Download Potential arrhythmogenic role of cyclic adenosine

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JACC Vol . 19. No. 7
Jun, 1 ., 1r82-37
BASIC CONCEPTS IN CARDIOLOGY
Arnold M . Katz, MD, FACC, Guest Editor
Potential Arrhythmogenic Role of Cyclic Adenosine Monophosphate
(AMP) and Cytosolic Calcium Overload- Implications for Prophylactic
Effects of Beta-Blockers in Myocardial Infarction and Proarrhythmic
Effects of Phosphodiesterase Inhibitors
WILHELM F. LUBBE, MD, FACC, THOMAS PODZUWEIT, PHD,5
LIONEL H . OPIE, MD, PHD, FACCt
Auckland, New Zealand: Cape Tonvr . Sourly Ali'ien, and Bad Naaheim, Gertnanv
Activation of the adrenergic nervous system appears to play a
crucial role in the genesis of fatal arrhythmias associated with the
very early stages of acute myocardial infarction. The second
messenger of beta-adrenergic catecholamine stimulation, cyclic
adenosine monophesphate (AMP), has established arrhyth .
magnate qualities, acting by an increase in cylosolic calcium,
which potentially has three adverse electrophysiologic effects .
First, stimulation of the transient inward current by excess
oscillations of cytosolic calcium can invoke delayed ufterdepoler .
tendons, so that triggered automaticity can develop in otherwise
quiescent ventricular muscle . Second, cyclic AMP can evoke
calcium-dependent slow responses in depolarized fibers, so that
conditions for reentry are favored . Third, excess cytosolic calcium
can cause intercellular uncoupling with conduction stowing .
Focal changes in cyclic AMP and cytosolic calcium promote the
development or ventricular fibrillation . Beta-adrenergic blockade
can limit the formation of cyclic AMP In Ischemic tissue . Furthermore, by reducing sinus tachycardia it can lessen cytosolic calcium
overload . Hence, beta-adrenergic blockade helps to prevent ven.
tricular fibrillation in the early stages of acute myocardial infarc(ion and protects from sudden death in the postinfaretion phase .
In congestive heart failure, abnormalities of cytosolic calcium
patterns exist with cytosolic calcium overload . It is proposed that
the adverse effects of phosphtdiesterase inhibitors on the mortality rate in patients with congestive heart failure can be explained
by increased rates of formation of cyclic AMP and the develop
meet of calcium-dependent arrhytbmias . Because calcium Is the
ultimate messenger of cyclic AMP-ioeduced arrbythmias and because rytosolic calcium Is increased in heart failure, it will be
difficult to develop positive inotropic agents that are free of the
risk of sudden death.
U Am Call Cordial 1992 ;19:1622-33)
Ventricular arrhythmias can arise because of abnormal impulse formation and disordered conduction within the myocardium . The cellular events that underlie cardiac automaticity and conduction disturbances have been well
characterized (I) . These include partial depolarization of
cells, leading to a decreased rest potential with consequent
enhancement of automaticity in Purkinje fibers, inhibition of
the rapid phase 0 depolarization (carried by the sodium
current) with facilitation of the slow response (mediated by
calcium ions), shortening of the action potential (with a
decrease in the refractory pdriod) and acceleration of phase
4 depolarization . When these changes are focal, they predispose to inhomogeneity of electrical activity, which has long
been recognized as the "anatomic substrate" for ventricular
fibrillation . Focal metabolic variations, probably reflecting
localized deficits in the microcirculation, also contribute to
inhomogeneity and thereby determine vulnerability to fibrillation . Thus, anatomic and metabolic substrates are mutually reinforcing rather than exclusive, thereby facilitating the
precipitation of ventricular fibrillation by a triggering arrhythmia .
Several metabolic consequences of ischemia have already
been shown to increase the vulnerability to fibrillation . Beck
(2) proposed that gradients of oxygen in the heart with
regional ischemia caused inhomogeneous metabolic and
electrical activity. Harris et al . (3) suggested that ventricular
From the Department of Medicine . Univc,vly
of
Auckland School of
Medicine. Auckland. New Zealand : the 'M o Planck Institute for Physiologand Clinical Research. Bad Nouheim . Germany. and the 'lcchaemic
Hea, Disease Research Unit otthe Medical Research Council and University
.,Cape Town Medical School . Cape Town. South Africa.
Manuscript received September 9, 1991 : revised manuscript received
December 12, 1991, accepted January 6 . 1992.
Address for eeerims: Lionel H . Opie. MD . Heart Research Unit . University of Cape Town. Medical School . Observatory
Africa.
©1992 by the
American College of Cardiology
7925 .
Cape Town . South
1,735-10971921s5 .00
1ACC Vni . 19. Nn 7
June 1992'.1922-11
CYi'LIC AMP . CALCtUsI
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catecholamine discharge, formation of cyclic adenosine monophoc-
Figure 2. The basic observations linking ischemic tissue cyclic
adenosine monephosphate (AMPI levels to the onset of ventrcular
fibrillatioo. Data from six baboons developing ventricular fibrillation
phate (AMP) and calcium ICa t '1-mediated triggered lachycardia .
within I h of ligation of the anterior descending coronary artery two
reentry and automaticity. The resultant ventricular ra:hycardia can
predispose to ventricular fibrillation . especially when there is electrical inhomogeneity . Note inhibition of this sequence by betaadrenergic blockers and aggravation by phosphodiesterase (PDEI
inhibitors . DADs = delayed afterdepolarizations : K' = potassium.
Na' = sodium : SA = sinoarial : SR = sarcoplasmic reticulum .
thirds from its origin . Data are plotted in relation to the onset of
ventricular fibrillation (0 min) . A specific symbol is assigned to each
baboon . Horizontal line = mean cyclic AMP levels in nonischemic
tissue . Ntte the consistent increase in cyclic AMP levels stoning
abut Ill min before the onset of ventricular fibrillation . Reprinted
from Podzaweit et at . (25), with permission of Academic Press .
