New functions and roles of the Na+-H+-exchanger NHE3.

The sodium/proton exchanger isoform 3 (NHE3) is expressed in the intestine and the kidney, where it contributes to hydrogen secretion and sodium (re)absorption. The roles of this transporter have been studied by the use of the respective knockout mice and by using pharmacological inhibitors. Whole-body NHE3 knockout mice suffer from a high mortality rate (with only ∼30% of mice surviving into adulthood), and based on the expression of NHE3 in both intestine and kidney, some conclusions that were originally derived were based on this rather complex phenotype. In the last decade, more refined models have been developed that added temporal and spatial control of NHE3 expression. For example, novel mouse models have been developed with a knockout of NHE3 in intestinal epithelial cells, tubule/collecting duct of the kidney, proximal tubule of the kidney, and thick ascending limb of the kidney. These refined models have significantly contributed to our understanding of the role of NHE3 in a tissue/cell type-specific manner. In addition, tenapanor was developed, which is a non-absorbable, intestine-specific NHE3 inhibitor. In rat and human studies, tenapanor lowered intestinal Pi uptake and was effective in lowering plasma Pi levels in patients on hemodialysis. Of note, diarrhea is seen as a side effect of tenapanor (with its indication for the treatment of constipation) and in intestine-specific NHE3 knockout mice; however, effects on plasma Pi were not supported by this mouse model which showed enhanced and not reduced intestinal Pi uptake. Further studies indicated that the gut microbiome in mice lacking intestinal NHE3 resembles an intestinal environment favoring the competitive advantage of inflammophilic over anti-inflammatory species, something similar seen in patients with inflammatory bowel disease. This review will highlight recent developments and summarize newly gained insight from these refined models.

Intravenous ferric carboxymaltose and ferric derisomaltose alter the intestinal microbiome in female iron-deficient anemic mice.

Iron deficiency anemia (IDA) is a leading global health concern affecting approximately 30% of the population. Treatment for IDA consists of replenishment of iron stores, either by oral or intravenous (IV) supplementation. There is a complex bidirectional interplay between the gut microbiota, the host's iron status, and dietary iron availability. Dietary iron deficiency and supplementation can influence the gut microbiome; however, the effect of IV iron on the gut microbiome is unknown. We studied how commonly used IV iron preparations, ferric carboxymaltose (FCM) and ferric derisomaltose (FDI), affected the gut microbiome in female iron-deficient anemic mice. At the phylum level, vehicle-treated mice showed an expansion in Verrucomicrobia, mostly because of the increased abundance of Akkermansia muciniphila, along with contraction in Firmicutes, resulting in a lower Firmicutes/Bacteroidetes ratio (indicator of dysbiosis). Treatment with either FCM or FDI restored the microbiome such that Firmicutes and Bacteroidetes were the dominant phyla. Interestingly, the phyla Proteobacteria and several members of Bacteroidetes (e.g., Alistipes) were expanded in mice treated with FCM compared with those treated with FDI. In contrast, several Clostridia class members were expanded in mice treated with FDI compared with FCM (e.g., Dorea spp., Eubacterium). Our data demonstrate that IV iron increases gut microbiome diversity independently of the iron preparation used; however, differences exist between FCM and FDI treatments. In conclusion, replenishing iron stores with IV iron preparations in clinical conditions, such as inflammatory bowel disease or chronic kidney disease, could affect gut microbiome composition and consequently contribute to an altered disease outcome.

Effectiveness of Near-Peer-Taught Case Reviews on Students' Confidence in National Board of Medical Examiners (NBME) Exams.

Introduction A key element to a first-year medical student's (MS1) education is guidance and practice in applying anatomy, pathophysiology, diagnosis, and treatment concepts to clinical vignettes. One potential solution to providing effective clinical reasoning training is the involvement of second-year medical students (MS2s) in small group sessions as teachers to provide more personalized instruction via case-based learning sessions. Near-peer teaching has been shown to benefit students' confidence in learning and improve test scores. Similarly, case-based learning is heavily associated with an improved understanding of complex topics. As such, this study assessed the efficacy of near-peer teaching with concomitant case-based style presentations on improving the comfort of MS1s with their understanding of the curriculum content and their comfort with applying their knowledge to clinical scenarios. Methods This randomized controlled crossover trial included several small-group study sessions, each consisting of five MS1s and led by an MS2 who reviewed selected clinical cases in a standardized slide decks. The control arm was provided the same slide decks but did not participate in the MS2-led sessions. During the first course, students were assigned to either the control or intervention group and then crossed over to the opposite group (control to intervention and vice versa) in every subsequent course. Comfort with the curriculum material was then assessed through pre and post surveys, with the post surveys administered after the MS1s took their final NBME examination for that course. Results The study was cut short due to COVID-19 precautions limiting in-person sessions. Nevertheless, the post survey demonstrated an increased understanding of pathological concepts for the intervention group compared to the control group. Conclusions Future work on near-peer group study sessions should enroll a larger sample size with measures to improve the response rate to better test whether near-peer-led case reviews had a significant effect on students' understanding of anatomical concepts and confidence during NBME examinations.

