A novel inhibitor of the Na+/H+ exchanger type 3 activates the central respiratory CO2 response and lowers the apneic threshold

Discovery and Optimization of 1-Phenoxy-2-aminoindanes as Potent, Selective, and Orally Bioavailable Inhibitors of the Na+/H+ Exchanger Type 3 (NHE3)

The design, synthesis, and structure-activity relationship of 1-phenoxy-2-aminoindanes as inhibitors of the Na⁺/H⁺ exchanger type 3 (NHE3) are described based on a hit from high-throughput screening (HTS). The chemical optimization resulted in the discovery of potent, selective, and orally bioavailable NHE3 inhibitors with 13d as best compound, showing high in vitro permeability and lacking CYP2D6 inhibition as main optimization parameters. Aligning 1-phenoxy-2-aminoindanes onto the X-ray structure of 13d then provided 3D-QSAR models for NHE3 inhibition capturing guidelines for optimization. These models showed good correlation coefficients and allowed for activity estimation. In silico ADMET models for Caco-2 permeability and CYP2D6 inhibition were also successfully applied for this series. Moreover, docking into the CYP2D6 X-ray structure provided a reliable alignment for 3D-QSAR models. Finally 13d, renamed as SAR197, was characterized in vitro and by in vivo pharmacokinetic (PK) and pharmacological studies to unveil its potential for reduction of obstructive sleep apneas.

Sodium/proton exchanger 3 (NHE3) and sudden infant death syndrome (SIDS)

The sodium/proton exchanger protein 3 (NHE3) is located in chemosensitive areas of the medulla oblongata and plays an important role in the central control of respiration. Overexpression of NHE3 is correlated with lower respiration and might therefore contribute to the vulnerability of infants dying suddenly and unexpected (sudden infant death syndrome, SIDS). Our aim in this study was to verify already reported genetic variations in the NHE3 gene in an independent SIDS cohort from Switzerland. Two single nucleotide polymorphisms (SNPs) in the promoter region (G1131A and C1197T) and one variation in the coding sequence of exon 16 (C2405T) in the NHE3 gene were analyzed in 160 Caucasian SIDS infants and 192 Swiss adult controls by using a single base extension method (SNaPshot multiplex). No significant differences were detected in the allelic frequencies of the three NHE3 polymorphisms between SIDS cases and controls. We conclude that the three investigated NHE3 SNPs are unlikely to play a major role in the pathogenesis of SIDS in Caucasian infants. However, further genetic investigations in different ethnicities are required to determine whether variations in NHE3 are associated with an increased SIDS risk.

Intracellular pH regulation by acid-base transporters in mammalian neurons

Intracellular pH (pH_i) regulation in the brain is important in both physiological and physiopathological conditions because changes in pH_i generally result in altered neuronal excitability. In this review, we will cover 4 major areas: (1) The effect of pH_i on cellular processes in the brain, including channel activity and neuronal excitability. (2) pH_i homeostasis and how it is determined by the balance between rates of acid loading (J_L) and extrusion (J_E). The balance between J_E and J_L determine steady-state pH_i, as well as the ability of the cell to defend pH_i in the face of extracellular acid-base disturbances (e.g., metabolic acidosis). (3) The properties and importance of members of the SLC4 and SLC9 families of acid-base transporters expressed in the brain that contribute to J_L (namely the Cl-HCO₃ exchanger AE3) and J_E (the Na-H exchangers NHE1, NHE3, and NHE5 as well as the Na⁺- coupled HCO₃⁻ transporters NBCe1, NBCn1, NDCBE, and NBCn2). (4) The effect of acid-base disturbances on neuronal function and the roles of acid-base transporters in defending neuronal pH_i under physiopathologic conditions.

