Thyroid hormone deficiency alters expression of acid-base transporters in rat kidney

The Role of the Endocrine System in the Regulation of Acid-Base Balance by the Kidney and the Progression of Chronic Kidney Disease

Systemic acid-base status is primarily determined by the interplay of net acid production (NEAP) arising from metabolism of ingested food stuffs, buffering of NEAP in tissues, generation of bicarbonate by the kidney, and capture of any bicarbonate filtered by the kidney. In chronic kidney disease (CKD), acid retention may occur when dietary acid production is not balanced by bicarbonate generation by the diseased kidney. Hormones including aldosterone, angiotensin II, endothelin, PTH, glucocorticoids, insulin, thyroid hormone, and growth hormone can affect acid-base balance in different ways. The levels of some hormones such as aldosterone, angiotensin II and endothelin are increased with acid accumulation and contribute to an adaptive increase in renal acid excretion and bicarbonate generation. However, the persistent elevated levels of these hormones can damage the kidney and accelerate progression of CKD. Measures to slow the progression of CKD have included administration of medications which inhibit the production or action of deleterious hormones. However, since metabolic acidosis accompanying CKD stimulates the secretion of several of these hormones, treatment of CKD should also include administration of base to correct the metabolic acidosis.

Pharmacology of Mammalian Na+-Dependent Transporters of Inorganic Phosphate

Inorganic phosphate (Pi) is an essential component of many biologically important molecules such as DNA, RNA, ATP, phospholipids, or apatite. It is required for intracellular phosphorylation signaling events and acts as pH buffer in intra- and extracellular compartments. Intestinal absorption, uptake into cells, and renal reabsorption depend on a set of different phosphate transporters from the SLC20 (PiT transporters) and SLC34 (NaPi transporters) gene families. The physiological relevance of these transporters is evident from rare monogenic disorders in humans affecting SLC20A2 (Fahr's disease, basal ganglia calcification), SLC34A1 (idiopathic infantile hypercalcemia), SLC34A2 (pulmonary alveolar microlithiasis), and SLC34A3 (hereditary hypophosphatemic rickets with hypercalciuria). SLC34 transporters are inhibited by millimolar concentrations of phosphonoformic acid or arsenate while SLC20 are relatively resistant to these compounds. More recently, a series of more specific and potent drugs have been developed to target SLC34A2 to reduce intestinal Pi absorption and to inhibit SLC34A1 and/or SLC34A3 to increase renal Pi excretion in patients with renal disease and incipient hyperphosphatemia. Also, SLC20 inhibitors have been developed with the same intention. Some of these substances are currently undergoing preclinical and clinical testing. Tenapanor, a non-absorbable Na+/H+-exchanger isoform 3 inhibitor, reduces intestinal Pi absorption likely by indirectly acting on the paracellular pathway for Pi and has been tested in several phase III trials for reducing Pi overload in patients with renal insufficiency and dialysis.

Clinical and Biochemical Characteristics of Patients with Renal Tubular Acidosis in Southern Part of West Bengal, India: A Retrospective Study

PURPOSE OF THE STUDY

Reversible proximal tubular dysfunction associated with distal renal tubular acidosis (dRTA) mimics type 3 RTA, a condition classically associated with features of both proximal RTA (pRTA) and dRTA. Proximal tubulopathy has been reported in children with primary dRTA, but the data in adults are lacking.

STUDY DESIGN

In this hospital record-based retrospective study, data from 66 consecutive cases of RTA, between January 2016 to December 2018, were retrieved and analyzed.

RESULTS

Mean age of the study population was 25.3 years (range: 3 months to 73 years). Six (9.1%) of them had pRTA, 58 (87.9%) had dRTA, 1 (1.5%) had type 3 RTA, and the remaining 1 (1.5%) had type 4 RTA. Ten patients (17.2%) with dRTA and 3 patients of pRTA (50%) had underlying secondary etiologies. Data on proximal tubular dysfunction were available for 30 patients with dRTA, of whom 1 had isolated dRTA, and the rest 29 patients had accompanying completely reversible proximal tubular dysfunction. Among the 10 cases of secondary dRTA, 6 were not evaluated for proximal tubular dysfunction. Of the remaining 4, 3 had reversible form of proximal tubular abnormality. Fifty-two patients with dRTA came from a population, indigenous to the "Rarh" region of India.

