Na+-H+ exchange in cardiac sarcolemmal vesicles

Enhanced Na+/H+ exchange during ischemia and reperfusion impairs mitochondrial bioenergetics and myocardial function

Inhibition of Na+/H+ exchange (NHE) during ischemia reduces cardiac injury due to reduced reverse mode Na+/Ca2+ exchange. We hypothesized that activating NHE-1 at buffer pH 8 during ischemia increases mitochondrial oxidation, Ca2+ overload, and reactive O2 species (ROS) levels and worsens functional recovery in isolated hearts and that NHE inhibition reverses these effects. Guinea pig hearts were perfused with buffer at pH 7.4 (control) or pH 8 +/- NHE inhibitor eniporide for 10 minutes before and for 10 minutes after 35- minute ischemia and then for 110 minutes with pH 7.4 buffer alone. Mitochondrial NADH and FAD, [Ca2+], and superoxide were measured by spectrophotofluorometry. NADH and FAD were more oxidized, and cardiac function was worse throughout reperfusion after pH 8 versus pH 7.4, Ca2+ overload was greater at 10-minute reperfusion, and superoxide generation was higher at 30-minute reperfusion. The pH 7.4 and eniporide groups exhibited similar mitochondrial function, and cardiac performance was most improved after pH 7.4+eniporide. Cardiac function on reperfusion after pH 8+eniporide was better than after pH 8. Percent infarction was largest after pH 8 and smallest after pH 7.4+eniporide. Activation of NHE with pH 8 buffer and the subsequent decline in redox state with greater ROS and Ca2+ loading underlie the poor functional recovery after ischemia and reperfusion.

Age-related differences in myocardial hydrogen ion buffering during ischemia

BACKGROUND

During myocardial ischemia, accumulation of end products from anaerobic glycolysis (hydrogen ions (H(+)), lactate) can cause cellular injury, consequently affecting organ function. The cells' ability to buffer H(+) (buffering capacity (BC)) plays an important role in ischemic tolerance. Age related differences in myocardial lactate and H(+) accumulation (one hour of ischemia) as well as differences in BC, myoglobin (Mb) and histidine (His) contents in the left (LV) and right (RV) ventricles were assessed in neonatal compared to adult pigs. The BC of the septum was also compared.

METHODS AND RESULTS

Neonatal RV and LV had lactate accumulations of 43% and 63% and significantly greater H(+) (p < 0.004) compared to the adult. In the neonate LV, BC was 17% significantly poorer (p = 0.0001), had 33% lower Mb (p = 0.0002) and 15% lower His content (p = 0.0004) when compared to the adult. In the RV, despite similar BC between the neonate and adult, myoglobin content was 36% (p = 0.0004) lower in the neonate. The neonate septum had a BC that was 11% lower than that of the adult. With maturation, the adult LV had a BC that was 10% greater (p < 0.01) than the RV while the septum mirrored that of the LV.

CONCLUSIONS

During maturation to adulthood, the BC of the septum begins to closely resemble the LV. Neonatal hearts have a potentially greater vulnerability to acid-base disturbances during ischemia in both ventricles when compared to hearts of adults. This is due to lower levels of myoglobin and histidine in the young, which could render them more susceptible to injury during ischemia. During myocardial ischemia, H(+) and lactate accumulation may pose deleterious effects on the heart. The ability to buffer H(+) (buffering capacity, BC) affects ischemic tolerance. Although lactate accumulation during 1 h of global ischemia was similar between ventricles of neonatal and adult swine, H(+) accumulation was greater and BC, Mb and His content were lower. With maturation, LV BC was higher than the RV while septum developmentally resembled the LV. Thus, hearts of neonates may be at a greater risk of ischemic injury compared to hearts of adults.

