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(Hypertension. 2005;46:1175.)
© 2005 American Heart Association, Inc.
Original Articles |
From the Hypertension and Vascular Research Division (H.W., J.L.G., Y.R., O.A.C.), Henry Ford Hospital, Detroit, Mich; Department of Biochemistry (J.R.F.), University of Texas, Southwestern Medical Center, Dallas, Tex; and the Department of Biostatistics and Research Epidemiology (S.S.S.), Henry Ford Hospital, Detroit, Mich.
Correspondence to Oscar A. Carretero, MD, Division of Hypertension and Vascular Research, Henry Ford Hospital, 2799 W Grand Blvd, Detroit, MI 48202. E-mail ocarret1{at}hfhs.org
| Abstract |
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Key Words: glomerulus cyclooxygenase-1 20-hydroxyeicosatetraenoic acid epoxyeicosatrienoic acid renal vascular resistance
| Introduction |
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BK B2 receptors are located in both the glomerulus and the Ef-Art and are involved in the vasodilator effect of BK on the Ef-Art via epoxyeicosatrienoic acids (EETs).1,3,4 In the glomerulus, activation of the B2 receptor by BK is known to stimulate the phospholipase A2 pathway, generating arachidonic acid. Glomerular cyclooxygenase (COX) metabolizes arachidonic acid to prostaglandin (PG) H2. This is followed by further metabolism into bioactive PG E2 and PG I2 via PGE synthases and prostacyclin synthase, respectively. Both PGE2 and PGI2 have been shown to have a vasodilator effect on renal microvessels.59
Previously, we determined that when Ef-Arts are perfused retrograde, bypassing the glomerulus, neither nitric oxide synthase inhibitors nor COX inhibitors alter BK-induced Ef-Art dilatation.10 However, the vasodilator effect of BK was completely blocked by inhibiting the synthesis of EETs, demonstrating that these arachidonic acid metabolites of the cytochrome P450 pathway totally account for BK-induced dilatation. In contrast, when Ef-Arts were perfused orthograde via the glomerulus, inhibition of COX with indomethacin blocked the Ef-Art dilatation caused by BK. In the presence of indomethacin, 20-HEDE, an antagonist of hydroxyeicosatetraenoic acid (20-HETE), completely restored the vasodilator effect of BK when Ef-Arts were perfused orthograde. Thus we concluded that the glomerulus releases both vasodilator prostaglandins and the vasoconstrictor 20-HETE. However, inhibiting COX may shunt arachidonic acid normally metabolized by COX into the cytochrome P450 pathway, thereby enhancing production of 20-HETE and EETs.10 Furthermore, it was not clear from our previous reports whether glomerular prostaglandins are released via the COX-1 or COX-2 pathway. Here we tested whether in the absence of COX inhibition the vasoconstrictor 20-HETE released by the glomerulus plays a role in regulating Ef-Art tone. In addition, we examined whether the vasodilator prostaglandins released by the glomeruli in response to BK are generated by COX-1 or COX-2.
| Methods |
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Microdissection and cannulation were completed within 90 minutes at 8°C, after which the bath was gradually warmed to 37°C for the rest of the experiment. Once temperature was stable, a 30-minute equilibration period was allowed before any measurements were taken. Images of the Ef-Art were displayed and recorded at magnifications up to 1980x. Diameter was measured with an image analysis system.
BK and norepinephrine were purchased from Sigma. Based on our previous study, concentrations of 20-HEDE and MS-PPOH were 1 µmol/L.10 A stock solution of N-[2-(cyclohexyloxy)-4-nitrophenyl]-methanesulfonamide (NS-398; Cayman) was prepared in dimethyl sulfoxide (DMSO). The final DMSO concentration was 0.01%, which had no effect by itself. NS-398 reportedly has a half-maximal inhibitory concentration (IC50) of 0.05 µmol/L for human COX-2 and 0.15 µmol/L for ovine COX-2.11,12 SC-58560 was a generous gift from Pfizer and reportedly has an IC50 of 0.0048 for recombinant human COX-1.13
Protocols
Perfused preparations were first pretreated with norepinephrine (3 µmol/L) to reduce basal Ef-Art diameter by &40%, and then BK (0.01 to 10 nmol/L) was added to the lumen to generate a dose-response curve. Luminal diameter at the site of maximum response was measured immediately before adding BK and 10 minutes after adding each dose of BK. After a 20-minute washout period, a second BK dose-response curve was generated in the presence of a 20-HETE antagonist (20-HEDE), an EET synthesis inhibitor (MS-PPOH), and/or COX inhibitors (SC-58560 for COX-1 and NS-398 for COX-2).
