Stimulation of the antigen receptor on WEHI-231 B lymphoma cells results in a voltage-independent increase in cytoplasmic calcium

WEHI-231, a lymphoma-derived murine B cell line, responded to anti-IgM antibodies by increasing the concentration of free calcium in the cytoplasm from 140 nM to 590 nM within 15 sec. This is very similar to the response observed previously in normal B cells (Pozzan et al., 1982, J. Cell Biol. 94:335). Only antibodies specific for mIgM stimulated this response; control antibodies had no effect. In addition, anti-IgM did not stimulate a response by a mutant with a greatly decreased amount of membrane IgM. The relationship of this increase in cytoplasmic calcium to the plasma membrane potential was examined. Anti-IgM did not cause a rapid depolarization of the cells, suggesting that a voltage-dependent calcium channel was not responsible for the calcium increase. Furthermore, experimental depolarization of WEHI-231 cells did not cause a calcium influx, and the calcium increase caused by anti-IgM was not greatly affected by previous depolarization or by prevention of depolarization. These experiments argue strongly that the increase in cytoplasmic calcium is not mediated by a depolarization-activated calcium channel, such as the one found in cardiac muscle and in some neurons. Indeed, a significant portion of the initial increase in cytoplasmic calcium was due to the release of calcium from internal stores, suggesting the involvement of a soluble mediator. Examination of these internal storage sites in permeabilized cells revealed that inositol 1,4,5-trisphosphate could induce the release of calcium. These results are consistent with the hypothesis that the calcium increase in B cells stimulated by anti-IgM is caused by breakdown of phosphatidylinositol 4,5-bisphosphate, generating diacylglycerol and inositol trisphosphate, with the latter compound mediating calcium mobilization.