STIMULATION OF CALCIUM INFLUX BY PLATELET-ACTIVATING-FACTOR IN DICTYOSTELIUM

Platelet activating factor (PAF) induces Ca2+ influx in Dictyostelium discoideum. In this investigation we used this activity to analyze the mechanism of PAF action, We found that PAF activity was confined to t he period of spike-shaped oscillations and suggest that the role of PA F is to augment cAMP relay, PAF seems to act only a few times during t his time period of two hours, since Ca2+ entry adapted to a subsequent stimulus for about 30 minutes, PAF showed a reduced response in the G protein beta- strain LW14 and was unable to induce Ca2+ influx in the G alpha 2(-) strains HC85 and JM1. The latter expresses the cAMP rece ptors cAR1 constitutively, and exhibits cAMP-induced Ca2+ influx, albe it at a reduced level, In order to decide whether the inability of PAP to elicit a Ca2+ response in JM1 cells was due to the lack of differe ntiation and/or the lack of G alpha 2, we inhibited the IP3-dependent pathway with compound U73122 and found that Ca2+ entry was blocked, wh ereas a closely related inactive compound, U73343, did not alter the r esponse. In agreement with this, NBD-Cl, an inhibitor of Ca2+ uptake i nto the IP3-sensitive store in Dictyostelium, also abolished PAF activ ity, The latter was not inhibited by the plasma membrane antagonists B N-52021 or WEB 2170, Therefore PAF seems to operate intracellularly vi a the IP3-signalling pathway at or upstream of the IP3-sensitive store .