Renal Chloride Channels in Relation to Sodium Chloride Transport

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

The Integral Role of Chloride & With-No-Lysine Kinases in Cell Volume Regulation & Hypertension

Chloride anions are the most abundant in humans. For many years, it has been believed that chloride is simply a counterion of all other cations, ensuring the electroneutrality of the extracellular space. Recent data suggests that chloride anions possess a broad spectrum of important activities that regulate vital cellular functions. It is now evident that, apart from its contribution to the electroneutrality of the extracellular space, it acts as an osmole and contributes to extracellular and intracellular volume regulation. Its anionic charge also contributes to the generation of cell membrane potential. The most interesting action of chloride anions is their ability to regulate the activity of with-no-lysine kinases, which in turn regulate the activity of sodium chloride and potassium chloride cotransporters and govern the reabsorption of salt and excretion of potassium by nephron epithelia. Chloride anions seem to play a crucial role in cell functions, such as cell volume regulation, sodium reabsorption in the distal nephron, potassium balance, and sodium sensitivity, which lead to hypertension. All of these functions are accomplished on a molecular level via complicated metabolic pathways, many of which remain poorly defined. We attempted to elucidate some of these pathways in light of recent advances in our knowledge, obtained mainly from experimental studies.

Directing two-way traffic in the kidney: A tale of two ions

The kidneys regulate levels of Na+ and K+ in the body by varying urinary excretion of the electrolytes. Since transport of each of the two ions can affect the other, controlling both at the same time is a complex task. The kidneys meet this challenge in two ways. Some tubular segments change the coupling between Na+ and K+ transport. In addition, transport of Na+ can shift between segments where it is coupled to K+ reabsorption and segments where it is coupled to K+ secretion. This permits the kidney to maintain electrolyte balance with large variations in dietary intake.

SGLT2 inhibition versus sulfonylurea treatment effects on electrolyte and acid-base balance: secondary analysis of a clinical trial reaching glycemic equipoise: Tubular effects of SGLT2 inhibition in Type 2 diabetes

Sodium-glucose transporter (SGLT)2 inhibitors increase plasma magnesium and plasma phosphate and may cause ketoacidosis, but the contribution of improved glycemic control to these observations as well as effects on other electrolytes and acid-base parameters remain unknown. Therefore, our objective was to compare the effects of SGLT2 inhibitors dapagliflozin and sulfonylurea gliclazide on plasma electrolytes, urinary electrolyte excretion, and acid-base balance in people with Type 2 diabetes (T2D). We assessed the effects of dapagliflozin and gliclazide treatment on plasma electrolytes and bicarbonate, 24-hour urinary pH and excretions of electrolytes, ammonium, citrate, and sulfate in 44 metformin-treated people with T2D and preserved kidney function. Compared with gliclazide, dapagliflozin increased plasma chloride by 1.4 mmol/l (95% CI 0.4-2.4), plasma magnesium by 0.03 mmol/l (95% CI 0.01-0.06), and plasma sulfate by 0.02 mmol/l (95% CI 0.01-0.04). Compared with baseline, dapagliflozin also significantly increased plasma phosphate, but the same trend was observed with gliclazide. From baseline to week 12, dapagliflozin increased the urinary excretion of citrate by 0.93 ± 1.72 mmol/day, acetoacetate by 48 μmol/day (IQR 17-138), and β-hydroxybutyrate by 59 μmol/day (IQR 0-336), without disturbing acid-base balance. In conclusion, dapagliflozin increases plasma magnesium, chloride, and sulfate compared with gliclazide, while reaching similar glucose-lowering in people with T2D. Dapagliflozin also increases urinary ketone excretion without changing acid-base balance. Therefore, the increase in urinary citrate excretion by dapagliflozin may reflect an effect on cellular metabolism including the tricarboxylic acid cycle. This potentially contributes to kidney protection.

