α₂-Adrenergic receptor-mediated sensitization of forskolin-stimulated cyclic AMP production

Preincubation of HT29 human colonic adenocarcinoma cells with α₂-adrenergic agonists resulted in a 10- to 20-fold increase in forskolin-stimulated cyclic AMP production as compared to cells preincubated without agonist. Similar results were obtained using either a [³H]adenine prelabeling assay or a cyclic AMP radioimmunoassay to measure cyclic AMP levels. This phenomenon, which is termed sensitization, is α₂-adrenergic receptor-mediated and rapid in onset and reversal. Yohimbine, an α₂-adrenergic receptor-selective antagonist, blocked norepinephrine-induced sensitization, whereas prazosin (α₁-adrenergic) and sotalol (β-adrenergic) did not. The time for half-maximal sensitization was 5 min and the half-time for reversal was 10 min. Only a 2-fold sensitization of cyclic AMP production stimulated by vasoactive intestinal peptide was observed, indicating that sensitization is relatively selective for forskolin. Sensitization reflects an increased production of cyclic AMP and not a decreased degradation of cyclic AMP, since incubation with a phosphodiesterase inhibitor and forskolin did not mimic sensitization. Increasing the levels of cyclic AMP during the preincubation (using a phosphodiesterase inhibitor) had no effect on sensitization, indicating that sensitization is not caused by decreased cyclic AMP levels during the preincubation. This rapid and dramatic sensitization of forskolin-stimulated cyclic AMP production is a previously unreported effect that can be added to the growing list of α₂-adrenergic responses that are not mediated by a decrease in cyclic AMP.