Store depletion by caffeine/ryanodine activates capacitative Ca2+ entry in nonexcitable A549 cells

Capacitative Ca2+ entry is essential for refilling intracellular Ca2+ stores and is thought to be regulated primarily by inositol 1, 4,5-trisphosphate (IP3)-sensitive stores in nonexcitable cells, In nonexcitable A549 cells, the application of caffeine or ryanodine induces Ca2+ release in the absence of extracellular Ca2+ similar to that induced by thapsigargin (Tg), and Ca2+ entry occurs upon the readdition of extracellular Ca2+. The channels thus activated are also permeable to Mn2+. The channels responsible for this effect appear to be activated by the depletion of caffeine/ryanodine-sensitive stores per se, as evidenced by the activation even in the absence of increased intracellular Ca2+ concentration. Tg pretreatment abrogates the response to caffeine/ryanodine, whereas Tg application subsequent to caffeine/ryanodine treatment induces further Ca2+ release. The response to caffeine/ryanodine is also abolished by initial ATP application, whereas ATP added subsequent to caffeine/ryanodine induces additional Ca2+ release. RT-PCR analyses showed the expression of a type 1 ryanodine receptor, two human homologues of transient receptor potential protein (hTrp1 and hTrp6), as well as all three types of the IP3 receptor. These results suggest that in A549 cells, (i) capacitative Ca2+ entry can also be regulated by caffeine/ryanodine-sensitive stores, and (ii) the RyR-gated stores interact functionally with those sensitive to IP3, probably via Ca2+-induced Ca2+ release.