Increase of [Ca2+]i via activation of ATP receptors in PC-Cl3 rat thyroid cell line

In PC-Cl3 rat thyroid cell line, ATP and UTP provoked a transient increase in [Ca²⁺]_i, followed by a lower sustained phase. Removal of extracellular Ca²⁺ reduced the initial transient response and completely abolished the plateau phase. Thapsigargin (TG) caused a rapid rise in [Ca²⁺]_i and subsequent addition of ATP was without effect. The transitory activation of [Ca²⁺]_i was dose-dependently attenuated in cells pretreated with the specific inhibitor of phospholipase C (PLC), U73122. These data suggest that the ATP-stimulated increment of [Ca²⁺]_i required InsP₃ formation and binding to its specific receptors in Ca²⁺ stores. Desensitisation was demonstrated with respect to the calcium response to ATP and UTP in Fura 2-loaded cells. Further studies were performed to investigate whether the effect of ATP on Ca²⁺ entry into PC-Cl3 cells was via L-type voltage-dependent Ca²⁺ channels (L-VDCC) and/or by the capacitative pathway. Nifedipine decreased ATP-induced increase on [Ca²⁺]_i. Addition of 2 mM Ca²⁺ induced a [Ca²⁺]_i rise after pretreatment of the cells with TG or with 100 μM ATP in Ca²⁺-free medium. These data indicate that Ca²⁺ entry into PC-Cl3 stimulated with ATP occurs through both an L-VDCC and through a capacitative pathway. Using buffers with differing Na⁺ concentrations, we found that the effects of ATP were dependent of extracellular Na⁺, suggesting that a Na⁺/Ca²⁺ exchange mechanism is also operative. These data suggest the existence, in PC-Cl3 cell line, of a P2Y purinergic receptor able to increase the [Ca²⁺]_i via PLC activation, Ca²⁺ store depletion, capacitative Ca²⁺ entry and L-VDCC activation.