GENETIC-EVIDENCE FOR INVOLVEMENT OF MULTIPLE EFFECTOR SYSTEMS IN ALPHA(2A)-ADRENERGIC RECEPTOR INHIBITION OF STIMULUS-SECRETION COUPLING

The alpha(2A)-adrenergic receptor (alpha(2A)AR), via its interaction w ith the pertussis toxin-sensitive G(i)/G(o) class of G proteins, modul ates multiple effector systems, including inhibition of adenylyl cycla se and Ca2+ channels and activation of K+ channels. Mutation of a memb rane-embedded aspartate residue, highly conserved among G protein-coup led receptors, in the alpha(2A)AR to asparagine (D79N alpha(2A)AR) res ults in selective uncoupling of the receptor to K+ currents but retent ion of inhibition of cAMP production and of voltage-sensitive Ca2+ cur rents when expressed in AtT20 anterior pituitary cells in culture. It is known that attenuation of cAMP synthesis alone cannot account for a lpha(2A)AR suppression of stimulus-secretion coupling; thus, the D79N alpha(2A)AR provides a unique tool with which to assess the relative c ontribution of K+ current activation and Ca2+ current suppression in m ediating the cellular responses of alpha(2A)AR. The wild-type alpha(2A )AR suppresses basal and secretagogue-evoked adrenocorticotropic hormo ne (ACTH) release in a manner indistinguishable from response to the e ndogenous somatostatin receptor. In contrast, the D79N alpha(2A)AR doe s not attenuate basal ACTH release and is only partially effective in suppressing ACTH secretion evoked by the secretagogue isoproterenol. R egulation of ACTH release evoked by 8-bromo-cAMP, which bypasses recep tor regulation of cAMP synthesis, suggests that attenuation of cAMP pr oduction, although not sufficient for inhibition of ACTH secretion, ne vertheless participates in a functionally relevant manner. Taken toget her, the present findings indicate that alpha(2A)AR-mediated suppressi on of neuropeptide secretion requires concomitant regulation of K+ and Ca2+ currents in parallel with attenuation of cAMP production.