Cells of Dictyostelium discoideum respond to extracellular cyclic AMP with marked changes in intracellular cyclic GMP levels and light scattering. In this work, defined temporal increases in cyclic AMP were produced by the continuous addition of cyclic AMP to agitated suspensions of cells; concomitant hydrolysis of cyclic AMP by the cells subsequently established a constant, steady state concentration. The cells responded to the initial increase in extracellular cyclic AMP with a rapid increase in the intracellular cyclic GMP concentration and a rapid decrease in light scattering. At cyclic AMP input rates of 0.5-5 nM X s-1, the fast reactions of cyclic GMP and light scattering had already relaxed while the cyclic AMP concentration in the cell suspension was still increasing. The cells responded to constant concentrations of cyclic AMP with constant elevated cyclic GMP concentrations and constant decreased levels of light scattering. Our results are consistent with the existence of two types of perception systems, one of which adapts to constant stimuli and one of which does not adapt.

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