Cardiac‐specific overexpression of human β2 adrenoceptors in mice exposes coupling to both Gs and Gi proteins

Left atrial strips from transgenic (TG4) mice with cardiac‐specific overexpression (∼200‐fold) of the β 2 adrenoceptor ( β 2 AR) were isolated, and their isometric force of contraction ( F c ) in response to electrical stimulation was measured. The β AR agonist isoprenaline elicited negative inotropic responses in all left atrial strips; in 6/11 preparations, it also had a small positive inotropic effect. This ‘up‐phase’ was observed from 0.1 to 10 n M , with the ‘down‐phase’ occurring at higher concentrations. Both phases were mediated by β 2 AR, as shown by their sensitivity to the β 2 AR antagonist ICI‐118,551 (100 n M ; p A 2 8.60±0.07, 8.45±0.19, for ‘up‐phase’ and ‘down‐phase,’ respectively), but not the β 1 AR antagonist CGP‐20712A (100 n M ). Conversely, nontransgenic littermate preparations responded to isoprenaline treatment solely by an increase in F c , which was β 1 AR‐mediated. Pretreatment of left atrial strips with either 10 n M isoprenaline or 1 m M 8‐bromo‐cAMP significantly attenuated the TG4 ‘up‐phase’, while having no effect on either the TG4 ‘down‐phase’ or the littermate controls' responses. B. pertussis toxin treatment of the animals prevented isoprenaline's negative inotropic effects in TG4 preparations, but had no effect in littermate controls. The findings imply that the responses of TG4 left atrium to isoprenaline are because of β 2 AR coupling to G s and G i proteins, consistent with the model of Daaka et al ., in which protein kinase A phosphorylation of the β 2 AR causes a switch from G s to G i protein coupling. British Journal of Pharmacology (2003) 138 , 1358–1366. doi: 10.1038/sj.bjp.0705191