Electrophysiologic effects of capsaicin on pacemaker cells in sinoatrial nodes of rabbits

· 826 ·
Cheng YP et al / Acta Pharmacol Sin 2003 Aug; 24 (8): 826-830
©2003, Acta Pharmacologica Sinica
Chinese Pharmacological Society
Shanghai Institute of Materia Medica
Chinese Academy of Sciences
http://www.ChinaPhar.com
Electrophysiologic effects of capsaicin on pacemaker cells
in sinoatrial nodes of rabbits
CHENG Yan-Ping, WANG Yi-He, CHENG Li-Ping1, HE Rui-Rong2
Department of Physiology, Institute of Basic Medicine, Hebei Medical University, Shijiazhuang 050017;
1the First Hospital of Handan, Handan 056002,China
KEY WORDS capsaicin; sinoatrial node; Bay-K-8644; ruthenium red; isoproterenol; eletrophysiology
ABSTRACT
AIM: To study the electrophysiologic effects of capsaicin on isolated pacemaker cells in sinoatrial (SA) nodes of
rabbits and its possible action mechanism(s). METHODS: Parameters of action potential (AP) in SA node were
recorded using intracellular microelectrode technique. RESULTS: By perfusion with capsaicin (10 µmol/L), the
amplitude of action potential (APA) and maximal rate of depolarization (V
) were decreased from (55±4) mV to
max
(49±4) mV (P<0.05) and from (2.4±0.5) V/s to (1.7±0.2) V/s (P<0.05). The velocity of diastolic (phase 4)
depolarization (VDD) and rate of pacemaker firing (RPF) were decreased from (91±34) mV/s to (70±30) mV/s (P
<0.01) and from (186±14) beat/min to (162±10) beat/min (P<0.01). The absolute value of maximal diastolic
potential (MDP) was decreased from (49±3) mV to (44±2) mV (P<0.01). However, the duration of 90 % repolar-
ization of action potential (APD ) was prolonged from (149±21) ms to (167±27) ms (P<0.01). Pretreatment with
90
ruthenium red (RR, 10 µmol/L), a vanilloid receptor (VR1) blocker, did not affect the effects of capsaicin on SA
node cells. Both elevation of calcium concentration (5 mmol/L) in superfusate and application of L-type Ca2+
channel agonist Bay-K-8644 ( 0.5 µmol/L) antagonized the effects of capsaicin on pacemaker cells. ?-adrenergic
agonist isoproterenol (Iso, 20 nmol/L) inhibited the capsaicin-induced prolongation of repolarization and decrease
of MDP. CONCLUSION: Capsaicin exerted a negative chronotropic action and induced a delayed repolarization
of pacemaker cells in SA nodes of rabbits. These effects were likely due to reduction in calcium influx and/or
potassium efflux, but were not mediated by VR1.
INTRODUCTION
tricular contractility, which was followed by a marked
stimulatory action[1]. Capsaicin produced a marked in-
Capsaicin, the main pungent ingredient in ‘hot’ chili
crease in coronary flow, a large positive chronotropic
peppers, has long been known to influence cardiac
effect and a significant reduction in contractile strength[2].
functions. Capsaicin caused a long-lasting, positive ino-
These actions on heart were mediated by its interaction
tropic and chronotropic effect on the spontaneously
with a specific vanilloid receptor (VR1) on sensory nerve
beating right atrium and a slight initial inhibition of ven-
endings in cardiac muscles and opening of a non-selec-
tive cation channel, inducing the liberation of
neuropeptides, most notably calcitonin gene-related pep-
2 Correspondence to Prof HE Rui-Rong.
Phn 86-311-604-4121, ext 5566. Fax 86-311-604-8177.
tide (CGRP), from the vanilloid-sensitive innervation of
E-mail syho@ Hebmu.edu.cn.
the heart[3-5]. Little is known, however, about the ef-
Received 2002-07-09
Accepted 2003-01-27
fects of capsaicin on pacemaker cells of isolated si-

---

Cheng YP et al / Acta Pharmacol Sin 2003 Aug; 24 (8): 826-830
· 827 ·
noatrial (SA) node. In the present study, we observed
capsaicin alone were observed firstly after application
the electrophysiologic effects of capsaicin on SA node
for 15 min. Then after superfusion of RR (10 µmol/L)
cells of rabbits and investigated the mechanism(s)
for 15 min, capsaicin 10 µmol/L was added to the
involved.
superfusate containing RR and AP were recorded; (3)
Effects of Bay-K-8644 (0.5 µmol/L) on capsaicin (10
MATERIALS AND METHODS
µmol/L)-induced changes on AP of pacemaker cells.
