Biologia Geral e Experimetal
ELECTROPHYSIOLOGICAL EFFECTS OF SODIUM THIOPENTAL ON THE
RIGHT ATRIUM OF THE RABBIT (ORYCTOLAGUS CUNICULUS)
Marcli Costa da Silveira Libório1
This study describes the effects of sodium thiopental (40 mg/l) on the rabbit atrium. The electrical endocardial signals and theintracellular action potentials were recorded. The results revealed a reduction in the atrial impulse velocity (from 75 ± 3 cm/secto 63 ± 7 cm/sec), disorganization of the propagated electrical front wave, reduction of the spontaneous atrial pacemaker rate,depolarization of atrial cells (quiescent: 3.46 ± 1.2 mV; under electrical stimulation: 3.1 ± 0.5 mV), and an increase of the atrialcellular refractory period (from 52 ± 5 msec to 117 ± 8 msec). Atropine sulfate (1 mg/l) did not prevent or abolished thebradicardia produced by the sodium thiopental. Keywords: Electrocardiophisiology, action potentials, atrial cells, sodium thiopental.
O estudo descreve os efeitos do tiopental sódico (40 mg/l) sobre átrios de coelho. Foram registrados os sinais elétricos endocárdicose os potenciais de ação intracelulares. Os resultados mostraram redução da velocidade de propagação do impulso atrial (75 ± 3 cm/sec a 63 ± 7 cm/sec), desorganização da frente de onda propagada, redução da freqüência espontânea do marcapasso atrial,despolarização de células atriais mantidas em repouso (3.46 ± 1.2 mV) ou sob estimulação elétrica (3.1 ± 0.5 mV), e aumento doperíodo refratário das células atriais (52 ± 5 msec a 117 ± 8 msec). O sulfato de atropina (1 mg/l) não preveniu ou aboliu abradicardia produzida pelo tiopental sódico. Palavras-chave: eletrocardiofisiologia, potencial de ação, células atriais, tiopental sódico.
those with high blood flow demands. In plasma, 85%
of NaTHIO is bound to albumin molecules and, as a
Sodium thiopental (NaTHIO) is a well-known
consequence, patients with severe hypoalbuminemia
barbiturate. Its rapid hypnotic properties derive from
may need less NaTHIO to lose consciousness during
its effective liposolubility, a property due to the
presence of a sulfur atom substituting an oxygen atom
Becker & Tonnesen (1978) reported some
in the ureic residue of the barbiturate ring. At blood
cardiovascular effects produced by NaTHIO in patients
concentrations of up to 16 mg/ml, conscience is lost
anesthetized exclusively by this drug. These effects
quickly (Evers & Crowder, 2001). Patients of normal
included an increase in heart rate during the induction
body weight who receive small doses of NaTHIO
phase, a decrease in systolic blood pressure, and an
normally wake up around 5 to 10 minutes later. Rapid
increase of the ventricular pre-ejection period.
recovery occurs as a consequence of the redistribution
Since Beattie et al. (1930) showed that chloroform
of NaTHIO among different tissues and organs, mainly
facilitates the appearance of ventricular fibrillation, the
1 Departamento de Fisiologia, Laboratório de Biofísica do Coração. Universidade Federal de Sergipe, Av. Marechal Rondon, s/n, Jardim RosaElze, São Cristóvão, Se, 49100-000. [email protected]
anesthetic side-effects have been investigated
mm, outer diameter: 1 mm) used as a grounded pole.
exhaustively. However, Price & Ohnishi (1980) have
The electrode was then mounted on a mechanical
emphasized the need for a better understanding of the
micromanipulator to allow it to be displaced smoothly
effects of NaTHIO on the conduction of electrical
(X-Y axes) along the endocardial surface. Electrical
impulses and myocardial excitability.
signals captured by this roving electrode were amplified
This paper describes the electrophysiological
differentially, monitored, and photographed by an
effects promoted by this thiobarbiturate. The following
oscilloscope camera (Differential Amplifier 5A22N, C-
parameters were studied: a) conduction velocity of the
50 Oscilloscope Camera, D44 Dual Beam Oscilloscope,
atrial electrical impulse, b) organization of the wave
TEKTRONIX, Inc. Beaverton, Oregon, USA).
