Signal transduction in Jurkat T lymphocytes. Evidence for early Ca2+ movements between the cytoplasm and the nucleoplasm in activated cells

Spatial analyses of the distribution of Ca2+in resting and activated T and B lymphocytes have shown that the bulk of increased [Ca2+]i appears to be associated with the nuclear region. These observations suggest that Ca2+ is released from the perinuclear space or that it diffuses to the nucleoplasm, or both. We have used laser scanning confocal microscopy to assess whether cytoplasmic diffusion of Ca2+ could contribute to the rise in nuclear Ca2+. We found that the activation of individual Jurkat cells by use of an anti‐Ti (β‐subunit) mAb induced a nucleus‐associated increase in [Ca2+]i. In cells loaded with the InsP3 receptor antagonist heparin, the nuclear Ca2+ response was abolished but not the response to thapsigargin. Evidence for a cytoplasmic Ca2+ response was obtained by loading Jurkat cells with a cytoplasm‐restricted Ca2+ probe (Calcium Green‐1‐Dextran). These observations suggested that a process of diffusion of cytoplasmic Ca2+ contributed to the rise of nuclear Ca2+ in Jurkat T cells. This interpretation was supported by the findings (1) that rapid scanning of thapsigargin‐released Ca2+ showed an inverse relationship between the levels of cytoplasmic and nuclear Ca2+ and (2) that modulation of the external concentration of Ca2+ in thapsigargin‐treated Jurkat cells showed a time‐dependent decrease of fluorescence from the nucleoplasm that was reversed by raising the concentration of external Ca2+. We conclude that Ca2+can rapidly diffuse between the cytoplasm and the nucleoplasm in activated Jurkat T lymphocytes and that hydrophilic Ca2+ probes largely partition to the nucleoplasm, thus giving rise to distorted nucleus‐to‐cytoplasm fluorescence ratios. J. Leukoc. Biol. 62: 874–884; 1997.