ARTICLES
NEWS IN PHYSIOLOGICAL SCIENCES
Skeletal and Cardiac Muscle Contractile Activation:
Tropomyosin “Rocks and Rolls”
A. M. Gordon, M. Regnier, and E. Homsher
O
n encountering a saber-tooth tiger, the caveman was
faced with two choices: to escape or to defend himself.
Either strategy required rapid activation of skeletal muscles and
adjustment of the performance of cardiac muscle to increase
blood flow to support an increased muscular effort. Although
saber-toothed tigers no longer exist, our physiological requirements are no less demanding for performing physical labor,
keeping trim through exercise, or competing in sports. The
explosive swing of the leg in striking a 130 km/h shot on goal
or the coordinated lifting of heavy loads requires rapid generation of high power output by skeletal muscles. In contrast, the
increased cardiac output needed for a 10 K run requires a
slower, subtler adaptation of the heart to increase blood flow
to skeletal muscles. Although the two muscle types appear
remarkably similar at the cellular and molecular levels, activation and regulation of each is fine-tuned to accomplishing its
different, highly controlled functions.
At the cellular level, an action potential triggers release of
Ca2+ from the sarcoplasmic reticulum, elevating intracellular
Ca2+ concentration ([Ca2+]) and rapidly activating skeletal muscle. This electrical activity is initiated and coordinated by the
nervous system to activate groups of muscle fibers as a motor
unit. Although the force from each motor unit varies somewhat
with frequency of nerve stimulation, gradation of force is
largely accomplished through controlling the recruitment of
motor units. Activation of the heart is also rapid, but in each
cardiac contraction all of the heart’s cells are activated. Electrical activity is spontaneous in cardiac pacemaker cells, and
coordination occurs through the spread of electrical activity
from cell to cell by specialized cells and structures, not by
direct neural control through motor units. Nevertheless, this
electrical activity still triggers Ca2+ release from the sarcoplasmic reticulum, elevating intracellular [Ca2+]. The heart’s output
is graded instead by controlling contraction frequency and
A. M. Gordon is in the Department of Physiology and Biophysics and
M. Regnier is in the Department of Bioengineering, University of Washington, Seattle, WA. E. Homsher is in the Department of Physiology, University
of California at Los Angeles, Los Angeles, CA.
0886-1714/99 5.00 © 2001 Int. Union Physiol. Sci./Am.Physiol. Soc.
modulating mechanical output of each cell, not the number of
activated cells. Contractions are controlled by intrinsic factors
such as heart rate and chamber volume (cell length, venous
return, the Frank-Starling relationship) and extrinsic factors
such as autonomic control of heart rate and intensity of cardiac
myocyte activation. Extrinsic control is exerted primarily
through phosphorylation of specific regulatory proteins.
Although the sarcomeric structures are the same for both
skeletal and cardiac muscle, the contractile protein isoforms
are different, giving rise to the different properties. Below, we
discuss recent observations that describe how Ca2+ regulates
contraction in striated muscle and the basis of the functional
differences between heart and skeletal muscle. A detailed
review of this topic has appeared recently and should be
consulted for supporting evidence and references to the
research literature (2).
Structural and biochemical basis of regulation
Contraction occurs when myosin S1 heads of the thick filament attach to and exert force on actin molecules in the thin
filament (Figs. 1 and 3) [see review by Geeves and Holmes
(1)]. This force causes the thin filament to slide over the thick
filament and the sarcomere to shorten and develop force
against a load the muscle must move. Ca2+ binding to troponin (Tn) on the thin filament initiates the force-generating
interaction of myosin and actin, and ATP hydrolysis provides
the energy for the molecular changes that drive force generation and muscle shortening (Fig. 3). Regulation by Ca2+ is
mediated through changes in the thin filament, although
modulation can occur through myosin.
