*2.1. Inhibition of Pump Activity of Na,K-ATPase by CTS*

Na,K-ATPase is a key player in osmotic equilibrium that transports Na<sup>+</sup> in exchange for K<sup>+</sup> by hydrolyzing ATP to establish an electrochemical gradient across the cell membrane. Electrochemical gradients can drive certain types of cotransporters or exchangers, such as the Na+/Ca2+ exchanger (NCX) [8]. In addition, ionic gradients are also indispensable for the regulation of membrane potential, electrical excitability, cell volume, renal reabsorption of ions, and nutrient transport [14]. Therefore, it has been reported that mutations of the sodium pump α1 are related to the development of secondary hypertension, endocrine syndrome, peripheral neuropathy, and neuromuscular disorders [15]. In addition, dysfunction of Na,K-ATPase substantially affects the osmotic equilibrium and several other essential functions that relate to the development of hypertension, cardiac hypertrophy, cataracts, diabetes, and other disorders [16–18].

Moreover, the extracellular domain of the sodium pump serves as a receptor for cardiac glycosides, such as ouabain, and binding of cardiac glycoside to the sodium pump induces inhibition of its enzymatic activity. In smooth muscle cells, astrocytes, and hippocampal neurons of rodents, CTSs such as ouabain inhibit α2- or α3-isoforms of Na,K-ATPase of the cell membrane that is in close proximity to the sarcoplasmic or endoplasmic reticulum (S/ER), and induces a transient elevation of sub-plasmalemma sodium concentration [10]. With the coupling of the Na,K-ATPase to that of NCX, the latter works in its reverse mode, which results in the local increase in cytosolic Ca2+ levels. This triggers Ca2+ release from the S/ER to the cytoplasm, which in turn initiates Ca2+-mediated intracellular signaling cascades [10]. The subtle elevation of cytosolic calcium may amplify the impact on intracellular Ca2+ stores, which induces considerable changes in the contractility of myocardial and smooth muscles [9]. The resultant arteriolar contraction is one of the mechanisms for essential hypertension [10,19].

Additionally, enhanced intracellular Ca2+ transients upon stimuli render vasomotor neurons and endothelial cells hyper-responsive. The synergistic action of neurotransmitter release by sympathetic neurons and elevated vascular reactivity increases arterial tone and peripheral vascular resistance, all of which are the traits of essential hypertension [19]. The involvement of the sodium pump in hypertension is corroborated by the fact that the inhibited expression of the smooth muscle-specific α2 subunit elevates basal blood pressure with enhanced sensitivity to angiotensin II (Ang II), whereas overexpression of this subunit reduces the basal BP and its sensitivity to Ang II [4]. In addition, abnormal elevation of sodium in the lens is related to the opacification of the lens cortex in age-related cataracts, and the alteration of Na,K-ATPase activity appears relevant to cataract formation in human and animal models [20].
