Losartan and enalapril therapies enhance vasodilatation in the mesenteric artery of spontaneously hypertensive rats

Endothelium-Dependent Hyperpolarization (EDH) in Hypertension: The Role of Endothelial Ion Channels

Upon stimulation with agonists and shear stress, the vascular endothelium of different vessels selectively releases several vasodilator factors such as nitric oxide and prostacyclin. In addition, vascular endothelial cells of many vessels regulate the contractility of the vascular smooth muscle cells through the generation of endothelium-dependent hyperpolarization (EDH). There is a general consensus that the opening of small- and intermediate-conductance Ca2+-activated K⁺ channels (SKCa and IKCa) is the initial mechanistic step for the generation of EDH. In animal models and humans, EDH and EDH-mediated relaxations are impaired during hypertension, and anti-hypertensive treatments restore such impairments. However, the underlying mechanisms of reduced EDH and its improvement by lowering blood pressure are poorly understood. Emerging evidence suggests that alterations of endothelial ion channels such as SKCa channels, inward rectifier K⁺ channels, Ca2+-activated Cl- channels, and transient receptor potential vanilloid type 4 channels contribute to the impaired EDH during hypertension. In this review, we attempt to summarize the accumulating evidence regarding the pathophysiological role of endothelial ion channels, focusing on their relationship with EDH during hypertension.

Vasopeptidase Inhibition Corrects the Structure and Function of the Small Arteries in Experimental Renal Insufficiency

BACKGROUND

We studied whether vasopeptidase inhibition corrects the structure and function of the small arteries in experimental chronic renal insufficiency (CRI).

METHODS

After 5/6 nephrectomy (NX) surgery was performed on rats, there was a 14-week follow-up, allowing CRI to become established. Omapatrilat (40 mg/kg/day in chow) was then given for 8 weeks, and the small mesenteric arterial rings were investigated in vitro using wire and pressure myographs.

RESULTS

Plasma and ventricular B-type natriuretic peptide (BNP) concentrations were increased 2- to 2.7-fold, while systolic blood pressure (BP) increased by 32 mm Hg after NX. Omapatrilat treatment normalized the BNP and reduced the BP by 45 mm Hg in the NX rats. Endothelium-dependent vasorelaxation was impaired but the response to acetylcholine was normalized after omapatrilat treatment. Vasorelaxations induced by nitroprusside, isoprenaline and levcromakalim were enhanced after omapatrilat, and the responses were even more pronounced than in untreated sham-operated rats. Arterial wall thickness and wall-to-lumen ratio were increased after NX, whereas omapatrilat normalized these structural features and improved the strain-stress relationship in the small arteries; this suggests improved arterial elastic properties.

CONCLUSION

Omapatrilat treatment reduced BP, normalized volume overload, improved vasorelaxation and corrected the dimensions and passive elastic properties of the small arteries in the NX rats. Therefore, we consider vasopeptidase inhibition to be an effective treatment for CRI-induced changes in the small arteries.

Palmitoylethanolamide treatment reduces blood pressure in spontaneously hypertensive rats: involvement of cytochrome p450-derived eicosanoids and renin angiotensin system

Palmitoylethanolamide (PEA), a peroxisome proliferator-activated receptor-α agonist, has been demonstrated to reduce blood pressure and kidney damage secondary to hypertension in spontaneously hypertensive rat (SHR). Currently, no information is available concerning the putative effect of PEA on modulating vascular tone. Here, we investigate the mechanisms underpinning PEA blood pressure lowering effect, exploring the contribution of epoxyeicosatrienoic acids, CYP-dependent arachidonic acid metabolites, as endothelium-derived hyperpolarizing factors (EDHF), and renin angiotensin system (RAS) modulation. To achieve this aim SHR and Wistar-Kyoto rats were treated with PEA (30 mg/kg/day) for five weeks. Functional evaluations on mesenteric bed were performed to analyze EDHF-mediated vasodilation. Moreover, mesenteric bed and carotid were harvested to measure CYP2C23 and CYP2J2, the key isoenzymes in the formation of epoxyeicosatrienoic acids, and the soluble epoxide hydrolase, which is responsible for their degradation in the corresponding diols. Effect of PEA on RAS modulation was investigated by analyzing angiotensin converting enzyme and angiotensin receptor 1 expression. Here, we showed that EDHF-mediated dilation in response to acetylcholine was increased in mesenteric beds of PEA-treated SHR. Western blot analysis revealed that the increase in CYP2C23 and CYP2J2 observed in SHR was significantly attenuated in mesenteric beds of PEA-treated SHR, but unchanged in the carotids. Interestingly, in both vascular tissues, PEA significantly decreased the soluble epoxide hydrolase protein level, accompanied by a reduced serum concentration of its metabolite 14-15 dihydroxyeicosatrienoic acid, implying a reduction in epoxyeicosatrienoic acid hydrolisis. Moreover, PEA treatment down-regulated angiotensin receptor 1 and angiotensin converting enzyme expression, indicating a reduction in angiotensin II-mediated effects. Consistently, a damping of the activation of angiotensin receptor 1 underlying pathways in mesenteric beds was shown in basal conditions in PEA-treated SHR. In conclusion, our data demonstrate the involvement of epoxyeicosatrienoic acids and renin angiotensin system in the blood pressure lowering effect of PEA.

