Inositol 1,4,5-trisphosphate and intracellular Ca²⁺ store sites in human peripheral lymphocytes

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.