Figure 1. Proposed sequence whereby myocardial ischemia leads Io
arrhythmias arose when potassium ions released by the
ischemic myocardium formed electrophysiologic gradients .
Likewise, mechanisms involving the accumulation of fatty
acids, acyl coenzyme A (4) and lysophosphoglycerides (5).
increased lactic acid (6) or a decrease in glycolytically
derived adenosine triphosphatc (ATP) (7) may ail predispose
to ventricular fibrillation in different models . However. in no
case is the evidence linking such metabolic changes to
ventricular fibrillation as compelling as that proposing that
increased adrenergic activity can initiate a series of cellular
events that ultisatcly result in ventricular fibrillation .
This review will first focas on the role of the adrenergic
nervous system and its second messenger, cyclic adenosine
monophosphate (AMP), in the causation of ventricular atrhythmics and expand the "cyclic AMP hypothesis" (8) to
identify abnormalities of cellular cytosolic calcium as the
ultimate mediator of increased adrenergic activity and the
generator of electrophysiologic abnormalities that could be
responsible for ventricular fibrillation associated with myocordial ischemia. The hypothesis also allows for the role of
other metabolic disturbances such as potassium loss from
the ischemic myocardium (Fig . I).
Evolution of the Calcium
Arrhythmogenesis Concept
The concept that mechanisms involving the adrenergic
nervous system are fundamental to the genesis of ventricular
fibrillation was based both on the early activation of the
adrenergic system in response to coronary artery occlusion
(91 and tho demonstration that carecholamine stimulation
increases the vulnerability of the heart to fibrillation (10) .
Sympathetic nervous system activation occurs soon after
coronary occlusion (I l) when ventricular fibrillation commonly occurs . However, the mechanism nr mechanisms
whcrchy adrenergic neurotransmitters released during sympathetic activation might exert their influence on the vulnerability of the heart to fibrillation has been obscure . The
cellular electrophysiologic responses to cyclic AMP were
demonstrated in 1973 by Tsien (12) : these resembled the
electrophysiologic effects of epinephrine . Explanations for
the involvement of cyclic AMP in pacemaker function (by
enhancement of phase 4 depolarization), in triggered automaticity (by formation of delayed aflerdepo!arizations) and
by facilitating reentry (stimulation of calcium-media,cd slow
responses) became available during the subsequent years
(13 .14).
The cyclic AMP hypothesis . Observations in baboons
(Fig . 2) and isolated rat heart models (Fig . 3) of acute
regional myocardial ischemia led to the cyclic AMP hypothesis (8.15 .16). whereby cyclic .AMP, the second messenger
of the adrenergic system, occupies a key role in the mediation of adrenergic effects on vulnerability to fibrillation . The
effects of cyclic AMP on electrical function of cardiac cells,
particularly when they are partially depolarized as in early
ischemia . could explain several of the electrophysiologic
phenomena encountered in arrhythmogenesis and ventricular fibrillation . The current concept is that calcium ions act
as a third messenger for adrenergic influences .
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LUBOP CT AI .
CYCLIC AMP. CALCIUM AND Ata0HYTHMIA5
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1ACC Vol . Iv, Nn. 7
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norepinephrine stores by reserpine or 6-hydruxydopamine
(22) and blockade of the beta-receptors (23,24) all protect
against the development of ventricular fibrillation in the
presence of mvocardialisct,va,a .
Cyclic AMP and Arrhythmogenesis
20
so
200
1000
Pedusala ad,enaline (nmol/o
5000
Figure 3. The basic observations linking beta-adrenergic blockade,
decreased tissue cyclic adenosine monophosphate (CAMP) and a
decreased ventricular fibrillation threshold IVFf) with converse
changes found during phosphodiesterase inhibition by Iheophylline .
The response curve of ventricular fibrillation threshold to epinephrine (Adrenaline) coincides with an increase in tissue cyclic AMP,
but not with changes in tissue phosphoereatine or tissue adenosine
Iriphosphate . The curve was shifted to the left by throphy fine
(1 m30 . with higher levels of tissue cyclic AMP achieved . Atenciol
(38 µM) shifted the ventricular fibrillation threshold curve upward
and to the right ; an increase in tissue cyclic AMP was no-,seen until
the reduction in ventricular fibrillation threshold occurred . Reprinted from Lubbe et al . (15„ with permission of the Joaraal of
Clinieal In, ratlgetimr.
Adrenergic Activity aad
Ventricular Fibrillation
Although sympathetic activation (9) may represent a
compensatory mechanism designed to maintain cardiac output by a damaged ventricle, it may also be causally related to
the development of ventricular fibrillation. Aortic wall and
carotid sinus stretch receptors have traditionally been
thought to initiate the adrenergic activation . There is also
activation of central noradrenergic centers in the hindbrain
in response to vagal afferent traffic traveling from receptors
within the ischemic myocardium (17) .
Carecholamine stimulation causes an increase in vulnerability of the heart to ventricular fibrillation (10) ; stimulation
of the left stellate ganglion is potent in the development of
ventricular fibrillation in the cat with coronary artery occlusion (18) . Administration of norepinephrine in the presence
of ischemia will elicit ventricular tachycardia or fibrillation in
a number of animal models: the isolated rat heart (15) or the
dog (19) or pig (20) . Conversely, when sympathetic activity
is inhibited, there is protection against ventricular fibrillation. Surgical denervation (21), depletion of myocardial
The elevation of myocardial cyclic AMP levels after
coronary artery occlusion has been linked to the development of ventricular fibrillation by Podzuweit et al . (25). In
baboons, the onset of ventricular fibrillation was preceded
by a progressive elevation of cyclic AMP in ischemic myocardium . Observations obtained in pigs supported this concept (16) . In an isolated heart system, an increase in vulnerability to fibrillation could be induced by epinephrine (Fig . 3)
as well as by dibulyryl cyclic AMP, a form of cyclic AMP
thought to penetrate the cell membrane (15).
