(Hypertension. 1997;30:948-952.)
© 1997 American Heart Association, Inc.
Articles |
From the Department of Anatomy, University of Heidelberg, Heidelberg, and the Department of Anatomy, Charité, Humboldt University, D-10098 Berlin, Germany.
Correspondence to Prof Dr S. Bachmann, A.G. Anatomie der HUB, Klinikum Charlottenburg, Haus 31, Spandauer Damm 130, D-14050 Berlin, Germany. E-mail sbachm{at}ukrv.de
| Abstract |
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Key Words: kidney macula mesangium nitric oxide nitrotyrosine
| Introduction |
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The present study was undertaken to demonstrate the release of and to localize possible pathways for NO at the juxtaglomerular apparatus and in the renal vasculature. The 2K1C Goldblatt model of renovascular hypertension has been chosen for investigation, since previous studies had revealed differential stimulation of both constitutive NOS isoforms.7
| Methods |
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To achieve renovascular hypertension by a unilateral stenosis of the renal artery, a total of 14 rats initially weighing 150±10.6 g (mean±SD) were used. Animals were divided into two groups, ie, a "clip" group that received a renal stenosis during 40 days (n=7) and an age-matched, sham-operated control group (n=7). For surgery, animals were anesthetized with ether, and then the right renal artery was isolated through flank incision; a 0.2-mm silver clip was placed in the animals designated for clipping. In sham-operated animals the right renal artery was partially exposed by removal of surrounding fat and connective tissue. Systolic arterial pressure was measured under light ether anesthesia using the tail-cuff method. Blood pressure values varied between 105 and 140 mm Hg in the sham-operated control group and 190 and 260 mm Hg in the clip group at the end of the experiment. All animals received standard laboratory chow and tap water ad libitum. Final body weights were 374.4±22.3 g for the control group and 325.4±28.3 g for the clip group.
For histochemical evaluation, kidneys were in vivo perfusion-fixed. Rats were anesthetized by pentobarbital sodium (40 mg/kg IP), and perfusion was performed for 90 seconds by cannulation of the abdominal aorta at a pressure of 220 mm Hg with 3% freshly prepared paraformaldehyde in PBS (pH 7.4). At reduced pressure (60 mm Hg), perfusion with the fixative was continued for another 4 minutes, followed by perfusion with a sucrose-PBS solution adjusted to 800 mOsm/kg H2O to prevent the formation of freezing artifacts. After perfusion, kidneys were removed. Pieces of part of the kidneys were routinely processed for paraffin embedding, and part of them immediately shock-frozen in liquid nitrogencooled isopentane.
Experimental Protocol
For histochemical demonstration of NADPH-d activity,
5-µm-thick cryostat sections were incubated in 0.1 mol/L
phosphate buffer containing nitroblue tetrazolium (NBT), ß-NADPH,
and Triton X-100.4 In control animals, the reaction was
stopped when the MD signal was clearly distinguishable and background
staining had not yet appeared; thereby the optimal exposure time was
set at 25 minutes (incubation at 37°C). No reaction product was
found when NADPH was replaced by NADH.
For detection of nitrotyrosine, a mouse monoclonal antibody directed against nitrotyrosine (gift from J. Beckman, Birmingham, Ala) was used on paraffin sections. After deparaffination, sections were incubated for 30 minutes with 5% normal rabbit serum diluted in phosphate buffer used as blocking medium, followed by incubation with specific antibody (1:10 in PBS) for 12 to 18 hours at 4°C. After sections were rinsed in PBS, bound antibody was detected by the PAP method with an anti-mouse PAP complex and diaminobenzidine and H2O2 for color development. Control experiments were made by blocking the specific antibody with 10 mmol/L nitrotyrosine as well as by omitting the first antibody and using nonimmune serum instead.
Quantification of Histochemical Signal
For histochemical evaluation, two sections from each animal were
viewed by two independent persons in a blinded fashion. For
semiquantitative evaluation of the degree of nitration of protein
tyrosine within the extraglomerular mesangium, the fraction
of identifiable glomerular vascular poles with a positive
staining was established. For the semiquantitative evaluation of
NADPH-d enzyme activity, the amount of MD cells with positive NADPH-d
reaction within a standard area was established as previously
shown.7
Chemical Reagents, Kits, and Instruments
ß-NADPH, NADH, and diaminobenzidine were from Sigma Chemical
Co. Biotin-streptavidin-peroxidase complex was from Amersham,
anti-mouse PAP complex was from Dianova, and nitroblue tetrazolium was
from Biomol. Slides were viewed in a Polyvar II light microscope
(Reichert) equipped with interference contrast optics.
