The Sarcolemmal Sodium-Calcium Exchange System

Optimal metabolic regulation of the mammalian heart Na(+)/Ca(2+) exchanger requires a spacial arrangements with a PtdIns(4)-5kinase

In inside-out bovine heart sarcolemmal vesicles, p-chloromercuribenzenesulfonate (PCMBS) and n-ethylmaleimide (NEM) fully inhibited MgATP up-regulation of the Na(+)/Ca(2+) exchanger (NCX1) and abolished the MgATP-dependent PtdIns-4,5P2 increase in the NCX1-PtdIns-4,5P2 complex; in addition, these compounds markedly reduced the activity of the PtdIns(4)-5kinase. After PCMBS or NEM treatment, addition of dithiothreitol (DTT) restored a large fraction of the MgATP stimulation of the exchange fluxes and almost fully restored PtdIns(4)-5kinase activity; however, in contrast to PCMBS, the effects of NEM did not seem related to the alkylation of protein SH groups. By itself DTT had no effect on the synthesis of PtdIns-4,5P2 but affected MgATP stimulation of NCX1: moderate inhibition at 1mM MgATP and 1μM Ca(2+) and full inhibition at 0.25mM MgATP and 0.2μM Ca(2+). In addition, DDT prevented coimmunoprecipitation of NCX1 and PtdIns(4)-5kinase. These results indicate that, for a proper MgATP up-regulation of NCX1, the enzyme responsible for PtdIns-4,5P2 synthesis must be (i) functionally competent and (ii) set in the NCX1 microenvironment closely associated to the exchanger. This kind of supramolecular structure is needed to optimize binding of the newly synthesized PtdIns-4,5P2 to its target region in the exchanger protein.

Measurements of mitochondrial pH in cultured cortical neurons clarify contribution of mitochondrial pore to the mechanism of glutamate-induced delayed Ca2+ deregulation

To clarify the role of the mitochondrial permeability transition pore (MPT) in the mechanism of the glutamate-induced delayed calcium deregulation (DCD) and mitochondrial depolarization (MD), we studied changes in cytosolic (pH(c)) and mitochondrial pH (pH(m)) induced by glutamate in cultured cortical neurons expressing pH-sensitive fluorescent proteins. We found that DCD and MD were associated with a prominent pH(m) decrease which presumably resulted from MPT opening. This pH(m) decrease occurred with some delay after the onset of DCD and MD. This argued against the hypothesis that MPT opening plays a dominant role in triggering of DCD. This conclusion was also supported by experiments in which Ca(2+) was replaced with antagonist of MPT opening Sr(2+). We found that in Sr(2+)-containing medium glutamate-induced delayed strontium deregulation (DSD), similar to DCD, which was accompanied by a profound MD. Analysis of the changes in pH(c) and pH(m) associated with DSD led us to conclude that MD in Sr(2+)-containing medium occurred without involvement of the pore. In contrast, in Ca(2+)-containing medium such "non-pore mechanism" was responsible only for MD initiation while in the final stages of MD development the MPT played a major role.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Maximal Ca2+i stimulation of cardiac Na+/Ca2+ exchange requires simultaneous alkalinization and binding of PtdIns-4,5-P2 to the exchanger

Using bovine heart sarcolemma vesicles we studied the effects of protons and phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2) on the affinity of the mammalian Na(+)/Ca(2+) exchanger (NCX1) for intracellular Ca(2+). By following the effects of extravesicular ligands in inside-out vesicles, their interactions with sites of NCX1 facing the intracellular medium were investigated. Two Na(+)-gradient-dependent fluxes were studied: Ca(2+) uptake and Ca(2+) release. PtdIns-4,5-P2 binding to NCX1 was investigated in parallel. Without MgATP (no 'de novo' synthesis of PtdIns-4,5-P2), alkalinization increased the affinity for Ca(2+) and the PtdIns-4,5-P2 bound to NCX1. Vesicles depleted of phosphoinositides were insensitive to alkalinization, but became responsive following addition of exogenous PtdIns-4,5-P2 or PtdIns plus MgATP. Acidification reduced the affinity for Ca(2+)(ev); this was only partially reversed by MgATP, despite the increase in bound PtdIns-4,5-P2 to levels observed with alkalinization. Inhibition of Ca(2+) uptake by increasing extravesicular [Na(+)] indicates that it is related to H(+)(i) and Na(+)(i) synergistic inhibition of the Ca(2+)(i) regulatory site. Therefore, the affinity of the NCX1 Ca(2+)(i) regulatory site for Ca(2+) was maximal when both intracellular alkalinization and an increase in PtdIns-4,5-P2 bound to NCX1 (not just of the total membrane PtdIns-4,5-P2) occurred simultaneously. In addition, protons influenced the distribution, or the exposure, of PtdIns-4,5-P2 molecules in the surroundings and/or on the exchanger protein.

In Bovine Heart Na+/Ca2+ Exchanger Maximal Ca2+i Affinity Requires Simultaneously High pHi and PtdIns-4,5-P2 Binding to the Carrier

Na+ i-dependent Ca2+ uptake, Na+-dependent Ca2+ release, and PtdIns-4,5-P2 binding to Na+/Ca2+ exchanger (NCX1) as a function of extravesicular (intracellular) [Ca2+] were measured. Alkalinization increases Ca2+ i affinity and PtdIns-4,5-P2 bound to NCX1; these effects are abolished by pretreatment with PtdIns-PLC and are insensitive to MgATP. Acidification reduces Ca2+ i affinity. MgATP reverts it only partially despite the fact that the PtdIns-4,5-P2 bound to NCX1 reaches the same levels as at pH 7.8. Extravesicular Na+-stimulated and Ca2+-dependent Ca2+ efflux indicate the Ca2+ regulatory site is involved. Therefore, to display maximal affinity to Ca2+ i, PtdIns-4,5-P2 binding and deprotonation of NCX1 are simultaneously need.

Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions

The Na(+)/Ca(2+) exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca(2+) from the cell. Two basic properties characterize them. 1) Their activity is not predicted by thermodynamic parameters of classical electrogenic countertransporters (dependence on ionic gradients and membrane potential), but is markedly regulated by transported (Na(+) and Ca(2+)) and nontransported ionic species (protons and other monovalent cations). These modulations take place at specific sites in the exchanger protein located at extra-, intra-, and transmembrane protein domains. 2) Exchange activity is also regulated by the metabolic state of the cell. The mammalian and invertebrate preparations share MgATP in that role; the squid has an additional compound, phosphoarginine. This review emphasizes the interrelationships between ionic and metabolic modulations of Na(+)/Ca(2+) exchange, focusing mainly in two preparations where most of the studies have been carried out: the mammalian heart and the squid giant axon. A surprising fact that emerges when comparing the MgATP-related pathways in these two systems is that although they are different (phosphatidylinositol bisphosphate in the cardiac and a soluble cytosolic regulatory protein in the squid), their final target effects are essentially similar: Na(+)-Ca(2+)-H(+) interactions with the exchanger. A model integrating both ionic and metabolic interactions in the regulation of the exchanger is discussed in detail as well as its relevance in cellular Ca(i)(2+) homeostasis.

Residues contributing to the Ca2+ and K+ binding pocket of the NCKX2 Na+/Ca2+-K+ exchanger

The Na(+)/Ca(2+)-K(+) exchanger (NCKX) extrudes Ca(2+) from cells utilizing both the inward Na(+) gradient and the outward K(+) gradient. NCKX is thought to operate by a consecutive mechanism in which a cation binding pocket accommodates both Ca(2+) and K(+) and alternates between inward and outward facing conformations. Here we developed a simple fluorometric method to analyze changes in K(+) and Ca(2+) dependences of mutant NCKX2 proteins in which candidate residues within membrane-spanning domains were substituted. The largest shifts in both K(+) and Ca(2+) dependences compared with wild-type NCKX2 were observed for the charge-conservative substitutions of Glu(188) and Asp(548), whereas the size-conservative substitutions resulted in nonfunctional proteins. Substitution of several other residues including two proline residues (Pro(187) and Pro(547)), three additional acidic residues (Asp(258), Glu(265), Glu(533)), and two hydroxyl-containing residues (Ser(185) and Ser(545)) showed smaller shifts, but shifts in Ca(2+) dependence were invariably accompanied by shifts in K(+) dependence. We conclude that Glu(188) and Asp(548) are the central residues of a single cation binding pocket that can accommodate both K(+) and Ca(2+). Furthermore, a single set of residues lines a transport pathway for both K(+) and Ca(2+).

Regulation of phosphatidylinositol-4,5-biphosphate bound to the bovine cardiac Na+/Ca2+ exchanger

Western blot and cross immunoprecipitation analysis with specific antibodies demonstrate that in bovine heart sarcolemmal vesicles phosphatidylinositol-4,5-biphosphate (PtdIns-4,5-P(2)) binds strongly to the Na(+)/Ca(2+) exchanger (NCX1). This binding is modulated by ATP, Ca(2+), vanadate, exchanger inhibitory peptide (XIP), and PLC-PtdIns specific in a way resembling the ATP regulation of the exchange fluxes. With 1 microM Ca(2+), 3 mM Mg(2+), and 0.4 mM vanadate, 1 mM ATP increased about twofold the bound PtdIns-4,5-P(2), reaching a steady state in 3-5 s at 37 degrees C. With 100 microM Ca(2+), ATP had no effect on the PtdIns-4,5-P(2) bound to NCX1 or on the exchange fluxes. Without vanadate the bound PtdIns-4,5-P(2) was largely reduced; under this condition ATP failed to increase it and did not stimulate the exchanger. XIP inhibits the exchanger, more noticeable in the absence of ATP. With XIP, ATP does not modify the levels of bound PtdIns-4,5-P(2); however there is a small but distinct ATP stimulation of the exchanger. Vesicles pretreated with PtdIns-PLC, showed no de novo, [(32)P]ATP-induced, production of PtdIns-4,5-P(2), but some ATP-stimulated increase in the bound PtdIns-4,5-P(2) was detected; however, that increase did not exceed the levels found with vanadate and no ATP. These results indicate that in bovine heart sarcolemmal vesicles, ATP upregulation of NCX1 is related to PtdIns-4,5-P(2) bound to the exchanger, perhaps over a "threshold" or "unspecific" amount. In addition, vanadate could influence the amount of detected PtdIns-4,5-P(2) either by inhibiting phosphoinositide-specific phosphatases and/or by inducing a redistribution of PtdIns-4,5-P(2) molecules associated with the Na(+)/Ca(2+) exchanger.

