Passive and active tension in single cardiac myofibrils

Abstract

Single myofibrils were isolated from chemically skinned rabbit heart and mounted in an apparatus described previously (Fearn et al., 1993; Linke et al., 1993). We measured the passive length-tension relation and active isometric force, both normalized to cross sectional area. Myofibrillar cross sectional area was calculated based on measurements of myofibril diameter from both phase-contrast images and electron micrographs. Passive tension values up to sarcomere lengths of approximately 2.2 microns were similar to those reported in larger cardiac muscle specimens. Thus, the element responsible for most, if not all, passive force of cardiac muscle at physiological sarcomere lengths appears to reside within the myofibrils. Above 2.2 microns, passive tension continued to rise, but not as steeply as reported in multicellular preparations. Apparently, structures other than the myofibrils become increasingly important in determining the magnitude of passive tension at these stretched lengths. Knowing the myofibrillar component of passive tension allowed us to infer the stress-strain relation of titin, the polypeptide thought to support passive force in the sarcomere. The elastic modulus of titin is 3.5 x 10(6) dyn cm-2, a value similar to that reported for elastin. Maximum active isometric tension in the single myofibril at sarcomere lengths of 2.1-2.3 microns was 145 +/- 35 mN/mm2 (mean +/- SD; n = 15). This value is comparable with that measured in fixed-end contractions of larger cardiac specimens, when the amount of nonmyofibrillar space in those preparations is considered. However, it is about 4 times lower than the maximum active tension previously measured in single skeletal myofibrils under similar conditions (Bartoo et al., 1993).

Passive properties of the diaphragm in COPD

“…We have chosen to analyze these results using similar methods to Prado et al (28) by applying a quadratic model to describe the force-length relationship, which fits the data well. This quadratic model is more consistent with the evidence available in the literature, where a roughly parabolic relationship is commonly described (16,19,28,34), compared with the linear relationship Ottenheijm et al (25) used in their recent paper. This method for analysis has allowed us to extract a single parameter, the AUC, from the length-tension relationship, which enables us to directly compare the passive tension properties of the muscle fibers between and within the patient groups.…”

“…Passive tension, or restoring force, is the force response of a muscle at rest to stretching. Much is known about passive tension in animal skeletal muscles, including the diaphragm (9,16,28,32), but largely due to the technical difficulties of obtaining diaphragm muscle samples there are few data on the passive properties of the human diaphragm in health or disease.…”

Molecular determinants of heart failure with normal left ventricular ejection fraction

“…When F passive values of control and HFNEF cardiomyocytes were pooled, significant correlations were found between F passive and indices of diastolic LV dysfunction, such as LVEDP, SM and s. The relations between F passive and indices of diastolic LV function all leveled off at higher values of LVEDP, s, and E. This could result from diuretic therapy to compensate patients before catheterization or from more intense interstitial fibrosis at severe diastolic LV dysfunction. Endomyocardial biopsy samples from patients with HFNEF had higher CVF than did controls, and in a bivariate linear regression analysis, both F passive and CVF were significantly correlated with LVEDP and s. A predominant effect of interstitial fibrosis upstream of diastolic LV dysfunction is consistent with previous experimental studies, which showed diastolic muscle stiffness to originate from structures within the sarcomere for sarcomere lengths up to 2.2 mm [26] and from perimysial fibers once filling pressures exceeded 30 mmHg [14].…”

Gigantic Business

“…At the level of the cardiomyocytes, the stiffness of titin depends on the expression ratio of compliant versus stiff isoforms, the phosphorylation status of some titin spring elements, and certain oxidative modifications of elastic titin domains ( Figure 1A), all of which are discussed in detail below. A general principle first proposed some 20 years ago 8 and confirmed various times since 9,10 is that titin/cardiomyocyte stiffness dominates total myocardial stiffness over most of the physiological sarcomere length (SL) range, whereas collagen/extracellular matrix stiffness becomes relatively more important beyond this range ( Figure 1B). However, the stiffness contribution from titin, measured in absolute values, also continues to grow beyond physiological SLs and cannot be neglected under disease conditions.…”