Vol. 281, Issue 4, H1667-H1674, October 2001
Effects of pituitary adenylate cyclase-activating polypeptide
on canine atrial electrophysiology
Masamichi
Hirose1,
Zeng
Leatmanoratn2,
Kenneth
R.
Laurita3, and
Mark D.
Carlson4
1 Department of Pharmacology, Shinshu University School of
Medicine, Matsumoto 390-8621, Japan; 2 Department of Biomedical
Engineering; 3 School of Medicine and MetroHealth Medical
Center, Cleveland 44109-1998; and 4 Department of Medicine,
School of Medicine and University Hospitals of Cleveland, Case Western
Reserve University, Cleveland, Ohio 44106
 |
ABSTRACT |
We hypothesized that
pituitary adenylate cyclase-activating polypeptide (PACAP)
activates intracardiac postganglionic parasympathetic nerves and has a
different effect than cervical vagal stimulation. We measured effective
refractory period (ERP) and conduction velocity at four atrial
sites [high right atrium (HRA), low right atrium (LRA), high left
atrium (HLA), and low left atrium (LLA)] and minimum atrial
fibrillation (AF) cycle length at 12 atrial sites during cervical vagal
stimulation and after PACAP in 26 autonomically decentralized,
open-chest, anesthetized dogs. PACAP shortened ERP to a similar extent
at all four sites (HRA, 58 ± 2.0 ms; LRA, 60 ± 6.3 ms; HLA,
68 ± 11.5 ms; and LLA, 60 ± 8.3 ms). Low- and high-intensity vagal stimulation shortened ERP at the HRA, but not in
the other atrial sites (low-intensity stimulation: HRA, 64 ± 4.0 ms; LRA, 126 ± 5.1 ms; HLA, 110 ± 9.5 ms; and LLA, 102 ± 11.5 ms; high-intensity stimulation: HRA, 58 ± 4.2 ms; and
HLA, 101 ± 4.0 ms). Conduction velocity was not altered by any
intervention. Minimum AF cycle length after PACAP was similar in both
atria but was shorter in the right atrium than in the left atrium
during vagal stimulation. After atropine administration, no
interventions changed ERP. These results suggest that PACAP shortens
atrial refractoriness uniformly in both atria through activation of
intrinsic cardiac nerves, not all of which are activated by
cervical vagal stimulation.
intrinsic cardiac nervous system; vagal nerve stimulation
| |
INTRODUCTION |
RECENT ANATOMIC AND
FUNCTIONAL DATA indicate that intracardiac ganglia possess a
population of neurons that are heterogeneous with regard to morphology
as well as electrophysiological and pharmacological properties
including associated neurotransmitters and phenotypes (12, 13,
16, 17, 37).
Pituitary adenylate cyclase-activating polypeptide (PACAP) is a
recently discovered neuropeptide that was isolated originally from the ovine hypothalamus (23). Interestingly, PACAP and
vasoactive intestinal peptide are members of a family of structurally
related regulatory neuropeptides (24). PACAP exhibits
multiple actions in the central nervous system and in various
peripheral organs including the endocrine glands, the airways, and the
cardiovascular and immune systems (32). For example, PACAP
stimulates pancreatic enzyme secretion in sheep via activation of vagal
cholinergic neurons (26) and also stimulates catecholamine
release from the adrenal gland in anesthetized dogs (11).
Several studies (5, 31, 35) have demonstrated that PACAP
and its receptors exist in the heart and in intrinsic cardiac nerves
and that PACAP increases cardiac ganglion neuron membrane excitability.
We previously demonstrated (14, 15) that PACAP evoked
negative chronotropic and inotropic responses in the canine heart. The
PACAP-induced negative cardiac effects were prevented by atropine
and tetrodotoxin but not by hexamethonium. These findings indicate that
ACh release from intracardiac postganglionic parasympathetic
nerves participates in the cardiac response to PACAP (14,
15). In addition, Braas et al. (5) recently
demonstrated that almost all postganglionic parasympathetic neurons in
the guinea pig heart express membrane-associated PACAP receptor
proteins. Therefore, PACAP may participate in the intrinsic neural
regulation of heart function.
