Dissociation between mechanical performance and the cost of isometric tension maintenance in Lethocerus flight muscle

Regulating the contraction of insect flight muscle

The rapid movement of the wings in small insects is powered by the indirect flight muscles. These muscles are capable of contracting at up to 1,000 Hz because they are activated mechanically by stretching. The mechanism is so efficient that it is also used in larger insects like the waterbug, Lethocerus. The oscillatory activity of the muscles occurs a low concentration of Ca(2+), which stays constant as the muscles contract and relax. Activation by stretch requires particular isoforms of tropomyosin and the troponin complex on the thin filament. We compare the tropomyosin and troponin of Lethocerus and Drosophila with that of vertebrates. The characteristics of the flight muscle regulatory proteins suggest ways in which stretch-activation works. There is evidence for bridges between troponin on thin filaments and myosin crossbridges on the thick filaments. Recent X-ray fibre diffraction results suggest that a pull on the bridges activates the thin filament by shifting tropomyosin from a blocking position on actin. The troponin bridges are likely to contain extended sequences of tropomyosin or troponin I (TnI). Flight muscle has two isoforms of TnC with different Ca(2+)-binding properties: F1 TnC is needed for stretch-activation and F2 TnC for isometric contractions. In this review, we describe the structural changes in both isoforms on binding Ca(2+) and TnI, and discuss how the steric model of muscle regulation can apply to insect flight muscle.

Regulation of oscillatory contraction in insect flight muscle by troponin

Insect indirect flight muscle is activated by sinusoidal length change, which enables the muscle to work at high frequencies, and contracts isometrically in response to Ca(2+). Indirect flight muscle has two TnC isoforms: F1 binding a single Ca(2+) in the C-domain, and F2 binding Ca(2+) in the N- and C-domains. Fibres substituted with F1 produce delayed force in response to a single rapid stretch, and those with F2 produce isometric force in response to Ca(2+). We have studied the effect of TnC isoforms on oscillatory work. In native Lethocerus indicus fibres, oscillatory work was superimposed on a level of isometric force that depended on Ca(2+) concentration. Maximum work was produced at pCa 6.1; at higher concentrations, work decreased as isometric force increased. In fibres substituted with F1 alone, work continued to rise as Ca(2+) was increased up to pCa 4.7. Fibres substituted with various F1:F2 ratios produced maximal work at a ratio of 100:1 or 50:1; a higher proportion of F2 increased isometric force at the expense of oscillatory work. The F1:F2 ratio was 9.8:1 in native fibres, as measured by immunofluorescence, using isoform-specific antibodies. The small amount of F2 needed to restore work to levels obtained for the native fibre is likely to be due to the relative affinity of F1 and F2 for TnH, the Lethocerus homologue of TnI. Affinity of TnC isoforms for a TnI fragment of TnH was measured by isothermal titration calorimetry. The K(d) was 1.01 muM for F1 binding and 22.7 nM for F2. The higher affinity of F2 can be attributed to two TnH binding sites on F2 and a single site on F1. Stretch may be sensed by an extended C-terminal domain of TnH, resulting in reversible dissociation of the inhibitory sequence from actin during the oscillatory cycle.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

A troponin switch that regulates muscle contraction by stretch instead of calcium

The flight muscles of many insects have a form of regulation enabling them to contract at high frequencies. The muscles are activated by periodic stretches at low Ca2+ levels. The same muscles also give isometric contractions in response to higher Ca2+. We show that the two activities are controlled by different isoforms of TnC (F1 and F2) within single myofibrils. F1 binds one Ca2+ with high affinity in the C-terminal domain and F2 binds one Ca2+ in the C-terminal domain and one exchangeable Ca2+ in the N-terminal domain. We have characterised the isoforms and determined their effect on the development of stretch-activated and Ca2+-activated tension by replacing endogenous TnC in Lethocerus flight muscle fibres with recombinant isoforms. Fibres with F1 gave stretch-activated tension and minimal isometric tension; those with F2 gave Ca2+-dependent isometric tension and minimal stretch-activated tension. Regulation by a TnC responding to stretch rather than Ca2+ is unprecedented and has resulted in the ability of insect flight muscle to perform oscillatory work at low Ca2+ concentrations, a property to which a large number of flying insects owe their evolutionary success.

