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Vol. 292, Issue 2, 752-760, February 2000
Center for Clinical Pharmacology, Departments of Medicine and Pharmacology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania.
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Abstract |
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 Top
 Abstract
 Introduction
Materials and Methods
Results
Discussion
References
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Adenosine A1 receptor antagonists are being developed for
use as diuretics in the treatment of hypertension, however, there is
relatively little data in hypertensive animal models regarding the
efficacy of these compounds. In addition, some controversy exists
surrounding the role of pertussis toxin (PT)-sensitive G-proteins in
the signaling pathway for receptors acted on by A1
antagonists. Our objectives for this study were 1) to compare the
diuretic, natriuretic, and cardiovascular effects of acute A1 receptor blockade in spontaneously hypertensive (SHR)
and normotensive Wistar-Kyoto rats (WKY); and 2) to determine whether
the diuretic effects are mediated through a PT-sensitive mechanism.
Acute administration of the selective A1 antagonist
1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 10 µg/kg/min) increased
urine output (410 ± 116 and 317 ± 86 µl/30 min/g kidney)
and sodium excretion (90.3 ± 25.6 and 76.8 ± 18.2 µmol/30
min/g kidney) similarly in WKY and SHR, respectively. DPCPX
significantly decreased mean arterial blood pressure in SHR
(
11.4 ± 2.7 mm Hg), but not WKY. Prior treatment with PT (30 µg/kg i.v.) abolished the diuretic response to DPCPX in both SHR and
WKY. In a subsequent experiment in PT-treated Sprague-Dawley rats,
DPCPX failed to evoke a diuretic response, whereas coinfusion of
furosemide with DPCPX induced marked diuresis. Our results indicate
that acute DPCPX administration produces similar natriuretic/diuretic effects in SHR and WKY, with beneficial effects on blood pressure in
SHR. PT abolishes the response to DPCPX, indicating that the natriuretic/diuretic response to DPCPX is mediated via blockade of
A1 receptors linked to tubular sodium transport through
PT-sensitive G-proteins.
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Introduction |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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The
pharmacological basis for the diuretic effect of caffeine and
structurally related xanthine compounds has been ascribed to the
ability of these drugs to block adenosine receptors in the kidney.
Although endogenous adenosine exerts cardiovascular and renal
hemodynamic effects through both A1 and
A2 receptors, selective A2
receptor antagonists do not induce diuresis (Suzuki et al., 1992
). In
contrast, selective A1 receptor antagonists (such
as 1,3-dipropyl-8-cyclopentylxanthine; DPCPX) have been shown to
produce diuresis and natriuresis in animal models (Collis et al., 1991;
Suzuki et al., 1992; Knight et al., 1993; Kuan et al., 1993).
Recognition of the diuretic efficacy of A1
antagonists has prompted the development and testing of these compounds
for use as diuretics in the treatment of hypertension and edema.
Adenosine receptor antagonists are being developed with essential
hypertension as an intended therapeutic target (van Buren et al.,
1993). However, there are relatively few published studies that have
examined the effects of these compounds in hypertensive models, thus
little is known regarding their antihypertensive efficacy. Beneficial
effects have been observed in the Dahl salt-sensitive rat, in which the
A1 antagonists KW-3902
[8-(noradamantan-3-yl)-1,3-dipropylxanthine] and FK-838
[6-oxo-3-(2-phenylpyrazolo(1,5-a)pyridin-3-yl)-1(6H)-pyridizinebutyric acid], attenuated the development of hypertension (Nomura et
al., 1995; Uehara et al., 1995). However, in normotensive rats, the nonselective adenosine antagonist 1,3-dipropyl-8-sulfophenylxanthine actually induced hypertension (Albino-Teixeira et al., 1991; Matias et
al., 1991; Rubino and Burnstock, 1995), and in renovascular hypertensive rats, the adenosine antagonist caffeine potentiated a
further increase of blood pressure (BP) (Ohnishi et al., 1986b, 1988;
Kohno et al., 1991; Choi et al., 1993; Kost et al., 1994). Based on
those observations, it is conceivable that A1
antagonists may prove to have little efficacy, or even worse, adversely
affect BP regulation in some hypertensive models. Accordingly, the
first aim of our study was to compare the acute diuretic/natriuretic and cardiovascular effects of a selective A1
receptor antagonist in normotensive (Wistar-Kyoto; WKY) and
spontaneously hypertensive rats (SHR).
