(Hypertension. 2001;37:129.)
© 2001 American Heart Association, Inc.
Scientific Contributions |
From the Departamento de Fisiología, Facultad de Medicina, Murcia, Spain.
Correspondence to F. Javier Salazar, Departamento de Fisiología, Facultad de Medicina, 30100 Murcia, Spain. E-mail salazar{at}um.es
| Abstract |
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Key Words: hemodynamics natriuresis prostaglandins cyclooxygenase nitric oxide bradykinin
| Introduction |
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The importance of NO in the renal changes induced by BK was suggested in one study showing that infusion of an NO synthesis inhibitor to dogs pretreated with meclofenamate completely prevents the renal effects of BK.4 These results suggest that NO plays a major role in mediating the renal effects of BK when both COX isoforms are inhibited. However, it remains to be demonstrated whether NO synthesis inhibition alone modifies the renal vasodilatation and natriuresis elicited by BK. It is also unknown whether NO synthesis inhibition completely prevents the renal effects of BK when only COX-2 activity is reduced. The second objective of the present study was to evaluate whether COX-2 inhibition potentiates the renal effects induced by NO synthesis blockade during BK infusion. Based on the results obtained in previous studies,10 11 the hypothesis is that NO mediates the decrease in RVR induced by BK by a direct effect and by increasing the production of COX-2derived metabolites.
| Methods |
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Experimental Groups
Group 1 (n=6)
After two 15-minute control clearances, BK (8 ng ·
kg-1 · min-1) was
infused into the right renal artery for the duration of the experiment.
Ten minutes after initiating BK infusion, 3 more 15-minute clearances
were obtained. In preliminary experiments, it was demonstrated that the
dose of BK used elicited a significant increase in renal blood flow
(RBF), natriuresis, and diuresis without inducing changes in
MAP and in the renal hemodynamic and excretory function
of the contralateral kidney. It was also found in preliminary
experiments (n=6) that urinary prostaglandin
E2 (PGE2) excretion
increased (P<0.05) from 23.5±3.9 to 48.2±9.9 ng/min in
response to BK infusion. Urinary concentration of
PGE2 was measured by use of a commercial
enzyme-linked immunoassay (Neogen Corp).
Group 2 (n=6)
Nimesulide was infused as a bolus (0.75 mg/kg) and then
continuously (5 µg · kg-1 ·
min-1) for the duration of the experiment.
Forty-five minutes after initiating this continuous infusion, two
15-minute control clearances were obtained. Ten minutes after
initiating BK (8 ng · kg-1 ·
min-1) infusion, three 15-minute clearances were
taken. The dose of nimesulide used does not modify the
arachidonic acidinduced platelet
aggregation.12 In preliminary experiments (n=3), it was
found that nimesulide reduced urinary PGE2
excretion (P<0.05) before BK infusion (23.5±3.9 to
13.2±1.0 ng/min). During BK infusion, urinary
PGE2 excretion was lower in dogs pretreated with
nimesulide (31.4±5.0 ng/min) than in those pretreated with saline
(48.2±9.9 ng/min).
Group 3 (n=6)
Forty-five minutes after initiating an infusion of meclofenamate
(5 µg · kg-1 ·
min-1), two 15-minute control clearances were
taken. Ten minutes after starting a continuous BK infusion (8 ng
· kg-1 · min-1),
three 15-minute clearances were obtained. The dose of meclofenamate
used blocked the arachidonic acidinduced platelet
aggregation.12 In preliminary experiments (n=3), it was
found that meclofenamate reduced urinary PGE2
excretion (P<0.05) before BK infusion (23.5±3.9 to
5.2±0.1 ng/min). During BK infusion, PGE2
excretion was lower in dogs pretreated with meclofenamate (12.7±4.8
ng/min) than in those pretreated with nimesulide (31.4±5.0 ng/min) or
pretreated with saline (48.2±9.9 ng/min).
Group 4 (n=6)
The protocol in this group was similar to that used in group 1,
with the only difference being that the 2 control clearances were
obtained 45 minutes after starting a continuous intrarenal infusion of
NG-nitro-L-arginine
methyl ester (L-NAME, 3 µg · kg-1
· min-1).
Group 5 (n=6)
The experimental protocol performed was similar to that used in
group 4, with the difference being that L-NAME (3 µg ·
kg-1 · min-1) was
simultaneously infused with nimesulide for 45 minutes
before starting the 2 control clearance periods. Nimesulide was
administered at the same dose used in group 2.
Group 6 (n=6)
The experimental protocol performed was similar to that used in
group 4, with the difference being that L-NAME (3 µg ·
kg-1 · min-1) was
simultaneously infused with meclofenamate (5 µg ·
kg-1 · min-1) for
45 minutes before starting the 2 control clearance periods.
