Cyclic 3'-5'-AMP Relay in Dictyostelium discoideum V. adaptation of the cAMP signaling response During cAMP stimulation

In Dictyostelium discoideum, extracellular cAMP activates adenylate cyclase, which leads to an increase in intracellular cAMP and the rate of cAMP secretion. The signaling response to a constant cAMP stimulus is terminated after several minutes by an adaptation mechanism. The time-course of adaptation to stimuli of 10^-6 or 10^-7 M cAMP was assessed. We used a perfusion technique to deliver defined cAMP stimuli to [³H]adenosine-labeled amoebae and monitored their secretion of [³H]cAMP. Amoebae were pretreated with 10^-6 or lO^-7 M cAMP for periods of 0.33-12 minutes, and then immediately given test stimuli of 10^-8 M to 2.5 × 10^-7 M cAMP. The response to a given test stimulus was progressively attenuated and finally extinguished as the duration of the pretreatment stimulus increased. During pretreatment with 10^-6 M cAMP, the rates of attenuation could be ranked according to the concentration of the test stimulus. The responses to test stimuli of 10^-8, 5 × 10^-8, 10^-7, or 2.5 × 10^-8 M cAMP were extinguished after ≈1, 2.25, 2.5, and 10 min, respectively. 1.5 min of stimulation with 10^-6 M cAMP was necessary to extinguish the response of a test stimulus of 10^-7 M cAMP. Our data suggest that adaptation begins within 20 s of stimulation, rises rapidly for ≈2.5 min, and reaches a plateau after ≈10 min. The absolute rate of rise was faster during pretreatment with 10^-8 than with 10^-8 M cAMP. These results support a working hypothesis in which the occupancy of surface cAMP receptors leads to changes in two opposing cellular processes, excitation and adaptation, that control the activity of D. discoideum adenylate cyclase.