Ion fluxes in giant excised cardiac membrane patches detected and quantified with ion-selective microelectrodes

We have used ion-selective electrodes (ISEs) to quantify ion fluxes across giant membrane patches by measuring and simulating ion gradients on both membrane sides. Experimental conditions are selected with low concentrations of the ions detected on the membrane side being monitored. For detection from the cytoplasmic (bath) side, the patch pipette is oscillated laterally in front of an ISE. For detection on the extracellular (pipette) side, ISEs are fabricated from flexible quartz capillary tubing (tip diameters, 2-3 microns), and an ISE is positioned carefully within the patch pipette with the tip at a controlled distance from the mouth of the patch pipette. Transport activity is then manipulated by solution changes on the cytoplasmic side. Ion fluxes can be quantified by simulating the ion gradients with appropriate diffusion models. For extracellular (intrapatch pipette) recordings, ion diffusion coefficients can be determined from the time courses of concentration changes. The sensitivity and utility of the methods are demonstrated with cardiac membrane patches by measuring (a) potassium fluxes via ion channels, valinomycin, and Na/K pumps; (b) calcium fluxes mediated by Na/Ca exchangers; (c) sodium fluxes mediated by gramicidin and Na/K pumps; and (d) proton fluxes mediated by an unknown electrogenic mechanism. The potassium flux-to-current ratio for the Na/K pump is approximately twice that determined for potassium channels and valinomycin, as expected for a 3Na/2K pump stoichiometery (i.e., 2K/charge moved). For valinomycin-mediated potassium currents and gramicidin-mediated sodium currents, the ion fluxes calculated from diffusion models are typically 10-15% smaller than expected from the membrane currents. As presently implemented, the ISE methods allow reliable detection of calcium and proton fluxes equivalent to monovalent cation currents <1 pA in magnitude, and they allow detection of sodium and potassium fluxes equivalent to <5 pA currents. The capability to monitor ion fluxes, independent of membrane currents, should facilitate studies of both electrogenic and electroneutral ion-coupled transporters in giant patches.

Effects of omega-3 polyunsaturated fatty acids on cardiac sarcolemmal Na(+)/H(+) exchange

Myocardial ischemia-reperfusion activates the Na(+)/H(+) exchanger, which induces arrhythmias, cell damage, and eventually cell death. Inhibition of the exchanger reduces cell damage and lowers the incidence of arrhythmias after ischemia-reperfusion. The omega-3 polyunsaturated fatty acids (PUFAs) are also known to be cardioprotective and antiarrhythmic during ischemia-reperfusion challenge. Some of the action of PUFAs may occur via inhibition of the Na(+)/H(+) exchanger. The purpose of our study was to determine the capacity for selected PUFAs to alter cardiac sarcolemmal (SL) Na(+)/H(+) exchange. Cardiac membranes highly enriched in SL vesicles were exposed to 10-100 microM eicosapentanoic acid (EPA) or docosahexanoic acid (DHA). H(+)-dependent (22)Na(+) uptake was inhibited by 30-50% after treatment with > or =50 microM EPA or > or =25 microM DHA. This was a specific effect of these PUFAs, because 50 microM linoleic acid or linolenic acid had no significant effect on Na(+)/H(+) exchange. The SL vesicles did not exhibit an increase in passive Na(+) efflux after PUFA treatment. In conclusion, EPA and DHA can potently inhibit cardiac SL Na(+)/H(+) exchange at physiologically relevant concentrations. This may explain, in part, their known cardioprotective effects and antiarrhythmic actions during ischemia-reperfusion.

Altered cardiac Na(+)/H(+) exchange in phospholipase D-treated sarcolemmal vesicles

Cardiac sarcolemmal Na(+)/H(+) exchange is critical for the regulation of intracellular pH, and its activity contributes to ischemia-reperfusion injury. It has been suggested that the membrane phospholipid environment does not modulate Na(+)/H(+) exchange. The present study was carried out to determine the effects on Na(+)/H(+) exchange of modifying the endogenous membrane phospholipids through the addition of exogenous phospholipase D. Incubation of 0.825 U of phospholipase D with 1 mg of porcine cardiac sarcolemmal vesicles hydrolyzed 34 +/- 2% of the sarcolemmal phosphatidylcholine and increased phosphatidic acid 10.2 +/- 0.5-fold. Treatment of vesicles with phospholipase D resulted in a 46 +/- 2% inhibition of Na(+)/H(+) exchange. Na(+)/H(+) exchange was measured as a function of reaction time, extravesicular pH, and extravesicular Na(+). All of these parameters of Na(+)/H(+) exchange were inhibited following phospholipase D treatment compared with untreated controls. Passive efflux of Na(+) was unaffected. Treatment of sarcolemmal vesicles with phospholipase C had no effect on Na(+)/H(+) exchange. We conclude that phospholipase D-induced changes in the cardiac sarcolemmal membrane phospholipid environment alter Na(+)/H(+) exchange.