front of the propagated electrical impulse, c)
Intracellular readings were taken with 3M KCl-
spontaneous pacemaker rate, d) cellular resting
filled glass microelectrodes (DC resistance equal to 40
potential in quiescent and electrically stimulated
MW, tip diameter about 0.1mm: Oliveira-Castro &
myocardial fibers, e) morphology of propagated action
Machado, 1969). Microelectrode signals were sent to a
potentials, and f) electrical refractory period of the atrial
high input impedance amplifier (M701, W-P Instruments,
cells. These parameters are especially important given
Inc., New Haven, Connecticut, USA) and then to a
the use of NaTHIO in animal research as a hypnotic
plug-in oscilloscope amplifier (5A48 Dual Trace
drug or as an auxiliary to prevent ischemic/reperfusion-
Amplifier, TEKTRONIX, Inc. Beaverton, Oregon, USA).
induced cardiac arrhythmias (Ruigrok et al., 1985;
Following the experimental protocol, the atrium was
Schultz et al., 1997; Conradie & Coetzee, 1999; Kato &
stimulated electrically using ungrounded electrical
current pulses (DS2 Isolator Unit, D4030 Pulse
Programmer, DIGITIMER Limited, Welwyn Garden City,
Hertfordshire, England). The stimuli were delivered
through a pair of stainless steal electrodes placed at
The present study was carried out on adult
rabbits (1.5-2.0 kg) of both sexes. Animals were
To evaluate the effect of thiobarbiturate on atrial
sacrificed by a blow applied to the base of the skull.
impulse velocity, the atrial pacemaker frequency was
Their hearts were removed immediately and immersed
set at a rate 20% higher than the spontaneous one.
in a modified Tyrode solution (in mM: NaCl 137, KCl
The conduction velocity was measured through
5.0, MgCl2 0.5, NaHCO3 12, CaCl2 1.8, Glucose 6.0,
previously selected pathways with uniform conduction
NaH2PO4 1.8). The right atrium was separated and
of electrical signals. They were placed 4 to 6 mm from
mounted in a chamber with its endocardial surface
the stimulus electrodes to avoid interference of the
facing upwards, superfused in Tyrode at 34.0±0.5°C
stimulation field. Atrial impulse speed was estimated
(UNITEMP, model 111, FANEM, Cumbica, Guarulhos,
by displacing the surface electrode at constant steps
Sao Paulo, SP), aerated and buffered with carbogen
(0.5 or 0.6 mm) and by simultaneous measurement of
mixture (95% oxygen plus 5% carbon dioxide, <1%
the time elapsed before the wave reached the electrode.
error). The test solution was prepared by adding
It was possible to estimate the velocity of the atrial
NaTHIO (Thionembutal, Abbott, Laboratórios do
impulse by plotting each displacement against its
Brasil, Ltda., São Paulo, SP) to the Tyrode.
corresponding time delay, based on the steepness of
Surface records were performed with the aid of a
the regression line used to fit the experimental data.
Teflon®-coated silver wire electrode (150 mm) inserted
To evaluate the effects of the barbiturate on atrial
into a hypodermic stainless steel needle (length:100
pacemaker activity, the electrical impulses recorded by
Electrophysiological effects of sodium tiopental
the surface electrode were counted, and the
were obtained for six other atria in which NaTHIO (40
spontaneous rate was determined in detail (D4030 Pulse
mg/l) decreased the impulse velocity from 75 ± 3 cm/
Programmer, DIGITIMER Limited, Welwyn Garden City,
sec to 63 ± 7 cm/sec (n = 18 trials, p < 0.001). This effect
Hertfordshire, England; 1830 Interval Generator, 1832
had not disappeared completely 30 minutes after
Preset Control, 1831 Pulse Control Module, W-P
removal of the barbiturate from the organ bath.
Instruments, Inc. New Haven, Connecticut, USA).
