Infusion of ANG II at a rate not
sufficient to evoke an immediate vasoconstrictor response, produces a
slow increase in blood pressure. Circulating levels of ANG II may be
within ranges found in normotensive individuals, although
inappropriately high with respect to sodium intake. When ANG II levels
are dissociated from sodium levels, oxidative stress (OXST) occurs,
which can increase blood pressure by several mechanisms. These
include inadequate production or reduction of bioavailability of nitric
oxide, alterations in metabolism of arachidonic acid, resulting in an
increase in vasoconstrictors and decrease in vasodilators, and
upregulation of endothelin. This cascade of events appears to be
linked, because ANG II hypertension can be blocked by inhibition of any
factor located distally, blockade of ANG II, OXST, or endothelin. Such characteristics are shared by other models of hypertension, such as
essential hypertension, hypertension induced by reduction in renal
mass, and renovascular hypertension. Thus these findings are clinically
important because they reveal 1) uncoupling between ANG II
and sodium, which can trigger pathological conditions; 2)
the various OXST mechanisms that may be involved in hypertension; and
3) therapeutic interventions for hypertension developed with the knowledge of the cascade involving OXST.
isoprostanes; endothelin; spontaneous hypertension; renovascular
hypertension; sodium balance
 |
INTRODUCTION |
THREE DECADES AGO, the production of
free radicals was thought to be the result of severe injuries or
pathological insults such as those resulting from exposure to
high-energy radiation or toxins such as carbon tetrachloride
(127). However, in 1968, the discovery of superoxide
dismutase (SOD) by McCord and Fridovich (96) strongly
suggested that all aerobically metabolizing cells are capable of
producing superoxide ions, which could play a role in normal metabolic
processes. The concomitant discovery of the biology of nitric oxide
(NO) strongly suggested that free radicals could constitute the basis
of metabolic regulation (67, 90, 118). One of the first
findings supporting such a notion was the demonstration that the
inhibition of the synthesis of NO produced by the administration
of an NO synthesis competitor, such as
N
-monomethyl-L- arginine] or
NG-nitro-L-arginine methyl ester
(L-NAME), produced a marked vasoconstriction (13,
136, 173), sodium retention (82, 83), and thereby a
sustained increase in mean arterial pressure (MAP) (13, 82, 83,
136, 173). The identification of a specific pathological situation in humans, where a decrease of NO may end up in an elevation of blood pressure was thought to be linked to endothelial dysfunction (105), such as those involved in aging, arteriosclerosis,
etc., but the specific metabolic alterations involved in this process were poorly understood. However, the relationship between a fall in NO
with oxidative stress (OXST) became apparent when Beckman et al.
(14) reported in 1990 that NO could combine with
superoxide with a high degree of affinity to form peroxynitrite and
that peroxynitrous acid was a constant source of active hydroxyl
radical with potent oxidative properties (14).
Furthermore, the study showed that superoxide and NO react together
with a rate constant that is as large or larger than those for the
reaction of superoxide with SOD or for NO with heme compounds
(97, 170). The role of OXST in the regulation of the
circulation was emphasized by the study of Rajagopalan et al.
(129), who showed that the administration of large doses
of ANG II to rats induced an increase in both blood pressure and the
formation of superoxide from isolated aortic strips. This response
could not be evoked by producing an equivalent elevation in blood
pressure with other vasoconstrictors, such as norepinephrine. The
specificity of ANG II to stimulate OXST, however, was questioned by an
experimental finding showing that the circulating levels of ANG II
during sodium restriction can be much higher than those necessary
to induce hypertension and other pathological effects
(138).
All these findings raise a number of important questions
with regard to the relationship of angiotensin, OXST, and NO in the regulation of renal blood flow (RBF), sodium excretion, and ultimately blood pressure. In this review we will try to answer these questions. Furthermore, we will examine first the effects of intravenous infusion
of pressor and subpressor doses of ANG II and the extent to which this
maneuver can be used as an experimental model to mimic different forms
of essential hypertension in humans. The understanding of this effect
of angiotensin necessitates an examination of the distribution of this
peptide in the intravascular, renal interstitial, and intracellular
compartment. We will also focus on the extent to which an imbalance
between the levels of angiotensin with respect to sodium intake and the
synthesis of NO will favor the production of OXST and also the
component of OXST that appears to be involved in the development of
hypertension (138). We will examine evidence showing that
the renal dysfunction generated by the disequilibrium between
angiotensin, NO, and OXST specifically affects RBF, the afferent and
efferent glomerular arteriolar tone, tubular glomerular feedback (TGF),
and medullary blood flow. We will also analyze evidence showing the
participation of these dysfunctions in models of experimental
hypertension such as in spontaneously hypertensive rats (SHR).
For reasons of space, we will not examine the alterations in
intracellular signaling by which the stimulation of AT1
angiotensin receptors lead to an exaggerated formation of oxygen by the
mitochondria as well as the intracellular components involved in other
pathological actions of angiotensin such as hypertrophy, arteriole
remodeling, inflammatory components, etc.
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CHARACTERISTICS OF THE VASOCONSTRICTOR RESPONSE EVOKED BY ANG II |
The vasopressor response to ANG II has been classified as both
fast and slow.
Fast pressor responses.
