The heart has two major modalities of hypertrophy in response to hemodynamic loads: concentric and eccentric hypertrophy caused by pressure and volume overload (VO), respectively. However, the molecular mechanism of eccentric hypertrophy remains poorly understood. Here we demonstrate that the Akt-mammalian target of rapamycin (mTOR) axis is a pivotal regulator of eccentric hypertrophy during VO. While mTOR in the heart was activated in a left ventricular end-diastolic pressure (LVEDP)-dependent manner, mTOR inhibition suppressed eccentric hypertrophy and induced cardiac atrophy even under VO. Notably, Akt was ubiquitinated and phosphorylated in response to VO, and blocking the recruitment of Akt to the membrane completely abolished mTOR activation. Various growth factors were upregulated during VO, suggesting that these might be involved in Akt-mTOR activation. Furthermore, the rate of eccentric hypertrophy progression was proportional to mTOR activity, which allowed accurate estimation of eccentric hypertrophy by time-integration of mTOR activity. These results suggested that the Akt-mTOR axis plays a pivotal role in eccentric hypertrophy, and mTOR activity quantitatively determines the rate of eccentric hypertrophy progression. As eccentric hypertrophy is an inherent system of the heart for regulating cardiac output and LVEDP, our findings provide a new mechanistic insight into the adaptive mechanism of the heart.The heart is a vital organ that maintains homeostasis in the body via blood circulation. To maintain blood circulation in the peripheral tissues, the heart is capable of remodeling in response to various stresses including hemodynamic load, neurohormones, oxidative stress, and cytokines [1][2][3] . Among those, mechanical loads are major inputs for the heart because the heart is incessantly subject to hemodynamic stresses. Given that mechanical load induces hypertrophy, mechanical stretching forces in systole and diastole in myocytes in vivo can be calculated as wall stresses in systole and diastole in accordance with Laplace's law. In 1975, Grossman et al. demonstrated that pressure overload (PO) increases systolic wall stress, resulting in concentric hypertrophy, which in turn normalizes systolic wall stress, and that volume overload (VO) increases diastolic wall stress, resulting in eccentric hypertrophy 1 . Based on this clinical observation, they proposed that the hypertrophic response was evoked by increased wall stress 1 . Currently, it is widely accepted that increased systolic and diastolic wall stresses lead to concentric and eccentric hypertrophy, respectively 4 . Many lines of evidence indicate that concentric and eccentric hypertrophy differ not only in terms of phenotype but also in the intracellular signaling pathways that are involved 5,6 . Various studies investigated the molecular mechanism of hypertrophy, especially in concentric hypertrophy caused by PO 7,8 ; however, the molecular mechanism of eccentric hypertrophy has yet to be fully elucidated.Mitral and aortic regurgitation are...