Prenatal testosterone induces sex-specific dysfunction in endothelium-dependent relaxation pathways in adult male and female rats

Prenatal testosterone (T) exposure impacts postnatal cardiovascular function, leading to increases in blood pressure with associated decreased endothelium-dependent vascular relaxation in adult females. Endothelial function in males is not known. Furthermore, which of the endothelial pathways contributes to endothelial dysfunction and if there exists sex differences are not known. The objective of this study was to characterize the relative contribution of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) to the impaired endothelium-dependent vasodilation in prenatal T-exposed adult males and females. Offspring of pregnant rats treated with T propionate or its vehicle were examined. Telemetric blood pressure levels and endothelium-dependent vascular reactivity were assessed with wire myography. Levels of nitric oxide synthase (NOS3) and Kcnn3 and Kcnn4 channel expression were examined in mesenteric arteries. Mean arterial pressure was significantly higher in T males and females than in controls. Endothelium-dependent acetylcholine relaxation was significantly lower in both T males and females. EDHF-mediated relaxation was specifically blunted in T males (Emax = 48.64% ± 3.73%) compared to that in control males (Emax = 81.71% ± 3.18%); however, NO-mediated relaxation was specifically impaired in T females (Emax = 36.01% ± 4.29%) compared with that in control females (Emax = 54.56% ± 6.37%). Relaxation to sodium nitroprusside and levcromakalim were unaffected with T-treatment. NOS3 protein was decreased in T females but not in T males. Kcnn3 expression was decreased in both T males and females compared to controls. These findings suggest that prenatal T leads to an increase in blood pressure in the adult offspring, associated with blunting of endothelial cell-associated relaxation and that the effects are sex-specific: EDHF-related in males and NO-related in females.

Angiotensin II-induced hypertension is associated with a selective inhibition of endothelium-derived hyperpolarizing factor-mediated responses in the rat mesenteric artery

Hypertension has been shown to be associated with impaired endothelium-derived hyperpolarizing factor (EDHF)-mediated arterial relaxation and hyperpolarization. Treatments of hypertensive rats with inhibitors of the renin-angiotensin system have been shown to restore both EDHF-mediated responses and the expression of connexins involved in the intercellular transfer of the hyperpolarization in mesenteric arteries. The present study was designed to determine whether chronic treatment of rats with angiotensin II impairs EDHF-mediated responses and the expression of connexins in the mesenteric arterial wall. Male Wistar rats were treated with angiotensin II (0.4 mg/kg/day) for 21 days using osmotic minipumps. Arterial pressure was measured by tail-cuff plethysmography. Contractile responses and membrane potential were measured in isolated mesenteric arteries. The expression of the three connexins (Cxs), Cx37, Cx40, and Cx43, was quantified in segments of mesenteric arteries by immunohistochemistry and quantitative real-time reverse transcriptase-polymerase chain reaction. Angiotensin II administration increased the mean systolic blood pressure. EDHF-mediated relaxation and hyperpolarization to acetylcholine and red wine polyphenols were significantly impaired in mesenteric arteries from angiotensin II-treated rats in comparison with control animals, whereas nitric oxide-mediated relaxation was unaltered. The expression of connexins Cx37, Cx40, and Cx43 was significantly decreased in the mesenteric artery from angiotensin II-treated rats. These findings indicate that angiotensin II-induced hypertension is associated with a selective impairment of EDHF-mediated relaxation and hyperpolarization in the rat mesenteric artery. The inhibition of EDHF-mediated responses is due, at least in part, to a decreased expression of connexins Cx37, Cx40, and Cx43 in the arterial wall.

