Coronary action of endothelin-1 and vasopressin during acute hypertension in anesthetized goats....

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Vascular Pharmacology

Coronary action of endothelin-1 and vasopressin during acute

hypertension in anesthetized goats. Role of nitric oxide and prostanoids

Nuria Fernandez, Marıa Angeles Martınez, Angel Luis Garcıa-Villalon,

Luis Monge, Godofredo Dieguez*

Departamento de Fisiologıa, Facultad de Medicina, Universidad Autonoma, Arzobispo Morcillo 2, 28029 Madrid, Spain

Received 27 May 2004; received in revised form 27 May 2004; accepted 30 June 2004

Abstract

Coronary reactivity to endothelin-1 and vasopressin during acute, moderate hypertension, and the role of nitric oxide (NO) and

prostanoids in this reactivity was examined in anesthetized goats. Left circumflex coronary flow was electromagnetically measured, and

hypertension was induced by constriction of the thoracic aorta in animals nontreated (7 goats) or treated with the inhibitor of NO synthesis

Nw-nitro-l-arginine methyl esther (l-NAME, 6 goats) or the cyclooxygenase inhibitor meclofenamate (6 goats). Under normotension (19

animals), basal mean values for mean arterial pressure and coronary vascular conductance (CVC) were 89F3 mm Hg and 0.36F0.038 ml/

min/mm Hg, respectively. Endothelin-1 (0.01–0.3 nmol) and vasopressin (0.03–1 Ag) dose-dependently decreased CVC, which, for

endothelin-1 ranged from 5F1% (0.01 nmol; Pb0.01) to 66F4% (0.3 nmol; Pb0.001) and for vasopressin ranged from 9F1% (0.03 AglPb0.01) to 41F3% (1 Ag; Pb0.001). During nontreated and treated hypertension, mean arterial pressure increased to ~130 mm Hg (Pb0.01),

and CVC decreased (17%) only during l-NAME-treated hypertension. The effects of endothelin-1 and vasopressin on CVC were decreased

by ~50% during nontreated hypertension, and this was abolished by l-NAME and was not affected by meclofenamate. Therefore, during

acute, moderate hypertension, the coronary vasoconstriction to endothelin-1 and vasopressin is attenuated, which may be related with

increased NO release but not with prostanoids.

D 2004 Elsevier Inc. All rights reserved.

Keywords: Coronary circulation; Coronary flow; Coronary vasoconstriction; Endothelium

1. Introduction

Recordings from normal subjects and hypertensive

patients show transient increases in arterial pressure

throughout the day (Mancia et al., 1983) and during

exercise (MacDougall et al., 1985). However, few studies

have examined the effects of acute increases in arterial

pressure on the coronary reactivity to vasoactive stimuli.

In blood-perfused dog hearts, acute increased perfusion

pressure in the left anterior descending coronary artery

produced endothelial damage and augmented coronary

response to serotonin but not to angiotensin (Lamping

1537-1891/$ - see front matter D 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.vph.2004.06.001

* Corresponding author. Tel.: +34 91 497 5424; fax: +34 91 497 5478.

E-mail address: [email protected] (G. Dieguez).

and Dole, 1987). One study performed in conscious

dogs shows that acute overload of left ventricular

pressure induces coronary endothelial stunning by

oxidant processes (Kinugawa et al., 2003). As coronary

endothelium release of nitric oxide (NO) may be affected

by changes in intravascular pressure and flow (Bassenge,

1995), and endothelin-1 and vasopressin may be of

significance in the regulation of the coronary circulation

at least under some pathological conditions, and their

coronary effects are modulated by NO, it could be of

interest to know how acute hypertension affects the

coronary reactivity to these two peptides and their

interrelation with NO.

Experimental observations show that endothelin-1 is

synthesized, stored and released in the human heart

(Russel and Molenaar, 2000), that it can produce a potent

41 (2005) 131–138

N. Fernandez et al. / Vascular Pharmacology 41 (2005) 131–138132

coronary vasoconstriction in vitro (Yanagisawa et al.,

1988) and in vivo (Ezra et al., 1989), which may be

modulated by NO but not by prostanoids (Ushio-Fukai et

al., 1992), that its plasma levels may increase during

some conditions (Miyauchi and Masaki, 1999) and that

this peptide may be involved in the regulation of

coronary flow under basal conditions and during exercise

(Merkus et al., 2003). On the other hand, vasopressin is

present in plasma under normal conditions and its plasma

levels can increase under some conditions (Cowley and

Liard, 1987) and this peptide can produce coronary

vasoconstriction (Heyndrickx et al., 1976; Maturi et al.,

1991; Garcıa-Villalon et al., 1996) which may be

modulated by NO (Garcıa-Villalon et al., 1996) but not

by prostanoids (Maturi et al., 1991).

