(Hypertension. 1996;27:584-590.)
© 1996 American Heart Association, Inc.
Articles |
Platelet Activation in Carotid Sinuses Triggers Reflex Sympathoinhibition and Hypotension
From the Department of Internal Medicine, University of Iowa College of Medicine, and the Department of Veterans Affairs Medical Center, Iowa City, Iowa.
Correspondence to Mark W. Chapleau, PhD, Assistant Professor, Department of Internal Medicine, E327 General Hospital, University of Iowa College of Medicine, 200 Hawkins Dr, Iowa City, IA 52242.
 |
Abstract |
|---|
 Top Abstract IntroductionMethods Results Discussion References |
|---|
Abstract The carotid sinuses, one of the major sites of baroreceptor innervation, are also a common site of atherosclerotic lesions and platelet aggregation. The goal of the present study was to determine whether platelet activation in carotid sinuses causes reflex-mediated changes in renal sympathetic nerve activity and arterial pressure. Rabbit platelets were isolated, resuspended in Krebs' buffer, and activated by thrombin. Injection of activated platelets (3x108 platelets/mL) into the vascularly isolated carotid sinuses of anesthetized rabbits essentially eliminated sympathetic nerve activity and acutely decreased mean arterial pressure from 126±5 to 53±4 mm Hg (n=16; P<.05). Sympathetic activity and arterial pressure returned to control levels over a period of minutes despite sustained exposure to activated platelets. Injection of U-46619, a thromboxane analogue and vasoconstrictor, into carotid sinuses did not alter sympathetic activity or arterial pressure. However, serotonin (5-hydroxytryptamine [5-HT]), which is known to be released from activated platelets, and the 5-HT3 receptor agonist phenylbiguanide mimicked the effect of platelets. Furthermore, the platelet-induced reflex inhibition of sympathetic activity and hypotension were not altered by the cyclooxygenase inhibitor indomethacin but were attenuated significantly by 5-HT receptor antagonists. Platelet activation inhibited sympathetic activity to 5±2% of control in the absence of antagonists but to only 35±11 and 76±4% of control after selective blockade of 5-HT2 and 5-HT3 receptors with ketanserin and MDL-72222, respectively. The results indicate that (1) platelet activation in carotid sinuses triggers reflex inhibition of sympathetic nerve activity and hypotension; (2) the reflex is not caused by carotid vasoconstriction and is not mediated by prostanoids; and (3) the reflex is mediated by 5-HT acting primarily on 5-HT3 and to a lesser extent on 5-HT2 receptors. We speculate that this reflex may contribute to arterial pressure lability and susceptibility to stroke in patients with carotid atherosclerotic disease.
Key Words: blood pressure • sympathetic nervous system • atherosclerosis • pressoreceptors • stroke • serotonin • carotid sinus
|
Introduction |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
The carotid sinuses are densely innervated by sensory nerves, including baroreceptor fibers, which have a major role in the regulation of arterial pressure and cardiovascular function.1 The carotid sinuses are also a common site of development of atherosclerotic lesions and platelet adhesion.2 3 4 5 Activation of platelets in carotid sinuses is a major cause of transient ischemic attacks and strokes.6 7 8 9 Fluctuation in arterial pressure would further increase the risk of cerebral ischemia and stroke.
We hypothesized that factors released from activated platelets alter the activity of carotid sinus sensory nerves and trigger significant reflex changes in SNA and arterial pressure. We previously demonstrated that factors released from aggregating platelets in the carotid sinus decrease the activity of type I baroreceptors with myelinated afferent fibers and increase the activity of type II baroreceptors.10 11 12 The major goal of the present study was to determine the effects of activation of rabbit platelets in isolated carotid sinuses on renal SNA, heart rate, and arterial pressure. In addition, we explored the possible roles of carotid vasoconstriction and of prostanoids and 5-HT released during platelet activation in triggering the reflex response.
|
Methods |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
Thirty New Zealand White rabbits of either sex were anesthetized with sodium pentobarbital (30 to 35 mg/kg) administered slowly through an ear vein. The rabbits were ventilated with room air supplemented with oxygen through a tracheotomy. The arterial blood gases and pH were kept within normal ranges by adjustment of the ventilation and administration of NaHCO3 when needed. Catheters were placed in the femoral artery and vein to enable measurement of arterial pressure and administration of anesthetic, respectively. Body temperature was maintained between 36°C and 38°C by external warming. Supplemental doses of anesthetic were administered as needed. All procedures were in accordance with institutional guidelines.
