Modulation of Na+-Ca2+ exchange and Ca2+ permeability in cardiac sarcolemmal vesicles by doxylstearic acids

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.

Conformational changes of the 120-kDa Na+/Ca2+ exchanger protein upon ligand binding: a Fourier transform infrared spectroscopy study

The 120-kDa Na+/Ca2+ exchanger was purified and reconstituted into lipid vesicles. The secondary structure composition of the exchanger was 39% alpha-helices, 20% beta-sheets, 25% beta-turns, and 16% random coils, as analyzed by Fourier transform infrared attenuated total reflection spectroscopy. The secondary structure composition of the COOH-terminal portion of the protein was compatible with a topology model containing 4-6 transmembrane segments. Furthermore, the secondary structure of the NH2-terminal portion of the cytoplasmic loop was analyzed and found to be different from that of the COOH-terminal portion. Ca2+ and/or the exchange inhibitory peptide (XIP) failed to affect the secondary structure of the 120-kDa protein. Tertiary structure modifications induced by Ca2+ and XIP were analyzed by monitoring the hydrogen/deuterium exchange rate for the reconstituted exchanger. In the absence of ligand, 51% of the protein was accessible to solvent. Ca2+ decreased accessibility to 40%, implicating the shielding of at least 103 amino acids. When both Ca2+ and XIP were added, accessibility increased to 66%. No modification was obtained when XIP was added alone. Likewise, in the presence of Ca2+, XIP failed to modify the tertiary structure of the 70-kDa protein, suggesting that XIP acts at the level of the COOH-terminal portion of the intracellular loop. The present data describe, for the first time, conformational changes of the Na+/Ca2+ exchanger induced by Ca2+ and XIP, compatible with an interaction model where regulatory Ca2+ and inhibitory XIP bind to distinct sites, and where XIP binding requires the presence of Ca2+.

Effects of diabetes and hypertension on myocardial Na+-Ca2+ exchange

Abnormalities in cardiac function have been extensively documented in experimental and clinical diabetes. These aberrations are well known to be exaggerated when hypertension and diabetes co-exist. The objective of the present study was to examine whether alterations in the activity of the myocardial Na+-Ca2+ exchanger (NCX) can account for the deleterious effects of diabetes and (or) hypertension on the heart. To this aim, the following experimental groups were studied: (i) control; (ii) diabetic; (iii) hypertensive; and (iv) hypertensive-diabetic. Wistar rats served as the control group (C) while Wistar rats injected with streptozotocin (STZ, 55 mg/kg) served as the diabetic (D) group. Spontaneously hypertensive (SH) rats were used as the hypertensive group (H) while SH rats injected with STZ served as the hypertensive-diabetic (HD) group. Sarcolemma was isolated from the ventricles of the C, D, H, and HD groups and NCX activity was examined using rapid quenching techniques to study initial rates over a [Ca2+]o range of 10-160 µM. The Vmax of NCX was lower in the D group when compared with the C group (D, 2.96 ± 0.26 vs. C, 4.0 ± 0.46 nmol·mgprot-1·s-1, P < 0.05), however combined diabetes and hypertension (HD) did not affect the Vmax of NCX activity (HD, 3.84 ± 0.88 vs. H, 3.59 ± 0.24 nmol·mgprot-1·s-1, P > 0.05). However, analysis of the Km values for Ca2+ indicated that both the D and HD groups exhibited a significantly lower Km when compared with their respective control groups (D, 42 ± 4 vs. C, 56 ± 4 µM,P < 0.05; HD, 33 ± 7 vs. H, 51 ± 8 µM, P < 0.05). Immunoblotting using polyclonal antibodies (against canine cardiac NCX) exhibited the typical banding of 160, 120, and 70 kDa. The 120 kDa band is believed to represent the native exchanger with its post-translational modifications. Examination of the blots revealed a lower intensity of the 120 kDa band in the D group when compared with the C group, however, no significant difference in the HD group was observed. We speculate that the lower Vmax in the D group may be due to a reduced concentration of exchanger protein in the membrane. The absence of this defect in the HD group may be a result of compensatory mechanisms to the overall hemodynamic overload, however, this remains to be determined. The increased affinity for Ca2+ in both the D and HD groups (determined by the lower Km values) is an interesting finding and may be due to changes in sarcolemmal lipid bilayer composition secondary to diabetes-induced hyperlipidemia.Key words: diabetes, hypertension, cardiac, Na+-Ca2+ exchange, contractility.

