Na(+)/H(+) exchange inhibition prevents endothelial dysfunction after I/R injury

Floralozone protects endothelial function in atherosclerosis by ameliorating NHE1

Endothelial dysfunction is the pathological basis of atherosclerosis. Incomplete understanding of endothelial dysfunction etiology has impeded drug development for this devastating disease despite the currently available therapies. Floralozone, an aroma flavor, specifically exists in rabbit ear grass. Recently, floralozone has been demonstrated to inhibit atherosclerosis, but the underlying mechanisms are undefined. The present study was undertaken to explore whether floralozone pharmacologically targets endothelial dysfunction and therefore exerts therapeutic effects on atherosclerosis. The Na+/H+ exchanger 1 (NHE1), a channel protein, plays a vital role in atherosclerosis. Whether NHE1 is involved in the therapeutic effects of floralozone on endothelial dysfunction has yet to be further answered. By performing oil red staining and hematoxylin-eosin staining, vascular functional study, and oxidative stress monitoring, we found that floralozone not only reduced the size of carotid atherosclerotic plaque but also prevented endothelial dysfunction in atherosclerotic rats. NHE1 expression was upregulated in the inner membrane of carotid arteries and H2O2-induced primary rat aortic endothelial cells. Inspiringly, floralozone prevented the upregulation of NHE1 in vivo and in vitro. Notably, the administration of NHE1 activator LiCl significantly weakened the protective effect of floralozone on endothelial dysfunction in vivo and in vitro. Our study demonstrated that floralozone exerted its protective effect on endothelial dysfunction in atherosclerosis by ameliorating NHE1. NHE1 maybe a drug target for the treatment of atherosclerosis, and floralozone may be an effective drug to meet the urgent needs of atherosclerosis patients by dampening NHE1.

Pathophysiology of skeletal muscle disturbances in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

Chronic Fatigue Syndrome or Myalgic Encephaloymelitis (ME/CFS) is a frequent debilitating disease with an enigmatic etiology. The finding of autoantibodies against ß2-adrenergic receptors (ß2AdR) prompted us to hypothesize that ß2AdR dysfunction is of critical importance in the pathophysiology of ME/CFS. Our hypothesis published previously considers ME/CFS as a disease caused by a dysfunctional autonomic nervous system (ANS) system: sympathetic overactivity in the presence of vascular dysregulation by ß2AdR dysfunction causes predominance of vasoconstrictor influences in brain and skeletal muscles, which in the latter is opposed by the metabolically stimulated release of endogenous vasodilators (functional sympatholysis). An enigmatic bioenergetic disturbance in skeletal muscle strongly contributes to this release. Excessive generation of these vasodilators with algesic properties and spillover into the systemic circulation could explain hypovolemia, suppression of renin (paradoxon) and the enigmatic symptoms. In this hypothesis paper the mechanisms underlying the energetic disturbance in muscles will be explained and merged with the first hypothesis. The key information is that ß2AdR also stimulates the Na+/K+-ATPase in skeletal muscles. Appropriate muscular perfusion as well as function of the Na+/K+-ATPase determine muscle fatigability. We presume that dysfunction of the ß2AdR also leads to an insufficient stimulation of the Na+/K+-ATPase causing sodium overload which reverses the transport direction of the sodium-calcium exchanger (NCX) to import calcium instead of exporting it as is also known from the ischemia-reperfusion paradigm. The ensuing calcium overload affects the mitochondria, cytoplasmatic metabolism and the endothelium which further worsens the energetic situation (vicious circle) to explain postexertional malaise, exercise intolerance and chronification. Reduced Na+/K+-ATPase activity is not the only cause for cellular sodium loading. In poor energetic situations increased proton production raises intracellular sodium via sodium-proton-exchanger subtype-1 (NHE1), the most important proton-extruder in skeletal muscle. Finally, sodium overload is due to diminished sodium outward transport and enhanced cellular sodium loading. As soon as this disturbance would have occurred in a severe manner the threshold for re-induction would be strongly lowered, mainly due to an upregulated NHE1, so that it could repeat at low levels of exercise, even by activities of everyday life, re-inducing mitochondrial, metabolic and vascular dysfunction to perpetuate the disease.

