Ca2+-activated K+ channels underlying the impaired acetylcholine-induced vasodilation in 2K-1C hypertensive rats

C-type natriuretic peptide-induced relaxation through cGMP-dependent protein kinase and SERCA activation is impaired in two kidney-one clip rat aorta

AIMS

Hypertension underlies endothelial dysfunction, and activation of vasorelaxation signaling with low dependence on nitric oxide (NO) represents a good alternative for vascular modulation. C-type natriuretic peptide (CNP) causes relaxation by increasing cyclic guanosine 3',5'-monophosphate (cGMP) or Gi-protein activation through its natriuretic peptide receptor-B or -C, respectively. We have hypothesized that CNP could exerts its effects and could overcome endothelial dysfunction in two kidney-one clip (2K-1C) hypertensive rat aorta. Here, we investigate the intracellular signaling involved in CNP effects in hypertension.

MATERIALS AND METHODS

The 2K-1C hypertension was induced in male Wistar rats (200 g). CNP-induced vascular relaxation and cGMP production were investigated in rat thoracic aortas. The natriuretic peptide receptor-B and -C localization was evaluated by immunofluorescence. Calcium mobilization was assessed in endothelial cells from rat aortas.

KEY FINDINGS

CNP induced similar relaxation in normotensive and 2K-1C hypertensive rat aortas, which increased after endothelium removal. CNP-induced relaxation involved natriuretic peptide receptor-B and -C activation in 2K-1C rats. Nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) counter-regulated CNP-particulate GC (pGC) activation in aortas. CNP reduced endothelial calcium and increased cGMP production, which was lower in 2K-1C. CNP-induced cGMP-dependent protein kinase (PKG) and sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA) activation was impaired in 2K-1C rat aorta.

SIGNIFICANCE

Our results indicated CNP triggered relaxation through its natriuretic peptide receptor-B and -C in 2K-1C rat aortas, and that CNP-induced relaxation overcomes endothelial dysfunction in hypertension. In addition, NOS and sGC activities counter-regulate CNP-pGC activation to induce vascular relaxation.

Equilin displays similar endothelium-independent vasodilator potential to 17β-estradiol regardless of lower potential to inhibit calcium entry

Conjugated equine estrogens (CEE) have been widely used by women who seek to relieve symptoms of menopause. Despite evidence describing protective effects against risk factors for cardiovascular diseases by naturally occurring estrogens, little is known about the vascular effects of equilin, one of the main components of CEE and not physiologically present in women. In this regard, the present study aims to compare the vascular effects of equilin in an experimental model of hypertension with those induced by 17β-estradiol. Resistance mesenteric arteries from female spontaneously hypertensive rats (SHR) were used for recording isometric tension in a small vessel myograph. As effectively as 17β-estradiol, equilin evoked a concentration-dependent relaxation in mesenteric arteries from female SHRs contracted with KCl, U46619, PDBu or ET-1. Equilin-induced vasodilation does not involve classical estrogen receptor activation, since the estrogen receptor antagonist (ICI 182,780) failed to inhibit relaxation in U46619-precontracted mesenteric arteries. Vasorelaxation was not affected by either endothelium removal or by inhibiting the release or action of endothelium-derived factors. Incubation with L-NAME (NOS inhibitor), ODQ (guanylyl cyclase inhibitor) or KT5823 (inhibitor of protein kinase G) did not affect equilin-induced relaxation. Similarly, indomethacin (COX inhibitor) or blockage of potassium channels with tetraethylammonium, glibenclamide, 4-aminopyridine, or ouabain did not affect equilin-induced relaxation. Inhibitors of adenylyl cyclase SQ22536 or protein kinase A (KT5720) also had no effects on equilin-induced relaxation. While 17β-estradiol inhibited calcium (Ca²⁺) -induced contractions in high-K⁺ depolarization medium in a concentration-dependent manner, equilin induced a slight rightward-shift in the contractile responses to Ca²⁺. Comparable pattern of responses were observed in the concentration-response curves to (S)-(-)-Bay K 8644, a L-type Ca²⁺ channel activator. Equilin was unable to block the transitory contraction produced by caffeine-induced Ca²⁺ release from intracellular stores. In conclusion, equilin blocks L-type Ca²⁺ channels less effectively than 17β-estradiol. Despite its lower effectiveness, equilin equally relaxes resistance mesenteric arteries by blocking Ca²⁺ entry on smooth muscle.

