303
Dynamic and Static Conditioning Pressures
Evoke Equivalent Rapid Resetting in Rat
Aortic Baroreceptors
Michael C. Andresen and Mingyong Yang
A recent study suggested that exposure of carotid sinus baroreceptors to pulsatile pressures for
a period of minutes can decrease the static threshold pressure at which they begin to discharge;
that is, the exposure can sensitize baroreceptors. Another study found that the rapid resetting
of carotid sinus baroreceptors to elevated conditioning pressures was reduced or eliminated by
pulsatile conditioning. In the present study, we tested the responses of aortic baroreceptors in
an in vitro preparation to a range of static and dynamic conditioning pressures lasting 15
minutes. Slow test ramps of increasing pressure were used to assess static discharge properties
(threshold and gain). To be accepted for analysis (n= 12), each baroreceptor had to successfully
complete static and dynamic test sequences for at least three different conditioning mean
arterial pressure levels. We measured aortic diameter simultaneously with pressure and
baroreceptor discharge. Generally, we found no significant difference between the static
pressure threshold measured before and after dynamic conditioning. The static pressure
threshold was linearly related to the mean level of the conditioning pressure, and no differences
in the slopes of these relations (a measure of the ability of a baroreceptor to rapidly reset) were
found between static and dynamic conditioning. After dynamic conditioning, discharge rates
returned to near control levels in all cases within a few seconds of the return to static
conditioning. Two baroreceptors exposed to the vasodilator nitroprusside throughout the
experiment showed similar results. Diameter measurements indicated no role of vessel
diameter changes during dynamic or static conditioning. In conclusion, we found no evidence
of a long-lasting sensitization of aortic baroreceptors by dynamic pressure inputs. The results
of this study of aortic baroreceptors during static and dynamic conditions together with
previous reports in conscious or intact animals suggest that the mean level of pressure is the
primary determinant of the magnitude of the shift in the baroreceptor discharge curve during
rapid resetting. (Circulation Research 1990;67:303-311)
A
rterial baroreceptors have long been known to
respond to both the static level of a stimulus
and the rate of change of the stimulus.1
However, some recent studies using dog carotid sinus
preparations have suggested that exposing baroreceptors to pulsatile conditioning pressures may have
two important additional effects on baroreceptors:
postconditioning sensitization and attenuation or
elimination of rapid resetting.2 In those studies,
addition of pulsation to a steady conditioning pressure substantially lowered the static pressure threshold (Pth) for discharge with some examples decreasing
From the Department of Physiology and Biophysics, University
of Texas Medical Branch, Galveston, Tex.
Supported in part by National Institutes of Health grant HL33436.
Address for correspondence: Dr. Michael C. Andresen, Department of Physiology and Biophysics, University of Texas Medical
Branch, Galveston, TX 77550.
Received August 15, 1989; accepted March 13, 1990.
threshold by more than 25 mm Hg.3 Under some
conditions, the sensitizing effect of pulsatile pressure on
arterial baroreceptors is reported to last for minutes
after a return to static pressures.2 As a result, discharge
rates were elevated relative to control static conditions
at identical pressures in the postconditioning period.
Effects of this magnitude are as large or larger than
modulation of arterial baroreceptor discharge by
smooth muscle or by a1-adrenergic excitation4 and
would have important functional consequences on the
reflex control of the circulation.5
Pulsatile conditioning has also been reported to
attenuate or eliminate rapid resetting of baroreceptors.3'6 Rapid resetting refers to the shifts found in
the pressure-discharge curves of baroreceptors after
changes in conditioning mean arterial pressure
(cMAP). These recent reports of a modification of
rapid resetting by dynamic conditioning contrast to
numerous earlier studies of baroreflexes and/or barore-
304
Circulation Research Vol 67, No 2, August 1990
ceptor afferents in animals7-9 and humans.10,1l Mean
pressure was changed in these earlier experiments with
a variety of techniques, but pressure stim-uli to the
baroreceptors were cardiac generated and, therefore,
pulsatile. The reason for the differences in findings is
unclear.