To study the effects of NaTHIO on cellular
2. Effects of sodium thiopental on the atrial pacemaker
resting potential, special care was taken with regard to
the grounding system of the organ bath. For this, the
At 60 mg/l, NaTHIO reduced the atrial pacemaker
silver electrode was covered with chloride, according
rate 16 to 47% (Figure 2). This negative chronotropic
to the technique described by Geddes (1972), in order
effect was eliminated partially or completely during the
to minimize junction potentials. This electrode,
washout (n = 13 atria, 25 trials, p < 0.001).
connected to the ground, was immersed in 3M KCl,
At a doseage of 40 mg/l, NaTHIO decreased the
with an agar/3M KCl bridge connecting it to the organ
spontaneous atrial rate progressively, and complete
bath. The aim was to obtain electrical stability, and no
asystole was observed in several experiments (see
electrical drift was recorded in the organ bath when the
Figure 3). Initial control rate was 158 bpm, but during
Tyrode voltage was monitored during 3 hours.
NaTHIO (40 mg/l), it decreased progressively,
Statistical Analysis: Results are presented as means
sometimes to zero (interruptions in the fitting line). This
± standard deviation. Student’s t-test was used to
effect was not altered by the application of atropine
sulfate (1 mg/l, Sigma Chemical Co., St. Louis, MO,
USA) 20 minutes before NaTHIO. However, removal of
NaTHIO from the perfusion solution resulted in a return
to the control rate. Similar results were obtained for
1. Effects of sodium thiopental on the propagated atrial
three other atria (n = 6 trials). In some experiments,
NaTHIO (40-60 mg/l) induced the appearance of
Figure 1 shows two sets of electrical waves
isolated extrasystoles or even activated rapid ectopic
recorded from the atrial endocardium by moving the
surface electrode at regular steps. The experiments were
carried out on a paced right atrium (2 Hz). In the control
3. Effects of sodium thiopental on the resting potential
(Figure 1A), the interval between successive waves
was highly regular (electrode displacement step = 0.6
Figure 4 shows an intracellular record obtained
mm), indicating uniform propagation. Conduction
from a quiescent atrial cell (resting potential = 81 mV).
velocity was calculated at 73 cm/sec (r2 = 0.9992). In the
Downward pointing arrows indicate the moment when
experimental procedure (Figure 1B), waves were
NaTHIO (40 mg/l) was added to the bath and upward
recorded in the presence of NaTHIO (40 mg/l). The
arrows indicate when the barbiturate was removed from.
surface electrode was displaced at steps of 0.5 mm and
NaTHIO promoted depolarization (3.46 ± 1.2 mV) when
the impulse velocity of the atrium decreased to 56 cm/
present in the solution bath. The depolarizing effect
sec (r2 = 0.9954, p < 0.001), 22% lower than the control
was not reverted completely by the washout. Similar
value. Wave morphology was also less regular, and in
effects were observed in five other atria (3.06 ± 1.4, n =
several cases, irregularities indicate loose organization
of atrial impulses. Figure 1C presents the regression
NaTHIO (40 mg/l) also promoted depolarization
lines for control and NaTHIO data points. Similar results
in electrically stimulated atrial tissue (1.2 Hz). Figure 5
Figure 1. Effect of NaTHIO (40mg/l) on the rabbit atrial conduction velocity. Superimposed electrical waves recorded on theendocadial surface by displacing a surface roving electrode at constant steps. A: control (step=0.6mm); B: test solution (Tyrode +NaTHIO 40mg/l, step=0.5mm); Atrial impulse velocities were determined (C) by the steepness of regression lines. NaTHIO reducedthe impulse velocity about 23 percent from 73cm/sec control (triangles, r2=0.99) to 56cm/sec (squares, r2=0.99, p<0.001). Theexperiment was carried out on paced atrium (2Hz, 34±0.1°C; Horizontal bar: 2msec).
Figure 2. Negative chronotropic effect produced by NaTHIO (40mg/l) on therabbit atrial pacemaker (n=13 atria). This effect ranged from 16 to 47 percentof the control rate. Experiments were performed on spontaneous beating atria(34±0.1°C, p<0.001). Electrophysiological effects of sodium tiopental
Figure 3. NaTHIO (40mg/l) induces asystole (interruption of the fitting line) in the rabbit right atrium. The asystole wasobserved in 4 of 13 rabbit atria assayed but bradycardia was present in all of them. This negative chronotropic effect couldnot be prevented by atropine sulfate (1mg/l) applied 20 minutes before and during the barbiturate action (2Hz, 34±0.1°C).