Preparation
The effects of capsaicin alone were observed firstly
Rabbits of either sex (n=30, weigh-
after application for 15 min. Then after pretreatment
ing 2.2 kg±0.2 kg, Grade II, Certificate No 04037, pro-
with Bay-K-8644 (0.5 µmol/L) for 10 min, capsaicin
vided by Experimental Animal Center of Hebei Province)
10 µmol/L was added and AP were recorded; (4) Ef-
were killed with a single blow on the head and the hearts
fects of high Ca2+ (5 mmol/L) on the actions of capsai-
were quickly excised. The region of the right atrium
cin (10 µmol/L). The effects of capsaicin alone were
bounded by the crista terminalis and the superior and
observed firstly after application for 15 min. Then nor-
inferior vena cava, and the interatrial septum were dis-
mal K-H solution was replaced by high Ca2+ (5 mmol/L)
sected free from the adjacent tissue[6] in Krebs-Henseleit
K-H solution for 15 min. Afterwards, capsaicin 10
(K-H) solution (0-4 ºC). The preparations were pinned
µmol/L was administered and AP were recorded; (5)
down on a thin silicon disc on the base of a perfusion
Effects of isoproterenol (Iso, 20 nmol/L) on the cap-
chamber and equilibrated for 1 h. The K-H solution
saicin-induced changes in MDP and repolarization of
was prepared with deionized, distilled water and had
SA node cells. The effects of capsaicin alone were
the following composition (mmol/L): NaCl 118.0,
observed firstly after application for 15 min. After pre-
NaHCO 25.0, KCl 4.7, MgSO 1.6, CaCl 2.5, KH PO
3
4
2
2
4
treatment with Iso 20 nmol/L for 10 min, capsaicin 10
1.2, and glucose 11.1. It was oxygenated with 95 %
µmol/L was applied and AP were recorded. In each
O and 5 % CO and maintained at (36.0±0.5) ºC with
2
2
experiment, the preparation was washed with K-H so-
pH of 7.40±0.03.
Electrical recording
lution after application of drugs to observe the recov-
The transmembrane poten-
ery of AP.
tials were recorded by KCl (3 mol/L )-filled micropi-
Drugs Drugs used in this study included capsaicin,
pettes (tip diameter less than 0.5 µm), coupled to a high
ruthenium red, and Bay- K- 8644 (Sigma Chemical Co,
input impedance amplifier (MEZ 8201, Nihon Kohden).
USA) and isoproterenol (Tianfeng Pharmacy Co, China).
The amplified signals were fed to the A/D converter
Capsaicin was dissolved in distilled water containing
and processed by a microcomputer. Maximal diastolic
10 % ethanol and 1 % Tween-80, and then diluted to
potential (MDP), amplitude of action potential (APA),
final concentration with saline. Bay-K-8644 was pre-
90 % of duration of action potential (APD ), maximal
90
pared as stock solution in alcohol and the final concen-
rate of depolarization (V
), rate of pacemaker firing
max
tration of alcohol was 0.1 %. Ruthenium red was dis-
(RPF), and velocity of diastolic (phase 4) depolariza-
solved in distilled water.
tion (VDD) were analyzed with the system of sampling
Statistical analysis All data were presented as
and processing cardiac transmembrane potential de-
mean±SD. Statistical comparisons were performed
signed by our department[7].
Experimental protocols
using t test. Statistical significance was set at P<0.05.
The experiment started
after the preparation was equilibrated in the K-H solu-
RESULTS
tion at a perfusion rate of 4 mL/min for 60 min . The
experiments consisted of 5 groups: (1) Electrophysio-
Effect of capsaicin on transmembrane action
logic effects of capsaicin on SA node pacemaker cells.
potentials Compared with control groups, capsaicin
The effects of capsaicin on AP were studied in a non-
(1-30 µmol/L) decreased VDD, RPF, and V
, and pro-
max
cumulative manner. Only one concentration of drug
longed APD in a concentration-dependent manner.
