Width and lattice spacing in radially compressed frog skinned muscle fibres at various pH values, magnesium ion concentrations and ionic strengths

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Abstract
The width (D) and the 1,0 lattice spacing (d 1,0) at various ionic compositions of mechanically skinned single fibres (from semitendinosus muscle ofRana catesbeiana) were measured at various concentrations of polyvinyl pyrrolidone (PVP K-30,M n =40000) from 0 to 6% at 20° C. In a standard relaxing solution (4mm MgATP2−, 1mm Mg2+, 4mm EGTA, ionic strength 150mm and pH 7),d 1,0 decreased exponentially as the PVP concentration increased:d 1,0), was 41.3 ± 0.4 (mean ±s.d.) nm at 0% PVP and 32.9 ± 0.4 nm at 6% PVP.D was proportional tod 1,0 except at very low PVP Concentrations, I.e. at 1% PVP,D decreased by 7%, whereasd 1,0 decreased by only 3%. At 0% PVP,D andd 1,0 decreased when either pH or ionic strength (Γ/2) was lowered. At 6% PVP,D andd 1,0 decreased with lowered pH or increased [Mg2+], but was independent of Γ/2. The radial stiffness, or degree of resistance to the changes ofD agaïnst the compressing force, increased considerably atd 1,0 < 35nm in a standard relaxing solution, but not at pH 5.5 or 30mm [Mg2+]. These effects of pH, [Mg2+] and Γ/2 onD ord 1,0 and on the radial stiffness can be explained by the modification of the properties of the elastic element and the hinge between subfragment-1 and -2 and/or the hinge between subfragment-2 and light meromyosin.