Download Regulation of myocardial contractility 1958–1983: An Odyssey

42
J AM COLL CARDIOL
\983.\ :42- 5 1
Regulation of Myocardial Contractility 1958-1983: An Odyssey
ARNOLD M. KATZ , MD , FACC
Farmington, Connecticut
The past 25 years have seen an unprecedented growth
in our understanding of the mechanisms responsible for
the regulation of myocardial contractility. Beginning with
a demonstration that myocardial contractility represents
an important determinant of cardiac function, this quarter century has witnessed a series of attempts to explain
length-independent changes in myocardial contractile
function. Alterations in the properties of the cardiac
contractile proteins and changes in the amount of calcium (Ca 2 +) available for binding to the contractile proteins during excitation-contraction coupling are now recognized as important biochemical bases for physiologic,
Sing in me, Muse. and through me tell me the story
of that man skilled in all ways of conte nding.
the wanderer. harried for years on end,
after he plundered the stronghold
of the proud heigh t of Troy.
He saw the townlands
and learned the minds of many men.
and weathered many bitter nights and days
in his deep heart at sea . . . .
Homer. The Odyssey . Bk I
As the wanderings of Odysseus revealed many wonders. so
have the pursuits of those who , over the past quarter century.
searched for an understanding of myocardial contractile
function and its regulation . Their intellectual journey to
uncover the " true " meaning of myocardia l contractility was
begun shortly afte r the collapse of a theory of cardiac regulation that had dom inated scie ntific thought for almost half
a century and has been no less fantastic than the legendary
wanderings of Od ysseus.
This brief and personal history begins in 1954 when. as
a junior medical student, I attended a sympos ium organized
on behalf of the American Physiological Society by my
father. Loui s N. Katz. Thi s meeting toppled the then held
view that the pumping action of the heart was controlled
From the Cardiology Division . Department of Medicine. University of
Connecticut Health Center, Farmington. Connectic ut. Supported by Research Grants HL-21812 and HL-22 l 35 from the National Institutes of
Health , Bethesda, Maryland .
Address for reprints : Arnold M. Katz, MD , Cardio logy Division . Department of Medicine , University of Connectic ut Health Center. Farmington , Connectic ut 06032.
~ 1983
by the American College of Cardiology
pharmacologic and pathologic changes in myocardial
contractility. Identification of the central role of Ca 2 +
in the initiation of the cardiac contractile process has
made possible the analysis of the mechanisms by which
several drugs alter myocardial contractile function, including the biochemical processes that allow beta-adrenergic agonists to enhance contractility. The rapid developments in this fieldsince 1958 promisean even greater
flow of new knowledge regarding the causes, prevention
and treatment of heart failure provided that there is
given adequate support for such research activities.
excl usively, or even primarily , by changes in the end-diastolic fiber length of the myocardial fibers (I) . As magnificent (and doomed) as the city of the Trojans. the view that
the regulation of cardiac work could be explained excl usively or primar ily by the Frank-Starling relation had dominated the thinking of cardiac physiologists since the classic
studies of Starling performed during the second decade of
this century . Althou gh data challengi ng the primac y of Starling ' s law had been publ ished. it remained generall y accepted that the major determin ant of cardiac work was the
end-diastolic volume in the vent ricles. Evidence presented
at the 1954 symposium. however. demonstrated conclusively that "end-diastolic volume , while playing an important role in the work performance of the heart. is only one
of a number of equally important factors adjusting the heart' s
power to changing conditions" (2) . Drawing on earlier co ncepts of Dow and McMichael , Sarnoff (3) bro ught into focus
the interrelations between Starling 's law and the new concept of myocardial contractility with the statement, " .. ,
it has not been genera lly apprecia ted that a single Starling
curve cannot always explain the observed phenomena: for
any give n heart there is a series or family of curves" (Fig.
I). With th is clear statement. the next generation of cardiovascular physiologists-my generation-was drawn to
the searc h for an understanding of myocardial contractility,
Energy Production
I might have made it safely home. that time,
but as I came round Malea the current
0735- 1097/83/0 10042-10$03.00
REGULATION OF MYOCARDIAL CONTRACTILITY
took me out to sea. and from the north
a fresh gale drove me on , past Kythera .
Nine days I drifted on the teeming sea
before dangerous high winds. Upon the tenth
we came to the coastline of the Lotos Eaters.
... who showed no will to do us harm. only
offerin g the sweet Lotos to our friendsbut those who ate this honeyed plant . the Lotos.
never cared to report , nor to return :
they longed to stay forever . browsing on
that native bloom forgetting their homeland .
