PERMISSIVE STIMULATION OF CA2-INDUCED PHOSPHOLIPASE A(2), BY AN ADENOSINE RECEPTOR AGONIST IN A PERTUSSIS-TOXIN-SENSITIVE MANNER IN FRTL-5 THYROID-CELLS - A NEW CROSS-TALK MECHANISM IN CA2+ SIGNALING()

We have described the pertussis toxin (PTX)-sensitive potentiation of P-2-purinergic agonist-induced phospholipase C activation, Ca2+ mobili zation and arachidonic acid release by an adenosine receptor agonist, N-6-(L-2-phenylisopropyl)adenosine (PIA), which alone cannot influence any of these cellular activities [Okajima, Sato, Nazarea, Sho and Kon do (1989) J. Biol. Chem. 264, 13029-13037]. In the present study we ha ve found that arachidonic acid release was associated with lysophospha tidylcholine production, and conclude that arachidonic acid is produce d by phospholipase A(2) in FRTL-5 thyroid cells. This led us to assume that PIA augments P-2-purinergic arachidonic acid release by increasi ng [Ca2+](i) which, in turn, activates Ca2+-sensitive phospholipase A( 2). The arachidonic acid-releasing response to PIA was, however, alway s considerably higher (3.1-fold increase) than the Ca2+ response (1.3- fold increase) to the adenosine derivative. In addition, arachidonic a cid release induced by the [Ca2+](i) increase caused by thapsigargin, an endoplasmic-reticulum Ca2+-ATPase inhibitor, or calcium ionophores was also potentiated by PIA without any effect on [Ca2+](i) and phosph olipase C activity. This action of PIA was also PTX-sensitive, but not affected by the forskolin- or cholera toxin-induced increase in the c ellular cyclic AMP (cAMP), suggesting that a PTX-sensitive G-protein(s ) and not cAMP mediates the PIA-induced potentiation of Ca2+-generated phospholipase A(2) activation. Although acute phorbol ester activatio n of protein kinase C induced arachidonic acid release, P-2-purinergic and alpha(1)-adrenergic stimulation of arachidonic acid release was m arkedly increased by the protein kinase C down-regulation caused by th e phorbol ester. This suggests a suppressive role for protein kinase C in the agonist-induced activation of arachidonic acid release. We con clude that PIA (and perhaps any of the G(i)-activating agonists) augme nts an agonist (maybe any of the Ca2+-mobilizing agents)-induced arach idonic acid release by activation of Ca2+-dependent phospholipase A(2) in addition to enhancement of agonist-induced phospholipase C followe d by an increase in [Ca2+](i).