The addition of theophylline causes greater reductions in
ventricular fibrillation threshold values accompanied by
higher levels of myocardial cyclic AMP . In dogs, toxic doses
of aminophylline cause marked elevation of tissue cyclic
AMP levels and the occurrence of spontaneous ventricular
fibrillation (26).
Pharmacologic agents that decrease myocardial cyclic AMP
levels protect against the occurrence of ventricular fibrillation .
In the isolated rat heart, propranolol (27), amiodarone (28),
labetalol (29) and adenosine (30) decrease myocardial cyclic
AMP levels in ischemic zones and prevent a decrease in
ventricular fibrillation threshold . Several of these agents also
prevent spontaneous ventricular fibrillation after coronary
artery ligation. In the pig, prostacyclin administration attenuates cyclic AMP levels after coronary artery ligation and
protects against the development of arrhythmias (31).
Choline esters prevent ventricular arrhythmias and cyclic
AMP accumulation in pig myocardium by an action that can
be blocked by atropine (32) . Adenosine inhibits spontaneous
arrhythmias in the isolated coronary artery-ligated rat heart
by an action opposed by phosphodiesterase inhibitors (33).
An interesting series of experiments was reported by
Podzuweit and coworkers (20,32,34,35) in which local stimulation of myocardial cyclic AMP was obtained . Intramyocardial infusion of norepinephrine, N 5',O2'-dibulyryl cyclic
AMP (DBcAMP) or N5-monobutyryl cyclic AMP elicited
arrhythmias. including ventricular tachycardia and fibrillation . Potent arrhythmogenic combinations involved infusions of norepinephrine or DBcAMP in combination with a
phosphodiesterase inhibitor and calcium chloride . Infusion
of the calcium chloride by itself did not cause arrhythmias .
The arrhythmias induced by infusion of norepinephrine were
blocked by coinfusion of propranolol . Biopsy specimens of
myocardium obtained from the infusion site at the onset of
ventricular tachycardia revealed elevated levels of cyclic
AMP .
(ACC V.I. 1'a. No. 7
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Figure 4.
t11tt
I .titttE ET AL .
'MP .CAtCtUsl ANn ARRHVTHMIAS
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Effect of calcium channel antagonists on the
ventricular fibrillation threshold
of
the rat heal . In an
isolated rat heart preparation, coronary artery ligation
decreases the ventricular fibrillation threshold I VFfl .
an effect further cnh.anced by the addition of epineph-
rine (udrenalincl (5 x 10 '.M) (column labeled No
drug') . The threshold reverts toward normal with vcrapamil and the I-form is more potent than the d-form .
Other active agents are nifedipiec fNl (10 ".MI .
diltiatem (D) 15 x Ill " .MI and tiapamil III) ',tn. For
methods and further discussion of the data for verapamil isomers, see Opie and Thandroyen
11041
and
Thandroyen el al. (100.105) . Note that the distinction
between the I- and d-isomers of verapamil is found only
in the presence of catecholamine stimulation
(441.
Re-
printed from Opie et aL (55), with permission of the
Aenn!s f the Nen York A, uderrr7 of Srirrrcc .c.
CORONARY LIGATION +
ADRENALINE
Evidence Against the Cyclic AMP Hypothesis
Nonetheless, evidence against a relation between tissue
cyclic AMP levels and the occurrence of ventricular fibrillation has been provided by two, sources . First, forskolininduced increases in cyclic AMP levels in isolated rat hearts
were not accompanied by ventricular fibrillation 136) . Forskotin activates adenylate cyclase directly, bypassing the
beta-receptors and membrane-based coupling proteins .
However, our current data (37) suggest that compartmentalization of cyclic AMP may provide the answer. Our subsequent work has established that only the generation of very
high levels of cyclic AMP by forskolin is accompanied by a
decrease in the ventricular fibrillation threshold, suggesting
an overflow phenomenon with some of the excess cyclic
AMP reaching that compartment able to generate arrhythmias.
A second objection to the cyclic- AMP hypothesis arose
from the failure of the beta-adrenergic blocking agent metoprolol to decrease ischemic myocardial levels of cyclic
AMP, although ventricular fibrillation was prevented in pigs
with coronary artery ligation (38) . There are alternative
explanations for this observation . Blood flow in the ischemic
myocardium was considerably higher in the meloprololtreated pigs than in pigs treated with propranolol or sotalol .
Also, the relatively high dose of the beta-blocker might have
had a negative inotropic effect, potentially annulling the
antiarrhythmic effect, as may be the case for propranolol
doses >0.1 mglkg body weight (24).
Cellular and Electrophysiologic Effects of
Cyclic AMP
Microiontophoresis of cyclic AMP into cardiac Purkinje
fibers caused shortening of the action potential, elevation of
the plateau level and enhancement of diastolic (phase 4)
depolarization (12) . In cells partially depolarized by elevated
extracellular potassium, the magnitude and frequency of low
5xtn°M
voltage oscillations were increased by cyclic AMP . Automaticity followed after the induction of slow responses by
cyclic AMP-induced inward calcium current (39) . Watan:tbe
and Besch (40) established the relation between cellular
levels of cyclic AMP, the ..aic'am-mediated inward current
and electrical excitability in partially depolarized guinea pig
hearts . The calcium channel antagonist D600 successfully
blocked cyclic AMP-mediated effects on clectrophysio logic
function and contractile activity (41) .