Blockade of Na+/H+ exchanger type 3 causes intracellular acidification and hyperexcitability via inhibition of pH-sensitive K+ channels in chemosensitive respiratory neurons of the dorsal vagal nucleus in rats

Extracellular pH (pHe) and intracellular pH (pHi) are important factors for the excitability of chemosensitive central respiratory neurons that play an important role in respiration and obstructive sleep apnea. It has been proposed that inhibition of central Na(+)/H(+) exchanger 3 (NHE-3), a key pHi regulator in the brainstem, decreases the pHi, leading to membrane depolarization for the maintenance of respiration. However, how intracellular pH affects the neuronal excitability of respiratory neurons remains largely unknown. In this study, we showed that NHE-3 mRNA is widely distributed in respiration-related neurons of the rat brainstem, including the dorsal vagal nucleus (DVN). Whole-cell patch clamp recordings from DVN neurons in brain slices revealed that the standing outward current (Iso) through pH-sensitive K(+) channels was inhibited in the presence of the specific NHE-3 inhibitor AVE0657 that decreased the pHi. Exposure of DVN neurons to an acidified pHe and AVE0657 (5 μmol/L) resulted in a stronger effect on firing rate and Iso than acidified pHe alone. Taken together, our results showed that intracellular acidification by blocking NHE-3 results in inhibition of a pH-sensitive K(+) current, leading to synergistic excitation of chemosensitive DVN neurons for the regulation of respiration.

The Na(+)/H (+) exchanger NHE5 is sorted to discrete intracellular vesicles in the central and peripheral nervous systems

The pH milieu of the central and peripheral nervous systems is an important determinant of neuronal excitability, function, and survival. In mammals, neural acid-base homeostasis is coordinately regulated by ion transporters belonging to the Na(+)/H(+) exchanger (NHE) and bicarbonate transporter gene families. However, the relative contributions of individual isoforms within the respective families are not fully understood. This report focuses on the NHE family, specifically the plasma membrane-type NHE5 which is preferentially transcribed in brain, but the distribution of the native protein has not been extensively characterized. To this end, we generated a rabbit polyclonal antibody that specifically recognizes NHE5. In both central (cortex, hippocampus) and peripheral (superior cervical ganglia, SCG) nervous tissue of mice, NHE5 immunostaining was punctate and highly concentrated in the somas and to lesser amounts in the dendrites of neurons. Very little signal was detected in axons. Similarly, in primary cultures of differentiated SCG neurons, NHE5 localized predominantly to vesicles in the somatodendritic compartment, though some immunostaining was also evident in punctate vesicles along the axons. NHE5 was also detected predominantly in intracellular vesicles of cultured SCG glial cells. Dual immunolabeling of SCG neurons showed that NHE5 did not colocalize with markers for early endosomes (EEA1) or synaptic vesicles (synaptophysin), but did partially colocalize with the transferrin receptor, a marker of recycling endosomes. Collectively, these data suggest that NHE5 partitions into a unique vesicular pool in neurons that shares some characteristics of recycling endosomes where it may serve as an important regulated store of functional transporters required to maintain cytoplasmic pH homeostasis.

CO2 chemoreception in cardiorespiratory control

Considerable progress has been made elucidating the cellular signals and ion channel targets involved in the response to increased CO2/H+ of brain stem neurons from chemosensitive regions. Intracellular pH (pHi) does not exhibit recovery from an acid load when extracellular pH (pHo) is also acid. This lack of pHi recovery is an essential but not unique feature of all chemosensitive neurons. These neurons have pH-regulating transporters, especially Na+/H+ exchangers, but some may also contain HCO3--dependent transporters as well. Studies in locus ceruleus (LC) neurons have shown that firing rate will increase in response to decreased pHi or pHo but not in response to increased CO2 alone. A number of K+ channels, as well as other channels, have been suggested to be targets of these pH changes with a fall of pH inhibiting these channels. In neurons from some regions it appears that multiple signals and multiple channels are involved in their chemosensitive response while in neurons from other regions a single signal and/or channel may be involved. Despite the progress, a number of key issues remain to be studied. A detailed study of chemosensitive signaling needs to be done in neurons from more brain stem regions. Fully describing the chemosensitive signaling pathways in brain stem neurons will offer new targets for therapies to alter the strength of central chemosensitivity and will yield new insights into the reason why there are multiple central chemoreceptive sites.

Impact of sodium/proton exchanger 3 gene variants on sudden infant death syndrome

OBJECTIVE

To determine the contribution of variations in the sodium/proton exchanger 3 (NHE3) gene in sudden infant death syndrome (SIDS).

