Hyposmolality stimulates Na(+)/H(+) exchange and HCO(3)(-) absorption in thick ascending limb via PI 3-kinase

Acid base homeostasis and serum bicarbonate concentration in syndrome of inappropriate anti-diuretic hormone secretion (SIADH) with hyponatremia

The Syndrome of Inappropriate ADH secretion (SIADH) presents with excess ADH release caused by a range of conditions; including pneumonia, brain tumors, certain lung cancers, and diseases of the hypothalamus. It presents with significant reduction in both sodium and chloride concentrations in the blood. However, reports examining the acid base status indicate a normal serum bicarbonate concentration and systemic acid base homeostasis. The mechanisms for the absence of abnormalities in acid base homeostasis remain speculative. This mini review is highlighting the recent advances in renal molecular physiology to provide answers for the maintenance of acid base status and serum bicarbonate in a physiological range.

Monophosphoryl lipid A induces protection against LPS in medullary thick ascending limb through induction of Tollip and negative regulation of IRAK-1

LPS inhibits HCO3- absorption in the medullary thick ascending limb (MTAL) through a Toll-like receptor 4 (TLR4)-myeloid differentiation factor 88 (MyD88)-extracellular signal-regulated kinase (ERK) pathway that is upregulated by sepsis. Pretreatment with the nontoxic immunomodulator monophosphoryl lipid A (MPLA) prevents inhibition by LPS through activation of a TLR4-TIR-domain-containing adaptor-inducing interferon-β (TRIF)-phosphatidylinositol 3-kinase (PI3K) pathway that prevents LPS-induced ERK activation. Here, we identified the molecular mechanisms that underlie the protective inhibitory interaction between the MPLA-PI3K and LPS-ERK pathways. Treatment of mouse MTALs with LPS in vitro increased phosphorylation of IL-1 receptor-associated kinase (IRAK)-1, a critical mediator of LPS signaling downstream of TLR4-MyD88. Activation of ERK by LPS was eliminated by a selective IRAK-1 inhibitor, establishing IRAK-1 as the upstream mediator of ERK activation. Pretreatment of MTALs with MPLA in vitro prevented LPS-induced IRAK-1 activation; this effect was dependent on PI3K. Treatment of MTALs with MPLA increased expression of Toll-interacting protein (Tollip), an inducible protein that negatively regulates LPS signaling by inhibiting IRAK-1. The MPLA-induced increase in Tollip protein level was prevented by PI3K inhibitors. In coimmunoprecipitation experiments, MPLA increased the amount of Tollip stably bound to IRAK-1, an interaction that inhibits IRAK-1 activation. These results support a mechanism whereby MPLA increases Tollip expression in the MTAL through a PI3K-dependent pathway. Tollip, in turn, inhibits LPS-induced TLR4 signaling by suppressing activation of IRAK-1, thereby preventing activation of ERK that inhibits HCO3- absorption. These studies show that MPLA induces reprogramming of MTAL cells that protects against LPS stimulation and identify IRAK-1 and Tollip as new therapeutic targets to prevent renal tubule dysfunction in response to infectious and inflammatory stimuli.

Monophosphoryl lipid A prevents impairment of medullary thick ascending limb [Formula: see text] absorption and improves plasma [Formula: see text] concentration in septic mice

Metabolic acidosis is the most common acid-base disorder in septic patients and is associated with increased mortality. Previously, we demonstrated that sepsis induced by cecal ligation and puncture (CLP) impairs [Formula: see text] absorption in the medullary thick ascending limb (MTAL) by 1) decreasing the intrinsic [Formula: see text] absorptive capacity and 2) enhancing inhibition of [Formula: see text] absorption by LPS through upregulation of Toll-like receptor (TLR) 4 signaling. Both effects depend on ERK activation. Monophosphoryl lipid A (MPLA) is a detoxified TLR4 agonist that enhances innate antimicrobial immunity and improves survival following sepsis. Pretreatment of MTALs with MPLA in vitro prevents LPS inhibition of [Formula: see text] absorption. Here we examined whether pretreatment with MPLA would protect the MTAL against sepsis. Vehicle or MPLA was administered to mice 48 h before sham or CLP surgery, and MTALs were studied in vitro 18 h postsurgery. Pretreatment with MPLA prevented the effects of sepsis to decrease the basal [Formula: see text] absorption rate and enhance inhibition by LPS. These protective effects were mediated through MPLA stimulation of a Toll/IL-1 receptor domain-containing adaptor-inducing IFN-β-(TRIF)-dependent phosphatidylinositol 3-kinase-Akt pathway that prevents sepsis- and LPS-induced ERK activation. The effects of MPLA to improve MTAL [Formula: see text] absorption were associated with marked improvement in plasma [Formula: see text] concentration, supporting a role for the kidneys in the pathogenesis of sepsis-induced metabolic acidosis. These studies support detoxified TLR4-based immunomodulators, such as MPLA, that enhance antimicrobial responses as a safe and effective approach to prevent or treat sepsis-induced renal tubule dysfunction and identify cell signaling pathways that can be targeted to preserve MTAL [Formula: see text] absorption and attenuate metabolic acidosis during sepsis.

Monophosphoryl lipid A induces protection against LPS in medullary thick ascending limb through a TLR4-TRIF-PI3K signaling pathway

Monophosphoryl lipid A (MPLA) is a detoxified derivative of LPS that induces tolerance to LPS and augments host resistance to bacterial infections. Previously, we demonstrated that LPS inhibits [Formula: see text] absorption in the medullary thick ascending limb (MTAL) through a basolateral Toll-like receptor 4 (TLR4)-myeloid differentiation factor 88 (MyD88)-ERK pathway. Here we examined whether pretreatment with MPLA would attenuate LPS inhibition. MTALs from rats were perfused in vitro with MPLA (1 µg/ml) in bath and lumen or bath alone for 2 h, and then LPS was added to (and MPLA removed from) the bath solution. Pretreatment with MPLA eliminated LPS-induced inhibition of [Formula: see text] absorption. In MTALs pretreated with MPLA plus a phosphatidylinositol 3-kinase (PI3K) or Akt inhibitor, LPS decreased [Formula: see text] absorption. MPLA increased Akt phosphorylation in dissected MTALs. The Akt activation was eliminated by a PI3K inhibitor and in MTALs from TLR4^-/- or Toll/IL-1 receptor domain-containing adaptor-inducing IFN-β (TRIF)^-/- mice. The effect of MPLA to prevent LPS inhibition of [Formula: see text] absorption also was TRIF dependent. Pretreatment with MPLA prevented LPS-induced ERK activation; this effect was dependent on PI3K. MPLA alone had no effect on [Formula: see text] absorption, and MPLA pretreatment did not prevent ERK-mediated inhibition of [Formula: see text] absorption by aldosterone, consistent with MPLA's low toxicity profile. These results demonstrate that pretreatment with MPLA prevents the effect of LPS to inhibit [Formula: see text] absorption in the MTAL. This protective effect is mediated directly through MPLA stimulation of a TLR4-TRIF-PI3K-Akt pathway that prevents LPS-induced ERK activation. These studies identify detoxified TLR4-based immunomodulators as novel potential therapeutic agents to prevent or treat renal tubule dysfunction in response to bacterial infections.