Phosphatidyl inositol-4,5-bisphosphate bound to bovine cardiac Na+/Ca2+ exchanger displays a MgATP regulation similar to that of the exchange fluxes

This work shows the existence of a phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5-P2) bound form of the cardiac sarcolemmal Na+/Ca2+ exchanger. That was demonstrated in Western blots and cross-immunoprecipitation by using specific antibodies against the NCX1 exchanger (NCX1) and against PtdIns-4,5-P2. In addition, PtdIns-4,5-P2 bound to the Na+/Ca2+ exchanger and the Na+/Ca2+ exchange fluxes displayed a similar MgATP regulation: (a) both increase by 100-130% when membrane vesicles are incubated (15-20 s at 37 degrees C) with 1 mM MgATP and 1 microM Ca2+ (b) in the presence of 100 microM Ca2+, MgATP fails to stimulate the exchange fluxes and does not modify the levels of PtdIns-4,5-P2 bound to the exchanger. In addition, in the absence of Ca2+, the net synthesis of total membrane PtdIns-4,5-P2 is greatly reduced compared with that in the presence of 1 microM Ca2+. Furthermore, in the absence of Ca2+ there is no effect of MgATP on the levels of PtdIns-4,5-P2 bound to the exchanger. These results indicate that, in bovine heart, MgATP-stimulation of Na+/Ca2+ exchange is associated with intracellular Ca2+-dependent levels of PtdIns-4,5-P2 bound to the exchanger molecule.

Na+/Ca2+ exchanger activity induces a slow DC potential after in vitro ischemia in rat hippocampal CA1 region

In rat hippocampal CA1 neurons recorded intracellularly from tissue slices, a rapid depolarization occurred approximately 5 min after application of ischemia-simulating medium. In extracellular recordings obtained from CA1 region, a rapid negative-going DC potential (rapid DC potential) was recorded, corresponding to a rapid depolarization. When oxygen and glucose were reintroduced after generating the rapid depolarization, the membrane further depolarized and the potential became 0 mV after 5 min. Contrary, the DC potential began to repolarize slowly and subsequently a slow negative-going DC potential (slow DC potential) occurred within 1 min. A prolonged application of ischemia-simulating medium suppressed the slow DC potential. Addition of a high concentration of ouabain in normoxic medium reproduced a rapid but not a slow DC potential. The slow DC potential was reduced in low Na+- or Co2+-containing medium, but was not affected in low Cl-, high K+ or K+-free medium, suggesting that the slow DC potential is Na+-and Ca2+-dependent. Ni2+ (Ca2+ channel blocker as well as the Na+/Ca2+ exchanger blocker) and benzamil hydrochloride (Na+/Ca2+ exchanger blocker) reduced the slow DC potential dose-dependently. These results suggest that the slow DC potential is mediated by forward mode operation of Na+/Ca2+ exchangers in non-neuronal cells, and that reactivation of Na+, K+-ATPase is necessary to the Na+/Ca2 +exchanger activity.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Expression of the Na+/Ca2+ exchanger emerges in hepatic stellate cells after activation in association with liver fibrosis

Activation of hepatic stellate (Ito) cells is a final common pathway of liver fibrosis. The findings presented in this paper indicate that expression of Na+/Ca2+ exchanger (NCX) emerges in rat hepatic stellate cells after activation in vitro during primary culture or in vivo in response to intoxication with CCl4. NCX mRNA became detectable by Northern blot analysis in cultured stellate cells on day 3, as was alpha-smooth muscle actin, an indicator not only of smooth muscle differentiation but also of stellate cell activation. Western blot analysis showed expression of the exchanger protein in the activated stellate cells. Functional expression of the exchanger, monitored by Ni2+-sensitive, verapamil-insensitive intracellular free Ca2+ increases in response to reduction of extracellular Na+ concentration, became sizable by using Fura-2 in stellate cells by 7 days in culture. Furthermore, increased expression of the exchanger mRNA was found predominantly in stellate cells freshly isolated from the CCl4 model rat of hepatic fibrosis. Thus, it is concluded that NCX expression is closely associated with activation of hepatic stellate cells in vitro and in vivo. Because, even at the whole liver level, increased expression of NCX mRNA became observable after induction of liver fibrosis, it is suggested that NCX expression serves a useful diagnostic marker of liver fibrosis or cirrhosis.

ATP stimulation of Na+/Ca2+ exchange in cardiac sarcolemmal vesicles

In cardiac sarcolemmal vesicles, MgATP stimulates Na+/Ca2+ exchange with the following characteristics: 1) increases 10-fold the apparent affinity for cytosolic Ca2+; 2) a Michaelis constant for ATP of approximately 500 microM; 3) requires micromolar vanadate while millimolar concentrations are inhibitory; 4) not observed in the presence of 20 microM eosin alone but reinstated when vanadate is added; 5) mimicked by adenosine 5'-O-(3-thiotriphosphate), without the need for vanadate, but not by beta,gamma-methyleneadenosine 5'-triphosphate; and 6) not affected by unspecific protein alkaline phosphatase but abolished by a phosphatidylinositol-specific phospholipase C (PI-PLC). The PI-PLC effect is counteracted by phosphatidylinositol. In addition, in the absence of ATP, L-alpha-phosphatidylinositol 4,5-bisphosphate (PIP2) was able to stimulate the exchanger activity in vesicles pretreated with PI-PLC. This MgATP stimulation is not related to phosphorylation of the carrier, whereas phosphorylation appeared in the phosphoinositides, mainly PIP2, that coimmunoprecipitate with the exchanger. Vesicles incubated with MgATP and no Ca2+ show a marked synthesis of L-alpha-phosphatidylinositol 4-monophosphate (PIP) with little production of PIP2; in the presence of 1 microM Ca2+, the net synthesis of PIP is smaller, whereas that of PIP2 increases ninefold. These results indicate that PIP2 is involved in the MgATP stimulation of the cardiac Na+/Ca2+ exchanger through a fast phosphorylation chain: a Ca(2+)-independent PIP formation followed by a Ca(2+)-dependent synthesis of PIP2.