Statistics
Values are expressed as mean±SE. ANOVA was used to assess overall change, with dose as the repeated measure. Each dose was compared with norepinephrine baseline using a paired t test. When significant interactions were found between dose and treatment group, paired t tests were performed to determine which dose(s) altered luminal diameter. Hochbergs method was used to adjust the
level of significance for multiple comparisons. P<0.05 was considered significant.
| Results |
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To test whether BK stimulates the release of glomerular 20-HETE in the absence of COX inhibition, we studied the effect of a selective 20-HETE antagonist, 20-HEDE, on BK-induced dilatation of the Ef-Art perfused orthograde through the glomerulus. 20-HEDE (1 µmol/L) added to the lumen and bath did not alter basal diameter; however, it significantly enhanced BK-induced dilatation (n=9; P<0.02, untreated versus 20-HEDE) (Figure 1). These data suggest that 20-HETE released from the glomerulus in response to BK plays a role in constricting the Ef-Art in the absence of COX inhibition.
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To study the role of EETs in BK-induced dilatation in the absence of COX inhibition, we examined the effect of adding the specific EET synthesis inhibitor MS-PPOH (1 µmol/L) to the lumen and bath. In the presence of MS-PPOH, BK-induced dilatation of the Ef-Art was significantly attenuated (n=6; P<0.02, untreated versus MS-PPOH) (Figure 2). We also examined the effect of MS-PPOH in the presence of 20-HEDE. Adding 1 µmol/L MS-PPOH to the lumen and bath together with 20-HEDE significantly attenuated the vasodilator effect of BK (n=6; P<0.01, with versus without MS-PPOH) (Figure 3). Taken together, these data indicate that EETs mediate at least part of the vasodilator effect of BK in the absence of COX inhibition.
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Previously, we have shown that prostaglandins released from the glomerulus also partially mediate the effect of BK on the Ef-Art. However, we did not study whether COX-1 or COX-2 was responsible. Consequently, we next tested the effect of a selective COX-1 inhibitor, SC-58560, on BK-induced dilatation. Ef-Arts were perfused orthograde in the presence of 20-HEDE and MS-PPOH. When 1 µmol/L SC-58560 was added in the presence of 20-HEDE and MS-PPOH, BK did not increase luminal diameter (n=6; P<0.001, with versus without SC-58560) (Figure 4). To test whether glomerular prostanoids released by COX-2 participate in BK-induced dilatation, we added 1 µmol/L NS-398 (a selective COX-2 inhibitor) to the lumen and bath in the presence of 20-HEDE and MS-PPOH. BK-induced dilatation of the Ef-Arts showed no difference with or without NS-398 (n=6). These data suggest that when the glomerulus is stimulated with BK, it releases vasodilator prostanoids via the COX-1 but not the COX-2 pathway.
