AT1 receptor antagonist combats oxidative stress and restores nitric oxide signaling in the SHR
2001; Elsevier BV; Volume: 59; Issue: 4 Linguagem: Inglês
10.1046/j.1523-1755.2001.0590041257.x
ISSN1523-1755
AutoresWilliam J. Welch, Christopher S. Wilcox,
Tópico(s)Renin-Angiotensin System Studies
ResumoAT1 receptor antagonist combats oxidative stress and restores nitric oxide signaling in the SHR. The tubuloglomerular feedback (TGF) responses of the spontaneously hypertensive rat (SHR) are under exaggerated regulation by angiotensin II (Ang II) type 1 receptors (AT1-R). Since AT1-Rs enhance oxygen radical (O2-) generation, we tested the hypothesis that the exaggerated TGF was due to a diminished blunting by macula densa (MD)-derived nitric oxide (NO) because of excessive AT1-R–dependent generation of O2-. Groups of SHR and control Wistar-Kyoto (WKY) rats received vehicle (Veh), the AT1-R antagonist candesartan (Cand; 3 mg · kg-1 · day-1), or nonspecific therapy with hydralazine + hydrochlorothiazide + reserpine (HHR) for two weeks. Compared with WKY rats, the elevated mean arterial pressure of SHR (WKY 125 ± 2 vs. SHR 163 to 779 mm Hg, P < 0.001) was reduced (P < 0.001) similarly in SHR by Cand and HHR (121 ± 5 and 116 ± 5 mm Hg, P = NS). The SHR had an increased maximal TGF response (change in stop flow pressure during luminal perfusion of fluid: SHR 11.2 ± 0.5 vs. WKY 8.3 ± 0.4 mm Hg, P < 0.01) and a reduced TGF response to blockade of neuroneal NO synthase (nNOS) in the MD with luminal 7-nitroindazole (7-NI: ΔTGF in WKY 2.8 ± 0.4 vs. SHR 1.1 ± 0.6 mm Hg, P < 0.05). Although the elevated TGF responses of SHR were normalized by both HHR and Cand, only Cand restored a normal TGF response to luminal perfusion of the MD with 7-NI (ΔTGF with 7-NI in SHR: Veh + 1.8 ± 0.4 vs. Cand + 3.4 ± 0.5 mm Hg, P < 0.05). To abrogate the local effects of O2-, tempol (a membrane-permeable superoxide dismutase mimetic) was perfused into the efferent arteriole. During tempol, SHR given vehicle or HHR had a much increased response to blockade of nNOS with 7-NI (ΔTGF in SHR with 7-NI during tempol: Veh 6.3 ± 1.0 and HHR 4.5 ± 0.8 mm Hg, P < 0.01 vs. no tempol for both), implying that the effects of NO had been prevented because of excessive O2-. In contrast, the TGF response to 7-NI in SHR given Cand was unaffected by tempol (ΔTGF with 7-NI during tempol: 2.9 ± 0.9, P = NS, compared with no tempol). In conclusion, TGF responses of SHR are exaggerated because of the effects of hypertension and AT1-R. AT1-R blockade specifically diminishes oxidative stress and restores NO signaling in the juxtaglomerular apparatus of the SHR. AT1 receptor antagonist combats oxidative stress and restores nitric oxide signaling in the SHR. The tubuloglomerular feedback (TGF) responses of the spontaneously hypertensive rat (SHR) are under exaggerated regulation by angiotensin II (Ang II) type 1 receptors (AT1-R). Since AT1-Rs enhance oxygen radical (O2-) generation, we tested the hypothesis that the exaggerated TGF was due to a diminished blunting by macula densa (MD)-derived nitric oxide (NO) because of excessive AT1-R–dependent generation of O2-. Groups of SHR and control Wistar-Kyoto (WKY) rats received vehicle (Veh), the AT1-R antagonist candesartan (Cand; 3 mg · kg-1 · day-1), or nonspecific therapy with hydralazine + hydrochlorothiazide + reserpine (HHR) for two weeks. Compared with WKY rats, the elevated mean arterial pressure of SHR (WKY 125 ± 2 vs. SHR 163 to 779 mm Hg, P < 0.001) was reduced (P < 0.001) similarly in SHR by Cand and HHR (121 ± 5 and 116 ± 5 mm Hg, P = NS). The SHR had an increased maximal TGF response (change in stop flow pressure during luminal perfusion of fluid: SHR 11.2 ± 0.5 vs. WKY 8.3 ± 0.4 mm Hg, P < 0.01) and a