STUDY DESIGN

Variations in the exons and promoter of the NHE3 gene were analyzed with direct sequencing analysis and mini sequencing (SNaPshot analysis) in 251 cases of SIDS, plus 50 infant control subjects who had died of other causes, and 170 healthy adults.

RESULTS

The C2405T variant (exon 16) and 2 polymorphisms in the promoter (G1131A and C1197T) were encountered significantly more frequently in cases of SIDS than in control subjects. At least 1 of these 3 variants was detected in 49% of SIDS cases, but only in 30% of control subjects.

CONCLUSIONS

Our findings suggest the involvement of polymorphisms in the NHE3 gene and promoter in cases of SIDS, which may result in an overexpression of NHE3 in the medulla oblongata and which possibly leads to a disturbance in breathing control. Furthermore, our results underline the heterogeneous character of SIDS.

pH regulating transporters in neurons from various chemosensitive brainstem regions in neonatal rats

We studied the membrane transporters that mediate intracellular pH (pH(i)) recovery from acidification in brainstem neurons from chemosensitive regions of neonatal rats. Individual neurons within brainstem slices from the retrotrapezoid nucleus (RTN), the nucleus tractus solitarii (NTS), and the locus coeruleus (LC) were studied using a pH-sensitive fluorescent dye and fluorescence imaging microscopy. The rate of pH(i) recovery from an NH(4)Cl-induced acidification was measured, and the effects of inhibitors of various pH-regulating transporters determined. Hypercapnia (15% CO(2)) resulted in a maintained acidification in neurons from all three regions. Recovery in RTN neurons was nearly entirely eliminated by amiloride, an inhibitor of Na(+)/H(+) exchange (NHE). Recovery in RTN neurons was blocked approximately 50% by inhibitors of isoform 1 of NHE (NHE-1) but very little by an inhibitor of NHE-3 or by DIDS (an inhibitor of HCO(3)-dependent transport). In NTS neurons, amiloride blocked over 80% of the recovery, which was also blocked approximately 65% by inhibitors of NHE-1 and 26% blocked by an inhibitor of NHE-3. Recovery in LC neurons, in contrast, was unaffected by amiloride or blockers of NHE isoforms but was dependent on Na(+) and increased by external HCO(3)(-). On the basis of these findings, pH(i) recovery from acidification appears to be largely mediated by NHE-1 in RTN neurons, by NHE-1 and NHE-3 in NTS neurons, and by a Na- and HCO(3)-dependent transporter in LC neurons. Thus, pH(i) recovery is mediated by different pH-regulating transporters in neurons from different chemosensitive regions, but recovery is suppressed by hypercapnia in all of the neurons.

Systemic inhibition of the Na(+)/H (+) exchanger type 3 in intact rats activates brainstem respiratory regions

Selective inhibition of the Na(+)/H(+) exchanger type 3 (NHE3) increases the firing rate of brainstem ventrolateral CO(2)/H(+) sensitive neurons, resembling the responses evoked by hypercapnic stimuli. In anesthetized animals, NHE3 inhibition has also been shown to stimulate the central chemosensitive drive. We aimed to analyze the respiratory-related brainstem regions affected by NHE3 inhibition in anaesthetized spontaneously-breathing rats with intact peripheral afferents. For that, c-Fos immunopositive cells were counted along the brainstem in rats intravenously infused with the selective NHE3 inhibitor AVE1599. A rostral extension of the ventral respiratory column which includes the pre-Bötzinger complex was activated by the NHE3 inhibitor. In addition, the number of c-Fos positive cells resulted significantly increased in the most rostral extension of the retrotrapezoid nucleus/parapyramidal region. In the pons, the intravenous infusion of AVE1599 activated the lateral parabrachial and Kölliker-Fuse nuclei. Thus, selective NHE3 inhibition in anaesthetized rats activates the respiratory network and evokes a pattern of c-Fos expressing cells similar to that induced by hypercapnia.

