Abstract Body (Do not enter title and authors here): BACKGROUND: Cardiac diseases such as heart failure (HF) are characterized by irregular cardiac contraction and relaxation due to repeated damage to the myocardium. β-adrenergic receptors (βAR) on cardiomyocytes (CMs) are predominant regulators of cardiac contractility and hence β-blockers are utilized in the treatment of HF. Biasing βAR-mediated signaling in CMs presents an opportunity for developing alternative therapies. We have recently discovered a new allosteric modulators (AMs) that preferentially diminish βAR-Gαs signaling, not affecting β-arrestin mediated signaling. Considering the functional significance of βAR-mediated Gαs- and β-arrestin signaling in CMs, our objective is to investigate the impact of our recently identified β₂AR AMs on CM contractility and heart function. We hypothesize that Negative AMs (NAMs) of β₂AR will inhibit the βAR-Gαs signaling induced by β-agonists thereby decreasing cardiac contractility in vitro and in vivo.
METHODS: In isolated adult ventricular murine CMs (AVCM), we assessed the effect of NAM on isoproterenol-induced contractility and calcium transients using the IonOptix System. A subset of cells were pre-treated with β₁AR-antagonist. To examine the effect in the whole heart, we treated perfused hearts in Langendorff Apparatus with ISO (100nM) and vehicle/NAM. Finally, to assess effects of AMs on heart functions in vivo, we performed electrocardiogram (ECG) and echocardiogram in mice treated with ISO.
RESULTS: In AVCMs, ISO augmented CM contractility and calcium. Concomitant NAM treatment significantly attenuated ISO-induced cell contractility, elevation of calcium peak and decreased time-to-relaxation. Interestingly, pretreatment of CMs with β₁AR-antagonist reversed the inhibitory effect of NAM on ISO-mediated contractility. Our ex vivo perfused heart experiments demonstrated an inhibition of ISO-induced cardiac contractility by the β₂AR NAM. ECG studies demonstrated an attenuation of ISO-induced increase in parameters including heart rate and PVCs. Echocardiogram recording confirmed the decrease in heart rate and revealed alleviation of ISO-induced reduction in ejection fraction and fractional shortening and inhibition of ISO-induced isovolumic contraction time and myocardial performance index.
CONCLUSIONS: These data collectively demonstrate that our recently discovered biased NAM could preferentially ameliorate cardiotoxic Gαs signaling and function in CMs.