The aim of the present study was to test whether titin is a calcium-dependent spring and whether it is the source of the passive force enhancement observed in muscle and single fiber preparations. We measured passive force enhancement in troponin C (TnC)-depleted myofibrils in which active force production was completely eliminated. The TnC-depleted construct allowed for the investigation of the effect of calcium concentration on passive force, without the confounding effects of actin-myosin cross-bridge formation and active force production. Passive forces in TnC-depleted myofibrils (n = 6) were 35.0 ± 2.9 nN/ μm2 when stretched to an average sarcomere length of 3.4 μm in a solution with low calcium concentration (pCa 8.0). Passive forces in the same myofibrils increased by 25% to 30% when stretches were performed in a solution with high calcium concentration (pCa 3.5). Since it is well accepted that titin is the primary source for passive force in rabbit psoas myofibrils and since the increase in passive force in TnC-depleted myofibrils was abolished after trypsin treatment, our results suggest that increasing calcium concentration is associated with increased titin stiffness. However, this calcium-induced titin stiffness accounted for only ∼25% of the passive force enhancement observed in intact myofibrils. Therefore, ∼75% of the normally occurring passive force enhancement remains unexplained. The findings of the present study suggest that passive force enhancement is partly caused by a calcium-induced increase in titin stiffness but also requires cross-bridge formation and/or active force production for full manifestation.
- residual force enhancement
when a contracting muscle is stretched, the steady-state force reached after the stretch is higher than the isometric force at the corresponding length; this phenomenon is referred to as residual force enhancement (1, 5–7, 13, 23, 30). Furthermore, passive force after deactivation of an actively stretched muscle is higher than the force produced after a purely passive stretch or after deactivation from an isometric contraction at the corresponding length (13, 14). This so-called “passive force enhancement” was first observed in the cat soleus muscle (13), and then in human muscle preparations (19), single fibers (18, 28, 30), and recently in single myofibrils (16).
The origin of the passive force enhancement remains unknown. Passive force enhancement occurs at long lengths, at which passive force is naturally occurring, it is long lasting (>25 s), and it increases with stretch magnitude and initial muscle length, but it is independent of the speed of stretch (13, 14). On the basis of these characteristics, it is likely that a structural protein may be responsible for the passive force enhancement.
Titin is a molecular spring that runs from the Z lines of sarcomeres to the M band, and it is the structure responsible for most of the passive force in fibers and myofibrils (10, 15, 22). In fact, myofibrillar passive elastic diversity has been related to different titin isoforms (4, 8, 10, 21, 35). For example, the higher passive forces observed in cardiac compared with skeletal muscle myofibrils have been associated with the expression of a short titin isoform in cardiac myofibrils (22). Different titin isoforms have also been used to explain differences in passive stiffness between psoas and soleus rabbit fibers (10, 22). Furthermore, degradation or extraction of titin from myofibrils leads to a rapid drop in passive force (12, 26, 35) in cardiac and skeletal myofibrils.
Titin has specific calcium binding sites (32), and calcium binding is thought to affect titin's stiffness and its binding to actin (17), thereby affecting titin's natural resting length. Since calcium concentration in the sarcoplasm increases dramatically when a muscle is activated, it has been suggested that titin is a spring whose stiffness may increase, or whose characteristic length may decrease, with activation and the associated increase in calcium concentration (13, 14, 31). Through these changes in mechanical properties, titin could be directly responsible for the passive force enhancement observed when a muscle is actively stretched.
The aim of the present study was to test the hypotheses that titin-based passive force is calcium dependent and that passive force enhancement is caused by a calcium-dependent increase in titin stiffness. To achieve this aim, tests were performed in myofibrils isolated from rabbit psoas muscle before and after elimination of cross-bridge formation and active force production through the deletion of troponin C (TnC) (2, 3, 9, 24, 27). In this way, the effects of increasing calcium concentrations on passive forces during and after stretching could be determined without the confounding effects of actin-myosin cross-bridge formation.
Since titin is the structure responsible for most of the passive force in myofibrils (4, 10, 15, 21, 35), any passive force enhancement is likely associated with titin. In addition, myofibrils are activated through a solution with a high calcium concentration, and, therefore, the effect of calcium on passive force can be directly studied after eliminating active force production. The term “active” in skeletal muscle contraction is typically associated with calcium-induced activation and force production. Here we modulated calcium concentrations in intact and TnC-depleted myofibrils. In the latter preparation, increases in calcium concentration do not lead to cross-bridge formation and active force; therefore, the term “active” becomes ambiguous. Here we define “active” by cross-bridge associated force and not by calcium concentration; thus we describe the “passive” stretching of TnC-depleted myofibrils at high calcium concentrations, and “passive” refers to the fact that cross-bridge attachment and the cross-bridge-associated forces are absent.