Numerous physiological studies have documented the nonuniform
effects of vagal stimulation on atrial tissue (1, 6, 25, 30,
38). However, whether the effects of PACAP on atrial tissue vary
in different regions of the atria is unknown. We hypothesized that
PACAP directly activates the intrinsic cardiac nerves and has a
different effect than cervical vagal stimulation. Therefore, the
present study was designed to assess the effects of PACAP on different
atrial sites.
| |
METHODS |
The experiments performed in this study were approved by the
Institutional Animal Care and Use Committee of Case Western Reserve University.
General methods.
Twenty-six mongrel dogs of either sex (weight 25-29 kg) were
anesthetized with pentobarbital sodium (30 mg/kg iv). Supplemental doses were given to maintain stable anesthesia. A tracheal cannula was
inserted and intermittent positive-pressure ventilation was administered by a respirator with room air (model 607; Harvard Apparatus; Millis, MA). Body temperature was maintained at 37.0°C. The chest was opened transversely at the fifth intercostal space and a
pericardial cradle was created. Cervical vagal nerves were isolated
bilaterally via a midline neck incision and crushed with tight
ligatures. Each stellate ganglion was isolated and crushed at its
junction with the ansa subclavia. These maneuvers remove almost all
tonic neural activity to the heart (21). Four bipolar electrodes were placed on four atrial epicardial sites. To stimulate the parasympathetic nerves to the heart, bipolar stainless steel wire
electrodes were inserted in the cardiac end of each cervical vagal
nerve, and the wires were connected to an electrical stimulator (model
S8800; Grass Instruments). Catheters were inserted into the right
femoral artery and vein to record the arterial blood pressure and to
inject drugs, respectively. A catheter was inserted into the left
atrium through the left atrial appendage to inject the PACAP. The heart
rate was derived from the standard electrocardiogram (ECG) lead II. The
ECG, heart rate, and femoral arterial blood pressure were monitored
continuously throughout the experiments (CardioLab; Prucka Engineering;
Houston, TX).
An electrode array containing 95 bipolar electrodes (48 pairs for the
right atrium, 36 pairs for the left atrium, and 11 pairs placed
separately on Bachmann's bundle) was used to record atrial electrical
activation (see Fig. 1). The
interelectrode distance of each bipolar electrode in the array was 1.2 mm, and the distance between the center of each bipolar electrode pair
and its neighbor was 7 mm longitudinally and 6 mm transversally. The
electrode array for Bachmann's bundle was placed under the aortic root
to cover the anterior aspect of the atrial appendages and Bachmann's bundle. The array containing 84 bipolar electrodes was placed around
the atria and secured with a hook-and-loop (Velcro) belt. Each
electrogram signal was filtered (0.3-500 Hz), digitized (12-bit resolution and 1-kHz sampling rate), transmitted into a microcomputer (Compaq), and saved to CD-ROM. Software developed in our laboratory was
used to analyze each electrogram signal and generate activation maps.
Each electrogram was analyzed with computer-determined peak-amplitude criteria and was verified manually.
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Fig. 1.
Two electrode templates. Bipolar electrode sites are
indicated by dots; a, b, c, and
d represent the high right atrium (HRA), low right atrium
(LRA), high left atrium (HLA), and low left atrium (LLA) pacing sites,
respectively. These pacing electrodes were used for effective
refractory period (ERP) and conduction velocity (CV) measurements.
Bipolar electrodes indicated by numbers were used for CV and minimum
atrial fibrillation cycle length (mAFCL) measurements. BB, Bachmann's
bundle; LAA, left atrial appendage; RAA, right atrial appendage; SVC,
superior vena cava; IVC, inferior vena cava; PV, pulmonary vein; AVR,
atrioventricular ring.
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Electrophysiological measurements.
