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 myrista te acetate (PMA) attenuated the ATP-induced increase in intracellular free Ca2+ ([Ca2+]i). In PKC down-regulated cells, the ATP-induced incr ease in [Ca2+]i was increased compared with control cells. This enhanc ed increase in [Ca2+]i was apparently dependent on extracellular Ca2+, as no difference was observed between control cells and PKC down-regu lated cells in Ca2+-free buffer. Addition of Ca2+ to cells stimulated with ATP in Ca2+-free buffer rapidly increased [Ca2+]i. The increase w as 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 [C a2+]i. In PKC down-regulated cells, addition of ATP after thapsigargin evoked a substantial increase in [Ca2+]i which was dependent on extra cellular Ca2+. The results indicate that PKC has a modulatory effect o n the ATP-induced entry of Ca2+ in FRTL-5 cells.