Fourier transform infrared spectroscopy study of the secondary and tertiary structure of the reconstituted Na+/Ca2+ exchanger 70-kDa polypeptide

The secondary structure of the purified 70-kDa protein Na+/Ca2+ exchanger, functionally reconstituted into asolectin lipid vesicles, was examined by Fourier transform infrared attenuated total reflection spectroscopy. Fourier transform infrared attenuated total reflection spectroscopy provided evidence that the protein is composed of 44% alpha-helices, 25% beta-sheets, 16% beta-turns, and 15% random structures, notably the proportion of alpha-helices is greater than that corresponding to the transmembrane domains predicted by exchanger hydropathy profile. Polarized infrared spectroscopy showed that the orientation of helices is almost perpendicular to the membrane. Tertiary structure modifications, induced by addition of Ca2+, were evaluated by deuterium/hydrogen exchange kinetic measurements for the reconstituted exchanger. This approach was previously proven as a useful tool for detection of tertiary structure modifications induced by an interaction between a protein and its specific ligand. Deuterium/hydrogen exchange kinetic measurements indicated that, in the absence of Ca2+, a large fraction of the protein (40%) is inaccessible to solvent. Addition of Ca2+ increased to 55% the inaccessibility to solvent, representing a major conformational change characterized by the shielding of at least 93 amino acids.

Membrane topology of the rat brain Na+-Ca2+ exchanger

To provide experimental evidence for the topology of the Na+-Ca2+ exchanger protein NCX1 in the membrane, indirect immunofluorescence studies using site specific anti-peptide antibodies and Flag-epitope insertion into chosen locations of the protein were carried out. Anti-peptide antibodies AbO-6 and AbO-8 were raised against peptide segments present in a large hydrophilic loop of about 500 amino acids, which separates the hydrophobic amino terminal part of the protein from the hydrophobic carboxy terminal. AbO-10 was raised against the C-terminal tail of the protein. All three antibodies bound to the exchanger protein expressed in transfected cells, in rat brain synaptic plasma membrane and in dog sarcolemmal preparations. The antibodies bound only to those NCX1 isoforms that contained the epitope against which they were raised. Detection of the exchanger protein in transfected cells in situ required the addition of permeabilizing agents suggesting an intracellular location of the epitopes to which AbO-6, AbO-8 and AbO-10 bind. The Flag epitope was inserted into ten putative extramembraneous segments along the exchanger protein. For topology studies, only the Flag-mutants that retained Na+-Ca2+ exchange activity in whole HeLa cells, were used. Immunofluorescence studies indicated, that the N-terminal of the protein is extracellular, the first hydrophilic loop separating transmembrane helices 1 and 2 as well as the C-terminal, are intracellular.

The selective activation of the cardiac sarcolemmal sodium-calcium exchanger by plasmalogenic phosphatidic acid produced by phospholipase D

Since plasmalogens are the predominant phospholipid of cardiac sarcolemma, the activation of the sodium-calcium exchanger by either plasmenylethanolamine or plasmalogenic phosphatidic acid generated by phospholipase D was explored. Sodium-calcium exchange activity was 7-fold greater in proteoliposomes comprised of plasmenylethanolamine compared to proteoliposomes comprised of only plasmenylcholine. Phospholipase D treatment of proteoliposomes resulted in 1 mol % conversion of plasmenylcholine or phosphatidylcholine to their respective phosphatidic acid molecular species with a concomitant 8-fold or 2-fold activation of sodium-calcium exchange activity, respectfully. Thus, phospholipase D-mediated hydrolysis of plasmalogens to phosphatidic acid may be an important mechanism for the regulation of the sodium-calcium exchanger.

Immunologic identification of Na/Ca exchange protein in rat brain synaptic plasma membrane

Polyclonal antibodies raised against partially purified dog cardiac Na/Ca exchanger react with cardiac sarcolemmal proteins of 160, 120 and 70 kDa on SDS-PAGE. Using the same specific antiserum, we detected three prominent immunoreactive bands of about 150, 120 and 70 kDa on immunoblots with rat forebrain synaptic plasma membrane proteins. These data indicate that the Na/Ca exchange protein in rat brain synaptic plasma membrane is structurally and antigenically similar to the exchange protein in dog cardiac sarcolemma.