CONCLUSIONS

Proximal tubular dysfunction often accompanies dRTA; 75% of the children with primary dRTA, at least 29% of adults with primary dRTA, and at least 30% of adults with secondary dRTA manifest such completely reversible form of proximal tubulopathy. "Rarh' region of India probably is a hotspot for endemic dRTA.

The ammonia transporter RhCG modulates urinary acidification by interacting with the vacuolar proton-ATPases in renal intercalated cells

Ammonium, stemming from renal ammoniagenesis, is a major urinary proton buffer and is excreted along the collecting duct. This process depends on the concomitant secretion of ammonia by the ammonia channel RhCG and of protons by the vacuolar-type proton-ATPase pump. Thus, urinary ammonium content and urinary acidification are tightly linked. However, mice lacking Rhcg excrete more alkaline urine despite lower urinary ammonium, suggesting an unexpected role of Rhcg in urinary acidification. RhCG and the B1 and B2 proton-ATPase subunits could be co-immunoprecipitated from kidney. In ex vivo microperfused cortical collecting ducts (CCD) proton-ATPase activity was drastically reduced in the absence of Rhcg. Conversely, overexpression of RhCG in HEK293 cells resulted in higher proton secretion rates and increased B1 proton-ATPase mRNA expression. However, in kidneys from Rhcg-/- mice the expression of only B1 and B2 subunits was altered. Immunolocalization of proton-ATPase subunits together with immuno-gold detection of the A proton-ATPase subunit showed similar localization and density of staining in kidneys from Rhcg+/+ and Rhcg-/-mice. In order to test for a reciprocal effect of intercalated cell proton-ATPases on Rhcg activity, we assessed Rhcg and proton-ATPase activities in microperfused CCD from Atp6v1b1-/- mice and showed reduced proton-ATPase activity without altering Rhcg activity. Thus, RhCG and proton-ATPase are located within the same cellular protein complex. RhCG may modulate proton-ATPase function and urinary acidification, whereas proton-ATPase activity does not affect RhCG function. This mechanism may help to coordinate ammonia and proton secretion beyond physicochemical driving forces.

Hypokalemic Paralysis: A Hidden Card of Several Autoimmune Diseases

Acute hypokalemic paralysis is a rare and potentially fatal condition, with few related causes, one of which highlights distal renal tubular acidosis (dRTA). Distal renal tubular acidosis is a rare complication of several autoimmune diseases such as systemic lupus erythematosus, Sjögren's syndrome, and Hashimoto thyroiditis. We report a case of a lupic patient who presented rapidly progressive quadriparesis in the context of active renal disease. Research revealed severe refractory hypokalemia, metabolic acidosis, and alkaline urine suggestive of dRTA. We diagnosed Sjögren's syndrome based on sicca symptoms, an abnormal salivary glands' nuclear scan and the presence of anti-Ro/SSA and anti-La/SSB. In addition, the finding of thyroid peroxidase, thyroglobulin antibodies, and hypothyroidism led us to the diagnosis of Hashimoto thyroiditis. Due to the active renal involvement on the context of systemic lupus erythematosus and Sjögren's syndrome, the patient received immunosuppression with rituximab, resulting in a progressive and complete improvement.

