Comparison of the activation of the Ca2+ release-activated Ca2+ current ICRAC 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-characterised store-operated Ca2+ current is the Ca2+ release-activated Ca2+ current (ICRAC). It is generally believed that high concentrations of intracellular Ca2+ buffer are required to measure ICRAC, due to Ca2+-dependent inactivation of the channels. Recently, we have recorded robust ICRAC 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 ICRAC to activate. To establish whether this is a specific feature of ICRAC 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, ICRAC 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 ICRAC 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 ICRAC was seen under the same conditions in RBL-1 cells. Our results show that ICRAC 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 ICRAC activation. Because ICRAC in pulmonary artery endothelia is, at best, very small, additional Ca2+ influx pathways may also contribute to agonist-induced Ca2+ entry.
Fierro
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L
Department of Physiology, University of Oxford, UK.
Lund
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PE
Parekh
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AB
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000502:physiology; D004553:Electric Conductivity; D004730:Endothelium, Vascular / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D019169:Jurkat Cells; D015471:Leukemia, Basophilic, Acute / Q000503:physiopathology*; D011651:Pulmonary Artery; D051381:Rats; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D013601:T-Lymphocytes / Q000502:physiology*; D013696:Temperature; D019284:Thapsigargin / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D015220:Calcium Channels; D004791:Enzyme Inhibitors; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D002118:Calcium