SUMMARY1. The calcium uptake capabilities of the sarcoplasmic reticulum (SR) of the fast-twitch muscles extensor digitorum longus (EDL) and tibialis anterior (TA) of the rat and the extensor digitorum longus of the cat have been compared with the same capabilities of the slow-twitch soleus muscles of the rat and cat.2. For the rat the Vmax values of sarcoplasmic reticulum from tibialis anterior, extensor digitorum longus and from soleus muscles were 50, 151, and 10 smole Ca2+/g per minute, respectively.3. For the extensor digitorum longus and soleus muscles of the cat the Vmax values were 34 and 5-6 ,mole Ca2+/g per minute, respectively. 4. These data were compared with mechanical data as reported in the literature for the same muscles. The relative calcium uptake capabilities of sarcoplasmic reticulum from slow and fast muscles corresponded closely to the relative rates of relaxation of these muscles.

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