(Hypertension. 1995;25:1212-1219.)
© 1995 American Heart Association, Inc.
Articles |
Nitric Oxide Modulates but Does Not Impair Myogenic Vasoconstriction of the Afferent Arteriole in Spontaneously Hypertensive Rats
Studies in the Isolated Perfused Hydronephrotic Kidney
From the Department of Internal Medicine, School of Medicine, Keio University, Tokyo, Japan.
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Abstract |
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Abstract Renal autoregulation curves are reset toward higher renal arterial pressure in spontaneously hypertensive rats (SHR) compared with those in Wistar-Kyoto rats (WKY). We previously demonstrated that myogenic afferent arteriolar constriction is shifted to higher renal arterial pressure. To investigate whether nitric oxide participates in the regulation of myogenic tone, we examined the effect of nitro-L-arginine on myogenic afferent arteriolar constriction in kidneys from SHR and WKY, using the isolated perfused hydronephrotic kidney. Elevating pressures from 40 to 80 mm Hg caused increases in afferent arteriolar diameter in WKY (from 18.2±0.4 to 19.0±0.3 µm) and SHR (from 17.3±0.6 to 18.4±0.6 µm). Further pressure elevation elicited constriction at 100 mm Hg in WKY (17.9±0.3 µm), but significant constriction was observed at 120 mm Hg in SHR (17.3±0.6 µm), indicating a resetting in myogenic responses to higher pressures. In WKY, after treatment with 10 µmol/L nitro-L-arginine, afferent arterioles exhibited pressure-dependent constriction, with a threshold pressure for constriction at 80 mm Hg. The addition of 100 µmol/L nitro-L-arginine had no further effect on myogenic responsiveness in WKY. In contrast, in SHR, nitro-L-arginine dose-dependently shifted the myogenic responses toward lower renal arterial pressure, with threshold pressures for constriction observed at 100 mm Hg (10 µmol/L) and 80 mm Hg (100 µmol/L). Finally, in the presence of 100 µmol/L nitro-L-arginine, afferent arterioles manifested 24±3% and 20±2% constriction at 180 mm Hg in WKY and SHR, respectively, not different from those in the absence of nitro-L-arginine (WKY, 21±1%; SHR, 19±2%). In conclusion, the present study demonstrates that nitric oxide modulates but does not impair myogenic afferent arteriolar contractility. Furthermore, elimination of the resetting in afferent arteriolar response by nitric oxide blockade suggests an augmented effect of nitric oxide on this vessel in SHR. The enhanced nitric oxide effect on afferent arterioles from SHR kidneys may account for the adaptive mechanisms of renal microvessels to hypertension.
Key Words: renal microcirculation • resetting • nitric oxide • rats, inbred SHR • renal hemodynamics
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Introduction |
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Sustained hypertension markedly alters renal hemodynamic responses to changes in renal perfusion pressure. Autoregulation curves of renal blood flow and glomerular filtration rate are impaired in both Dahl salt-sensitive rats1 and Goldblatt (two-kidney, one clip) hypertensive rats.2 In parallel with the whole-kidney hemodynamics, myogenic afferent arteriolar vasoconstriction, which constitutes a pivotal determinant of renal autoregulation,3 4 is reported to be diminished in these hypertensive models.5 6 In striking contrast, in spontaneously hypertensive rats (SHR), a genetic model of essential hypertension, renal autoregulation is unimpaired but reset toward higher renal perfusion pressure.7 In the renal microvasculature, juxtamedullary afferent arterioles from SHR manifest greater vascular tone than those from Wistar-Kyoto rats (WKY).8 Furthermore, we previously demonstrated that myogenic vasoconstriction of superficial afferent arterioles is well preserved but shifted toward higher renal perfusion pressure compared with that in WKY.9 Although these adaptive changes in renal responses may prevent the kidney from glomerular barotrauma,10 mechanisms for the resetting of myogenic vasoconstriction of this vessel remain undetermined.