90
was given to a preparation. After recording of 3 con-
Capsaicin 10 and 30 µmol/L also induced a significant
trol AP, capsaicin 1, 3, 10, and 30 µmol/L were sepa-
reduction in APA and MDP (Tab 1, Fig 1). The changes
rately applied. AP were then recorded at 1, 5, 10, 20,
in RPF induced by capsaicin paralleled to those of VDD.
and 30 min after application of capsaicin; (2) Effects of
The above effects occurred after 10 min of superfusion
ruthenium red (RR, 10 µmol/L) on the response of SA
of capsaicin and reached the peak within 20-30 min.
node cells to capsaicin (10 µmol/L). The effects of
The vehicle of capsaicin had no effect on parameters

---

· 828 ·
Cheng YP et al / Acta Pharmacol Sin 2003 Aug; 24 (8): 826-830
Tab 1. Effects of capsaicin on transmembrane action potentials of rabbit sinoatrial node cells. n=6. Mean±SD. aP>0.05,
bP<0.05, cP<0.01 vs control.
MDP/mV APA/mV V
/V·s-1
VDD/mV·s-1 APD /ms RPF/beat·min-1
max
90
Control
-49±3
55±4
2.4±0.5
91±34
149±21
186±14
Capsaicin (µmol/L)
1
-48±4a
53±5a
2.1±0.3a
84±34b
154±19b
178±12a
3
-47±3a
51±4a
1.9±0.3b
77±35b
159±23c
171±11b
10
-44±2c
49±4b
1.7±0.2b
70±30c
167±27c
162±10c
30
-43±2c
47±4c
1.5±0.4b
59±29c
182±42b
151±17c
MDP: maximal diastolic potential; APA: amplitude of action potential; V
: maximal rate of depolarization; VDD: velocity of diastolic
max
(phase 4) depolarization ; RPF: rate of pacemaker firing; APD : 90 % of duration of action potential.
90
changes of AP L-type calcium channel agonist Bay-
K-8644 (0.5 µmol/L) significantly increased V
, RPF,
max
and VDD. Upon the application of Bay-K-8644, the
effects of capsaicin (10 µmol/L) in V
, RPF, and VDD
max
were inhibited (Tab 2). The vehicle of Bay-K-8644
(0.1 % alcohol in superfusate) had no effect on param-
eters of AP of pacemaker cells.
Effects of high Ca2+ on the actions of capsaicin
Elevation of Ca2+ concentration (5 mmol/L) in
superfusate increased VDD, V
, and APA, and blocked
max
the inhibitory action of capsaicin 10 µmol/L (Tab 2).
Effects of isoproterenol on capsaicin-induced
changes in MDP and repolarization of SA node cells
?-adrenergic agonist Iso (20 nmol/L) not only increased
V , RPF, and APA, but also significantly shortened
max
APD and increased MDP. After pretreatment with
90
Iso 20 nmol/L, the APD -prolonging and MDP-decreas-
90
ing effects of capsaicin (10 µmol/L) were antagonized
(Tab 2).
DISCUSSION
The present study showed that capsaicin concen-
tration-dependently decreased V
, VDD, RPF, APA
max
Fig 1. Effects of capsaicin on transmembrane action poten-
and MDP, and prolonged APD of pacemaker cells in
90
tials of rabbit sinoatrial node cells. A: Control; B: Capsai-
SA nodes of rabbits. It has been generally accepted
cin 3 µmol/L; C: Capsaicin 30 µmol/L; D: Wash out.
that many of the capsaicin effects are mediated by
vanilloid receptor (VR1). VR1, a distant relative of the
of AP of pacemaker cells.
transient release potential (TRP) family of stored-oper-
Effects of ruthenium red on the response of
ated calcium channels, is expressed almost exclusively
SA node cells to capsaicin Vanilloid receptor (VR1)
by primary sensory neurons involved in nociceptions
blocker RR (10 µmol/L) had no effect on AP. Pretreat-
and inflammation[8]. On the other hand, some capsai-
ment with RR 10 µmol/L failed to affect the above men-
cin-induced effects did not follow typical features of
tioned effects induced by capsaicin 10 µmol/L (Tab 2).
vanilloid receptors[9]. The presence of a VR1-like ex-
Effects of Bay-K-8644 on capsaicin-induced
pressed sequence tags (EST) clone in heart is in accor-

---

Cheng YP et al / Acta Pharmacol Sin 2003 Aug; 24 (8): 826-830
· 829 ·
Tab 2. Effects of ruthenium red (RR, 10 µmol/L), Bay-K-8644 (Bay, 0.5 µmol/L), high Ca2+ (5 mmol/L), and isoproterenol (Iso,
20 nmol/L) on capsaicin (10 µmol/L)-induced changes on transmembrane action potentials of rabbit sinoatrial node cells.
n=24. Mean±SD. aP>0.05, bP<0.05, cP<0.01 vs control.