Dietary potassium and the kidney: lifesaving physiology

Potassium often has a negative connotation in Nephrology as patients with chronic kidney disease (CKD) are prone to develop hyperkalaemia. Approaches to the management of chronic hyperkalaemia include a low potassium diet or potassium binders. Yet, emerging data indicate that dietary potassium may be beneficial for patients with CKD. Epidemiological studies have shown that a higher urinary potassium excretion (as proxy for higher dietary potassium intake) is associated with lower blood pressure (BP) and lower cardiovascular risk, as well as better kidney outcomes. Considering that the composition of our current diet is characterized by a high sodium and low potassium content, increasing dietary potassium may be equally important as reducing sodium. Recent studies have revealed that dietary potassium modulates the activity of the thiazide-sensitive sodium-chloride cotransporter in the distal convoluted tubule (DCT). The DCT acts as a potassium sensor to control the delivery of sodium to the collecting duct, the potassium-secreting portion of the kidney. Physiologically, this allows immediate kaliuresis after a potassium load, and conservation of potassium during potassium deficiency. Clinically, it provides a novel explanation for the inverse relationship between dietary potassium and BP. Moreover, increasing dietary potassium intake can exert BP-independent effects on the kidney by relieving the deleterious effects of a low potassium diet (inflammation, oxidative stress and fibrosis). The aim of this comprehensive review is to link physiology with clinical medicine by proposing that the same mechanisms that allow us to excrete an acute potassium load also protect us from hypertension, cardiovascular disease and CKD.

PGF2α stimulates the 10-pS Cl− channel and thiazide-sensitive Na+-Cl− cotransporter in the distal convoluted tubule

We used patch-clamp and Western blot analysis to test whether PGF2α stimulates the basolateral 10-pS Cl− channel and thiazide-sensitive Na+-Cl− cotransporter (NCC) in the distal convoluted tubule (DCT) via a prostaglandin F receptor (FP-R). Single channel and whole cell recordings demonstrated that PGF2α stimulated the 10-pS Cl− channel in the DCT. The stimulatory effect of PGF2α on the Cl− channel was mimicked by a FP-R agonist, latanoprost, but was abrogated by blocking FP-R with AL8810. Also, the effect of PGF2α on the Cl− channel in the DCT was recapitulated by stimulating PKC but was blocked by inhibiting PKC. Furthermore, inhibition of p38 MAPK but not ERK blocked the effect of PGF2α on the 10-pS Cl− channel. Inhibition of NADPH oxidase also abrogated the stimulatory effect of PGF2α on the 10-pS Cl− channel, while the addition of 10 μM H2O2 mimicked the stimulatory effect of PGF2α on the 10-pS Cl− channel. Moreover, superoxide-related species may mediate the stimulatory effect of PGF2α on the 10-pS Cl− channel because the stimulatory effect of PGF2α and H2O2 was not additive. Western blot analysis showed that infusion of PGF2α in vivo not only increased the expression of FP-R but also increased the expression of total NCC and phosphorylated NCC. We conclude that PGF2α stimulates the basolateral 10-pS Cl− channel in the DCT by activating FP-R through PKC/p38 MAPK and NADPH oxidase-dependent pathways. The stimulatory effects of PGF2α on the Cl− channel and NCC may contribute to PGF2α-induced increases in NaCl reabsorption in the DCT.

Effects of Hypotonic and Isotonic Enteral Electrolyte Solutions Administered in Continuous Flow in Weaned Foals

The effects of fluid therapy with maintenance enteral electrolytic solutions administered by nasogastric route in continuous flow have not previously been studied in weaned foals. This study primary goal was to compare the effects of two maintenance enteral electrolytic solutions administered by nasogastric route in continuous flow on the hydro electrolytic balance in weaned foals. This paper was a controlled trial in a cross-over design (6 × 2) performed in six foals with a mean age of 7.3 ± 1.4 months; each animal received two treatments, IsoES and HypoES, with an interval of 7 days between treatments. After 12 h of fasting, the animals were treated with enteral electrolyte solutions administered via nasogastric route in continuous flow in a volume of 15 mL/kg/h for 12 h. The evaluations were performed at T-12h (the beginning of the fasting), T0h (end of fasting and beginning of fluid therapy), T4h (4 h of fluid therapy), T8h (8 h of fluid therapy), T12h (end of fluid therapy), and T24h (12 h after the end of fluid therapy). Twelve hours of fasting resulted in a reduction (P < 0.05) in body weight, abdominal circumference, whereas serum sodium, SID and enophthalmos increased. Twelve hours of fluid therapy normalized these parameters and promoted increased urinary volume and decreased urinary density without causing electrolyte imbalances. Both enteral electrolytic solutions were effective in reestablishing clinical and laboratorial variables without causing electrolyte imbalances.