Bilateral Isolation of Carotid Sinuses
Both carotid sinuses
were vascularly isolated, as described
previously.10 11 12 13 14
Catheters were placed in the common
carotid and lingual arteries. The carotid sinuses were filled with
Krebs-Henseleit buffer of the following composition (in mmol/L): NaCl
118.0, KCl 4.7, NaHCO3 24.0, MgSO4 1.2,
CaCl2 1.1, KH2PO4 1.1, and glucose
10.0. Krebs' solution was bubbled before the experiment with a 95%
O2/5% CO2 gas mixture, resulting in pH
7.3 to 7.4, PO2 of >200 mm Hg, and
PCO2 of 30 to 40 mm Hg, and was kept in a
sealed glass container placed in a temperature-controlled water
bath (37°C). The carotid sinuses were periodically refilled with
fresh Krebs' buffer.
The common carotid catheters were connected to a pressure bottle partially filled with Krebs' buffer. Pressure in the carotid sinuses was controlled by altering the inflow of air into the bottle from a pressurized air source and was measured with a transducer (model P23ID; Statham) connected to one of the lingual artery catheters. Vascular isolation of the carotid sinuses was confirmed by the ability to maintain a constant carotid sinus pressure over time after pressurization via the pressure bottle and occlusion of the common carotid catheter and by the absence of blood leaking into the carotid sinuses at a pressure of 0 mm Hg. The cervical sympathetic and aortic depressor nerves were sectioned to eliminate sympathetic influences on baroreceptor sensitivity and buffering of reflex responses by the aortic baroreceptors. In the majority of experiments, the vagus nerves were also sectioned. Decamethonium bromide (0.3 mg/kg IV) was administered before beginning the protocol to eliminate skeletal muscle contraction and associated interference with nerve recordings.
Measurement of Renal SNA
The left kidney was exposed through
a flank incision, and a
renal nerve was isolated and carefully placed across a platinum
electrode. The nerve and electrode were encased in silicone gel. Nerve
activity was recorded with a high-impedance probe (30- to
100-Hz to 3-kHz band width, model HIP511J; Grass Instrument Co). The
filtered neurogram was displayed on a Tektronix dual-beam storage
oscilloscope (model 5113), and we listened to the neural activity
through a loudspeaker. Renal SNA was quantified by counting the
frequency of action potentials that exceeded a selected voltage level
set just above the electrical noise with a nerve traffic
analyzer (model 706C; Department of Bioengineering, University
of Iowa [Iowa City]). In a few experiments, SNA was measured by both
spike counting and integrating the voltage of the neurogram with an
integrator amplifier (model 13-4615-70; Gould Inc). There were no
differences in the sympathetic nerve responses as measured by the two
methods. Measurements of carotid sinus, pulsatile, and mean systemic
arterial pressures and mean and integrated renal SNAs were
recorded continuously with a pen recorder (model R611, Beckman
Co, or model 11-1202-25, Gould Inc) and were analyzed manually
from the chart recordings. In some experiments, heart rate was
measured with a cardiotachometer triggered by the arterial
pressure pulse.
Isolation of Platelets
Blood was withdrawn from anesthetized
rabbits and
platelets were isolated as described
previously.10 11 15 16 In
brief, the blood was
anticoagulated with acid citrate dextrose, and platelets were
isolated by multiple centrifugation and washing. The
platelets were initially prepared in modified Tyrode's solution
and then either resuspended or diluted into oxygenated
Krebs' buffer, creating a final concentration of 3x108
platelets/mL. Platelets were maintained at 37°C in a water
bath before use and often were stored at 4°C for use in experiments
on subsequent days. The reactivity of platelets was confirmed by
measurement of thrombin (0.1 U/mL)-induced aggregation in a
dual-chamber aggregometer on the day of each experiment.
Experimental Protocols
Reflex responses to activated
platelets and various
pharmacological agents were measured while maintaining carotid sinus
pressure constant at 
80 mm Hg. Bovine thrombin (0.4 U/mL) was added
to the platelet suspension (3x108 platelets/mL) in
a syringe to activate platelets. The suspension was then
rapidly injected into the isolated carotid sinuses, with care taken to
restore carotid sinus pressure to the same level as before the
injection. The injection of platelets was completed within 20
seconds. The response to activated platelets was measured
at least until the maximum response began to wane (n=16), and in some
experiments the platelet suspension was left in the carotid sinuses
for 15 to 20 minutes (n=8). At this time, the suspension was washed out
of the sinuses, and the carotid sinuses were refilled with fresh
Krebs' buffer in an attempt to reverse the effects of platelets.
In some experiments, arterial pressure and renal SNA were
measured before and after the injection of thrombin alone (0.4 U/mL)
into the carotid sinuses (n=7).