In the present study, we have evaluated the influence of pulse pressure on the ability of aortic baroreceptors to rapidly reset in our in vitro preparation.
This preparation allowed us to carefully control
experimental conditions and simultaneously record
pressure, action potentials, and aortic diameter. Vessel wall mechanics clearly affect the absolute value of
the baroreceptor discharge threshold during rapid
resetting,12 and thus changes in diameter might contribute to baroreceptor responses during dynamic
conditioning. Previous studies suggested that the
resetting ratio (APth/AcMAP), a measure of the
ability of a given baroreceptor to rapidly reset, differs
substantially from baroreceptor to baroreceptor,
even within the same animal.12 Therefore, we sought
to define the rapid resetting process over a range of
several cMAP levels for both static and dynamic
cMAPs in each baroreceptor.
Materials and Methods
Male Sprague-Dawley rats (200-350 g; Harlan
Sprague-Dawley, Inc., Indianapolis) were anesthetized with pentobarbital sodium (40 mg/kg i.p.). The
methods for isolation and perfusion of rat aortic arch
baroreceptors were identical to those described
previously.'314 Briefly, after a midline thoracotomy,
the left aortic nerve and aortic arch were surgically
isolated. The aortic arch was cannulated at the
innominate and descending aorta, and the preparation was removed from the animal and mounted in a
temperature-regulated (370 C) Plexiglas perfusion
dish. The aortic arch was covered with warm mineral
oil and perfused with an oxygenated, buffered KrebsHenseleit solution13 at a mean pressure of 80 mm Hg.
A roller pump produced a steady mean flow of 3
ml/min. An air-filled reservoir damped pump-induced
pressure oscillations and regulated the mean level of
perfusion pressure by a modified Starling resistor on
the outflow. To measure vessel diameter, the aorta was
positioned over a central glass window in the floor of
the perfusion dish for projection of the aortic profile
onto photodiode arrays of a custom-made highresolution (0.02%) photoelectronic caliper.1516 Aortic
diameter just distal to the left subclavian artery was
measured continuously throughout in all but four of the
experiments (five baroreceptors).
The aortic nerve was desheathed and split into fine
filaments containing one or, in two cases, two active
baroreceptors. Only regularly firing baroreceptors
were tested further. The discharge properties of
aortic baroreceptors were tested with slow ramps of
pressure (<2 mm Hg/sec) from 20 to 150-180
mm Hg. When a single baroreceptor was isolated,
perfusion was halted and pressure was lowered to 20
mm Hg for 30 seconds followed by a test ramp using
the shaker-bellows-driver system (model 411, Ling
Dynamic Systems, Yalesville, Conn.). We have previously shown that the pressure-discharge curves
evoked by this ramp rate are very similar to the
adapted discharge responses measured 30 seconds
after step changes in pressure,13 and we use these
ramp responses as a measure of the static discharge
properties of the baroreceptor. By using ramps, we
were able to precisely and repeatedly assess the
threshold, suprathreshold gain, and saturation characteristics of each baroreceptor in the shortest possible time.16 Pressure, diameter, and the electroneurogram were recorded on FM magnetic tape for later
analysis. Discharge frequency was measured as the
instantaneous frequency (reciprocal of the interspike
interval) by microcomputer (PDP 11/23, MDB Systems, Inc., Orange, Calif.). Discharge rate was measured as the number of spikes divided by a given time
interval of measurement (generally 1 second or longer). For recordings from pairs of baroreceptors,
spikes belonging to each neuron were sorted electronically (BAK, Rockville, Md.) and then processed
by the computer.17 Pressure and diameter were sampled every 10 msec. Mean pressures, discharge frequencies/rates, and diameters were determined by
computer averaging over 1-5-second time bins.
