cAMP controls cytosolic Ca(2+ )levels in Dictyostelium discoideum

BACKGROUND: Differentiating Dictyostelium discoideum amoebae respond upon cAMP-stimulation with an increase in the cytosolic free Ca(2+ )concentration ([Ca(2+)](i)) that is composed of liberation of stored Ca(2+ )and extracellular Ca(2+)-influx. In this study we investigated whether intracellular cAMP is involved in the control of [Ca(2+)](i). RESULTS: We analyzed Ca(2+)-fluxes in a mutant that is devoid of the main cAMP-phosphodiesterase (PDE) RegA and displays an altered cAMP metabolism. In suspensions of developing cells cAMP-activated influx of extracellular Ca(2+ )was reduced as compared to wild type. Yet, single cell [Ca(2+)](i)-imaging of regA(- )amoebae revealed a cAMP-induced [Ca(2+)](i )increase even in the absence of extracellular Ca(2+). The cytosolic presence of the cAMP PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX) induced elevated basal [Ca(2+)](i )in both, mutant and wild type cells. Under this condition wild type cells displayed cAMP-activated [Ca(2+)](i)-transients also in nominally Ca(2+)-free medium. In the mutant strain the amplitude of light scattering oscillations and of accompanying cAMP oscillations were strongly reduced to almost basal levels. In addition, chemotactic performance during challenge with a cAMP-filled glass capillary was altered by EGTA-incubation. Cells were more sensitive to EGTA treatment than wild type: already at 2 mM EGTA only small pseudopods were extended and chemotactic speed was reduced. CONCLUSION: We conclude that there is a link between the second messengers cAMP and Ca(2+). cAMP-dependent protein kinase (PKA) could provide for this link as a membrane-permeable PKA-activator also increased basal [Ca(2+)](i )of regA(- )cells. Intracellular cAMP levels control [Ca(2+)](i )by regulating Ca(2+)-fluxes of stores which in turn affect Ca(2+)-influx, light scattering oscillations and chemotactic performance.