Hydrogen peroxide increases nerve-evoked contractions in mouse tail artery by an endothelium-dependent mechanism

Reactive oxygen species contribute to regulating the excitability of vascular smooth muscle. This study investigated the actions of the relatively stable reactive oxygen species, H₂O₂, on nerve-evoked contractions of mouse distal tail artery. H₂O₂ (10–100 μM) increased nerve-evoked contractions of isometrically mounted segments of tail artery. Endothelium denudation increased nerve-evoked contractions and abolished the facilitatory effect of H₂O₂. Inhibition of nitric oxide synthase with l-nitroarginine methyl ester (0.1 mM) also increased nerve-evoked contractions and reduced the late phase of H₂O₂-induced facilitation. H₂O₂-induced facilitation of nerve-evoked contractions depended, in part, on synthesis of prostanoids and was reduced by the cyclooxygenase inhibitor indomethacin (1 μM) and the thromboxane A₂ receptor antagonist SQ 29548 (1 μM). H₂O₂ increased sensitivity of nerve-evoked contractions to the α₂-adrenoceptor antagonist idazoxan (0.1 μM) but not to the α₁-adrenoceptor antagonist prazosin (10 nM). Idazoxan and the α_2C-adrenoceptor antagonist JP 1302 (0.5–1 μM) reduced H₂O₂-induced facilitation. H₂O₂ induced facilitation of nerve-evoked contractions was abolished by the non-selective cation channel blocker SKF-96365 (10 μM), suggesting it depends on Ca²⁺ influx. In conclusion, H₂O₂-induced increases in nerve-evoked contractions depended on an intact endothelium and were mediated by activating thromboxane A₂ receptors and by increasing the contribution of α₂-adrenoceptors to these responses.     

Hydrogen peroxide and endothelium-dependent hyperpolarization in the guinea-pig carotid artery

This study was designed to determine whether or not endothelium-dependent hyperpolarizations evoked by acetylcholine in the isolated guinea-pig carotid artery involve hydrogen peroxide. Membrane potential was recorded in the vascular smooth muscle cells of that artery. Under control conditions, acetylcholine induced endothelium-dependent hyperpolarization of the vascular smooth muscle cells which was not affected by the presence of catalase, superoxide dismutase or their combination. Neither the superoxide dismutase mimetic, tiron nor the thiol-reducing agent N-acetyl-l-cysteine modified the hyperpolarization evoked by 0.1 μM acetylcholine but each produced a partial and significant inhibition of the hyperpolarization induced by 1 μM acetylcholine. Neither 10 nor 100 μM hydrogen peroxide influenced the resting membrane potential of the smooth muscle cells and the higher concentration did not significantly influence the hyperpolarization elicited by acetylcholine. These data indicate that, in the guinea-pig isolated carotid artery, hydrogen peroxide is unlikely to contribute to the endothelium-dependent hyperpolarization evoked by acetylcholine.     

Hydrogen peroxide is an endothelium-derived hyperpolarizing factor in animals and humans

Vascular endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several vasodilating factors, such as prostacyclin, nitric oxide (NO), and a yet unidentified endothelium-derived hyperpolarizing factor (EDHF). Possible candidates for EDHF include epoxyeicosatrienoic acids, endothelium-derived K(+) ions, and as we have recently identified, hydrogen peroxide (H(2)O(2)). Electrical communication between endothelial and smooth muscle cells through gap junctions has also been suggested to be involved in endothelium-dependent hyperpolarization. Among the above candidates, the H(2)O(2) hypothesis well explains the pathophysiological interactions between NO and EDHF and re-highlights the physiological roles of the reactive oxygen species in endothelium-dependent vascular responses. This brief review summarizes our current knowledge about H(2)O(2) as an EDHF, with special reference to its production by the endothelium, its action on membrane potentials and its pathophysiological roles.     

