Inhibition of tension development and actomyosin ATPase activity in barnacle muscle by the Ca2+-indicator dye antipyrylazo III

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.

Characterization of troponin-C interactions in skinned barnacle muscle: comparison with troponin-C from rabbit striated muscle

Previously it was shown that when troponin-C (TnC) is extracted from barnacle myofibrillar bundles they lose their Ca2+ sensitivity, which can be restored by adding back barnacle TnC (either isoform, BTnC1 or BTnC2). Thus barnacle muscle shows thin filament regulation, as does rabbit psoas skeletal muscle. In this paper we compare the interactions of barnacle and rabbit fast muscle TnC in their respective muscles. We demonstrate that muscle fibres from the giant barnacle, Balanus nubilus, contain about 186 microM kg-1 muscle tissue of BTnC1 plus BTnC2 compared to about 91 microM kg-1 of TnC in rabbit psoas muscle fibres. Extraction of BTnC is achieved using similar low ionic strength, low divalent ion Ca(2+)-low Mg2+ conditions which are required for TnC extraction in rabbit psoas skinned muscle fibres; extraction was prevented by 1 mM Mg2+. Full reconstitution of Ca(2+)-sensitivity was achieved by adding back BTnC (1 + 2, or 2). Reconstitution of barnacle muscle with rabbit fast skeletal TnC (RTnC) was more complex, with partial recovery of Ca(2+)-sensitivity with reconstitution in the presence of 3 mM Mg2+ and more fully with reconstitution in the presence of activating Ca2+ (pCa 4.0). This suggests that the barnacle TnC-TnI (troponin I) recognition sites may be more complex than in rabbit because the barnacle sites appear to have at least two different conformations or types, in which one recognizes RTnC in the presence of Mg2+ and the other only in the presence of Ca2+ and Mg2+. This is consistent with the presence of several TnI isoforms in barnacle striated myofibrils. RTnC has two C-terminal Ca(2+)-Mg2+ binding sites that are thought to be involved in the Mg(2+)-sensitive binding of RTnC in rabbit muscle, yet it has been suggested that this site in barnacle muscle does not bind Mg2+, even though Mg2+ stabilizes BTnC binding in barnacle muscle. Consistent with this stabilizing action of Mg2+, using fluorescent probes IAANS or IAE on isolated BTnC2 we demonstrate that BTnC2 binds both Ca2+ and Mg2+, but the data do not suggest direct competition. Consistent with the C-terminal sites on BTnC being Ca(2+)-specific, BTnC1 + 2 could only reconstitute low levels of force (about 1/3) in TnC-extracted rabbit skinned muscle fibers in the presence of pCa 4.0 (not just Mg2+) and only at low ionic strengths (0.09 M). Ca(2+)-activation of contraction was further examined using fluorescently labelled BTnC2 (labelled with IANBD) incorporated into skinned barnacle myofibrillar bundles. Maximal Ca2+ binding produced structural changes in BTnC which resulted in a 45% decrease in the fluorescence compared to the value at pCa 9.2. The magnitude of the fluorescence decrease paralleled the increase in force with increasing Ca2+. The Hill fits to the data gave pCa1/2 and n of 5.61 +/- 0.02 and 2.06 +/- 0.12 for force, and 5.52 +/- 0.02 and 1.88 +/- 0.10 for fluorescence. Removing MgATP to induce rigor in the fibre decreased BTnC2-NBD fluorescence only about 11%, but the addition of Ca2+ in rigor further decreased the fluorescence to a slightly larger extent than under maximal Ca2+ activating conditions. These fluorescence changes are qualitatively similar to the fluorescence enhancement seen with Ca(2+)-activation and rigor with RTnCDanz exchanged into rabbit psoas skinned muscle fibres. The data support a similar model for Ca(2+)-activation of force in barnacle muscle and in rabbit psoas skeletal muscle fibres.

