The Sarcolemmal Sodium-Calcium Exchange System

Temperature dependence of Na+/Ca2+ exchange activity in beef-heart sarcolemmal vesicles and proteoliposomes

Temperature dependence of Na+/Ca2+ exchange activity was studied in beef cardiac sarcolemmal vesicles in the absence and presence of the inhibitor amiloride and in proteoliposomes reconstituted with different lipid mixtures. Arrhenius plots for Na+/Ca2+ exchange activity in both control and amiloride-treated vesicles revealed an apparent energy of activation of 9665 +/- 585 (SE, n = 4) cal/mol, corresponding to a temperature coefficient (Q10) value of 1.70 +/- 0.05 (SE, n = 4) over the range 25-37 degrees C. When Na+/Ca2+ exchange was reconstituted into phosphatidylcholine (PC):phosphatidylserine (PS) (52:48, mol/mol), PC:PS:cholesterol (25:39:36, mol/mol), and PC:PS:distearoylphosphatidylcholine (DSPC) (31:48:21, mol/mol) proteoliposomes, the highest activity was found in PC:PS:cholesterol proteoliposomes. Arrhenius plots of Na+/Ca2+ exchange activity exhibited breakpoints at 23 degrees C (PC:PS), 33 degrees C (PC:PS:cholesterol), and 23 degrees C (PC:PS:DSPC). The increase in the thermotropic transition temperature with cholesterol could result from the condensing effect of this sterol, whereas the breaks observed with PC:PS and PC:PS:DSPC could be caused by a non-lipid-mediated membrane protein conformational change. These results indicate that the lipid microenvironment around the Na+/Ca2+ exchanger and the nature of the specific lipid-protein interactions influence the activity of this antiporter. Further evidence supporting the hypothesis that cholesterol behaves as a specific positive effector for the exchanger is also given.

Effect of Quin-2 on 45Ca2+ uptake mediated by Na+i/Ca2+o exchange and 45Ca2+ efflux in rat brain synaptosomes: a requirement for [Ca2+]i

The Na+/Ca2+ exchanger of squid axons, barnacle muscle and sarcolemma requires micromolar intracellular calcium for activation in the Na+i/Ca2+o exchange mode ('reverse' Na+/Ca2+ exchange). The requirement for [Ca2+]i has been demonstrated with the use of intracellular calcium buffers, such as Quin-2, to inhibit Na+i/Ca2+o exchange. However, the inhibition of Na+i/Ca2+o exchange in mammalian nerve terminals loaded with Quin-2 has not been observed [7], suggesting a lower sensitivity to low [Ca2+]i for this system. In contrast, the results reported herein indicate that 45Ca2+ uptake in synaptosomes through Na+i/Ca2+o exchange is inhibited by Quin-2 much in the same way as it is in the squid, provided that synaptosomes are preincubated in low Ca2+ medium to avoid saturation of Quin-2. Under these conditions, 45Ca2+ efflux via Ca2+i/Ca2+o exchange is also inhibited. Our results indicate that the Na+i/Ca2+o and Ca2+i/Ca2+o modes of the Na+/Ca2+ exchanger from rat brain synaptosomes require intracellular calcium for activation. However, because no clear relationship between the observed [Ca2+]i values and the inhibition of Na+i/Ca2+o exchange has been found, it is suggested that localised submembrane calcium concentrations not detected by the [Ca2+]i probe might regulate the exchanger.

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.

Inhibition of Na+/Ca2+ exchange by amiloride acting from opposite sides of cardiac sarcolemma

