Proposed signaling role of arachidonic acid in human myometrium.

The objective of this study was to test the hypothesis that, in human myometrial cells (HMC), PGF2 alpha and oxytocin promote the release of arachidonic acid (AA) which, in turn, acts to mobilize intracellular Ca2+. Primary monolayer cultures of HMC were labeled with [3H]arachidonic acid ([3H]AA) to isotopic equilibrium before exposure to PGF2 alpha or oxytocin. Radiolabeled phospholipids were separated on thin layer chromatography and quantitated by scintillation counting. Prostanoids were analyzed by high performance liquid chromatography. Calcium release was quantitated in digitonin-permeabilized myocytes preloaded with 45Ca, in the presence of ATP and ruthenium red. PGF2 alpha (10(-7) M) caused a rapid (peaking at 2 min), and significant (P < 0.01) increase in [3H]AA release that was derived selectively from phosphatidylethanolamine (PE), indicative of phospholipase A2 activation. Oxytocin caused a rapid (30 s) and significant increase in diacylglycerol, concomitant with a drop in phosphoinositides, as well as an increase in [3H]AA and a fall in PE and phosphatidylcholine. Exogenous AA caused a rapid and dose-related efflux of 45Ca2+, which was not inhibited by blockers of AA metabolism, or by heparin that abolished inositol 1,4,5-trisphosphate-induced 45Ca2+ release. It is concluded that PGF2 alpha and oxytocin promote, by different mechanisms, the release of AA, which in turn may amplify their action by enhancing Ca2+ mobilization from the sarcoplasmic reticulum, thereby fulfilling the role of intracellular signaling molecule in human myometrium.