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Department of Medical Physiology, Cardiovascular Research Institute, College of Medicine, Texas A&M University System Health Science Center, College Station, Texas 77843-1114
Submitted 28 June 2003 ; accepted in final form 2 July 2003
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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oxidative stress; prostaglandins; smooth muscle; hyperpolarization
Recent studies suggested that H2O2 released from the endothelium can serve as a mediator for acetylcholine- and bradykinin-induced vasodilation in mouse intestinal arterioles (25) and in piglet cerebral circulation (22), respectively. Vascular insults associated with cardiac ischemia and reperfusion can result in a release of H2O2 from the endothelium and leukocytes (1, 38). In the context of neutrophil-endothelial interaction, the local H2O2 concentration may reach 100–400 µM (41). H2O2 has been shown to cause dilation of large conduit coronary arteries with an intact endothelium (9, 34) but produce transient contraction in the absence of functional endothelium (12). A recent study demonstrated that H2O2 released from the endothelium during NO synthase (NOS) uncoupling (i.e., cofactor deficiency) can serve as a mediator for endothelium-dependent relaxations (9). This vasodilation may present an important compensatory mechanism for NO deficiency in the coronary circulation. Furthermore, the study on human coronary microvessels indicated that H2O2 released from the endothelium causes vasodilation in response to increased flow (26). It appears that H2O2 is a vasoactive agent and may play an important role as an endogenous mediator contributing to the regulation of vascular tone and possibly local blood flow in the coronary microcirculation under physiological and/or pathophysiological conditions. However, the underlying mechanism responsible for the H2O2-induced vasomotor response in the coronary microvessels remains unclear. Because coronary microvascular tone is the predominant determinant of coronary resistance (7), and thus of coronary blood flow, it is imperative to understand the direct effect of H2O2 and its vasomotor signaling pathway in these microvessels. In the present study, to avoid the confounding influences from hemodynamic changes and local vasoregulatory mechanisms, the role of endothelial prostaglandin, NO, cytochrome P-450-derived metabolites, and potassium channels in H2O2-induced response was examined in the isolated and pressurized coronary resistance vessels.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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Isolation and cannulation of coronary microvessels. To eliminate confounding influences from hemodynamic, neurohumoral, and metabolic factors on vasomotor function, individual subepicardial coronary arterioles (50–100 µm internal diameter in situ) were dissected out for in vitro study as previously described (16). Vessels were cannulated with glass micropipettes and pressurized to 60 cmH2O intraluminal pressure. The cannulated vessel was bathed in physiological salt solution (PSS) containing bovine serum albumin (1%; USB, Cleveland, OH) at 37°C (21). Internal diameters of the vessel were measured throughout the experiment using video microscopic techniques incorporated into the MacLab (ADInstruments, Milford, MA) data acquisition system (21).
Role of endothelium in H2O2-induced coronary arteriolar response. After a stable basal tone (
60 min) was developed, the vasomotor reaction of coronary arterioles to extraluminal administration of H2O2 (100 nM to 100 µM) was examined. The role of endothelium in mediating H2O2-induced response was determined after endothelial removal by perfusion of a nonionic detergent, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS, 0.4%) to the lumen of the vessels as described in previous studies (18). The efficacy of endothelial denudation was verified by the absence of vasodilation to an endothelium-dependent vasodilator bradykinin (1 nM; 90% dilation in the intact vessel). To ensure that the vascular smooth muscle function was not compromised by the CHAPS treatment, dose-dependent dilation of the vessel to an endothelium-independent vasodilator sodium nitroprusside (1 nM to 10 µM) was examined. Only vessels that exhibited normal basal tone showed no vasodilation to bradykinin and showed unaltered response to sodium nitroprusside after endothelium removal were accepted for data analysis. The specificity of the H2O2-induced response was confirmed by the absence of vasomotor response to H2O2 in the presence of H2O2 scavenger catalase (1,200 U/ml).
Involvement of endothelial enzymes in the H2O2-induced coronary arteriolar response. The role of NOS, cytochrome P-450 monooxygenase, and cyclooxygenase (COX) in H2O2-induced response was examined after treatment of the respective inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 10 µM) (16), miconazole (30 µM) (22), and indomethacin (10 µM) (16) for 30 min. In some experiments, the combination of above inhibitors was used to probe the possible activation of multiple pathways by H2O2.
