Dietary potassium deficiency causes stimulation of sodium reabsorption leading to an increased risk in blood pressure elevation. The distal convoluted tubule (DCT) is the main rheostat linking plasma K+ levels to the activity of the Na-Cl cotransporter (NCC). This occurs through basolateral membrane potential sensing by inwardly rectifying K+ channels (Kir4.1/5.1); decrease in intracellular Cl-; activation of WNK4 and interaction and phosphorylation of STE20/SPS1-related proline/alanine-rich kinase (SPAK); binding of calcium-binding protein 39 (cab39) adaptor protein to SPAK, leading to its trafficking to the apical membrane; and SPAK binding, phosphorylation, and activation of NCC. As kidney-specific with-no-lysine kinase 1 (WNK1) isoform (KS-WNK1) is another participant in this pathway, we examined its function in NCC regulation. We eliminated KS-WNK1 specifically in the DCT and demonstrated increased expression of WNK4 and long WNK1 (L-WNK1) and increased phosphorylation of NCC. As in other KS-WNK1 models, the mice were not hyperkalemic. Although wild-type mice under low-dietary K+ conditions demonstrated increased NCC phosphorylation, the phosphorylation levels of the transporter, already high in KS-WNK1, did not change under the low-K+ diet. Thus, in the absence of KS-WNK1, the transporter lost its sensitivity to low plasma K+. We also show that under low K+ conditions, in the absence of KS-WNK1, there was no formation of WNK bodies. These bodies were observed in adjacent segments, not affected by the targeting of KS-WNK1. As our data are overall consistent with those of the global KS-WNK1 knockout, they indicate that the DCT is the predominant segment affecting the salt transport regulated by KS-WNK1.NEW & NOTEWORTHY In this paper, we show that KS-WNK1 is a critical component of the distal convoluted tubule (DCT) K+ switch pathway. Its deletion results in an inability of the DCT to sense changes in plasma potassium. Absence of KS-WNK1 leads to abnormally high levels of WNK4 and L-WNK1 in the DCT, resulting in increased Na-Cl phosphorylation and function. Our data are consistent with KS-WNK1 targeting WNK4 and L-WNK1 to degradation.

Excessive salt intake can induce a variety of diseases, such as hypertension, cardiovascular disease, kidney disease and so on，it is also one of the factors promoting bone resorption. The mechanism of osteoporosis-induced exacerbations of high salt diet is not well-defined. In this study, we used ovariectomized 6-month-old Sprague Dawley rats to construct a high bone turnover model, and then administrated with high sodium chloride diet (2.0% w/w NaCl, 8.0% w/w NaCl) for 12 weeks to observe the effect of high salt diet on bone metabolism. The results showed that high salt diet could lead to the destruction of bone microstructure, promote the excretion of urinary calcium and phosphorus and accelerate the bone turnover, as well as cause the pathologic structural abnormalities in renal tubular. At the same time, it was accompanied by the up-regulated expression of the epithelial sodium channel (ENaCα), voltage-gated chloride channels (ClC)- 3 and the down-regulated expression of Na-Cl cotransporter (NCC), sodium calcium exchanger (NCX1) in femoral tissue and renal tubules. These findings confirm that high salt diet can destroy the microstructure of bone by increasing bone resorption and affect some ion channels of bone tissue and renal tubule in ovariectomized rats.

The inwardly rectifying potassium channel (Kir) 4.1 (encoded by KCNJ10) interacts with Kir5.1 (encoded by KCNJ16) to form a major basolateral K+ channel in the renal distal convoluted tubule (DCT), connecting tubule (CNT), and the cortical collecting duct (CCD). Kir4.1/Kir5.1 heterotetramer plays an important role in regulating Na+ and K+ transport in the DCT, CNT, and CCD. A recent development in the field has firmly established the role of Kir4.1/Kir5.1 heterotetramer of the DCT in the regulation of thiazide-sensitive Na-Cl cotransporter (NCC). Changes in Kir4.1/Kir5.1 activity of the DCT are an essential step for the regulation of NCC expression/activity induced by dietary K+ and Na+ intakes and play a role in modulating NCC by type 2 angiotensin II receptor (AT2R), bradykinin type II receptor (BK2R), and β-adrenergic receptor. Since NCC activity determines the Na+ delivery rate to the aldosterone-sensitive distal nephron (ASDN), a distal nephron segment from late DCT to CCD, Kir4.1/Kir5.1 activity plays a critical role not only in the regulation of renal Na+ absorption but also in modulating renal K+ excretion and maintaining K+ homeostasis. Thus, Kir4.1/Kir5.1 activity serves as an important component of renal K+ sensing mechanism. The main focus of this review is to provide an overview regarding the role of Kir4.1 and Kir5.1 of the DCT and CCD in the regulation of renal K+ excretion and Na+ absorption.

