Effect of aminophylline and verapamil ... force generation in dogs

Eur Respir J
1990, 3, 456--462
Effect of aminophylline and verapamil upon diaphragmatic
force generation in dogs
E.L. De Vito, A.J. Roncoroni
Effect of aminophylline and verapamil upon diaphragmatic force generation
in dogs. EL. De Vito, AJ. Roncoroni.
ABSTRACT: The effects on the diaphragm of verapamll (VPM) and
aminophylline (AMP) were studied In dogs during stimulation of phrenic
nerves at the 5th cervical roots (Sth-PS) or transvenously at tbe trunk
(T-PS). Transdlaphragmatlc pressure (Pdi)/frequency curves were constructed. Our findings were: 1) AMP Increased Pdl at all stimulation
frequencies (p<O.Ol) during Sth-PS but only at 10-30 Hz during T-PS;
2) In other dogs infusion of VPM (0.14 mg·kg- 1·mln'1) decreased Pdl at all
frequencies (p<0.025) without change In dlap bragmatlc blood flow; 3) the
effects of VPM were completely reverted by AMP; 4) after a larger amount
of AMP, Infusion of VPM (0.21 mg·kg' 1·mln' 1) decreased Pdl at all
frequencies. Since these drugs have several mechanisms of action and do
not show mutual blocking effect, different action sites are suggested.
Eur Respir J., 1990, 3, 456-462.
Aminophylline (AMP) is one of the few drugs used for
treatment of chronic obstructive pulmonary disease
(COPD). In the past, a stimulatory effect upon the central
respiratory controllers and bronchodilation were the
favoured mechanisms of its action. Since the establishment of its beneficial effects upon force developed by
the fresh or fatigued diaphragm its use has increased,
as inspiratory muscle fatigue may compromise further
ventilatory incapacity in COPD. The mechanism of this
effect remains, however, unclear. In isolated rat hemidiaphragm V ARAGIC and KENTERA [1] found that AMP
increases twitch tension and maximal velocity of tension
increase (dp/dt) max, an effect markedly dependent
upon Ca++ concentration in the bath. This effect was not
observed when Ca++ was absent or verapamil (VPM) was
added. Similarly, AUBIER et al. [2] showed that AMP
infusion in dogs increased tension developed by fresh
diaphragms stimulated in situ at 20 Hz. This effect was
not obtained under previous VPM infusion. Other
investigators [3] also working with diaphragms of dogs
in situ, stimulated through the nerves at 1-40Hz, did not
find that the beneficial effe.cts of AMP upon muscle contractility were aCfected by VPM infusion. Therefore, they
concluded that AMP aclion upon the diaphragm does not
require normal operation of calcium channels. Recent
evidence [4, 5] suggests that AMP stimulates the Na-K
pump.
Our study was conducted to investigate lhe effects and
interaction of AMP and VPM upon diaphragmatic
response to phrenic stimulation by single twitch and
lnstituto de lnvestigaciones Medicas "Alfredo Lanari",
Universidad de Buenos Aires, Av. Dooato Alvarez
3150, 1427 Buenos Aires, Argentina.
Correspondence: Dr A.J. Roncoroni, lstituto de
lnvestigaciones Medicas "Alfredo l..anari", Universidad
de Buenos Aires, Av. Donate Alvarez 3150, 1427
Buenos Aires, Argentina.
Keywords: Aminophylline; diaphragmatic force
generation; Pdi/frequency curve; verapamil.
Received: August 8, 1988; accepted after revision
August 28, 1989.
tetanic pulses at several frequencies. The drugs were used
singly and combined at two different dosages. Sirce both
are vasoactive drugs diaphragmatic blood flow (Qdi) was
measured to evaluate possible changes. The study was
intended to clarify the previously quoted [2, 3] converse
results also obtained, in dog diaphragm in situ, with
similar methods.
Materials and methods
Thirty dogs from 7.5-30 kg body weight (BW) were
anaesthetized with sodium pentobarbilal (20 mg·kg-1) i.v.,
intubated and mechanically ventilaLcd during the
surgical procedure. Anaesthesia was maintained with
constant pentobarbital venous infusion with a Harvard
pump. The femoral artery was catheterized for blood
pressure monitoring and sampling. Rectal temperature
was monitored and maintained constant.
