Stretch activation is an intrinsic length-sensing mechanism that allows muscle to
function with an autonomous regulation that reduces reliance on extrinsic regulatory
systems. This autonomous regulation is most dramatic in asynchronous insect flight
muscle and gives rise to wing beat frequencies that exceed the frequency capacity
of neural motor control systems. Stretch activation in insect flight muscle allows
the contractile features of the flight muscle to be matched and tuned to the wing-thorax-aerodynamic
load to ensure proper muscle contraction frequency and effort for flight (Pringle,
1977); a role for which intrinsic autonomous regulation is especially suited. In stretch-sensitive
insect flight muscles, neurally controlled intracellular calcium plays a permissive
role (it needs to be present at adequate levels to allow the intrinsic stretch activation
mechanisms to operate) but it is not the dominant player in force generation or in
work production. That role belongs to stretch itself, which activates the myofilament
system in such a way (i.e., with appropriate phase delay) to generate force and perform
rhythmic work.
The function of stretch activation is less obvious in muscles, such as cardiac muscle,
that rely heavily on rising and falling intracellular calcium to modulate force generation.
Although stretch activation has been demonstrated in cardiac muscle (Steiger, 1971,
1977; Vemuri et. al., 1999), intracellular activator calcium is the primary determinant
of force-generating capacity. However, two unique features of cardiac muscle function
suggest that stretch activation could be important: (1) the rhythmic nature of cardiac
muscle contraction begs a functional analogy for stretch activation in heart muscle
with stretch activation in insect flight muscle; and (2) the steep length–tension
relationship in cardiac muscle (relative to skeletal muscle, where stretch activation
is much less pronounced) is necessary for the valuable function that the heart gains
from the Frank-Starling relationship (Allen and Kentish, 1985). It is likely that
stretch activation contributes significantly to this steepness. Thus, the relative
contribution of stretch activation and calcium activation to cardiac muscle function
is an important issue that remains largely unresolved.
In this issue, Stelzer et al. (p. 95) provide the first definitive study that dissects
the relative contributions of calcium and stretch activation to cardiac muscle force
generation. Stelzer et al. make use of the force response to a quick stretch in mouse
myocardium where they separate the initial response (phase 1) and the rapid recovery
from elastic distortion (phase 2) from the slower part of the response (phase 3),
in which force rises to a higher steady-state level as a result of stretch-induced
recruitment of new force-generating cross-bridges. Phase 3 is the expression of stretch
activation and Stelzer et al. use both the amplitude of force rise and the apparent
rate constant that governs the rate of force rise during phase 3 as measures of stretch
activation. To explore the interaction between calcium activation and stretch activation,
calcium activation levels were varied and the force response to quick stretch was
evaluated. The authors demonstrate that stretch activation is most pronounced at low
levels of calcium activation, where there are ample sites available on the thin filament
for the formation of additional strong-binding cross-bridges, XBs. To test the hypothesis
that strong-binding XBs cooperatively recruit more strong-binding XBs during stretch
activation, they used a chemically modified myosin S1 subfragment (NEM-S1, which binds
strongly to actin with no force generation) to occupy some fraction of thin-filament
myosin binding sites. When the thin-filament binding sites are occupied with NEM-S1,
stretch activation is much blunted, and they conclude that stretch activation involves
strong-binding XBs cooperatively promoting further strong binding of XBs through an
XB-based activation of the thin filament. This cooperative XB activation both increases
the relative magnitude of the force response to stretch and slows the approach to
the eventual steady state.
A second valuable contribution of the study by Stelzer et al. is that they give a
lucid verbal account of the mechanisms by which XBs participate in stretch activation.
The clarity of their account, in combination with an equally clear account given earlier
by the same group (Moss et. al., 2004), allowed us, with very few additions of our
own, to formulate a simple mathematical model of the myofilament system, a model in
which both calcium and strong-binding XBs each cause myofilament activation, and in
which these two activation mechanisms could be treated separately in ways that could
not be done experimentally.