Sodium (Na+) electrochemical gradients established by Na+/K+ ATPase activity drives the transport of ions, minerals, and sugars in both excitable and non-excitable cells. Na+-dependent transporters can move these solutes in the same direction (cotransport) or in opposite directions (exchanger) across both the apical and basolateral plasma membranes of polarized epithelia. In addition to maintaining physiological homeostasis of these solutes, increases and decreases in sodium may also initiate, directly or indirectly, signaling cascades that regulate a variety of intracellular post-translational events. In this review, we will describe how the Na+/K+ ATPase maintains a Na+ gradient utilized by multiple sodium-dependent transport mechanisms to regulate glucose uptake, excitatory neurotransmitters, calcium signaling, acid-base balance, salt-wasting disorders, fluid volume, and magnesium transport. We will discuss how several Na+-dependent cotransporters and Na+-dependent exchangers have significant roles in human health and disease. Finally, we will discuss how each of these Na+-dependent transport mechanisms have either been shown or have the potential to use Na+ in a secondary role as a signaling molecule.

Reduced renal Na+ reabsorption along with restricted dietary Na+ depletes intravascular plasma volume which can then result in hypotension. Whether hypotension occurs and the magnitude of hypotension depends in part on compensatory angiotensin II-mediated increased vascular resistance. We investigated whether the ability of vascular resistance to mitigate the hypotension was compromised by decreased contractile reactivity. In vitro reactivity was investigated in aorta from mouse models of reduced renal Na+ reabsorption and restricted dietary Na+ associated with considerable hypotension and renin-angiotensin system activation: (1) the Na+-Cl--Co-transporter (NCC) knockout (KO) with Na+ restricted diet (0.1%, 2 weeks) and (2) the relatively more severe pendrin (apical chloride/bicarbonate exchanger) and NCC double KO. Contractile sensitivity to KCl, phenylephrine, and/or U46619 remained unaltered in aorta from both models. Maximal KCl and phenylephrine contraction expressed as force/aorta length from NCC KO with Na+-restricted diet remained unaltered, while in pendrin/NCC double KO were reduced to 49 and 64%, respectively. Wet weight of aorta from NCC KO with Na+-restricted diet remained unaltered, while pendrin/NCC double KO was reduced to 67%, consistent with decreased medial width determined with Verhoeff-Van Gieson stain. These findings suggest that hypotension associated with severe intravascular volume depletion, as the result of decreased renal Na+ reabsorption, may in part be due to decreased contractile reactivity as a consequence of reduced vascular hypertrophy.

Epithelial K+ channels are essential for maintaining electrolyte and fluid homeostasis in the kidney. It is recognized that basolateral inward-rectifying K+ (Kir ) channels play an important role in the control of resting membrane potential and transepithelial voltage, thereby modulating water and electrolyte transport in the distal part of nephron and collecting duct. Monomeric Kir 4.1 (encoded by Kcnj10 gene) and heteromeric Kir 4.1/Kir 5.1 (Kir 4.1 together with Kir 5.1 (Kcnj16)) channels are abundantly expressed at the basolateral membranes of the distal convoluted tubule and the cortical collecting duct cells. Loss-of-function mutations in KCNJ10 cause EAST/SeSAME tubulopathy in humans associated with salt wasting, hypomagnesaemia, metabolic alkalosis and hypokalaemia. In contrast, mice lacking Kir 5.1 have severe renal phenotype that, apart from hypokalaemia, is the opposite of the phenotype seen in EAST/SeSAME syndrome. Experimental advances using genetic animal models provided critical insights into the physiological role of these channels in electrolyte homeostasis and the control of kidney function. Here, we discuss current knowledge about K+ channels at the basolateral membrane of the distal tubules with specific focus on the homomeric Kir 4.1 and heteromeric Kir 4.1/Kir 5.1 channels. Recently identified molecular mechanisms regulating expression and activity of these channels, such as cell acidification, dopamine, insulin and insulin-like growth factor-1, Src family protein tyrosine kinases, as well as the role of these channels in NCC-mediated transport in the distal convoluted tubules, are also described.

Mutations in WNK1 and WNK4 kinase genes have been shown to cause a human hereditary hypertensive disease, pseudohypoaldosteronism type II (PHAII). We previously discovered that WNK kinases phosphorylate and activate OSR1/SPAK kinases that regulate renal SLC12A family transporters such as NKCC2 and NCC, and clarified that the constitutive activation of this cascade causes PHAII. WNK3, another member of the WNK kinase family, was reported to be a strong activator of NCC/NKCC2 when assayed in Xenopus oocytes, suggesting that WNK3 also plays a major role in regulating blood pressure and sodium reabsorption in the kidney. However, it remains to be determined whether WNK3 is in fact involved in the regulation of these transporters in vivo. To clarify this issue, we generated and analyzed WNK3 knockout mice. Surprisingly, phosphorylation and expression of OSR1, SPAK, NKCC2 and NCC did not decrease in knockout mouse kidney under normal and low-salt diets. Similarly, expression of epithelial Na channel and Na/H exchanger 3 were not affected in knockout mice. Na(+) and K(+) excretion in urine in WNK3 knockout mice was not affected under different salt diets. Blood pressure in WNK3 knockout mice was not lower under normal diet. However, lower blood pressure was observed in WNK3 knockout mice fed low-salt diet. WNK4 and WNK1 expression was slightly elevated in the knockout mice under low-salt diet, suggesting compensation for WNK3 knockout by these WNKs. Thus, WNK3 may have some role in the WNK-OSR1/SPAK-NCC/NKCC2 signal cascade in the kidney, but its contribution to total WNK kinase activity may be minimal.