Subcellular localization of the Na+/H+ exchanger NHE1 in rat myocardium

The Na+/H+ exchanger NHE1 isoform is an integral component of cardiac intracellular pH homeostasis that is critically important for myocardial contractility. To gain further insight into its physiological significance, we determined its cellular distribution in adult rat heart by using immunohistochemistry and confocal microscopy. NHE1 was localized predominantly at the intercalated disk regions in close proximity to the gap junction protein connexin 43 of atrial and ventricular muscle cells. Significant labeling of NHE1 was also observed along the transverse tubular systems, but not the lateral sarcolemmal membranes, of both cell types. In contrast, the Na+-K+-ATPase alpha1-subunit was readily labeled by a specific mouse monoclonal antibody (McK1) along the entire ventricular sarcolemma and intercalated disks and, to a lesser extent, in the transverse tubules. These results indicate that NHE1 has a distinct distribution in heart and may fulfill specialized roles by selectively regulating the pH microenvironment of pH-sensitive proteins at the intercalated disks (e.g., connexin 43) and near the cytosolic surface of sarcoplasmic reticulum cisternae (e.g., ryanodine receptor), thereby influencing impulse conduction and excitation-contraction coupling.

Myocardial dysfunction is associated with activation of Na+/H+ exchange immediately during reperfusion

Amiloride analogs block Na+/H+ exchange and thereby protect the heart from myocardial ischemia-reperfusion injury. It is unclear whether drugs must be present before ischemia to be cardioprotective. After 60 min of global ischemia in the coronary-perfused right ventricular wall (RVW), as little as 1 min of exposure to dimethyl amiloride (DMA) immediately at the time of reperfusion protected the RVW. Delaying the drug attenuated the cardioprotection. If DMA was introduced in an ischemic solution near the end of ischemia, the cardioprotective effects were augmented. If the drug was washed out of the RVW vascular space before ischemia, cardioprotection was not observed. In contrast, in whole hearts, preischemic perfusion of the drug was necessary for cardioprotection and the cardioprotection remained even if the drug was washed out before ischemia. We conclude that Na+/H+ exchange is active and contributes to contractile dysfunction during the first seconds of reperfusion. This is difficult to detect in the perfused whole heart, and the washout data suggest that this may be due to a limitation in drug delivery across the vascular wall. The data also suggest that the exchanger is not as active during ischemia itself as it is during reperfusion.

Effects of aprindine on ischemia/reperfusion-induced cardiac contractile dysfunction of perfused rat heart

The present study was undertaken to determine whether aprindine, a class Ib antiarrythymic agent, exerts beneficial effects on ischemia/reperfusion-induced cardiac contractile dysfunction and metabolic derangement. Isolated rat hearts were subjected to 35-min global ischemia, followed by 60-min reperfusion, and functional and metabolic alterations of the heart were determined with or without aprindine-treatment. Ischemia induced a cessation of left ventricular developed pressure (LVDP), a rise in left ventricular end-diastolic pressure (LVEDP), and an increase in myocardial sodium content and a decrease in myocardial potassium content. When the hearts were reperfused, little recovery of LVDP and sustained rise in LVEDP and perfusion pressure were observed. Ischemia/reperfusion resulted in a release of ATP metabolites and creatine kinase from perfused hearts, an increase in myocardial sodium and calcium contents, and a decrease in myocardial potassium and magnesium contents. Treatment of the perfused heart with either 10 or 30 microM aprindine for the last 3 min of pre-ischemia improved contractile recovery during reperfusion and suppressed changes in myocardial ion content during ischemia and reperfusion. Treatment with the agent also attenuated the release of ATP metabolites and creatine kinase from the heart. However, treatment with high concentrations of aprindine (70 and 100 microM) improved neither cardiac contractile dysfunction, myocardial ionic disturbance nor the release of ATP metabolites and creatine kinase during reperfusion. Two possible mechanisms for the cardioprotection by the agent have been suggested: suppression of transmembrane flux of substrates and enzymes, and prevention of accumulation of myocardial sodium during ischemia.