It is well known that the intravenous administration of a relatively
large dose of ANG II
(>ng · kg
1 · min1)
given as a bolus produces a rapid contraction of the smooth muscle
through the phosphoinositide-Ca2+-protein kinase C (PKC)
effector system (4, 53, 110, 158). The response achieves a
maximal pressor effect in seconds and returns to normal levels in
2-3 min (60, 109). Comparative studies suggest that
the fast responses to ANG II may only constitute a pharmacological
phenomenon, because a sustained increase in the circulating levels of
ANG II that is necessary to evoke a rapid elevation of blood pressure
of this magnitude is higher than 2,500 pg/ml of plasma
(23). Concentrations of this high level of circulating ANG
II are never seen in a physiological (severe sodium deprivation) or in
pathological (severe renovascular hypertension or hemorrhage) situations.
Slow pressor responses.
Another type of vasopressor response induced by ANG II consists in the
progressive elevation of MAP induced by the continuous administration
of a subpressor dose of ANG II; that is to say, at doses of ANG II that
do not evoke an immediate pressor response. For example, in rats, Hu et
al. (62) showed that the continuous administration of a
dose as low as 3.5 ng/min of ANG II produced hypertension almost 1 wk
after commencing the infusion. This kind of "delayed or slow pressor
response" was first demonstrated by Dickinson and Lawrence in 1963 (38). They observed that a continuous infusion of an
amount of ANG II below the threshold of the direct vasoconstrictor
effect, elicited a gradual increase in blood pressure. Two years later,
McCubbin et al. (98) reported similar findings in dogs.
Slow pressor responses have also been demonstrated in a studies
conducted in different species, such as rabbits (38), dogs
(98), rats (23, 62), swine (55),
and humans (5). At present, the minimal amount of ANG II
capable of producing a slow response (delayed elevation of blood
pressure) in humans and animals has not been determined, although the
slow pressor response to ANG II appears to be evoked at much lower
doses in humans (30, 48, 49, 152) than in animals
(5, 23, 38, 55, 62, 98), as discussed below.
A more detailed analysis of the characteristics of the slow pressor
response is pertinent because this model of hypertension resembles most
of the characteristics of essential hypertension found in humans:
1) the levels of circulating ANG II necessary to produce the
development of delayed hypertension can be within the range of those
found in normotensive individuals; 2) the hypertension evolves with an early increase in intrarenal resistance that coexists with sodium retention (this latter effect subsides after hypertension has been achieved); and 3) there is a significant elevation
of peripheral resistance that plays an important role in the elevation of blood pressure since cardiac output remains normal.
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PERIPHERAL LEVELS OF ANG II NEEDED TO PRODUCE HYPERTENSION |
The levels of circulating ANG II in humans with essential
hypertension remains undefined. In these patients, determination of
plasma renin activity (PRA) has been routinely measured rather than
circulating levels of ANG II (25). However, it is
difficult to compare studies, because the levels of circulating ANG II
cannot actually be derived from measurements of PRA, although under
most conditions there is a correlation between the two determinations. However, when PRA is measured and plotted against urinary sodium excretion, 50% of essential hypertensive patients exhibit levels of
PRA no different from that seen in normotensive individuals (25). Furthermore, a subset of the population of
hypertensive individuals exists (25%) in whom the level of circulating
PRA is significantly less than that measured in normotensive
individuals (25). ANG II does contribute to the
maintenance of hypertension in these essential hypertensive individuals
because blood pressure is markedly reduced by the administration of
either converting enzyme inhibitors (72a, 42) or ANG II receptor
antagonists (44). Furthermore, this is particularly true
in the diabetic low renin state (126) and in reduced renal
mass hypertension (73) in which circulating levels of ANG
II are not different from normal (33, 81, 190).
These observations raise the question of whether hypertension can be
produced or even maintained with normal levels of ANG II. This issue
can be better understood by examining the peripheral levels of ANG II
that are actually needed to produce hypertension through slow pressor
responses. In the above mentioned study performed by Brown et al.
(23), it was found that the continuous intravenous infusion of 20 ng · kg1 · min1
of ANG II to conscious rats failed to elicit any detectable pressor response over the control values (103 ± 4 mmHg) during the first hour of the infusion. However, the following day, MAP increased by 15 mmHg. On day 7 of infusion, MAP reached 153 ± 6 mmHg.
The level of circulating ANG II in these hypertensive animals on
day 7 was 249 ± 25 pg/ml of plasma, which is threefold
higher than the level recorded in normotensive controls infused with
dextrose (80 ± 19 pg/ml). It was also found that the amount of
ANG II needed to produce, during 1 h (i.e., a fast response), an
elevation of MAP to levels comparable to that seen on the 7th day in
the chronically infused hypertensive animals was 2,700 pg · kg1 · min1.
This finding proved that continuous infusion of subpressor doses of ANG
II, which is accompanied by small increments in plasma ANG II (see
later), is capable of producing the same increment in blood pressure as
that induced by the fast infusion of a dose of ANG II that is ~10
times higher than that needed to produce the slow response. This
observation led Dickinson and Lawrence (38) to state that
"ANG II may bring into action some secondary mechanism which sustains
arterial pressure by means other than general arterial vasoconstriction
due directly to the hormone."
