804
Adaptation to Prolonged /^-Blockade of
Rabbit Atrial, Purkinje, and Ventricular
Potentials, and of Papillary Muscle
Contraction
Time-Course of Development of and Recovery from
Adaptation
A.E.G. RAINE AND E.M. VAUGHAN WILLIAMS
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SUMMARY Groups of littermate rabbits were treated for various periods up to 6 weeks with twice
daily subcutaneous injections of saline, D-propranolol, DL-propranolol, or metoprolol, the latter two at
doses equivalent to those used in clinical therapy. Investigations were made at a sufficient time (20-24
hours after the most recent dose) to ensure that the drugs would have been eliminated from the body,
so that any observed changes would represent an adaptation to treatment, not effects due to the
presence of the drugs. The ECG was recorded in vivo at regular intervals during treatment. After
several days, Q-Tc was prolonged by the /J-blockers, reaching a peak effect at about 3 weeks from the
start of treatment, and returned to control values at 3 weeks after cessation of treatment. Action
potential duration, measured in vitro by intracellular recording, was also prolonged uniformly in atria
and ventricles over a similar time-course, unrelated to cardiac frequency, but shortened in distal
Purkinje cells. Peak tension was not altered in propranolol-adapted papillary muscles, but the relationship of rate of rise of tension to peak tension was steeper. It is concluded that these effects represent
a myocardial adaptation to prolonged /^-blockade. Circ Res 48: 804-812, 1981
DESPITE the widespread clinical use of/3-adrenergic antagonists, many questions about their mode
of action remain unanswered. It is often difficult to
correlate their therapeutic action with acute yS-receptor blockade, and this has led to suggestions that
factors other than /S-adrenergic antagonism may be
important (Koch-Weser, 1975). An alternative possibility is that part of the beneficial effects of j3blockade may be slow in onset, developing after the
first week or two of treatment. It is well known, for
example, that the fall in vascular resistance which
may accompany and partly account for the hypotensive effect of propranolol is delayed.
Investigations in animals of the cellular effects of
prolonged /J-blockade were begun several years ago,
and it was shown that treatment of young rabbits
for several weeks with /?-adrenoceptor blocking
drugs at doses comparable with those used clinically
had a number of secondary effects which could be
distinguished from the acute actions of the drugs
(Vaughan Williams et al., 1975; Raine and Vaughan
Williams, 1980). Atrial transmembrane action po-
tential duration was greatly prolonged, a surprising
finding since, in acute experiments, /8-blockers
either have no effect on action potential duration
or shorten it, in both atrial (Papp and Vaughan
Williams, 1969) and ventricular muscle (Davis and
Temte, 1968). Chronic treatment of young rabbits
with /?-blockers also caused a reduction of heart
weight in relation to body weight and produced
some morphological changes, notably an increase in
the relative volume of vascular elements and interfibrillar fluid (Vaughan Williams et al., 1977).
In the light of this evidence that prolonged /?blockade might produce adaptational changes
within cardiac muscle, we have now studied changes
in action potential duration (APD) and in contractility during chronic treatment in atrial, ventricular
and Purkinje tissue. The time course of development of these changes and of their regression after
propranolol withdrawal has been investigated, and
has been found to parallel the time course of reduction in biochemical indices of sympathetic activity
during long-term ^-blockade, which we have also
previously reported (Raine and Chubb, 1977).
From the Oxford University Department of Pharmacology, Oxford,
England.
Dr. Raine: Rhodes Scholar. Current address: Nuffield Department of
Medicine, John Radcliffe Hospital, Oxford.
Address for reprints: Dr. E.M. Vaughan Williams, Oxford University,
Department of Pharmacology, South Parks Road, Oxford 0X1 3QT
England.
Received June 9, 1980; accepted for publication December 4, 1980.
Methods
Littermate rabbits of either sex weighing 1.5-2.0
kg were injected twice daily subcutaneously with
equal volumes of DL-propranolol, 4 mg/kg (n = 10);
D-propranolol, 4 mg/kg (n = 4); metoprolol, 6 mg/
ADAPTATION TO PROLONGED ^-BLOCKADE/flame and Vaughan Williams
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kg (n = 6); or saline, 1 ml/kg (n = 6), for a total of
24 days. To monitor electrophysiological changes in
vivo, electrocardiograms were recorded by limb
leads (fine needle electrodes) with a Devices ACl
preamplifier and DC5 pen recorder at a paper speed
of 100 mm/sec. All electrocardiograms were recorded 20 hours after the previous dose of drug, by
which time negligible amounts would remain in the
body, so that any observed changes could be attributed to a secondary adaptation to the prolonged
blockade and not to the blockade itself. Recordings
were made before treatment began, and on days 5,
9, 15, and 22 of treatment. RR, PR, QRS, and QT
intervals were measured and QTC derived from the
equation, Q-Tc = Q-T(sec)/VR-R (sec).