We previously demonstrated that injection of rabbit platelets into the isolated carotid sinus causes significant vasoconstriction.11 To determine whether vasoconstriction of carotid sinuses triggers reflex changes in arterial pressure and SNA, we measured responses to injection of the thromboxane analogue U-46619 (100 nmol/L) into the isolated carotid sinuses (n=6). We previously showed that this concentration of U-46619 causes a magnitude of carotid vasoconstriction similar to rabbit platelets.11
To further explore the mechanism of the platelet-induced reflex, several additional protocols were performed. In one group, the carotid sinuses and platelets were pretreated with indomethacin (40 µmol/L) for 5 to 15 minutes before injection of platelets into carotid sinuses to test for a possible role of prostanoids in mediating the reflex response (n=5). The reflex response to platelets was also measured before and after exposure of carotid sinuses and platelets to 5-HT receptor antagonists. Ketanserin (n=5) and MDL-72222 (n=8) (100 nmol/L) were used to selectively block 5-HT2 and 5-HT3 receptors, respectively.17 18 Neither indomethacin, ketanserin, nor MDL-72222 altered the magnitude of thrombin-induced platelet aggregation measured in the aggregometer. The reflex responses to injections of 5-HT (10 µmol/L) and the selective 5-HT3 receptor agonist phenylbiguanide (10 µmol/L) into carotid sinuses were also determined before and after ketanserin or MDL-72222. All of these pharmacological agents were confined to the isolated carotid sinuses and therefore could influence SNA and arterial pressure only by altering the activity of carotid sinus sensory nerves.
Intact carotid sinus innervation and central neurotransmission were confirmed at the beginning of and periodically throughout each experiment through measurement of the reflex response to a ramp increase in carotid sinus pressure from 0 to 175 mm Hg in the absence of platelets or drugs in the isolated sinuses. Failure of baroreceptor stimulation to effectively inhibit SNA was cause to exclude an experiment from data analysis.
Drugs
Bovine thrombin, indomethacin, and 5-HT were
obtained from Sigma Chemical Co. Ketanserin tartrate, MDL-72222, and
1-phenylbiguanide were purchased from Research Biochemicals
International. U-46619 was obtained from Cayman Chemical Co.
Data Analysis
The absolute amount of nerve activity recorded
from whole
nerve preparations depends on the recording conditions and
varies among preparations. Therefore, platelet- and
drug-induced changes in renal SNA are expressed as a percentage of
the baseline level measured just before the injection of
activated platelets or a particular pharmacological agent
into the isolated carotid sinuses.
Values of SNA and arterial pressure measured 0.5, 1, 1.5, 2, 4, 6, 8, 10, and 15 to 20 minutes after the injection of activated platelets into the isolated carotid sinuses were compared with preinjection control values using one-factor ANOVA.19 When the ANOVA value was significant, differences in response at specific time points were determined with Fisher's protected least-significant differences test.19 The values of SNA, heart rate, and arterial pressure measured at the time of the maximum reflex response were compared with the corresponding baseline values by means of a paired t test.19 The magnitudes of the platelet- and drug-induced changes in SNA and arterial pressure before versus after blockade of 5-HT receptors were compared with the use of a paired t test. An unpaired t test was used to compare the level of SNA during platelet activation in the presence of the 5-HT2 receptor antagonist ketanserin with that of SNA during platelet activation in the presence of the 5-HT3 receptor antagonist MDL-72222. Data are presented as mean±SEM. Differences were considered significant at P<.05.
|
Results |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
Reflex Response to Platelet Activation in Carotid Sinuses
Injection of thrombin-activated platelets into the carotid sinuses essentially eliminated SNA (5±2% of control) and decreased arterial pressure significantly from 126±5 to 53±4 mm Hg (n=16, Fig 1
). Heart rate was also
decreased
significantly from 238±11 to 205±9 bpm (-33±4 bpm,
n=10). The
maximum inhibition of SNA usually occurred within 30 seconds and the
maximum fall in pressure usually occurred within the first minute after
injection of platelets (Figs 1 and 2). SNA and
arterial pressure returned toward control levels over a
period of minutes despite sustained exposure of carotid sinuses to
platelets, with recovery of SNA occurring more rapidly than that of
arterial pressure (n=8, Figs 1 and 2).
Reflex responses to
intracarotid injection of activated platelets were
abolished after the sectioning of carotid sinus nerves (data not
shown).
|
|
Injection of thrombin alone into the isolated carotid sinuses, in the absence of platelets, failed to trigger reflex responses. Arterial pressure averaged 113±10 and 117±10 mm Hg before and after 1 minute of exposure to thrombin, respectively (n=7). SNA averaged 102±3% of control during exposure to thrombin (n=7).
