Theory and observation of spontaneous oscillatory contractions in skeletal myofibrils

Calcium and strontium contractile activation properties of single skinned skeletal muscle fibres from elderly women 66-90 years of age

The single freshly skinned muscle fibre technique was used to investigate Ca²⁺- and Sr²⁺-activation properties of skeletal muscle fibres from elderly women (66-90 years). Muscle biopsies were obtained from the vastus lateralis muscle. Three populations of muscle fibres were identified according to their specific Sr²⁺-activation properties: slow-twitch (type I), fast-twitch (type II) and hybrid (type I/II) fibres. All three fibre types were sampled from the biopsies of 66 to 72 years old women, but the muscle biopsies of women older than 80 years yielded only slow-twitch (type I) fibres. The proportion of hybrid fibres in the vastus lateralis muscle of women of circa 70 years of age (24%) was several-fold greater than in the same muscle of adults (< 10%), suggesting that muscle remodelling occurs around this age. There were no differences between the Ca²⁺- and Sr²⁺-activation properties of slow-twitch fibres from the two groups of elderly women, but there were differences compared with muscle fibres from young adults with respect to sensitivity to Ca²⁺, steepness of the activation curves, and characteristics of the fibre-type dependent phenomenon of spontaneous oscillatory contractions (SPOC) (or force oscillations) occurring at submaximal levels of activation. The maximal Ca²⁺ activated specific force from all the fibres collected from the seven old women use in the present study was significantly lower by 20% than in the same muscle of adults. Taken together these results show there are qualitative and quantitative changes in the activation properties of the contractile apparatus of muscle fibres from the vastus lateralis muscle of women with advancing age, and that these changes need to be considered when explaining observed changes in women's mobility with aging.

SPontaneous Oscillatory Contraction (SPOC): auto-oscillations observed in striated muscle at partial activation

Striated muscle is well known to exist in either of two states-contraction or relaxation-under the regulation of Ca2+ concentration. Described here is a less well-known third, intermediate state induced under conditions of partial activation, known as SPOC (SPontaneous Oscillatory Contraction). This state is characterised by auto-oscillation between rapid-lengthening and slow-shortening phases. Notably, SPOC occurs in skinned muscle fibres and is therefore not the result of fluctuating Ca2+ levels, but is rather an intrinsic and fundamental phenomenon of the actomyosin motor. Summarised in this review are the experimental data on SPOC and its fundamental mechanism. SPOC presents a novel technique for studying independent communication and coordination between sarcomeres. In cardiac muscle, this auto-oscillatory property may work in concert with electro-chemical signalling to coordinate the heartbeat. Further, SPOC may represent a new way of demonstrating functional defects of sarcomeres in human heart failure.

The mechanism of spontaneous oscillatory contractions in skeletal muscle

Most striated muscles generate steady contractile tension when activated, but some preparations, notably cardiac myocytes and slow-twitch fibers, may show spontaneous oscillatory contractions (SPOC) at low levels of activation. We have provided what we believe is new evidence that SPOC is a property of the contractile system at low actin-myosin affinity, whether caused by a thin-filament regulatory system or by other means. We present a quantitative single-sarcomere model for isotonic SPOC in skeletal muscle with three basic ingredients: i), actin and myosin filaments initially in partial overlap, ii), stretch activation by length-dependent changes in the lattice spacing, and iii), viscoelastic passive tension. Modeling examples are given for slow-twitch and fast-twitch fibers, with periods of 10 s and 4 s respectively. Isotonic SPOC occurs in a narrow domain of parameter values, with small minimum and maximum values for actin-myosin affinity, a minimum amount of passive tension, and a maximum transient response rate that explains why SPOC is favored in slow-twitch fibers. The model also predicts the contractile, relaxed and SPOC phases as a function of phosphate and ADP levels. The single-sarcomere model can also be applied to a whole fiber under auxotonic and fixed-end conditions if the remaining sarcomeres are treated as a viscoelastic load. Here the model predicts an upper limit for the load stiffness that leads to SPOC; this limit lies above the equivalent loads expected from the rest of the fiber.

Rippling muscle disease may be caused by ?silent? action potentials in the tubular system of skeletal muscle fibers

Rippling muscle disease (RMD) is a generally benign, myotonic-like myopathy associated with rapid rolling contractions and percussion-induced contractions. These contractions are electrically silent in electromyographic recordings, which is taken as evidence that action potentials are not involved in the phenomena. The pathophysiological mechanisms underlying the symptoms have not been elucidated. Many cases of RMD are caused by mutations in caveolin-3, and aberrations in the tubular system are commonly observed. Here, recent data are discussed showing that action potentials can travel over substantial distances entirely within the transverse and longitudinal tubular systems of a muscle fiber and that stretch can induce such action potentials. Action potentials travelling in the tubular system in most circumstances probably cannot excite the sarcolemma and hence would not be detected. It is suggested that the distinctive contractions in RMD may be due to stretch-induced generation of action potentials within the tubular system.

Synchronization of active mechanical oscillators by an inertial load

Motivated by the operation of myogenic (self-oscillatory) insect flight muscle, we study a model consisting of a large number of identical oscillatory contractile elements joined in a chain, whose end is attached to a damped mass-spring oscillator. When the inertial load is small, the serial coupling favors an antisynchronous state in which the extension of one oscillator is compensated by the contraction of another, in order to preserve the total length. However, a sufficiently massive load can synchronize the oscillators and can even induce oscillation in situations where isolated elements would be stable. The system has a complex phase diagram displaying quiescent, synchronous and antisynchronous phases, as well as an unusual asynchronous phase in which the total length of the chain oscillates at a different frequency from the individual active elements.

