CYCLIC AMP-INDUCED AND INS(1,4,5)P3-INDUCED CA(2-DISCOIDEUM - CGMP REGULATES CA(2+) ENTRY ACROSS THE PLASMA-MEMBRANE()FLUXES IN PERMEABILIZED CELLS OF DICTYOSTELIUM)

Transduction of chemotactic signals in Dictyostelium discoideum appare ntly involves a precise regulation of the cytosolic Ca2+ concentration . Cyclic AMP stimulation causes Ca2+ influx followed by Ca2+ extrusion , the magnitude of extrusion depending on the state of differentiation . Here, we show that the cAMP receptor controls Ca2+ influx both at th e level of entry across the plasma membrane and at the level of transp ort into Ca2+-sequestering organelles. The use of permeabilised cells allowed us to discriminate between both fluxes. Permeabilised cells st ill showed the cAMP-induced Ca2+ uptake. The flux across the plasma me mbrane was more sensitive to Ba2+ and Mn2+, respectively, than Ca2+ se questration. We have shown previously, using stmF mutants, that cGMP r egulates Ca2+ inflUX. We confirmed this result with the membrane-perme ant cGMP-analogue, Sp-8-Br-cGMPS, which enhanced the cAMP-induced Ca2 influx in intact cells but not the uptake in permeabilised cells, ind icating that cGMP regulates Ca2+ influx across the plasma membrane. Oc casionally, a fast transient Ca2+ efflux, preceding the influx, occurr ed in intact cells. A small cAMP-induced Ca2+ release was also found i n permeabilised cells. A similarly sized Ca2+ release was elicited by Ins(1,4,5)P3 and could be substituted for by GTP or GTPgammaS. This re sult suggests that rapid Ca2+ release can be mediated by Ins(1,4,5)P3.