ABSTRACT
1. Left atrial strips from transgenic (TG4) mice with cardiac-specific overexpression ( approximately 200-fold) of the beta(2) adrenoceptor (beta(2)AR) were isolated, and their isometric force of contraction (F(c)) in response to electrical stimulation was measured. 2. The betaAR 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 nM, with the 'down-phase' occurring at higher concentrations. Both phases were mediated by beta(2)AR, as shown by their sensitivity to the beta(2)AR antagonist ICI-118,551 (100 nM; pA(2) 8.60+/-0.07, 8.45+/-0.19, for 'up-phase' and 'down-phase,' respectively), but not the beta(1)AR antagonist CGP-20712A (100 nM). Conversely, nontransgenic littermate preparations responded to isoprenaline treatment solely by an increase in F(c), which was beta(1)AR-mediated. 3. Pretreatment of left atrial strips with either 10 nM isoprenaline or 1 mM 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. 4. The findings imply that the responses of TG4 left atrium to isoprenaline are because of beta(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 beta(2)AR causes a switch from G(s) to G(i) protein coupling.
