Activation of Na(+)-H+ antiporter (NHE-1) gene expression during growth, hypertrophy and proliferation of the rabbit cardiovascular system

The specific inhibition of the cardiac electrogenic sodium/bicarbonate cotransporter leads to cardiac hypertrophy

Two alkalinizing mechanisms coexist in cardiac myocytes to maintain intracellular pH: sodium/bicarbonate cotransporter (electroneutral isoform NBCn1 and electrogenic isoform NBCe1) and sodium/proton exchanger (NHE1). Dysfunction of these transporters has previously been reported to be responsible for the development of cardiovascular diseases. The aim of this study was to evaluate the contribution of the downregulation of the NBCe1 to the development of cardiac hypertrophy. To specifically reduce NBCe1 expression, we cloned shRNA into a cardiotropic adeno-associated vector (AAV9-shNBCe1). After 28 days of being injected with AAV9-shNBCe1, the expression and the activity of NBCe1 in the rat heart were reduced. Strikingly, downregulation of NBCe1 causes significant hypertrophic heart growth, lengthening of the action potential in isolated myocytes, an increase in the duration of the QT interval and an increase in the frequency of Ca²⁺ waves without any significant changes in Ca²⁺ transients. An increased compensatory expression of NBCn1 and NHE1 was also observed. We conclude that reduction of NBCe1 is sufficient to induce cardiac hypertrophy and modify the electrical features of the rat heart.

In vivo Overexpression of Electrogenic Sodium/Bicarbonate Cotransporter (NBCe1) by AAV9 Modifies the Cardiac Action Potential and the QT Interval in Mice

Cardiac cells depend on specific sarcolemmal ion transporters to assure the correct intracellular pH regulation. The sodium/bicarbonate cotransporter (NBC) is one of the major alkalinizing mechanisms. In the heart two different NBC isoforms have been described: the electroneutral NBCn1 (1Na⁺:1HCO3-) and the electrogenic NBCe1 (1Na⁺:2HCO3-). NBCe1 generates an anionic repolarizing current that modulates the action potential duration (APD). In addition to regulating the pH, the NBC is a source of sodium influx. It has been postulated that NBC could play a role in the development of hypertrophy. The aim of this research was to study the contribution of NBCe1 in heart electrophysiology and in the development of heart hypertrophy in an in vivo mouse model with overexpression of NBCe1. Heart NBCe1 overexpression was achieved by a recombinant cardiotropic adeno-associated virus (AAV9) and was evidenced by western-blot and qPCR. AAV9-mCherry was used as a transduction control. NBCe1 overexpression fails to increase heart growth. Patch clamp and electrocardiogram were performed. We observed a reduction on both, ventricular myocytes APD and electrocardiogram QT interval corrected by cardiac rate, emphasizing for the first time NBCe1 relevance for the electrical activity of the heart.

SGLT2 Inhibitors Play a Salutary Role in Heart Failure via Modulation of the Mitochondrial Function

Three cardiovascular outcome trials of sodium glucose cotransporter 2 (SGLT2) inhibitors, including the EMPA-REG OUTCOME trial, CANVAS Program, and DECLARE TIMI 58 trial, revealed that SGLT2 inhibitors were superior to a matching placebo in reducing cardiovascular events, including mortality and hospitalization for heart failure, in patients with type 2 diabetes. However, the detailed mechanism underlying the beneficial effects that SGLT2 inhibitors exert on cardiovascular diseases remains to be elucidated. We herein review the latest findings of the salutary mechanisms of SGLT2 inhibitors in cardiomyocytes, especially focusing on their mitochondrial function-mediated beneficial effects. The administration of SGLT2 inhibitors leads to the elevation of plasma levels of ketone bodies, which are an efficient energy source in the failing heart, by promoting oxidation of the mitochondrial coenzyme Q couple and enhancing the free energy of cytosolic ATP hydrolysis. SGLT2 inhibitors also promote sodium metabolism-mediated cardioprotective effects. These compounds could reduce the intracellular sodium overload to improve mitochondrial energetics and oxidative defense in the heart through binding with NHE and/or SMIT1. Furthermore, SGLT2 inhibitors could modulate mitochondrial dynamics by regulating the fusion and fission of mitochondria. Together with ongoing large-scale clinical trials to evaluate the efficacy of SGLT2 inhibitors in patients with heart failure, intensive investigations regarding the mechanism through which SGLT2 inhibitors promote the restoration in cases of heart failure would lead to the establishment of these drugs as potent anti-heart failure drugs.

Cardiac effects of SGLT2 inhibitors: the sodium hypothesis

The effects of intense glycaemic control on macrovascular complications in patients with type 2 diabetes are incompletely resolved, and many glucose-lowering medications negatively affect cardiovascular outcomes. Recently, the EMPA-REG OUTCOME trial revealed that empagliflozin, an inhibitor of the sodium-glucose cotransporter 2 (SGLT2), substantially reduced the risk of hospitalization for heart failure, death from cardiovascular causes, and all-cause mortality in patients with type 2 diabetes mellitus at high cardiovascular risk. Although several mechanisms may explain this benefit, plasma volume contraction and a metabolic switch favouring cardiac ketone bodies oxidation have recently been proposed as the major drivers. Recent experimental work has prompted a novel and intriguing hypothesis, according to which empagliflozin may reduce intracellular sodium (Na+) load observed in failing cardiac myocytes by inhibiting the sarcolemmal Na+/H+ exchanger. Since elevated intracellular Na+ hampers mitochondrial Ca2+ handling and thereby, deteriorates energy supply and demand matching and the mitochondrial antioxidative defence systems, empagliflozin may positively affect cardiac function by restoring mitochondrial function, and redox state in the failing heart. Here, we review the current evidence for such a third mechanistic hypothesis, which may foster heart failure and diabetes research into a new direction which harbours several potential targets for therapeutic intervention.

Na+/H+ exchanger isoform 1 induced cardiomyocyte hypertrophy involves activation of p90 ribosomal s6 kinase

Studies using pharmacological and genetic approaches have shown that increased activity/expression of the Na⁺/H⁺ exchanger isoform 1 (NHE1) play a critical role in the pathogenesis of cardiac hypertrophy. Despite the importance of NHE1 in cardiac hypertrophy, severe cerebrovascular side effects were associated with the use of NHE1 inhibitors when administered to patients with myocardial infarctions. p90 ribosomal S6 Kinase (RSK), a downstream regulator of the mitogen-activated protein kinase pathway, has also been implicated in cardiac hypertrophy. We hypothesized that RSK plays a role in the NHE1 induced cardiomyocyte hypertrophic response. Infection of H9c2 cardiomyoblasts with the active form of the NHE1 adenovirus induced hypertrophy and was associated with an increase in the phosphorylation of RSK (P<0.05). Parameters of hypertrophy such as cell area, protein content and atrial natriuretic mRNA expression were significantly reduced in H9c2 cardiomyoblasts infected with active NHE1 in the presence of dominant negative RSK (DN-RSK) (P<0.05). These results confirm that NHE1 lies upstream of RSK. Increased phosphorylation and activation of GATA4 at Ser²⁶¹ was correlated with increased RSK phosphorylation. This increase was reversed upon inhibition of RSK or NHE1. These findings demonstrate for the first time that the NHE1 mediated hypertrophy is accounted for by increased activation and phosphorylation of RSK, which subsequently increased the phosphorylation of GATA4; eventually activating fetal gene transcriptional machinery.

Resistance to cardiomyocyte hypertrophy in ae3-/- mice, deficient in the AE3 Cl-/HCO3- exchanger

Background

Cardiac hypertrophy is central to the etiology of heart failure. Understanding the molecular pathways promoting cardiac hypertrophy may identify new targets for therapeutic intervention. Sodium-proton exchanger (NHE1) activity and expression levels in the heart are elevated in many models of hypertrophy through protein kinase C (PKC)/MAPK/ERK/p90^RSK pathway stimulation. Sustained NHE1 activity, however, requires an acid-loading pathway. Evidence suggests that the Cl⁻/HCO₃⁻ exchanger, AE3, provides this acid load. Here we explored the role of AE3 in the hypertrophic growth cascade of cardiomyocytes.

Methods

AE3-deficient (ae3^−/−) mice were compared to wildtype (WT) littermates to examine the role of AE3 protein in the development of cardiomyocyte hypertrophy. Mouse hearts were assessed by echocardiography. As well, responses of cultured cardiomyocytes to hypertrophic stimuli were measured. pH regulation capacity of ae3^−/− and WT cardiomyocytes was assessed in cultured cells loaded with the pH-sensitive dye, BCECF-AM.