reduced TGF response to blockade of neuroneal NO synthase (nNOS) in the MD with luminal 7-nitroindazole (7-NI: ΔTGF in WKY 2.8 ± 0.4 vs. SHR 1.1 ± 0.6 mm Hg, P < 0.05). Although the elevated TGF responses of SHR were normalized by both HHR and Cand, only Cand restored a normal TGF response to luminal perfusion of the MD with 7-NI (ΔTGF with 7-NI in SHR: Veh + 1.8 ± 0.4 vs. Cand + 3.4 ± 0.5 mm Hg, P < 0.05). To abrogate the local effects of O2-, tempol (a membrane-permeable superoxide dismutase mimetic) was perfused into the efferent arteriole. During tempol, SHR given vehicle or HHR had a much increased response to blockade of nNOS with 7-NI (ΔTGF in SHR with 7-NI during tempol: Veh 6.3 ± 1.0 and HHR 4.5 ± 0.8 mm Hg, P < 0.01 vs. no tempol for both), implying that the effects of NO had been prevented because of excessive O2-. In contrast, the TGF response to 7-NI in SHR given Cand was unaffected by tempol (ΔTGF with 7-NI during tempol: 2.9 ± 0.9, P = NS, compared with no tempol). In conclusion, TGF responses of SHR are exaggerated because of the effects of hypertension and AT1-R. AT1-R blockade specifically diminishes oxidative stress and restores NO signaling in the juxtaglomerular apparatus of the SHR. angiotensin II artificial plasma angiotensin II type 1 angiotensin II type 1 receptor artificial tubular fluid candesartan efferent arteriole glomerular filtration rate hydralazine + hydrochlorothiazide + reserpine heart rate juxtaglomerular apparatus loop of Henle mean arterial pressure macula densa 7-nitroindazole neuronal nitric oxide synthase oxygen radical proximal stop flow pressure spontaneously hypertensive rat superoxide dismutase tubuloglomerular feedback vehicle Wistar Kyoto rat Hypertension is accompanied by oxidative stress of large conduit blood vessels and certain organs, including the kidney1McIntyre M. Bohr D.F. Dominiczak A.F. Endothelial function in hypertension.Hypertension. 1999; 34: 539-545Crossref PubMed Scopus (304) Google Scholar. Presently, neither the cause of the oxidative stress nor its role in the regulation of the tone of the major renal resistance vessels is clear. Studies with a cell membrane-permeable form of superoxide dismutase (SOD) or with the nitroxide SOD mimetic tempol have implicated oxygen radicals (O2-) in the hypertension2Schnackenberg C. Wilcox C.S. Two-week administration of tempol attenuates both hypertension and renal excretion of 8-iso prostaglandin F2α.Hypertension. 1999; 33: 424-428Crossref PubMed Google Scholar,3Nakazono K. Watanabe N. Matsuno K. Does superoxide underlie the pathogenesis of hypertension?.Proc Natl Acad Sci USA. 1991; 88: 10045-10048Crossref PubMed Scopus (675) Google Scholar, renal vasoconstriction4Schnackenberg C.G. Welch W.J. Wilcox C.S. Normalization of blood pressure and renal vascular resistance in SHR with a membrane-permeable superoxide dismutase mimetic: Role of nitric oxide.Hypertension. 1998; 32: 59-64Crossref PubMed Scopus (453) Google Scholar, and enhanced tubuloglomerular feedback (TGF)-dependent tone of the renal afferent arteriole5Welch W.J. Tojo A. Wilcox C.S. Roles of NO and oxygen radicals in tubuloglomerular feedback in SHR.Am J Physiol. 2000; 278: F769-F776PubMed Google Scholar of the spontaneously hypertensive rat (SHR) model. Angiotensin II (Ang II) induces oxidative stress, as indicated by enhanced levels of 8-iso prostaglandin F2α (8-iso-PGF2α)6Romero J.C. Reckelhoff J.F. Role of angiotensin and oxidative stress in essential hypertension.Hypertension. 1999; 34: 943-949Crossref PubMed Google Scholar. There is experimental evidence of excessive Ang II type 1 receptor (AT1-R)-dependent vasoconstriction specifically in the kidney of the SHRs. Thus, although the circulating levels of Ang II and intrarenal levels of Ang I, Ang II, and Ang 1–7 are normal or low in the SHR7Campbell D.J. Duncan A.