The Drosophila Malpighian tubule as a model for mammalian tubule function

PURPOSE OF REVIEW

Studies of the genetic model organism, Drosophila melanogaster, have unraveled molecular pathways relevant to human physiology and disease. The Malpighian tubule, the Drosophila renal epithelium, is described here, including tools available to study transport; conserved transporters, channels, and the signaling pathways regulating them; and fly models of kidney stone disease.

RECENT FINDINGS

Tools to measure Malpighian tubule transport continue to advance, including use of a transgenic sensor to quantify intracellular pH and proton fluxes. A recent study generated an RNA-sequencing-based atlas of tissue-specific gene expression, with resulting insights into Malpighian tubule gene expression of transporters and channels. Advances have been made in understanding the molecular physiology of the With No Lysine kinase-Ste20-related proline/alanine rich kinase/oxidative stress response kinase cascade that regulates epithelial ion transport in flies and mammals. New studies in Drosophila kidney stone models have characterized zinc transporters and used Malpighian tubules to study the efficacy of a plant metabolite in decreasing stone burden.

SUMMARY

Study of the Drosophila Malpighian tubule affords opportunities to better characterize the molecular physiology of epithelial transport mechanisms relevant to mammalian renal physiology.

Intracellular chloride: a regulator of transepithelial transport in the distal nephron

PURPOSE OF REVIEW

This review focuses on the role of intracellular chloride in regulating transepithelial ion transport in the distal convoluted tubule (DCT) in response to perturbations in plasma potassium homeostasis.

RECENT FINDINGS

Low dietary potassium increases the phosphorylation and activity of the sodium chloride cotransporter (NCC) in the DCT, and vice versa, affecting sodium-dependent potassium secretion in the downstream aldosterone-sensitive distal nephron. In cells, NCC phosphorylation is increased by lowering of intracellular chloride, via activation of the chloride-sensitive with no lysine (WNK)-SPAK/OSR1 (Ste20-related proline/alanine-rich kinase/oxidative stress response) kinase cascade. In-vivo studies have demonstrated pathway activation in the kidney in response to low dietary potassium. A possible mechanism is lowering of DCT intracellular chloride in response to low potassium because of parallel basolateral potassium and chloride channels. Recent studies support a role for these channels in the response of NCC to varying potassium. Studies examining chloride-insensitive WNK mutants, in the Drosophila renal tubule and in the mouse, lend further support to a role for chloride in regulating WNK activity and transepithelial ion transport. Caveats, alternatives, and future directions are also discussed.

SUMMARY

Chloride sensing by WNK kinase provides a mechanism to allow coupling of extracellular potassium with NCC phosphorylation and activity to maintain potassium homeostasis.

Analysis of CLCNKB mutations at dimer-interface, calcium-binding site, and pore reveals a variety of functional alterations in ClC-Kb channel leading to Bartter syndrome

Pathological missense mutations in CLCNKB gene give a wide spectrum of clinical phenotypes in Bartter syndrome type III patients. Molecular analysis of the mutated ClC-Kb channels can be helpful to classify the mutations according to their functional alteration. We investigated the functional consequences of nine mutations in the CLCNKB gene causing Bartter syndrome. We first established that all tested mutations lead to decreased ClC-Kb currents. Combining electrophysiological and biochemical methods in Xenopus laevis oocytes and in MDCKII cells, we identified three classes of mutations. One class is characterized by altered channel trafficking. p.A210V, p.P216L, p.G424R, and p.G437R are totally or partially retained in the endoplasmic reticulum. p.S218N is characterized by reduced channel insertion at the plasma membrane and altered pH-sensitivity; thus, it falls in the second class of mutations. Finally, we found a novel class of functionally inactivated mutants normally present at the plasma membrane. Indeed, we found that p.A204T alters the pH-sensitivity, p.A254V abolishes the calcium-sensitivity. p.G219C and p.G465R are probably partially inactive at the plasma membrane. In conclusion, most pathogenic mutants accumulate partly or totally in intracellular compartments, but some mutants are normally present at the membrane surface and simultaneously show a large range of altered channel gating properties.