An additional aim of our study was to determine whether the
diuretic/natriuretic response to A1 receptor
blockade is mediated via a pertussis toxin (PT)-sensitive mechanism.
Our interest in the A1 receptor-effector coupling
in the diuretic response was prompted by an earlier report in which PT
was shown to have no effect on the diuretic and natriuretic response to
the selective A1 receptor antagonists DPCPX and
KW-3902 (Mizumoto et al., 1993). The findings challenge the classic
paradigm in which A1 receptors are thought to be
coupled to effector systems (e.g., inhibition of adenylyl cyclase and
activation of phospholipase C) via the Gi/Go family of G-proteins,
and in nearly all cases, responses to A1 receptor
activation have been shown to be inhibitable by PT (Fredholm et al.,
1994). The data of Mizumoto et al. (1993) indicate that
A1 receptor antagonists produce
diuresis/natriuresis independent of A1 receptor
blockade or through blockade of A1 receptors that
are not coupled to effector systems via the
Gi/Go family of proteins.
Accordingly, we felt it was important to further investigate the
mechanism whereby DPCPX exerts its diuretic effect.
In this study, we compared the natriuretic/diuretic and cardiovascular responses in SHR and WKY during i.v. infusions of DPCPX (1 and 10 µg/kg/min). We also compared the effects of DPCPX (10 µg/kg/min i.v.) in naive and PT-pretreated SHR and WKY. We found that DPCPX produced similar natriuretic/diuretic effects in the two strains, and that the diuretic effect was accompanied by a reduction of BP in SHR. We also found that PT abolished the natriuretic/diuretic response to DPCPX in both strains.
Our findings that PT abolished the diuretic response to DPCPX in WKY
and SHR were in contrast to those of Mizumoto et al. (1993) in Wistar
rats. Because we were concerned that our observations may represent a
strain-specific phenomenon, we performed a supplementary study
comparing the diuretic effect of DPCPX in control and PT-treated Sprague-Dawley (SD) rats. An additional concern was that the PT pretreatment may have affected the rats in some way so as to render them unable to respond to any diuretic treatment. To explore this possibility, the response to the loop diuretic furosemide was examined
in control and PT-treated SD rats. We found that PT abolished the
diuretic/natriuretic response to DPCPX, but not to a coinfusion of
furosemide and DPCPX. Collectively, our data indicate that the diuretic
response to DPCPX is mediated via blockade of A1 receptors linked to tubular sodium transport through PT-sensitive G-proteins.
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Materials and Methods |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Male WKY (11-13 weeks of age) and age-matched SHR were obtained from Taconic Farms (Germantown, NY), and male SD rats (12-13 weeks of age) were obtained from Charles River Laboratories (Wilmington, MA). Rats were allowed to acclimate to the University of Pittsburgh Animal Facility for at least 1 week before initiation of the experimental protocols. Protocols were approved by the Institutional Animal Care and Use Committee.
Protocol A: Response to DPCPX in SHR and WKY.
The acute
responses to DPCPX infusions at 1 and 10 µg/kg/min were measured in
SHR and WKY (318 ± 17 and 389 ± 10 g, respectively). These infusion rates of DPCPX (1 and 10 µg/kg/min) have been shown to
block the bradycardic response to the selective
A1 agonist N6-cyclopentyladenosine (0.1 µg/min), without altering the hypotensive response to the selective
A2A agonist CGS21680C (1 µg/min) in anesthetized rats (Kuan et al., 1992). Kuan et al. (1992) also demonstrated that lower infusion rates of DPCPX (0.1 and 0.3 µg/kg/min) provided only partial blockade of A1
receptors, whereas higher infusion rates (30 and 100 µg/kg/min)
partially blocked A2A receptors.