Involvement of COX enzymes and PGE2 receptors in H2O2-induced response. To further investigate the involvement of COX enzyme isoforms, the vasomotor response to H2O2 was examined before and after a 30-min incubation of the vessels with a selective COX-1 inhibitor 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole (SC-560; 0.1 µM; Calbiochem; San Diego, CA) (30, 35) or a selective COX-2 inhibitor N-(2-cyclohexyloxy-4-nitrophenyl)-methanesulfonamide (NS-398; 10 µM; Calbiochem) (30, 35). The involvement of PGE2 receptor (EP) in the H2O2-induced response was examined by treating the vessels with AH-6809, a non-selective PGE2 receptor (EP1 and EP2) antagonist (10, 48). The efficacy of EP receptor inhibition was verified by the absence of vasomotor response to the exogenous PGE2 (10 pM to 10 nM; Calbiochem).
Measurement of prostaglandin release from coronary microvessels. Porcine coronary arterioles (10 segments, 70–100 µm in diameter with 1 to 2 mm in length) were isolated and placed in a microcentrifuge tube containing 450 µl PSS at 37°C. After a 30-min equilibrium period, H2O2 (50 µl, 100 µM final concentration) was added to the vessel bath for 30 min, and then the bathing solution was collected for the measurement of 6-keto-prostaglandin F1
(a stable metabolite of PGI2) and bicyclo-PGE2 (a stable catabolite of PGE2) using enzyme immunoassay kits according to the manufacturer's protocol (Cayman Chemical; Ann Arbor, MI). In another set of experiments, vessels were treated with indomethacin (10 µM) for 20 min during the initial incubation period. The PGE2 and PGI2 production was then assayed after a 30-min incubation of the vessels with H2O2 (100 µM). Vehicle solution (i.e., PSS), instead of H2O2 and indomethacin, was added in the parallel experiments as control. The background level of arachidonic acid metabolites in the solution was measured from the tube containing PSS only, and this value was subsequently subtracted from the sampled solution to obtain prostaglandin production. The protein levels in each vessel tube were quantified by bicinchoninic acid protein assay (Pierce; Rockford, IL) and were used to normalize the prostaglandin production.
Role of smooth muscle potassium channels in vasomotor response to H2O2. To determine whether smooth muscle hyperpolarization and activation of specific potassium channels were involved in the vasomotor response to H2O2, the reaction of denuded vessels to H2O2 was examined in the presence of a depolarizing agent KCl (35 mM) and specific potassium channel inhibitors, respectively. The role of ATP-sensitive potassium (KATP) channels, calcium-activated potassium (KCa) channels, and inward rectifier potassium (Kir) channels in the vasomotor response to H2O2 was determined by pretreating the denuded vessels with glibenclamide (5 µM) (18), iberiotoxin (0.1 µM) (28), and barium chloride (30 µM) (28), respectively, for 30 min.
Role of smooth muscle guanylyl cyclase and lipoxygenase in vasomotor response to H2O2. Recent studies in endothelium-denuded coronary arteries suggested the involvement of vascular smooth muscle lipoxygenase (3) and guanylyl cyclase (14) in H2O2-induced vasorelaxation. To determine whether activation of these pathways mediated smooth muscle relaxation to H2O2, the reaction of denuded coronary arterioles to H2O2 was examined in the presence of either lipoxygenase inhibitor eicosatriynoic acid (5 µM) (3) or guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ; 0.1 µM) (15) for 30 min.
Chemicals. Drugs were obtained from Sigma-Aldrich (St. Louis, MO), except as otherwise stated. Barium chloride, bradykinin, catalase, H2O2, iberiotoxin, L-NAME, and sodium nitroprusside were dissolved in PSS. Indomethacin, NS-398, PGE2, and pinacidil were dissolved in ethanol, and the subsequent concentrations were diluted in PSS. The final concentration of ethanol in the vessel bath was 0.1%. AH-6809, eicosatriynoic acid, glibenclamide, miconazole, ODQ, and SC-560 were dissolved in DMSO, and the subsequent concentration was diluted in PSS. The final concentration of DMSO in the vessels bath was 0.03%. Vehicle control studies indicated that the final concentration of ethanol and DMSO had no effect on arteriolar function.