| Results |
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With the use of anti-nitrotyrosine antibody, extensive
immunoreactivity was detected in the EGM of the stenotic
kidneys adjacent to the NADPH-dpositive MD, whereas in nonclipped
contralateral kidneys as well as in kidneys from sham-operated control
kidneys, no such staining was observed (Figs 1
and 2
). Cells of the MD
were not stained by anti-nitrotyrosine antibody in all specimens
investigated. Semiquantitative evaluation revealed that approximately
two thirds of the total of identified vascular poles of the
stenotic kidneys were clearly positive for nitrotyrosine
immunostaining. The proportion of stained EGMs varied
considerably between individuals (values ranging between 39% and
84%), and in some specimens evaluation was impaired by nonspecific
background staining. The most intensive nitrotyrosine staining was
found in those animals that showed the highest blood pressure values,
but a significant correlation between these two parameters
could not be established. Control incubations for the histochemical
reaction, which comprised omission of the second antibody or
coincubation of specific antibody with 10 mmol/L
nitrotyrosine, respectively, were lacking EGM staining (Fig 1
).
Renal Vasculature
In the sham-operated control kidneys, the
endothelium of cortical renal arterioles was moderately
positive, with NADPH-d staining that has previously been identified as
endothelial NOS by colocalization with antiNOS III
antibody.4 Under renovascular hypertension, contrary
to juxtaglomerular staining, endothelial
NADPH-d staining in the clipped kidneys was often weaker than in
control kidneys, whereas in the nonclipped contralateral kidney there
was a marked increase in staining (Fig 3
). In parallel, a strong signal was
encountered in the arteriolar wall and periarteriolar connective tissue
of interlobular and glomerular afferent arterioles of the
nonclipped kidneys when anti-nitrotyrosine antibody staining was
applied (Fig 3
). Other portions of the renal vasculature showed no such
staining. Arterioles were identified using interference contrast
microscopy; the small arterioles have only a single muscle cell layer,
and the afferent can be distinguished from the efferent arteriole by
the drastic difference in number of endothelial cells
protruding into the lumen. Structural resolution of nitrotyrosine
staining was not quite sufficient to permit a distinction between
endothelial and vascular smooth muscle signals, but it
seemed rather that both structures were positive. No arteriolar
nitrotyrosine staining was re- corded in sham-operated control
kidneys. As in the EGM, controls for specificity of the nitrotyrosine
antibody staining resulted in a total absence of signal in the
arteriolar wall and surrounding tissue (Fig 3
).
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| Discussion |
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Increased tyrosine nitration has been detected in diseases associated with oxidative stress, ie, in atherosclerotic lesions of human vasculature,12 in infants with acute lung injury,14 and in the aorta15 and lung16 of rats injected with bacterial endotoxin. These results suggested that large amounts of NO originating from the induction of cytokine-dependent inducible type II NOS reacted with locally generated superoxide to form ONOO-, which in turn can hydroxylate and nitrate phenols such as tyrosine.12 17
In the present study, however, NO release was not likely to be derived from NOS II because no histological indication of an inflammatory process in either kidney from the 2K1C group was encountered in or near sites of nitrotyrosine immunoreactivity. A "constitutive" presence of inducible NOS expression in interlobular arteries or glomeruli, as proposed by Mohaupt and associates,18 could contribute to vascular or juxtaglomerular NO release, but we could not confirm these findings using in situ hybridization (unpublished results from our laboratory), and the transcription rate for inducible NOS may therefore be small. More likely, as shown in a previous histochemical investigation that identified the local distribution of NOS isoforms,4 release of NO was based on the activation of endothelial NOS III and epithelial NOS I, respectively, that is to say, from sources that normally produce NO to tonically act within physiological ranges.19 20 Under the specific conditions of chronic 2K1C-Goldblatt hypertension, enhanced stimulation of constitutive NOS isoforms may have led to the release of particularly high levels of NO comparable to the amount of NO released under inflammatory conditions.
This stimulation may have been effective not only at the level of calcium/calmodulindependent activation of NOS but also may have been reinforced by means of increased de novo synthesis of an NOS enzyme. Augmented NOS I mRNA levels were detected in the MD of the clipped kidney in response to a chronic decrease of the transport rate at the MD,7 which suggests additional formation of NOS up to amounts that may equal induction of type II isoform. Likewise, an augmentation of endothelial NOS under chronic stimuli such as increased shear stress has been reported.21 22 Apart from new enzyme synthesis, altered supply with substrate and/or cofactors and changes in degradation of the enzyme may well be involved in the enhancement of constitutive NOS. It should be noted in this respect that a principal difference in the amount of NO produced per quantity of a given isoform of NOS has not been demonstrated so far.
