Single Channel Properties and Regulated Expression of Ca²⁺ Release-Activated Ca²⁺ (Crac) Channels in Human T Cells

Although the crucial role of Ca²⁺ influx in lymphocyte activation has been well documented, little is known about the properties or expression levels of Ca²⁺ channels in normal human T lymphocytes. The use of Na⁺ as the permeant ion in divalent-free solution permitted Ca²⁺ release-activated Ca²⁺ (CRAC) channel activation, kinetic properties, and functional expression levels to be investigated with single channel resolution in resting and phytohemagglutinin (PHA)-activated human T cells. Passive Ca²⁺ store depletion resulted in the opening of 41-pS CRAC channels characterized by high open probabilities, voltage-dependent block by extracellular Ca²⁺ in the micromolar range, selective Ca²⁺ permeation in the millimolar range, and inactivation that depended upon intracellular Mg²⁺ ions. The number of CRAC channels per cell increased greatly from ∼15 in resting T cells to ∼140 in activated T cells. Treatment with the phorbol ester PMA also increased CRAC channel expression to ∼60 channels per cell, whereas the immunosuppressive drug cyclosporin A (1 μM) suppressed the PHA-induced increase in functional channel expression. Capacitative Ca²⁺ influx induced by thapsigargin was also significantly enhanced in activated T cells. We conclude that a surprisingly low number of CRAC channels are sufficient to mediate Ca²⁺ influx in human resting T cells, and that the expression of CRAC channels increases ∼10-fold during activation, resulting in enhanced Ca²⁺ signaling.