Low-affinity state beta1-adrenoceptor-induced vasodilation in SHR

Low-affinity state beta1-adrenoceptor (beta1-AR) was functionally expressed in some blood vessels and was different from beta1, beta2 and beta3-AR. In rat aorta, low-affinity state beta1-AR activation produced an endothelium-independent relaxation which was impaired in spontaneously hypertensive rats (SHRs). In the present work, we investigated whether renin-angiotensin system was involved in this alteration by evaluating the effects of enalapril, an angiotensin converting enzyme (ACE) inhibitor or losartan, an AT1 angiotensin receptor antagonist. Cumulative concentration-response curves to low-affinity state beta1-AR agonists (CGP 12177, cyanopindolol or alprenolol) and to NS 1619, a large conductance Ca2+-activated K+ channels (BK) agonist were performed in denuded aortic rings isolated from control or treated Wistar Kyoto (WKY) rats or SHRs in different experimental conditions. The low-affinity state beta1-AR-mediated aortic vasodilation was impaired in 5 and 12 weeks old SHRs when compared to age-matched WKY. Twelve days enalapril (5 mg/kg/day) or losartan (15 mg/kg/day) treatments reduced systolic blood pressure (SBP) only in 12 weeks old SHRs whereas no significant change was observed in other groups. These treatments improved low-affinity state beta1-AR effect only in SHRs groups. In 12 weeks old WKY rats, CGP 12177-induced relaxation was insensitive to glibenclamide, a K(ATP)+ channel blocker, but was reduced by TEA or iberiotoxin, two large conductance Ca2+-activated K+ channel (BK) blockers. The impairment of NS 1619-induced vasodilation in both 5 and 12 weeks old SHRs was restored by enalapril or losartan. These results suggested that improvement of the low-affinity state beta1-AR-mediated vasodilation in 5 and 12 weeks old SHRs could be attributed to enhanced BK channels-induced hyperpolarization in SHRs independently of lowering of SBP.

Changes in endothelium-derived hyperpolarizing factor in hypertension and ageing: response to chronic treatment with renin-angiotensin system inhibitors

1. Endothelial function is impaired in hypertension and ageing and this may be associated with an increase in cardiovascular disease. Several clinical studies have shown that blocking the renin-angiotensin system (RAS) improves endothelial function not only in hypertensive patients, but also in normotensive patients with cardiovascular disease. 2. The aim of the present study was to test whether endothelium-derived hyperpolarizing factor (EDHF)-mediated smooth muscle hyperpolarization and relaxation are altered in hypertension and ageing and, if so, whether chronic treatment with RAS inhibitors (the angiotensin-converting enzyme inhibitor enalapril and the angiotensin AT1 receptor antagonist candesartan) would correct such changes. 3. Endothelium-derived hyperpolarizing factor-mediated responses were examined in mesenteric arteries from 12-month-old spontaneously hypertensive rats (SHR) and 3-, 6-, 12- and 24-month-old normotensive Wistar-Kyoto (WKY) rats. Furthermore, both strains were treated for 3 months with either RAS blockers or a conventional therapy with hydralazine and hydrochlorothiazide from 9 to 12 months of age. 4. In arteries of 12-month-old SHR, EDHF-mediated responses were impaired compared with age-matched WKY rats. In SHR, all antihypertensive treatments improved the impairment of EDHF-mediated responses; however, RAS inhibitors tended to improve these responses to a greater extent compared with conventional therapy with hydralazine and hydrochlorothiazide. 5. In arteries of WKY rats, EDHF-mediated responses were impaired at the age of 12 and 24 months compared with 3- and 6-month-old rats, with the response tending to be impaired to a greater extent in 24-month-old rats. 6. Three months of treatment of WKY rats, until 12 months of age, with RAS inhibitors, but not with conventional therapy with hydralazine and hydrochlorothiazide, improved the age-related impairment of EDHF-mediated responses, despite a similar reduction in blood pressure by both treatments. 7. These findings suggest that: (i) EDHF-mediated hyperpolarization and relaxation decline with hypertension and ageing in rat mesenteric arteries; (ii) antihypertensive treatment restores the impaired EDHF-mediated responses in hypertension; (iii) RAS inhibitors may be more efficacious in improving endothelial dysfunction associated with hypertension; and (iv) chronic treatment with RAS inhibitors improves the age-related impairment of EDHF-mediated responses, presumably through the blockade of RAS but not blood pressure lowering alone.