The underlying kinetic scheme for the model is shown in Fig. 1. In brief, thin filament
regulatory units (RUs) are distributed between “Blocked” (R*off
and Ro
off
), “Closed” (R*on
and Ro
on
), and “Open” (A* and Ao
) states. Each state has calcium bound, * (left Ca-bound column in Fig.1), or not,
o (right Ca-not-bound column in Fig. 1). Myosin XBs cycle between a nonforce bearing
state, D (Fig. 1, middle), and a force-bearing state, A (Fig. 1, bottom). Transition
from the Blocked to the Closed RU states represents activation of the myofilament
system, which is required for XBs to cycle between the D and A states. Activation
occurs as a result of calcium binding: Ro
off
+ Ca2+ → R*off
→ R*on
with the last step representing the activation step, which is governed by the kinetic
constant kon
. Calcium activation takes place only in the left Ca-bound column in Fig. 1. Alternatively,
activation could result from the action of strong-binding XBs, numerically equal to
A, causing allosteric change in neighboring RU and transition from Closed to Open
states. This kinetic step, which is governed by the nonlinear rate coefficient kXB
, does not require calcium to be bound to the RUs and, thus, takes place in both the
Ca-bound and Ca-not-bound columns in Fig. 1. Cross-bridge binding to the thin filament,
which is governed by the kinetic rate constant f, occurs as long as the RU is in the
Closed (Ro
on
and R*on
) states, independent of whether calcium is bound to the RU.
Figure 1.
Kinetic scheme for model of myofilament activation and myosin cross-bridge cycling.
Myofilament activation occurs by steps responsible for the transition between Blocked
and Closed states. This scheme allows myofilament activation by both calcium binding
and by cooperative cross-bridge mechanisms. See text for explanation and definition
of symbols.
Both the RUs and the XBs obey conservation constraints with the total number of XBs
(including detached and attached states) equal to three times the total number of
RUs. Further, calcium binding to the RUs is considered to be in equilibrium with the
binding constant, Keq
. As a result, Ro
on
and R*on
can be combined into a single Ron
state and the Ao
and A* states can be combined into a single A state. The force, F, generated by this
model myofilament system is equal to the number of XBs in state A times the average
force generated by a single XB (Eq. 1). The average XB force is equal to XB stiffness,
ε, times the average elastic distortion, x, among attached XBs. With this, the model
myofilament kinetic system in Fig. 1 can be represented by the following set of four
equations:
(1)
\documentclass[10pt]{article}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\usepackage{pmc}
\usepackage[Euler]{upgreek}
\pagestyle{empty}
\oddsidemargin -1.0in
\begin{document}
\begin{equation*}F=A({\epsilon}x)\end{equation*}\end{document}
(2)
\documentclass[10pt]{article}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\usepackage{pmc}
\usepackage[Euler]{upgreek}
\pagestyle{empty}
\oddsidemargin -1.0in
\begin{document}
\begin{gather*}\frac{dR_{on}}{dt}=- \left \left[\frac{k_{off}+k_{XB}+K_{eq}Ca\hspace{.167em}k_{on}}{1+K_{eq}Ca}+fD\right]
\right R_{on}
\\
+ \left \left[g-\frac{k_{XB}+K_{eq}Ca\hspace{.167em}k_{on}}{1+K_{eq}Ca}\right] \right
A
\\
+ \left \left[\frac{k_{XB}+K_{eq}Ca\hspace{.167em}k_{on}}{1+K_{eq}Ca}\right] \right
[{\beta}(L-L_{0})]\end{gather*}\end{document}
(3)
\documentclass[10pt]{article}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\usepackage{pmc}
\usepackage[Euler]{upgreek}
\pagestyle{empty}
\oddsidemargin -1.0in
\begin{document}
\begin{equation*}\frac{dA}{dt}=[fD]R_{on}-[g]A\end{equation*}\end{document}
(4)
\documentclass[10pt]{article}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\usepackage{pmc}
\usepackage[Euler]{upgreek}
\pagestyle{empty}
\oddsidemargin -1.0in
\begin{document}
\begin{equation*}\frac{dx}{dt}=-[g][x-x_{0}]+\frac{dL}{dt}\end{equation*}\end{document}
Eq. 4 describes the dynamic changes in the XB elastic distortion when muscle length
changes at a velocity, dL/dt. Eqs. 2 and 3 describe the dynamic processes of both
RU state change and XB recruitment. The term β(L − L
0) in Eq. 2 consists of muscle length, L, a reference muscle length, L0
, and a multiplying coefficient, β. This term represents the total number of RU in
the filament overlap region of the sarcomere where structural features allow XB cycling.