Identification of nitric oxide (NO) as an
endothelium-derived relaxing factor reveals that NO
plays an important role in the regulation of renal hemodynamics and
natriuresis. The inhibition of NO synthesis by
NG-monomethyl-L-arginine
or N
-nitro-L-arginine (N-Arg)
causes prominent reduction of renal blood flow and natriuresis in
vivo.11 12 13 A growing amount of evidence indicates that
hypertension alters the NO system in various organs.14 15 16 17 18 19 20 21 22 23 24
In the kidney, Ikenaga et al22 have recently demonstrated
that L-arginine, a precursor of NO, restores the blunted
pressure-natriuresis curves observed in SHR and suggested that
diminished NO action is responsible for the impaired pressure
natriuresis. In contrast, renal vascular actions of NO are demonstrated
to be similar in magnitude18 23 or even greater in SHR
than in normotensive WKY kidneys.19 20 Thus, it has not
been determined whether sustained hypertension alters the renal
hemodynamic actions of NO in SHR. Furthermore, there has been no
investigation to assess the effects of NO on the myogenic
vasoconstriction of the afferent arteriole in hypertensive animals.
In the present study, we investigated the effects of NO blockade on the myogenic vasoconstriction of the afferent arteriole in SHR kidneys to assess the role of NO in this constrictor response. To directly evaluate the renal microvascular action, we used the isolated perfused hydronephrotic rat kidney.9 25 26 27 Our present study demonstrates that NO does not impair the myogenic contractility of the afferent arteriole in either WKY or SHR kidneys but plays an important role in renal hemodynamics as a modulator of renal vascular tone. Furthermore, we demonstrate that blockade of NO synthesis restores the resetting of myogenic afferent arteriolar vasoconstriction observed in SHR kidneys, suggesting that NO is responsible for the resetting of the myogenic response of this microvessel in SHR kidneys.
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Methods |
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Preparation of Donor Rats
Chronic hydronephrosis was established to facilitate subsequent visualization of the renal microcirculation in isolated perfused kidneys as described previously.9 25 26 27 Six-week-old male WKY and SHR were anesthetized with ether. The right ureter of each animal was ligated through a midabdominal incision. After 8 to 10 weeks, at which time renal tubular atrophy had progressed to a stage that allowed direct microscopic visualization of renal microvessels,28 the kidneys were removed for perfusion study.
On the day of harvesting the hydronephrotic kidneys, the systolic pressures of the conscious donor rats were measured by tail-cuff sphygmomanometry (model KN-210-1, Natsume Co). To minimize experimental errors, we obtained averages of at least five measurements.
All procedures involving this study were performed following the instructions of the Animal Care Committee of Keio University. The rats had free access to water and chow throughout the study.
Perfusion of Hydronephrotic Kidneys
Donor rats were anesthetized with ether, and the abdominal
cavity was exposed by a midline incision. The renal artery of the
hydronephrotic kidney was cannulated in situ across the aorta through
the superior mesenteric artery. Warm oxygenated medium was perfused
throughout the cannulation procedure. The hydronephrotic kidney was
excised and placed on the stage of an inverted microscope (IMT-2,
Olympus) modified to accommodate a heated chamber equipped with a thin
glass viewing port on the bottom surface. Kidneys were allowed to
equilibrate for at least 30 minutes before experimental manipulations
were begun.
Kidneys were perfused with medium consisting of a Krebs-Ringer bicarbonate buffer containing 5 mmol/L D-glucose, 7.5% bovine serum albumin (Sigma Chemical Co), and a complement of amino acids as described previously.29 The perfusion apparatus has been illustrated previously.27 The perfusion medium was saturated with a gas mixture of 95% O2/5% CO2 within a pressurized reservoir. The perfusion pressure, monitored at the level of the renal artery, was altered by adjusting the back-pressure-type regulator (model 10BP, Fairchild Industrial Products Co), which controlled the exit of gas from the medium reservoir. Perfusate flow was monitored by means of an extracorporeal electromagnetic flow probe (model FF-015T, Nihon-Kohden) placed in the perfusion circuit immediately proximal to the kidney.
Vessel diameters were measured as detailed previously.9 25 26 27 In brief, video images from a video camera (model XC-77, Sony) were recorded with a videocassette recorder and transmitted to a computer (PS55/model 5551, IBM Japan) equipped with a video acquisition and display board (Targa 16+, Truevision Inc). Vessel diameters were estimated with an automated program custom designed to permit determination of the mean distance between parallel edges of the selected microvessels.9 25 26 27 A segment of interlobular arteries and the adjoining afferent arterioles approximately 50 µm in length was scanned at 1- to 3-second intervals. Mean vessel diameter was determined by averaging all measurements obtained during the plateau of the response.
