Limits of titin extension in single cardiac myofibrils

Abstract

Passive force and dynamic stiffness were measured in relaxed, single myofibrils from rabbit ventricle over a wide range of sarcomere lengths, from approximately 2-5 microns. Myofibril stretch up to sarcomere lengths of approximately 3 microns resulted in a steady increase in both force and stiffness. The shape of the length-force and the length-stiffness curves remained fully reproducible for repeated extensions to a sarcomere length of approximately 2.7 microns. Above this length, myofibrillar viscoelastic properties were apparently changed irreversibly, likely due to structural alterations within the titin (connectin) filaments. Stretch beyond approximately 3 microns sarcomere length resulted in a markedly reduced slope of the passive force curve, while the stiffness curve became flat. Thus, cardiac sarcomeres apparently reach a strain limit near a length of 3 microns. Above the strain limit, both curve types frequently showed a series of inflections, which we assumed to result from the disruption of titin-thick filament bonds and consequent addition of previously bound A-band titin segments to the elastic I-band titin portion. Indeed, we confirmed in immunofluorescence microscopic studies, using a monoclonal antibody against titin near the A/I junction, that upon sarcomere stretch beyond the strain limit length, the previously stationary antibody epitopes suddenly moved into the I-band, indicating A-band titin release. Altogether, the passive force/stiffness-length relation of cardiac myofibrils was qualitatively similar to, but quantitatively different from, that reported for skeletal myofibrils. From these results, we inferred that cardiac myofibrils have an approximately two times greater relative I-band titin extensibility than skeletal myofibrils. This could hint at differences in the maximum passive force-bearing capacity of titin filaments in the two muscle types.

Passive properties of the diaphragm in COPD

“…After the 60-s interval, the microscope camera captured the image of the fiber for analysis of sarcomere length and fiber width, using the methods described above. This time interval allowed the restoring force to reach a peak and decay to a relatively constant plateau as described previously (15,32). In pilot studies within this laboratory, 60 s were sufficient to allow plateau tension to be achieved (Fig.…”

“…Titin is a giant protein with a molecular mass of Ͼ3,000 kDa whose molecules span from the Z disk to the M line of the muscle sarcomere (11), and it is abundant in skeletal muscle cells. In animal models, it has been shown that muscle cells can modulate their stiffness by selective expression of different isoforms of titin (9,15,32), but the evidence as to whether the skeletal muscle of human diaphragm can modulate its titin expression into more than one isoform under different conditions is scarce. Using SDS-agarose gels, we have demonstrated that there is a difference in the titin molecule in the diaphragm muscle of COPD patients compared with non-COPD control patients, seen as a difference in the migration of titin bands between the two groups (Fig.…”

Effects of BDM, [Ca2+]o, and temperature on the dynamic stiffness of quiescent cardiac trabeculae from rat

“…Recently, the majority of passive tension developed by cardiac tissue at low sarcomere length extensions has been attributed to the intracellular elastic protein titin (14,44). Ca 2ϩ -dependent titin-actin interactions (15,26,41), as well as Ca 2ϩ -sensitive molecular domains within titin (12,23), are reported to modulate passive stiffness. Thus the possibility exists that BDM modifies these interactions and results in the frequency-dependent alterations of dynamic stiffness observed in Fig.…”

Strain softening is not present during axial extensions of rat intact right ventricular trabeculae in the presence or absence of 2,3-butanedione monoxime

“…Likewise, Granzier and Irving (17) performed axial extensions of skinned cardiac myocytes from the rat and found no evidence of strain softening between SLs of 1.85 and 2.4 m. Upon axial extension, isolated rabbit (29) and rat (27) myofibrils have demonstrated strain softening, but only when the SL was extended above 3 m. Because the skinning process removed the entire extracellular matrix and collagen, Linke et al (28) attributed the resulting development of passive force and strain softening at large extension to titin.…”

“…Both these studies reported strain softening well within the physiological range. Linke et al (27) found strain softening in isolated rabbit cardiac myofibrils but only at sarcomere length (SL) extensions above 2.7 m, which is greater than the normal physiological SL extension. Because all of the collagen had been removed, this strain softening was attributed to titin being nonreversibly altered.…”