Effect of Carotid Vasoconstriction on SNA and
Arterial Pressure
The purpose of this experimental group was to
determine whether
carotid vasoconstriction, which we have shown to occur after injection
of rabbit platelets,11 mechanically triggers
baroreceptor reflex–mediated inhibition of SNA. Injection of the
thromboxane analogue U-46619, a powerful vasoconstrictor in
this preparation,11 into the isolated carotid sinuses
failed to influence SNA or arterial pressure.
Arterial pressure averaged 113±6 and 116±7 mm Hg before
and 1 minute after injection of U-46619 (n=6), respectively. SNA
averaged 101±4% of control during exposure to U-46619 (n=6).
Role of Prostanoids in Platelet-Induced Reflex
A possible
role of prostanoids in mediating the reflex response to
platelet activation in carotid sinuses was examined by testing
responses to platelets after inhibition of prostanoid formation
with indomethacin. The maximum reflex changes in SNA
and arterial pressure in response to platelets are
shown in Fig 3 for control experiments (n=16) and for
experiments in which carotid sinuses and platelets were treated
with indomethacin (n=5). Platelet activation in
carotid sinuses triggered equivalent and reversible inhibition of SNA
and decreases in arterial pressure in both groups (Fig 3).
Baseline levels of SNA and arterial pressure were not
significantly different in the control and
indomethacin-treated groups (Fig 3).
|
Role of 5-HT in Platelet-Induced Reflex
Selective blockade of
either 5-HT2 or
5-HT3 receptors with ketanserin (n=5) or MDL-72222
(n=8),
respectively, significantly attenuated the reflex response to
platelet activation in carotid sinuses (Figs 4 and
5). MDL-72222 was considerably more
effective than ketanserin in inhibiting the
platelet-induced reflex (Fig 5). SNA was inhibited by
platelet activation to a significantly greater extent in the
presence of ketanserin than in the presence of MDL-72222 (SNA, 35±11%
and 76±4% of baseline, respectively). Although the reflex was
markedly attenuated by MDL-72222, there was still significant reflex
inhibition of SNA and a decrease in arterial pressure in
response to platelets (Figs 4 and 5). Neither
ketanserin nor
MDL-72222 significantly influenced baseline SNA or arterial
pressure (Fig 5). The attenuation of the platelet-induced
reflex by 5-HT receptor antagonists was not caused by
desensitization of the response because the reflex was reproducible
with repeated injections of activated platelets in the
absence of antagonists (data not shown).
|
|
Magnitude of
response to platelets was significantly less after compared with
before 5-HT receptor antagonist (paired t
test).
Injection of exogenous 5-HT
into the isolated carotid sinus mimicked
the effects of activated platelets on SNA and
arterial pressure (Figs 4
and 6).
5-HT–induced inhibition of SNA was significantly attenuated by
ketanserin (n=5; Fig 6, left). Similar to that observed
with
platelets, MDL-72222 was more effective than ketanserin in
inhibiting both inhibition of SNA and the hypotension caused by 5-HT
(n=5; Fig 6, right). Injection of the 5-HT3
receptor
agonist phenylbiguanide into the sinuses significantly inhibited SNA to
57±12% of control and decreased arterial pressure from
119±7 to 79±11 mm Hg (n=7).
|
|
Discussion |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
The major new findings of the present study are that (1) injection of thrombin-activated rabbit platelets, but not thrombin alone, into carotid sinuses triggers reflex-mediated inhibition of SNA and hypotension; (2) the thromboxane analogue U-46619 injected into the isolated carotid sinus does not influence SNA or arterial pressure, whereas 5-HT and the 5-HT3 receptor agonist phenylbiguanide mimic the effect of platelets; and (3) the platelet-induced reflex is not altered after inhibition of prostanoid formation with indomethacin but rather is attenuated significantly by either the 5-HT3 receptor antagonist MDL-72222 or the 5-HT2 receptor antagonist ketanserin. MDL-72222 is more effective than ketanserin in attenuating the reflex.
These results suggest that 5-HT released from activated platelets acts directly on carotid sinus sensory nerve endings, most likely baroreceptors, via both 5-HT3 and 5-HT2 receptors to trigger reflex inhibition of SNA and hypotension. We discuss the results of the present study in relation to previous studies that have investigated the effects of aggregating platelets and 5-HT on sensory nerves and the possible pathophysiological implications of our findings.
Effects of Activated Platelets on Sensory
Nerves
We previously demonstrated that sustained exposure of the
isolated
carotid sinus of rabbits to thrombin-activated
platelets for 10 to 20 minutes suppresses baroreceptor activity
recorded from either the whole carotid sinus nerve or from single
baroreceptor fibers classified as type I
baroreceptors10 11 14 based on criteria
described by
Seagard et al.20 The suppression of baroreceptor activity
was mediated by a stable, diffusible factor14 that was not
thromboxane, 5-HT, or adenosine
diphosphate.10 11 Suppression of baroreceptor
activity
would predict that platelet activation in carotid sinuses should
increase instead of decrease SNA and arterial pressure.