Characterization of myocardial Na(+)-Ca2+ exchange in rainbow trout

This study compared Na(+)-Ca2+ exchange from the hearts of rainbow trout with that from canines. In several respects, trout cardiac Na(+)-Ca2+ exchange is functionally similar to that from dogs and other mammals. Trout cardiac Na(+)-Ca2+ exchange is stimulated approximately 200% after 30-min incubation with 10 micrograms/ml chymotrypsin at 21 degrees C, similar to mammals. On the other hand, both the temperature and pH dependencies are strikingly different between the trout and canine myocardial Na(+)-Ca2+ exchange. While canine heart Na(+)-Ca2+ exchange exhibits a Q10 of greater than 2 (similar to values observed in other mammals), that from trout is relatively insensitive to temperature with a Q10 of approximately 1.2. The absolute rates of Na(+)-Ca2+ exchange in trout heart are four- to sixfold higher than that in mammals when measured at 7 degrees C. Furthermore, the temperature insensitivity of trout myocardial Na(+)-Ca2+ exchange is retained when the exchanger is reconstituted into an asolectin bilayer, suggesting that this property is intrinsic to the protein and not dependent on species differences in lipid bilayer composition. Trout Na(+)-Ca2+ exchange is not markedly stimulated by alkaline pH, in contrast to mammals, and this characteristic is also maintained after reconstitution. Western blots of trout cardiac sarcolemma run on 7.5% sodium dodecyl sulfate-polyacrylamide gel electrophoresis react with antibodies raised against the canine Na(+)-Ca2+ exchanger with a similar pattern of bands (70, 120, and 160 kDa). Furthermore, a cDNA probe from canine Na(+)-Ca2+ exchanger hybridizes on Northern blots of trout heart mRNA to a 7-kb band, similar to that in mammals. Thus, while important functional differences in Na(+)-Ca2+ exchange exist between trout and mammalian hearts, the molecular basis is not yet known.

Na(+)-Ca2+ exchangers from rod outer segments and cardiac sarcolemma: comparison of properties

The properties of the Na(+)-Ca2+ exchangers from cardiac sarcolemma (SL) and rod outer segments (ROS) were studied in parallel by measuring the Na+ gradient-dependent Ca2+ uptake into SL or ROS vesicles. The ROS exchanger, but not the SL exchanger, has a striking specific dependence on K+. The ROS exchanger is stimulated by K+ with an apparent concentration at half-maximum of 1 mM. The addition of valinomycin, to produce an inside-positive membrane potential, stimulates the SL exchanger 1.8-fold and the ROS exchanger 1.2-fold. The Michaelis constant for half-maximal transport rate for Ca2+ and the maximal transport rate for the exchangers, in the absence of valinomycin, are estimated to be 20 microM and 8 nmol.mg-1.2.2 s-1 in SL and 5 microM and 1 nmol.mg-1.2.2 s-1 in ROS. Both exchangers are modulated by the same regulatory influences. For example, both are stimulated by proteases, phospholipase D, and intravesicular Ca2+.

Effect of temperature on sodium-calcium exchange in sarcolemma from mammalian and amphibian hearts

We have investigated temperature dependence of Ca2+ uptake by the cardiac sarcolemmal Na(+)-Ca2+ exchanger from dog, rabbit and bullfrog. In native rabbit sarcolemmal vesicles, Ca2+ affinity of the Na(+)-Ca2+ exchanger is unchanged from 7 to 37 degrees C; however, the initial velocity of Ca2+ uptake declines much more steeply below 22 degrees C than above 22 degrees C. In native dog sarcolemma, the temperature dependence of Na(+)-Ca2+ exchange velocity is similar to that of native rabbit. However, in frog heart the velocity of Na(+)-Ca2+ exchange declines much more slowly with decreasing temperature at both temperature ranges. Reconstitution of the Na(+)-Ca2+ exchanger into artificial lipid vesicles consisting of either asolectin or phosphatidylserine, phosphatidylcholine, and cholesterol has little effect on temperature dependence of Na(+)-Ca2+ exchange velocity in any of the three species. We conclude that the lesser temperature sensitivity of the cardiac sarcolemmal Na(+)-Ca2+ exchanger of a poikilothermic species is at least partly an intrinsic property of the transport protein.

