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Introduction
Clinical reports have shown that arterial hypertension is
correlated with a high risk of sudden cardiac death due to
fatal ventricular arrhythmias[1,2]. However, few experimental
studies were performed regarding the relationship between
hypertension and the risk of ventricular arrhythmias, thus
little information is available regarding the underlying
mechanisms.
Arterial baroreflex (ABR) is very important in the
regulation of cardiovascular activities. It was found that ABR
function, expressed as baroreflex sensitivity (BRS), was
impaired in hypertension[3,4].
In our previous studies, it was found that impaired ABR function, induced by anesthesia
and sinoaortic denervation, led to an increased
susceptibility of ventricular arrhythmia to aconitine, and an intact ABR
function is necessary for preventing drug-induced
ventricular arrhythmias[5]. In addition, ABR function is an important
determinant of the magnitude of blood pressure (BP)
fluctuation, and a sound ABR function helps to maintain a
stable BP[6]. Therefore, it is logical to expect that the
dysfunction of ABR and the abnormal hemodynamics may be
associated with the susceptibility of ventricular arrhythmia
to an arrhythmogenic agent in hypertensive rats. Aconitine
is a highly cardiotoxic agent which is known to suppress the
inactivation of voltage-dependent Na+ channels by binding
to neurotoxin binding site 2 of the α-subunit of the channel
protein. It is widely used in animals to produce experimental
arrhythmia, and plays an important role in evaluating the
antiarrhythmic effects of new
drugs[7,8]. Accordingly, the present work was designed to investigate the relationship
between BP, blood pressure variability (BPV), BRS, and the
susceptibility of ventricular arrhythmia to aconitine in Lyon
hypertensive rats (LH). Considering the fact that the
baroreflex function was largely inhibited by
anesthesia[9,10], the present experiments were carried out in conscious
animals.
Materials and methods
Animals Male 20-week-old LH and Lyon low blood
pressure rats (LL) were provided by the Animal Center at our
institute. All the rats were housed within controlled
temperature (23_25 °C) and lighting (8:00_20:00 light, 20:00_8:00
dark), and with food and tap water available ad
libitum. All the animals used in this work received humane care in
compliance with institutional animal care guidelines.
BP measurement Systolic BP (SBP), diastolic BP (DBP)
and the heart period (HP) of the rats were continuously recorded using the previously described
technique[11,12]. Briefly, the rats were anesthetized with a combination of
ketamine (40 mg/kg) and diazepam (6 mg/kg). A floating
polyethylene catheter was inserted into the lower abdominal aorta
via the left femoral artery for BP measurement, while another
catheter was placed into the left femoral vein for intravenous
injection. The catheters were exteriorized through the
interscapular skin. After a 3 d recovery period, the animals were
placed in individual cylindrical cages containing food and
water for BP recording. The aortic catheter was connected
to a BP transducer via a rotating swivel that allowed the
animals to move freely in the cage. After about 4 h
habitua-tion, the BP signal was digitized by a microcomputer. SBP,
DBP and HP values from every heartbeat were determined
online. The mean values and standard deviation of these
parameters during a period of 4 h were calculated. The
standard deviation was defined as the quantitative parameter of
BPV, that is, systolic BPV (SBPV), diastolic BPV (DBPV),
and HP variability (HPV).
BRS measurement BRS was measured using the
previously described method[3,13]. Briefly, the principle of this
method is to measure the prolongation of HP in response to
an elevation of BP. A bolus injection of phenylephrine was
used to induce an elevation of SBP between 20_40 mmHg.
HP was plotted against SBP for linear regression analysis
and the slope of SBP-HP was expressed as BRS (ms/mmHg).
Arrhythmia induced by aconitine in anesthetized
LH As described
previously[5], under anesthesia with a
combination of ketamine (50 mg/kg) and diazepam (5 mg/kg), the
cathe-ters were inserted into the femoral artery and femoral vein
using aforementioned means. Four electrodes were fixed
subcutaneously at 4 extremities for electrocardiogram (ECG)
recording. The aortic catheter and electrocardiogram
electrodes were connected to the computerized BP and ECG
monitoring systems. Aconitine (provided by E. Merck, AG
Darmstadt, Germany) was then infused via the femoral vein
catheter with a micro-infusion pump at a velocity of 0.2
mL/min (10 mg/mL in saline). BP and ECG were simultaneously
recorded during the period of aconitine infusion until death
of the animal[5,14]. The time when aconitine was administered
and arrhythmias occurred was recorded. The threshold of
aconitine required for producing ventricular premature beat
(VPB), ventricular fibrillation (VF) and cardiac arrest (CA)
were determined according to the following formula:
threshold (µg/kg) for VPB=10 µg/mL×0.2 mL/min×time required for
inducing VPB (min)/bodyweight (kg)=2 µg/min×time
(min)/bodyweight (kg). The representative tracings of SBP and
ECG in the anesthetized LH rats are shown in Figure 1.
Arrhythmia induced by aconitine in conscious
rats BP was recorded with the technique mentioned in the BP
measurement section. When BP was stable, aconitine was infused via the fermoral vein catheter with a micro-infusion
pump at a speed of 0.2 mL/min (10 µg/mL in saline) in
cons-cious, unrestrained rats[5,14]. BP was monitored during the
entire period of aconitine infusion. The threshold of aconitine required for producing cardiac VPB, VF, and CA
was determined using the same formula as for the
anesthetized rats. The characteristic of premature beat, ventricular
fibrillation and cardiac arrest obtained in blood pressure
tracings of the conscious rats was verified with that of
electrocardiograms in anaesthetized rats.
