**10. Endocytosis of Na/K-ATPase and NHE3 in salt sensitivity**

In male Sprague-Dawley rats, compared to a normal salt (0.4% NaCl, 7 days) diet, a high-salt (4% NaCl, 7 days) diet increased urinary sodium and MBG excretion. In isolated proximal tubules, a high-salt diet inhibits the Na/K-ATPase ionexchange activity and enzymatic activity, which is accompanied by a decreased Na/K-ATPase α1 content in heavy membrane fraction and an increased Na/K-ATPase α1 content in both early and late endosomes. These high-salt diet-mediated changes were ameliorated by administration of an antibody against MBG [156]. Results indicate that a high-salt diet increased MBG production, activated RPT Na/K-ATPase signaling, and induced endocytosis of Na/K-ATPase.

The Dahl R and S rat strains were developed from Sprague-Dawley rats by selective breeding, depending on the resistance or susceptibility to the hypertensive effects of high dietary sodium [247]. In these two strains, the RPT sodium handling is an essential determinant of their different BP responses [248–251]. At the cost of elevated systolic BP, Dahl S rats get rid of excess sodium primarily via pressure natriuresis. In contrast, Dahl R rats get rid of excess sodium primarily via a significant reduction of renal sodium reabsorption without increasing the BP. *In* vivo study indicates that impaired RPT Na/K-ATPase signaling appears to be causative of experimental Dahl salt sensitivity [79]. In vivo studies with Dahl R and S rats (Jr strains) demonstrated that impairment of RPT Na/K-ATPase signaling is a causative factor of experimental Dahl salt sensitivity [79]. In Dahl R but not S rats, a high-salt (2% NaCl, 1 week) diet activated RPT Na/K-ATPase signaling and stimulated coordinated redistribution of the Na/K-ATPase and NHE3, leading to increased total and fractional urinary sodium excretion as well as normal BP. However, there are still questions about the underlying mechanism(s) that need to be further investigated, such as the difference of Na/K-ATPase signaling function between Dahl R and S rats, as well as the translation of Na/K-ATPase signaling to NHE3 regulation. Furthermore, low concentration of ouabain causes hypertrophic response both in the heart and kidney, by concentrating the Na/K-ATPase, Src, EGFR, and MAPKs within rat caveolae, and activates the Na/K-ATPase/Src/MAPK signaling pathway [64]. However, there is no simple explanation for this occurrence. First, the α1 subunit is essentially the only α isoform expressed in RPT, and genes coding α1 subunit and NHE3 (in rat chromosomes 1 and 2, respectively) are not located in identified and/or proposed BP quantitative trait loci [252]. Second, there is no difference in α1 gene (*Atp1a1*) coding [251], α1 ouabain sensitivity [253], and α1 expression [79] between these two strains. Third, acute salt loading increases circulating CTS (ouabain and MBG) in both S and R rats [162]. These observations suggest that there must be resistance to CTS signaling in the Dahl S rat, a phenomenon that we only partially understand. As discussed above, the carbonylation/decarbonylation process could be another new regulatory mechanism of Na/K-ATPase signaling. It is reasonable to postulate that prolonged excessive α1-carbonylation in Dahl saltsensitive rats might overcome the decarbonylation capacity, leading to desensitization or termination of the Na/K-ATPase signaling function.

#### **11. Perspective**

As pointed out by Guyton many years ago [254], the kidney is the most important organ in the regulation of Na+ handling and BP. Dietary salt intake *vs.* renal sodium handling is a key determinant of long-term BP regulation and plays an important role in the pathogenesis of hypertension, with more pronounced effects seen in salt-sensitive patients. Consequently, modest restriction of dietary salt and

#### *The Na/K-ATPase Signaling Regulates Natriuresis in Renal Proximal Tubule DOI: http://dx.doi.org/10.5772/intechopen.92968*

diuretic therapy are often recommended for the treatment of resistant hypertension, particularly with the salt-sensitive subgroup [254–258].

Although the relationships among CTS, renal Na<sup>+</sup> handling, and hypertension were proposed many years ago, there has been an explosion of reports supporting this idea. As discussed, reports from Lingrel's laboratory clearly demonstrated a specific role of the isoforms of the Na/K-ATPase and its interaction with endogenous CTS in the regulation of Na+ excretion and BP in intact animals [75–77]. From the ligand perspective, studies have demonstrated that CTS are present in measurable amounts under normal physiological conditions and that several disease states are associated with elevations in the circulating levels of CTS. The new concept that the Na/K-ATPase has an ion-pumping-independent receptor function (induced by both CTS and ROS) that can confer the agonist-like effects of CTS on intracellular signal transduction is a new mechanism for RPT sodium handling. Moreover, this newly discovered signaling mechanism operates in intact animals in response to CTS stimulation. The Na/K-ATPase has recently emerged as a therapeutic target [259, 260]. A clearer understanding of the mechanisms, in which a CTS-ROS-Na/K-ATPase signaling axis counterbalancing salt retention, would not only have major pathophysiological and therapeutic implications, but also further explain the progressive impairment of renal sodium handling under excessive oxidative stresses such as hypertension, aging, obesity, and diabetes.
