Vol. 59, Issue 2, 331-338, February 2001

Partial Agonist Clonidine Mediates ₂-AR Subtypes Specific Regulation of cAMP Accumulation in Adenylyl Cyclase II Transfected DDT1-MF2 Cells

Isabelle Limon-Boulez, Rachel Bouet-Alard, Tom W. Gettys, Stephen M. Lanier, Jean-Paul Maltier, and Chantal Legrand

Laboratoire de Physiologie de la Reproduction, Centre National de la Recherche Scientifique ESA 7080, Université Pierre et Marie Curie, Paris, France (I.L.-B., R.B.-A., J.-P.M., C.L.); Division of Gastroenterology, Department of Medicine (T.W.G.) and Department of Pharmacology (S.M.L.), Medical University of South Carolina, Charleston, South Carolina

2-Adrenergic receptor (₂-AR) activation in the pregnant rat myometrium at midterm potentiates ₂-AR stimulation of adenylyl cyclase (AC) via G regulation of the type II isoform of adenylyl cyclase. However, at term, ₂-AR activation inhibits ₂-AR stimulation of AC. This phenomenon is associated with changes in ₂-AR subtype expression (midterm _2A/D-AR _2B-AR; term _2B  2A/D-AR), without any change in ACII mRNA, suggesting that _2A/D- and _2B-AR differentially regulate ₂-cAMP production. To address this issue, we have stably expressed the same density of _2A/D- or _2B-AR with AC II in DDT1-MF2 cells. Clonidine (partial agonist) increased ₂-AR-stimulated cAMP production in _2A/D-AR-ACII transfectants but inhibited it in _2B-AR-ACII transfectants. In contrast, epinephrine (full agonist) enhanced ₂-stimulated ACII in both _2A- and _2B-ACII clonal cell lines. 4-Azidoanilido-[-³²P]GTP-labeling of activated G proteins indicated that, in _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 ₂-AR stimulation of AC by _2A/D and _2B-AR in DDT1-MF2 cells. This may reflect an issue of the amount of G released upon receptor activation and/or composition of Gi₃ versus Gi₂.