Oesophageal and gastric pressures were measured by
latex catheters and balloons (5 cm long) placed in the
middle third of the oesophagus and in the stomach.
Each catheter was connected to one side of a differential
pressure manometer (Validyne MP-45) to obtain transdiaphragmatic pressure (Pdi). They were also connected to
two similar differential pressure transducers recording
transpulmonary pressure (Ptp, oesophageal vs tracheal)
and gastric pressure (Pga) against atmospheric.
Phrenic nerve stimulation was carried out: a) in 18
dogs at the 5lh cervical roots (5lh-PS); and b) in 12 dogs
AMINOPHYLLINE, VERAPAMlL AND DIAPHRAGMATIC FORCE GENERATION
at the intrathoracic nerve trunks (T -PS). Both phrenic
nerves at the fifth cervical root (Sth-PS) were isolated
and platinum e lectrodes under oil were placed and
protected with gauze pads. Fish hooks were placed,
through superior laparotomy, in the anterior muscle part
of both diaphragms to record muscle potential (EMG).
The surgical incision was carefully closed and residual
air aspirated.
The nerves were stimulated by 0.1 ms pulses during
about 2 sat frequencies of 10, 30, 60, 100Hz while Pdi
was being measured. After the voltage provoking
maximal Pili at 100 Hz was determined, current intensity
was raised 10-20% (usually to 10-12.5 volts) to ensure
supramaximal stimulation of the muscle innervated by
the 5th cervical roots. Single twitch Pdi were also
measured with supramaximal stimuli of 0.1 ms duration.
Peak twitch tension (PTI), maximal velocity of tension
increase (dp/dt max) and re laxation speed (MxRx) were
measured. Since changes in EMG were minimal during
tetanus it was a ssumed that s timulating electrode
position was maintained. The study was conducted
during spontaneous breathing and positive corneal
reflex.
It is well known that the Pdi developed depends on the
initial d.iaphragmatic length. This is influenced by changes
in the lung volume and abdominal pressure under the
effect of the abdominal muscle's state of contraction.
Electrical stimulation was applied at functional residual
capac ity (FRC) with the airway occluded. The constancy
of FRC was evaluated by Ptp level at end expiration and
abdominal muscle contraction by monitoring Pga. Both
Ptp and Pga were stable before stimulation. Since only
the 5th cervical roots were stimulated, diaphragmatic
shortening was limited to anterior parts so that stimulation was submaximal, in relation to the whole muscle.
The consistency of diaphragmatic activation (Edi)
ensured that the responses after drug administration were
corn parable.
In 12 d ogs, bil a te ral T-PS was obtained by
introducing 2 catheters into the internal left jugular
vein and advancing them to the poim of maximal Pdi
response at 100Hz [6). To register spontaneous electrical activity of the anterior rectus abdominis fish hooks
were placed through a small incision. A cast was placed
covering the lower thorax and abdomen.
In 6 other dogs, the jugular vein was catheterized and
a Swan-Ganz catheter placed in the pulmonary artery
uunk. A Cordis shepherd hook catheter was advanced,
through tl1e femoral vein under fluoroscopy, into the left
diaphragmatic vein. Cardiac output (CO) was measured
by thermodilution. Left hcmidiaphragmatic blood flow
was measured by blood collection in a calibrated vessel.
In order to compensate for catheter resistance internal
diameter (ID): 1.2 mm) a negative pressure was
produced placing the catheter tip 45 cm below the
horizontal plane via the dog's back. The means of three
measurements, with a variation coefficient less than 10%,
were used for calculation of the two variables.
Diaphragm blood flow (Qdi) was reproducible. Specific
vascular diaphragmatic conductance (VCdi) was estimated
by the following equation:
457
Qdi ml·min·1·g·'Jmean arterial pressure (MAP) mmHg
At the end of the experiment the venous catheter
position was confirmed by fluoroscopy and india ink was
injected tl1rough it. At post mone m examination it was
observed Lhat the whole le ft hemid iapluagm was
coloured. The muscle was carefully d issected and
weighed with ~d without the central tendon in order
to ca lculate Qdi per gram of muscle. The result
obtained, 4.91±0.16% BW, is similar to published
results [7].
