The extensibility of the sarcolemma of single myofibers can be reduced locally by leaving a segment covered by a sleeve of surrounding tissue composed of cut myofibers, blood vessels, and connective tissue, hereafter referred to as "the splint" Splinted fibers from frog semitendinosus muscle were used to study mechanical connections (transverse coupling) between myofibrillar components and sarcolemma The transverse coupling is strong enough to insure a tight correlation between myofibril length and overlying sarcolemma length in both resting and activated fibers and to transmit nearly maximum isometric tension to the splint Lateral transmission of active tension was demonstrated with a preparation which had the distal two-thirds of an intact fiber covered by a splint and the proximal third dissected clean When the outer end of the splint was pinned down and only the distal tendon was held, tension generated in the splinted fiber was transmitted to, and recorded from, the splint Parameters of isometric tension transmitted laterally were not significantly different from those of tension transmitted longitudinally Myofibrils branch profusely and form a network that may act as a unitary force generator and transmitter In splinted fibers its output is possibly picked up circumferentially and transmitted across the sarcolemma by a microfilament network A cap of relatively inextensible sarcolemma "splints" myofiber ends Resting tension is transmitted to and from the myofibrils by transverse coupling beyond the cap and the region of short sarcomere spacing it covers Transverse cytoskeletal connections at Z and M regions are described Immobilization of the sarcolemma allows study of myofibril-sarcolemma linkage in intact fibers Both active and resting tension were transmitted laterally

Lateral transmission of tension in frog myofibers: A myofibrillar network and transverse cytoskeletal connections are possible transmitters

The force-velocity relation has been studied in sixteen single fibres from frog semitendinosus muscle with particular attention to the high-force portion of the curve. The force-velocity curve was hyperbolic except for a reversal of curvature near 80% measured isometric tension (PO). Rectangular hyperbolas fitted (linear, least-squares method) these data well only when values below 0.78 PO were considered. Extrapolation of these hyperbolas above 0.78 PO gave predicted isometric tensions (P*O) which averaged 32+/-6% above the measured PO values. Hill's constants (1.84 degrees C) for these hyperbolas were: a/P*O=0.177+/-0.021, b=0.329+/- 0.035 M.L./sec, Vmax=1.91+/-0.074 M.L./sec. The reversal of curvature persisted when force-velocity data were obtained using: 1 or 60 min response intervals, afterloaded isotonic responses, grid stimulation, electrically induced contractures and bundles of fibres. The reversal of curvature diminished when force-velocity data were obtained from slightly stretched fibres (about 2.3 mum sarcomere length as compared to 2.1 mum in the control). The results indicate that sarcomere length redistributions probably do not account for the non-hyperbolic force-velocity relation. An explanation for the behavior based on the geometry of the contractile filament lattice is discussed.

Non-Hyperbolic Force-Velocity Relationship in Single Muscle Fibres1

It has long been known that the contractile performance of striated muscle is strongly dependent upon the length of the muscle (e.g. Ramsey & Street, 1940) and the sliding filament theory of muscle contraction offers a ready explanation for many features of the isometric length-force relation of fully activated skeletal muscle fibres (Gordon et al., 1966b). Over the past decade, however, it has become increasingly more apparent that differences in the mechanical behaviour of striated muscle at different lengths, particularly if submaximally activated, often cannot be satisfactorily explained on the basis of filament overlap alone. There is growing evidence that changing the length of a muscle fibre alters not only the physical overlap of the filaments (i.e. the striation spacing or sarcomere length) but also modifies other processes involved in the activation of contraction. In particular it appears that both the characteristics of the sarcoplasmic Ca 2+ concentration change, which occurs upon excitation of the fibre, and the apparent sensitivity of the contractile proteins to Ca 2+ can vary with muscle length. This article will concern itself principally with the length dependence of these two aspects of contractile activation.

