Electron Microscopical Study of Skeletal Muscle During Isotonic (Afterload) and Isometric Contraction

Abstract

Observations by light microscopy on contracted muscle have been made chiefly under isometric conditions: on striated muscle of frog (9,14,5,2,4,15), of Astracusfluviatilis and Carcinus maenas (17), and guinea pig (12); and on frog cardiac muscle (18).Studies dealing with isotonic contraction are those of Htirthle (13) (11) on the flight muscle of the blowfly.For both isometric and isotonic contraction the published results are contradictory. In part this may result from the fact that during isotonic contraction the shortening of the fibre may bring the length of the anisotropic and isotropic bands below the resolving power of the light microscope. The changes in length that occur in the anisotropic and isotropic bands of skeletal muscle during stretch were described in a previous study (6). The present electron microscopic study was undertaken to determine the length of the anisotropic and isotropic bands during isotonic (afterload) and isometric contraction. MethodBundles (75 to 100 fibres) from the curarized semitendinosus muscle of Rana temporaria and Rana esculenta were isolated and the equilibrium length determined. They were stretched to 30 to 40 per cent above the equilibrium length (L0 --100) for about 30 minutes in Ringer's solution at 0--4°C. The fibre bundle was then placed in a myograph (3), adjusted to the desired length (25 per cent stretch for aftefload and 0 to 50 per cent stretch for isometric contraction). The term "afterload contraction" denotes a contraction in a muscle initially stretched to a desired length by an external load. Upon stimulation the muscle first contracts isometrically until the developed tension equals the external force, and then it shortens against this "aftefload." The muscle was maintained at the desired length until the myograph gave constant readings for at least 30 minutes. The Ringer's solution was then removed, and the muscle stimulated directly before and for some time during fixation of the muscle in 10 per cent ice cooled neutral formalin solution. The stimulating square wave pulses had a duration of I0 201

Elasticity of the Fibrous Muscle Proteins*

“…This result contrasts with the microscopic results (Huxley and Hanson, 1954;Huxley and Niedergerke, 1954) obtained with light optics for contraction under physiologic conditions, for which the A-band has been rejiorted to remain constant in length, whereas the Z-Z distance decreases. However, the results of Knappeis and Carlsen (1956). obtained by electron microscopy of muscle contracted under physiologic conditions and staining techniques similar to ours, indicate contraction of the A-bands also.…”

Studies on the Endoplasmic Reticulum

“…Some special significance probably attaches to the fact that these triadic differentiations are always located opposite the I band or some part of it, for in isotonic contraction it is the I band that disappears (35)(36)(37). Whether this results from the incorporation of the I band myofilaments into the A, as Huxley and Hanson have proposed (37), or from some other process, it does seem that the isotropic band is the especially "active" segment of the fibril and that associated structures must be involved in its activity.…”

Effect of temperature on post-mortem structural changes in rabbit, bovine and porcine skeletal muscle

“…It has, however, been difficult to obtain detailed information on the nature of the interaction of the crossbridges with actin during contraction. Recently Reedy (Reedy, 1967;Reedy et al, 1955) A few years ago, several workers (Carlsen et al, 1961;deVillafranca, 1961;deVillafranca and Marschhaus, 1963;Knappeis and Carlsen, 1956) reported that the A-band and there fore, the thick filaments, shorten during contraction. Huxley and coworkers (Page, 1964;Page and Huxley, 1963) conducted an extensive examination of filament lengths at different stages of contraction and found that filaments do not change in length during the contraction process.…”