Because the focus is on the ascending limb of the length–tension relationship, the
total RU is proportional to muscle length via the β(L − L
0) term. This length effect on total RU is independent of the activation state of
the muscle; passive stretch and active stretch had the same effect: bringing more
RU into the overlap zone where XB cycling could occur. All other mechanisms for recruiting
force-generating XBs reside in the terms contained within the square brackets that
multiply each variable in Eqs. 2 and 3.
Of all possible XB recruitment mechanisms that could be implemented using terms within
the square brackets, those of interest in this exercise are just Ca and kXB
because these are the only terms that bring about myofilament activation (Fig. 1).
Ca and Keq
were combined into a single term, α. With a typical value of Keq
= 10−6, α and pCa are related as follows: α = 0.1, pCa = 7; α = 1, pCa = 6; α = 100,
pCa = 4.
The manner in which the XB activating coefficient, kXB
, depends on the number of strong binding XB is important. To keep things simple,
and to ensure that zero calcium resulted in zero force, while XB activation increases
monotonically with A, we made the assignment:
(5)
\documentclass[10pt]{article}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\usepackage{pmc}
\usepackage[Euler]{upgreek}
\pagestyle{empty}
\oddsidemargin -1.0in
\begin{document}
\begin{equation*}k_{xb}=k_{a}A\end{equation*}\end{document}
Thus, kXB
increases linearly with A. Although this assignment is not what normally is encountered
in a cooperative process, we use the term “cooperative” because strong-binding XBs
enhance myofilament activation by increasing kXB
, which enhances the formation of more strong-binding XBs. With this assignment for
cooperative XB activation, it was possible to vary XB activation within the model
by changing ka
.
One difference between the Stelzer et al. dialogue and the model presented here is
in the stoichiometric relationship between the RU Open states and attached force-generating
XB. For simplicity, we set this stoichiometric relationship to 1, and, thus, A represents
both the number of RUs in the open state and the number of force-generating XBs. In
actuality, as Stelzer et al. discuss, the span of the RU over the thin filament dictates
that there can be more force-generating XBs than RUs in the Open state. However, spatial
considerations as required to represent an RU–XB stoichiometry different from 1 would
quickly lead to a model that would be mathematically and computationally much more
complicated. The qualitative and quantitative differences between the simple model
presented here and a more complicated but realistic model were judged unimportant
to our argument.
To recreate the experimental results of Stelzer et. al., model muscle length, L, was
set at a reference value and Ca, α, was varied among the values 0.1, 1.0, and 100
(corresponding to pCa's of 7, 6, and 4, respectively). After obtaining steady force
for the given value of α, the muscle length was changed in the manner of a step equal
to 1% of the initial length and the force response to this sudden 1% stretch was predicted.
To roughly simulate the experiments of Stelzer et al. with NEM-S1, these varied levels
of Ca activation were used to predict the force when ka
equaled zero (no XB activating effects as in the presence of NEM-S1) and when ka
equaled 20 (an arbitrary number giving modest XB activation). The model-predicted
results, Fig. 2, emulate the experimental findings in the following essential respects.
Figure 2.
Model-predicted force response to sudden muscle stretch at varying levels of calcium
activation. Model predictions were when cross-bridge activation was operative (A–C)
and when it was not (a–c). Equal levels of calcium activation for the pairs: A-a,
B-b, and C-c. Force was normalized to force at maximal activation.