Figure 1 shows the salient structural features of thin filament regulation. Helically arranged actins form the backbone of the thin filament, with the regulatory proteins
tropomyosin (Tm) and Tn attached to actin (A) in a 7:1:1
(A:Tm:Tn) ratio. Tm, a long, flexible molecule, binds to seven
actin monomers in the thin filament helix and overlaps the
adjacent Tms (Fig. 1B). Tn attaches to two actins in the
absence of Ca2+ through its TnI subunit and to Tm through the
TnT subunit at the Tm/Tm overlap zone (Fig. 1C). Ca2+ binding
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Changes in thin filament structure induced by Ca2+ binding to troponin and subsequent strong cross-bridge
binding regulate additional strong cross-bridge attachment, force development, and dependence of
force on sarcomere length in skeletal and cardiac muscle. Variations in activation properties
account for functional differences between these muscle types.
to the TnC subunit strengthens the TnC-TnI interaction and
detaches TnI from its contacts with actin (Fig. 1C) (2, 9, 10).
Recent structural studies (11, 15) show that this Ca2+-mediated
detachment of TnI from actin allows Tm to move over the surface of the thin filament. On the actin surface, there are sites
for weak (mainly electrostatic) and strong myosin binding
(Fig. 2A). Tm either rolls around its axis or slides from a position near the outer edge of the thin filament [where it covers
many of the myosin binding sites on actin (Fig. 2B)] to a position allowing increased weak and some strong myosin head
binding (Fig. 2C). Tm is a flexible molecule, and its positioning should be considered dynamic; Tm does not occupy
a single fixed position in the presence of elevated [Ca2+] but
“rocks and rolls” or “slips and slides” back and forth over the
actin surface. The Tm positions shown in Fig. 2, B–D should
therefore be considered average positions. When myosin
cross-bridge is strongly bound to actin, Tm is locally stabilized in a position that makes both weak and strong myosin
binding sites available on nearby actin monomers (Fig. 2D).
Thus structural data suggests that thin filament activation is
achieved by the movement of Tm over the actin surface,
which is controlled both by Ca2+ binding to TnC and initial
cross-bridge binding to actin to allow additional strong
cross-bridge binding. This Tm motion permits force generation and shortening.
Biochemical and structural studies suggest the existence of
three states of the thin filament (6): one in the absence of Ca2+
in which Tm “blocks” cross-bridge access to thin filament
strong binding sites (Fig. 2B), a second in which cross-bridges
can weakly bind to actin [called the “closed” state (Fig. 2C)],
and a third in which myosin can strongly bind to actin [called
an “open” state (Fig. 2D)]. In the presence of saturating Ca2+,
the position of Tm is such that ~20% of the actin sites at any
time are in the open state and ~80% are in the closed state
(6), corresponding to an average position of Tm, indicated in
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Fig. 2C. Ca2+ binding to TnC partially activates the thin filament by allowing Tm to move to a position in which a fraction of the cross-bridges can bind strongly and generate
force. Because cross-bridges bind strongly, they hold the Tm
in a position that exposes strong binding sites on actin for a
longer duration and more fully activate the thin filament. Biochemical data suggest that Ca2+ binding and subsequent
strong cross-bridge attachment spatially extends activation
along the thin filament to 12–14 actins, i.e., beyond the 7 actins
covered by a single Tm (5). Thus there is good correspondence between the structural and biochemical states of thin
filament activation as defined by Tm position and the ability
of cross-bridges to bind either weakly or strongly to actin.
This cooperative model of activation by Ca2+ and strongly
attached cross-bridges is the basis for the discussion below of
regulation in physiological preparations.
Mechanical correlates of the cross-bridge cycle and its
regulation
Understanding how actin-myosin interaction and ATP
hydrolysis are regulated requires knowledge of the chemomechanical cross-bridge cycle. Although the cycle is the same for
skeletal and cardiac muscle, the rate constants controlling
cross-bridge intermediate transitions differ. The values given in
the following discussion and Fig. 3 are for fast skeletal muscle
myosin; in many cases analogous rates for slow skeletal and
cardiac myosin isoforms remain to be determined. Figure 3
shows the cross-bridge cycle in terms of the various reactants
and products (A) and the corresponding structural changes (B).
