in vivo pharmacodynamic interactions between two drugs used in orthostatic hypotension – midodrine...

doi: 10.1111/j.1472-8206.2006.00450.x

O R I G I N A L

A R T I C L E

In vivo pharmacodynamic interactionsbetween two drugs used in orthostatichypotension – midodrine anddihydroergotamine

Geraldine Jourdana,b,d*, Patrick Verwaerdea,b,d, Atul Pathaka,c,d,Marie-Antoinette Trana,c,d, Jean-Louis Montastrucc,d, Jean-MichelSenarda,c,d

aInserm, U586, Unite de Recherches sur les Obesites, F-31432 Toulouse, FrancebEcole Nationale Veterinaire de Toulouse, UP d’anesthesie-reanimation-urgences, F-31076 Toulouse, FrancecFaculte de Medecine Purpan, Laboratoire de Pharmacologie Medicale et Clinique, F-31000 Toulouse, FrancedInstitut Louis Bugnard IFR31, Universite Paul Sabatier, F-31432 Toulouse, France

I N T R O D U C T I O N

Orthostatic hypotension (OH) is defined as a fall in blood

pressure (BP) of at least 20 mmHg systolic and/or

10 mmHg diastolic during standing [1]. It occurs often

in the elderly (up to 20% of patients aged ‡65 years

[2,3]) probably because of a decrease in baroreflex

sensitivity and concomitant drug use. However, OH is a

more common finding in autonomic nervous system

diseases, which often include alterations of baroreflex

Keywords

dihydroergotamine,

midodrine,

orthostatic hypotension

Received 31 December 2005;

revised 10 February 2006;

accepted 5 October 2006

*Correspondence and reprints:

[email protected]

A B S T R A C T

A combination of midodrine and dihydroergotamine (DHE) is frequently used

clinically in patients suffering from severe orthostatic hypotension (OH). Whereas

midodrine acts as a selective, peripheral alpha1-receptor agonist, DHE displays

complex pharmacology and can behave as an alpha-adrenergic receptor agonist or

antagonist. Surprisingly, the consequences of such a combination on blood pressure

have never been investigated. The present study was performed in order to evaluate

the pressor effects induced by the administration of both midodrine and DHE in old

conscious dogs (n ¼ 6) in experimental condition reproducing autonomic failure-

related baroreflex dysfunction (atropine 0.1 mg/kg). For this purpose, we first studied

the relative potency and intrinsic activity of each agonist and noradrenaline (NA)

for the alpha1-adrenergic receptor. The orders of potency obtained in our study were

0.35, 11 and 400 lg/kg for NA, DHE and midodrine, and intrinsic activity:

NA > midodrine > DHE. These results strongly suggest that DHE really acts in vivo

as an alpha1-adrenoceptor partial agonist. Afterwards, the pressor effects of

coadministration of midodrine (0.4 mg/kg) and DHE (15 lg/kg) were investigated:

in one setting, midodrine was first administered, followed by DHE; in another, DHE

was first administered, followed by midodrine. Our results show that in conscious

dogs, the combination of midodrine and DHE leads to near-complete abolition

of the pressor effect induced by the first administered drug. This in vivo proof of

such antagonistic effects on blood pressure could explain clinical observations

of worsening of OH in humans administered midodrine plus DHE. Although in vivo

results obtained in conscious healthy dogs need to be experimentally and clinically

confirmed in humans suffering from OH, these results strongly suggest that a

midodrine–DHE combined treatment should be avoided in clinical practice.

Journal compilation ª 2006 Blackwell Publishing Ltd. No claims to original French government works Fundamental & Clinical Pharmacology 21 (2007) 45–53 45

pressor responses [4–6]. In addition to the risk of fall and

syncope, OH is also implicated in cognitive decline [7],

cerebrovascular diseases [8] and represents an inde-

pendent predictive factor of mortality [9].

