We used a Ca'-sensitive electrode to measure changes in extracellular Ca" concentration in cell suspensions of Dictyostelium discoideum during differentiation and attractant stimulation . The cells maintained an external level of 3-8 AM Ca" until the beginning of aggregation and then started to take up Ca". The attractants, folic acid, cyclic AMP, and cyclic GMP, induced a transient uptake of Ca" by the cells. The response was detectable within 6 s and peaked at 30 s. Half-maximal uptake occurred at 5 nM cyclic AMP or 0.2

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