Isometrically Contracting Research Articles

Optical diffraction study of muscle fibers. I. A theoretical basis.

Abstract We present a simple theoretical model of optical diffraction by striated muscle fibers. The model accounts for (1) changes in diffraction intensity during isometric contraction, (2) spectra of quasielastically scattered light from isometrically contracting muscle, and (3) an electro-optical effect or intensity modulation of diffraction lines by applied electric field. Items (1) and (2) are concerned with the fluctuations of the sarcomere structure during isometric contraction, and item (3) is concerned with the lateral motions of, or the flexibility of, thin filaments in striated muscle at rest.

Relaxation of heart muscle by catecholamines and by dibutyryl cyclic adenosine 3',5'-monophosphate. Similarity of beta-adrenoceptors mediating contractile and relaxant effects of catecholamines in kitten pipillary muscle.

1. In isometrically contracting kitten papillary muscles, dibutyryl cyclic AMP (DBcAMP) enhanced peak tension, increased rates of contraction and relaxation, decreased tension of a phasic but not of a small tonic component of KCl-contractures, and caused aftercontractions. These effects resemble closely those of catecholamines. 2. The effects of DBcAMP on kitten papillary muscle were not influenced by (-)-bupranolol, a beta-adrenoceptor antagonist. 3. DBcAMP decreased KCl-contractures in strips of frog ventricle. 4. Phasic KCl-contractures in kitten papillary muscles were decreased by (-)- and (+)-isoprenaline. For similar effects, 100-fold higher concentrations of (+)-isoprenaline than of (-)-isoprenaline were required. 5. Increases in maximum rates of contraction and relaxation, increases in peak tension of isometric contractions and reduction of phasic KCl-contractures by catecholamines were antagonized competitively to a similar extent by (-)-bupranolol. Mean apparent equilibrium constants for the beta-adrenoceptor-(-)-bupranolol complex of 0.46-0.70 nM were estimated. These constants were quite similar irrespective of whether (-)-isoprenaline, (+)-isoprenaline or (-)-noradrenaline were used as agonists. 6. Increases in contractile strength, maximum rates of contraction and of relaxation of isometric contractions and decreases in KCl-contractures by (-)-isoprenaline were surmountably blocked by (+)-bupranolol. Mean apparent equilibrium constants for the receptor-(+)-bupranolol complex were 40-50 nM. 7. The equilibrium constants of (-)- and (+)-bupranolol for the receptors mediating positive inotropic and relaxant effects of catecholamines were not significantly different from constants for bupranolol-receptor complexes in cell-free membrane particles of kitten heart ventricle. It is suggested that the same beta-adrenoceptor triggers positive inotropic, relaxant and adenylyl cyclase-activating effects of catecholamines in kitten papillary muscle. 8. The partial agonist (-)-dichloroisoprenaline (DCI) (1 muM) reduced by 79% the phasic KCl-contractures of the kitten papillary muscles. DCI stimulates adenylyl cyclase activity of ventricle membranes to less than 1/4 of maximum stimulation by (-)-isoprenaline. If cyclic AMP produced by DCI is involved in the decrease of phasic KCl-contracture, small increase in cyclic AMP should be sufficient to induce this effect.

Structural dynamics of frog muscle during isometric contraction.

Intensity fluctuation autocorrelation functions of laser light scattered by actively contracting muscle were measured at points in the scattered field. They were reproducible and showed characteristics which depended on the physiological state of the muscle and the parameters of the scattering geometry. The autocorrelation functions had large amplitudes and decay rates that varied significantly with the phase of the contraction-relaxation cycle. The dependence of the autocorrelation function on scattering geometry indicated many elements with diameters on the order of 0.5 mum (presumed to be myofibrillar sarcomeres or their A bands or I bands) undergo independent random changes in their axial positions and their internal distribution of optical polarizability during the plateau of an isometric tetanus. The experimental results are interpreted in terms of a model in which most of the scattering elements in isometrically contracting muscle have random fluctuating axial velocities of average magnitude 20 nm/ms that persist for a few milliseconds at least. In addition to these axial motions there are local fluctuations in polarizability. Similar intensity fluctuation autocorrelation functions were observed throughout the active state on two muscle preparations, whole sartorius muscle and small bundles of single fibers (three to eight) of semitendinosus muscle. These results imply that the tension developed during an isometric tetanus contains a fluctuating component as well as a constant component.

Role of acetylstrophanthidin in augmenting myocardial oxygen consumption. Relation of increased O-2 consumption to changes in velocity of contraction.

A polarographic method was used to determine the effects of acetylstrophanthidin on myocardial oxygen consumption (MV · O 2 ) of 19 cat papillary muscles contracting under both afterloaded and isometric conditions. Under afterloaded conditions, acetylstrophanthidin shifted the force-velocity relation to the right and produced increments in both the extent and velocity of shortening at constant levels of developed force. These changes in myocardial mechanical behavior after strophanthidin were always associated with an increased MV · O 2 . Since both the extent and velocity of shortening increased following augmentation of the contractile state with glycoside, consideration of the individual effect of these variables in myocardial mechanical behavior on MV · O 2 was precluded. Therefore, experiments were performed on isometrically contracting muscle to evaluate the effect of changes in the velocity of contraction on MV · O 2 independent of changes in contractile element shortening. Comparison of the MV · O 2 of isometric contractions at equal levels of developed tension (and thus equivalent amounts of internal contractile element shortening) was accomplished by decreasing initial muscle length after augmentation of the contractile state by acetylstrophanthidin. Under these conditions, augmentation of the contractile state, characterized by an increase in the velocity of contraction, was associated with an increased MV · O 2 . Thus it is concluded that the effect of acetylstrophanthidin is to increase MV · O 2 at a constant load and that this augmentation of MV · O 2 can be related to the change in contractile state of the muscle.

The series elasticity of active taenia coli in vitro.

AbstractThe series elastic components (SEC) of the smooth muscle taenia coli of the guinea‐pig have been investigated.The rate of isotonic shortening after release of an isometrically contracting taenia coli showed that the muscle behaved like a model with contractile elements in series with elastic components. Thus taenia coli has SEC in the same sense as it has been described for striated muscles and heart muscles. At 37°C and full isometric tension the SEC in taenia coli in vitro were measured to be extended on the average 13.1 ± 2.7 per cent of Lo. Maximum “isometric” work developed during an isometric contraction was approximately 30 per cent of the maximum work that the muscle could perform during an isotonic shortening.