FUNCTIONAL EXPRESSION OF ADRENERGIC AND OPIOID RECEPTORS IN XENOPUS-OOCYTES - INTERACTION BETWEEN ALPHA(2)-ADRENERGIC AND BETA(2)-ADRENERGIC RECEPTORS

We functionally expressed alpha(2)-adrenergic, beta(2)-adrenergic, and delta-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. Go-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 beta(2)-adrenergic receptors and CFTR, and co-administration of propranolol antagonized these responses. Similarly, the alpha(2)-adrenergic agonist UK14304 increased IBMX-induced currents only in oocytes injected with mRNA encoding alpha(2)-adrenergic receptors and CFTR, and idazoxan antagonized these enhancements. The delta-opioid agonist DADLE produced concentration-related, naloxone-reversible increases in IBMX- and forskolin-induced currents only in oocytes injected with mRNA encoding delta-opioid receptors and CFTR. In oocytes co-injected with alpha(2), beta(2), and CFTR mRNAs, isobolographic analysis revealed an additive interaction between alpha(2)- and beta(2)-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 alpha(2)-adrenergic and delta-opioid receptors to G proteins, possibly G(s) proteins, other than G(i) proteins.