After isolation of a single baroreceptor, ramp tests
were repeated every 5 minutes to measure stability of
the discharge properties. Thus, all experiments began
at a static cMAP of 80 mm Hg. After at least three
test ramps were completed during conditioning at a
steady pressure of 80 mm Hg, a 4-Hz sinusoidal
oscillation was added to the mean perfusion pressure
using the shaker system. Pulse pressure was 40
mm Hg. The dynamic cMAP was continued for a
total of 15 minutes. In half the cases, test ramps
interrupted conditioning every 5 minutes. In the
remainder, test ramps were given after 10 and 15
minutes. Following completion of both static and
dynamic conditioning at a given cMAP, pressure was
then stepped to a new cMAP at which the static and
dynamic conditioning sequence was repeated. The
range of cMAPs was from 50 to 140 mm Hg and
included pressures in the subthreshold and maximum
discharge regions of the steady-state pressure-discharge
relations of the individual baroreceptors.'8 The order
of cMAP presentation was random except that all
experiments began at 80 mm Hg, and static conditioning always preceded dynamic conditioning at any
given level. No cMAP steps were smaller than 20
mm Hg or larger than 60 mm Hg. To be accepted for
analysis, each baroreceptor had to successfully complete static and dynamic test sequences for at least
three different cMAP levels. Thus, all successful
experiments were based on a minimum of 12 measurements of the complete baroreceptor discharge
curve over 90 minutes.
Rapid resetting of arterial baroreceptors results in
parallel shifts of both the pressure-discharge and
diameter-discharge curves.12,19-21 We used two methods to measure these curve shifts: 1) comparison of the
Andresen and Yang Pulsatile Resetting of Aortic Baroreceptors
entire pressure-discharge curves,7,8 or 2) comparison of
threshold values when the shape of the pressuredischarge curve was unchanged. When threshold values
were used, the first 10 points were averaged to avoid
basing this important measure on the location of a
single action potential.16 Equivalent comparisons
were made on the basis of diameter-discharge curves.
These threshold or location values were then plotted
against cMAP, and these resetting relations were fitted
with a linear function by least-squares regression.
Resetting relations were described well by this linear
function (r' exceeded 0.9 in all cases except one). The
slope of these lines fit to the rapid resetting relation
(APth/AcMAP) as used as a measure of the ability of a
given baroreceptor to reset rapidly.12 For each baroreceptor, a relation was constructed for responses to
static cMAPs and another for dynamic cMAPs. The
slopes and locations of the two regressions in each
experiment (static versus dynamic) were compared by
analysis of covariance.22
Results
Perfusion at suprathreshold, nonpulsatile, "static"
pressures evoked a steady rate of action potentials
from regularly discharging aortic baroreceptors (Figure 1). The even spacing of intervals between successive action potentials resulted in very small standard
deviations of activity rates whether expressed as
discharge rate (counts divided by counting time interval) or as the average instantaneous frequency17
(Figure 1). Addition of pulsations during the conditioning period generally either did not change or
slightly increased the time-averaged, mean discharge
rate of the baroreceptor (Figure 1). The addition of
pulsations usually increased the average instantaneous frequency of baroreceptor action potentials
(Figure 1), an effect resulting from the altered temporal distribution of action potentials during pulsation (Figure 2). On average in the case of Figures 1
and 2, simple counts across time showed no difference in discharge rate. The time-averaged instantaneous frequency of action potentials increased substantially after switching from static to dynamic
conditioning.
In other cases, however, discharge rate was elevated during dynamic conditioning (Figures 3 and 4).
As has been observed previously,23-25 baroreceptor
discharge responses to pulsatile pressures often
occurred at pressure excursions somewhat below the
Pth of that baroreceptor (Figure 3, lowest dynamic
cMAP). However, at cMAPs well above Pth, discharge rates were often similar during static and
dynamic stimuli at the same mean pressure (Figures
1 and 3). We never observed decreases in baroreceptor activity when we switched from static to dynamic
stimuli, whether activity was expressed as mean
instantaneous frequency or as mean discharge rate
(counts). Return to static conditioning promptly
returned the discharge rate and the instantaneous
frequency to levels similar to those preceding dynamic
conditioning (Figures 1-4).