Barnacle muscle: Ca2+, activation and mechanics

In this review, aspects of the ways in which Ca2+ is transported and regulated within muscle cells have been considered, with particular reference to crustacean muscle fibres. The large size of these fibres permits easy access to the internal environment of the cell, allowing it to be altered by microinjection or microperfusion. At rest, Ca2+ is not in equilibrium across the cell membrane, it enters the cell down a steep electrochemical gradient. The free [Ca2+] at rest is maintained at a value close to 200 nM by a combination of internal buffering systems, mainly the SR, mitochondria, and the fixed and diffusible Ca(2+)-binding proteins, as well as by an energy-dependent extrusion system operating across the external cell membrane. This system relies upon the inward movement of Na+ down its own electrochemical gradient to provide the energy for the extrusion of Ca2+ ions. As a result of electrical excitation, voltage-sensitive channels for Ca2+ are activated and permit Ca2+ to enter the cell more rapidly than at rest. It has been possible to determine both the amount of Ca2+ entering by this step, and what part this externally derived Ca2+ plays in the development of force as well as in the free Ca2+ change. The latter can be determined directly by Ca(2+)-sensitive indicators introduced into the cell sarcoplasm. A combination of techniques, allowing both the total and free Ca2+ changes to be assessed during electrical excitation, has provided valuable information as to how muscle cells buffer their Ca2+ in order to regulate the extent of the change in the free Ca2+ concentration. The data indicate that the entering Ca2+ can only make a small direct contribution to the force developed by the cell. The implication here is that the major source of Ca2+ for contraction must be derived from the internal Ca2+ storage sites within the SR system, a view reinforced by caged Ca2+ methods. The ability to measure the free Ca2+ concentration changes within a single cell during activation has also provided the opportunity to analyse, in detail, the likely relations between free Ca2+ and the process of force development in muscle. The fact that the free Ca2+ change precedes the development of force implies that there are delays in the mechanism, either at the site of Ca2+ attachment on the myofibril, or at some later stage in the process of force development that were not previously anticipated.(ABSTRACT TRUNCATED AT 400 WORDS)

Cross-bridges affect both TnC structure and calcium affinity in muscle fibers

In vertebrate striated muscle, calcium binding to troponin initiates contraction, a strong interaction of actin and myosin. In isolated proteins and skinned fibers, the strong interaction of myosin with actin also affects troponin. Fluorescent labels attached to troponin C show structural changes in the TnC environment with cross-bridge attachment and also with calcium binding. Evidence that this effect of crossbridges also occurs in intact striated muscle comes from studies in partially activated cardiac or skeletal muscle by others and in barnacle muscle by us. Length changes which detach myosin cross-bridges produce a brief burst of extra calcium that can be detected by aequorin in activated, voltage clamped single barnacle muscle fibers. That this calcium is coming from calcium bound to the activating site (troponin-C) is supported by several pieces of evidence. Studies on the dependence of the extra calcium on force and the time of the length change are consistent with the amplitude of the extra calcium being proportional to the bound calcium (CaTnC) and with increased cross-bridge attachment and force increasing calcium binding to troponin-C by up to a factor of 10. Importantly, stretch of active muscle (which first detaches cross-bridges and then enhances steady force) gives a biphasic response: first extra calcium (presumably due to cross-bridge detachment) and then, decreased calcium (presumably due to enhanced calcium binding to TnC). The enhanced calcium binding we see with elevated force (via strained cross-bridges) implies that calcium binding to TnC is enhanced not only be cross-bridge attachment but also by crossbridge (or thin filament) strain. This effect of cross-bridge attachment/force on calcium binding is consistent with a dual mechanism of calcium activation of contraction. First, calcium binds to troponin in the thin filament activating strong myosin binding to the thin filament. Then, strong myosin binding in turn provides additional activation either by increasing calcium binding or by changing the thin filament structure directly allowing additional cross-bridge attachment.

Functional characterization of the two isoforms of troponin C from the arthropod Balanus nubilus

Two isoforms of troponin C (BTnC1 and BTnC2) from the striated muscle of the arthropod Balanus nubilus Darwin (giant barnacle) have been purified (Potter et al., 1987; Collins et al., 1991). Both isoforms were present in all of the white striated muscle fibres studied but not in the red fibres. The ratio of BTnC2 to BTnC1 in different fibre types varied between 3:1 and 1:1. Both forms of TnC could be readily extracted from myofibrillar bundles of barnacle muscle in low ionic strength EDTA solutions, reducing force activation to less than 10%. Both forms either separately or together reassociated with the TnC-depleted fibres in a relaxing (LR) solution (pCa greater than 8.0, [Mg2+] free = 1 mM, I = 0.15 M), and the reconstituted fibres could be subsequently activated in contraction (LA) solution (pCa = less than 3.8, [Mg2+] free = 1 mM, I = 0.15 M). The dissociation of BTnC 1 + 2 is blocked in low ionic strength solutions containing Mg2+ (greater than or equal to 10 mM). The two isoforms of crayfish TnC (CrTnC1 and CrTnC2) were also found to be equivalent to the barnacle TnCs in their ability to reactivate TnC-depleted barnacle myofibrillar bundles. Similar experiments using rabbit skeletal muscle TnC (STnC) (I = 0.15 M) in BTnC-depleted myofibrillar bundles of barnacle showed considerable variability. STnC could associate, although weakly, with the depleted bundles in either LR or LA, and force could be partially restored. In neither situation was it as effective as either BTnC or CrTnC. Interestingly, bovine cardiac TnC (CTnC), although it did not associate at pCa greater than 7.0, did associate and effectively activate force at pCa less than 3.8, but dissociated on return to pCa greater than 7.0 (LR). Neither barnacle TnC isoform associated with TnC-depleted skinned fibres from rabbit skeletal muscle at pCa greater than 7.0, but did associate and activate these fibres at pCa less than 3.8. Once these fibres were returned to LR and then placed in LA at pCa 3.8 all BTnC-restored force was lost, indicating a dissociation of BTnC once the Ca2+ is lowered, as observed with CTnC in barnacle myofibrillar bundles. Finally, the inhibitory effect of BTnI on force and the absence of an effect of calmodulin, trifluoperazine or ATP-gamma-S on force were all taken as evidence for a thin filament regulated Ca2+ control system.

