Effects of Ca2+ on the kinetics of phosphate release in skeletal muscle

Cooperative effect of calcium binding to adjacent troponin molecules on the thin filament-myosin subfragment 1 MgATPase rate

The myosin subfragment 1 (S1) MgATPase rate was measured using thin filaments with known extents of Ca2+ binding controlled by varying the ratio of native cardiac troponin versus an inhibitory troponin with a mutation in the sole regulatory Ca2+ binding site of troponin C. Fractional MgATPase activation was less than the fraction of troponins that bound Ca2+, implying a cooperative effect of bound Ca2+ on cross-bridge cycling. Addition of phalloidin did not alter cooperative effects between bound Ca2+ molecules in the presence or absence of myosin S1. When the myosin S1 concentration was raised sufficiently to introduce cooperative myosin-myosin effects, lower Ca2+ concentrations were needed to activate the MgATPase rate. MgATPase activation remained less than Ca2+ binding, implying a true, not just an apparent, increase in Ca2+ affinity. MgATPase activation by Ca2+ was more cooperative than could be explained by cooperativeness of overall Ca2+ binding, the discrepancy between Ca2+ binding and MgATPase activation, or interactions between myosins. The results suggest the thin filament-myosin S1 MgATPase cycle requires calcium binding to adjacent troponin molecules and that this binding is cooperatively promoted by a single cycling cross-bridge. This mechanism is a potential explanation for Ca2+-mediated regulation of cross-bridge kinetics in muscle fibers.

Troponin C - troponin I interactions and molecular signalling in cardiac myofilaments

This chapter describes a current perception of the molecular interactions regulating myofilament activity in heart cells. The focus is on the interaction between troponin-C (TnC), the Ca(2+)-receptor and troponin I (TnI), an inhibitory protein. It is this interaction that appears to form a molecular switch that turns on the thin filament. It will be seen that control of the actin-myosin reaction is not only through Ca(2+)-binding to TnC, but also through steric, cooperative and allosteric processes involving all of the main myofilament proteins-actin, myosin, tropomyosin (Tm), troponin T (TnT), TnC, and TnI. The process is modulated by covalent and non-covalent mechanisms. The process is altered in diverse myopathies and pathologies of the heart and is a target for pharmacological manipulation by a new class of inotropic agents, the "Ca(2+)-sensitizers".

Effect of series elasticity on delay in development of tension relative to stiffness during muscle activation

Experimental data have indicated that during activation, the attachment of myosin to actin, measured by mechanical stiffness, precedes tension generation by 10-30 ms. Using computer simulation, we have investigated the effect of a series elastic element on the lag between stiffness and tension development during muscle activation. Two versions of the two-state cross-bridge model originally proposed by Huxley and a three-state model were considered. After simulated activation, stiffness and tension increased with rates that were strongly dependent on the series elastic strain. In the absence of a series elastic element, the rise in stiffness preceded, lagged, or was coincident with the increase in tension, depending on the model. For large elastic strains, tension lagged stiffness for all models. Lags of 10-30 ms could be obtained with elastic strains of 0.3-1% of the muscle length. This is a realistic value in experiments without sarcomere length servocontrol, suggesting that series elasticity may be an important contributor to the experimentally observed lag between tension and stiffness.

Activation dependence and kinetics of force and stiffness inhibition by aluminiofluoride, a slowly dissociating analogue of inorganic phosphate, in chemically skinned fibres from rabbit psoas muscle

To examine the mechanism by which aluminiofluoride, a tightly binding analogue of inorganic phosphate, inhibits force in single, chemically skinned fibres from rabbit psoas muscle, we measured the Ca(2+)-dependence of the kinetics of inhibitor dissociation and the kinetics of actomyosin interactions when aluminiofluoride was bound to the crossbridges. The relation between stiffness and the speed of stretch during small amplitude ramp stretches (< 5 nm per h.s.) was used to characterize the kinetic properties of crossbridges attached to actin; sarcomere length was assessed with HeNe laser diffraction. During maximum Ca(2+)-activation at physiological ionic strength (pCa 4.0, 0.2 M gamma/2), stiffness exhibited a steep dependence on the rate of stretch; aluminiofluoride inhibition at pCa 4.0 (0.2 M gamma/2) resulted in an overall decrease in stiffness, with stiffness at high rates of stretch (10(3)-10(4) nm per h.s. per s) being disproportionately reduced. Thus the slope of the stiffness-speed relation was reduced during aluminiofluoride inhibition of activated fibres. Relaxation of inhibited fibres (pCa 9.2, 0.2 M gamma/2) resulted in aluminiofluoride being 'trapped' and was accompanied by a further decrease in stiffness at all rates of stretch which was comparable to that found in control relaxed fibres. In relaxed, low ionic strength conditions (pCa 9.2, 0.02 M gamma/2) which promote weak crossbridge binding, stiffness at all rates of stretch was significantly inhibited by aluminiofluoride 'trapped' in the fibre. To determine the Ca(2+)-dependence of inhibitor dissociation, force was regulated independent of Ca2+ using an activating troponin C (aTnC). Results obtained with a TnC-activated fibres confirmed that there is no absolute requirement for Ca2+ for recovery from force inhibition by inorganic phosphate analogues in skinned fibres; the only requirement is thin filament activation which enables active crossbridge cycling. These results indicate that aluminiofluoride preferentially inhibits rapid equilibrium or weak crossbridge attachment to actin, that aluminiofluoride-bound crossbridges attach tightly to the activated thin filament, and that, at maximal (or near-maximal) activation, crossbridge attachment to actin prior to inorganic phosphate analogue dissociation is the primary event regulated by Ca2+.

Effects of cycling and rigor crossbridges on the conformation of cardiac troponin C

The results of work by several investigators indicate that crossbridge attachment serves as a positive feedback mechanism that transiently increases the Ca2+ affinity of troponin C (TnC) during each normal heartbeat. To monitor structural changes in the cardiac isoform of TnC (cTnC) associated with Ca2+ binding and crossbridge attachment in muscle, we labeled cTnC with the sulfhydryl-specific fluorescent probe 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid (IAANS). When IAANS-labeled cTnC (cTnCIAANS) was substituted for endogenous TnC, the fluorescence intensity of cardiac and skeletal muscle preparations increased substantially during rigor crossbridge attachment in the absence of Ca2+ (pCa 9.2). In cardiac muscle, the fluorescence signal increased the same amount in rigor and maximal activation, whereas in skeletal muscle, it was higher in rigor (rigor: cardiac and skeletal = 1; pCa 4.0: cardiac = 0.98 +/- 0.13, skeletal = 0.59 +/- 0.05). This indicates that crossbridge attachment alone is capable of influencing the structure of cTnCIAANS. Because the relative fluorescence intensity of cTnCIAANS was more sensitive to Ca2+ than was force in both preparations (cardiac: pCa50 fluorescence = 6.05 +/- 0.05, pCa50 force = 5.51 +/- 0.11; skeletal: pCa50 fluorescence = 5.94 +/- 0.13, pCa50 force = 5.65 +/- 0.14), we measured the Ca2+ sensitivity of the strong crossbridge attachment (sinusoidal stiffness was measured by imposing 1 kHz at 0.1-0.2% muscle length) in rat trabeculae.(ABSTRACT TRUNCATED AT 250 WORDS)