Homer , The Odvssey, Bk IX
Because cardiac contraction is an energy-consuming process, it was logical that early attempts to understand the
mechanism for the regulation of myocardial contractility
focused on changes in the availability of chemical energy
for the contractile process . As early as 1949, evidence had
been obtained that high ener gy phosphate content remained
normal during the acute failure of isolated heart preparati ons
(4), although a decline in both adenosine tripho sphate (ATP)
and phosphocreatine concentrations was noted in the chronically overloaded heart (5) . While a decrease in high energy
phosphate compounds remains a plausible explanation for
some form s of heart failure. notably ischemi c and hypoxic
(6), it now seems a fair generalization that under most physiologic conditions, changes in total high energy phosphate
content are the result of alterations in the level of cardiac
work (7) rather than a primary determ inant of the work
capacity of the myocardium. In the case of the powerful
positive inotropic effect of the catecholamines (Fig. I), for
Figure 1. Effect of epinephrine (suprarenin) on the work of the heart.
illustrating the effect of the beta-adrenergic agonist on the Starling curve.
GM . M. = gram-meter; LT. = left; L.V. = left ventricular. (Reprinted
from Sarnoff SJ [3], with permission.)
60
.-CONTROL
X-SUPRARENIN 1.36 j.Lg/KG .lMIN.
50
L.v.
40
STROKE
WORK
GM.M.
O-CONTROL
J AM CaLL CARDIaL
1983;1:42- 51
43
example , it is clear that the enhancement of myocardial
contractility is not due to a primary effect to increase energy
production (8). A similar conclusion regarding the positive
inotropic effect of the cardiac glycosides was report ed by
R. Bing (9) in the first volume of the American Journal of
Cardiology . He wrote : "[Digitalis] increases the force of
myocardial contraction . . . . How this is accompli shed is
not clear, j ust as the fundamental reasons for impaired muscular funct ion in clinical heart failure are obscure . . . . It
does not affect the (chemical) ener gy forming mechani sm
of the cardi ac cell .. . it improves the dimini shed response
of the total contractile mass of the failing heart to an undimin ished supply of chemical energy. . . ."
Heart Failure Myosin
In the next land we found were Kyklopes.
giants. louts. without a law to bless them.
. . . Kyklopes have no muster and no meeting.
no consultation or old tribal ways .
but each one dwell s in his own mountain cave
dea ling out rough justice to wife and child.
indifferent to what the others do.
Homer . The Odyssey. Bk IX
The demonstration that the work of the heart could be regulated by length-independent change s occurring within the
myocardium led a number of investigators to examine the
properties of the cardiac contractile protein s under conditions of altered myocardi al contra ctility. During the late
1950s and early 1960s. chan ges in the structure of the cardiac myosin molecule were reported to occur in the failing
heart , and one group ( 10) reported that the positive inotropic
effect of cardiac glyco sides was accomp anied by a rever sal
of the physicochemical abnormalities in "heart failure
myosin ." Controversial when first described (11), these
obser vations were shown to be incorrect by subsequent work
that dem onstrated that the overall molecular characteri stics
of the myosin molecule were not altered in a manner that
could explain either the effects of heart failure or drugs on
myocardial contractility (for review, see reference 12).
Role of Myosin Adenosine Triphosphatase in
Determining Muscle Shortening Velocity
30
20
When Dawn spread out her finger tips of rose
we turned out marvelling , to tour the isle.
while Zeus's shy nymph daughters flushed wild goats
down from the heights-a breakfast for my men.
Homer, The Odyssey. Bk IX
10
o~-"""'_"""_-""---"""'----l"---~------'
10
20
LT. AURICLE MEAN PRESSURE
(CM. H20)
In the late 1950s , a time of intense controvers y regardin g
the exi stence of physicochemical abnormalities in heart failure myosin, several investigators noted that the adeno sine
J AM cou, CARDIOl
1983:I:42-51
44
KATZ
triphosphatase (ATPase) activity of cardiac myosin was less
than that of rabbit fast (white) skeletal muscle, the "standard" source of this contractile protein. In 1966, my colleagues and I (13) confirmed these reports that cardiac myosin
had a lower ATPase activity than rabbit skeletal muscle
myosin, and obtained evidence that the low enzymatic activity of cardiac myosin might be related to the low shortening velocity of the intact myocardium. A now classic
study by Barany (14) related myosin ATPase activity to the
mechanical expression of this enzymatic rate by showing a
close correlation between the rate of ATP hydrolysis by
myosin and the maximal shortening velocity in a variety of
different muscles (Fig. 2). At a time when studies of cardiac
mechanics had come to dominate our view of the physiologic
expression of myocardial contractility (see later), and shortly
after A.F. Huxley (15) had provided a theoretical basis for
understanding the relation between the rate of cross bridge
turnover and muscle shortening velocity, Barany's observations provided a clear biochemical explanation for the
early observations of A.V. Hill (16) that "the rate at which
chemical transformation would occur, and therefore at which
energy would be liberated, would ... [increase) as the force
diminished. "
The direct relation between myosin ATPase activity and
the maximal shortening velocity of unloaded muscle has
Figure 2. Relation between myosin adenosine triphosphatase (A'l'Pasc)
activity and maximal shortening velocity of a variety of muscles, based
on data of Barany (14). (Reprinted with permission from Katz AM. Contractile proteins in normal and failing myocardium. Hosp Pract 1972:7:5769.)
