Constitutive inhibitory G protein activity upon adenylyl cyclase-dependent cardiac contractility is limited to adenylyl cyclase type 6

PURPOSE

We previously reported that inhibitory G protein (Gi) exerts intrinsic receptor-independent inhibitory activity upon adenylyl cyclase (AC) that regulates contractile force in rat ventricle. The two major subtypes of AC in the heart are AC5 and AC6. The aim of this study was to determine if this intrinsic Gi inhibition regulating contractile force is AC subtype selective.

METHODS

Wild-type (WT), AC5 knockout (AC5KO) and AC6 knockout (AC6KO) mice were injected with pertussis toxin (PTX) to inactivate Gi or saline (control).Three days after injection, we evaluated the effect of simultaneous inhibition of phosphodiesterases (PDE) 3 and 4 with cilostamide and rolipram respectively upon in vivo and ex vivo left ventricular (LV) contractile function. Also, changes in the level of cAMP were measured in left ventricular homogenates and at the membrane surface in cardiomyocytes obtained from the same mouse strains expressing the cAMP sensor pmEPAC1 using fluorescence resonance energy transfer (FRET).

RESULTS

Simultaneous PDE3 and PDE4 inhibition increased in vivo and ex vivo rate of LV contractility only in PTX-treated WT and AC5KO mice but not in saline-treated controls. Likewise, Simultaneous PDE3 and PDE4 inhibition elevated total cAMP levels in PTX-treated WT and AC5KO mice compared to saline-treated controls. In contrast, simultaneous PDE3 and PDE4 inhibition did not increase in vivo or ex vivo rate of LV contractility or cAMP levels in PTX-treated AC6KO mice compared to saline-treated controls. Using FRET analysis, an increase of cAMP level was detected at the membrane of cardiomyocytes after simultaneous PDE3 and PDE4 inhibition in WT and AC5KO but not AC6KO. These FRET data are consistent with the functional data indicating that AC6 activity and PTX inhibition of Gi is necessary for simultaneous inhibition of PDE3 and PDE4 to elicit an increase in contractility.

CONCLUSIONS

Together, these data suggest that AC6 is tightly regulated by intrinsic receptor-independent Gi activity, thus providing a mechanism for maintaining low basal cAMP levels in the functional compartment that regulates contractility.

Non-classical regulation of β1- and β 2-adrenoceptor-mediated inotropic responses in rat heart ventricle by the G protein Gi

Studies suggest that increased activity of Gi contributes to the reduced β-adrenoceptor-mediated inotropic response (βAR-IR) in failing cardiomyocytes and that β2AR-IR but not β1AR-IR is blunted by dual coupling to Gs and Gi. We aimed to clarify the role of Gi upon the β1AR-IR and β2AR-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 β2AR-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 β1AR- or β2AR-stimulation were reduced. Inactivation of Gi with pertussis toxin (PTX) did not restore the β1AR-IR or β2AR-IR in HF to Sham levels but did enhance the maximal β2AR-IR. PTX increased both β1AR- and β2AR-evoked cAMP accumulation more in Sham than that in HF, and HF levels approached those in untreated Sham. The potency of agonists at β1 and at β2ARs (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 β2AR-IR, not potency. We conclude that Gi regulates both β1AR- and β2AR-IR independent of receptor coupling with Gi. Gi together with PDE3&4 tonically restrict the β2AR-IR. Gi inhibition did not restore the βAR-IR in HF despite increasing cAMP levels, suggesting that the mechanism of impairment resides downstream to cAMP signalling.

Gi proteins regulate adenylyl cyclase activity independent of receptor activation

Background and purpose

Despite the view that only β₂- as opposed to β₁-adrenoceptors (βARs) couple to G_i, some data indicate that the β₁AR-evoked inotropic response is also influenced by the inhibition of G_i. Therefore, we wanted to determine if G_i exerts tonic receptor-independent inhibition upon basal adenylyl cyclase (AC) activity in cardiomyocytes.

Experimental approach

We used the G_s-selective (R,R)- and the G_s- and G_i-activating (R,S)-fenoterol to selectively activate β₂ARs (β₁AR blockade present) in combination with G_i inactivation with pertussis toxin (PTX). We also determined the effect of PTX upon basal and forskolin-mediated responses. Contractility was measured ex vivo in left ventricular strips and cAMP accumulation was measured in isolated ventricular cardiomyocytes from adult Wistar rats.

Key results

PTX amplified both the (R,R)- and (R,S)-fenoterol-evoked maximal inotropic response and concentration-dependent increases in cAMP accumulation. The EC₅₀ values of fenoterol matched published binding affinities. The PTX enhancement of the G_s-selective (R,R)-fenoterol-mediated responses suggests that G_i regulates AC activity independent of receptor coupling to G_i protein. Consistent with this hypothesis, forskolin-evoked cAMP accumulation was increased and inotropic responses to forskolin were potentiated by PTX treatment. In non-PTX-treated tissue, phosphodiesterase (PDE) 3 and 4 inhibition or removal of either constitutive muscarinic receptor activation of G_i with atropine or removal of constitutive adenosine receptor activation with CGS 15943 had no effect upon contractility. However, in PTX-treated tissue, PDE3 and 4 inhibition alone increased basal levels of cAMP and accordingly evoked a large inotropic response.

Conclusions and implications

Together, these data indicate that G_i exerts intrinsic receptor-independent inhibitory activity upon AC. We propose that PTX treatment shifts the balance of intrinsic G_i and G_s activity upon AC towards G_s, enhancing the effect of all cAMP-mediated inotropic agents.