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FIGURE 1. Baroreceptor activity during static and dynamic
conditioning pressures. This figure displays 1-second averages
of both the perfusion pressure (PRESS.) (bottom panel) and
of two measures of baroreceptor action potential activity: the
mean discharge rate (DISCH.) (counts of action potentials per
1-second bin, top panel) and the mean instantaneous frequency (FREQ.) (1-second averages of the reciprocals of the
interspike intervals, middle panel). Pressure and frequency
points are mean+±SD. Beginning at about 70 seconds, the
aortic arch was pulsed sinusoidally around the mean conditioning pressure (4 Hz, 40 mm Hg pulse pressure) for a little
more than 5 minutes. Note that, in this case, the average
discharge rate was unchanged from the original static level
both during and after dynamic conditioning. During the
dynamic stimulus, however, mean instantaneous frequency
was increased.
Without careful attention, the precision of the
measurement and manipulation of conditioning pressure may affect the conclusions drawn. Controlling
pressure precisely enough was sometimes difficult
across transitions between steady and dynamic conditioning periods. This may have resulted from
imprecision in pressure measurements during the
experiments or from small differences in the efficacy
of the Starling resistor in controlling mean pressure
at all levels of cMAP during dynamic stimuli. After
digitization, when precision was better than 0.1%,
small differences in MAP were sometimes clearly
evident. Thus, some differences in discharge rate
during conditioning occurred that could be attributed
simply to small differences in MAP (Figure 5).
Dynamic cMAP levels were set during the experiment by monitoring the minimum, maximum, and
mean pressures on a pen recorder (filtered with a
3.2-second time constant) (Gould, Cleveland). At a
chart paper scale of 4 mm Hg/mm, the practical
pressure resolution was only about 2 mm Hg. Actual
mean dynamic pressures measured by the computer
were generally found to be within ±2 mm Hg of the
static cMAP. For a baroreceptor with a static gain of
1 spike/sec/mm Hg measured with a slow ramp
Circulation Research Vol 67, No 2, August 1990
306
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FIGURE 2. Details of the response of a single baroreceptor to
the onset (panel A) and offset (panel B) of dynamic conditioning pressure (PRESS.). This figure replots portions of the
data of Figure 1. Here, discharge (DISCH.) is the rate of
action potential firing expressed as counts per second over
successive 300-msec periods. Frequency (FREQ.) points are
values (unaveraged) of the instantaneous frequency of action
potential firing (reciprocal of the interval between successive
pairs of action potentials). Note in the lower panels that
baroreceptor retums to activity levels similar to those preceding
dynamic conditioning within 1 second.
(Table 1), the dynamic gain at 4 Hz would be
expected to be at least two to three times higher.26
Thus, this small difference in mean pressure would
be expected to produce a substantial change in mean
discharge rate of ±4 to 6 spikes/sec but not as large
as those observed in the carotid sinus.2 In practice, in
our best experiments for which the dynamic cMAP
was within 1 mm Hg of the static cMAP, mean
discharge rates were the same or somewhat greater
during dynamic cMAPs than at the equivalent static
cMAP.
Generally, we found very little difference between
the Pth measured before and after dynamic conditioning. The value of these differences in Pth mea-
sured after static conditioning compared with that
measured after dynamic conditioning ranged from
-5.4 to +4.8 mm Hg and averaged +0.11±0.40
mm Hg (mean+SEM) across all baroreceptor tests
(n=38). The overwhelming majority (73.7%) of the
differences in Pth produced by transitions between
static and dynamic conditioning were less than 3
mm Hg (Figure 6). The baroreceptor discharge
curves were superimposable in 24% of the observed
cases.
Twelve baroreceptors met or exceeded our minimum criteria (see "Materials and Methods") for a
successful determination of rapid resetting ability to
both static and dynamic conditioning (Table 1). In
five cases tested, one cMAP level was sufficiently
subthreshold for the particular baroreceptor that no
discharge occurred during dynamic or static conditioning. All experiments included at least one cMAP level
within the linear portion of the quasi-steady-state
pressure-discharge curve generated by the control test
ramp at the initial cMAP of 80 mm Hg. Five baroreceptors were exposed to one cMAP that was within the
maximum discharge region of the pressure-response
curve.