Sodium phosphate cotransporter 2a inhibitors: potential therapeutic uses.

PURPOSE OF REVIEW: Targeting sodium phosphate cotransporter 2a (Npt2a) offers a novel strategy for treating hyperphosphatemia in chronic kidney disease (CKD). Here we review recent studies on the efficacy of Npt2a inhibition, its plasma phosphate (Pi)-lowering effects, as well as potential "off-target" beneficial effects on cardiovascular consequences. RECENT FINDINGS: Two novel Npt2a-selective inhibitors (PF-06869206 and BAY-767) have been developed. Pharmacological Npt2a inhibition shows a significant phosphaturic effect and consequently lowers plasma Pi and parathyroid hormone (PTH) levels regardless of CKD. However, plasma fibroblast growth factor 23 (FGF23), a master regulator of Pi homeostasis, shows inconsistent responses between these two inhibitors (no effect by PF-06869206 vs. reduction by BAY-767). In addition to the effects on Pi homeostasis, Npt2a inhibition also enhances urinary excretions of Na+, Cl-, and Ca2+, which is recapitulated in animal models with reduced kidney function. The effect of Npt2a inhibition by BAY-767 on vascular calcification has been studied, with positive results showing that oral treatment with BAY-767 (10 mg kg-1) attenuated the increases in plasma Pi and Ca2+ content in the aorta under the setting of vascular calcification induced by a pan-FGF receptor inhibitor. Together, Npt2a inhibition offers a promising therapeutic approach for treating hyperphosphatemia and reducing cardiovascular complications in CKD. SUMMARY: Npt2a inhibition significantly increases urinary Pi excretion and lowers plasma Pi and PTH levels; moreover, it exerts pleiotropic "off-target" effects, providing a novel treatment for hyperphosphatemia and exhibiting beneficial potential for cardiovascular complications in CKD.

Intestine-Specific NHE3 Deletion in Adulthood Causes Microbial Dysbiosis.

In the intestine, the Na+/H+ exchanger 3 (NHE3) plays a critical role for Na+ and fluid absorption. NHE3 deficiency predisposes patients to inflammatory bowel disease (IBD). In mice, selective deletion of intestinal NHE3 causes various local and systemic pathologies due to dramatic changes in the intestinal environment, which can influence microbiota colonization. By using metagenome shotgun sequencing, we determined the effect of inducible intestinal epithelial cell-specific deletion of NHE3 (NHE3IEC-KO) in adulthood on the gut microbiome in mice. Compared with control mice, NHE3IEC-KO mice show a significantly different gut microbiome signature, with an unexpected greater diversity. At the phylum level, NHE3IEC-KO mice showed a significant expansion in Proteobacteria and a tendency for lower Firmicutes/Bacteroidetes (F/B) ratio, an indicator of dysbiosis. At the family level, NHE3IEC-KO mice showed significant expansions in Bacteroidaceae, Rikenellaceae, Tannerellaceae, Flavobacteriaceae and Erysipelotrichaceae, but had contractions in Lachnospiraceae, Prevotellaceae and Eubacteriaceae. At the species level, after removing those with lowest occurrence and abundance, we identified 23 species that were significantly expanded (several of which are established pro-inflammatory pathobionts); whereas another 23 species were found to be contracted (some of which are potential anti-inflammatory probiotics) in NHE3IEC-KO mice. These results reveal that intestinal NHE3 deletion creates an intestinal environment favoring the competitive advantage of inflammophilic over anti-inflammatory species, which is commonly featured in conventional NHE3 knockout mice and patients with IBD. In conclusion, our study emphasizes the importance of intestinal NHE3 for gut microbiota homeostasis, and provides a deeper understanding regarding interactions between NHE3, dysbiosis, and IBD.