Programmed atrial stimulation was performed from four atrial sites: the
high right atrium (HRA), the low right atrium (LRA), the high left
atrium (HLA), and the low left atrium (LLA) (see Fig. 1). Pacing was
performed using 2-ms square pulses at twice the diastolic threshold. An
eight-beat drive train (S1) at a basic cycle length (BCL)
of 300 ms was followed by a premature stimulus (S2). The
coupling interval was progressively increased by 10-ms increments until
atrial capture occurred. The effective refractory period (ERP) was
defined as the longest S1-S2 interval that
failed to produce a propagated response. The ERP measurement was
started 30 s after the onset of vagal stimulation or 60 s
after PACAP injection (see Fig. 2). The
ERP measurement was started at a coupling interval close to the ERP and
was performed in 15 s. If the ERP measurement was not determined
in 15 s, the measurement was repeated after adequate recovery
time. Heart rate was measured immediately before programmed stimulation
was started (see Fig. 2). In addition, we determined the minimum atrial
fibrillation (AF) cycle length at each of 12 electrode sites (6 right
atrial sites and 6 left atrial sites). This measurement can be used to
predict the distribution of atrial refractoriness (19) and
was calculated by averaging the two shortest electrogram intervals
recorded from an electrode during a 2-s interval of AF. Electrograms
were carefully analyzed to reject low-amplitude potentials and to
detect double potentials associated with block. Correlation with the
surface ECG was used to eliminate ventricular electrograms. AF was
induced by a single premature extra stimulus delivered at 10 ms longer
than the ERP at each site used for ERP determination. The initiation of
AF was defined as a rapid (>400 beats/min) irregular atrial rhythm persisting spontaneously for >1 min.
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Fig. 2.
Time course for ERP measurement during vagal stimulation
(VS) or after pituitary adenylate cyclase-activating polypeptide
(PACAP) injection. During VS, ERP measurement was performed 30-45
s after VS was started (top). ERP measurement was performed
60-75 s after PACAP was injected (bottom). Heart rate
(HR) was measured immediately before ERP measurement began.
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Conduction velocity was measured during constant pacing at 300 ms. The
regional conduction velocity was determined in the vicinity of the
stimulating electrode (beginning at least 14 mm from the pacing site to
avoid virtual cathode effects). We analyzed activation times at a
series of electrode sites in each atrium (see Fig. 1). The distance of
each site from the first of the series of electrodes was plotted
against activation time, and conduction velocity was determined using
linear regression. The activation sequence of the paced beat was
analyzed to ensure that impulse propagation was linear. A minimum of
three electrode sites was used for each analysis, and the correlation
coefficient between activation time and distance always exceeded 0.99. The average of two beats was used to calculate conduction velocity for
each determination. For each experiment, the same sites were used to measure conduction velocity.
Nerve stimulation and PACAP administration.
In the present experiments, the right and left cervical vagal nerves
were stimulated simultaneously to decrease the sinus rate by ~30 and
50%; such stimulations were defined as low- and high-intensity vagal
stimulations, respectively. Low-intensity cervical vagal nerve
stimulation was performed with 10-mA pulse amplitude at a frequency of
3 Hz. Pulse durations of 0.03 or 0.04 ms were used to achieve the
desired heart rate decrease (30%). High-intensity vagal stimulation
was performed with a 12-mA pulse amplitude at a frequency of 5 Hz.
Pulse durations of 0.04 or 0.05 ms were used to achieve the desired
heart rate decrease (50%). PACAP (5-10 nmol) was injected into
the left atrium to decrease the sinus rate by ~30%. The changes in
heart rate in response to vagal stimulation and PACAP administration
were stable during the time course of ERP measurement. The intensity of
vagal nerve stimulation and the dose of PACAP were adjusted before each
experiment to achieve consistent changes in heart rate in response to
those interventions through the experiments. The effects of vagal
stimulation or PACAP administration on heart rate were stable through
the experiment.
Experimental protocol.
Experiments were performed in 26 autonomically decentralized hearts of
open-chest anesthetized dogs. After 30 min of postsurgical stabilization, two series of experiments were performed. In one series
of experiments, we examined the effects of low-intensity cervical vagal
stimulation (n = 5) and PACAP administration
(n = 5) on the ERP and conduction velocity at 4 atrial
sites in 10 dogs. A 2- to 4-s interval of AF was recorded from the
atrial electrode array to measure the minimum AF cycle length (mAFCL) 2 min after vagal stimulation started or after PACAP administration (n = 8) at 12 atrial sites. In another six dogs, we
examined the effects of high-intensity cervical vagal stimulation on
the ERP at two atrial sites, namely, the HRA and the HLA.