24
M~. ,~...Io
gito rum
'0..1//
20
/
Rat Extensor Digitorum Longus
/
2
/--
Cat Fast
/
8
i/
Mytilus
Posterior
Adductor
V----.
/
/
/
V
Mouse Soleus
c---- Rat DiaPhralgm
/
Frog Sartorius
v·~""''''
.-Human
E1bo1 Flexor
_ _ Cat Soleus
Pecten IStrlated
I
Adduct~r
Frog Sartorius
Dogfish
Corac~hyold
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o
10
A1Pase Activrtv
In
Presence of
15
20
Ca"" (,LLmoles Plfgm/secl
25
been amply confirmed during the past decade, and now
represents a firm basis for an understanding of one aspect
of myocardial contractility and its tonic regulation (17). In
the failing, chronically overloaded heart, for example, a
decrease in maximal shortening velocity can be explained
by a redirection of protein synthesis that leads to the appearance of a myosin isozyme having a low ATPase activity
(18) and subtle alterations in amino acid sequence (19).
Similarly, changes in maximal shortening velocity that occur in the hearts of hyperthyroid and hypothyroid animals
are paralleled by changes in myosin ATPase activity (20).
Cardiac Mechanics
. . . inside her quiet house
they heard the goddess Kirke.
Low she sang
in her beguiling voice. while on her loom
she wove ambrosial fabric sheer and bright,
by that craft known to the goddesses of heaven.
... On thrones she seated them. and lounging chairs.
while she prepared a meal of cheese and barley
and amber honey mixed with Pramnian wine.
adding her own vile punch to make them lose
desire or thought of our dear father land.
Scarce had they drunk when she flew after them
with her long stick and shut them in a pigstybodies, voices. heads and bristles. all
swinish now ....
Homer. The Odvssev, Bk X
Force-velocity relation. Shortly before the role of myosin
ATPase activity in determining the shortening velocity of
cardiac muscle had become the subject of intensive study,
there appeared an approach to the characterization of myocardial contractility that came to dominate the thinking of
cardiovascular physiologists for more than a decade. This
was the utilization of the classic force-velocity relation,
which describes the relatively stable tetanic contractions of
frog sartorius muscle (16.21) as characterizing the more
dynamic situation during contraction of cardiac muscle.
In 1959, Abbott and Mommaerts (22) published the first
characterization of the mechanical behavior of cardiac muscle that used the force-velocity relation as a basis for data
analysis. This report, however, noted several possible pitfalls in the application of the muscle mechanics observed
in tetanized skeletal muscle to the much more complex,
twitchlike contraction of the heart. Three years later, Sonnenblick (23) presented a more extensive study of this subject at the annual meeting of the American Physiological
Society (Fig. 3). Shortly after the importance of changing
myocardial contractility in regulating cardiac performance
had been recognized, Sonnenblick's report played a seminal
role in bringing together the work then in progress on the
biochemistry of the contractile proteins and efforts to define
the basis for the changing hemodynamic properties of the
J AM cou. CARDIOL
1983:1:42-51
REGULATION OF MYOCARDIAL CONTRACTILITY
EFFECTS OF INTERVENTIONS ON THE FORCE
VELOCITY RELATIONS OF THE CAT PAPILLARY MUSCLE
V mall
INCREASING LENGTH
INCREASING RATE
NOREPINEPHRINE
t
tt
Po
tt
tar -
t
Figure 3. Diagrammatic representation of force-velocity relation of cardiac muscle. (--) control: ( - - - - ) increasing Po: (...... ) increasing
Vm " . (Reprinted from Sonnenblick EH [23]. with permission.)
heart in the intact animal and human beings. Although subsequent developments have generally supported the earlier
caution of Abbott and Mommaerts, Sonnenblick's presentation stimulated important efforts to integrate biochemical
and physiologic approaches to an understanding of myocardial contractility.