Figure 4. Effect of NaTHIO (40mg/l) on the quiescent rabbit atrial cell (initialresting potential=81mV). Downward and upward arrows mark when NaTHIOwas added or removed, respectively, from the organ bath. Note that it producedsmall depolarizations (3.46±1.2mV). This effect was not completely abolishedduring washout (34±0.1°C).
Figure 5. Effect of NaTHIO (40mg/l) on the resting potential of a rabbit atrialpaced cell (1.2Hz). Thiobarbiturate depolarized the cell (downward arrows). Similar result was seen in other 5 atria (3.1±0.5mV, 34±0.1°C). Upward arrowsstand for the washout.
shows intracellular records obtained from a stimulated
atrium. The amplitude of action potentials is truncated
due to the high gain needed for monitoring resting
potential. Up and down arrows indicate, respectively,
when NaTHIO (40 mg/l) was added to or removed from
the bath. Note that NaTHIO induced depolarization.
Similar results were obtained from five other atria (n =
12 trials) in which NaTHIO depolarized atrial cells 3.1 ±
0.5 mV (p < 0.001). Nevertheless, in contrast to the
quiescent myocardium, recovery of control resting
potential was faster in the paced atrium, and residual
4. Effects of sodium thiopental on the morphology of
the propagated action potential in atrial tissue
Figure 6 shows superimposed traces of action
potentials that were randomly obtained from atrial cells
located in a previously selected myocardium area
(Zoom Stereo Microscope, model SZ-III, Olympus
Optical Co., Ltd., Tokyo, Japan, ocular with embedded
Figure 6. Superimposed traces of propagated action potentialsobtained in different cell of the atrial endocardial suface. A: six
reticle). Control action potentials are seen in the Figure
action potentials recorded on control solution. All of them
6A, and Figure 6B presents action potentials recorded
showed a well-developed fast component (depolarization phase). B: four action potential were recorded in the same atrial area,
when NaTHIO (20 mg/l) was added to the external
but after adding NaTHIO (40mg/l). Under the barbiturateaction, several action potentials showed depressed fast
medium. Arrows indicate the maximum amplitude of
components (arrows), suggesting a partial inhibition of the
the fast component of the myocardial action potentials.
sodium current (34±0.1°C, stimuli: 30mV, 1ms, 2Hz).
In the presence of NaTHIO, the amplitude of the fast
component was reduced and was exceeded by that of
the slow component (Paes de Carvalho et al., 1966,
5. Effects of sodium thiopental on the cellular refractory
Figure 7 shows action potentials obtained from
an atrial cell. The upper panel presents the control, and
the lower panel depicts the effect of NaTHIO (40 mg/l)
on the cellular refractory period. Extrasystolic stimuli,
applied at different coupling intervals (between normal
Figure 7. Effect of NaTHIO (40mg/l) on the cellular refractory
and premature stimuli), permitted measurement of the
period determined by applying extrasystolic stimuli withdifferent coupling interval. Upper panel: control (refractory
cellular refractory period. In the control, the refractory
period equal to 73msec); lower panel: test with 40mg/l of NaTHIO
period was 73 msec, but it increased to 127 msec when
(refractory period equal to 127msec). Experiment carried outat 34±0.1°C. Calibration bars: 20mV (vertical), 20msec (hori-
NaTHIO was added to the bath. Similar results were
Electrophysiological effects of sodium tiopental
obtained in fifteen other cells (n = 4 atria; control : 52 ±
patients anesthetized with NaTHIO. However, in the
5 msec; test: 117 ± 8 msec; p < 0.001).
isolated rabbit right atrium, this barbiturate promoted
bradycardia, sometimes followed by an asystole. The
tachycardic effect related by these authors could be
In spite of the development of new hypnotic
explained by the depression of myocardial contractility,
agents, NaTHIO is still employed as an anesthetic in
given that NaTHIO is known to reduce inward calcium
many experimental procedures with animals. This paper
flow in myocardial cells (Komai & Rusy, 1991, 1994a,
contributes to the understanding of its effects on the
1994b; Park & Lynch, 1992; Housmans et al., 1995;
mammalian myocardium. The results showed that
Bettens et al., 1996; Descorps et al., 2001). This entry
NaTHIO promotes arrhythmogenic effects similar to
of calcium ions is important for the promotion of
electrical wave front fragmentation, reduction of the
calcium-induced calcium release in the myocardial cells
fast component of action potential amplitude, and
(Sitsapesan & Williams, 1994; Bassani et al., 1995;
cellular depolarization. On the other hand, the increase
López-López et al., 1995; Sipido et al., 1998; Wier &
of the refractory period may have an anti-
Balke, 1999; Shannon et al., 2000; Bers, 2002) and
arrhythmogenic effect associated with NaTHIO.
