Calcium stores in electropermeabilized HL-60 cells before and after differentiation

Non-induced HL-60 cells (N-IND) and HL-60 cells induced to differentiate with 2 microM retinoic acid (IND) were electropermeabilized with electrical discharges, and the intracellular Ca2+ stores were measured in each type of cell. Both N-IND and IND cells accumulate Ca2+ in the presence of ATP after electropermeabilization. The Ca2+ is stored in at least two different compartments; accumulation in one of the compartments is inhibited by oligomycin and CCCP, and it is not releasable by Ins(1,4,5)P3. The maximal accumulation of Ca2+ by the Ins(1,4,5)P3 sensitive pool is about 0.3 nmol/10(6) cells and 0.9 nmol/10(6) cells for the N-IND and for the IND cells, respectively, and the half-maximal value occurs at a free Ca2+ concentration of 0.23 microM and 0.63 microM, respectively. The oligomycin + CCCP sensitive pool hardly accumulates any Ca2+ at this level of free Ca2+, but at higher free [Ca2+] (greater than microM) its maximal capacity is 80-100-fold higher than the Ins(1,4,5)P3-sensitive pool (about 17-18 nmol/10(6) cells). It is concluded that at physiological free Ca2+ concentrations, the non-mitochondrial Ca2+ pool is regulating the intracellular free Ca2+ in N-IND and IND HL-60 cells, and that this Ca2+ pool can be mobilized by Ins(1,4,5)P3. Furthermore, the capacity of this pool increases about 3-fold when the cells are induced to differentiate with retinoic acid.