The predicted force response to sudden stretch had three well-defined phases (compare
our Figs. 2 and 3 with Figs. 1, 2, and 8 in Stelzer et al.). Phase 1 in the model
response is due to the sudden stretch of attached XBs. The amplitude of phase 1 is
directly proportional to the prestretch force; a relationship that can be derived
from Eqs. 1 and 4. Phase 2 is a rapid recovery from the sudden XB stretch as the stretched
XBs, which were distorted by the imposed length step, detach and are replaced by new
attached XBs that had not been stretched by the imposed length change. In the model,
as in Stelzer et al., the time course of phase 2 was only weakly related to activation
or prestretch force. The phase of primary interest, phase 3, is due to the recruitment
of more force-bearing XBs as a result of the increase in muscle length. Like the findings
of Stelzer et al., the predicted amplitude and time course of phase 3 depend strongly
on the prestretch force (or level of calcium activation). The relative amplitude was
much higher, and the force approached a new steady state much slower, at low levels
than at high levels of calcium activation. Furthermore, whereas phases 1 and 2 are
largely independent of whether XB activation was operative, phase 3 depends strongly
on XB activation. Without XB activation (ka
= 0), phase 3 is relatively rapid and independent of the degree of calcium activation,
as is its relative amplitude (normalized to prestretch force). In contrast, with XB
activation (ka
= 20), phase 3 is relatively slow and of large amplitude relative to the prestretch
force at low activation, but much faster and of smaller relative amplitude at high
activation. In fact, at saturating levels of calcium the prestretch force amplitude,
the speed of phase 3, and the amplitude of phase 3 do not differ between conditions
with vs. without XB activation. This is because at maximal activation all potential
XBs are recruited into the cycling population and there is no opportunity to recruit
more XBs with either calcium activation or cooperative XB activation. Our model results
and mechanisms for obtaining these results agree with the explanations for experimental
findings given by Stelzer et al. in terms of cooperative XB activation determining
the characteristics of phase 3, i.e., the stretch activation response.
Figure 3.
Activation-dependent time course and amplitude of step response. Step responses shown
in Fig. 2 normalized to prestretch force. Responses for conditions in which cross-bridge
activation was operative (A–C) and when it was not (a–c). Effect of cross-bridge activation
to slow the time course and to increase the amplitude of phase 3 at lowest level of
calcium activation is shown in right panel.
We used the model to extend the work of Stelzer et al. into functional domains they
did not study by predicting the effects of XB activating mechanisms on the force–length
relationship. We systematically varied muscle length before imposing a constant, midlevel
(α = 1.0) calcium activation signal and calculated the corresponding steady-state
force both with and without XB activation. This relationship between muscle length
and steady-state force was used to construct a force–length relationship. The force–length
relationship was twofold steeper with XB activation (ka
= 20) than without (ka
= 0). Because the muscle force–length relationship is transformed, through the cardiac
geometry, into the left ventricular chamber pressure–volume relationship, our finding
means that stretch activation via XB activation is important in determining the steepness
of the Frank-Starling relationship of the heart even though the stretch in question
is applied before calcium activation, at a time when the muscle is relaxed. A steep
Frank-Starling relationship enables the heart to respond sensitively to changes in
venous return, i.e., it is an autonomous means for matching the heart with its preloading
system.
However, a steeper force–length relation, as a consequence of stretch activation,
is a two-edged sword. On the one hand, it enhances force generation with increased
muscle length favoring a steep Frank-Starling relationship, but this same property
also means that muscle shortening, as during ejection, would strongly reduce the force;
an undesirable outcome if the maintenance of force is required to overcome the afterload
against which shortening occurs. Countering the reduction in force with muscle shortening
is the second major effect of XB activation, which is to slow the attainment of a
new force after a change in length. This is just as true for muscle shortening as
it is for muscle lengthening. We used the model to demonstrate the slow reduction
in force after a sudden 1% reduction in muscle length (Fig. 4). In this example, the
velocity effects on force reduction are confined to just the onset of the response,
allowing the slow actions of stretch activation to be observed without the obfuscations
by velocity-related force reductions. Thus, although the muscle has shortened to a
length that would sustain only a much reduced steady-state force, because XB activation
dissipates slowly, force continues to be greater than the steady-state level for a
considerable time. That is, the muscle effectively remembers the higher force associated
with the longer length from which the shortening began. Hence, the dynamical aspects
of stretch activation are functionally important even during muscle shortening and
act to sustain force during shortening.
Figure 4.
