title: Gi proteins regulate adenylyl cyclase activity independent of receptor activation.
abstract: OBJECTIVE
  Despite the view that only β2- as opposed to β1-adrenoceptors (βARs) couple to G(i), some data indicate that the β1AR-evoked inotropic response is also influenced by the inhibition of Gi. Therefore, we wanted to determine if Gi exerts tonic receptor-independent inhibition upon basal adenylyl cyclase (AC) activity in cardiomyocytes.
  
  
  METHODS
  We used the Gs-selective (R,R)- and the Gs- and G(i)-activating (R,S)-fenoterol to selectively activate β2ARs (β1AR blockade present) in combination with Gi inactivation with pertussis toxin (PTX). We also determined the effect of PTX upon basal and forskolin-mediated responses. Contractility was measured ex vivo in left ventricular strips and cAMP accumulation was measured in isolated ventricular cardiomyocytes from adult Wistar rats.
  
  
  RESULTS
  PTX amplified both the (R,R)- and (R,S)-fenoterol-evoked maximal inotropic response and concentration-dependent increases in cAMP accumulation. The EC50 values of fenoterol matched published binding affinities. The PTX enhancement of the Gs-selective (R,R)-fenoterol-mediated responses suggests that Gi regulates AC activity independent of receptor coupling to Gi protein. Consistent with this hypothesis, forskolin-evoked cAMP accumulation was increased and inotropic responses to forskolin were potentiated by PTX treatment. In non-PTX-treated tissue, phosphodiesterase (PDE) 3 and 4 inhibition or removal of either constitutive muscarinic receptor activation of Gi with atropine or removal of constitutive adenosine receptor activation with CGS 15943 had no effect upon contractility. However, in PTX-treated tissue, PDE3 and 4 inhibition alone increased basal levels of cAMP and accordingly evoked a large inotropic response.
  
  
  CONCLUSIONS
  Together, these data indicate that Gi exerts intrinsic receptor-independent inhibitory activity upon AC. We propose that PTX treatment shifts the balance of intrinsic G(i) and Gs activity upon AC towards Gs, enhancing the effect of all cAMP-mediated inotropic agents.
authors.affiliation: Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre and Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Caroline Bull
authors.initials: CB
authors.lastname: Melsom
authors.affiliation: Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre and Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Øivind
authors.initials: Ø
authors.lastname: Ørstavik
authors.affiliation: Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre and Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Jan-Bjørn
authors.initials: JB
authors.lastname: Osnes
authors.affiliation: Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre and Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Tor
authors.initials: T
authors.lastname: Skomedal
authors.affiliation: Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre and Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Finn Olav
authors.initials: FO
authors.lastname: Levy
authors.affiliation: Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre and Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Kurt Allen
authors.initials: KA
authors.lastname: Krobert
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D000242:Cyclic AMP / Q000378:metabolism; D054703:Cyclic Nucleotide Phosphodiesterases, Type 4 / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D005280:Fenoterol / Q000737:chemistry / Q000494:pharmacology; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000378:metabolism*; D008297:Male; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism; D037342:Pertussis Toxin / Q000494:pharmacology; D058987:Phosphodiesterase 3 Inhibitors / Q000494:pharmacology; D058988:Phosphodiesterase 4 Inhibitors / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction / Q000187:drug effects; D013237:Stereoisomerism
chemical_list: D058987:Phosphodiesterase 3 Inhibitors; D058988:Phosphodiesterase 4 Inhibitors; D005280:Fenoterol; D000242:Cyclic AMP; D037342:Pertussis Toxin; D054703:Cyclic Nucleotide Phosphodiesterases, Type 4; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D000262:Adenylyl Cyclases