Mechanisms underlying ATP-induced Ca2+ mobilization in human neutrophils

The effect of ATP on Ca2+ mobilization in human neutrophils was examined by using fura-2 as a Ca2+ indicator. ATP (0.1-100 muM) caused a significant [Ca2+](i) increase in a concentration-dependent manner. The [Ca2+](i) signal comprised an initial rise followed by a plateau. Removal of external Ca2+ diminished the peak value of the [Ca2+](i) signal. In Ca2+-free medium, pretreatment with an endoplasmic reticulum Ca2+ pump inhibitor, thapsigargin, prevented ATP from releasing Ca2+. In contrast, thapsigargin still increased [Ca2+](i) after pretreatment with 10 muM ATP. These results indicate that 10 muM ATP released Ca2+ mainly from thapsigargin-sensitive stores. Adding 3 mM Ca2+ induced a concentration-dependent increase in [Ca2+](i) after pretreatment with ATP or thapsigargin in Ca2+-free medium, suggesting ATP induced Ca2+ influx via capacitative Ca2+ entry. ATP (10 muM)-induced Ca2+ release was abolished by inhibiting phospholipase C with 2 muM U73122, indicating that inositol-1,4,5-trisphosphate (IP3) mediates ATP-induced Ca2+ release. Conversely, ATP-induced [Ca2+], increase was abolished by activating protein kinase C (PKC) with 10 nM phorbol myristate acetate (PMA), but was not altered by inhibiting PKC with 2 muM GF 109203X. This implies ATP-induced [Ca2+](i) increase is a PMA-linked event. Together, the results suggest ATP increases [Ca2+](i) in human neutrophils by releasing Ca2+ from IP3-coupled, thapsigargin-sensitive Ca2+ stores, and inducing Ca2+ influx via the process of capacitative Ca2+ entry. The ATP-induced Ca2+ signal is a PMA-linked event.