Amiloride inhibited the Na+Ca2+ exchange activity of cardiac sarcolemmal vesicles with similar affinities at the cis and trans sides of the membrane, estimated apparent Ki on both sides of the sarcolemma being similar. The extent of amiloride inhibition on Na+/Ca2+ exchange activity was decreased by alkaline pH only when the drug was acting from the external side of the vesicle sarcolemma, whereas when vesicles were preincubated with the drug at different pH values, amiloride appeared to act as a weak permeant base, being a more effective inhibitor at alkaline pH values. In fact, a rise in the pH of the preincubation medium may favour the entry and consequently the effect of the drug on the exchanger. The pH dependence of the inhibition of Na+/Ca2+ exchange activity by either extravesicular or intravesicular amiloride was consistent with the hypothesis that in both cases the protonated drug was the active form. Evidence is presented that the pattern of interaction of amiloride on the Na+/Ca2+ exchange system strictly depended on the sidedness of drug action. In fact, while Na+ protected against inhibition by amiloride when it was acting on the same side of the vesicle membrane as the drug, it synergically interacted with amiloride to inhibit exchange activity when it was acting on the opposite side of the sarcolemma as the drug. Furthermore, only extravesicular amiloride removed the stimulation of Na+/Ca2+ exchange activity in Ca2+-treated vesicles.

Na-Ca exchanger as a calcium influx pathway in adrenal glomerulosa cells

When aequorin-loaded glomerulosa cells were incubated in isotonic Na2+-free medium containing N-methyl-D-glucamine instead of NaCl, there was an increase in cytoplasmic free calcium concentration, [Ca2+] c, which was not observed when extracellular calcium concentration was reduced to 1 microM. Upon removal of extracellular sodium, there was nearly five-fold increase in fractional efflux ratio of calcium. The reduction of extracellular sodium resulted in a stimulation of calcium influx rate, the magnitude of which was dependent on extracellular sodium concentration. Similar stimulation of calcium influx was observed when extracellular sodium was replaced with lithium. Nitrendipine did not affect the calcium influx induced by the reduction of extracellular sodium while a derivative of amiloride 3',4'-dichlorobenzamil, which inhibits Na-Ca exchange, attenuated calcium influx observed in sodium-free medium. These results indicate that removal of extracellular sodium leads to an increase in [Ca2+] c by stimulating calcium influx and that calcium enters the cell via Na-Ca exchanger.

Molecular cardiology: new avenues for the diagnosis and treatment of cardiovascular disease

This review summarizes some of the major advances in the investigation of molecular mechanisms underlying both normal and abnormal cardiovascular function. Four major areas are highlighted including cardiac muscle, the blood vessel, atherosclerosis and thrombosis/thrombolysis. The remarkable strides in understanding multifactorial diseases such as atherosclerosis, and the development of innovative new therapies such as the use of thrombolytic agents produced by recombinant deoxyribonucleic acid (DNA) technology, are noted. Moreover, it is concluded that the past decade of basic research has provided a solid framework for improvements in the diagnosis and therapy of other forms of cardiovascular disease as well. An evaluation of current trends in basic cardiovascular research suggests that diagnostic and therapeutic approaches to disease will increasingly target specific molecular processes underlying the pathophysiologic state.

Relationship between cytosolic free Ca2+ and Na+-Ca2+ exchange in aortic muscle cells

We examined the relationship between extracellular Na+ ([Na+]o) and cytosolic free Ca2+ ([Ca2+]i) in primary and passaged cultures of aortic muscle cells. Removing [Na+]o increased [Ca2+]i by approximately 10-fold in cells that were Na+ loaded. Decreasing [Na+]o from 140 to 32 mM caused the half-maximal increase in [Ca2+]i. [Ca2+]i exhibited a sigmoidal dependence on [Na+]o. Mg2+, a competitive inhibitor of Na2+-Ca2+ antiport in these cells, antagonized the increase in [Ca2+]i produced by lowering [Na+]o. High K+ decreased the potency of Mg2+ 6.5-fold as previously reported for Na+ gradient-dependent 45Ca2+ influx. In contrast to the Na+-loaded cells, removing [Na+]o caused no detectable change in [Ca2+]i in cells with normal Na+ even though the calculated electrochemical driving force for Na+-Ca2+ exchange was large enough to almost maximally increase [Ca2+]i in the Na+-loaded cells. We conclude that Na+-Ca2+ antiport activity is latent in the unstimulated cell at basal intracellular Na+ and Ca2+.