MDP/mV APA/mV V
/V·s-1
VDD/mV·s-1 APD /ms RPF/beat·min-1
max
90
Control
-48±7
60±8
3.0±0.7
81±17
124±14
196±28
Capsaicin
-44±7c
53±8c
1.9±0.5c
59±22c
143±16c
166±21c
RR
-49±7a
60±7a
2.8±0.5b
77±12a
127±11a
189±23a
RR+capsaicin
-45±5a
54±7b
1.9±0.4b
59±19c
145±19c
164±18c
Control
-44±3
50±4
1.8±0.4
103±20
136±7
216±13
Capsaicin
-41±3c
45±4b
1.58±0.24b
75±4b
146±6c
193±9c
Bay
-44±3a
55±3a
2.5±0.4c
145±31b
134±5a
236±14c
Bay+capsaicin
-43±2b
49±3a
1.9±0.3a
107±15a
145±3a
212±13a
Control
-47±5
50±5
2.3±0.6
67±10
132±10
229±27
Capsaicin
-43±4b
47±5a
1.9±0.3b
49±10c
146±12b
188±30b
High Ca2+
-48±5a
52±5b
3.4±1.0b
79±8b
129±12a
235±28a
High Ca2++capsaicin
-47±5a
51±4a
2.5±0.7a
71±9a
129±10a
230±24a
Control
-49±9
58±13
3.9±1.4
57±7
116±9
221±27
Capsaicin
-46±7b
55±14b
2.9±0.9b
50±4a
140±7c
169±34b
Iso
-54±9c
64±12b
6.3±2.5b
75±19b
104±6c
255±32c
Iso+capsaicin
-48±8a 58±13a 4.5±1.7a 63±14a 114±8a 238±28a
MDP: maximal diastolic potential; APA: amplitude of action potential; V
: maximal rate of depolarization; VDD: velocity of diastolic
max
(phase 4) depolarization ; RPF: rate of pacemaker firing; APD : 90 % of duration of action potential
90
dance with the recognition of nonneuronal VRs[10,11].
increased. It has been well known that I is the main
K
The negative inotropic actions on cardiac muscles of
ionic current which participates in repolarization of SA
high-dose (in µmol/L) of capsaicin was not due to the
node[13], so the above action might be related to a re-
toxic effect of capsaicin[12]. In our experiment, RR, a
duction of potassium current. Castle reported that cap-
vanilloid receptor blocker, failed to abolish the
saicin increased the action potential duration of rat ven-
electrophysiologic effects of capsaicin on SA node,
tricular myocytes. The effect was associated with an
[15].
suggesting that VR1 might not mediate the inhibitory
inhibition of three distinct K+ currents, I , I , and I
to
K
Ki
effects of capsaicin. High-dose of capsaicin might ex-
In our study, ?-adrenergic agonist isoproterenol in-
ert its action through the nonspecific mechanism.
creased V
, VDD, and RPF of SA node cells. At the
max
It has been widely accepted that calcium currents
same time, Iso significantly shortened APD and in-
90
play important roles in action potential upstroke and
creased MDP. The latter effects were due to the acti-
[16,17].
pacemaker depolarization of SA node cells[13,14].
vation of I
Iso inhibited the capsaicin-induced
K
Therefore, the inhibitory effects of capsaicin on V
,
decrease of MDP and prolongation of repolarization of
max
VDD, RPF, and APA might be attributed to the reduc-
SA node cells, suggesting that capsaicin might possess
tion of I . Our presumption was substantiated by the
the ability to inhibit I in SA node.
K
Ca
following findings that elevation of calcium concentra-
In summary, capsaicin exhibited inhibitory effects
tion in superfusate or application of L-type Ca2+ chan-
on pacemaker cells in SA nodes of rabbits, which may
nel agonist Bay-K-8644 blocked the inhibitory effects
be attributed to reduction in calcium influx and/or po-
of capsaicin.
tassium efflux and may be not mediated by VR1.