Measurement of Rapid Amiloride-Dependent pH Changes at the Cell Surface Using a Proton-Sensitive Field-Effect Transistor

We present a novel method for the rapid measurement of pH fluxes at close proximity to the surface of the plasma membrane in mammalian cells using an ion-sensitive field-effect transistor (ISFET). In conjuction with an efficient continuous superfusion system, the ISFET sensor was capable of recording rapid changes in pH at the cells’ surface induced by intervals of ammonia loading and unloading, even when using highly buffered solutions. Furthermore, the system was able to isolate physiologically relevant signals by not only detecting the transients caused by ammonia loading and unloading, but display steady-state signals as would be expected by a proton transport-mediated influence on the extracellular proton-gradient. Proof of concept was demonstrated through the use of 5-(N-ethyl-N-isopropyl)amiloride (EIPA), a small molecule inhibitor of sodium/hydrogen exchangers (NHE). As the primary transporter responsible for proton balance during cellular regulation of pH, non-electrogenic NHE transport is notoriously difficult to detect with traditional methods. Using the NHE positive cell lines, Chinese hamster ovary (CHO) cells and NHE3-reconstituted mouse skin fibroblasts (MSF), the sensor exhibited a significant response to EIPA inhibition, whereas NHE-deficient MSF cells were unaffected by application of the inhibitor.

Sodium/hydrogen exchange inhibition with cariporide reduces leukocyte adhesion via P-selectin suppression during inflammation

BACKGROUND AND PURPOSE

The Na(+)/H(+) exchange (NHE) inhibitor cariporide is known to ameliorate ischaemia/reperfusion (I/R) injury by reduction of cytosolic Ca(2+) overload. Leukocyte activation and infiltration also mediates I/R injury but whether cariporide reduces I/R injury by affecting leukocyte activation is unknown. We studied the effect of cariporide on thrombin and I/R induced leukocyte activation and infiltration models and examined P-selectin expression as a potential mechanism for any identified effects.

EXPERIMENTAL APPROACH

An in vivo rat mesenteric microcirculation microscopy model was used with stimulation by thrombin (0.5 micro ml(-1)) superfusion or ischaemia (by haemorrhagic shock for 60 min) and reperfusion (90 min).

KEY RESULTS

Treatment with cariporide (10 mg kg(-1) i.v.) significantly reduced leukocyte rolling, adhesion and extravasation after thrombin exposure. Similarly, cariporide reduced leukocyte rolling (54+/-6.2 to 2.4+/-1.0 cells min(-1), P<0.01), adherence (6.3+/-1.9 to 1.2+/-0.4 cells 100 microm(-1), P<0.01) and extravasation (9.1+/-2.1 to 2.4+/-1.1 cells per 20 x 100 microm perivascular space, P<0.05), following haemorrhagic shock induced systemic ischaemia and reperfusion. The cell adhesion molecule P-selectin showed a profound decrease in endothelial expression following cariporide administration in both thrombin and I/R stimulated groups (35.4+/-3.2 vs 14.2+/-4.1% P-selectin positive cells per tissue section, P<0.01).

CONCLUSIONS AND IMPLICATIONS

The NHE inhibitor cariporide is known to limit reperfusion injury by controlling Ca(2+) overload but these data are novel evidence for a vasculoprotective effect of NHE inhibition at all levels of leukocyte activation, an effect which is likely to be mediated at least in part by a reduction of P-selectin expression.

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.

Na+/H+ exchangers: physiology and link to hypertension and organ ischemia

PURPOSE OF REVIEW

Na/H exchangers (NHEs) are ubiquitous proteins with a very wide array of physiological functions, and they are summarized in this paper in view of the most recent advances. Hypertension and organ ischemia are two disease states of paramount importance in which NHEs have been implicated. The involvement of NHEs in the pathophysiology of these disorders is incompletely understood. This paper reviews the principal findings and current hypotheses linking NHE dysfunction to hypertension and ischemia.

RECENT FINDINGS

With the advent of large-scale sequencing projects and powerful in-silico analyses, we have come to know what is most likely the entire mammalian NHE gene family. Recent advances have detailed the roles of NHE proteins, exploring new functions such as anchoring, scaffolding and pH regulation of intracellular compartments. Studies of NHEs in disease models, even though not conclusive to date, have contributed new evidence on the interplay of ion transporters and the delicate ion balances that may become disrupted.

SUMMARY

This paper provides the interested reader with a concise overview of NHE physiology, and aims to address the implication of NHEs in the pathophysiology of hypertension and organ ischemia in light of the most recent literature.