High-mobility group box 1 inhibits HCO3- absorption in the medullary thick ascending limb through RAGE-Rho-ROCK-mediated inhibition of basolateral Na+/H+ exchange

High-mobility group box 1 (HMGB1) is a nuclear protein released extracellularly in response to infection or injury, where it activates immune responses and contributes to the pathogenesis of kidney dysfunction in sepsis and sterile inflammatory disorders. Recently, we demonstrated that HMGB1 inhibits HCO3 (-) absorption in perfused rat medullary thick ascending limbs (MTAL) through a basolateral receptor for advanced glycation end products (RAGE)-dependent pathway that is additive to Toll-like receptor 4 (TLR4)-ERK-mediated inhibition by LPS (Good DW, George T, Watts BA III. Am J Physiol Renal Physiol 309: F720-F730, 2015). Here, we examined signaling and transport mechanisms that mediate inhibition by HMGB1. Inhibition of HCO3 (-) absorption by HMGB1 was eliminated by the Rho-associated kinase (ROCK) inhibitor Y27632 and by a specific inhibitor of Rho, the major upstream activator of ROCK. HMGB1 increased RhoA and ROCK1 activity. HMGB1-induced ROCK1 activation was eliminated by the RAGE antagonist FPS-ZM1 and by inhibition of Rho. The Rho and ROCK inhibitors had no effect on inhibition of HCO3 (-) absorption by bath LPS. Inhibition of HCO3 (-) absorption by HMGB1 was eliminated by bath amiloride, 0 Na(+) bath, and the F-actin stabilizer jasplakinolide, three conditions that selectively prevent inhibition of MTAL HCO3 (-) absorption mediated through NHE1. HMGB1 decreased basolateral Na(+)/H(+) exchange activity through activation of ROCK. We conclude that HMGB1 inhibits HCO3 (-) absorption in the MTAL through a RAGE-RhoA-ROCK1 signaling pathway coupled to inhibition of NHE1. The HMGB1-RAGE-RhoA-ROCK1 pathway thus represents a potential target to attenuate MTAL dysfunction during sepsis and other inflammatory disorders. HMGB1 and LPS inhibit HCO3 (-) absorption through different receptor signaling and transport mechanisms, which enables these pathogenic mediators to act directly and independently to impair MTAL function.

Vasopressin regulation of sodium transport in the distal nephron and collecting duct

Arginine vasopressin (AVP) is released from the posterior pituitary gland during states of hyperosmolality or hypovolemia. AVP is a peptide hormone, with antidiuretic and antinatriuretic properties. It allows the kidneys to increase body water retention predominantly by increasing the cell surface expression of aquaporin water channels in the collecting duct alongside increasing the osmotic driving forces for water reabsorption. The antinatriuretic effects of AVP are mediated by the regulation of sodium transport throughout the distal nephron, from the thick ascending limb through to the collecting duct, which in turn partially facilitates osmotic movement of water. In this review, we will discuss the regulatory role of AVP in sodium transport and summarize the effects of AVP on various molecular targets, including the sodium-potassium-chloride cotransporter NKCC2, the thiazide-sensitive sodium-chloride cotransporter NCC, and the epithelial sodium channel ENaC.

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.

High-mobility group box 1 inhibits HCO(3)(-) absorption in medullary thick ascending limb through a basolateral receptor for advanced glycation end products pathway

High-mobility group box 1 (HMGB1) is a damage-associated molecule implicated in mediating kidney dysfunction in sepsis and sterile inflammatory disorders. HMGB1 is a nuclear protein released extracellularly in response to infection or injury, where it interacts with Toll-like receptor 4 (TLR4) and other receptors to mediate inflammation. Previously, we demonstrated that LPS inhibits HCO(3)(-) absorption in the medullary thick ascending limb (MTAL) through a basolateral TLR4-ERK pathway (Watts BA III, George T, Sherwood ER, Good DW. Am J Physiol Cell Physiol 301: C1296-C1306, 2011). Here, we examined whether HMGB1 could inhibit HCO(3)(-) absorption through the same pathway. Adding HMGB1 to the bath decreased HCO(3)(-) absorption by 24% in isolated, perfused rat and mouse MTALs. In contrast to LPS, inhibition by HMGB1 was preserved in MTALs from TLR4(-/-) mice and was unaffected by ERK inhibitors. Inhibition by HMGB1 was eliminated by the receptor for advanced glycation end products (RAGE) antagonist FPS-ZM1 and by neutralizing anti-RAGE antibody. Confocal immunofluorescence showed expression of RAGE in the basolateral membrane domain. Inhibition of HCO(3)(-) absorption by HMGB1 through RAGE was additive to inhibition by LPS through TLR4 and to inhibition by Gram-positive bacterial molecules through TLR2. Bath amiloride, which selectively prevents inhibition of MTAL HCO(3)(-) absorption mediated through Na⁺/H⁺ exchanger 1 (NHE1), eliminated inhibition by HMGB1. We conclude that HMGB1 inhibits MTAL HCO(3)(-) absorption through a RAGE-dependent pathway distinct from TLR4-mediated inhibition by LPS. These studies provide new evidence that HMGB1-RAGE signaling acts directly to impair the transport function of renal tubules. They reveal a novel paradigm for sepsis-induced renal tubule dysfunction, whereby exogenous pathogen-associated molecules and endogenous damage-associated molecules act directly and independently to inhibit MTAL HCO(3)(-) absorption through different receptor signaling pathways.

P2X receptors trigger intracellular alkalization in isolated perfused mouse medullary thick ascending limb

Aims

Extracellular ATP is an important regulator of renal tubular transport. Recently, we found that basolateral ATP markedly inhibits Na⁺ and Cl⁻ absorption in mouse medullary thick ascending limb (mTAL) via a P2X receptor. The underlying mechanism that mediates this ATP-dependent transport inhibition in mTAL is, however, unclear. The renal outer medullary K⁺ channel (ROMK) is sensitive to intracellular pH where a reduction leads to closing of ROMK. We speculated that P2X receptor stimulation in the TAL could lead to changes in pH_i, leading to a reduction in NaCl transport.

Methods

To test this hypothesis, we measured pH_i in single perfused mouse mTALs using the fluorescent ratiometric dye 2′,7′-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethylester.

Results

Interestingly, basolateral ATP (100 μm) caused a prominent, reversible intracellular alkalization of mTAL, with an average pH_i increase of 0.14 ± 0.02 (n = 14). This was completely abolished by the P2X receptor antagonist periodate-oxidized ATP (50 μm). The P2X receptor-mediated intracellular alkalization required the activity of the apical Na⁺/H⁺ exchanger (NHE3). Typically, G_q-coupled receptors cause a significant acidification of tubular epithelial cells, which was confirmed in this study, by P2Y₂ and Ca²⁺ sensing receptor stimulation.

