including spontaneous tone developed in vascular smooth muscle, and the depressing effect of the endothelium is impaired in the preparations from SHR. The endothelium of blood vessels of SHR are structurally injured as demonstrated by scanning electron microscopy. Antihypertensive treatment prevented these functional and structural changes. Chronic treatment with inhibitors of NO production in normotensive rats impaired EDR and elevated blood pressure. The impairment of EDR is a secondary change due to continued hypertension, and early initiation of antihypertensive therapy is recommended.

Endothelial cell dysfunction in mice after transgenic knockout of type 2, but not type 1, 11beta-hydroxysteroid dehydrogenase

BACKGROUND

11beta-Hydroxysteroid dehydrogenase (11betaHSD) isozymes catalyze the interconversion of active and inactive glucocorticoids, allowing local regulation of corticosteroid receptor activation. Both are present in the vessel wall; here, using mice with selective inactivation of 11betaHSD isozymes, we test the hypothesis that 11betaHSDs influence vascular function.

METHODS AND RESULTS

Thoracic aortas were obtained from weight-matched male wild-type (MF1x129 cross(+/+)), 11betaHSD1(-/-), and 11betaHSD2(-/-) mice. mRNA for both isozymes was detected in wild-type aortas by RT-PCR. 11betaHSD activity in aortic homogenates (48.81+/-4.65% conversion) was reduced in both 11betaHSD1(-/-) (6.36+/-2.47% conversion; P<0.0002) and 11betaHSD2(-/-) (24.71+/-3.69; P=0.002) mice. Functional responses were unaffected in aortic rings isolated from 11betaHSD1(-/-) mice. In contrast, aortas from 11betaHSD2(-/-) mice demonstrated selectively enhanced constriction to norepinephrine (E(max) 4.28+/-0.56 versus 1.72+/-0.47 mN/mm; P=0.004) attributable to impaired endothelium-derived nitric oxide activity. Relaxation responses to endothelium-dependent and -independent vasodilators were also impaired. To control for chronic renal mineralocorticoid excess, MF1 mice were treated with fludrocortisone (16 weeks) but did not reproduce the functional changes observed in 11betaHSD2(-/-) mice.

CONCLUSIONS

Although both 11betaHSD isozymes are present in the vascular wall, reactivation of glucocorticoids by 11betaHSD1 does not influence aortic function. Mice with 11betaHSD2 knockout, however, have endothelial dysfunction causing enhanced norepinephrine-mediated contraction. This appears to be independent of renal sodium retention and may contribute to hypertension in 11betaHSD2 deficiency.

Clonidine-induced nitric oxide-dependent vasorelaxation mediated by endothelial alpha(2)-adrenoceptor activation

1. To assess the involvement of endothelial alpha(2)-adrenoceptors in the clonidine-induced vasodilatation, the mesenteric artery of Sprague Dawley rats was cannulated and perfused with Tyrode solution (2 ml min(-1)). We measured perfusion pressure, nitric oxide (NO) in the perfusate using chemiluminescence, and tissue cyclic GMP by RIA. 2. In phenylephrine-precontracted mesenteries, clonidine elicited concentration-dependent vasodilatations associated to a rise in luminal NO. One hundred nM rauwolscine or 100 microM L(omega)-nitro-L-arginine antagonized the clonidine-induced vasodilatation. Guanabenz, guanfacine, and oxymetazoline mimicked the clonidine-induced vasorelaxation. 3. In non-contracted mesenteries, 100 nM clonidine elicited a maximal rise of NO (123+/-13 pmol); associated to a peak in tissue cyclic GMP. Endothelium removal, L(omega)-nitro-L-arginine, or rauwolscine ablated the rise in NO. One hundred nM aminoclonidine, guanfacine, guanabenz, UK14,304 and oxymetazoline mimicked the clonidine-induced surge of NO. Ten microM ODQ obliterated the clonidine-induced vasorelaxation and the associated tissue cyclic GMP accumulation; 10 - 100 nM sildenafil increased tissue cyclic GMP accumulation without altering the clonidine-induced NO release. 4. alpha(2)-Adrenergic blockers antagonized the clonidine-induced rise in NO. Consistent with a preferential alpha(2D)-adrenoceptor activation, the K(B)s for yohimbine, rauwolscine, phentolamine, WB-4101, and prazosin were: 6.8, 24, 19, 165, and 1489 nM, respectively. 5. Rat pretreatment with 100 mg kg(-1) 6-hydroxydopamine reduced 95% tissue noradrenaline and 60% neuropeptide Y. In these preparations, 100 nM clonidine elicited a rise of 91.9+/-15.5 pmol NO. Perfusion with 1 microM guanethidine or 1 microM guanethidine plus 1 microM atropine did not modify the NO surge evoked by 100 nM clonidine. 6. Clonidine and congeners activate endothelial alpha(2D)-adrenoceptors coupled to the L-arginine pathway, suggesting that the antihypertensive action of clonidine involves an endothelial vasorelaxation mediated by NO release, in addition to presynaptic mechanisms.