The correct renal function evaluation in patients with thyroid dysfunction

Thyroid dysfunction induces several renal derangements involving all nephron portions. Furthermore, dysthyroidism is a recognized risk factor associated with the development of chronic kidney disease. Current data, in fact, demonstrate that either subclinical or overt thyroid disease is associated with significant changes in creatinine, estimated glomerular filtration rate, measured glomerular filtration rate and Cystatin C. Herein, we systematically reviewed several relevant studies aiming at the identification of the most sensitive and specific parameter for the correct renal function evaluation in patients with thyroid dysfunction, that are usually treated as outpatients. Our systematic review indicates that estimated glomerular filtration rate, preferably with CKD-EPI equation, appears to be the most reliable and wieldy renal function parameter. Instead, Cystatin C should be better used in the grading of thyroid dysfunction severity.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Impaired expression of key molecules of ammoniagenesis underlies renal acidosis in a rat model of chronic kidney disease

BACKGROUND

Advanced chronic kidney disease (CKD) is associated with the development of renal metabolic acidosis. Metabolic acidosis per se may represent a trigger for progression of CKD. Renal acidosis of CKD is characterized by low urinary ammonium excretion with preserved urinary acidification indicating a defect in renal ammoniagenesis, ammonia excretion or both. The underlying molecular mechanisms, however, have not been addressed to date.

METHODS

We examined the Han:SPRD rat model and used a combination of metabolic studies, mRNA and protein analysis of renal molecules involved in acid-base handling.

RESULTS

We demonstrate that rats with reduced kidney function as evident from lower creatinine clearance, lower haematocrit, higher plasma blood urea nitrogen, creatinine, phosphate and potassium had metabolic acidosis that could be aggravated by HCl acid loading. Urinary ammonium excretion was highly reduced whereas urinary pH was more acidic in CKD compared with control animals. The abundance of key enzymes and transporters of proximal tubular ammoniagenesis (phosphate-dependent glutaminase, PEPCK and SNAT3) and bicarbonate transport (NBCe1) was reduced in CKD compared with control animals. In the collecting duct, normal expression of the B1 H(+)-ATPase subunit is in agreement with low urinary pH. In contrast, the RhCG ammonia transporter, critical for the final secretion of ammonia into urine was strongly down-regulated in CKD animals.

CONCLUSION

In the Han:SPRD rat model for CKD, key molecules required for renal ammoniagenesis and ammonia excretion are highly down-regulated providing a possible molecular explanation for the development and maintenance of renal acidosis in CKD patients.

NBCe1 as a model carrier for understanding the structure-function properties of Na⁺ -coupled SLC4 transporters in health and disease

SLC4 transporters are membrane proteins that in general mediate the coupled transport of bicarbonate (carbonate) and share amino acid sequence homology. These proteins differ as to whether they also transport Na(+) and/or Cl(-), in addition to their charge transport stoichiometry, membrane targeting, substrate affinities, developmental expression, regulatory motifs, and protein-protein interactions. These differences account in part for the fact that functionally, SLC4 transporters have various physiological roles in mammals including transepithelial bicarbonate transport, intracellular pH regulation, transport of Na(+) and/or Cl(-), and possibly water. Bicarbonate transport is not unique to the SLC4 family since the structurally unrelated SLC26 family has at least three proteins that mediate anion exchange. The present review focuses on the first of the sodium-dependent SLC4 transporters that was identified whose structure has been most extensively studied: the electrogenic Na(+)-base cotransporter NBCe1. Mutations in NBCe1 cause proximal renal tubular acidosis (pRTA) with neurologic and ophthalmologic extrarenal manifestations. Recent studies have characterized the important structure-function properties of the transporter and how they are perturbed as a result of mutations that cause pRTA. It has become increasingly apparent that the structure of NBCe1 differs in several key features from the SLC4 Cl(-)-HCO3 (-) exchanger AE1 whose structural properties have been well-studied. In this review, the structure-function properties and regulation of NBCe1 will be highlighted, and its role in health and disease will be reviewed in detail.