Na+/H+ exchanger and reperfusion-induced ventricular arrhythmias in isolated perfused heart: possible role of amiloride

The roles of the Na+/H+ exchange system in the development and cessation of reperfusion induced ventricular arrhythmias were studied in the isolated perfused rat heart. The hearts were perfused in the working heart mode with modified Krebs Henseleit bicarbonate (KHB) buffer and whole heart ischemia was induced by a one-way ball valve with 330 beat/min pacing. Ischemia was continued for 15 min followed by 20 min of aerobic reperfusion (control). Amiloride (1.0 mM), an inhibitor of the Na+/H+ exchange system, was added to the KHB buffer only during reperfusion (group B) or only during ischemic periods (group C). Electrocardiographic and hemodynamic parameters were monitored throughout the perfusion. Coronary effluent was collected through pulmonary artery cannulation and PO2, PCO2, HCO3- and pH were measured by blood-gas analyzer. The incidence of reperfusion induced ventricular arrhythmias was 100%, 100% and 0% in control, group B and group C, respectively. The mean onset time of termination of reperfusion arrhythmias was significantly shorter in group B than in control. PCO2 increased from 39.0 +/- 0.9 to 89.3 +/- 6.0 mmHg at the end of ischemia in control and from 40.6 +/- 0.4 to 60.5 +/- 5.8 in group C, the difference between groups was statistically significant. HCO3- level decreased from 21.8 +/- 0.1 to 18.3 +/- 0.5 mmol/l in control, however, this decrease was significantly inhibited in group C (from 22.0 +/- 0.5 to 20.3 +/- 0.2). The increase in PCO2 and the decrease in HCO3- in group B were similar over time to those observed in control.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of reperfusion-induced ionic imbalance by free radical scavengers in spontaneously hypertensive rat retina

We studied the effects of free radical scavengers, superoxide dismutase (SOD), vitamin E, and EGB 761, on ion shifts (Na+, K+, and Ca2+) induced by ischemia reperfusion in rat retina obtained from spontaneously hypertensive rats. Eyes were subjected to 90 min of retinal ischemia followed by 24 h of reperfusion. Two basic protocols were used: (1) chronic application, in which rats received SOD (7500, 15,000, and 30,000 U/kg, i.v.), vitamin E (50, 100, and 200 mg/kg, i.v.), and EGB 671 (50, 100, and 200 mg/kg, orally) for 10 d, respectively; and (2) acute administration, in which 7500, 15,000, and 30,000 U/kg of SOD, 50, 100, and 200 mg/kg of vitamin E, and 50, 100, and 200 mg/kg of EGB 761 were administered after an ischemic episode, at the onset of reperfusion, respectively. In the drug-free control group, 90 min ischemia followed by 24 h of reperfusion resulted in an accumulation of retinal sodium and calcium from their nonischemic control values of 76 +/- 4 and 3.2 +/- 0.1 mumol/g dry weight to 112 +/- 6 (p < .001) and 6.2 (p < .001) mumol/g dry weight, respectively. Tissue potassium loss was also observed in this model of retinal ischemia reperfusion, and after 90 min ischemia followed by 24 h of reperfusion potassium content was significantly reduced from its nonischemic control value of 266 +/- 5 to 207 +/- 6 (p < .001) mumol/g dry weight. The chronic administration of SOD, vitamin E, and EGB 761 dose dependently reduced the reperfusion-induced ionic imbalance and improved the recovery of retinal ion contents.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischaemia- and reperfusion-induced Na+, K+, Ca2+ and Mg2+ shifts in rat retina: effects of two free radical scavengers, SOD and EGB 761