Decreased sensitivity to nitric oxide in the aorta of severely hypercholesterolemic apolipoprotein E-deficient mice

A normal response to nitric oxide donors has been cited as evidence that impaired endothelium-dependent vasodilation during hypercholesterolemia is due to decreased synthesis of nitric oxide. This tenet was examined by determining responses to nitric oxide gas as well as to acetylcholine and sodium nitroprusside in the isolated aorta of apolipoprotein E-deficient mice fed normal or Western-type cholesterol-rich diet until 21 or 35 weeks of age. In mice fed normal chow, relaxation to all agents remained comparable to that obtained in wild-type mice. In mice fed Western diet, the relaxation to acetylcholine as well as to nitric oxide was decreased at 35 weeks of age. At 21 weeks of age, decreased sensitivity to nitric oxide was observed despite a normal response to acetylcholine. The response to sodium nitroprusside was normal in all groups. A decrease in aortic superoxide dismutase activity as well as an increase in aortic superoxide anion generated in the presence of NADH as measured by lucigenin chemiluminescence was observed in the group fed Western diet at 35 weeks. This provides evidence that altered superoxide anion could contribute to the deterioration in nitric oxide sensitivity that underlies the impaired endothelium-dependent relaxation. These data indicate that decreased sensitivity to nitric oxide may contribute to the development of impaired endothelium-dependent relaxation in hypercholesterolemia. The response to sodium nitroprusside appears not to reflect the decreased sensitivity of vascular smooth muscle to authentic nitric oxide.

Altered coronary dilation in deoxycorticosterone acetate-salt hypertension

OBJECTIVE

To compare coronary dilation in uninephrectomized hypertensive deoxycorticosterone acetate (DOCA)-salt rats (HTRs), treated for 2 or 4 weeks, with age-matched uninephrectomized normotensive rats (NTRs). DESIGN AND METHODS Coronary perfusion pressure was recorded in isolated hearts perfused at a constant flow rate to evaluate coronary resistance.

RESULT

A decreased vasoconstriction due to NG-nitro-Larginine (NNLA, 30 pmol/I) in hearts from HTRs suggested a reduced basal nitric oxide (NO) release. In contrast, coronary vasodilation due to the NO donor, sodium nitroprusside (3 pmol/I), remained unaffected in 2-week HTRs, and was enhanced in 4-week HTRs. Cumulative dose-response curves to bradykinin induced an important vasodilation in NTRs, with a maximal response that remained unaffected in the presence of either NNLA (30 pmol/I), indomethacin (10 pmol/l) or the two combined. In contrast, hearts from HTRs showed a diminished maximal relaxation to bradykinin, suggesting an altered endothelium-dependent relaxation. The presence of NNLA or indomethacin had no effect on the weak relaxation observed in HTRs. However, NNLA and indomethacin combined unmasked an important relaxation due to bradykinin in HTRs. The addition of clotrimazole (1 pmol/I) to NNLA and indomethacin blunted the relaxation due to bradykinin in both NTRs and HTRs. Perfusion with superoxide dismutase (120 IU/ml) restored most of the coronary relaxation due to bradykinin in hearts from HTRs. Bradykinin-induced prostaglandin 12 (PGI2) and E2 (PGE2) production was unaffected by hypertension. No increase in thromboxane A2 (TXA2) due to bradykinin was detected. Finally, reduced reactivity to papaverine and forskolin was observed in hearts from HTRs.