MATERIALS AND METHODS
Samples and myofibrils were prepared as described previously (29, 36). Single myofibrils isolated from skinned rabbit psoas samples were fixed to a glass needle at one end and to a silicon nitride nanolever (stiffness = 22 pN/nm) at the other end, allowing for length change control and force measurement, respectively. The striation pattern of the myofibrils was projected onto a linear photodiode array (10,680 elements), which generated a light intensity profile representing the sarcomere banding pattern. The centroids of the A bands were determined, and sarcomere lengths were calculated as the distance between adjacent A-band centroids by using an algorithm that tracked the signal peak positions continuously during the experiments (29, 36). The experiments were visualized and recorded with the use of a charge-coupled device camera and video cassette recorder. Images of the myofibrils were subsequently used for measurements of the myofibril diameter and force calculation. The displacement (and therefore the force) of the nanolever was evaluated by using Motion Capture Plus software.
TnC extraction protocol.
To determine the effects of calcium concentration on passive (titin) stiffness and passive force enhancement without inducing cross-bridge formation and active force production, TnC was extracted from actin by using standard procedures (2, 3, 9, 24, 27).
Myofibrils were incubated with a low ionic strength rigor-EDTA solution (pH 7.8) for 10 min at 15 ± 3°C, and they were subsequently washed with relaxing solution. To confirm TnC extraction, control and TnC-extracted samples were subjected to SDS-polyacrylamide electrophoresis and then Western blot analyses by using a mouse monoclonal antibody for TnC (33). Possible degradation of titin resulting from sample preparation and TnC deletion was also evaluated by using agarose-strengthened SDS-polyacrylamide electrophoresis gels (25).
To verify whether active force production was eliminated following TnC extraction, control myofibrils were activated at sarcomere lengths of 2.4 and 3.4 μm before and after TnC extraction. Activation was induced by the introduction of a high calcium concentration-activating solution.
Myofibrils (n = 6) were fixed at an average sarcomere length of 2.4 μm and were then stretched passively, in a relaxing solution (pCa 8.0) by 1 μm/sarcomere at a speed of 0.1 μm ·s−1·sarcomere−1. The myofibril was held isometrically for 1 min after stretch and then released. TnC was then extracted by incubating the myofibrils in the rigor-EDTA solution for 10 min. After TnC extraction, myofibrils were washed with a relaxing solution and the same stretch protocol was imposed. A second stretch of 1 μm/sarcomere was performed at a speed of 0.05 μm·s−1·sarcomere−1. After the activating solution (pCa 3.5) was added, the same stretch protocol was performed (two stretches at speeds of 0.1 μm·s−1·sarcomere−1 and 0.05 μm·s−1·sarcomere−1). All tests were performed twice, with a rest period of 10 min.
To test for titin-based passive force, myofibrils were subsequently treated with trypsin at a concentration of 0.05 μg/ml for 5 min and were stretched in the relaxing and activating solutions by 1 μm/sarcomere at a speed of 0.1 μm·s−1·sarcomere−1, held isometrically for 1 min, and then released.
Force produced in myofibrils when stretched in the absence of active force production (i.e., at all calcium concentrations in the TnC-depleted myofibrils and at low calcium concentration in the intact myofibrils) was called passive force. All forces were normalized by myofibril cross-sectional area and are expressed as stress (in nN/μm2). The calcium-dependent passive force enhancement was determined as the difference between the passive isometric steady-state forces following stretching of TnC-depleted myofibrils in the activating (Fact) and relaxing (Frel) solutions (Fig. 1). Frel and Fact were determined as the mean force recorded during the last 10 s of the isometric phase following stretching in the relaxing and activating solutions, respectively. For each myofibril, Frel and Fact were compared by using a paired t-test. For comparison across myofibrils, the nonparametric Mann-Whitney U-test was used.
The passive sarcomere force-length relationship was determined from the slow stretch-shortening experiments in TnC-depleted myofibrils performed in the relaxing and the activating solutions.
The rigor, relaxing, and activating solutions were identical to those described previously (29).
TnC extraction protocol.
Figure 1 shows the active forces produced by myofibrils before and after extraction of TnC, at sarcomere lengths of 2.4 and 3.4 μm. At both sarcomere lengths, active force was completely abolished following TnC extraction. SDS-electrophoresis confirmed that TnC had been successfully extracted to levels of <1% compared with normal (Fig. 2A). Furthermore, titin was found to be degraded by 18% because of the skinning protocol and conservation. Titin degradation increased further by 4.3% because of the extraction of TnC (Fig. 2B).
Passive forces in the relaxing solution and after TnC extraction were similar to those produced by intact myofibrils, which indicates that TnC extraction had little, if any, effect on passive force (Fig. 3).
