T cell receptor‐mediated Ca 2+ signaling: Release and influx are independent events linked to different Ca 2+ entry pathways in the plasma membrane

In this study, we showed that cross‐linking CD3 molecules on the T cell surface resulted in Ca 2+ release from the intracellular stores followed by a sustained Ca 2+ influx. Inhibition of release with TMB‐8 did not block the influx. However, inhibition of phospholipase C activity suppressed both Ca 2+ release and influx. Once activated, the influx pathway remained open in the absence of further hydrolysis of PIP2. Thapsigargin, a microsomal Ca 2+ ‐ATPase inhibitor, stimulated Ca 2+ entry into the cells by a mechanism other than emptying Ca 2+ stores. In addition, Ca 2+ entry into the Ca 2+ ‐depleted cells was stimulated by low basal level of cytosolic Ca 2+ , not by the emptying of intracellular Ca 2+ stores. Both the Ca 2+ release and influx were dependent on high and low concentrations of extracellular Ca 2+ . At low concentrations, Mn 2+ entered the cell through the Ca 2+ influx pathway and quenched the sustained phase of fluorescence; whereas, at higher Mn 2+ concentration both the transient and the sustained phases of fluorescence were quenched. Moreover, Ca 2+ release was inhibited by low concentrations of Ni 2+ , La 3+ , and EGTA, while Ca 2+ influx was inhibited by high concentrations. Thus, in T cells Ca 2+ influx occurs independently of IP3‐dependent Ca 2+ release. However, some other PIP2 hydrolysis‐dependent event was involved in prolonged activation of Ca 2+ influx. Extracellular Ca 2+ influenced Ca 2+ release and influx through the action of two plasma membrane Ca 2+ entry pathways with different pharmacological and biochemical properties.