Feedback regulation of ATP-induced Ca²⁺ signaling in HL-60 cells is mediated by protein kinase A- and C-mediated changes in capacitative Ca²⁺ entry

Extracellular ATP increases intracellular Ca²⁺ ([Ca²⁺](i)) in HL-60 cells. When cells are stimulated with supramaximal concentrations of ATP, although the initial [Ca²⁺](i) increase is similar over a range of 30, 100, and 300 μM ATP, the rate of the return to basal [Ca²⁺](i) level is faster in cells treated with higher concentrations of ATP. This probably results from differences in Ca²⁺ influx rather than Ca²⁺ release, since the influx of the unidirectional Ca²⁺ surrogates Ba²⁺ and Mn²⁺ also exhibit similar responses. Furthermore, while 300 μM ATP had an inhibitory effect on the thapsigargin-induced capacitative Ca²⁺ entry, 30 μM ATP potentiated the response. However, the inhibitory action of 300 μM ATP was blocked by protein kinase C (PKC) inhibitors, such as GF 109203X and chelerythrine, and the potentiating action of 30 μM ATP was blocked by protein kinase A (PKA) inhibitors H89 and Rp-cAMPS. The PKC inhibitors also slowed the decay rate of the Ca²⁺ response induced by 300 μM ATP, and the PKA inhibitors increased it when induced by 30 μM ATP. In the measurements of PKA and PKC activity, 30 μM ATP activates only PKA, while 300 μM ATP activates both kinases. Taken together, these data suggest that the changes in the ATP-induced Ca²⁺ response result from differential modulation of ATP-induced capacitative Ca²⁺ entry by PKC and PKA in HL-60 cells.