Conclusion

This study reports that stimulation of basolateral P2X receptors causes a substantial intracellular alkalization in the isolated perfused mouse mTAL. This intracellular alkalization is mediated through an increased apical NHE3 activity, similar to what we previously observed when tubular transport is inhibited with furosemide. This increased NHE3 activity causes H⁺ secretion in the mTAL and provides further support that the TAL is a site of urinary acidification.

Functional role of glucose metabolism, osmotic stress, and sodium-glucose cotransporter isoform-mediated transport on Na+/H+ exchanger isoform 3 activity in the renal proximal tubule

Na(+)-glucose cotransporter 1 (SGLT1)-mediated glucose uptake leads to activation of Na(+)-H(+) exchanger 3 (NHE3) in the intestine by a process that is not dependent on glucose metabolism. This coactivation may be important for postprandial nutrient uptake. However, it remains to be determined whether SGLT-mediated glucose uptake regulates NHE3-mediated NaHCO3 reabsorption in the renal proximal tubule. Considering that this nephron segment also expresses SGLT2 and that the kidneys and intestine show significant variations in daily glucose availability, the goal of this study was to determine the effect of SGLT-mediated glucose uptake on NHE3 activity in the renal proximal tubule. Stationary in vivo microperfusion experiments showed that luminal perfusion with 5 mM glucose stimulates NHE3-mediated bicarbonate reabsorption. This stimulatory effect was mediated by glycolytic metabolism but not through ATP production. Conversely, luminal perfusion with 40 mM glucose inhibited NHE3 because of cell swelling. Notably, pharmacologic inhibition of SGLT activity by Phlorizin produced a marked inhibition of NHE3, even in the absence of glucose. Furthermore, immunofluorescence experiments showed that NHE3 colocalizes with SGLT2 but not SGLT1 in the rat renal proximal tubule. Collectively, these findings show that glucose exerts a bimodal effect on NHE3. The physiologic metabolism of glucose stimulates NHE3 transport activity, whereas, supraphysiologic glucose concentrations inhibit this exchanger. Additionally, Phlorizin-sensitive SGLT transporters and NHE3 interact functionally in the proximal tubule.

Structural dynamics and regulation of the mammalian SLC9A family of Na⁺/H⁺ exchangers

Mammalian Na⁺/H⁺ exchangers of the SLC9A family are widely expressed and involved in numerous essential physiological processes. Their primary function is to mediate the 1:1 exchange of Na⁺ for H⁺ across the membrane in which they reside, and they play central roles in regulation of body, cellular, and organellar pH. Their function is tightly regulated through mechanisms involving interactions with multiple protein and lipid-binding partners, phosphorylations, and other posttranslational modifications. Biochemical and mutational analyses indicate that the SLC9As have a short intracellular N-terminus, 12 transmembrane (TM) helices necessary and sufficient for ion transport, and a C-terminal cytoplasmic tail region with essential regulatory roles. No high-resolution structures of the SLC9As exist; however, models based on crystal structures of the bacterial NhaAs support the 12 TM organization and suggest that TMIV and XI may form a central part of the ion-translocation pathway, whereas pH sensing may involve TMII, TMIX, and several intracellular loops. Similar to most ion transporters studied, SLC9As likely exist as coupled dimers in the membrane, and this appears to be important for the well-studied cooperativity of H⁺ binding. The aim of this work is to summarize and critically discuss the currently available evidence on the structural dynamics, regulation, and binding partner interactions of SLC9As, focusing in particular on the most widely studied isoform, SLC9A1/NHE1. Further, novel bioinformatic and structural analyses are provided that to some extent challenge the existing paradigm on how ions are transported by mammalian SLC9As.

Lumen LPS inhibits HCO3(-) absorption in the medullary thick ascending limb through TLR4-PI3K-Akt-mTOR-dependent inhibition of basolateral Na+/H+ exchange

Sepsis and endotoxemia induce defects in renal tubule function, but the mechanisms are poorly understood. Recently, we demonstrated that lipopolysaccharide (LPS) inhibits HCO3(-) absorption in the medullary thick ascending limb (MTAL) through activation of different Toll-like receptor 4 (TLR4) signaling pathways in the basolateral and apical membranes. Basolateral LPS inhibits HCO3(-) absorption through ERK-dependent inhibition of the apical Na(+)/H(+) exchanger NHE3. Here, we examined the mechanisms of inhibition by lumen LPS. Adding LPS to the lumen decreased HCO3(-) absorption by 29% in rat and mouse MTALs perfused in vitro. Inhibitors of phosphoinositide 3-kinase (PI3K) or its effectors Akt and mammalian target of rapamycin (mTOR) eliminated inhibition of HCO3(-) absorption by lumen LPS but had no effect on inhibition by bath LPS. Exposure to LPS for 15 min induced increases in phosphorylation of Akt and mTOR in microdissected MTALs that were blocked by wortmannin, consistent with activation of Akt and mTOR downstream of PI3K. The effects of lumen LPS to activate Akt and inhibit HCO3(-) absorption were eliminated in MTALs from TLR4(-/-) and MyD88(-/-) mice but preserved in tubules lacking Trif or CD14. Inhibition of HCO3(-) absorption by lumen LPS was eliminated under conditions that inhibit basolateral Na(+)/H(+) exchange and prevent inhibition of HCO3(-) absorption mediated through NHE1. Lumen LPS decreased basolateral Na(+)/H(+) exchange activity through PI3K. We conclude that lumen LPS inhibits HCO3(-) absorption in the MTAL through TLR4/MyD88-dependent activation of a PI3K-Akt-mTOR pathway coupled to inhibition of NHE1. Molecular components of the TLR4-PI3K-mTOR pathway represent potential therapeutic targets for sepsis-induced renal tubule dysfunction.

Toll-like receptor 2 is required for LPS-induced Toll-like receptor 4 signaling and inhibition of ion transport in renal thick ascending limb

Previously we demonstrated that basolateral LPS inhibits HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through TLR4-dependent ERK activation. Here we report that the response of the MTAL to basolateral LPS requires TLR2 in addition to TLR4. The basolateral addition of LPS (ultrapure Escherichia coli K12) decreased HCO(3)(-) absorption in isolated, perfused MTALs from wild-type mice but had no effect in MTALs from TLR2(-/-) mice. In contrast, inhibition of HCO(3)(-) absorption by lumen LPS was preserved in TLR2(-/-) MTALs, indicating that TLR2 is involved specifically in mediating the basolateral LPS response. LPS also did not increase ERK phosphorylation in MTALs from TLR2(-/-) mice. TLR2 deficiency had no effect on expression of TLR4, MD-2, or MyD88. However, LPS-induced recruitment of MyD88 to the basolateral membrane was impaired in TLR2(-/-) MTALs. Inhibition of HCO(3)(-) absorption by LPS did not require CD14. Co-immunoprecipitation studies demonstrated an association between TLR4 and TLR2. Inhibition of HCO(3)(-) absorption by TLR2-specific ligands was preserved in MTALs from TLR4(-/-) mice. These results indicate that the effect of basolateral LPS to inhibit HCO(3)(-) absorption in the MTAL through MyD88-dependent ERK activation depends on a novel interaction between TLR4 and TLR2. TLR2 plays a dual role in the induction of intracellular signals that impair MTAL function, both through cooperation with TLR4 to mediate ERK signaling by LPS and through a TLR4-independent signaling pathway activated by Gram-positive bacterial ligands. Regulation of TLR2 expression and its interaction with TLR4 may provide new mechanisms for controlling and therapeutic targeting of TLR4-mediated LPS responses.