-M. Kladis A. Angiotensin peptides in spontaneously hypertensive and normotensive donryu rats.Hypertension. 1995; 25: 928-934Crossref PubMed Scopus (76) Google Scholar, there is an exaggerated blood pressure (BP) response to renin-angiotensin-aldosterone system (RAAS) blockade8Wood J.M. Mah S.C. Schnell C. Comparison of the acute hypotensive effects of renin inhibition, converting enzyme inhibition, and angiotensin II antagonsism in rats.J Cardiovasc Pharmacol. 1990; 16: S60-S64Crossref PubMed Scopus (32) Google Scholar and an exaggerated renal vasoconstrictor response to Ang II9Chatziantoniou C. Arendshorst W.J. Angiotensin and thromboxane in genetically hypertensive rats: Renal blood flow and receptor studies.Am J Physiol. 1991; 261: F238-F247Google Scholar,10Kost C.K. Herzer W.A. Jackson E.K. Vascular reactivity to angiotensin II is selectively enhanced in the kidneys of spontaneously hypertensive rats.J Pharmacol Exp Ther. 1994; 269: 82-88PubMed Google Scholar that cannot be ascribed to enhanced renal or glomerular expression of AT1 or AT2 receptors9Chatziantoniou C. Arendshorst W.J. Angiotensin and thromboxane in genetically hypertensive rats: Renal blood flow and receptor studies.Am J Physiol. 1991; 261: F238-F247Google Scholar, 11Wu J.-N. Edwards D. Berecek K.H. Changes in renal angiotensin II receptors in spontaneously hypertensive rats by early treatment with the angiotensin-converting enzyme inhibitor captopril.Hypertension. 1994; 23: 819-822Crossref PubMed Google Scholar, 12Chatziantoniou C. Arendshorst W.J. Angiotensin receptor sites in renal vasculature of rats developing genetic hypertension.Am J Physiol. 1993; 265: F853-F862PubMed Google Scholar. The enhanced response to Ang II is specific for the kidney10Kost C.K. Herzer W.A. Jackson E.K. Vascular reactivity to angiotensin II is selectively enhanced in the kidneys of spontaneously hypertensive rats.J Pharmacol Exp Ther. 1994; 269: 82-88PubMed Google Scholar and may underlie an enhanced hypertensinogenic action of Ang II in the SHR13Li P. Jackson E.K. Enhanced slow-pressor response to angiotensin II in spontaneously hypertensive rats.J Pharmacol Exp Ther. 1989; 251: 909-921PubMed Google Scholar. Nitric oxide (NO) generated from a neuroneal or type 1 constitutive nitric oxide synthase (nNOS) that is heavily expressed in macula densa (MD) cells is activated during MD solute reabsorption. It normally blunts the expression of the TGF response14Wilcox C.S. Welch W.J. Schmidt H.H.H.W. Nitric oxide synthase in macula densa regulates glomerular capillary pressure.Proc Natl Acad Sci USA. 1992; 89: 11993-11997Crossref PubMed Scopus (464) Google Scholar. We found evidence that production of O2- in the juxtaglomerular apparatus (JGA) opposes the normal buffering of TGF by MD-derived NO15Wilcox C.S. Welch W.J. Interactions between nitric oxide and oxygen radicals in regulation of tubuloglomerular feedback.Acta Physiol Scand. 2000; 168: 119-124Crossref PubMed Google Scholar. This effect of O2- is exaggerated in the JGA of the SHR5Welch W.J. Tojo A. Wilcox C.S. Roles of NO and oxygen radicals in tubuloglomerular feedback in SHR.Am J Physiol. 2000; 278: F769-F776PubMed Google Scholar. The functional effects of O2- could be corrected by microperfusion of tempol, which is a membrane-permeable nitroxide SOD mimetic16Mitchell J.B. Samuni A. Krishna M.C. Biologically active metal-independent superoxide dismutase mimetics.Biochemistry. 1990; 29: 2802-2807Crossref PubMed Scopus (427) Google Scholar,17Mitchell J.B. De Graff W. Kaufman D. Inhibition of oxygen-dependent radiation-induced damage by the nitroxide superoxide dismutase mimetic, tempol.Arch Biochem Biophys. 