In this study, maximal diastolic potential was de-
REFERENCES
creased and action potential duration of pacemaker cells
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· 826 ·
Cheng YP et al / Acta Pharmacol Sin 2003 Aug; 24 (8): 826-830
©2003, Acta Pharmacologica Sinica
Chinese Pharmacological Society
Shanghai Institute of Materia Medica
Chinese Academy of Sciences
http://www.ChinaPhar.com
Electrophysiologic effects of capsaicin on pacemaker cells
in sinoatrial nodes of rabbits
CHENG Yan-Ping, WANG Yi-He, CHENG Li-Ping1, HE Rui-Rong2
Department of Physiology, Institute of Basic Medicine, Hebei Medical University, Shijiazhuang 050017;
1the First Hospital of Handan, Handan 056002,China
KEY WORDS capsaicin; sinoatrial node; Bay-K-8644; ruthenium red; isoproterenol; eletrophysiology
ABSTRACT
AIM: To study the electrophysiologic effects of capsaicin on isolated pacemaker cells in sinoatrial (SA) nodes of
rabbits and its possible action mechanism(s). METHODS: Parameters of action potential (AP) in SA node were
recorded using intracellular microelectrode technique. RESULTS: By perfusion with capsaicin (10 µmol/L), the
amplitude of action potential (APA) and maximal rate of depolarization (V
) were decreased from (55±4) mV to
max
(49±4) mV (P<0.05) and from (2.4±0.5) V/s to (1.7±0.2) V/s (P<0.05). The velocity of diastolic (phase 4)
depolarization (VDD) and rate of pacemaker firing (RPF) were decreased from (91±34) mV/s to (70±30) mV/s (P
<0.01) and from (186±14) beat/min to (162±10) beat/min (P<0.01). The absolute value of maximal diastolic
potential (MDP) was decreased from (49±3) mV to (44±2) mV (P<0.01). However, the duration of 90 % repolar-
ization of action potential (APD ) was prolonged from (149±21) ms to (167±27) ms (P<0.01). Pretreatment with
90
ruthenium red (RR, 10 µmol/L), a vanilloid receptor (VR1) blocker, did not affect the effects of capsaicin on SA
node cells. Both elevation of calcium concentration (5 mmol/L) in superfusate and application of L-type Ca2+
channel agonist Bay-K-8644 ( 0.5 µmol/L) antagonized the effects of capsaicin on pacemaker cells. ?-adrenergic
agonist isoproterenol (Iso, 20 nmol/L) inhibited the capsaicin-induced prolongation of repolarization and decrease
of MDP. CONCLUSION: Capsaicin exerted a negative chronotropic action and induced a delayed repolarization
of pacemaker cells in SA nodes of rabbits. These effects were likely due to reduction in calcium influx and/or
potassium efflux, but were not mediated by VR1.
INTRODUCTION
tricular contractility, which was followed by a marked
stimulatory action[1]. Capsaicin produced a marked in-
Capsaicin, the main pungent ingredient in ‘hot’ chili
crease in coronary flow, a large positive chronotropic
peppers, has long been known to influence cardiac
effect and a significant reduction in contractile strength[2].
functions. Capsaicin caused a long-lasting, positive ino-
These actions on heart were mediated by its interaction
tropic and chronotropic effect on the spontaneously
with a specific vanilloid receptor (VR1) on sensory nerve
beating right atrium and a slight initial inhibition of ven-
endings in cardiac muscles and opening of a non-selec-
tive cation channel, inducing the liberation of
neuropeptides, most notably calcitonin gene-related pep-
2 Correspondence to Prof HE Rui-Rong.
Phn 86-311-604-4121, ext 5566. Fax 86-311-604-8177.
tide (CGRP), from the vanilloid-sensitive innervation of
E-mail syho@ Hebmu.edu.cn.
the heart[3-5]. Little is known, however, about the ef-
Received 2002-07-09
Accepted 2003-01-27
fects of capsaicin on pacemaker cells of isolated si-

---

Cheng YP et al / Acta Pharmacol Sin 2003 Aug; 24 (8): 826-830
· 827 ·
noatrial (SA) node. In the present study, we observed
capsaicin alone were observed firstly after application
the electrophysiologic effects of capsaicin on SA node
for 15 min. Then after superfusion of RR (10 µmol/L)
cells of rabbits and investigated the mechanism(s)
for 15 min, capsaicin 10 µmol/L was added to the
involved.
superfusate containing RR and AP were recorded; (3)
Effects of Bay-K-8644 (0.5 µmol/L) on capsaicin (10
MATERIALS AND METHODS
µmol/L)-induced changes on AP of pacemaker cells.