Down to the shore and ship at last we went
bowed with anguish. cheeks all wet with tears.
to find that Kirke had been there before us
and tied nearby a black ewe and a ram:
for she had gone by like air.
For who could see the passage of a goddess
unless she wished his mortal eyes aware?
Homer. The Odyssey. Bk X
Controversies concerning Vmax' A review of the controversies regarding the interpretation of the force and velocity data obtained in cardiac muscle is beyond the scope
of this article, but a delightful tongue-in-cheek description
45
of these early years by Harris (24) is highly recommended.
These controversies focused on three issues: I) the validity
of the extrapolations of velocity measurements made at various muscle loads to the shortening velocity at zero load
(V maJ (25). 2) the ability of such extrapolated values for
V max to serve as quantitative measurements of myocardial
contractility (26), and 3) the relation of these V max determinations to the level of myosin ATPase activity (27). Although none of these issues has been settled to the satisfaction of all workers in these fields. these controversies
have gradually drifted to the background as the complexity
of cardiac muscle mechanics has become recognized. These
complexities, and the ambiguities in the interpretation of
velocity data calculated by the use of various equations
applied to pressure measurements obtained during isovolumic contractions. have led to a general abandonment of
Vmax and related indexes for quantifying the state of the
myocardium in patients with heart disease.
Ejection fraction, The failure of cardiac mechanics as
a clinical tool is eloquently demonstrated by the fact that
cardiologists have recently turned to the ejection fraction,
an index obtained readily by noninvasive methods, as a
means to evaluate the overall pump function of the heart.
Although the ejection fraction is of proved value in the
classification of patients with heart disease, our continuing
inability to quantify myocardial contractility is evidenced
by the current (and to this writer, distressing) tendency to
use ejection fraction as a measure of the contractile state of
the heart muscle despite the obvious and major effects of
extramyocardial variables, such as heart rate and peripheral
resistance, on the ratio between stroke volume and enddiastolic volume.
Role of Potassium in Determining
Myocardial Contractility
Swiftly she turned and led him to her cave.
and they went in, the mortal and immortal.
... where the divine Kalypso placed before him
victuals and drink of men: then she sat down
facing Odysseus, while her serving maids
brought nectar and ambrosia to her side.
Then each one' s hands went out on each one' s feast
until they had had their pleasure; and she said:
'Son of Laertes, versatile Odysseus,
after these years with me, you still desire
your old home? Even so, I wish you well.
If you could see it all, before you goall the adversity you face at seayou would stay here and guard this house. and be
immortal. .. .'
Homer. The Odyssey. Bk V
Twenty-five years ago, attempts by physiologists to relate
changes in potassium (K +) fluxes across the myocardial cell
membrane to variations in the contractile state of the myocardium represented an interesting but erroneous effort to
46
J AM COLL CARDIOL
19R3;I ''+2- S1
ex plain bea t to bea t changes in myocardial contractility . Th e
idea that changing K + level s wit hin the ce ll could influence
myocardial contrac tility had its origi ns iii the classic studie s
of Szent-Gyorgyi (28) who obse rved that high K + concentration s reduced the interaction s between actin and myosin
in vitro. The often cited study by Hajdu (29), in which the
force ge nerated from isol ated ca rdiac muscle prep aration s
was found to decrease by interventions that led to K + uptake
by the heart and to incre ase when K + left the myocard ial
ce lls , coupled with Szent -Gyorgyi' s observations , served as
the basis for a widely held hypothesis that changes in cytosol ic K + were directl y responsible for changes in myoca rdial contractility. However, prop onent s of this theory had
overloo ked the fact that Hajdu (29) had also found that K +
efflux was accompanied by a slight net influx of sodium
(Na + ) and, more importantl y . a corresponding loss of water,
and that intracellular K + concentration hardl y changed. Furtherm ore, the inhibitory effe cts of K + and Na + on the
interaction s between the contrac tile prote ins were alm ost
ident ical (30) . Th ese obse rvations , coupled with the rapid
grow th of our understanding of the primary role of calcium
(Ca~ + ) in the control of the contractile process , led to the
gradual aband onment of the conce pt that myocardial contract ility was importantly regul ated by changes in cellular
K + content.
Calcium and Activation of the Cardiac
Contractile Proteins
A seco nd course
lies betwee n the headlan ds. One is a sharp mountain
piercing the sky, with stormcl oud round the peak .. , ,
No mortal man could scale it. nor so much
as land the re . not with twenty hands and feet ,
so sheer the cliffs are as of pol ished stone.