Differential participation of calcium-activated potassium channel in endothelium-dependent hyperpolarization-type relaxation in superior mesenteric arteries of spontaneously hypertensive rats

The purpose of this study was to determine the relationship of K_Ca channels to endothelium-dependent hyperpolarizing factor (EDHF)-mediated relaxation induced by acetylcholine (ACh) in the superior mesenteric arteries of 7-month-old spontaneously hypertensive rats (SHR). Upon inhibition of nitric oxide synthase and cyclooxygenase, ACh-induced EDHF-mediated relaxation was found to be weaker in SHR than in age-matched Wistar Kyoto rats (WKY). These relaxations in both group were attenuated by combined treatment with small-conductance and intermediate-conductance Ca²⁺-activated K⁺ channels (SK_Ca and IK_Ca) inhibitors, with the exception of relaxation resistant to inhibition of these channels in SHR (vs. WKY). Treatment with large-conductance Ca²⁺-activated K⁺ channels (BK_Ca) inhibitor specifically attenuated relaxation in SHR, but not in WKY. Protein expression of IK_Ca and SK_Ca in the arteries did not differ between the 2 groups, whereas ratio of sloβ1 subunit to α subunit of BK_Ca was increased in SHR (vs. WKY). These results suggest that EDHF-mediated relaxations in superior mesenteric arteries are impaired in SHR, and utilize components of BK_Ca in addition to SK_Ca/IK_Ca channel activities, that the increased participation of BK_Ca may be attributable to alterations in α and sloβ1 subunit ratio, and that components unrelated to K_Ca activity may also contribute to the difference between SHR and WKY arteries.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

In vitro Treatment with cis-[Ru(H-dcbpy-)2(Cl)(NO)] Improves the Endothelial Function in Aortic Rings with Endothelial Dysfunction

PURPOSE

The ruthenium complex cis-[Ru(H-dcbpy-)2(Cl)(NO)] (DCBPY) is a nitric oxide (NO) donor and studies suggested that the ruthenium compounds can inactivate O2-. The aim of this study is to test if DCBPY can revert and/or prevent the endothelial dysfunction.

METHODS

Normotensive (2K) and hypertensive (2K-1C) wistar rats were used. To vascular reactivity study, thoracic aortas were isolated, rings with intact endothelium were incubated with: DCBPY: 0.1; 1 and 10μM, DCBPY plus hydroxocobalin (NO scavenger) or tempol during 30 minutes, and concentration effect curves to acetylcholine were performed. The potency values (pD2) and maximum effect (ME) were analyzed. The O2- was generated using hypoxantine xantine oxidase and the reduction of cytochrome c, NO consumption by O2- and the effect in avoid NO consumption was measured.

RESULTS

In 2K-1C DCBPY at 0.1; 1 or 10μM improved the relaxation endothelium dependent induced by acetylcholine in aortic rings compared to control 2K-1C, and also improved ME. In rings from 2K incubation with DCBPY (0.1; 1.0 and 10 μM) did not change pD2 or ME. Incubation with 0.1 μM of DCBPY plus hydroxocobalamin did not modify the potency and ME in 2K-1C compared to DCBPY (0.1 μM). DCBPY and SOD inhibits the reduction of cytochrome c and inhibited the NO consumption by O2-, showing that O2- has been removed from the solution.

CONCLUSION

Our results suggest that DCBPY at a lower concentration (0.1 µM) is not an NO generator, but can inactivate superoxide and improves the endothelial function.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

Low-level lead exposure changes endothelial modulation in rat resistance pulmonary arteries

Lead exposure induces hypertension and endothelial dysfunction. However, the effects on the pulmonary vasculature have not been explored. In this study, rats exposed to lead acetate for seven days (4μg/100g on the 1st day and 0.05μg/100g/day i.m. subsequently) had lead blood level of 3.9±0.7μg/dL and increased right ventricular pressures. There was an increased Pb deposition and superoxide anions production in the pulmonary arteries, associated with reduced vasoconstriction but unchanged endothelium-dependent vasodilatation to acetylcholine (ACh). In both groups, inhibition of the nitric oxide (NO) synthase with L-NAME blocked the response to ACh, while indomethacin (cycloxygenase inhibitor) had no effect. Incubation with nonspecific potassium channel blocker (tetraethylammonium) reduced the ACh-induced vasodilatation only in the Pb group. Apamin (SKCa channel blocker) and 4-aminopyridine (Kv channel blocker), but not iberiotoxin (BKCa channel blocker), also inhibited this response in the Pb group. The vasodilatation to exogenous NO was reduced by Pb, while relaxation to the cGMP analogue was similar between groups. Concordantly, the protein level of soluble guanylate cyclase (sGC) was reduced. In conclusion, short-term and low-level exposure to Pb changes pulmonary haemodynamic and increases oxidative stress. The pulmonary vasculature exhibited increased hyperpolarization by the Kv and SKCa channels, probably as a compensatory mechanism to the decreased responsiveness to NO.