Hydrogen peroxide as an endothelium-derived hyperpolarizing factor

Vascular endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several vasodilating factors, such as prostacyclin, nitric oxide (NO), and a yet unidentified endothelium-derived hyperpolarizing factor (EDHF). Possible candidates for EDHF include epoxyeicosatrienoic acids (EETs), endothelium-derived potassium ions (K(+)), and as we have recently identified, hydrogen peroxide (H2O2). Electrical communication between endothelial and smooth muscle cells through gap junctions has also been suggested to be involved in endothelium-dependent hyperpolarization. Among the above candidates, the H2O2 hypothesis well explains the pathophysiological interactions between NO and EDHF and re-highlights the physiological roles of the reactive oxygen species (ROS) in endothelium-dependent vascular responses. This brief review summarizes our current knowledge about H2O2 as an EDHF, with special reference to its production by the endothelium, its action on membrane potentials and its pathophysiological roles.     

Nitric oxide and inactivation of the endothelium-dependent contracting factor released by acetylcholine in spontaneously hypertensive rat

In the aorta of the spontaneously hypertensive rat (SHR), endothelium-dependent contractions are enhanced by inhibitors of NO synthase and scavengers of NO, but not by methylene blue, an inhibitor of guanylyl cyclase, suggesting that the endothelium-derived contracting factor (EDCF) interacts chemically with NO and is inactivated by the latter. However, in view of the relative lack of specificity of methylene blue this hypothesis was re-examined. Acetylcholine-induced endothelium-dependent contractions of isolated rings of SHR aorta were significantly and similarly potentiated by two NOS inhibitors, by two structurally different NO scavengers, by two inhibitors of guanylate cyclase ODQ and NS2028, but to a lesser extent by methylene blue. The contraction of the isolated rat trachea in response to methacholine and the contraction of the rat aorta in response to both 8-isoprostane and KCl were inhibited significantly by methylene blue. Methylene blue binds to the M3 muscarinic receptor subtype but not to the TP receptor. Therefore, methylene blue is an antagonist of the M3 muscarinic receptor subtype, involved in the release of EDCF, and a non-specific inhibitor of TP receptor-mediated contractions, the receptor involved in the action of EDCF. These inhibitory effects of methylene blue are likely to counteract the effect of the inhibition of soluble guanylate cyclase. These results rule out the hypothesis according to which NO would chemically inactivate EDCF.     

Human urotensin-II is an endothelium-dependent vasodilator in rat small arteries

The possible role of the endothelium in modulating responses to human urotensin-II (U-II) was investigated using isolated segments of rat thoracic aorta, small mesenteric artery, left anterior descending coronary artery and basilar artery. Human U-II was a potent vasoconstrictor of endothelium-intact isolated rat thoracic aorta (EC(50)=3.5+/-1.1 nM, R(max)=103+/-10% of control contraction induced by 60 mM KCl and 1 microM noradrenaline). However the contractile response was not significantly altered by removal of the endothelium or inhibition of nitric oxide synthesis with L-NAME (100 microM). Human U-II did not cause relaxation of noradrenaline-precontracted, endothelium-intact rat aortae. Human U-II contracted endothelium-intact rat isolated left anterior descending coronary arteries (EC(50)=1.3+/-0.8 nM, R(max)=20.1+/-4.9% of control contraction induced by 10 microM 5-HT). The contractile response was significantly enhanced by removal of the endothelium (R(max)=55.4+/-16.1%). Moreover, human U-II caused concentration-dependent relaxation of 5-HT-precontracted arteries, which was abolished by L-NAME or removal of the endothelium. No contractile effects of human U-II were found in rat small mesenteric arteries. However the peptide caused potent, concentration- and endothelium-dependent relaxations of methoxamine-precontracted vessels. The relaxant responses were potentiated by L-NAME (300 microM) but abolished in the additional presence of 25 mM KCl (which inhibits the actions of endothelium-derived hyperpolarizing factor). The present study is the first to show that human U-II is a potent endothelium-dependent vasodilator in some rat resistance vessels, and acts through release of EDHF as well as nitric oxide. Our findings have also highlighted clear anatomical differences in the responses of different vascular beds to human U-II which are likely to be important in determining the overall cardiovascular activity of this peptide.     

Important role of endothelium-derived hyperpolarizing factor in shear stress--induced endothelium-dependent relaxations in the rat mesenteric artery.

