P₂-purinergic stimulation of iodide efflux in FRTL-5 rat thyroid cells involves parallel activation of PLC and PLA₂

Extracellular ATP increases inositol phosphates, cytosolic Ca²⁺ concentration ([Ca²⁺](i)), arachidonic acid (AA) release, and iodide efflux in FRTL-5 cells. To examine the sequence of events in P₂-purinergic receptor activation by ATP, a phospholipase C (PLC) inhibitor (U-73122), and a phospholipase A₂ (PLA₂) inhibitor (U-26384), as well as 1,2-bis(2- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and downregulation of protein kinase C (PKC) were used. ATP increased inositol trisphosphate (IP₃), [Ca²⁺](i), AA release, and ¹²⁵I efflux dose dependently. U- 73122 inhibited the IP₃ and calcium increase but not AA; U-26384 prevented AA release but not the increase in calcium. Both agents inhibited iodide efflux. BAPTA prevented any ATP-induced increase in [Ca²⁺](i) without affecting AA release or ¹²⁵I efflux. PKC downregulation had no effect on ATP stimulated AA release, but reduced ¹²⁵I efflux: We conclude that ATP- induced iodide efflux involves parallel, not sequential, activation of PLC and PLA₂. No increase in [Ca²⁺](i) or PKC activity is required for PLA₂ activation. In contrast, an increase in ¹²⁵I efflux depends on PKC and PLA₂ activities, but not an increase in [Ca²⁺](i).