Experimental Protocols
We used kidneys from WKY and SHR to investigate the effects of
NO on myogenic vasoconstriction of renal microvessels. Initially, renal
arterial pressure (RAP) of each preparation was maintained at 80
mm Hg. Thereafter, the pressure was reduced to 40 mm Hg and was
subsequently raised in a stepwise fashion by increments of 20 mm Hg up
to a maximum of 180 mm Hg. The diameters of afferent arterioles and
interlobular arteries were determined for at least 1 minute at each RAP
level. Based on the initial diameter, the portions of the interlobular
arteries are thought to represent the terminal portions of the
interlobular artery. Likewise, the afferent arterioles are thought to
be originally located in the superficial cortical region.
After the observation of baseline vasoconstrictor responses (described above), the effects of either 10 or 100 µmol/L N-Arg (Sigma) or both doses of N-Arg on myogenic afferent arteriolar vasoconstriction were assessed. The same regions of the vessels (ie, as previously observed in the absence of N-Arg) were measured 30 minutes after the administration of each dose of N-Arg.
To further confirm whether the effects of N-Arg were mediated by the inhibition of NO synthesis and the subsequent production of cGMP, the reversal by L-arginine and nitroprusside of the N-Arg–induced changes in myogenic response was assessed. After the evaluation of the effects of N-Arg, L-arginine (3 mmol/L, Sigma) or nitroprusside (10 µmol/L, Sigma) was subsequently administered into the perfusate. The vasoconstrictor responses of the afferent arteriole were assessed at the RAP values described above.
Analysis of Data
All data are expressed as mean±SEM. Data were analyzed by
two-way ANOVA followed by Newman-Keuls post hoc test. A value of
P<.05 was considered statistically significant.
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Results |
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Tail-cuff blood pressure on the day of renal hemodynamic studies was measured in the conscious donor rats. Mean blood pressure was 134±4 mm Hg in WKY (n=10) and 199±4 mm Hg in SHR (n=10, P<.001).
Afferent Arteriole
Fig 1 shows representative tracings
illustrating the effects of N-Arg on myogenic responses of an afferent
arteriole in a WKY kidney. A temporary RAP reduction from 80 to 40
mm Hg caused a prompt decrease in vessel diameter. The addition of 100
µmol/L N-Arg resulted in a decrease in diameter. The subsequent RAP
reduction caused a paradoxical vasodilation. The treatment with
L-arginine (3 mmol/L) restored both basal diameter and
the response to a RAP reduction. When five afferent arteriolar
responses were summarized, it was evident that N-Arg converted a
decrease (from 19.1±0.9 to 18.2±0.8 µm, P<.001) to an
increase (from 15.6±1.2 to 16.2±0.9 µm, P<.001) in
diameter in response to a RAP reduction from 80 to 40
mm Hg.
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Figs 2 and 3 summarize the myogenic responses of afferent arterioles from WKY and SHR kidneys in the absence or presence of N-Arg (10 or 100 µmol/L). In WKY (Fig 2, left), in the absence of N-Arg (circles), a RAP elevation from 40 to 80 mm Hg elicited an increase in diameter from 18.2±0.4 to 19.0±0.3 µm (P<.01). Further RAP elevations converted dilation to constriction; a significant constriction was observed at 100 mm Hg (17.9±0.3 µm, P<.01). As RAP was increased further to 160 mm Hg, vessel diameter decreased in a pressure-dependent manner (120 mm Hg, 17.0±0.3 µm, P<.01; 140 mm Hg, 16.1±0.3 µm, P<.01; 160 mm Hg, 15.4±0.3 µm, P<.01). Increasing RAP from 160 to 180 mm Hg elicited no further vasoconstriction (ie, 15.1±0.3 µm).
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The addition of 10 µmol/L N-Arg resulted in decreases in afferent arteriolar diameter at all RAP values examined (squares, Fig 2). In contrast to the responses in the absence of N-Arg, afferent arterioles did not manifest dilator responses to elevated RAP; vessel diameter was maintained constant as RAP was raised from 40 mm Hg (16.5±1.1 µm, n=5) to 60 mm Hg (16.5±0.9 µm). A significant vasoconstriction was observed at 80 mm Hg (15.8±1.0 µm, P<.05), a RAP lower than that in the absence of N-Arg (ie, 100 mm Hg). At 180 mm Hg, afferent arterioles constricted to 12.9±0.8 µm (P<.01).