Data analysis. At the end of each experiment, vessels were relaxed with sodium nitroprusside (100 µM) to obtain maximal diameter at 60 cmH2O intraluminal pressure (21). All diameter changes in response to agonists were normalized to the vasodilation induced by 100 µM sodium nitroprusside and expressed as a percentage of maximal dilation. All data are presented as means ± SE. Statistical comparisons of vasomotor responses under different treatments were performed with two-way ANOVA and tested with Fisher's protected least-significant difference multiple-range test. Differences in resting diameter before and after pharmacological interventions were compared by paired Student's t-test and P values <0.05 were considered to be significant.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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Contribution of NOS, cytochrome P-450 monooxygenase, and COX. Coronary arterioles dilated significantly to 30 µM H2O2 as shown in Fig. 1B. This response was not affected by either NOS inhibitor L-NAME (10 µM) or the cytochrome P-450 blocker miconazole (30 µM). However, consistent with the results presented in Fig. 1A, the dilation was inhibited by either indomethacin (10 µM) or denudation (Fig. 1B). In the presence of indomethacin, addition of L-NAME did not further inhibit vasodilation to H2O2 (Fig. 1B). These results indicate that H2O2-induced vasodilation is mediated in part by the release of COX metabolites from endothelial cells. It should be noted that these inhibitors did not alter resting tone or the vasodilation to sodium nitroprusside (Table 1).
Involvement of COX-1 and COX-2. To identify the isoform of COX involved in H2O2-induced dilation, the vascular response to H2O2 was examined in the presence of selective COX-1 and COX-2 inhibitors. In the intact vessels, the vasodilation to H2O2 was significantly attenuated by the COX-1 inhibitor SC-560 (0.1 µM) but not by the COX-2 inhibitor NS-398 (10 µM) (Fig. 2). The inhibitory effect of SC-560 is identical to that produced by endothelial removal (Fig. 1A). In contrast to the intact vessels, SC-560 did not exert an inhibitory effect on the H2O2-induced dilation of denuded vessels (n = 3, data not shown).
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Involvement of prostaglandin release and EP receptor activation. In the absence of H2O2, resting PGE2 and PGI2 production from coronary arterioles were 6.2 ± 3.0 and 5.8 ± 0.9 pg/mg protein, respectively (Fig. 3, A and B). The addition of H2O2 (100 µM, 30 min) to the vessels produced a threefold increase (17.7 ± 5.4 vs. 6.2 ± 3.0 pg/mg protein) in PGE2 production (Fig. 3A). In contrast, H2O2 had no effect on the PGI2 production (6.6 ± 0.9 vs. 5.8 ± 0.9 pg/mg protein) (Fig. 3B). Indomethacin (10 µM) inhibited H2O2-induced PGE2 production (Fig. 3A) and also reduced the basal release of PGI2 (Fig. 3B). To investigate the involvement of PGE2 receptors in the vasodilatory response to H2O2, coronary arteriolar dilation was examined in the presence of an EP1/EP2 receptor antagonist AH-6809. AH-6809 significantly inhibited the vasodilation in response to an exogenous PGE2 (Fig. 4A) and also attenuated vasodilation to H2O2 (Fig. 4B). The extent of attenuation is identical to that produced by either denudation or indomethacin (Fig. 1A). Furthermore, AH-6809 did not exert an inhibitory effect on the H2O2-induced dilation of denuded vessels (n = 3, data not shown).
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Role of vascular smooth muscle hyperpolarization and potassium channel activation. The involvement of smooth muscle cell hyperpolarization and potassium channel activation in vasodilation to H2O2 is elucidated in Fig. 5A. In this series of studies, the endothelium was removed to eliminate its contribution to vasodilation. Disruption of endothelium attenuated H2O2-induced vasodilation, which is consistent with the data shown in Fig. 1A. In the presence of a depolarizing agent KCl (35 mM), there was a tendency to increase vascular tone but not in a significant manner. This concentration of KCl significantly inhibited vasodilation in response to H2O2 (Fig. 5) but had no effect on sodium nitroprusside-induced vasodilation (Table 1). Inhibition of KCa channel by iberiotoxin (0.1 µM) significantly attenuated vasodilation to H2O2 in an identical manner to that produced by KCl (Fig. 5). However, inhibition of KATP channels and Kir channels by glibenclamide (5 µM; Fig. 5) and BaCl2 (n = 5; data not shown), respectively, had no effect on the H2O2-induced vasodilation. Inhibition of lipoxygenase and guanylyl cyclase by eicosatriynoic acid and ODQ, respectively, also had no effect on H2O2-induced dilation of these denuded vessels (Fig. 5B).