Results

ae3^−/− mice were indistinguishable from wild type (WT) mice in terms of cardiovascular performance. Stimulation of ae3^−/− cardiomyocytes with hypertrophic agonists did not increase cardiac growth or reactivate the fetal gene program. ae3^−/− mice are thus protected from pro-hypertrophic stimulation. Steady state intracellular pH (pH_i) in ae3^−/− cardiomyocytes was not significantly different from WT, but the rate of recovery of pH_i from imposed alkalosis was significantly slower in ae3^−/− cardiomyocytes.

Conclusions

These data reveal the importance of AE3-mediated Cl⁻/HCO₃⁻ exchange in cardiovascular pH regulation and the development of cardiomyocyte hypertrophy. Pharmacological antagonism of AE3 is an attractive approach in the treatment of cardiac hypertrophy.

High sodium augments angiotensin II-induced vascular smooth muscle cell proliferation through the ERK 1/2-dependent pathway

Angiotensin II (Ang II)-induced vascular injury is exacerbated by high-salt diets. This study examined the effects of high-sodium level on Ang II-induced cell proliferation in rat vascular smooth muscle cells (VSMCs). The cells were cultured in a standard medium containing 137.5 mmol l(-1) of sodium. The high-sodium medium (140 mmol l(-1)) contained additional sodium chloride. Extracellular signal-regulated kinase (ERK) 1/2 phosphorylation was determined by western blot analysis. Cell proliferation was evaluated by [(3)H]-thymidine incorporation. Ang II (100 nmol l(-1)) significantly increased ERK 1/2 phosphorylation and cell proliferation in the both medium containing standard sodium and high sodium. High-sodium level augmented Ang II-induced ERK 1/2 phosphorylation and cell proliferation compared with standard sodium. Pre-treatment with candesartan (1 μmol l(-1), Ang II type 1 receptor blocker) or PD98095 (10 μmol l(-1), ERK kinase iinhibitor) abolished the proliferative effect induced by high sodium/Ang II. Pre-treatment with 5-N,N-hexamethylene amiloride (30 μmol l(-1), Na(+)/H(+) exchanger type 1 (NHE-1) inhibitor), but not SN-6 (10 μmol l(-1), Na(+)/Ca(2+) exchanger inhibitor) or ouabain (1 mmol l(-1), Na(+)/K(+)-ATPase inhibitor) attenuated ERK 1/2 phosphorylation or cell proliferation. Osmotic pressure or chloride had no effect on Ang II-induced proliferative changes. High-sodium level did not affect Ang II receptor expression. Ang II increased intracellular pH via NHE-1 activation, and high-sodium level augmented the pH increase induced by Ang II. These data suggest that high-sodium level directly augments Ang II-induced VSMC proliferation through NHE-1- and ERK 1/2-dependent pathways and may offer new insights into the mechanisms of vascular remodeling by high-sodium/Ang II.

Intracellular Na⁺ and cardiac metabolism

In heart failure, alterations of excitation-contraction underlie contractile dysfunction. One important defect is an elevation of the intracellular Na(+) concentration in cardiac myocytes ([Na(+)]i), which has an important impact on cytosolic and mitochondrial Ca(2+) homeostasis. While elevated [Na(+)]i is thought to compensate for decreased Ca(2+) load of the sarcoplasmic reticulum (SR), it yet negatively affects energy supply-and-demand matching and can even induce mitochondrial oxidative stress. Here, we review the mechanisms underlying these pathophysiological changes. The chain of events may constitute a vicious cycle of ion dysregulation, oxidative stress and energetic deficit, resembling characteristic cellular deficits that are considered key hallmarks of the failing heart. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

The effects of amiloride, a Na+-H+ exchange inhibitor, on iliac artery stenosis after balloon injury in rabbits

This study was designed to explore the effects of amiloride, a Na+-H+ exchange (NHE) inhibitor, on vessel stenosis by observing the expression of NHE-1 protein in vascular smooth muscle (VSM) after balloon injury and the effects of amiloride on VSM cell proliferation, migration, and excretion of extracellular matrices (ECMs). A total of 32 adult male New Zealand white rabbits were randomly divided into a balloon injury group (BG), an amiloride-treated group (AG), and a sham-operated group (SG). The left iliac artery was injured by inflating a 2.5 mm x 20 mm Foley catheter in BG and AG rabbits; in SG rabbits, the Foley catheter was inserted but not inflated. Amiloride (5 mg x kg(-1) x d(-1)) was injected intraperitoneally in AG and the same volume of distilled water was used in BG 3 days before balloon injury and for 28 days after the injury. The left iliac artery was stained by hematoxylin-eosin, alpha-actin, and Masson's trichrome to observe the vessel cava, neointima, media layer, and ECMs. NHE-1 proteins of the VSM were detected by Western blotting. A narrowing of the arterial cava, neointima formation, and thickened VSM layer were observed 28 days after balloon injury in BG and AG. However, in AG, the vessel cava was not as narrowed as that of BG and the intimal areas were to a lesser extent than in BG. In AG, the alpha-actin-positive areas and the ECM areas in the neointima were increased compared with SG, but to a lesser extent than in BG. The expression of NHE-1 protein in VSM was increased in BG and AG after balloon injury; however, the levels in AG were significantly less than in BG. In conclusion, VSM cell proliferation, migration, and excretion of ECMs contributed to vessel stenosis in the BG and AG rabbits. The expression of NHE-1 protein in VSM increased after balloon injury. Amiloride, an inhibitor of NHE-1, can limit the development of vessel stenosis through inhibition of VSM cell proliferation, migration, and excretion of ECMs.

Altered expression of Na+ transporters at the mRNA level in rat normal and hypertrophic myocardium

Intracellular Na(+) ([Na(+)](i)) regulation plays a crucial role in the structural, mechanical, and electrical properties of myocardium. It is assumed that the [Na(+)](i) handling system may differ not only between normal and diseased hearts but also regionally within a heart. To gain new insight concerning disease- and region-dependent differences in the [Na(+)](i)-regulatory system, we investigated mRNA expression of Na+ transporters, the principal determinants of [Na(+)](i). Nonischemic pressure-overloaded hypertrophy was created by suprarenal abdominal aortic constriction of 50% for 7 weeks. mRNA abundances of Na(+)-Ca(2+) exchanger (NCX1), Na(+)-H(+) exchanger (NHE1), Na(+)-K(+)-2Cl(-) exchanger (NKCC1) and Na(+), K(+)-ATPase multigene family(alpha(1), alpha(2), alpha(3), and beta(1) isoforms) were measured by the real-time quantitative polymerase chain reaction method. mRNA abundance of all transporters mediating Na(+) influx (NCX1, NHE1, and NKCC1) was significantly upregulated as compared to normal. In contrast, Na(+)-efflux-mediating transporter (Na(+), K(+)-ATPase) mRNA expression was unaltered between normal and hypertrophic hearts. Losartan, an angiotensin II AT1 receptor antagonist, significantly attenuated upregulation of Na(+)-influx-mediating transporters induced by aortic constriction. The onset of Na(+)-influx-mediating transporter upregulation occurred within 5 days following constriction. In normal and hypertrophied hearts, mRNA of all Na(+)-influx-mediating transporters was expressed in order of abundance as: apex > septum approximately free wall of left ventricles. A transmural gradient in expression was also evident in normal hearts (midcardium > endo- and epicardium), which was attenuated under hypertrophic development. Myocardial hypertrophy is associated with significant changes in the spatial distribution and expression levels of Na(+) transporters. The upregulation of Na influx transporters during hypertrophy may contribute to the remodeling process, modulate contractility and promote arrhythmias.

Chronic inhibition of the Na+/H+ - exchanger causes regression of hypertrophy, heart failure, and ionic and electrophysiological remodelling

BACKGROUND AND PURPOSE

Increased activity of the Na+/H+ -exchanger (NHE-1) in heart failure underlies raised [Na+]i causing disturbances of calcium handling. Inhibition of NHE-1, initiated at the onset of pressure/volume overload, prevents development of hypertrophy, heart failure and remodelling. We hypothesized that chronic inhibition of NHE-1, initiated at a later stage, would induce regression of hypertrophy, heart failure, and ionic and electrophysiological remodelling.

EXPERIMENTAL APPROACH

Development of heart failure in rabbits was monitored electrocardiographically and echocardiographically, after one or three months. Cardiac myocytes were also isolated. One group of animals were treated with cariporide (inhibitor of NHE-1) in the diet after one month. Cytoplasmic calcium, sodium and action potentials were measured with fluorescent markers and sarcoplasmic reticulum calcium content by rapid cooling. Calcium after-transients were elicited after rapid pacing. Sodium channel current (INa) was measured using patch-clamp techniques.