Our previous experiments (14, 15) showed that in addition
to releasing ACh from parasympathetic nerves, PACAP directly activates
the sinus node cells and atrial muscle and causes positive chronotropic
and inotropic effects in dog heart. Because of this, the second series
of experiments was designed to determine whether direct cardiac effects
of PACAP participate in the changes in atrial ERP. In a separate group
of 10 dogs, at each of the four sites we tested the effects of each
intervention (n = 5) on ERP after atropine treatment
(0.3 mg · kg
1 · h1 iv). In
this group of experiments, to confirm the effects of vagal stimulation
and PACAP administration on the heart rate, cervical vagal nerves were
stimulated and PACAP was also administered before atropine treatment.
During ERP measurement, the P-R interval, QRS complex duration, and Q-T
interval were measured from ECG lead II during constant pacing at a BCL
of 300 ms in four dogs. In addition, the mean arterial blood pressure
(MABP) was measured immediately before the ERP measurement was started
in 20 experiments.
Because PACAP induces tachyphylaxis in neonatal pig hearts
(27), adequate recovery times were allowed between PACAP
administrations. To minimize time-dependent changes in atrial ERP, each
dog was randomly assigned to undergo programmed stimulation at only two of the four atrial sites. The basal ERP and arterial blood pressure of
the control conditions did not change during the protocol.
Data analysis.
All data are shown as means ± SE. An ANOVA with Bonferroni's
test was used for the statistical analysis of multiple comparisons of
data. Student's t-test for paired or unpaired data was used to compare the two groups. P values <0.05 were considered
statistically significant.
Drugs.
Drugs were mixed just before each experiment. PACAP (PACAP-27;
Peninsula Laboratories) was dissolved in distilled water. Atropine sulfate (Sigma; St. Louis, MO) was dissolved and diluted in 0.9% NaCl. Drugs were injected into the left atrium or the right femoral vein.
| |
RESULTS |
Effects of vagal stimulation and PACAP on heart rate.
Low-intensity vagal stimulation decreased the mean heart rate from
116 ± 2.5 to 77 ± 2.3 beats/min (P < 0.001). High-intensity vagal stimulation decreased the mean heart rate
from 114 ± 4.6 to 56 ± 2.4 beats/min (P < 0.001). PACAP decreased the mean heart rate from 116 ± 2.5 to
79 ± 3.3 beats/min (P < 0.001). The mean heart
rate measurements during low-intensity vagal stimulation and after
PACAP administration were not statistically different. Atropine
prevented the decreases in heart rate in response to vagal stimulation
and PACAP administration (P < 0.001). After atropine
treatment, PACAP increased the heart rate from 114 ± 6 to
181 ± 7 beats/min (P < 0.001).
Effects of PACAP on atrial ERP and conduction velocity.
In the control state, the ERP at the four sites tested were not
significantly different. During low-intensity vagal stimulation, the
ERP shortened in the HRA but not in the LRA, HLA, or LLA (see Fig.
3A). During high-intensity
vagal stimulation, the ERP shortened more in the HRA than in the HLA
(P < 0.001; see Fig. 3B). In contrast, after PACAP administration, the ERP shortened to a similar extent at
all atrial sites tested (see Fig. 3C). These results
indicate that cervical vagal stimulation had nonuniform effects on the atrial ERP, whereas the effect of PACAP on the ERP was uniform in both
atria. As an additional measure of ERP, mAFCL was calculated for 12 atrial sites. The mAFCL was shorter in the HRA and right atrial
appendage than in the left atrium (P < 0.05) 2 min
after the low-intensity vagal stimulation was started (see Fig.
4A). In contrast, the mAFCL in
the right atrium was similar to that in the left atrium 2 min after the
PACAP injection (see Fig. 4B). In addition, AF terminated
~4 min after PACAP was injected. These data are consistent with the
ERP measurements and suggest that the duration of the effect of PACAP
on atrial refractoriness is probably ~4 min.
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Fig. 3.
Effects of low-intensity bilateral cervical VS
(A; n = 5) at each of four atrial sites;
high-intensity bilateral cervical VS (B; n = 6) at each of two atrial sites; and PACAP (C;
n = 5) at each of four atrial sites on atrial ERP. Analysis
is based on a total of 16 autonomically decentralized hearts of
open-chest anesthetized dogs. Vertical bars, SE. **P < 0.01 and ***P < 0.001 vs. control.