At physiological ATP concentrations (3–5 mM), ATP binding
to myosin (step 1) is very rapid and irreversible. The subsequent detachment of actin from the actin-myosin·ATP
(A~M·ATP) complex (step 2) is similarly rapid and is caused by
an opening between myosin’s upper and lower 50-kDa
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FIGURE 1. A: sarcomere showing overlapping thin and thick filaments and myosin cross-bridges. B: expanded portion of thin filament showing troponin (Tn) subunits TnC, TnI, and TnT and the overlapping tropomyosins (Tms) along the actin helix. C: changes in Tn subunit interactions with Ca2+ binding to TnC, TnI movement
away from actin (A), and Tm movement over the actin filament. Connecting line width signifies interaction strength. Figure adapted from Gordon et al. (2).
regions (Fig. 3B) like the opening of jaws. A “flexing” or bending of the myosin neck region (Fig. 3B) accompanies step 3,
the hydrolytic cleavage of ATP, whose equilibrium constant
(K3, defined as k+3/k–3) is only ~10. Following ATP cleavage,
myosin again binds weakly to actin at a high rate, but in the
absence of Ca2+ Tm sterically blocks access of the myosin
head to strong binding sites on actin (Fig. 3B). However, when
Ca2+ is bound to TnC, TnI detaches from actin, allowing the
Tm/Tn complex to roll or slide over the thin filament surface.
This exposes weak binding sites on actin and transiently
exposes strong binding sites on actin (Fig. 3B) for binding to
the complementary regions in myosin’s 50-kDa domain. The
greater the [Ca2+], the greater the fraction of time the Tm/Tn
complex allows myosin access to strong binding sites on
actin. Consequently, the rate of strong cross-bridge attachment, the flux through step 5, is dependent on [Ca2+] and Tm
position (i.e., in the simplest case, the value of k+5 is proportional to the fraction of Tn having bound calcium). Strong
binding of myosin to actin (Fig. 3B) is associated with movement of the upper and lower 50-kDa subdomains toward each
other (or closing the jaws). This movement may allow the neck
region of myosin to extend, opening a pathway for inorganic
phosphate release from the ATP binding pocket in myosin.
Alternatively, closing the jaws might promote inorganic phosphate release from the binding pocket, which then allows the
extension of myosin’s neck region. In any event, myosin neck
extension, step 6, is the power stroke that, in isometric muscle, stretches an elastic element (represented here as the S2
segment) by some 10 nm and produces a force of ~2
pN/cross-bridge (7). In nonisometric conditions, shortening of
the neck extension causes the thick and thin filaments to slide
past each other. Step 7 is an irreversible isomerization and is
strain sensitive; i.e., when the force on the cross-bridge is
large as in isometric contractions, k+7 is slow (3–10 s–1) and is
the rate-limiting step for the cross-bridge cycle. However,
when the strain on the cross-bridge is low, as during rapid
shortening, k+7 rises to >500 s–1. Finally, ADP is released from
A·Mf·ADP (where f is a cross-bridge exerting force) in the
reversible step 8 to form the rigor state, A·Mf. During isometric contractions, the slowness of k+7 causes the population of
cross-bridges in the initial force-bearing (A·Mf*·ADP) state to
rise and with it force. Cross-bridges attach and exert force
constantly during steps 7, 8, and 1 during isometric contraction, and force drops to zero when the cross-bridges detach in
step 2. During shortening contractions the filaments slide past
each other, the strain on the cross-bridge is reduced, and step
7 occurs more rapidly. This accounts for the Fenn effect (an
increased rate of energy liberation above the isometric rate as
shortening velocity increases).
The chemomechanical mechanism shown in Fig. 3
implies that during an isometric contraction, a cross-bridge
remains strongly attached to actin for a relatively long time
(>100 ms/cycle). Strongly bound cross-bridges prevent Tm/Tn
from returning to its blocked or closed position, maintaining
the thin filament in a “switched on” position (Fig. 3B). In the
absence of Ca2+, cross-bridge detachment at the end of the
cycle allows Tm/Tn to cover the strong myosin binding sites
on actin and deactivate the thin filament (Fig. 3A).
Ca2+ regulation of contraction
Ca2+ control of steady-state force generation and the rate of
force redevelopment (termed ktr) can be understood from the
foregoing structural and kinetic considerations.