NHE3 in the thick ascending limb is required for sustained but not acute furosemide-induced urinary acidification.

Na+/H+ exchanger isoform 3 (NHE3) facilitates Na+ reabsorption and H+ secretion by the kidneys. Despite stronger NHE3 abundance in the thick ascending limb (TAL) compared with the S1 and S2 segments of the proximal tubule, the role of NHE3 in the TAL is poorly understood. To investigate the role of NHE3 in the TAL, we generated and phenotyped TAL-specific NHE3 knockout (NHE3TAL-KO) mice. Compared with control mice, NHE3TAL-KO mice did not show significant differences in body weight, blood pH, or plasma Na+, K+, or Cl- levels. Fluid intake trended to be higher and urine osmolality was significantly lower in NHE3TAL-KO mice. Despite a similar glomerular filtration rate, NHE3TAL-KO mice had a greater urinary K+-to-creatinine ratio. One proposed role of NHE3 relates to furosemide-induced urinary acidification. Acute bolus treatment with furosemide under anesthesia did not result in differences in the dose dependence of urinary flow rate, Cl- excretion, or maximal urinary acidification between genotypes; however, in contrast with control mice, urinary pH returned immediately toward baseline levels in NHE3TAL-KO mice. Chronic furosemide treatment reduced urine osmolality similarly in both genotypes but metabolic alkalosis, hypokalemia, and calciuresis were absent in NHE3TAL-KO mice. Compared with vehicle, chronic furosemide treatment resulted in greater Na+-K+-2Cl- abundance regardless of genotype. Na+-phosphate cotransporter 2a abundance was also greater in furosemide-treated control mice compared with vehicle treatment, an effect that was absent in NHE3TAL-KO mice. In summary, NHE3 in the TAL plays a role in the sustained acidification effect of furosemide. Consistent with this, long-term treatment with furosemide did not result in metabolic alkalosis in NHE3TAL-KO mice.NEW & NOTEWORTHY Na+/H+ exchanger isoform 3 (NHE3) is very abundant in the thick ascending limb (TAL) compared with the proximal tubule. Much has been learned about the role of NHE3 in the proximal tubule; however, the function of NHE3 in the TAL remains elusive. A novel mouse model that lacks NHE3 selectively in the TAL not only shows a phenotype under baseline conditions but also identifies that NHE3 is required for sustained but not acute furosemide-induced urinary acidification.

Npt2a as a target for treating hyperphosphatemia.

Hyperphosphatemia results from an imbalance in phosphate (Pi) homeostasis. In patients with and without reduced kidney function, hyperphosphatemia is associated with cardiovascular complications. The current mainstays in the management of hyperphosphatemia are oral Pi binder and dietary Pi restriction. Although these options are employed in patients with chronic kidney disease (CKD), they seem inadequate to correct elevated plasma Pi levels. In addition, a paradoxical increase in expression of intestinal Pi transporter and uptake may occur. Recently, studies in rodents targeting the renal Na+/Pi cotransporter 2a (Npt2a), responsible for ∼70% of Pi reabsorption, have been proposed as a potential treatment option. Two compounds (PF-06869206 and BAY-767) have been developed which are selective for Npt2a. These Npt2a inhibitors significantly increased urinary Pi excretion consequently lowering plasma Pi and PTH levels. Additionally, increases in urinary excretions of Na+, Cl- and Ca2+ have been observed. Some of these results are also seen in models of reduced kidney function. Responses of FGF23, a phosphaturic hormone that has been linked to the development of left ventricular hypertrophy in CKD, are ambiguous. In this review, we discuss the recent advances on the role of Npt2a inhibition on Pi homeostasis as well as other pleiotropic effects observed with Npt2a inhibition.

Enhanced phosphate absorption in intestinal epithelial cell-specific NHE3 knockout mice.