Passive force enhancement in TnC-depleted myofibrils.
TnC-depleted myofibrils had greater passive forces following stretching in the activating solution (pCa 3.5) than in the relaxing solution (Fig. 4). The difference in these passive forces became greater with increasing sarcomere lengths (Fig. 4B). Calcium-dependent passive force enhancement ranged from 25% to 30% of the passive force produced in the relaxing solution.
When titin was degraded, the passive forces decreased dramatically (to ∼20% of the force produced by intact myofibrils), and the difference between the passive forces in the relaxing and activating solution was abolished (Fig. 5).
The purpose of the present study was to test the hypotheses that titin-based passive force is calcium dependent and that passive force enhancement is caused by a calcium-dependent increase in the stiffness of titin. We investigated passive forces in single TnC-depleted myofibrils, in which active force production was eliminated, so that the effects of increasing calcium concentrations on passive forces during and after stretching could be determined without the confounding effects of actin-myosin cross-bridge formation and active force production.
TnC extraction and active force inhibition.
TnC was removed by using a low ionic strength-EDTA solution previously used to partially extract TnC from fibers and myofibrils (24, 27). Extending the extraction time to 10 min resulted in a complete TnC extraction (Fig. 2A). In addition, this treatment resulted in a complete suppression of active force production in the presence of calcium at sarcomere lengths of 2.4 and 3.4 μm (Fig. 1). Since the passive forces of TnC-depleted myofibrils stretched in the relaxing solution were similar to the passive forces produced by intact myofibrils after passive stretching (Fig. 3), we are confident that the TnC extraction protocol successfully eliminated actin-myosin interactions and force production without altering the passive properties of the myofibrils.
Passive force enhancement in TnC-depleted myofibrils.
Passive forces in the TnC-depleted myofibrils were increased by ∼25% to 30% when stretched in the activating compared with the relaxing solution. We also observed an upward shift in the passive force-sarcomere length relationship when calcium concentration was increased (Fig. 4B). This result is in accordance with findings by Labeit et al. (17), who showed increases in passive forces of skinned fibers (in which actin was depleted) in the presence of calcium. Since we are testing myofibrils in which titin is the primary source of passive force, the increase in passive force in the presence of calcium is likely related to titin. Furthermore, the calcium-dependent increase in passive force in the TnC-depleted myofibrils was abolished when titin was eliminated by trypsin (Fig. 5), which suggests that it was caused by changes in the mechanical properties of titin.
The level of calcium-dependent passive force enhancement in TnC-depleted myofibrils was between 25% and 30%. In a previous study (16), we measured passive force enhancement in intact (i.e., TnC was not depleted) rabbit psoas myofibrils under the exact same stretch conditions as used in the present study. Passive force enhancement in the intact myofibrils ranged from 85% to 145% of the passive force, averaging 118% or about four times greater than the Ca-dependent increase in passive forces observed here in the TnC-depleted myofibrils. In other words, the calcium-induced titin stiffness observed here can account for only ∼25% of the passive force enhancement observed in intact myofibrils. Conceptually, there are two possibilities on how to explain the missing 75% of the passive force enhancement. First, it could be associated with cross-bridge interactions that were eliminated in the TnC-depleted myofibrils, and, second, it could be related to a calcium-mediated change in actin-titin interaction that is force, cross bridge, or TnC dependent. Titin interacts with actin (11, 20, 34); thus a TnC- or force-dependent change in actin-titin interactions is a real possibility. Although direct evidence for either of these proposed mechanisms is lacking, we prefer the latter because it has been shown in whole muscle preparations that passive force enhancement changes with muscle length and is retained with repeat activation and deactivation of the muscle (13). These properties could be explained within the framework of activation-dependent changes of a passive structural element, but they would be hard to reconcile with a “cross-bridge”-related mechanism.
To address the question concerning the origin of the unaccounted passive force enhancement, studies on titin-actin affinity in the presence and absence of TnC and calcium, as well as studies in TnC-depleted myofibrils in which actin filaments strained without cross-bridge attachments, need to be performed. Such studies would shed light on the role of actin-titin interactions and the changes in the effective stiffness of titin and thus could provide novel information on the role of cross-bridge formation and force production in controlling actin-titin interactions.
The nanolevers used in this work were built at the Cornell NanoScale Facility, which is supported by the National Science Foundation (Grant ECS 03-35765). The financial support from National Sciences and Engineering Research Council of Canada, Canadian Institutes of Health Research, and the Canada Research Chair Program is acknowledged.
The authors thank Dr. L. C. Rome for helpful discussions and A. Jinha for writing the analysis program.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
- Copyright © 2008 the American Physiological Society