Knockout of adenylyl cyclase isoform 5 or 6 differentially modifies the β1-adrenoceptor-mediated inotropic response

Although only beta(2)-adrenergic receptors (beta AR) dually couple with stimulatory G protein (G(s)) and inhibitory G protein (G(i)), inactivation of G(i) enhances both beta 1AR and beta(2)AR responsiveness. We hypothesize that G(i) restrains spontaneous adenylyl cyclase (AC) activity independent of receptor activation. Subcellular localization of the AC5/6 subtypes varies contributing to the compartmentation of beta AR signaling. The primary objectives were to determine: (1) if beta(1)AR-mediated inotropic responses were dependent upon either AC5 or AC6; (2) if intrinsic G(i) inhibition is AC subtype selective and (3) the role of phosphodiesterases (PDE) 3/4 to regulate beta(1)AR responsiveness. beta(1)AR-mediated increases in contractile force and cAMP accumulation in cardiomyocytes were measured from wild type, AC5 and AC6 knockout (KO) mice, with or without pertussis toxin (PTX) pretreatment to inactivate G(i) and/or after selective inhibition of PDEs 3/4. Noradrenaline potency at beta(1)ARs was increased in AC6 KO. PDE4 inhibition increased noradrenaline potency in wild type and AC5 KO, but not AC6 KO. PTX increased noradrenaline potency only in wild type but increased the maximal beta(1)AR response in all mouse strains. PDE3 inhibition increased noradrenaline potency only in AC5 KO that was treated prior with PTX. beta(1)AR-evoked cAMP accumulation was increased more by PDE4 inhibition than PDE3 inhibition in wild type and AC5 KO that was amplified by G(i) inhibition. These data indicate that beta(1)AR-mediated inotropic responses are not dependent upon either AC5 or AC6 alone. Inactivation of G(i) enhanced beta(1)AR-mediated inotropic responses despite not coupling to G(i), consistent with G i exerting a tonic receptor independent inhibition upon AC5/6. PDE4 seems the primary regulator of beta(1)AR signaling through AC6 in wild type. AC6 KO results in a reorganization of beta(1)AR compartmentation characterized by signaling through AC5 regulated by G(i), PDE3 and PDE4 that maintains normal contractile function.