Respiratory response to systemic inhibition of the Na+/H+ exchanger type 3 in intact rats

The Na+/H+ exchangers (NHEs) are a family of antiporters involved in the maintenance of neural steady-state intracellular pH. The NHE3 seems to be the predominant subtype in central chemosensitive cells. We aimed to analyze the effect of a selective NHE3 inhibition on the respiratory pattern in spontaneously breathing rats with intact vagi. Rats were intravenously infused for 10 min with the selective NHE3 inhibitor AVE1599 (Aventis Pharma Deustchland, 0.5 and 2 mg/kg) or with phosphate buffer. Whole-body plethysmography was used to monitor breathing pattern before, during, and up to 30 min after the drug infusion. Immunohistochemistry for the c-Fos protein was performed in the animal brains and c-Fos-positive cells were counted along the brainstem. Selective NHE3 inhibition induced a significant increase in the respiratory frequency and in the number of c-Fos immunopositive cells in the lateral parabrachial nucleus, the pre-Bötzinger complex and a rostral extension of the retrotrapezoid nucleus/parapyramidal region (p<0.05, ANOVA). We conclude that systemic administration of AVE1599 increases respiratory frequency and activates ponto-medullary areas implicated in the central control of breathing and chemoreception.

Role of Brainstem Sodium/Proton Exchanger 3 for Breathing Control during Chronic Acid–Base Imbalance

RATIONALE

The sodium/proton exchanger (NHE) 3 is expressed in brainstem areas with prevalence for central chemosensitivity. Selective NHE3 inhibitors can evoke CO(2) mimetic responses both in vitro and in vivo, demonstrating the functional significance of this pH-regulating protein. Moreover, levels of NHE3 expression are inversely correlated to interindividual differences of baseline ventilation in conscious rabbits.

OBJECTIVES

We explored the influence of chronic acid-base disturbances on mRNA levels of brainstem NHE3 in relation to breathing control.

METHODS

Alveolar ventilation (Va), blood gases, systemic base excess (BE), and metabolic Vco(2) were determined in rabbits shortly after exposure to either CO(2)-enriched air for 3 days (n = 5) or to ammonium chloride with drinking water for 2 days (n = 6). Untreated animals served as controls (n = 24). NHE3 mRNA within the obex region was quantified by real-time reverse transcription-polymerase chain reaction.

MEASUREMENTS AND MAIN RESULTS

After chronic hypercapnia, we found a compensatory rise of BE (mean +/- SEM) to 5.3 +/- 0.5 mmol x L(-1) with slightly elevated Pa(CO(2)). Brainstem NHE3 mRNA as well as Va were not significantly different from control levels. In the NH(4)Cl group, arterial pH was approximately 0.09 units lower than control, and BE decreased to -6.5 +/- 1.6 mmol x L(-1) with slightly decreased Pa(CO(2)), but considerably reduced Va (by approximately 25%; P < 0.05) and Vco(2). Concomitantly, brainstem NHE3 mRNA had increased from control level of 1.45 +/- 0.19 to 3.64 +/- 0.37 fg cDNA/mug RNA; P < 0.01.

CONCLUSIONS

Expression of brainstem NHE3 is up-regulated by chronic metabolic acidosis but not by prolonged hypercapnia. It is proposed that elevated brainstem NHE3 expression contributes to limit maladaptive hyperventilation during metabolic acidosis.

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.

Impaired intestinal NHE3 activity in the PDK1 hypomorphic mouse

In vitro experiments have demonstrated the stimulating effect of serum- and glucocorticoid-inducible kinase (SGK)1 on the activity of the Na+/H+ exchanger (NHE3). SGK1 requires activation by phosphoinositide-dependent kinase (PDK)1, which may thus similarly play a role in the regulation of NHE3-dependent epithelial electrolyte transport. The present study was performed to explore the role of PDK1 in the regulation of NHE3 activity. Because mice completely lacking functional PDK1 are not viable, hypomorphic mice expressing approximately 20% of PDK1 (pdk1(hm)) were compared with their wild-type littermates (pdk1(wt)). NHE3 activity in the intestine and PDK1-overexpressing HEK-293 cells was estimated by utilizing 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein fluorescence for the determination of intracellular pH. NHE activity was reflected by the Na+-dependent pH recovery from an ammonium prepulse (DeltapH(NHE)). The pH changes after an ammonium pulse allowed the calculation of cellular buffer capacity, which was not significantly different between pdk1(hm) and pdk1(wt) mice. DeltapH(NHE) was in pdk1(hm) mice, only 30 +/- 6% of the value obtained in pdk1(wt) mice. Conversely, DeltapH(NHE) was 32 +/- 7% larger in PDK1-overexpressing HEK-293 cells than in HEK-293 cells expressing the empty vector. The difference between pdk1(hm) and pdk1(wt) mice and between PDK1-overexpressing and empty vector-transfected HEK cells, respectively, was completely abolished in the presence of the NHE3 inhibitor S3226 (10 microM). In conclusion, defective PDK1 expression leads to significant impairment of NHE3 activity in the intestine, pointing to a role of PDK1-dependent signaling in the regulation of NHE-mediated electrolyte transport.