Shear stress is one of the most important stimulators for the release of endothelium-derived relaxing factors. Although shear stress-induced release of nitric oxide (NO) has been extensively investigated, it remains to be elucidated whether endothelium-derived hyperpolarizing factor (EDHF) contributes to the endothelium-dependent relaxations to shear stress. This study was designed to address this point in the isolated rat mesenteric artery. Large mesenteric arteries (400-500 microm) and resistance mesenteric arteries (150-250 microm) of the rat were precontracted with phenylephrine (at 80 mm Hg of perfusion pressure), and the changes in vessel diameter in response to variable flow (0-300 microl/min) were continuously examined. The relative contributions of vasodilator prostaglandins, NO, and EDHF were analyzed by the inhibitory effects of indomethacin (10(-5) M), N(G)-nitro-L-arginine (L-NNA, 10(-4) M), and KCl (40 mM), respectively. The shear stress-induced relaxations were totally endothelium dependent in both-sized blood vessels, and the contribution of NO was more prominent in large arteries than in resistance arteries, whereas that of EDHF was noted in both-sized blood vessels. Tetrabutylammonium (a nonselective inhibitor of K channels) almost abolished, whereas the combination of charybdotoxin (an inhibitor of both large- and intermediate-conductance Ca2+ -activated K channels) and apamin (an inhibitor of small-conductance Ca2+ -activated K channels) significantly inhibited the EDHF-mediated component of the shear stress-induced relaxations. These results indicate that EDHF plays an important role in shear stress-induced endothelium-dependent relaxations, where K channels, especially calcium-activated K channels, appear to be involved.     

Thapsigargin- and cyclopiazonic acid-induced endothelium-dependent hyperpolarization in rat mesenteric artery.

1. The present study was designed to determine whether putative, selective inhibitors of the Ca(2+)-pump ATPase of endoplasmic reticulum, thapsigargin (TSG) and cyclopiazonic acid (CPA), induce endothelium-dependent hyperpolarization in the rat isolated mesenteric artery. The membrane potentials of smooth muscle cells of main superior mesenteric arteries were measured by the microelectrode technique. 2. In tissues with endothelium, TSG (10(-8)-10(-5) M) caused sustained hyperpolarization in a concentration-dependent manner. In tissues without endothelium, TSG did not cause any change in membrane potential. CPA (10(-5) M) also hyperpolarized the smooth muscle membrane, an effect that was endothelium-dependent and long-lasting. 3. The hyperpolarizing responses to these agents were not affected by indomethacin or NG-nitro-L-arginine (L-NOARG). 4. In Ca(2+)-free medium, neither TSG nor CPA elicited hyperpolarization, in contrast to acetylcholine which generated a transient hyperpolarizing response. 5. In rings of mesenteric artery precontracted with phenylephrine, TSG and CPA produced endothelium-dependent relaxations. L-NOARG significantly inhibited the relaxations to these agents, but about 40-60% of the total relaxation was resistant to L-NOARG. The L-NOARG-resistant relaxations were abolished by potassium depolarization. 6. These results indicate that TSG and CPA can cause endothelium-dependent hyperpolarization in rat mesenteric artery possibly by releasing endothelium-derived hyperpolarizing factor and that membrane hyperpolarization can contribute to the endothelium-dependent relaxations to these agents. The mechanism of hyperpolarization may be related to increased Ca2+ influx into endothelial cells triggered by depletion of intracellular Ca2+ stores due to inhibition of endoplasmic reticulum Ca(2+)-pump ATPase activity.     

