Both cAMP production by adenylyl cyclase and cAMP degradation by phosphodiesterases account for intracellular cAMP levels. We previously demonstrated an increased phosphodiesterase activity in GH-secreting adenomas bearing the gsp oncogene. Here we characterize both the activity and the expression of cAMP-specific phosphodiesterase genes in the human pituitary and in gsp+ and gsp- GH-secreting adenomas and analyze the impact of this intracellular feedback mechanism on the levels of cAMP-responsive element-binding protein phosphorylation. Normal pituitary and gsp- GH-secreting adenomas showed similar phosphodiesterase activities, and 7-fold higher levels were observed in gsp+ tumors. In these tumors the increased activity was mainly owing to isobutyl-methyl-xanthine-sensitive phosphodiesterase 4 and to isobutyl-methyl-xanthine-insensitive isoforms. By semiquantitative RT-PCR, all phosphodiesterase 4 transcripts were expressed in the normal and tumoral pituitary. However, the levels of phosphodiesterase 4C and 4D messenger RNAs were significantly higher in gsp+ than in gsp- GH-secreting adenomas and normal pituitary. Expression of the thyroid-specific isobutyl-methyl-xanthine-insensitive phosphodiesterase 8B was absent in the normal pituitary but detectable in almost all GH-secreting adenomas and higher in gsp+ (P < 0.02). Therefore, this study provides a characterization of phosphodiesterase expression in human pituitary and demonstrates a dramatic induction of the cAMP-specific phosphodiesterases 4C and phosphodiesterases 4D and phosphodiesterases 8B in gsp+ GH-secreting adenomas. Similar levels of cAMP-responsive element-binding protein phosphorylation were observed in gsp- and gsp+ GH-secreting adenomas; however, phosphodiesterase blockade caused an increase in cAMP-responsive element-binding protein phosphorylation that was significantly higher in gsp+ than in gsp- adenomas. Because cAMP-responsive element-binding protein represents the principal end point of the cAMP pathway, these results suggest that the enhanced phosphodiesterase activity may have a significant impact on the phenotypic expression of gsp mutations.