After the addition of 100 µmol/L N-Arg (triangles, Fig 2), afferent arteriolar diameter tended to be less compared with that in the presence of 10 µmol/L N-Arg, although the differences did not attain statistical significance. The myogenic response was also similar to that observed in the presence of 10 µmol/L N-Arg; the elevation of RAP from 40 to 60 mm Hg did not alter vessel diameter (40 mm Hg, 15.5±0.5 µm; 60 mm Hg, 15.3±0.5 µm, n=11). When RAP was increased to 80 mm Hg, a significant constriction was obtained (14.5±0.4 µm, P<.05). At 180 mm Hg, afferent arteriolar diameter was reduced to 11.7±0.5 µm (P<.01).
When afferent arteriolar responses to pressures were expressed as changes from the diameters at 40 mm Hg (Fig 2, right), it was apparent that N-Arg did not alter the magnitude of myogenic constrictor responses. In the absence of N-Arg (circles), elevated RAP elicited 4.7±1.3% increments in diameter at 80 mm Hg, followed by 16.7±1.6% decrements at 180 mm Hg; when evaluated from a maximal diameter (at 80 mm Hg), afferent arterioles exhibited 20.5±1.3% constriction. After treatment with 10 and 100 µmol/L N-Arg (squares and triangles, respectively), elevation of RAP from 40 to 180 mm Hg elicited 21.7±3.0% and 23.7±2.8% constriction, respectively, a value nearly identical with that observed in the absence of N-Arg. Furthermore, N-Arg caused a leftward shift in myogenic responses; the RAP that elicited half-maximal constriction was lower in the presence of N-Arg (10 µmol/L, 105±3 mm Hg; 100 µmol/L, 103±4 mm Hg) than that in the absence of N-Arg (116±3 mm Hg, P<.05). The myogenic response curves were, however, similar in the presence of 10 and 100 µmol/L N-Arg.
Fig 3 illustrates the effects of N-Arg on myogenic afferent arteriolar responses in SHR kidneys. In the absence of N-Arg (circles), basal diameters did not differ in SHR (17.3±0.6 µm, n=15) and WKY (18.2±0.4 µm, n=12, P>.2). Elevation of RAP to 80 mm Hg caused an increase in diameter to 18.4±0.6 µm (P<.01). In contrast to the responses in WKY, the increase of RAP from 80 to 100 mm Hg failed to produce contraction (ie, 18.3±0.6 µm); a significant constriction was observed at 120 mm Hg (17.3±0.6 µm, P<.01). Further RAP elevations elicited progressive constriction (16.4±0.6, 15.5±0.6, and 15.0±0.6 µm for 140, 160, and 180 mm Hg, respectively, P<.01).
The addition of N-Arg dose-dependently shifted myogenic afferent arteriolar responses in SHR kidneys. Thus, in the presence of 10 µmol/L N-Arg (squares), following a vasodilator tendency (from 16.3±0.9 µm at 40 mm Hg to 16.8±0.8 µm at 60 mm Hg, n=10), afferent arterioles exhibited a significant constriction at 100 mm Hg (15.6±0.7 µm, P<.05). At 180 mm Hg, vessel diameter decreased to 13.0±0.9 µm (P<.01). After the addition of 100 µmol/L N-Arg (triangles), elevated RAP failed to increase afferent arteriolar diameter but elicited progressive constriction; a significant constriction was obtained at 80 mm Hg (from 16.6±0.9 µm at 40 mm Hg to 15.8±0.8 µm at 80 mm Hg, n=11, P<.01). Further RAP elevations to 180 mm Hg decreased vessel diameter to 13.3±0.8 µm (P<.01). Thus, N-Arg caused a leftward shift in threshold pressures that elicited significant constriction (120, 100, and 80 mm Hg for control, 10 µmol/L, and 100 µmol/L N-Arg, respectively). Moreover, RAP values at which half-maximal constrictor responses were observed were shifted toward the lower end (control, 134±3 mm Hg; 10 µmol/L, 118±4 mm Hg; 100 µmol/L, 106±4 mm Hg). Of note, when afferent arteriolar responses were expressed as percent changes from maximal diameters, the myogenic responsiveness in the presence of 10 µmol/L (-23.7±3.4%) or 100 µmol/L (-19.8±1.6%) N-Arg was nearly identical with that in the absence of N-Arg (-18.6±1.6%).
