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. Th e time-course of sarcomere shortening and re-lengthening was recorded by microcinephotography. Application of Ruthenium Red, a potent and sp ecific inhibitor of Ca2+ uptake by mitochondria, did not affect the co ntraction-relaxation cycles of typical glycolytic white fibres (frog s artorius, pigeon breast). By contrast, Ruthenium Red greatly;slowed do wn the relaxation rate in mitochondria-rich fibres (rat soleus and rab bit masseter). In these fibres, Ca2+ uptake by mitochondria seems to p lay an active role in promoting relaxation.