The present study was performed to analyze the

coronary vascular reactivity to endothelin-1 and vaso-

pressin during acute, moderate hypertension, as well as the

role of NO and prostanoids in this reactivity. The experi-

ments were made in anesthetized, open-chest goats where

blood flow through the left circumflex coronary artery was

electromagnetically measured, and hypertension was

induced by constriction of the thoracic aorta. Endothelin-

1 and vasopressin were directly injected into this artery

before (normotension) and during hypertension in animals

non-treated or treated with the inhibitor of NO synthesis

Nw-nitro-l-arginine methyl esther (l-NAME) or the cyclo-

oxygenase inhibitor meclofenamate. The goat has been

considered as a good model for cardiovascular studies

(Lipovetsky et al., 1983) and its coronary circulation is

similar to that of humans by having a small development

of collaterals (Brown et al., 1991).

2. Material and methods

2.1. Experimental preparation

In this study, 19 adult, female goats (34–58 kg) were

used. Anesthesia of the animals was induced with intra-

muscular injection of 10 mg/kg ketamine hydrochloride and

i.v. administration of 2% thiopental sodium; supplemental

doses were given as necessary for maintenance. After

orotracheal intubation, artificial respiration with room air

was instituted by use of Harvard respirator. A left

thoracotomy in the fourth intercostal space was performed

and the pericardium was opened. The proximal segment of

the left circumflex coronary artery was dissected, and an

electromagnetic flow transducer (Biotronex) was placed on

that artery to measure blood flow. A snare-type occluder

was also placed around the artery, just distal to the flow

probe, to obtain baseline flow. A needle, connected to a

polyethylene catheter, pierced the left circumflex coronary

artery between the flow probe and the occluder, and allowed

endothelin-1 and vasopressin to be injected into the

coronary vasculature.

Systemic arterial pressure was measured through a

polyethylene catheter placed in one temporal artery and

connected to a Statham transducer. In every animal, blood

flow, systemic arterial pressure and heart rate were simulta-

neously recorded on a Grass model 7 polygraph. In five of

the 7 goats subjected to nontreated hypertension, left

ventricular pressure was measured by implanting a micro-

transducer catheter (MTC, Hugo Sachs Elektronic) intro-

duced through the left ventricle wall, and the first derivative

of the left ventricular pressure (dP/dt max) was obtained

with a tachograph preamplifier for evaluating myocardial

performance.

2.2. Experimental protocol

Endothelin-1 (0.01, 0.03, 0.1 and 0.3 nmol) and vaso-

pressin (0.03, 0.1, 0.3 and 1 Ag), prepared in isotonic saline,

were directly injected into the left circumflex coronary

artery using volumes of 0.3 ml injected over 5–10 s. Both

peptides were injected in the animals under normotension

(control conditions) and after increased systemic arterial

pressure. Arterial hypertension was induced by gradual

constriction of the thoracic aorta with a mechanical occluder

until mean arterial pressure reached a level of ~130 mm Hg,

which was performed in animals nontreated (7 goats) and in

animals after treatment with the inhibitor of NO synthesis

Nw-nitro-l-arginine methyl esther (l-NAME, 6 goats) or

with the inhibitor of cyclooxygenase meclofenamate (6

goats). l-NAME and meclofenamate, prepared in isotonic

saline at a concentration of 10 mg/ml, were administered by

i.v. route at 47 and 6–8 mg/kg, respectively, over 10–15

min, before inducing hypertension. The experiments were

performed as follows: a) in the group of nontreated animals,

endothelin-1 and vasopressin were firstly tested under

normotension and then during hypertension, and b) in the

group of treated animals, endothelin-1 and vasopressin were

firstly tested under normotension, then l-NAME or

meclofenamate was administered, and after inducing hyper-

tension in the presence of these treatments the two peptides

were tested again. In every case, the effects of endothelin-1

and vasopressin during hypertension were tested when

arterial pressure (and coronary flow) reached a new steady

state.

