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 perme ant ion in divalent-free solution permitted Ca2+ release-activated Ca2+ (CR AC) channel activation, kinetic properties, and functional expression level s to be investigated with single channel resolution in resting and phytohem agglutinin (PHA)-activated human T cells. Passive Ca2+ store depletion resu lted in the opening of 41-pS CRAC channels characterized by high open proba bilities, voltage-dependent block by extracellular Ca2+ in the micromolar r ange, selective Ca2+ permeation in the millimolar range, and inactivation t hat depended upon intracellular Mg2+ ions. The number of CRAC channels per cell increased greatly from similar to 15 in resting T cells to similar to 140 in activated T cells. Treatment with the phorbol ester PMA also increas ed CRAC channel expression to similar to 60 channels per cell, whereas the immunosuppressive drug cyclosporin A (1 mu M) suppressed the PHA-induced in crease in functional channel expression. Capacitative Ca2+ influx induced b y thapsigargin was also significantly enhanced in activated T cells. We con clude that a surprisingly low number of CRAC channels are sufficient to med iate Ca2+ influx in human resting T cells, and that the expression of CRAC channels increases similar to 10-fold during activation, resulting in enhan ced Ca2+ signaling.