Anti-immunoglobulin, cytoplasmic free calcium, and capping in B lymphocytes.

This paper examines, in mouse spleen lymphocytes, the effect of anti-immunoglobulin (anti-Ig) on the cytoplasmic free calcium concentration, Ca2+i, measured with the fluorescent indicator quin2, and the relationship of Ca2+i to the capping of surface Ig. Anti-Ig causes a rapid rise of Ca2+, which precedes capping. Assuming that only those 40-50% of the cells which can bind anti-Ig (the B cells) undergo a Ca2+i response, Ca2+i in those cells approaches 500 nM. It declines to resting levels over many minutes, roughly paralleling the formation of caps. Part of the Ca2+i signal is due to stimulated influx across the plasma membrane, since in Ca2+-free medium, anti-Ig gives a smaller and shorter Ca2+i rise. The amplitude of this reduced transient now varies inversely with quin2 content, as if some 0.25 mmol Ca per liter of cells was released into the cytoplasm from internal stores. These stores are probably sequestered in organelles since A23187 in Ca2+-free medium also causes a transient Ca2+i rise after which anti-Ig has no effect. These organelles seem not to be mitochondria because uncouplers have hardly any effect on Ca2+i. Though anti-Ig normally raises Ca2+i before causing capping, there seems to be no causal link between the two events. Cells in Ca2+-free medium whose stores have been emptied by A23187, still cap with anti-Ig even though there is no Ca2+i rise. Cells loaded with quin2 in the absence of external Ca2+ still cap anti-Ig normally even though their Ca2+i remains steady at below 30 nM, four times lower than normal resting Ca2+i.