Chronic intermittent hypobaric hypoxia antagonizes renal vascular hypertension by enhancement of vasorelaxation via activating BKCa

AIM

The purpose of the present study was to explore anti-hypertensive effect of chronic intermittent hypobaric hypoxia (CIHH) in renovascular hypertension (RVH) rats, as well as the role of large-conductance calcium-activated potassium channel (BKCa) in anti-hypertensive effect of CIHH.

MAIN METHODS

Male adult age- and body weight-matched Sprague-Dawley rats were divided into SHAM, CIHH, RVH and RVH+CIHH groups. Hypertension was induced by two-kidney-1-clip method (2K1C) in RVH rats. CIHH rats were exposed to 28-days hypobaric hypoxia simulating 5000m altitude, 6h daily. SHAM rats got an operation without 2K1C, and RVH+CIHH rats received CIHH treatment after 2K1C. The endothelium-dependent vasorelaxation induced by acetylcholine (ACh), BKCa currents in smooth muscle cells (VSMCs) of mesenteric arteries and the protein expression of BKCa in mesenteric arteries was examined.

KEY FINDINGS

The systolic arterial blood pressure (SAP) in RVH rats was higher than that in SHAM rats and CIHH treatment significantly decreased SAP in RVH rats. The enhanced vasorelaxation of mesenteric artery in CIHH-treated RVH rats was cancelled by BKCa blocker IBTX. The vasorelaxation induced by BKCa activator was reduced in RVH rats and the decreased vasorelaxation was improved by CIHH treatment. The β1 subunit of BKCa in mesenteric artery was upregulated and BKCa current in VSMCs was increased in CIHH-treated RVH rats compared with RVH rats.

SIGNIFICANCE

In conclusion, CIHH treatment enhances the relaxation of mesenteric artery through activation of BKCavia up-regulating β1 subunit of BKCa, which might be one of mechanisms for anti-hypertensive effect of CIHH in RVH rats.

The Labdane Ent-3-Acetoxy-Labda-8(17), 13-Dien-15-Oic Decreases Blood Pressure In Hypertensive Rats

Background

Labdane-type diterpenes induce lower blood pressure via relaxation of vascular smooth muscle; however, there are no studies describing the effects of labdanes in hypertensive rats.

Objective

The present study was designed to investigate the cardiovascular actions of the labdane-type diterpene ent-3-acetoxy-labda-8(17), 13-dien-15-oic acid (labda-15-oic acid) in two-kidney 1 clip (2K-1C) renal hypertension.

Methods

Vascular reactivity experiments were performed in aortic rings isolated from 2K-1C and normotensive (2K) male Wistar rats. Nitrate/nitrite (NOx) measurement was performed in aortas by colorimetric assay. Blood pressure measurements were performed in conscious rats.

Results

Labda-15-oic acid (0.1-300 µmol/l) and forskolin (0.1 nmol/l - 1 µmol/l) relaxed endothelium-intact and endothelium-denuded aortas from both 2K-1C and 2K rats. Labda-15-oic acid was more effective at inducing relaxation in endothelium-intact aortas from 2K pre-contracted with phenylephrine when compared to the endothelium-denuded ones. Forskolin was more potent than labda-15-oic acid at inducing vascular relaxation in arteries from both 2K and 2K-1C rats. Labda-15-oic acid-induced increase in NOx levels was lower in arteries from 2K-1C rats when compared to 2K rats. Intravenous administration of labda-15-oic acid (0.3-3 mg/kg) or forskolin (0.1-1 mg/kg) induced hypotension in conscious 2K-1C and 2K rats.

Conclusion

The present findings show that labda-15-oic acid induces vascular relaxation and hypotension in hypertensive rats.

Na+K+-ATPase activity and K+ channels differently contribute to vascular relaxation in male and female rats

Gender associated differences in vascular reactivity regulation might contribute to the low incidence of cardiovascular disease in women. Cardiovascular protection is suggested to depend on female sex hormones' effects on endothelial function and vascular tone regulation. We tested the hypothesis that potassium (K+) channels and Na+K+-ATPase may be involved in the gender-based vascular reactivity differences. Aortic rings from female and male rats were used to examine the involvement of K+ channels and Na+K+-ATPase in vascular reactivity. Acetylcholine (ACh)-induced relaxation was analyzed in the presence of L-NAME (100 µM) and the following K+ channels blockers: tetraethylammonium (TEA, 2 mM), 4-aminopyridine (4-AP, 5 mM), iberiotoxin (IbTX, 30 nM), apamin (0.5 µM) and charybdotoxin (ChTX, 0.1 µM). The ACh-induced relaxation sensitivity was greater in the female group. After incubation with 4-AP the ACh-dependent relaxation was reduced in both groups. However, the dAUC was greater in males, suggesting that the voltage-dependent K+ channel (Kv) participates more in males. Inhibition of the three types of Ca2+-activated K+ channels induced a greater reduction in Rmax in females than in males. The functional activity of the Na+K+-ATPase was evaluated by KCl-induced relaxation after L-NAME and OUA incubation. OUA reduced K+-induced relaxation in female and male groups, however, it was greater in males, suggesting a greater Na+K+-ATPase functional activity. L-NAME reduced K+-induced relaxation only in the female group, suggesting that nitric oxide (NO) participates more in their functional Na+K+-ATPase activity. These results suggest that the K+ channels involved in the gender-based vascular relaxation differences are the large conductance Ca2+-activated K+ channels (BKCa) in females and Kv in males and in the K+-induced relaxation and the Na+K+-ATPase vascular functional activity is greater in males.

