Passive and active tension in single cardiac myofibrils

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).

Spontaneous sarcomeric oscillations at intermediate activation levels in single isolated cardiac myofibrils

Spontaneous oscillations observed in various heart muscle preparations are widely thought to be triggered by spontaneous release of Ca2+ from the sarcoplasmic reticulum (SR). Here, we report undamped propagated oscillations that occur in the absence of SR. In single cardiac myofibrils treated with Triton X-100 to remove SR and held isometrically, partial activation initiated periodic fluctuations of sarcomere length persisting up to 1 hour. Oscillation characteristics could be readily quantitated by virtue of the small size of the preparation. In an individual sarcomere, the oscillation cycle generally consisted of a slow shortening phase, followed by a phase of rapid lengthening. Oscillations usually propagated along the myofibril--frequently along the entire specimen--in a wavelike fashion (average velocity, 12.3 microns/s at 10 degrees C; Q10, approximately 1.3). The oscillation period was 2.30 and 1.72 seconds at 10 degrees and 20 degrees C, respectively, and was insensitive to stretch. The average oscillation amplitude, which was temperature independent, decreased with stretch from more than 20% of the mean sarcomere length at lengths below 2 microns to zero beyond a sarcomere length of 3 microns. Stiffening of the Z line by labeling with anti-alpha-actinin resulted in a dose-dependent decrease of oscillation amplitude, while the period was not affected. Tension oscillations could not be detected in single myofibrils but were frequently detectable in myofibril doublets, where the oscillation magnitude (approximately 1 microgram) was above the noise floor. Addition of 10 mumol/L ryanodine to the activating solution did not alter oscillation characteristics, as expected, since the oscillations are unrelated to SR calcium release. On the basis of our results, we consider a mechanism for the oscillations in which a length dependence of myofibrillar Ca2+ sensitivity and a dynamic Z-line structure are essential.