Intracellular beta-blockade: overexpression of G alpha(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 P-adrenergic response is largely unknown. Methods: To evaluate the functional consequences of Gait 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 muM 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 (1=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. (C) 2002 Elsevier Science B.V. All rights reserved.