1991; 289: 62-70Crossref PubMed Scopus (212) Google Scholar into the efferent arteriole (EA) that supplies the peritubular capillaries and the interstitium around the MD region. Oxidative stress is enhanced by Ang II acting on AT1 receptors18Pagano P.J. Chanock S.J. Siwik D.A. Angiotensin II induces p67phox mRNA expression and NADPH oxidase superoxide generation in rabbit aortic adventitial fibroblasts.Hypertension. 1998; 32: 331-337Crossref PubMed Scopus (198) Google Scholar, 19Bachmann S. Ramasubbu K. Immunohistochemical colocalization of the α-subunit of neutrophil NADPH oxidase and ecto-5′-nucleotidase in kidney and liver.Kidney Int. 1997; 51: 479-482Abstract Full Text PDF PubMed Scopus (29) Google Scholar, 20Bosse H.M. Bachmann S. Immunohistochemically detected protein nitration indicates sites of renal nitric oxide release in Goldblatt hypertension.Hypertension. 1997; 30: 948-952Crossref PubMed Scopus (65) Google Scholar. Recent studies have shown that short-term AT1-R blockade can restore a normal TGF response in the SHR21Brannstrom K. Morsing P. Arendshorst W.J. Candesartan normalizes exaggerated tubuloglomerular feedback activity in young spontaneously hypertensive rats.J Am Soc Nephrol. 1999; 10: S213-S219Google Scholar. Therefore, the present studies were designed to test the hypothesis that AT1-R–dependent oxidative stress in the JGA of the SHR model eclipses the blunting of the TGF response by MD-derived NO. These experiments utilized groups of SHR and control Wistar-Kyoto (WKY) normotensive rats treated for two weeks with a vehicle or with a long-acting AT1-R antagonist, candesartan. Additional groups received equi-antihypertensive therapy with hydralazine, hydrochlorothiazide, and reserpine (HHR) that was designed to lower BP similarly to candesartan but leave intact the AT1-R and stimulate the RAAS22Anderson S. Rennke H.G. Brenner B.M. Therapeutic advantage of converting enzyme inhibitors in arresting progressive renal disease associated with systemic hypertension in the rat.J Clin Invest. 1986; 77: 1993-2000Crossref PubMed Scopus (920) Google Scholar. The effects of MD-derived NO were deduced from the responses to luminal microperfusions of the relatively nNOS-specific antagonist 7-nitroindazole (7-NI)23Wolff D.J. Gribin B.J. The inhibition of the constitutive and inducible nitric oxide synthase isoforms by indazole agents.Arch Biochem Biophys. 1994; 311: 300-306Crossref PubMed Scopus (101) Google Scholar. The effects of oxidative stress mediated by O2- in preventing the blunting of TGF by MD-derived NO were deduced from the effects of microperfusion of tempol into the EA supplying the test nephron on the TGF response to luminal microperfusion of 7-NI. These experiments obviate the confounding effects produced by systemic administration of these antagonists and directly target the JGA. Studies were undertaken on male SHRs and WKY rats whose initial weights were 200 to 250 g (obtained from Harlan-Sprague-Dawley, Madison, WI, USA). They were maintained on a standard rat chow (Purina, St. Louis, MO, USA) with a sodium content of 0.3 g-1 · 100 g-1 for one to two weeks before study. They were allowed free access to food and water until the day of study. Groups of SHRs and age-matched WKY rats were prepared for in vivo clearance, micropuncture, microperfusion, and TGF studies as described in detail previously24Welch W.J. Wilcox C.S. Potentiation of tubuloglomerular feedback in the rat by thromboxane mimetic: Role of macula densa.J Clin Invest. 