When non-pharmacological therapy fails to prevent

symptoms of OH [10], pharmacological drugs are admin-

istered [11,12] despite the fact that the efficacy of most of

them has not been established in randomized, placebo-

controlled clinical trials [13,14]. Midodrine, a selective

peripheral-acting alpha1-receptor agonist, is a unique

agent in the armamentarium against OH to have shown

successfull efficacy in several recent randomized, con-

trolled studies [15–19]. However, its use is limited to

hospital specialists because of regulatory restrictions. By

contrast, dihydroergotamine (DHE), an ergot derivative

with alpha-adrenergic agonist activity, is widely used by

general practitioners despite the fact that its usefulness

remains to be demonstrated. In fact, it displays a complex

pharmacology because it can behave as an alpha-

adrenergic receptor agonist or antagonist [20]. Mono-

therapy is only effective in a reduced set of patients

needing antihypotensive drugs [21]. The use of a

combination of midodrine and DHE is frequent in severely

disabled patients (J.M. Senard, A. Pathak, unpublished

data) despite the fact that consequences of midodrine and

DHE combination on BP have never been investigated.

The aim of this study was to investigate in conscious

dogs, the pressor effects induced by administration of

both midodrine and DHE in conditions reproducing

baroreflex dysfunction observed in autonomic failure.

M A T E R I A L S A N D M E T H O D S

Animals

Experiments were performed on six Beagle-Harrier male

dogs weighing 10–16 kg, aged 8–9 years. The dogs were

made to fast on the morning of the experiment but had

free access to water ad libitum. All procedures were

conducted in strict compliance with approved French

Agriculture Department for Animal Use for Research and

Education groups.

General procedure

Two days before pharmacological testing

Two days before pharmacological testing, all dogs under-

went a 24-h ambulatory echocardiogram (ECG) record-

ing. Briefly, two bipolar ECG leads were placed in a

diagonal lead placement as described in other studies [22].

The recordings were performed at a 200-Hz sampling rate.

The ELATEC Holter analysis QT software (ELA Medical,

Montrouge, France) was used for analysis. Recordings

were excluded if they only lasted <20 h, if they were of

poor quality, if atrial fibrillation was present or if T-wave

amplitude was <0.15 mV. The 24-h ECG Holter data were

converted into 2880 templates obtained at 30-s intervals.

Only intervals in which >80% of electrocardiographic

complexes were eligible were included. Heart rate (HR)

was analyzed during day, night and 24-h period. Fresh

blood for catecholamine assay was collected from femoral

arterial catheter. Blood samples were obtained from

conscious animals in a supine position and at the end of

the resting period of cardiovascular stabilization, just

before signal acquisition for HR and BP analysis.

On the day of pharmacological testing

Cardiovascular monitoring. Blood pressure and HR were

recorded by means of a catheter introduced into the

abdominal aorta via the femoral artery, connected to

a Baxter Corporation transducer (Baxter Healthcare

Corporation, Irvine, CA, USA) on a Plugsys recorder

(Hugo Sachs Elektronik, Gruenstrasse, Germany) and

stored in a compatible IBM-PC computer. HR was

obtained using a heart period meter triggered by BP.

The BP signal was digitized at 500 Hz. Systolic blood

presure (SBP), diastolic blood presure (DBP) and HR were

computed for each cycle and extracted at regular intervals

of 500 ms. They were then stored in a compatible IBM-PC

computer. Data were the mean of several beats and

corresponded to the average of 1-min recording.

Plasma catecholamine assay. After cannulation of the

saphenous vein, samples were drawn into heparinized

tubes, stored on ice and centrifuged (2000 g, 10 min,

0 �C). Catecholamines were separated by high-pressure

liquid chromatography and quantified by electrochemical

detection. The sensitivity of the method was 10 pg/mL

[23]. Blood samples were obtained from conscious

animals at the end of the resting period of cardiovascular

stabilization before pharmacological testing.