Calcium handling by the sarcoplasmic reticulum during oscillatory contractions of skinned skeletal muscle fibres

Isometric ATP consumption and force were investigated in mechanically skinned fibres from iliofibularis muscle of Xenopus laevis. Measurements were performed at different [Ca2+], in the presence and absence of caffeine (5 nM). In weakly Ca2+-buffered solutions without caffeine, spontaneous oscillations in force and ATPase activity occurred. The repetition frequency was [Ca2+]-and temperature-dependent. The Ca2+ threshold (+/- SEM) for the oscillations corresponded to a pCa of 6.5 +/- 0.1. The maximum ATP consumption associated with calcium uptake by the sarcoplasmic reticulum (SR) reached during the oscillations was similar to the activity under steady-state conditions at saturating calcium concentrations in the presence of caffeine. Maximum activity was reached when the force relaxation was almost complete. The calculated amount of Ca2+ taken up by the SR during a complete cycle corresponded to 5.4 +/ 0.4 mmol per litre cell volume. In strongly Ca2+-buffered solutions, caffeine enhanced the calcium sensitivity of the contractile apparatus and, at low calcium concentrations, SR Ca uptake. These results suggest that when the SR is heavily loaded by net Ca uptake, there is a massive calcium-induced calcium release. Subsequent net Ca uptake by the SR then gives rise to the periodic nature of the calcium transient.

Potassium current oscillations across the rabbit lens epithelium

Rabbit lenses expressing spontaneous oscillations in translens short-circuit current (Isc) are obtained somewhat frequently, with this phenomenon observed in approximately 30% of isolated lenses as described earlier (Exp. Eye Res. 61, 129-140, 1995). Since pharmacological protocols to consistently elicit Isc oscillations were not found, characterizations of the underlying transport processes have been limited to the application of various inhibitors on the spontaneous phenomenon. The present report extends the initial observations by confirming that oscillations are immediately inhibited upon the anterior addition of the Ca2+ channel blocker nifedipine (10 microM), and by demonstrating that other treatments which should affect epithelial Ca2+ homeostasis are also inhibitory (e.g., Bay K 8644 (10 microM), diltiazem (10 microM), EGTA (2 mm), and Ca2+-free media). Furthermore, Isc oscillations are immediately inhibited by the K+ channel blocker, Ba2+, but not by the Na+-K+ pump inhibitor, ouabain. The intracellular Ca2+ mobilizing agents thapsigargin (0.1 microM) or acetylcholine (1 microM) modified but did not permanently inhibit the oscillations, confirming earlier observations. At 50 microM, however, acetylcholine addition was inhibitory, but reversible, for oscillations restarted upon its subsequent removal. In addition, lens oscillations were also characterized under open-circuit conditions with microelectrodes inserted in the superficial cells near the equator of lenses isolated in a divided chamber. The potential difference (PD) across each lens face was recorded, as was the translens PD (PDt), which equals the difference between the PDs across each lens surface. Oscillations in PDt were obtained in 7 of 26 lenses. The oscillations arose only from an oscillation in the PD across the anterior face (PDa). While PDa and PDt oscillated with the same amplitude (approximately 12 mV) and period (approximately 70 sec), the PD across the posterior surface remained stable. During these oscillations the conductance of the anterior surface was maximal at the most positive voltage of the anterior bath with respect to the lens interior (46 mV), whereas, minimal conductance occurred at the least positive PDa (34 mV). Overall, these observations are consistent with the likely presence of voltage-operated Ca2+ channels in parallel with various Ca2+-sensitive K+ channels in the epithelial basolateral membrane. A model to explain the oscillatory pattern across the anterior face while the PD across the posterior face remains unaltered is presented.

Molecular cogs in machina carnis

1. This review explores the complexity of skeletal muscle function mainly from the perspective of work performed by the author over the past two decades.

Synchronous behavior of spontaneous oscillations of sarcomeres in skeletal myofibrils under isotonic conditions

An isotonic control system for studying dynamic properties of single myofibrils was developed to evaluate the change of sarcomere lengths in glycerinated skeletal myofibrils under conditions of spontaneous oscillatory contraction (SPOC) in the presence of inorganic phosphate and a high ADP-to-ATP ratio. Sarcomere length oscillated spontaneously with a peak-to-peak amplitude of about 0.5 microns under isotonic conditions in which the external loads were maintained constant at values between 1.5 x 10(4) and 3.5 x 10(4) N/m2. The shortening and yielding of sarcomeres occurred in concert, in contrast to the previously reported conditions (isomeric or auxotonic) under which the myofibrillar tension is allowed to oscillate. This synchronous SPOC appears to be at a higher level of synchrony than in the organized state of SPOC previously observed under auxotonic conditions. The period of sarcomere length oscillation did not largely depend on external load. The active tension under SPOC conditions increased as the sarcomere length increased from 2.1 to 3.2 microns, although it was still smaller than the tension under normal Ca2+ contraction (which is on the order of 10(5) N/m2). The synchronous SPOC implies that there is a mechanism for transmitting information between sarcomeres such that the state of activation of sarcomeres is affected by the state of adjacent sarcomeres. We conclude that the change of myofibrillar tension is not responsible for the SPOC of each sarcomere but that it affects the level of synchrony of sarcomere oscillations.