Length dependence of changes in sarcoplasmic calcium concentration and myofibrillar calcium sensitivity in striated muscle fibres

The time course of the relationship between force, shortening velocity, and length during shortening was examined in mammalian heart muscle in calcium (Ca)-containing solutions and solutions with strontium (Sr) substituted for Ca by imposing load clamps of various amplitudes at various times during external shortening. The onset of this force-velocity-length relationship was measured by abruptly unloading the muscle at different times to nearly zero external load--by zero load clamping the muscle--under conditions of optimal damping; the subsequent unloaded active muscle shortening velocity was then analyzed with respect to time and length. In Ca- containing solution, the unloaded velocity in the length range around 90% of L max rose within 16% of the time required to reach peak isometric force to a maximum level appropriate to a given contractile state. Subsequently, the force-velocity-length relationship was independent of time at any load over a well-determined portion of external shortening. In Sr-containing solution, unloaded active velocity rose much slower to reach a steady state after 30-35% of the time to peak force, and the time independence of the force-velocity-length relationship was delayed until after this time. In Na-free Sr-containing solution, maximum unloaded shortening velocity and the time-independent portion of the force-velocity-length relationship were achieved again within 20% of the time to peak force. The effects of substitution of Sr for Ca thus resemble those of caffeine on mammalian heart muscle. These findings suggest a close relationship between the time course of the force-velocity-length relationship and the degree of activation.

Onset of Mechanical Activation of Mammalian Heart Muscle in Calcium- and Strontium-Containing Solutions

1. Male rats were undernourished either during the gestational and suckling periods or for a period of time immediately following weaning. Some rats were killed at the end of the period of undernutrition; others were nutritionally rehabilitated for lengthy periods of time before examination. Two muscles, the extensor digitorum longus (EDL) and soleus (SOL) were studied from each rat. Histochemically-stained transverse sections of these muscles were used to determine total number of fibres, the fibre cross-sectional areas and the relative frequency of the various fibre types. 2. All rats killed immediately following undernutrition showed significant deficits in body-weight, muscle weight and fibre cross-sectional area compared to age-matched controls. 3. Animals undernourished during gestation and suckling and then fed normally for 5 months showed persistent and significant deficits in body-weight, muscle weight, muscle weight and total fibre number. There were also significant deficits in mean fibre cross-sectional area of each fibre type except for red fibres in the EDL. No difference in the volume proportion of connective tissue was found. 4. Rats undernourished after weaning and then fed ad lib. for approximately 7 months had normal body-and muscle weights. Their muscles showed no significant differences in total fibre number, relative frequency of the various fibre types, fibre size or volume proportion of connective tissue. 5. These results indicate that, although the effects on rat skeletal muscle of a period of undernutrition after weaning can be rectified, undernutrition before weaning causes lasting deficits.

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Early life undernutrition in rats. 1. Quantitative histology of skeletal muscles from underfed young and refed adult animals.

1. The latency relaxation has been examined in single fibres from frog striated muscle with particular attention given to its possible relation to Ca(2+) release during excitation-contraction coupling.2. Latency relaxations were recorded at 19-23 degrees C from massively stimulated (0.2 msec pulses) single fibres using two selected RCA 5734 transducer tubes in a bridge circuit.3. The depth of the latency relaxation has its full value when stimulus strength is between 40 and 400% above twitch threshold. Stronger stimuli reversibly diminish the latency relaxation.4. The variation in depth of latency relaxation with sarcomere length was found similar to that reported previously for multifibre preparations but in single fibres the peak of the curve consists of a plateau between sarcomere lengths of 2.8 mu and 3.2 mu.5. Sucrose hypertonicity increases the depth of the latency relaxation at sarcomere lengths below 2.8 mu but above this length it has either no effect or a depressant effect depending on the degree of hypertonicity.6. The maximal depth of the latency relaxation (measured at 3 mu) averaged 0.23% of the maximal tetanus tension (measured at 2.2 mu) and was strongly correlated (r = 0.87) with the latter in forty-five single fibres.7. The maximal depth of the latency relaxation is not correlated with the number of sarcomeres in series in a fibre.8. The results of this study are shown to fully support and extend Sandow's (1966) hypothesis that the latency relaxation is caused by release of activator Ca(2+) from the sarcoplasmic reticulum.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1972.sp009850

Louis A. Mulieri

Papers

The dependence of the latency relaxation on sarcomere length and other characteristics of isolated muscle fibres