["Freezing" of the conformation of thin filaments in a single skinned fiber of the rabbit muscle by glutaraldehyde].

It is shown that short treatment of a single skinned rigor fibre from rabbit m X psoas with 0.05% glutaraldehyde in the absence of Ca ions leads to a modified state of the contractile apparatus. After the addition of 5 mM MgATP in the absence of Ca ions to the fibre a sharp rise and subsequent slow decay of tension were observed in contrast to the tension drop in case of the control (unmodified) specimen. The tension transients following quick stretch (L 0.5%) were similar to those for Ca-activated tension. In case of the modified relaxed fibre such a phenomenon was not observed. These results can be explained by "freezing" with glutaraldehyde the thin filament structure either in the "on" or "off" states. The relation of these results to the cooperativity in the regulation mechanism of contraction is discussed.