initiatiation of the contractile process. Depression of
myocardial contractility would lead to a decrease in
electrical changes in the myocardium that can promote
arterial blood pressure, triggering a reflex response from
and sustain cardiac arrhythmias. This is because it
the aortic pressure baroreceptors. Under such
reduces the myocardium impulse velocity and
conditions, the sympathetic tonus of the heart would
simultaneously disorganizes the propagated wave front
be enhanced and the cardiac rate increased. The
of the electrical impulse. This could facilitate the
depressor effect of NaTHIO on the atrial pacemaker
establishment of a chaotic state in electrical wave
cells does not seem to be mediated by release of
propagation. It is important to note that the irregular
acetylcholine from the parasympathetic nervous
morphology observed in the surface records during
endings, given that the muscarinic blockade with
NaTHIO action (Figure 1B) represents a form of wave
atropine sulfate did not have any effect.
front fragmentation that is a consequence of micro-
accelerations and micro-deaccelerations of the
myocardial performance by acting on the ionic cellular
propagated impulse. This facilitates re-entry
currents responsible for electrogenesis in the cardiac
mechanisms in the myocardium tissue, leading to the
tissue. To study this, resting potentials from quiescent
appearance of cardiac arrhythmia. The thiobarbiturate
and non-quiescent myocardial cells were measured
also decreased impulse propagation velocity, probably
(Figs. 4 and 5). In both cases, thiopental depolarized
due to the decrease of the fast sodium currents that are
the myocardium. It is known that the resting potential
responsible for the depolarization phase of propagated
is maintained by a complex balance between
action potentials. This effect became clear because
depolarizing currents, which are mainly carried by
NaTHIO reduced clearly the fast component of the
sodium and calcium ions, and hyperpolarizing currents,
myocardial action potential (Figure 6B). Similar sodium
carried by potassium ions. The depolarizing effect of
current inhibition has been recorded for sodium
thiopental should thus be due either to an increase of
pentobarbital (Wartenberg et al., 2001) – a close
the inward sodium-calcium current or to a decrease of
the ouward potassium current. The depressor effect of
Becker & Tonnesen (1978) observed a cardiac
NaTHIO on potassium channels was described recently
rate increase during the sleep induction phase in
(Pancrazio et al., 1993; Carnes et al., 1997; Martynyuk
et al., 1999). In fact, NaTHIO, as demonstrated elegantly
of the pro- and anti-arrhythmogenic effects that are
by Heath & Terrar (1996), is a selective blocker of the
K potassium channels (a subtype of the delayed
Acknowledgements ELETROBRÁS - Centrais Elétricas
potassium channel that is not sensitive to sotalol, a
Brasileiras (Process Number 23113.009351/03-67) FAP-SE,
beta-adrenergic agonist). However, this channel is not
FUNTEC FNS-MS - Fundação de Apoio à Pesquisa do Estado de
involved in diastolic depolarization and thus does not
Sergipe, Fundo Nacional de Saúde do Ministério da Saúde,
appear to be related to the chronotropic effects of
Brasília/DF, Brazil (Process Number 01/2003) CNPq –Brazilian
thiopental. It remains to be understood whether
Research Council, Ministério da Ciência e Tecnologia, Federal
NaTHIO also enhances the inward rectifier conductance
of the potassium channel (K ) or reduces the slow
inward sodium and calcium currents during the
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Lightweight Lithium Ion starter battery, 1.98 lb / 900 gram, specially developed for motorcycles, jet-skis, snowmobiles, ATVs This newly developed battery is designed to replace the much heavier 10 to 12 Ah lead/acid battery. The super B 5200 is based on the safe Lithium Iron Phosphate technology (LiFePO4), better known as Lithium Ion. The super B 5200 delivers up to 300 Cranking Amps. The sup