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

Abstract

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