KEY RESULTS

Hypertrophy and heart failure developed after one month and progressed during the next two months. After one month, dietary treatment with cariporide was initiated. Two months of treatment reduced hypertrophy and heart failure, duration of action potential QT-interval and QRS, and restored sodium and calcium handling and the incidence of calcium after-transients. In cardiac myocytes, parameters of INa were not changed by cariporide.

CONCLUSION AND IMPLICATIONS

In rabbit hearts with hypertrophy and signs of heart failure one month after induction of pressure/volume overload, two months of dietary treatment with the NHE-1 inhibitor cariporide caused regression of hypertrophy, heart failure and ionic and electrophysiological remodelling.

Enhanced activity of the myocardial Na+/H+ exchanger contributes to left ventricular hypertrophy in the Goto-Kakizaki rat model of type 2 diabetes: critical role of Akt

AIMS/HYPOTHESIS

Diabetes mellitus is a strong risk factor for the development of heart failure, and left ventricular (LV) hypertrophy has been detected in a significant proportion of diabetic patients. Because several studies have suggested that the Na(+)/H(+) exchanger (NHE1) plays a part in the molecular mechanisms involved in cardiac hypertrophy, we investigated its activity and its role in LV myocytes from the Goto-Kakizaki (GK) rat model of type 2 diabetes.

MATERIALS AND METHODS

Fluorometric measurements were used to assess sarcolemmal NHE1 activity in isolated myocytes. NHE1 levels and the possible molecular pathways driving and/or related to NHE1 activity were investigated in relation to the diabetic LV phenotype.

RESULTS

Enhanced NHE1 activity was associated with LV myocyte hypertrophy. This occurred in the absence of any change in NHE1 protein levels; however, activation of several molecular pathways related to NHE1 activity was demonstrated. Thus, phosphorylation of the extracellular signal-regulated protein kinase (Erk), of the protein kinase Akt (also known as protein kinase B) and of the Ca(2+)/calmodulin-dependent kinase II was increased in GK LV myocytes. Intracellular Ca(2+) levels were also increased. Chronic treatment (10-12 weeks) with the NHE1 inhibitor cariporide normalised NHE1 activity, decreased [Formula: see text] levels and reduced LV myocyte hypertrophy. Moreover, among the various activated pathways, cariporide treatment markedly reduced Akt activity only.

CONCLUSIONS/INTERPRETATION

These findings indicate that activation of the Akt pathway represents a likely mechanism mediating the hypertrophic effect of increased NHE1 activity in the GK model of type 2 diabetes.

Regulation of mitogen-activated protein kinase pathways by the plasma membrane Na+/H+ exchanger, NHE1

The mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, play a major role in the regulation of pivotal cellular processes such as cell death/survival balance, cell cycle progression, and cell migration. MAPK activity is regulated by a three-tiered phosphorelay system, which is in turn regulated by a complex network of signaling events and scaffolding proteins. The ubiquitous plasma membrane Na(+)/H(+) exchanger NHE1 is activated by, and implicated in, the physiological/pathophysiological responses to many of the same stimuli that modulate MAPK activity. While under some conditions, NHE1 is regulated by MAPKs, a number of studies have, conversely, implicated NHE1 in the regulation of MAPK activity. Here, we discuss the current evidence indicating the involvement of NHE1 in MAPK regulation, the mechanisms by which this may occur, and the possible physiological and pathophysiological relevance of this phenomenon.

Enhanced activity of ventricular Na+-HCO3- cotransport in pressure overload hypertrophy

The Na(+)-HCO(3)(-) cotransporter (NBC) plays a key role in intracellular pH (pH(i)) regulation in normal ventricular muscle. However, the state of NBC in nonischemic hypertrophied hearts is unresolved. In this study, we examined functional and molecular properties of NBC in adult rat ventricular myocytes. The cells were enzymatically isolated from both normal and hypertrophied hearts. Ventricular hypertrophy was induced by pressure overload created by suprarenal abdominal aortic constriction of 50% for 7 wk. pH(i) was measured in single cells using the fluorescent pH indicator 2',7'-bis(2-carboxyethyl)5-(6)carboxyfluorescein. Real-time PCR analysis was used to quantitatively assess expression of NBC-encoding mRNA, including SLC4A4 (encoding electrogenic NBC, NBCe1) and SLC4A7 (electroneutral NBC, NBCn1). Our results demonstrate that: 1) mRNA levels of both the electrogenic NBCe1 (SLC4A4) and electroneutral NBCn1 (SLC4A7) forms of NBC were increased by aortic constriction, 2) the onset of NBC upregulation occurred within 3 days after constriction, 3) normal and hypertrophied ventricles displayed regional differences in NBC expression, 4) acid extrusion via NBC (J(NBC)) was increased significantly in hypertrophied myocytes, 5) although acid extrusion via Na(+)/H(+) exchange was also increased in hypertrophied myocytes, the relative enhancement of J(NBC) was larger, 6) membrane depolarization markedly increased J(NBC) in hypertrophied myocytes, and 7) losartan, an ANG II AT(1) receptor antagonist, significantly attenuated the upregulation of both NBCs induced by 3 wk of aortic constriction. Enhanced NBC activity during hypertrophic development provides a mechanism for intracellular Na(+) overload, which may render the ventricles more vulnerable to Ca(2+) overload during ischemia-reperfusion.

Expression and characterization of the Na+/H+ exchanger in the mammalian myocardium

We examined two expression systems for studying the Na(+)/H(+) exchanger in the mammalian myocardium. Mammalian NHE1 with a hemagglutinin (HA) tag and was cloned behind the alpha myosin heavy chain promoter. Transgenic mice were made with wild type NHE1 protein or with a hyperactive NHE1 protein mutated at the calmodulin-binding domain. Three lines of transgenic mice were made of each cDNA with expression levels of each type varying from high to low. Higher levels and activity of the Na(+)/H(+) exchanger were associated with decreased long-term survival of mice, and with dilated or hypertrophic cardiomyopathy. The exogenous NHE1 protein was present in freshly made cardiomyocytes from transgenic mice, however, expression from the alpha myosin heavy chain promoter declined rapidly and little exogenous NHE1 was apparent on the fourth day after cardiomyocyte isolation. To express NHE1 protein in isolated cardiomyocytes, we transferred a mutated form of the protein into an adenoviral expression system. Infection of neonatal rat cardiomyocytes resulted in robust expression of the exogenous NHE1 protein. The mutant form of the NHE1 protein could be distinguished from the endogenous Na(+)/H(+) exchanger by its resistance to inhibition by amiloride analogs. Our results suggest that for in vivo studies on intact hearts and animals, expression in transgenic mice is an appropriate system, however for long-term studies on cardiomyocytes, this model is inappropriate due to waning expression from the alpha myosin heavy chain promoter. Therefore, infection by adenovirus is a superior system for long-term studies on cardiomyocytes in culture.

Mechanisms of Ca2+-Dependent Calcineurin Activation in Mechanical Stretch-Induced Hypertrophy

Pressure overload is the major stimulus for cardiac hypertrophy. Accumulating evidence suggests an important role for calcium-induced activation of calcineurin in mediating hypertrophic signaling. Hypertrophy is an important risk factor for cardiovascular morbidity and mortality. We therefore employed an in vitro mechanical stretch model of cultured neonatal cardiomyocytes to evaluate proposed mechanisms of calcium-induced calcineurin activation in terms of inhibition of calcineurin activity and hypertrophy. The protein/DNA ratio and ANP gene expression were used as markers for stretch-induced hypertrophy. Stretch increased the calcineurin activity, MCIP1 gene expression and DNA binding of NFATc as well as the protein/DNA ratio and ANP mRNA in a significant manner. The specific inhibitor of calcineurin, cyclosporin A, inhibited the stretch-induced increase in calcineurin activity, MCIP1 gene expression and hypertrophy. The L-type Ca2+ channel blocker nifedipine and a blocker of the Na+/H+ exchanger (cariporide) both suppressed stretch-dependent enhanced calcineurin activity and hypertrophy. Also application of a blocker of the Na+/Ca2+ exchanger (KB-R7943) was effective in preventing calcineurin activation and increases in the protein/DNA ratio. Inhibition of capacitative Ca2+ entry with SKF 96365 was also sufficient to abrogate calcineurin activation and hypertrophy. The blocker of stretch-activated ion channels, streptomycin, was without effect on stretch-induced hypertrophy and calcineurin activity. The present work suggests that of the proposed mechanisms for the calcium-induced activation of calcineurin (L-type Ca2+ channels, capacitative Ca2+ entry, Na+/H+ exchanger, Na+/Ca2+ exchanger and stretch-activated channels) all but stretch-activated channels are possible targets for the inhibition of hypertrophy.