Immunoreactivity for nitrotyrosine, as observed in the present study, may be based on tyrosine nitration by NO alone forming 4-nitrosophenol, a reaction that is assumed not to be very effective since it implies the slow reaction of NO with oxygen to form nitrogen dioxide.13 Alternatively, ONOO-, which is readily formed by the reaction of NO and superoxide anion (O2·-), was suggested to be the more likely compound for inducing nitration of tyrosine residues at a rapid rate.13 In the present model, superoxide, which is required for this reaction, may have been generated by flavoprotein-containing oxidoreductases such as NADH oxidase or NADPH oxidase, which are present in the endothelium23 and may also be localized in the EGM and vascular wall; these enzymes were described as major sources of vascular O2·- release.23 24 Elevated angiotensin II levels in this model may have contributed to enhanced O2·- release, since it has recently been demonstrated that angiotensin II can increase both NADH and NADPH oxidase activity in cultured vascular smooth muscle.25 Since in sham-operated controls no protein nitration was found in the EGM despite significant NOS activity, it must be questioned whether increased NOS activity alone has led to protein nitration in the stenotic kidney. Previous semiquantitative histochemical evaluation of NOS I mRNA expression and NADPH-d in the MD under similar conditions has revealed significant increases in NOS I mRNA but less marked changes in NOS activity in stenotic compared with sham-operated control kidneys.7 It may therefore be considered that other factors, such as local superoxide formation, could be selectively active in the clipped kidney with an impact on protein nitration in the EGM. However, since the previous quantification analysis referred to the number of histochemically labeled MD cells rather than to the intensity of label of the individual MD cells directly adjacent to the EGM,7 the output of NO in the stenotic kidney may be higher than can be estimated from the presently available evidence.
When the particular microanatomy of the EGM is considered, which is a closed, avascular compartment in which little washout of intercellular compounds may occur compared with the "regular" renal cortical interstitium, local concentrations of NO might reach relatively high levels that in turn may have caused the observed intensive immunoreactivity for nitrotyrosine. The absence of nitrotyrosine immunoreactivity in MD cells proper may be related to a lack or low quantity of O2·- formation in the MDcontrary to the EGMthus preventing intraepithelial ONOO- formation.
The abundant presence of G6PD, as previously observed in MD and EGM,26 and unpublished observations from our laboratory may be functionally related to the high local output of NO. G6PD is known to play a prominent role in the defense against oxidative stress27 and may therefore serve as a local scavenger for NO. Conditions that provoke an increase in juxtaglomerular NOS I activity/transcription also induce an increased activity of G6PD.7 26 Thus, upregulation of G6PD in the stenotic kidney could actually decrease the effect of NO on protein nitration in the EGM. However, since G6PD may as well function to supply NOS with its cosubstrate NADPH, an enhancing effect is equally well possible.
Nitrotyrosine immunoreactivity in the vascular walls of the nonclipped contralateral kidney suggests an enhanced tyrosine nitration caused by endothelial release of NO in the cortical vasculature. One of the principal factors that determine endothelial NO synthesis is shear stress. Since it can be anticipated that blood flow in the contralateral kidney was markedly increased, activation of an NO-dependent vasorelaxing response is likely to occur in the present model. In addition, others have shown that NO-induced vasodilatation is an important regulatory response to maintain contralateral renal perfusion despite elevated angiotensin in 2K1C hypertension.2
Apart from the use of nitrotyrosine immunoreactivity as a footprint for the local release of NO, possible functional or pathological roles of tyrosine nitration on protein function are poorly elucidated. It has been suggested that via the inhibition of tyrosine phosphorylation, cellular regulation and signal transduction may be altered,28 and general protein conformation and function may be changed by nitration. It is unknown whether such effects play a role in the current model.
In conclusion, the documentation of nitrotyrosine immunoreactivity in parallel with enhanced staining for NOS suggests the local formation of NO. The notion that NO may be released into the EGM in response to a stimulation of MD NOS I may help to direct future work toward the role of NO in tubulovascular signaling. NO with its broad impact on biological functions may primarily exert an effect in the EGM that has yet to be defined; from there a secondary stimulus could be transmitted to the glomerular vasculature, which is in intimate contact with the EGM. Vascular effects of NO in the nonclipped contralateral kidney may document reactive changes in response to altered blood flow and/or to stimulation of the renin-angiotensin system.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received August 1, 1996; first decision August 29, 1996; accepted February 27, 1997.
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