Protocol B: Effect of PT on Response to DPCPX in SHR and WKY. The acute response to DPCPX was measured in SHR and WKY pretreated with PT or saline. SHR and WKY (322 ± 4 and 486 ± 7 g, respectively) were anesthetized with halothane and a small (~1 cm) incision was made in the skin of each rat to expose the right jugular vein. Each rat was randomly assigned to one of two treatment groups and received a bolus i.v. injection of either PT (30 µg/kg) or 0.9% saline (150 µl/kg). The number of rats injected with saline was approximately double that of the PT-treated rats so that a subgroup of saline-injected rats could serve as a time control for the effects of DPCPX as described below. The i.v. injection was made through a 28-gauge needle, and a cotton-tip applicator was used to apply pressure to the vein halting the flow of blood for 30 s following removal of the needle. The skin incision was closed with wound clips, and on recovery from anesthesia, the rats were returned to their cages.
The acute responses to A1 receptor blockade were measured in the anesthetized saline- and PT-treated rats 3 to 5 days following the i.v. injection. Each rat was anesthetized with Inactin (100 mg/kg i.p.), and instrumented as described in protocol A. Following the surgical preparation, infusions of DMSO (0.65 µl/min) and [14C]inulin (0.035 µCi/100 µl of 0.9% saline/min) were initiated, and a 2-h stabilization period was permitted. Following the stabilization period, MABP, HR, and RBF were recorded at 5-min intervals, and a urine sample and mid-point blood sample were collected during a 30-min baseline period (period 1; DMSO). The DMSO infusion was replaced with an infusion of DPCPX at 10 µg/kg/min, and a 45-min stabilization period was permitted. Then, MABP, HR, and RBF were recorded, and a urine sample and mid-point blood sample were collected during an additional 30-min DPCPX period (period 2). Following the first infusion period, approximately half of the saline-injected rats from each strain were randomly assigned as time-controls to permit comparison of the effects of time versus DPCPX in SHR and WKY. These rats received DMSO infusion during both periods 1 and 2. On completion of period 2, rats were euthanatized and the left kidneys were weighed. Samples were analyzed as described in protocol A. Values obtained before initiation of the treatment regimen (i.e., DPCPX versus DMSO) from a subset of the rats used in protocol B were previously reported (Kost et al., 1999), and results from protocol B
have been presented in abstract form (Kost et al., 1998).
Protocol C: Effect of PT on Response to DPCPX and Furosemide in SD Rats. The diuretic/natriuretic responses to DPCPX and to the loop diuretic furosemide were compared in SD rats pretreated with either PT or saline. Male SD rats (338 ± 3 g) were randomly assigned to one of two treatment groups and received a bolus i.v. injection of either PT (30 µg/kg) or 0.9% saline (150 µl/kg). The i.v. injection was performed under halothane anesthesia as described above.
The acute diuretic responses to DPCPX and furosemide were measured in the anesthetized saline- and PT-treated SD rats 3 to 5 days following the i.v. injection. Each rat was anesthetized with Inactin (100 mg/kg i.p.), and instrumented as described in protocol A. Following the surgical preparation, infusions of DMSO (0.65 µl/min) and [14C]inulin (0.035 µCi/100 µl of 0.9% saline/min) were initiated, and a 90-min stabilization period was permitted. Following the stabilization period, MABP, HR and RBF were recorded at 5-min intervals, and a urine sample and mid-point blood sample were collected during a 30-min baseline period (period 1; DMSO). The DMSO infusion was replaced with an infusion of DPCPX at 10 µg/kg/min, and a 30-min stabilization period was permitted. Then, MABP, HR and RBF were recorded at 5-min intervals, and a urine sample and mid-point blood sample were collected during an additional 30-min DPCPX period (period 2; DPCPX). On completion of period 2, infusion of a solution containing both DPCPX and furosemide (each at 10 µg/kg/min) was initiated. Following a 30-min stabilization period, MABP, HR and RBF were recorded, and urine and blood samples were collected for an additional 30-min period (period 3; furosemide + DPCPX). After completion of period 3, rats were euthanatized and the left kidneys were weighed. Samples were analyzed as described in protocol A.Chemicals. PT, furosemide, and DMSO were purchased from Sigma Chemical Co. (St. Louis, MO), and halothane was purchased from A.J. Buck & Son (Owings Mills, MD). DPCPX and Inactin were purchased from Research Biochemicals (Natick, MA), and [14C]inulin was purchased from NEN (Boston, MA).