CONCLUSION

DOCA-salt hypertension is associated with alterations in coronary reactivity. Basal NO formation appears to be reduced in HTRs, but the intact relaxation to exogenous NO suggests a preserved guanylate cyclase pathway. In addition, alteration in adenylate cyclase activity, and not in prostaglandin production, may explain the blunted cAMP-mediated responses in HTRs. The combined nitric-oxide synthase (NOS) and cyclo-oxygenase (COX) inhibition unmasked an endothelium-derived hyperpolarizing factor (EDHF) involvement in the coronary dilation due to bradykinin in hearts from HTRs, suggesting that endothelial NO and PGI2, although unable to induce coronary smooth-muscle relaxation, can inhibit EDHF production in HTRs. Impairment in the adenylate cyclase pathway and the suppression of NO by free radicals may explain the blunted vasodilation in DOCA-salt hypertension.

Dietary chloride does not correlate with urinary thromboxane in deoxycorticosterone acetate-treated rats

Renal vascular resistance in deoxycorticosterone acetate (DOCA) salt-treated uninephrectomized rats is increased by high dietary chloride. Because DOCA salt-hypertensive rats exhibit an increased urinary excretion of thromboxane B2 (TXB2), a metabolite of thromboxane A2 (TXA2), the increased TXB2 excretion by DOCA salt-treated rats could relate to elevated dietary chloride, increased blood pressure, and/or the presence of intact renal tubules. We hypothesized that high NaCl intake, resulting in an elevated tubular chloride excretion, stimulates TXA2 production. A result of that production could be renal vasoconstriction. Baseline blood pressures were measured for 10 days, and then the rats were treated with DOCA (30 mg/kg) and fed (1) normal NaCl, (2) normal sodium with high chloride, or (3) high sodium chloride (NaCl) for 4.5 weeks. Next, the rats were uninephrectomized (1K) or unihydronephrectomized (1KHK) to yield one kidney without an intact tubular system and therefore no macula densa. Two and a half weeks later, urinary excretion of TXB2 was determined. DOCA-high NaCl-fed 1KHK or 1K rats had significant increases in systemic blood pressure to 172 +/- 12 and 190 +/- 5 mm Hg, respectively, compared with no significant increase in blood pressure among the other groups. Urinary TXB2 excretion was increased to 29 +/- 4 pg per 24 hours per gram of body weight in all DOCA-treated 1KHK and 1K animals regardless of diet compared with DOCA-treated animals with two intact kidneys (13 +/- 2 pg per 24 hours per gram of body weight). DOCA treatment in rats with one functional kidney results in the excretion of high levels of urinary TXB2 unrelated to dietary chloride load, blood pressure, or intact renal tubules.

Cardiac and vascular responses in deoxycorticosterone acetate-salt hypertensive rats

1. Hypertension leads to ventricular hypertrophy and, eventually, to heart failure. The present study has investigated the functional consequences of deoxycorticosterone acetate (DOCA)-salt hypertension in rats by defining the inotropic, chronotropic and vascular responses to noradrenaline (NA; beta1-adrenoceptor agonist), forskolin (adenylate cyclase activator) and theophylline (phosphodiesterase inhibitor). 2. Administration of DOCA (25 mg, s.c., every 4th day) and excess salt (1% NaCl in drinking water) to uninephrectomized rats increased left ventricular wet weight by 35 and 71% after 4 and 8 weeks, respectively. Addition of KCl (0.4%) or CaCl2 (1%) in the drinking water for 4 weeks attenuated blood pressure increases, but not ventricular weight increases (46 and 28%, respectively). 3. Positive inotropic responses in papillary muscles from uninephrectomized rats to NA (-log EC50 6.73+/-0.38; n = 7), forskolin (-log EC50 6.15+/-0.31; n = 7) and CaCl2 (-log EC50 2.40+/-0.02; n = 14) were unchanged in hypertrophied left ventricles of DOCA and DOCA-CaCl2 rats, although maximal responses to NA were decreased in DOCA-KCI rats (1.2+/-0.6 mN, n = 8; DOCA-salt 2.9+/-0.5 mN, n = 6); theophylline was less potent in DOCA-salt rats. Positive chronotropic responses to NA, forskolin and theophylline in right atria and negative inotropic responses to carbachol in papillary muscles were unchanged. 4. Maximal vasoconstrictor responses to NA in thoracic aortic rings were reduced in DOCA-KCI rats to 2.4+/-0.9 mN (n = 5), but were increased in DOCA-CaCl2 rats to 26.6+/-2.2 mN (n = 7; DOCA-salt 7.8+/-2.2 mN, n = 9). Vasorelaxant responses to forskolin and theophylline were unchanged. 5. These results show that cardiac responses are only minimally affected during the development of DOCA-salt hypertension-induced hypertrophy, despite the reported decreases in adenylate cyclase activity, in these rats. This is in contrast with the decreased responses reported in other rat models of cardiac hypertrophy and in the failing human heart. Thus, hypertrophy in hearts of DOCA-salt hypertensive rats does not produce similar changes to the failing human heart.