Pharmacological testing

All experimental protocols were performed after a

15-min resting period of cardiovascular stabilization

(i.e. 20 min after placement of femoral catheter). In the

first part of the study, we assessed the cardiovascular

dose–response effects of three intravenous doses of

noradrenaline (NA) (0.5, 1 and 2 lg/kg) with or without

atropine pretreatment (0.1 mg/kg). Atropine pretreat-

ment was performed to antagonize the baroreflex-

induced changes in HR in order to reproduce in dogs,

46 G. Jourdan et al.

Journal compilation ª 2006 Blackwell Publishing Ltd. No claims to original French government works Fundamental & Clinical Pharmacology 21 (2007) 45–53

the usual decrease in parasympathetic nervous system

activity observed in humans associated with autonomic

failure. The 0.1 mg/kg atropine dose was defined after

preliminary experiments indicating a rather complete

suppression of HR changes induced by NA (data not

shown) and duration of blockade longer enough to allow

experiments lasting for 1 h.

The effects of midodrine alone, DHE alone or midodrine

plus DHE combination on SBP, DBP and HR were analyzed

at differents times after intravenous administration. First,

preliminary studies were carried out to establish the

kinetic of pressor effects induced by 0.4 mg/kg midodrine

(Figure 1a) and 15 lg/kg DHE (Figure 1b) administered

alone. These doses were chosen in accordance to data

from literature [24–27], and from two preliminary studies

(not shown).

A dose–response experiment was thereafter performed

using six i.v. cumulative doses of midodrine (0.15, 0.3,

0.4, 0.45, 0.8 and 1.0 mg/kg; n ¼ 6) administered at

10-min intervals. Another dose–pressor response curve

with four i.v. cumulative doses of DHE (5, 10, 15 and

20 lg/kg; n ¼ 6), administered at 6 minutes intervals

was also carried out. Cardiovascular parameters were

analyzed before any drug administration, 4 min after

atropine (0.1 mg/kg) and 9 min after each midodrine

dose or 5 min after each DHE dose (i.e. at the time of

maximal BP increase as determined in the experiments

investigating the kinetic of the pressor effect).

Finally, we assessed the pressor effects of midodrine

plus DHE combination in two other experimental settings.

In the first setting (A), DHE (15 lg/kg) was administered

first and 5 min later, two cumulative doses of midodrine

(0.4 and 0.8 mg/kg) were injected at 10-min intervals

(experimental group A; n ¼ 6). Another group (control

group A; n ¼ 6) just received two cumulative doses of

midodrine (0.4 and 0.8 mg/kg). In the second setting (B),

midodrine (0.4 mg/kg) was administered first and

10 min later, a single dose of DHE (15 lg/kg) was

injected (experimental group B; n ¼ 6). Another group

(control group B; n ¼ 6) received a single dose of DHE

(15 lg/kg). Cardiovascular parameters were analyzed as

indicated above. In the last part of this study, we assessed

in two dogs the pressor effects of DHE (15 lg/kg) after

alpha1-adrenoceptor blockade by pretreatment with

prazosin (0.1 mg/kg) (without atropine pretreatment).

Statistical methods and data analysis

All results are presented as mean ± standard error of the

mean (SEM). Dose–response curves were fitted by a

nonlinear regression to a sigmoidal using the program

Sigmaplot (Sigmaplot 2002 for Windows version 8.02

SPSS Inc. 1986–2001). All statistical comparisons were

performed after examination of homoscedasticity. Intra-

group comparisons were performed using one-way

ANOVA followed when required by a Dunnett’s post hoc

0

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100(a)

(b)

0 5 10 15 20 25 30

Time after midodrine injection (min)

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Time after DHE injection (min)

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Figure 1 (a) Effects of a single i.v. injection of midodrine (0.4 mg/kg)

on systolic (j) and diastolic (d) BP changes. Maximal effect

was reached after 9–12 min (SBP, +53 ± 7 mmHg and DBP,

+43 ± 4 mmHg) and remained stable for at least 30 min. (b) Effects

of a single i.v. injection of DHE (15 lg/kg) on systolic (j) and

diastolic (d) BP changes. Maximal effect was reached after 4–6 min

(SBP, +84 ± 8 mmHg and DBP, +70 ± 5 mmHg). Changes in

BP represented the difference between BP value at any time and

BP value after atropine pretreatment. Values are expressed as

mean ± SEM (n ¼ 6).

