The necessity of using two parameters to describe isotonic shortening velocity of muscle tissues: the effect of various interventions upon initial shortening velocity (vi) and curvature (b)
- 42 Downloads
In skinned skeletal muscle fibers and skinned preparations of myocardium or smooth muscle, for all conditions studied, the length traces during isotonic shortening are always found to be significantly curved. It is demonstrated that the observed curvature is not simply due to inhomogeneities on the sarcomere level in striated muscle, damaged ends of the preparations, double overlap and collision of filaments, or depletion of MgATP during the period of isotonic shortening. It is shown that the velocity of shortening can be described by an exponential function: v=vi exp (-b[SLi-SL]) with SLi: sarcomere length at the start of the release; SL: sarcomere length during isotonic shortening. Thus, instantaneous shortening velocity (v) is determined by two parameters: vi, the initial shortening velocity for SL=SLi, and b, a constant characterizing the decrease in velocity during isotonic shortening. Factors which affect isotonic shortening can do this by affecting vi, or by changing b, or both. Therefore, when analysing the effects of interventions which affect instantaneous shortening velocity, these two possibilities have to be distinguished. Since curvature of the length traces might be caused by noncrossbridge components, only vi, the initial speed of shortening, is a parameter which directly reflects kinetics of the cross-bridge cycle while instantaneous speed of shortening might also be affected by noncrossbridge factors.
Analysing isotonic shortening in terms of vi and b, the effects of ionic strength, free Ca++ concentration, MgATP/MgADP ratio and temperature on unloaded isotonic shortening have been studied. For the conditions used, it can be shown that ionic strength and free Ca++ concentration only affect b without significant effect on vi, whereas MgATP/MgADP ratio and temperature affect both vi and b. This means that of these factors only MgATP/MgADP ratio and temperature affect the crossbridge kinetics which determine the maximum speed of shortening while ionic strength and free Ca++ concentration have no such effect within the experimental error.
Key wordsisotonic shortening velocity initial speed of shortening (vi) curvature (b) striated muscle smooth muscle
Unable to display preview. Download preview PDF.
- 1.Arner A (1982) Mechanical characteristics of chemically skinned guinea-pig Taenia coli. Pflüg Arch 395:277–284Google Scholar
- 2.Brenner B (1980) Effect of free sarcoplasmic Ca2+ concentration on maximum unloaded shortening velocity: measurements on single glycerinated rabbit psoas muscle fibres. J Musc Res Cell Mot 1:409–428Google Scholar
- 5.Eastwood AB, Wood DS, Bock KL, Sorenson MM (1979) Chemically skinned mammalian skeletal muscle. I. The structure of skinned rabbit psoas. Tissue & Cell 11:553–566Google Scholar
- 6.Ebrecht G, Rupp H, Jacob R (1982) Alterations of mechanical parameters in chemically skinned preparations of rat myocardium as a function of isoenzyme pattern of myosin. Basic Res Cardiol 77:221–234Google Scholar
- 9.Eisenberg E, Greene LE (1980) The relation of muscle biochemistry to muscle physiology. Ann Rev Physiol 42:293–309Google Scholar
- 15.Hill AV (1970) First and last experiments in muscle mechanics. University Press, CambridgeGoogle Scholar
- 16.Huxley AF (1957) Molecular structure and theories of contraction. Prog Biophys Molec Biol 7:255–318Google Scholar
- 17.Huxley AF, Julian FJ (1965) Speed of unloaded shortening in frog striated muscle fibres. J Physiol Lond 177:60PGoogle Scholar
- 22.Moss RL (1982) The effect of calcium on the maximum velocity of shortening in skinned skeletal muscle fibres of the rabbit. J Muscle Res Cell Mot 3:295–311Google Scholar