Basolateral LPS inhibits NHE3 and HCOFormula absorption through TLR4/MyD88-dependent ERK activation in medullary thick ascending limb

Sepsis is associated with defects in renal tubule function, but the underlying mechanisms are incompletely understood. Recently, we demonstrated that Gram-negative bacterial lipopolysaccharide (LPS) inhibits HCO(3)(-) absorption in the medullary thick ascending limb (MTAL) through activation of Toll-like receptor 4 (TLR4). Here, we examined the mechanisms responsible for inhibition of HCO(3)(-) absorption by basolateral LPS. Adding LPS to the bath decreased HCO(3)(-) absorption by 30% in rat and mouse MTALs perfused in vitro. The inhibition of HCO(3)(-) absorption was eliminated by the mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK)/ERK inhibitors U0126 and PD98059. LPS induced a rapid (<15 min) and sustained (up to 60 min) increase in ERK phosphorylation in microdissected MTALs that was blocked by PD98059. The effects of basolateral LPS to activate ERK and inhibit HCO(3)(-) absorption were eliminated in MTALs from TLR4(-/-) and myeloid differentiation factor 88 (MyD88)(-/-) mice but were preserved in MTALs from TIR (Toll/interleukin-1 receptor) domain-containing adapter-inducing interferon-β (Trif)(-/-) mice. Basolateral LPS decreased apical Na(+)/H(+) exchanger 3 NHE3 activity through a decrease in maximal velocity (V(max)). The inhibition of NHE3 by LPS was eliminated by MEK/ERK inhibitors. LPS inhibited HCO(3)(-) absorption despite the presence of physiological stimuli that activate ERK in the MTAL. We conclude that basolateral LPS inhibits HCO(3)(-) absorption in the MTAL through activation of a TLR4/MyD88/MEK/ERK pathway coupled to inhibition of NHE3. These studies identify NHE3 as a target of TLR4 signaling in the MTAL and show that bacterial molecules can impair the absorptive functions of renal tubules through inhibition of this exchanger. The ERK pathway links TLR4 to downstream modulation of ion transport proteins and represents a potential target for treatment of sepsis-induced renal tubule dysfunction.

High sodium intake increases HCO(3)- absorption in medullary thick ascending limb through adaptations in basolateral and apical Na+/H+ exchangers

A high sodium intake increases the capacity of the medullary thick ascending limb (MTAL) to absorb HCO(3)(-). Here, we examined the role of the apical NHE3 and basolateral NHE1 Na(+)/H(+) exchangers in this adaptation. MTALs from rats drinking H(2)O or 0.28 M NaCl for 5-7 days were perfused in vitro. High sodium intake increased HCO(3)(-) absorption rate by 60%. The increased HCO(3)(-) absorptive capacity was mediated by an increase in apical NHE3 activity. Inhibiting basolateral NHE1 with bath amiloride eliminated 60% of the adaptive increase in HCO(3)(-) absorption. Thus the majority of the increase in NHE3 activity was dependent on NHE1. A high sodium intake increased basolateral Na(+)/H(+) exchange activity by 89% in association with an increase in NHE1 expression. High sodium intake increased apical Na(+)/H(+) exchange activity by 30% under conditions in which basolateral Na(+)/H(+) exchange was inhibited but did not change NHE3 abundance. These results suggest that high sodium intake increases HCO(3)(-) absorptive capacity in the MTAL through 1) an adaptive increase in basolateral NHE1 activity that results secondarily in an increase in apical NHE3 activity; and 2) an adaptive increase in NHE3 activity, independent of NHE1 activity. These studies support a role for NHE1 in the long-term regulation of renal tubule function and suggest that the regulatory interaction whereby NHE1 enhances the activity of NHE3 in the MTAL plays a role in the chronic regulation of HCO(3)(-) absorption. The adaptive increases in Na(+)/H(+) exchange activity and HCO(3)(-) absorption in the MTAL may play a role in enabling the kidneys to regulate acid-base balance during changes in sodium and volume balance.

Membrane surface charge dictates the structure and function of the epithelial Na+/H+ exchanger

The Na(+)/H(+) exchanger NHE3 plays a central role in intravascular volume and acid-base homeostasis. Ion exchange activity is conferred by its transmembrane domain, while regulation of the rate of transport by a variety of stimuli is dependent on its cytosolic C-terminal region. Liposome- and cell-based assays employing synthetic or recombinant segments of the cytosolic tail demonstrated preferential association with anionic membranes, which was abrogated by perturbations that interfere with electrostatic interactions. Resonance energy transfer measurements indicated that segments of the C-terminal domain approach the bilayer. In intact cells, neutralization of basic residues in the cytosolic tail by mutagenesis or disruption of electrostatic interactions inhibited Na(+)/H(+) exchange activity. An electrostatic switch model is proposed to account for multiple aspects of the regulation of NHE3 activity.

Changes of renal AQP2, ENaC, and NHE3 in experimentally induced heart failure: response to angiotensin II AT1 receptor blockade

Heart failure (HF) was induced by ligation of the left anterior descending artery (LAD). Left ventricular end-diastolic pressure (LVEDP) >25 mmHg (at day 23 after LAD ligation) was the inclusion criterion. The rats were divided into three groups: sham-operated (Sham, n = 23, LVEDP: 5.6 +/- 0.6 mmHg), HF (n = 14, LVEDP: 29.4 +/- 1.4 mmHg), and candesartan (1 mg.kg(-1).day(-1) sc)-treated HF (HF + Can, n = 9, LVEDP: 29.2 +/- 1.2 mmHg). After 7 days (i.e., 29 days after LAD ligation) semiquantitative immunoblotting revealed increased abundance of inner medulla aquaporin-2 (AQP2) and AQP2 phosphorylated at Ser(256) (p-AQP2) in HF. There was also markedly enhanced apical targeting of AQP2 and p-AQP2 in inner medullary collecting duct (IMCD) in HF compared with Sham rats, shown by immunocytochemistry. Candesartan treatment significantly reversed the increases in both AQP2 and p-AQP2 expression and targeting. In contrast, there were only modest changes in other collecting duct segments. Semiquantitative immunoblots revealed increased expression of type 3 Na(+)/H(+) exchanger (NHE3) and Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) in kidneys from HF compared with Sham rats: both effects were reversed or prevented by candesartan treatment. The protein abundance of alpha-epithelial sodium channel (alpha-ENaC) was increased while beta-ENaC and gamma-ENaC expression was decreased in the cortex and outer stripe of the outer medulla in HF compared with Sham rats, which was partially reversed by candesartan treatment. These findings strongly support an important role of angiotensin II in the pathophysiology of renal water and sodium retention associated with HF.