The effects of vasopressin and endothelin-1 on coronary

vasculature under the different conditions tested were

evaluated as changes in coronary vascular conductance at

the maximal effects on coronary flow. Coronary vascular

conductance was calculated by dividing coronary blood

flow in ml/min by mean systemic arterial pressure in mm

Hg.

Blood samples from the temporal artery were taken

periodically to measure pH, pCO2 and pO2 by standard

electrometric methods (Radiometer, ABLTM 5, Copenhagen,

Denmark). After termination of the experiments, the goats

were killed with an overdose of i.v. thiopental sodium and

potassium chloride.

N. Fernandez et al. / Vascular Pharmacology 41 (2005) 131–138 133

2.3. Drugs

[Arg8] vasopressin acetate and Nw-nitro-l-arginine

methyl ester (l-NAME) from Sigma; endothelin-1 (human,

porcine) from Peninsula Laboratories Europe and meclofe-

namate was from Parke Davis.

2.4. Statistical analysis

The data are expressed as meansFS.E.M. The effects

of constriction of the thoracic aorta, and of i.v.

administration of Nw-nitro-l-arginine methyl ester (l-

NAME) and meclofenamate on resting coronary blood

flow, systemic arterial pressure, coronary vascular con-

ductance, heart rate, left ventricular pressure and dP/dt

max, and blood gases and pH were analyzed using data

in absolute values by applying the Student’s t-test for

paired data; in each case, the animal was used as its own

control. The effects of vasopressin and endothelin-1 on

coronary vascular conductance and on the other hemody-

namic parameters were compared using changes in

absolute values by applying two-way, repeated measures

ANOVA, followed by the Student’s t-test for paired data;

in each case, the animal was also used as its own control

as follows: control vs. hypertension nontreated, control

vs. hypertension pretreated with l-NAME and control vs.

hypertension pretreated with meclofenamate. To compare

the hemodynamic values obtained during hypertension in

animals nontreated and treated with l-NAME and

meclofenamate, one-way ANOVA was applied, followed

by the Dunnett’s test. The coronary effects of endothelin-

1 and vasopressin during hypertension in animals non-

treated and treated with l-NAME or meclofenamate were

compared using two-way ANOVA, followed by the

Dunnett’s test. In each case, Pb0.05 was considered

statistically significant.

The investigation conformed with the Guide for the Care

and Use of Laboratory Animals published by the US

National Institutes of Health (NIH Publication No. 85-23,

revised 1996), and the experimental procedure used in the

Table 1

Resting hemodynamic and blood gases and pH values obtained under normotensio

treated with l-NAME (6 goats) or meclofenamate (meclo, 6 goats)

Hypertension non-treated Hyp

Control Hypertension Con

CBF (ml/min) 29F3 41F5a 32

MAP (mm Hg) 88F4 128F5a 90

CVC (ml/min/mm Hg) 0.35F0.039 0.33F0.041 0.37

HR (beats/min) 75F6 65F6b 65

( pO2) (mm Hg) 92F5 93F5 94

( pCO2) (mm Hg) 27F4 28F3 28

pH 7.39F0.01 7.40F0.02 7.40

Values are meanFS.E.M. CBF=coronary blood flow; MAP=mean systemic arteria Pb0.01 compared with its control conditions.b Pb0.05 compared with its control conditions.

present study was approved by the local Animal Research

Committee.

3. Results

3.1. Hemodynamic changes

Table 1 summarizes the values for hemodynamic

parameters and blood gases and pH obtained in anesthe-

tized goats before (normotension) and during hypertension

nontreated, pretreated with l-NAME and pretreated with

meclofenamate. Table 2 summarizes the hemodynamic

effects and levels of blood gases and pH after l-NAME

and meclofenamate under normotension. l-NAME by

itself, administered in 6 goats under normotension, reduced

resting coronary blood flow by 12F3% ( pb0.05),

increased mean arterial pressure by 19F3% (Pb0.01)

and decreased heart rate by 15F3% ( pb0.05), without

changing blood gases and pH. Meclofenamate by itself,

administered in 6 goats under normotension, did not

change significantly the hemodynamic parameters meas-

ured and blood gases and pH.

