INTRACELLULAR CA2-LYMPHOCYTES - CHANGES INDUCED BY IN-VITRO AND IN-VIVO ACTIVATION( STORES OF T)

Intracellular Ca2+ stores were investigated in resting and activated s plenic T lymphocytes from Lewis rats. Activation was obtained either i n vitro (spleen cells isolated from ''naive'' rats exposed to concanav alin A for 24 h) or in vivo (spleen cells from rats with fully develop ed symptoms of experimental allergic encephalomyelitis). In both exper imental conditions several changes of Ca2+ homeostasis were observed w ith respect to resting lymphocytes: (1) a threefold increase of the to tal intracellular calcium (from 1.15 to 3.5 mmol/l); (2) a moderate in crease of the pool sensitive to inositol 1,4,5-trisphosphate (IP3), in vestigated both in intact T lymphocytes (fura-2 and Ca-45(2+)-release techniques in cells challenged with phytohemagglutinin) and in T lymph ocytes permeabilized with beta-escin ((45)Ca2(+) release induced by sa turating concentrations of IP3); and (3) the appearance of a pool rele ased by the endoplasmic reticulum (ER) Ca2+ ATPase inhibitor thapsigar gin (Tg), but insensitive to IP3, which, therefore, appears to be loca lized in areas of the ER devoid of the cognate receptor. The latter tw o findings were paralleled in activated lymphocytes by an increase of expression of ER markers, involved (calreticulin; Ca2(+) ATPase) or no t (protein disulfide isomerase) in the regulation of Ca2+ homeostasis. In contrast, calnexin (another ER marker) and the receptor for IP3 we re increased to only a moderate extent. Finally, an enlargement of non -ER Ca2+ pools was observed in the cells pretreated with Tg in which C a-45(2+) release was induced by the Ca2+ ionophore ionomycin. Our resu lts document structural and functional changes of intracellular Ca2+ s tores which might play an important regulatory role in activated T lym phocytes.