Luminal Na(+)/H (+) exchange in the proximal tubule

The proximal tubule is critical for whole-organism volume and acid-base homeostasis by reabsorbing filtered water, NaCl, bicarbonate, and citrate, as well as by excreting acid in the form of hydrogen and ammonium ions and producing new bicarbonate in the process. Filtered organic solutes such as amino acids, oligopeptides, and proteins are also retrieved by the proximal tubule. Luminal membrane Na(+)/H(+) exchangers either directly mediate or indirectly contribute to each of these processes. Na(+)/H(+) exchangers are a family of secondary active transporters with diverse tissue and subcellular distributions. Two isoforms, NHE3 and NHE8, are expressed at the luminal membrane of the proximal tubule. NHE3 is the prevalent isoform in adults, is the most extensively studied, and is tightly regulated by a large number of agonists and physiological conditions acting via partially defined molecular mechanisms. Comparatively little is known about NHE8, which is highly expressed at the lumen of the neonatal proximal tubule and is mostly intracellular in adults. This article discusses the physiology of proximal Na(+)/H(+) exchange, the multiple mechanisms of NHE3 regulation, and the reciprocal relationship between NHE3 and NHE8 at the lumen of the proximal tubule.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Nerve growth factor inhibits Na+/H+ exchange and formula absorption through parallel phosphatidylinositol 3-kinase-mTOR and ERK pathways in thick ascending limb

In the medullary thick ascending limb, inhibiting the basolateral NHE1 Na(+)/H(+) exchanger with nerve growth factor (NGF) induces actin cytoskeleton remodeling that secondarily inhibits apical NHE3 and transepithelial HCO(3)(-) absorption. The inhibition by NGF is mediated 50% through activation of extracellular signal-regulated kinase (ERK). Here we examined the signaling pathway responsible for the remainder of the NGF-induced inhibition. Inhibition of HCO(3)(-) absorption was reduced 45% by the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin or LY294002 and 50% by rapamycin, a specific inhibitor of mammalian target of rapamycin (mTOR), a downstream effector of PI3K. The combination of a PI3K inhibitor plus rapamycin did not cause a further reduction in the inhibition by NGF. In contrast, the combination of a PI3K inhibitor plus the MEK/ERK inhibitor U0126 completely eliminated inhibition by NGF. Rapamycin decreased NGF-induced inhibition of basolateral NHE1 by 45%. NGF induced a 2-fold increase in phosphorylation of Akt, a PI3K target linked to mTOR activation, and a 2.2-fold increase in the activity of p70 S6 kinase, a downstream effector of mTOR. p70 S6 kinase activation was blocked by wortmannin and rapamycin, consistent with PI3K, mTOR, and p70 S6 kinase in a linear pathway. Rapamycin-sensitive inhibition of NHE1 by NGF was associated with an increased level of phosphorylated mTOR in the basolateral membrane domain. These findings indicate that NGF inhibits HCO(3)(-) absorption in the medullary thick ascending limb through the parallel activation of PI3K-mTOR and ERK signaling pathways, which converge to inhibit NHE1. The results identify a role for mTOR in the regulation of Na(+)/H(+) exchange activity and implicate NHE1 as a possible downstream effector contributing to mTOR's effects on cell growth, proliferation, survival, and tumorigenesis.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Phosphatidylinositol 3-kinase is involved in prostaglandin E2-mediated murine duodenal bicarbonate secretion

Prostaglandin E(2) (PGE(2)) plays an important role in the regulation of duodenal bicarbonate (HCO(3)(-)) secretion, but its signaling pathway(s) are not fully understood. In the present study, we investigated the signaling pathways involved in PGE(2)-mediated duodenal HCO(3)(-) secretion. Murine duodenal mucosal HCO(3)(-) secretion was examined in vitro in Ussing chambers by pH-stat titration in the presence of a variety of signal transduction modulators. Phosphatidylinositol 3-kinase (PI3K) activity was measured by immunoprecipitation of PI3K and ELISA, and Akt phosphorylation was measured by Western analysis with anti-phospho-Akt and anti-Akt antibodies. PGE(2)-stimulated duodenal HCO(3)(-) secretion was reduced by the cAMP-dependent signaling pathway inhibitors MDL-12330A and KT-5720 by 23% and 20%, respectively; the Ca(2+)-influx inhibitor verapamil by 26%; and the calmodulin antagonist W-13 by 24%; whereas the PI3K inhibitors wortmannin and LY-294002 reduced PGE(2)-stimulated HCO(3)(-) secretion by 51% and 47%, respectively. Neither the MAPK inhibitor PD-98059 nor the tyrosine kinase inhibitor genistein altered PGE(2)-stimulated HCO(3)(-) secretion. PGE(2) application caused a rapid and concentration-dependent increase in duodenal mucosal PI3K activity and Akt phosphorylation. These results demonstrated that PGE(2) activates PI3K in duodenal mucosa and stimulates duodenal HCO(3)(-) secretion via cAMP-, Ca(2+)-, and PI3K-dependent signaling pathways.

Membrane curvature alters the activation kinetics of the epithelial Na+/H+ exchanger, NHE3

The epithelial Na(+)/H(+) exchanger, NHE3, was found to activate slowly following an acute cytosolic acidification. The sigmoidal course of activation could not be explained by the conventional two-state model, which postulates that activation results from protonation of an allosteric modifier site. Instead, mathematical modeling predicted the existence of three distinct states of the exchanger: two different inactive states plus an active form. The interconversion of the inactive states is rapid and dependent on pH, whereas the conversion between the second inactive state and the active conformation is slow and pH-independent but subject to regulation by other stimuli. Accordingly, exposure of epithelial cells to hypoosmolar solutions activated NHE3 by accelerating this latter transition. The number of surface-exposed exchangers and their association with the cytoskeleton were not affected by hypoosmolarity. Instead, NHE3 is activated by the membrane deformation, a result of cell swelling. This was suggested by the stimulatory effects of amphiphiles that induce a comparable positive (convex) deformation of the membrane. We conclude that NHE3 exists in multiple states and that different physiological parameters control the transitions between them.

Metabolic Alkalosis, Bedside and Bench

Although significant contributions to the understanding of metabolic alkalosis have been made recently, much of our knowledge rests on data from clearance studies performed in humans and animals many years ago. This article reviews the contributions of these studies, as well as more recent work relating to the control of renal acid-base transport by mineralocorticoid hormones, angiotensin, endothelin, nitric oxide, and potassium balance. Finally, clinical aspects of metabolic alkalosis are considered.