Age-related decrease in endothelium-dependent dilator response to histamine in rat mesenteric artery

The effect of aging on the vasodilator responses to histamine, 2-pyridylethylamine and 4-methylhistamine of ring segments of rat mesenteric arteries were investigated. The maximal extent of histamine-induced dilatation of the arteries previously contracted with norepinephrine was greatest for arteries from rats aged 2 and 8 weeks. The maximal response decreased progressively with an increase in age to 13 and 56 weeks. Arteries from 99 week old rats scarcely responded to histamine. Under these conditions, the dilatation induced by papaverine showed no change with age. The vasodilatation caused by 2-pyridylethylamine and 4-methylhistamine also decreased age dependently. The dilatation of the arteries induced by these agents was inhibited by the H1-antagonist chlorpheniramine, but not by the H2-antagonist cimetidine. Removal of the endothelium completely abolished the vasodilator effect of histamine, leaving the effect of papaverine unaffected. Hydroquinone and methylene blue reversed the dilatation induced by histamine, without affecting that caused by papaverine. These results suggest that the age-related decrease in dilatation of rat mesenteric artery in response to histamine is mainly due to a decrease in the ability of the endothelium to liberate a mediator(s).     

Thapsigargin-induced endothelium-dependent triphasic regulation of vascular tone in the porcine renal artery.

1. To elucidate the role of thapsigargin-induced Ca2+ entry in endothelial cells in the regulation of vascular tone, changes in Ca2+ and force of smooth muscle were simultaneously monitored in fura-2-loaded strips of porcine renal artery. 2. During phenylephrine-induced sustained contraction, thapsigargin caused an endothelium-dependent triphasic response; an initial relaxation, a subsequent transient contraction, and a sustained relaxation. The initial relaxation and the contraction were associated with a decrease and an increase in [Ca2+]i, respectively. There was no apparent [Ca2+]i decrease during the sustained relaxation. Thapsigargin-induced responses were observed at 10-8 M and higher concentrations, with the maximum response observed at 10-6 M. 3. The transient contraction was inhibited by a cyclo-oxygenase inhibitor (10-5 M indomethacin), a thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptor antagonist (10-5 M ONO-3708), and a TXA2 synthase inhibitor (10-5 M OKY-046). 4. During the phenylephrine-induced contraction in the presence of indomethacin, thapsigargin caused an initial, but not a sustained relaxation, in the presence of Nomega-nitro-L-arginine methylester (L-NAME). During the contraction induced by phenylephrine plus 40 mM K+-depolarization in the presence of indomethacin, thapsigargin induced both a transient and a sustained relaxation. However, these relaxations were completely abolished in the presence of L-NAME. 5. Thapsigargin caused a large Ca2+ elevation in cultured endothelial cells of the renal artery. The concentration-response relation was thus similar to that for force development in the arterial strips. 6. In conclusion, thapsigargin-induced Ca2+ entry in endothelial cells led to triphasic changes in the tone of the porcine renal artery. The endothelium-dependent contraction was mediated mainly by TXA2. Nitric oxide and hyperpolarizing factor are both involved in the initial relaxation. However, a sustained relaxation was observed which mainly depended on nitric oxide.     

Relationship between NaF- and thapsigargin-induced endothelium-dependent hyperpolarization in rat mesenteric artery.

1. In isolated rat mesenteric artery with endothelium, NaF caused slowly developing hyperpolarization. The hyperpolarizing effect was unchanged in the presence of N(G)-nitro-L-arginine (L-NOARG) and indomethacin, but was markedly reduced by high K+. In Ca2+ -free medium or in the presence of Ni2+, NaF failed to produce hyperpolarization. 2. NaF-induced hyperpolarization was substantially unaffected by deferoxamine, an Al3+ chelator, okadaic acid and calyculin A, phosphatase inhibitors, and preincubation with pertussis toxin, suggesting that neither the action of fluoroaluminates as a G protein activator nor inhibition of phosphatase activity contributes to the hyperpolarizing effect. 3. The selective inhibitors of the Ca2+ -pump ATPase of endoplasmic reticulum, thapsigargin and cyclopiazonic acid, elicited hyperpolarization, whose properties were very similar to those of NaF. When intracellular Ca2+ stores had been depleted with these inhibitors, NaF no longer generated hyperpolarization. 4. In Ca2+ -free medium, NaF (or thapsigargin) caused a transient increase in the cytosolic Ca2+ concentration ([Ca2+]i) in cultured porcine aortic endothelial cells, and subsequent application of thapsigargin (or NaF) failed to increase [Ca2+]i. 5. In arterial rings precontracted with phenylephrine, NaF produced endothelium-dependent relaxation followed by sustained contraction even in the presence of L-NOARG and indomethacin. The relaxant response was abolished by high K+ or cyclopiazonic acid. 6. These results indicate that NaF causes endothelium-dependent hyperpolarization, thereby leading to smooth muscle relaxation of rat mesenteric artery. This action appears to be mediated by the promotion of Ca2+ influx into endothelial cells that can be triggered by the emptying of intracellular Ca2+ stores, as proposed for those of thapsigargin and cyclopiazonic acid.     