, . , that is the den of Skylla, where she yaps
abomi nably. a newborn whelp ' s cry. ' , . ,
The opposite point seem s more a tongue of land
you 'd touch with a good bowshot at the narrows.
A great wild fig. a shaggy mass of leaves.
grow s on it. and Khary bdis lurks below
to swa llow dow n the dark sea tide .
Ho mer. The Odvssey , S k XII
Our present understanding of the prim ary role that Ca 2 +
plays in the regul ation of myocard ial contractilit y arose from
a series of observations that began a ce ntury ago when
Ringer (3 1) found that this cation was esse ntial for cardiac
contraction. Th e finding of He ilbrunn and Wiercinski (32)
that injec tion of Ca2+ into sing le muscle fibers caused co ntrac tion pro vided concrete evide nce that this esse ntial role
of Ca2 + in myocardial contract ion was due to an action
within the cell. Thi s led Sand ow (33) in 1952 to formulate
a hypothesis in which Ca 2 + release d from the cell membrane
activated the contractile pro cess.
KATZ
"Soluble relaxing factor." The critical role of Ca 2 + in
contrac tile activation found additional ex perimental support
in obse rvations made dur ing the 1950s with a "soluble
relaxing factor" prepared fro m ske letal muscle homogenates. Th is factor ca used relaxation in a variety of actomyosin preparations. an effec t that co uld be reversed by the
additio n of sma ll amounts of Ca 2 + . At that time , however.
knowledge of both the nature of the relaxing factor and the
actions of Ca 2 + on the co ntractile protein s was in a state
of confus ion and controve rsy.
Mechanisms. Th e mechanism by which Ca2 + regulates
mu scle contraction and the nature of the action of the "soluble relaxing factor" were clarified by two observations.
both described in the early 1960s. The first to be defined
was the nature of the "soluble relaxing factor," which turned
out not to be soluble. but instead to consist of membrane
vesicles pre sent in the supernatants obtained after the relatively low speed centrifugations generally used in the 1950s.
These membrane ve sicles. derived from an intrace llular
membrane structure . the sarcoplasmic reticulum . were found
to transport Ca 2 + into their interior . thereby ex plaining both
their actio n to relax actomyosi n preparation s and the ability
of added Ca 2 + to reverse their relaxing effect. As was the
rule in the field of muscle biochemistry, these observation s
were first made in skeletal muscle (34.35) and shortly thereafter in ca rdiac muscle (36 -40) .
The second observation that defined the role of Ca 2 + in
regulating myocardial contractility stemmed from an understand ing of the interaction between Ca 2 + and the co ntracti le proteins. A critica l observation was made by Weber
and Winicur (4 1). who clar ified the confusing effects of
Ca2 + on skeletal muscle acto myosi n by defining a role for
micromo lar Ca 2 + conc ent ration in activating its Mg 2 + -activated ATPase activity. Th eir paper also introduced the use
of the Ca 2 + -chelator (EGTA). whose high specificity for
Ca 2 + -binding in the pre sence of magnesium (Mg 2 + ) allowed Ca2 + concentration to be regulated experimentall y
in the micromolar range.
. . . we rowed into the strait. Skylla to port and on our starboa rd
beam Kharybd is .. ..
Homer. The Odyssey . S k XU
Ca 2 + and the troponin-tropomyosin complex.
Characterization of the Ca 2 + -sensitivity of actomyosin, which
of course ex pla ined the Ca2 + -dependent action of the ' 'soluble relaxing factor," led to intensive inves tigation of the
mechani sm by which Ca 2 + activa ted the contractile prote ins
(see reference 12 for review) . Within a few years I had
found (42) that trop omyosin. a protein described in 1948.
was able to regulate the interaction between actin and myosin.
More important was the discovery by Ebashi and co-workers
(43) that troponin, a newl y discovered muscle protein. along
with tropomyosin could sen sitize actomyosin to changes in
J AM cou, CARDIOl
1983;I:42- 51
REGULATION OF MYOCARDIAL CONTRACTILITY
B
A
~
C
0.400
=t
E
o
<0
<0
Figure 4. Effects of regulatory proteins on the EGTA
(GEDTAl -sensitivity of superpre cipitation (an in vitro model
of contraction) by reconstituted actom yosin. (e l low Ca~ + ;
(0) Ca ~ + present. A, Troponin alone added . B, Tropom yosin
alone added . C, Troponin plus tropom yosin added . (Reprinted
from Ebashi S. Kodama A [45], with permission.)