Force-pCa relationship. The force exerted by the muscle in
the isometric state depends on the number of strongly
attached cross-bridges and the force developed by each
cross-bridge. In turn, this depends on the number of actin
binding sites open for strong myosin binding. The maximum
number of cross-bridges attached during contraction is
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FIGURE 2. Tm movement over the surface of the actin filament at various degrees of activation. In each panel, three sequential actin monomers of one strand of
the thin filament are shown [actin structures from Lorenz and Holmes (4)]. Actin residues are light gray, except for residues (1–4, 24–25, 95–100; shown in light
blue) making weak electrostatic interaction with myosin and residues (144–148, 340–346, 332–334; shown in red) forming stronger attachments with myosin. In
B–D, a surface rendering of a cardiac Tm segment (residues 61–112 of each Tm strand) in dark gray [coordinates from Whitby and Phillips (14)], positioned according to Lehman and coworkers (15) is shown. Arg90 of Tm in each strand is shown in yellow to illustrate the putative rolling motion of Tm.
uncertain [see Gordon et al. (2)]. Structural considerations
suggest that no more than four myosin S1 heads can attach
per seven-actin unit. During isometric contraction it is likely
that only 20–40% of the available cross-bridges attach at one
time, meaning ~1–2 myosin S1 heads per 7 actins.
The chemomechanical model of the cross-bridge cycle
describes the interaction of one myosin with one actin in the
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thin filament, yet the sarcomeric structure implies that activation involves many potential myosin interactions with
actins along the length of each thin filament (Fig. 1A). Thus
activation could involve cooperativity within and between
regulatory units (A7TnTm). The model of activation shown in
Figs. 1, 2, and 3 implies that there are four mechanisms
whereby Ca2+ binding to TnC and subsequent binding of
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FIGURE 3. A: the cross-bridge cycle is composed of 8 basic events. Strong binding is designated by “·” between two species and weak binding by “~”. A, actin; M,
myosin; Pi, inorganic phosphate. Cycle begins at the top, with a cross-bridge strongly bound to actin (A·Mf, where f is a cross-bridge exerting force) and its neck
region in an extended position (see B). The rates of steps 2 and 4 are >1,000/s, whereas ATP cleavage rates (k+3 + k–3) are only ~100/s. Estimates of (k+5 + k–5) at maximal Ca2+ concentration ([Ca2+]) range from 30 to 150/s, with the ratio k+5 / k–5 near 3–5, and k+5 increases with increases in [Ca2+]. The rate of the force-generating
transition (k+6) in the isometric case is readily reversible and ~20–30/s. ADP release occurs in step 8 at >100/s with a dissociation constant for ADP binding to actinmyosin of ~100 µM. B: same cross-bridge cycle with structural changes to convey how force and motion occur. Noninteracting cross-bridge-actin pairs are shown
as gray actin and green myosin, weak interactions as yellow actin and light blue myosin, and strong interactions as green actin and red myosin.
cross-bridges create an allosteric or cooperative increase in
cross-bridge binding along the thin filament (see Fig. 4).
First, Ca2+ binding to one binding site on skeletal TnC
increases Ca2+ binding to the other site on that TnC or to the
nearest neighbor (n-n) TnC (Fig. 4A). This effect on the Ca2+ sensitivity of force is moderate, maximally increasing the Hill coefficient (n) to 2. Second, strong binding of cross-bridges in one
A7TmTn unit increases the Ca2+ binding in that unit or in n-n
units (Fig. 4B). This is small in skeletal muscle under physiological conditions but may be important in cardiac muscle (12).
Third, strong binding of cross-bridges in one A7TmTn unit
increases strong binding of cross-bridges in that unit and in n-n
units (Fig. 4C). This occurs primarily through interactions
between Tms along the thin filament (Fig. 4C) linked by head-totail connections, but may also occur through interactions
between actins (Fig. 4D). This source of cooperativity helps
explain the steep relationship between force and [Ca2+] but plays
less of a role in regulating the rate of force redevelopment (2).
In the simplest model of activation, the value of k+5 (the
rate of strong cross-bridge attachment) increases in proportion to Ca2+ bound to Tn. Cooperative activation mechanisms
can modify this controlling step (step 5; Fig. 3) of the chemomechanical cross-bridge cycle. Kinetic analysis of the mechanism in Fig. 3A shows that the number of force-generating
cross-bridges is a hyperbolic function of the value of k+5,
approaching an asymptote when (k+5/k–5) > 3.
