Comparison of the activation of the Ca2+ release-activated Ca2+ current I(CRAC) to InsP3 in Jurkat T-lymphocytes, pulmonary artery endothelia and RBL-1 cells

In many electrically non-excitable cells, Ca2+ entry is mediated predominantly by the store-operated Ca2+ influx pathway. The best- characterized store-operated Ca2+ current is the Ca2+ release-activated Ca2+ current (I(CRAC)). It is generally believed that high concentrations of intracellular Ca2+ buffer are required to measure I(CRAC), due to Ca2+-dependent inactivation of the channels. Recently, we have recorded robust I(CRAC) in rat basophilic leukaemia (RBL-1) cells at physiological levels of Ca2+ buffering when stores were depleted by inhibition of the sarcoplasmic/endoplasmic reticulum Ca2+-activated adenosine triphosphatase (SERCA) pumps. However, the second messenger inositol 1,4,5-trisphosphate (InsP3) was not able to evoke the current under such conditions, despite inducing substantial Ca2+ release. We have therefore suggested that a threshold exists within the Ca2+ stores which has to be overcome for macroscopic I(CRAC) to activate. To establish whether this is a specific feature of I(CRAC) in RBL-1 cells or whether it is a more general phenomenon, we investigated whether a threshold is also seen in other cell-types used to study store-operated Ca2+ entry. In Jurkat-T lymphocytes, I(CRAC) is activated weakly by InsP3 in the presence of low concentrations of Ca2+ buffer, whereas the current is large when SERCA pumps are blocked simultaneously, as in RBL-1 cells. Although the electrophysiological properties of I(CRAC) in the Jurkat cell are very similar to those of RBL-1 cells, the Na+ conductance in the absence of external divalent cations is quite different. Unexpectedly, we failed consistently to record any store- operated Ca2+ current in macrovascular pulmonary artery endothelia whereas robust I(CRAC) was seen under the same conditions in RBL-1 cells. Our results show that I(CRAC) has a similar profile of activation in the presence of physiological levels of Ca2+ buffering for Jurkat T-lymphocytes and RBL-1 cells, indicating that the threshold mechanism may be a general feature of I(CRAC) activation. Because I(CRAC) in pulmonary artery endothelia is, at best, very small, additional Ca2+ influx pathways may also contribute to agonist-induced Ca2+ entry.