Enhanced c-Fos expression in the rostral ventral respiratory complex and rostral parapyramidal region by inhibition of the Na+/H+ exchanger type 3

Previous studies have shown that selective inhibition of Na+/H+ exchanger type 3 (NHE3) induces intracellular acidification and activates CO2/H+-sensitive medullary neurons, mimicking the responses evoked by hypercapnic stimuli. In addition, NHE3 blockers administration decreases the duration of apnoea induced by laryngeal stimulation, presumably by means of central chemoreceptor activation. To test the hypothesis that the central chemoreceptor network may be affected by NHE3 inhibition, brainstem c-Fos immunoreactive cell counting was performed after systemic administration of the NHE3 blocker AVE1599 (Aventis Pharma Deutschland GmbH) (2 mg/kg). The rostro-caudal quantitative c-Fos analysis showed a significant increase in the number of c-Fos positive cells in the rostral part of the ventral respiratory complex (VRC) as well as in the rostral part of the parapyramidal (Ppy) region. The VRC activated region (-4.2 to -3.2mm interaural) included the pre-Bötzinger complex, the rostral ventral respiratory group and the rostral ventrolateral medulla, all of them involved in cardiorespiratory control. The activated Ppy region corresponded with the rostral chemosensitive area, which elicits the strongest ventilatory response upon ventral medullary surface stimulation with H+/CO2. Most cells activated in Ppy after NHE3 inhibition were serotonergic. Hence, systemic application of NHE3 blockers may induce central chemoreceptors activation and an increase in the respiratory network activity in a similar way to known physiological stimuli such as hypercapnia. On the other hand, selective NHE3 blockers could be excellent tools for treatment of pathological states where central chemoreceptor function is diminished or absent, such as central hypoventilation syndrome or sudden infant death syndrome.

Sodium/Proton Exchanger 3 in the Medulla Oblongata and Set Point of Breathing Control

RATIONALE

In vivo inhibition of the sodium/proton exchanger 3 (NHE3) in chemosensitive neurons of the ventrolateral brainstem augments central respiratory drive in anesthetized rabbits.

OBJECTIVES

To further explore the possible role of this exchanger for the control of breathing, we examined the individual relationship between brainstem NHE3 abundance and ventilation in rabbits during wakefulness.

METHODS

In 32 adult male rabbits on standard nutritional alkali load, alveolar ventilation, metabolic CO2 production, and blood gases were determined, together with arterial and urinary acid-base status and renal base control functions. Expression of NHE3 in brainstem tissue from the obex region was determined by quantitative real-time reverse-transcription polymerase chain reaction analysis.

MEASUREMENTS AND MAIN RESULTS

Regarding the distribution above and below the median, we classified high and low brainstem NHE3 animals, expressing a mean (+/- SEM) NHE3 mRNA of 2.08 +/- 0.28 and 0.72 +/- 0.06 fg cDNA/mg RNA, respectively. Alveolar ventilation of high brainstem NHE3 animals was lower than that of low brainstem NHE3 animals (715 +/- 36 vs. 919 +/- 41 ml . minute(-1); p < 0.01), a finding also reflected by a marked difference in Pa(CO2) (5.24 +/- 0.16 vs. 4.44 +/- 0.15 kPa; p < 0.01). Among possible secondary factors, CO2 production, systemic base excess, and fractional renal base reabsorption were not found to be different.