Interaction between nitric oxide and mineralocorticoids in the long-term control of blood pressure

We analyzed the effects of a possible interaction between nitric oxide deficiency and mineralocorticoids on the long-term control of blood pressure and renal and endocrine variables. Six groups of uninephrectomized male Wistar rats were used: control animals and rats that received (1) N(G)-nitro-L-arginine methyl ester (L-NAME) subpressor (0.5 mg/100 mL drinking fluid), (2) L-NAME pressor (35 mg/100 mL drinking fluid), (3) deoxycorticosterone acetate (DOCA; 12. 5 mg/wk per rat), (4) DOCA plus L-NAME subpressor, or (5) L-NAME pressor plus DOCA. For all groups, the drinking fluid was tap water or 1% NaCl solution. We measured the time course of tail systolic blood pressure (SBP) and body weight for 3 weeks in all rats. At the end of the experimental period, we measured mean arterial pressure (direct recording) and endocrine and renal variables. Tail SBP rose significantly in the DOCA plus L-NAME subpressor-treated group but remained at normotensive levels in the DOCA-treated group. The addition of L-NAME to the subpressor dose accelerated the blood pressure increase in DOCA-salt hypertensive rats. The simultaneous administration of DOCA and L-NAME increased blood pressure and mortality rates in rats that drank water or saline compared with the rats treated with L-NAME alone. The subpressor dose of L-NAME did not increase blood pressure in saline-drinking rats. We conclude that impaired NO synthesis results in increased sensitivity to the pressor effect of mineralocorticoids in the presence or absence of an increased saline intake. Hence, nitric oxide contributes to the adaptative response to mineralocorticoid excess, perhaps through the facilitation of natriuresis and, thus, control of blood pressure.

Angiotensin II and connective tissue: homeostasis and reciprocal regulation

As a concept traditionally applied to integrative organ physiology, homeostasis likewise applies to self-regulated growth and structure of loose, dense and specialized connective tissues. De novo generation and co-induction of signals, either stimulatory or inhibitory to the formation of these tissues, provide for a reciprocal regulation of their composition; angiotensin (Ang) II is a growth stimulator. Components involved in AngII generation and its biological activity, including angiotensin converting enzyme (ACE) and AngII receptors, are expressed by mesenchymal cells responsible for connective tissue turnover. ACE inhibition or AT1 receptor antagonism attenuate the formation of these connective tissues. The concept of circulatory homeostasis, and the endocrine properties of plasma AngII involved in maintaining same, need each be broadened to encompass auto- and paracrine effects of AngII produced within connective tissues, where it contributes to their homeostatic regulation of structure and composition.

Effect of dietary salt loading and high-calcium diet on vascular smooth muscle responses and endothelium function in rats

1. The present study examined the effects of concurrent manipulation of dietary calcium and salt on contractile responses of vascular smooth muscle (VSM) and endothelial function of aortic rings from Sprague-Dawley rats. 2. Salt loading enhanced the contractile response of the aortic rings to noradrenaline (NA), an effect that was blunted by a high calcium intake. 3. Removal of the endothelium and incubation of aortic rings in physiological salt solution containing methylene blue increased the sensitivity of the rings to NA. 4. The increase in the sensitivity of aortic rings induced by endothelium removal was more pronounced in aortic rings from salt-loaded rats. 5. Acetylcholine caused similar degrees of relaxation in all experimental groups, but the relaxation to histamine was smaller (P < 0.05) in salt-loaded rats than in other groups of rats; however, after removal of the endothelium, the contractile response to histamine was higher in salt-loaded rats. 6. The results indicate that the hypersensitivity of isolated aortic rings to agonists, as observed in salt-loaded rats, is due to altered responses of the VSM and not as a result of changes in the endothelium. In addition, salt loading tends to increase the synthesis of endothelium-dependent relaxing factor. The ability of salt loading to enhance the contractile responses of VSM to agonists can be prevented by supplementing the diet with high calcium.

