Calcium-Induced Calcium Release from Sarcoplasmic Reticulum Vesicles1

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Abstract
Passive permeability of sarcoplasmic reticulum vesicles for Ca2+ was studied by the Millipore filtration method using 45Ca. The efflux rate of Ca2+ increased in the presence of micromolar concentrations of Ca2+ in the medium when the Mg2+ concentration was low. The dependence of the Ca2+ release rate on the extrave-sicular Ca2+ concentration was not affected by the intravesicular Ca2+ concentration. The Ca2+ release was enhanced by caffeine, but reduced by procaine and Mg+. These properties suggest that the Ca2+ release is a similar phenomenon to the Ca2+-induced Ca2+ release observed in skinned muscle fiber. Kinetic analysis of the efflux curves suggests that two types of vesicles exist in our preparation; about 2/3 of the vesicles release Ca2+ as described above and the remainder does not.