Platelet-Activating Factor Activates the Phosphoinositide Cycle and Promotes Ca2+Mobilization in Human Myometrial Cells

Abstract

Several recent studies have implicated platelet-activating factor (PAF) in the physiological control of parturition. Because Ca2+ plays a critical role in the pharmacomechanical coupling in smooth muscle, we evaluated the effects of PAF on Ca2+ mobilization in permeabilized human myometrial cells (HMC) and compared this to the action of prostaglandin F2α, (PGF2α), a well-characterized uterotonic agent. Digitonin-permeabilized HMC in the presence of ATP and ruthenium red rapidly sequestered 45Ca to extramitochondrial compartment(s). Exposure of such cells to 10–9–10–6 M PAF or PGF2α caused a rapid, biphasic, and dose-related release of Ca. The action of PAF, but not that of PGF2α, was associated with a similarly rapid, and dose-dependent generation of [3H]-inositol phosphates. The Ca2+-mobilizing effect of PAF, but not that of PGF2α, was blocked by inhibitors of phospholipase C or heparin. Moreover, the combined effect of PGF2α and PAF or PGF2α and Ins(1,4,5)P3 on 45Ca efflux were additive, whereas no such enhancement of PAF action was observed when combined with Ins(1,4,5)P3. Finally, PAF-promoted 45Ca release was inhibited by a PAF receptor antagonist. It is concluded that PAF-induced Ca2+ mobilization is mediated by the activation of phospholipase C and the release of Ins(1,4,5)P3, whereas PGF2α-promoted Ca efflux does not require the interplay of the phosphoinositol cycle. Nonetheless, both of these agonists are effective in the physiological range, and therefore may play functional roles in the activation of the pregnant uterus at term.