Role of vascular nitric oxide in physiological and pathological conditions

This review describes the ability of certain diseases, such as essential hypertension, atherosclerosis, angina, and vasospasm, to reduce vascular nitric oxide (NO) formation or to increase its metabolism. In contrast, others, such as hypotension, sepsis, stroke, myocardial depression, and inflammatory responses, increase NO synthesis. The mechanism implicated in the changes in the formation and metabolism of NO are described. To prevent or treat these pathological processes, in which a deficiency in vascular NO formation plays a causative role, NO may be provided through methods such as direct NO administration or indirect NO supply through either NO donors or L-arginine, which facilitates NO formation.

Differential contribution of endothelial function to vascular reactivity in conduit and resistance arteries from deoxycorticosterone-salt hypertensive rats

The purpose of these studies was to compare changes in conduit and resistance artery function in deoxycorticosterone-salt hypertensive rats. We hypothesized that if there was a common mechanism producing changes in vascular function in hypertension, then there would be similar alterations in reactivity of conduit and resistance arteries. Helically cut strips of common carotid artery were prepared for measurement of isometric force generation, and segments of small mesenteric arteries were pressurized for video dimension analysis. Sensitivity of arteries to phenylephrine and acetylcholine was determined. Carotid arteries from deoxycorticosterone-salt hypertensive rats were more sensitive to phenylephrine than arteries from control rats, whereas mesenteric resistance arteries from hypertensive rats were less sensitive to phenylephrine. In carotid arteries, endothelial denudation or incubation with N psi-nitro-L-arginine increased phenylephrine sensitivity in control rats to the level seen in deoxycorticosterone-salt rats. These manipulations had no effect on phenylephrine sensitivity in arteries from deoxycorticosterone-salt rats. In mesenteric resistance arteries, endothelium denudation normalized the depressed phenylephrine sensitivity in arteries from hypertensive rats but had no effect on arteries from normotensive rats. This depressed phenylephrine sensitivity in deoxycorticosterone-salt mesenteric arteries was not reversed by incubation with Npsi-nitro-L-arginine. Acetylcholine-induced relaxation was depressed in carotid arteries from deoxycorticosterone-salt hypertensive rats, and Npsi-nitro-L-arginine blocked these relaxations. In contrast, acetylcholine relaxation in the mesenteric arteries from normotensive and hypertensive rats did not differ. N psi-nitro-L-arginine slightly but significantly attenuated acetylcholine dilation only in mesenteric resistance arteries from the hypertensive rats. We conclude that qualitatively different changes in vasoconstrictor sensitivity to phenylephrine occur in carotid arteries and mesenteric resistance arteries of deoxycorticosterone-salt hypertensive rats. The increased phenylephrine sensitivity in carotid arteries in this model of hypertension is due to the loss of endothelium-derived nitric oxide production. In contrast, the decreased phenylephrine sensitivity in mesenteric resistance arteries from deoxy-corticosterone-salt rats is due to a non-nitric oxide-mediated influence of the endothelium that is absent in arteries from normotensive rats.

Endothelial function in deoxycorticosterone-NaCl hypertension: effect of calcium supplementation

BACKGROUND

Dietary calcium intake has been suggested to correlate inversely with blood pressure in humans and experimental animals. However, the effects of calcium supplementation on hypertensive disturbances of the endothelium have not been well characterized.

METHODS AND RESULTS

Wistar-Kyoto rats made hypertensive by deoxycorticosterone (DOC)-NaCl treatment, but a concurrent increase in chow calcium content from 1.1% to 2.5% markedly attenuated the rise in blood pressure. The function of isolated mesenteric arterial rings in vitro was investigated at the close of the 10-week study. In norepinephrine-precontracted rings, the relaxations to acetylcholine (ACh) and ADP, as well as to nitroprusside, 3-morpholinosydnonimine, and isoproterenol were attenuated in hypertensive rats on 1.1% calcium supplementation. In the presence of NG-nitro-L-arginine methyl ester (L-NAME), the relaxations to ACh in hypertensive animals on normal calcium were practically absent, whereas in normotensive rats and calcium-supplemented hypertensive rats, distinct relaxations to higher concentrations of ACh were still present. These responses were reduced by 30% to 50% with apamin, a blocker of Ca2+-activated K+ channels, and were further inhibited by blockade of ATP-dependent K+ channels with glyburide. Interestingly, relaxations elicited by ACh and ADP during precontraction with 60 mmol/L KCl (preventing endothelium-dependent hyperpolarization) were not impaired in hypertensive animals. The contractile sensitivity of endothelium-intact arterial rings to 5-hydroxytryptamine and norepinephrine was higher in hypertensive rats on either normal or high-calcium diet, whereas the increase in contractile sensitivity caused by L-NAME corresponded in all groups.