A role of intracellular Na+ in the regulation of synaptic transmission and turnover of the vesicular pool in cultured hippocampal cells

Propagation of action potentials in axons and dendrites increases intracellular Na+ ([Na+]i) and Ca2+ concentrations ([Ca2+]i). While the importance of [Ca2+]i in synaptic transmission is well established, a possible functional role of [Na+]i is unclear. In cultured hippocampal cells, [Na+]i was increased by veratridine. We have then measured spontaneous excitatory postsynaptic currents (sEPSCs) and, by means of fluorescent dyes, changes in [Na+]i, in [Ca2+]i, and in the turnover of the vesicular pool of individual boutons. An elevation of [Na+]i and a concomitant rise in [Ca2+]i, led to a large increase in sEPSC frequency and in the turnover of the presynaptic vesicular pool. Extracellular Ca2+ was essential for these effects of elevated [Na+]i on synaptic transmission. They probably occur via Na+/Ca2+ exchange.

Regulation of cytosolic calcium levels in vascular smooth muscle

Ca2+ plays an important role in the contraction of skeletal, cardiac, and smooth muscle, as well as in a number of important processes, such as secretion and neuronal activity. In this review, I focus on the various mechanisms by which cytosolic Ca2+ concentration is regulated in vascular smooth muscle, in the resting state and during activation. Particular attention is paid to the calcium pumps of the plasmalemma and the sarcoplasmic reticulum, to the inositol 1,4,5-trisphosphate- and ryanodine-sensitive calcium channels of the sarcoplasmic reticulum, and to voltage-dependent and voltage-independent calcium channels of the plasmalemma.

Inhibition and acceleration of Na+/Ca2+/K+ exchange fluxes by Ag+ in bovine retinal rod outer segments

The effect of Ag+ on Ca2+ fluxes mediated by the retinal rod Na+/Ca2+/K+ exchanger was investigated in intact bovine rod outer segments (ROS). Intracellular Na+ concentration ([Na+]in)-dependent Ca2+ influx and extracellular Na+ concentration ([Na+]out)-dependent Ca2+ efflux were monitored by changes in cytosolic free Ca2+ measured with the fluorescent Ca(2+)-indicating dye fluo 3. Ag+ was the most effective inhibitor of Na+/Ca2+/K+ exchange fluxes described to date, with half-maximal inhibition observed at 2-8 microM Ag+. Inhibition by Ag+ could be reversed by addition of beta-mercaptoethanol but not by addition of cysteine. Reversal by beta-mercaptoethanol resulted in a marked acceleration of [Na+]out-dependent lowering of cytosolic free Ca2+ but not of [Na+]in-dependent Ca2+ influx. We suggest that Ag+ inhibits and accelerates Na+/Ca2+/K+ exchange fluxes by binding to cysteine residues on the cytosolic surface of the exchanger protein.

The Na(+)-Ca2+ exchange system in rat glial cells in culture: activation by external monovalent cations

Cultured rat glial cells display a Na(+)-Ca2+ exchange system located at the plasma membrane levels. This was evidenced by the Na+ (i)-dependency of a Na+ (o)-inhibitable influx of Ca2+, or reversal exchange mode. This antiporter has an external site where monovalent cations (K+, Li+, and Na+ were investigated) stimulate the exchange by a chemical action. The monovalent cation is not transported during the exchange cycle. The mechanism of that stimulation agrees with an increase in the apparent affinity of the carrier for Ca2+(o) without effect on the maximal translocation rate. Two models can equally well account for the data: i) the formation of ECa(o) is essential for the binding of the monovalent cation, or ii) the activating cation can bind even when the carrier is free of Ca2+(o). The cations K+ and Li+ produced only stimulation, although that of K+ seem to require actions other than the chemical effect. The response to Na+ was biphasic; this can be fully explained considering that at low concentrations, Na+(o) binds preferentially to the activating monovalent site while at high concentrations it displaces Ca2+ from its external transporting site. Pure type I astrocytes displayed the same Na(+)-Ca2+ exchange mechanism.

Effects of vanadate on MgATP stimulation of Na-Ca exchange support kinase-phosphatase modulation in squid axons

We have proposed that in squid axons MgATP stimulation of Na-Ca exchange involves a phosphorylation-dephosphorylation process catalyzed by a kinase-phosphatase system. In the present work, we used vanadate as a tool to gather further evidence about the mechanism of metabolic control of the Na-Ca exchanger in internally dialyzed and voltage-clamped squid axons. Vanadate, at concentrations up to 100 microM, stimulated extracellular Na (Nao)-dependent Ca efflux only in the presence of MgATP but failed to do so when the axons were dialyzed with the nonhydrolyzable ATP analogue beta, gamma-methyleneadenosine 5'-triphosphate or with CrATP, a MgATP analogue that completely abolishes MgATP stimulation of the Na-Ca exchange. In axons fully activated by Mg-adenosine 5'-O-(3-thiotriphosphate), vanadate had no effect on Na-Ca exchange. The dose-response curve for vanadate stimulation followed Michaelian kinetics with a Km of 5.6 +/- 0.4 microM and a maximum velocity of 216 +/- 10 fmol.cm-2.s-1 (intracellular Ca concentration = 0.8 microM). This coincides with the high affinity of vanadate in inhibiting the in vitro phosphatase activity of an alkaline phosphatase extracted from rat liver. In addition, vanadate increased fivefold the apparent affinity for MgATP (Km from 220 +/- 14 to 40 +/- 4 microM). Concentrations of vanadate in the millimolar range inhibited the MgATP-stimulated Na-Ca exchange (apparent Ki of 5.7 +/- 0.3 mM) and the in vitro phosphorylation by the catalytic subunit of a adenosine 3',5'-cyclic monophosphate protein kinase (apparent Ki 2.64 +/- 0.04 mM). We conclude that MgATP stimulation of Na-Ca exchange is proportional to the levels of phosphorylation that result from the balance of the activity of a kinase and a phosphatase activity.

