Role of phosphatidylinositol in cardiac sarcolemmal membrane sodium-calcium exchange

A new cell-penetrating peptide that blocks the autoinhibitory XIP domain of NCX1 and enhances antiporter activity

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) is a high-capacity ionic transporter that exchanges 3Na(+) ions for 1Ca(2+) ion. The first 20 amino acids of the f-loop, named exchanger inhibitory peptide (XIP(NCX1)), represent an autoinhibitory region involved in the Na(+)-dependent inactivation of the exchanger. Previous research has shown that an exogenous peptide having the same amino acid sequence as the XIP(NCX1) region exerts an inhibitory effect on NCX activity. In this study, we identified another regulatory peptide, named P1, which corresponds to the 562-688aa region of the exchanger. Patch-clamp analysis revealed that P1 increased the activity of the exchanger, whereas the XIP inhibited it. Furthermore, P1 colocalized with NCX1 thus suggesting a direct binding interaction. In addition, site-directed mutagenesis experiments revealed that the binding and the stimulatory effect of P1 requires a functional XIP(NCX1) domain on NCX1 thereby suggesting that P1 increases the exchanger activity by counteracting the action of this autoinhibitory sequence. Taken together, these results open a new strategy for developing peptidomimetic compounds that, by mimicking the functional pharmacophore of P1, might increase NCX1 activity and thus exert a therapeutic action in those diseases in which an increase in NCX1 activity might be helpful.

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.

Alterations of sarcolemmal phospholipase D and phosphatidate phosphohydrolase in congestive heart failure

Phospholipase D 2 (PLD2) is the major PLD isozyme associated with the cardiac sarcolemmal (SL) membrane. Hydrolysis of SL phosphatidylcholine (PC) by PLD2 produces phosphatidic acid (PA), which is then converted to 1,2 diacylglycerol (DAG) by the action of phosphatidate phosphohydrolase type 2 (PAP2). In view of the role of both PA and DAG in the regulation of Ca(2+) movements and the association of abnormal Ca(2+) homeostasis with congestive heart failure (CHF), we examined the status of both PLD2 and PAP2 in SL membranes in the infarcted heart upon occluding the left coronary artery in rats for 1, 2, 4, 8 and 16 weeks. A time-dependent increase in both SL PLD2 and PAP2 activities was observed in the non-infarcted left ventricular tissue following myocardial infarction (MI); however, the increase in PAP2 activity was greater than that in PLD2 activity. Furthermore, the contents of both PA and PC were reduced, whereas that of DAG was increased in the failing heart SL membrane. Treatment of the CHF animals with imidapril, an angiotensin-converting enzyme (ACE) inhibitor, attenuated the observed changes in heart function, SL PLD2 and PAP2 activities, as well as SL PA, PC and DAG contents. The results suggest that heart failure is associated with increased activities of both PLD2 and PAP2 in the SL membrane and the beneficial effect of imidapril on heart function may be due to its ability to prevent these changes in the phospholipid signaling molecules in the cardiac SL membrane.

Na(+)-K(+)-ATPase gene expression in the avian eggshell gland: distinct regulation in different cell types

The avian eggshell gland (ESG) is a tissue specialized in transporting the Ca(2+) required for eggshell formation and represents a unique biological system in which the calcification process takes place in a circadian fashion. With the use of RNA fingerprinting, a set of genes differentially induced at the time of calcification was detected, one of which was identified as the alpha(1)-subunit of Na(+)-K(+)-ATPase. The gene was expressed in a circadian manner in both cell types populating the ESG, but in different temporal patterns, suggesting distinct mechanisms of regulation. Ca(2+) flux and mechanical strain were found to regulate gene expression in the inner glandular epithelium and the pseudostratified epithelium facing the lumen, respectively. Mechanical strain also affected gene expression in cell layers facing the lumen in other parts of the oviduct. Only the alpha(1)-isoform, not the alpha(2)- or alpha(3)-isoform, of Na(+)-K(+)-ATPase was expressed in the ESG. In summary, we demonstrate that the alpha(1)-subunit Na(+)-K(+)-ATPase gene is expressed in different epithelial cell types in the ESG and is regulated by various mechanisms, which may reflect the disparity in the physiological roles of the cells in the process of eggshell formation.

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.