Therefore, we were surprised at first by the pronounced inhibition of
SNA that occurred when activated platelets were injected
into the carotid sinuses.
Preliminary results from recent experiments provide a possible explanation for these findings.12 In contrast to that observed with type I baroreceptors, platelet activation in the carotid sinus increased the activity of type II baroreceptors.12 Furthermore, the increase in activity was evident soon after injection of platelets before the delayed inhibition of activity of type I baroreceptors. The results suggest that type II baroreceptors may have a predominant role in mediating the rapid inhibition of SNA during platelet activation.
We noticed that the
increased activity of type II baroreceptors
persisted throughout the period of exposure to activated
platelets.12 Therefore, the question remains why SNA
and arterial pressure returned to control levels despite
the sustained exposure of carotid sinuses to the platelets (see
Figs 1 and 2). We speculate that the recovery of
SNA and
arterial pressure may reflect the opposing influence of the
delayed decrease in activity of type I baroreceptors or adaptation
within the central nervous system to the increased type II baroreceptor
activity.
It is also possible that activation of unmyelinated afferent C-fibers contributes to the platelet-induced reflex inhibition of SNA and hypotension. Activated human platelets have been shown to excite nociceptor C-fiber afferents innervating skin.21
The mechanisms responsible for causing reflex inhibition of SNA during localized activation of platelets in carotid sinuses may also be applicable during systemic thrombotic disorders associated with hypotension. Wiggins et al22 found that intravenous administration of dextran sulfate to rabbits triggered hypotension and bradycardia associated with release of 5-HT from degranulating platelets before but not after depletion of circulating platelets. The hypotension in response to either dextran or 5-HT was markedly attenuated by sectioning the vagus and aortic depressor nerves and was nearly abolished after additional carotid artery ligation, indicating the reflex nature of the response.22
Role of Serotonin in Mediating Reflex
Sympathoinhibition
The conclusion that 5-HT is responsible for
triggering the
platelet-induced inhibition of SNA is based on the findings
that the reflex was attenuated by 5-HT receptor antagonists
and mimicked by 5-HT and phenylbiguanide. Furthermore, the facts that
MDL-72222 did not totally block the reflex and that ketanserin
attenuated the reflex suggest that both 5-HT2 and
5-HT3 receptors contribute to the response to
platelets. These conclusions rely heavily on the effectiveness and
selectivity of the antagonists.
Selectivity of Antagonists
Based on the literature,
ketanserin and MDL-72222 at the
concentration that we used (100 nmol/L) should effectively block
5-HT2 and 5-HT3 receptors, respectively, in a
selective manner.17 18 The reflex inhibition of SNA
and
decrease in arterial pressure caused by the specific
5-HT3 receptor agonist phenylbiguanide were inhibited by
>85% by MDL-72222 (n=2) but were not influenced by ketanserin
(n=4).
Phenylbiguanide decreased SNA to 59±21% and 59±19% of control
before and after ketanserin, respectively. Phenylbiguanide decreased
arterial pressure from 120±12 to 70±16 mm Hg
(-50±24 mm Hg) before ketanserin and from 129±11 to
76±16 mm
Hg (-53±21 mm Hg) after ketanserin (n=4). These data
demonstrate
the effectiveness of MDL-72222 in blocking 5-HT3 receptors
and the selective blockade of 5-HT2 receptors by ketanserin
in our preparation.
We also considered the possibility that the marked attenuation of the platelet-induced reflex by MDL-72222 may have resulted from nonspecific interference with impulse conduction in carotid sinus afferent nerves. To address this possibility, we examined whether MDL-72222 would also inhibit baroreflex-mediated inhibition of SNA. We found that a ramp increase in nonpulsatile carotid sinus pressure to 175 mm Hg inhibited SNA to a similar extent before and after MDL-72222; at 175 mm Hg of pressure, SNA averaged 10±6% and 5±4% of control before and after MDL-72222, respectively (n=6). Therefore, the attenuation of platelet-induced inhibition of SNA by MDL-72222 was not the result of nonspecific inhibition of afferent nerve activity.