Oral supplementation with the rutin improves cardiovagal baroreflex sensitivity and vascular reactivity in hypertensive rats

The hypothesis that oral supplementation with the flavonoid rutin improves baroreflex sensitivity and vascular reactivity in hypertensive (2-kidney-1-clip (2K1C)) rats was tested. Sixty-four rats were divided in 4 groups: sham + saline; sham + rutin; 2K1C + saline, and 2K1C + rutin. Six weeks after 2K1C surgery, the animals were treated with saline or rutin (40 mg·kg(-1)·day(-1)) by gavage for 7 days. Baroreflex sensitivity test using phenylephrine (8 μg·kg(-1), iv) and sodium nitroprusside (25 μg·kg(-1), iv), vascular reactivity, and thiobarbituric acid reactive substances assay were performed. Baroreflex sensitivity in hypertensive rats was impaired and compared with sham (-2.77 ± 0.15 vs. -1.53 ± 0.27 beats·min(-1)·mm Hg(-1); n = 8; p < 0.05). Oral supplementation with rutin restored baroreflex sensitivity in 2K1C rats (-2.40 ± 0.24 vs. -2.77 ± 0.15 beats·min(-1)·mm Hg(-1); n = 8; p > 0.05). Besides, hypertensive rats have greater contraction to phenylephrine (129.49% ± 4.46% vs. 99.50% ± 11.36%; n = 8; p < 0.05), which was restored by rutin (99.10% ± 1.77% vs. 99.50% ± 11.36%; n = 8; p > 0.05). Furthermore, vasorelaxation to acetylcholine was diminished in hypertensive rats (96.42% ± 2.80% vs. 119.35% ± 5.60%; n = 8; p < 0.05), which was also restored by rutin (117.55% ± 6.94% vs. 119.35% ± 5.60%; n = 8; p > 0.05). Finally, oxidative stress was greater in hypertensive rats (1.54 ± 0.12 vs. 0.53 ± 0.12 nmol MDA·mL(-1); n = 8; p < 0.05) and rutin supplementation significantly decreased oxidative stress in those animals (0.70 ± 0.13 vs. 1.54 ± 0.12 nmol MDA·mL(-1); n = 8; p < 0.05). We concluded that oral supplementation with rutin restores impaired baroreflex sensitivity and vascular reactivity in hypertensive rats by decreasing oxidative stress.

Contribution of oxidative stress to endothelial dysfunction in hypertension

Endothelial dysfunction is the hallmark of hypertension, which is a multifactorial disorder. In the cardiovascular system reactive oxygen species play a pivotal role in controlling the endothelial function and vascular tone. Physiologically, the endothelium-derived relaxing factors (EDRFs) and endothelium-derived contractile factors (EDCFs) that have functions on the vascular smooth muscle cells. The relaxation induced by the EDRFs nitric oxide (NO), prostacyclin, and the endothelium-derived hyperpolarization factor (EDHF) could be impaired in hypertension. The impaired ability of endothelial cells to release NO along with enhanced EDCFs production has been described to contribute to the endothelium dysfunction, which appears to lead to several cardiovascular diseases. The present review discusses the role of oxidative stress, vascular endothelium, and vascular tone control by EDRFs, mainly NO, and EDCFs in different models of experimental hypertension.

NO donors-relaxation is impaired in aorta from hypertensive rats due to a reduced involvement of K(+) channels and sarcoplasmic reticulum Ca(2+)-ATPase

AIMS

To examine the vasodilatation induce by the NO donors, [Ru(terpy)(bdq)NO](3+) (TERPY) and sodium nitroprusside (SNP), and to compare their effects in aortic rings from hypertensive 2K-1C and normotensive 2K rats.

MAIN METHODS

Vascular reactivity was performed in aortic rings pre-contracted with phenylephrine (Phe 100nM). We have analyzed the maximal relaxation (Emax) and potency (pD(2)) of NO donors.

