Extracellular ATP activates mast cells via a mechanism that is different from the activation induced by the cross-linking of Fc receptors

In this study, the extracellular ATP (ATPo)-induced biochemical events were elucidated by comparing them with either the FcεRI- or FcγR-induced events in the mouse mast cell line MC9. The omission of extracellular Ca²⁺ almost completely abolished the elevation of intracellular Ca²⁺ ([Ca²⁺](i)) in the ATPo-stimulated cells, but only suppressed the second phase of the increase of [Ca²⁺](i) in FcR-stimulated cells, thus suggesting that the ATPo-induced elevation of [Ca²⁺](i) is totally dependent on the entry of extracellular Ca²⁺. Pretreatment with genistein, which inhibits protein kinases, especially protein tyrosine kinase, inhibited the FcR- triggered increase of [Ca²⁺2+](i), but not the ATPo-triggered one; however, such pretreatment did suppress both ATPo- and FcR-mediated β-hexosaminidase release. An immunoblot analysis revealed that both ATPo and the cross- linking of FcRs led to tyrosine phosphorylation of 44- and 110-kDa proteins, which thus suggested that these tyrosine-phosphorylated proteins are involved in a modulation of the degranulation process following an elevation of [Ca²⁺](i). Pretreatment with PMA inhibited the FcR-induced [Ca²⁺](i) increase, while not inhibiting the ATPo-induced one, thus suggesting that ATPo can mobilize [Ca²⁺](i) even when protein kinase C (PKC) has already been activated. Pretreatment of calphostin C, a specific PKC inhibitor, had little effect on the ATPo-mediated β-hexosaminidase secretion, thus indicating that the ATPo-induced degranulation is not mediated by PKC. Taken together, these results demonstrate that ATPo activates MC9 mast cells by a mechanism that is different from the activation induced by the cross-linking of FcRs.