Inhibition of Na+/H+-exchanger with sabiporide attenuates the downregulation and uncoupling of the myocardial beta-adrenoceptor system in failing rabbit hearts

Chronic heart failure (HF) is characterized by left ventricular (LV) structural remodeling, impaired function, increased circulating noradrenaline (NA) levels and impaired responsiveness of the myocardial beta-adrenoceptor (betaAR)-adenylyl cyclase (AC) system. In failing hearts, inhibition of the sodium/proton-exchanger (NHE)-1 attenuates LV remodeling and improves LV function. The mechanism(s) involved in these cardioprotective effects remain(s) unclear, but might involve effects on the impaired betaAR-AC system. Therefore, we investigated whether NHE-1 inhibition with sabiporide (SABI; 30 mg kg(-1) day(-1) p.o.) might affect myocardial betaAR density and AC activity in relation to changes in LV end-diastolic diameter (LVEDD) and LV systolic fractional shortening (LVS-FS) after 3 weeks of rapid LV pacing in rabbits. After 3 weeks of rapid LV pacing LVEDD was significantly increased (Shams 17+/-0.2 mm, n=9 vs 3 wksHF 20+/-0.5 mm, n=8; P<0.05) and LVS-FS decreased (Shams 31+/-1%, n=9 vs 3 wksHF 10+/-1%, n=8; P<0.05). SABI treatment significantly improved LV function independent of whether rabbits were treated after 1 week of pacing (3 wksHF+2 wksSABI (n=7): LVEDD 18+/-1 mm; LVS-FS 16+/-4%) or before pacing (3 wksHF+3wksSABI (n=9): LVEDD 18+/-1 mm; LVS-FS 18+/-6%). After 3 weeks of rapid LV pacing, SABI treatment significantly attenuated increases in serum NA content (Shams 0.83+/-0.19, 3 wksHF 2.68+/-0.38, 3 wksHF+2 wksSABI 1.22+/-0.32, 3 wksHF+3wksSABI 1.38+/-0.33 ng ml(-1)). Moreover, betaAR density (Shams 64+/-5, 3 wksHF 38+/-3, 3 wksHF+2 wksSABI 48+/-4, 3 wksHF+3 wksSABI 55+/-3 fmol mg(-1) protein) and responsiveness (isoprenaline-stimulated AC activity. (Shams 57.6+/-4.9, 3 wksHF 36.3+/-6.0, 3 wksHF+2 wksSABI 56.9+/-6.0, 3 wksHF+3 wksSABI 54.5+/-4.8 pmol cyclic AMP mg(-1) protein(-1) min(-1)) were significantly improved in SABI-treated rabbits. From the present data we cannot address whether the improved betaAR-AC system permitted improved LV function and/or whether the improved LV function resulted in less activation of the sympathetic nervous system and by this in a reduced stimulation of the betaAR-AC system. Accordingly, additional studies are needed to fully establish the cause-and-effect relationship between NHE-1 inhibition and the restoration of the myocardial betaAR system.

Intracellular acidification delays hormonal G2/M transition and inhibits G2/M transition triggered by thiophosphorylated MAPK in Xenopus oocytes

Xenopus oocyte maturation is analogous to G2/M transition and characterized by germinal vesicle breakdown (GVBD), spindle formation, activation of MPF and Mos-Xp42(Mpk1) pathways. It is accompanied prior to GVBD by a transient increase in intracellular pH. We determined that a well known acidifying compound, NH(4)Cl, delayed progesterone-induced GVBD in a dose-dependent manner. GVBD(50) was delayed up to 2.3-fold by 10 mM NH(4)Cl. Cyclin B2 phosphorylation, Cdk1 Tyr15 dephosphorylation as well as p39(Mos) accumulation, Xp42(Mpk1) and p90(Rsk) phosphorylation induced by progesterone were also delayed by incubation of oocyte in NH(4)Cl. The delay induced by NH(4)Cl was prevented by injection of MOPS buffer pH 7.7. In contrast to acidifying medium, alkalyzing treatment such as Tris buffer pH 9 injections, accelerated GVBD, MPF and Xp42(Mpk1) activation, indicating that pHi changes control early steps of G2/M dynamics. When injected in an immature recipient oocyte, egg cytoplasm triggers GVBD through MPF auto-amplification, independently of protein synthesis. In these conditions, GVBD and Xp42(Mpk1) activation were delayed by high concentration of NH(4)Cl, which never prevented or delayed MPF activation. Strickingly, NH(4)Cl strongly inhibited thiophosphorylated active MAPK-induced GVBD and MPF activation. Nevertheless, Tris pH 9 did not have any effects on egg cytoplasm- or active MAPK-induced GVBD. Taken together, our results suggest that dynamic of early events driving Xp42(Mpk1) and MPF activation induced by progesterone may be negatively or positively regulated by pH(i) changes. However Xp42(Mpk1) pathway was inhibited by acidification alone. Finally, MPF auto-amplification loop was not sensitive to pH(i) changes.

The Na+/H+ exchanger NHE1 in stress-induced signal transduction: implications for cell proliferation and cell death

The ubiquitous plasma membrane Na+/H+ exchanger NHE1 is highly conserved across vertebrate species and is extensively characterized as a major membrane transport mechanism in the regulation of cellular pH and volume. In recent years, the understanding of the role of NHE1 in regulating cell function has expanded from one of a household protein involved in ion homeostasis to that of a multifaceted regulator and/or modulator of a wide variety of cell functions. NHE1 plays pivotal roles in response to a number of important physiological stress conditions which, in addition to cell shrinkage and acidification, include hypoxia and mechanical stimuli, such as cell stretch. It has recently become apparent that NHE1-mediated modulation of, e.g., cell migration, morphology, proliferation, and death results not only from NHE1-mediated changes in pHi, cell volume, and/or [Na+]i, but also from direct protein-protein interactions with, e.g., ezrin/radixin/moesin (ERM) proteins and regulation of cellular signaling events, including the activity of mitogen-activated protein kinases (MAPKs) and Akt/protein kinase B (PKB). The aim of this review is to present and discuss new findings implicating NHE1 activation as a central signaling event activated by stress conditions and modulating cell proliferation and death. The pathophysiological importance of NHE1 in modulating the balance between cell proliferation and cell death in cancer and in ischemia/severe hypoxia will also be briefly addressed.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

Intracellular pH as a determinant of vascular smooth muscle function

Intracellular pH (pHi) is a physiological parameter that is intimately linked to contractility, growth and proliferation of vascular smooth muscle (VSM). Regarding contractility, no general unifying concept of pHi regulation but a rather complex relation between pHi signals and vascular tone has been revealed so far. The modulation of vasotone by pHi depends on the type of blood vessel as well as on the pattern of regulatory input signals. In addition, changes in pHi have been recognized as an important cellular signal to determine the fate of cells in terms of proliferation or apoptosis. Cellular sensors for pHi include a variety of ion transport systems which control intracellular Ca2+ gradients and are likely to serve as a link between pHi and cell functions. Here we provide an overview on the potential targets and mechanisms that transduce pHi signals in VSM. The role of pHi-sensing signaling complexes and localized pHi signaling as the basis of diversity of pHi regulation of VSM function is discussed.