Vascular Versus Neural Mechanism of Action of 5-HT
An
important question is whether the reflex inhibition of SNA
caused by 5-HT (or by activated platelets) is the result of
a direct action of 5-HT on sensory nerve endings or the result of
mechanical activation of baroreceptors secondary to vascular
contraction. Local exposure of carotid sinuses to
norepinephrine, epinephrine, and vasopressin may
trigger significant reflex bradycardia and
hypotension.23 24 This reflex has been attributed to
mechanical deformation of baroreceptor nerve endings, particularly
those with unmyelinated C-fiber
afferents.23 24 25 It is important to
note, however, that
direct excitation of sensory nerves by these factors was not ruled out
in these studies. We recently demonstrated that rabbit platelets
and U-46619 (100 nmol/L) injected into the isolated carotid sinus of
rabbits cause significant and equivalent
vasoconstriction.11 The striking difference in the reflex
responses to platelets and U-46619 despite similar vascular
responses strongly suggests that the platelet-induced reflex
inhibition of SNA was not the result of carotid vasoconstriction per
se. Furthermore, if the platelet- or 5-HT–induced reflex were the
result of mechanical activation of baroreceptors, one would not expect
it to be blocked by the 5-HT3 receptor
antagonist MDL-72222. 5-HT3 receptors are not
present on vascular muscle; vascular contraction caused by 5-HT is
mediated through 5-HT2 and 5-HT1
receptors26 and therefore should not be blocked by
MDL-72222. In addition, MDL-72222 did not attenuate
baroreflex-mediated inhibition of SNA in response to increased
carotid sinus pressure in our experiments, indicating that it does not
interfere with mechanoelectrical transduction and therefore should also
not interfere with vascular contraction–induced mechanical
activation of baroreceptors. Thus, several lines of evidence suggest
that the reflex-mediated inhibition of SNA triggered by
platelet activation and 5-HT involves a direct action of 5-HT on
the sensory nerve endings.
5-HT has been shown to enhance membrane excitability and increase spike discharge frequency of many types of sensory nerves, including vagal afferents innervating the heart and lungs27 28 and type II baroreceptors29 and chemoreceptors30 31 32 located in the carotid sinus region. Activation of chemoreceptors reflexly increases SNA and therefore is not responsible for our finding of reflex inhibition of SNA during exposure of carotid sinuses to platelets or 5-HT. Injection of 5-HT or phenylbiguanide into blood perfusing the nodose ganglion in cats causes reflex apnea, bradycardia, and hypotension that are thought to result from a direct action of these substances on the cell soma of nodose neurons.33 34 In our experiments, activation of carotid sinus sensory nerves by 5-HT was responsible for triggering the reflex response to activated platelets because the sectioning of carotid sinus nerves abolished the reflex.
Intravenous administration of 5-HT or phenylbiguanide evokes reflex inhibition of SNA, bradycardia, and hypotension.27 35 This reflex is essentially eliminated by vagotomy or intrapericardial procaine (for an exception see Reference 22), indicating the absence of a contribution of carotid sinus afferents in mediating the reflex.27 35 This finding is not surprising considering that the concentration of the intravenously administered agonists likely falls considerably before reaching the carotid sinuses and that these compounds activate numerous types of afferents with potentially opposing influences on the circulation. Our results indicate that 5-HT produced locally from activated platelets as well as phenylbiguanide in carotid sinuses triggers reflex sympathoinhibition and hypotension.
Two different ionic mechanisms most likely mediate the increase in sensory nerve activity caused by activation of 5-HT3 and 5-HT2 receptors. The 5-HT3 receptor is a receptor/ion channel complex.36 37 Opening this nonselective cation channel leads to depolarization of sensory nerves.28 36 Activation of 5-HT2 receptors inhibits K+ channel activity,38 39 which may depolarize and/or block spike after–hyperpolarization in visceral sensory neurons.40 41 These results along with those of the present study suggest that the ionic mechanism of platelet-induced activation of carotid sinus sensory nerves may involve both opening of the 5-HT3 cation channel and inhibition of K+ channels.
Pathophysiological
Implications
The carotid sinuses are particularly prone to development
of
atherosclerotic lesions and platelet
adhesion.2 3 4 5 The
proximity of platelets aggregating in carotid sinuses to sensory
nerve endings increases the likelihood that factors released from
platelets in vivo may produce significant changes in sensory nerve
activity.
It is established that platelets aggregating in carotid sinuses are an important cause of transient cerebral ischemia and strokes.6 7 8 9 The pronounced reflex decrease in arterial pressure resulting from inappropriate activation of carotid sinus sensory nerves would further compromise cerebral blood flow by reducing cerebral perfusion pressure. We speculate that fluctuations in SNA in response to episodic activation of platelets may also provide a neural mechanism of increased arterial pressure lability in atherosclerotic and thrombotic states.
|
Selected Abbreviations and Acronyms |
|---|
|
|
Acknowledgments |
|---|
This work was supported by research funds from the Department of Veterans Affairs and grant PO1-HL-14388 from the National Institutes of Health. Dr Mao was the recipient of an Institutional National Research Service Award from the National Institutes of Health when this study was performed. The authors thank Shawn M. Roach and Debra J. Schiek for preparation of figures and Colleen Chapleau and Jackie Schneider for typing the manuscript.
|
References |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
1. Kirchheim HR. Systemic arterial baroreceptor reflexes. Physiol Rev. 1976;56:100-176.
2.
