D-MYOINOSITOL 1,3,4,5-TETRAKISPHOSPHATE RELEASES CA2+ FROM CRUDE MICROSOMES AND ENRICHED VESICULAR PLASMA-MEMBRANES, BUT NOT FROM INTRACELLULAR STORES OF PERMEABILIZED T-LYMPHOCYTES AND MONOCYTES

In the human T-lymphocyte cell lines Jurkat and HPB.ALL and the human monocytoid cell line U937, Ins(1,3,4,5)P4 triggers a dose-dependent release of Ca2+ from crude microsomal preparations, with a half-maximal effective concentration (EC50) of 1.2-2.3 muM. Similar results were obtained with enriched vesicular plasma membranes from U937 cells. However, in permeabilized preparations of the same cell types only Ins(1,4,5)P, was able to release Ca2+ from intracellular stores, with EC50 values in the range 0.11-0.84 muM. In crude microsomes the effects of Ins(1,3,4,5)P4 and Ins(2,4,5)P3, a non-metabolizable InsP3 isomer, occurred independently of each other, indicating subpopulations of Ins(1,3,4,5)P4- and Ins(1,4,5)P3-sensitive vesicles. The Ins(1,3,4,5)P4 preparation used for the Ca2+-release experiments contains neither Ca2+ nor contaminating Ins(1,4,5)P4 and was not metabolized to Ins(1,4,5)P3 during the Ca2+-release experiments. We conclude that Ins(1,3,4,5)P4 independently of Ins(1,4,5)P3 induces a Ca2+ flux via a membrane compartment, most likely the plasma membrane, that is functionally destroyed during the permeabilization of the cells.