Extracellular zinc triggers ERK-dependent activation of Na+/H+ exchange in colonocytes mediated by the zinc-sensing receptor

Extracellular zinc promotes cell proliferation and its deficiency leads to impairment of this process, which is particularly important in epithelial cells. We have recently characterized a zinc-sensing receptor (ZnR) linking extracellular zinc to intracellular release of calcium. In the present study, we addressed the role of extracellular zinc, acting via the ZnR, in regulating the MAP kinase pathway and Na+/H+ exchange in colonocytes. We demonstrate that Ca2+ release, mediated by the ZnR, induces phosphorylation of ERK1/2, which is highly metal-specific, mediated by physiological concentrations of extracellular Zn2+ but not by Cd2+, Fe2+, Ni2+, or Mn2+. Desensitization of the ZnR by Zn2+, is followed by approximately 90% inhibition of the Zn2+ -dependent ERK1/2 phosphorylation, indicating that the ZnR is a principal link between extracellular Zn2+ and ERK1/2 activation. Application of both the IP3 pathway and PI 3-kinase antagonists largely inhibited Zn2+ -dependent ERK1/2 phosphorylation. The physiological significance of the Zn2+ -dependent activation of ERK1/2 was addressed by monitoring Na+/H+ exchanger activity in HT29 cells and in native colon epithelium. Preincubation of the cells with zinc was followed by robust activation of Na+/H+ exchange, which was eliminated by cariporide (0.5 microm); indicating that zinc enhances the activity of NHE1. Activation of NHE1 by zinc was totally blocked by the ERK1/2 inhibitor, U0126. Prolonged acidification, in contrast, stimulates NHE1 by a distinct pathway that is not affected by extracellular Zn2+ or inhibitors of the MAP kinase pathway. Desensitization of ZnR activity eliminates the Zn2+ -dependent, but not the prolonged acidification-dependent activation of NHE1, indicating that Zn2+ -dependent activation of H+ extrusion is specifically mediated by the ZnR. Our results support a role for extracellular zinc, acting through the ZnR, in regulating multiple signaling pathways that affect pH homeostasis in colonocytes. Furthermore activation of both, ERK and NHE1, by extracellular zinc may provide the mechanism linking zinc to enhanced cell proliferation.

Hypertrophy of the heart: a new therapeutic target?

Recent studies call into question the necessity of hypertrophic growth of the heart as a "compensatory" response to hemodynamic stress. These findings, coupled with recent progress in dissecting the molecular bases of hypertrophy, raise the prospect of suppressing hypertrophy without provoking circulatory insufficiency. In this article, we focus on signaling pathways that hold promise as potential targets for therapeutic intervention. We also summarize observations from animal models and clinical trials that suggest benefit from an antihypertrophic strategy.

Na+/H+ exchange inhibition attenuates left ventricular remodeling and preserves systolic function in pressure-overloaded hearts

Cardiac hypertrophy is a homeostatic response to elevated afterload. Na+/H+ exchanger (NHE) inhibition reduces the hypertrophic response in animal models of left ventricular hypertrophy (LVH) and myocardial infarction. We examined the effect of chronic treatment with cariporide, a selective inhibitor of Na+/H+ exchanger isoform 1 (NHE-1), on left ventricular (LV) systolic and diastolic function under pressure overload conditions. Male CD-1 mice were randomized to receive either a control diet or an identical diet supplemented with 6000 p.p.m. of cariporide. Cardiac pressure overload was induced by thoracic aortic banding. LV dimension and systolic and diastolic function were assessed in sham and banded mice by echocardiography and cardiac catheterization 2 and 5 weeks after surgery. Histological analysis was also performed. After 2 weeks of pressure overload, the vehicle-treated banded mice (Veh-Bd) had enhanced normalized LV weight (about +50%) and normal chamber size and function, whereas cariporide-treated banded mice (Car-Bd) showed a preserved contractility and systolic function despite a marked attenuation of LVH. Diastolic function did not differ significantly among groups. After 5 weeks, the Veh-Bd developed LV chamber enlargement and systolic dysfunction as evidenced by a 16% increase in LV end-diastolic diameter, a 36% decrease in myocardial contractility, and a 26% reduction in percent fractional shortening. In contrast, Car-Bd showed an attenuated increase in LV mass, normal chamber size, and a maintained systolic function. A distinct histological feature was that in banded mice, cariporide attenuated the development of cardiomyocyte hypertrophy but not the attendant myocardial fibrosis. In conclusion, the results of the present study indicate that (i) the hypertrophic response to pressure overload is dependent on NHE-1 activity, and (ii) at the 5-week stage, banding-induced deterioration of LV performance is prevented by NHE-1 inhibition.British Journal of Pharmacology (2004) 141, 526-532. doi:10.1038/sj.bjp.0705631

Regulation of Expression of the Na+/H+ Exchanger by Thyroid Hormone

The Na+/H+ exchanger is a pH regulatory protein with a ubiquitous distribution in eukaryotic cells. Several isoforms of the Na+/H+ exchanger are known. The first isoform to be characterized and cloned, NHE1, is present on the plasma membrane of cells and functions to remove one intracellular proton in exchange for one extracellular sodium ion. It is involved in pH regulation, cell growth, differentiation, and cell migration. NHE1 is also involved in the cycle of damage that occurs in the heart with ischemic heart disease. Recent studies have shown that the Na+/H+ exchanger is regulated in response to thyroid hormone. Reduction in circulating thyroid hormone levels reduces the amount of both protein and mRNA of NHE1. Conversely, an elevation of thyroid hormone levels has the opposite effects. Transcriptional regulation of NHE1 expression has been demonstrated. The NHE1 promoter contains a TR alpha(1) binding site located between -841 to -800 bp. This element responds positively to TR alpha(1). This regulation of the NHE1 promoter by thyroid hormone is proposed to be responsible for postnatal changes in expression of the Na+/H+ exchanger.

ERK Is Regulated by Sodium-Proton Exchanger in Rat Aortic Vascular Smooth Muscle Cells

The purposes of this study were to test 1) the relationship between two widely studied mitogenic effector pathways, and 2) the hypothesis that sodium-proton exchanger type 1 (NHE-1) is a regulator of extracellular signal-regulated protein kinase (ERK) activation in rat aortic smooth muscle (RASM) cells. Angiotensin II (Ang II) and 5-hydroxytryptamine (5-HT) stimulated both ERK and NHE-1 activities, with activation of NHE-1 preceding that of ERK. The concentration-response curves for 5-HT and Ang II were superimposable for both processes. Inhibition of NHE-1 with pharmacological agents or by isotonic replacement of sodium in the perfusate with choline or tetramethylammonium greatly attenuated ERK activation by 5-HT or Ang II. Similar maneuvers significantly attenuated 5-HT- or Ang II-mediated activation of MEK and Ras but not transphosphorylation of the epidermal growth factor (EGF) receptor. EGF receptor blockade attenuated ERK activation, but not NHE-1 activation by 5-HT and Ang II, suggesting that the EGF receptor and NHE-1 work in parallel to stimulate ERK activity in RASM cells, converging distal to the EGF receptor but at or above the level of Ras in the Ras-MEK-ERK pathway. Receptor-independent activation of NHE-1 by acute acid loading of RASM cells resulted in the rapid phosphorylation of ERK, which could be blocked by pharmacological inhibitors of NHE-1 or by isotonic replacement of sodium, closely linking the proton transport function of NHE-1 to ERK activation. These studies identify NHE as a new regulator of ERK activity in RASM cells.

Cardiac effects of amiloride and of enalapril in the spontaneously hypertensive rat

OBJECTIVES

To compare the effects of the sodium-hydrogen exchange blocker, amiloride, with those of the angiotensin-converting enzyme inhibitor, enalapril, on cardiac structure and function and intracardiomyocyte calcium concentration ([Ca2+]i) and pH (pHi), in spontaneously hypertensive rats (SHRs).

METHODS

Experiments were performed in SHRs treated for 4 weeks with amiloride 7.5 mg/kg per day, enalapril 6.0 mg/kg per day or vehicle, and in Sprague-Dawley rats (SDRs). After haemodynamic measurements were taken, the heart was removed and weighed and hydroxyproline (a marker of collagen content) was assayed. In separate rats, ventricular myocytes were isolated, their size determined, and [Ca2+]i and pHi examined using fluo-3 acetoxymethyl ester and 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein tetrakis acetoxymethyl ester fluorescence, respectively.

RESULTS

Left ventricular end-diastolic pressure was increased, and the maximal rates of increase and of decrease in pressure with time in the left ventricle were decreased in SHRs compared with SDRs. Myocytes were larger and hydroxyproline was increased in the left ventricle, but not in the right ventricle of SHRs compared with SDRs. Amiloride and enalapril decreased systolic blood pressure in SHRs similarly, and improved diastolic function in these rats, enalapril more than amiloride. Both agents decreased left ventricular myocyte size to similar extents; however, whereas enalapril decreased the left ventricular hydroxyproline content, amiloride did not. Left ventricular myocytes from SHRs exhibited greater [Ca2+]i and pHi than those from SDRs; enalapril decreased [Ca2+]i more than amiloride, but amiloride decreased pHi more than enalapril.

CONCLUSIONS

In SHRs, enalapril prevents left ventricular hypertrophy, collagen deposition, diastolic dysfunction, and increases in [Ca2+]i more effectively than does amiloride. In contrast, the latter prevents the increase in pHi more effectively than enalapril, despite similar reductions in blood pressure. These findings suggest that their effects do not depend solely on blood pressure reduction.

