Na+/Ca2+ antiport in cultured arterial smooth muscle cells. Inhibition by magnesium and other divalent cations

Dietary potassium regulates vascular calcification and arterial stiffness

Vascular calcification is a risk factor that predicts adverse cardiovascular complications of several diseases including atherosclerosis. Reduced dietary potassium intake has been linked to cardiovascular diseases such as hypertension and incidental stroke, although the underlying molecular mechanisms remain largely unknown. Using the ApoE-deficient mouse model, we demonstrated for the first time to our knowledge that reduced dietary potassium (0.3%) promoted atherosclerotic vascular calcification and increased aortic stiffness, compared with normal (0.7%) potassium-fed mice. In contrast, increased dietary potassium (2.1%) attenuated vascular calcification and aortic stiffness. Mechanistically, reduction in the potassium concentration to the lower limit of the physiological range increased intracellular calcium, which activated a cAMP response element-binding protein (CREB) signal that subsequently enhanced autophagy and promoted vascular smooth muscle cell (VSMC) calcification. Inhibition of calcium signals and knockdown of either CREB or ATG7, an autophagy regulator, attenuated VSMC calcification induced by low potassium. Consistently, elevated autophagy and CREB signaling were demonstrated in the calcified arteries from low potassium diet-fed mice as well as aortic arteries exposed to low potassium ex vivo. These studies established a potentially novel causative role of dietary potassium intake in regulating atherosclerotic vascular calcification and stiffness, and uncovered mechanisms that offer opportunities to develop therapeutic strategies to control vascular disease.

Cell culture alters Ca2+ entry pathways activated by store-depletion or hypoxia in canine pulmonary arterial smooth muscle cells

Previous studies have shown that, in acutely dispersed canine pulmonary artery smooth muscle cells (PASMCs), depletion of both functionally independent inositol 1,4,5-trisphosphate (IP(3))- and ryanodine-sensitive Ca(2+) stores activates capacitative Ca(2+) entry (CCE). The present study aimed to determine if cell culture modifies intracellular Ca(2+) stores and alters Ca(2+) entry pathways caused by store depletion and hypoxia in canine PASMCs. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured in fura 2-loaded cells. Mn(2+) quench of fura 2 signal was performed to study divalent cation entry, and the effects of hypoxia were examined under oxygen tension of 15-18 mmHg. In acutely isolated PASMCs, depletion of IP(3)-sensitive Ca(2+) stores with cyclopiazonic acid (CPA) did not affect initial caffeine-induced intracellular Ca(2+) transients but abolished 5-HT-induced Ca(2+) transients. In contrast, CPA significantly reduced caffeine- and 5-HT-induced Ca(2+) transients in cultured PASMCs. In cultured PASMCs, store depletion or hypoxia caused a transient followed by a sustained rise in [Ca(2+)](i). The transient rise in [Ca(2+)](i) was partially inhibited by nifedipine, whereas the nifedipine-insensitive transient rise in [Ca(2+)](i) was inhibited by KB-R7943, a selective inhibitor of reverse mode Na(+)/Ca(2+) exchanger (NCX). The nifedipine-insensitive sustained rise in [Ca(2+)](i) was inhibited by SKF-96365, Ni(2+), La(3+), and Gd(3+). In addition, store depletion or hypoxia increased the rate of Mn(2+) quench of fura 2 fluorescence that was also inhibited by these blockers, exhibiting pharmacological properties characteristic of CCE. We conclude that cell culture of canine PASMCs reorganizes IP(3) and ryanodine receptors into a common intracellular Ca(2+) compartment, and depletion of this store or hypoxia activates voltage-operated Ca(2+) entry, reverse mode NCX, and CCE.

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.

Sodium-Calcium Exchanger in Pulmonary Artery Smooth Muscle Cells

The expression and function of the Na+/Ca2+ exchanger (NCX) in the regulation of intracellular Ca2+ homeostasis have been well studied in cardiac, skeletal, and systemic vascular myocytes, but not in pulmonary artery smooth muscle cells (SMCs). We have recently demonstrated that the NCX current is present in freshly isolated pulmonary artery SMCs using the patch-clamp technique. The current has a mean amplitude of 13 pA under near physiological resting conditions. The NCX may function in the forward mode to make a significant contribution to the decay of intracellular Ca2+ following Ca2+ release and/or depolarization. Hypoxic stimulation inhibits the NCX current, reduces the removal of intracellular Ca2+, and enhances Ca2+ release from the sarcoplasmic reticulum. Using RT-PCR, subcloning and sequence analysis, we have shown that three NCX1 splice variants: NCX1.2 (containing exons B, C, and D), NCX1.3 (exons B and D), and NCX1.7 (exons B, D, and F) are expressed in pulmonary artery smooth muscle. Each of these splice variants expressed in HEK293 cells it likely to show a distinct activity in the removal of intracellular Ca2+. Taken together, we provide clear evidence that NCX1 is functionally and molecularly expressed and plays a physiological role in pulmonary artery SMCs.

