Br. J. Pharmac. (1979), 67, 283-292
EFFECTS OF CLONIDINE, PRAZOSIN AND PHENTOLAMINE ON
HEART RATE AND CORONARY SINUS CATECHOLAMINE CONCEN-
TRATION DURING CARDIOACCELERATOR NERVE STIMULATION IN
I. CAVERO, T. DENNIS*, FRAN4OISE LEFtVRE-BORG, P. PERROT,
A.G. ROACH1 & B. SCATTON*
Biology Department SYNTHELABO (L.E.R.S.), Cardiovascular Group & Neurochemistry
Unit* 58, rue de la Glaciere, 75013 Paris, France
sient rise in aortic blood pressure and a sustained increase in both heart rate and coronary sinus
blood flow. The latter effects were accompanied by a significant elevation in the coronary sinus
plasma noradrenaline concentration without significant changes in the levels of dopamine and
adrenaline. The concentrations of the three catecholamines in thoracic aorta plasma were not signifi-
cantly changed by cardioaccelerator nerve stimulation.
Clonidine (20 pg/kg, i.v.), given during cardioaccelerator nerve stimulation, increased both mean
aortic blood pressure and coronary sinus blood flow and decreased heart rate and coronary sinus
venous plasma noradrenaline overflow.
Phentolamine (0.3 mg/kg, i.v.) completely antagonized these effects of clonidine. Prazosin (0.3
mg/kg, i.v.) inhibited by only 43 and 38% the respective reductions in heart rate and noradrenaline
overflow elicited by clonidine.
On termination of cardioaccelerator stimulation (about 10 min after either prazosin or phentol-
amine), heart rate and coronary sinus noradrenaline overflow returned to control prestimulation
Phentolamine or prazosin, administered alone during stimulation of.the cardioaccelerator nerve,
increased heart rate and noradrenaline overflow into the coronary sinus plasma. However, intra-
venous phentolamine and prazosin, in contrast to desipramine (0.3 mg/kg,
mg, i.a.), failed to change the tachycardia resulting from the local administration of noradrenaline
into the sinus node artery (i.a.).
These results show that in spinal dogs the clonidine-induced reduction in heart rate (elevated
by electrical stimulation of the cardioaccelerator nerve) is accompanied by a fall in the quantity
of noradrenaline overflowing into the coronary sinus plasma. The latter effect is presumably the
result of an action of clonidine on cardiac presynaptic a-adrenoceptors, the activation of which
is followed by a reduction in the release of noradrenaline per nerve impulse. Phentolamine and
prazosin are both antagonists of cardiac presynaptic a-adrenoceptors in spinal dogs, as suggested
by their action against clonidine and by their positive chronotropic effect when administered during
stimulation of the cardioaccelerator nerve.
In spinal dogs, continuous electrical stimulation of the cardioaccelerator nerve produced a tran-
i.v.) or tyramine (1.0
Much evidence favours the view that a-adrenoceptors
are present on sympathetic nerve terminals and
modulate neurotransmitter release via a classical feed-
back mechanism, operating in response to the nor-
adrenaline concentration within the synapse.
a-adrenoceptors results in a diminution of noradren-
aline liberated per nerve impulse and this effect in
turn is accompanied by a smaller end organ response.
The inverse occurs when the modulatory function of
presynaptic a-adrenoceptors is impaired with specific
antagonists (see reviews: Langer, 1977; Starke, 1977;
Westfall, 1977). This feed-back mechanism has been
'Present address: Reckitt & Colman, Pharmaceutical Divi-
sion, Dansom Lane, Kingston-upon-Hull, HU8 7DS.
%I,FI%Macmillan Journals Ltd 1979
I. CAVERO et al.
extensively studied by measurement of the release of
tritiated noradrenaline in numerous isolated perfused
organs (see above reviews). However, due to the diffi-
culty of both maintaining intact animal preparations
for a prolonged period and measuring endogenous
noradrenaline overflow, in vivo techniques have been
sparingly used to investigate presynaptic mechanisms.
Recently, however, Yamaguchi, De Champlain &
Nadeau (1977) found that in anaesthetized dogs, heart
rate increases elicited by short term electrical stimu-
lation of cardiac sympathetic fibres were accompanied
by a rise in the total catecholamine concentration
overflowing into coronary sinus blood. Clonidine in-
hibited both these effects.