KEY FINDINGS

Potency of SNP was greater than TERPY in both arterial groups. The vasodilatation induced by TERPY was greater in 2K than in 2K-1C, and it was inhibited by sGC inhibitor ODQ in 2K and in 2K-1C aortic rings. ODQ did not alter the efficacy to SNP, but it reduced its potency in 2K and 2K-1C. The blockade of K(+) channels reduced the potency of TERPY only in aortic rings of 2K. On the other hand, the potency of SNP was reduced in both 2K and 2K-1C. The combination of ODQ and TEA reduced the relaxation induced by TERPY and SNP in 2K and reduced the efficacy to SNP in 2K-1C aortic rings but it had no additional effect on the TERPY relaxation in 2K-1C aortas. The production of cGMP induced by TERPY was greater than that produced by SNP, which was similarly increased in 2K and 2K-1C. Sarcoplasmic reticulum Ca-ATPase inhibition only impaired the relaxation induced by SNP in 2K aortic rings.

SIGNIFICANCE

Taken together, our results provide evidences that in this model of hypertension, impaired K(+) channels activation by TERPY and SERCA activation by SNP may contribute to decreased vasodilatation.

Antioxidant treatment with tempol and apocynin prevents endothelial dysfunction and development of renovascular hypertension

BACKGROUND

Two-kidney-one-clip (2K-1C) rats develop renovascular hypertension associated with endothelial dysfunction and elevated levels of oxidative stress. The role of oxidative damage is unknown in vascular dysfunction coupled with 2K-1C hypertension. The aims of this study were to evaluate the effects of chronic treatment with a superoxide dismutase (SOD) mimetic (tempol) and an inhibitor of nicotinamide adenine dinucleotide phosphate (NADPH)-dependent oxidase (apocynin) on the development of hypertension, endothelial dysfunction, and oxidative damage in 2K-1C rats.

METHODS

2K-1C rats and sham-operated rats were treated with tempol or apocynin for 40 days, while the corresponding nontreated groups received tap water. Blood pressure (BP), mesenteric arterial reactivity, plasma and mesentery oxidative damage, mesenteric protein expression, and antioxidant activities were compared among the four groups.

RESULTS

Chronic treatment with tempol (1 mmol/l) or apocynin (33 microg/kg/day) impaired the development of hypertension in 2K-1C rats and did not change the BP in control animals. The reduction in vasodilatory effect induced by acetylcholine (ACh) in the mesenteric arterial beds (MABs) of 2K-1C rats was restored by tempol and apocynin. Plasma and mesentery levels of malondialdehyde (MDA) were higher in 2K-1C rats, and these levels were significantly reduced by the administration of tempol and apocynin. Mesenteric SOD activity and expression were higher in 2K-1C rats than in the controls, and treatment with tempol resulted in a reduction in SOD activity.

CONCLUSIONS

The data suggest that a compromised mechanism of antioxidant defense and an increase in oxidative damage contribute to the development of hypertension and associated vascular dysfunction in 2K-1C rats, and that tempol and apocynin prevent these effects.

Decreased number of caveolae in endothelial cells impairs the relaxation induced by acetylcholine in hypertensive rat aortas

The present study was designed to investigate the contribution of endothelial cell caveolae to vascular relaxation in aortas from a normotensive (2K) and renal hypertensive (2K-1C) rat. For that purpose, concentration-effect curves to acetylcholine were constructed in 2K and 2K-1C intact endothelium aortic rings, in the absence or in the presence of the caveolae disassembler methyl-beta-ciclodextrin. The potency (pD(2)) and the maximum relaxant effect to acetylcholine were greater in 2K than in 2K-1C aortas. Methyl-beta-ciclodextrin reduced the pD(2) in 2K and the maximum relaxant effect in both 2K and 2K-1C. The quantification of the caveolae number by electronic microscopy has shown a larger number of caveolae in 2K than in 2K-1C endothelial cells, which was reduced by methyl-beta-ciclodextrin in both 2K and 2K-1C. The production of NO stimulated with acetylcholine was greater in 2K than in 2K-1C endothelial cells, and this effect was impaired by methyl-beta-ciclodextrin in both 2K and 2K-1C. The cytosolic Ca(2+) concentration ([Ca(2+)]c) was simultaneously measured in endothelial and smooth muscle cells stimulated with acetylcholine by confocal image of aortic slices. Acetylcholine produced a greater [Ca(2+)]c increase in 2K than in 2K-1C endothelial cells, which response was inhibited by methyl-beta-ciclodextrin only in 2K cells. In smooth muscle cells the reduction of [Ca(2+)]c was higher in 2K than in 2K-1C. This effect was inhibited by methyl-beta-ciclodextrin only in 2K cells. Taken together, our results suggest that the decreased number of caveolae in the endothelial cells from 2K-1C rat aortas is involved in the impaired effect of acetylcholine on [Ca(2+)]c and NO.