Hydrogen peroxide reduces beta-adrenoceptor function in the rat small intestine.

Incubation of isolated rat intestinal segments with hydrogen peroxide (H2O2) led to a decreased beta-adrenoceptor response. The maximal relaxation induced by isoprenaline was lowered while the EC50 remained unaffected. The effect of H2O2 in the small intestine increased slightly from duodenum to ileum. In the ileum, 10(-4) M H2O2 led to a 10% decrease of the maximal relaxation due to isoprenaline and 1 mM decreased the maximal response to about 50%. We further investigated the level at which the isoprenaline response was impaired. The relaxation caused by the stable cAMP analog, dibutyryl-cAMP, or by the adenylate cyclase activator, forskolin, was not affected or affected less than by isoprenaline. When the response to isoprenaline was expressed relative to the maximal response to dibutyryl-cAMP or forskolin, pretreatment with H2O2 led to a decreased isoprenaline response relative to the response to dibutyryl-cAMP or forskolin. This might indicate that exposure to H2O2 leads to a disturbance in receptor-mediated cAMP production. The adenylate cyclase unit is probably not affected since the response to forskolin is relatively resistant to H2O2. Our conclusion is that pretreatment of isolated intestinal segments with H2O2 leads to disturbed beta-adrenoceptor coupling, probably due to altered membrane integrity.     

Hydrogen peroxide affects contractile activity and anti-oxidant enzymes in rat uterus

Background and purpose:

The effects of hydrogen peroxide (H₂O₂) on uterine smooth muscle are not well studied. We have investigated the effect and the mechanism of action of exogenous hydrogen peroxide on rat uteri contractile activity [spontaneous and calcium ion (Ca²⁺)-induced] and the effect of such treatment on anti-oxidative enzyme activities.

Experimental approach:

Uteri were isolated from virgin Wistar rats and suspended in an organ bath. Uteri were allowed to contract spontaneously or in the presence of Ca²⁺ (6 mM) and treated with H₂O₂ (2 µM–3 mM) over 2 h. Anti-oxidative enzyme activities (manganese superoxide dismutase-MnSOD, copper-zinc superoxide dismutase-CuZnSOD, catalase-CAT, glutathione peroxidase-GSHPx and glutathione reductase-GR) in H₂O₂-treated uteri were compared with those in uteri immediately frozen after isolation or undergoing spontaneous or Ca²⁺-induced contractions, without treatment with H₂O₂. The effect of inhibitors (propranolol, methylene blue, L-NAME, tetraethylamonium, glibenclamide and 4-aminopyridine) on H₂O₂-mediated relaxation was explored.

Key results:

H₂O₂ caused concentration-dependent relaxation of both spontaneous and Ca²⁺-induced uterine contractions. After H₂O₂ treatment, GSHPx and MnSOD activities were increased, while CuZnSOD and GR (In Ca²⁺-induced rat uteri) were decreased. N^ω-nitro-L-arginine methyl ester antagonized the effect of H₂O₂ on Ca²⁺-induced contractions. H₂O₂-induced relaxation was not affected by propranolol, potentiated by methylene blue and antagonized by tetraethylamonium, 4-aminopyridine and glibenclamide, with the last compound being the least effective.

Conclusions and implications:

H₂O₂ induced dose-dependent relaxation of isolated rat uteri mainly via changes in voltage-dependent potassium channels. Decreasing generation of reactive oxygen species by stimulation of anti-oxidative pathways may lead to new approaches to the management of dysfunctional uteri.     