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Fig. 4.
Effects of low VS (A; n = 8)
and PACAP (B; n = 8) on mAFCL at each of 12 electrode sites (6 right and 6 left atrial sites). Analysis is based on
a total of 10 autonomically decentralized hearts of open-chest
anesthetized dogs. Vertical bars, SE.
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In the control condition, conduction velocity did not differ between
pacing sites. PACAP tended to increase conduction velocity at the right
atrium and HLA, but these responses were not statistically significant
(see Fig. 5). Vagal stimulation did not
change conduction velocity regardless of the atrial site tested (see
Fig. 5).
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Fig. 5.
Effects of low VS (n = 5) and PACAP
(n = 5) on intra-atrial CV at each of four atrial
sites. Analysis is based on a total of 10 autonomically decentralized
hearts of open-chest anesthetized dogs. Vertical bars, SE.
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Low-intensity vagal stimulation did not shorten the ERP at any site
after atropine administration (see Fig.
6A). After atropine treatment,
PACAP tended to shorten the ERP in the HRA (P = 0.065) and the HLA (P = 0.199), but these responses were not
statistically significant (see Fig. 6B). PACAP did not
shorten the ERP in the other two sites.
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Fig. 6.
Effects of low VS (A; n = 5)
and PACAP (B; n = 5) on atrial ERP after
atropine treatment at each of four atrial sites. Analysis is based on a
total of 10 autonomically decentralized hearts of open-chest
anesthetized dogs. Vertical bars, SE.
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Effects of PACAP on ECG intervals and MABP.
P-R intervals during low-intensity vagal stimulation and after PACAP
administration were significantly longer than those measured in the
control condition in four non-atropine-treated dogs (see Table
1). In addition, the QRS complex duration
was shorter after PACAP administration than during low-intensity vagal
stimulation and during control condition in the same four dogs.
However, the Q-T interval was the same regardless of the intervention.
MABP did not differ regardless of the interventions in 10 non-atropine-treated dogs, whereas it increased significantly after
PACAP administration in 10 atropine-treated dogs (see Table 1).
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Table 1.
Electrocardiogram parameters and mean blood pressure during control
conditions, low-intensity VS, and after PACAP administration
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DISCUSSION |
In this study, we found that cervical vagal stimulation caused
nonuniform effects on atrial ERP, whereas PACAP caused uniform effects
on atrial ERP (see Fig. 3). The effects of vagal stimulation on ERP
were similar to those of PACAP in the HRA, but not in the other three
sites (see Fig. 3). The effects of PACAP on atrial refractoriness
during AF were uniform in both atria but those of vagal stimulation
were not (see Fig. 4). Conduction velocity did not change during any
intervention regardless of the atrial site tested (see Fig. 5). After
atropine treatment, none of these interventions shortened the atrial
ERP in any of the four sites tested (see Fig. 6). These results
demonstrate that PACAP shortens atrial refractoriness uniformly in
different regions of both atria.
Effect of PACAP on ERP.
Previous studies have shown that cervical vagal stimulation elicits
nonuniform effects on atrial ERP (1, 6, 38). Zipes et al.
(38) showed that when supramaximal cervical vagal
stimulation diminished the sinus rate to a value <30 per minute, it
shortened the atrial ERP mainly in the right atrium. However, in the
present study, administration of PACAP caused uniform shortening of the ERP in both atria (see Fig. 3). In addition, after PACAP
administration, mAFCL at 12 atrial sites were similar in both atria
(see Fig. 4). This finding provides additional evidence for spatial
uniformity in atrial refractory properties after PACAP administration.
Our previous results (14, 15) suggested that the
PACAP-induced negative chronotropic and inotropic responses in the
canine right atrium were caused by the release of ACh from intracardiac
postganglionic parasympathetic nerves. The results of the present study
suggest that PACAP causes uniform atrial ERP shortening by activating intracardiac postganglionic nerves that are not activated by cervical vagal stimulation.