Cardiac hypertrophy: the good, the bad, and the ugly

Cardiac hypertrophy is the heart's response to a variety of extrinsic and intrinsic stimuli that impose increased biomechanical stress. While hypertrophy can eventually normalize wall tension, it is associated with an unfavorable outcome and threatens affected patients with sudden death or progression to overt heart failure. Accumulating evidence from studies in human patients and animal models suggests that in most instances hypertrophy is not a compensatory response to the change in mechanical load, but rather is a maladaptive process. Accordingly, modulation of myocardial growth without adversely affecting contractile function is increasingly recognized as a potentially auspicious approach in the prevention and treatment of heart failure. In this review, we summarize recent insights into hypertrophic signaling and consider several novel antihypertrophic strategies.

Regression of isoproterenol-induced cardiac hypertrophy by Na+/H+ exchanger inhibition

Cardiac hypertrophy is often associated with an increased sympathetic drive, and both in vitro and in vivo studies have demonstrated the development of cardiomyocyte hypertrophy in response to either alpha- or beta-adrenergic stimulation. Because an association between the Na+/H+ exchanger and cellular growth has been proposed, this study aimed to analyze the possible role of the antiporter in isoproterenol-induced cardiac hypertrophy. Isoproterenol alone (5 mg/kg IP once daily) or combined with a selective inhibitor of the Na+/H+ exchanger activity (3 mg x kg(-1) x d(-1) BIIB723) was given to male Wistar rats for 30 days. Sex- and age-matched rats that received 0.9% saline IP daily served as controls. Echocardiographic follow-up showed a 33% increase in left ventricular mass in the isoproterenol-treated group, whereas it did not increase in the isoproterenol+BIIB723-treated group. Heart weight-to-body weight ratio at necropsy was 2.44+/-0.11 in controls and increased to 3.35+/-0.10 (P<0.05) with isoproterenol, an effect that was markedly attenuated by BIIB723 (2.82+/-0.07). Intense cardiomyocyte enlargement and severe subendocardial fibrosis were found in isoproterenol-treated rats, and both effects were attenuated by BIIB723. Myocardial Na+/H+ exchanger activity and protein expression significantly increased in isoproterenol-treated rats compared with the control group (1.45+/-0.11 vs 0.91+/-0.05 arbitrary units, P<0.05). This effect was significantly reduced by BIIB723 (1.17+/-0.02, P<0.05). In conclusion, our results show that Na+/H+ exchanger inhibition prevented the development of isoproterenol-induced hypertrophy and fibrosis, providing strong evidence in favor of a key role played by the antiporter in this model of cardiac hypertrophy.

Structure and function of the NHE1 isoform of the Na+/H+ exchanger

The Na+/H+ exchanger is a ubiquitous, integral membrane protein involved in pH regulation. It removes intracellular acid, exchanging a proton for an extracellular sodium ion. There are seven known isoforms of this protein that are the products of distinct genes. The first isoform discovered (NHE1) is ubiquitously distributed throughout the plasma membrane of virtually all tissues. It plays many different physiological roles in mammals, including important functions in regulation of intracellular pH, in heart disease, and in cytoskeletal organization. The first 500 amino acids of the protein are believed to consist of 12 transmembrane helices, a membrane-associated segment, and two reentrant loops. A C-terminal regulatory domain of approximately 315 amino acids regulates the protein and mediates cytoskeletal interactions. Studies are underway to determine the amino acid residues important in NHE1 function. At present, it is clear that transmembrane segment IV is important in NHE1 function and that transmembrane segments VII and IX are also involved in transport. Further experiments are required to elucidate the mechanism of transport and regulation of this multifunctional protein.

Na/H exchange isoform 1 is involved in mineralocorticoid/salt-induced cardiac injury

Long-term exposure of uninephrectomized rats to desoxycorticosterone acetate (DOCA)/salt induces cardiac fibrosis and hypertrophy through mineralocorticoid receptors (MRs). However, the underlying cellular mechanisms remain unclear. To determine whether Na/H exchange isoform 1 (NHE1) is involved in the cellular mechanisms, we examined the effects of a specific NHE1 inhibitor, cariporide, and an MR antagonist, spironolactone, on DOCA/salt-induced cardiac fibrosis and hypertrophy. Uninephrectomized rats were given 20 mg of DOCA (single subcutaneous injection) plus 0.9% NaCl/0.3% KCl to drink and were killed at 8 days. Two groups of rats given DOCA/salt were treated with either spironolactone (50 mg/kg per day SC) or cariporide (30 mg/kg per day PO) for 8 days. Control rats were treated with only high salt after the operation. The DOCA/salt-induced perivascular collagen deposition was completely abolished by cariporide and spironolactone. DOCA/salt-induced interstitial collagen deposition was partially and completely suppressed by spironolactone and cariporide, respectively. The rats exposed to DOCA/salt had cardiocyte hypertrophy in the subendocardial and subepicardial regions, a finding that was completely inhibited by cariporide but not by spironolactone. In rats given DOCA/salt, NHE1 protein expression was markedly increased. This was partially and completely reversed by spironolactone and cariporide, respectively. We concluded that cardiac NHE1 contributes to DOCA/salt-induced cardiac fibrosis and hypertrophy and that the NHE1 inhibitor cariporide completely prevents the detrimental effects of DOCA/salt on the heart. We also demonstrated that DOCA/salt-induced cardiac injury through the MRs partly occurs through NHE1 activation.

Regression of hypertensive myocardial fibrosis by Na(+)/H(+) exchange inhibition

We have recently reported that the inhibition of the Na(+)/H(+) exchanger (NHE) during 1 month in spontaneously hypertensive rats (SHR) is followed by regression of cardiomyocyte hypertrophy but not of myocardial fibrosis. The aim of this study was to evaluate whether a treatment of longer duration could reduce myocardial fibrosis and stiffness. SHR received 3.0 mg/kg per day of the specific NHE-1 inhibitor cariporide; the effect on cardiomyocyte cross-sectional area, myocardial collagen volume fraction, collagen synthesis, and myocardial stiffness (length-tension relation in left papillary muscles) was evaluated at several time points (after 1, 2, or 3 months). A slight decrease of approximately 5 mm Hg in systolic blood pressure was observed after 1 month of treatment with no further changes. After 2 and 3 months of treatment, the size of cardiomyocytes remained within normal values and myocardial fibrosis progressively decreased to normal level. Accordingly, myocardial stiffness and the serum levels of the carboxyterminal propeptide of procollagen type I, a marker of collagen type I synthesis, were normalized after 3 months. Left ventricular weight decreased from 910+/-43 (in untreated SHR) to 781+/-21 mg (treated SHR) after 3 months of treatment. No difference in body weight between treated and untreated SHR was observed after this period of treatment. The present data allow us to conclude that in the SHR the administration of an NHE-1 inhibitor for 2 or 3 months leads to the normalization of collagen type I synthesis, myocardial collagen volume fraction, and stiffness.

Upregulation of NHE3 is associated with compensatory cell growth response in young uninephrectomized rats

It is well established that after removal of renal mass, the remaining tissue undergoes compensatory growth. Several laboratories have reported that the activity of the apical membrane Na+ - H+ exchanger (NHE3) is increased after a reduction in renal mass. These studies were designed to determine whether NHE3 expression is altered early after loss of renal mass and to investigate the possible role of NHE3 activation in the compensatory tissue growth response. Experiments were performed in young male Wistar rats submitted to left nephrectomy or sham operation. At either 4 or 24 h after the surgery, the right kidney from each animal was removed and weighed. Significant increases in the wet weight of the remaining kidney were only observed 24 h after uninephrectomy (UNX). Western blot analysis of brush-border membranes and Northern blot analysis of cortex RNA showed that NHE3 protein abundance and NHE3 mRNA were greatly enhanced 4 and 24 h after UNX in relation to the sham kidney. To identify which growth pattern was mostly responsible for the enlargement of the remained kidney in our experimental models, we measured 5-bromo-2-deoxyuridine incorporation (BrdU) and protein-to-DNA ratio (protein/DNA ratio). The number of BrdU-positive nuclei increased and protein/DNA ratio slightly decreased, indicating that a hyperplastic response was the main component involved in the early compensatory renal growth in our animals. BrdU incorporation and protein/DNA were also assessed in rats treated with S3226, a selective blocker of NHE3. Neither the number of BrdU-positive nuclei nor the protein/DNA ratio was significantly altered 4 and 24 h after UNX in rats treated with S3226. In conclusion, UNX induced an upregulation of NHE3, which was evidenced at both functional and expression levels. The compensatory growth response in young UNX rats could be blocked by inhibiting NHE3 activity, suggesting that NHE3 activation may result in a facilitator state for the cell growth response in the renal proximal tubule.

