Dissociation of unidirectional influx of external Ca2+ and release from internal stores in activated human T lymphocytes.

Addition of lectin or antibody to the T cell receptor complex of human T cells results in a rapid increase in the concentration of cytoplasmic free Ca2+ ([Ca2+]i). This response is biphasic and results from contributions of Ca2+ from internal stores, uptake of Ca2+ across the plasma membrane and possibly a decrease in Ca2+ efflux. These responses have been linked through the activity of inositol 1,4,5-trisphosphate in releasing Ca2+ from internal stores and potentially mediating Ca2+ uptake across the plasma membrane. Following addition of phytohemagglutinin or anti-CD3 antibody to resting T cells or Jurkat cells, we have been able to dissociate the [Ca2+]i responses by loading cells with the Ca2+ chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA). In BAPTA-loaded T cells, we have shown that Ca2+ mobilized from intracellular stores following activation is effectively buffered, while stimulated Ca2+ uptake and associated changes in [Ca2+]i were relatively unaffected. In this report, we show that the sustained increase in [Ca2+]i is due to increased unidirectional influx of external Ca2+ without changes in efflux and that it is the entry of extracellular Ca2+ which is sensitive to the transmembrane potential.