Some studies show that a high concentration of catecholamines is the triggering factor of Takotsubo cardiomyopathy (TCM), particularly epinephrine (Epi) [1,2]. A high concentration of Epi switches G protein from Gs to Gi protein signal pathway, increasing negative inotropic effect [3,4]. The patients of TCM have a clear gender predilection, most of whom are primarily post-menopausal women [5]. It is unknown whether low estrogen level following menopause is associated with G protein or not in TCM. Female Sprague-Dawley rats were ovariectomized (OVX). Six weeks later, ventricular cardiomyocytes were isolated. We found that the shortening amplitude of cardiomyocytes increased in the OVX group, compared with the Sham group (Sham vs. OVX, p<0.05). The decrease of shortening amplitude in cardiomyocytes of Sham group was eliminated by estrogen antagonist (Sham+ ICI182 vs. OVX, p>0.05). Further, we isolated Sham and OVX rat ventricular cardiomyocytes and subjected to Epi for making TCM model in vitro to explore the relationship between estrogen and G protein in TCM. We found that the percentage of rod-shaped cells and shortening amplitudes of the above cardiomyocytes were all decreased, compared without Epi group (Sham vs. Sham + Epi, OVX vs. OVX + Epi, p<0.05), more seriously in cardiomyocytes from OVX group (Sham + Epi vs. OVX + Epi, P<0.05). But, pertussis toxin, the antagonist of Gi protein, abolished all the differences of the above groups. We also measured the concentration of cAMP, a substance of GS downstream signaling. We found that the concentration of cAMP was increased in the Sham group and decreased in the OVX group, if the cardiomyocytes were pretreated with Epi (Sham vs. Sham + Epi, OVX vs. OVX + Epi, p<0.05). And the concentration of cAMP in Sham group was lower than that in OVX group without Epi state (Sham vs. OVX, p<0.05). Our results indicated that the estrogen activated Gi protein signal pathway, producing negative inotropic effect at physiological state. And the estrogen activated Gs protein signal pathway rapidly when the concentration of Epi in plasma increased sharply, which limited the negative inotropic effect of Gi protein. Our results suggested that estrogen reduced the occurrence of TCM via balancing Gs /Gi protein signal pathway. In low estrogen levels, such as postmenopausal women, high concentration of Epi doesn’t activate the Gs protein signal pathway effectively and the Gi protein signal pathway is strengthened. This may be the reason why TCM usually occurs in postmenopausal women.