Aldosterone inhibits apical NHE3 and HCO3- absorption via a nongenomic ERK-dependent pathway in medullary thick ascending limb

Although aldosterone influences a variety of cellular processes through nongenomic mechanisms, the significance of nongenomic pathways for aldosterone-induced regulation of epithelial function is not understood. Recently, we demonstrated that aldosterone inhibits transepithelial HCO(3)(-) absorption in the medullary thick ascending limb (MTAL) through a nongenomic pathway. This inhibition is mediated through a direct cellular action of aldosterone to inhibit the apical membrane NHE3 Na(+)/H(+) exchanger. The present study was designed to identify the intracellular signaling pathway(s) responsible for this aldosterone-induced transport regulation. In rat MTALs perfused in vitro, addition of 1 nM aldosterone to the bath decreased HCO(3)(-) absorption by 30%. This inhibition was not mediated by cAMP/PKA and was not prevented by inhibitors of PKC or PI3-K, pertussis toxin, or rapamycin. The inhibition of HCO(3)(-) absorption by aldosterone was largely eliminated by the MEK/ERK inhibitors U-0126 and PD-98059. Aldosterone increased ERK activity 1.8-fold in microdissected MTALs. This ERK activation is rapid (</=5 min) and is blocked by U-0126 or PD-98059 but is unaffected by spironolactone or actinomycin D. Pretreatment with U-0126 to block ERK activation prevented the effect of aldosterone to inhibit apical NHE3. These data demonstrate that aldosterone inhibits NHE3 and HCO(3)(-) absorption in the MTAL through rapid activation of the ERK signaling pathway. The results identify NHE3 as a target for nongenomic regulation by aldosterone and establish a role for ERK in the acute regulation of NHE3 and its epithelial absorptive functions.

Na+/H+ exchangers and the regulation of volume

The regulation of volume is fundamental to life. There exist numerous conditions that can produce perturbations of cell volume. The cell has developed mechanisms to directly counteract these perturbations so as to maintain its physiological volume. Directed influx of the major extracellular cation, sodium, serves to counteract a decreased cell volume through the subsequent osmotically coupled movement of water to the intracellular space. This process, termed regulatory volume increase is often mediated by the ubiquitous sodium/hydrogen ion exchanger, NHE1. Similarly, the maintenance of intravascular volume is essential for the maintenance of blood pressure and consequently the proper perfusion of vital organs. Numerous mechanisms exist to counterbalance alterations in intravascular volume, not the least of which is the renal absorption of sodium filtered at the glomerulus. Two-thirds of filtered sodium and water are absorbed in the renal proximal tubule, a mechanism that intimately involves the apical sodium/hydrogen ion exchanger, NHE3. This isoform is fundamental to the maintenance and regulation of intravascular volume and blood pressure. In this article, the effects of cell volume on the activity of these different isoforms, NHE1 and NHE3, will be described and the consequences of their activity on intracellular and intravascular volume will be explored.

Nongenomic regulation by aldosterone of the epithelial NHE3 Na(+)/H(+) exchanger

The relevance of nongenomic pathways to regulation of epithelial function by aldosterone is poorly understood. Recently, we demonstrated that aldosterone inhibits transepithelial HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through a nongenomic pathway. Here, we examined the transport mechanism(s) responsible for this regulation, focusing on Na(+)/H(+) exchangers (NHE). In the MTAL, apical NHE3 mediates H(+) secretion necessary for HCO(3)(-) absorption; basolateral NHE1 influences HCO(3)(-) absorption by regulating apical NHE3 activity. In microperfused rat MTALs, the addition of 1 nM aldosterone rapidly decreased HCO(3)(-) absorption by 30%. This inhibition was unaffected by three maneuvers that inhibit basolateral Na(+)/H(+) exchange and was preserved in MTALs from NHE1 knockout mice, ruling out the involvement of NHE1. In contrast, exposure to aldosterone for 15 min caused a 30% decrease in apical Na(+)/H(+) exchange activity over the intracellular pH range from 6.5 to 7.7, due to a decrease in V(max). Inhibition of HCO(3)(-) absorption by aldosterone was not affected by 0.1 mM lumen Zn(2+) or 1 mM lumen DIDS, arguing against the involvement of an apical H(+) conductance or apical K(+)-HCO(3)(-) cotransport. These results demonstrate that aldosterone inhibits HCO(3)(-) absorption in the MTAL through inhibition of apical NHE3, and identify NHE3 as a target for nongenomic regulation by aldosterone. Aldosterone may influence a broad range of epithelial transport functions important for extracellular fluid volume and acid-base homeostasis through direct regulation of this exchanger.

Contribution of the PI 3-kinase/Akt survival pathway toward osmotic preconditioning

Osmolytes are rapidly lost from the ischemic heart, an effect thought to benefit the heart by reducing the osmotic load. However, the observation that chronic lowering of one of the prominent osmolytes, taurine, is more beneficial to the ischemic heart than acute taurine loss suggests that osmotic stress may benefit the ischemic heart through multiple mechanisms. The present study examines the possibility that chronic osmotic stress preconditions the heart in part by stimulating a cardioprotective, osmotic-linked signaling pathway. Hyperosmotic stress was produced by treating rat neonatal cardiomyocytes during the pre-hypoxic period with either the taurine depleting agent, beta-alanine (5 mM), or with medium containing 25 mM mannitol. The cells were then subjected to chemical hypoxia in medium containing 3 mM Amytal and 10 mM deoxyglucose but lacking beta-alanine and mannitol. Cells that had been pretreated with either 5 mM beta-alanine or 25 mM mannitol exhibited resistance against hypoxia-induced apoptosis and necrosis. Associated with the osmotically preconditioned state was the activation of Akt and the inactivation of the pro-apoptotic factor, Bad, both events blocked by the inhibition of PI 3-kinase. However, preconditioning the cardiomyocyte with mannitol had no effect on the generation of free radicals during the hypoxic period. Osmotic stress also promoted the upregulation of the anti-apoptotic factor, Bcl-2. Since inhibition of PI 3-kinase with Wortmannin also prevents osmotic-mediated cardioprotection, we conclude that hyperosmotic-mediated activation of the PI 3-kinase/Akt pathway contributes to osmotic preconditioning.

Na+ / H+ exchanger-3 is involved in mouse blastocyst formation

The mouse blastocyst consists of the trophectoderm, the inner cell mass, and a fluid-filled cavity, the blastocoel. Formation and subsequent expansion of this cavity is important for further differentiation of the inner cell mass and successful implantation. Previous work provided evidence that vectorial transport of Na+ and CL- ions through the trophectoderm into the blastocoel generates an osmotic gradient that drives fluid across this epithelium. As the activity of the Na+ / H+ exchanger (NHE) has been implicated as the exchanger responsible for facilitating the transtrophectodermal Na+ flux, the functional role of NHE in mouse blastocoel development was determined. Embryos were cultured in the presence of subtype-specific NHE inhibitors to examine the role of NHEs in blastocoel development. When 2-cell stage embryos were treated continuously with a specific inhibitor of NHE-1, cariporide, the embryos passed beyond the 8-cell stage and became blastocysts. However, in the presence of a specific inhibitor of NHE-3, S3226, the 2-cell stage embryos developed to the morula stage but formation of the blastocyst were inhibited in a dose-dependent manner. Cariporide did not inhibit the formation of the blastocoel cavity from the morula stage whereas S3226 did inhibit that process. S3226 also reduced the rate of re-expansion of blastocysts collapsed by cytochalasin D upon transfer to the control medium. An immunofluorescence study showed that NHE-3 was detected in the vicinity of the cell membrane of the trophectoderm, especially in the apical cell margins of the trophectoderm. These results suggest that NHE-3 is likely involved in blastocyst formation.

