(Hypertension. 1997;30:897-904.)
© 1997 American Heart Association, Inc.
Articles |
From the Unidad de Regulación Neurohumoral, Departamento de Ciencias Fisiológicas, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago (Chile).
| Abstract |
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Key Words: pepsanurin bradykinin receptor blocker natriuretic peptides kininogens
| Introduction |
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| Methods |
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Preparation of PU-D1
The starting material was fresh human plasma obtained from the
blood bank of the Hospital of Pontificia Universidad Católica de
Chile. Plasma was processed immediately to obtain the kininogens, as
described by Johnson et al.10 Following affinity
chromatography on carboxymethyl papain Sepharose,
kininogens were concentrated by ultrafiltration on Amicon PM-30
membrane and chromatographed on a semi-preparative HPLC ionic
exchange DEAE column. The separated LMW kininogen was approximately
95% pure by SDS-PAGE and Western blot analysis and was
submitted to the hydrolytic action of pepsin, at pH 2.5, 37°C for 4
hours. One mol of pepsin was added per 10 mol of kininogen. At the end
of the incubation, the pH of the mixture was adjusted to 6.1, and after
heating to 70°C for a few minutes it was centrifuged at
10 000g for 20 minutes. Adequate volumes of the supernatant
were fractionated in Sep-PakC18 columns equilibrated with
H2O and eluted in two steps with 2 mL of 15%
CH3CN and 30% CH3CN in 0.1% TFA. Each
collected fraction was lyophilized, resuspended, and tested in the
bioassay described below. The material eluted by 30% CH3CN
exhibited strong inhibition of ANP-stimulated urinary excretion,
particularly on Na. Thereafter this fraction was purified and isolated
using an HPLC Lichrocart 125-4 Supersphere 100 RP-18 column
equilibrated with 7.5% CH3CN and 0.1% TFA. To obtain
separation of the active compound, a gradient was employed from 5%
CH3CN to 35% CH3CN in 0.1% TFA, with a flow
rate of 1 mL/min. The total volume obtained was 60 mL. The fluids
corresponding to the different peaks were lyophilized separately. The
material showing the highest activity was submitted to an additional
purification using a Hibar 250-4 Lichrosorb RP-18 column equilibrated
with 7.5% CH3CN and 0.1% TFA. The sample was eluted using
a gradient from 7.5% to 50% of CH3CN in 0.1% TFA with a
flow rate of 1 mL/min. The corresponding chromatogram of the last
fraction is shown in Fig 1
. The pure
fraction was lyophilized and submitted to a sequence
analysis at Bios Chile by Edmond degradation determining the
N-terminus. The sequence KYEIKEGDCPVQSGKTWQDC was obtained and
identified as corresponding to residues 57 to 76 of domain 1 of the
heavy chain of human kininogens. Thereafter, this 20amino acid
peptide was synthetized using Fast-Moc chemistry in a commercially
available peptide synthesizer (Applied Biosystems). The peptide is
water soluble, colorless, and odorless, and its purity was
characterized by compositional analysis (total HCl hydrolysis),
mass spectroscopy, and sequencing.
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Bioassay to Test the Anti-ANP Effects
A previously reported bioassay was used.3 4 5 6 8 All
experimental procedures were in accordance with institutional and
international guidelines for the welfare of animals. In brief, fasted
adult female rats, body weight 0.235 ± 0.002 kg (mean±SEM) were
anesthetized with sodium pentobarbital, 40 mg/kg IP, and
heparinized. Polyethylene cannulas were inserted in a femoral artery, a
jugular and femoral vein, and trachea. Arterial blood
pressure was monitored by connecting a femoral artery to a transducer
and a Grass polygraph. A constant infusion of IGS was started through
the jugular vein (0.6 mL/h at time 0). Urine was collected during 10
periods of 20 minutes each by means of a silastic catheter introduced
in the bladder. Two intravenous boluses of 209 pmol (0.5
µg) of ANP were administered at the start of the fourth and ninth
periods. PU-D1 was given either IV or ID, 3 minutes and 40 minutes
before the second ANP bolus, respectively. To inject ID, a small
incision in the abdominal wall was performed ahead of time. PU-D1 was
injected ID 2 to 5 cm from the pylorus. IGS was used as the vehicle for
PU-D1, BK, and Hoe 140 throughout. All doses are expressed in pmol per
rat. Urinary volume was determined gravimetrically, and Na and K levels
were measured in an Eppendorf flame spectrophotometer. For a
quantitative expression of the anti-ANP effects of PU-D1, the urinary
excretion of sodium, potassium, and volume brought about by the second
bolus of ANP were compared with the respective values following the
first bolus, considered the control.