Haemodynamic effects of the selective phosphodiesterase 5 inhibitor, UK-357,903, in conscious SHR

1. Regional haemodynamic responses to a continuous, 4-day infusion of the selective phosphodiesterase type 5 inhibitor, UK-357,903 (0.133 or 1.33 mg x kg(-1) h(-1)) were measured in conscious spontaneously hypertensive rats, and compared with those of enalapril (1 mg x kg(-1) h(-1)). 2. Both doses of UK-357,903 caused modest reductions in mean blood pressure that were not dose-dependent and only significantly different from the vehicle effects on Day 1 of the study (mean -11.8 and -15.3 mmHg for low and high doses, respectively). UK-357,903 had mesenteric and hindquarters vasodilator effects, which were, again, similar for both dose levels and only significantly different from vehicle on Day 1. Neither dose of UK-357,903 affected renal vascular conductance or heart rate. 3. Although the haemodynamic effects of UK-357,903 were not clearly dose-related and some appeared to wane with time, geometric mean plasma levels of UK-357,903 increased in proportion to dose, and were sustained throughout the infusion period. Furthermore, plasma cyclic guanosine monophosphate, a biomarker of phosphodiesterase 5 inhibition, was persistently elevated, and increased with increasing dose. 4. Enalapril caused a fall in mean blood pressure on day 1 (-14.1 mmHg) that was associated with dilatation in renal, mesenteric and hindquarters vascular beds. The haemodynamic effects of enalapril were sustained or increased over the 4-day infusion, although plasma free drug levels were stable. 5. In conclusion, we have shown regional and temporal changes in the haemodynamic effects of UK-357,903, which may be due to activation of compensatory mechanisms, but there were no signs of functional compensation to the cardiovascular effects of enalapril.

Aged spontaneously hypertensive rats exhibit a selective loss of EDHF-mediated relaxation in the renal artery

Endothelium-dependent relaxation is frequently attenuated in hypertension. We hypothesized that the contribution of the endothelium-derived hyperpolarizing factor (EDHF) to the acetylcholine (ACh)-induced, endothelium-dependent relaxation is attenuated with aging in the renal artery of spontaneously hypertensive rats (SHR) compared with age-matched Wistar-Kyoto (WKY) rats. ACh-induced, NO-mediated relaxation was identical in young (8-week-old) WKY and SHR, whereas EDHF-mediated relaxations (assessed in the presence of Nomega-nitro-l-arginine and diclofenac) were much more pronounced in SHR than WKY. KCl-induced relaxations were more pronounced in vessels from young WKY rats than from young SHR. The cytochrome P450 inhibitor sulfaphenazole significantly inhibited EDHF-mediated relaxation in vessels from young SHR but not WKY. Vessels from old (22 months) SHR exhibited a slightly reduced NO-mediated relaxation but a complete loss of EDHF-mediated responses. In contrast, aging did not affect EDHF-mediated responses in WKY. Moreover, ACh-induced hyperpolarization and resting membrane potential were decreased in old SHR but not in WKY. KCl-induced relaxation increased with age in WKY, whereas no response to KCl was recorded in arteries from aged SHR. In vessels from old WKY but not old SHR, mRNA expression of the Na-K-ATPase subunit alpha2 was increased by 2-fold compared with young animals. These data indicate that the increase in EDHF responses in renal arteries from aged WKY can be attributed to the release of K+ ions from the endothelium, whereas increased EDHF responses in renal arteries from young SHR can be attributed to a sulfaphenazole-sensitive cytochrome P450-dependent EDHF.

Impaired endothelial alpha-2 adrenergic receptor-mediated vascular relaxation in the fructose-fed rat

To investigate the vascular endothelial dysfunction in the insulin resistance syndrome, muscarinic and alpha2-adrenergic mediated relaxations were studied in the fructose-fed rat. Male Sprague-Dawley rats were fed either fructose-rich chow (FFR, n=14) or normal chow (CNT, n=13) for 8 weeks. Systolic blood pressure (SBP) was measured by the tail-cuff method. A 3 mm segment of mesenteric artery was cannulated and pressurized, pretreated with prazosin (10(-6) mol/l) and propranolol (3x10(-6) mol/l), then pre-contracted with serotonin (10(-6) mol/l). Endothelium-dependent relaxation was induced by addition of acetylcholine (ACh, 10(-9)-10(-4) mol/l) or a selective alpha2-agonist, B-HT 920 (10(-9)-10(-5) mol/l), with or without the nitric oxide (NO) synthase inhibitor, L-NAME (10(-4) mol/l). SBP was significantly elevated in FFR but not in CNT. Plasma triglyceride in FFT (241+/-115 mg/dl) was significantly (p<0.01) higher than in CNT (84+/-34 mg/dl). Insulin and insulin/glucose ratio were higher but not significantly. Plasma glucose was not different between the two groups. In the dose-response curves to ACh, maximum relaxation and ED50 were similar between FFR and CNT. Moreover, L-NAME shifted the dose-response curves similarly to the right in both groups. Dose-response curves to B-HT 920, however, showed less relaxation in FFR than in CNT (p<0.05). B-HT 920-induced relaxations were mostly abolished by L-NAME. It is concluded that endothelial alpha2-adrenergic relaxation, predominantly mediated by NO, is likely more sensitive to the development of insulin resistance than muscarinic receptor relaxation in this 8-weeks FFR model. This early impairment of endothelial alpha2-adrenergic relaxation may contribute to the development of hypertension and insulin resistance in the FFR.