The phosphate transporter NaPi-IIa determines the rapid renal adaptation to dietary phosphate intake in mouse irrespective of persistently high FGF23 levels

Renal reabsorption of inorganic phosphate (Pi) is mediated by the phosphate transporters NaPi-IIa, NaPi-IIc, and Pit-2 in the proximal tubule brush border membrane (BBM). Dietary Pi intake regulates these transporters; however, the contribution of the specific isoforms to the rapid and slow phase is not fully clarified. Moreover, the regulation of PTH and FGF23, two major phosphaturic hormones, during the adaptive phase has not been correlated. C57/BL6 and NaPi-IIa(-/-) mice received 5 days either 1.2 % (HPD) or 0.1 % (LPD) Pi-containing diets. Thereafter, some mice were acutely switched to LPD or HPD. Plasma Pi concentrations were similar under chronic diets, but lower when mice were acutely switched to LPD. Urinary Pi excretion was similar in C57/BL6 and NaPi-IIa(-/-) mice under HPD. During chronic LPD, NaPi-IIa(-/-) mice lost phosphate in urine compensated by higher intestinal Pi absorption. During the acute HPD-to-LPD switch, NaPi-IIa(-/-) mice exhibited a delayed decrease in urinary Pi excretion. PTH was acutely regulated by low dietary Pi intake. FGF23 did not respond to low Pi intake within 8 h whereas the phospho-adaptator protein FRS2α necessary for FGF-receptor cell signaling was downregulated. BBM Pi transport activity and NaPi-IIa but not NaPi-IIc and Pit-2 abundance acutely adapted to diets in C57/BL6 mice. In NaPi-IIa(-/-), Pi transport activity was low and did not adapt. Thus, NaPi-IIa mediates the fast adaptation to Pi intake and is upregulated during the adaptation to low Pi despite persistently high FGF23 levels. The sensitivity to FGF23 may be regulated by adapting FRS2α abundance and phosphorylation.

PTH-independent regulation of blood calcium concentration by the calcium-sensing receptor

Tight regulation of calcium levels is required for many critical biological functions. The Ca2+-sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.

PTH-independent regulation of blood calcium concentration by the calcium-sensing receptor

Tight regulation of calcium levels is required for many critical biological functions. The Ca2+-sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.

The anion exchanger pendrin (SLC26A4) and renal acid-base homeostasis

The anion exchanger pendrin (Pds, SLC26A4) transports various anions including bicarbonate, chloride and iodide. In the kidney, pendrin is exclusively expressed on the luminal pole of bicarbonate-secretory type B intercalated cells. Genetic ablation of pendrin in mice abolishes luminal chloride-bicarbonate exchanger activity from type B intercalated cells suggesting that pendrin is the apical bicarbonate extruding pathway. The renal expression of pendrin is developmentally adapted and pendrin positive cells originate from both the uretric bud and mesenchyme. In adult kidney, pendrin expression and activity is regulated by systemic acid-base status, dietary electrolyte intake (mostly chloride), and hormones such as angiotensin II and aldosterone which can affect subcellular localization, the relative number of pendrin expressing cells, and the overall abundance consistent with a role of pendrin in maintaining normal acid-base homeostasis. This review summarizes recent findings on the role and regulation of pendrin in the context of the kidneys role in acid-base homeostasis in health and disease.

Localization of members of MCT monocarboxylate transporter family Slc16 in the kidney and regulation during metabolic acidosis

The monocarboxylate transporter family (MCT) comprises 14 members with distinct transport properties and tissue distribution. The kidney expresses several members of the MCT family, but only little is known about their exact distribution and function. Here, we investigated selected members of the MCT family in the mouse kidney. MCT1, MCT2, MCT7, and MCT8 localized to basolateral membranes of the epithelial cells lining the nephron. MCT1 and MCT8 were detected in proximal tubule cells whereas MCT7 and MCT2 were located in the thick ascending limb and the distal tubule. CD147, a β-subunit of MCT1 and MCT4, showed partially overlapping expression with MCT1 and MCT2. However, CD147 was also found in intercalated cells. We also detected SMCT1 and SMCT2, two Na+-dependent monocarboxylate cotransporters, on the luminal membrane of type A intercalated cells. Moreover, mice were given an acid load for 2 and 7 days. Acidotic animals showed a marked but transient increase in urinary lactate excretion. During acidosis, a downregulation of MCT1, MCT8, and SMCT2 was observed at the mRNA level, whereas MCT7 and SMCT1 showed increased mRNA abundance. Only MCT7 showed lower protein abundance whereas all other transporters remained unchanged. In summary, we describe for the first time the localization of various MCT transporters in mammalian kidney and demonstrate that metabolic acidosis induces a transient increase in urinary lactate excretion paralleled by lower MCT7 protein expression.