Experimental Protocols
A similar setup reported in previous experiments to test the
anti-ANP effects of BK and of other kinins4 5 6 7 was carried
out to evaluate PU-D1 as an ANP blocking agent.
Effect of PU-D1 Given IV
Single injections of 2.16, 4.32, 10.8, 21.6, 43.2, 108, and 216
pmol (5, 10, 25, 50, 100, 250, and 500 ng) PU-D1 were assayed in order
to obtain dose-response curves and to find the most effective dose of
PU-D1 as an ANP blocking agent. Each dose dissolved in 50 µL of IGS
was injected 3 minutes before the second ANP bolus. A control group
received 50 µL IGS injected 3 minutes before the second ANP
bolus.
Effect of PU-D1 Given ID
Single doses of 2.16, 4.32, 10.8, 21.6, and 108 pmol PU-D1 were
given ID to test whether this peptide might reproduce the
inhibitory effect of PU-15 given by this
route.6 Each dose was injected in 250 µL of IGS, 40
minutes before the second ANP bolus.
Effect of Hoe 140 and Anti-ANP Action of PU-D1
Having evidence that the most effective dose of PU-D1 as an ANP
blocker was 10.8 pmol (25 ng) per rat, we investigated whether a dose
of 1.92 nmol per rat of Hoe 140 (2.5 µg) that abolished the anti-ANP
effect of BK was able to counteract PU-D1, either given IV or ID. The
effect of Hoe 140 was assayed in three groups of rats. In the first
one, Hoe 140 and PU-D1 were administered IV 40 minutes and 3 minutes,
respectively, before the second ANP bolus. In the second group, Hoe 140
was given IV, 45 minutes before, and PU-D1 was given ID, 40 minutes
before the second ANP bolus. To test for the effect of a local blockade
of BK receptors, the third group was given a smaller dose of Hoe 140
(77 pmol, 100 ng) ID 45 minutes before, and PU-D1 10.8 pmol (25 ng) ID
40 minutes before the second ANP bolus.
Durability of the Blocking Effect of PU-D1
To determine the duration of the blockade on ANP-mediated
increase of renal excretory function, each rat received 3 ANP boluses
at regular intervals of 60 minutes, and 3 minutes before the first
bolus, a single dose of 10.8 pmol (25 ng) PU-D1 was given IV. The
response of the urinary parameters following each ANP bolus
was evaluated. The control group received the same doses of ANP, the
first one being preceded by the vehicle of PU-D1.
Additive Effect of Subthreshold Doses of PU-D1 and BK
Three modalities were assayed. (1) In one group of rats, 4.32
pmol (10 ng) of PU-D1 and 47 pmol (50 ng) of BK each dissolved in 50
µL of IGS were injected IV 3 minutes and 1.5 minutes, respectively,
prior to the second ANP bolus. (2) In another group, the order of the
injections of PU-D1 and BK was reversed. (3) Two mixtures containing
different amounts of PU-D1 and BK were tested, each in a different
group of rats. One mixture had the same amount of peptides as described
above and the other had 2.16 pmol (5 ng) of PU-D1 and 47 pmol (50 ng)
of BK. Each mixture was given IV in 50 µL of IGS, 3 minutes before
the second ANP bolus.