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.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Abnormal fluidity state in membranes of malignant hyperthermia pig skeletal muscle

The fluidity state was analyzed on sarcoplasmic reticulum membranes and phospholipid vesicles prepared from normal and malignant hyperthermia susceptible pig muscle. Electron spin resonance studies were performed to determine the fluidity state at the region near the polar headgroups and in the central core of the bilayer using 5-nitroxide (5-NS) and 16-nitroxide stearic acid (16-NS), respectively. With the 5-NS label, no differences were found between normal and malignant hyperthermia sarcoplasmic reticulum (MH SR) membranes whereas with the 16-NS label, a significant increase of the activation energy was shown with MH membranes. Lower values of fluorescence anisotropy observed with DPH-labeled MH membranes as compared with normal ones, confirmed the higher abnormal fluidity state of these membranes. The fluidizing effect of halothane, a triggering agent of malignant hyperthermia syndrome, was also studied in these membranes. We show that a relatively low concentration of the drug destabilized not only the diseased sarcoplasmic reticulum membranes but also the vesicles made of total phospholipids extracted from MH skeletal muscle. Together, these findings strongly suggest that an overall increase in membrane fluidity may be implied in the MH disease, improving the general membrane defect hypothesis for this syndrome.

Influence of phospholipid fatty acyl composition on sarcolemmal and sarcoplasmic reticular cation transporters

Using solubilization/reconstitution techniques, we have investigated the influence of membrane fatty acyl composition on the activities of sarcolemmal and sarcoplasmic reticular transporters. The sarcolemmal Na(+)-Ca2+ exchanger and Na+, K(+)-ATPase and the sarcoplasmic reticular Ca2(+)-ATPase were reconstituted into phosphatidylcholine:phosphatidylserine:cholesterol (30:50:20% by weight) proteoliposomes of defined fatty acyl composition. Transport activities varied considerably with phospholipid fatty acyl composition. Quite strikingly, the dependence on membrane fatty acyl composition for all three transporters was identical.

Functional characteristics of the cardiac sarcolemmal monocarboxylate transporter

We have previously shown that a mechanism for transporting L-lactate is located in cardiac sarcolemmal membranes (Am. J. Physiol. 252:C483-C489, 1987). This mechanism has now been shown to transport pyruvate also. The transporter recognizes a wide range of monocarboxylic acids with chain lengths of three to six carbons, as evidenced by their ability to inhibit L-lactate uptake into sarcolemmal vesicles. The ability of the monocarboxylate analogues to inhibit depends strongly on the nature of substituents, particularly at the second carbon. L-lactate and pyruvate transport are not affected by dicarboxylates other than oxaloacetate. The transporter is inhibited by the protein modifiers diethylpyrocarbonate, dinitrofluorobenzene, and phenylisothiocyanate. Diethylpyrocarbonate inhibition is not reversed by hydroxylamine, nor is dinitrofluorobenzene inhibition reversed by thiol reagents, suggesting that the target residues are not histidine, or tyrosine or cysteine, respectively. Several monocarboxylates effectively protect the transporter from inhibition by the modifying reagents, suggesting that the modified residue(s) may be at or near the binding site. Alternatively, the target amino acid(s) in the transport protein may become inaccessible due to a conformation change triggered by the substrate analogues. Overall, the results suggest that a sensitive free amino group, associated with substrate binding, is attacked by the protein-modifying reagents.

Effects of active oxygen generated by DTT/Fe2+ on cardiac Na+/Ca2+ exchange and membrane permeability to Ca2+

Sarcolemmal vesicles isolated from bovine heart were preincubated at 37 degrees C with an oxygen radical generating system consisting of 1 mM dithiothreitol (DTT) and 50 microM FeSO4. Exposure of the vesicles for 1 to 40 mins stimulated Na+/Ca2+ exchange about 2.5-fold. The DTT/Fe2+ treatment decreased the apparent Km for Ca2+ of Nai+-dependent Ca2+ uptake by 80% (from 63 to 13 microM). The effect on Vmax was much smaller however. The resulting stimulation of exchange activity was diminished by the presence of desferrioxamine (95%) or catalase (60%). In contrast, superoxide dismutase and sodium formate did not prevent the effects of DTT/Fe2+ on the exchanger. Neither Zn2+ nor Ga3+ could replace Fe2+ in the stimulation of Na+/Ca2+ exchange. Passive Ca2+ efflux was determined by first allowing Na+/Ca2+ exchange to continue to plateau values and then diluting the loaded vesicles in the presence of EGTA. Ca2+ leakage from the vesicles was slightly but significantly (P less than 0.05) increased by the action of DTT/Fe2+, the rate constants for the passive Ca2+ efflux being 0.22 and 0.26/min in control and treated groups, respectively. The calcium loading observed in myocytes in ischemia/reperfusion injury suggests that the stimulation of Na+/Ca2+ exchange by active oxygen may moderate the myocardial response to oxygen mediated injuries including ischemia/reperfusion injury. However, the clinical relevance of these phenomena is far from clear as the stimulation depends in part on the Km for Ca2+ prior to treatment.

