α2A-Adrenergic Receptor Stimulated Calcium Release Is Transduced by Gi-Associated Gβγ-Mediated Activation of Phospholipase C

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Abstract
Proposed mechanisms by which α2-adrenergic receptors (α2AR) regulate intracellular calcium ([Ca2+]i) include stimulation and inhibition of cell surface calcium channels, stimulation of calcium release via receptor coupling to Gq with subsequent activation of phospholipase C and release of IP3, or stimulation of calcium release via coupling to Gi in an IP3-independent manner. These potential mechanisms were explored in cells that expressed α2AAR endogenously (HEL cells), permanently transfected CHO cells, and transiently transfected COS-7 cells. Each cell type displayed agonist (UK14304)-dependent increases in [Ca2+]i that were blocked by yohimbine, ablated by pertussis toxin, and largely unaffected by chelation of extracellular calcium. Furthermore, calcium release was associated with IP3 accumulation and was blocked by an inhibitor of phospholipase C (PLC). When expressed in CHO cells, a mutated α2AAR which has the amino and carboxyl termini of the third intracellular loop substituted with β2AR sequence poorly coupled to Gi in adenylyl cyclase assays, and likewise displayed virtually no coupling to increased [Ca2+]i. These results all point toward a Gi- versus a Gq-mediated coupling pathway triggering release of intracellular calcium stores. The possibility that Gβγ subunits released from α2AAR−Gi coupling is the mechanism of PLC activation was explored in COS-7 cells by coexpressing α2AAR with the Gβγ inhibitors transducin or a carboxy-terminal portion of the βAR kinase. Both βγ inhibitors markedly inhibited α2AAR modulation of [Ca2+]i while not affecting thromboxane A2 receptor mediated stimulation of [Ca2+]i via Gq coupling. Thus, α2AAR couple to calcium release via Gi-associated Gβγ subunits. This coupling is present in multiple cell types and should be considered a major signal transduction pathway of this receptor.