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Because H2O2 stimulates a release of PGE2 (Fig. 3A), and the KCa channel activation is involved in the smooth muscle relaxation to H2O2 (Fig. 5), we examined whether PGE2-induced dilation was mediated through KCa channel activation. As shown in Fig. 6, the dilation of intact coronary arterioles to PGE2 was insensitive to iberiotoxin, suggesting that the vasodilatory action of PGE2 is independent of KCa channel activation. However, the combined inhibition of the PGE2 pathway (by AH-6809) and KCa channels (by iberiotoxin) abolished the dilation to H2O2, except at the highest concentration (100 µM; 20% dilation) (Fig. 6B).
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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Accumulating evidence suggests that oxidant stress influences many functions of the endothelium, including the vasomotor tone. Various enzymes such as endothelial NOS, cyclooxygenase, lipoxygenase, cytochrome P-450 enzymes, and NAD(P)H oxidases have been shown to produce superoxide anions and other ROS, including H2O2. Superoxide anions can directly inactivate NO and subsequently lead to the reduction of NO bioavailability and the impairment of NO-mediated vasodilatory response. Interestingly, H2O2 derived from superoxide anions has been shown to participate in the regulation of vasomotor tone. Although the vasodilatory effect of H2O2 in various microvascular beds has been demonstrated in many in vivo studies (44, 46, 47), the mechanisms underlying this response remain inconclusive possibly due to the confounding influences from neurohumoral and/or local control mechanisms. In contrast to the in vivo studies, the direct effect of H2O2 on vascular tone in vitro has been reported with different results. For example, H2O2 elicits vasoconstriction in the canine basilar artery (20) and rat aorta (51) but produces dilation in the rabbit aorta (52). A biphasic response consisting of a transient contraction, followed by relaxation was observed in the rat pulmonary artery (33). The contradictory results, either constriction (37) or dilation (5) of rabbit intrapulmonary arteries, were also reported. In the heart, H2O2 evokes relaxation of canine (34) and porcine (3) large conduit coronary arteries. Although these discrepancies may have resulted from the organ and/or species differences, the confounding effects derived from the use of various preconstrictors (KCl, endothelin, PGF2a, and thromboxane, etc.) in large vessel preparations (i.e., vascular strip and ring) also cannot be discounted. Regardless of these considerations and in contrast to the wealth of information from large conduit vessels, the research on the direct effect of H2O2 on coronary microvasculature is sparse. In the present study using isolated and pressurized coronary arterioles, without confounding influences from the changes in flow, pressure, and other vasoregulatory mechanisms, we demonstrated that coronary arterioles dilated to H2O2 in a concentration-dependent manner. This is consistent with the findings in large conduit coronary vessels. Regarding the role of the endothelium in vasorelaxation to H2O2, various results have been reported in large vessels. A previous study has shown that H2O2-induced dilation is independent of endothelium in porcine coronary artery rings (3), but a partial dependence on endothelium has been also reported in rabbit aorta (52) and in canine coronary (34) and basilar (50) arteries. Our present study demonstrated that the vasodilation to H2O2 is partially dependent on the endothelium, given the fact that the vasodilatory response was not abolished by endothelial removal. In contrast to our results, the coronary arterioles isolated from the atrial appendage of patients under cardiopulmonary bypass procedures exhibited endothelium-independent vasodilation to H2O2 (26). The reason for this discrepancy is unclear but may be related to the differences in species or vessel locations (atrial appendage vs. ventricular tissue). Furthermore, the possible alteration of vasodilatory mechanisms by an added pharmacological preconstrictor and/or by the apparent coronary vascular disease in the above human study also cannot be excluded.