Ouabain treatment of cardiac cells induces enhanced Na+-Ca2+ exchange activity

Inhibition of the cardiac Na+-K+-ATPase with cardiac glycosides causes a rise in internal Na+ and a subsequent increase in cellular Ca2+ due to the sarcolemmal Na+-Ca2+ exchange mechanism. We investigated the adaptation of cultured cardiac cells to prolonged elevation of internal Ca2+ after exposure to ouabain. Cultured neonatal rat heart cells were treated with 100 microM ouabain for 4-48 h. This ouabain concentration inhibited Na+-K+-ATPase activity by approximately 45% and caused modest cellular Ca2+ loading. We found that cells adapted to ouabain treatment by increasing the amount of sarcolemmal Na+-Ca2+ exchange activity by 50-90% over a 24-h period. Kinetic and immunological data indicate that the increase was due to increased numbers of functional exchangers. Neither total cellular nor total sarcolemmal protein content was affected by the ouabain treatment. These results may be relevant toward understanding the effects of therapeutic use of cardiac glycosides.

Uncoupling of cardiac cells by doxyl stearic acids specificity and mechanism of action

The influence of doxyl stearic acids (DSAs) on gap junctional conductance (gj) between pairs of neonatal rat heart cells was studied. DSAs are spin probes that perturb the membrane at different depths depending on position of the doxyl group on the fatty acyl chain. 16-DSA and 12-DSA rapidly and reversibly reduced gj to unmeasureable levels in a dose- and time-dependent manner. Single channel events observed when gj was low were of the same unitary size as those observed under control conditions. The methyl esters of 16- and 12-DSA, stearic acid itself, and TEMPO, an analogue of the doxyl group that has no fatty acyl chain, had no effect on gj. Protonation of the carboxyl head group (by acidifying the solution) reduced the potency of 16- or 12-DSA. Spontaneous beating activity and action potentials were observed at concentrations of the DSAs 15-20 times that necessary for uncoupling. These results indicate that uncoupling by the DSAs requires the presence of the charged carboxyl group and localized perturbation of the channel at the lipid-channel interface by the doxyl group. Furthermore, they predict that unsaturated free fatty acids, which accumulate during ischemia, may exert their arrhythmogenic effect by reducing gj, and thereby slowing conduction.

Na+-Ca2+ exchange in cardiac sarcolemma: modulation of Ca2+ affinity by exercise

The high activity of the cardiac Na+-Ca2+ exchanger has led to the suggestion that it plays an important role in the regulation of myocardial contractility. We have proposed that exercise training increases stroke volume as a consequence of an enhanced contractility caused by an adaptation in Ca2+ transport across the cardiac plasma membrane (sarcolemma). The present study examined the possibility that the Na+-Ca2+ exchanger in heart muscle is modified in response to training. Sprague-Dawley rats (female, n = 72) were randomly divided into exercise-trained (T) and sedentary control (C) groups. As a result of the 11-wk treadmill-training paradigm, group T had a 7.6% higher (P less than 0.005) heart-to-body weight ratio and a 36% increase (P less than 0.01) in gastrocnemius mitochondrial enzyme activity. Na+-Ca2+ exchange was studied in highly purified sarcolemmal vesicles using rapid-quenching techniques. The absolute initial rate of uptake was significantly higher in T vs. C at calcium concentrations [( Ca2+]) ranging from 10 to 80 microM. This increased uptake appears to be due solely to the fact that the apparent Km of the myocardial Na+-Ca2+ exchanger for Ca2+ was significantly lower in T vs. C (15.7 +/- 1.1 vs. 36.1 +/- 2.6 microM), since the maximum velocity was unchanged. The observed increase in the affinity of the exchanger for Ca2+ is not attributable to group differences in vesicular purity, cross-contamination, or passive Ca2+ efflux. This observation is consistent with observed alterations in sarcolemmal composition in response to exercise training. We propose that the modification of the Na+-Ca2+ exchanger may play an important role in the adaptation of the heart to exercise.