Relative isometric force is given by the following relationship: relative isometric force = 1/[1 + αβ + (γ/k+5)], where α
= k+1k+7k+8/(k+1k+7 + k+1k+8 + k+7k+8), β = [1/k+2 + (k–3 +
k+4)/k+3k+4 + 1/k+4 + 1/k+6], and γ = [α + α(k–5/k+6)]. This follows from the differential equations describing the crossbridge cycle and reasonable assumptions about rates and
reversibility. See Regnier and Homsher (8) for a discussion of
how cross-bridge cycle kinetics affect force and ktr.
If one considers only the control of k+5 by Ca2+ binding to
TnC, with no contribution to activation from strongly attached
cross-bridges and no other cooperativity, the dependence of
force on [Ca2+] can be calculated. Assuming that k+5 is proportional to the fraction of TnC bound to Ca2+, the relationship
between the relative force (F/Fmax) and pCa will be given by
F/Fmax = 1/{1 + 10[n(pCa–pCa50)]} where pCa50 = –log Kd for Ca2+
binding to TnC (where Kd is the dissociation constant) and
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FIGURE 4. A–D: cooperative activation mechanisms within one A7TmTn regulatory unit (left) and the nearest-neighbor (n-n) A7TmTn unit (right), with the property enhanced shown as a hatched structure. Actins in the closed state are yellow, in the open state green, in the blocked state gray (none shown). Tm position is
closed or open. Strong cross-bridges are red. Ca2+ binding induces increased Ca2+ affinity in that and n-n TnC (A). Strong cross-bridge binding increases Ca2+ binding to TnC in that unit and the n-n unit (B). Strong cross-bridge binding increases strong cross-bridge binding in that and the n-n unit (C). Strong cross-bridge binding influences actin structure in that and n-n unit to increase cross-bridge binding (D). E: force-pCa relationship in skinned rabbit psoas skeletal muscle fiber with
Hill fit to data. pCa50 = 5.82 ± 0.01, n = 3.65 ± 0.18. F: relationship of force to the rate of force redevelopment (ktr) for skinned rabbit psoas skeletal muscle fiber.
“Of additional importance is the increase in Ca2+
sensitivity seen with increasing sarcomere length….”
constant for force development. This is normally measured as ktr
after an activated, demembranated muscle preparation has been
allowed to shorten (to detach cross-bridges), restretched rapidly
to the initial length, and then held isometric while cross-bridges
reattach and force redevelops. [Fig. 4F, plotted here as the relationship between ktr and initial isometric force to demonstrate
clearly how ktr relates to the number of attached, force-generating cross-bridges (proportional to force); see Gordon et al. (2)].
The best models to date support the idea that Ca2+ binding and
activation of the weakly to strongly bound cross-bridge transition
(step 5; Fig. 3) can account for the relationship between ktr and
force. For skeletal muscle, the force-ktr relationship changes little
until F/Fmax > 0.5 but increases rapidly thereafter. This seems to
suggest cooperative behavior (Fig. 4E). However, this behavior is
explained by the cross-bridge cycle shown in Fig. 3 and Ca2+dependent control of k+5 without hypothesizing any cooperative
behavior. The mechanism given in Fig. 3A predicts that ktr can be
approximated as [k+7 + k+6(K5/(K5 + 1)]. As K5 rises from zero at
low [Ca2+] to higher values with increasing [Ca2+], [K5/(K5 + 1)]
will initially be small (<0.2, at which isometric force is 50% of
maximal) and ktr will be dominated by k+7. However, as K5
increases to >0.2, ktr will increase dramatically as k+6[K5/(K5 + 1)]
begins to dominate ktr, even though force continues to increase
linearly. This behavior is shown experimentally in Fig. 4F. Thus
this steep relationship between force and ktr follows directly
from the regulation of strong attachment at individual crossbridges and does not require a cooperativity between regulatory
units, as was required to account for the steep steady-state isometric force-pCa relationship (Fig. 4E) in striated muscle.
Force-sarcomere length relationship. Sarcomere length
affects the maximum force and Ca2+ sensitivity of force in skele54
News Physiol. Sci. • Volume 16 • April 2001
tal and cardiac muscle. The dependence of maximum tetanic
tension in skeletal muscle, particularly the decline at long sarcomere lengths, has been used to support the cross-bridge
model of muscle contraction (3), whereas the decline at short
sarcomere lengths (the so-called ascending limb) has been less
precisely explained. The ascending limb of the length-tension
relationship is of great importance in cardiac muscle because it
is the sarcomere length range over which the heart normally
operates, giving rise to the Frank-Starling relationship.