Developmental regulation of Na(+)/H(+) exchanger expression in fetal and neonatal mice

We examined the hypothesis that Na(+)/H(+) exchanger expression is regulated during fetal and neonatal development and differentiation. To examine transcriptional regulation of the NHE1 isoform of the Na(+)/H(+) exchanger, transgenic mice were created that contained the mouse NHE1 promoter driving expression of green fluorescent protein. The level of NHE1 transcription varied between tissues and with the stage of embryonic development. The highest expression was in the heart and liver of 12- to 15-day-old mice, and this declined with age. To examine Na(+)/H(+) exchanger protein levels, we immunoblotted mouse tissues from 18-day-old embryos, neonates, and adults. Protein levels increased after embryonic day 18 and peaked at 14 days of age in the heart, lung, liver, kidney, and brain. The greatest rise in NHE1 protein expression occurred in the heart, whereas the smallest increase was in the brain. The results suggest that Na(+)/H(+) exchanger transcription and protein levels are controlled in a tissue-specific and time-dependent manner during development.

ERK mediates inhibition of Na(+)/H(+) exchange and HCO(3)(-) absorption by nerve growth factor in MTAL

Mitogen-activated protein (MAP) kinases mediate a variety of critical cellular events, but their role in the regulation of epithelial transport is largely undefined. Recently, we demonstrated that nerve growth factor (NGF) inhibits HCO(3)(-) absorption in the rat medullary thick ascending limb (MTAL) through an unusual mechanism: 1) NGF inhibits basolateral membrane Na(+)/H(+) exchange activity, an effect opposite to the stimulation of Na(+)/H(+) exchange by growth factors in other cells; and 2) inhibition of basolateral Na(+)/H(+) exchange results secondarily in inhibition of apical Na(+)/H(+) exchange, thereby inhibiting HCO(3)(-) absorption. In this study, we examined the role of MAP kinases in mediating inhibition by NGF. In tissue strips from the inner stripe of the outer medulla and in microdissected MTALs, NGF increased extracellular signal-regulated kinase (ERK) activity twofold but had no effect on c-Jun NH(2)-terminal kinase (JNK) or p38 MAP kinase activity. The selective MAP kinase kinase (MEK1/2) inhibitors U0126 and PD-98059 abolished the NGF-induced ERK activation and largely eliminated (> or = 60%) the effects of NGF to inhibit basolateral Na(+)/H(+) exchange activity and transepithelial HCO absorption in perfused MTALs. The MEK1/2 inhibitors did not affect inhibition of HCO(3)(-) absorption by bath ethylisopropyl amiloride, indicating that ERK activation is not involved in mediating interaction between the basolateral and apical Na(+)/H(+) exchangers. These results demonstrate that NGF inhibits basolateral Na(+)/H(+) exchange activity and HCO(3)(-) absorption in the MTAL through activation of the ERK signaling pathway. These findings identify a novel action of ERK to inhibit Na(+)/H(+) exchange activity and establish a role for MAP kinase pathways in the acute regulation of Na(+)/H(+) exchange activity and transepithelial acid secretion in renal tubules.

Inhibition of Na(+)-H(+) exchange prevents hypertrophy, fibrosis, and heart failure in beta(1)-adrenergic receptor transgenic mice

Chronic stimulation of the beta(1)-adrenergic receptor leads to hypertrophy and heart failure in beta(1)-adrenergic receptor transgenic mice and contributes to disease progression in heart failure patients. The cellular mechanisms underlying these detrimental effects are largely unknown. In this study, we have identified the cardiac Na(+)-H(+) exchanger (NHE1) as a novel mediator of adrenergically induced heart failure. beta(1)-Adrenergic receptor transgenic mice showed upregulation of both NHE1 mRNA (+140+/-6%) and protein (+42+/-19%). In order to test whether increased NHE1 is causally related to beta(1)-adrenergic-induced hypertrophy, fibrosis, and heart failure, beta(1)-adrenergic receptor transgenic (TG) and wild-type (WT) littermates were treated with a diet containing 6000 ppm of the NHE1 inhibitor cariporide or control chow for 8 months. There was significant hypertrophy of cardiac myocytes in beta(1)-adrenergic receptor transgenic mice (2.3-fold increase in myocyte cross-sectional area), which was virtually absent in cariporide-fed animals. Interstitial fibrosis was prominent throughout the left ventricular wall in nontreated beta(1)-adrenergic receptor transgenic mice (4.8-fold increase in collagen volume fraction); cariporide treatment completely prevented this development of fibrosis. Left ventricular catheterization showed that cariporide also prevented the loss of contractile function in beta(1)-adrenergic receptor transgenic mice: whereas untreated transgenic mice showed a significant decrease in left ventricular contractility (5250+/-570 mm Hg/s TG versus 7360+/-540 mm Hg/s WT, dp/dt(max)), this decrease was completely prevented by cariporide (8150+/-520 mm Hg/s TG cariporide). Inhibition of NHE1 prevented the development of heart failure in beta(1)-receptor transgenic mice. We conclude that the cardiac Na(+)-H(+) exchanger 1 is essential for the detrimental cardiac effects of chronic beta(1)-receptor stimulation in the heart.

Platelet sodium-hydrogen exchanger activity and left ventricular mass

INTRODUCTION

The sodium-hydrogen exchanger (NHE) is integral to the processes that facilitate cell growth and may contribute to the development of left ventricular hypertrophy. The aim of this study was to examine the relationship between platelet sodium-hydrogen exchanger activity and left ventricular mass index (LVMI).

METHODS

Twenty male untreated Caucasians (mean age +/- s.d.: 48 +/- 13; body mass index: 29 +/- 4 kg/m(2)) with a wide range of blood pressures were studied (mean BP: 152 +/- 22/93 +/- 15 mm Hg; range: 115-190/61-117 mm Hg). Sodium-hydrogen exchanger activity was determined as the rate of sodium-dependent recovery of intracellular pH in isolated platelets loaded with BCECF and acidified to pH 6.25 using nigericin. LVMI was calculated from measurements made by M-mode echocardiography. Associations between continuous variables were examined using parametric tests.

RESULTS

The mean rate of pHi recovery was 0.15 +/- 0.03 dpHi/s (range: 0.09-0.21). Mean LVMI was 120 +/- 32 g/m(2) (range: 56-178) and was not significantly correlated with either systolic (r = 0.39, P = 0.09) or diastolic blood pressure (r = 0.27, P = 0.3). Platelet NHE activity was not significantly correlated with LVMI (r = 0.06; P = 0.8). Platelet NHE activity was not significantly different between subjects with (n = 7 with LVMI >131 g/m(2)) and without left ventricular hypertrophy (n = 13).

CONCLUSIONS

The results of this study show that platelet NHE activity is not significantly correlated with LVMI. These data contrast with previously described correlations of LVMI with exchanger activity measured in leucocytes and erythrocytes; and indicate that the relationship between LVMI and exchanger activity may be specific to the cell type in which exchanger activity is measured.

Targeted disruption of the Nhe1 gene fails to inhibit beta(1)-adrenergic receptor-induced parotid gland hypertrophy

Chronic beta(1)-adrenergic receptor activation results in hypertrophy and hyperplasia of rodent salivary gland acinar cells. Na(+)/H(+) exchanger isoform 1 (NHE1) regulates cell volume and the induction of cell proliferation in many tissues. To investigate the relationship between NHE1 and the response of parotid glands to beta(1)-adrenergic agonists, we examined by Northern blot analysis NHE1 expression in saline-treated mice and mice 30 min and 2, 6, and 24 h after isoproterenol injection. NHE1 transcripts increased approximately 50% by 2 h, and a more than twofold increase was noted at 24 h. Isoproterenol did not acutely increase Na(+)/H(+) exchanger activity; however, exchanger activity was significantly elevated by 24 h. To test whether NHE1 activity is essential for inducing salivary gland hypertrophy in vivo, mice with targeted disruption of Nhe1 were treated with isoproterenol. Na(+)/H(+) exchanger activity was absent in acinar cells from Nhe1(-/-) mice, nevertheless, the lack of NHE1 failed to inhibit isoproterenol-induced hypertrophy. These data directly demonstrate that acinar cell hypertrophy induced by chronic beta(1)-adrenergic receptor stimulation occurs independently of NHE1 activity.

