Inhibition of mitochondrial calcium uptake slows down relaxation in mitochondria-rich skeletal muscles

Isolated fibres from various muscles were skinned mechanically in oil. From a Ca2+-loaded micropipette, local applications of Ca2+ were made. These produced a limited contraction which relaxed spontaneously. The time-course of sarcomere shortening and re-lengthening was recorded by microcinephotography. Application of Ruthenium Red, a potent and specific inhibitor of Ca2+ uptake by mitochondria, did not affect the contraction-relaxation cycles of typical glycolytic white fibres (frog sartorius, pigeon breast). By contrast, Ruthenium Red greatly;slowed down the relaxation rate in mitochondria-rich fibres (rat soleus and rabbit masseter). In these fibres, Ca2+ uptake by mitochondria seems to play an active role in promoting relaxation.