Of additional importance is the increase in Ca2+ sensitivity
seen with increasing sarcomere length in both skeletal and
cardiac muscle. This effect is greater in cardiac muscle and
contributes to its greater length dependency of activation,
enhancing the Frank-Starling relationship. The reduction in
Ca2+ sensitivity with decreasing sarcomere length may be
most easily explained by increased distance between thick
and thin filament (lattice spacing), in effect decreasing k+5.
Strongly attached cross-bridges contribute to activation,
along with Ca2+ binding, and the probability of these attachments (determined by k+5) at a given [Ca2+] decreases with
increasing lattice spacing. The differences in the relative
effect of sarcomere length in skeletal and cardiac muscle
could result from different dependencies on strongly attached
cross-bridges to activate and maintain activation of the thin
filament, as discussed below.
Differences between skeletal and cardiac muscle
regulation
The differences in regulation of skeletal and cardiac muscle, introduced above, are mainly due to differences in regulation of contraction. Both types of muscle are activated
rapidly by Ca2+ binding to TnC and the consequent movement
of Tm. This in turn controls the transition of weak to strong
attachment for cross-bridges that have already hydrolyzed ATP
and are ready to release energy in the power stroke (Fig. 3).
The major difference between cardiac and skeletal muscle
is modulation of the extent of thin filament activation. Force
development must be controlled mainly at the cellular level
in cardiac muscle because each cardiac cell is activated on
each beat. Thus each cell must be able to undergo the full
dynamic range shown by the cardiac output. Contributing to
this behavior in cardiac muscle cells are 1) incomplete thin
filament activation with each transient increase in intracellular [Ca2+], 2) a less steep force-pCa relationship, 3) a greater
change in force achieved during either submaximal or maximal Ca2+ activation with neural or hormonal modulation, and
4) a greater dependence of force on sarcomere length for a
given level of Ca2+ activation.
The first of these properties arises in part from the phasic
electrical activity of cardiac muscle, but incomplete thin filament activation and the other three important properties
probably arise from the different regulatory protein isoforms
in the two muscle types. The basic activation mechanisms in
cardiac and skeletal muscle are the same, with some minor
differences resulting from the properties of individual proteins.
The cross-bridge scheme (Fig. 3) is the same (with differences
in rate constants still to be determined), and the Ca2+-regulated
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n will be ~1. However, the relationship between F/Fmax and
pCa measured in a skinned muscle preparation is much
steeper (Fig. 4E), with n much greater than 1 (as much as 6)
and a pCa50 (termed the Ca2+ sensitivity) of 5–6.5, depending
on the preparation. Thus the control of step 5 in the cycle must
involve a highly cooperative activation of the thin filament to
increase the fraction of cross-bridges that strongly attach. Of
the four processes that may contribute to cooperative activation of the thin filament (Fig. 4), the most important is the
effect of strongly attached cross-bridges to increase activation
of the regulatory unit and neighboring units (Fig. 4C), as
shown in Fig. 2D. Functionally this means that strongly
attached cross-bridges have the effect of increasing the value
of k+5 at any given [Ca2+]. Factors that modify attachment or
availability of cross-bridges (such as decreased spacing
between thick and thin filaments as sarcomere length
increases or movement of myosin heads away from the thick
filament backbone following myosin light chain phosphorylation) can increase Ca2+ sensitivity and/or steepness of the
force-pCa curve by increasing the effective k+5.
Rate of force redevelopment. In addition to steady-state force,
there is also a steep relationship between [Ca2+] and the rate
Summary
In resting skeletal and cardiac muscle, Tm on the thin filament blocks strong binding of myosin to actin. Ca2+ binding
to Tn allows Tm to move over the thin filament, exposing
some myosin binding sites. Strong attachment of myosin stabilizes Tm position, allowing further strong myosin binding
and force development. The kinetic step in the cross-bridge
cycle regulated by Tn/Tm is the strong myosin attachment
step, and both Ca2+ binding and strong myosin binding cooperatively activate it. Control of this step explains both the
dependence of isometric force on Ca2+ and the force dependence of ktr. This also helps explain the effect of sarcomere
length on force in the ascending limb of the length-tension
curve and serves as a basis for understanding the differences
in regulation of skeletal and cardiac muscle.