Solberg LA, Eggen DA. Localization and sequence
of development of atherosclerotic lesions in the carotid and vertebral
arteries. Circulation. 1971;43:711-724.
3. Heath D, Smith P, Harris P, Winson M. The atherosclerotic human carotid sinus. J Pathol. 1973;110:49-58. [Medline] [Order article via Infotrieve]
4. Meyer WW, Noll M. Gross patterns of early lipid deposits in the carotid artery and their relation to the preformed arterial structures. Artery. 1974;1:31-45.
5. Isaka Y, Kimura K, Uehara A, Hashikawa K, Mieno M, Matsumoto M, Handa N, Nakabayashi S, Imaizumi M, Kamada T. Platelet aggregability and in vivo platelet deposition in patients with ischemic cerebrovascular disease: evaluation by indium-111-platelet scintigraphy. Thromb Res. 1989;56:739-749. [Medline] [Order article via Infotrieve]
6.
Imparato AM, Riles TS, Gorstein F. The carotid
bifurcation plaque: pathologic findings associated with cerebral
ischemia. Stroke. 1979;10:238-245.
7.
Lusby RJ, Ferrell LD, Ehrenfeld WK, Stoney RJ, Wylie
EJ. Carotid plaque hemorrhage: its role in
production of cerebral ischemia. Arch
Surg. 1982;117:1479-1488.
8. Born GVR. Aspirin and platelets in relation to cerebrovascular disease. Stroke. 1990;21(suppl IV):IV-29-IV-31.
9.
Norris JW, Zhu CZ, Bornstein NM, Chambers BR.
Vascular risks of asymptomatic carotid
stenosis. Stroke. 1991;22:1485-1490.
10.
Li Z, Abboud FM, Chapleau MW. Aggregating human
platelets in carotid sinus of rabbits decrease sensitivity of
baroreceptors. Circ Res. 1992;70:644-650.
11. Li Z, Su X, Chapleau MW. Role of cyclooxygenase metabolites in mediating platelet-induced baroreceptor dysfunction. Am J Physiol. 1995;269(Heart Circ Physiol 38):H599-H608.
12. Chapleau MW, Su X, Li Z. Platelets aggregating in carotid sinus selectively modulate activity of baroreceptor fiber types. FASEB J. 1995;9:A7. Abstract.
13.
Chen HI, Chapleau MW, McDowell TS, Abboud FM.
Prostaglandins contribute to activation of baroreceptors in
rabbits: possible paracrine influence of
endothelium. Circ Res. 1990;67:1394-1404.
14. Li Z, Chapleau MW. Platelet-induced suppression of baroreceptor activity is mediated by a stable diffusible factor. J Auton Nerv Syst. 1995;51:59-65. [Medline] [Order article via Infotrieve]
15. Mustard JF, Perry DW, Ardlie NG, Packham MA. Preparation of suspensions of washed platelets from humans. Br J Haematol. 1972;22:193-204. [Medline] [Order article via Infotrieve]
16. Czervionke RL, Hoak JC, Fry GL. Effect of aspirin on thrombin-induced adherence of platelets to cultured cells from the blood vessel wall. J Clin Invest. 1978;62:847-856.
17. Fozard JR. MDL 72222: a potent and highly selective antagonist at neuronal 5-hydroxytryptamine receptors. Naunyn Schmiedebergs Arch Pharmacol. 1984;326:36-44. [Medline] [Order article via Infotrieve]
18. Zifa E, Fillion G. 5-Hydroxytryptamine receptors. Pharmacol Rev. 1992;44:401-458. [Medline] [Order article via Infotrieve]
19. Winer BJ. Statistical Principles in Experimental Design. New York, NY: McGraw-Hill Book Co; 1971:419-582, chaps 6 and 7.
20.
Seagard JL, van Brederode JFM, Dean C, Hopp FA,
Gallenberg LA, Kampine JP. Firing characteristics of
single-fiber carotid sinus baroreceptors.
Circ Res. 1990;66:1499-1509.
21. Ringkamp M, Schmelz M, Kress M, Allwang M, Ogilvie A, Reeh PW. Activated human platelets in plasma excite nociceptors in rat skin in vitro. Neurosci Lett. 1994;170:103-106. [Medline] [Order article via Infotrieve]
22.
Wiggins RC, Glatfelter A, Campbell AM, Kunkel RG,
Ulevitch RJ. Acute hypotension due to platelet
serotonin-induced chemoreflexes after
intravenous injection of dextran sulfate in the
rabbit. Circ Res. 1985;57:262-277.