Extra calcium on shortening in barnacle muscle. Is the decrease in calcium binding related to decreased cross-bridge attachment, force, or length?

Barnacle single muscle fibers were microinjected with the calcium-specific photoprotein aequorin. We have previously shown (Ridgway, E. B., and A. M. Gordon, 1984, Journal of General Physiology, 83:75-104) that when barnacle fibers are stimulated under voltage clamp and length control and allowed to shorten during the declining phase of the calcium transient, extra myoplasmic calcium is observed. The time course of the extra calcium for shortening steps at different times during the calcium transient is intermediate between those of free calcium and muscle force. Furthermore, the amplitude increases with an increased stimulus, calcium transient, and force. Therefore, the extra calcium probably comes from the activating sites on the myofilaments, possibly as a result of changes in calcium binding by the activating sites. The change in calcium binding may be due, in turn, to the change in muscle length and/or muscle force and/or cross-bridge attachment per se. In the present article, we show that the amount of the extra calcium depends on the initial muscle length, declining at shorter lengths. This suggests length-dependent calcium binding. The relation between initial length and extra calcium, however, parallels that between initial length and peak active force. The ratio of extra calcium to active force is therefore virtually independent of initial length. These data do not distinguish between a direct effect of length on calcium binding and an indirect effect owing to changes in cross-bridge attachment and force through some geometrical factor. The amount of extra calcium increases with the size of the shortening step, tending toward saturation for steps of greater than or equal to 10%. This experiment suggests that calcium binding depends on muscle force or cross-bridge attachment, not just length (if at all). There is much less extra calcium seen with shortening steps at high force when the high force results from stretch of the active muscle than when it results from increased stimulation of muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of tension development and actomyosin ATPase activity in barnacle muscle by the Ca2+-indicator dye antipyrylazo III

We have investigated the effects of the Ca2+-indicator dye antipyrylazo III on: (1) tension development in myofibrillar preparations from barnacle depressor muscle; and (2) actomyosin ATPase activity in myofibrils/native actomyosin isolated from barnacle muscle and in actomyosin hybrids prepared from pure (unregulated) rabbit F-actin and purified barnacle myosin. In all solutions, pCa was either heavily buffered with suitable [CaEGTA]/[EGTA] and/or measured with a calcium-electrode so as to offset the appreciable Ca2+-buffering effects of the dyes. Antipyrylazo III produced a rapid, reversible and concentration-dependent inhibition of: (1) tension development by isolated barnacle myofibrils; (2) calcium-regulated ATPase activity in barnacle myofibrils and native actomyosin; and (3) calcium-independent actin-activated ATPase activity in hybrid actomyosins prepared from purified barnacle myosin and rabbit actin. This latter observation indicated that the inhibitory effect of the dye on calcium-regulated tension and ATPase in intact myofibrils is due to specific repression of active crossbridge formation rather than modification of calcium-regulatory mechanisms. The hypothesis that antipyrylazo III specifically represses active crossbridge formation was supported by the observation that the dye had no effect on rigor tension development. Specific and saturable binding of the dye to these myofibrils was characterized by a maximum capacity of 1.3 mumol dye g-1 myofibrillar protein, consistent with a calculated 1 dye: 1 myosin stoichiometry. These various biological effects were observed with both commercially available antipyrylazo III and highly purified dye preparations. Preliminary studies using myofibrillar preparations from rabbit psoas muscle, guinea-pig portal vein smooth muscle, and scallop adductor muscle have indicated that contractile function in these muscle types does not appear to be inhibited by antipyrylazo III.