Analytical Methods
Renal clearances were taken during each experimental period to
determine the glomerular filtration rate (GFR), sodium,
potassium, and lithium excretion, and urine flow rate (UV). Blood
samples for plasma sodium, potassium, lithium, and inulin
concentrations were also obtained. GFR was measured by the clearance of
inulin. Inulin concentrations were analyzed by the anthrone
method. Concentrations of sodium and potassium were measured by flame
photometry (Corning 435). Proximal tubule sodium reabsorption was
estimated by the lithium clearance technique. Lithium concentrations
(µmol/L) were measured only in groups 1 and 4 by flame emission
spectrophotometry (Perkin-Elmer, model 5500). That lithium is a marker
for changes in proximal tubule sodium reabsorption is suggested by
results reported by Thomsen et al.15
Statistical Analysis
The data for the 2 control clearance periods were averaged for
statistical comparisons, because the fluid and solute excretions were
in steady-state conditions. Data are expressed as mean±SE. The
significance of differences between values of each period in the same
group was evaluated by using a 1-way ANOVA and the Fisher test. The
significance of differences between the values obtained in different
groups was calculated by using a 2-way ANOVA and the Duncan test. A
value of P<0.05 was considered significant.
| Results |
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Group 2
Pretreatment with nimesulide reduced the BK-induced renal
vasodilatation (Figure 1). The elevation in RBF (from 147±24 to
169±25 mL/min, P<0.05) was smaller than that found in the
control group (Figure 1). BK also induced an increase
(P<0.05) in UNaV (from 31±6 to 129±26 µmol/min)
and UV (from 0.20±0.04 to 1.16±0.18 mL/min) that remained significant
until the end of the experiment (Figure 4). This excretory response to BK was not
significantly different than that found in the control group.
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Group 3
BK infusion elicited an increase in RBF (from 155±13 to 183±20
mL/min, P<0.05). This increment was similar to that found
in nimesulide-pretreated dogs (group 2) and smaller
(P<0.05) than that found in group 1, in which PG synthesis
was not inhibited (Figure 1). Meclofenamate pretreatment did not
inhibit the excretory response to BK (Figure 4). UNaV and UV
increased (P<0.05) from 24±5 µmol/min and
0.15±0.04 mL/min to 103±14 µmol/min and 0.78±0.20 mL/min,
respectively, during the first clearance obtained during BK infusion.
These increments remained (P<0.05) until the end of the
experiment (Figure 4). No significant differences were found
between the increments in UNaV and UV in this group, in which a
nonisozyme-specific COX inhibitor was administered, and
in the control group.
Group 4
Previous inhibition of NO synthesis completely prevented the
BK-induced renal vasodilatation observed in the control group (Figure 1). As can be observed in Figure 2, BK elicited an
increase (P<0.05) in UNaV (from 32±5 to 81±9
µmol/min) and UV (from 0.26±0.02 to 0.94±0.21 mL/min) in dogs
pretreated with L-NAME. These increments remained significant
throughout the experiment (Figure 2). Although the BK-induced
increments in UNaV and UV tended to be smaller in this group than in
the vehicle-treated animals (group 1), the increase in renal excretory
ability elicited by BK was not significantly modified by previous NO
synthesis inhibition. As shown in Figure 3, BK infusion did not
induce a significant change in FeLi when NO synthesis was inhibited,
because FeLi was similar before and after BK infusion.
Group 5
As occurred in group 4, in which only L-NAME was administered,
intrarenal BK infusion did not induce changes of RBF in dogs pretreated
with nimesulide and L-NAME (130±16 versus 129±16 mL/min in the basal
period). However, in contrast to what was found in group 6, in which
both COX isoforms were inhibited, selective COX-2 inhibition did not
prevent the increase in renal excretory ability induced by BK (Figure 5). UNaV and UV increased
(P<0.05) from 14±2 µmol/min and 0.14±0.03 mL/min
to 77±13 µmol/min and 0.64±0.11 mL/min, respectively, during
the first clearance obtained during BK infusion. Both excretory
parameters remained elevated (P<0.05)
throughout the experiment during BK infusion.
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Group 6
The rise in RBF elicited by BK was also prevented by the
simultaneous infusion of L-NAME and the nonselective COX
inhibitor (meclofenamate) (122±20 versus 118±17 mL/min in
the basal period). Figure 5 shows that BK infusion was not able
to modify significantly UNaV and UV when endogenous
synthesis of NO and PG were inhibited with L-NAME and
meclofenamate.
| Discussion |
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Several studies have proposed that the renal effects induced by endothelium-dependent vasodilators are mediated by different vasoactive substances, such as NO, COX-derived metabolites, and cytochrome P-450 monooxygenase.1 2 3 4 5 6 An interaction between NO and PG in mediating the renal effects elicited by acetylcholine has been reported in studies performed in anesthetized dogs.16 It has been shown that administration of either a COX or NO synthesis inhibitor does not modify the renal effects of acetylcholine. However, simultaneous inhibition of COX and NO synthesis completely prevents the renal effects of acetylcholine.16 With respect to BK, it is unknown whether there is also an interaction between NO and PG in mediating its renal effects.