1992; 89: 1857-1865Crossref PubMed Scopus (71) Google Scholar. In brief, animals were anesthetized with thiobarbital (Inactin, 100 mg · kg-1 intraperitoneally; Research Biochemicals, Inc., Natick, MA, USA). A catheter was placed in a jugular vein for fluid infusion and in a femoral artery for the recording of mean arterial pressure (MAP) from the electrically damped output of a pressure transducer (Statham, Gould Inc., Oxnard, CA, USA). A tracheotomy tube was inserted. The animals were allowed to breathe spontaneously. A catheter was placed in the bladder to collect urine from both kidneys. The left kidney was exposed by a flank incision, cleaned of connective tissue, and stabilized in a Lucite cup. It was bathed in 0.154 mol/L NaCl maintained at 37°C. After completion of surgery, rats were infused with a solution of 0.154 mol/L NaCl and 1% albumin at 1.5 mL · h-1 to maintain a euvolemic state24Welch W.J. Wilcox C.S. Potentiation of tubuloglomerular feedback in the rat by thromboxane mimetic: Role of macula densa.J Clin Invest. 1992; 89: 1857-1865Crossref PubMed Scopus (71) Google Scholar. Micropuncture studies were begun after 60 minutes for stabilization. Clearance studies were undertaken during the micropuncture measurements. [3H]-inulin (0.1 μCi · mL-1; ICN Biochemicals, Costa Mesa, CA, USA) was added to the intravenous fluid infusion. Urine was obtained from the bladder, and blood was sampled at the midpoint. The micropuncture period lasted 60 to 90 minutes, during which a renal clearance was undertaken with a blood sample drawn at the midpoint. Values of MAP and heart rate (HR) were obtained over 15-minute intervals during this period and were averaged. For orthograde microperfusion of the loop of Henle (LH), a micropipette (8 μm OD) containing artificial tubular fluid (ATF) stained with FD&C dye was inserted into a late proximal tubule24Welch W.J. Wilcox C.S. Potentiation of tubuloglomerular feedback in the rat by thromboxane mimetic: Role of macula densa.J Clin Invest. 1992; 89: 1857-1865Crossref PubMed Scopus (71) Google Scholar. Injections of the colored ATF identified the nephron and the direction of flow. An immobile bone wax block was inserted into this micropuncture site via a micropipette (10 to 15 μm) connected to a hydraulic drive (Trent Wells, Inc., La Jolla, CA, USA) to halt tubular fluid flow. A perfusion micropipette (6 to 8 μm) containing ATF with test compounds or vehicle was inserted into the proximal tubule downstream from the wax block and connected to a nanoliter perfusion pump (WPI, Sarasota, FL, USA). A pressure micropipette (1 to 2 μm) was inserted into the proximal tubule upstream from the wax block to measure proximal stop flow pressure (PSF). The pressure was recorded by a servo-null instrument (Instruments for Physiology and Medicine, La Jolla, CA, USA). Changes in PSF are an index of changes in glomerular capillary hydraulic pressure. Measurements of PSF were made in each nephron during zero loop perfusion and during perfusion with ATF at 40 nL · min-1. This produces a maximal TGF response, defined as the difference between PSF values recorded during perfusion of the loop with ATF at 0 and 40 nL · min-1. Maximal rat TGF responses were determined in one to three nephrons. Measurements were made during perfusion of the LH with ATF + vehicle and were contrasted with ATF + 7-NI (10-4 mol/L). We had found previously that this dose of 7-NI was maximally effective and produced a rapid and reversible increase in TGF responses25Welch W.J. Tojo A. Lee J.-U. Nitric oxide synthase in the JGA of the SHR. expression and role in tubuloglomerular feedback.Am J Physiol. 1999; 277: F130-F138PubMed Google Scholar. In separate groups of rats, the EA supplying the test nephron was perfused at 15 nL · min-1 with artificial plasma (AP) + tempol (10-4 mol/L)24Welch W.J. Wilcox C.S. Potentiation of tubuloglomerular feedback in the rat by thromboxane mimetic: Role of macula densa.J Clin Invest. 