L-Arginine (3 mmol/L) completely restored the N-Arg–induced shifts in myogenic afferent arteriolar responses in both WKY and SHR kidneys (Fig 4). Thus, in WKY in the presence of both N-Arg and L-arginine (squares, left panel), elevation in RAP elicited 3.2±1.4% increments in diameter at 80 mm Hg (from 18.5±0.7 to 19.2±0.9 µm, n=5), followed by pressure-dependent constriction, with 18.5±2.4% decrements observed at 180 mm Hg. Similarly, in SHR following a maximal (ie, 5.8±2.1%) increment in diameter at 100 mm Hg, afferent arterioles manifested 13.4±0.7% constrictor responses to elevated RAP at 180 mm Hg (squares, right panel).
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We further assessed the effects of nitroprusside on N-Arg–induced changes in myogenic afferent arteriolar responses (Fig 5). Nitroprusside (10 µmol/L) returned the N-Arg–induced shifts in myogenic responsiveness in both rat strains. Thus, although N-Arg caused leftward shifts in myogenic constriction, the subsequent addition of nitroprusside (ie, N-Arg plus nitroprusside) elicited rightward shifts in the responsiveness in both WKY and SHR; the response curve in the presence of both N-Arg and nitroprusside coincided with that observed before addition of N-Arg and nitroprusside (ie, control).
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We previously demonstrated that the myogenic afferent arteriolar response in SHR kidneys was reset to higher RAP values compared with that in WKY.9 To clarify the role of NO in the resetting of myogenic afferent arteriolar constriction in SHR, we compared the response curves in WKY and SHR during NO inhibition (Fig 6). Afferent arterioles in SHR kidneys manifested rightward resetting in myogenic responses (left). In contrast, 100 µmol/L N-Arg elicited a greater leftward shift in the response in SHR than in WKY, resulting in an overlap of the response curves (right).
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Interlobular Artery
In the absence of N-Arg (Fig 7, circles), initial
diameters did not differ in WKY (30.9±4.0 µm, n=8) and SHR
(28.5±2.6 µm, n=16, P>.5). Elevation of RAP from 40 to
80 mm Hg elicited similar increments in diameter (WKY, 2.0±1.0%;
SHR, 4.8±1.8%). In WKY, further RAP elevations tended to constrict
interlobular arteries at 100 mm Hg (31.5±4.1 to 30.5±4.1 µm) and
elicited a significant constriction at 120 mm Hg (28.8±3.6
µm, P<.01). In SHR, significant vasoconstriction was
observed at 100 mm Hg (29.8±2.6 to 29.1±2.6 µm,
P<.01). At 180 mm Hg, interlobular arteries manifested
18.3±2.4% (25.0±3.2 µm, P<.01) and 14.3±1.8%
(24.4±3.0 µm, P<.01) constriction in WKY and SHR,
respectively.
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The addition of N-Arg shifted the myogenic constrictor curves toward lower RAP values in both rat strains. In WKY, 10 µmol/L N-Arg caused a leftward shift in myogenic responsiveness, and 100 µmol/L N-Arg exerted no additional shifts, whereas in SHR N-Arg dose-dependently shifted the myogenic responsiveness. Thus, in WKY, interlobular arteries exhibited a significant constrictor response to elevation in RAP from 40 to 80 mm Hg in the presence of 10 µmol/L (from 29.3±3.2 to 28.2±3.1 µm, n=7, P<.05) and 100 µmol/L (from 27.4±3.8 to 25.5±3.5 µm, n=8, P<.01) N-Arg. Furthermore, the RAP values that elicited half-maximal constriction were identical in the presence of 10 µmol/L (100±5 mm Hg) and 100 µmol/L (97±5 mm Hg) N-Arg. In contrast, in SHR, a significant vasoconstriction was observed at 100 mm Hg in the presence of 10 µmol/L N-Arg (from 27.7±2.6 to 26.9±2.5 µm, n=15, P<.05) and at 80 mm Hg in the presence of 100 µmol/L N-Arg (from 26.2±2.8 to 25.6±2.8 µm, n=13, P<.05). Additionally, N-Arg tended to shift the RAP that elicited half-maximal constriction to the lower end (132±5, 124±4, and 118±4 mm Hg for control, 10 µmol/L, and 100 µmol/L, respectively).
