The Mechanism of the Length-Tension Relation in Cardiac Muscle of Rana Catesbeiana
A recently developed instrument permits tension measurement and length control at such high resolution and frequency bandwidth that it is now possible to obtain static and dynamic mechanical properties of single myofibrils consisting of only 20 to 30 sarcomeres. Isometric tension vs. sarcomere length curves of bull-frog single or double myofibrils were constructed with intracellular solutions having pCa values of 9.0, 6.0, 5.5 and 5.0 at several sarcomere lengths from 2.2 µm to 3.6 µm. Every sarcomere of the myofibril preparation was observable and if a noticeable abnormality in sarcomere length uniformity and A-band misregistration developed, the data were rejected. Good quality single myofibrils produced up to 50 µg active tension which corresponds to about 5 Kg/cm2 (or 50 N/cm2). All active isometric tension vs. sarcomere length curves had positive slopes, i.e. active tension increased with sarcomere length up to 3.6 µm except when pCa was 5.0 in which case the tension measurements at greater sarcomere lengths than 3.0 µm were not possible without compromising the sarcomere uniformity and A-band registration. It was concluded that sarcomeres (both cardiac and skeletal) have an intrinsic property that the active tension increases with sarcomere length within the range of myofilament overlap so long as sarcomeres are activated by submaximal levels of Ca2+ concentration, and that excessive activation leads to misregistration of the A-band which appears to occur due to functional failure of myosin molecules.
KeywordsSarcomere Length Active Tension Length Curve Myosin Molecule Cardiac Sarcomere
Unable to display preview. Download preview PDF.
- 4.Fabiato A (1981). Myoplasmic free calcium concentration reached during the twitch of an intact isolated cardiac cell and during calcium-induced release of calcium from the sarcoplasmic reticulum of a skinned cardiac cell from the adult rat or rabbit ventricle. J Gen Physiol 78: 457–197.PubMedCrossRefGoogle Scholar
- 6.Iwazumi T (1970). A new field theory of muscle contraction. PhD Thesis, University of Pennsylvania. Univ Microfilms, Inc.Google Scholar
- 7.Iwazumi T (1978). Molecular mechanism of muscle contraction: another view. In: Cardiovascular system dynamics. Baan, Noordergraff and Raines (eds), MIT Press, 11–21.Google Scholar
- 8.Iwazumi T (1979a). A new field theory of muscle contraction. In: Crossbridge mechanism in muscle contraction. Pollack and Sugi, (eds), Univ. of Tokyo press: 611–632.Google Scholar
- 9.Iwazumi T (1979b). A theory of sarcomere dynamics. In: The cardiac cycle. MIM Noble, Blackwell: 54–89.Google Scholar