1992; 89: 1857-1865Crossref PubMed Scopus (71) Google Scholar 2,2,6,6-tetramethyl-1-piperidinyloxyl2Schnackenberg C. Wilcox C.S. Two-week administration of tempol attenuates both hypertension and renal excretion of 8-iso prostaglandin F2α.Hypertension. 1999; 33: 424-428Crossref PubMed Google Scholar, 16Mitchell J.B. Samuni A. Krishna M.C. Biologically active metal-independent superoxide dismutase mimetics.Biochemistry. 1990; 29: 2802-2807Crossref PubMed Scopus (427) Google Scholar, 17Mitchell J.B. De Graff W. Kaufman D. Inhibition of oxygen-dependent radiation-induced damage by the nitroxide superoxide dismutase mimetic, tempol.Arch Biochem Biophys. 1991; 289: 62-70Crossref PubMed Scopus (212) Google Scholar and stained with FD&C green dye. After five minutes, with the EA microperfusion continuing, the TGF was tested with luminal perfusions of ATF + vehicle and ATF + 7-NI at 0 and 40 nL · min-1. The order of luminal perfusions was randomized. We have shown previously that EA perfusion with AP at 15 to 20 nL · min-1 does not perturb the TGF responses24Welch W.J. Wilcox C.S. Potentiation of tubuloglomerular feedback in the rat by thromboxane mimetic: Role of macula densa.J Clin Invest. 1992; 89: 1857-1865Crossref PubMed Scopus (71) Google Scholar. Only nephrons in which the surrounding interstitium was tinted with the colored AP perfusion solution were included in the study. For 14 to 18 days, groups (N = 6 to 8) of SHRs and WKY rats received a daily gavage with vehicle (Veh), candesartan (Cand; 3 mg · kg-1 · day-1) or hydralazine + hydrochlorothiazide + reserpine (HHR; 30 + 10 + 0.2 mg · kg-1 · day-1). Drugs were withheld on the day of study. The soluble salt of 7-NI was obtained from Tocris Cookson, Inc. (St. Louis, MO, USA). Tempol was obtained from Aldrich Chemical Co. (Milwaukee, WI, USA). Other chemicals were obtained from Sigma (St. Louis, MO, USA). Values are reported as mean ± SEM. A repeated-measures analysis of variance (ANOVA) was applied to data of SHRs and WKY rats administered vehicle, candesartan, or HHR. Where appropriate, a post hoc Dunnett's t test was applied thereafter. Values are taken as statistically significant if P < 0.05. Data for whole kidney function in rats of series 1 are shown in Table 1. Data for series 2 were strictly similar, but clearance measurements of glomerular filtration rate (GFR) were made only in series 1. The body weights for WKY rats at the time of study were significantly higher than SHR, reflecting their better rate of growth. The MAP was increased in the vehicle-administered SHR compared with equivalent WKY. Both Cand and HHR reduced the MAP of the SHRs to levels that were not significantly different between the two treatments or from the values of WKY rats given vehicle. Therefore, both treatments effectively normalized the MAP of the SHR. These drugs also reduced the MAP of WKY rats. The HRs and rates of urine flow were similar between groups. The mean values for the GFR were significantly (P < 0.05) lower by 31% in vehicle-treated SHRs compared with WKY rats. The GFR of SHRs was not modified by treatment with HHR but was increased significantly (P < 0.05) by Cand to levels of vehicle-treated WKY rats.Table 1Body weights, mean arterial pressure (MAP), heart rate (HR), urine flow (UV) and glomerular filtration rate (GFR)GroupNBody wt gMAP mm HgHR beats·min-1UV μL·min-1·100 g-1GFR mL·min-1·100 g-11. WKY, Veh6358±17125±2368±51.7±0.30.73±0.062. WKY, HHR5342±1298±5355±81.8±0.30.70±0.07 P values vs. #1NS<0.01NSNSNS3. WKY, Cand6358±12103±2363±52.1±0.40.79±0.08 P values vs. #1NS<0.01NSNSNS4. SHR, Veh6296±10163±9370±112.0±0.40.51±0.08 P values vs. #1<0.01<0.01NSNS<0.055. SHR, HHR5307±9116±5375±121.7±0.30.54±0.10 P values vs. #4NS<0.01NSNSNS6. SHR, Cand6285±10121±5362±41.9±0.30.79±0.09 P values vs. #4NS<0.01NSNS<0.05Values are mean ± SEM (N = number of rats studied) for rats of series 1. Abbreviations are in the Appendix. Open table in a new tab Values are mean ± SEM (N = number of rats studied) for rats of series 1. Abbreviations are in the Appendix. Tubuloglomerular feedback parameters are shown in Table 2. The PSF at zero LH perfusion was elevated (P < 0.05) by approximately 2 mm Hg in hypertensive SHR given vehicle compared with equivalent WKY rats. This was anticipated from a prior study that had shown that the PSF at zero loop perfusion is dependent on the BP26Schnermann J. Briggs J.P. Effects of angiotensin and other pressor agents on tubuloglomerular feedback responses.Kidney Int. 1990; 38: S77-S80Google Scholar. The maximal TGF responses during loop perfusion with ATF + vehicle were enhanced significantly (P < 0.001) by 35% in vehicle-administered SHR compared with comparable WKY. Both Cand and HHR reduced the exaggerated TGF responses of SHR significantly (P < 0.001) to values comparable to the vehicle-treated WKY, whereas these treatments did not modify the TGF responses of the WKY.Table 2Proximal stop flow pressure (PSF) during luminal microperfusion of artificial tubular fluid (ATF) containing vehicle (Veh) or 7-nitroindazole (7-NI)PSF (mm Hg) during LH microperfusion at (nL·min-1)GroupAdded to ATF0400–40WKY1. WKY, Veh (N=6)Veh38.5±0.430.2±0.88.3±0.47-NI38.2±0.427.1±0.611.2±0.4 Δ+0.1±0.3-2.9±0.9+2.8±0.4 P valueNS<0.01<0.0012. WKY, HHR (N=5)Veh37.5±1.029.0±1.08.4±0.87-NI38.0±1.126.6±0.711.2±0.8 Δ+0.4±0.4-2.4±0.8+2.8±0.6 P valueNS<0.001<0.0013. WKY, Cand (N=6)Veh36.8±0.528.7±0.57.8±0.37-NI36.8±0.626.5±0.810.3±0.5 Δ0.0±0.3-2.5±0.7+2.4±0.4 P valueNS<0.001<0.001SHR4. SHR, Veh (N=6)Veh40.0±0.829.2±0.510.9±0.77-NI40.0±0.727.5±0.712.0±0.7 Δ-0.2±0.3-1.7±0.7+1.1±0.6 P valueNS<0.05NS5. SHR, HHR (N=5)Veh37.2±0.729.7±0.67.3±0.77-NI37.2±0.828.1±0.79.1±0.6 Δ0.0±0.3-1.5±0.4+1.8±0.4 P valueNS<0.001<0.016. SHR, Cand (N=6)Veh37.3±0.829.0±0.57.8±0.57-NI37.2±0.826.0±0.411.4±0.7 Δ+0.1±0.3-3.0±0.7+3.4±0.5 P valueNS<0.001<0.001Data are mean ± SEM (N = number of rats studied). ATF is artificial tubular fluid perfused orthograde through the loop of Henle. Open table in a new tab Data are mean ± SEM (N = number of rats studied). ATF is artificial tubular fluid perfused orthograde through the loop of Henle. The role of MD-derived NO in blunting TGF responses was assessed from the increase in TGF during blockade of nNOS by luminal microperfusion of 7-NI Table 2. Whereas WKY had a consistent increase in maximal TGF responses with 7-NI of 2.8 ± 0.4 mm Hg, there was a significantly (P < 0.05) blunted rise of 1.1 ± 0.6 mm Hg in vehicle-treated SHR. The increase in TGF with 7-NI was not increased significantly above vehicle treatment in WKY rats given HHR (2.8 ± 0.6 mm Hg) or Cand (2.4 ± 0.4 mm Hg). SHRs receiving HHR had only a modest increase in TGF with 7-NI of 1.8 ± 0.4 mm Hg that was not significantly different from the increase in vehicle-administered SHRs of 1.1 ± 0.6 mm Hg. However, SHRs receiving Cand had an increase in TGF with luminal 7-NI of 3.4 ± 0.5 mm Hg that was significantly (P < 0.05) greater than in SHRs receiving HHR and significantly (P < 0.01) greater than in SHRs receiving vehicle. Cand effectively normalized the TGF response to 7-NI; during candesartan, the response of SHRs to 7-NI was not significantly different from vehicle-treated WKY rats. We conclude that SHRs have a diminished blunting of TGF by MD-derived NO that is restored in full by candesartan, but not by HHR. Table 3 details the TGF responses to loop perfusion of ATF + 7-NI compared with ATF + vehicle during EA perfusion of tempol. As described previously, tempol blunted TGF responses to ATF + vehicle more in SHR than WKY rat nephrons5Welch W.J. Tojo A. Wilcox C.S. Roles of NO and oxygen radicals in tubuloglomerular feedback in SHR.Am J Physiol. 