Renal Vascular Resistance
The effects of N-Arg on pressure-induced changes of renal vascular
resistance (RVR) were assessed in kidneys from WKY (Fig 8,
left) and SHR (right). In the absence of N-Arg
(circles), initial RVR (at 40 mm Hg) did not differ in WKY [5.8±0.6
mm Hg/(mL/min), n=10] and SHR [6.0±0.9 mm Hg/(mL/min), n=10]. In
both strains, RVR increased in response to elevated RAP. In WKY,
significant RVR increases were observed at 140 mm Hg [6.5±0.9
mm Hg/(mL/min), P<.05], and elevation of RAP to 180
mm Hg elicited 24±4% increments in RVR. In SHR, a significant
increase in RVR was observed at 180 mm Hg [6.9±0.9 mm Hg/(mL/min),
P<.05], corresponding to 12±3% increments from initial
RVR.
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The addition of N-Arg markedly altered the initial RVR and the RVR responses to pressures. Thus, 10 µmol/L (squares) and 100 µmol/L (triangles) N-Arg increased the initial RVR in WKY kidneys [7.3±0.8 mm Hg/(mL/min), n=5 and 8.0±1.0 mm Hg/(mL/min), n=10, respectively] and SHR kidneys [7.2±1.3 mm Hg/(mL/min), n=6 and 8.3±1.2 mm Hg/(mL/min), n=10, respectively]. In WKY, elevated RAP to 100 mm Hg elicited a significant increase in RVR in the presence of 10 µmol/L N-Arg [8.1±1.0 mm Hg/(mL/min), P<.05] and 100 µmol/L N-Arg [9.0±1.1 mm Hg/(mL/min), P<.05]. Further RAP elevations produced pressure-dependent increases in RVR. At 180 mm Hg, 23±3% (10 µmol/L) and 28±4% (100 µmol/L) increments in RVR were observed. In SHR kidneys, by contrast, N-Arg caused a leftward shift in RVR responses to pressures in a dose-dependent manner. Thus, RAP that elicited significant increases in RVR was 140 mm Hg [8.2±1.2 mm Hg/(mL/min), P<.05] and 100 mm Hg [9.1±1.0 mm Hg/(mL/min), P<.05] for 10 and 100 µmol/L N-Arg, respectively. At 180 mm Hg, RVR increments were 19±4% and 24±4% in the presence of 10 and 100 µmol/L N-Arg, respectively.
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Discussion |
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NO plays an important role in the regulation of renal hemodynamics. Inhibition of NO synthesis causes a marked decrease in renal blood flow.11 12 Furthermore, at the renal microvascular level, acetylcholine reverses both norepinephrine- and angiotensin II–induced afferent arteriolar constriction, and this reversal is inhibited by N-Arg.30 31 32 These observations suggest that NO inhibits substantially the vasoconstrictor tone of the renal vasculature. In contrast, it has also been documented that inhibition of NO does not alter renal autoregulatory capability.13 33 34 Since myogenic afferent arteriolar vasoconstriction contributes importantly to renal autoregulation,3 4 it is reasonable to infer that NO does not impair the myogenic mechanism of the afferent arteriolar vasoconstriction. Nevertheless, the effects of NO on the myogenic response of renal microvessels have not been fully delineated. Furthermore, in hypertensive animals, the NO system is reported to be diminished,14 15 augmented,19 20 21 or unaltered.16 17 18 Although hypertension is shown to be associated with both the resetting of renal autoregulation7 35 and shifting of myogenic afferent arteriolar vasoconstriction,9 it has not been examined whether NO modulates the myogenic vasoconstriction of renal microvessels in hypertensive animals.
The present study has demonstrated that NO modifies the myogenic responsiveness of the afferent arteriole. Thus, N-Arg did not alter the magnitude of myogenic afferent arteriolar vasoconstriction in either WKY kidneys (control, 20.5±1.3%; 10 µmol/L, 21.7±3.0%; 100 µmol/L, 23.7±2.8%; expressed as percent changes from maximal diameters) or SHR kidneys (control, 18.6±1.6%; 10 µmol/L, 23.7±3.4%; 100 µmol/L, 19.8±1.6%). Alternatively, N-Arg causes marked shifts in myogenic vasoconstrictor responses toward lower RAP values in both WKY and SHR kidneys. The shifts in the myogenic responses are reversed by two different NO donors, L-arginine and nitroprusside. Furthermore, whereas the myogenic afferent arteriolar response is reset to higher RAP values in SHR, treatment with N-Arg abolishes this relative shift. Thus, our present findings demonstrate that NO does not impair the myogenic afferent arteriolar contractility but modulates the sensitivity of this response. Additionally, the elimination of the relative shift by blockade of NO synthesis and the reversal of the shift by two NO donors suggest that an enhanced action of NO is responsible for the relative shift in the myogenic afferent arteriolar responses in SHR kidneys.