Recent insights pertaining to sarcolemmal phospholipid alterations underlying arrhythmogenesis in the ischemic heart

Myocardial ischemia in vivo is associated with dramatic electrophysiologic alterations that occur within minutes of cessation of coronary flow and are rapidly reversible with reperfusion. This suggests that subtle and reversible biochemical alterations within or near the sarcolemma may contribute to the electrophysiologic derangements. Our studies have concentrated on two amphipathic metabolites, long-chain acylcarnitines and lysophosphatidylcholine (LPC), which have been shown to increase rapidly in ischemic tissue in vivo and to elicit electrophysiologic derangements in normoxic tissue in vitro. Incorporation of these amphiphiles into the sarcolemma at concentrations of 1 to 2 mole%, elicits profound electrophysiologic derangements analogous to those observed in ischemic myocardium in vivo. The pathophysiological effects of the accumulation of these amphiphiles are thought to be mediated by alterations in the biophysical properties of the sarcolemmal membrane, although there is a possibility of a direct effect upon ion channels. Inhibition of carnitine acyltransferase I (CAT-I) in the ischemic cat heart was found to prevent the increase in long-chain acylcarnitines and LPC and to significantly reduce the incidence of malignant arrhythmias including ventricular tachycardia and fibrillation. This review focuses on the electrophysiologic derangements that are observed during early ischemia and presents data supporting the concept that accumulation of these amphiphiles within the sarcolemma contributes to these changes. The potential contribution of these amphiphiles to the increases in extracellular potassium and intracellular calcium are examined. Finally, recent data pertaining to the accumulation of long-chain acylcarnitines on cell-to-cell uncoupling are presented. In addition to the events reviewed here, there are many other alterations that occur during early myocardial ischemia, but the results from multiple studies over the past two decades indicate that the accumulation of these amphiphiles contributes importantly to arrhythmogenesis and that development of specific inhibitors of CAT-I or phospholipase A2 may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease in man.

Ca2+ influx via Na(+)-Ca2+ exchange in immortalized aortic myocytes. II. Feedback inhibition by [Ca2+]i

Depolarization with 50 mM K+ evoked a spike in cytosolic free Ca2+ ([Ca2+]i) and increased 45Ca2+ uptake in immortalized aortic myocytes. The following evidence indicates that the electrogenic Na(+)-Ca2+ exchanger caused the Ca2+ influx that was evoked by K+ depolarization. First, K+ depolarization had no effect on [Ca2+]i and 45Ca2+ uptake in cells with basal Na+ but strikingly increased both in Na(+)-loaded cells. Second, the [Ca2+]i increases produced by K+ depolarization depended hyperbolically on external Ca2+ (50% maximum concentration = 1.5 mM). Third, the increases in [Ca2+]i and 45Ca2+ uptake were greater when external Na+ was replaced with K+ rather than with N-methyl-D-glucamine or choline. A series of K+ depolarizations elicited a sequence of [Ca2+]i spikes, provided there was a short incubation at 5 mM K+ between the depolarizations. A prior K+ depolarization almost abolished the 45Ca2+ uptake response to K+ depolarization. The inhibition of exchange activity by a prior K+ depolarization required external Ca2+ and was completely reversible. A prior incubation with angiotensin II, platelet-derived growth factor, or ionomycin also inhibited exchange activity. Moderate [Ca2+]i increases probably feedback inhibit Ca2+ influx via the exchanger by a kinetic mechanism. Inactivation of the exchanger, together with Ca2+ extrusion or sequestration, causes the rapid decrease in [Ca2+]i from the peak evoked by depolarization.

Demonstration of sodium/calcium exchange in rodent osteoblasts

Based on the inhibition of stimulated Ca release from cultured bone by several different agents that alter Na transport, we proposed that hormonally stimulated bone resorption requires Na/Ca exchange. Calcemic hormones appear to interact primarily directly with the osteoblast, which then mediates the activation of osteoclast activity. In organ culture it is not possible to determine whether Na/Ca exchange is involved in this initiating step in the osteoblast or directly in osteoclast-mediated Ca release, and there have been no prior direct measurements of Na/Ca exchange in bone or bone cells. The purpose of this study was to demonstrate the presence of Na/Ca exchange transport in the osteoblast. Thus, we characterized Na-dependent Ca transport in osteoblast-like rat osteosarcoma cells (UMR-106) and primary bone cells isolated from neonatal mouse calvaria. Cells were loaded with the Ca-sensitive dye fura-2 in the presence of physiologic NaCl and the absence of Ca with or without 0.3 mM ouabain. Changes in free cytosolic Ca after the extracellular addition of 1.5 mM Ca were measured spectrofluorimetrically. An outward Na gradient was generated by decreasing extracellular Na while maintaining isotonicity. UMR-106 cells that were Na loaded by ouabain inhibition of Na,K-ATPase activity exhibited 30% greater Ca uptake than control cells. Similar results were obtained with primary bone cells. This uptake required extracellular Ca, was not inhibited by 200 microM verapamil, and was reversible upon reversal of the Na gradient. These data demonstrate the presence of a Na/Ca exchange transport system in osteoblasts.