Static conditioning produced a linear rapid resetting relation (Figure 7), as reported previously.12'18
The relations for rapid resetting to dynamic conditioning were similar (Figure 7). The slopes of the
relations, the resetting ratios (APth/AcMAP), and
the goodness of the linear fit (r2 values) were similar
(p>0.50). In individual experiments, when small
differences in Pth were induced by dynamic conditioning (Figure 6), similar changes in Pth tended to
be produced at all cMAPs for that baroreceptor unit.
Thus, when present, these small differences in Pth
shifted the rapid resetting relation in a parallel manner to slightly higher or lower pressures (p<0.05), but
they did not affect the resetting ratio, that is, the
ability of the baroreceptor to reset.
In only a few cases were there measurable changes
in any of the pressure-diameter curves during conditioning. Diameter changes, when they occurred, followed no trends with cMAP or dynamic versus static
conditioning and were generally not consistent or
maintained during any given condition. Some of
these changes in diameter may give rise to a portion
of the observed variability in the rapid resetting
relations (Figure 7, Table 1). To test whether subtle,
immeasurable changes in smooth muscle activation
could affect baroreceptors during dynamic conditioning, we tested rapid resetting to static and dynamic
conditioning in two baroreceptors in the presence of
the vasodilator nitroprusside (10-6 M). No differences in the resetting relations of these baroreceptors were found between static and dynamic conditioning (p>0.25).
Discussion
The ability of arterial baroreceptors and baroreflexes
to rapidly reset after changes in the prevailing pressure
has attracted considerable attention recently. For the
Andresen and Yang Pulsatile Resetting of Aortic Baroreceptors
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most part, these reports describe the rapid resetting
in baroreceptor afferents as a parallel shifting
of the pressure-discharge curve with no change in
slope.7'18'19'21'27'28 The earliest afferent studies used a
variety of anesthetized animals (rabbits,7 rats,29 and
dogs27) and different techniques for manipulating blood
pressure, including blood volume changes (hemorrhage
or infusion), vasoactive drugs, aortic banding, and reservoir control systems. Similar parallel shifts have been
observed in baroreflex relations for control of heart rate
in conscious rabbits,7'8 rats,9 and humans.10"1 Under
these circumstances, pressures were generated by the
pumping action of the heart and, therefore, had pulse
pressures of about 40 mm Hg and frequencies in the
range of heart rates for the various species, roughly 1-6
Hz. These studies in relatively intact preparations have
a rapid resetting process, which is similar in form and
magnitude to that in studies using static conditioning in
isolated in vitro preparations.12"18
In the past two years, several studies have reported
that switching between static and dynamic pressures
process
307
FIGURE 3. Relation between the rate of discharge found
during dynamic conditioning and the static pressure-activity
curve for that baroreceptor. For this case, the baroreceptor was
exposed to dynamic conditioning mean arterial pressures
(cMAPs) of 81, 115, and 145 mm Hg. Panel A: Three traces
of connected points are given in each panel and represent the
three different levels of dynamic cMAP. Mean arterialpressure
(lower panel) and average discharge rate (upper panel, counts
of action potentials every 2 seconds) are plotted against time.