Evidence for high molecular weight Na-Ca exchange in cardiac sarcolemmal vesicles

Cardiac sarcolemma (SL) vesicles were subjected to irradiation inactivation-target sizing analyses and gel permeation high performance liquid chromatography (HPLC) to ascertain the weight range of native Na-Ca exchange. Frozen SL vesicle preparations were irradiated by electron bombardment and assayed for Na-Ca exchange activity. When applied to classical target sizing theory, the results yielded a minimum molecular weight (Mr) of approximately 226,000 +/- 20,000 SD (n = 6). SL vesicle proteins were solubilized in 6% sodium cholate in the presence of exogenous phospholipid and fractionated by size on a TSK 30XL HPLC column. Eluted proteins were mixed 1:1 with mobile phase buffer containing 50 mg/ml soybean phospholipid and reconstituted by detergent dilution. The resulting proteoliposomes were assayed for Na-Ca exchange activity. Na-Ca exchange activity eluted in early fractions containing larger proteins as revealed by SDS-PAGE. Recovery of total protein and Na-Ca exchange activity were 91 +/- 7 and 68 +/- 11%, respectively. In the peak fraction, Na-Ca exchange specific activity increased two- to threefold compared to reconstituted controls. Compared to the elution profile of protein standards under identical column conditions, sodium cholate solubilized exchange activity had a minimum Mr of 224,000 Da. Specific 45Ca2+-binding SL proteins with Mr of 234,000, 112,000, and 90,000 Da were detected by autoradiography of proteins transferred electrophoretically to nitrocellulose. These data suggest that native cardiac Na-Ca exchange is approximately 225,000 Da or larger. The exact identification and purification of cardiac Na-Ca exchange protein(s) remains incomplete.

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.

Measurement of the cytosolic sodium ion concentration in rat brain synaptosomes by a fluorescence method

A method for the measurement of the cytosolic Na+ concentration in intact synaptosomes is described. This method makes use of a pH sensitive dye (BCECF) that can be loaded into the cytosol and a relatively specific ionophore (monensin) that can exchange Na+ for H+ across the synaptosomal membrane. By setting conditions such that there is no electrochemical potential difference for H+ across the membrane (no membrane potential and pHi = pHo), addition of ionophore would induce a H+ flux only if there is a concentration difference for Na+. Thus, when there is no fluorescence change (no cytosolic pH change) extracellular [Na+] equals intrasynaptosomal [Na+]. The intrasynaptosomal [Na+] concentration was determined to be 7 +/- 3 mM (n = 5; mean +/- S.E.). The results obtained with this fluorescence method are compared with estimates obtained by atomic absorption spectrometry. Limitations and applications of the method are discussed.

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.

Experimental and theoretical work on excitation and excitation-contraction coupling in the heart

A combination of experimental and theoretical work has been used to investigate the movements of calcium during cardiac excitation. In addition to calcium entry through several types of calcium channel, calcium efflux occurs to balance the entry during each cycle of activity. Measurements of net membrane calcium movements have been made with the right time resolution by Don Hilgemann in Los Angeles by investigating fast extracellular calcium transients. This work shows that, in mammalian cardiac cells, net calcium exit occurs quite early during repolarization and is nearly complete by the time the resting potential is re-established. These results correlate very well indeed with measurements made in the Oxford laboratory of calcium-activated inward current in single cardiac myocytes. Both approaches are consistent with the view that calcium efflux occurs largely through the sodium-calcium exchange process. Modelling of this process in equations developed recently with Dario DiFrancesco, Susan Noble and Don Hilgemann succeeds in reproducing both the ionic current changes and the fast extracellular calcium transients.