Using Sprague-Dawley rats with transient (90-min) regional ischaemia induced by retinal artery occlusion in the eye, we have shown that superoxide dismutase (SOD) and EGB 761 (IPSEN, France), two free radical scavengers, can dramatically reduce the reperfusion-induced sodium and calcium gains, and potassium loss in retinal tissue. Investigating whether this was a 'direct' protective effect, operating during reperfusion, or an 'indirect' effect arising from the action of SOD or EBG 761 on the tissue during ischaemia. SOD (15,000 U kg-1) and EGB 761 (100 mg kg-1) were added to the rats at the moment of reperfusion (after an ischaemic insult). Eyes were subjected to 90 min ischaemia followed by 4 and 24 hr of reperfusion, respectively. In the drug-free control group, 90 min of ischaemia resulted in an accumulation of retinal sodium (2-fold) and calcium (3-fold), and a loss of cell potassium (by 40%) and magnesium (by 40%). During the first 4 hr of reperfusion the ionic imbalance was unchanged, while after 24 hrs of reperfusion a normalization was observed and the ion content of the retina almost returned to their preischaemic values. SOD and EGB 761 treatment significantly reduced the reperfusion-induced ionic imbalance (magnesium was an exception) and improved the recovery of retinal ion contents. Our results indicate that the elimination of oxygen radicals by free radical scavengers may reduce the reperfusion-induced ionic imbalance and improve the ionic homeostasis in the injured retinal cells.

Identification of the protein and cDNA of the cardiac Na+/H+ exchanger

We examined the myocardial form of the Na+/H+ exchanger. A partial length cDNA clone was isolated from a rabbit cardiac library and it encoded for a Na+/H+ exchange protein. In comparison with the human Na+/H+ exchanger, the sequence of the 5' end of the cDNA was highly conserved, much more than the 3' region, while the deduced amino acid sequence was also highly conserved. To further characterize the myocardial Na+/H+ exchange protein, we examined Western blots of isolated sarcolemma with antibody produced against a fusion protein of the Na+/H+ exchanger. The antibodies reacted with a sarcolemma protein of 50 kDa and with a protein of 70 kDa. The results show that the rabbit myocardium does possess a Na+/H+ exchanger protein homologous to the known human Na+/H+ exchanger.

Oxidation of membrane cholesterol alters active and passive transsarcolemmal calcium movement

Oxygen free radicals have the ability to oxidize cholesterol. However, nothing is known about the effects of cholesterol oxidation on ion transport in isolated myocardial membranes. The purpose of the present study was to investigate the effects of in situ oxidative modification of sarcolemmal cholesterol on Ca2+ flux. Cholesterol oxidase was used to oxidatively modify membrane cholesterol. After incubation of cardiac sarcolemmal vesicles with cholesterol oxidase, cholest-4-en-3-one (cholestenone) was the predominant species of oxidated cholesterol produced. Cholesterol oxidase inhibited sarcolemmal Na(+)-Ca2+ exchange in a concentration-dependent manner. Both the Vmax and Km of the reaction were altered after cholesterol oxidase treatment. Extensive treatment of the sarcolemmal membranes with cholesterol oxidase increased the passive permeability characteristics of the membrane. Passive Ca2+ efflux from the sarcolemmal vesicles was stimulated by increasing the concentration of cholesterol oxidase. ATP-dependent Ca2+ uptake was also inhibited after cholesterol oxidase treatment, but it was not as sensitive as the Na(+)-Ca2+ exchange. Conversely, passive Ca2+ binding to sarcolemmal vesicles was strikingly stimulated by cholesterol oxidase treatment. The results demonstrate that oxidative modification of sarcolemmal membrane cholesterol can directly affect ionic interactions with the sarcolemmal vesicle and provide potentially important mechanistic information for the molecular basis of the effects of free radicals on ion flux and function in the heart.