Effects of angiotensin II receptor antagonist on endothelial vasomotor function and urinary albumin excretion in type 2 diabetic patients with microalbuminuria

BACKGROUND

Microalbuminuria is associated with dysfunction of the vascular endothelium in patients with diabetes mellitus. The objective of the present study was to determine whether treatment with losartan at a dose sufficient to lower urinary albumin excretion was accompanied by an improvement in endothelial function in type 2 diabetic patients with microalbuminuria.

METHODS

Endothelial function was measured in 80 type 2 diabetic patients with microalbuminuria and 68 non-diabetic controls using high-resolution vascular ultrasound. The diabetic patients were randomised to receive either losartan 50 mg daily or placebo in a 6-month double-blind study. Urinary albumin excretion and endothelial function were assessed at baseline, 3 and 6 months.

RESULTS

Both endothelium-dependent (p<0.01) and -independent vasodilation (p<0.01) were significantly impaired in diabetic patients with or without history of hypertension compared to the non-diabetic controls. At baseline, the losartan- and placebo-treated groups were comparable in their clinical characteristics. Blood pressure did not change significantly in either group throughout the study. Urinary mean albumin excretion rate (MAER) decreased in the losartan-treated group (p<0.01) whereas an increase was observed in the placebo group (p<0.05). At 6 months, the losartan-treated group had significantly lower MAER than the placebo-treated group [54.5 (58.3) vs 78.5 (100.5) microg/min, p<0.05; median (interquartile range)]. No significant differences were found in endothelium-dependent or -independent vasodilation.

CONCLUSIONS

Type 2 diabetic patients with microalbuminuria have impaired endothelium-dependent and -independent vasodilation. Treatment with low-dose losartan is sufficient to reduce microalbuminuria in these patients without alteration in endothelial function and systemic blood pressure.

Long-term fructose feeding impairs vascular relaxation in rat mesenteric arteries

To investigate the long-term influence of insulin resistance and hyperinsulinemia on vascular reactivity, both muscarinic and alpha2-receptor-mediated relaxations and the contribution of nitric oxide to these mechanisms were studied in the fructose-fed rat. Male Sprague-Dawley rats were fed either fructose-rich chow (FFR, n = 6) or normal chow (CNT, n = 6) for 40 weeks. Systolic blood pressure was measured by tail-cuff method. A 3-mm segment of mesenteric artery was excised, cannulated and pressurized, pretreated with prazosin (10(-6) mol/L) and propranolol (3 x 10(-6) mol/L), then precontracted with serotonin (10(-6) mol/L). Endothelium dependent relaxation was induced by addition of acetylcholine (10(-9) to 10(-4) mol/L), or a selective alpha2-agonist B-HT 920 (10(-9) to 10(-5) mol/L), with or without the nitric oxide synthase inhibitor L-NAME (10(-4) mol/L). Systolic blood pressure was significantly higher in FFR at the early period; however, there was no difference at the end of 40 weeks compared to CNT. Fasting plasma insulin was much higher in FFR than in CNT (110+/-62 v 41+/-11 microU/mL, P < .05), whereas plasma glucose was not different. Maximum relaxation to acetylcholine was attained at 10(-6) mol/L in FFR but at 3 x 10(-7) mol/L in CNT. The degree of maximum relaxation attained with acetylcholine was similar in FFR and CNT (89+/-9 and 94+/-4% of precontraction), although attenuated (P < .01) by the addition of L-NAME only in FFR (to 34+/-22%, P < .05) but not in CNT (to 82+/-25%). The half-maximal relaxation dose of acetylcholine was greater in FFR (P < .01) compared with CNT and was significantly increased (P < .05) by L-NAME in both groups. B-HT 920 at 10(-5) mol/L induced a greater relaxation in CNT (36+/-10% of serotonin constriction) than in FFR (19+/-14%, P < .05). These responses were significantly blunted by L-NAME. Thus, muscarinic receptor-mediated vascular relaxation is less sensitive and more nitric oxide dependent in FFR versus CNT. Alpha2-adrenergic-mediated relaxation, predominantly mediated by nitric oxide, is also impaired in FFR. It is possible that prolonged insulin resistance and hyperinsulinemia in FFR could alter endothelial-dependent vasodilatory mechanisms, thereby contributing to the increase in blood pressure seen in this model.