One of the conceptual problems that one encounters when attempting to
model the development of essential hypertension with the infusion of
subpressor doses of ANG II is that a mild increase of ANG II
presupposes the requirement of a concomitant increase in PRA. This
implies an excessive release of renin without the modification of
factors that control the velocity of the renin-angiotensinogen reaction
(134), particularly a rapid depletion of renin substrate or even the absence of changes of renin binding by extrarenal renin
receptors (28, 150). An important study to define this problem was undertaken by Hu et al. (62), who showed that
slow responses can be developed in the rat by chronic administration of
either ANG II (3.5 ng/min) or rat recombinant renin (0.6 ng/min), compared with vehicle control (Fig. 1).
The results are somewhat similar to those reported by Brown et al.
(23) in that the control levels of plasma ANG II were
statistically elevated by 2.5-fold from ~4.5 ± 0.8 up to
10.7 ± 0.7 pg/ml (23). However, it differs in that
on days 12-13, the concentrations of ANG II in plasma return to levels similar to those recorded during the control period
(Fig. 1). The renin-infused animals experienced a slow and progressive
increase in blood pressure very similar to that found in ANG II-infused
animals. The studies of Hu et al. (62) and Brown et al.
(23) clearly show that the administration of subpressor
doses of ANG II or renin induced an increase in blood pressure that was
sustained, despite the fact that the concentration of ANG II in plasma
was never increased by more than a small amount (23) or
was only transiently increased (62).
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Fig. 1.
Plasma ANG II levels during renin (solid bars) or ANG II
(open bars) infusions. Each time point represents results from equal
volume of plasma from a 3-day period. C, control days; E, experimental
days; R, recovery days. Values of blood pressure (bottom)
are average obtained from both groups during the 3-day period.
*P < 0.05 vs. control period;  P < 0.05 vs. recovery period. (Modified from Ref. 62.)
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Highly relevant to the subject under discussion are the studies of Ames
et al. (5), who were the first to investigate the chronic
effects (6-11 days) of ANG II in humans. These investigators infused in normal volunteers an amount of ANG II (~25
ng · kg1 · min1)
that was capable of producing an initial increase in blood pressure of
20-30 mmHg (fast pressor response). However, as soon as sodium retention occurred, the subjects became increasingly sensitive to the
pressor effect of ANG II in a manner comparable to that seen during the
development of slow pressor responses (Fig.
2). Under these conditions blood pressure
was maintained as ANG II doses were progressively diminished to
one-fourth of the initial dose (Fig. 2). Similar observations were
later on made by Laragh et al. (85). In later studies
Gandhi et al. (49), Cargill et al. (30),
Shoback et al. (152), and Gaboury et al. (48) reported that ANG II infusions as low as 2-3
ng · kg1 · min1
over 20-30 min caused acute increases in blood pressure in
normotensive individuals on a normal salt diet. These studies show
first that much lower doses of ANG II are typically required to
increase blood pressure in humans than in experimental animals. The
doses of ANG II that produced slow pressor effect in experimental
animals were below 50 ng · kg1 · min1
(23, 38, 55, 62, 98). These doses produce fast pressor responses in humans (30, 48, 49, 152). Second, as shown by
Ames et al. (5) and Laragh et al. (85), a
potentiating pressor effect similar to that seen in slow pressor
responses developed on top of the fast pressor responses. The chronic
pressor potentiation in response to ANG II is then an effect that
occurs whenever ANG II is infused (or increased above normal) in a
chronic fashion. The doses of ANG II that are within the subpressor
range in humans have not yet been established, although Hollenberg et al. (60) reported that ANG II infusion of 1 ng · kg1 · min1
had no effect on MAP in humans but caused a marked reduction in RBF.
Unfortunately, the infusion was not continued beyond the acute phase,
so the development of slow pressor responses was not evaluated. In any
case, the consistent delay of small subpressor doses of ANG II to
produce a chronic increase in blood pressure is indicative of a time
requirement necessary for the activation of additional vasoconstrictor
processes, which can then trigger an autocatalytic reaction that
accelerates or potentiates the vasoconstrictor effect of ANG II.
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Fig. 2.
Effects of continuous angiotensin infusion (11 days) on
blood pressure (A) and urine sodium excretion
(B). Dotted lines separate infusion period from control and
postinfusion. Note that angiotensin infusion dose (C) was
significantly decreased from day 3 to keep the increase in
blood pressure constant. (Modified from Ref. 84.)
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CIRCULATING AND INTRARENAL LEVELS OF ANG II |
Before investigating the vasoconstrictor mechanism(s) that could
potentiate the effect of ANG II, it will be important to understand the
manner in which peripheral levels of ANG II relate to the
concentrations in the interstitial fluid and in the intracellular compartment of some organs. It has been known for years that the kidney
releases renin into the general circulation where it acts upon a
microglobulin (103, 157) from which several active
peptides are produced, ANG II being the one that exerts the most
prominent pressor effect (21, 128). ANG II can also be
generated by renin within the kidney. In fact, it has been emphasized
in several studies that renin, angiotensinogen, ANG I, or ANG II are
localized in secretory granules around the glomerular vascular pole and macula densa (65, 68, 70, 169). Hence, ANG I and II are probably cosecreted with renin by these cells (65, 68).
The intrarenal renin-ANG II system appears to be important because high
concentrations of ANG II have been detected in renal interstitial fluid
(156) and in proximal tubular fluid (22,
151). Furthermore, measurement of renin in renal lymph indicated
that renal interstitial fluid contains high renin concentrations
(10, 88), and the interstitial fluid compartment is
thought to be the site of intrarenal ANG II production. All these
findings suggest that there should be a gradient of ANG II
concentration that favors diffusion from the kidney to plasma and from
there to other organs, their interstitial spaces, and intracellular
compartments. This may not be the case, however. Navar et al.
