Partial Agonist Clonidine Mediates α₂-AR Subtypes Specific Regulation of cAMP Accumulation in Adenylyl Cyclase II Transfected DDT1-MF2 Cells

Abstract

α2-Adrenergic receptor (α₂-AR) activation in the pregnant rat myometrium at midterm potentiates β₂-AR stimulation of adenylyl cyclase (AC) via Gβγ regulation of the type II isoform of adenylyl cyclase. However, at term, α₂-AR activation inhibits β₂-AR stimulation of AC. This phenomenon is associated with changes in α₂-AR subtype expression (midterm α_2A/D-AR ≫ α_2B-AR; term α_2B ≥ α_2A/D-AR), without any change in ACII mRNA, suggesting that α_2A/D- and α_2B-AR differentially regulate β₂-cAMP production. To address this issue, we have stably expressed the same density of α_2A/D- or α_2B-AR with AC II in DDT1-MF2 cells. Clonidine (partial agonist) increased β₂-AR-stimulated cAMP production in α_2A/D-AR-ACII transfectants but inhibited it in α_2B-AR-ACII transfectants. In contrast, epinephrine (full agonist) enhanced β₂-stimulated ACII in both α_2A- and α_2B-ACII clonal cell lines. 4-Azidoanilido-[α-³²P]GTP-labeling of activated G proteins indicated that, in α_2B-AR transfectants, clonidine activated only Gi₂, whereas epinephrine, the full agonist, effectively coupled to Gi₂ and Gi₃. Thus, partial and full agonists selectively activate G proteins that lead to drug specific effects on effectors. Moreover, these data indicate that Gi₃ activation is required for potentiation of β₂-AR stimulation of AC by α_2A/D and α_2B-AR in DDT1-MF2 cells. This may reflect an issue of the amount of Gβγ released upon receptor activation and/or βγ composition of Gi₃ versus Gi₂.