Failure of endothelin-1 to activate store-operated Ca2+ channels by lack of mobilization from intracellular Ca2+ stores in cultured bovine adrenal chromaffin cells

We have recently shown that in addition to L-type voltage-operated Ca2+ channel (VOC), endothelin-1 (ET-1) stimulation opens two types of Ca2+-permeable nonselective cation channels [designated nonselective cation channel-1 (NSCC-1) and NSCC-2]. However, in this Ca2+ entry, the involvement of store-operated Ca2+ channel (SOCC), which is suggested to exist in chromaffin cells, was unclear. Those NSCCs as well as SOCC can be pharmacologically discriminated using Ca2+ channel blockers such as SK&F 96365 and LOE 908. To clarify whether SOCC should actually exist and play a role in Ca2+ entry in chromaffin cells stimulated with ET-1, we examined the effects of removal of extracellular Ca2+, thapsigargin (TG, an inhibitor of endoplasmic reticulum Ca2+-ATPase), LOE 908 and SK&F 96365 on cytosolic free Ca2+ concentrations ([Ca2+]i) in cultured bovine adrenal chromaffin cells. After the cells were exposed to Ca2+-free medium followed by exposure to TG to deplete Ca2+ from the intracellular Ca2+ store, restoration of extracellular Ca2+ caused a gradual increase in [Ca2+]i (to about 200% of control). The increase was unaffected by LOE 908, but completely abolished by SK&F 96365. In the Ca2+-free medium, no increase in [Ca2+]i by ET-1 was observed, but the subsequent restoration of extracellular Ca2+ induced a rapid increase in [Ca2+]i (to the same level of [Ca2+]i as that evoked by ET-1 in the normal medium (1.0 mM Ca2+)). Since SK&F 96365 is also a blocker of SOCC, these results indicate that in bovine adrenal chromaffin cells, Ca2+ entry through SOCC (Ca2+ influx through the capacitative Ca2+ entry system) occurs but is comparably weak, and that it virtually does not work on the stimulation of ET-1.