Thrombin causes endothelium-dependent biphasic regulation of vascular tone in the porcine renal interlobar artery

Using a method employing front-surface fura-2 fluorometry to measure the cytosolic Ca(2+) concentration, [Ca(2+)](i), the mechanism of endothelium-dependent regulation of vascular tone by thrombin was studied in porcine renal interlobar arterial strips. At concentrations lower than 3 u ml(-1), thrombin evoked only early transient relaxation, while at 3 u ml(-1) and higher concentrations, thrombin caused an early relaxation and a subsequent transient contraction. Both thrombin-induced relaxation and contraction were abolished by removing the endothelium. Similar biphasic responses were observed with a protease-activated receptor-1-activating peptide. Early relaxation was associated with a decrease in [Ca(2+)](i), while the transient contraction was not associated with a change in [Ca(2+)](i) of smooth muscle cells. A thromboxane A(2) (TXA(2))/prostaglandin H(2) (PGH(2)) receptor antagonist (10(-5) M ONO-3708) completely inhibited the thrombin-induced contraction, whereas a thromboxane A(2) synthase inhibitor (10(-5) M OKY-046) only partly inhibited it. When the thrombin-induced contraction was inhibited by ONO-3708, either pretreatment with N(omega)-nitro-L-arginine methylester (L-NAME) or an increase in the amount of external K(+) to 40 mM did not abolish thrombin-induced relaxation during phenylephrine-induced sustained contraction. However, the combination of pretreatment with L-NAME and an elevation of external K(+) to 40 mM completely abolished the relaxation. There was no significant difference in the concentration-dependent effects of thrombin on the initial early relaxation between conditions in which the contractile components either were or were not inhibited. Thrombin is thus considered to mainly activate protease-activated receptor-1 and cause a biphasic response, early relaxation and a transient contraction, in the porcine renal interlobar artery in an endothelium-dependent manner. The thrombin-induced endothelium-dependent relaxation was mediated by nitric oxide and hyperpolarizing factors, while the contraction was mediated by TXA(2) and PGH(2).     

The mechanism of bradykinin-induced endothelium-dependent contraction and relaxation in the porcine interlobar renal artery

The mechanism of endothelium-dependent regulation of vascular tone of bradykinin was investigated by simultaneously monitoring the changes in the cytosolic Ca(2+) concentration and the force of smooth muscle in fura-2-loaded strips of the porcine renal artery with endothelium. During phenylephrine-induced sustained contraction, bradykinin (>3x10(-9) M) caused endothelium-dependent triphasic changes in the force of the strips, composed of an initial relaxation, a subsequent transient contraction and a late sustained relaxation. At low concentrations (10(-10) - 10(-9) M), bradykinin caused an endothelium-dependent biphasic relaxation with no contraction. A thromboxane A(2) (TXA(2))/prostaglandin H(2) (PGH(2)) receptor antagonist (10(-5) M ONO-3708) completely inhibited, while a TXA(2) synthase inhibitor (10(-5) M OKY-046) only partially inhibited, the transient contraction induced by bradykinin. Under conditions where the bradykinin-induced contraction was inhibited by ONO-3708 during the phenylephrine-induced contraction, bradykinin induced only a transient relaxation in the presence of N(Omega)-nitro-L-arginine methyl ester (L-NAME). This transient relaxation was inhibited when the precontraction was initiated by phenylephrine plus 40 mM extracellular K(+). The removal of L-NAME from this condition caused a partial reappearance of the initial relaxation and a complete reappearance of the sustained relaxation. In conclusion, bradykinin caused the endothelium-dependent triphasic regulation of vascular tone in the porcine renal artery. The concentrations of bradykinin required to induce a contraction was higher than that required to induce relaxation. Both TXA(2) and PGH(2) were involved in the bradykinin-induced contraction. The initial relaxation was mediated by nitric oxide and hyperpolarizing factors while the sustained relaxation depended on nitric oxide.     