Twenty-four hours after the final dose, the animals were injected with 100 units of heparin iv,
stunned, and their hearts were removed rapidly.
The superior cervical ganglia were excised for separate biochemical analysis of tyrosine hydroxylase
and dopamine-/?-hydroxylase activity, as an index
of sympathetic function (Thoenen, 1972). The atria
were suspended horizontally in modified Locke's
solution gassed with 95% O2 and 5% CO2 at 32 °C
for recording of transmembrane potentials. The
solution had a composition of (mM) Na+ = 140, K+
= 5.6, Ca2+ = 2.17, HCO"3 = 25, Cl~ = 125, glucose
= 11, and pH = 7.4. A right ventricular papillary
muscle was also suspended in the tissue bath by a
silk loop tied around the chordae tendineae and a
second loop threaded through the base of the muscle. The atria were paced at a frequency 10% above
the spontaneous frequency and the papillary muscle
at a frequency of 1 Hz, by rectangular stimuli of
twice threshold strength 2 msec in duration. Action
potentials were recorded from atrial and ventricular
myocardium by 3M-KCl-filled glass microelectrodes connected to a negative capacitance amplifier (WP Instruments). The differentiated action
potential upstroke was also displayed.
In further studies, rabbits in groups of four were
injected with DL-propranolol, 4 mg/kg, twice daily
s.c. for 3, 6,12, or 24 days with one animal per group
from each of four litters. The remaining six littermates were killed on day zero, to serve as controls.
Animals were stunned 20-24 hours after the final
injection, the left and right superior cervical ganglia
were removed for separate biochemical analysis,
and the atria and papillary muscle were set up for
the recording of transmembrane potentials as described above. In addition, the effective refractory
period of the ventricular muscle was measured directly. This was defined as the minimum interval
(msec) between two stimuli Si and S2 for which the
second stimulus elicited a propagated action potential, during continuous impalement of a single papillary muscle cell.
Recording from Ventricular Conducting
System
Rabbits weighing 1.5-2.0 kg were injected either
with propranolol, 2 mg/kg, sc b.d., or saline, 1 ml/
805
kg, sc b.d., for a period of 6 weeks. Twenty-four
hours after the final dose, the animals were stunned,
the hearts removed, and the ventricles separated
from the atria. The right ventricular conducting
system was then dissected and set up in the tissue
bath for recording of intracellular potentials, as
described previously (Salako et al., 1976). The His
bundle was stimulated at a frequency of 1.0 Hz and
potentials were recorded at 12 successive intervals
measured in mm, along the His bundle, proximal
right bundle, and false tendon.
Action potentials were also recorded from right
ventricular endocardium and at intervals back
along the false tendon for 2-3 mm. A mean value
was obtained for APD in each area, defined by its
distance in mm either from the proximal His bundle
or from ventricular muscle.
Measurement of Contractions
Papillary muscles were suspended horizontally in
the tissue bath as described above. The base of the
muscle was fixed and the free end connected to an
RCA 5734 transducer mounted on a millimeter
vernier. Developed tension (T) and dT/dt were
displayed on a Tektronix D13 oscilloscope, and
length-tension curves obtained by increasing the
length in steps by adjustment of the vernier to Lmax,
the length at which developed tension was maximal.
Time-to-peak tension was also measured.
Results have been given as means ± SEM, and
statistical significance of differences calculated by
Student's f-test.
Results
Electrophysiological Effects of Chronic
Treatment with DL-Propranolol,
D-Propranolol, and Metoprolol
Groups of 4-6 rabbits were dosed for 24 days as
described. Twenty-four hours after the final injection, lack of residual ^-blocking effect was confirmed functionally by the absence of any reduction
in the chronotropic response to isoproterenol measured by ECG recording in vivo. In previous experiments it had been shown that, at this time, samples
of plasma and of ventricular tissue contained no
measurable concentration of propranolol (Raine
and Vaughan Williams, 1980). Although there was
no remaining acute drug effect, there were marked
changes in the transmembrane action potentials
recorded in vitro from cardiac muscle after chronic
treatment. Similar effects on the action potential
were seen in both atrial and ventricular muscle and
these are illustrated in Figure 1. In each experiment
records were obtained from 7-10 different areas and
the results for each treatment group were pooled
and analyzed statistically (Table 1). The notable
feature is that 24 days' treatment with both propranolol (which is nonselective) and metoprolol
(which is selective for /?i -receptors) caused a consistent (P < 0.001) prolongation of action potential
duration, in both atria (+23%) and ventricles
CIRCULATION RESEARCH
806
VENTRICLE
ATRIUM
VOL. 48, No. 6, JUNE 1981
(+25%). In contrast, cardiac action potential duration of animals treated with dextropropranolol was
similar to that of controls. Resting membrane potential and action potential amplitude were unchanged in all the groups, and there were negligible
differences in the maximum rate of depolarization
(MRD).