Links Between Cyclic AMP and Calcium :
Mechanisms of Arrhythmias
Effect of calcium antagonists . Cyclic AMP, when injected
into calcium-overloaded myocyles, can evoke large depolarizations that are calcium dependent (14) . If cyclic AMP
produced early ischemic arrhythmias by a mechanism involving calcium, it would be logical that calcium antagonists
(calcium ion entry Mockers) should inhibit such arrhythmias .
'Thus, verapami! given to dogs with coronary artery ligation
protected against spontaneous ventricular arrhythmias (42)
and prevented the lowering of ventricular fibrillation threshold 143) . In such experiments, however, it was difficult to
rule out a primary effect of the calcium antagonists on
peripheral hemodynumic variables. Thandroyen (44) and
Lubbe et al . (45) reported the protective action of calcium
antagonists in isolated rat hearts. However, relatively high
concentrations were required to protect against ventricular
fibrillation. The active L(-) isomer of vempamil but not the
inactive D(+) isomer prevented the decrease in the ventricular fibrillation threshold (Fig . 4) and in guinea pig papillary
muscle also inhibited calcium-dependent slow responses .
The antifibrillatory effect could be found in the absence of
changes in tissue cyclic AMP and supported the argument
for a specific role of calcium channel activity in early
ventricular fibrillation .
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IACC vni . 19, No . 7
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Lt1RRR ET AL .
CYCLIC AMP, CALCIUM AND ARRNYTHMIAS
VT
VF
' REENTRY
EcEcnnCAL
nuaanounsEnv
"Slow response" action potentials . These can be demonstrated by the addition of cateche :amines or other agents
that increase cyclic AMP in myocardial tissue (Fig . 5) in the
presence of extracellular potassium concentrations sufficiently elevated to cause depolarization (46,47) . Although
the rate of microreentry in ventricular fibrillation and ventricular tachycardia is too fast to be dependent on slow
responses (48.49), a slow response action potential could
create an area of extremely slow conduction that, in turn,
could predispose to reentrant circuits . Such a circuit could
be relatively rapid . Slow responses are particularly likely to
occur when high cyclic AMP levels are combined with high
external potassium levels, as would occur in zones with
severe ischemia .
"Flow of Injury" Current and Calcium
Jansc et al . (48,49) suggested that the origin of ischemiarelated ventricular tachycardia and fibrillation was located at
the "normal" side of the ischemic border, possibly caused
by a "current of injury" that flowed from the ischemic to the
normal zone . Such injury currents could flow as a result of
differences in the electrical potential between these zones, as
shown in a sucrose gap preparation (50) . In ischemia, there
is a gradient of extracellular potassium from the ischemic
zone, where the level is high, to the nonischemic zone,
where it is less elevated or normal . The current resulting
from this potassium gradient can lead to a phenomenon
known as depolarization-induced automaticity (51) . Such
automatieity occurring in depolarized myocardium is partially due to the calcium current (52) and is influenced by
changes in the external calcium concentration (53) . Therefore, inhibition of the calcium current by calcium antagonists
should reduce depolarization-induced automaticity . In addition, calcium antagonists may lessen the release of potassium, decreasing the severity of depolarization and reducing
the depolarization-induced current (54).
Janse et al . (48) made another interesting predictionnamely, that the current of injury could induce afterdepolarizations in Purkinje fibers, especially in the presence of
Figure 5. Links between cyclic adenosine monophosphate (cAMPI. eytosoiic calcium and specific
calcium-mediated electrophysiologic abnormalities
thought to predispose to ventricular tachycardia
(VTIendfibrillation (VF) .Fordetails,sccOpicctal .
(55). Abbreviations a5 in Figure I .
epinephrine stimulation . Hence, it became appropriate to
examine the possible contribution of afterdepolarizations to
ischemic ventricular arrhythmias .
Delayed Afterdepolarizations
Cyclic AMP, internal calcium and afterdepolarizations .
The elevation of cyclic AMP in ischemic tissue may provoke
delayed afterdepolarizations and, hence, ventricular automaticity (55). We found (56) that delayed afterdepolarizations were precipitated by dibutyryl cyclic AMP or isoproterenol under control conditions but not during ischemic
perfusion of guinea pig papillary muscles ; then, paradoxically, delayed afterdepolarizations could be precipitated
during reperfusion. These findings could serve as an explanation for the observations that increased levels of cyclic
AMP in ischemic myocardium predispose to reperfusion
arrhythmias (32,57) and for the antiarrhythmic effert of
severe myocardial ischemia on cyclic AMP-mediated arrhythmias (35). The proposed explanation for the lack of
precipitation of delayed afterdepolarizations in severely ischemic preparations rests on the requirement for ATP in the
cellular cycling of calcium .
Aferdepolarizations, transient inward current and energy
requirements. Delayed afterdepolarizations are the electrophysiologic abnormalities associated with rhythmic internal
:alcium oscillations . Delayed afterdepolarizations can be
evoked by a variety of conditions (Fig . 6) that induce
intracellular calcium overload, including fast pacing, betaadrenergie stimulation, the addition of dibutyryl cyclic AMP
and intracellular injection of calcium . Because calcium overload may occur during ischemia and reperfusion, a reasonable hypothesis would be that delayed afterdepolarizations
could play a role in ischemic and reperfusion arrhythmias
(55). The factors capable of provoking and inhibiting delayed
afterdepolarizations in guinea pig papillary muscles were
studied (58); delayed afterdepolarizations were abolished by
"simulated" ischemia consisting of the combined presence
of increased extracellular potassium, high lactate, low pH
and hypoxia. In contrast. reoxygenation after a hypoxic
JACC
Vnl. 19. Nn. 7
June 1992 :11,22-11
A
MV
CYCLIC
Extracellular potassium
54mM 10BmM 162mM
I
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Figure
6.