In the three groups of animals, constriction of the

thoracic aorta increased mean systemic arterial pressure

from ~90 (control) to ~130 mm Hg, and the level of

hypertension was similar in animals non-treated and treated

with l-NAME or meclofenamate. During nontreated hyper-

tension coronary flow increased by 42% ( pb0.05), heart

rate decreased by 14% ( pb0.05) and coronary vascular

conductance did not change significantly. Something similar

occurred during hypertension pretreated with meclofena-

mate where coronary flow increased by 35% ( pb0.05),

heart rate decreased by 16% ( pb0.05) and coronary

vascular conductance did not change significantly. During

l-NAME-treated hypertension coronary flow increased by

26% ( pb0.05), heart rate did not change significantly, and

coronary vascular conductance decreased by 17% ( pb0.05).

The increase in coronary flow during hypertension was

comparable ( pN0.05) in animals non-treated and treated

n (control) and hypertension in anesthetized goats nontreated (7 goats) and

ertension+l-NAME Hypertension+meclo

trol Hypertension Control Hypertension

F4 40F5b 35F5 47F5a

F4 131F5a 92F5 130F6a

F0.040 0.31F0.038b 0.39F0.042 0.37F0.040

F5 61F5 68F6 57F5b

F4 96F4 95F5 93F6

F4 30F4 29F4 30F4

F0.01 7.41F0.02 7.39F0.02 7.38F0.02

al pressure; CVC=coronary vascular conductance; HR=heart rate.

Table 2

Resting hemodynamic values and blood gases and pH obtained in anesthetized goats before (basal) and after treatment with l-NAME (6 goats) or with

meclofenamate (6 goats)

l-NAME treatment Meclofenamate treatment

Basal l-NAME Basal meclofenamate

CBF (ml/min) 32F4 28F3a 35F5 37F5

MAP (mm Hg) 90F4 107F5b 92F5 93F4

CVC (ml/min/mm Hg) 0.37F0.039 0.27F0.30a 0.39F0.042 0.40F0.039

HR (beats/min) 65F5 53F4a 68F6 65F5

pO2 (mm Hg) 94F4 95F5 95F5 94F4

pCO2 (mm Hg) 28F4 29F4 30F4 31F4

pH 7.40F0.01 7.41F0.02 7.40F0.02 7.39F0.02

Values are meanFS.E.M. CBF=coronary blood flow; MAP=mean systemic arterial pressure; CVC=coronary vascular conductance; HR=heart rate.a Pb0.05 compared with its basal conditions.b Pb0.01 compared with its basal conditions.


with meclofenamate, and it was significantly lower

( pb0.05) in animals treated with l-NAME.

In 5 of the 7 animals subjected to nontreated hyper-

tension, left ventricular systolic pressure increased from

105F5 to 147F9 mm Hg ( pb0.05) and dP/dt max

increased from 1375F105 to 1650F142 mm Hg/s

( pb0.05).

Fig. 1. Summary of the effects on coronary vascular conductance induced by intracoronary injections of endothelin-1 (left) and vasopressin (right) in

anesthetized goats under normotension (top, averages of the effects under normotension in the three groups of animals) and under acute hypertension (bottom

nontreated (., 7 goats) and treated with l-NAME (n, 6 goats) or with meclofenamate (D, 6 goats). *Pb0.05 and **Pb0.01 compared with nontreated

hypertension.

3.2. Coronary effects of endothelin-1 and vasopressin

(a) During nontreated hypertension (7 animals, mean

arterial pressure=128F5 mm Hg), endothelin-1 (0.01–0.3

nmol) and vasopressin (0.03–1 Ag) caused dose-dependent

decreases in coronary vascular conductance, but these

effects for the two peptides were significantly lower than

)


under normotension (control; Fig. 1). After releasing aortic

constriction and normalizing arterial pressure and coronary

flow, the effects of these two peptides on coronary vascular

conductance in 5 of these 7 goats were not significantly

distinct from those found during control conditions (these

data are not shown).

The two higher doses of endothelin-1 and the highest

dose of vasopressin caused also moderate increases in mean

arterial pressure under normotension and this effect was

evident after their maximal effects on coronary flow. In 5 of

these 7 goats during normotension, the highest dose of

vasopressin (1 Ag) caused decreases of 6F2 mm Hg

( pb0.05) in left ventricular systolic pressure without

affecting significantly dP/dt max, and the two higher doses

of endothelin-1 (0.1 and 0.3 nmol) caused decreases of 4F1

and 8F2 mm Hg ( pb0.05), respectively, in left ventricular

systolic pressure, and 0.3 nmol of endothelin-1 caused also

decreases of 119F16 mm Hg/s ( pb0.05) in dP/dt max,

coinciding with their maximal effects on coronary flow. The

effects of vasopressin and endothelin-1 on mean systemic

arterial pressure, left ventricular systolic pressure and dP/dt

max were also present under hypertension, and they were

not significantly different from those under normotension

(control).

