(CRAC) Channels in Human T Cells

Although the crucial role of Ca 2 1 influx in lymphocyte activation has been well documented, little is known about the properties or expression levels of Ca 2 1 channels in normal human T lymphocytes. The use of Na 1 as the permeant ion in divalent-free solution permitted Ca 2 1 release-activated Ca 2 1 (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 Ca 2 1 store depletion resulted in the opening of 41-pS CRAC channels characterized by high open probabilities, voltage-dependent block by extracellular Ca 2 1 in the micromolar range, selective Ca 2 1 permeation in the millimolar range, and inactivation that depended upon intracellular Mg 2 1 ions. The number of CRAC channels per cell increased greatly from z 15 in resting T cells to z 140 in activated T cells. Treatment with the phorbol ester PMA also increased CRAC channel expression to z 60 channels per cell, whereas the immunosuppressive drug cyclosporin A (1 m M) suppressed the PHA-induced increase in functional channel expression. Capacitative Ca 2 1 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 Ca 2 1 influx in human resting T cells, and that the expression of CRAC channels increases z 10-fold during activation, resulting in enhanced Ca 2 1 signaling.