Role of Na+/Ca2+ exchange in endothelin-1-induced increases in Ca2+ transient and contractility in rabbit ventricular myocytes: pharmacological analysis with KB-R7943

1. The effects of endothelin-1 (ET-1) on intracellular Ca2+ ion level and cell contraction were simultaneously investigated in rabbit ventricular cardiac myocytes loaded with indo-1/A1. The role of Na+/Ca2+ exchange in ET-1-induced positive inotropic effect (PIE) was examined by using KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulphonate), a selective inhibitor of reverse mode Na+/Ca2+ exchange. 2. ET-1 at 0.3 pM - 1 nM increased cell contraction and Ca2+ transient (CaT) with EC50 values of 2.9 pM and 1.2 pM, respectively, and the increase in amplitude of CaT was much smaller relative to the PIE: ET-1 at 1 nM increased peak cell shortening by 237%, while it increased peak CaT by 167%. For a given level of PIE, ET-1-induced increase in CaT was much smaller than that induced by elevation of [Ca2+]o and by isoprenaline. Therefore, ET-1 shifted the relationship between peak CaT and cell shortening to the left relative to the relationship for increase in [Ca2+]o, an indication that ET-1 increased myofibrillar Ca2+ sensitivity. 3. KB-R7943 at 0.1 microM and higher inhibited contraction and CaT induced by 0.1 nM ET-1 and at 0.3 microM it abolished the increase in CaT while inhibiting the PIE by 48.1%. Over concentration range of 0.1-0.3 microM, KB-R7943 neither inhibited baseline contraction and CaT nor the isoprenaline-induced response, although at 1 microM and higher it had a significant inhibitory action on these responses. 4. These results indicate that in rabbit ventricular myocytes both increases in CaT and myofibrillar Ca2+ sensitivity contribute to the ET-induced PIE, and the activation of reverse mode Na+/Ca2+ exchange may play a crucial role in increase in CaT induced by ET-1 in rabbit ventricular cardiac myocytes.

Effect of angiotensin II on Ca2+ efflux from freshly isolated adult rat cardiomyocytes: possible involvement of Na+/Ca2+ exchanger

In the present study, we examined the effect of angiotensin II on Ca2+ efflux from freshly isolated adult rat cardiomyocytes. Angiotensin II stimulated the efflux of 45Ca2+ from the cells in a concentration-dependent manner, at least in pharmacological doses of 10(-8) M to 10(-5) M. The 45Ca2+ efflux was inhibited by the type 1 angiotensin II receptor antagonist losartan, but not by the type 2 antagonist PD 123319. Angiotensin II also induced an increase in cytosolic free calcium ([Ca2+]i) and inositol trisphosphate formation within the cardiomyocytes. Angiotensin II-induced 45Ca2+ efflux and the increase in [Ca2+]i were both inhibited by thapsigargin, a specific inhibitor of the sarcoplasmic reticulum Ca2+ pump. The 45Ca2+ efflux was not affected by removal of the extracellular Ca2+ but was dependent on the presence of extracellular Na+. In addition, angiotensin II caused 22Na+ influx into the cells. These results indicate that angiotensin II stimulates Na+-dependent 45Ca2+ efflux from freshly isolated adult rat cardiomyocytes, probably through its stimulatory effect on the plasma membrane type 1 angiotensin II receptors. Angiotensin II-induced increase in [Ca2+]i may cause an activation of Na+/Ca2+ exchange which finally results in the stimulation of 45Ca2+ efflux from the cells. Since it is reported that Na+/Ca2+ exchange is important in calcium homeostasis within the cells, angiotensin II may play some role in the reduction of intracellular Ca2+ from isolated adult rat cardiomyocytes.

Influence of dietary myoinositol on myocardial vulnerability and norepinephrine release in a diabetic animal model

In a canine model of diabetes enhanced ventricular vulnerability (VFT) has been associated with reduced myocardial myoinositol and increased release of norepinephrine (NE). To assess the role of the polyol, a dietary supplement of myoinositol was fed for 1 year to a diabetic group. Diabetes was induced with alloxan, 30 mg/kg. Controls (Group 1) were compared with diabetics without (Group 2) and with the inositol supplement (Group 3). After 1 year the animals were anesthetized to assess VFT. Basal heart rate and arterial pressure were comparable. The VFT in Group 1 was 43 +/- 2.6 ma, 26.7 +/- 2.8 ma in Group 2 (P < 0.02) and 39 +/- 3.5 ma in Group 3 (P < 0.02 vs. Group 2). Since the cardiac sympathetic system may promote arrhythmogenesis, the release of NE into the coronary sinus (CS) has been determined. To assess basal NE release serial arterial (A) and (CS) samples were taken at 5 min intervals for 20 min during infusion of 3H-NE. There was no significant difference between the diabetic groups in the level of arterial NE (HPLC). The mean for NEA-CS was higher in Group 2 (-228 +/- 33 pg/ml) compared to normals (-75 +/- 19 P < 0.02). In Group 3 the mean NE in the coronary venous effluent was -33 +/- 9 pg/ml, significantly less than Group 2. Infusion of 3H-NE was attended by significantly higher 3H-NEcs levels in Group 2. While dihydroxyphenylglycol (DHPG) was increased, 3H DHPG was not, suggesting that an impaired uptake mechanism contributed to the increased NEcs.(ABSTRACT TRUNCATED AT 250 WORDS)