CONCLUSION

High-calcium diet markedly opposed experimental DOC-NaCl hypertension, an effect associated with improved arterial relaxation, while abnormalities of vascular contractile properties remained unaffected. In particular, the hyperpolarization-related component of endothelium-dependent arterial relaxation, mediated via opening of arterial K+ channels, could be augmented by calcium supplementation in DOC-NaCl hypertension.

Blood pressure-related changes of endothelium-dependent relaxation in the aorta from SHRSP at developmental ages of hypertension

1. Shear stress and flow influence endothelial functions to stimulate the release of endothelium-derived relaxing factor (EDRF). However, it is not well defined how hypertension affects endothelial functions. In this study, blood pressure-related changes of endothelium-dependent relaxation (EDR) in the aorta were examined in SHRSP, WKY and F1 hybrid at developmental ages of hypertension, and hypotensively-treated SHRSP. 2. Acetylcholine-induced EDR of aortic rings was significantly enhanced in 8 week old SHRSP compared with that of age-matched WKY. 3. NG-monomethyl L-arginine (L-NMMA) (1 mmol/L), an inhibitor of nitric oxide, greatly reduced the relaxation in both strains. Indomethacin (10(-5) mol/L), a cyclo-oxygenase inhibitor, did not affect the relaxation at this age. 4. EDR was positively correlated (r = 0.81, n = 22) with blood pressure at 9 weeks of age in WKY, SHRSP and F1 hybrid. 5. A five week hypotensive treatment of SHRSP caused a significant reduction in EDR with decreasing blood pressure. 6. It was concluded that the endothelium releases more EDRF in response to increasing blood pressure to regulate vascular tone at developmental ages of hypertension.

The role of nitric oxide and other endothelium-derived vasoactive substances in vascular disease

Alteration in the release and action of endothelium-derived vasoactive factors is responsible for changes in vascular reactivity early in the course of vascular disease. These factors include nitric oxide, eicosanoids, endothelium-derived hyperpolarizing factor, endothelin, and angiotensin II. Because endothelial dysfunction occurs at early stages of disease, it may reflect physiological changes that, if allowed to become chronic, are responsible for changes in vascular structure and growth and adhesivity to platelets and leukocytes, ultimately leading to atherosclerosis and thrombosis. Each of the major risk factors predisposing to vascular disease are associated with endothelial cell dysfunction, suggesting a direct etiologic link between the effects of the risk factors on the endothelium and their propensity to accelerate vascular disease. Restoration or replacement of endothelium-derived factors such as nitric oxide and prostacyclin, which impede the progression of vascular disease, or preventing the action of mediators such as vasoconstrictor eicosanoids, angiotensin II, or endothelin, which accelerate the progression of vascular disease, has become a useful paradigm in the treatment and prevention of vascular disease. Thus, understanding the physiology of endothelium-derived vasoactive factors is a necessary part of every physician's education.

Lack of evidence for linkage of the endothelial cell nitric oxide synthase gene to essential hypertension

BACKGROUND

The basal release of nitric oxide by the endothelium plays an important role in regulating blood flow and pressure and mediates most of the endothelium-dependent vasodilation. Impairment of nitric oxide production by specific inhibitors increases blood pressure in humans, and several reports suggest that hypertensive subjects have a blunted endothelium-dependent vasodilatation that might be secondary to decreased nitric oxide production from the vessel wall.

METHODS AND RESULTS

To determine whether the endothelial nitric oxide synthase gene is involved in human essential hypertension, we identified informative biallelic and multiallelic markers of this locus and performed case-control and linkage studies in hypertensive subjects and normotensive control subjects. We used the affected sib pair method to test for potential linkage in 145 hypertensive pedigrees (269 sib pairs, 346 subjects) with a highly polymorphic marker of the nitric oxide synthase gene (polymorphism information content of 92%). There was no evidence for linkage among affected siblings. The 95% upper confidence limit of this value suggests that at most 1% of alleles in excess of expected are shared. We also identified two informative biallelic markers of this gene to perform a case-control study on white hypertensive and normotensive subjects. Similar genotype distributions between the two groups were noted for both markers. Estimated haplotype frequencies by maximum likelihood methods combining the two biallelic markers were also similar in both groups.