External calcium sensitivity of low sodium contractures in the control and hypertrophied right ventricle of the ferret

The existence of possible differences of calcium (Ca2+) fluxes through the sarcolemmal sodium-calcium (Na+/Ca2+) exchanger during hypertrophy has been tested by comparing the characteristics of the contracture--as an indicator of the intracellular Ca2+ concentration--induced by partial or total withdrawal of external sodium (Na+), in the absence of external potassium, in the right ventricular trabeculae of adult ferret hearts. Pressure-overload was induced by pulmonary artery clipping and led to an increase of the right ventricular weight of 60%. At an external Ca2+ concentration ([Ca2+]o) of 3 mM, the dependence of the contractures on extracellular sodium concentration ([Na+]o), the rate of tension development, the time course of spontaneous relaxation and the time course for the repriming of the contracture were unchanged by hypertrophy. However, the relationship between [Ca2+]o and contracture amplitude at various [Na+]o showed that the apparent affinity of the contracture for [Ca2+]o was decreased in hypertrophied preparations. Thus, in 0 mM [Na+]o, half-maximal contracture was induced at a [Ca2+]o of 0.012 +/- 0.016 mM and 0.171 +/- 0.021 mM in control (n = 11) and hypertrophy (n = 12) respectively (P less than 0.001). Although these data may be indicative of a decreased Ca2+ influx through the Na+/Ca2+ exchanger, it cannot be excluded that intracellular buffering mechanism may also be involved in this differential response to [Na+]o withdrawal.

Calcium fluxes in rat thyroid FRTL-5 cells. Evidence for a functional Na+/Ca2+ exchange mechanism

The effect of extracellular Na+ on cytosolic free Ca2+ and on influx and efflux of Ca2+ was investigated in FRTL-5 thyroid cells. Stimulating the cells with the purinergic agonist ATP induced a rapid efflux of 45Ca2+ from cells loaded with 45Ca2+. Replacement of extracellular Na+ with choline+, significantly decreased the adenosine triphosphate-induced efflux of 45Ca2+. Furthermore, adenosine triphosphate-induced uptake of 45Ca2+ was increased when extracellular Na+ was replaced with choline+, compared with the uptake seen in Na+ buffer. Replacing extracellular Na+ with choline+, increased resting levels of cytosolic free Ca2+ from 50 +/- 2 nM (mean +/- SE) to 81 +/- 3 nM (P less than 0.05) in Fura 2 loaded cells. In cells preincubated with 1 mM ouabain for 30 min, resting cytosolic free Ca2+ increased to 73 +/- 3 nM (P less than 0.05). In a Na+ buffer, the adenosine triphosphate-induced transient increase in cytosolic free Ca2+ was 872 +/- 59 nM, compared with 1070 +/- 63 nM in choline+ buffer (P less than 0.05). The plateau level of cytosolic free Ca2+ in response to adenosine triphosphate was 130 +/- 16 nM in Na+ buffer, compared with 209 +/- 9 nM in choline+ buffer (P less than 0.05). Readdition of Na+ to the plateau phase decreased cytosolic free Ca2+ to 152 +/- 5 nM. Stimulating the cells with 10 microM of the Na(+)-selective monovalent ionophore monensin increased cytosolic free Ca2+ from 53 +/- 9 nM to 124 +/- 16 nM (P less than 0.05). This increase in cytosolic free Ca2+ was dependent on both extracellular Na+ and extracellular Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of opiate agonists and antagonists on Na(+)-Ca2+ exchange in cardiac sarcolemma vesicles

Opiate agonists and antagonists inhibit Na(+)-Ca2+ exchange in the isolated cardiac sarcolemma vesicles. Non-opioid stereoisomers (dextrorphan, Mr 1542MS, WIN 44,441-3) display effects similar to their opioid isomers (levorphanol, Mr 1543MS, WIN 44,441-2) suggesting that inhibition is not mediated by opiate receptors. Naloxone (permeable) and methylnaloxone (impermeable) inhibit the Na(+)-Ca2+ exchange similarly, suggesting an extravesicular location of inhibitory site. The inhibitory potency of naloxone is pH-independent in the range of 7.4-9.1, suggesting that the charge-carrying properties of drug-protein interactions are not altered under the tested conditions. Opiates display similar dose-response relationships for Na(+)-Ca2+ exchange and its partial reaction, the Ca(2+)-Ca2+ exchange. The opiate-induced inhibition is complete and noncompetitive in regard to extravesicular calcium. These data suggest that opiates do not bind to the Ca(2+)-binding domain (A-site), but they may interest either with the Na(+)-binding site (B-site) or with a putative opiate-binding site, presumably located outside of the ion-binding vicinity. Further studies on structure-activity relationship might lead to the discovery of potent and more specific inhibitors of cardiac Na(+)-Ca2+ exchanger. A possible relevance of these findings to some non-opioid pharmacological effects of naloxone on the cardiac muscle is suggested.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

Kinetic models of Na-Ca exchange in ferret red blood cells. Interaction of intracellular Na, extracellular Ca, Cd, and Mn

The kinetic equation that best describes the intracellular Na dependence of Ca influx into ferret red cells is sequential; whether this implies that there is a conformation of the protein that has both Na and Ca ions bound remains to be determined. Cd and Mn substitute very well for Ca on the exchanger in ferret red cells; this suggests that the Ca-binding site does not contain an important thiol and that the one of the Na steps may be rate limiting.