In the present study we have used the spinalized
dog preparation for two reasons. Firstly, spinalization
removes the major homeostatic mechanisms regulat-
ing the cardiovascular system, thus avoiding elevated
levels of circulating catecholamines as observed in the
preparation used by Yamaguchi
Secondly, we have shown that in the spinalized prep-
cardioaccelerator nerve produces a stable, sustained
steady-state conditions in which to investigate cardiac
The purpose of this study was to assess the effects
of clonidine, a known potent agonist of presynaptic
a-adrenoceptors, on the amounts of adrenaline, dopa-
mine and noradrenaline overflowing into coronary
sinus blood during sustained electrical stimulation of
postganglionic cardioaccelerator fibres. In addition,
the effects of phentolamine, a classical a-adrenoceptor
antagonist (blocking both
a-adrenoceptors; see above reviews), and prazosin, a
compound reported to possess cardiac presynaptic
a-adrenoceptor blocking properties in the dog (Roach
et al., 1978a; Constantine, Weeks & McShane, 1978)
but not in the rat, as assessed against clonidine
(Cavero, Lefevre & Roach, 1977; Roach et al., 1978a;
Hua & Moulds, 1978), were examined on the tachy-
cardia and noradrenaline overflow which result from
cardiac sympathetic nerve stimulation.
pre- and postsynaptic
Mongrel dogs of either sex weighing 15 to 18 kg were
anaesthetized with sodium pentobarbitone (35 mg/kg,
i.v.) and after tracheal intubation were ventilated with
room air by means of a Bird Mark 7 respirator.
Catheters were placed in the thoracic aorta, via a
brachial artery, and a brachial vein in order respect-
ively to measure aortic blood pressure with a Statham
P23Db pressure transducer and administer drugs in-
travenously. Heart rate was calculated with a cardio-
tachometer (custom made with 1% maximal error)
f 7 rI
10 15 20 25 30
B1-Blo indicate the times at which thoracic aortic and
coronary sinus blood samples were taken.
Flow-chart of the experimental procedure.
triggered from the electrocardiogram (lead II). The
aortic blood pressure (Hellige MA preamplifier), heart
rate and coronary sinus blood flow (Carolina Medical
Electronics) were recorded on an electrostatic writer
(Varian, Statos IV).
The spinal cord was exposed between the second
and third cervical vertebrae and transected 5 min
after local administration of lignocaine (2% w/v). The
animals were then bivagotomized and the common
carotid arteries ligated.
After thoracotomy (second intercostal space) the
right stellate ganglion was fully decentralized and its
most caudal post-ganglionic branch was placed on
a bipolar platinum electrode. Then, a suitable period
was allowed for blood pressure and heart rate stabili-
zation before the administration of heparin (750 u/kg,
i.v.). The coronary sinus was catheterized with a rub-
ber cannula (french size 16 with closed round tip and
2 large eyes on the lateral wall by the tip) via the
jugular vein. Coronary sinus blood passed through
an extracorporeal (non cannulating) electromagnetic
flow probe (the accuracy of which was verified by
collecting blood samples over a minute period into
a graduated cylinder) and drained into a container
placed in a water bath maintained at 40°C. Coronary
sinus blood samples could, therefore, be easily taken
when required. Dextran (6% w/v) was added to re-
place the blood samples taken as described below.
A roller pump (Sarns) was used to return the blood
to the animal via a femoral vein.
In a group of spinal dogs the sinus node artery
was dissected (Hashimoto, Tanaka, Hirata & Chiba,
1967) and a shunt was made between it and the caro-
tid artery. The shunting circuit contained an extracor-
poreal electromagnetic flow probe and a rubber con-
nector to allow drug administration into the sinus
Measurement ofplasma catecholamines
Blood samples (2 to 3 ml) were collected from the
thoracic aorta and the coronary sinus and then placed
in ice-cooled plastic tubes. The blood was centrifuged
at 500 g for about 15 min in a refrigerated centrifuge.
The plasma was transferred into vials and frozen at
I. CAVERO et al.
or other agonists should not be envisaged as the only
test toassess the cardiac presynaptic
properties of a compound.
The present results suggest that the measurement
of noradrenaline concentration in the coronary sinus
plasma of spinal dogs may be a useful
approach for studying cardiac presynaptic mechan-
The authors thank Mrs J. Lorrain for her excellent techni-
cal assistance in some of the experiments. Thanks are also
extended to Dr R. Gomeni for his help with the statistical
analysis of the data and to Mrs N. Sieller for secretarial
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(Received January 30, 1979.
Revised March 1, 1979.)