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Effect of Frequency of Stimulation on APD
Cardiac action potential duration is frequencydependent (Hoffman and Suckling, 1954), and drugs
which produce bradycardia would also prolong
APD. To measure the extent to which changes in
heart rate would influence repolarization, papillary
muscle cells were impaled and the stimulus cycle
length was decreased from 2000 to 200 msec during
a continuous impalement. As shown in Figure 2,
action potential duration shortened only at cycle
lengths less than 500 msec in myocardium from
both control and treated animals. However, the 50
msec prolongation of APD, induced as an adaptation to prolonged /J-blockade, was maintained at all
frequencies.
FIGURE 1 Intracellular
potentials from the right
atrium and right ventricular papillary muscle of rabbits
killed 24 hours after prolonged treatment with saline,
racemic propranolol, dextro-propranolol, or metoprolol.
In each panel the horizontal trace gives the zero potential, the second trace gives the intracellular record, and
the bottom trace the differentiated potential (dV/dt). In
the atrial preparations contractions also were recorded.
TABLE 1 Repolarization
Changes in APD in the Ventricular
Conducting System after Chronic /?-Blockade
Action potentials were recorded at intervals
along the length of the right ventricular conducting
system dissected from hearts of four rabbits treated
with propranolol 2 mg/kg b.d. sc, for 6 weeks, and
six controls given saline. The His bundle was stimulated at a frequency of 1.0 Hz. The resting potential and action potential amplitudes were not altered by propranolol treatment, but there were
striking effects on APD. Normally, APD in the
conducting tissue lengthens progressively from the
bundle of His as far as the most distal false tendons,
then shortens abruptly by 100 msec or more as
ventricular muscle is reached (Myerburg et al.,
1970). The controls showed this pattern (Fig. 3). In
contrast, in the propranolol-treated rabbits, action
potential duration was much more uniform
Times (msec) from Peak to 20%, 50%, and 90%
Atrium
No. of
animals
No. of
fibers
Saline
1 ml/kg b.d.
6
D-Propranolol
4 mg/kg b.d.
Treatment
Ventricle
APDa,
APD50
APD M
41
30.6
±1.1
54.0
±1.0
97.0
±1.5
4
42
30.6
±1.1
55.1
±1.4
DL-Propranolol
4 mg/kg b.d.
4
36
36.7
±0.9
Metoprolol
6 mg/kg b.d.
6
50
39.9
±1.5
No. of
fibers
APD M
APDW
APD.,,
48
84.3
±2.5
128.9
±2.9
163.6
±2.6
94.7
±1.8
40
82.0
±2.7
126.6
±2.7
160.1
±2.0
66.3
±1.5
113.7
±2.4
41
110.5
±2.8
167.3
±3.4
201.6
±3.7
66.8
±1.8
105.2
±1.7
50
106.4
±3.0
161.1
±3.4
195.7
±3.1
D-propranolol, which has the same local anesthetic activity as DL-propranolol, but only l/u» the /2-blocking potency, did not alter action potential
duration. The cardioselective drag metoprolol, and the non-selective propranolol, caused highly significant prolongations (P < 0.001) in APDM, A P D K , and
807
ADAPTATION TO PROLONGED /J-BLOCKADE/Rame and Vaughan Williams
•
day in the group on D-propranolol was not statistically significant.
Control
A
Ptopranolol
•
Mcloprolol
APD
Cycle length (msec)
2 Frequency-dependence of action potential
duration. Ordinate: APDgo, msec. Abscissa: stimulation
cycle length, msec. Intracellular records made on papillary muscles taken from animals treated for 24 days,
and killed 24 hours after the final dose. The controls
were saline-treated littermates of the rabbits given racemic propranolol, 4 mg/kg; or metoprolol, 6 mg/kg (sc
b.d.).
FIGURE
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throughout the conducting system; the APD of the
ventricular free wall was 50 msec longer than in the
controls [as already demonstrated in the isolated
papillary muscle (Table 1)]. Similarly, at the proximal end of the conducting system, APD was also
longer in the bundle of His taken from treated
rabbits, than in the controls. In Purkinje cells in the
central region, however, APD was shorter than in
the controls.
