Coupling of the expressed α_1B-adrenergic receptor to the phospholipase C pathway in Xenopus oocytes: The role of G_o

α_1B-Adrenergic receptor mRNA was injected into Xenopus oocytes, resulting in a norepinephrine-evoked Cl^- current. The response was proportional to norepinephrine concentration, blocked by prazosin, and dependent on intracellular Ca²⁺ derived from inositol trisphosphate-sensitive stores. Oocytes treated with 2 μg/ml pertussis toxin showed a time-dependent decrease of the norepinephrine response, taking up to 72 h to show an 80% decrease. Overnight treatment with 10 μg/ml pertussis toxin also resulted in 80% reduction. Responses to two other cloned receptors (M₁-muscarinic and serotonin-1c) expressed in oocytes were also reduced 50% or more by 72 h of pertussis toxin treatment. Pertussis toxin labeling of the cloned Xenopus α_o-subunit translated in vitro showed that it was a significantly poorer substrate for pertussis toxin than the two mammalian α_o-subunits expressed and assayed under identical conditions. This unexpected biochemical behavior of the Xenopus α_o-subunit is in agreement with the rather unusual treatment conditions required to observe the effects of pertussis toxin on the receptor-evoked Cl^- current in the oocyte. Injection of mammalian heterotrimeric G_o but not G_i3 significantly enhanced the norepinephrine-evoked Cl^- current in oocytes. Injection of mixtures of anti-sense oligonucleotides to the Xenopus α_o-subunit reduced the norepinephrine-evoked Cl^- current by 60% within 24 h, compared with oocytes injected with the oligonucleotides encoding sense sequences. These studies indicate that the expressed α_1B-adrenergic receptor, like the native muscarinic receptor, utilizes G_o to couple to the phospholipase C-mediated Cl^- current in Xenopus oocytes.