Enhanced G _i Signaling Selectively Negates β ₂ -Adrenergic Receptor (AR)– but Not β ₁ -AR–Mediated Positive Inotropic Effect in Myocytes From Failing Rat Hearts

Abstract

Background-Myocardial contractile response to ␤ 1 -and ␤ 2 -adrenergic receptor (AR) stimulation is severely impaired in chronic heart failure, in which G i signaling and the ratio of ␤ 2 /␤ 1 are often increased. Because ␤ 2 -AR but not ␤ 1 -AR couples to G s and G i with the G i coupling negating the G s -mediated contractile response, we determined whether the heart failure-associated augmentation of G i signaling contributes differentially to the defects of these ␤-AR subtypes and, if so, whether inhibition of G i or selective activation of ␤ 2 -AR/G s by ligands restores ␤ 2 -AR contractile response in the failing heart. Methods and Results-Cardiomyocytes were isolated from 18-to 24-month-old failing spontaneously hypertensive (SHR) or age-matched Wistar-Kyoto (WKY) rat hearts. In SHR cardiomyocytes, either ␤-AR subtype-mediated inotropic effect was markedly diminished, whereas G i proteins and the ␤ 2 /␤ 1 ratio were increased. Disruption of G i signaling by pertussis toxin (PTX) enabled ␤ 2 -but not ␤ 1 -AR to induce a full positive inotropic response in SHR myocytes. Furthermore, screening of a panel of ␤ 2 -AR ligands revealed that the contractile response mediated by most ␤ 2 -AR agonists, including zinterol, salbutamol, and procaterol, was potentiated by PTX, indicating concurrent G s and G i activation. In contrast, fenoterol, another ␤ 2 -AR agonist, induced a full positive inotropic effect in SHR myocytes even in the absence of PTX. Conclusions-We conclude that enhanced G i signaling is selectively involved in the dysfunction of ␤ 2 -but not ␤ 1 -AR in failing SHR hearts and that disruption of G i signaling by PTX or selective activation of ␤ 2 -AR/G s signaling by fenoterol restores the blunted ␤ 2 -AR contractile response in the failing heart. (Circulation. 2003;108:1633-1639.)

Epinephrine Activates Both Gsand GiPathways, but Norepinephrine Activates Only the GsPathway through Human β2-Adrenoceptors Overexpressed in Mouse Heart

“…Interestingly, epinephrine exhibits a ϳ200-fold greater potency for activating the G i pathway than activating the G s pathway at recombinant ␣ 2C(10) -adrenoceptors, whereas the imidazoline agonist oxymetazoline only stimulates the G i pathway (Eason et al, 1994). Furthermore, during submission of this work, an article supporting the concept of agonist-dependent selectivity for coupling of rat cardiac ␤ 2 -adrenoceptors was published (Xiao et al, 2003). These authors showed that the positive inotropic effects of the ␤ 2 -selective agonists salbutamol, zinterol, and procaterol, but not of fenoterol, are enhanced by PTX.…”

In vivo and in vitro assessment of cardiac β-adrenergic receptors in larval zebrafish (Danio rerio)

“…Also, the stimulation of 2ARs by isoproterenol causes an initial increase in contraction rate followed by a sustained decrease in murine cardiomyocytes (Devic et al, 2001;Wang et al, 2008), suggesting some cardioinhibitory role for the receptor. This is further supported by the observation that disruption of G i activity by pertussis toxin (PTX) enhances the 2AR-mediated contractile response of murine (for reviews, see Xiao, 2001;Xiao et al, 2003) and canine (Kuschel et al, 1999) cardiomyocytes. Indeed, while the 2AR-G i complex does not seem to directly inhibit global cAMP production, it does seem to affect downstream PKA activity and also the association of 2ARs with G s proteins.…”

“…Many factors including subcellular localization (e.g. caveolae) (Rybin et al, 2000), changes in conformation, dual coupling to G s and G i proteins, and agonist-mediated internalization of the receptor can all play a role in how 2AR affects different cell signalling pathways (for reviews, see Xiao et al, 2003;Zheng et al, 2004).…”

Cardiac Adrenoceptors: Physiological and Pathophysiological Relevance

“…Thus, in isolated ventricular cardiomyocytes from spontaneously hypertensive rats (SHR) with heart failure, the contractile response to several β 2 AR-agonists, including terbutaline, salbutamol, zinterol, and procaterol, could be markedly enhanced when the cardiomyocytes were pretreated with pertussis toxin (PTX), thereby inactivating the G iprotein. Interestingly, however, the contractile response to another β 2 AR-agonist, fenoterol, was not affected by PTX-treatment (11). Similarly, in rat ventricular cardiomyocytes, fenoterol fully inhibited phenylephrineevoked hypertrophic response [evoked by α 1A AR stimulation (12)], while terbutaline and salbutamol evoked only partial inhibition.…”