Time Course of Adaptation to Prolonged
y3-Blockade
A prolongation of ventricular APD, provided it is
uniform and unaccompanied by changes of conduction pathway, should be detectable as a prolongation of Q-T interval in the electrocardiogram. ECG
records were made on a number of rabbits, which
were then separated into four groups on the basis
of Q-Tc calculation, so that the mean Q-Tc was
approximately the same in each group. They were
injected (sc) twice daily for 24 days with saline,
lml/kg (n = 6); D-propranolol, 4 mg/kg (4); DLpropranolol, 4 mg/kg (10); or metropolol, 6 mg/kg
(6). ECG records were made on the 5th, 9th, 15th,
and 22nd day after the start of treatment, 20 hours
after the previous injection. There was no significant change in the P-R and R-R intervals, confirming the absence of acute blockade at the time of
measurement, or of any adaptational bradycardia.
QRS width was also unaltered, but in the groups
receiving propranolol and metoprolol, Q-T interval
was lengthened significantly (P < 0.05) on the 9th,
but not on the 5th day, and remained at the prolonged level (Fig. 4) on the 15th and 22nd day (P
< 0.005). A slight prolongation of Q-T on the 22nd
Correlation between Q-T, APD, and
Refractory Period
To establish whether the time-course of the adaptation to ^-blockade observed in vivo as a prolongation of Q-T was paralleled by the prolongation of
APD observed in vitro, four litters of rabbits were
divided into groups and injected twice daily s.c.
with 4 mg/kg DL-propranolol, and the animals were
killed after 3, 6, 12, and 24 days of treatment, one
rabbit from each of the litters being in each of the
groups. The remaining six littermates were killed
on day 0, to serve as controls. The ECG was recorded before the rabbits were killed, and the hearts
were then removed for measurement in vitro of
atrial and ventricular intracellular potentials and
refractory period.
As depicted in Figure 5, the ventricular APD90
was actually reduced after 3 days (P < 0.01), but
increased progressively thereafter, being 40 msec
longer than controls after 24 days (P < 0.001). The
Q-Tc changes paralleled those of ventricular APD.
The effective refractory period was not significantly
different after 3 days from that of control littermates killed at the start of the experiments, but
increased progressively thereafter (+36 msec at 24
days, P < 0.001). Adaptation of APD in the atria
was more rapid than in the ventricles, the prolongation being highly significant after 6 days of treatment, and lengthening by only a further few milliseconds thereafter.
Regression of Q- T Lengthening after Cessation of
^Blockade
Adaptive changes were defined as effects induced
by treatment, and persisting after elimination from
310
Action
• Control n- 6
A Propranolol n=4
R>tential 290
Duration
(msec) 2W
230
200
no
140
SEPTAL PURKINJE
FALSE TENDON
y
MUSCLE
110
FIGURE 3 Effects of prolonged ^-blockade on APD in
the ventricular conducting system. Ordinate: APD90,
msec. Abscissa: distance of recording electrode (mm)
from the proximal cut end of the His bundle (left) or from
the right ventricular muscle (right). The hatched regions
denote the SEM.
CIRCULATION RESEARCH
808
O-310
•
Controls
•
Metoprolol
A
Propranolol
VOL. 48, No. 6, JUNE 1981
(6)
(6)
(10)
d-Propranok>l(4)
O-290
O-270
O-2S0
10
Days of
20
15
25
treatment
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FIGURE 4 Time-course of adaptation of Q-T interval to fi-blockade. Ordinate: Q-Tc(= Q.T. (sec)/jR-R (sec)).
Abscissa: duration of treatment. By the 22nd day of treatment, the rabbits receiving fi-blockers had significantly
prolonged Q-Tc's (P < 0.005), but in the group receiving D-propranolol, Q-Tc did not differ significantly from the
control.
the plasma and tissues of the drugs used (Raine and
Vaughan Williams, 1980). To answer the question,
therefore, for how long such changes would persist
after cessation of therapy, a group of four rabbits
was treated with propranolol, 4 mg/kg, for 28 days,
and at this time the Q-Tc was prolonged by 0.041
above the value observed in saline-treated littermate controls (n = 2). Treatment was stopped, and
it was found that 10 days later, the Q-Tc had not
altered. Gradually, however, the Q-Tc shortened
again, until, at 3 weeks after cessation of treatment,
it was no longer significantly different from the
controls (Fig. 6).
Adaptive Changes in Contractions of
Papillary Muscles after Prolonged
/^-Blockade
Length-tension curves were obtained from isolated papillary muscles of approximately uniform
width (1.5 mm) taken from rabbits treated with
propranolol for 12 or 24 days, or with metoprolol
for 24 days, or with saline for 24 days. The lengths
A Refractory period
# Action potential duration
- - -m ECG QTc interval
1
i
t
I
..--} •
0.30
A
•320
o
Propranolol
—
OTr
Control
-300
0.28
•280
-260
0.26
T
i
0
0
6
12
18
24
Days of propranolol treatment
5 Time-course of adaptation to /?- blockade of
APD and refractory period in vitro correlated with Q-Tc.