CONCLUSIONS

These findings do not suggest that common molecular variants of the endothelial nitric oxide synthase gene are involved in essential hypertension.

Mechanisms of enhanced angiotensin II-stimulated signal transduction in vascular smooth muscle by aldosterone

We tested the hypothesis that mineralocorticoids potentiate angiotensin II-stimulated phospholipase C activation through an increased number of angiotensin II receptors in cultured rat aortic vascular smooth muscle cells. Exposure of cells to aldosterone for 24 h resulted in concentration-dependent increases in angiotensin II receptor binding. Via studies of angiotensin II displacement by non-peptide receptor antagonists, both basal and upregulated angiotensin II receptors were found to be of the AT1 subtype. Incubation with 1 microM aldosterone resulted in 50-100% enhancement of angiotensin II (100 nM)-stimulated diacylglycerol formation and intracellular calcium mobilization. Exposure to 100 nM 1,25-(OH)2VitD3, which did not upregulate angiotensin II receptors, did not potentiate stimulated inositol phosphate formation. Incubation with aldosterone resulted in potentiation of inositol phosphate formation upon receptor occupation (100 nM angiotensin II) but not upon post-receptor stimulation (25 mM NaF/10 microM AlCl3). Aldosterone did not increase basal phospholipase C activity or content of the inositol trisphosphate precursor phosphatidylinositol-4,5-bisphosphate. These data are consistent with the hypothesis that aldosterone potentiates angiotensin II-stimulated, phospholipase C-dependent intracellular signals solely by coupling to an increased number of angiotensin II receptors. This mechanism may contribute to the sensitized vascular responses to angiotensin II observed in states of mineralocorticoid excess.

Enhancement of endothelium-dependent relaxation in the aorta from stroke-prone spontaneously hypertensive rats at developmental stages of hypertension

1. Endothelium-dependent relaxation (EDR) in aortic rings from young (8 weeks) and adult (16 weeks and 20 weeks) stroke-prone spontaneously hypertensive rats (SHRSP) was investigated in comparison with age-matched Wistar-Kyoto rats (WKY). 2. At 8 weeks, acetylcholine (3 x 10(-9)-10(-5) mol/L) and ionomycin (4 x 10(-8)-10(-6) mol/L)-induced EDR in SHRSP aortae was significantly enhanced compared to that in WKY aortae. Mechanical denudation of the endothelium completely abolished, and pretreatment of aortae with NG-monomethyl L-arginine (1 mmol/L), an inhibitor of nitric oxide formation, greatly reduced the relaxation in both strains. Indomethacin (10(-5) mol/L), a cyclo-oxygenase inhibitor that blocks the production of endothelium-derived contracting factors, did not significantly alter the relaxation by acetylcholine at this age. There was no difference in endothelium-independent relaxation of denuded aortae by sodium nitroprusside (10(-9)-10(-6) mol/L) and 8-bromoguanosine 3', 5'-cyclic monophosphate (10(-6)-10(-3) mol/L). 3. In adult SHRSP with established hypertension, however, the acetylcholine (10(-8)-10(-5) mol/L)-induced relaxation markedly diminished at any of the concentrations tested compared to that observed in 8 weeks old SHRSP and WKY at 8-20 weeks of age. This finding differed from other observations where the relaxation in SHRSP was impaired only at higher concentrations of acetylcholine. Indomethacin pretreatment of aortae from 20 week old SHRSP restored acetylcholine-induced EDR to a level comparable with that in age-matched WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin-mediated skeletal muscle vasodilation is nitric oxide dependent. A novel action of insulin to increase nitric oxide release

The purpose of this study was to examine whether insulin's effect to vasodilate skeletal muscle vasculature is mediated by endothelium-derived nitric oxide (EDNO). N-monomethyl-L-arginine (L-NMMA), a specific inhibitor of NO synthase, was administered directly into the femoral artery of normal subjects at a dose of 16 mg/min and leg blood flow (LBF) was measured during an infusion of saline (NS) or during a euglycemic hyperinsulinemic clamp (HIC) designed to approximately double LBF. In response to the intrafemoral artery infusion of L-NMMA, LBF decreased from 0.296 +/- 0.032 to 0.235 +/- 0.022 liters/min during NS and from 0.479 +/- 0.118 to 0.266 +/- 0.052 liters/min during HIC, P < 0.03. The proportion of NO-dependent LBF during NS and HIC was approximately 20% and approximately 40%, respectively, P < 0.003 (NS vs. HIC). To elucidate whether insulin increases EDNO synthesis/release or EDNO action, vasodilative responses to graded intrafemoral artery infusions of the endothelium-dependent vasodilator methacholine chloride (MCh) or the endothelium-independent vasodilator sodium nitroprusside (SNP) were studied in normal subjects during either NS or HIC. LBF increments in response to intrafemoral artery infusions of MCh but not SNP were augmented during HIC versus NS, P < 0.03. In summary, insulin-mediated vasodilation is EDNO dependent. Insulin vasodilation of skeletal muscle vasculature most likely occurs via increasing EDNO synthesis/release. Thus, insulin appears to be a novel modulator of the EDNO system.