Relaxation induced by calcium ionophore is impaired in carotid arteries from 2K-1C rats due to failed effect of nitric oxide on the smooth muscle cells

Vascular endothelium generates nitric oxide (NO) in large vessels and induces relaxation of vascular smooth muscle cells (VSMC). The aim of this study was to evaluate the contribution of NO produced in the endothelial cells (EC) to the relaxation induced by the Ca2+ ionophore A23187 and whether this relaxation is impaired in renal hypertensive (2K-1C) rat arteries. Concentration-effect curves for A23187 were constructed in intact endothelium isolated carotid rings from 2K-1C and normotensive (2K) in the absence or in the presence of the extracellular NO scavenger haemoglobin or inhibitors of NO-synthase (NOS, L-NOARG), guanylyl-cyclase (GC, ODQ). In carotid rings loaded with Fluo-3AM, both EC and VSMC were simultaneously imaged by a confocal microscope and [Ca2+]c was derived from fluorescence intensities (IF). The maximal relaxation (ME) induced by A23187 was lower in 2K-1C than in 2K arteries. A23187-induced relaxation was abolished by haemoglobin and L-NOARG in both groups. ODQ reduced the ME to A23187 in 2K and abolished its relaxation in 2K-1C. A23187 increased [Ca2+]c in a similar way in 2K and 2K-1C EC, and decreased [Ca2+]c in VSMC, which effect was higher in 2K than in 2K-1C arteries. L-NOARG inhibited the effect of A23187 in VSMC from 2K and abolished it in 2K-1C rats. On the other hand, L-NOARG did not modify the effect of A23187 in EC from 2K and 2K-1C rats. The basal content of cGMP was higher in 2K than in 2K-1C arterial rings that was similarly increased by A23187. In conclusion, the Ca2+ ionophore A23187 increases Ca2+, activates NOS and NO production in the EC activating GC in VSMC and [Ca2+]c decrease. All these effects are higher in 2K, which contribute to the impaired relaxation to A23187 in 2K-1C rat arteries.

Caveolae dysfunction contributes to impaired relaxation induced by nitric oxide donor in aorta from renal hypertensive rats

Relaxation induced by nitric oxide (NO) donors is impaired in renal hypertensive two kidney-one clip (2K-1C) rat aortas. It has been proposed that caveolae are important in signal transduction and Ca2+ homeostasis. Therefore, in the present study we investigate the integrity of caveolae in vascular smooth muscle cells (VSMCs), as well as their influence on the effects produced by NO released from both the new NO donor [Ru(NH.NHq) (terpy)NO+]3+ (TERPY) and sodium nitroprusside (SNP) on 2K-1C rat aorta. The potency of both TERPY and SNP was lower in the 2K-1C aorta that in the normotensive aorta [two kidney (2K)], whereas the maximal relaxant effect (ME) was similar in both 2K-1C and 2K aortas. In the 2K aorta, methyl-beta-cyclodextrin (CD) reduced both the potency of TERPY and SNP, and their ME compared with the control, but it had no effect on the potency and ME of these NO donors in 2K-1C aortas. The decrease in cytosolic Ca2+ concentration ([Ca2+]c) induced by TERPY was larger in 2K than in 2K-1C cells, and this effect was inhibited by CD in 2K cells only. Aortic VSMCs from 2K rats presented a larger number of caveolae than those from 2K-1C rats. Treatment with CD reduced the number of caveolae in both 2K and 2K-1C aortic VSMCs. Our results support the idea that caveolae play a critical role in the relaxant effect and in the decrease in [Ca2+]c induced by NO, and they could be responsible for impaired aorta relaxation by NO in renal hypertensive rats.

The endothelium-derived hyperpolarising factor (EDHF) in isolated bovine choroidal arteries

