Cardiac Oxidative Signaling And Physiological Hypertrophy In The Na/K-Atpase Alpha 1(S/S)Alpha 2(S/S) Mouse Model Of High Affinity For Cardiotonic Steroids

The Na/K-ATPase is the specific receptor for cardiotonic steroids (CTS) such as ouabain and digoxin. At pharmacological concentrations used in the treatment of cardiac conditions, CTS inhibit the ion-pumping function of Na/K-ATPase. At much lower concentrations, in the range of those reported for endogenous CTS in the blood, they stimulate hypertrophic growth of cultured cardiac myocytes through initiation of a Na/K-ATPase-mediated and reactive oxygen species (ROS)-dependent signaling. To examine a possible effect of endogenous concentrations of CTS on cardiac structure and function in vivo, we compared mice expressing the naturally resistant Na/K-ATPase alpha 1 and age-matched mice genetically engineered to express a mutated Na/K-ATPase alpha 1 with high affinity for CTS. In this model, total cardiac Na/K-ATPase activity, alpha 1, alpha 2, and beta 1 protein content remained unchanged, and the cardiac Na/K-ATPase dose-response curve to ouabain shifted to the left as expected. In males aged 3-6 months, increased alpha 1 sensitivity to CTS resulted in a significant increase in cardiac carbonylated protein content, suggesting that ROS production was elevated. A moderate but significant increase of about 15% of the heart-weight-to-tibia-length ratio accompanied by an increase in the myocyte cross-sectional area was detected. Echocardiographic analyses did not reveal any change in cardiac function, and there was no fibrosis or re-expression of the fetal gene program. RNA sequencing analysis indicated that pathways related to energy metabolism were upregulated, while those related to extracellular matrix organization were downregulated. Consistent with a functional role of the latter, an angiotensin-II challenge that triggered fibrosis in the alpha 1(r/r)alpha 2(s/s) mouse failed to do so in the alpha 1(s/s)alpha 2(s/s). Taken together, these results are indicative of a link between circulating CTS, Na/K-ATPase alpha 1, ROS, and physiological cardiac hypertrophy in mice under baseline laboratory conditions.