Comparison of the Effects of Added Orthophosphate on Calcium Uptake and Release by Bovine and Rabbit Muscle Sarcoplasmic Reticulum

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Abstract
In respect of Ca accumulating capacity in the absence of added inorganic orthophosphate (Pi) and maximum rate of Ca uptake, sarcoplasmic reticulum (SR) prepared from beef sternomandibularis muscles is intermediate between SR from rabbit white muscles and SR from rabbit red muscles. In the presence of added Pi, the Ca accumulating capacity of SR, whatever the source, is increased and a constant rate of uptake is reached. This constant rate is greater the higher the Pi concentration. The cold‐induced release of Ca from beef muscle SR is only partly reduced by including up to 50 mM Pi in the medium, whereas the release from rabbit white muscle SR is completely prevented by only about 20 mM Pi. This last finding is discussed in terms of the possible importance of postmortem accumulation of Pi in relation to cold shortening.