Effect of increasing estracellulur potassium c
1627
Metabolic maps, cyclic AMP and ATP . The preceding
data indicate that it is logical to expect that delayed afterdepolarizations can be induced by intracellular cytosolic cal .
clam overload in conditions where there is -combination of
an elevated tissue cyclic AMP level and tissue ATP levels
that have not yet fallen to <2 m,M (approximately 2 ;,moles/
p1. 1 n early regional ischemia . 20 min after coronary ligation,
it is possible to find the combination of a localized increase
in cyclic AMP with a relatively well maintained ATP level
>2 1amoles'g (32) . This combination is. however, highly
focal . in keeping with the role of metabolic heterogeneity in
prcinitating ventricular arrhythmias . All these observations
do not directly prove that delayed afterdepolarizations occur
in ischemic myocardium . but they provide strong indirect
evidence .
40
~'ll9lz • .
Ltmar. FT AL.
AMP, CALCIUM AND ARRHYTHMIAS
cntri-
tions on delayed afterdepolarizations (DAD,). A, A delayed afteedepolarization was elicited by using a train of 30 pulses at a
frequency of 5 Hz in the presence of isoproterenol (10-7, and then
abruptly stopping the train . Increasing the extracellular concentration of potassium (K'1 to 10 .8 mM or 162 m,bf depolarized the
membrane potential and decreased the amplitude of the afterdcpolarization . B, Proposed influences of different components of ischemia on delayed afterdepoladzations . Note that delayed afterdepolarizations are unlikely to develop in severely ischemic zones with
high external potassium and low intracellular adenosine triphosphate (ATP). 0 = betaadrenergic receptor : Na = sodium . Data
provided by W. A . Coetaee .
glucose-free period led to an increased amplitude of delayed
afterdepolarizations with associated extrasystoles(59) . Similarly, Bhattachaiyya and Vassalle (60) found that digitalisinduced delayed afterdepolarizations were abolished ey metabolic inhibition .
Mechanism underlying delayed afterdepolarizations. This
mechanism is the calcium-induced transient inward current
(T i or I,). It is proposed that rhythmic internal calcium
oscillations cause rhythmic switching on and off of the
transient inward current, thus predisposing to arrhythmias
mediated by this current . The mechanism of abolition of the
transient inward current by metabolic inhibition (58) was
investigated in guinea pig ventricular myocytes by the voltage clamp technique (61,62) . The blocking effect of metabolic inhibition could not be explained solely by either an
increased potassium current or changes in the inward calcium current (lea ) . Internal dialysis of isolated myocytes
showed that ATP depletion to <40% of control values
blocked the transient inward current, probably by interfering
with calcium uptake by the sarcoplasmic reticulum . Repletion of ATP by internal dialysis showed that levels of ATP of
2 to 5 mM were required for the transient inward current to
be operative (61) . The presumed mechanism of the effects of
ATP was by restoration of energy-dependent internal calcium movements, probably at the level of the energydependent uptake of calcium by the sarcoplasmic reticulum .
Other Calcium-Dependent Mechanisms
Possibly Contributing to Early
Ischemic Arrhythmias
1. Early aferdepolarizatiom- Although early afterdepoiarizations are also calcium dependent !63) and arrhythmogenic . these abnormalities are precipitated under conditions associated with lengthening of the action potential
duration, as may occur in ischemia (64) . In contrast, delayed
afterdepolarizations are provoked when calcium overload is
accompanied by iachycard Early afterdepolarizations are
more likely to cause torsade de pointer than the regular
ventricular tachycardia precipitated by delayed afterdepolarizations . Among patients who died suddenly while wearing a
Hotter ECG monitor, 8% had tarsade de pointes and 68%p
had some form of ventricular tachycardia (65) .
2 . Delayed intercellular conduction and altered cable propenles. Both cyclic AMP 166) and calcium (67) augment the
intercellular resistance, thereby decreasing conduction velocity and predisposing to reentry arrhythmias. Conduction
velocity in myocardial ischemia may also be changed by
altered cable properties . Thus, hypoxia increases the internal longitudinal resistance (R i) (68,69). This effect is also
seen with increased cyclic AMP (66)- increased external
calcium or intracellular acidosis (67) . All these effects are
probably mediated by elevated cytosolic calcium because
the junctional conductance is much more dependent on the
internal calcium ion concentration than on the internal
proton concentration (70l.
In addition, an increase in extracellular potassium concentration, as in ischemia, will depolarize the membrane
potential, moving it into a range of potentials where the
sodium current is partly inactivated . Within a certain range
of ischemia-induced increases in the external potassium
concentration, the upstroke of such a "depressed fast response" might actually consist of two phases, the first of
which can be blocked by sodium channel blockers and the
second of which is calcium dependent (47) .
1628
LueeeeTAtCYCLIC AMP, CALCIUM AND ARRHYTHMIA .
3. Direct evidence linking cylwulie valcium and ventricular
tachycardia or fibrillation . More direct evidence for link,
between increased cytosolic calcium and ventricular alrhythmias is threefold . First, Lee et al . (711 measured local
variations in cytosolic calcium among various sites in the
ischemic zone . They proposed f?21 that such local variations
in cytosolic calcium may explain the nlternnns pattern in the
T wave that precedes ventricular fibrillation in the dug.
Second, Merillat et al . 173) induced ventricular fibrillation
in the rabbit heart by sodium pump inhibition. Decreasing
external calcium to 80 kM or adding verapamil or nitrendipine consistently prevented the initiation of such ventricular
fibrillation . Ventricular fibrillation, once induced . may result
in abnormalities of cytosolic calcium, including enhanced
entry through the voltage-dependent calcium channel (74) .
