Na,K-ATPase also known as the sodium pump, is an oligomeric membrane bound enzyme that catalyzes ATP-dependent transport of three Na
+
ions out and two K
+
ions into the cell. It plays an important role in cardiac function by influencing the intracellular Ca
2+
levels via the Na
+
/Ca
2+
exchanger and thereby cardiac contractility. Na,K-ATPase is composed of two essential subunits, alpha-subunit and beta-subunit, both of which have multiple isoforms. The alpha subunit, which is also the catalytic subunit, is the receptor for cardiac glycosides such as digitalis, used for the treatment of congestive heart failure. Evidence indicates that Na,K-ATPase alpha1, alpha2 and beta1 isoforms as well as the Na,K-ATPase enzymatic activity is reduced in the failing human heart. Heterozygous knockout mice of the alpha-subunit isoforms had altered cardiac contractility. However, nothing much is known about the role of the beta-subunit in cardiac function. To investigate the role of beta-subunit in the myocardium, we used Cre/loxP technology to inactivate the beta-subunit gene exclusively in ventricular cardiac myocytes. Animals with homozygous ventricular myocyte beta-subunit gene excision were born at the expected Mendelian ratio, grew into adulthood, and appeared to be healthy until 9 months of age. These animals had ~20% and ~50% of control levels of beta-subunit and alpha-subunit protein in the heart respectively. At 13–14 months of age, these mice had 24% higher heart/body weight ratios, elevated levels of markers of cardiac hypertrophy such as ANP, BNP, alpha MHC, alpha skeletal actin, and reduced contractility by echocardiography (56.7% versus 66.4% EF in the WT, n = 7 each) as compared to their WT littermates. Pressure overload by transverse aortic constriction in younger mice, resulted in compensated hypertrophy in WT mice, but 50% of Na,K-beta knockout mice died soon after TAC. The few survivors exhibited decreased contractile function (50% as compared to 71% EF in WT, n = 4 each) and mimicked the effects of Na,K-beta knockout in old mice. In conclusion, our results suggest that loss of beta-subunit leads to significant pathophysiology from altered ionic homeostasis in the heart.
Role of a pituitary-specific transcription factor (pit-1/GHF-1) or a closely related protein in cAMP regulation of human thyrotropin-beta subunit gene expression.
