EVIDENCE FOR A PERTUSSIS-TOXIN-SENSITIVE CALCIUM-ENTRY PATHWAY IN THYROID FRTL-5 CELLS

Receptor-mediated calcium entry was investigated in Fura 2 loaded FRTL -5 cells. The purinergic agonist ATP activated the release of sequeste red calcium and the entry of extracellular calcium. Down regulation of protein kinase C (PKC) substantially enhanced the ATP-evoked calcium entry. Pretreatment of the cells with pertussis toxin (Ptx) decreased the ATP-evoked calcium entry by 56% and the release of sequestered cal cium by 34%. In PKC-downregulated cells, the effect of Ptx treatment o n the ATP-evoked increase in [Ca2+](i) was 73% and 44%, respectively. Phorbol myristic acetate (PMA) decreased the ATP-evoked calcium entry to the same extent as Ptx. In Ptx-treated cells, the ATP-evoked influx of Ca-45(2+) was attenuated. Stimulation of the cells with P-2p-purin ergic agonist GTP evoked no entry of calcium, although GTP released th e same amount of sequestered calcium as did ATP. PKC downregulation or pretreatment with Ptx had no effects on the GTP-evoked responses, whe reas PMA decreased the GTP-evoked release of calcium. We conclude that the ATP-activated rapid calcium entry pathway is a second messenger-o perated calcium channel. (C) 1995 Wiley-Liss, Inc.