Preparation
The effects of capsaicin alone were observed firstly
Rabbits of either sex (n=30, weigh-
after application for 15 min. Then after pretreatment
ing 2.2 kg±0.2 kg, Grade II, Certificate No 04037, pro-
with Bay-K-8644 (0.5 µmol/L) for 10 min, capsaicin
vided by Experimental Animal Center of Hebei Province)
10 µmol/L was added and AP were recorded; (4) Ef-
were killed with a single blow on the head and the hearts
fects of high Ca2+ (5 mmol/L) on the actions of capsai-
were quickly excised. The region of the right atrium
cin (10 µmol/L). The effects of capsaicin alone were
bounded by the crista terminalis and the superior and
observed firstly after application for 15 min. Then nor-
inferior vena cava, and the interatrial septum were dis-
mal K-H solution was replaced by high Ca2+ (5 mmol/L)
sected free from the adjacent tissue[6] in Krebs-Henseleit
K-H solution for 15 min. Afterwards, capsaicin 10
(K-H) solution (0-4 ºC). The preparations were pinned
µmol/L was administered and AP were recorded; (5)
down on a thin silicon disc on the base of a perfusion
Effects of isoproterenol (Iso, 20 nmol/L) on the cap-
chamber and equilibrated for 1 h. The K-H solution
saicin-induced changes in MDP and repolarization of
was prepared with deionized, distilled water and had
SA node cells. The effects of capsaicin alone were
the following composition (mmol/L): NaCl 118.0,
observed firstly after application for 15 min. After pre-
NaHCO 25.0, KCl 4.7, MgSO 1.6, CaCl 2.5, KH PO
3
4
2
2
4
treatment with Iso 20 nmol/L for 10 min, capsaicin 10
1.2, and glucose 11.1. It was oxygenated with 95 %
µmol/L was applied and AP were recorded. In each
O and 5 % CO and maintained at (36.0±0.5) ºC with
2
2
experiment, the preparation was washed with K-H so-
pH of 7.40±0.03.
Electrical recording
lution after application of drugs to observe the recov-
The transmembrane poten-
ery of AP.
tials were recorded by KCl (3 mol/L )-filled micropi-
Drugs Drugs used in this study included capsaicin,
pettes (tip diameter less than 0.5 µm), coupled to a high
ruthenium red, and Bay- K- 8644 (Sigma Chemical Co,
input impedance amplifier (MEZ 8201, Nihon Kohden).
USA) and isoproterenol (Tianfeng Pharmacy Co, China).
The amplified signals were fed to the A/D converter
Capsaicin was dissolved in distilled water containing
and processed by a microcomputer. Maximal diastolic
10 % ethanol and 1 % Tween-80, and then diluted to
potential (MDP), amplitude of action potential (APA),
final concentration with saline. Bay-K-8644 was pre-
90 % of duration of action potential (APD ), maximal
90
pared as stock solution in alcohol and the final concen-
rate of depolarization (V
), rate of pacemaker firing
max
tration of alcohol was 0.1 %. Ruthenium red was dis-
(RPF), and velocity of diastolic (phase 4) depolariza-
solved in distilled water.
tion (VDD) were analyzed with the system of sampling
Statistical analysis All data were presented as
and processing cardiac transmembrane potential de-
mean±SD. Statistical comparisons were performed
signed by our department[7].
Experimental protocols
using t test. Statistical significance was set at P<0.05.
The experiment started
after the preparation was equilibrated in the K-H solu-
RESULTS
tion at a perfusion rate of 4 mL/min for 60 min . The
experiments consisted of 5 groups: (1) Electrophysio-
Effect of capsaicin on transmembrane action
logic effects of capsaicin on SA node pacemaker cells.
potentials Compared with control groups, capsaicin
The effects of capsaicin on AP were studied in a non-
(1-30 µmol/L) decreased VDD, RPF, and V
, and pro-
max
cumulative manner. Only one concentration of drug
longed APD in a concentration-dependent manner.