The present study reports of an endothelium-dependent and NO- and prostanoid-independent relaxation in isolated choroidal arteries, and evaluates the hypothesis of an endothelium-derived hyperpolarising factor (EDHF) playing a role in the choroidal circulation. Choroidal arteries were isolated from bovine eyes and mounted in a small vessel wire-myograph for isometric tension recording. Concentration-response curves for acetylcholine (0.1nM-10microM) were constructed in isolated choroidal arteries contracted with 10microM norepinephrine. Acetylcholine induced a concentration-dependent relaxation in the choroidal arteries. The presence of the NO-synthase inhibitor L-NA and the cyclo-oxygenase inhibitor indomethacin only had a limited effect on this relaxation. All further experiments were performed in the presence of L-NA and indomethacin, in order to study the NO- and prostanoid-independent part of the acetylcholine-relaxations. Both removal of the vascular endothelium or the presence of an increased K(+) concentration in the organ bath abolished the NO- and prostanoid-independent part of the acetylcholine-relaxations. The presence of TEA, a rather non-specific K(+) channel blocker, significantly reduced the acetylcholine-relaxations. Simultaneous application of apamin (an inhibitor of small-conductance Ca(2+)-activated K(+) channels) and charybdotoxin (an inhibitor of intermediate- and large-conductance Ca(2+)-activated K(+) channels) abolished the acetylcholine-induced relaxation and even resulted in a concentration-dependent contraction. Transmembrane potential recordings in isolated choroidal arteries revealed a clear membrane hyperpolarisation in the vascular smooth muscle cells of isolated choroidal arteries. It was therefore concluded that the acetylcholine-induced relaxation of choroidal arteries in the presence of NO-synthase and cyclo-oxygenase inhibitors is mediated by an endothelium-derived hyperpolarising factor. This EDHF seems to be of more importance than endothelium-derived NO or prostanoids.

Renovascular BKCachannels are not activated in vivo under resting conditions and during agonist stimulation

We investigated the role of large-conductance Ca2+-activated K+(BKCa) channels for the basal renal vascular tone in vivo. Furthermore, the possible buffering by BKCaof the vasoconstriction elicited by angiotensin II (ANG II) or norepinephrine (NE) was investigated. The possible activation of renal vascular BKCachannels by cAMP was investigated by infusing forskolin. Renal blood flow (RBF) was measured in vivo using electromagnetic flowmetry or ultrasonic Doppler. Renal preinfusion of tetraethylammonium (TEA; 3.0 μmol/min) caused a small reduction of baseline RBF, but iberiotoxin (IBT; 0.3 nmol/min) did not have any effect. Renal injection of ANG II (1–4 ng) or NE (10–40 ng) produced a transient decrease in RBF. These responses were not affected by preinfusion of TEA or IBT. Renal infusion of the BKCaopener NS-1619 (90.0 nmol/min) did not affect basal RBF or the response to NE, but it attenuated the response to ANG II. Coadministration of NS-1619 with TEA or IBT abolished this effect. Forskolin caused renal vasodilation that was not inhibited by IBT. The presence of BKCachannels in the preglomerular vessels was confirmed by immunohistochemistry. Despite their presence, there is no indication for a major role for BKCachannels in the control of basal renal tone in vivo. Furthermore, BKCachannels do not have a buffering effect on the rat renal vascular responses to ANG II and NE. The fact that NS-1619 attenuates the ANG II response indicates that the renal vascular BKCachannels can be activated under certain conditions.

Nitric oxide pathway counteracts enhanced contraction to membrane depolarization in aortic rings of rats on high-sodium diet

Vascular smooth muscle cell contraction and endothelium-dependent relaxation was evaluated in aortic rings isolated from weaned, 5-mo-old Sprague-Dawley rats fed a normal (NS; 0.8% NaCl) or high (HS; 8% NaCl) sodium diet. Arterial pressure was 140 +/- 6 (NS) and 145 +/- 6 mmHg (HS). In endothelium-denuded rings, the response to phenylephrine (PE) was not modified by the sodium diet, while that of depolarizing agent KCl and intracellular calcium releasing agent caffeine increased in the HS group. When endothelium was preserved, PE-evoked contraction was reduced in both NS and HS groups, the contraction being yet lower in the HS group. This effect was partially obliterated by addition of N(G)-nitro-L-arginine methyl ester (L-NAME), independently of the sodium diet. Relaxation to ACh in intact rings and to sodium nitroprusside (SNP) and 8-bromoadenosine 3'5' cyclic guanosine monophosphate (8-BrcGMP) in the absence of endothelium was enhanced in rings isolated from HS rats. In addition, the dose-response curve to 8-BrcGMP was shifted to the right in the presence of iberiotoxin, an inhibitor of large conductance, voltage-dependent, and calcium-sensitive potassium channel (BK(Ca)). However, shift was more marked in rings from HS rats. Present results provide evidence that response of vascular smooth muscle cell to nitric oxide/cGMP-related compounds is increased in HS rings and is associated with a greater activation of the repolarizing BK(Ca) channels. Such changes might counterbalance enhanced contractile response to membrane depolarization and thus participate in maintenance of arterial pressure in the present model of early and long-term HS feeding in rats.