<; 0.300
Control
~
r
•
)-
':::
C/l
z
47
Control
0 .200
...J
<l:
GEDTA
o
f=
g;
0.100
_--'-_~!_~I-
2
3
I
I
I
246
I
8
TIM E (minutes)
Ca2 + concentration in the micromolar range (Fig. 4) (44,45).
An interesting reflection of Ebashi ' s attitude during this
phase in the development of our knowledge of the contractile
proteins can be seen in a letter that I received in 1965 from
Ebashi, who was aware of my effort s to explain why highly
purified tropomyosin failed to sensitize the reconstituted
actomyosins made from highly purified actin and myosin to
Ca 2 + (46). In this letter, Ebashi suggested that a protein
mixture I had found to sensitize actomyosin to Ca 2 + contained troponin , and he also sent a draft of his unpublished
description of this protein , giving me permission to use his
unpubli shed methods. His characterization of the activating
effect of Ca2+ on skeletal muscle contractile protein s as a
result of an interaction of this cation with the troponintropomyosin complex made it possible for us to show that
a similar effect also occurred in the cardiac contractile proteins (Fig . 5) (47).
We rowed on.
The rocks were now behind ; Kharybdis too .
and Skylla dropped astern.
Then we were coasting
the noble island of the god. where grazed
those cattle with wide brows. and bounteous flocks
of Helios, lord of noon . who rides high heaven.
Horn er, The Odyssey . Bk XII
Thi s rapidly unfolding series of discoveries, which occurred during the early 1960s, established that the appearance of CaB in the cytosol represented the biochemical
basis for the physiologic activat ion of cardiac contraction.
A more far-reaching consequence of this advance in our
knowledge of the control of cardiac contraction was to provide a new understanding that variations in the availability
of Ca 2 + for binding to troponin could be responsible for
.4,---.-- - - - -- - - - - - ,
a. Skeleta
.02'- r-- - - - - - - - - - .....,
b Car diac
l M yos in
M y 0 51 n
• Sxeteta I Actin
(~
E
c
E
Figure 5. Effects of Ca" + (plotted versus pCa. which
is -log[C a! +]) on the adenosine triphosphatase
(A'I'Pase) activity of skeletal (a) and cardiac (h) myosins ( - - - - ) and on the corre sponding actornyos ins
prepared with actins containing troponin and tropomyosin from skeletal (e) and cardiac (0) muscle.
(Reprinted from Katz AM . Repke DI. Cohen BR
[47], by permissio n of the American Heart Association . lnc .)
0 """, Actm (~
o
a,
a:Cl>
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j
'0
~ .2
.01
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e
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_
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_
o -I-z z-,- -,.--,---,-----i
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Jc--r-----r---------.----l
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48
J AM COLL CARDIOL
I<.:xrz
1983:1:42-5 1
impo rtant phasic , beat to beat. changes in myocardial contractil ity ( 17).
Control of Calcium Availability for
Activation of the Cardiac Contractile
Proteins
Now six full days my galla nt crew cou ld feast
upon the prime beef they had marked for slaughter
from Hellos" herd ; and Ze us, the son of Kronos,
added one fine morn ing.
All the gales
had cease d, blown out, and with an offshore breeze
we launched again , steppin g the mast and sail.
to make for the open sea . . . .
Homer. The Odyssey. Bk XII
Reco gnition of the mechanism by which Ca 2 + activated the
ca rdiac co ntractile proteins provided a clear direction for
studies of the phasic regulation of myocardial contractility.
The se studies were guided in part by the recognition that
two Ca2 + pools could be used for this activation: the extracellul ar space and the sarco plasmic reticulum. Thu s. an
incre ase in myocardial contractility could be attributed to
an increased Ca 2 + influx into the cell across the sarcolemma.
an incre ase in the amount of Ca 2 + released from the sarcoplasmic reticulum , or both. Research into these two mechanisms differed in approach, however. Fluxes of Ca 2 + across
the sarcolemma were ge nerally studied by measurements of
tracer Ca movements into or out of intact cardiac cells, or
by electroph ysiologic studies that characterized the charge
movements associated with these Ca2 + fluxes. In contrast.
Ca 2 + fluxes into and out of the sarco plasmic reticulum were
measured by biochemical techn ique s, using isolated sarcopl asmic reticulum vesicles pre pared from cardiac muscle
homogenates .