Na+/H+ Exchanger Inhibitor Prevented Endothelial Dysfunction Induced by High Glucose

The aims of this study were to examine whether cariporide, a selective Na+/H+ exchanger inhibitor, has protective effects against endothelial dysfunction induced by high glucose in vitro and to investigate the potential mechanisms. Exposure of rat aorta rings to high glucose (44 mmol/L) for 6 hours caused an inhibition of acetylcholine-induced endothelium-dependent relaxation but had no effect on sodium nitroprusside-induced endothelium-independent relaxation. Treatment with cariporide (0.01, 0.1, 1 micromol/L) of aortic rings incubated with high-glucose medium attenuated the impaired endothelium-dependent relaxation in a dose-dependent manner. Furthermore, high glucose resulted in an increase of malondialdehyde and a decrease of superoxide dismutase activity in rat aorta rings, and these effects were reversed by cariporide. In addition, cariporide was able to inhibit the activation of Na+/H+ exchanger induced by high glucose in cultured endothelial cells. These findings suggest that the endothelial dysfunction induced by high glucose in vitro is caused by the activation of Na+/H+ exchanger.

Effect of the Na+/H+ exchange inhibitor eniporide on cardiac performance and myocardial high energy phosphates in pigs subjected to cardioplegic arrest

BACKGROUND

Pharmacologic Na(+)/H(+) exchange inhibition has been suggested to ameliorate cardiac performance depression associated with myocardial ischemia/reperfusion. The purpose of our experimental study was to investigate the impact of the novel Na(+)/H(+) exchange inhibitor Eniporide (EMD 96785) on cardiac performance and high energy phosphate content in a clinically relevant pig model of cardioplegic arrest.

METHODS

We subjected 21 pigs (47 +/- 12 [SD] kg) to cardiopulmonary bypass (CPB) and 60 minutes cold (4 degrees C) crystalloid cardioplegic arrest (Bretschneider). The pigs were randomized to receive either systemic infusion of 3 mg/kg Eniporide before cardioplegia with added 2 micromol/L Eniporide (ENI-CP+iv; n = 7); 3 mg/kg Eniporide in cardioplegia only (ENI-CP; n = 7); or no Eniporide (control; n = 7). For cardiac performance determination we measured preload recruitable stroke work and Tau, the time constant of left ventricular (LV) isovolumic relaxation using sonomicrometry and micromanometry before CPB as well as 30, 60, and 120 minutes after weaning off CPB. LV and right ventricular myocardial adenine nucleotides (ATP, ADP, and AMP), glycogen, and water content were determined at the end of the experiments.

RESULTS

Neither for standard hemodynamics including vascular pressures and cardiac index nor for cardiac performance factors did we find statistically significant differences between the groups. Similarly, myocardial adenine nucleotides, glycogene, and water content did not differ significantly between the groups.

CONCLUSIONS

In this acute study we did not find significant effects of the Na(+)/H(+) exchange inhibitor Eniporide on cardiac performance and high energy phosphate content in healthy pig hearts subjected to ischemia/reperfusion induced by crystalloid cardioplegic arrest.

Na+ overload during ischemia and reperfusion in rat hearts: comparison of the Na+/H+ exchange blockers EIPA, cariporide and eniporide

Intracellular myocardial Na+ overload during ischemia is an important cause of reperfusion injury via reversed Na+/Ca2+ exchange. Prevention of this Na+ overload can be accomplished by blocking the different Na+ influx routes. In this study the effect of ischemic inhibition of the Na+/H+ exchanger (NHE) on [Na+]i, pH, and post-ischemic contractile recovery was tested, using three different NHE-blockers: EIPA, cariporide and eniporide. pHi and [Na+]i were measured using simultaneous 31P and 23Na NMR spectroscopy, respectively, in paced (5 Hz) isolated, Langendorff perfused rat hearts while contractility was assessed by an intraventricular balloon. NHE-blockers (3 microM) were administered during 5 min prior to 30 min of global ischemia followed by 30 min drug-free reperfusion. NHE blockade markedly reduced ischemic Na+ overload; after 30 min of ischemia [Na+]i had increased to 293 +/- 26, 212 +/- 6, 157 +/- 5 and 146 +/- 6% of baseline values in untreated and EIPA (p < 0.01 vs. untreated), cariporide (p < 0.01 vs. untreated) and eniporide (p < 0.01 vs. untreated) treated hearts, respectively. Ischemic acidosis did not differ significantly between groups. During reperfusion, however, recovery of pH, was significantly delayed in treated hearts. The rate pressure product recovered to 12.0 +/- 1.9, 12.1 +/- 2.1, 19.5 +/- 2.8 and 20.4 +/- 2.5 x 10(3) mmHg/min in untreated and EIPA, cariporide (p < 0.01 vs. untreated) and eniporide (p < 0.01 vs. untreated) treated hearts, respectively. In conclusion, blocking the NHE reduced ischemic Na+ overload and improved post-ischemic contractile recovery. EIPA, however, was less effective and exhibited more side effects than cariporide and eniporide in the concentrations used.