Midodrine and DHE in vivo interactions 47


test and between-group comparisons, using Student’s

t-test or Aspin–Welsh test. A P-value <0.05 was

considered significant.

Drugs

Atropine sulfate was obtained from Aguettant (Lyon,

France). Noradrenaline bitartrate and prazosin were from

Sigma Chemical Co. (Lyon, France). Midodrine hydrochlo-

ride was a generous gift from Nycomed (Linz, Austria) and

DHE from Novartis Pharma (Rueil-Malmaison, France).

R E S U L T S

Holter monitoring

Data from 24-h Holter ECG recording indicate that dogs

had 24-h HR (98 ± 4.6 bpm) similar to that observed at

the beginning of any midodrine–DHE experiment and

obtained through BP recording post-processing. How-

ever, the expected reduction in HR at night was not

observed in our old dogs (HR day vs. night: 100.4 ± 5.2

vs. 95 ± 4.7 bpm, ns).

Plasma catecholamines

In the resting period, in our old conscious dogs,

noradrenaline and adrenaline levels were 626.5 ±

56.9 and 345.5 ± 45.6 pg/mL, respectively.

Effects of atropine on BP and HR

Atropine alone (0.1 mg/kg i.v.) drastically increased HR

(+139 ± 11 bpm) and decreased SBP by 35 ± 8 mmHg.

When administered after atropine, at the moment of the

midodrine- or DHE-induced maximal BP increase, a non-

significant decrease in HR by 5 ± 5 and 7 ± 8 bpm for

midodrine and DHE, respectively, was observed. More-

over, a non-significant decrease in HR was also observed

between the beginning and the end of each experiment

(data not shown).

Agonist dose–response experiments

A 140-mmHg increase in SBP was considered as the

maximal increase acceptable in our conscious dogs

(Figure 2). Dose–response curve graphic analysis showed

that the dose required to produce a 50-mmHg increase in

SBP was 0.35 lg/kg for NA, 11 lg/kg for DHE and

400 lg/kg for midodrine.

Pressor effects of midodrine plus DHE combination

Midodrine pressor effects after DHE

During baseline periods (i.e. before and after atropine

pretreatment), mean values of SBP and DBP were not

significantly different in control versus experimental

group A (P > 0.05) (Table I). In controls, i.v. injections

of midodrine (0.4 or 0.8 mg/kg) induced a significant

increase in both SBP and DBP. In experimental group A

(midodrine administered after DHE), midodrine induced

Log dose (µg/kg)–2

0

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100

120

140

160

–1 0 1 2 3 4

Sys

tolic

blo

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pre

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han

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(m

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g)

Figure 2 Cumulative dose–response curves for NA (–––), DHE (- - -)

and midodrine (...) injected by i.v. route on systolic BP changes in

conscious dogs after atropine pretreatment (0.1 mg/kg). Systolic BP

changes represented the difference between maximal systolic BP

after any dose of agonist and baseline systolic BP after atropine

pretreatment. Values (d) are expressed as mean ± SEM (n ¼ 6).

The dose of agonist required to produce a 50-mmHg systolic BP

increase was obtained by graphic analysis (…).

Table I baseline values of HR, systolic and diastolic BP in control

and experimental groups in the setting midodrine after DHE

combination (A).

Control

group A

Experimental

group A

Student’s

t-test

Systolic BP (mmHg)

Before any drug administration 216 ± 5 227 ± 3 NS

After atropine pretreatment 190 ± 7 191 ± 5 NS

After DHE pretreatment (15 lg/kg) – 259 ± 9 –

Diastolic BP (mmHg)




HR (bpm)




Values are expressed as mean ± SEM.

