The cytosolic free calcium in anti‐μ‐stimulated human B cells is derived partly from extracellular medium and partly from intracellular stores

The inositol phospholipid metabolism and the increase in cytosolic free Ca2+ concentration ([Ca2+]i) into the cell are recognized as two important events in the anti-μ-induced B cell activation. The anti-μ stimulation caused the [3H]inositol incorporation and also a rapid increase in [Ca2+]i from 85 nM to 285 nM. This signal returned to baseline a few minutes after stimulation. By using the fluorescent indicator quin-2 we demonstrated that this [Ca2+]i uptake was derived part from extracellular medium and part from intracellular stores. Both EGTA (a calcium chelator) and TMB.8 (a drug which interferes with Ca2+ sequestration by smooth endoplasmic retiulum) partially suppressed the intracellular Ca2+ uptake and were fully inhibitory when added together. The role of Ca2+ from intracellular stores may also be evidenced in calcium-free experiments, or in permeabilized experiments using exogenous inositol 1,4,5-trisphosphate (IP3, the putative mobilizer of intracellular Ca2+). Preventing the increase in [Ca2+]i also prevents the apparition of early activation markers. These results are consistent with the hypothesis that the Ca2+ increase in B cells stimulated by anti-μ is caused by the generation of IP3 during the phosphatidyl-inositol metabolism and also by the entry of extracellular Ca2+ through the plasma membrane.