Solid circles display values during the final minute of a
5-minute period of dynamic conditioning (4 Hz, 40 mm Hg
pulse pressure). At the arrow, the sinusoidal pulsation was
tumed off, but pulseless MAP was maintained to record any
transient changes after dynamic conditioning (open circles). At
the lowest two cMAPs, discharge immediately dropped to zero
at the end of sinusoidal conditioning. At the highest cMAP,
discharge was about 40 spikes/sec, a rate that prevailed before,
during, and after dynamic conditioning. Panel B: Increases in
dynamic cMAP caused progressive resetting (parallel shifts) in
the static pressure-activity curve of the baroreceptor. Pressureactivity curves (open circles) immediately after 10 minutes of
conditioning at each of the three cMAPs are plotted as mean
instantaneous frequency points (averaged over 0.5-second time
bins) vs. mean pressure. For slow pressure ramps in which
action potentials are evenly spaced, the mean instantaneous
frequency is identical to the mean discharge rate expressed as
counts per 1-second intervals.17 Large, solid circles are the
mean discharge rates observed during the conditioning pulsatile perfusion at the three cMAP levels (primary data shown in
Panel A, solid circles). At the two lowest dynamic cMAPs,
activity values during perfusion were far to the left of the
postdynamic pressure-activity curves for those cMAPs. The
activity value during dynamic perfusion for the highest cMAP
fell near the midpoint and directly on the pressure-activity
curve for that cMAP (far right curve). If expressed as mean
instantaneous frequency values during pulsatile perfusion,
these activity values would have been higher (40, 105, and 110
spikes/sec, respectively) than the plotted discharge rate values.
can have long-lasting effects on baroreceptor discharge properties and that these effects can modify
the shifts in pressure threshold found during rapid
resetting.2,3'30 Failure of some recent baroreflex studies to find rapid resetting in response to dynamic
conditioning6'31 has led to speculation that afferent
mechanisms might be involved. Other sites in the
baroreflex arc, however, offer plausible potential
mechanisms that might prevent the expression of
resetting of afferent characteristics in baroreflex
responses.5'32
The results of our study about the responses of rat
aortic baroreceptors to dynamic stimuli present a
clear contrast in several important aspects to observations recently reported for carotid sinus baroreceptors in dogs.3'23 First, the dynamic sensitivity of aortic
baroreceptors at moderate frequencies was higher
than the static sensitivity (slope of the pressureactivity curve) so that baroreceptor activity tended to
increase during pulsatile conditioning. Thus, these
aortic baroreceptor responses were similar to condi-
Circulation Research Vol 67, No 2, August 1990
308
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FIGURE 5. Transition between dynamic and static conditioning mean arterial pressures in a single baroreceptor where
mean pressure was not constant. Conditioning with a sinuso-
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discharge rate and pressure for the final seconds of dynamic
conditioning. At the point marked 40 seconds, the sinusoidal
pressure pulse was tumed off Mean pressure and discharge
rate increased slightly when pulsations ceased.
TIME (sec)
FIGURE 4. Transition between dynamic and static conditioning mean arterial pressures (cMAPs) after 10 minutes of
conditioning with a sinusoidal pressure in a single baroreceptor. This baroreceptor had the largest increase in instantaneous
frequency observed in this study when sinusoidal pressure was
added to the static cMAP ofperfusion. The figure displays the
1-second means of instantaneous frequency, discharge rate
(counts/sec), and pressure for the final 30 seconds of the
10-minute dynamic conditioning period and the first 12
seconds after a return to static perfusion pressures. Note that
mean pressure did not change, but discharge immediately fell
to a lower steady rate when pulsation ceased.
tions I, II, and III for the carotid sinus work23 in
which some or all of the input pressure pulse was
below Pth. However, we never observed a decreased
mean discharge rate after the addition of our sinusoidal pulse at any cMAP, as was observed in carotid
sinus baroreceptors when the minimum pressure of
the dynamic stimulus ("diastolic" pressure) exceeded
Pth (condition IV23). Those decreases in activity during
pulsatile stimulation at moderately high mean pressures reduced the slope of the multifiber pressureactivity curve (Figure 7 in Reference 23) and of singlefiber baroreceptor curves (Figure 9 in Reference 23).
For multifiber baroreceptor activity, it is not clear
whether, or how much, the activity decreases reported
(-20% to -25% in Reference 5) represent true
declines in baroreceptor activity or simply failure to
detect action potentials in an electroneurogram
degraded by increases in voltage signal interactions
during pulsatile inputs.17
Second, we found no substantial sensitization of
aortic baroreceptors after dynamic conditioning.