Previous investigators (6, 38) showed that the nonuniform
atrial ERP shortening induced by vagal stimulation was caused by
regional differences in the pattern of vagal innervation. In addition,
Yuan et al. (37) demonstrated anatomically that
1) the majority of canine cardiac ganglionated plexuses
are located in four atrial and three ventricular regions, 2)
the number of ganglia in each plexus is variable, and 3)
many neurons have features (plentiful cytoplasm and many organelles)
that are typical of autonomic neurons. Therefore, there are regional
differences in nerve innervation on the atria. Given the heterogeneous
distribution of the ganglionated plexuses and the nonuniform shortening
of atrial ERP caused by vagal stimulation, one might expect PACAP to
exert a heterogeneous effect on atrial refractoriness. However, we
found that compared with vagal stimulation, PACAP caused relatively uniform ERP shortening. Braas et al. (5) have shown that
in the guinea pig heart, cardiac ganglia neuronal plasma membranes express PACAP receptor proteins, and PACAP increases cardiac ganglion neuron membrane excitability. Recent physiological studies indicate that cardiac ganglia contain [in addition to efferent parasympathetic postganglionic neurons (3)] afferent neurons
(2) and local circuit neurons (4). In
addition, Yuan et al. (37) inferred from their anatomic
data that the canine intrinsic cardiac nervous system contains a
variety of neurons interconnected via nerve plexuses and complex
interactions can occur within intrinsic cardiac ganglia. They concluded
that canine intrinsic cardiac neurons are more numerous and widely
distributed than has been thought previously. Therefore, PACAP may
cause relatively uniform ERP shortening in both atria through
interactions of a variety of neurons in the cardiac ganglia.
In the present study, we observed that atropine inhibited the atrial
ERP shortening in response to PACAP (see Fig. 6). We cannot exclude the
possibility that PACAP caused uniform shortening of the ERPs in both
atria via the activation of intracardiac muscarinic receptors. However,
our previous studies (14) showed that the negative
chronotropic and inotropic responses to PACAP injected directly into
the sinus node artery were blocked by tetrodotoxin but not by
hexamethonium. In addition, AF induction by PACAP was also abolished by
tetrodotoxin but not by hexamethonium, propranolol, and phentolamine
(15). These results suggest that PACAP-induced neural
release of ACh causes the negative chronotropic and inotropic effects
and the induction of AF. It is well known that the induction of AF is
associated with atrial ERP shortening (33). Therefore, it
is most likely that PACAP shortened the atrial ERP as a result of ACh
released from intracardiac postganglionic nerves.
Direct cardiac effects of PACAP.
In our previous studies, PACAP directly activated the atrial muscle and
increased atrial contractility after atropine treatment in the isolated
canine right atrial preparation (14). However, whether
PACAP shortens atrial ERP by direct effects on atrial tissue is not
known. Several studies have indicated that the direct effects on
cardiac tissue are mediated by cAMP. Ross-Ascuitto et al.
(27) showed that 3-isobutyl-1-methylxanthine, a
nonspecific phosphodiesterase inhibitor, augmented the increase in the
maximal rate of change of left ventricular pressure in response to
PACAP in neonatal pig hearts. In addition, PACAP increased cAMP
in several tissues (23, 28). 
-Adrenergic stimulation
increases cAMP and shortens atrial ERP (22, 30). The
shortening of atrial ERP is related to the augmentation of relaxation
after the increase in cAMP. Therefore, PACAP may shorten atrial ERP
directly by increasing tissue cAMP. To confirm whether PACAP shortens
atrial ERP via a direct effect on atrial tissue, we studied the effects
of PACAP on atrial ERP after atropine treatment. PACAP did not shorten atrial ERP after atropine treatment in our study (see Fig. 6). This
demonstrates that PACAP does not shorten ERP by a direct effect on
atrial tissue. Our results indicate that atrial ERP shortening in
response to PACAP is associated with the activation of intracardiac
postganglionic parasympathetic nerves rather than a direct effect on
atrial tissue.
Intrinsic cardiac nervous system and PACAP.
