Genetic evidence for involvement of multiple effector systems in alpha 2A-adrenergic receptor inhibition of stimulus-secretion coupling.

The alpha 2A-adrenergic receptor (alpha 2AAR), via its interaction with the pertussis toxin-sensitive Gi/G(o) class of G proteins, modulates multiple effector systems, including inhibition of adenylyl cyclase and Ca2+ channels and activation of K+ channels. Mutation of a membrane-embedded aspartate residue, highly conserved among G protein-coupled receptors, in the alpha 2AAR to asparagine (D79N alpha 2AAR) results in selective uncoupling of the receptor to K+ currents but retention of inhibition of cAMP production and of voltage-sensitive Ca2+ currents when expressed in AtT20 anterior pituitary cells in culture. It is known that attenuation of cAMP synthesis alone cannot account for alpha 2AAR suppression of stimulus-secretion coupling; thus, the D79N alpha 2AAR provides a unique tool with which to assess the relative contribution of K+ current activation and Ca2+ current suppression in mediating the cellular responses of alpha 2AAR. The wild-type alpha 2AAR suppresses basal and secretagogue-evoked adrenocorticotropic hormone (ACTH) release in a manner indistinguishable from response to the endogenous somatostatin receptor. In contrast, the D79N alpha 2AAR does not attenuate basal ACTH release and is only partially effective in suppressing ACTH secretion evoked by the secretagogue isoproterenol. Regulation of ACTH release evoked by 8-bromo-cAMP, which bypasses receptor regulation of cAMP synthesis, suggests that attenuation of cAMP production, although not sufficient for inhibition of ACTH secretion, nevertheless participates in a functionally relevant manner. Taken together, the present findings indicate that alpha 2AAR-mediated suppression of neuropeptide secretion requires concomitant regulation of K+ and Ca2+ currents in parallel with attenuation of cAMP production.