The basolateral NHE1 Na+/H+ exchanger regulates transepithelial HCO3- absorption through actin cytoskeleton remodeling in renal thick ascending limb

In the renal medullary thick ascending limb (MTAL), inhibiting the basolateral NHE1 Na(+)/H(+) exchanger with amiloride or nerve growth factor (NGF) results secondarily in inhibition of the apical NHE3 Na(+)/H(+) exchanger, thereby decreasing transepithelial HCO3- absorption. MTALs from rats were studied by in vitro microperfusion to identify the mechanism underlying cross-talk between the two exchangers. The basolateral addition of 10 microM amiloride or 0.7 nM NGF decreased HCO3- absorption by 27-32%. Jasplakinolide, which stabilizes F-actin, or latrunculin B, which disrupts F-actin, decreased basal HCO3- absorption by 30% and prevented the inhibition by amiloride or NGF. Jasplakinolide had no effect on HCO3- absorption in tubules bathed with amiloride or a Na(+)-free bath to inhibit NHE1. Jasplakinolide and latrunculin B did not prevent inhibition of HCO3- absorption by vasopressin or stimulation by hyposmolality, factors that regulate HCO3- absorption through primary effects on apical Na(+)/H(+) exchange. Treatment of MTALs with amiloride or NGF for 15 min decreased polymerized actin with no change in total cell actin, as assessed both by fluorescence microscopy and by actin Triton X-100 solubility. Jasplakinolide prevented amiloride-induced actin remodeling. Vasopressin, which inhibits HCO3- absorption by an amount similar to that observed with amiloride and NGF but does not act via NHE1, did not affect cellular F-actin content. These results indicate that basolateral NHE1 regulates apical NHE3 and HCO3- absorption in the MTAL by controlling the organization of the actin cytoskeleton.

Transepithelial HCO3- absorption is defective in renal thick ascending limbs from Na+/H+ exchanger NHE1 null mutant mice

In the medullary thick ascending limb (MTAL) of rat kidney, inhibiting basolateral Na(+)/H(+) exchange with either amiloride or nerve growth factor (NGF) results secondarily in inhibition of apical Na(+)/H(+) exchange, thereby decreasing transepithelial HCO(3)(-) absorption. To assess the possible role of the Na(+)/H(+) exchanger NHE1 in this regulatory process, MTALs from wild-type and NHE1 knockout (NHE1(-/-)) mice were studied using in vitro microperfusion. The rate of HCO(3)(-) absorption was decreased 60% in NHE1(-/-) MTALs (15.4 +/- 0.5 pmol.min(-1).mm(-1) wild-type vs. 6.0 +/- 0.5 pmol.min(-1).mm(-1) NHE1(-/-)). Transepithelial voltage, an index of the NaCl absorption rate, did not differ in wild-type and NHE1(-/-) MTALs. Basolateral addition of 10 microM amiloride or 0.7 nM NGF decreased HCO(3)(-) absorption by 45-49% in wild-type MTALs but had no effect on HCO(3)(-) absorption in NHE1(-/-) MTALs. Inhibition of HCO(3)(-) absorption by vasopressin and stimulation by hyposmolality, both of which regulate MTAL HCO(3)(-) absorption through primary effects on apical Na(+)/H(+) exchange, were similar in wild-type and NHE1(-/-) MTALs. Thus the regulatory defect in NHE1(-/-) MTALs is specific for factors (bath amiloride and NGF) shown previously to inhibit HCO(3)(-) absorption through primary effects on basolateral Na(+)/H(+) exchange. These findings demonstrate a novel role for NHE1 in transepithelial HCO(3)(-) absorption in the MTAL, in which basolateral NHE1 controls the activity of apical NHE3. Paradoxically, a reduction in NHE1-mediated H(+) extrusion across the basolateral membrane leads to a decrease in apical Na(+)/H(+) exchange activity that reduces HCO(3)(-) absorption.

Endothelin stimulates endothelial nitric oxide synthase expression in the thick ascending limb

Endothelin-1 (ET-1) acutely inhibits NaCl reabsorption by the thick ascending limb (THAL) by activating the ET(B) receptor, stimulating endothelial nitric oxide synthase (eNOS), and releasing nitric oxide (NO). In nonrenal tissue, chronic exposure to ET-1 stimulates eNOS expression via the ET(B) receptor and activation of phosphatidylinositol 3-kinase (PI3K). We hypothesized that ET-1 increases eNOS expression in the THAL by binding to ET(B) receptors and stimulating PI3K. In primary cultures of medullary THALs treated for 24 h, eNOS expression increased by 36 +/- 18% with 0.01 nM ET-1, 123 +/- 30% with 0.1 nM (P < 0.05; n = 5), and 71 +/- 30% with 1 nM, whereas 10 nM had no effect. BQ-788, a selective ET(B) receptor antagonist, completely blocked stimulation of eNOS expression caused by 0.1 nM ET-1 (12 +/- 25 vs. 120 +/- 40% for ET-1 alone; P < 0.05; n = 5). BQ-123, a selective ET(A) receptor antagonist, did not affect the increase in eNOS caused by 0.1 nM ET-1. Sarafotoxin c (S6c; 0.1 microM), a selective ET(B) receptor agonist, increased eNOS expression by 77 +/- 30% (P < 0.05; n = 6). Wortmannin (0.01 microM), a PI3K inhibitor, completely blocked the stimulatory effect of 0.1 microM S6c (77 +/- 30 vs. -28 +/- 9%; P < 0.05; n = 6). To test whether the increase in eNOS expression heightens activity, we measured NO release in response to simultaneous treatment with l-arginine, ionomycin, and clonidine using a NO-sensitive electrode. NO release by control cells was 337 +/- 61 and 690 +/- 126 pA in ET-1-treated cells (P < 0.05; n = 5). Taken together, these data suggest that ET-1 stimulates THAL eNOS, activating ET(B) receptors and PI3K and thereby increasing NO production.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Angiotensin II stimulates NHE3 activity by exocytic insertion of the transporter: role of PI 3-kinase

BACKGROUND

Low-concentration angiotensin II (Ang II) stimulates Na+/H+ exchanger 3 (NHE3) activity in renal proximal tubule mainly via angiotensin II type 1 (AT1) receptors. The mechanisms that mediate the increase in NHE3 activity elicited by Ang II remain incompletely settled.

METHODS

To assess a potential role of NHE3 trafficking in the Ang II effect, NHE3 activity was measured by H+-driven initial rate of 22Na uptake resistant to 50 micromol/L of the Na+/H+ exchange inhibitor cariporide (HOE642), and sensitive to 300 micromol/L ethyl isopropyl amiloride (EIPA), in a model of cultured proximal tubular cells (MKCC), in which functional apical NHE3 and AT receptors are normally present. Apical expression of NHE3 protein was determined by cell surface biotinylation and immunoblotting.

RESULTS

Ang II (10-10 mol/L, 43 minutes) increased NHE3 activity and biotinylated NHE3 protein without any change in total amount of NHE3 protein. Both effects were suppressed by specific AT1 receptor antagonists. When 2-mercaptoethanesulphonic acid (MESNA) was used to cleave biotin from all apical proteins, intracellular biotinylated NHE3 protein remained unchanged after Ang II incubation compared to control. When sulfo-N-hydrosuccinimide (NHS)-acetate was used first to block all apical reactive sites, an increase in biotinylated NHE3 protein was observed following Ang II incubation. To evaluate the role of phosphatidylinositol 3-kinase (PI 3-kinase), the specific inhibitor wortmannin was used. It suppressed Ang II-induced increase in NHE3 activity and trafficking. Furthermore, latrunculin B, inhibitor of actin filament polymerization, prevented both Ang II stimulatory effects.