Cardiac function is regulated by the autonomic nervous system. The
magnitude of the cardiac response to stimulation of the stellate
ganglia and vagus nerves parallels the density of sympathetic and
parasympathetic nerves in the heart, respectively (20). This relatively simple description of autonomic cardiac control has
been complicated by recent demonstrations of various neuropeptides (neuropeptide Y, somatostatin, substance P, and vasoactive intestinal peptide), which have been associated with intrinsic cardiac neurons histochemically (9, 10, 29, 36). PACAP is a newly
discovered neuropeptide that was isolated originally from the ovine
hypothalamus (23). Several studies (5, 31,
35) demonstrated that PACAP and its receptor exist in the heart
and in intrinsic cardiac nerves and that PACAP increases cardiac
ganglion neuron membrane excitability. Our present results indicate
that PACAP directly activates the intrinsic cardiac nerves and has a
different effect on atrial ERP than does cervical vagal stimulation.
Therefore, PACAP may act as a modulator for the regulation of heart
function through the interactions that occur within the intrinsic
cardiac nervous system.
Our previous study (15) demonstrated that PACAP
spontaneously induced AF in anesthetized dogs. ERP shortening is an
important factor influencing induction of AF (33).
The present study demonstrates that PACAP shortens atrial ERP in both
atria. These effects of PACAP on atrial ERP probably participate in the
induction of AF. Coumel (7, 8) has shown that in humans,
some paroxysmal AF is associated with high parasympathetic tone. PACAP
may have some role in generating the arrhythmia in these patients.
Study limitations.
Because PACAP was injected into the left atrium, we cannot be certain
that the drug was distributed evenly within the coronary vascular tree
and the atrial myocardium. Thus we cannot exclude the possibility that
PACAP was distributed preferentially to certain atrial sites. However,
left atrial injection provided a simple, reproducible means for
infusing PACAP. Aortic or coronary artery injection would have been
cumbersome and perhaps less reliable. In addition, the response to
PACAP injection was consistent among dogs. Hence the drug appeared to
be distributed evenly throughout the coronary circulation and the
atrial myocardium.
In the present study, MABP did not differ regardless of the
interventions in 10 non-atropine-treated dogs, whereas it increased significantly after PACAP administration in 10 atropine-treated dogs
(see Table 1). In addition, PACAP caused changes in ECG parameters
including the P-R interval and the QRS complex duration (see Table 1).
Furthermore, PACAP has several cardiovascular effects in the dog such
as a vasodilatation, catecholamine release from the adrenal gland, and
increased blood pressure (11, 15, 18). Therefore,
although PACAP is likely to cause atrial ERP shortening primarily due
to interactions with a variety of neurons in the intracardiac ganglia,
we cannot exclude the possibility that other cardiovascular effects of
PACAP participate in the atrial ERP shortening in the present study.
Although the normal serum concentration (i.e., circulating levels) of
PACAP in the dog has not yet been established, in humans it is a few
picomoles (34). Therefore, the high doses of PACAP used in
the present study may not be physiologically relevant in the normal
condition. In addition, circulating PACAP may not affect the atrial ERP
under normal physiological conditions. However, Braas et al.
(5) demonstrated that PACAP and its receptors were
localized to intrinsic postganglionic cardiac neurons in the guinea pig
heart. Therefore, it is possible that the doses of PACAP affect cardiac
function through the paracrine system in normal physiological and
pathophysiological conditions. The doses of PACAP used in the present
study may not represent physiologically relevant serum concentrations
of PACAP, but such doses are likely to be required to achieve
physiologically relevant concentrations at the receptor level.
In this study, PACAP shortened the atrial refractory period uniformly
in the right and left atria. This response to PACAP is distinct from
the response to bilateral cervical vagal nerve stimulation, which
shortens the refractory period to a greater extent in the right than
the left atrium. Evidence from this study and from previous studies
indicates that PACAP shortens atrial refractoriness by causing ACh to
be released from postganglionic parasympathetic nerves. From these data
and those from previous studies, we believe that PACAP most likely
stimulates intracardiac parasympathetic nerves to both atria, not all
of which are activated by cervical vagal stimulation.
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ACKNOWLEDGEMENTS |
The authors thank Matthew E. Joseph for skilled technical
assistance and Dr. Matthew N. Levy for editorial assistance.
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FOOTNOTES |
Address for reprint requests and other correspondence: M. D. Carlson, Dept. of Medicine, Univ. Hospitals of Cleveland, 11100 Euclid Ave., Cleveland, OH 44106 (E-mail:
mdc2{at}po.cwru.edu).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 13 July 2000; accepted in final form 31 May 2001.
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