(b) During hypertension pretreated with l-NAME (6

animals, mean arterial pressure=131F5 mm Hg), endothe-

lin-1 (0.01–0.3 nmol) and vasopressin (0.03–1 Ag)decreased coronary vascular conductance in a dose-

dependent manner (Fig. 1). These decreases for the two

peptides were not significantly different from those

recorded under control conditions (normotension), and

they were significantly higher than those found in the

animals during non-treated or meclofenamate-treated

hypertension (Fig. 1). The two higher doses of endothe-

lin-1 and the highest dose of vasopressin produced also

moderate increases in mean systemic arterial pressure

similarly under normotension and hypertension, and this

was evident after their maximal effects on coronary flow

as occurred in nontreated animals.

(c) During hypertension pretreated with meclofenamate

(6 animals, mean arterial pressure=130F6 mm Hg),

endothelin-1 (0.01–0.3 nmol) and vasopressin (0.03–1 Ag)also decreased coronary vascular conductance in a dose-

dependent way (Fig. 1). These decreases for the two

peptides were significantly lower than under normotension

(control), and they were not significantly different from

those recorded in the animals under nontreated hyper-

tension. The two higher doses of endothelin-1 and the

highest dose of vasopressin produced also moderate

increases in mean systemic arterial pressure similarly under

normotension and hypertension, and this was evident after

their maximal effects on coronary blood flow as occurred in

nontreated animals.

During hypertension, the effects of endothelin-1 and

vasopressin on systemic arterial pressure were comparable

in nontreated and treated animals (these results are not show).

4. Discussion

The present study in normotensive animals confirms

previous observations from our laboratory indicating that

endothelin-1 (Dieguez et al., 1992; Garcıa et al., 1996;

Martınez et al., 2004) and vasopressin (Fernandez et al.,

1998; Martınez et al., 2004) produce coronary vasocon-

striction in anesthetized goats as also occurs in other species

(Heyndrickx et al., 1976; Yanagisawa et al., 1988; Ezra et

al., 1989; Maturi et al., 1991; Garcıa-Villalon et al., 1996).

In addition, we found that l-NAME reduced resting

coronary flow and caused hypertension as it has been

described elsewhere (Garcıa et al., 1992, 1996; Fernandez et

al., 1998), supporting the idea that NO produces a basal

vasodilator tone in the coronary circulation (Bassenge,

1995). The role of prostanoids in regulating the coronary

circulation is less clear, and although prostacyclin can be

released in coronary vasculature (Karwatowska-Prokopczuk

and Wennmalm, 1990), the administration of inhibitors of

cyclooxygenase have failed to modify resting coronary flow

or vascular resistance (Wang et al., 1994). As occurred in

previous studies from our laboratory (Garcıa et al., 1992,

1996; Fernandez et al., 1998), in the present study,

meclofenamate in doses showed to be effective to inhibit

cyclooxygenase (Walker et al., 1988) also failed to change

resting coronary hemodynamics, suggesting that prostanoids

are not involved in the regulation of coronary flow under

basal conditions.

Before commenting the coronary effects of endothelin-1

and vasopressin during hypertension, we should make some

methodological considerations. Using experimental prepa-

rations where ventricular pressure, cardiac contractility and

heart rate are controlled, an autoregulatory response has

been found in the coronary circulation within changes of

coronary perfusion pressure in the range of 60–140 mm Hg

(Ganz and Braunwald, 1997). This autoregulatory response

is, however, difficult to demonstrate in intact animals

because modifications of arterial pressure also change

myocardial metabolism and produce extrinsic compression

of coronary vessels (Ganz and Braunwald, 1997), and they

generally induce parallel directional changes in coronary

flow (Berne and Levi, 1998). Therefore, under normal

conditions, increases in arterial pressure may induce

increases in coronary flow as occurred in our experiments.