It has been demonstrated that BK increases the PG synthesis derived from both COX isoforms,8 but the relative contribution of each isoform in producing the PGs involved in mediating the renal vasodilatation and excretory response to BK has not been elucidated. To evaluate the role of COX-2derived metabolites in the renal effects elicited by BK, we have used an inhibitor (nimesulide) with a high COX-2 selectivity.17 18 Nevertheless, the possibility cannot be completely ruled out that the effects of nimesulide are partly due to COX-1 inhibition. Supporting the possibility that the renal effects elicited by the dose of nimesulide used are not secondary to an inhibition of COX-1, we have reported that platelet aggregation elicited by arachidonic acid is absent in plasma from dogs treated with meclofenamate and not significantly altered in plasma from nimesulide-treated dogs.12 These results are relevant because the thromboxane A2 involved in platelet aggregation is COX-1 dependent.19 We have also found that (1) nimesulide reduces urinary PGE2 excretion to a lower extent than does meclofenamate,12 and (2) meclofenamate, but not nimesulide, potentiates the effects induced by NO synthesis inhibition on renal hemodynamic and excretory function.20
The decrease in RVR induced by BK in the present study occurred without changes in GFR and MAP. As previously proposed by Edwards,21 our results suggest that BK has a greater effect on the efferent than on the afferent arteriole. As far as we know, to date, the role of NO in mediating the BK-induced renal vasodilatation in the intact animal has been examined in only one study, in which this hemodynamic effect was prevented by reducing NO synthesis in meclofenamate-pretreated dogs.4 Our results suggest that NO is mainly responsible for the BK-induced renal vasodilatation because, without PG inhibition, the elevation in RBF was completely prevented by reducing NO synthesis. The fact that the administration of a selective COX-2 inhibitor or a nonisozyme-specific COX inhibitor reduces to the same extent the renal vasodilatation elicited by BK suggests that COX-2derived metabolites are also involved in the renal hemodynamic response to BK. Taken together with the results obtained during L-NAME administration, it can be proposed that BK induces an increase in NO that elevates the production of COX-2derived metabolites. Then, NO and COX-2 derived metabolites are finally responsible for the decrease in RVR secondary to BK infusion (Figure 6). The existence of an interaction between NO and COX-2 has been proposed previously by several studies,10 11 22 in which it has been suggested that NO seems to be an endogenous regulator of the COX-2 activity. It also has been shown that BK induces the expression of COX-2.8 9
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The BK-induced natriuretic and diuretic response decreased progressively in the control group, inasmuch as the increments in UNaV and UV at the end of the experiments were lower than those found during the first clearance obtained after the initiation of BK infusion (Figure 2). In 5 dogs, it was observed that UNaV and UV decreased even more significantly when BK infusion was maintained for 120 minutes. Supporting the idea that BK infusion induces a natriuretic and diuretic response that decreases and even disappears with time, Granger and Hall23 have found that UNaV and UV are not elevated when BK infusion is maintained for several hours or days. The renal excretory changes elicited by BK have been proposed to be secondary to an indirect hemodynamic effect24 and a direct action on sodium reabsorption in several tubular segments.3 In the present study, the renal excretory response to BK seems to be mediated mainly by changes in tubular reabsorption, inasmuch as the renal vasodilatation, but not the natriuresis and diuresis elicited by BK, was prevented by NO synthesis inhibition. The results of the present study suggest that proximal sodium reabsorption decreases only transitorily during BK infusion and that this change is mediated by NO, because it was prevented by the previous L-NAME administration. This notion is supported by a recent study proposing that BK and acetylcholine, through an NO-mediated mechanism, decrease sodium transport in proximal tubule epithelial cells.25
As previously mentioned, the role of endogenous PGs in mediating the natriuretic and diuretic responses to BK has been suggested by studies showing that BK infusion elevates PG synthesis7 and that the COX inhibition reduces the renal excretory changes elicited by BK.1 Based on studies demonstrating the presence of COX-2 in the renal tubules26 and in results showing that BK elevates COX-2 expression,8 9 it could be proposed that COX-2derived metabolites are involved in mediating the renal excretory response to BK. The results of the present study show that the mean increments in UNaV and UV elicited by BK tended to be lower in dogs pretreated with nimesulide or meclofenamate than in the control group. However, there were not significant differences between the BK-induced natriuresis and diuresis in these experimental groups. In support of the notion that PGs are involved in the BK-induced natriuresis,4 we have confirmed that meclofenamate prevents the changes in renal excretory function elicited BK in L-NAMEtreated dogs (Figure 5). A new finding of the present study is that the administration of a COX-2 inhibitor does not modify the renal excretory response to BK in dogs in which NO synthesis is reduced (Figure 5). These results support the concept that the PGs mediating the BK-induced natriuresis and diuresis are mainly derived from COX-1 activity (Figure 6).
In summary, from the results obtained in the present study, the following can be proposed: (1) NO is primary responsible of the renal vasodilatation elicited by BK. (2) COX-2derived metabolites are also involved in the renal hemodynamic response to BK infusion. However, this involvement is evident only when NO synthesis is not modified. (3) NO and COX-1derived metabolites are mainly responsible for the renal excretory response to BK administration.
| Acknowledgments |
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Received May 18, 2000; first decision June 19, 2000; accepted July 3, 2000.
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