The width (D) and the 1,0 lattice spacing (d1,0) at various ionic compositions of mechanically skinned single fibres (from semitendinosus muscle of Rana catesbeiana) were measured at various concentrations of polyvinyl pyrrolidone (PVP K-30, Mn = 40 000) from 0 to 6% at 20 degrees C. In a standard relaxing solution (4 mM MgATP2-, 1 mM Mg2+, 4 mM EGTA, ionic strength 150 mM and pH 7), d1,0 decreased exponentially as the PVP concentration increased: d1,0 was 41.3 +/- 0.4 (mean +/- S.D.) nm at 0% PVP and 32.9 +/- 0.4 nm at 6% PVP. D was proportional to d1,0 except at very low PVP concentrations, i.e. at 1% PVP, D decreased by 7%, whereas d1,0 decreased by only 3%. At 0% PVP, D and d1,0 decreased when either pH or ionic strength (gamma/2) was lowered. At 6% PVP, D and d1,0 decreased with lowered pH or increased [Mg2+], but was independent of gamma/2. The radial stiffness, or degree of resistance to the changes of D against the compressing force, increased considerably at d1,0 less than or equal to 35 nm in a standard relaxing solution, but not at pH 5.5 or 30 mM [Mg2+]. These effects of pH, [Mg2+] and gamma/2 on D or d1,0 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.
