Independent regulation of Ca2+ entry and release from internal stores in activated B cells

Abstract

Addition of crosslinking antibody to B lymphocytes results in a rapid rise in cytoplasmic-free Ca2+ ([Ca2+](i)) due to release of Ca2+ from internal stores and uptake of Ca2+ across the plasma membrane. Inositol 1,4,5-triphosphate is believed to mediate the release of internal Ca2+ stores and has also been proposed to mediate extracellular Ca2+ entry. We have compared the properties of these two pathways for Ca2+ mobilization by dissociating the [Ca2+](i) changes in ligand-activated human B cells after loading of the cells with the Ca2+ chelator BAPTA. In the present paper we show that: (a) the sustained increase in [Ca2+](i) is due to increased unidirectional influx of external [Ca2+](i); (b) entry of extracellular Ca2+, but not release of internal stores is sensitive to the transmembrane potential; and (c) entry of extracellular Ca2+, but not release of internal stores, is inhibited by increasing [Ca2+](i). These findings suggest that the permeation pathways mediating the translocation of Ca2+ across the plasma membrane and endoplasmic reticulum membrane are not identical.