Functional expression of adrenergic and opioid receptors in Xenopus oocytes: interaction between α₂- and β₂-adrenergic receptors

We functionally expressed α₂-adrenergic, β₂-adrenergic, and δ-opioid receptors in Xenopus laevis oocytes. We detected receptor function as changes in currents carried by adenosine 3′,5′-cyclic monophosphate (cAMP)-regulated chloride channels provided by the cystic fibrosis transmembrane conductance regulator (CFTR) and recorded by two-electrode voltage clamp. Co-application of forskolin and isobutylmethylxanthine (IBMX) or IBMX alone produced currents with a reversal potential indicative of chloride ions only in oocytes previously injected with mRNA encoding CFTR. Isoproterenol produced concentration-dependent responses in oocytes injected with mRNA encoding β₂-adrenergic receptors and CFTR, and co-administration of propranolol antagonized these responses. Similarly, the α₂-adrenergic agonist UK14304 increased IBMX-induced currents only in oocytes injected with mRNA encoding α₂-adrenergic receptors and CFTR, and idazoxan antagonized these enhancements. The δ-opioid agonist DADLE produced concentration-related, naloxonc-reversible increases in IBMX- and forskolin-induced currents only in oocytes injected with mRNA encoding δ-opioid receptors and CFTR. In oocytes co-injected with α₂, β₂, and CFTR mRNAs, isobolographic analysis revealed an additive interaction between α₂ - and β₂-adrenergic receptors. These studies establish the oocyte as a cell system for studying the interactions among cAMP-modulating G protein-coupled receptors and provide another example of alternative coupling of α₂-adrenergic and δ-opioid receptors to G proteins, possibly G_s proteins, other than G_i proteins.