ATP-induced entry of calcium in thyroid FRTL-5 cells. Studies with phorbol myristate acetate and thapsigargin.

Receptor-mediated Ca2+ entry was investigated in fura-2-loaded thyroid FRTL-5 cells. Activation of protein kinase C (PKC) by phorbol myristate acetate (PMA) attenuated the ATP-induced increase in intracellular free Ca2+ ([Ca2+]i). In PKC down-regulated cells, the ATP-induced increase in [Ca2+]i was increased compared with control cells. This enhanced increase in [Ca2+]i was apparently dependent on extracellular Ca2+, as no difference was observed between control cells and PKC down-regulated cells in Ca(2+)-free buffer. Addition of Ca2+ to cells stimulated with ATP in Ca(2+)-free buffer rapidly increased [Ca2+]i. The increase was blocked by PMA. However, PKC down-regulation had no effect on the [Ca2+]i response. Stimulating FRTL-5 cells with thapsigargin increased [Ca2+]i. Addition of ATP after thapsigargin had almost no effect on [Ca2+]i. In PKC down-regulated cells, addition of ATP after thapsigargin evoked a substantial increase in [Ca2+]i which was dependent on extracellular Ca2+. The results indicate that PKC has a modulatory effect on the ATP-induced entry of Ca2+ in FRTL-5 cells.