Differences between control and experimental groups were tested for

statistical significance by Student’s t-test (P < 0.05 was considered

significant).



an increase in SBP and DBP reaching only a statistically

significant level for the 0.8 mg/kg dose (one-way ANOVA,

Dunnett’s post hoc test: P < 0.05). The changes in SBP

and DBP were significantly different for any dose of

midodrine between control and experimental groups A

(P < 0.05) (Figure 3).

DHE pressor effects after midodrine

During baseline periods (i.e. before and after atropine

pretreatment), mean values of SBP and DBP were not

significantly different in controls and in experimental

group B (P > 0.05) (Table II). In control group B (DHE

alone) as in experimental group B (DHE administered

after midodrine), a single i.v. injection of DHE (15 lg/kg)

induced a significant increase in SBP and DBP at any

time of evaluation (one-way ANOVA, Dunnett’s post hoc

test: P < 0.05). Between-group analysis showed that the

increases in DBP were significantly (P < 0.05) different

5, 10 and 15 min after DHE administration and

increases in SBP only 5 and 10 min after (Figure 4).

Pressor effects of DHE after alpha1-adrenoceptor

blockade

Prazosin (0.1 mg/kg) pretreatment only partially blocked

the pressor response induced by DHE (Figure 5).

D I S C U S S I O N

In the present study, the pressor effects of a coadminis-

tration of two antihypotensive drugs, midodrine and DHE,

were investigated in vivo in old conscious dogs. Our

results show that DHE acts in vivo as an alpha1-

adrenoceptor partial agonist in contrast to midodrine

which acts as a full agonist. Consequently, the combina-

tion of these two agonists in old conscious dogs (>8 years

old) do not potentiate each other’s pressor effect but lead

to a drastic reduction of blood pressure level.

First and foremost, the choice of our canine model

needs comments. In fact, we deliberately decided to study

old animals (age >8 years old) with high resting BP

010

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405060

7080

90100

0 0.4 0.8

Cumulative doses of midodrine (mg/kg)

Dia

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d p

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ure

chan

ges

(m

mh

g)

#

#

#*

*

0102030405060708090

100

0 0.4 0.8

Cumulative doses of midodrine (mg/kg)

Sys

tolic

blo

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pre

ssu

rech

ang

es (

mm

Hg

)

#

#

#*

*

Figure 3 Effects of two cumulative doses of midodrine (0.4 and

0.8 mg/kg) administered alone (–––) and after DHE pretreatment

(15 lg/kg) (- - -) on systolic (j) and diastolic (d) BP changes.

Values are expressed as mean ± SEM (n ¼ 6). Differences between

control and experimental groups were tested for statistical signifi-

cance by Student’s t-test (*P < 0.05 was considered significant).

Differences in control and experimental groups were tested for

statistical significance by one way ANOVA followed by Dunnett’s

post hoc test (#P < 0.05) was considered significant versus baseline

values. Atropine pretreatment and DHE pretreatment values

(maximal values obtained) were considered as baseline value for

control and experimental group, respectively.

Table II baseline values of HR, systolic and diastolic BP in control

and experimental groups in the setting DHE after midodrine

combination (B).

Control

group B

Experimental

group B

Student’s

t-test

Systolic BP (mmHg)



After midodrine pretreatment

(0.4 mg/kg)

– 231 ± 5 –

Diastolic BP (mmHg)




(0.4 mg/kg)

– 144 ± 4 –

HR (bpm)




(0.4 mg/kg)

– 233 ± 8 –

Values are expressed as mean ± SEM.

