CARDIOVASCULAR JOURNAL OF AFRICA • Vol 22, No 1, January/February 2011
AFRICA
15
the femoral artery. This was done for the purpose of blood pres-
sure and heart rate recording. The catheters were made of 4-cm
segments of PE-10 polyethylene (Clay Adams, USA), heat-
bound to a 13-cm segment of PE-50. The catheters were tunneled
under the skin and exteriorised at the animal’s dorsum.
11
Approximately 24 hours after surgery the animals were kept
in individual cages used in the transport to the experimental
room. Animals were allowed 20 min to adapt to the conditions of
the experimental room, such as sound and illumination, before
starting blood pressure and heart rate recording. The experimen-
tal room was acoustically isolated and had constant background
noise produced by an air exhauster. At least another 15-min
period was allowed before beginning the experiment.
Pulsatile arterial pressure (PAP) of the freely moving animals
was recorded using an HP-7754A pre-amplifier (Hewlett
Packard, USA) and an acquisition board (model Powerlab 16SP,
ADInstruments, Colorado Springs, CO, USA) connected to a
computer.Meanarterialpressure(MAP)andheartrate(HR)values
were derived from the PAP recordings and processed on-line.
11
The baroreflex was tested with a pressor dose of phenyle-
phrine (PE bolus: 8
μ
g/kg i.v.; Sigma Chemical) and a depressor
dose of sodium nitroprusside (SNP bolus: 50
μ
g/kg i.v.; Sigma
Chemical). The baroreflex was calculated as the derivation of HR
as a function of the MAP variation (
Δ
HR/
Δ
MAP). There was an
interval of at least 15 minutes between the infusions to allow the
recovery of basal values. We also measured the bradycardic and
tachycardic peak and the HR range – the difference between the
bradycardic and tachycardic peak.
11
We divided the rats into groups according to the baroreflex
gain (BG): (1) low bradycardic baroreflex (LB) group: BG
between 0 and –1 bpm/mmHg tested with PE; (2) high brady-
cardic baroreflex (HB) group: BG
<
–1 bpm/mmHg tested with
PE; (3) low tachycardic baroreflex group (LT): BG between 0
and 3 bpm/mmHg tested with SNP; and (4) high tachycardic
baroreflex group (HT): BG
>
3 bpm/mmHg tested with SNP.
We compared the LB group with the HB group and the LT
group with the HT group. We defined the values for bradycardic
and tachycardic baroreflex gain according to a previous study.
12
Statistical analysis
Values are reported as the means
±
standard error of means
(SEM). HR, MAP,
Δ
HR,
Δ
MAP, bradycardic and tachycardic
peak, HR range and
Δ
HR/
Δ
MAP were compared between HB
and LB groups, as well as between HT and LT groups. After
the distributions were evaluated with the Kolmogorov normality
test, the unpaired Student’s
t-
test was used to verify differences
between normal distributions, and the Mann-Whitney test was
applied to assess differences between non-parametric distribu-
tions. Differences were considered significant when the prob-
ability of a type I error was less than 5% (
p
<
0.05).
Results
Among all the 19 SHRs evaluated (based on baroreflex gain test-
ed with PHE), approximately 37% presented with a higher para-
sympathetic baroreflex gain (HB group:
<
–1 bpm/mmHg). The
majority of the animals who received PHE demonstrated lower
baroreflex gain (LB group: between 0 and –1 bpm/mmHg).
In order to investigate the possibility that another cardiovascu-
lar parameter may have differed between the LB and HB groups,
we compared baseline MAP and HR, the bradycardic and tachy-
cardic peak, the HR range and baroreflex gain, tested with both
PHE and SNP. No significant differences were noted between the
two groups regarding the basal MAP and HR, the bradycardic
peak and the sympathetic component of baroreflex gain (Table
1). However, there were significant differences in relation to the
HR range, the tachycardic peak and the parasympathetic compo-
nent of baroreflex gain.
PHE-induced increases in the MAP did not differ between
the HB and LB groups (
p
=
0.33). However, bradycardic reflex
responses to intravenous PHE were significantly decreased in the
LB group (
p
=
0.0001) (Fig. 1).
We also compared SNP-induced decreases in MAP and the
tachycardic response to i.v. SNP between the HB and LB groups.
MAP decreases in response to SNP tended to be reduced in the
LB group (
p
=
0.0709), however, they did not reach statistical
significance. With regard to the tachycardic reflex response, we
did not note significant differences between the two groups (
p
=
0.7229) (Fig. 2).
When baroreflex gain was tested with SNP, we noted that
among all the 19 SHRs analysed, approximately 27% presented
with a higher baroreflex gain (HT group:
>
3 bpm/mmHg), while
the majority (approximately 73%) presented with a lower barore-
flex gain (LT group: between 0 and 3 bpm/mmHg).
We observed significant differences with regard to the sympa-
thetic component of the baroreflex gain (Table 2). There were no
significant differences between the two groups regarding basal
MAP and HR, the bradycardic and tachycardic peak, HR range
or the parasympathetic component of the baroreflex gain.
When comparing the HT and LT groups with regard to
TABLE 1. BASELINE LEVEL OF MEANARTERIAL PRES-
SURE (MAP) AND HEART RATE (HR), BRADYCARDICAND
TACHYCARDIC PEAK, HR RANGEAND BAROREFLEX GAIN
(BG) IN HB (
n
=
7) AND LB (
n
=
12) GROUPS. MEAN
±
SEM
Variable
Group 1
Group 2
p
-value
MAP (mmHg)
166.14
±
4.3
161
±
3.5 0.3736
HR (bpm)
372.3
±
12.8 337
±
10.6 0.0527
Bradycardic peak (bpm) 319.6
±
17.13 309.92
±
11.6 0.6355
Tachycardic peak (bpm) 519.7
±
11.7 471.1
±
9.2 0.0.0048
HR range (bpm)
218.14
±
18.4 162.45
±
15.7 0.0375
BG (bpm.mmHg
-1
) PHE –1.25
±
0.09 –0.61
±
0.064
<
0.0001
BG (bpm.mmHg
-1
) SNP –1.94
±
0.31 –2.87
±
0.34 0.0817
50
40
30
20
10
0
Group 1 Group 2
∆
MAP (mmHg)
0
–10
–20
–30
–40
–50
–60
Group 1 Group 2
∆
HR (bpm)
Fig. 1. Increase in mean arterial pressure (MAP, mmHg)
and decrease in heart rate (HR, bpm) in response to
phenylephrine (PHE, 8
m
g/kg i.v.) in HB (
n
=
7) and LB (
n
=
12) groups. *
p
<
0.0005: different from LB. Mean
±
SEM.