Protocol 1
In 12 dogs, 6 (Sth-PS) and 6 (T-PS), AMP was
injected intravenously in a bolus dose of 15 mg·kg-', after
basal stimulated Pdi determinations. Twenty minutes later
Pdi/Frequency curves were obtained.
Protocol 2
In 18 dogs, a continuous venous infusion of VPM
(0.14 mg·kg·•·min·1) was started after basal measurements
of Pdi. When mean arterial pressure (MAP) and heart
rate (HR) were about half of basal values during 15 m in,
Pdi measurements were repeated. Immediately afterwards,
AMP was given in a bolus injection of IS mg·kg-1• After
20 min samples for AMP blood levels were obtained and
new Pdi measurements carried out while VPM infusion
was continued. When VPM infusion was stopped HR
recovered basal values but MAP remained low. In 12
dogs two hours later, AMP bolus injection was repeated
in the same amoun t ( AMP 2) and new Pdi values
(Sth-PS) and AMP plasma levels were obtained 20 min
later. Infusion of VPM was reinstated, now at 0.21
mg·kg·• ·min ·•, and after a fall of HR new Pdi
measurements were carried out
Protocol 3
In 6 dogs, CO and Qdi were measured before and after
VPM and AMP infusions, reproducing the procedure
described in Protocol 2 with only the first dosage
schedule.
The animals were studied in supine position breathing
air, with normal arterial oxygen tension (Pao2 ) and
acid-base levels. Each Pdi is the mean of the three
measurements. All variables were recorded in Physiograph
MK-IV P and abdominal muscle EMG also amplified on
audio-system.
Results are shown as mean±standard error of mean
(sEM). Statistical analysis was performed by Student's test
for paired data or analysis of variance when appropriate.
Results
With 5th-PS, basal Pd.i at 100Hz was 26.8±1.5 cmH.p
while with T-PS the corresponding value was 73.8±2.9
cmH.p, similar to published results [8, 9].
458
E.L. DE YITO, A.J. RONCORONI
Table 1. - Effect of AMP upon Pdi/frequency relation of the fresh diaphragm
T-PS
Sth-PS
Hz
10
30
60
100
10/60
Basal
AMP
p<
21.2±2.7
80.8±3.0
94.7±1.3
100
0.22±0.03
27.9±2.8
98.8±4.2
110.3±4.3
116.9±4.3
0.25±0.03
0.025
0.025
0.01
0.01
0.05
Basal
19.7±3.4
81.1±2.3
92.9±1.4
100
0.21±0.04
AMP
p<
25.5±4.3
84.2±2.3
95.7±0.9
101.7±1.5
0.27±0.04
0.0025
0.05
NS
NS
0.01
Pdi values expressed as % basal Pdi at 100 Hz. Results are shown as mean±sEM. Ns: not
significant; AMP: aminophylline; Sth-PS: stimulation of phrenic nerves at 5th cervical roots; T-PS:
stimulation of phrenic nerve at the trunk.
Table 2. - Effects of VPM 1 and AMP 1 upon Pdilfrequency relations
5th-PS
Hz
10
30
60
100
10/60
T-PS
Basal
VPMI
VPMI+AMPI
22.2±2.0
86.4±2.1
94.5±1.0
18.0±1.8
67.6±3.7
75.4±3.3
79.2±3.0
0.24±0.02
22.0±2.5
81.6±3.8
88.3±3.7
94.1±3.9
0.25±0.02
lOO
0.24±0.02
Basal
21.9±2.4
83.7±1.9
93.7±1.0
100
0.24±0.03
VPMI
VPMI+AMPI
14.3±1.6
71.0±1.8
77.2±2.0
77.7±2.4
0.19±0.02
20.9±2.4
87.4±1.9
93.5±2.0
95.8±2.2
0.22±0.03
Sth-PS: Basal vs VPM 1 p<0.0005; VPM 1 liS VPM1+AMP 1 p<0.025; Basal vs VPM 1+AMP1 Ns; 10/60 Hz Ns. T-PS:
Basal vs VPM 1 p<0.0025 ; VPM 1 liS VPM 1+AMP 1 p<0.0025; Basal vs VPM 1+AMP1 Ns; 10/60 Hz NS. Pdi values
expressed as% basal Pdi at 100 Hz. Results are shown as mean±:SHM. NS: not significant; VPM: veraparnil. For other
abbreviations see legend to table 1.