Ordinate: left, time in msec: right Q-Tc. Abscissa: duration of treatment. The treated animals were littermates of the controls (n = 6) killed on day zero.
FIGURE
5
10
15
Days after Propranolol
20
25
withdrawal
FIGURE 6 Time-course after cessation of treatment of
return of prolonged Q-T to control values. Ordinate: QTc. Abscissa: time after cessation of treatment. The
treated animals had received racemic propranolol, 4
mg/kg b.d. sc, for 28 days, and the littermate controls
had received saline.
ADAPTATION TO PROLONGED /}-BLOCKADE/.Rame and Vaughan Williams
Controls
Propranotol
PropranoloJ
Metoprolol
f
2
12 days
24 days
24 days
FIGURE 7 Length-tension
relation in papillary muscle. Ordinate:
tension, mg/mm. Abscissa: length
at which tension was measured, on
a scale from the length at which
developed tension was just detectable (Lo = O) to that at which it was
maximal (Xmal = 100). fi-Blockade
had no adaptational effect on tension developed over the whole range
of muscle lengths.
30
20
10
20
40
60
Muscle length ( % length at peak tension)
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obtained by stretching from the completely relaxed
length have been expressed as a percentage of the
range of lengths between that at which developed
tension was maximal (Lmax), and that at which
developed tension was just detectable (Lo). (% =
100 X (L-Lo)/(Lmax -Lo)). The peak tension developed at Lmax was similar (28-32 mg/mm) in all
groups, and the complete length-tension curves
were almost identical for the papillary muscles
taken from saline-treated and drug-treated animals
(Fig. 7). Thus there was no evidence of any negative
inotropic effect after prolonged treatment, confirming our initial observations on atrial muscle;
(Vaughan Williams et al., 1975); indeed, it was noted
that treated muscles appeared to have a more rapid
maximum rate of rise of tension (dT/dt max) than
controls. This observation was examined in greater
detail with isolated papillary muscles from 17 controls and 16 propranolol-treated animals. Peak tension at Lmax was similar in the two groups, but dT/
dt max was greater and time-to-peak tension
shorter in the treated group, although these differences did not attain statistical significance (Table
2). However, a significant linear correlation (P =
0.001) was demonstrated between peak tension and
dT/dt max in each group (Fig. 8), and when the
gradients of the regression lines were compared,
that for the treated group was steeper (P = 0.01).
This suggests that, in the propranolol-adapted papillary muscles, there was a more rapid rate of at-
80
100
tainment of peak tension, particularly marked in
the more strongly contracting preparations (Fig. 8).
Discussion
These studies have shown that in rabbits treated
twice daily (sc) with doses of /6-blockers equivalent
to those used clinically (Raine and Vaughan Williams, 1980), action potential duration gradually
increases as an adaptation to treatment during the
first 3 weeks. The increase occurs uniformly in both
atrial and ventricular muscle. In the ventricular
conducting system, although APD is lengthened in
the His bundle, it is shortened in preterminal Purkinje fibres, so that the usual disparity of APD in
these regions is reduced. It may be concluded that
the primary effect leading to these changes is repeated, though intermittent, blockade of cardiac
/?-adrenoceptors, since similar effects were produced both by propranolol, which blocks both /?i
and /?2 receptors, and by metoprolol which is selective for /Si receptors. APD was not prolonged by Dpropranolol, which is 100 times less active than DLpropranolol as a yS-blocker, but is equipotent with
DL-propranolol in restricting fast inward current in
cardiac muscle and as a local anaesthetic in nerve
(Dohadwalla et al., 1969).
Prolonged treatment with /S-blockers also caused
a lengthening of Q-T interval in vivo, measured 20
hours after the previous injection of drug. The in
vitro measurements of APD were made in animals
TABLE 2 Parameters of Contraction in Isolated Papillary Muscle
No. of
animals
Peak tension
(mg)
Time-to-peak
tension
(msec)
dT/dt max
(g/sec)
Saline
1 ml/kg b.d.
17
176 ± 2 8
172 ± 8
0.59 ± 0.07
DL-Propranolol
4 mg/kg b.d.
16
186 ± 36
150 ± 7
0.73 ±0.13
0.818
0.066
0.373
Treatment
P value
809
There was clearly no significant change in peak tension as a result of prolonged /3-blockade, but there appeared
to be a trend towards a shortening of time-to-peak, only just not significant.