Stretch activation during quick release. Effect of stretch activation when muscle
shortens with quick release (heavy solid line) compared with response to quick stretch
(light solid line). Steady-state force before length change and eventual steady-state
force after length change are shown as broken lines. Double-headed arrows between
eventual steady-state force and the response transient after quick release demonstrate
the slow loss of memory of the earlier higher force at each of the identified times.
This effect, whereby initial length of muscle continues to exert positive effects
on force generation during the shortening interval of cardiac ejection, has been observed
previously by Hunter (1989) in left ventricular pressure–volume behavior during ejection
. This effect gave rise to what Hunter called “positive effects of ejection.” Hunter
correctly described the underlying phenomena in terms of memory effects persisting
throughout ejection. This seminal observation has gone unexplained for more than 15
years. Thanks to Stelzer et al., we now have a myofilament mechanism that explains
this functionally important phenomenon in the whole heart.
Thus, stretch activation exerts a positive effect on function when muscle is stretched
by increasing the force-producing capability as muscle is stretched to longer lengths
and it exerts a second positive effect on function when muscle shortens by prolonging
the force-generating capacity through a dynamic effect that remembers the greater
force-producing capacity of previous longer lengths. We elucidated these effects using
constant calcium activation. But, how do stretch activation and its XB activating
mechanisms interact with calcium activation mechanisms when calcium is not constant?
To approximate changing activator calcium as in a normal heartbeat, we created a time-varying
α(t) signal that rose to peak level (α(t)
max
= 1.0) consistent with midlevel calcium activation (Fig. 5). We then predicted and
compared the time course of force generation when the cooperative XB activating mechanisms
of stretch activation were operative and when they were not. The first comparison
is with the amplitude of force generation. When XB activating mechanisms were operative,
the model predicted a force amplitude 35% higher than when they were not (Fig. 5,
left). The second comparison is with respect to the time course of force production.
When the XB activating mechanisms were operative, the model-predicted force transient
is maintained above its half maximal value for a period (t
1/2) that was 30% longer than when they were not operative (Fig. 5, right). Thus,
XB-based stretch activation interacts with calcium activation to significantly enhance
force generation during the pulse of normal cardiac activation and, further, stretch
activation interacts with calcium activation to prolong calcium-initiated force transients.
Stretch activation acts to slow force transients, whether these transients are due
to muscle shortening or due to calcium activation pulses. Both of these slowing actions
help to sustain force during normal cardiac ejection.
Figure 5.
Effect of stretch activation mechanisms when there has been no muscle stretch or shortening.
Calcium transient emulating that of a muscle twitch is shown as light dashed line
in left panel and labeled α(t). Predicted force response to calcium transient when
cross-bridge activation is operative (solid line) and when it is not (dashed line).
Cross-bridge activation resulted in 35% increase in amplitude of force transient (left,
force normalized to peak of twitch with cross-bridge activation) and a 30% increase
in duration above 1/2 peak value, t
1/2 (right, force of both twitches normalized to each peak value).
We suggest that stretch activation in heart muscle functions as an intrinsic force-regulating
mechanism to help match the heart with its preload by increasing the steepness of
the Frank-Starling relationship and to help match the heart with its afterload by
enhancing and sustaining force after a brief calcium activation pulse and during shortening.
Thus, stretch activation in heart muscle plays a role similar to that in insect flight
muscle in that it functions to match the contractile organ with its mechanical load.
The possible role of stretch activation in heart muscle in establishing optimal contraction
frequencies (heart rates) has yet to be explored. Such explorations are likely to
involve mathematical models much like the one we used here. Further, a myocardial
model similar to the one used here will be useful when incorporated into a spatially
distributed global heart model. For instance, a heart model constructed from modeled
myocardium in counterhelically oriented epicardium and endocardium, each helix with
different stretch activation features, will be useful in exploring the means whereby
torsional motions and transmural inhomogeneity in stretch activation function to support
ejection (Davis et. al., 2001). In summary, our understanding of the functional consequences
of stretch activation in the heart will require representation of XB-based stretch
activation mechanisms as described by Stelzer et al. in mathematical models of cardiac
muscle and the whole heart.