ENHANCEMENT OF CALCIUM SIGNALING AND PROLIFERATION RESPONSES IN ACTIVATED HUMAN T-LYMPHOCYTES - INHIBITORY EFFECTS OF K-CELL ACTIVATION STATE( CHANNEL BLOCK BY CHARYBDOTOXIN DEPEND ON THE T)

T cell receptor (TCR) stimulation, leading to T cell activation and ul timately to cell proliferation and differentiation, evokes elevations of [Ca2+](i) with a high variability between individual T lymphocytes. We have used Ca2+-imaging of Fura-2 loaded cells to study the origin of the variation in Ca2+ signals and its consequences for the final ce llular response. We found that, compared to resting cells, the percent age of responding cells and the average amplitude of the Ca2+ signal u pon TCR re-stimulation by PHA increases in the first 5 days of T cell activation and declines thereafter, with more pronounced [Ca2+](i) osc illations in later stages. In parallel, an enhancement of T cell proli feration is observed. Stronger stimulation of the TCR/CD3 complex by c o-crosslinking CD3 with CD4/CD8 molecules evokes oscillating Ca2+ resp onses irrespective of the activation state, indicating that the basic capacity for Ca2+ signaling is essentially the same in resting and act ivated cells. Nevertheless, also the amplitude of the CD3+CD4/8 respon se shows a transient additional increase during the first days of T ce ll activation. Experiments with the K+ channel blocker charybdotoxin ( CTX) indicate that [Ca2+](i) oscillations depend critically on K+ chan nel functioning, but suppression of these oscillations by CTX does not significantly affect the average amplitude of the Ca2+ signal nor PHA -induced proliferation. However, when applied during the first 4-5 day s of activation, CTX reduces in addition the average level of the TCR evoked Ca2+ response and inhibits subsequent proliferation.