CONCLUSION

Ang II stimulates NHE3 activity, at least in part, by exocytic insertion of the protein into the apical membrane. This effect is mediated by PI 3-kinase and required integrity of actin cytoskeleton.

P2X7 receptor activates extracellular signal-regulated kinases ERK1 and ERK2 independently of Ca2+ influx

P2X7 nucleotide receptors modulate a spectrum of cellular events in various cells including epithelia, such as exocrine pancreas. Although the pharmacology and channel properties of the P2X7 receptors have been studied intensively, signal transduction pathways are relatively unknown. In this study we applied a heterologous expression system of rat P2X7 receptors in HEK-293 cells. We followed the receptor expression and function using the enhanced green fluorescent protein (EGFP) tag, activation of intracellular proteins and increases in cellular Ca2+. EGFP-P2X7 receptors localized to the plasma membrane, clusters within the membrane and intracellularly. Stimulation of P2X7 receptors in HEK-293 cells led to an activation of extracellular signal-regulated kinases ERK1 and ERK2 and this activation was seen after just 1 min of stimulation with ATP. Using C- and N-terminal P2X7-receptor mutants we show that the N-terminus is important in activation of ERKs, whereas deletion of the last 230 amino acids in the C-terminus did not effect ERK activation. On the other hand, Ca2+ entry was impaired in C-terminal but not in N-terminal mutants. In cell suspensions prepared from rat pancreas we show that P2X7 receptors also activate ERK1 and ERK2, indicating that these signalling pathways are also turned on in native epithelium.

P-gp-induced modulation of regulatory volume increase occurs via PKC in mouse proximal tubule

The present study examined the role of protein kinase C (PKC) in the P-glycoprotein (P-gp)-induced modulation of regulatory volume increase (RVI) in the isolated nonperfused proximal tubule S2 segments from mice lacking both mdr1a and mdr1b genes (KO) and wild-type (WT) mice. The hyperosmotic solution (500 mosmol/kgH(2)O) involving 200 mM mannitol activated PKC and elicited RVI in the tubules from KO mice but not from WT mice. The addition of the hyperosmotic solution including the PKC activator phorbol 12-myristate 13-acetate (PMA) to the tubules of the WT mice activated PKC and elicited RVI. The hyperosmotic solution in the presence of the P-gp inhibitors (verapamil or cyclosporin A) elicited RVI in the tubules from the WT mice but not from the KO mice. The PMA- and the P-gp inhibitors-induced RVI was abolished by cotreatment with the PKC inhibitors (staurosporine or calphostin C). In the tubules of the KO mice, the PKC inhibitors abolished RVI, but PMA did not. In the tubules of the WT mice, the microtubule disruptor (colchicine), the microfilament disruptor (cytochalasin B), the phosphatidylinositol 3-kinase (PI 3-kinase) blocker (wortmannin), but not another PI 3-kinase blocker (LY-294002), inhibited the PMA-induced RVI. In the tubules of the KO mice, colchicine, cytochalsin B, and wortmannin abolished RVI, but LY-294002 did not. We conclude that 1) in the mouse proximal tubule, P-gp-induced modulation of RVI occurs via PKC; and 2) the microtubule, microfilament, and wortmannin-sensitive, LY-294002-insensitive PI 3-kinase contribute to the PKC-induced RVI.

Neurotrophin-3 inhibits HCO absorption via a cAMP-dependent pathway in renal thick ascending limb

Neurotrophins are expressed in the adult kidney, but their significance is unclear. We showed previously that nerve growth factor (NGF) inhibits HCO absorption in the rat medullary thick ascending limb (MTAL) via an extracellular signal-regulated kinase (ERK)-dependent pathway. Here we examined whether other neurotrophic factors affect MTAL HCO absorption. Brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor had no effect. In contrast, neurotrophin-3 (NT-3, 0.7 nM) inhibited HCO absorption by 40% (half-maximal inhibition at approximately 0.4 nM). Inhibition by NT-3 was additive to inhibition by NGF. Inhibitors of ERK activation that block inhibition by NGF had no effect on inhibition by NT-3. In contrast, 8-bromo-cAMP or forskolin pretreatment blocked inhibition by NT-3 but not NGF. Inhibition by NT-3 was also blocked by the specific protein kinase A (PKA) inhibitor myristoylated PKI(14-22) amide and by vasopressin, which inhibits HCO absorption via cAMP. Inhibitors of phosphatidylinositol 3-kinase or protein kinase C did not affect NT-3-induced inhibition, but inhibition by NT-3 was eliminated by genistein, consistent with involvement of a receptor tyrosine kinase. These results demonstrate that NT-3 inhibits HCO absorption via a cAMP- and PKA-dependent pathway. NT-3 and NGF regulate MTAL ion transport through different signal transduction mechanisms. These studies establish a direct role for NT-3 in regulation of renal tubule transport and identify the MTAL as an important target for neurotrophins, which may be involved in the control of renal acid excretion.

The expression of the gamma subunit of Na-K-ATPase is regulated by osmolality via C-terminal Jun kinase and phosphatidylinositol 3-kinase-dependent mechanisms

The alpha and beta subunits of Na-K-ATPase are up-regulated by hypertonicity in inner-medullary collecting duct cells adapted to survive in hypertonic conditions. We examined the regulation of the gamma subunit by hypertonicity. Although cultured inner-medullary collecting duct cells lacked the gamma subunits, both variants gamma(a) and gamma(b) were expressed in cells adapted to 600 and 900 mosmol/KgH(2)O. This expression was reversible with a half-time of 17.2 +/- 0.5 h. The message of the gamma subunit was absent in isotonic conditions and increased with higher tonicity in adapted cells. In acute experiments the appearance of the gamma subunit was found to be both time-dependent (> or =24 h) and osmolality-dependent (> or =500 mosmol/KgH(2)O). No induction was noted with urea and only minimal induction with mannitol. Increasing concentrations of the phosphatidylinositol 3-kinase inhibitor LY294002 resulted in a dose-dependent decrement in the expression of the gamma subunit with total abolition at 10 microM. This was associated with a decrease in cell viability as <20% survived the treatment with 10 microM of LY294002. Neither inhibition of extracellular response kinase nor p38 mitogen-activated protein kinase inhibited osmotic induction of the gamma subunit. In contrast, cells transfected with a dominant negative c-Jun N-terminal kinase 2-APF construct displayed complete inhibition of the gamma subunit. Such cells have accelerated loss of viability in hypertonic conditions. This study describes the regulation of the gamma subunit of Na-K-ATPase by hypertonicity. This regulation is transcriptionally regulated and involves signaling mediated by phosphatidylinositol 3-kinase and c-Jun N-terminal kinase 2 pathways.