Use of electron spin resonance spectroscopy of spin labels for studying drug-induced membrane perturbation

The use of electron spin resonance spectroscopy of spin labels is reviewed in the context of drug-induced membrane perturbation. The correlation between membrane perturbation and biological effects is also considered.

Purification of the cardiac Na+-Ca2+ exchange protein

We have used fractionation procedures to enrich solubilized cardiac sarcolemma in the Na+-Ca2+ exchange protein. Sarcolemma is extracted with an alkaline medium to remove peripheral proteins and is then solubilized with decylmaltoside. Next, the exchanger is applied to DEAE-Sepharose and eluted with high salt. The DEAE fraction is applied to WGA-agarose, and a small fraction of protein, enriched in the exchanger, can be eluted by changing the detergent to Triton X-100. This fraction is reconstituted into asolectin proteoliposomes for measurement of Na+-Ca2+ exchange activity and gel electrophoresis. The purified fraction has a Na+-Ca2+ exchange activity of 600 nmol Ca2+/mg of protein per s at 10 microM Ca2+ and a purification factor of about 30 as compared with control reconstituted sarcolemmal vesicles. Ca2+-Ca2+ exchange and Na+-Ca2+ exchange activities were both present in the same final reconstituted vesicles indicating that the same protein is responsible for both transport activities. SDS-PAGE reveals two prominent protein bands at 70 and 120 kDa. After mild chymotrypsin treatment (1 microgram/ml), there is no loss of exchange activity, but the 120 kDa band disappears and the 70 kDa band becomes more dense. This suggests that the 70 kDa band is due to an active proteolytic fragment of the 120 kDa protein. Under non-reducing gel conditions, only a single protein band is seen with an apparent molecular weight of 160 kDa. Antibodies to the purified exchanger preparation are able to immunoprecipitate exchange activity and confirm that the 70 kDa protein derives from the 120 kDa protein. We propose that both the 70 and 120 kDa proteins are associated with the Na+-Ca2+ exchanger.

Sarcolemmal phospholipid fatty acid composition and permeability

In this study, the mechanism of ischaemia-induced increased sarcolemmal permeability, as manifested by release of intracellular enzymes, was investigated. The role of changes in the sarcolemmal phospholipid bilayer in this process was evaluated by experimental modulation of the phospholipid fatty acid composition. The isolated perfused rat heart subjected to low-flow hypoxia, was used as a model of global ischaemia. Glucose as well as saturated (palmitate) and unsaturated (linoleate) long-chain fatty acids were used as substrates. Hearts perfused with palmitate or linoleate (1.5 mM, fatty acid/albumin ratio, 3.4) showed a significantly higher rate of lactate dehydrogenase release in both control and ischaemic conditions than hearts perfused with glucose (10 mM). Lactate dehydrogenase release in the fatty acid-perfused hearts was associated with a significant increase in the percentage unsaturation of the sarcolemmal phospholipid fatty acids. Glucose-perfused hearts, on the other hand, showed only minor changes in the sarcolemmal phospholipid fatty acid composition. Attempts to correlate enzyme release directly with an increase in the percentage unsaturation of phospholipid fatty acids failed, since enzyme release was also stimulated in control fatty-acid-perfused hearts which (when compared with glucose) contained a higher percentage saturated phospholipid fatty acids. The results suggest that myocardial ischaemia, apart from changes in the sarcolemmal phospholipid fatty acid composition, also induces several other changes in sarcolemmal composition (e.g., cholesterol loss) which may affect is permeability for macromolecules.