Consequences of monocarboxylate transporter 8 deficiency for renal transport and metabolism of thyroid hormones in mice

Patients carrying inactivating mutations in the gene encoding the thyroid hormone transporting monocarboxylate transporter (MCT)-8 suffer from a severe form of psychomotor retardation and exhibit abnormal serum thyroid hormone levels. The thyroidal phenotype characterized by high-serum T(3) and low-serum T(4) levels is also found in mice mutants deficient in MCT8 although the cause of these abnormalities is still unknown. Here we describe the consequences of MCT8 deficiency for renal thyroid hormone transport, metabolism, and function by studying MCT8 null mice and wild-type littermates. Whereas serum and urinary parameters do not indicate a strongly altered renal function, a pronounced induction of iodothyronine deiodinase type 1 expression together with increased renal T(3) and T(4) content point to a general hyperthyroid state of the kidneys in the absence of MCT8. Surprisingly, accumulation of peripherally injected T(4) and T(3) into the kidneys was found to be enhanced in the absence of MCT8, indicating that MCT8 deficiency either directly interferes with the renal efflux of thyroid hormones or activates indirectly other renal thyroid hormone transporters that preferentially mediate the renal uptake of thyroid hormones. Our findings indicate that the enhanced uptake and accumulation of T(4) in the kidneys of MCT8 null mice together with the increased renal conversion of T(4) into T(3) by increased renal deiodinase type 1 activities contributes to the generation of the low-serum T(4) and the increase in circulating T(3) levels, a hallmark of MCT8 deficiency.

The calcineurin inhibitor FK506 (tacrolimus) is associated with transient metabolic acidosis and altered expression of renal acid-base transport proteins

Calcineurin inhibitors like FK506 (tacrolimus) are routinely used for immunosuppression following transplantation. Its use is limited by many side effects, including renal tubular acidosis (RTA), mainly of the distal type. In this study, rats were treated with FK506 and at baseline (after 9 days) systemic acid-base status was similar to that in control animals. However, FK506-treated rats given NH4Cl in the drinking water for 2 days developed a more severe metabolic acidosis than control animals. Urine pH was more alkaline, but net acid excretion was normal. After 7 days of acid load, all differences related to acid-base homeostasis were equalized in both groups. Protein abundance of type IIa Na-Pi cotransporter, type 3 Na+/H+ exchanger, and electrogenic Na+-bicarbonate cotransporter, and both a4 and B2 subunits of the vacuolar H+-ATPase were reduced under baseline conditions, while induction of metabolic acidosis enhanced protein abundance of these transporters in FK506-treated animals. In parallel, protein expression of AE1 was reduced at baseline and increased together with pendrin during NH4Cl loading in FK506 rats. Protein abundance of the Na+-bicarbonate cotransporter NBCn1 was reduced under baseline conditions but remained downregulated during metabolic acidosis. Morphological analysis revealed an increase in the relative number of non-type A intercalated cells in the connecting tubule and cortical collecting duct at the expense of principal cells. Additionally, subcellular distribution of the a4 subunit of the vacuolar H+-ATPase was affected by FK506 with less luminal localization in the connecting tubule and outer medullary collecting duct. These data suggest that FK506 impacts on several major acid-base transport proteins in the kidney, and its use is associated with transient metabolic acidosis and altered expression of key renal acid-base transport proteins.