Distinctive properties of the purified Na-Ca exchanger from rod outer segments

The Na-Ca exchanger of rod outer segments plays an important role in the regulation of Ca levels in photoreceptor cells. While this transporter shares functional properties with other Na-Ca exchangers, it has several unique features. The purified ROS exchanger migrates as a single band at 220 kDa in SDS-polyacrylamide gels, indicating that the unit size of its polypeptide is larger than other known Na-Ca exchangers (and most transporters). A specific antiserum to the ROS exchanger does not bind to the Na-Ca exchangers found in sarcolemmal vesicles or brain synaptic plasma membranes. Similarly, polyclonal antiserum specific for the cardiac exchanger does not react with ROS or brain proteins. The ROS exchanger requires K for transport activity. By incorporating the purified exchanger into proteoliposomes and measuring the sequestration of K, the actual transport of K is demonstrated. A stoichiometry of 4Na:1Ca,1K for the exchanger of ROS has been measured.

Role of sarcolemmal membrane sodium-calcium exchange in vascular smooth muscle tension

A body of information obtained by experiments with intact tissues, isolated cells, and sarcolemmal vesicles indicates, beyond a reasonable doubt, that a specific Na(+)-Ca2+ exchange system exists in vascular smooth muscle. However, its role in the regulation of cytosolic free-Ca2+ concentration and cell tension under physiological conditions remains unclear. Under pharmacological conditions in which the Na(+)-K+ pump is inhibited either by digitalis glycosides or K(+)-free medium, Na(+)-Ca2+ exchange may be modulated by increases in cytosolic free Na+ to increase the cytosolic free-Ca2+ concentration and cell tension. Under pathological conditions in which the cytosolic Na+ concentration is increased as a result of inhibition of the Na(+)-K+ pump by endogenous ouabain or a digitalis-like factor, or activation of the Na(+)-H+ exchange or passive permeability of Na+, the Na(+)-Ca2+ exchange activity of vascular smooth muscle and the nerve terminal may play an important role in the development and/or maintenance of hypertension. These and other premises remain to be confirmed or discounted.

Sodium-calcium exchange in renal epithelial cells: dependence on cell sodium and competitive inhibition by magnesium

Kinetic properties of Na(+)-Ca2+ exchange in a renal epithelial cell line (LLC-MK2) were assessed by measuring cytosolic free Ca2+ with fura-2 and 45Ca2+ influx. Replacing external Na+ with K+ produced relatively small increases in free Ca2+ and 45Ca2+ uptake unless the cells were incubated with ouabain. Ouabain markedly increased cell Na+ and strongly potentiated the effect of replacing external Na+ with K+ on free Ca2+ and 45Ca2+ uptake. 45Ca2+ influx in 140 mM K+ or N-methyl-D-glucamine minus influx in 140 mM Na+ was used to quantify Na(+)-Ca2+ exchange activity of Na(+)-loaded cells. The dependence of exchange on cell Na+ was sigmoidal; the K0.5 was 26 +/- 3 mmol/liter cell water space, and the Hill coefficient was 3.1 +/- 0.2. The kinetic features of the dependence of exchange on cell Na+ partly account for the small increase in Ca2+ influx when all external Na+ is replaced by K+. Besides raising cell Na+ ouabain appears to activate the exchanger. Magnesium competitively inhibited exchange activity. The potency of Mg2+ was 8.2-fold lower with potassium instead of N-methyl-D-glucamine or choline as the replacement for external Na+. Potassium also increased the Vmax of exchange by 86% and had no effect on the Km for Ca2+. The exchanger does not cause detectable 22Na(+)-Mg2+ exchange and does not appear to require K+ or transport 86Rb+. Although exchange activity was plentiful in the epithelial cells from monkey kidney, others from amphibian, canine, opossum, and porcine kidney had no detectable exchange activity. All of the measured kinetic properties of Na(+)-Ca2+ exchange in the renal epithelial cells are very similar to those of the exchanger in rat aortic myocytes.

Na-Ca exchange: evidence against a ping-pong mechanism and against a Ca pool in ferret red blood cells

To determine the mechanism of Na-Ca exchange, we estimated the ratio of maximum velocity to Michaelis constant for extra-cellular Ca by measuring the rate of Ca uptake at very low extracellular Ca. In a Ping-Pong mechanism, one set of sites alternatively transports Ca and Na. In a sequential mechanism, Ca and Na sites are both filled during part of the transport cycle. In each set of experiments, two intracellular Na concentrations were studied. The Ca uptake rate (at low Ca) increased as Na increased; this is consistent with a sequential model, as has been found in other cells. We also examined the alternative hypothesis that the exchanger followed Ping-Pong kinetics and that the red blood cells had a submembrane pool for Ca that limited mixing with the cytosol. In these experiments Ca pump activity was monitored by measuring ATP hydrolysis. This model was disproven by experiments that indicated that greater than 80% of the Ca that entered the cell became bound to EGTA and less than 20% resulted in Ca efflux by the Ca pump.

