Cyclic AMP regulation of arachidonic acid (AA) release and phospholipid metabolism in human monocytes: modulation by intracellular calcium

Stimulation of inflammatory cells by specific ligands results in activation of phospholipase(s) and production of oxygenation products of AA. The authors have employed (3H)AA labeled monocytes to examine the involvement of cAMP in regulating phospholipase activity as measured by percent of incorporated (3H)AA released and TLC analysis of (3H)AA cellular lipids. Maximum release of radiolabel (31 +/- 5%) occurred upon challenge with the calcium ionophore A23187- (10μM), while FMLP (1μM) yielded 15 +/- 1% and untreated cells 8 +/- 1%. Pretreatment of monocytes with isobutyl methyl xanthine-(IBMX) or dibutyrl cyclic AMP (d-cAMP) inhibited FMLP stimulated release with IC50's of 2.5 x 10-5M and 8 x 10-5M respectively. Exposure of monocytes to maximal levels of IBMX (5 x 10-4M) or d-cAMP (10-3M) also reduced release from controls by 40%, while A23187 induced release was uneffected by either. Examination of (3H) AA labeled phospholipids showed that phosphatidylcholine (PC) and phosphatidylinositol were the major pools labeled and that stimulation by FMLP or A23187 appeared to deplete the PC pool exclusively. Prior exposure to IBMX or d-cAMP inhibited the loss from the PC pool only in untreated or FMLP stimulated cells. The data suggests that a phospholipase A2 activity, directly primarily towards PC, is regulated by cAMP possibly by inhibiting receptor mediated increases in intracellular calcium levels