Statistics. Data from protocol A were analyzed by repeated-measures, two-factor (2F) ANOVA where factor 1 is strain (SHR versus WKY) and factor 2 is DPCPX infusion rate (0, 1, or 10 µg/kg/min). In protocol B, baseline data from saline- and PT-injected rats were compared by 2F-ANOVA where factor 1 is strain (SHR versus WKY) and factor 2 is treatment (saline versus PT). Values measured during period 1 (baseline) were subtracted from period 2 to obtain DPCPX- or vehicle-induced changes from baseline. The DPCPX- and vehicle-induced changes from baseline were compared in saline-injected rats by 2F-ANOVA where factor 1 is strain (SHR versus WKY) and factor 2 is treatment (DPCPX versus vehicle). The DPCPX-induced changes from baseline were compared in saline- and PT-injected rats by 2F-ANOVA where factor 1 is strain (SHR versus WKY) and factor 2 is treatment (saline versus PT). Data from protocol C in SD rats were analyzed by repeated-measures, 2F-ANOVA where factor 1 is treatment group (control versus PT-treated) and factor 2 is diuretic infusion period (1, basal; 2, DPCPX; 3, furosemide + DPCPX). Values measured in period 1 (basal) were subtracted from period 2 (DPCPX) to determine the effect of DPCPX, and values measured in period 2 were subtracted from period 3 (furosemide + DPCPX) to determine the effect of furosemide. When 2F-ANOVA detected significance (P < .05), data were further analyzed by post hoc comparison using a 2F-Fisher's least-significant difference (LSD) test.
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Results |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Our first study (protocol A) was designed to compare the effects of DPCPX (1 and 10 µg/kg/min i.v.) on renal excretory and hemodynamic function in SHR and WKY (Table 1). DPCPX increased UV and UNaV in both SHR and WKY, and the responses did not significantly differ between strains (i.e., interaction terms for UV and UNaV; P = .25 and .56, respectively). The diuretic/natriuretic response to DPCPX was not accompanied by potassium wasting in either strain (i.e., DPCPX decreased UKV in both SHR and WKY). DPCPX did not significantly alter GFR in either strain, and although the compound tended to increase RBF in SHR, the effect did not reach significance. In WKY, DPCPX decreased RBF. HR was increased in both strains during the higher infusion rate of DPCPX. DPCPX decreased MABP and renal vascular resistance (RVR) in SHR, but not in WKY.
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The purpose of our second study (protocol B) was to evaluate the
effect of PT on the response to DPCPX in both SHR and WKY. We reasoned
that if renal A1 receptors are linked to second
messenger systems via a PT-sensitive G-protein, then pretreatment with
PT should abolish the response to DPCPX. Baseline renal excretory and
hemodynamic values were measured in SHR and WKY at 3 to 5 days
following injection of saline or PT (Table
2). Although PT treatment did not produce
significant alterations in UV or UNaV, other
baseline parameters were affected, some in a strain-specific manner, by
prior treatment with PT (Kost et al., 1999).
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Effects of DPCPX versus vehicle (DMSO; time control) on renal
excretory function were compared in the saline-injected rats (Fig.
1). DPCPX (10 µg/kg/min i.v.) increased
UV and UNaV 2- to 3-fold more than DMSO, and
similarly in SHR and WKY. Compared with vehicle, DPCPX did not
significantly alter UKV in the rats used in this
protocol, although there was a tendency for DPCPX to increase
UKV in WKY. DPCPX increased HR (P < .05; overall treatment effect; 2F-ANOVA) in the saline-injected WKY
and SHR (increase of 25 ± 3 and 30 ± 5 bpm, respectively)
compared with the effect of DMSO on HR in time-control WKY and SHR
(increase of 13 ± 3 and 21 ± 6 bpm, respectively). DPCPX
produced strain-dependent hemodynamic responses in the saline-injected
rats (Fig. 2). In SHR, DPCPX tended to
increase RBF, significantly decreased MABP, and tended to decrease RVR.
