Calcium Binding Activity of Microsomal Fraction of Rabbit Red Muscle*

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Abstract
1. A microsomal fraction which showed a considerable Ca-binding activity even in the absence of oxalate was prepared from rabbit red muscle, viz., soleus and semitendinosus muscle. The capacity of Ca-binding reached 56 mμmoles/mg protein which correspond to one third that of white muscle. 2. The Ca-uptake was not inhibited by various metabolic inhibitors, such as azide, oligomycin and 2, 4-dinitrophenol, which have been known to depress the Ca-uptake of mitochondria. It is very probable that the Ca-uptake of red muscle microsomes is due to the fragmented sarcoplasmic reticulum contained in this fraction. 3. This microsomal fraction inhibited the superprecipitation of myosin B and myofibrils and also the myofibrillar ATPase [EC 3.6.1.3] activity. 4. The properties of the ATPase activity of red muscle microsomes were different from those of white muscle in its response to Ca, GEDTA and metabolic inhibitors. 5. The rate of Ca-binding of red muscle microsomes was determined by the double beam spectrophotometric method. In view of this result together with the yield and the Ca-binding capacity of the microsomal fraction, the role of the sarcoplasmic reticulum in the contraction-relaxation cycle was discussed. It was suggested that the sarcoplasmic reticulum of red muscle might play essentially the same role as that in white muscle.

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