Is timing everything? Therapeutic potential of modulators of cardiac Na(+) transporters

Sodium ion (Na(+)) transporters have roles in the modulation of cardiomyocyte pH and Na(+) and Ca(2+) handling. Activation of the cardiac Na(+)-H(+) exchanger 1 (NHE1) during ischaemia induces arrhythmias, myocardial stunning and irreversible cell injury. As the benefits of NHE1 inhibitors (e.g., amiloride, cariporide) in models of myocardial infarction are usually much greater when used as pretreatment, rather than during or after ischaemia, it is probably not surprising that clinical trials with cariporide in ischaemia have shown little shortterm benefit. NHE1 inhibitors have been shown to be beneficial in animal models of ventricular fibrillation and resuscitation, cardioplegia, hypertrophy and heart failure, and their therapeutic potential in these conditions should be further developed. The Na(+)-HCO(3)(-) cotransporter (NBC) is also stimulated by intracellular acidification, and part of the benefit of angiotensin-converting enzyme inhibitors after myocardial infarction may be due to inhibition of the NBC. Selective inhibitors of the NBC are required to determine the therapeutic potential of this mechanism. The Na(+)-Ca(2+) exchanger (NCX) has a major role in cardiac Na(+) and Ca(2+) homeostasis and influences cardiac electrical activity. The NCX also has a role in ischaemia/infarction, arrhythmias, hypertrophy and heart failure. NCX inhibitors may have beneficial effects in animal models of ischaemia and reperfusion injury and the therapeutic benefit of these should be further studied in animal models.

Is microvascular protection by cariporide and ischemic preconditioning causally linked to myocardial salvage?

Two independent cardioprotective interventions, Na(+)/H(+) exchange inhibition and ischemic preconditioning (PC), were investigated with respect to differential effects on microvascular and myocardial salvage in anesthetized rabbits (30 min of ischemia, 180 min of reperfusion). Cariporide (Car, 300 microg/kg) administered before occlusion and PC reduced infarct size (IS) as measured by triphenyltetrazolium staining [control, 46.0 +/- 4.2% of risk area (RA); Car, 17.6 +/- 3.7% (P < 0.01); PC, 27.5 +/- 4.1% (P < 0.01)] and concomitantly decreased the area of anatomic no reflow (ANR) as measured by thioflavin S staining [control, 40.4 +/- 3.7%; Car, 19.0 +/- 2.9% (P < 0.01); PC, 26.9 +/- 3.4% (P < 0.05)]. Regional myocardial blood flow (RMBF, measured by radioactive microspheres) in the RA, which deteriorated between 30 and 180 min of reperfusion (control, from 79 +/- 6 to 26 +/- 2% of nonischemic flow), was shifted to higher values with both treatments [Car, from 110 +/- 12 to 49 +/- 7% (P < 0.05); PC, from 109 +/- 8 to 38 +/- 6% (P < 0.05)]. However, neither intervention uncoupled the close relationship between IS and ANR (r = 0.92-0.95) or RMBF. Car given at reperfusion did not alter IS, ANR, RMBF, or the close interrelationships. Because size and spatial distribution of no reflow and myocardial necrosis remained closely coupled with independent cardioprotective interventions, a potential causal connection between microvascular and myocardial salvage is discussed.

Intracellular sodium hydrogen exchange inhibition and clinical myocardial protection

Although the mechanisms underlying ischemia/reperfusion injury remain elusive, evidence supports the etiologic role of intracellular calcium overload and oxidative stress induced by reactive oxygen species. Activation of the sodium hydrogen exchanger (NHE) is associated with intracellular calcium accumulation. Inhibition of the NHE-1 isoform may attenuate the consequences of this injury. Although there is strong preclinical and early clinical evidence that NHE inhibitors may be cardioprotective, definitive proof of this concept in humans awaits the results of ongoing clinical trials.