Non-classical regulation of β1- and β2-adrenoceptor-mediated inotropic responses in rat heart ventricle by the G protein Gi

Studies suggest that increased activity of G(i) contributes to the reduced beta-adrenoceptor-mediated inotropic response (beta AR-IR) in failing cardiomyocytes and that beta(2)AR-IR but not beta(1)AR-IR is blunted by dual coupling to G(s) and G(i). We aimed to clarify the role of G(i) upon the beta(1)AR-IR and beta(2)AR-IR in Sham and failing myocardium by directly measuring contractile force and cAMP accumulation. Contractility was measured ex vivo in left ventricular strips and cAMP accumulation in cardiomyocytes from rats with post-infarction heart failure (HF) or sham operates (Sham). The beta(2)AR-IR in Sham and HF was small and was amplified by simultaneously inhibiting phosphodiesterases 3 and 4 (PDE3&4). In HF, the inotropic response and cAMP accumulation evoked by beta(1)AR- or beta(2)AR-stimulation were reduced. Inactivation of G(i) with pertussis toxin (PTX) did not restore the beta(1)AR-IR or beta(2)AR-IR in HF to Sham levels but did enhance the maximal beta(2)AR-IR. PTX increased both beta(1)AR- and beta(2)AR-evoked cAMP accumulation more in Sham than that in HF, and HF levels approached those in untreated Sham. The potency of agonists at beta(1) and at beta(2)ARs (only under PDE3&4 inhibition) was increased in HF and by PTX in both HF and Sham. Without PDE3&4 inhibition, PTX increased only the maximal beta(2)AR-IR, not potency. We conclude that G(i) regulates both beta(1)AR- and beta(2)AR-IR independent of receptor coupling with G(i). G(i) together with PDE3&4 tonically restrict the beta(2)AR-IR. G(i) inhibition did not restore the beta AR-IR in HF despite increasing cAMP levels, suggesting that the mechanism of impairment resides downstream to cAMP signalling.