In WKY, DPCPX tended to decrease RBF, tended to increase MABP, and
significantly increased RVR. DPCPX did not significantly alter GFR in
the saline-injected SHR and WKY (data not shown, 2F-ANOVA: strain,
P = .78; treatment, P = .22;
interaction, P = .58). Prior treatment with PT
abolished the diuretic/natriuretic response to DPCPX in both SHR and
WKY (Fig. 3), and abolished the
strain-dependent hemodynamic responses to DPCPX (Fig.
4).
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In our third study (protocol C), we compared the effects of DPCPX and furosemide in control and PT-treated SD rats. DPCPX significantly increased UV and UNaV by greater than 3-fold in control SD rats, but had no effect on UV and UNaV in PT-treated SD rats (Table 3, Fig. 5). In contrast, furosemide coinfused with DPCPX, markedly increased UV and UNaV in both control and PT-treated SD rats.
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Discussion |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Results from our study indicate that adenosine plays a comparable role in renal regulation of fluid homeostasis in normotension and in genetic hypertension. The adenosine A1 antagonist DPCPX produced similar diuretic and natriuretic effects in WKY and SHR without significantly increasing potassium excretion in SHR. In addition, a decrease of MABP accompanied the diuretic effect in SHR. Our findings suggest that A1 antagonists may have utility as isokaliuric diuretics in a variety of clinical states involving hypertensive patients.
Another significant finding of this study was that PT-sensitive
G-proteins appear to be involved in the signal transduction linking
antagonism of A1 receptors to
diuresis/natriuresis. In this regard, we observed that pretreatment
with PT completely abolished the diuretic/natriuretic response to DPCPX
in SHR and WKY. In support of our findings, Hayslett et al. (1995)
reported that both PT and DPCPX blocked A1
agonist-stimulated epithelial sodium transport (assessed by measuring
short-circuit current in cultured amphibian A6
cells), suggesting that A1 receptors are linked
to tubular sodium transport through PT-sensitive
Gi/Go-proteins. However,
our findings are at odds with an earlier published study in which the
authors did not observe attenuation of the diuretic/natriuretic effects
of A1 receptor blockade by PT-pretreatment in
rats (Mizumoto et al., 1993).
The contrasting results regarding the ability of PT to inhibit the diuretic response to A1 antagonists necessitated further experimentation on our part. One of our concerns was that the PT-sensitive mechanism linking A1 receptors to natriuresis/diuresis in rats may be present in a strain-dependent manner (i.e., in WKY and genetically related SHR). To address this concern, we compared the natriuretic/diuretic response to DPCPX in control and PT-treated SD rats, a strain unrelated to WKY and SHR. We found that DPCPX produced a profound diuresis in the control SD rats, but had no effect on urine and sodium excretion in the PT-treated SD rats. An additional concern was that PT may have produced some alterations in the rats so as to inhibit their response to any diuretic. To assess this concern, we compared the responses to furosemide and DPCPX in PT-treated SD rats. In the PT-treated SD rats, DPCPX failed to evoke a diuretic response. However, coinfusion of furosemide with DPCPX resulted in marked diuresis, indicating that the PT-treated rats are able to excrete sodium and water in response to a diuretic that acts through a mechanism other than A1 blockade.
There were several differences in the technical aspects of our study
compared with that of Mizumoto et al. (1993). Rats in our study were
anesthetized with thiobutabarbital, whereas rats in Mizumoto's study
were anesthetized with urethane. In addition, the level of
extracellular fluid expansion achieved during the clearance periods
differed in that Mizumoto et al. (1993) infused saline at a rate of 2 ml/h compared with 6 ml/h in this study. Whether these technical
aspects impacted A1-linked signal transduction pathways differently is not known.
Further differences between our study and that of Mizumoto et al.
(1993) include the dose of PT administered, and the period of time
between the PT injection and the acute experiment. In the study by
Mizumoto et al. (1993), rats were pretreated with 10 µg/kg PT and
were studied 7 days later. In this study, rats were pretreated with 30 µg/kg PT and were studied 3 to 5 days later. We chose to perform our
studies within the window of 3 to 5 days following the PT injection
because a minimum of 48 to 72 h is required to observe signs of
maximal ADP ribosylation of G-protein 
-subunits following in vivo
treatment (Komatsu et al., 1995), and because we were concerned that
ADP-ribosylated G-proteins might be replaced de novo if animals were
studied much beyond 5 days following injection. Although it is
conceivable that the 10-µg/kg dose of PT did not produce adequate
inhibition, or that G-proteins were replenished by 7 days, we propose
it is unlikely that this PT dosing difference could account for our dissimilar findings. The 10-µg/kg dose of PT used by Mizumoto et al.