In the isolated rabbit aorta, H2O2 elicits NO-mediated vasorelaxation (52). Although the endothelium-dependent relaxation to H2O2 has been reported in the canine coronary artery (34), the factors involved in this vasomotor response was not characterized. We found that H2O2-induced arteriolar dilation is resistant to L-NAME and miconazole, indicating that H2O2-induced vasodilation is not mediated by the release of NO or cytochrome P-450-derived metabolites. Because H2O2-induced coronary arteriolar dilation was attenuated by indomethacin to the same extent as endothelial removal (Fig. 1A), endothelial production/release of prostaglandins is believed to be responsible, in part, for the H2O2-induced response. This contention is directly supported by the result of prostaglandin measurements, which showed that PGE2 production was increased after incubation of the coronary arterioles with H2O2. It is worth noting that the dilation of intact coronary arterioles occurred at a lower threshold concentration of H2O2 (1 µM) compared with that at a higher concentration (i.e., 10 µM) in denuded vessels (Fig. 1). Because vasodilation to H2O2 at low micromolar concentrations (
10 µM) was abolished by indomethacin, it appears that the endothelial COX pathway amplifies the signaling of H2O2 for vasodilation.
In the present study, COX-1 inhibitor SC-560, but not COX-2 inhibitor NS-398, mimicked the inhibitory effect of indomethacin in H2O2-induced vasodilation, suggesting the possible role of COX-1 activation in this vasodilatory process. Although the enzymatic study of COX indicates that prostaglandin biosynthesis can be activated by a variety of peroxides (17), the specific COX downstream products responsible for H2O2-induced vasodilation has not been determined. It has been shown that hydroperoxide (2 µM) rapidly activates COX to produce various prostaglandins, including PGG2, PGF2, PGD2, and PGH2, with PGE2 as a major product (17). This is consistent with our finding that the PGE2 was significantly elevated by H2O2 in the coronary arterioles. Although a recent study suggested that COX-1 is the primary source of PGE2 and PGI2 in coronary microvascular endothelial cells (13), it seems that H2O2 selectively activates PGE2 production in the intact coronary arterioles because the release of PGI2 was not affected in the present study.
It is well documented that PGE2 elicits its autocrine/paracrine effects on target cells through interaction with the prostaglandin E receptor series, designated as EP1, EP2, EP3, and EP4 (8). Activation of EP2 and EP4 receptors is generally associated with smooth muscle relaxation, whereas activation of EP1 and EP3 receptors leads to smooth muscle contraction (40). Although the exact receptor subtypes responsible for the PGE2-induced dilation cannot be determined due to the lack of selective antagonists for each of these receptors, it is very likely that the PGE2-induced dilation in the coronary arterioles is mediated by the activation of EP2 receptors. This contention is supported by the observation that an EP1/EP2 receptor blocker AH-6809 (10, 48) abolished the vasodilation elicited by the exogenous PGE2. Because SC-560 and AH-6809 exerted a similar blocking effect as indomethacin in the H2O2-induced vasodilation, PGE2 is likely to be the major vasodilatory metabolite derived from COX-1 under H2O2 stimulation, leading to the activation of possibly EP2 receptors for vasodilation.
At the higher concentration of H2O2 (
30 µM), the contribution of endothelium to vasodilation is diminished, and the direct action of H2O2 on smooth muscle predominates. H2O2 has been shown to stimulate a variety of signal transduction pathways, including phospholipase A2 (32), protein kinase C (6), and c-fos (31) in vascular smooth muscle cells. However, activation of these pathways is either relatively slow (few minutes to hours) or requires a very high concentration of H2O2 (e.g., at millimolar range). Because the vasomotor response to H2O2 takes place at lower concentrations (10 to 100 µM) within 30 s in denuded coronary arterioles, it is unlikely that activation of these pathways is responsible for the observed dilation in the present study. Interestingly, H2O2 at 100 µM has been shown to cause hyperpolarization and relaxation of denuded porcine coronary vascular strip (4), which may offer a possible mechanism underlying endothelium-independent vasodilation of the downstream small coronary arterioles. Our results support this contention because H2O2-induced dilation in the denuded vessel was significantly inhibited by a depolarizing agent KCl.