(113) found that there is a significant renal uptake of
ANG II from plasma. This observation was confirmed by van Kats et al.
(174), who observed that continuous infusion of a very
small amount of 125I-labeled ANG II into the left ventricle
of rats resulted in an accumulation of this peptide in the renal
cortical tissue and was 4.1 ± 0.6 fold higher than in arterial
plasma, whereas the medullary tissue/arterial plasma concentration
ratio was 1.8 ± 0.2 (Fig. 3). The
kidney was capable of clearing 88% of the ANG II that got into the
organ through the renal artery. Furthermore, it was also found that
during the infusion of 125I-labeled ANG II, the
concentration of endogenous (naturally synthesized ANG II) in the renal
cortex was 102 ± 30 times higher than in plasma, whereas the
concentration in the renal medulla was 64.4 ± 14 times higher
than in the arterial plasma (Fig. 3). The concentrations of endogenous
(not labeled) ANG II in renal vein was 0.4 nmol. Thus the
tissue-to-blood plasma concentration ratio was much higher for
endogenous ANG II than it was for the infused 125I-ANG II.
These results essentially show that although there is an uptake of ANG
II by the kidney, ~85% of renal tissue ANG II originates from local
production in the kidney.
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Fig. 3.
Schematic representation of the relative
concentration of infused 125I-labeled ANG II (solid line)
and endogenously formed ANG II (dashed line) in renal cortex and
medulla and in arterial and venous plasma. For simplicity, the amounts
of both forms of ANG II are given in arbitrary units (pg/ml) assuming
that during the infusion of ANG II, the concentration achieved in
plasma was 1 pg/ml.
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An additional finding of van Kats et al. (174) was that
when infusion of ANG II was repeated in the presence of converting enzyme inhibitors, the tissue-to-plasma ratio of infused labeled ANG II
or endogenous ANG II was not significantly altered. However, the
administration of ANG II receptor antagonist significantly decreased
the renal uptake of labeled ANG II, the tissue concentration of which
reached levels below that recorded in plasma. ANG II receptor
antagonists also resulted in a 75-80% decrease in the concentration ratio of endogenous ANG II, although endogenous ANG II
remained higher in renal tissue than in plasma. This latter part of the
study confirms previous observations by other investigators that
cellular uptake of ANG II in the kidney depends on binding to cell
membrane AT1 receptors. This process is then followed by
receptor-mediated endocytosis. Cell-associated, blood-derived ANG II
has a longer half-life than ANG II in the circulation
(175), but the function of internalized ANG II is not
completely known. The finding that ANG II receptor antagonist has a
lowering effect on renal ANG II uptake and decreases endogenous
concentrations of renal ANG II, effects that are not obtained by
converting enzyme inhibitors, may have clinical repercussions that
should be further investigated.
These results of van Kats (174) agree with those published
by Nishiyama et al. (115), who conducted a study to
determine the existing endogeneous levels of ANG I and ANG II in plasma and renal interstitium by means of microdialysis probes implanted in
the renal cortex of rats. It was observed that the concentration of ANG
I and ANG II in the renal interstitium were, respectively, 0.84 ± 0.04 and 3.07 ± 0.43 nmol/g (Fig. 4).
These values were much higher than the respective plasma levels (ANG I
0.112 ± 0.014 nmol/l; ANG II 0.106 ± 0.008 nmol/l). This is
to say that, in renal interstitium, the concentration of ANG II was
3.65-fold higher than ANG I, whereas interstitial levels of ANG II were 29-fold higher than in plasma. ANG I in renal interstitium was only
7.5-fold higher than in plasma (Fig. 4). These investigations also
found that the administration of an angiotensin-converting enzyme
inhibitor decreased the concentration of ANG II in plasma but failed to
change the concentration of ANG II in the renal interstitium.
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Fig. 4.
ANG I and II levels measured in plasma and in renal
interstitial fluid with microdialysis probes implanted in the kidneys
of anesthetized rats. Values of ANG I and ANG II are modified
from Ref. 115.
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Thus from these studies it can be concluded that the intrarenal
concentration of ANG II may not necessarily be related to the
peripheral levels in plasma, because ANG II can accumulate in the renal
interstitium and renal cells by continuous formation within the kidney
and this concentration can be further enhanced by renal uptake from
peripheral circulation. These findings have important physiological and
pathological connotations, because they may indicate that the levels of
ANG II that matter are not those found in plasma but those contained
within the kidney itself. In fact, as we will see later, the kidney
could be the most indicated place for the organism "to monitor"
changes in the release of renin in relation to changes in extracellular
fluid volumes.
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THE EFFECTS OF SODIUM INTAKE ON THE PRESSOR RESPONSE TO ANG II |
As mentioned above, the physiological levels of PRA correlate
inversely with the amount of sodium intake. Brunner et al.
