D. C. S. White

Physiological properties of the dorsal longitudinal flight muscle and the tergal depressor of the trochanter muscle of Drosophila melanogaster.

SummaryA prerequisite for using muscle mutants to study contraction inDrosophila melanogaster is a description of the mechanics of wild-type muscles. Here we describe the mechanics of two different wild-type muscles; the dorsal longitudinal flight muscle which is asynchronous (nerve impulses are not synchronised with each contraction), and a leg muscle, the tergal depressor of the trochanter, which is synchronous. We have compared their mechanics to those of the asynchronous flight and the synchronous leg muscle from the giant waterbugLethocerus indicus.We found that the mechanics of the asynchronous flight muscles from the two species were similar. At rest length both muscles had a high relaxed stiffness, were partially activated by Ca2+ (low steady-state active tension) and, once activated, had a large delayed increase in tension, which was well maintained, in response to a rapid stretch. The rate constant for the delayed increase in tension was about 10 times greater forD. melanogaster than forL. indicus under the same conditions. The mechanics of the synchronous leg muscles from both species were different from those of the flight muscles and resembled those of other synchronous muscles such as vertebrate striated muscle. At rest length, both muscles had a lower relaxed stiffness than the flight muscles, were fully activated by Ca2+ (high steady-state active tension) and, once activated, had a small delayed increase in tension, which was less well maintained, in response to a rapid stretch. The rate constant for the delayed increase in tension was similar for the leg muscles of both species.The different mechanical properties of the flight and leg muscles must arise from differences in their contractile proteins. The demonstration that satisfactory mechanical responses can be obtained from the small (less than 1 mm long) muscles ofD. melanogaster will enable future responses from mutant muscles to be tested.

The kinetics of muscle contraction

The kinetics of muscle contraction , The kinetics of muscle contraction , کتابخانه مرکزی دانشگاه علوم پزشکی ایران

Distributed Representations for Actin-Myosin Interaction in the Oscillatory Contraction of Muscle

In this paper we suggest and test a specific hypothesis relating the attachment-detachment cycle of cross bridges between actin (I) and myosin (A) filaments to the measured length-tension dynamics of active insect fibrillar flight muscle. It is first shown that if local A-filament strain perturbs the rate constants in the cross-bridge cycle appropriately, then exponentially delayed tension changes can follow imposed changes of length; the latter phenomenon is sufficient for the work-producing property of fibrillar muscle, as measured with small-signal forcing of length and at low Ca(2+) concentration, and possibly for related effects described recently in frog striated muscle. It is not clear a priori that the above explanation of work production by fibrillar muscle will remain tenable when the viscoelastic complexity of the heterogeneous sarcomere is taken into account. However, Whites (1967) recent mechanical and electron microscope study of the passive dynamics of glycerinated fibrillar muscle has produced a model of the distributed viscoeleastic structure sufficiently explicit that alternative schemes for cross-bridge force generation in this muscle can now be tested more critically than previously. Therefore, we derive and solve third-order partial-differential equations which relate local interfilament shear forces associated with the perturbed cross-bridge cycles to the over-all length-tension dynamics of an idealized sarcomere. We then show (a) that the starting hypothesis can account approximately for the small-signal dynamics of glycerinated muscle in the work-producing state over two decades of frequency and (b) that the rate constants for cross-bridge formation and breakage, restricted solely by fitting of the model to the mechanical data, determine a cycling rate of cross bridges in the model compatible with recent measurements of ATP hydrolysis rate vs. stretch in this muscle. Finally, the formulation is extended tentatively to the large-signal nonlinear case, and shown to compare favorably with previous suggestions for the origin of the work-producing dynamics of fibrillar flight muscle.

The role of magnesium adenosine triphosphate in the contractile kinetics of insect fibrillar flight muscle

SummaryThe changes in tension produced by small step or sinusoidal changes of length have been measured for chemically skinned flight muscle fibres of the giant tropical water bugLethocerus at MgATP concentrations in the range 0.1–10 mM. In the presence of calcium ion concentrations of about 20 µm, the rates of the rapid mechanical processes observed were found to increase with increasing MgATP, exhibiting saturation with an apparent half-saturation constant lying between 0.1 and 1mM MgATP, depending upon the conditions used. Under the same conditions, an increase in MgATP concentration was found to lead to a slight decrease in the isometric tension generated by the preparations. The results are discussed with reference to some current crossbridge models of muscle contraction.

Dynamic Force Measurement at the Microgram Level, with Application to Myofibrils of Striated Muscle

We describe a system for measurement of the dynamic mechanical properties of small bundles of myofibrils of muscle. Underwater fiber-optic sensing of tension via displacement of a quartz beam has been accomplished with a noise level of about 100 nanoNewtons in a bandwidth of 1 to 40 Hz.

Analysis of the ATPase mechanism of myosin subfragment 1 from insect fibrillar flight muscle in the presence and absence of actin, using phosphate-water oxygen exchange measurements

SummaryPhosphate-water oxygen exchange was measured during ATPase activity of myosin subfragment 1, isolated fromLethocerus flight muscle. The result supports a mechanism including a rapid reversible ATP cleavage step followed by slow Pi release, so that extensive exchange occurs by multiple reversals of the cleavage step. Interaction with actin accelerates Pi release and reduces the extent of exchange. These properties are similar to vertebrate skeletal muscle subfragment 1. Differences in kinetic properties between insect flight and rabbit skeletal muscle exhibited in the fibres, particularly in respect of the strain activation of insect flight muscle, are not exhibited in the isolated myosin heads and are therefore probably due to organization within the fibre lattice.

Kinetics of ATP release and Pi binding during the ATPase cycle of Lethocerus flight muscle fibres, using phosphate-water oxygen exchange

SummaryRate constants have been obtained using oxygen isotope exchange techniques for steps controlling ATP release and Pi binding in the ATPase cycle of insect flight muscle fibres from the giant waterbugLethecerus. The new exchange data for Pi binding and ATP release are compatible with a model developed previously in which only the rate constants controlling Pi and ATP release change during fibre activation.Phosphate-water oxygen exchange occurs into ATP remaining after partial hydrolysis by chemically skinned fibres in (18O) water. For fully activated fibres, the results are compatible with a single set of rate constants controlling this exchange and give a rate constant for ATP release of 1 s−1 (21° C, pH 7.0I=120mm). Oxygen exchange also occurs between (18O4)Pi in the medium and water during ATP hydrolysis. There is a strong correlation between the measured rate constant of exchange and the value ofkcat for the ATPase activity at different levels of activation. For fibres fully activated by oscillation or strain, the rate constant for Pi binding to an actomyosin. ADP state is >960m−1 s−1.