Sodium-calcium exchange in sarcolemmal vesicles from tracheal smooth muscle

Sarcolemmal vesicles prepared by a new procedure from bovine tracheal smooth muscle were found to have a Na-Ca exchange activity that is significantly higher than that reported for different preparations from other types of smooth muscle. The exchange process system co-purified with 5'-nucleotidase, a plasma membrane marker enzyme, and was significantly enriched (over 100-fold) compared to mitochondria (cytochrome-c oxidase) but only slightly enriched (4-fold) compared to sarcoplasmic reticulum (NADPH-cytochrome-c reductase). The Na+ dependence of Ca2+ transport was demonstrated through both uptake and efflux procedures. The uptake profile with respect to Ca2+ was monotonic with a linear vo VS. vo.S-1 plot. The resultant Km of Ca2+ from the airway sarcolemmal vesicles (20 microM) was similar in magnitude to the Km of cardiac sarcolemmal vesicles (30 microM). Tracheal vesicles demonstrated a Vmax of 0.3-0.5 nmol.mg-1.s-1 which is significantly higher than that reported in preparations from other smooth muscle types. Furthermore, two processes found to stimulate cardiac Na-Ca exchange, pretreatment with either a mixture of dithiothreitol and Fe2+ or with chymotrypsin, were ineffective on the tracheal smooth muscle. Thus, the Na-Ca exchanger identified in tracheal smooth muscle appears to be different from that observed in cardiac muscle, implying that regulation of this activity may also be different.

The squid axon as a model for studying plasma membrane mechanisms for calcium regulation

Calcium movement across plasma membranes occurs mainly by three routes: voltage-dependent calcium channels, adenosine 5'-triphosphate-driven calcium pump, and Na+-Ca2 exchange. The regulation of the intracellular ionized calcium is the consequence of two parallel calcium transport mechanisms: a high affinity, low capacity system responsible for extruding calcium during resting conditions (calcium pump) and a low affinity and high capacity system (Na+-Ca2 antiporter). This last system is designed to extrude calcium ions when intracellular calcium rises above certain levels and also to lead calcium ions into the cell under conditions that favor the reverse mode of operation of the exchanger. This short review provides an analysis of the most conspicuous features of the two membrane transport mechanisms determined in dialyzed squid axons with special emphasis on both the complexity of the Na+-Ca2+ exchange system and its marked asymmetry.

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.

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

We examine the effects of 5-, 12- and 16-doxylstearic acids on the Na+-Ca2+ exchange and passive Ca2+ permeability of cardiac sarcolemmal vesicles. Stearic acid is a weak stimulator of Na+-Ca2+ exchange. A doxyl moiety potentiates stimulation with the order of increasing potency being 5-, 12- and then 16-doxylstearic acid. Stearic acid has little effect on vesicle Ca2+ permeability but again the doxylstearates are more effective. The sequence of potency is reversed, however, from that for increasing Na+-Ca2+ exchange. 5-Doxylstearic acid most markedly exchanges passive Ca2+ flux followed by the 12-, and then 16-doxylstearic acids. Methyl esters of the doxylstearates have no effect on either Na+-Ca2+ exchange or Ca2+ permeability. We model the results as follows. For a fatty acid to stimulate Na+-Ca2+ exchange activity, an anionic charge is required to interact with the exchanger protein at the membrane surface. Stimulation is potentiated by a perturbation (such as provided by a doxyl group) within the lipid bilayer. The perturbation is most effective at a location towards the center of the bilayer. To increase passive Ca2+ permeability an anionic charge is again essential. Disorder within the bilayer is also important, but now the most important site is near the membrane surface. Results of experiments with linolenic and gamma-linolenic acid and previous studies with other fatty acids also support this model.

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.

Alterations in intracellular calcium activity and contractility of isolated perfused rabbit hearts by ionophores and adrenergic agents

The fluorescent calcium indicator quin2 has been used for the first continuous measurement of the effects of pharmacological agents on intracellular calcium activity in isolated, perfused rabbit hearts. The average intracellular calcium activity was elevated after the infusion of norepinephrine, concurrent with increases in left ventricular pressure and heart rate. These changes were abolished by pretreatment of the heart with phentolamine and nadolol, alpha and beta adrenergic receptor antagonists, respectively. Pretreatment with phentolamine and nadolol did not eliminate the increases in left ventricular pressure and intracellular calcium activity caused by the infusion of the monovalent carboxylic ionophores monensin and salinomycin. It is concluded that the ionophores cause these effects by elevating intracellular sodium activity, which then raises the intracellular calcium activity of the myocardium through intracellular displacement and/or transcellular exchange. It is suggested that the use of fluorescent calcium indicators in intact organs could be useful in evaluating the role of calcium in a variety of pathological states.