23.
Heymans C, Delaunois AL. Fundamental role of the
tone and resistance to stretch of the carotid sinus arteries in the
reflex regulation of blood pressure. Science. 1951;114:546-547.
24.
Heymans C, Delaunois AL. Action of drugs on
carotid body and sinus. Pharmacol Rev. 1955;7:119-142.
25. Landgren S, Neil E, Zotterman Y. The response of the carotid baroceptors to the local administration of drugs. Acta Physiol Scand. 1951;25:24-37.
26. Martin GR. Vascular receptors for 5-hydroxytryptamine: distribution, function and classification. Pharmacol Ther. 1994;62:283-324. [Medline] [Order article via Infotrieve]
27. Douglas WW, Ritchie JM. On excitation of nonmedullated afferent fibres in the vagus and aortic nerves by pharmacological agents. J Physiol (Lond). 1957;138:31-43.
28. Ireland SJ, Tyers MB. Pharmacological characterization of 5-hydroxytryptamine-induced depolarization of the rat isolated vagus nerve. Br J Pharmacol. 1987;90:229-238. [Medline] [Order article via Infotrieve]
29.
Qu L, Stuesse SL.
Low-pressure–sensitive baroreceptor fibers recorded from
rabbit carotid sinus nerves. Circ Res. 1991;69:1608-1615.
30.
Comroe JH Jr, Mortimer L. Respiratory and
cardiovascular responses of temporally separated aortic
and carotid bodies to cyanide, nicotine, phenyldiguanide and
serotonin. J Pharmacol Exp
Ther. 1964;146:33-41.
31. Nishi K. The action of 5-hydroxytryptamine on chemoreceptor discharges of the cat's carotid body. Br J Pharmacol. 1975;55:27-40. [Medline] [Order article via Infotrieve]
32. Kirby GC, McQueen DS. Effects of the antagonists MDL 72222 and ketanserin on responses of cat carotid body chemoreceptors to 5-hydroxytryptamine. Br J Pharmacol. 1984;83:259-269. [Medline] [Order article via Infotrieve]
33.
Jacobs L, Comroe JH Jr. Reflex apnea, bradycardia, and
hypotension produced by serotonin and phenyldiguanide
acting on the nodose ganglia of the cat. Circ Res. 1971;29:145-155.
34. Sampson SR, Jaffe RA. Excitatory effects of 5-hydroxytryptamine, veratridine and phenyldiguanide on sensory ganglion cells of the nodose ganglion of the cat. Life Sci. 1975;15:2157-2165.
35. Evans RG, Ludbrook J, Michalicek J. Characteristics of cardiovascular reflexes originating from 5-HT3 receptors in the heart and lungs of unanaesthetized rabbits. Clin Exp Pharmacol Physiol. 1990;17:665-679. [Medline] [Order article via Infotrieve]
36. Peters JA, Malone HM, Lambert JJ. An electrophysiological investigation of the properties of 5-HT3 receptors of rabbit nodose ganglion neurones in culture. Br J Pharmacol. 1993;110:665-676. [Medline] [Order article via Infotrieve]
37.
Maricq AV, Peterson AS, Brake AJ, Myers RM, Julius
D. Primary structure and functional expression of the
5HT3 receptor, a serotonin-gated ion
channel. Science. 1991;254:432-437.
38. Kavanaugh MP, Christie MJ, Osborne PB, Busch AE, Shen K-Z, Wu Y, Seeburg PH, Adelman JP, North RA. Transmitter regulation of voltage-dependent K+ channels expressed in Xenopus oocytes. Biochem J. 1991;277:899-902.
39. Attali B, Honore E, Lesage F, Lazdunski M, Barhanin J. Regulation of a major cloned K+ channel from human T lymphocytes. FEBS Lett. 1992;303:229-232. [Medline] [Order article via Infotrieve]
40. Yoshioka M, Ikeda T, Abe M, Togashi H, Minami M, Saito H. Pharmacological characterization of 5-hydroxytryptamine-induced excitation of afferent cervical vagus nerve in anaesthetized rats. Br J Pharmacol. 1992;106:544-549. [Medline] [Order article via Infotrieve]
41.
Christian EP, Taylor GE, Weinreich D.
Serotonin increases excitability of rabbit C-fiber neurons
by two distinct mechanisms. J Appl
Physiol. 1989;67:584-591.
This article has been cited by other articles:
 |
|
 |
|
  D. O. Arnar, D. Xing, H.-C. Lee, and J. B. Martins Prevention of ischemic ventricular tachycardia of Purkinje origin: role for {alpha}2-adrenoceptors in Purkinje? Am J Physiol Heart Circ Physiol, March 1, 2001; 280(3): H1182 - H1190. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||