2000; 278: F769-F776PubMed Google Scholar. We interpret any effect of tempol to increase the effectiveness of 7-NI in enhancing TGF to represent an effect of short-term abrogation of oxidative stress to restore NO signaling in the JGA. As shown in Figure 1, the increases in TGF produced by luminal 7-NI in groups of WKY administered vehicle, HHR, or Cand were not significantly different from those seen in the absence of tempol. In contrast, in vehicle-treated SHR microperfused with tempol, there was a robust enhancement of TGF by 7-NI of 6.3 ± 1.0 mm Hg that was sixfold greater (P < 0.001) than the response to 7-NI of 1.1 ± 0.6 mm Hg seen in the absence of tempol. During the administration of HHR, SHR nephrons microperfused with tempol had an increase in TGF with luminal 7-NI of 4.5 ± 0.8 mm Hg that was threefold greater (P < 0.01) than the increase of 1.8 ± 0.4 mm Hg seen without tempol. This increase in TGF with 7-NI during tempol in SHRs treated with HHR was not significantly different from SHRs treated with vehicle. In contrast, during the administration of Cand, SHR nephrons microperfused with tempol had an increase in TGF with 7-NI of 2.9 ± 0.9 mm Hg that was not significantly different from the increase of 3.4 ± 0.5 mm Hg seen without tempol. We conclude that the effects of O2- to prevent blunting of TGF responses by NO were exaggerated in the SHRs. This effect was prevented in full only by candesartan.Table 3Proximal stop flow pressure (PSF) to luminal perfusion of artificial tubular fluid (ATF) containing vehicle (Veh) or 7-nitroindazole (7-NF) during microperfusion of tempol into the efferent arteriolePSF (mm Hg) during LH microperfusion at (nL·min-1)GroupAdded to ATF0400–40WKY1. WKY, Veh (N=6)Veh39.4±0.532.6±0.37.0±0.37-NI39.5±0.527.4±0.910.2±0.5 Δ+0.1±0.3-5.3±0.9+3.2±0.5 P value—0.0010.0012. WKY, HHR (N=6)Veh38.2±0.531.8±0.36.4±0.47-NI38.6±0.629.1±0.49.6±0.4 Δ+0.5±0.3-2.6±0.5+3.2±0.5 P valueNS0.0010.0013. WKY, Cand (N=8)Veh38.9±0.432.1±0.36.9±0.57-NI38.7±0.329.2±0.49.6±0.4 Δ-0.2±0.3-2.9±0.6+3.2±0.7 P valueNS0.0010.001SHR4. SHR, Veh (N=7)Veh41.1±0.433.4±0.57.5±0.57-NI40.9±0.527.0±0.613.8±0.4 Δ-0.2±0.3-6.5±0.7+6.3±1.0 P valueNS0.0010.0015. SHR, HHR (N=7)Veh39.7±0.732.9±0.56.9±0.57-NI39.2±1.127.8±0.511.5±1.0 Δ-0.5±0.3-3.1±0.5+4.5±0.8 P valueNS0.0010.0016. SHR, Cand (N=9)Veh37.2±0.929.7±0.57.9±0.57-NI37.9±0.927.2±0.410.7±0.4 Δ+0.7±0.5-2.6±0.5+2.9±0.9 P valueNS0.0010.001Data are mean ± SEM values (N = number of rats studied). ATF is the artificial tubular fluid perfused orthograde and through the loop of Henle. Open table in a new tab Data are mean ± SEM values (N = number of rats studied). ATF is the artificial tubular fluid perfused orthograde and through the loop of Henle. Brannstrom, Morsing, and Arendshorst reported that a single intravenous bolus of candesartan normalizes the exaggerated TGF responses of young SHR rat nephrons21Brannstrom K. Morsing P. Arendshorst W.J. Candesartan normalizes exaggerated tubuloglomerular feedback activity in young spontaneously hypertensive rats.J Am Soc Nephrol. 1999; 10: S213-S219Google Scholar. This was confirmed in the present study, which utilized adult SHRs that were administered candesartan over two weeks. In the prior study, candesartan normalized both the enhanced TGF-induced changes in PSF and SNGFR, and the responsiveness and the set point (sensitivity) of the TGF response. Our results extend the prior evidence for an exaggerated role for AT1-R in the kidneys of the SHR to the JGA8Wood J.M. Mah S.C. Schnell C. Comparison of the acute hypotensive effects of renin inhibition, converting enzyme inhibition, and angiotensin II antag
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