06 µM, 37°C) found here,
seems to be in reasonable agreement with about 320 kPa resulting from measurements with a fast-twitch mouse fiber at 25°C [37]. The extrapolated value of maximal shortening velocity, vmaxHS = 1.95 µm × HS−1 × s−1 ([Ca2+]= 1.06 µM, AL = 0 J/mol, HS = half-sarcomere) compares to 1.6 µm × HS−1 × s−1 of frog fibers at 0°C [12]. A value of ηmax of about 50% at about 0.18 vmax ([Ca2+] = 1.06µM) results from adjustment. It compares to the experimental values of 35–45 % for the same value of v for frog muscles at 0 °C [12]. All these Inhibitors,research,lifescience,medical parameters of contractile performance may, however, be reduced to a certain extent by dissipative frictional processes associated with v which are not addressed in the present simulation. Such dissipation during fiber shortening may be produced mainly by viscous deformations of membranes and the filament lattice. 2.6. [H+], [Mg2+], and Fatigue Enzyme-catalysed ATP splitting by myosin heads is formulated here with respect to Inhibitors,research,lifescience,medical the ATP species MgATP2−.
By using a reference constant and binding polynomials, an [H+] and [Mg2+] dependent K’ATP of this reaction can be formulated (see A6 and A7). In simulations of fatigue, in addition to [H+], [Mg2+] has also been included as a variable, especially Inhibitors,research,lifescience,medical because this ion may interfere with ATP species and so may influence JEn through a change in [MgATP2−], which in turn would alter [CB]. Changes in [H+] in the sarcosol are brought Inhibitors,research,lifescience,medical about mainly by two different mechanisms, which are both related to metabolic activity. One source of protons is manifest when metabolism is switched from rest to high power output. Fluxes in ATP consumption and production, JATPCon and JATPPro, respectively, must then both increase to the same extent to reach a new steady state. During the adjustment, a phase of disturbed steady state occurs, during which both fluxes do not match. When power output increases, JATPCon
always leads JATPPro, i.e., there is an uncompensated ATP splitting until a new steady state is reached, at which point ATP production again Inhibitors,research,lifescience,medical equals ATP consumption. According to Alberty [20], this reaction is associated with proton production in dependence of [H+] and [Mg2+] (see (A6) and (A7)) for derivation of [H+] changes and pH buffering). In addition, the CK and adenylate kinase (AK) reactions are involved, because these equilibria are also changed under these DNA Synthesis animal study conditions and, as with ATP splitting, H+ and Mg2+ binding species are involved. Buffering of both ion concentrations old is brought about mainly by sites intrinsic to the sarcosol. For Mg2+ binding sites, an additional release of Mg2+ by interfering [H+] has to be expected. Figure 5A shows the time courses of rates of [H+] changes. Interestingly, [H+] production by ATP splitting is practically compensated by [H+] consumption by the CK reaction. The contribution by the AK reaction is negligible. A similar behavior is found for Mg2+ (Figure 5B).
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