AIMS: The kidneys play a major role in maintaining Pi homeostasis. Patients in later stages of CKD develop hyperphosphatemia. One novel treatment option is tenapanor, an intestinal-specific NHE3 inhibitor. To gain mechanistic insight into the role of intestinal NHE3 in Pi homeostasis, we studied tamoxifen-inducible intestinal epithelial cell-specific NHE3 knockout (NHE3IEC-KO ) mice. METHODS: Mice underwent dietary Pi challenges, and hormones as well as urinary/plasma Pi were determined. Intestinal 33 P uptake studies were conducted in vivo to compare the effects of tenapanor and NHE3IEC-KO . Ex vivo Pi transport was measured in everted gut sacs and brush border membrane vesicles. Intestinal and renal protein expression of Pi transporters were determined. RESULTS: On the control diet, NHE3IEC-KO mice had similar Pi homeostasis, but a ~25% reduction in FGF23 compared with control mice. Everted gut sacs and brush border membrane vesicles showed enhanced Pi uptake associated with increased Npt2b expression in NHE3IEC-KO mice. Acute oral Pi loading resulted in higher plasma Pi in NHE3IEC-KO mice. Tenapanor inhibited intestinal 33 P uptake acutely but then led to hyper-absorption at later time points compared to vehicle. In response to high dietary Pi , plasma Pi and FGF23 increased to higher levels in NHE3IEC-KO mice which was associated with greater Npt2b expression. Reduced renal Npt2c and a trend for reduced Npt2a expression were unable to correct for higher plasma Pi . CONCLUSION: Intestinal NHE3 has a significant contribution to Pi homeostasis. In contrast to effects described for tenapanor on Pi homeostasis, NHE3IEC-KO mice show enhanced, rather than reduced, intestinal Pi uptake.

Genetic deletion of connexin 37 causes polyuria and polydipsia.

The connexin 37 (Cx37) channel is clustered at gap junctions between cells in the renal vasculature or the renal tubule where it is abundant in basolateral cell interdigitations and infoldings of epithelial cells in the proximal tubule, thick ascending limb, distal convoluted tubule and collecting duct; however, physiological data regarding its role are limited. In this study, we investigated the role of Cx37 in fluid homeostasis using mice with a global deletion of Cx37 (Cx37-/- mice). Under baseline conditions, Cx37-/- had ~40% higher fluid intake associated with ~40% lower urine osmolality compared to wild-type (WT) mice. No differences were observed between genotypes in urinary adenosine triphosphate or prostaglandin E2, paracrine factors that alter renal water handling. After 18-hours of water deprivation, plasma aldosterone and urine osmolality increased significantly in Cx37-/- and WT mice; however, the latter remained ~375 mmol/kg lower in Cx37-/- mice, an effect associated with a more pronounced body weight loss despite higher urinary AVP/creatinine ratios compared to WT mice. Consistent with this, fluid intake in the first 3 hours after water deprivation was 37% greater in Cx37-/- vs WT mice. Cx37-/- mice showed significantly lower renal AQP2 abundance and AQP2 phosphorylation at serine 256 than WT mice in response to vehicle or dDAVP, suggesting a partial contribution of the kidney to the lower urine osmolality. The abundance and responses of the vasopressin V2 receptor, AQP3, NHE3, NKCC2, NCC, H+-ATPase, αENaC, γENaC or Na+/K+-ATPase were not significantly different between genotypes. In summary, these results demonstrate that Cx37 is important for body water handling.

PF-06869206 is a selective inhibitor of renal Pi transport: evidence from in vitro and in vivo studies.

Plasma phosphate (Pi) levels are tightly controlled, and elevated plasma Pi levels are associated with an increased risk of cardiovascular complications and death. Two renal transport proteins mediate the majority of Pi reabsorption: Na+-phosphate cotransporters Npt2a and Npt2c, with Npt2a accounting for 70-80% of Pi reabsorption. The aim of the present study was to determine the in vitro effects of a novel Npt2a inhibitor (PF-06869206) in opossum kidney (OK) cells as well as determine its selectivity in vivo in Npt2a knockout (Npt2a-/-) mice. In OK cells, Npt2a inhibitor caused dose-dependent reductions of Na+-dependent Pi uptake (IC50: ~1.4 µmol/L), whereas the unselective Npt2 inhibitor phosphonoformic acid (PFA) resulted in an ~20% stronger inhibition of Pi uptake. The dose-dependent inhibitory effects were present after 24 h of incubation with both low- and high-Pi media. Michaelis-Menten kinetics in OK cells identified an ~2.4-fold higher Km for Pi in response to Npt2a inhibition with no significant change in apparent Vmax. Higher parathyroid hormone concentrations decreased Pi uptake equivalent to the maximal inhibitory effect of Npt2a inhibitor. In vivo, the Npt2a inhibitor induced a dose-dependent increase in urinary Pi excretion in wild-type mice (ED50: ~23 mg/kg), which was completely absent in Npt2a-/- mice, alongside a lack of decrease in plasma Pi. Of note, the Npt2a inhibitor-induced dose-dependent increase in urinary Na+ excretion was still present in Npt2a-/- mice, a response possibly mediated by an off-target acute inhibitory effect of the Npt2a inhibitor on open probability of the epithelial Na+ channel in the cortical collecting duct.

