Reversible increase in light scattering during recovery from fatigue in Xenopus muscle fibres

During recovery from fatigue produced by repeated tetanic stimulation, isolated Xenopus muscle fibres often enter a reversible state of severely depressed tension production. This post-contractile depression has previously been found to be accompanied by an altered light microscopical appearance of fibres (increased light scattering), which suggests some morphological change as the cause of the force reduction. In the present study the relation between tension production and light scattering (measured as optical density) has been studied in more detail. It was found that fibres developing a marked post-contractile depression exhibited the largest increase in light scattering. When fibres recovered from post-contractile depression, an increased optical density remained in the central region of fibres, and this region also produced less tension. The tension output of fibres recovering without post-contractile depression declined markedly when the tonicity of the bathing solution was raised, and this decline was accompanied by increased optical density. However, a close temporal correlation between increased light scattering and tension depression was not always found: in some fibres the time course of changes in light scattering differed from that of the tension depression. In summary, the present results show that reversible structural changes may develop after fatiguing stimulation, and some form of coupling between these changes and the force depression during post-contractile depression is suggested.