Third, Kihara and Morgan (75) showed that spontaneous
ventricular fibrillation rapidly developed in an isolated ferret
heart concomitant with large irregular oscillations in internal
calcium . These oscillations probably represented calcium
ions cycling in and out of the sarcoplasmic reticulum because the oscillations were reduced by caffeine, an inhibitor
of calcium release from the sarcoplasmic reliculum . Pretreatment with ryanodine, a highly specific inhibitor of
rdciu: cycling that acts on the - aauline channel of the
sarcoplasmic rciicuium, prevented the induction of ventriculne fibrillation .
Fourth, Thandroyen et al . (76) found that spontaneous
fibrillation in calcium-overloaded isolated myocytes was
preceded by an increase in cytosolic calcium from nartomelar to micromolar levels . Furthermore, the tnehyarrhythmia
itself promoted severe calcium overload, which is thought to
he important in the perpetuation and degeneration of the
arrhylhntias .
Nonetheless . a is clear that more data are required on
cytosolic calcium levels in the whole heart preceding the
induction of ventricularftbrillation by calecholamines, cyclic
AMP and calcium .
Clinical Evidence Supporting the Hypothesis :
Protection Against Sudden Cardiac Death by
Beta-Adrenergic Blockade
Most instances of sudden cardiac death in developed
countries are believed to be due to ventricular fibrillation,
generally preceded by monomorphic ventricular lachycardia
(6S) . In animal experiments, as we just reviewed, administration or some beta-blockers before coronary artery ligation
lowers the ischemic myocardial cyclic AMP levels and
reduces the incidence of ventricular fibrillation (24) . There is
now firm evidence that heta-blocker treatment offers protection against sudden cardiac death in victims of acute myocatdial infarction (77), but thus far no convincing evidence
has been produced for a primary preventive action in hypertensive patients .
. 19. Na. 7
IACC 1'M
I- 9v2 1v22-13
Primary prevention . The risk of all forms of eorenary
heart disease, including sudden cardiac death . is substantially increased in hyperlensive patients . Propranolul in the
Medical Research Council (MRC) trial, exprcnolel in the
international Prospective Primary Prevention Study in Hypertension (IPPPSHI rind atenolol of metoprolol in a third
trial have been studied (791 . No convincing primary preventive effect of beta-blocker treatment against sudden cardiac
death auld be demonstrated in these trials (7&). Subsequently, in the Metoprolol Atherosclerosis Prevention in
Hypertensive (MAPIIYI trial (79), io which 3,234 hypertca.
sive patients were randomized to metoprolol or diuretic
therapy, a significant reduction in the incidence of sudden
corehary death was suggested for subjects randomized to
metoprolol. There were 99 deaths that were classified as due
to cardiovascular causes and 78% of these were sudden
(within 24 h of the onset of symptoms). The 32 sudden deaths
in the metoprolol group were fewer than the 45 sudden deaths
in rile diuretic group (p < 0 .02) . However, there was no
placebo control group and it is possible that the number of
sudden deaths was increased in the diuretic-treated group
rather than reduced in the metoprolol -treated cohort .
Secondary prevention, In secondary prevention trials,
there is convincing evidence for a protective effect of betablockers against sudden cardiac death . Analysis of the
pooled data (80) from trials in which beta-blocker treatment
was started early after hospital admission and was maintained for al year suggests that beta-blocker treatment
reduces the risk of sudden death by about 30% . In prolonged
postinforction follow-up therapy with metoprolol, the sudden death rate was reduced by 50% (81) . At least some of
these beuelits tiny be explained by the combined effects of
beta-blockade in preventing ischemic changes and the elcclrophysielegic changes mediated by ischemia .
Early Intravenous bela.bluckude fw acute myocardial inferctfon . In 27 trials (77) in which beta-blockers were administered intravenously' on admission to patients with myocardial infarction (before the thrombolytic era), a I3%
reduction in the mortality rate was demonstrted in 27,040
subjects . Analysis of the pooled data for the 27 trials showed
a 23 *_ 8%a reduction in the risk of dying in the Ist 24 h in
patients with myocardial infarction treated with betablo.kers versus I I ± 11% in the 2nd 4& h and a I ± I t%
reduction in the period between days 4 and 7 . Thu importance of very early administration of beta-blockers for obtaining the protective effect of beta-blockade against sudden
death is well illustrated by comparing the results of the First
International Study of Infarct Survival (ISISI) (82) and the
trial reported by Norris et al . (83). When atenolol was
administered within 8 h of symptom onset in the ISIS-1 (82)
to 81% of 8,037 subjects, the incidence rate of ventricular
fibrillation was 2 .4% versus 2 .5% in control subjects . Administration of intravenous propranolol within 4 h of the
onset of symptoms of myocardial infarction decreased the
incidence of ventricular fibrillation from 5 .7% to 0 .8% (p <
0 .01). 'the maximal early beueftt of beta-blockers in the
JACC V01. 19 . Nu . 7
.unc 1992:102:-33
prevention of ventricular fibrillation in acute myocardial
infarction supports observations in animal experiments i'-51
that show a distinct lime phase when ischemic myocardial
cyclic AMP levels are elevated .
Factors enhancing the protective effect of beta-blockers .
Other factors based on experimental data may also he
relevant for the clinicall y important protective effect of
beta-blockers to become manifest . First . exercise scathing,
for 10 weeks in rats (84) previously subjected to coronary
artery ligation resulted in a blunted increase in myocardial
cyclic AMP levels (at i0 s) after a second coronary artery
ligation. Second, the dosage of the hem-hlorker administered intravenously may also he a determining factor . In pigs
(24) . 0 .1 mg/kg of propranolol administered 30 min before
and 10 min after coronary artery ligation prevented ventricular fibrillation, whereas perhaps unexpectedly, doses of 0 .5
and 3 mg/kg did not. It was proposed that the protective
effect shown at the lower dose was offset by the negative
inotropic effect encountered with the higher doses . Third .
hypokalemia associated with acute adrenergic overactivity
may be of paramount importance. The reduction in scrum
potassium is mediated by beta,-adrenoceptors, and betablockers with beta_ antagonist activity may attenuate the
severity of hypokalemia (85) . Reduced extmcellular levels of
potassium are associated with greater elevation of myocardial cyclic AMP levels after coronary artery ligation (86).