Table 3. - Effect of VPM upon Pdilfrequency
relation of AMP pretreated dogs
Hz
10
30
60
100
10/60
AMP2
VPM2
28.7±2.1
89.1±1.3
97.7±1.5
100
0.29±0.02
24.3±2.4
76.4±4.4
83.6±4.8
83.9±4.1
0.29±0.02
p<
0.01
0.0025
0.0025
0.0025
NS
Pdi value expressed as % basal Pdi at 100 Hz.
Results are shown as mean±sHM. For abbreviations
see legends to tables 1 and 2.
Protocol I
The Pdi/frequency curves obtained after AMP
injection (plasma level: 19.0±1.8 mg·l·1) with 5th-PS
technique showed a significant increase at all stimulation
frequencies (p<0.01) (table 1). The relation at 10/60 Hz
increased from 0.22±0.03 to 0.25±0.03 (p<0.05).
However, during T-PS, Pdi increased only at 10-30Hz
(table 1). Peak twitch tension speed of tension
development and relaxation rate were increased by AMP
to 117.0±6.3 (p<0.025), 121.4±8.6 (p<0.05) and
126.2±10.1 (p<0.05) % of basal, respectively.
Protocol 2
Infusion of VPM 1 (5th-PS) induced a markedly
significant fall in MAP from 135.0±4.3 to 77 .5±3.8 mmHg
(p<0.0005) unchanged by AMP1 or AMP2• After VPM2
infusion, MAP decreased to 55.5±5.0 mmHg (p<0.0005).
Basal HR was 179±11.6 beats·min·1 , decreased to 102±7.2
beats·min·1 (p<0.0005) during VPM1 and remained unchanged after AMP1 injection. With higher AMP concentration HR was 167±11.9 beats·min· 1 (similar to basal)
and fell to 119±9.1 beats·min· 1 (p<0.0005) during VP~
infusion. Similar results were found during T-PS.
Infusion of VPM 1 (5th-PS) decreased Pdi at all
stimulation frequencies (p<0.0005) while AMP 1
injection (plasma level: 19.4±4.2 mg·l- 1) completely
reverted this effect (p<0.0025) so that Pdi/frequency
curves were no different from basal (fig. 1). Neither VPM1
nor AMP1 changed the 10/60 Hz relation (table 2). Results
after T-PS were similar (table 2). After injection of AMP2
(plasma level: 35.4±6.8 mg·l-1) VP~ infusion decreased
Pdi at all frequencies (p<0.05) (fig. 2). The Pdi relations
at 10/60 Hz were not modified by AMP2 or VP~. Peak
twitch tension and speed of tension development were
decreased by VPM 1 to 85.3±5.7% (p<0.05) and
84.6±3.5% (p<0.01) of basal while relaxation was not
modified. After AMP 1 these changes were completely
reverted so that basal values were recovered (table 4).
With T-PS, peak twitch tension (PTT), dp/dt max and
459
AMINOPHYLLINE, VERAPAMIL AND DIAPHRAGMATIC FORCE GENERATION
100
100
n:12
····· .... ············t
1
··········
80
/
(---
-- --
~
0
~
• I
60
E
/ I
:: I
'
:'1
:,
40
.,ft
I
I
I
I
I
-AMP
----VPM
I
40
I
I
I
:,
20
I
;:::.
~
---~
p<O.Ot
I
60
~
-BASAL
"·······AMP+VMP
••••VMP
:I
...... -r-----,r--,
80
------------1
... -t
n=12
I
/I
20
10
30
100
60
1
10
30
Hz
100
60
Hz
Fig. 1. - Effects of VPM and AMP upon Pdi/frequency curves (SthPS). Plasma level of AMP 1: 19.4±4.2 mg·t-', VPM 1 infusion rate: 0.14
mg·kg· 1·min·1•
Fig. 2. - Effects of VPM and AMP upon Pdi/frequency curves (SthPS). Plasma level of AMP1 : 35.4±6.8 mg·l-1, VPM. infusion rate: 0.21
mg·kg·'·min·•.
