ACTIVATION OF CA2-CELLS FOLLOWING THE DEPLETION OF CA2+ STORES BY MICROSOMAL CA2+-ATPASE INHIBITORS( CURRENT IN JURKAT T)

The mechanism of TCR-stimulated Ca2+ influx was studied in the Jurkat human T cell line using Ca2+ indicator dyes and whole-cell patch clamp . Ca2+ influx induced by inositol 1,4,5-triphosphate (IP3)-coupled sur face receptors (either the TCR or a heterologous muscarinic receptor) was compared with Ca2+ influx induced by inhibitors of the microsomal Ca2+-ATPase (thapsigargin, cyclopiazonic acid, di-tert-butylhydroquino ne), which release stored Ca2+ without production of IP3. The same Ca2 + influx pathway could be activated by IP3-dependent or IP3-independen t means, and therefore appeared to be regulated by the fullness of the microsomal Ca2+ stores rather than by the direct action of IP3. Deple tion of stored Ca2+ by either receptor stimulation or microsomal Ca2+- ATPase inhibition activated a low conductance, Ca2+-selective, non-vol tage-activated membrane current. Ca2+ currents induced by receptor sti mulation and Ca2+-ATPase inhibition were not additive. Several propert ies of the depletion-activated Ca2+ current suggest that it is carried by a novel type of Ca2+ channel rather than an electrogenic carrier o r pump. The conductance saturated when external Ca2+ was raised (K-d a pproximate to 2 mM) and became highly permeable to monovalent cations when external Ca2+ was lowered to below 100 nM, much as has been obser ved for some voltage-gated Ca2+ channels. The Ca2+ current was reversi bly blocked by >90% with 0.3 mM Cd2+, whereas the same concentration o f Ni2+ or Co2+ blocked only 50 to 60% of the current. However, the abs ence of voltage-dependent activation, relative conductance sequence fo r divalent cations (Ca2+>Ba(2+)approximate to Sr2+>>Mn2+), and lack of inhibition by nifedipine, D600, diltiazem, delta conotoxin, or aga-IV a were unlike that of voltage-gated Ca2+ channels.