(25) measured the levels of PRA at different levels of
sodium intake in hypertensive and normotensive patients. This allowed
the investigators to construct a nomogram in which low urinary
excretion of sodium (from 0 to 30 meq/day) was associated with
12-13
ng · ml1 · h1
of PRA, whereas urinary excretion of sodium in the range of 300 meq/day
resulted in suppression of PRA to 0.5-1
ng · ml1 · h1
(Fig. 5). This relationship indicates
that for any given level of sodium intake, the existing levels of ANG
II and NO are within the range needed to maintain sodium balance and
normal arterial pressure. Therefore, an increase of ANG II to levels
that are capable of producing a slow pressor effect should necessarily be above those concentrations that are at that moment determined by the
existing amount of sodium intake or volume repletion (ANG II-fluid
volume coupling). This relationship can be monitored by the kidney,
because there is an inverse relationship between sodium intake and
renal synthesis of renin (52, 69) while renal interstitial
pressure correlates well with volume expansion (27). One
important corollary of this relationship between renin and sodium
intake is that the levels of ANG II that may be excessive for the
existing fluid volume when sodium intake is high (300 meq/day) and are
capable of triggering slow pressor responses may be much lower than the
physiological levels that are spontaneously produced when sodium intake
is reduced to very low levels. Consequently, any plasma or renal level
of PRA within the normal range can induce hypertension if they are
inappropriately high with respect to fluid volume. Studies to determine
the consequences of uncoupling body fluid volume from the level of ANG
II were initially conducted by DeClue et al. (36). These
investigators showed (Fig. 6) that in
volume-depleted animals the extracellular fluid volume expansion induced by progressive increases of sodium intake from 3 meq · kg1 · day1
(a total of 63 meq/dog) to 25 meq · kg1 · day1
(a total of 525 meq/dog) produced a small 3% increase in MAP. Extracellular fluid volume was measured with radiolabeled
22Na. However, when a comparable sodium load was given in
dogs in which ANG II was "clamped" by continuous infusion of small
subpressor doses of ANG II (5 ng · kg1 · min1),
blood pressure was increased by 45 ± 3.8 mmHg or 46% over the control period (see Fig. 5). This finding was interpreted to indicate that although the infusions of ANG II were too small to produce any
change in blood pressure in volume-depleted animals, it was sufficient
to shift pressure natriuresis impairing sodium excretion during volume
expansion inducing hypertension. Therefore, when ANG II was clamped,
the progressive increase in body fluid volume and renal interstitial
pressure became excessive (or inappropriate) to the existing plasma or
renal levels of ANG II. The word inappropriate may be more pertinent
because the hypertensinogenic effect of ANG II was produced in the
presence of volume expansion that was not different from animals that
did not receive ANG II and remained normotensive, although many other
investigators have shown that increased sodium intake causes a
marked potentiation of ANG II pressor effect (6, 35, 74,
108, 142, 154). DeClue et al. (36) was the first to
demonstrate that a dissociation between extracellular fluid volume and
the plasma or perhaps tissue levels of ANG II may be an important
alteration underlying the development of high blood pressure. Such
dissociation (or uncoupling) can be set by either inducing a
progressive volume expansion in the presence of a fixed level of ANG II
or, vice versa, by increasing the level of ANG II in the presence of a
fixed volume. For example, a progressive infusion of small doses of ANG
II that do not produce a pressor effect in dogs in normal conditions
become hypertensive when infused into volume-expanded animals
(36). This latter study was performed in dogs fed with 544 meq of sodium a day for 5 days, a sodium load similar to the highest
intake given to the dogs in the experiment of DeClue et al.
(36). In these dogs, progressively increasing infusions of
subpressor doses of ANG II from 0.25 to 1, 2.5, and finally 5 ng · kg1 · min1
resulted in 10, 20, and 40% increments in MAP, respectively
(36).
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Fig. 5.
Relation of the ambulatory plasma renin activity (PRA;
vertical axis) to the concurrent daily rate of urinary sodium excretion
(horizontal axis) in essential hypertensive patients. Dashed lines
define the range of PRA values found in normotensive patients. Open
circle, low renin essential hypertension; gray circle, normal renin
essential hypertension; solid circle, high renin essential
hypertension. (Modified from Ref. 25.)
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Fig. 6.
Effects of progressively increased sodium intake
(horizontal axis) on mean arterial pressure (MAP; vertical axis) of
dogs in which circulating levels of angiotensin were clamped by an
infusion of 5 ng · kg1 · min1
ANG II. (Modified from Ref. 36.)
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Finally, an additional observation that corroborates the importance of
the dissociation between the levels of ANG II and body fluid volume
required to produce hypertension is that in dogs made hypertensive by a
large sodium load (500 meq/day) on top of ANG II infusions (5 ng · kg1 · min1),
the administration of a diuretic (furosemide) tends to normalize blood
pressure by reducing fluid volumes to levels that are presumably "appropriate" to the concentration of ANG II (Fig.
7) (36). This observation
may explain the therapeutic success in hypertensive patients of both
ANG II inhibitors (because it renders the activity of ANG II
appropriate to fluid volumes) or diuretics, which render fluid volume
appropriate to the levels of ANG II (36). However, diuretics are less effective in normalizing blood pressure than an ANG
II antagonist because volume contraction stimulates the release of
renin, thus increasing the levels of renal and circulating ANG II.