Influence of endothelium on Mg(2+)-induced relaxation in noradrenaline-contracted aorta from DOCA-salt hypertensive rat

The aim of this study was to examine the influence of vascular endothelium on the relaxation induced by increased extracellular Mg2+ concentrations on isolated and noradrenaline-precontracted aorta from deoxycorticosterone acetate-salt (DOCA-salt) hypertensive and normotensive rats. In Mg(2+)-free physiologic salt solution (PSS), addition of Mg2+ (0.1-6.0 nM) caused concentration-dependent relaxation of noradrenaline-precontracted aorta with intact or disrupted endothelium. Mg(2+)-induced relaxation in intact aorta, however, was less in DOCA-salt hypertensive rats than in normotensive rats. When endothelium was disrupted, Mg(2+)-induced relaxation was depressed in aorta from both DOCA-salt hypertensive and normotensive rats. The same observations were made in presence of N-nitro-L-arginine methyl ester (L-NAME), an inhibitor of endothelium-derived relaxing factor nitric oxide (EDRF/NO) biosynthesis. Mg(2+)-induced relaxation following contraction with noradrenaline was significantly less in intact aorta treated with L-NAME from DOCA-salt hypertensive rats than in intact aorta from normotensive rats. Indomethacin did not affect Mg(2+)-induced relaxation in intact aorta from normotensive rats whereas indomethacin significantly increased it in DOCA-salt hypertensive rats. It is concluded that (1) Mg(2+)-induced relaxation can be mediated by endothelium-dependent mechanisms implicating EDRF/NO; (2) the influence of EDRF/NO is more pronounced on the impaired Mg(2+)-induced relaxation of aorta from DOCA-salt hypertensive rats; (3) Mg(2+)-induced relaxation seems masked by vasoconstrictor prostaglandin release in DOCA-salt hypertensive rats; (4) these differences between normotensive and hypertensive rats could be related to the impaired endothelial function in aorta from DOCA-salt hypertensive rats.

Interactions between insulin and norepinephrine on blood pressure and insulin sensitivity. Studies in lean and obese men

To explore the interactions between insulin action and norepinephrine (NE) on blood pressure and muscle vascular resistance, we studied seven lean (66 +/- 1 kg) sensitive and seven age-matched obese (96 +/- 3 kg) insulin-resistant men after an overnight fast. Both groups were normotensive; however, the obese exhibited higher basal blood pressure, 90.8 +/- 2.2 vs. 83.4 +/- 1.6 mmHg, P < 0.04. Each subject was studied on two separate days during either saline (S) infusion or a euglycemic hyperinsulinemic clamp (I) achieving insulin concentrations of approximately 70 microU/ml. After 180 min of either S or I, NE was infused systemically at rates of approximately 50, 75, and 100 pg/kg per min. Glucose uptake was measured in whole body ([3-3H]glucose) and in leg by the balance technique. The results indicate: (a) the NE/pressor dose-response curve was decreased (shifted to the right) during I in lean but not in obese subjects, (b) I enhanced NE metabolic clearance by 20% in lean but not in obese, (c) NE decreases leg vascular resistance more in lean than in obese, and (d) NE causes a approximately 20% increase in insulin-mediated glucose uptake in both groups. In conclusion, insulin resistance of obesity is associated with an apparent augmented NE pressor sensitivity and decreased NE metabolic clearance. Both of these mechanisms can potentially contribute to the higher incidence of hypertension in obese man. Insulin resistance is likely to be a predisposing but not sufficient factor in the pathogenesis of hypertension. Because the obese group exhibited higher basal blood pressure, it is possible that our results reflect this difference. Further studies will be required to clarify this issue.