Transient appearance of vacuoles in fatigued Xenopus muscle fibres.

In the preceding paper we showed that post-contractile depression is accompanied by an increased light scattering in the light microscope, which suggests an association between morphological changes and the force reduction. In the present paper the morphology of fatigued fibres has been studied using electron microscopical techniques. Fibres fixed in glutaraldehyde during maximum post-contractile depression (about 20 min after fatiguing stimulation) contained a large number of vacuoles. Fibres fixed earlier displayed generally swollen and in some cases vesiculated mitochondria, but only a few vacuoles. Fixation methods aiming at visualizing the T-tubular system revealed apparent communications between T-tubules and vacuoles; apart from this the T-tubular system, as well as the triadic junctions, appeared to be normal. We consider it most likely that the vacuoles primarily originate from damaged mitochondria, but other possibilities cannot be excluded. Further, a simple causal relation between the observed ultrastructural changes and the force depression is not obvious. Rather we suggest that post-contractile depression is caused by additional changes in the triadic junctions, which were not detected with the present techniques.