90
was given to a preparation. After recording of 3 con-
Capsaicin 10 and 30 µmol/L also induced a significant
trol AP, capsaicin 1, 3, 10, and 30 µmol/L were sepa-
reduction in APA and MDP (Tab 1, Fig 1). The changes
rately applied. AP were then recorded at 1, 5, 10, 20,
in RPF induced by capsaicin paralleled to those of VDD.
and 30 min after application of capsaicin; (2) Effects of
The above effects occurred after 10 min of superfusion
ruthenium red (RR, 10 µmol/L) on the response of SA
of capsaicin and reached the peak within 20-30 min.
node cells to capsaicin (10 µmol/L). The effects of
The vehicle of capsaicin had no effect on parameters

---

· 828 ·
Cheng YP et al / Acta Pharmacol Sin 2003 Aug; 24 (8): 826-830
Tab 1. Effects of capsaicin on transmembrane action potentials of rabbit sinoatrial node cells. n=6. Mean±SD. aP>0.05,
bP<0.05, cP<0.01 vs control.
MDP/mV APA/mV V
/V·s-1
VDD/mV·s-1 APD /ms RPF/beat·min-1
max
90
Control
-49±3
55±4
2.4±0.5
91±34
149±21
186±14
Capsaicin (µmol/L)
1
-48±4a
53±5a
2.1±0.3a
84±34b
154±19b
178±12a
3
-47±3a
51±4a
1.9±0.3b
77±35b
159±23c
171±11b
10
-44±2c
49±4b
1.7±0.2b
70±30c
167±27c
162±10c
30
-43±2c
47±4c
1.5±0.4b
59±29c
182±42b
151±17c
MDP: maximal diastolic potential; APA: amplitude of action potential; V
: maximal rate of depolarization; VDD: velocity of diastolic
max
(phase 4) depolarization ; RPF: rate of pacemaker firing; APD : 90 % of duration of action potential.
90
changes of AP L-type calcium channel agonist Bay-
K-8644 (0.5 µmol/L) significantly increased V
, RPF,
max
and VDD. Upon the application of Bay-K-8644, the
effects of capsaicin (10 µmol/L) in V
, RPF, and VDD
max
were inhibited (Tab 2). The vehicle of Bay-K-8644
(0.1 % alcohol in superfusate) had no effect on param-
eters of AP of pacemaker cells.
Effects of high Ca2+ on the actions of capsaicin
Elevation of Ca2+ concentration (5 mmol/L) in
superfusate increased VDD, V
, and APA, and blocked
max
the inhibitory action of capsaicin 10 µmol/L (Tab 2).
Effects of isoproterenol on capsaicin-induced
changes in MDP and repolarization of SA node cells
?-adrenergic agonist Iso (20 nmol/L) not only increased
V , RPF, and APA, but also significantly shortened
max
APD and increased MDP. After pretreatment with
90
Iso 20 nmol/L, the APD -prolonging and MDP-decreas-
90
ing effects of capsaicin (10 µmol/L) were antagonized
(Tab 2).
DISCUSSION
The present study showed that capsaicin concen-
tration-dependently decreased V
, VDD, RPF, APA
max
Fig 1. Effects of capsaicin on transmembrane action poten-
and MDP, and prolonged APD of pacemaker cells in
90
tials of rabbit sinoatrial node cells. A: Control; B: Capsai-
SA nodes of rabbits. It has been generally accepted
cin 3 µmol/L; C: Capsaicin 30 µmol/L; D: Wash out.
that many of the capsaicin effects are mediated by
vanilloid receptor (VR1). VR1, a distant relative of the
of AP of pacemaker cells.
transient release potential (TRP) family of stored-oper-
Effects of ruthenium red on the response of
ated calcium channels, is expressed almost exclusively
SA node cells to capsaicin Vanilloid receptor (VR1)
by primary sensory neurons involved in nociceptions
blocker RR (10 µmol/L) had no effect on AP. Pretreat-
and inflammation[8]. On the other hand, some capsai-
ment with RR 10 µmol/L failed to affect the above men-
cin-induced effects did not follow typical features of
tioned effects induced by capsaicin 10 µmol/L (Tab 2).
vanilloid receptors[9]. The presence of a VR1-like ex-
Effects of Bay-K-8644 on capsaicin-induced
pressed sequence tags (EST) clone in heart is in accor-

---

Cheng YP et al / Acta Pharmacol Sin 2003 Aug; 24 (8): 826-830
· 829 ·
Tab 2. Effects of ruthenium red (RR, 10 µmol/L), Bay-K-8644 (Bay, 0.5 µmol/L), high Ca2+ (5 mmol/L), and isoproterenol (Iso,
20 nmol/L) on capsaicin (10 µmol/L)-induced changes on transmembrane action potentials of rabbit sinoatrial node cells.
n=24. Mean±SD. aP>0.05, bP<0.05, cP<0.01 vs control.
