Selective inhibition of adenylyl cyclase by octopamine via a human cloned α_2A‐adrenoceptor

Abstract

1 In this study we have compared the abilities of the enantiomers of the structural isomers of the phenolamines, octopamine and synephrine, and the catecholamines, noradrenaline and adrenaline, to couple selectively a human cloned a 2A -adrenoceptor, stably expressed in a Chinese hamster ovary (CHO) cell line, to G-protein linked second messenger pathways mediating an increase and a decrease in cyclic AMP production. 2 The catecholamines couple the a 2A -adrenoceptor to both an increase and a decrease in the rate of cyclic AMP production. In the absence of pertussis toxin pretreatment both catecholamines tested showed a dose-dependent decrease with a maximum at 100 nM. After pertussis toxin pretreatment they both produced a dose-dependent increase in cyclic AMP production with a maximum at 10 mM. 3 The phenolamines, octopamine and synephrine were only able to couple the a 2A -adrenoceptor to a dose-dependent decrease in cyclic AMP production at concentrations up to 1 mM, with the synephrine isomers being more potent than the corresponding octopamine isomers. The meta-isomers of both phenolamines were more potent than the corresponding para-isomers and the (7)-enantiomers were more potent than the (+)-enantiomers. Thus, (7)-meta-synephrine [ (7)-phenylephrine] was the most e ective isomer tested with an observable decrease occurring between 100 nM and 1 mM.4 The e ects of octopamine and the catecholamines on the decrease in cyclic AMP production were additive at submaximal concentrations, whilst octopamine reduced the stimulant e ect of submaximal concentrations of noradrenaline on cyclic AMP production after pertussis toxin pretreatment. 5 The time courses of the inhibitory e ects of both meta-octopamine and noradrenaline were parallel and peaked after a 1 min exposure to the agonist. In contrast, the stimulant e ects of noradrenaline after pertussis toxin pretreatment were of a much slower time course with a maximum e ect occurring after a 5 min incubation period. 6 Since octopamine and synephrine occur naturally in, and are co-released with catecholamines from, mammalian tissues, the results of the present study suggest that the human cloned a 2A -adrenoceptor can be coupled selectively by di erent endogenous agonists to G-protein pathways mediating the regulation of adenylyl cyclase activity.

A comparison of agonist‐specific coupling of cloned human α2‐adrenoceptor subtypes

“…At higher concentrations of (7)-noradrenaline the inhibitory e ects of the agonist were not signi®cantly altered. In contrast to some previous studies (Fraser et al, 1989;Eason et al, 1992;Airriess et al, 1997;Rudling et al, 1999) in the transfected cell line used in this study, with a lower expression level, the coupling of the a 2A -adrenoceptor to cyclic AMP production did not display a biphasic appearance. However, after 24 h pre-incubation with PTX, the inhibition of cyclic AMP production was abolished and replaced by a signi®cant stimulation (F=7.24; d.f.=6,31; P50.001) with a maximum e ect at an agonist concentration of 1 mM (161.1+10.8% of control value; n=5) ( Figure 1A).…”

“…However, the results di er to previous studies involving CHO, HEK293, PC-12 and JEG-3 cells, where activation of a 2A -adrenoceptors by noradrenaline resulted in a biphasic response in cyclic AMP production (Fraser et al, 1989;Duzic & Lanier, 1992;Eason et al, 1992;Pepperl & Regan, 1993;Svensson et al, 1996;Airriess et al, 1997;Jasper et al, 1998;Rudling et al, 1999). The receptor-mediated stimulation of adenylyl cyclase activity in CHO cells has previously been attributed to high receptor expression levels (Eason et al, 1992).…”

Rapid, Nongenomic Responses to Ecdysteroids and Catecholamines Mediated by a NovelDrosophilaG-Protein-Coupled Receptor

“…Both the PI3K pathway (Scheid and Woodgate, 2001) and catecholamines (Pendelton et al, 1997) are known to have important roles in the control of Drosophila development, and, in addition, the activation of the MAPK pathway is an important regulator of cellular differentiation in Drosophila (Bier, 1998). A wide range of other Drosophila GPCRs, including the OA/TYR receptor (Robb et al, 1994), the DopR99B D 1 -like DA receptor (Reale et al, 1997), and the short neuropeptide F receptor , have also been shown to exhibit various forms of agonist-specific coupling, as have several vertebrate adrenergic and neuropeptide GPCRs (Spengler et al, 1993;Airriess et al, 1997). It remains to be determined whether both DA and the ecdysteroids can signal or modulate each other's actions through DmDopEcR in vivo.…”

“…cAMP assays. CHO cells stably expressing DmDopEcR or the Drosophila Oct/Tyr receptor (Arakawa et al, 1990) and wild-type cells were incubated with agonist in the presence of forskolin and isobutyl-3-methylxanthine (IBMX), as described previously (Airriess et al, 1997). Sf9 cells infected with DmDopEcR were harvested 48 h after infection and incubated with various concentrations of agonists in the presence of forskolin and IBMX, as described previously (Bandoh et al, 1999).…”

Agonist Regulation of D2 Dopamine Receptor/G Protein Interaction

“…noradrenaline, lead to stimulation of both G i -and G s -dependent pathways, whereas monohydroxylated agonists, e.g. octopamine, lead only to activation of G i -dependent pathways (47).…”