A possible Na/Ca exchange in the follicle cells of Xenopus oocyte

In manually dissected Xenopus oocytes, we found that the replacement of external sodium by Tris, choline, or lithium induced a large membrane depolarization and, in voltage clamp, a large inward current. This current appears to be due to activation of a calcium-dependent chloride conductance since it is reversed near ECl, increased by the removal of external chloride, and can be abolished by an injection of BAPTA or by the removal of external Ca2+. Using the Ca-dependent Cl current as a monitor of Ca concentration at the inner surface of the oocyte membrane, we are led to propose that the removal of external Na+ induces an increase in internal Ca2+ via the activation of a Na/Ca exchanger operating in the reverse mode. This interpretation is supported by the finding that the chloride current is diminished in either 3',4'-dichlorobenzamyl (DCB) or high external [Mg2+]o, both of which are known to block the Na/Ca exchanger, whereas it is increased when Li+, rather than Tris or choline, is used as the substitute for Na. The effect of zero [Na+]o was not obtained in oocytes from which follicular cells were removed by enzymatic treatment. This observation led us to test the possibility that the Na/Ca exchanger was present in the follicle cells and not in the oocyte membrane, assuming that entering Ca2+ could pass into the oocyte through gap junctions. Octanol, which blocks gap junctions, or a high [Ca2+]o both considerably reduced the inward current. While octanol probably blocked the gap junctions directly, we propose that the block by high [Ca2+] was due to an excessive rise of [Ca2+]i in the follicular cells. These results, taken together, indirectly suggest the presence of a Na/Ca exchanger in the follicular cells. These results, taken together, indirectly suggest the presence of a Na/Ca exchanger in the follicle cells of Xenopus oocyte which could contribute to the regulation of the internal Ca concentration of the oocyte before fertilization.

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.

Diamide: positive inotropic effect in isolated atria and inhibition of Na+/Ca2+ exchange in cardiomyocytes

The influence of frequency of stimulation and external calcium on the positive inotropic response of guinea-pig left atria to diamide and the inhibitory action on Na+/Ca2+ exchange activity of rat cardiomyocytes by this oxidant of sulphhydryl groups have been investigated. Diamide (50-500 microM) induces a concentration-dependent positive inotropic effect which is more pronounced when atria are driven at 1.0 Hz rather than at 0.5 and 0.1 Hz, and are bathed in 2.72 mM rather than in 1.36 mM external calcium. A decrease in the positive inotropic effect at 35 degrees C with respect to 29 degrees C is also observed. In addition, diamide in positive inotropic concentrations (100-300 microM) significantly reduces Na+/Ca2+ exchange activity and cytoplasmic glutathione levels in adult rat cardiomyocytes. The thiol reducing agent dithiothreitol either reverses or prevents diamide effects both in isolated atria and cardiomyocytes, suggesting that the actions of diamide are correlated to its property to oxidize sulphhydryl groups to disulphides. In view of the functional importance of Na+/Ca2+ exchange in myocardial contractility, it is proposed that diamide may increase the heart force of contraction by an inhibition of the sarcolemmal Na+/Ca2+ exchange activity.

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.

Na(+)-ionophore, monensin-induced rise in cytoplasmic free calcium depends on the presence of extracellular calcium in FRTL-5 rat thyroid cells

Calcium is an important regulator of cell function, and may be influenced by the intracellular sodium content. In the present study, the Na(+)-ionophore, monensin, was used to investigate the interrelationship between changes in intracellular Na+ concentration ([Na+]i) and elevation of cytosolic Ca2+ concentration ([Ca2+]i) in FRTL-5 thyroid cells. Cytoplasmic Ca2+ levels were measured using the fluorescent dye, indo-1. Monensin induced a dose-dependent increase in [Ca2+]i in FRTL-5 cells. Inhibitors of intracellular Ca2+ release, TMB-8 and ryanodine, were unable to prevent the monensin effect on [Ca2+]i. The alpha 1-receptor antagonist, prazosin, did not block the monensin-stimulated increase in [Ca2+]i. In the absence of extracellular calcium there was a marked diminution in the monensin effect on [Ca2+]i, yet calcium channel antagonists (nifedipine, diltiazem and verapamil) did not inhibit the response. Replacement of Na+ by choline chloride in the medium depressed the monensin-evoked rise in [Ca2+]i by up to 84%. Furthermore, addition of the Na(+)-channel agonist, veratridine, elicited an increase in [Ca2+]i, even though less dramatic than that caused by monensin. Ouabain increased the resting cytosolic Ca2+ concentration as well as the magnitude of the monensin effect on [Ca2+]i. The absence of any effect on the Na(+)-ionophore evoked increase in [Ca2+]i upon addition of tetrodotoxin (TTX) excluded a possible involvement of TTX-sensitive Na+ channels. These data show that the rise in [Ca2+]i induced by increasing [Na+]i is largely dependent on both external Na+ and Ca2+. Calcium entry appears not to involve voltage-dependent or alpha 1-receptor sensitive Ca2+ channels, but may result from activation of an Na(+)-Ca2+ exchange system.

Cell calcium, oncogenes, and hypertrophy

The cellular mechanisms of cardiac hypertrophy remain unclear despite tantalizing clues gleaned from a variety of experimental approaches. Here we examine the hypothesis that an increase in cytosolic free Ca2+ concentration ([Ca2+]i) triggers the expression of proto-oncogenes, which in turn direct the characteristic increase in protein synthesis. New results from perfused ferret hearts are presented demonstrating that [Ca2+]i increases as a direct consequence of an elevation in perfusion pressure. It therefore seems plausible that [Ca2+]i constitutes the crucial link between the initial stimulus for hypertensive hypertrophy (elevated perfusion pressure) and the secondary alterations in gene expression. Nevertheless, further investigation will be required to establish whether changes in [Ca2+]i are necessary or sufficient to stimulate myocardial cell growth.