We gratefully acknowledge the assistance of Martha Mathiason in the
preparation of the figures and the manuscript and of Emilie Warner in providing editorial comments.
This work was supported by grants from the National Institutes of Health [NS08384 (A. M. Gordon), AR-30988 (E. Homsher), and HL-61683 (M. Regnier)].
References
1. Geeves MA and Holmes KC. Structural mechanism of muscle contraction.
Annu Rev Biochem 68: 687–728, 1999.
2. Gordon AM, Homsher E, and Regnier M. Regulation of contraction in striated muscle. Physiol Rev 80: 853–924, 2000.
3. Gordon AM, Huxley AF, and Julian FJ. The variation in isometric tension
with sarcomere length in vertebrate muscle fibres. J Physiol (Lond) 184:
170–192, 1966.
4. Lorenz M, Popp D, and Holmes KC. Refinement of the F-actin model
against X-ray fiber diffraction data by the use of a directed mutation algorithm. J Mol Biol 234: 826–836, 1993.
5. Maytum R, Lehrer SS, and Geeves MA. Cooperativity and switching
within the three-state model of muscle regulation. Biochemistry 38:
1102–1110, 1999.
6. McKillop DF and Geeves MA. Regulation of the interaction between actin
and myosin subfragment 1: evidence for three states of the thin filament.
Biophys J 65: 693–701, 1993.
7. Molloy JE, Burns JE, Kendrick JJ, Tregear RT, and White DC. Movement and
force produced by a single myosin head. Nature 378: 209–212, 1995.
8. Regnier M and Homsher E. The effect of ATP analogs on posthydrolytic
and force development steps in skinned skeletal muscle fibers. Biophys J
74: 3059–3071, 1998.
9. Solaro RJ and Rarick HM. Troponin and tropomyosin: proteins that
switch on and tune in the activity of cardiac myofilaments. Circ Res 83:
471–480, 1998.
10. Tobacman LS. Thin filament-mediated regulation of cardiac contraction.
Annu Rev Physiol 58: 447–481, 1996.
11. Vibert P, Craig R, and Lehman W. Steric-model for activation of muscle
thin filaments. J Mol Biol 266: 8–14, 1997.
12. Wang YP and Fuchs F. Length, force, and Ca2+-troponin C affinity in
cardiac and slow skeletal muscle. Am J Physiol Cell Physiol 266:
C1077–C1082, 1994.
13. Weisberg A and Winegrad S. Relation between crossbridge structure and
actomyosin ATPase activity in rat heart. Circ Res 83: 60–72, 1998.
14. Whitby FG and Phillips GN Jr. Crystal structure of tropomyosin at 7 Angstroms
resolution. Proteins 38: 49–59, 2000.
15. Xu C, Craig R, Tobacman L, Horowitz R, and Lehman W. Tropomyosin
positions in regulated thin filaments revealed by cryoelectron microscopy.
Biophys J 77: 985–992, 1999.
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step is probably the same. As discussed, Tm movement and
activation depend on both Ca2+ binding to TnC (with Tn dissociation from actin) and strong binding of myosin to actin,
amplified by the cooperativity mechanisms shown in Fig. 4.
The direct activating effect of Ca2+ appears to be less in cardiac cells, so that they rely more on strong cross-bridge
attachment for activation. In fact, cycling cross-bridge
enhancement of Ca2+ binding is observed most prominently
in cardiac muscle (12).
The result of greater reliance on cross-bridge attachment
for activation in the heart is that factors that modulate strong
(and perhaps weak) attachment of cross-bridges can have a
greater effect on submaximal and maximal activation. This
would include factors such as sarcomere length (discussed
above) and modulation of cross-bridge structure through
phosphorylation of the myosin regulatory light chain or
myosin binding protein C, under either adrenergic or Ca2+
control. Phosphorylation of the myosin regulatory light chain
and myosin binding protein C both result in movement of the
myosin heads away from the thick filament and toward the
thin filament, thus increasing the probability of attachment
(13) and enhancing both the force-generating and activating
effects of cross-bridges in the heart. This would allow greater
modulation of contraction at the cellular level. The molecular properties of the regulatory protein isoforms responsible
for these functional differences in skeletal and cardiac muscle regulation are currently under investigation.