Statistical Analysis of Data
All values are given as mean±SEM. Excretion of Na and K is
expressed in µmol/kg and volume in mL/kg body weight per
20 minutes. MAP is expressed in mm Hg. Statistical significance
of the differences between the first and second ANP response was
determined for each experimental group by using a paired t
test. In addition, to compare the extent of the inhibition induced by
different doses of PU-D1, the ANP response ratio, defined as the ratio
between the second and first response for each excretory
parameter, was calculated. ANP response ratios were
submitted to an arcsin transformation and analyzed by unpaired
t test using tables for multiple comparisons against a
single control (vehicle injection was used as control).
| Results |
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Effect of PU-D1 Given ID
The effect of PU-D1 given ID 40 minutes before the second ANP
bolus is shown in Table 2
. In control
animals not injected with PU-D1, the second ANP response was moderately
but significantly larger than the first response (P<.01
paired t test), similar to that obtained after IV vehicle
injections. In general, ID injection of PU-D1 reproduced the blunting
actions found by the intravenous route, although the
inhibitory effect was less intense (Fig 3
) and was maintained through a wider
dose range. The doses of 10.8 and 21.6 pmol (25 and 50 ng) inhibited
ANP-induced Na excretion by 69% and 44%, respectively, whereas volume
excretion dropped by 57% and 32%, respectively. In rats injected with
the largest dose of 108 pmol PU-D1 (250 ng), the ANP response ratio for
Na and volume was significantly lower than the control, although the
second response was not significantly different from the first one. In
contrast, the smallest dose of 4.32 pmol (10 ng) PU-D1 induced a
significant increase in Na and volume excretion, giving an ANP response
ratio significantly larger than that of the control. In general, PU-D1
given either IV or ID inhibits ANP-induced renal excretion, following
the already reported pattern described for kinins.6 7
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Effect of Hoe 140 on the Anti-ANP Action of PU-D1
The administration of Hoe 140 (1.92 nmol, 2.5 µg) was highly
effective for hindering the anti-ANP action of 10.8 pmol (25 ng) PU-D1
given IV or ID. In rats treated with Hoe 140, Na, K, and volume
excretion in response to first and second ANP bolus did not differ in
spite of PU-D1 IV administration before the second ANP bolus (Fig 2
).
Similar obliteration of ANP inhibition by PU-D1 was obtained with an IV
injection of Hoe 140 5 minutes prior to the administration of 10.8 pmol
PU-D1 ID (Fig 3
). These findings reproduced the blockade by Hoe 140 of
the anti-ANP action of BK and kinins of 15, 16, and 18 amino acids
irrespective of the route of administration.5 6 7 8 In another
experiment, the introduction of 77 pmol (100 ng) of Hoe 140 in the
duodenum, 5 minutes before the injection of 10.8 pmol PU-D1 in the same
cavity, also counteracted the blunting effect of this later peptide on
ANP excretion. In this group, the urinary parameters
following the second ANP bolus were similar to the respective values
obtained after the first ANP bolus (+36.6% for Na, +4.0% for K, and
+16.7% for volume, n=4).
Durability of the Anti-ANP Effect of PU-D1
A single 10.8-pmol (25 ng) PU-D1 injection inhibited the renal
excretory response induced by ANP for a period of 140 minutes (Fig 4
). In the control animals, successive
injections of 209 pmol (0.5 µg) ANP, given 60 minutes apart, produced
similar excretory responses for the three urinary
parameters. In PU-D1treated rats, the ANP responses also
were constant over time but were dramatically smaller than controls
(P<.001, unpaired t test). In another experiment
(not shown) the inhibitory effect was prolonged for more
than 3 hours.
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Effect of Subthreshold Doses of PU-D1 and BK Given
Successively or Simultaneously to Inhibit ANP Renal
Excretory Action
Doses of 4.32 pmol (10 ng) PU-D1 (Table 1
) or 47 pmol (50 ng) BK
were ineffective5 in the inhibition of ANP renal excretory
action. However, when the same doses were given successively in the
same rat at 3 minutes (PU-D1) and 1.5 minutes (BK) before the second
ANP bolus, the diuretic response was blocked, particularly
sodium excretion (by 94.4%), as shown in Fig 5
. The inhibitory effect was
also obtained when the order of the injections was reversed (not
shown). However, when the same doses were mixed and injected together,
the blockade was not observed (Fig 6
).
Anti-ANP effectiveness was recovered when the dose of PU-D1 associated
with 47 pmol of BK was reduced to 2.16 pmol (5 ng), as can be seen in
Fig 6
. The three urinary parameters decreased
significantly, particularly Na (by 74.3%, P<.01).
