Arachidonic acid for loading induced prostacyclin and prostaglandin E-2 release from osteoblasts and osteocytes is derived from the activities of different forms of phospholipase A(2)

Mechanical loading of bone stimulates resident bone cells to produce prosta cyclin (PGI(2)) and prostaglandin (PG)E-2 by a mechanism that can be differ entially regulated by ion channel blockers. We have investigated difference s in the loading-related PC production mechanisms in rat ulnae explants loa ded ex vivo, Loading and aluminium fluoride (AlF3, a nonselective activator of C-proteins) both increased PGI(2) and PGE, release into culture medium. Pertussis toxin (PTX) blocked loading-related release of PGE(2), but not P GI(2), while isotetrandrine, an inhibitor of G-protein-mediated activation of phospholipase (PL)A(2), abolished the loading-related release of both PG s, This suggests both PTX-sensitive and -insensitive G-protein-dependent, P LA(2)-mediated mechanisms for loading-related PG production, Blockade of se cretory (s)PLA(2) activity prevented loading-related release of PGE2 and PG I2, whereas inhibition of cytosolic (c)PLA(2) activity blocked loading-rela ted release of PGE(2) alone. cPLA(2) was localized immuno-cytochemically to osteoblasts, but not to osteocytes, sPLA(2) was localized to osteocytes an d osteoblasts, Exogenous type-IA sPLA(2) and type-IB sPLA(2) stimulated sig nificant increases in PGE(2) and PGI(2) release. PTX reduced the release of both PGs stimulated by type IA PLA(2), but not type IB. Furthermore, inhib ition of protein kinase C (PKC) activity blocked loading-related release of PGE(2), but not that of PGI(2). These data suggest that loading-related re lease of PCI2 and PGE(2) utilizes arachidonic acid derived from the activit y of different PLA(2). In osteocytes and osteoblasts, arachidonic acid for PGI(2) synthesis is liberated by PTX-insensitive G-protein dependent sPLA(2 ) alone. In osteoblasts, arachidonic acid for PGE, synthesis is released by PTX-sensitive, G-protein-dependent, cPLA(2)-mediated activity, which also requires upstream sPLA(2) and PKC activities. (C) 2000 by Elsevier Science Inc. All rights reserved.