Table 4. - Effects of VPM upon Pdi single twitch
T-PS
5th-PS
PTT
dp/dt max
MxRx
VPMI
AMPI+VPMI
p
VPMI
AMPI+VPMI
p
85.3±5.7
84.6±3.5
82.8±9.4
99.1±5.8
103.8±4.5
96.8±11.6
<0.05
<0.05
NS
82.2±5.8
85.3±5.6
90.8±8.9
119.0±5.9
127.7±12.4
128.6±11.8
<0.0005
<0.0025
<0.01
PTT: peak twitch tension; dp/dt max: maximal speed of Pdi generation; MxRx: maximal relaxation
speed. Values are expressed as % of basal. Results are as mean±SilM. For other abbreviations see
legends to tables 1 and 2.
MxRx were decreased by VPM1 (p<0.025) while AMP1
provoked an increase over basal values (table 4).
Table 5. - Haemodynamic data (Protocol 3)
Basal
CO
146±27
ml·kg· 1-min·1
MAP
151.5±15.4
mmHg
HR
177.2±21.7
beats·min· 1
Qdi
0.12±0.01
ml·min·1·g·1
VCdi
8.5±1 .6
ml·min·1·g·1·mmHg·1
150±52
130±25
87.4±10.6
72.1±6.7
83.3±11.0
63 .2±8.6
0.13±0.02
0.15±0.03
15.9±2.9
21.7±4.5
X 1Q-4
Results are shown as mean±sllM. CO: ~ardiac output; MAP:
mean arterial pressure; HR: heart rate; Qdi: diaphragm blood
flow; VCdi: vascular/diaphragmatic conductance. For other
abbreviations see legends to tables 1 and 2. Statistical analysis .
MAP: B vs VPMI p<0.0025; HR: B vs VPMI p<0.0025; B VS
VPMI+AMPI p<0.05.VCdi: B vs VPMI p<0.0125; VPMI VS
VPM 1+AMP1 p<0.025. All others NS.
Protocol 3
Basal CO (variance analysis: F=0.08) did not change
during VPM infusion or after AMP (table 5). Mean arterial
blood pressure showed similar changes to those
observed in Protocol2, i.e. the decrease induced by VPM
was not corrected by AMP. Mean basal specific Qdi,
0.12±0.01 ml·min · 1 ·g · 1 coincided with results
obtained collecting venous outflow fro'!l diaphragmatic
muscle strips [10]. Control Qdi was not
significantly modified by VPM or AMP (variance
analysis: F=0.46) (table 5). Mean control specific VCdi
was 8.5±1.6 ml·min· 1 ·g· 1 -mmHg· 1x10· 4 • Basal VCdi
was increased by VPM (p<0.0125) and further
more by AMP (p<0.025). Pressure/VCdi curve
showed a slope of -0.14±0.04 ml -min· 1·mmHg· 1
(r=0.69, p<O.Ol).
460
E.L. DB VITO, A.J. RONCORONI
Discussion
Evaluation of the method
Force developed by the diaphragm under tetanus or
single twitch stimulus was estimated by Pdi determinations. Before accepting modification in diaphragmatic
contractility (DC) as the cause of Pdi alterations,
possible changes in lung volume or chest-abdomen
configuration should be considered, otherwise different
muscle fibre length may be the reason for Pdi changes.
Electrical stimulation at the 5th phrenic roots was carried
out at end-expiratory level (FRC) with constant recording of transpulmonary pressure (Ptp) which did not change
more than 2 cmHp during the experiment. Expiratory
gastric pressure (Pga) just before stimulation did not
change more than 2 cmHp either. We believe that
abdominal muscle contraction, if present, was probably
insufficient to modify diaphragm length. Partial
contraction of the muscle may result from this technique,
presumably inducing lengthening of unstimulated
portions. In order to clarify some of these problems we
performed T-PS, restricted lower thoracic and abdominal
expansion with a cast and recorded EMG of the rectus
abdominis. In these conditions Pdi at 100 Hz was much
higher and similar to published values [9] reflecting
complete diaphragmatic stimulation. Since Pga and Pdi
at end-expiration were unchanged and the EMG of rectus
abdominis muscle was silent during expiration, we
excluded significant diaphragm lengthening after AMP.
