ARY ulation of calcium influx by platelet activating fa

Platele 3-phos patholo Barnes Kato e cellula IP3, via 1987; Snyder, 1989). Modulatory activities linked to intracellular receptors have also been suggested, based on differential effects of PAF antagonists, but they are less well characterized (Stewart et al., 1990; Hwang, 1990; Marcheselli and Bazan, 1994; Kato et al., 1994). PAF has been found also in lower eukaryotes, such as Tetrahymena pyriformis (Lekka et al., 1990) and Dictyostelium discoideum (Bussolino et al., 1991). D. discoideum cells grow as free-living amoebae, by feeding on bacteria. Development is triggered by starvation, and results in the formation of multicellular aggregates which undergo a series of morphogenetic changes (Bonner, 1967; Loomis, 1982). A major regulatory role in aggregation is played by cAMP, which is secreted by the cells a few hours after starvation and acts as a chemoattractant. In Dictyostelium, the intracellular concentration of PAF increases during development, and in response to cAMP (Bussolino et al., 1991). Spontaneous, periodic cellular oscilcur s, 19 n of leve lar et itan e ab s n guanylate cyclase. In contrast, PAF induces extracellular Ca2+ influx, independently of the presence of endogenous or exogenously added cAMP (Sordano et al., 1993). We have further characterized this effect of PAF, and we show that a specific, intracellular acceptor for PAF exists in these cells that appears during spike-shaped oscillations.