Influence of myosin isoforms on tension cost and crossbridge kinetics in skinned rat cardiac muscle

1. In attempting to consolidate the role of ventricular isomyosins in regulating the contractility of the myocardium, actomyosin ATPase and crossbridge kinetics were obtained at 24 degrees C in chemically skinned isometrically contracting cardiac muscles containing V1 and V3 isomyosins. 2. The ATPase activity was measured at various levels of Ca2+ activation by the enzymatic coupling of ATP hydrolysis with the conversion of NADH to NAD+. The crossbridge kinetics were inferred from small-amplitude perturbations of muscle length and muscle tension, and characterized by the frequency-domain parameter fmin. 3. The ATPase rates of V1 and V3 muscles obtained at various levels of Ca2+ activation were plotted against the corresponding proportional tensions. The ATPase vs tension plots were linear with slopes of 4.92 nmol/min-1 per mm per mN and 1.98 nmol/min-1 per mm per mN, respectively for, V1 and V3 muscles. Individual calculations of ATPase-to-tension ratios (nmol/min-1 per mm per mN) gave corresponding averages of 4.98 +/- 0.12 (s.e.m., n = 12) and 2.16 +/- 0.12 (s.e.m., n = 10). The myosin isoform induced proportional change in tension cost was accompanied by a similar change in fmin (4.1 +/- 0.1 Hz and 1.95 +/- 0.03 Hz, means +/- s.e.m., for V1 and V3 muscles, respectively). 4. The observations and other published kinetic data are discussed in the context of models of crossbridge cycling. It is suggested that the tension economy of V3 muscle arises principally from an increase in the fraction of time, during the crossbridge cycle, when the crossbridge is exerting force.

Troponin of asynchronous flight muscle

Troponin has been prepared from the asynchronous flight muscle of Lethocerus (water bug) taking special care to prevent proteolysis. The regulatory complex contained tropomyosin and troponin components. The troponin components were Tn-C (18,000 Mr), Tn-T (apparent Mr 53,000) and a heavy component, Tn-H (apparent Mr 80,000). The troponin was tightly bound to tropomyosin and could not be dissociated from it in non-denaturing conditions. A complex of Tn-T, Tn-H and tropomyosin inhibited actomyosin ATPase activity and the inhibition was relieved by Tn-C from vertebrate striated muscle in the presence of Ca2+. However, unlike vertebrate Tn-I, Tn-H by itself was not inhibitory. Monoclonal antibodies were obtained to Tn-T and Tn-H. Antibody to Tn-T was used to screen an expression library of Drosophila cDNA cloned in lambda phage. The sequence of cDNA coding for the protein was determined and hence the amino acid sequence. The Drosophila protein has a sequence similar to that of vertebrate skeletal and cardiac Tn-T. The sequence extends beyond the carboxyl end of the vertebrate sequences, and the last 40 residues are acidic. Part of the sequence of Drosophila Tn-T is homologous to the carboxyl end of the Drosophila myosin light chain MLC-2 and one anti-Tn-T antibody cross-reacted with the light chain. Lethocerus Tn-H is related to the large tropomyosins of Drosophila flight muscle, for which the amino acid sequence is known, since antibodies that recognize this component also recognize the large tropomyosins. Tn-H is easily digested by calpain, suggesting that part of the molecule has an extended configuration. Electron micrographs of negatively stained specimens showed that Lethocerus thin filaments have projections at about 39 nm intervals, which are not seen on thin filaments from vertebrate striated muscle and are probably due to the relatively large troponin complex. Decoration of the thin filaments with myosin subfragment-1 in rigor conditions appeared not to be affected by the troponin. The troponin of asynchronous flight muscle lacks the Tn-I component of vertebrate striated muscle. Tn-H occurs only in the flight muscle and may be involved in the activation of this muscle by stretch.

The role of calcium ions in the activation of rabbit psoas muscle

This study tested the effects of free Ca++ on both the small-amplitude mechanical behaviour (dynamic stiffness and phase between 1 and 500 Hz) and the large-scale filament-sliding behaviour (Vmax) of single fibres of chemically-activated glycerol-extracted rabbit psoas muscle. Small-amplitude vibrations (0.1% peak-to-peak of initial length L0) were used to elicit fw, the frequency for maximum oscillatory work-production per cycle. The unloaded contraction velocity Vmax was measured during the same contractions, using the slack-test method with shortening length steps of up to 10% L0. These produced nonlinear length-time plots demonstrating that the unloaded contraction velocity was not constant as contraction progressed, but fell with time. This behaviour was approximated by two velocities, V1 the velocity observed for about the first 15 ms and V2 the velocity after this time. V1 and V2 were found to have different sensitivities to Ca++. The value of V2 fell as the level of [Ca++] was reduced, and was linearly proportional to the active tension over the range 0.2 Pmax to Pmax (where Pmax is the isometric tension for saturating amounts of Ca++). In contrast to this V1 remained insensitive to changes in [Ca++] for levels of activation corresponding to active tensions ranging from Pmax to 0.6 Pmax, and then fell as the level of activation was further reduced. It was found that the level of [Ca++] did not affect the magnitude of fw over the range of concentrations yielding active tensions from 0.2 Pmax to Pmax. These results are discussed in terms of the kinetic processes underlying transient readjustments to perturbations from isometric equilibrium.