Decreased contraction to phenylephrine by ouabain in 2K-1C rat aorta is modulated by the endothelium

The effects of ouabain were studied on the contraction stimulated with phenylephrine or KCl in intact endothelium and denuded aortic rings isolated from normotensive (2K) and renal hypertensive 2 kidney-1clip (2K-1C) rats. Ouabain did not change the basal tone of aortic rings. Ouabain (1 nmol/l) had no effect on the contraction to phenylephrine in all the artery groups studied. Ouabain (10 nmol/l) decreased the E(max) to phenylephrine in intact endothelium arteries from 2K-1C. By contrast, ouabain (10 nmol/l) had no effect on the contraction to KCl. Ouabain induced membrane depolarization measured by confocal image with Di-4-ANEPPS dye, that was greater in 2K than in 2K-1C rat aorta smooth muscle cells. In conclusion, ouabain (10 nmol/l) decreased the contractile responses to phenylephrine only in 2K-1C rat aortic rings with intact endothelium. Interestingly, 10 nmol/l ouabain depolarizes the smooth muscle cells but this depolarization level is not enough to alter the phenylephrine or KCl-induced contractions. Our results indicate that the endothelium modulates the vascular action of ouabain.

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.

DECREASED VASODILATION INDUCED BY A NEW NITRIC OXIDE DONOR IN TWO KIDNEY, ONE CLIP HYPERTENSIVE RATS IS DUE TO IMPAIRED K+ CHANNEL ACTIVATION

1. We studied the effect of the new compound trans-[RuCl([15]aneN(4))NO](2+) (15-ane) in denuded aortic rings of two kidney (2K) normotensive and two kidney, one clip (2K-1C) hypertensive rats. 2. The compound 15-ane releases nitric oxide (NO) when reduced by a catecholamine (noradrenaline). 3. Oxyhemoglobin (HbO(2)), an NO scavenger, completely abolished the effect of 15-ane in both 2K and 2K-1C rats, indicating that the relaxation is really due to NO release. 4. We tested the hypothesis that an impairment of K(+) channels plays an important role in the vasodilation induced by 15-ane. 5. The selective inhibitor of soluble guanylyl-cyclase, namely 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ; 1 micromol/L) reduced the relaxation induced by 15-ane. In 2K-1C rat aortic rings, ODQ reduced the maximum effect (E(max)) of 15-ane, whereas in 2K rat aortic rings ODQ reduced E(max) and pD(2) values to 15-ane. 6. The selective K(+) channel blockers glibenclamide (blocks K(ATP); 3 micromol/L), 4-aminopyridine (blocks K(V); 1 mmol/L) and the small conductance K(Ca) channel blocker apamin (1 micromol/L) reduced E(max) and pD(2) values for 15-ane-induced relaxation responses of aortas from 2K rats. However, iberiotoxin, a blocker of large conductance K(Ca) channels, reduced only the E(max) to 15-ane. None of these K(+) channel blockers had any effect on the relaxation induced by 15-ane of aortas from 2K-1C rats. 7. These data indicate that an impaired functional activity of K(+) channels contributes to the deficient relaxation induced by the NO donor 15-ane in renal hypertensive 2K-1C rat aortas.

Clonidine induces rat aorta relaxation by nitric oxide-dependent and -independent mechanisms

The alpha1- and alpha2-adrenoceptors coexist in vascular smooth muscle cells producing vascular contraction and relaxation. This study was designed to investigate which is the mechanism activated by clonidine in the rat aorta, and the endothelial factors possibly involved in the relaxation induced by clonidine. The alpha2-adrenoceptors agonist clonidine relaxed rat aortas pre-contracted with phenylephrine, with or without endothelium. In non-contracted denuded arteries, clonidine produced contractions instead of relaxation. In intact endothelium aortic rings, clonidine induced greater relaxation than in denuded aortic rings. In aortas with intact endothelium, the NO-synthase inhibitor L-NAME (10 micromol/L) and the NO-scavenger hemoglobin (10 micromol/L) reduced the relaxation to clonidine. On the other hand, indomethacin (10 micromol/L) failed to alter the relaxation induced by clonidine. These results suggest the participation of NO, but not prostacyclin in clonidine-induced relaxation. In aortic rings pre-contracted with KCl (60 mmol/L) the relaxation induced by clonidine was abolished; however, the K+ channel blockers glibenclamide (K(ATP)), tetraethylamonium (K(Ca)), and the combination of apamin and charybdotoxin (K(Ca)) did not change the relaxation induced by clonidine. The relaxation induced by clonidine on PGF2alpha-contracted arteries was not affected by prazosin. However, in the absence of prazosin, clonidine had an additional contractile effect in PGF2alpha-contracted arteries. In conclusion, our results show that in rat aorta clonidine can activate alpha2-adrenoceptors in the smooth muscle cells and alpha2-adrenoceptors in the endothelial cells that activates NO production, but not prostacyclin and/or EDHF. In the absence of phenylephrine and prazosin, clonidine can also activate alpha1-adrenoceptors and rat aorta contraction.