Estrogen deficiency compromised the β₂AR-Gs/Gi coupling: implications for arrhythmia and cardiac injury

Estrogen and β₂-adrenergic receptors (β₂AR) play important roles in the processes that protect the heart. Here, we investigated how ovariectomy influenced the β₂AR downstream pathways in the context of catecholaminergic stress. In vivo and in vitro stress models were developed in female Sprague-Dawley (SD) rats by epinephrine (Epi) treatments. The cardiac function was evaluated at in vivo and in vitro levels in terms of contraction, rhythm, and injury. We found that myocardial contractility was not significantly different between Sham and ovariectomized (OVX) group rats in the normal state. However, Epi pretreatment decreased the contractility and increased abnormal rhythms especially in OVX group, which were attributed to lack of estrogen. Inhibition of the β₂AR-Gi-PI3K/p38MAPK pathway with ICI118,551, PTX or LY294002 increased contractility and aggravated Epi-induced injury on cardiomyocytes, decreased p38MAPK phosphorylation, and only increased arrhythmia in Sham group. These results indicated that OVX exacerbated cardiac injury and abnormal rhythms through β₂AR-Gi-PI3K and β₂AR-Gi-p38MAPK pathways, respectively. In normal state, the levels of activated Gi were similar in both groups, but those of cAMP and activated Gs were higher in OVX group. Epi treatment increased activated Gi (especially in Sham group) and activated Gs and cAMP in Sham group but decreased it in OVX group. These results suggested that estrogen increased the Gi activity in normal and stress states and Gs activity in stress state. These results indicated that lack of estrogen impaired the β₂AR-Gs/Gi coupling during stress which compromised cardiac contractility and increased abnormal rhythms.