CIRCULATION RESEARCH
810
Controls
15
(n-17).
y-0-27x + 0-15 . r-0-924
Propranolol (n-16).
y-O-39oc + 004 . r-0-963
FIGURE 8 Rate of rise of tension (dT/dt)
as a function of peak tension at Lml,x,
compared in separate papillary muscles
from 16 propranoloI-treated animals and
17 controls. Ordinate: maximum rate of
rise of T g/sec X 0.4. Abscissa: peak tension, g X 10~l. Treatment for 24 days with
propranolol, 4 mg/kg, induced an adaptive increase in rate of rise of papillary
muscle tension in comparison with salinetreated littermates. Linear regressions
gave highly significantly slopes (P <
0.001) for both groups, and the slopes were
significantly different from each other (P
10
0-5
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20
10
Peak
tension
30
(gm>10
VOL. 48, No. 6, JUNE 1981
40
)
killed not less than 24 hours after the last dose of
drug. The effects represented, therefore, an adaptation to the treatment, because negligible amounts
of drug would have been present in the tissues at
the time the observations were made (Raine and
Vaughan Williams, 1980). The prolongation of Q-T
and APD developed gradually after a period of a
few days, becoming maximal at 3 weeks. Q-T remained prolonged for 10 days after cessation of
treatment, and declined to control values at 3
weeks. In vitro APD was uniformly prolonged in
comparison with controls over a wide range of stimulation frequencies and, in vivo, the prolonged Q-T
was not associated with any adaptational bradycardia.
The prolongation of APD as an adaptational
response to prolonged /3-blockade contrasts with
the effects of acute blockade, which either does not
alter or shortens APD (Papp and Vaughan Williams, 1969; Davis and Temte, 1968). In the presence
of propranolol, norepinephrine lengthens APD in
guinea pig atria (Pappano, 1971) and in Purkinje
fibers (Giotte et al., 1973), presumably as a result of
stimulation of a-adrenoceptors. The adaptational
prolongation of APD observed in our experiments,
however, was much greater than could have been
accounted for by a-adrenergic stimulation (Rosen
et al., 1977). Furthermore, it was shown previously
(Raine and Chubb, 1977) that prolonged ^-blockade
reduced the activities of enzymes involved in norepinephrine synthesis, and the time-course of the
development of that decrease has been shown in
this study to parallel that of the adaptive changes
in APD. In any complex control system, repeated
interruption of the effector output at one point will
also affect afferent input, and could lead to long-
term readjustments, which might include a reduction of sympathetic efferent traffic (Lewis, 1976;
Vaughan Williams, 1977).
The adaptational lengthening of APD after prolonged /^-blockade involved primarily an increase in
plateau height and duration, rather than of the
"tail" of the action potential, and it may be associated with an increased rate of development of tension. One explanation for these changes might be
an increased activation of slow inward current, since
dT/dt is intimately associated with the release of
intracellular calcium from the sarcoplasmic reticulum, which in turn is triggered by calcium entering
the cell as slow inward current (Fabiato and Fabiato, 1977). It must be stressed that the relationship between activation of slow inward current,
duration of the plateau, and activation of contraction is complex, and as yet poorly understood; increases in tension development may be associated
with either lengthening or shortening of APD (Allen, 1977). Nevertheless, changes similar to those
we report are produced by epinephrine, which increases plateau duration and contractile force, and
shortens time to peak contraction (Beresiwicz and
Reuter, 1977) and increases slow inward current
(Vassort et al., 1969). It may appear paradoxical to
suggest that prolonged /^-blockade produces effects
similar to those of epinephrine. However, although
voltage-dependent slow channels operate in the absence of adrenergic influence, slow inward current
is normally augmented in vivo by ^-receptor stimulation. One might speculate that prolonged fiblockade leads to a compensatory increase in the
number of non-dependent channels relative to that
of catecholamine-dependent channels, perhaps by
inducing synthesis of new channels, which would
ADAPTATION TO PROLONGED /3-BLOCKADE/flame and Vaughan Williams
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account for the delay in onset of the adaptation. A
similar delay is observed in the appearance of increased myocardial RNA content and protein synthesis in response to hypoxia or elevated work-load
(Meerson, 1975). The biochemical basis for shortening of APD in hypoxia and its amelioration by
glucose (McDonald and McLeod, 1973) is unknown,
but it is of interest that APD in ischemia is correlated with intracellular glycogen content (Cowan
and Vaughan Williams, 1980), and that myocardial
glycogen content is greatly increased (+40%) by /?blockade (Smithen et al., 1975).