Fourth, ancillary properties of beta-blockers may also be
important (87) . Propranolol, which is fat soluble . is concentrated in cardiac cells and some actions may not strictly
reflect plasma levels . In the isolated rat heart '27) . propranolol (d- and I-isomers) prevented the early inerea0e in cyclic
AMP after coronary artery ligation, whereas atenolol did
not. However, controlled clinical trials will be required to
establish whether propranolol has a more powerful protective effect than other beta-blockers against ischemia-related
ventricular fibrillation .
LUHHE ET AL.
CYCLIC AMP. C .-'.LCIUM AND ARRHVTHMIAS
1629
Heart Failure, C;'ti:c AMP-Calcium,
Phosphodiesterase Inhibitors and Sudden
Cardiac Death
Systolic and diastolic dysfunction in heart failure . Derangements in cellular levels of cyclic AMP and cytosolic
calcium are pivotal to the disordered systolic and diastolic
function in heart failure (93,94) . The overactivity of the
adrenergic nervous system. with enhanced neurotransmitter
release and circulating catecholamines, is paradoxically associated with reduced intracellular levels of cyclic AMP,
chiefly because of downregulation of myocardial beta,
adrcnergic receptors or increased expression of the inhibitory G-protein (Fig . 7 and 8), or both (95) . Subcellularly .
there are abnormalities in patterns of rise and fall of cytosolic calcium levels . The systolic calcium transients are
reduced (as a consequence of inadequate cyclic AMPmediated calcium channel activation), resulting in reduced
force generation and lower stroke output . Impaired interaction of calcium with troponin-actomyosin also contributes to
the systolic dysfunction . Uptake of cytosolic calcium by the
sarcoplasmic reticulum is impaired with consequent prolonged diastolic calcium transients, which may account for
the diastolic dysfunction in heart failure .
Arrhythmias in cungeslise heart failure (Table I) . Patients
with congestive heart failure have a poor prognosis, and
about 50% of the deaths are sudden and probably arrhythmic
in origin (96) . In patients with severe heart failure (functional
classes III and IV), Maskin et al . (97) found a high prevalence of arrhythmias ; 92% of the patients in their study had
multifocal premature ventricular complexes and 71% had
nonsustained runs of ventricular tachycardia. Treatment of
heart failure with inotropic agents oith proaehythmic potential may therefore improve hemodynamic function but
increase mortality.
Arrhythmias due to inotropic therapy . The time-honored
approach to inotropic therapy of heart failure involves
increasing intracellular levels of calcium by digitalis glycosides . which act by a cyclic AMP-independent mechanism .
Stress and Ventricular Fibrillation
The proarrhythmic potential of toxic doses of glycosides is
established, with the role of calcium in such ventricular
Although postinfarction death is not linked to a type A
tachyarrhythmias well supported by laboratory observapersonality (88), the most common triggers of acute myocardial infarction are emotional upset (18%) and physical activtions . Alternative approaches whereby systolic function can
he boosted involve increasing intracellular levels of cyclic
ity (14%) (89), both associated with catecholamine disAMP either by stimulation of available beta-receptors or
charge . Of interest is the experimental finding (90) that
reducing cyclic AMP breakdown by inhibition of the phosphointracerebral administration of propranolol acts to reduce
diesterase responsible for cyclic AMP breakdown. All posithe incidence of ventricular fibrillation in stressed pigs with
tively inotropic agents ultimately acting through an increase in
regional ischemia . It remains possible that beta-blockade
cytosolic calcium are likely to be proarrhythmic (Fig . 7).
may act in part against sudden death by a centrally mediated
reduction of stress-stimulated pathways . In the BetaTreatment with beta ; agonists: prenalterol and dubs.
famine. The results of long-term treatment of heart failure
Blockade Heart AttackTrial (BHAT) study (91). propranolol
reduced the incidence of early morning sudden cardiac
with the use of beta,-agonists (prenalterol, dohutamine) have
been disappointing. The initial improvement seen in cardiac
death, which is thought to be mediated by increased sympafunction is not maintained, suggesting further downregulathetic activity. In patients with spontaneous ventricular
tion of beta-receptors . A meta-analysis (98) of nine trials
tachycardia or fibrillation, there is evidence for selective
involving the use of beta-receptor agonists shows a distressing
(92)
.
cardiac sympathetic activation
LUBBE ET AL.
CYCLIC AMP, CALCIUM AND ARRHYTHMIAS
1630
JACC VAl 19 . No. 7
Jrme 1991d02-33
°'
a
0,
--fFigure 7. Calcium (Cae i), inotropes and arrhythmias
seclraflu
Three positively inotropic principles include the use of
digitali.., beta-adrcnergic agents or phosphodiesterase
(PDE) inhibitors . in each case, the ultimate effect is an
increase in cytosolic calcium with risk of arrhythmias. AC
J
,nnrcas cAMP oraa c-
h .. 0AD5
.Slow re
pro arrhy
= adenylate cyctase . G, = stimulatory G-protein; I, ;
transient inward current; other abbreviations as in Figures
I and 6.
ses
Deaen~cells
surcease by 2 .07 in the relative risk of dying during such
therapy . It is possible that intracellular calcium overload,
related to persistent beta-receptor stimulation and fluctuations
in intracellular cyclic AMP levels determined by variable
degrees of phosphodiesterase inhibition by ischemia and cellular acidosis, may be responsible for fatal arrhythmias (Fig. 8) .