Hence, volume depletion should be driven to almost a maximum to drop
MAP close to normal levels. As shown in Fig. 7 the dose of furosemide
necessary to lower MAP was 85 mg/day. Relevant to the role of volume in
ANG II-induced hypertension are the studies of Krieger and Cowley
(78), who demonstrated that ANG II-induced hypertension
can be prevented if volume expansion is servocontrolled and maintained
within normal levels. The concept of excessive levels of ANG II with
respect to the levels of sodium intake was introduced by Hollenberg et
al. (60) when they observed that there was a human
population of hypertensive individuals who were incapable of lowering
the levels of PRA during fluid volume expansion. They called this group
"nonmodulators." This group of investigators also found that this
alteration was corrected by converting enzyme inhibitors. It should be
mentioned here that Dickinson and Yu (39) strongly
emphasized that sodium retention and/or volume expansion is not an
important condition to elicit slow responses to ANG II. These
investigators showed that a progressive rise of blood pressure of
~20-30 mmHg obtained by the infusion of 0.01 µg · kg1 · min1
of ANG II for 3 days was not accompanied by any significant change in
urinary excretion of sodium, urine volume, or body weight. We concur
with such a conclusion that slow responses to angiotensin are not
dependent on a measurable volume expansion. However, the results of
Dickinson and Yu (39) can be reinterpreted as indicating that ANG II exerted a strong antinatriuretic effect because the increase in blood pressure did not induce the habitual pressure natriuresis. Under these conditions, it can be safely assumed that the
levels of sodium excretion and/or blood pressure were inappropriate to
the levels of angiotensin in plasma or in the kidney.
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Fig. 7.
Decreasing effects of furosemide on blood pressure in dogs made
hypertensive by the combination of a high intake of sodium chloride
intake (500 meq/day) plus continuous angiotensin infusion (5 ng · kg1 · min1).
C, control period; A, experimental period; subscript nos., days in
period.
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SALT SENSITIVITY INDUCED BY ALTERATIONS IN THE CONCENTRATION OF
INTRARENAL NO WITH RESPECT TO ANG II |
As it is apparent, the study of DeClue et al. (36)
constitutes the first salt-sensitive model obtained in a dog by
interfering with the lowering effect that volume expansion has on the
level of ANG II. Under these conditions, the level of blood pressure is
essentially determined by the amount of sodium intake. A related model
of salt sensitivity also reported long ago was caused by the partial
inhibition of NO. Lahera et al. (83) first observed that
the acute and progressive inhibition of NO in rats obtained by the
successive infusions of 0.1, 1, and 10 mg of a NO synthesis inhibitor,
L-NAME, produced significant decrements in urine volume, urine sodium excretion, RBF, and glomerular filtration rate (GFR) before the elevation of blood pressure was detected. With the passage
of time MAP went up, bringing urine sodium excretion back to normal.
This sequence of events was also produced in 180 min during the
intravenous infusion of 10 µg · kg1 · min1
(Fig. 8). This finding clearly suggests
that the most sensitive parameter affected during partial systemic
inhibition of NO is a decrease of sodium and water excretion and an
elevation of intrarenal vascular resistance whose threshold of
activation is higher than that of systemic blood pressure
(136). This sequence of events resembles those obtained
during the infusion of small doses of ANG II, which produces an
increase in intrarenal resistance and a fall in water and sodium
excretion before the overall elevation of peripheral resistance and
blood pressure (56, 60, 62). This observation is
consistent with the idea that the effects of NO are continuously
countervailed by the vasoconstrictor effect of ANG II. Therefore, much
of the effects seen during the progressive inhibition of NO are, in
fact, produced by ANG II, whose antinatriuretic and vasoconstrictor
effects are left unbalanced (136). A finding that supports
this concept is the study of Salazar et al. (140), who
showed that the continuous administration (for 5 days) of 0.1 g · kg1 · min1
of L-NAME to dogs produces a marked decrease in water and
sodium excretion, although this small dose was not sufficient to alter blood pressure. Under these conditions, the administration of a high
sodium intake, which ranged from 80 to 300 meq/day, produced a very
significant increase in blood pressure. This study shows that partial
inhibition of NO synthesis produces salt sensitivity with similar
characteristics as that obtained by clamping the levels of ANG II
(36).
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Fig. 8.
Changes in MAP, glomerular filtration rate (GFR), and
urine sodium excretion (UNaV) at 60, 120, and 180 min of infusion of
NG-nitro-L-arginine methyl ester
(L-NAME) 10 µg · kg1 · min1.
Early decrements in GFR and UnaV (120 min) were corrected when
MAP reached 135 mmHg. (Modified from Ref. 83.)
*P < 0.05 compared with control (time 0).
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INTRARENAL FUNCTIONS REGULATED BY ANG II AND NO |
It is pertinent to briefly comment here on those effects of
intrarenal ANG II that are modulated by NO (Fig.
9) because an imbalance between NO and
ANG II produced by OXST could explain a specific renal dysfunction that
fosters slow pressor responses and thereby hypertension. The first of
these processes involves the so-called TGF mechanism that operates at a
single-nephron level to maintain a balance between the reabsorptive
function of each nephron and the amount of solids and fluids filtered
at this glomerulus (17, 111, 147, 184). The macula densa
cells of the thick ascending limb of the loop of Henle detect changes in the composition of tubular fluid entering the terminal portion of
the thick ascending limb of the loop of Henle and transmit signals that
alter glomerular vascular resistance, glomerular capillary pressure,
and thereby single-nephron GFR. Specifically, increases in the sodium
chloride or total solid concentration of the tubular fluid flowing past
the macula densa cells in response to increases in fluid flow into the
ascending limb of the loop of Henle lead to increases in glomerular
vascular resistance and decreases in single GFR (17, 111, 147,
184). Although the available evidence indicates that ANG II does
not directly mediate TGF responses, it is clear that the prevailing ANG
II levels do exert an important modulatory influence on the overall
sensitivity of this mechanism (102, 112, 114, 146, 147).