It is not clear why prolonged ^-blockade caused
shortening of APD in Purkinje cells, in contrast to
ventricular muscle. Propranolol acutely shortens
APD in Purkinje cells (Davis and Temte, 1968) but
this acute effect could not explain our findings
because no propranolol remained in the tissue at
the time recordings were made. APD in Purkinje
cells is inherently longer than in ventricular muscle,
and is determined mainly by outward potassium
currents, as slow inward current is of small amplitude (Noble, 1975). There are so many factors which
could be involved in a change of APD that further
experiments would be required to elucidate their
individual contributions to the APD shortening in
the preterminal Purkinje cells. The possibility that
these cells are more susceptible to hypoxia could be
considered; also, the activation of outward K+ current by a possibly increased inward Ca2+ current
might be involved (Bassingthwaite et al., 1976).
There are grounds for believing that the adaptational prolongation of APD reported here is not
irrelevant to the use of /S-blockers in humans. It has
recently been shown that similar plasma propranolol concentrations block /^-receptors to the same
extent in rabbits and humans (McDevitt and
Shand, 1975; Raine and Vaughan Williams, 1980)
and long-term /S-blockade causes a lengthening of
human APD as measured with intracardiac suction
electrodes (Edvardsson and Olsson, 1978). The Q-T
is longer, especially in exercise, at a given rate than
in pretreatment controls (Vaughan Williams et al.,
1980), and in comparison with matched patients not
receiving /6-blockers (Raine and Pickering, 1977). A
prolongation of APD, provided that it is uniform
and not associated with great heterogeneity within
the myocardium such as occurs in the "long Q-T
syndrome," should have an antiarrhythmic effect
(Raine and Vaughan Williams, 1978; Vaughan Williams, 1980), as might also the increased uniformity
of APD within the ventricular conducting system.
It has been noted that there is similarity between
patients adapted to prolonged /S-blockade (Brundin
et al., 1976) and athletes at a high level of fitness, in
that the usual hemodynamic effects of acute fiblockade are much attenuated. The implication is
that in both groups excitation-contraction coupling
currents are less dependent on adrenergically controlled slow channels. Abrupt cessation of /S-block-
811
ade could add back the adrenergically controlled
channels, to the now larger number of non-adrenergically dependent channels, and in patients with
narrowed coronary arteries could provoke an excesssive oxygen demand, i.e., the "rebound" phenomenon. Our own finding that the electrophysiological adaptation to prolonged /S-blockade persists
for 10 days, and that the Q-T interval returns to
normal only after about 3 weeks, suggests that if
/^-blockade has to be discontinued in anginal patients, dosage should be reduced gradually during
at least a similar period.
Acknowledgments
We wish to thank Dr. J. Wittig for his assistance with the
recording from Purkinje fibers.
References
Allen DG (1977) On the relationship between action potential
duration and tension in cat papillary muscle. Cardiovasc Res
11: 210-218
Bassingthwaite JB, Fry CH, McGuigan JAS (1976) Relationship
between internal calcium and outward current in mammalian
ventricular muscle; a mechanism for the control of the action
potential duration? J Physiol (Lond) 262: 15-38
Beresewicz A, Reuter H (1977) The effects of adrenaline and
theophylline on action potential and contraction of mammalian ventricular muscle under "rested-state" and "steadystate" stimulation. Arch Pharmacol 301: 99-107
Brundin T, Edhag O, Lundman T (1976) Effects remaining after
withdrawal of long-term beta-receptor blockade. Br Heart J
38: 1065-1072
Cowan JC, Vaughan Williams, EM (1980) The effects of various
fatty acids on action potential shortening during sequential
periods of ischaemia and reperfusion. J Mol Cell Cardiol 12:
347-369
Davis LD, Temte JV (1968) Effects of propranolol on the transmembrane potentials of ventricular muscle and Purkinje fibres
of the dog. Circ Res 22: 661-677
Dohadwalla AN, Freedberg AS, Vaughan Williams, EM (1969)
The relevance of ^-receptor blockade to ouabain-induced cardiac arrhythmias. Br J Pharmacol 36: 257-267
Edvardsson N, Olsson SB (1978) Inverkan av akut och kronisk
Beta-blocked pa repolarisationsfasen i hoger kammare (abstr).
Acta Scietatis Medicorum Suecanae. 87: 111
Fabiato A, Fabiato F (1977) Calcium release from the sarcoplasmic reticulum. Circ Res 40: 119-129
Florio, RA, Nayler WG, Slade A, Vaughan Williams EM, Yepez
LE (1980) Effect of prolonged /8-adrenoceptor blockade on
heart weight and ultrastructure in young rabbits. Br J Pharmacol 68: 485-498
Giotti A, Ledda F, Mannaioni PF (1973) Effects of noradrenaline
and isoprenaline in combination with a- and /J-receptor blocking substances on the action potential of cardiac Purkinje
fibres. J Physiol (Lond) 229: 99-113
Hoffman, BF, Suckling EE (1954) Effect of heart rate on cardiac
membrane potentials and the unipolar electrogram. Am J
Physiol 179: 123-130
Koch-Weser, J. (1975) Non-beta-blocking actions of propranolol.