Occurrence and functions of the phosphatidylinositol cycle in the myocardium

In the last decade a great deal of attention was awarded to a signal transduction pathway which is utilized primarily by 'Ca2+ mobilizing' signal molecules and which involves the hydrolysis of a quantitatively minor phospholipid, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) by a PtdIns-specific phospholipase C (PLC). The evidence for the existence of receptor-mediated GTP binding protein-coupled PLC in myocardium and its possible functions are briefly summarized. The minireview is concentrated on the following aspects: 1) cellular localization and synthesis of polyphospho-PtdIns from PtdIns, 2) desensitization of the alpha 1-adrenergic agonist and endothelin-1 mediated PtdIns responses, 3) oscillatory Ca2+ transients initiated by PtdIns(4,5)P2 hydrolysis, 4) polyunsaturated fatty acids as constituents of polyphospho-PtdIns and of the protein kinase C activator 1,2-diacylglycerol (DAG), 5) source other than PtdIns(4,5)P2 contributing to the stimulated DAG, 6) role of the PtdIns pathway in cardiomyocyte growth and gene expression during the hypertrophic response.

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 hypothermic ischemia and reperfusion on calcium transport by myocardial sarcolemma and sarcoplasmic reticulum

The effects of hypothermic ischemia and reperfusion on sarcolemma and sarcoplasmic reticulum Ca2+ transport were studied in vesicles isolated from rabbit hearts. Hypothermic global ischemia was produced by immersing hearts in saline at 4 degrees C for 3 h. Following hypothermic ischemia, reperfusion was carried out for 40 min using a Langendorff perfusion system for the working heart. Na+,K(+)-ATPase activity of sarcolemmal vesicles (SL), was not depressed by hypothermic ischemia nor by ischemia and reperfusion. The initial rate of Na(+)-Ca2+ exchange in SL vesicles was not depressed, but the maximum amount of Ca2+ uptake was increased both after hypothermic ischemia and after reperfusion. Ca2+ uptake activity of sarcoplasmic reticulum vesicles (SR) isolated from hearts subjected to hypothermic ischemia was slightly lower than that of control, and was further reduced following reperfusion. Ca(2+)-ATPase activity of SR was unaffected by hypothermic ischemia, while it was markedly lowered after reperfusion. Although the phosphoenzyme level in SR vesicles was slightly decreased, the turnover rate was reduced after reperfusion. Reperfusion injury thus took place mainly in SR while SL appeared to be tolerant to ischemia and reperfusion.

Two major antigens of heart sarcolemma are Ca2(+)-binding glycoproteins that copurify with the dihydropyridine receptor

Ca2+ binding has been studied in isolated heart sarcolemmal membranes using the 45Ca overlay technique. 45Ca bound to two sarcolemmal polypeptides of 125 kDa and 97 kDa in preparations from dog, rabbit, cow and pig. During fractionation on DEAE ion-exchange and wheat-germ lectin affinity columns, the two Ca2(+)-binding polypeptides copurified with the dihydropyridine receptor associated with the voltage gated Ca2+ channel. These polypeptides were the major proteins in the isolated fraction as judged by silver staining in SDS-PAGE. Antisera raised against purified dog heart, sarcolemma indicated that the 125 and 97 kDa polypeptides were highly antigenic components of this membrane. The antisera cross-reacted with similar polypeptides in cardiac sarcolemmal preparations from rabbit, cow and pig, but not sarcoplasmic reticulum membranes. Purified antibodies against the 125 kDa polypeptide did not cross-react with the 97 kDa polypeptide, while antibodies against the 97 kDa polypeptide did not cross-react with the 125 kDa polypeptide. Both the 125 kDa and 97 kDa polypeptides bound wheat-germ lectin, suggesting both were glycoproteins. It is unlikely that these Ca2+ binding glycoproteins represent subunits of the dihydropyridine receptor-Ca2+ channel in this membrane.

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.