Treatment with phosphodiesterase inhibitors: amrinone,
milrinone and enoximune . A substantial improvement in
hemodynamic function can be obtained in heart failure with
the short-term use of inhibitors of phosphodiesterase type
III, such as amrinone, milrinone and enoximone. These
agents selectively increase cellular levels of cyclic AMP and
have the potentially beneficial additional action of vasodilation . Their use as therapeutic agents in heart failure is
Figure 8 . Proposed prow rhythmic mechanisms in left ventricular
(LV) failure and arrhythmias, most of which are thought to act by
means of an increase in cellular calcium . Impaired function of the
sarcoplasmic reticulum may account for the prolonged calcium
transients (93) . In left ventricular failure, rom ation of cyclic AMP is
impaired and myocardial function can be improved by the combined
administration of both phosphodiesterase (PDE) inhibitors and
beta-stimulation with, however, risks of ventricular arrhythmias .
G; = inhibitory G-protein ; other abbreviations as in Figures I and 6.
p-afire ergic
drive
~~PIE
MP 7
lJ
Inho.mrs
downgraded
captors
fnonrensn
digaalls
- '
chanoterrors
DADS
responses
uncoupling
slow
low
arterial
x+
therefore based on rational principles . However, according
to the cyclic AMP hypothesis, phosphodiesterase type III
inhibition have the potential to cause ventricular tachycardia
and fibrillation . Analysis of 12 trials (98) in which phosphodiesterase inhibitors were used has indicated an increased
relative risk (1 .58) of dying during treatment with such
agents . The Prospective Randomized Milrinone Survival
Evaluation (PROMIS) study, in which milrinone was used in
patients refractory to conventional therapy of heart failure,
was terminated because of the increased cardiovascular
mortality in treated patients (99) . Earlier uncontrolled studies (100) with milrinone revealed an increase in complex
ventricular ectopic activity and instances of sudden death
despite hemodynamic improvements .
Caffeine and arrhythmias . Caffeine, a phosphodiesterase
inhibitor, will elevate myocardial cyclic AMP (101), yet no
clear links have emerged between heavy coffee drinking and
Table l . Factors Influencing Development of Potentially Lethal
Arrhythmias in Congestive Heart Failure
Prearrhythmic factors
Circulating factors
Increased circulating catecholamines
Increased reninangiotensin activation
Decreased arterial potassium
Cardiac factors
Taahycardia
Cardiac dilation
Cardiac sanellular factors
Combination of PDE inhibition and beta-agonism thigh cAMP)
Inhibition of sodium pump by digitalis glycosides
Prolonged calcium transients
Antiarrhylhmic factors
Cardiac cellular factors
Decreased rates of formation of cAMP theta-receptor
downgmding. adenylate cycluse inhibition . excess G, protein)
Cardiac therapy
Beta adrenergic blockade
Angiutensirvconverting enzyme inhibiaan
For data on calcium transients and cyclic adenosinemonophosphare
VENTRICULAR TACHYCARDIA
AND FIBRILLATION
lcAMPI. see Stonier 193). G, = inhibitory G-protein ; PDE = phosphodi-
esterase .
IACC Vol. 19. No. 7
June 19''9622-33
sudden cardiac death . Probably in contrast to the other
phosphodieslerase inhibitors_ cuffai a, extremely complex additional effects on calcium uptake and release by the
sarcoplasmic reticulum . These latter effects probably explain why caffeine decreases the incidence of ischemic and
ventricular arrhythmias in the rat heart (101) . Caffeine acts
on the calcium release channel of the sarcoplasmic reticulum
to increase the open probability of this channel and therefore
to discharge calcium from the sarcoplasmic reticulum 11021 .
leading to decreased calcium availability for calciuminducea calcium release (103) and consequent inhibition of
calcium recycling in and out of the sarcoplasmic reticulum .
The latter effect is antiarrhythmic in contrast to tae early
initial proarrhythmie discharge of calcium from the sarcoplasmic reticulum (104).
Conclusions : Cyclic AMP, Calcium and
Ventricular Arrhythmias
Although there is considerable indirect evidence suggesting a role for cytosoiic calcium excess in mediating the
effects of an increased cellular cyclic AMP level, cytosolic
calcium concentrations can increase as part of the to tabolic
changes in ischemia through mechanisms that are not necessarily directly linked to an increased tissue level of cyclic
AMP . In general, catecholamine stimulation will increase
the severity of ischemia and the rate of accumulation of
cyclic AMP and cytosoiic calcium . Crucial experiments
linking beta-receptor stimulation with levels of cyclic AMP
and levels of cytosolic calcium must still be undertaken
before it can be stated with confidence that cytosolic calcium
is the ultimate mediator of catecholamine activity in causing
early ischemic ventricular arrhythmias.
However, the cyclic AMP hypothesis provides a rational
scientific basis for the effect of beta-blockade in decreasing
the incidence of ventricular fibrillation after the onset of
myocardial infarction and reducing sudden death in the
postinfarction period, as well as explaining why phosphodiesterase inhibitors can increase the mortality rate in patients
with congestive heart failure. Nonetheless, it must not be
supposed that cyclic AMP itself is the "prime mover" ; rather.
the ultimate messenger seems to be elevation of cytosolic
calcium. Because cytosolic calcium is increased in heart
failure (106), it will be difficult to develop positive inotropic
agents that are free of the risk of sudden death .
We are grateful to our colleagues who have advanced these concepts by
numerous discussions and collaborative work including Dr . F . Thandreyen
and Dr . W. A. Coetzee. Dr. Curlier kindly provided Figures 5 and 6 . We also
thank the Medical Research Council of South Africa . the Chris Barnad Fund
and the University of Cape Town for support. This repair was written during
the tenure of a Visiting professorship held by W . F. Lubbe aI the University
of Cape Town.
LUPuF Fa a,t_
C ve rIC AMP . CALCIUM AND anneroHMIAs
1691
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