Activation of vascular potassium channels leading to smooth muscle relaxation is one of the major signaling mechanisms underlying vasomotor regulation (28). There is substantial evidence that H2O2 is capable of producing membrane hyperpolarization by activating diverse potassium channels in various cell types, including vascular smooth muscle cells (2, 3, 22, 25). Although KATP channels have been demonstrated to play a role in H2O2-induced dilation of cat (44) and porcine (22) cerebral arteries, the dilation of rat cerebral arteries (39), mouse mesenteric arteries (25), porcine coronary arteries (2, 3, 14), and human atrial arterioles (26) was suggested to be mediated by KCa channels. Our results support the latter view because inhibition of KCa channels, but not KATP or Kir channels, attenuated vasodilation to H2O2. Although KCa channels have been shown to be highly expressed in the porcine (2, 3) and human coronary myocytes (11), the signaling cascade for KCa channel activation by H2O2 in the coronary microvessel remains unclear. A number of studies have suggested that KCa channels can be modulated by a number of intracellular second messengers (28). For example, activation of KCa channels by cGMP has been reported in vascular smooth muscle cells (28, 45). Interestingly, activation of guanylyl cyclase also has been suggested to mediate H2O2-induced relaxation of denuded pulmonary arteries (5). It is likely that the guanylyl cyclase-cGMP pathway is involved in the H2O2-induced dilation of coronary arterioles through KCa channel activation. However, inhibition of guanylyl cyclase by ODQ, which effectively blocked vasodilation to sodium nitroprusside (Table 1), had no effect on the dilation of denuded vessels to H2O2 (Fig. 5B). This is consistent with the recent report in human coronary arterioles that H2O2-induced dilation is insensitive to ODQ (26). Thus these findings do not support the idea that the guanylyl cyclase-cGMP signaling is involved in the smooth muscle relaxation to H2O2. Interestingly, a recent pharmacological and electrophysiological study in isolated conduit coronary arteries demonstrated that KCa channel-mediated H2O2-induced vasorelaxation is through a signal transduction cascade involving PLA2 activation and arachidonic acid production in smooth muscle cells (2). This transduction pathway is independent of the COX and cytochrome P-450 cascades but involves activation of lipoxygenase (2). In agreement with these findings, the dilation of denuded coronary arterioles to H2O2 in the present study was insensitive to a cytochrome P-450 inhibitor and indomethacin, suggesting that the activation of smooth muscle KCa channels by H2O2 is independent of the cytochrome P-450 pathway and COX. In contrast to the conduit coronary artery, our results do not support the role of lipoxygenase/leukotrienes because inhibition of the lipoxygenase pathway by an effective dose of eicosatriynoic acid (3) did not affect H2O2-induced dilation in either intact or denuded coronary arterioles. It is worth noting that PGE2, the prostanoid involved in the vasodilation to H2O2 as shown in the present study, has been reported to activate KCa channels in cultured vascular smooth muscle cells (36, 53). Therefore, it is possible that the PGE2 synthesized in the vascular cells might activate KCa channels for vasodilation. However, the dilation of coronary arterioles to PGE2 was insensitive to a KCa channel inhibitor (Fig. 6A), suggesting that PGE2 does not contribute to the KCa channel-mediated vasodilation to H2O2 in the present study. Interestingly, a recent study indicated that KCa channels activity in an inside-out patch of porcine coronary artery smooth muscle cell can be increased by a relatively low concentration of H2O2 (10 µM) (14). This electrophysiological study suggests that H2O2 is capable of activating KCa channel without the participation of cellular second messengers. Therefore, it is plausible that the H2O2-elicited dilation of denuded coronary arterioles in the present study is through direct activation of smooth muscle KCa channels.