Impaired [Ca(2+)](i) and pH(i) responses to kappa-opioid receptor stimulation in the heart of chronically hypoxic rats

kappa-Opioid receptor (kappa-OR) stimulation with U50,488H, a selective kappa-OR agonist, or activation of protein kinase C (PKC) with 4-phorbol 12-myristate 13-acetate (PMA), an activator of PKC, decreased the electrically induced intracellular Ca(2+) ([Ca(2+)](i)) transient and increased the intracellular pH (pH(i)) in single ventricular myocytes of rats subjected to 10% oxygen for 4 wk. The effects of U50,488H were abolished by nor-binaltorphimine, a selective kappa-OR antagonist, and calphostin C, a specific inhibitor of PKC, while the effects of PMA were abolished by calphostin C and ethylisopropylamiloride (EIPA), a potent Na(+)/H(+) exchange blocker. In both right hypertrophied and left nonhypertrophied ventricles of chronically hypoxic rats, the effects of U50,488H or PMA on [Ca(2+)](i) transient and pH(i) were significantly attenuated and completely abolished, respectively. Results are first evidence that the [Ca(2+)](i) and pH(i) responses to kappa-OR stimulation are attenuated in the chronically hypoxic rat heart, which may be due to reduced responses to PKC activation. Responses to all treatments were the same for right and left ventricles, indicating that the functional impairment is independent of hypertrophy. kappa-OR mRNA expression was the same in right and left ventricles of both normoxic and hypoxic rats, indicating no regional specificity.

Mechanisms underlying hypoxia-induced neuronal apoptosis

In vivo models of cerebral hypoxia-ischemia have shown that neuronal death may occur via necrosis or apoptosis. Necrosis is, in general, a rapidly occurring form of cell death that has been attributed, in part, to alterations in ionic homeostasis. In contrast, apoptosis is a delayed form of cell death that occurs as the result of activation of a genetic program. In the past decade, we have learned considerably about the mechanisms underlying apoptotic neuronal death following cerebral hypoxia-ischemia. With this growth in knowledge, we are coming to the realization that apoptosis and necrosis, although morphologically distinct, are likely part of a continuum of cell death with similar operative mechanisms. For example, following hypoxia-ischemia, excitatory amino acid release and alterations in ionic homeostasis contribute to both necrotic and apoptotic neuronal death. However, apoptosis is distinguished from necrosis in that gene activation is the predominant mechanism regulating cell survival. Following hypoxic-ischemic episodes in the brain, genes that promote as well as inhibit apoptosis are activated. It is the balance in the expression of pro- and anti-apoptotic genes that likely determines the fate of neurons exposed to hypoxia. The balance in expression of pro- and anti-apoptotic genes may also account for the regional differences in vulnerability to hypoxic insults. In this review, we will examine the known mechanisms underlying apoptosis in neurons exposed to hypoxia and hypoxia-ischemia.

Regulation of sarcolemmal Na(+)/H(+) exchanger activity by angiotensin II in adult rat ventricular myocytes: opposing actions via AT(1) versus AT(2) receptors

Increased sarcolemmal Na(+)/H(+) exchanger activity has been implicated as a mediator of the cardiac actions of angiotensin II. We studied the receptor subtypes and signaling pathways involved in the regulation of sarcolemmal Na(+)/H(+) exchanger activity by angiotensin II in adult rat ventricular myocytes. Cells were loaded with the pH-sensitive fluoroprobe carboxy-seminaphthorhodafluor-1, and acid efflux rates estimated during recovery from intracellular acidosis were used to quantify exchanger activity. Sarcolemmal Na(+)/H(+) exchanger activity was not affected by angiotensin II alone but was increased by angiotensin II plus PD123319 (AT(2) antagonist). In contrast, angiotensin II plus losartan (AT(1) antagonist) or CGP42112A (AT(2) agonist) did not affect exchanger activity. The increase in Na(+)/H(+) exchanger activity induced by angiotensin II plus PD123319 was blocked by losartan, PD98059 (extracellular signal-regulated kinase inhibitor), GF109203X (protein kinase C inhibitor), and tyrphostin AG1478 (epidermal growth factor receptor kinase inhibitor). Extracellular signal-regulated kinase phosphorylation and activity, measured by immunoblot analysis and an immune-complex kinase assay, respectively, were increased significantly by angiotensin II plus PD123319; these increases were blocked by losartan and PD98059. The increase in extracellular signal-regulated kinase phosphorylation induced by angiotensin II plus PD123319 was blocked also by GF109203X and tyrphostin AG1478. These data show that AT(1) stimulation increases sarcolemmal Na(+)/H(+) exchanger activity in adult rat ventricular myocytes and that this response requires extracellular signal-regulated kinase activation through a protein kinase C- and epidermal growth factor receptor-mediated mechanism. The positive effect of AT(1) stimulation on Na(+)/H(+) exchanger activity is counteracted by simultaneous AT(2) stimulation through a mechanism that does not involve direct inhibition of the exchanger or attenuation of extracellular signal-regulated kinase activation.

Na(+)influx via Na(+)/H(+)exchange activates protein kinase C isozymes delta and epsilon in cultured neonatal rat cardiac myocytes

Protein kinase C (PKC) is one of the important signaling molecules in the development of the cardiac hypertrophic response, and activation of Na(+)/H(+)exchange is caused by PKC in myocytes. In this study we examined the contribution of Na(+)/H(+)exchange in cardiac hypertrophy induced by the activation of PKC and its mechanism using cultured neonatal rat cardiac myocytes. Phenylephrine (PE), endothelin-1 (ET-1) and phorbol 12-myristate 13-acetate (PMA) increased cytoplasmic pH in myocytes, and this effect was strongly inhibited by treatment with HOE694, an inhibitor of Na(+)/H(+)exchange. These substances increased the [(3)H]phenylalanine incorporation, total protein content and beta -myosin heavy chain protein content in myocytes. These hypertrophic responses were also attenuated by HOE694. To clarify the role of Na(+)influx through activation of Na(+)/H(+)exchange in cardiac hypertrophy, we next examined the hypertrophic responses to veratridine and ouabain, which increase the intracellular Na(+)content. Veratridine and ouabain increased the [(3)H]phenylalanine incorporation. Staurosporine, a PKC inhibitor, completely abolished veratridine-induced hypertrophic response, but did not affect increment of intracellular Na(+)concentration by veratridine. PMA caused increases of alpha -, delta -and epsilon -PKC in the particulate fraction, but PE, ET-1 and veratridine affected only those of delta - and epsilon -PKC. HOE694 significantly inhibited only increases of delta - and epsilon -PKC caused by PE, ET-1 or PMA, but not those by veratridine. These results demonstrate that Na(+)influx via activation of Na(+)/H(+)exchange reactivates PKC in myocytes. delta - and epsilon -PKC appear to be involved in the signal mechanism of the hypertrophic response induced by Na(+)influx through Na(+)/H(+)exchange in myocytes.

Functional and cellular regulation of the myocardial Na+/H+ exchanger

The Na(+)/H(+) exchanger is a pH-regulatory protein present in the plasma membrane of cardiomyocytes and other cell types. In response to intracellular acidosis, the protein removes one intracellular proton in exchange for an extracellular sodium. The protein consists of a membrane transporting domain and a regulatory cytosolic domain. The regulatory cytosolic domain mediates the stimulation of the membrane domain. Hormonal stimulation of myocardial cells results in activation of the antiporter, possibly through protein kinases and other regulatory proteins. Several hormones and growth factors have been shown to stimulate the antiporter in the myocardium, including endothelin, thrombin, angiotensin II, and alpha(1)-adrenergic stimulation. The exact mechanisms involved in this stimulation are as yet unclear, and may be important in regulation of the Na(+)/H(+) exchanger during ischemia and reperfusion.

Regulation of cardiac sarcolemmal Na+/H+ exchanger activity: potential pathophysiological significance of endogenous mediators and oxidant stress

The cardiac sarcolemmal Na(+)/H(+) exchanger (NHE) extrudes one H(+) in exchange for one Na(+) entering the myocyte, utilizing for its driving force the inwardly directed Na(+) gradient maintained by the Na(+), K(+)-ATPase. The exchanger is quiescent at physiological values of intracellular pH but becomes activated in response to intracellular acidosis. Recent evidence suggests that a variety of extracellular signals (e.g., adrenergic agonists, thrombin, endothelin, and oxidant stress) also modulate sarcolemmal NHE activity by altering its sensitivity to intracellular H(+). Because sarcolemmal NHE activity is believed to be an important determinant of the extent of myocardial injury during ischemia and reperfusion, regulation of exchanger activity by factors that are associated with ischemia is likely to be pathophysiological importance.