Slow inward Ca 2 + current and level of myocardial
contractility. By the earl y 1960s. Hodgkin and Huxle y' s
magn ificent characterization of the ionic currents respon sible for the action potential in the squid axo n had been
applied to the heart , and a model of the more compl ex
cardiac action potential had been constructed assuming that.
as in the nerve . Na + ions contributed the only import ant
inward curre nt (48) . However , subsequent work showed that
although the initial depolarization of the myocard ium could
be explained by a fast inward Na 2 + current. a second slow
inward current carried largely by Ca 2 + ions was responsible
for the plateau phase of the cardiac action potential (49.50).
and that the extent of Ca 2 + ent ry by way of this slow inward
current was directl y related to the level of myocardial contractility (50) . Thu s, the discovery of the slow inward current provided a major brid ge linkin g the physiolog y of excitation to the biochemistry of contraction. Identification of
these sarcolemmal Ca 2 + channels provided the basis for
several importan t therapeuti c adva nces in treatment of the
patient with heart disease. notably the development of the
calcium channel blockers and an understandin g of their interpl ay with the beta-adr energic receptor blocking age nts.
Since the discovery that the " soluble relaxing factor "
was a sarcoplasmic reticulum membrane fraction having its
relaxing ability due to the active transport of Ca 2 + into the
vesicle interior. a large bod y of know ledge characterizing
the Ca 2 + pump ATPase protein in both skeletal and card iac
muscle sarcoplasmic reticulum has been accumulated (5 1,52).
Much less is known of the biochem ical mechanisms by
which Ca 2 + mo ves down its elec trochemical grad ient out
of these membranes to initiate the contractile proce ss, although we have obtained ev idence that the Ca 2 + pump
ATP ase protein also can medi ate Ca 2 + release from the
sarcoplasmic reticulum (53.54).
Molecular Basis for the Positive Inotropic
Action of Beta-Adrenergic Agonists
Now shrugging off his rags the wiliest fighter of the islands
leapt and stood on the broad door still. his ow n bow in his hand.
He poured out at his feet a rain of arrows from the quive r
and spoke to the crowd :
.So much for that. Your clea ncut game
is over.
Now watch me hit a target that no man has hit before.
if I can make this shot. Help me. Apollo'
Homer. The Odyssev, Bk XXII
Recogniti on of the central role of Ca 2 + in the initiation of
the cardiac contractile process and of the existence of two
pools from which this activator Ca 2 + could be deri ved has
made it possible to anal yze mechanisms by which several
drugs modify myocardial cont ractil ity. This is illustrated by
the rapid development in our understanding of the mechanism by which beta-adrenergic receptor agonists enh ance
myocardi al co ntractility.
Cyclic AMP and activator Ca 2 + . As show n in Figure
I . the powerful inotropic effec t of the beta-adrene rgic agonists was used by Sarnoff (3) in his demonstration of the
variabi lity of myocardial co ntrac tility. That the sympathetic
nervous system also has marked effec ts on the duration of
systole , first suggested over a century ago by Baxt (cited in
reference 55) . was established in 1920 by Wiggers and Katz
(the latter was then a sophomore medical student) (55).
Based on knowledge of the role of Ca 2 + delivery to and
removal from the cardiac co ntract ile proteins. and on use
of both electrophys iologic and biochem ical approaches.
identification of the mech anism for these actions of the betaadrenergic agonists was finally made possible by the discovery of the role of cyclic adenosine monophosphate (AMP)
as an intracellular messenger (56) . Together. these approaches allowed the response of the heart to beta-adrenergic
J AM cou, CARDIOL
1983:1 :42-51
REGULATION OF MYOCARDIAL CONTRACTILITY
agonists to be defined in terms of effects of cyclic AMP on
both Ca 2 + entry by way of the slow inward (Ca 2 +) current
and Ca 2 + fluxes across the sarcoplasmic reticulum.
Electrophysiologic mechanism (cyclic AMP and the
slow inward current). It is now established that through
an action of cyclic AMP, beta-adrenergic agonists increase
the slow inward current (57); this effect can be described
electrophysiologically as due to an increase in the number
ofCa 2 + channels that open during the action potential (58).
The resulting increase in the amount of activator Ca 2 + within
the myocardial cell thus plays a central role in the enhancement of tension development.
Biochemical mechanism (cyclic AMP phosphorylation
and calcium fluxes across sarcoplasmic reticulum). A
biochemical explanation of these effects. especially an acceleration of Ca 2 + fluxes across the sarcoplasmic reticulum,
required yet another advance in our understanding of the
mechanism by which beta-adrenergic agonists and cyclic
AMP, the second messenger, regulate cellular processes.