Several studies have shown that systemic administration of ANG II
receptor antagonist or ACE inhibitors attenuates the TGF in response to
increase in distal nephron perfusion rate (63, 122, 123).
When these alterations are induced by converting enzyme inhibitors, the
mechanism can be restored by the infusion of ANG II (146,
148). All together, these findings indicate that ANG II acts to
enhance the sensitivity of the vascular element that mediates TGF,
inducing alterations in single-nephron hemodynamic function (112,
114, 146, 147). Interestingly, the intensity of the response is
modulated by the production of NO because the blockade of neuronal NO
synthase produces an increase in tubuloglomerular response similar to
that observed when angiotensin is infused (181). The
physiological importance of this dualistic mechanism is that during
volume repletion produced by excessive sodium intake there is a
decrease of intrarenal synthesis of ANG II (181) with a
relative enhancement of NO. This decreases the sensitivity of the TGF,
which allows an increased natriuresis by permitting the passage of a
bigger distal delivery of solutes through the distal nephrons. The
opposite effect occurs during volume depletion where the increased
synthesis of ANG II will increase the sensitivity of TGF, producing
afferent arteriole vasoconstriction. This latter effect is important
because it could explain the early renal vasoconstriction in essential
hypertension that occurs when there is an excessive amount of ANG II.
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Fig. 9.
Schematic representation of the major renal
functions (right) that are influenced by the
equilibrium of ANG vs. nitric oxide (NO). BF, blood flow; GFR,
glomerular filtration rate; interst, interstitial.
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A second important effect of ANG II is the interaction with NO in the
regulation of glomerular vascular resistance. Under normal conditions,
the infusion of angiotensin into the kidney at low doses produces a
more pronounced vasoconstriction in the efferent than in the afferent
arteriole; however, after the blockade of NO with L-NAME,
ANG II-induced vasoconstriction of the afferent arteriole is comparable
to the vasoconstriction of the efferent. It is interpreted that the
selective efferent vasoconstriction will protect GFR from excessive
amounts of angiotensin produced within the kidney during volume
depletion. These effects have been demonstrated by Ito et al.
(72) in the isolated glomeruli with attached afferent and
efferent arteriole and by Granger et al. (145) in the
whole animal.
A third important effect of intrarenal angiotensin is the stimulation
of proximal tubular sodium reabsorption. This action takes place at
real minimum doses of ANG II that are not sufficient to alter MAP
(60, 112) or aldosterone secretion. Under these conditions, ANG II produces a marked sodium retention that leads to an
increase in arterial pressure after a few days of initiating the
infusion (112). This antinatriuretic effect can be largely explained by a direct tubular effect, which, in micropuncture studies,
was shown to be exerted at doses of ANG II that range from
1010 to 108 (58, 64, 121,
161). Such an effect, which has been discussed in other
publications (135), appears important because it is quantitatively larger than those exerted by aldosterone
(56).
The antinatriuretic effects of ANG II contrast with the natriuretic
actions of NO, which appears to be exerted by stimulation of cGMP in
distal nephrons, thus modifying amelioride NO-sensitive channels
(71, 149, 162). Moreover, NO appears to exert a tonic vasodilation of the vasa recta in the renal medulla. Variations in NO
synthesis will be followed by proportional changes in renomedullary blood flow and thereby in renal interstitial pressure and will produce
an inversely related change in sodium reabsorption (132). We will examine this mechanism later on.
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ANG II AND NO EQUILIBRIUM IN THE CONTROL OF RENAL FUNCTION AND
BLOOD PRESSURE |
The experiments discussed above suggest that a balance between the
synthesis of NO and the formation of ANG II is necessary for the
appropriate modulation of renal function and blood pressure (136,
138). This concept has been well demonstrated by studies of
several investigators showing that most of the effect produced by NO
synthesis inhibition are minimized or abolished by the simultaneous blockade of ANG II and, vice versa, that many of the consequences that
result from the blockade of ANG II (vasodilatation, fall in blood
pressure, natriuresis) are diminished by previous blockades of NO
(100, 124, 153). This interaction between ANG II and NO
indicates that the enhancement of the vasoconstrictor response of ANG
II seen during the development of slow pressor responses could be
produced by the activation of a process that impairs the activity of
NO. A process that exhibits such characteristics is OXST, because it
has the ability to quench NO and decrease its concentration in
interstitial fluid. However, OXST also involves other pressor
mechanisms, which are commented on below.
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THE MAJOR COMPONENTS OF OXST: STIMULATION BY ANG II |
Two of the major components of OXST are superoxide and hydrogen
peroxide. Superoxide and hydrogen peroxide
(H2O2) can be generated in many vascular cells
and are derived from NADPH oxidase, cyclooxygenase, lipooxygenase, heme
oxygenases, peroxidases, and hemoproteins, such as heme and hematin.