Statistical analysis Data are expressed as mean±SD.
Comparisons between groups were made by
Student's t-test. The relationships between the threshold of aconitine
and hemodynamic parameters were analyzed by classic
univariate correlation analysis. P<0.05 was considered
statistically significant.
Results
Hemodynamic parameters in conscious LL and LH
rats Compared with the LL rats, the LH rats possessed
signi-ficantly higher SBP (+34%, P<0.01), DBP (+37%,
P<0.01), SBPV (+42%, P<0.01) and DBPV (+35%,
P<0.01) and lower BRS (-38%, P<0.01). No obvious change was found in body
weight, HP and HPV between the LL and LH rats (Table 1).
Threshold of aconitine required for arrhythmia in
conscious LL and LH rats Compared with the LL rats, the LH
rats displayed a significantly lower threshold of aconitine
dose required for VF (LH: 70.5±16 vs LL: 109.0±41.2
mg/kg; P<0.01) and CA (LH: 92.0±13.9
vs LL: 132±40.4 mg/kg; P<0.01). The threshold of aconitine required for ventricular
premature beat was not different between the LL and LH groups
(29.7±10.8 vs 36.5±6.60 mg/kg;
P>0.05) (Figure 1).
Relationships between hemodynamic parameters and the
threshold of aconitine required for arrhythmia in conscious
LH rats Relationships between hemodynamic parameters
and the threshold of aconitine required for arrhythmia in
conscious LH rats are shown in Table 2. It was found that all
hemodynamic parameters studied were not correlated with
the threshold of aconitine required for arrhythmia, with the
exception of BRS, which was positively related to the
threshold of aconitine required for VPB (r=0.567,
P<0.01). Some examples of the correlations are shown in Figure 2.
Discussion
The present work demonstrated that the LH rats
possessed significantly higher BP and BPV, lower BRS, and
greater susceptibility of ventricular arrhythmias to aconitine
than the LL rats.
It is well known that elevated BP contributes to a wide
variety of complications associated with hypertension
including stroke, heart failure and renal failure. During the
early phase of such complications, end-organ
damage, such as left ventricular hypertrophy and renal lesion are present.
Indeed, BP level is a major determinant for hypertensive
end-organ damage. In addition, clinical studies demonstrated
that left ventricular hypertrophy in hypertensive subjects
was associated with an increased prevalence of ventricular
arrhythmias[15,16]. Therefore, we speculated that the elevated
BP might contribute to the greater susceptibility of
ventricular arrhythmias to aconitine in the LH rats. However, the
present work showed that the BP level was not correlated
with the threshold of aconitine required for arrhythmia in the
LH rats. This result indicated that the BP level was not an
independent determinant for ventricular arrhythmia onset in
the LH rats.
It was found that the LH rats possessed significantly
higher BPV when compared with the LL rats in the present
work. Clinical observations suggest that BPV is related to
organ damage in hypertensive
patients[17_19]. In animal studies, organ damage induced by sinoaortic denervation
were related to the high BPV, but not to the BP
level[11,20]. As mentioned earlier, end-organ damage such as left ventricular
hypertrophy in hypertensive subjects was associated with
an increased prevalence of ventricular
arrhyth-mias[15,16]. Accordingly, the role of BPV in hypertension-related
abnormalities is imperative[21]. However, in the present work, BPV
was not correlated with the threshold of aconitine required
for arrhythmia in the LH rats. Therefore, BPV was not an
independent determinant associated with ventricular
arrhythmias in the LH rats.
Arterial baroreflex dysfunction is another characteristic
of hypertension. It has been well recognized that BRS is
impaired in hypertensive humans and
animals[22_25]. Accord-ingly, we demonstrated that the LH rats had lower BRS than
the LL rats. Previous studies suggest that BRS is an
independent variable related to end-organ damage in
hypertension[26]. Furthermore, we found that impaired arterial
baroreflex function, induced by anesthesia and sinoaortic
denervation, led to increased susceptibility of ventricular
arrhythmia to aconitine, and intact baroreflex function is
necessary for preventing drug-induced ventricular
arrhythmias[5]. Nevertheless, the current study showed that BRS was only
positively related to the threshold of aconitine required for
ventricular premature beat, but not to those required for
ventricular fibrillation and cardiac arrest. Hence, BRS was not
one of the independent variables related to the
susceptibility of ventricular arrhythmias to aconitine in the LH rats.
In conclusion, the LH rats possessed greater
susceptibility to aconitine-induced ventricular arrhythmias when
compared with LL rats. This greater susceptibility could
not be attributed to any one of the hemodynamic
parameters alone studied in LH rats. It is proposed that various
hypertension-associated abnormalities, including the
abnormal hemodynamics, may co-contribute to this
vulnerability to ventricular arrhythmias.
Acknowledgement
The authors thank Prof Jean SASSARD at the University
Claude Bernard (Lyon, France) for providing founder rats of
Lyon hypertensive rats and Lyon low blood pressure rats.
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