Tubular effects of sodium-glucose cotransporter 2 inhibitors: intended and unintended consequences.

PURPOSE OF REVIEW: Sodium-glucose cotransporter 2 (SGLT2) inhibitors are antihyperglycemic drugs that act by inhibiting renal sodium-glucose cotransport. Here we present new insights into 'off target', or indirect, effects of SGLT2 inhibitors. RECENT FINDINGS: SGLT2 inhibition causes an acute increase in urinary glucose excretion. In addition to lowering blood glucose, there are several other effects that contribute to the overall beneficial renal and cardiovascular effects. Reabsorption of about 66% of sodium is accomplished in the proximal tubule and dependent on the sodium-hydrogen exchanger isoform 3 (NHE3). SGLT2 colocalizes with NHE3, and high glucose levels reduce NHE3 activity. The proximal tubule is also responsible for the majority of phosphate (Pi) reabsorption. SGLT2 inhibition is associated with increases in plasma Pi, fibroblast growth factor 23 and parathyroid hormone levels in nondiabetics and type 2 diabetes mellitus. Studies in humans identified a urate-lowering effect by SGLT2 inhibition which is possibly mediated by urate transporter 1 (URAT1) and/or glucose transporter member 9 in the proximal tubule. Of note, magnesium levels were also found to increase under SGLT2 inhibition, an effect that was preserved in nondiabetic patients with hypomagnesemia. SUMMARY: Cardiorenal effects of SGLT2 inhibition might involve, in addition to direct effects on glucose homeostasis, effects on NHE3, phosphate, urate, and magnesium homeostasis.

An inducible intestinal epithelial cell-specific NHE3 knockout mouse model mimicking congenital sodium diarrhea.

The sodium-hydrogen exchanger isoform 3 (NHE3, SLC9A3) is abundantly expressed in the gastrointestinal tract and is proposed to play essential roles in Na+ and fluid absorption as well as acid-base homeostasis. Mutations in the SLC9A3 gene can cause congenital sodium diarrhea (CSD). However, understanding the precise role of intestinal NHE3 has been severely hampered due to the lack of a suitable animal model. To navigate this problem and better understand the role of intestinal NHE3, we generated a tamoxifen-inducible intestinal epithelial cell-specific NHE3 knockout mouse model (NHE3IEC-KO). Before tamoxifen administration, the phenotype and blood parameters of NHE3IEC-KO were unremarkable compared with control mice. After tamoxifen administration, NHE3IEC-KO mice have undetectable levels of NHE3 in the intestine. NHE3IEC-KO mice develop watery, alkaline diarrhea in combination with a swollen small intestine, cecum and colon. The persistent diarrhea results in higher fluid intake. After 3 weeks, NHE3IEC-KO mice show a ∼25% mortality rate. The contribution of intestinal NHE3 to acid-base and Na+ homeostasis under normal conditions becomes evident in NHE3IEC-KO mice that have metabolic acidosis, lower blood bicarbonate levels, hyponatremia and hyperkalemia associated with drastically elevated plasma aldosterone levels. These results demonstrate that intestinal NHE3 has a significant contribution to acid-base, Na+ and volume homeostasis, and lack of intestinal NHE3 has consequences on intestinal structural integrity. This mouse model mimics and explains the phenotype of individuals with CSD carrying SLC9A3 mutations.

Pharmacological Npt2a Inhibition Causes Phosphaturia and Reduces Plasma Phosphate in Mice with Normal and Reduced Kidney Function.