The ability of NO to dilate renal microvessels may vary, depending on the types of underlying vasoconstrictor tone. In a study that directly assessed renal microvascular responses, Ohishi et al36 reported that N-Arg constricted the afferent arteriole in blood-perfused juxtamedullary nephrons. Furthermore, in isolated rabbit renal afferent arterioles, N-Arg potentiated the endothelin-induced30 and angiotensin II–induced31 constriction. We have recently reported that N-Arg inhibits the acetylcholine-induced dilation during norepinephrine- and KCl-induced afferent arteriolar constriction in the same setting as that used in the present study.32 In contrast to a marked dilator action of NO, our present study reveals that endogenously released NO does not impair afferent arteriolar contractility when the vessel is constricted by pressure. The inability of NO to relax myogenic constriction would be explained by a selective inhibitory action of cGMP-dependent vasodilators, because the dilator action of NO is mediated largely by the activation of guanylate cyclase and the subsequent production of cGMP in vascular smooth muscle.12 Thus, we have recently demonstrated that cGMP-dependent vasodilators, including atrial natriuretic peptide, nitroprusside, and 8-bromo-cGMP, only partially inhibit myogenic afferent arteriolar constriction26 ; in an identical experimental setting, these vasodilators completely inhibit norepinephrine-induced afferent arteriolar constriction. Thus, cGMP-dependent vasodilators selectively inhibit receptor-mediated constriction, whereas myogenic afferent arteriolar constriction is refractory to these vasodilators. Together these findings indicate that NO is capable of dilating the afferent arteriole, but the ability to relax the afferent arteriole is greatly diminished when the vessel is constricted by pressure.
Several investigations have reported the effects of NO on the myogenic response of the renal microcirculation. A number of in vivo observations reveal that NO inhibition fails to alter the autoregulatory efficiency of renal blood flow whereas basal blood flow is markedly reduced.13 33 34 37 38 These results indicate that renal autoregulation is preserved during continuous release of endogenous NO and suggest that NO does not impair the myogenic constrictor mechanism of the afferent arteriole, a determinant of renal autoregulation.3 4 More direct observations by Imig and Roman39 indicated that N-Arg did not alter the magnitude of myogenic afferent arteriolar constriction in juxtamedullary nephron preparations. Furthermore, Hoffend et al40 have demonstrated that in the in vivo hydronephrotic kidney, NO inhibition does not affect the pressure-induced responses of cortical afferent arterioles. In contrast, Hoffend et al also reported in the same study that NO blockade augments the pressure-induced response of juxtamedullary afferent arterioles. Furthermore, Juncos et al41 reported that N-Arg caused an enhanced myogenic tone in isolated microperfused rabbit afferent arterioles. Although the reason for these conflicting observations is unclear, the divergent results may be ascribed to the types of experimental settings used. Juncos et al reported that when intraluminal flow through the afferent arteriole was stopped, myogenic afferent arteriolar constriction was markedly augmented within the pressure range observed (ie, 30 to 120 mm Hg), suggesting that flow-induced shear stress constitutes a determinant of the myogenic responsiveness. Moreover, Hoffend et al observed that glomerular blood flow in juxtamedullary nephrons was profoundly greater than that in cortical nephrons. Since greater shear stress results in a more abundant release of NO, afferent arteriolar tone in juxtamedullary nephrons may be more dependent on tonic release of NO than that in cortical nephrons.42 We observed myogenic responses of cortical afferent arterioles to increased RAP from 40 to 180 mm Hg, and under this condition cGMP-dependent vasodilators exerted only a modest dilator action.26 Clearly, further studies are required to elucidate the interaction between NO and myogenic tone within the renal microvasculature.