In nontreated animals, we found that increase of mean

arterial pressure to ~130 mm Hg was accompanied by

increased coronary flow by 42% without changing coronary

vascular conductance, along with bradycardia and increased

myocardial contractility, and without changes in systemic

blood gases and pH. The observed increased ventricular

overload and dP/dt is probably related to the induced

hypertension, and coronary autoregulation was not pre-

served in the present experimental conditions. The brady-

cardia recorded during hypertension may related to the

baroreceptor reflex. After meclofenamate treatment, hyper-

tension induced similar changes in coronary hemodynamics,


whereas after l-NAME treatment, it induced lower

increases in coronary flow (and decreased coronary vascular

conductance) than under nontreated hypertension. Treat-

ment with l-NAME and meclofenamate did not change the

relation between hypertension and blood gases and pH

found under nontreated hypertension, and only l-NAME

treatment modified the relation between hypertension and

heart rate because in this latter case bradycardia was not

present. This absence of bradycardia might be related with

the effects of l-NAME itself and/or the degree of

anesthesia, which might have been more pronounced in

the l-NAME-treated animals than in nontreated animals.

The data with l-NAME and meclofenamate suggest that

NO but not prostanoids may be involved in the coronary

flow–pressure relationship during hypertension in our

experimental conditions. Experiments performed in anes-

thetized dogs also suggest that NO may be involved in the

relation between coronary pressure and flow during

ischemia (Smith and Canty, 1993). We cannot exclude,

however, that in our experiments myocardial metabolic

factors, myocardial CO2 and pH, neurohumoral factors and

coronary myogenic factors may be also involved in the

observed coronary flow–pressure relationship.

Most of studies to examine the effects of changes in

pressure and flow on coronary reactivity have been

performed by cannulating and perfusing a coronary artery,

modifying its pressure and flow without affecting systemic

arterial pressure and ventricular function. These prepara-

tions have advantages because they allow to analyze

coronary reactivity without interference of systemic and

myocardial factors, but they are far from the conditions in

intact animals. In our preparation, systemic and myocardial

factors are probably interfering with coronary reactivity, but

despite this limitation, the present study may be useful to

know how systemic hypertension affect coronary reactivity

to vasoactive stimuli. To analyze the coronary effects of

endothelin-1 and vasopressin, we have selected the changes

in coronary vascular conductance because they probably

reflect better the in vivo vascular effects, especially when

blood flow is the variable mainly affected (Lautt, 1989).

Our results show that acute, moderate hypertension with

moderate increased coronary flow blunts the coronary

response to endothelin-1 and vasopressin, and that this

anticonstrictor effect of hypertension reversed when arterial

pressure and coronary flow returned to control values after

releasing aortic constriction. We also found that endothelin-

1 decreased slightly left ventricle systolic pressure and dP/

dt max when higher doses of this peptide were applied,

which occurred coinciding with their maximal effects on

coronary flow, confirming previous results from our

laboratory (Garcıa et al., 1996). This suggests that the

reduction of myocardial contractility by endothelin-1 may

result at least in part from coronary ischemia. The effects of

vasopressin on myocardial contractility are not clear as this

peptide decreased left ventricular systolic pressure but not

dP/dt max in spite of the marked reduction of coronary

flow. Data from literature show discrepant results as

endothelin-1 can produce a negative inotropic effect as

consequence of coronary ischemia (Karwatowska-Prokopc-

zuk and Wennmalm, 1990; Wang et al., 1994) and that it can

also produce a direct positive inotropic effect (Hu et al.,

1988). Moreover, conflicting data have been reported with

respect to vasopressin as this peptide can produce a direct

positive or negative inotropic action (Sakuma et al., 1993).

The effects of these peptides on myocardial function were

also present during acute hypertension, and they were

similar to those recorded under normotension.

During hypertension pretreated with meclofenamate, the

reactivity to endothelin-1 and vasopressin was lower than

under normotension, and it was comparable to that

recorded under non-treated hypertension. This indicates

that meclofenamate did not modify the effects of acute

hypertension on the coronary response to these two

peptides. Previous studies performed in anesthetized,

normotensive goats (Garcıa et al., 1996; Fernandez et al.,

1998) and in other species (Maturi et al., 1991; Garcıa-

Villalon et al., 1996) suggest that prostanoids are not

involved in the coronary effects of endothelin-1 and

vasopressin. Therefore, our present observations suggest

that prostanoids are not involved in the coronary effects of

these two peptides during acute hypertension as occurs

under normotension.

During hypertension pretreated with l-NAME, the

effects of endothelin-1 and vasopressin on coronary vascular

conductance were comparable to those found in the same

animals under normotension, and they were higher than

under nontreated or meclofenamate-treated hypertension.