The discovery that cyclic AMP could modify cell function
by initiating phosphorylation of various intracellular proteins by cyclic AMP-dependent protein kinases (59,60) allowed my colleagues and me (61) to describe the ability of
a cardiac cyclic AMP-dependent protein kinase to stimulate
Ca2+ transport by cardiac sarcoplasmic reticulum vesicles.
This effect of the cyclic AMP-dependent protein kinase is
mediated by the phosphorylation of phospholamban. a small
protein in these membranes (62,63) that leads to an increase
in the rates of both calcium uptake and calcium release by
the sarcoplasmic reticulum (Fig. 6) (8). These effects appear
to be of importance to the ability of the beta-adrenergic
agonists to accelerate both relaxation and contraction in the
intact heart. There is growing, but still inconclusive. evidence that a similar cyclic AMP-induced phosphorylation
is also responsible for the ability of these agents to increase
Ca 2 + fluxes across the sarcolemma (64). Thus. research in
this area has begun to combine the electrophysiologic and
biochemical approaches to the regulation of myocardial
contractility.
Conclusions and Predictions
I am a part of all that I have met:
Yet all experience is an arch wherethrough
Gleams that untravelled world. whose margin fades
For ever and ever when I move.
How dull it is to pause. to make an end.
To rust unburnished. not to shine in use.
Tennyson. "Ulysses"
Looking back over the past 25 years of research on the
mechanisms responsible for the regulation of myocardial
contractility, I am struck by the large number of questions
that could not even have been asked in 1958, but which
Phosphoprotein
Phosphatase
II
49
Cyclic AMP-Dependent
Protein Kinase
CH2
I
C-H
I
oI
P
Figure 6, Schematic representation of the effects of phospholamban phosphorylation on the calcium pump of the cardiac sarcoplasmic reticulum.
Top: Phospholamban in the dephospho-form interacts with the calcium
pump adenosine triphosphatase (ATf'ase) protein to confer positive cooperativity between its two Ca 2T-binding sites and to slow pump turnover
rate. Bottom: Phosphorylation of phospholamban reduces its interaction
with the calcium pump ATPase protein, allowing each Ca 2T-binding site
to act independently with Ca 2T. thereby increasing both Ca'T -sensitivity
and turnover of the calcium pump. MW = molecular weight. (Reprinted
from Hicks, Shigekawa and Katz. Circ Res 1979;44:384-91, by permission
of the American Heart Association. Inc.)
have now been answered. There also emerges a clear sense
of an accelerating flow of knowledge, of a steady building
on the discoveries and concepts that appeared rapidly during
this period. Those of us who had the privilege of pursuing
these challenges during this exciting quarter century have
been fortunate indeed!
In many ways. our experiences since 1958 resemble those
of my father 25 years before. In 1933, he had just left
Wiggers' laboratory in Cleveland to become Director of
50
J AM COLL CARDIOL
KATZ
}983:1:42-51
Cardiovascular Research at Michael Reese Hospital in Chicago. At the time. his studies of cardiac hemodynamics in
dogs had little impact on the way cardiologists treated their
patients. Yet, with the introduction of the clinical use of
cardiac catheterization and the many advances that made it
possible to operate safely on the human heart (65), the
foundations of basic knowledge provided by the great hemodynamic physiologists of my father's generation made possible our modern era of open heart surgery.
To what can we look forward, if our present generation
of young investigators is allowed to continue the progress
in cardiovascular research at its current pace? It is within
the realm of possibility that means will be found to promote
the regeneration of heart muscle, so as to overcome the
disability of patients whose cardiac function has been compromised by myocardial infarction. For others, new approaches to improving myocardial contractile function promise
relief of the lost contractility in patients with heart failure.
Knowledge of the basis for the cardiomyopathies may allow
these disease processes to be arrested or reversed. Research
now progressing rapidly in the fields of membrane biochemistry and cellular electrophysiology may allow identification of cellular and membrane abnormalities responsible for clinical cardiac arrhythmias. Coupled with rapidly
evolving approaches to antiarrhythmic therapy based on an
understanding of the molecular interactions between drugs
and membranes, this knowledge can be expected to provide
specific. nontoxic treatment for patients suffering from electrophysiologic disorders or at risk for sudden cardiac death.
While such predictions may seem fantastic, one need only
to look back over the past quarter century to see that they
are, in fact, rather modest.
From the book THE ODYSSEY by Homer. Translated by Robert
Fitzgerald. Copyright © 1961 by Robert Fitzgerald. Published by
Doubleday and Co., Inc., New York. I thank Dr. Phyllis B. Katz for
help with this text.
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