Ca2+ storage by vesicles in human lymphocytes was studied using saponin-permeabilized cells. Saponin-treated lymphocytes could accumulate Ca2+ in the presence of ATP. In the presence of NaN3 (10 mM), the maximal storages of Ca2+ (0.87 nmol/4 x 106 cells) was obtained at free Ca2+ concentration of 10-5 M, and the apparent affinity constant for Ca2+ was approximately 1.4 x 106 M-1. Stored Ca2+ was rapidly released by A23187. Ca2+ accumulation was markedly enhanced by oxalate, and precipates of Ca-oxalate were electron-microscopically observed inside the endoplasmic reticulum-like structures. In the absence of NaN3, the maximal amount of intracellular Ca2+ was about 30 nmol/4 x 106 cells, while the affinity for Ca2+ was much lower. Application of inositol 1,4,5-trisphosphate to saponin-permeabilized cells which had accumulated Ca2+ by ATP in the presence of NaN3 rapidly released 35% of stored Ca2+ within 1 min. However, when the concentration of free Ca2+ was higher than 1.5 x 10-6 M, Ca2+ release by this agent was inhibited. The results suggest that inositol 1,4,5-trisphosphate may function as a mediator in lymphocytes, linking activation of surface membrane receptors with mobilization of internal Ca2+, which plays an important role in lymphocyte responses involved in proliferation.
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