Effects of different tetra-n-alkylammonium ions on acetylcholine-induced endothelium-dependent hyperpolarization in rat mesenteric artery

The effects of a series of symmetric tetra-n-alkylammonium (TAA) compounds with alkyl side chains of one to six carbons in length on acetylcholine-induced endothelium-dependent hyperpolarization were examined in rat mesenteric artery. All TAA compounds caused a concentration-dependent inhibition of the hyperpolarizing response to acetylcholine. The potency of TAAs showed a general trend to increase with the lengths of the alkyl side chains. The inhibitory effects of TAAs, excepting the smallest compound, on the acetylcholine response were reversible. However, TAAs with long alkyl side chains may act as antagonists at muscarinic receptors, because the suppressive effect on A23187-induced endothelium-dependent hyperpolarization was more marked with TAAs having smaller alkyl side chains. Conversely, the hyperpolarizing response to pinacidil, an ATP-sensitive K+ channel opener, was significantly prevented only by TAA compounds with long alkyl side chains. TAA compounds with one-to three-carbon alkyl side chains caused a modest and reversible depolarization of the membrane, whereas the depolarizing effects of the compounds with four-to six-carbon alkyl side chains were marked and irreversible. These results suggest that TAAs could gain access to the target K+ channels for endothelium-derived hyperpolarizing factor, the ATP-sensitive K+ channels, and the K+ channels responsible for the regulation of the resting membrane potential in different ways.     

Vascular smooth muscle relaxation induced by epidermal growth factor is endothelium-dependent

Epidermal growth factor (EGF) is released from platelets during aggregation. Because we thought that EGF played a role in vascular tone, we investigated its vascular reactivity using isolated rat aortic strips with and without the endothelium. In the presence of endothelium, EGF relaxed vascular smooth muscle precontracted with 40 mM K⁺, 10⁻⁵ M prostaglandin F₂ or 10⁻⁶ M norepinephrine. The relaxation induced by EGF was more prominent on the prostaglandin F₂- and norepinephrine-induced contractions than on the K⁺-induced contraction. Atropine (10⁻⁵ M) and aspirin (10⁻⁵ M) had no effect on the EGF-induced relaxation, but methylene blue (10⁻⁵ M) partly abolished the relaxation evoked by EGF. These results suggest that EGF relaxes vascular smooth muscle in the presence of the endothelium. They also suggest that EGF has an effect on the endothelium to produce relaxing factor independent of cyclooxygenase; the releasing factor activates soluble guanylate cyclase, resulting in relaxation of vascular smooth muscle through the production of cyclic GMP.     

Heat stress alters G-protein coupled receptor-mediated function and endothelium-dependent relaxation in rat mesenteric artery

Heat stress has been demonstrated to have strong cardiovascular effects. However, the underlying mechanism-mediated cardiovascular effects are still not fully understood. The present study was designed to examine if heat stress alters vascular G-protein coupled receptor-mediated vasomotion and endothelium function in rat mesenteric artery. Rats were divided into two groups, heat stress rats and control. The G-protein coupled receptors of endothelin type B (ETB) receptor-, endothelin type A (ETA) receptor-, 5-hydroxytryptamine (5-HT) receptor-, calcitonin gene-related peptide (CGRP) receptor-, alpha-adrenoceptor-mediated vosoactivity and endothelium-dependent relaxation on rat mesenteric artery ring segments were monitored by a myograph system. The plasma level of CGRP was determined by radioimmunological assay. Compared with control arterial segments, the contractile response curves of sarafotoxin 6c, a selective ETB receptor agonist and 5-HT in the arterial segments from heat stress rats were shifted towards left. An increased maximum contraction (Emax) induced by sarafotoxin 6c, but not 5-HT, was seen in the arterial segments from heat stress rats. CGRP-induced relaxation in endothelium-denuded arterial segments from heat stress rats was enhanced. The relaxation in endothelium-intact arterial segments induced by acetylcholine was significantly decreased in heat stress rats. In addition, the plasma concentration of CGRP was increased in heat stress rats. The endothelium-dependent relaxation was characterized and shown there was a decrease in nitric oxide and endothelium-derived hyperpolarizing factor-mediated relaxation in the arterial segments from heat stress rats. In conclusion, heat stress induces an enhanced vascular endothelin ETB-, 5-HT-receptors-mediated contraction, an enhanced CGRP-receptor-induced relaxation and damage to endothelium-dependent relaxation.