Protein methylation inhibits Na+-Ca2+ exchange activity in cardiac sarcolemmal vesicles

We have examined the effect of membrane methylation on the Na+-Ca2+ exchange activity of canine cardiac sarcolemmal vesicles using S-adenosyl-L-methionine as methyl donor. Methylation leads to approximately 40% inhibition of the initial rate of Nai+-dependent Ca2+ uptake. The inhibition is due to a lowering of the Vmax for the reaction. The inhibition is not due to an effect on membrane permeability and is blocked by S-adenosyl-L-homocysteine, an inhibitor of methylation reactions. The following experiments indicated that inhibition of Na+-Ca2+ exchange was due to methylation of membrane protein and not due to methylated phosphatidylethanolamine (PE) compounds (i.e., phosphatidyl-N-monomethylethanolamine (PMME) or phosphatidyl-N,N'-dimethylethanolamine (PDME]: (1) We solubilized sarcolemma and reconstituted activity into vesicles containing no PE. The inhibition by S-adenosyl-L-methionine was not diminished in this environment. (2) We reconstituted sarcolemma into vesicles containing PMME or PDME. These methylated lipid components had no effect on Na+-Ca2+ exchange activity. (3) We verified that many membrane proteins, probably including the exchanger, become methylated.

Phospholipid composition modulates the Na+-Ca2+ exchange activity of cardiac sarcolemma in reconstituted vesicles

Na+-Ca2+ exchange activity in cardiac sarcolemmal vesicles is known to be sensitive to charged, membrane lipid components. To examine the interactions between membrane components and the exchanger in more detail, we have solubilized and reconstituted the Na+-Ca2+ exchanger into membranes of defined lipid composition. Our results indicate that optimal Na+-Ca2+ exchange activity requires the presence of certain anionic phospholipids. In particular, phosphatidylserine (PS), cardiolipin, or phosphatidic acid at 50% by weight results in high Na+-Ca2+ exchange activity, whereas phosphatidylinositol and phosphatidylglycerol provide a poor environment for exchange. In addition, incorporation of cholesterol at 20% by weight greatly facilitates Na+-Ca2+ exchange activity. Thus, for example, an optimal lipid environment for Na+-Ca2+ exchange is phosphatidylcholine (PC, 30%)/PS (50%)/cholesterol (20%). Na+-Ca2+ exchange activity is also high when cardiac sarcolemma is solubilized and then reconstituted into asolectin liposomes. We fractionated the lipids of asolectin into subclasses for further reconstitution studies. When sarcolemma is reconstituted into vesicles formed from the phospholipid component of asolectin, Na+-Ca2+ exchange activity is low. When the neutral lipid fraction of asolectin (including sterols) is also included in the reconstitution medium, Na+-Ca2+ exchange activity is greatly stimulated. This result is consistent with the requirement for cholesterol described above. Proteinase treatment, high pH, intravesicular Ca2+ and dodecyl sulfate all stimulate Na+-Ca2+ exchange in native sarcolemmal vesicles. We examined the effects of these interventions on exchange activity in reconstituted vesicles of varying lipid composition. In general, Na+-Ca2+ exchange could be stimulated only when reconstituted into vesicles of a suboptimal lipid composition. That is, when reconstituted into asolectin or PC/PS/cholesterol (30:50:20), the exchanger is already in an activated state and can no longer be stimulated. The one exception was that the Na+-Ca2+ exchanger responded to altered pH in an identical manner, independent of vesicle lipid composition. The mechanism of action of altered pH on the exchanger thus appears to be different from other interventions.

Enrichment of Na+-Ca2+ exchange in cardiac sarcolemmal vesicles by alkaline extraction

Exposure of canine cardiac sarcolemmal vesicles to alkaline media (greater than or equal to pH 12) results in the extraction of 33% of the protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that specific proteins are being solubilized. Most of the phospholipid and sialic acid remains with the pellet after centrifugation. Electron microscopy reveals that alkaline treatment does not cause gross morphological damage to the vesicles, although freeze-fracture demonstrates some aggregation of intramembrane particles. The data indicate that high pH probably removes peripheral proteins and leaves the integral proteins in place. We find complete recovery of Na+-Ca2+ exchange activity in alkaline-extracted membranes after solubilization and reconstitution. These vesicles contain only 50% of the protein of vesicles reconstituted from control sarcolemma. Thus, the specific activity of Na+-Ca2+ exchange is doubled. Alkaline extraction is a useful and reproducible procedure for enrichment of the Na+-Ca2+ exchange protein. (Na+ + K+)-ATPase is completely inactivated by exposure to pH 12 medium though immunodetection shows that the (Na+ + K+)-ATPase proteins are not extracted. We detect both alpha and alpha + forms of (Na+ + K+)-ATPase and deduce that the Na+ pump proteins do not comprise a major fraction of sarcolemmal protein.