Relaxing effects of AMP on the diaphragm have not, to
our knowledge, been described. Conversely increase in
resting tension was found above 60 mg·/·1 concentration
in isolated fibre preparations [11].
Shortening, which decreases Pdi generation, is reduced
but not suppressed by the cast [12]. For that reason the
Pdi obtained are the result of the contractile properties of
the muscle and the force-length characteristics.
Aminophylline effects
After AMP with 5th-PS, Pdi increased at all frequencies while with T-PS the increase was limited to 10-30
Hz (Protocol 1). We believe this difference may be
attributed to submaximal stimulation with the 5th-PS
technique [13). With the latter procedure the increase at
10 Hz was 3 1.6% and with T-PS, 29.8% (table 1).
Incrcnse in force generation (FG) after AMP has been reported in similar experimental models. At 24 mg·/·1 AMP
plasma level single twitch increased 21% and at 20Hz,
29% [3]; at 20-30 mg·/·1 Pdi at 10Hz increased 31.3%
[8].
There is general agreement that action of AMP upon
con tractility is due to a direct action on muscle [1 I, 14];
the mechanism is, however, unclear. Several in vitro
studies were addressed to c larify this point. fn rat
diaphragm, AMP action was dependem on optimal Ca..
concentration and functionally operative Cat+ channels
since VPM (4.5 mol·l·1) blocked its eiTccts. TL was thought
that CaH channel influx was facilitated by AMP. This
effect, however, was obtained at AMP concentrations
(0.2-2 mmol·i- 1=36- 180 mg·/· 1) much higher than
therapeutic levels [1]. Recently, in isolated diaphragmatic
fibres, an increase in peak twitch tension was found
[11) at therapeutic concentration levels (15 mg·l·1). Since
provoked hypocalcaemia decreases Pdi/frequency curves
[15] and VPM prevents AMP enhancemem of contractility (2] it was accepted that AMP effects are dependent
on ea++ influx.
In isolated hamster diaphragmatic strips, AMP at
100 mg·/· 1 increased tension and resting membrane
potential. This hyperpolarization was attributed to
increased K+ influx and Na+ extrusion [4, 5]. It is
possible that the latter effect is ea+• dependent.
Verapamil effects
Decrease in FG during VPM infusion (0.14
mg·kg·1·min·1) is in agreement with peak twitch tension
and dp/dt max decrease observed in isolated rat diaphragm
[1]. A slight decrease in Pdi at 20Hz with VPM infusion
(0.1 mg·kg·1·min·1) was reported by AUBIER et al. [2).
Since the animals had received AMP previously and VPM
infusion rate was lower than ours, it is possible to explain the more marked effect obtained in our dogs. Since
we induced a marked decrease in arterial pressure and
heart rate the possible lowering effect on cardiac output
should be ascertained. Skeletal muscle force depends on
adequate energy provision and muscle blood flow is
relatyd to cardiac output [7]. Neither this last variable
nor Qdi were changed by VPM or AMP. On the other
hand, a marked decrease in systemic arterial resistance
and increase in VCdi was provoked by VPM, an effect
enhanced by Al\.1P. The slope of the inverse correlation
between MAP and VCdi coincides with that obtained
with an electromagnetic now probe in .the phrenic artery
[16] and suggests Lhe existence of v!lscular autoregulation. Changes in FG dependent on Qdi were ruled out
(table 6) and, anyway, they would influence endurance
rather than response to short isolated tetanus.
Recently, VPM in isolated diaphragm of hamsters was
shown to decrease tension and resting membrane potential [4]. Slow inward ea++ currents of sarcolemma are
inhibited by VPM, without affecting sarcoplasmic
reticulum ea•+ channels in the heart [17). Both calcium
influx and efflux are decreased. Extracellular Ca++ levels
have a competitive effect on this action and activation of
unblocked channels does not seem to be affected [18].