MDP/mV APA/mV V
/V·s-1
VDD/mV·s-1 APD /ms RPF/beat·min-1
max
90
Control
-48±7
60±8
3.0±0.7
81±17
124±14
196±28
Capsaicin
-44±7c
53±8c
1.9±0.5c
59±22c
143±16c
166±21c
RR
-49±7a
60±7a
2.8±0.5b
77±12a
127±11a
189±23a
RR+capsaicin
-45±5a
54±7b
1.9±0.4b
59±19c
145±19c
164±18c
Control
-44±3
50±4
1.8±0.4
103±20
136±7
216±13
Capsaicin
-41±3c
45±4b
1.58±0.24b
75±4b
146±6c
193±9c
Bay
-44±3a
55±3a
2.5±0.4c
145±31b
134±5a
236±14c
Bay+capsaicin
-43±2b
49±3a
1.9±0.3a
107±15a
145±3a
212±13a
Control
-47±5
50±5
2.3±0.6
67±10
132±10
229±27
Capsaicin
-43±4b
47±5a
1.9±0.3b
49±10c
146±12b
188±30b
High Ca2+
-48±5a
52±5b
3.4±1.0b
79±8b
129±12a
235±28a
High Ca2++capsaicin
-47±5a
51±4a
2.5±0.7a
71±9a
129±10a
230±24a
Control
-49±9
58±13
3.9±1.4
57±7
116±9
221±27
Capsaicin
-46±7b
55±14b
2.9±0.9b
50±4a
140±7c
169±34b
Iso
-54±9c
64±12b
6.3±2.5b
75±19b
104±6c
255±32c
Iso+capsaicin
-48±8a 58±13a 4.5±1.7a 63±14a 114±8a 238±28a
MDP: maximal diastolic potential; APA: amplitude of action potential; V
: maximal rate of depolarization; VDD: velocity of diastolic
max
(phase 4) depolarization ; RPF: rate of pacemaker firing; APD : 90 % of duration of action potential
90
dance with the recognition of nonneuronal VRs[10,11].
increased. It has been well known that I is the main
K
The negative inotropic actions on cardiac muscles of
ionic current which participates in repolarization of SA
high-dose (in µmol/L) of capsaicin was not due to the
node[13], so the above action might be related to a re-
toxic effect of capsaicin[12]. In our experiment, RR, a
duction of potassium current. Castle reported that cap-
vanilloid receptor blocker, failed to abolish the
saicin increased the action potential duration of rat ven-
electrophysiologic effects of capsaicin on SA node,
tricular myocytes. The effect was associated with an
[15].
suggesting that VR1 might not mediate the inhibitory
inhibition of three distinct K+ currents, I , I , and I
to
K
Ki
effects of capsaicin. High-dose of capsaicin might ex-
In our study, ?-adrenergic agonist isoproterenol in-
ert its action through the nonspecific mechanism.
creased V
, VDD, and RPF of SA node cells. At the
max
It has been widely accepted that calcium currents
same time, Iso significantly shortened APD and in-
90
play important roles in action potential upstroke and
creased MDP. The latter effects were due to the acti-
[16,17].
pacemaker depolarization of SA node cells[13,14].
vation of I
Iso inhibited the capsaicin-induced
K
Therefore, the inhibitory effects of capsaicin on V
,
decrease of MDP and prolongation of repolarization of
max
VDD, RPF, and APA might be attributed to the reduc-
SA node cells, suggesting that capsaicin might possess
tion of I . Our presumption was substantiated by the
the ability to inhibit I in SA node.
K
Ca
following findings that elevation of calcium concentra-
In summary, capsaicin exhibited inhibitory effects
tion in superfusate or application of L-type Ca2+ chan-
on pacemaker cells in SA nodes of rabbits, which may
nel agonist Bay-K-8644 blocked the inhibitory effects
be attributed to reduction in calcium influx and/or po-
of capsaicin.
tassium efflux and may be not mediated by VR1.
In this study, maximal diastolic poten