(1993) was shown to attenuate the Gi-protein-mediated bradycardic response to A1 receptor activation at 7 days
after administration, and it is improbable that cardiac G-proteins
would be affected differently, or replenished more slowly, than tubular
G-proteins. At present, it is unclear why our study and that of
Mizumoto et al. (1993) produced contrasting results regarding the
ability of PT to alter diuretic responses evoked by
A1 antagonists.
Results from our study indicate that the diuretic effects of
A1 blockade are primarily mediated at the level
of the tubule. It is well recognized that adenosine has multiple renal
hemodynamic effects (Osswald et al., 1978, Spielman and Thompson, 1982;
Murray and Churchill, 1984; for review, see Jackson, 1997), and it is conceivable that the diuresis/natriuresis produced by
A1 receptor blockade could be secondary to
alterations in renal hemodynamics. However, we did not observe a
significant DPCPX-induced increase of GFR or RBF, indicating that the
diuretic/natriuretic effect was not secondary to hemodynamic changes.
Our results are consistent with earlier clearance studies (Suzuki et
al., 1992; Knight et al., 1993; Kuan et al., 1993) that used the
A1 receptor antagonists DPCPX, KW-3902, and
FK453. In addition, Munger and Jackson (1994) used renal
micropuncture techniques to demonstrate that DPCPX induced diuretic
effects without significantly altering GFR, and Zou et al. (1999)
demonstrated that infusion of DPCPX directly into the renal medullary
interstitial tissue increased UV and UNaV without
altering GFR or medullary blood flow. Collectively, these studies
provide compelling evidence that A1 blockade
induces diuresis/natriuresis primarily via a tubular effect, rather
than secondary to a renal hemodynamic effect.
The major tubular site of action for
A1-antagonists is not currently known. DPCPX has
been shown to increase fractional excretion of lithium (a marker of
fluid delivery from the proximal tubule) in vivo in the anesthetized
rat (Knight et al., 1993), and to inhibit
Na+/3HCO3
symport activity in vitro in the microperfused rabbit proximal convoluted tubule (Takeda et al., 1993), indicating that the proximal tubule may be an important site of natriuretic action for the A1 antagonists. Given that the natriuretic effect
of DPCPX may be mediated via inhibition of proximal tubular sodium
reabsorption, one might expect a reduction of GFR during the increased
distal sodium delivery due to activation of tubuloglomerular feedback (TGF). However, GFR was not decreased in our study during DPCPX-induced natriuresis. Our data support the concept that adenosine plays a role
in mediating TGF (Osswald et al., 1982), and are consistent with
prior studies showing that blockade of A1
receptors inhibits TGF, thus uncoupling GFR from the rate of downstream
sodium reabsorption (Kawabata et al., 1998; Wilcox et al., 1999).
Despite qualitatively consistent and quantitatively significant
increases (in the range of 2-3-fold) of both UV and
UNaV during infusion of DPCPX,
UKV was either unaffected, modestly increased, or
decreased by DPCPX in our studies. Previous studies (Knight et al.,
1993; Kuan et al., 1993; Mizumoto et al., 1993; Gellai et al., 1998)
have demonstrated that blockade of A1 receptors produces natriuresis with minimal impact on potassium excretion, thus
our findings are consistent with previously published data, and
indicate that A1 antagonists may prove useful as
isokaliuric diuretics.
We observed a significant reduction of BP (~10-20 mm Hg) in SHR, but not WKY, during acute treatment with DPCPX. It is important to note that our study may have underestimated the diuretic/natriuretic effect of DPCPX in SHR relative to WKY. The acute hypotensive response in SHR may have partially opposed the direct natriuretic effects of DPCPX by: 1) activating renal sympathetic activity thereby increasing sympathetically mediated sodium reabsorption; and 2) reducing renal perfusion pressure thereby decreasing the driving force for sodium excretion (i.e., altered pressure-natriuresis). In the absence of the hypotensive effect, it may be anticipated that the diuretic/natriuretic response to DPCPX in SHR could exceed that observed in WKY.