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| Discussion |
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Previously, we found that when Ef-Arts are perfused retrograde, inhibition of either nitric oxide synthase or COX does not alter the vasodilator effect of BK, indicating that Ef-Arts do not release nitric oxide or prostaglandins in response to BK. In the present study, we found that when Ef-Arts were perfused orthograde in the presence of 20-HEDE and MS-PPOH, the vasodilator effect of BK was completely blocked by a selective COX-1 inhibitor (SC-58560), whereas COX-2 inhibition had no such effect. We conclude that glomerular COX-1 releases vasodilator prostaglandins in response to BK that partially mediate dilatation of the Ef-Art when it is perfused in a physiological manner (orthograde). The selectivity of the COX-1 inhibitor we used (SC-58560) has already been demonstrated.16 Using an in vitro perfused rat juxtamedullary nephron preparation, Imig and Deichmann reported that a selective COX-2 inhibitor, NS-398, did not affect the Af-Art response to angiotensin II, but enhanced the Af-Art vasoconstrictor response to norepinephrine.17 Based on our previous10 and present work, it appears that eicosanoids both mediate and antagonize the effects of BK in the Ef-Art. The EETs and prostaglandins that mediate the vasodilator effect of BK originate in the Ef-Art and the glomeruli, respectively, whereas the vasoconstrictor 20-HETE that antagonizes the vasodilator effect of BK originates mainly in the glomerulus. Whether EETs are also produced by the glomerulus cannot be established by our studies, because during retrograde perfusion the Ef-Art produces EETs that mediate BK-induced dilatation. In the kidney, arachidonic acid metabolites produced by cytochrome P450 enzymes play a major role in the regulation of not only renal vascular tone but also sodium transport, inflammation, and tubuloglomerular feedback.1820 Cytochrome P450 enzymes metabolize arachidonic acid to a series of EETs and HETEs.21 20-HETE is a potent constrictor of renal arterioles.22,23 The glomerulus produces 20-HETE, dihydroxyeicosatrienoic acids (diHETEs), and 12-HETE when incubated with arachidonic acid.24 On the other hand, EETs are vasodilators,25,26 which may be important in the control of glomerular hemodynamics. Imig et al27 demonstrated that when renal microvessels are stimulated by bradykinin, they produce EETs that in turn dilate the Af-Art. We have shown that when the Ef-Art is perfused orthograde via the glomerulus, BK induces release of 20-HETE and EETs, constrictor and dilator cytochrome P450 metabolites of arachidonic acid that mediate its effects both in the presence10 and absence of COX inhibition (this study). Thus, our data support the contention that these compounds are important regulators of the renal microcirculation.
COX enzymes are necessary for synthesis of prostaglandins from arachidonic acid. There are 2 different isoforms of COX, referred to as COX-1 and COX-2, which are encoded by different genes.28 Recent studies have demonstrated that both isoforms are present in the kidney.29 COX-1 is constitutively expressed in the glomerulus, the thick ascending limb of Henles loop, and the collecting duct,30 and is thought to be responsible for continuous generation of prostaglandins that regulate renal hemodynamics and tubular transport.31 Recent studies showed that PGE2 EP2 and EP4 receptors are located in both cortical preglomerular resistance vessels and outer medullary vasa recta bundles,32,33 and PGI2 IP receptor mRNA has been detected in both Af-Arts and vasa recta.34,35 These receptors might participate in the vasodilator effect of COX-1derived prostaglandins on the Ef-Art. Consistent with previous reports, we conclude from our present data that the vasodilator prostaglandins that mediate BK-induced dilatation of the Ef-Art are produced by COX-1 rather than COX-2.
Perspectives
BK is considered to be an endothelial nitric oxide-dependent vasodilator; however, clearly in the Ef-Art its vasodilator effect is mediated not by nitric oxide but rather by a balance between arachidonic acid metabolites of cytochrome P450 and COX-1 that are vasoconstrictors and vasodilators. The eicosanoids released by the glomerulus and Ef-Art may be important in regulating not only Ef-Art tone but also glomerular filtration rate by altering resistance in the Ef-Art and consequently glomerular capillary pressure. One could speculate that eicosanoids released by the glomerulus and Ef-Art may regulate not only Ef-Art resistance but also the downstream renal circulation, and thereby participate in the regulation of water and sodium excretion. The present studies illustrate the complexity of the pathways that regulate the renal microcirculation and suggest that inhibition of eicosanoid synthesis may have unexpected effects on renal function.
| Acknowledgments |
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Received May 18, 2005; first decision June 3, 2005; accepted September 13, 2005.
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