Increases in action potential rates did not change
TABLE 1. Mean Baroreceptor Resetting Parameters
Mean
Pressure threshold*
104.0
(mm Hg)
SEM
2.35
Baroreceptor gain*
0.983
0.097
(spikes/sec/mm Hg)
Static resetting
0.2047
0.0252
APth/AcMAP*
r2
0.9531
0.0099
4.9093
ADth/AcMAP (x 1,000)t
2.4591
r2
0.8385
0.1016
Dynamic resetting
APth/AcMAP*
0.2316
0.0214
r2
0.9531
0.0111
ADth/AcMAP (x 1,000)t
4.2414
1.4095
r2
0.9033
0.0427
*Means based on 12 single-fiber baroreceptors.
tAll diameter threshold (Dth) measures based on seven singlefiber baroreceptors.
Andresen and Yang Pulsatile Resetting of Aortic Baroreceptors
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FIGURE 6. Differences in baroreceptor pressure threshold
(APth) between measurements following static and dynamic
conditioning mean arterial pressures (cMAPs). Pth was measured in each condition with an identical slow ramp of
increasing pressure (see "Materials and Methods"). APth was
calculated as the difference between Pth measured after
dynamic conditioning with sine waves minus Pth measured
after static conditioning. Since multiple measurements were
made at each cMAP, we compared means of these measurements for a given condition. Borders of the histogram bars
mark the range of case counting so that, for example, the bar
at the far left represents those transitions where APth was a
relative decrease in threshold of between -6 and -4 mm Hg
during dynamic conditioning relative to static cAL4P.
over time during the dynamic conditioning period
and activity rates promptly fell to predynamic conditioning levels after a return to steady pressures. In
fact, the distribution of the small changes in pressure
threshold between postdynamic and poststatic conditions that we did observe suggests that desensitization (an increase in threshold) was as likely to result
from dynamic conditioning as sensitization in rat
aortic baroreceptors.
Third, our rapid resetting relations show that
equivalent rapid resetting occurs whether static or
dynamic cMAP is used. The magnitude of the shift in
pressure threshold over the range of pressures spanning subthreshold to maximum frequency portions of
the baroreceptor response curves was linearly related
to the mean pressure, as reported earlier.7,12"18 We
found no consistent changes in pressure-diameter
relations during baroreceptor resetting to static and
dynamic cMAPs, and in experiments using preparations maximally dilated with nitroprusside, the results
were similar.
Several potentially important differences in experimental conditions exist between our study and those
reporting postpulsatile sensitization of carotid sinus
baroreceptors. First, we studied aortic arch baroreceptors of the rat. While some differences between
these two sites in terms of smooth muscle content
and distensibility may exist, direct comparisons of
rapid resetting of simultaneously recorded aortic and
carotid sinus baroreceptors to dynamic pressures in
dogs showed no differences.28 Second, we used con-
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FIGURE 7. Rapid resetting relations for two different baroreceptors (panels A and B) for static and dynamic conditioning.
Plots are ofpressure threshold (Pth) vs. the conditioning mean
arterial pressure (cMAP). Pth was measured in response to a
slow ramp of pressure. Points represent a single measurement
of Pth after a period of static cMAP (open circles) or dynamic
cMAP (solid circles). Dynamic conditioning used a 4-Hz sine
wave with a 40 mm Hg pulse pressure at the mean. Flow
through the aortic arch was maintained throughout the conditioning period. In these examples, three measurements were
made under each condition, but overlap obscures many points
where similar Pth values were found at a given cMAP. Lines
are least-square fits to the data (static cMAP, solid line;
dynamic cMAP, broken line).