Differences between control and experimental groups were tested for

statistical significance by Student’s t-test (P < 0.05 was considered

significant).



levels in order to compare with findings in humans

which that during autonomic failure, supine arterial

hypertension is found in around 70% of patients and is

frequently associated with neurogenic OH [28]. Cate-

cholamine plasma levels were obtained in these condi-

tions of high BP measurement, i.e. in conscious animals

and at the end of the resting period. These levels are

indeed slightly greater than those obtained in similar

conditions by our team; however, the subjects were

younger (<2 years old) [29]. These findings exclude

abnormal sympathetic activation as well as any rela-

tionship between high BP, stress and sympathetic

activation. To the best of our knowledge, modification

of age-related catecholamine plasma levels are not well

defined in dogs but a slight increase is a common finding

in elderly humans [30]. Taken together, these data

suggest that high BP levels in our dogs can be related

to age-impaired baroreflex activity. Moreover, additional

data coming from 24-h Holter ECG recording indicate

that animals had 24-h HR similar to those observed in

midodrine-DHE experiments (98 ± 4.6 bpm) but that

the normal reduction in HR at night is no longer

observed [31]. This lower HR variability (night/day) is

also a common finding in old patients suffering from

severe OH [32, 33]. Finally, in order to reproduce the

decreased parasympathetic nervous system regulation of

HR frequently reported in autonomic nervous system

diseases, dogs were also pretreated with atropine. As

reported in the literature and in accordance with our

experimental observations (no significant decrease in HR

between the beginning and the end of experiment),

atropine remained effective for 1–112

h, i.e. the maximal

duration of our experiments and allowed us to be sure of

complete blockade of parasympathetic nervous system

during all the procedures. Taken together, these data

suggest that our canine model closely reproduces chan-

ges observed in old human patients suffering from

autonomic failure, especially supine high BP.

The first part of this study was designed to determine

potency and intrinsic activity of both midodrine and

DHE. The potency of an agonist is usually defined as the

drug concentration or dose required to produce a half-

maximal response (ED50). However, defining ED50 would

require using very large doses of drugs with potential

deleterious adverse reactions linked to excessive increase

in BP. This is the reason why we decided to define the

dose necessary to produce a reasonable increase in BP,

50 mmHg. The potency order obtained in our study was

0.35, 11 and 400 lg/kg for NA, DHE and midodrine,

respectively. Efficacy or intrinsic activity of a drug is

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100

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Time (min)

#*

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#

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#

# #

##

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100

0 5 10 15 20 25

Time (min)

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)#

#*

#

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##

#

#

#

Dia

sto

lic b

loo

d p

ress

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chan

ges

(m

mh

g)

Figure 4 Effects of a single dose of DHE (15 lg/kg) administered

alone (–––) and after midodrine pretreatment (0.4 mg/kg) (- - -) on

systolic (j) and diastolic (m) BP changes. Values are expressed as

mean ± SEM (n ¼ 6). Differences between control and experimen-

tal groups were tested for statistical significance by Student’s t-test

(*P < 0.05 was considered significant). Differences in control and

experimental groups were tested for statistical significance by one

way ANOVA followed by Dunnett’s post hoc test (#P < 0.05 was

considered significant versus baseline values. Atropine pretreatment

and midodrine pretreatment values (maximal values obtained) were

considered as baseline value for control and experimental group,

respectively.)

Prazosin DHE

Blood pressure

Heart rate

Systolic

MeanDiastolic

32392879251921591800144010807203600

32392879251921591800144010807203600

0

0

40

80

120

160

200

70

140

210

280

350

Time (s)

Time (s)

Figure 5 Typical record of pressor effects of DHE (15 lg/kg) after

prazosin pretreatment on BP and HR on one dog.



characterized by the magnitude of the maximal response.

It is reflected as a plateau in the log dose–response curve.

In our experimental conditions, the maximal pressor

effect (Emax) was obtained only for DHE but it appeared

to be the lowest of the three values. According to these

data and to the graphic analysis (Figure 2), the order of

intrinsic activity between each agonist on the pressor

response was: NA > midodrine > DHE. Consequently,

our study strongly suggests that DHE acts as an alpha1-

adrenoceptor partial agonist. It binds to the alpha1-

adrenergic receptor with higher affinity than midodrine

but its efficacy is lower than that of midodrine. To the

best of our knowledge, there is no other in vivo study

assessing this alpha1-adrenoceptor partial agonist beha-

vior of DHE. This result is in good agreement with in

vitro studies and other data from literature [27].