Negative inotropic effect induced by diethylamiloride (DEA) in rabbit myocardium

The effects of the Na+/H+ exchange blocking drug diethylamiloride (DEA) on mechanical function have been studied in the rabbit isolated, arterially perfused interventricular septum. At concentrations of 10(-6)-10(-5) M, DEA induced a significant, dose-dependent, negative inotropic effect (a 54% decrease from control values at the highest concentration), which was slow to develop. After a 45 min washout, recovery was almost complete (95 +/- 3.4%). At concentrations greater than 5 x 10(-5) M, DEA induced a rapid and marked decrease in developed tension, associated with a progressive decrease in excitability and incomplete recovery. Resting tension was not significantly modified at any of the concentrations tested. At greater than 10(-6) M DEA enhanced significantly the transient negative inotropic effect of the brief intracellular acidosis induced by removal of NH4Cl perfusion, both by decreasing the minimal value of developed tension and by increasing the time required to produce this effect. These effects suggest that the dose-dependent DEA negative inotropic effect could be mediated by a progressive intracellular acidosis produced by inhibition of the Na+/H+ exchange system.

Effects of proton buffering and of amiloride derivatives on reperfusion arrhythmias in isolated rat hearts. Possible evidence for an arrhythmogenic role of Na(+)-H+ exchange

We investigated the hypothesis that an accelerated Na+o-H+i exchange on reperfusion may lead to a displacement of the 3[Na+] [Ca2+]i/o equilibrium in favor of an arrhythmogenic rise in cytosolic [Ca2+]. Supporting evidence was obtained by subjection of isolated rat hearts to 15 minutes of low-flow (5% of control) ischemia and 2 minutes of reperfusion in the presence of a Krebs-Henseleit HEPES buffer (pH 7.4) containing lactate (10 mM). At first, the [HEPES] was fixed at 5 mM; then, 2 minutes before reflow, either the [HEPES] was varied from 50 to 1 mM to slow H+o washout, or increasing concentrations of 5-(N,N-dimethyl)-amiloride (Ki 7 microM) or 5-(N,N-hexamethylene)-amiloride (Ki 0.2 microM) were added for inhibition of Na(+)-H+ exchange. In each case, reperfusion ventricular arrhythmias were reduced by 69-73% (p less than 0.001).

Role for sulfur-containing groups in the Na+-Ca2+ exchange of cardiac sarcolemmal vesicles

Different amino acid residues in cardiac sarcolemmal vesicles were modified by incubation with various chemical reagents. The effects of these modifications on sarcolemmal Na+-Ca2+ exchange were examined. Dithiothreitol, an agent that maintains sulfur-containing residues in a reduced state, caused a time- and concentration-dependent decrease in Na+-Ca2+ exchange. The treatment with dithiothreitol resulted in a decrease in Vmax values but did not alter the Km for Ca2+ for the Na2+-Ca2+ exchange reaction. If Na+ replaced K+ as the ion present during the modification of sarcolemmal membranes with dithiothreitol, there was substantially less of an inhibitor effect on Na+-Ca2+ exchange. Similar results were obtained with reduced glutathione, a reagent that also maintains sulfur-containing residues in a reduced state. Two sulfhydryl modifying reagents, methylmethanethiosulfonate and N'-ethylmaleimide, were capable of altering Na+-Ca2+ exchange, and the type of ion present during modification significantly affected the extent of this alteration. Almost all of the chemical reagents investigated that modified other amino acid resides (carboxyl, lysyl, histidyl, tyrosyl, tryptophanyl, arginyl and hydroxyl) had the capacity to alter Na+-Ca2+ exchange after preincubation with the sarcolemmal membrane vesicles. However, the sulfur residue-modifying reagents were the only compounds to exhibit significant differences in their action on Na+-Ca2+ exchange, depending on whether Na+ or K+ was present in the preincubation modification medium. The tryptophan modifier, N-bromosuccinimide, was the sole reagent that elicited a substantial increase in membrane permeability. The evidence is consistent with the hypothesis that sulfur-containing residues interact with a Na+-binding site for Na+-Ca2+ exchange in cardiac sarcolemmal vesicles.