Whereas NO fails to alter renal autoregulatory efficiency13 33 34 37 38 and our present study indicates intact myogenic afferent arteriolar contractility in the presence of endogenous NO, NO may modulate the myogenic tone of renal microvessels. Beierwaltes et al33 showed that N-Arg modestly, albeit not significantly, reduced lower pressure limits of renal blood flow autoregulation. Recently, Hoffend et al40 have reported that whereas under an intact NO system glomerular blood flow autoregulation is lost at RAP values less than 90 mm Hg, N-Arg treatment renders the kidney well autoregulated within RAP ranges of 80 to 110 mm Hg. The latter observation indicates that in the presence of N-Arg, a lower limit for glomerular flow autoregulation is less than 80 mm Hg and suggests that NO inhibition elicits a leftward shift in a threshold pressure for autoregulation. The present study showed that NO inhibition caused a leftward shift in myogenic afferent arteriolar responses; the threshold RAP values at which significant contraction was observed were lower in the presence of N-Arg than in the absence of N-Arg in both WKY and SHR. Furthermore, the pressures that elicited half-maximal contraction were also shifted to lower RAP values after N-Arg treatment. Similar responses to N-Arg were observed in interlobular arteries and RVR. Finally, the N-Arg–induced shift in myogenic afferent arteriolar responses was reversed by two NO donors, L-arginine and nitroprusside. These findings thus suggest that NO modulates the sensitivity of myogenic responsiveness of renal microvessels. The pharmacological manipulations that alter cGMP levels in the vascular smooth muscle could modulate calcium sensitivity during myogenic contraction.43
The present study further documents that the resetting of myogenic responsiveness in SHR kidneys is associated with NO-induced changes in renal microvascular responsiveness. We previously demonstrated that myogenic afferent arteriolar responses in SHR were reset toward higher RAP compared with those in WKY kidneys.9 In the present study, we found that N-Arg abolished the relative shift in the myogenic afferent arteriolar response observed in SHR. Furthermore, the N-Arg–induced alteration was restored by the subsequent addition of L-arginine or nitroprusside. Together these findings suggest that the resetting in myogenic afferent arteriolar response in SHR kidneys is attributed to enhanced actions of NO on this renal microvessel. Although changes in pressures at the level of the renal artery may produce differing microvascular transmural pressures between WKY and SHR, our additional findings that N-Arg–induced alterations in RVR (Fig 6) paralleled the observed shift in afferent arterioles (Fig 4) support the contention that an enhanced action of NO is responsible for the resetting of the renal autoregulatory response to pressures in SHR kidneys.
Several divergent observations have been reported regarding the vascular effects of NO in hypertensive animals. For example, endothelium-dependent relaxation is reported to be attenuated in basilar arteries from SHR.14 In addition, Malinski et al15 found impaired NO synthase activity in cultured endothelial cells from SHR. In contrast, recent studies have demonstrated that the effects of NO are not impaired in various vascular beds, including mesenteric resistance vessels17 and isolated perfused kidneys.23 Moreover, a growing amount of evidence shows that the effects of NO are rather increased in SHR.19 20 21 24 In SHR aortic rings, inhibitory effects of NG-monomethyl-L-arginine on acetylcholine-induced relaxation have been shown to be greater than those in WKY rings.24 In addition, Ito and Carretero20 have demonstrated that N-Arg elicits greater constriction of afferent arterioles isolated from SHR than from WKY, suggesting that the effect of NO was enhanced in SHR. The present study shows that N-Arg maximally shifts the myogenic responses of both afferent arterioles and interlobular arteries at 10 µmol/L in WKY and at 100 µmol/L in SHR. Thus, it appears that a higher N-Arg concentration is required to obtain maximal inhibition of NO-induced shifts in myogenic responses in SHR than in WKY. Both afferent arterioles and interlobular arteries may produce greater NO or exhibit enhanced responsiveness to NO in SHR than in WKY. Nevertheless, further investigations are required to explain the reason why the relative shift in myogenic responsiveness is restricted to the afferent arteriole and is not observed at the interlobular artery.
In conclusion, the present study has demonstrated that inhibition of NO does not alter the ability of the afferent arteriole to constrict in response to pressures but causes a shift in this responsiveness toward lower RAP in both WKY and SHR kidneys. Furthermore, the myogenic responsiveness in SHR kidneys is reset toward higher RAP compared with that in WKY, and inhibition of endogenous NO abolishes this resetting. Together the results indicate that NO modulates but does not impair myogenic afferent arteriolar contractility. Furthermore, alterations in the NO system within the afferent arteriole may contribute to the resetting of myogenic responsiveness in SHR. The augmented NO action on the afferent arteriole in SHR may explain the adaptive mechanisms of renal microvessels to hypertension, ie, a rightward shift in renal autoregulation and an intact myogenic preglomerular constriction, which could protect glomeruli from barotrauma.
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Footnotes |
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Reprint requests to Takao Saruta, MD, Department of Internal Medicine, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku-ku, Tokyo 160, Japan.
Received September 9, 1994; first decision November 9, 1994; accepted February 6, 1995.
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