Down-regulation of Cell Surface Cyclic AMP Receptors and Desensitization of Cyclic AMP-stimulated Adenylate Cyclase by Cyclic AMP in Dictyostelium discoideum. Kinetics and Concentration Dependence

cAMP binds to Dictyostelium discoideum surface receptors and induces a transient activation of adenylate cyclase, which is followed by desensitization. cAMP also induces a loss of detectable surface receptors (down-regulation). Cells were incubated with constant cAMP concentrations, washed free of cAMP, and cAMP binding to surface receptors and cAMP-induced activation of adenylate cyclase were measured. 1) cAMP could induce maximally 65% loss of binding activity and complete desensitization of cAMP-stimulated adenylate cyclase activity. 2) Half-maximal effects for down-regulation were observed at 50 nM cAMP and for desensitization at 5 nM cAMP. 3) Down-regulation was rapid with half-times of 4, 2.5, and 1 min at 0.1, 1, and 10 µM cAMP, respectively. Similar kinetic data have been reported for desensitization. 4) Down-regulation and desensitization were not reversible at 0 °C. 5) Down-regulation reversed slowly at 20 °C with a half-time of about 1 h. 6) Resensitization of adenylate cyclase was biphasic showing half-times of 4 min and about 1 h, respectively; the contribution of the rapidly resensitizing component was diminished when down-regulation of receptors was enhanced. These results suggest that cAMP-induced down-regulation of receptors and desensitization of adenylate cyclase stimulation proceed by at least two steps. One step is rapidly reversible, occurs at low cAMP concentrations, and induces desensitization without down-regulation, while the second step is slowly reversible, requires higher cAMP concentrations, and also induces down-regulation