In conclusion, porcine coronary arteriolar dilation to H2O2 is mediated by at least two mechanisms. First, H2O2, at low concentrations (10 µM), selectively activates the endothelial COX-1 pathway for PGE2 production, which subsequently activates smooth muscle EP receptors for vasodilation. Second, H2O2, at high concentrations (30 µM), selectively activates smooth muscle KCa channels leading to hyperpolarization and smooth muscle relaxation. It appears that the latter vasodilatory pathway is independent of COX-1-PGE2 signaling. Although the endogenous role of H2O2 in the cardiovascular system remains obscure, accumulating evidence suggests that H2O2 can play a critical role in vasoregulation under physiological and pathophysiological conditions. For example, H2O2 may act as an endothelium-derived hyperpolarizing factor in some vascular beds in terms of vasodilation in response to acetylcholine (25), bradykinin (22), and shear stress (26) stimulation. H2O2 has been also reported to mimic reactive hyperemia in the skeletal muscle microcirculation (47). Furthermore, a recent in vivo study suggested that H2O2 plays an important role in autoregulation of coronary blood flow (49). In patients with coronary artery disease, H2O2 may also play a critical role in compensating for impaired NO-mediated dilation in the coronary microcirculation (26). It is speculated that compromising of endothelial function related to the COX-1-PGE2 pathway (29) and/or alteration of smooth muscle KCa channels (23) during disease states may potentially mitigate the role of H2O2 in coronary flow regulation and exacerbate the already deteriorated cardiac function.
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DISCLOSURES |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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 M. Wareing, X. Bai, F. Seghier, C. M. Turner, S. L. Greenwood, P. N. Baker, M. J. Taggart, and G. K. Fyfe Expression and function of potassium channels in the human placental vasculature Am J Physiol Regulatory Integrative Comp Physiol, August 1, 2006; 291(2): R437 - R446. [Abstract] [Full Text] [PDF]
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 N. Ardanaz and P. J. Pagano Hydrogen peroxide as a paracrine vascular mediator: regulation and signaling leading to dysfunction. Experimental Biology and Medicine, March 1, 2006; 231(3): 237 - 251. [Abstract] [Full Text] [PDF]
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 H. Cai Hydrogen peroxide regulation of endothelial function: Origins, mechanisms, and consequences Cardiovasc Res, October 1, 2005; 68(1): 26 - 36. [Abstract] [Full Text] [PDF]
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R. D. Roghair, J. L. Segar, R. V. Sharma, M. C. Zimmerman, D. K. Jagadeesha, E. M. Segar, T. D. Scholz, and F. S. Lamb Newborn lamb coronary artery reactivity is programmed by early gestation dexamethasone before the onset of systemic hypertension Am J Physiol Regulatory Integrative Comp Physiol, October 1, 2005; 289(4): R1169 - R1176. [Abstract] [Full Text] [PDF]
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D. D. Gutterman Mitochondria and Reactive Oxygen Species: An Evolution in Function Circ. Res., August 19, 2005; 97(4): 302 - 304. [Full Text] [PDF]
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T. Fujiki, H. Shimokawa, K. Morikawa, H. Kubota, M. Hatanaka, M.A. H. Talukder, T. Matoba, A. Takeshita, and K. Sunagawa Endothelium-Derived Hydrogen Peroxide Accounts for the Enhancing Effect of an Angiotensin-Converting Enzyme Inhibitor on Endothelium-Derived Hyperpolarizing Factor-Mediated Responses in Mice Arterioscler. Thromb. Vasc. Biol., April 1, 2005; 25(4): 766 - 771. [Abstract] [Full Text] [PDF]
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D. D. Gutterman, H. Miura, and Y. Liu Redox Modulation of Vascular Tone: Focus of Potassium Channel Mechanisms of Dilation Arterioscler. Thromb. Vasc. Biol., April 1, 2005; 25(4): 671 - 678. [Abstract] [Full Text] [PDF]
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P. V. G. Katakam, C. D. Tulbert, J. A. Snipes, B. Erdos, A. W. Miller, and D. W. Busija Impaired insulin-induced vasodilation in small coronary arteries of Zucker obese rats is mediated by reactive oxygen species Am J Physiol Heart Circ Physiol, February 1, 2005; 288(2): H854 - H860. [Abstract] [Full Text] [PDF]
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 C. Cseko, Z. Bagi, and A. Koller Biphasic effect of hydrogen peroxide on skeletal muscle arteriolar tone via activation of endothelial and smooth muscle signaling pathways J Appl Physiol, September 1, 2004; 97(3): 1130 - 1137. [Abstract] [Full Text] [PDF]
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B. Kalyanaraman and D. D. Gutterman Prologue: Vascular effects of free radicals Am J Physiol Heart Circ Physiol, December 1, 2003; 285(6): H2253 - H2254. [Full Text] [PDF]
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P < 0.05 Indo + H2O2 vs. H2O2.