stant flow perfusion throughout the conditioning
periods and had only brief intervals of zero flow
during ramp tests lasting roughly 2 minutes. By
constantly perfusing the lumen of the aorta with new,
freshly gassed saline, we feel confident that vital
extracellular conditions were maintained constant
during the course of our experiments. It is not clear
what effects might arise from nonperfusion for
lengthy periods, although some studies suggest that
baroreceptor discharge properties might be affected
by accumulation of substances secreted by the
endothelium33-35 or by flow per se.36 Third, we did
not use acetate in our perfusate. Acetate is a normal
constituent of arterial plasma but generally at low
concentrations (about 0.07 mM in humans37) compared with the 20 mM used by Chapleau et al.3'23 At
310
Circulation Research Vol 67, No 2, August 1990
these levels, acetate would be expected to have three
potentially important effects on baroreceptor, smooth
muscle, or endothelial cells by the following mechanisms: 1) intracellular acidification by diffusion of the
undissociated form across biological membranes along
its concentration gradient and subsequent intracellular dissociation with proton release,38 2) decreases in
calcium activity levels by complexing with acetate,39
and 3) vasodilation.37,40 Tests of equimolar substitution of 20 mM acetate for NaCl in our KrebsHenseleit solution on one aortic baroreceptor for 20
minutes showed, however, only minor vasodilation,
modest decreases in pressure threshold (<5 mm Hg),
and small (<20%) increases in the maximum discharge rate (unpublished observations). It is difficult
to conclude whether any or all of these environmental factors are important to our negative findings or
to the expression of postpulsatile sensitization and
attenuation of rapid resetting.
Some technical/analytical differences may also
affect conclusions. We have measured the magnitude
of rapid resetting by examining the entire pressureresponse curve of a given baroreceptor. Measures of
the shifts in these baroreceptor response curves were
based on either 10 points at the pressure threshold or
shifts of the entire response curve consisting of
between 1,200 and 3,000 data points. We made
replicate measurements at each cMAP over 15 minutes and used at least three different cMAPs in each
experiment. We believe the combination of replications, lengthy conditioning periods, and use of complete baroreceptor response curves assures us of
reliable estimates of potential experimental variation
and error in each experiment. In the resetting studies
of Chapleau et al,3 in which attenuated resetting was
observed, pressure ramps were used to elicit only
threshold discharge, and replicate measures were not
mentioned. Threshold pressure appears to have been
measured as the pressure at which the first action
potential occurred.
We found that the method of measuring baroreceptor "activity" rates was critically important even in
these single baroreceptors. Apparent rates of baroreceptor activity were often higher during dynamic stimuli
when the same action potential records were analyzed
by averaging instantaneous frequencies rather than by
counting spikes over a fixed period. For instance, we
found clear examples in which shifting from steady
pressures to pulsatile pressures did not change the
mean discharge rate measured as counts of action
potentials per unit time, but average instantaneous
frequency was increased. Others have also reported
that average counts of spikes per second are unchanged over a range of pressure input frequencies.25,26 Because the temporal pattern of afferent
action potentials invading central nervous system
synapses may be a critical factor determining the
efficacy of action potential trains to generate reflex
responses,41-43 instantaneous frequency may better
reflect important differences in baroreceptor input
patterns to baroreflexes than averaged counts of
discharge over time.
In conclusion, our study does not provide support
for the idea that pressure pulsations induce longlasting (seconds or longer) changes in static baroreceptor discharge properties. None of the differences
in technique, protocol, or conditions appear to offer
a ready explanation for the differences in the findings
between this study and those reporting postpulsatile
sensitization. It is possible that there may be unique
differences in the carotid sinus of dogs that provide a
sensitizing mechanism in that preparation. However,
this study, together with studies in more intact systems, suggests that sensitization is not a fundamental
or general property shared by all baroreceptors and
species. Rapid resetting of aortic baroreceptors
clearly occurred with exposure to dynamic conditioning and was quantitatively similar to static conditioning. Our study reaffirms that the level of mean
arterial pressure is the most important determinant
of rapid resetting and, therefore, is a critical influence on the absolute location of the baroreceptor
pressure threshold.
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KEY WORDS * baroreceptors * pressoreceptors
resetting * baroreflex * blood pressure
rapid