CONCLUSIONS

We conclude that the level of brainstem NHE3 expression-most likely via intracellular pH modulation-contributes to the individual control of breathing and Pa(CO2) in conscious rabbits by adjusting the set point and the loop gain of the system.

Distribution of pH regulators in the rat laryngeal nerve: the spatial relationship between Na+/HCO3− cotransporters and Na+/H+ exchanger type 3

We studied the distribution of Na(+)/HCO(3)(-) cotransporters (NBC) and Na(+)/H(+) exchanger type 3 (NHE3) in the laryngeal nerve by immunohistochemistry to elucidate the spatial relationship of pH regulation system in the peripheral nerves. The nervous components, i.e., the nerve cells in the nodose and local ganglia and nerve fibers, were immunoreactive for NBC. Glial components such as Schwann cells and satellite cells surrounding nerve fibers and nerve cell bodies were also immunoreactive for NBC in most cases, while the cells comprising the perineurium and endoneurium were immunoreactive for NHE3. These results suggest that NBC-dependent pH regulation systems are present in the laryngeal nerve. Whereas, NHE3 may regulate extracellular pH rather than intracellular pH. In conclusion, spatial relationship of NBC and NHE3 in the laryngeal nerve would be important for pH regulation. Perineural lymph may have key role for acid-induced modulation of axons and Schwann cells.

CO2 transduction in avian intrapulmonary chemoreceptors is critically dependent on transmembrane Na+/H+ exchange

Avian intrapulmonary chemoreceptors (IPC) are vagal respiratory afferents that are inhibited by high lung Pco(2) and excited by low lung Pco(2). Previous work suggests that increased CO(2) inhibits IPC by acidifying intracellular pH (pH(i)) and that pH(i) is determined by a kinetic balance between the rate of intracellular carbonic anhydrase-catalyzed CO(2) hydration/dehydration and transmembrane extrusion of acids and/or bases by various exchangers. Here, the role of amiloride-sensitive Na(+)/H(+) exchange (NHE) in the IPC CO(2) response was tested by recording single-unit action potentials from IPC in anesthetized ducks, Anas platyrhynchos. For each of the IPC tested, blockade of the NHE using dimethyl amiloride (DMA) elicited a marked (>50%) dose-dependent decrease in mean IPC discharge (P < 0.05), suggesting that NHE is important for pH(i) regulation and CO(2) transduction in IPC. In addition, activation of the NHE using 12-O-tetradecanoylphorbol 13-acetate stimulated six of the seven IPC tested, although the overall effect was not statistically significantly (P = 0.07). Taken together, these findings suggest that CO(2) transduction in IPC is dependent on transmembrane NHE although it is likely to be much slower than carbonic anhydrase-catalyzed hydration-dehydration of CO(2).

Clathrin-mediated endocytosis and recycling of the neuron-specific Na+/H+ exchanger NHE5 isoform. Regulation by phosphatidylinositol 3'-kinase and the actin cytoskeleton

Mammalian Na+/H+ exchangers (NHEs) are a family of integral membrane proteins that play central roles in sodium, acid-base, and cell volume homeostasis. The recently cloned NHE5 isoform is expressed predominantly in brain, but its functional and cellular properties are poorly understood. To facilitate its characterization, an epitope-tagged construct of NHE5 was ectopically expressed in nonneuronal and neuronal cells. In NHE-deficient Chinese hamster ovary AP-1 cells, NHE5 localized at the plasmalemma, but a significant fraction accumulated intracellularly in vesicles that concentrated in a juxtanuclear region. Similarly, in nerve growth factor-differentiated neuroendocrine PC12 cells and primary hippocampal neurons, immunolabeling of NHE5 was detected in endomembrane vesicles in the perinuclear region of the cell body but also along the processes. More detailed characterization in AP-1 cells using organelle-specific markers showed that NHE5 co-localized with internalized transferrin, a marker of recycling endosomes. Transient transfection of a dominant negative mutant of dynamin-1, which inhibits clathrin-mediated endocytosis, blocked uptake of transferrin as well as internalization of NHE5. Likewise, wortmannin inhibition of phosphatidylinositol 3'-kinase, a lipid kinase implicated in endosomal traffic, induced coalescence of vesicles containing NHE5 and caused a pronounced inhibition of plasmalemmal Na+/H+ exchange. By contrast, disruption of the F-actin cytoskeleton with cytochalasin D increased cell surface NHE5 activity and abundance. These observations demonstrate that NHE5 is localized to the recycling endosomal pathway and is dynamically regulated by phosphatidylinositol 3'-kinase and by the state of F-actin assembly.