N Engl J Med 293: 988-989
Lewis P (1976) The essential action of propranolol in hypertension. Am J Med 60: 837-852
McDevitt DG, Shand DG (1975) Plasma concentrations and the
time course of beta blockade due to propranolol. Clin Pharmacol Therapeut 18: 708-713
McDonald TF, McLeod DP (1973) Metabolism and electrical
activity of anoxic ventricular muscle. J Physiol (Lond) 229:
559-582
Meerson FZ (1975) Role of synthesis of nucleic acid and protein
812
CIRCULATION RESEARCH
Downloaded from http://circres.ahajournals.org/ by guest on June 15, 2017
in adaptation to the external environment. Physiol Rev 55:
79-123
Myerburg RJ, Steward JS, Hoffman BF (1970 Electrophysiological effects of canine peripheral A-V conducting system. Circ
Res 26: 361-378
Noble D (1975) The initiation of the heart beat. Oxford, Oxford
University Press
Papp JG, Vaughan Williams EM (1969) A comparison of the
antiarrhythmic actions of ICI 50172 and (—)-propranolol and
their effects on intracellular cardiac action potentials. Br J
Pharmacol 37: 391-399
Pappano AJ (1971) Propranolol-insensitive effects of epinephrine on action potential repolarisation in electrically driven
atria of the guinea pig. J Pharmacol Exp Ther 177: 85-95
Raine AEG, Chubb IW (1977) Long term /?-adrenergic blockade
reduces tyrosine hydroxylase and dopamine-/?-hydroxylase activities in sympathetic ganglia. Nature 267: 265-267
Raine AEG, Pickering TG (1977) Cardiovascular and sympathetic response to exercise after long-term beta-adrenergic
blockade. Br Med J 2: 90-92
Raine AEG, Vaughan Williams EM (1980) Adaptational responses to prolonged y8-adrenoceptor blockade in adult rabbits.
Br J Pharmacol 70: 205-218
Rosen MR, Hordof AJ, Ilvento JP, Danilo P (1977) Effects of
adrenergic amines on electrophysiological properties and automaticity of neonatal and adult canine Purkinje fibers. Circ
Res 40: 390-400
Salako LA, Vaughan Williams EM, Wittig JH (1976) Investigations to characterize a new anti-arrhythmic drug, ORG 6001,
including a simple test for calcium antagonism. Br J Phar-
VOL. 48, No. 6, JUNE 1981
macol 57:251-262
Smithen C, Christodoulou J, Brachfeld N (1975) Protective role
of increased glycogen stores induced by propranolol. Recent
Adv Stud Cardiac Struct Metab 8: 141-152
Thoenen H (1972) Neuronally mediated enzyme induction in
adrenergic neurons and adrenal chromaffin cells. Biochem Soc
Symp 36: 3-15
Vassort G, Rougier O, Garnier D, Sauviat MP, Coraboeuf E,
Gargouil YM (1969) Effects of adrenaline on membrane inward currents during the cardiac action potential. Pfluegers
Arch 309: 70-81
Vaughan Williams EM (1977) Adaptation of the heart and
sympathetic system to prolonged /3-adrenoceptor blockade.
Proc R Soc Med 70 (suppl 11): 49-61
Vaughan Williams EM (1980) Antiarrhythmic Action. London,
Academic Press
Vaughan Williams EM, Raine AEG, Cabrera AA, Whyte JM
(1975) The effects of prolonged /?-adrenoceptor blockade on
heart weight and cardiac intracellular potentials in rabbits.
Cardiovasc Res 9: 579-592
Vaughan Williams EM, Tasgal J, Raine AEG (1977) Morphometric changes in ventricular myocardium produced by prolonged beta-adrenoceptor blockade in rabbits. Lancet 2: 850852
Vaughan Williams EM, Hassan MO, Floras JS, Sleight P, Jones
JV (1980) Adaptation of hypertensives to treatment with
cardioselective and non-selective beta blockers. Absence of
correlation between bradycardia and blood-pressure control,
and reduction in the slope of the Q-T/R-R relation. Br Heart
J 44: 473-487
Adaptation to prolonged beta-blockade of rabbit atrial, purkinje, and ventricular potentials,
and of papillary muscle contraction. Time-course of development of and recovery from
adaptation.
A E Raine and E M Vaughan Williams
Downloaded from http://circres.ahajournals.org/ by guest on June 15, 2017
Circ Res. 1981;48:804-812
doi: 10.1161/01.RES.48.6.804
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