Comparison of the activation of the Ca2+ release-activated Ca2+ current I-CRAC to InsP(3) in Jurkat T-lymphocytes, pulmonary artery endothelia and RBL-1 cells

In many electrically non-excitable cells, Ca2+ entry is mediated predominan tly by the store-operated Ca2+ influx pathway. The best-characterised store -operated Ca2+ current is the Ca2+ release-activated Ca2+ current (I-CRAC) It is generally believed that high concentrations of intracellular Ca2+ buf fer are required to measure I-CRAC, due to Ca2+-dependent inactivation of t he channels. Recently, we have recorded robust ICRAC in rat basophilic leuk aemia (RBL-1) cells at physiological levels of Ca2+ buffering when stores w ere depleted by inhibition of the sarcoplasmic/ endoplasmic reticulum Ca2+- activated adenosine triphosphatase (SERCA) pumps. However, the second messe nger inositol 1,4,5-trisphosphate (InsP(3)) was not able to evoke the curre nt under such conditions, despite inducing substantial Ca2+ release. We hav e 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 mor e general phenomenon, we investigated whether a threshold is also seen in o ther cell-types used to study store-operated Ca2+ entry. In Jurkat T-lympho cytes, I-CRAC is activated weakly by InsP(3) in the presence of low concent rations of Ca2+ buffer, whereas the current is large when SERCA pumps are b locked 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-o perated Ca2+ current in macrovascular pulmonary artery endothelia whereas r obust 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 ph ysiological levels of Ca2+ buffering for Jurkat T-lymphocytes and RBL-1 cel ls, indicating that the threshold mechanism may be a general feature of I-C RAC 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.