Despite the strain-specific effect on MABP, HR was increased during
DPCPX infusion in both SHR and WKY. The increase in HR during DPCPX
infusion may be attributable to blockade of cardiac A1 receptors. The mechanism by which the acute BP
reduction occurs in SHR is unclear. In SHR, injections of adenosine
(Ohnishi et al., 1986a) or selective adenosine receptor agonists
(Casati et al., 1994) may lower BP via activation of vascular
A2 receptors (resulting in decreased peripheral
resistance), via activation of cardiac A1
receptors (resulting in bradycardia and decreased cardiac output), and
via activation of presynaptic inhibitory A1
receptors (resulting in decreased sympathetic tone). Further studies,
perhaps using a chronic treatment regimen in conscious SHR, are
required to determine whether the beneficial BP-lowering effects of
A1 antagonists are sustained, and through what
mechanism they occur.
In summary, the A1 antagonist DPCPX produced similar acute natriuretic/diuretic effects in WKY and SHR, with a strain-specific reduction of BP in SHR. Inhibition of Gi/Go-proteins via PT abolished the diuretic/natriuretic response to DPCPX in both SHR and WKY. In PT-treated SD rats, DPCPX failed to evoke a significant diuresis, whereas coinfusion of furosemide with DPCPX resulted in marked diuresis. Collectively, our data indicate that A1 antagonists may be effective diuretics in the treatment of genetic hypertension, and that the mechanism of action involves blockade of A1 receptors linked to tubular sodium transport via PT-sensitive G-proteins.
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Footnotes |
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Accepted for publication November 4, 1999.
Received for publication July 22, 1999.
1 This work was supported in part by National Heart, Lung, and Blood Institute Grants HL-35909 and HL-55314.
Send reprint requests to: Curtis K. Kost, Jr. Ph.D., 623 Scaife Hall, Center for Clinical Pharmacology, 200 Lothrop St., University of Pittsburgh Medical Center, Pittsburgh, PA 15213. E-mail: Kost{at}med1.dept-med.pitt.edu
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Abbreviations |
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DPCPX, 1,3-dipropyl-8-cyclopentylxanthine; BP, blood pressure; WKY, Wistar-Kyoto rats; SHR, spontaneously hypertensive rats; PT, pertussis toxin; SD, Sprague Dawley; MABP, mean arterial blood pressure; HR, heart rate; DMSO, dimethyl sulfoxide; RBF, renal blood flow; UV, urine volume; GFR, glomerular filtration rate; 2F, two factor; LSD, least-significant difference; TGF, tubuloglomerular feedback; RVR, renal vascular resistance; g kid, gram of kidney weight.
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References |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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cotransport in rabbit proximal convoluted tubule via adenosine A1 receptor.
Am J Physiol
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A. Deng, C. M. Miracle, M. Lortie, J. Satriano, F. B. Gabbai, K. A. Munger, S. C. Thomson, and R. C. Blantz Kidney oxygen consumption, carbonic anhydrase, and proton secretion Am J Physiol Renal Physiol, May 1, 2006; 290(5): F1009 - F1015. [Abstract] [Full Text] [PDF]
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T. Rieg, H. Steigele, J. Schnermann, K. Richter, H. Osswald, and V. Vallon Requirement of Intact Adenosine A1 Receptors for the Diuretic and Natriuretic Action of the Methylxanthines Theophylline and Caffeine J. Pharmacol. Exp. Ther., April 1, 2005; 313(1): 403 - 409. [Abstract] [Full Text] [PDF]
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L. Gao, C. Zhu, and E. K. Jackson alpha 2-Adrenoceptors Potentiate Angiotensin II- and Vasopressin-Induced Renal Vasoconstriction in Spontaneously Hypertensive Rats J. Pharmacol. Exp. Ther., May 1, 2003; 305(2): 581 - 586. [Abstract] [Full Text] [PDF]
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E. K. Jackson and R. K. Dubey Role of the extracellular cAMP-adenosine pathway in renal physiology Am J Physiol Renal Physiol, October 1, 2001; 281(4): F597 - F612. [Abstract] [Full Text] [PDF]
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