Inhibition of Na(+)/H(+) exchanger type 3 reduces duration of apnea induced by laryngeal stimulation in piglets

Reflexes from the larynx induce cessation of breathing in newborn animals. The magnitude of respiratory inhibition is inversely related to the level of central chemical input. Recent studies indicate that selective inhibition of Na(+)/H(+) exchanger type 3 (NHE3) activates CO(2)/H(+)-sensitive neurons, resembling the responses evoked by hypercapnic stimuli. Hence, the use of NHE3 inhibitors may reduce reflexly mediated respiratory depression and duration of apnea in the neonatal period. This possibility was examined in decerebrate, vagotomized, ventilated, and paralyzed piglets by testing the effects of i.v. administration of NHE3 blocker S8218 on the response of phrenic nerve amplitude, frequency, and duration of apnea induced by graded electrical stimulation of the superior laryngeal nerve. Superior laryngeal nerve stimulation caused a significant decrease in phrenic nerve amplitude, frequency, minute phrenic activity, and inspiratory time (all p < 0.01) that was proportional to the level of electrical stimulation. Increased levels of stimulation were more likely to induce apnea both during and after cessation of stimulation. NHE3 blocker S8218 reduced the superior laryngeal nerve stimulation-induced decrease in phrenic nerve amplitude, minute phrenic activity, and phrenic nerve frequency (all p < 0.05) and reduced superior laryngeal nerve stimulation-induced apnea and duration of poststimulation apnea (p < 0.05). In six other pigs the brain concentrations of S8218 were measured at different intervals after i.v. administration of the drug and were found to be higher in the brain tissue than plasma at all intervals. These findings suggest that the use of NHE3 blockers may decrease the duration of apnea and possibly reduce the pathophysiologic consequences of potentially life-threatening apnea in infants.

Role of intracellular and extracellular pH in the chemosensitive response of rat locus coeruleus neurones

The chemosensitive response of locus coeruleus (LC) neurones to changes in intracellular pH (pH(i)), extracellular pH (pH(o)) and molecular CO(2) were investigated using neonatal rat brainstem slices. A new technique was developed that involves the use of perforated patch recordings in combination with fluorescence imaging microscopy to simultaneously measure pH(i) and membrane potential (V(m)). Hypercapnic acidosis (15 % CO(2), pH(o) 6.8) resulted in a maintained fall in pH(i) of 0.31 pH units and a 93 % increase in the firing rate of LC neurones. On the other hand, isohydric hypercapnia (15 % CO(2), 77 mM HCO(3)(-), pH(o) 7.45) resulted in a smaller and transient fall in pH(i) of about 0.17 pH units and an increase in firing rate of 76 %. Acidified Hepes (N-2-hydroxyethylpiperazine-N'-2- ethanesulfonic acid)-buffered medium (pH(o) 6.8) resulted in a progressive fall in pH(i) of over 0.43 pH units and an increase in firing rate of 126 %. Isosmotic addition of 50 mM propionate to the standard HCO(3)(-)-buffered medium (5 % CO(2), 26 mM HCO(3)(-), pH(o) 7.45) resulted in a transient fall in pH(i) of 0.18 pH units but little increase in firing rate. Isocapnic acidosis (5 % CO(2), 7 mM HCO(3)(-), pH(o) 6.8) resulted in a slow intracellular acidification to a maximum fall of about 0.26 pH units and a 72 % increase in firing rate. For all treatments, the changes in pH(i) preceded or occurred simultaneously with the changes in firing rate and were considerably slower than the changes in pH(o). In conclusion, an increased firing rate of LC neurones in response to acid challenges was best correlated with the magnitude and the rate of fall in pH(i), indicating that a decrease in pH(i) is a major part of the intracellular signalling pathway that transduces an acid challenge into an increased firing rate in LC neurones.