This suggests that inhibition of NO synthesis with l-NAME

abolishes the anticonstrictor action of acute hypertension on

the coronary effects of endothelin-1 and vasopressin.

Previous studies from our laboratory in anesthetized,

normotensive goats suggest that NO modulates the coronary

vasoconstriction to endothelin-1 (Garcıa et al., 1996) and

vasopressin (Fernandez et al., 1998), which agree with that

reported in other studies (Ushio-Fukai et al., 1992; Garcıa-

Villalon et al., 1996). In addition, it has been suggested that

the coronary endothelium may release NO, which may be

affected by changes in blood flow and intravascular pressure

(Bassenge, 1995). Therefore, our data with l-NAME

suggest that augmented coronary intravascular pressure

and flow increases the release of NO from the coronary

endothelium and that this may be involved in the

anticonstrictor effects of acute hypertension on the coronary

reactivity to endothelin-1 and vasopressin. Ueeda et al.

(1992) reported that the coronary reactivity to vasopressin is

blunted when coronary flow increases after perfusion

coronary pressure increments above 55 mm Hg, and the

authors suggest that it is related to NO release. Moreover, it

has been observed in anesthetized dogs that increases of

coronary flow produced endothelium-dependent coronary

vasodilatation and reduced coronary vasoconstriction to

serotonin (Lamping and Dole, 1988), and that acute


hypertension with increased coronary flow produced endo-

thelial damage and augmented coronary response to

serotonin but not to angiotensin (Lamping and Dole,

1987). In this latter study (Lamping and Dole, 1987),

perfusion pressure of the left descending coronary was

increased to 200 mm Hg, a hypertension more pronounced

than in the present study, and this difference in the degree of

hypertension may underlie the different results between this

study and ours. Recent studies in dogs subjected to acute,

transient ventricular pressure overload induced by constric-

tion of the ascending aorta show that after releasing this

constriction, the NO-dependent coronary vasodilatation is

depressed, and nitrite production from isolated coronary

microvessels is not altered (Kinugawa et al., 2003). The

authors of this study conclude that ventricular pressure

overload induced endothelial stunning which is caused by

oxidant processes (Kinugawa et al., 2003). In this study

(Kinugawa et al., 2003), left ventricular systolic pressure

was increased to N200 mm Hg, and in our experiments, it

was increased to about ~147 mm Hg; this difference in the

level of hypertension may produce different effects on the

coronary endothelium. A pronounced hypertension may

damage the coronary endothelium (Lamping and Dole,

1987; Kinugawa et al., 2003), whereas moderate hyper-

tension may stimulate the release of vasodilators (e.g., NO)

from the coronary endothelium. Our present data suggest

that during and after acute hypertension, endothelial

stunning may be not present because if this had occurred,

the coronary effects of endothelin-1 and vasopressin would

have not changed or have increased, not decreased as

occurred in our case, and l-NAME probably would have

not modified the effects of hypertension on the coronary

response to these two peptides. From studies in chronically

instrumented swines, it has been reported that the ETA-

mediated coronary vasoconstrictor effects of endothelin-1

are decreased during exercise, where coronary flow was

augmented (Merkus et al., 2003). The authors of this study

(Merkus et al., 2003) suggest that these effects of exercise

may be related to decreased ETA receptor sensitivity through

an increased release of NO, adenosine or both, thereby

facilitating coronary metabolic vasodilatation. In our experi-

ments, acute hypertension was accompanied by increased

myocardial contractility and probably by increased myo-

cardial metabolic activity and O2 consumption. Therefore,

we cannot exclude that, in addition to increased NO release,

other factors such as adenosine, neurohormonal factors,

coronary myogenic factors and downregulation of specific

receptors for endothelin-1 and vasopressin are also involved

in the decreased coronary response to these peptides found

during acute, moderate hypertension.

In conclusion, the present study provides data suggesting

that acute, moderate hypertension, with moderate increased

coronary flow, attenuates the coronary vasoconstriction to

endothelin-1 and vasopressin, and that this attenuation may

be related, at least in part, with increased NO release and not

with prostanoids.

Acknowledgements

This work was supported, in part, by FundacionMAPFRE

Medicina, FIS (96/0474) and CICYT (PM95/0032).

We are grateful to Ms. H. Fernandez-Lomana and E.

Martınez for their technical assistance.

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