Effects of VPM, however, are not limited to blocking
of calcium channels. This drug induces a local anestheticlilce action in frog sartorius muscle, decrease in Na• and
K• conductance and membrane excitabi lity [19]. In
isolated rat soleus muscle, VPM (0.1 mmol) gradually
reduces action potential amplitude during repetitive
stimulation, more evidcmly at high frequencies [20j. In
isolated hamster diaphragm VPM decreased resting
membrane potential [4]. Fall in Pdi at all stimulation
frequencies with preservation of the 10/60 Hz relation,
which we found during VPM infusion, suggests global
depression in contractility. This is perhaps the result of
AMINOPHYLLINE , VERAPAMIL AND DIAPHRAGMATIC FORCE GENERAT ION
local anaesthesia, instead of a selective action upon slow
Ca++ channels, in which case only low frequency fatigue
should be expected. The depressing effect upon twitch
tension and speed of force development which we found,
cannot be explained by slow CaH channels inactivation,
considering the shorter contraction Lime [21]. These effects
may depend on depression of Na• and K• currents of the
muscle cellular membrane [20].
Combined effects of AMP and VPM
In our dogs AMP completely reverted VPM effects
upon FG aL low and high stimulation frequencies and
single twitch. These results differ from those of VARAGIC
and KENTERA [ 11 with single twitch and AuumR et al. [2)
during tetanic stimulation only at 20 Hz. On the other
hand, in dogs prctremed with AMP aL high plasma levels,
it was also necessary for a higher VPM infusion rate in
order to decrease FG . These results are opposed to those
of Dr MARco et al. [3] but they used a much lower VPM
infusion rate (0.02 mg·kg·1·min·1). In isolated hamster
diaphragm, when both VPM and AMP were present in
the bath, no changes were found in resting membrane
potential and maximal tetanic tension at 100 Hz [4).
The action site of the drugs we used is incompletely
known and action mechanisms cannot be explored by
were present in the bath, no changes were found in resting
membrane potential and maximal tetanic tension at 100
Hz [4].
The action site of the drugs we used is incompletely
known and action mechanisms cannot be explored by
were present in the bath, no changes were found in resting
membrane potential and maximal tetanic tension at 100
Hz [4].
The action site of the drugs we used is incompletely
known and action mechanisms cannot be explored by
studies of this type. Effects of VPM upon muscle may be
attributed to calcium channel blocking or
anaesthetic-like action. On the other hand, AMP effects
apparently depend on increased calcium inflow or
calcium-dependent increased K• inflow provoking
hyperpolarization. This last effect has only been observed
in vitro at very high supratherapeutic levels for humans
[4]. Our results could be explained by adequate
combination of the previous three possible mechanisms.
Acknowledgments: The writers wish to
thank M. Rodrfguez and A. Brentta for
technical assistance and N. Montoya for
preparation of typescript.
References
1. Varagic VM, Kentera D. - Interactions of calcium,
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Ejfets de l' amirwphylline et du verapamil sur la production de
force diaphragmatique ·chez les chiens. E.L. De Vito, A.J.
Roncoroni.
RESUME:Les effets du verapamil (VPM) et de
!'aminophylline (AMP) sur le diaphragme ont ete eiudies chez
462
E .L. OI! VITO , A.J. RONCORONI
le chiens au cours de la s~imulation des nerfs phreniques au
nivcau des 5emes racines cervicales (5th-PS) ou par voie
transveineuse au niveau du lronc (T-PS). L'on a conslruit des
courbes pression transdiaphragmatique (Pdi) sur frequence. Nos
observations sont les s uivantes: 1) !'aminophylline
augmente Pdi Atoutes les frequences de stimulation (p<O.O I) si
!'on stirnule 5lh-PS mais uniquement a la frequence de 10 a 30
Hz pour la stimulation T -PS; 2; chcz d'autrcs chicns,
l'infusion de VPM (0.14 mg·kg· 1 ·min-1 ) diminue Pdi a
toutes !es frequences (p<0.025) sans modifier le debit
sanguin diaphragmatique; 3) Les effets de VPM sont
completement armihiMs par AMP; 4) apres administration d'une
quantite plus elevee d'AMP, !'injection de VPM (0.21
mg·kg·1·min·1) diminue Pdi a toutes les frequences. Le fait que
ces medicaments aient plusieurs mecanismes d'action et
n'entrainent aucun effet bloquant mutuel suggere des sites
d'action differents.
Eur Respir J., 1990, 3, 456-462.