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| Discussion |
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Sodium excretion is controlled by several factors that maintain homeostasis and blood pressure level. This is of paramount importance in many physiological and pathological conditions. For this reason we cannot disregard a factor such as PU-D1 that might modify the role of ANP, one of the hormones that participates constantly as a fine regulator of blood volume. It is possible that BK or PU-D1 affects ANP excretory action by modifying the number or activity of ANP clearance receptors, which play a modulatory role in ANP function by decreasing the availability of ANP to stimulate guanylate cyclase receptors.12 The blunted ANP response also may correspond to an increased phosphodiesterase activity in medullary collecting duct cells, leading to a faster cGMP turnover, such as that observed in rats with sodium-retaining disorders.13 We have shown that blockade of ANP excretory response induced by exogenous BK,9 or by inhibition of kininase II,5 is paralleled with a reduced urinary cGMP excretion. In this context, the peculiar finding that ANP inhibition is observed only in a small dose range of either kinins5 6 7 or PU-D1 may be the result of two or more interplaying mechanisms acting independently on ANP clearance and guanylate cyclase receptors, or on intracellular phosphodiesterase activity. Several other possible mechanisms not involving ANP receptors may be considered, like changes in the activity of endogenous proteases, such as the neutral endopeptidase 24.11 enzyme that degrades both kinins and ANP,14 or modifications in prostaglandin E2 production, which inhibits Na reabsorption in renal collecting tubule cells.15 The observation that 216 pmol (500 ng) PU-D1 indeed facilitate rather than inhibit ANP-induced Na and volume excretion closely resembles what is observed with the 15amino acid kinin PU-15 when injected at doses of 300 to 600 pmol ID or IV.6 These findings suggest that, in addition to the ANP inhibitory effect, PU-D1 shares with kinins the ability to increase diuresis-natriuresis at higher doses; however, this hypothesis requires confirmation.
The multidomain nature of kininogens has been interpreted as a highly refined mechanism for BK release.16 The release of BK by the enzymatic action of kallikreins is the best known function of kininogen domains. Actually, six domains have been described in kininogens and specific activities in a large variety of biochemical processes have been identified for all of them, except for domain 1.17 Domain 2 is multifunctional, capable of inhibiting Ca-activated cysteine proteases, calpain, papain, and cathepsin L, and domain 3 is the most potent inhibitor of cathepsin L and papain.18 In addition, an epitope has been found that inhibits thrombin binding to platelets; domain 4 has the BK sequence; domain 5 binds to surfaces such as endothelium and to a region that binds to Zn and anionic surfaces; domain 6 possesses overlapping sites for kallikrein. Because, to our knowledge, no specific activity has been described for domain 1, it is tempting to speculate that it could be implicated in either a protective antiprotease function to prevent a too-fast BK inactivation, or the inducement of an up-regulation of ANP clearance receptors.12 One of the most provocative questions is whether PU-D1 can be split from its mother molecule, in some physiological or pathological conditions, to exert an anti-ANP action associated or not with BK. We have to take into account that kininogens are part of the kallikrein-kinin system, phylogenetically one of the oldest in living organisms involved in processes such as inflammation, pain, blood coagulation, smooth-muscle contraction and relaxation, vascular endothelial permeability, NO release, blood pressure, and natriuresis. Therefore, we speculate that PU-D1 might share some of the attributes of BK for a fine regulation of diuresis-natriuresis. Another question deals with the physiological events related to blood volume maintenance that might require an inhibition of ANP action on renal excretion. We assume that slowing down the stimulatory effect of ANP on renal excretory functions is a physiological requirement during strenuous muscular exercise, prandial periods,7 and pregnancy.19 The release of ANP from the atria cannot be stopped because ANP participates in the extravascular distribution of fluids during diverse physiological situations.20 During digestion, for instance, a large amount of water and electrolytes is taken from blood plasma by the exocrine glands that produce the digestive juices necessary to maintain the fluidity of the intestinal contents. The remarkable effects of BK (not published), other kinins,6 7 and PU-D1 of inhibiting ANP-mediated diuresis-natriuresis following their introduction in a small dose into the duodenum lends credence to this hypothesis. The kallikrein-kinin system is highly represented in the digestive tract21 and gastric pepsin secretion could release active peptide fragments from kininogens, which could exert a temporal blockade of ANP-stimulated renal excretory functions.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| Footnotes |
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Received September 17, 1996; first decision October 10, 1996; accepted March 13, 1997.
| References |
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