BACKGROUND: The kidneys play an important role in phosphate homeostasis. Patients with CKD develop hyperphosphatemia in the later stages of the disease. Currently, treatment options are limited to dietary phosphate restriction and oral phosphate binders. The sodium-phosphate cotransporter Npt2a, which mediates a large proportion of phosphate reabsorption in the kidney, might be a good therapeutic target for new medications for hyperphosphatemia. METHODS: The authors assessed the effects of the first orally bioavailable Npt2a inhibitor (Npt2a-I) PF-06869206 in normal mice and mice that had undergone subtotal nephrectomy (5/6 Nx), a mouse model of CKD. Dose-response relationships of sodium, chloride, potassium, phosphate, and calcium excretion were assessed in response to the Npt2a inhibitor in both groups of mice. Expression and localization of Npt2a/c and levels of plasma phosphate, calcium, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23) were studied up to 24-hours after Npt2a-I treatment. RESULTS: In normal mice, Npt2a inhibition caused a dose-dependent increase in urinary phosphate (ED50 approximately 21 mg/kg), calcium, sodium and chloride excretion. In contrast, urinary potassium excretion, flow rate and urinary pH were not affected dose dependently. Plasma phosphate and PTH significantly decreased after 3 hours, with both returning to near baseline levels after 24 hours. Similar effects were observed in the mouse model of CKD but were reduced in magnitude. CONCLUSIONS: Npt2a inhibition causes a dose-dependent increase in phosphate, sodium and chloride excretion associated with reductions in plasma phosphate and PTH levels in normal mice and in a CKD mouse model.

What does sodium-glucose co-transporter 1 inhibition add: Prospects for dual inhibition.

Epithelial glucose transport is accomplished by Na+ -glucose co-transporters, SGLT1 and SGLT2. In the intestine, uptake of dietary glucose is for its majority mediated by SGLT1, and humans with mutations in the SGLT1 gene show glucose/galactose malabsorption. In the kidney, both transporters, SGLT1 and SGLT2, are expressed and recent studies identified that SGLT2 mediates up to 97% of glucose reabsorption. Humans with mutations in the SGLT2 gene show familial renal glucosuria. In the last three decades, significant progress was made in understanding the physiology of these transporters and their potential as therapeutic targets. Based on the structure of phlorizin, a natural compound acting as a SGLT1/2 inhibitor, initially several SGLT2, and later SGLT1 and dual SGLT1/2 inhibitors have been developed. Interestingly, SGLT2 knockout or treatment with SGLT2 selective inhibitors only causes a fractional glucose excretion in the magnitude of ∼60%, an effect mediated by up-regulation of renal SGLT1. Based on these findings the hypothesis was brought forward that dual SGLT1/2 inhibition might further improve glycaemic control via targeting two distinct organs that express SGLT1: the intestine and the kidney. Of note, SGLT1/2 double knockout mice completely lack renal glucose reabsorption. This review will address the rationale for the development of SGLT1 and dual SGLT1/2 inhibitors and potential benefits compared to sole SGLT2 inhibition.

Adenylyl Cyclase 6 Expression Is Essential for Cholera Toxin-Induced Diarrhea.

BACKGROUND: Cholera toxin (CT)-induced diarrhea is mediated by cyclic adenosine monophosphate (cAMP)-mediated active Cl- secretion via the cystic fibrosis transmembrane conductance regulator (CFTR). Although the constitutive activation of adenylyl cyclase (AC) in response to CT is due to adenosine diphosphate ribosylation of the small G protein α-subunit activating CFTR with consequent secretory diarrhea, the AC isoform(s) involved remain unknown. METHODS: We generated intestinal epithelial cell-specific adenylyl cyclase 6 (AC6) knockout mice to study its role in CT-induced diarrhea. RESULTS: AC6 messenger RNA levels were the highest of all 9 membrane-bound AC isoforms in mouse intestinal epithelial cells. Intestinal epithelial-specific AC6 knockout mice (AC6loxloxVillinCre) had undetectable AC6 levels in small intestinal and colonic epithelial cells. No significant differences in fluid and food intake, plasma electrolytes, intestinal/colon anatomy and morphology, or fecal water content were observed between genotypes. Nevertheless, CT-induced fluid accumulation in vivo was completely absent in AC6loxloxVillinCre mice, associated with a lack of forskolin- and CT-induced changes in the short-circuit current (ISC) of the intestinal mucosa, impaired cAMP generation in acutely isolated small intestinal epithelial cells, and significantly impaired apical CFTR levels in response to forskolin. CONCLUSIONS: AC6 is a novel target for the treatment of CT-induced diarrhea.

Contribution of NHE3 and dietary phosphate to lithium pharmacokinetics.