The mechanism(s) by which ANG II causes production of superoxide has
not been entirely elucidated. However, Mollnau and colleagues
(104) found that chronic (7 days) ANG II infusion
increased expression of nox 1, gp91(phox), and p22(phox) subunits of
NADPH oxidase via a PKC mechanism (Fig.
10). This ANG II-mediated increase in
the production of superoxide sets into motion a series of events that
may play important roles in hypertension.
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Fig. 10.
ANG II has been shown to upregulate synthesis of NAD(P)H
oxidase subunits to produce superoxide. Under normal conditions,
superoxide will be converted by superoxide dismutase (SOD) to hydrogen
peroxide (H2O2). Hydrogen peroxide is converted
to water via the actions of catalase and glutathione peroxidase (GPx).
Hydrogen peroxide can also be converted to hydroxyl radical
(OH). GSH, reduced glutathione; GSSG, oxidized
glutathione.
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Under normal conditions, the superoxide that is produced in the cell
combines with SOD that dismutates it to H2O2
(Fig. 10). H2O2 is further metabolized to
H2O by the action of catalase and glutathione peroxidase,
but under pathological conditions when an oxidative environment is
present in cells, H2O2 can be a source of
hydroxyl radical. Also under oxidative conditions, tetrahydrobiopterin (BH4), a cofactor required for the NO synthase activity,
can be oxidized to dihydrobiopterin (BH2) (Fig.
11). In this case, NO synthase will
produce superoxide rather than NO (163, 176, 187), making
this another mechanism by which reactive oxygen species (ROS)
can be produced and NO concentrations can be reduced.
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Fig. 11.
In the presence of tetrahydrobiopterin
(BH4), nitric oxide synthase (NOS) produces NO
(A). Under oxidative conditions, BH4 is
converted to dihydrobiopterin (BH2), and NOS produces
superoxide (O ·) (B).
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In 1990, Beckman et al. (14) reported that the product of
the combination of superoxide and NO radicals is peroxynitrite, which
is in equilibrium with peroxynitrous acid (Fig.
12A).
Peroxynitrite/peroxynitrous acid are potent oxidants, and peroxynitrous
acid can be a source of the biologically active hydroxyl radical as can
H2O2 (14). Peroxynitrite has been
shown to upregulate cyclooxygenase (COX)-mediated production of
prostaglandin E2 in macrophages from old mice
(15). In addition to stimulation of COX activity,
peroxynitrite can inhibit prostacyclin synthase activity as shown in an
endothelial cell line (195), which could lead to a
reduction in the vasodilator prostacyclin (Fig. 12B). In
addition to the above-mentioned systems, ROS have also been shown in
numerous studies to play a role in signal transduction in various cell
types (32, 188). Therefore, in a situation where ROS, such
as superoxide, H2O2, hydroxyl radical, are
produced in concentrations that are not able to be controlled by the
usual mechanisms employed by the cell, the increase in oxidation can
produce a variety of negative effects on cellular function, including
alteration of transcription factors, kinases, protein synthesis, and
redox sensitive genes, which, in turn, could influence hypertrophy,
migration, proliferation, endothelial dysfunction, and inflammation. In
addition, the overproduction of ROS could set into motion mechanisms
that could cause vasoconstriction and thereby influence blood pressure,
as mediated by reduction in vasodilators NO and prostacyclin, or
increases in vasoconstrictors, F2-isoprostane, and
endothelin (Fig. 12B). Most of the results of the studies
discussed here suggest the importance of having appropriate methods to
evaluate the magnitude of OXST to determine the correlation with
hypertension and organ damage. This constitutes an extensive chapter
that is beyond the scope of this review. Readers interested in this
subject can consult Tarpey and Fridovich (168).
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Fig. 12.
A: combination of NO and superoxide is
peroxynitrite (ONOO), which is in equilibrium with peroxynitrous acid
(ONOOH). Peroxynitrous acid is a source of hydroxyl radical
(OH). B: peroxynitrite, itself a vasodilator,
produces vasoconstriction by converting arachidonic acid (aa) to
F2-isoprostanes and by increasing thromboxane
A2 (TxA2). Peroxynitrite also causes
vasoconstriction by inhibiting prostacyclin (PGI) synthase and thereby
decreasing vasodilator prostacyclin (PGI2).
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ROLE OF OXST IN ANG II HYPERTENSION |
A number of studies have been conducted to elucidate the specific
components of OXST that are involved in the development of ANG
II-induced hypertension. Haas et al. (55), in pigs, and Reckelhoff et al. (130), in rats, were among the first to
demonstrate that the slow hypertensive response to ANG II (10 ng · kg1 · min1)
was accompanied by a significant elevation of OXST as estimated indirectly by increases in plasma F2-isoprostanes, an
oxidative metabolite of arachidonic acid (106). Nishiyama
et al. (115) also demonstrated that a prolonged infusion
of ANG II in rats stimulates OXST. In this study, the administration of
tempol, a SOD mimetic, reversed the vasoconstriction and produced
vasodilation via an NO-dependent mechanism. Ortiz et al.
(116) found in rats that the development of slow pressor
responses to ANG II (10 ng · kg1 · min1)
could be inhibited by the administration by antioxidants such as tempol
and vitamin E. As a result of antioxidant treatment, there was a fall
in RBF and GFR, whereas the indexes of OXST, thiobarbituric acid
reactive substances, and isoprostanes were found to be
decreased in peripheral circulation as well as the </
TOP
ABSTRACT
INTRODUCTION