Our results also show that combination of two alpha1-

adrenoceptor agonists, midodrine (a full agonist) with

DHE (a partial agonist) leads to near-complete abolition

of the pressor effect of the first administered drug. In

order to demonstrate a possible pharmacological antag-

onism between these two agonists, we tried to find the

best experimental conditions: (i) doses of agonists indu-

cing maximal systolic and diastolic pressor increases

were used (DHE 15 lg/kg and midodrine 0.8 mg/kg)

and (ii) in experiments with both drugs, the second

agonist was always administered at the moment of the

maximal pressor effects of the first one supposing that a

maximal percentage of the total number of alpha1-

adrenoceptors was then occupied.

In these experimental conditions, our results showed a

significant decrease in midodrine pressor responses when

administered after DHE (Figure 3). These results could be

interpreted as follows. DHE interacts with alpha1-

adrenoceptors and then reduces the total number of

available sites for midodrine. Despite higher affinity, DHE

possesses a lower degree of efficacy than midodrine, a full

agonist. As a consequence, the maximal pressor effect of

midodrine after DHE pretreatment is reduced. These first

experimental results demonstrated that a partial agonist

could antagonize in vivo action of a full agonist [34].

The results of the second combined treatment show

that, after midodrine pretreatment, DHE hypertensive

response was just partially decreased 5 and 10 min after

administration. We have no satisfactory explanation for

this result. Considering our experimental doses, it is

possible that DHE, despite higher affinity does not

completely succeed in removing midodrine from its

receptor sites. Some pharmacokinetic interactions be-

tween midodrine and DHE can also be hypothesized

(unexplored in our experimental work). Our results

showing that prazosin, an alpha1-adrenoceptor blocker,

only partially prevented the pressor effects of DHE

strongly suggest that, similar to other ergot derivatives,

DHE interacts with other receptors than alpha1-adreno-

ceptors, especially 5-hydroxytryptamine (5-HT) recep-

tors [35,36]. The affinity of DHE for several receptors

(a1-HT, 5-HT receptors, etc.) explains that prevision of

its interactions with another drug, as midodrine, remains

speculative. Further studies could then be necessary inclu-

ding midodrine and DHE dose–response curves under

blockade of the different subtypes of 5-HT receptors.

Limitations of the study

This canine model did not reflect all changes in the

autonomic nervous system activity encountered during

human autonomic failure. Hence, modification of postsy-

naptic alpha-adrenoceptors and altered responses to

vasopressor drugs response have also been described

[37]. As a consequence, the clinical relevance of our

experimental results obtained in old dogs remained

unpredictable concerning coadministration of midodrine

and DHE for the treatment of OH in humans. Further

studies of experimental models of autonomic failure and

OH [38] on the one hand and relevant clinical trials

including both healthy volunteers and autonomic failure

patients on the other hand, should then be performed to

confirm these promising initial in vivo outcomes.

C O N C L U S I O N

Our study demonstrated in vivo in dogs a potential

antagonism between midodrine and DHE, suggesting a

risk for a combined treatment with midodrine plus

DHE. Indeed, some clinical observations (J.M. Senard,

A. Pathak, unpublished data) have shown that OH

worsening is an adverse reaction of such combined

treatment in humans. Although in vivo results obtained

in conscious old dogs need to be clinically confirmed in

humans suffering from OH, these results strongly

suggest that a combined treatment of midodrine-DHE

has to be avoided in practice.

R E F E R E N C E S

1 The Consensus Committee of the American Autonomic Society

and The American Academy of Neurology. Consensus statement

on the definition of orthostatic hypotension, pure autonomic

failure and multiple system atrophy. Neurology (1996) 46

1470.



2 Rutan G.H., Hermanson B., Bild D.E. et al. Orthostatic hypo-

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in vivo pharmacodynamic interactions between two drugs used in orthostatic hypotension – midodrine...

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