Partial agonist clonidine mediates alpha(2)-AR subtypes specific regulation of cAMP accumulation in adenylyl cyclase II transfected DDT1-MF2

alpha2-Adrenergic receptor (alpha (2)-AR) activation in the pregnant rat my ometrium at midterm potentiates beta (2)-AR stimulation of adenylyl cyclase (AC) via G beta gamma regulation of the type II isoform of adenylyl cyclas e. However, at term, alpha (2)-AR activation inhibits beta (2)-AR stimulati on of AC. This phenomenon is associated with changes in alpha (2)-AR subtyp e expression (midterm alpha (2A/D)-AR >> alpha (2B)-AR; term alpha (2B) gre ater than or equal to alpha (2A/D)-AR), without any change in ACII mRNA, su ggesting that alpha (2A/D)- and alpha (2B)-AR differentially regulate beta (2)-cAMP production. To address this issue, we have stably expressed the sa me density of alpha (2A/D)- or alpha (2B)-AR with AC II in DDT1-MF2 cells. Clonidine (partial agonist) increased beta (2)-AR-stimulated cAMP productio n in alpha (2A/D)-AR-ACII transfectants but inhibited it in alpha (2B)-AR-A CII transfectants. In contrast, epinephrine (full agonist) enhanced beta (2 )-stimulated ACII in both alpha (2A)- and alpha (2B)-ACII clonal cell lines . 4-Azidoanilido-[alpha-P-32]GTP-labeling of activated G proteins indicated that, in alpha (2B)-AR transfectants, clonidine activated only Gi(2), wher eas epinephrine, the full agonist, effectively coupled to Gi(2) and Gi(3). Thus, partial and full agonists selectively activate G proteins that lead t o drug specific effects on effectors. Moreover, these data indicate that Gi (3) activation is required for potentiation of beta (2)-AR stimulation of A C by alpha (2A/D) and alpha (2B)-AR in DDT1-MF2 cells. This may reflect an issue of the amount of Gbg released upon receptor activation and/or beta ga mma composition of Gi(3) versus Gi(2).