Lithium is one of the mainstays for the treatment of bipolar disorder despite its side effects on the endocrine, neurological, and renal systems. Experimentally, lithium has been used as a measure to determine proximal tubule reabsorption based on the assumption that lithium and sodium transport go in parallel in the proximal tubule. However, the exact mechanism by which lithium is reabsorbed remains elusive. The majority of proximal tubule sodium reabsorption is directly or indirectly mediated by the sodium-hydrogen exchanger 3 (NHE3). In addition, sodium-phosphate cotransporters have been implicated in renal lithium reabsorption. In order to better understand the role of sodium-phosphate cotransporters involved in lithium (re)absorption, we studied lithium pharmacokinetics in: i) tubule-specific NHE3 knockout mice (NHE3loxloxPax8Cre), and ii) mice challenged with low or high phosphate diets. Intravenous or oral administration of lithium did not result in differences in lithium bioavailability, half-life, maximum plasma concentrations, area under the curve, lithium clearance, or urinary lithium/creatinine ratios between control and NHE3loxloxPax8Cre mice. After one week of dietary phosphate challenges, lithium bioavailability was ~30% lower on low versus high dietary phosphate, possibly the consequence of a smaller area under the curve after oral administration. This was associated with higher apparent lithium clearance after oral administration and lower urinary lithium/creatinine ratios on low versus high dietary phosphate. Collectively, renal NHE3 does not play a role in lithium pharmacokinetics; however, dietary phosphate could have an indirect effect on lithium bioavailability and lithium disposition.

Renal tubular NHE3 is required in the maintenance of water and sodium chloride homeostasis.

The sodium/proton exchanger isoform 3 (NHE3) is expressed in the intestine and the kidney, where it facilitates sodium (re)absorption and proton secretion. The importance of NHE3 in the kidney for sodium chloride homeostasis, relative to the intestine, is unknown. Constitutive tubule-specific NHE3 knockout mice (NHE3loxloxCre) did not show significant differences compared to control mice in body weight, blood pH or bicarbonate and plasma sodium, potassium, or aldosterone levels. Fluid intake, urinary flow rate, urinary sodium/creatinine, and pH were significantly elevated in NHE3loxloxCre mice, while urine osmolality and GFR were significantly lower. Water deprivation revealed a small urinary concentrating defect in NHE3loxloxCre mice on a control diet, exaggerated on low sodium chloride. Ten days of low or high sodium chloride diet did not affect plasma sodium in control mice; however, NHE3loxloxCre mice were susceptible to low sodium chloride (about -4 mM) or high sodium chloride intake (about +2 mM) versus baseline, effects without differences in plasma aldosterone between groups. Blood pressure was significantly lower in NHE3loxloxCre mice and was sodium chloride sensitive. In control mice, the expression of the sodium/phosphate co-transporter Npt2c was sodium chloride sensitive. However, lack of tubular NHE3 blunted Npt2c expression. Alterations in the abundances of sodium/chloride cotransporter and its phosphorylation at threonine 58 as well as the abundances of the α-subunit of the epithelial sodium channel, and its cleaved form, were also apparent in NHE3loxloxCre mice. Thus, renal NHE3 is required to maintain blood pressure and steady-state plasma sodium levels when dietary sodium chloride intake is modified.

Novel developments in differentiating the role of renal and intestinal sodium hydrogen exchanger 3.

The Na+/H+ exchanger isoform 3 (NHE3) facilitates Na+ absorption and H+ secretion and is expressed in the intestine, proximal tubule, and thick ascending limb of the kidney. While the function of NHE3 for Na+ and [Formula: see text](re)absorption has been defined using conventional NHE3 knockout mice (NHE3-/-), the recent generation of conditional NHE3 knockout mice started to give critical new insight into the role of this protein by allowing for temporal and spatial control of NHE3 expression. For example, in contrast to NHE3-/- mice, knockout of NHE3 in the S1 and S2 segments of the proximal tubule or along the entire tubule/collecting duct does not cause any lethality. Nonabsorbable NHE3 inhibitors have been developed, and preclinical as well as clinical trials indicate possible pharmacological use in fluid overload, hypertension, chronic kidney disease, hyperphosphatemia, and constipation. Some of the therapeutic considerations seem to be directly related to the pharmacodynamic properties of these drugs; however, little is known about the effects of these nonabsorbable NHE3 inhibitors on intestinal phosphate transport and the mechanisms so far remain elusive. This review focuses on novel findings of NHE3 in the intestine and the kidney as well as novel drug developments targeting NHE3.