Intracellular beta-blockade: overexpression of Galpha(i2) depresses the beta-adrenergic response in intact myocardium.
OBJECTIVE
Increased levels of inhibitory G proteins have been observed in heart failure, but their physiological relevance in mediating the reduced beta-adrenergic response is largely unknown.


METHODS
To evaluate the functional consequences of Galpha(i2) overexpression, we studied myocardial contraction in intact isometric contracting cardiac rabbit trabeculae and isolated myocytes after adenovirus-mediated gene transfer of Galpha(i2).


RESULTS
Neither Galpha(i2) nor lacZ (control) overexpression altered baseline contractile force. After 72 h of continuous contractions, developed force (F(dev)) increased after addition of 1 microM isoproterenol by 28.5+/-9.7 mN/mm(2) in the control group, which was unchanged from the initial response at t=0 h (23.7+/-3.8 mN/mm(2)). In sharp contrast, in preparations transfected with AdGalpha(i2), the response to isoproterenol was significantly attenuated (5.9+/-2.0 vs. 27.6+/-4.2 mN/mm(2), t=72 vs. 0 h, respectively, P<0.01). In a primary culture of transfected isolated myocytes from a nearly identical baseline, isoproterenol increased cell shortening by 3.1+/-0.6% in the lacZ transfected myocytes, but only by 1.3+/-0.5% in Galpha(i2) transfected myocytes (t=72 h, P<0.01). In Galpha(i2) transfected myocytes, pertussis toxin restored beta-adrenergic responsiveness, indicating specificity of attenuation by the transgene.


CONCLUSIONS
Overexpression of Galpha(i2) attenuates the positive inotropic effects of beta-adrenergic stimulation in myocardium. In addition, the method we developed allows investigation of a causal link between altered protein expression and subsequent alterations in contractile function in a physiological relevant in vitro manner.
Janssen
Paul M L
PM
Department Cardiology and Pneumology, University of Göttingen, Göttingen, Germany. janssen.10@osu.edu
Schillinger
Wolfgang
W
Donahue
J Kevin
JK
Zeitz
Oliver
O
Emami
Shahriyar
S
Lehnart
Stephan E
SE
Weil
Joachim
J
Eschenhagen
Thomas
T
Hasenfuss
Gerd
G
Prestle
Juergen
J
D000256:Adenoviridae; D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D002316:Cardiotonic Agents / Q000494:pharmacology*; D018929:Cell Culture Techniques; D048429:Cell Size / Q000187:drug effects; D051960:GTP-Binding Protein alpha Subunit, Gi2; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000235:genetics / Q000502:physiology*; D005822:Genetic Vectors; D007545:Isoproterenol / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D037342:Pertussis Toxin; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000502:physiology*; D011817:Rabbits; D014162:Transfection; D019076:Transgenes; D015854:Up-Regulation; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D000318:Adrenergic beta-Agonists; D002316:Cardiotonic Agents; D011518:Proto-Oncogene Proteins; D010566:Virulence Factors, Bordetella; D037342:Pertussis Toxin; D051960:GTP-Binding Protein alpha Subunit, Gi2; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D007545:Isoproterenol