Long-term regulation of vacuolar H(+)-ATPase by angiotensin II in proximal tubule cells

Long-term effects of angiotensin II (Ang II) on vacuolar H(+)-ATPase were studied in a SV40-transformed cell line derived from rat proximal tubules (IRPTC). Using pH(i) measurements with the fluorescent dye BCECF, the hormone increased Na(+)-independent pH recovery rate from an NH(4)Cl pulse from 0.066 +/- 0.014 pH U/min (n = 7) to 0.14 +/- 0.021 pH U/min (n = 13; p < 0.05) in 10 h Ang II (10(-9) M)-treated cells. The increased activity of H(+)-ATPase did not involve changes in mRNA or protein abundance of the B2 subunit but increased cell surface expression of the V-ATPase. Inhibition of tyrosine kinase by genistein blocked Ang II-dependent stimulation of H(+)-ATPase. Inhibition of phosphatidylinositol-3-kinase (PI3K) by wortmannin and of p38 mitogen-activated protein kinase (MAPK) by SB 203580 also blocked this effect. Thus, long-term exposure of IRPTC cells to Ang II causes upregulation of H(+)-ATPase activity due, at least in part, to increased B2 cell surface expression. This regulatory pathway is dependent on mechanisms involving tyrosine kinase, p38 MAPK, and PI3K activation.

Functional regulation of H+-ATPase-rich cells in zebrafish embryos acclimated to an acidic environment

It is important to maintain internal pH homeostasis in biological systems. In our previous studies, H(+)-ATPase-rich (HR) cells were found to be responsible for proton secretion in the skin of zebrafish embryos during development. In this study, zebrafish embryos were exposed to acidic and basic waters to investigate the regulation of HR cell acid secretion during pH disturbances. Our results showed that the function of HR cells on the skin of zebrafish embryos can be upregulated in pH 4 water not only by increasing the cell number but also by enlarging the acid-secreting function of single cells. We also identified an "alveolar-type" apical opening under scanning electron microscopy observations of the apical membrane of HR cells, and the density and size of the alveolar type of apical openings were also increased in pH 4 water. p63 and PCNA immunostaining results also showed that additional HR cells in pH 4 water may be differentiated not only from ionocyte precursor cells but also newly proliferating epithelial stem cells.

Effect of thyroid hormone on the postnatal renal expression of NHE8

We previously demonstrated that there are developmental changes in proximal tubule Na(+)/H(+) exchanger (NHE) activity. There is a maturational increase in postnatal brush-border membrane (BBM) vesicle NHE3 protein abundance and decrease in NHE8 protein abundance. The purpose of this study was to determine whether thyroid hormone plays a role in the rat renal maturational isoform switch from NHE8 to NHE3 and whether thyroid hormone regulates NHE8. Administration of thyroid hormone to neonatal rats, before the normal postnatal increase in serum thyroid hormone levels at 3 wk of age, resulted in a premature increase in NHE3/beta-actin BBM protein abundance and mRNA abundance. Thyroid hormone also caused a premature decrease in BBM NHE8/beta-actin protein abundance, whereas there was no change in mRNA expression (standardized to 28s). Rats made hypothyroid from birth were studied at 28 days, after the normal maturational increase in thyroid hormone. In these hypothyroid adult rats, the maturational increase in BBM NHE3 protein abundance and NHE3 mRNA expression was prevented. In contrast, the developmental decrease in BBM NHE8 protein abundance was prevented in hypothyroid adults, but mRNA expression was unchanged in hypothyroid rats. To determine whether the effect of thyroid hormone was due to a direct epithelial effect, we studied normal rat kidney cells in culture. We recently showed that this cell line expresses NHE8, but does not express NHE3. Thyroid hormone caused a decrease in surface expression of NHE8, determined by biotinylation, but total cellular abundance remained unchanged. NHE8 activity, measured as the sodium-dependent rate of intracellular pH recovery from an acid load, was less with thyroid treatment than control. In conclusion, thyroid hormone plays a potential role in the developmental isoform change from NHE8 to NHE3 and decreases NHE8 activity.