Paradoxical nifedipine facilitation of 45Ca uptake into rat hippocampal synaptosomes

Nifedipine has a high incidence of neurologic adverse reactions as compared with other dihydropyridine Cav1 (L-type) channel blockers used for treating cardiovascular diseases. The mechanism mediating neuronal excitation by nifedipine is still in debate. Nifedipine caused a dual role on veratridine-induced 45Ca uptake by rat hippocampal synaptosomes. In the nanomolar range (0.001-0.3 microM), nifedipine decreased 45Ca uptake in a cadmium-sensitive manner. In contrast with nitrendipine (0.001-10 microM), nifedipine consistently facilitated 45Ca accumulation when used in low micromolar concentrations (0.3-10 microM). The cadmium-insensitive nifedipine facilitation became less evident upon increasing veratridine concentration from 5 to 20 microM and was not detected when the synaptosomes where depolarised with 30 mM KCl. Na+ substitution by N-methyl-D-glucamine (132 mM) or blockade of Na+ currents with tetrodotoxin (1 microM) both prevented nifedipine excitation. The Na+/Ca2+-exchanger inhibitor, KB-R7943 (3-50 microM), did not reproduce nifedipine actions. Data suggest that tetrodotoxin-sensitive Na+ channels may operate paradoxical nifedipine facilitation of 45Ca uptake by rat hippocampal synaptosomes.

Functional role of the Na+/H+ exchanger in the regulation of cerebral arteriolar tone in rats

OBJECT

In vascular smooth-muscle cells, the Na+/H+ exchanger (NHE) is involved in the regulation of [Na+]i, pHi through [H+], and cell volume. Recently, investigations have determined that this exchanger contributes to ischemia and reperfusion injury in coronary circulation. Nonetheless, there is limited information on this glycoprotein in cerebral circulation, especially microcirculation. Thus, the authors in the present study examined the role of NHE in the regulation of cerebral arteriolar tone and its related mechanisms in vitro.

METHODS

The internal diameter of isolated pressurized intracerebral arterioles in rats was monitored with the aid of a microscope. To examine the basal activity of NHE two kinds of Na+/H+ exchange inhibitors (FR183998 and 5-[N,N-hexamethylene]amiloride) were administered in the arterioles. Furthermore the authors studied the effects of nitric oxide (NO) synthase inhibitor (NG methyl-L-arginine), Na+/K+ -adenosine triphosphatase (NKA) inhibitor (ouabain), and the Na+/Ca++ exchange inhibitor (SEA0400) on the vascular response induced by either of the Na+/H+ exchange inhibitors. Both of the Na+/H+ exchange inhibitors constricted the arteriole. Subsequent application of NO synthase inhibitor further decreased the diameter of the arterioles. The Na+/H+ exchange inhibitor-induced constriction was completely abolished in the presence of ouabain and SEA0400.

CONCLUSIONS

The NHE is active in the basal condition and regulates cerebral arteriolar tone through NKA and the Na+/Ca++ exchanger. Endogenous NO is not related to the activity of NHE in basal conditions.

Role of the choline exchanger in Na(+)-independent Mg(2+) efflux from rat erythrocytes

Two types of Na(+)-independent Mg(2+) efflux exist in erythrocytes: (1) Mg(2+) efflux in sucrose medium and (2) Mg(2+) efflux in high Cl(-) media such as KCl-, LiCl- or choline Cl-medium. The mechanism of Na(+)-independent Mg(2+) efflux in choline Cl medium was investigated in this study. Non-selective transport by the following transport mechanisms has been excluded: K(+),Cl(-)- and Na(+),K(+),Cl(-)-symport, Na(+)/H(+)-, Na(+)/Mg(2+)-, Na(+)/Ca(2+)- and K(+)(Na(+))/H(+) antiport, Ca(2+)-activated K(+) channel and Mg(2+) leak flux. We suggest that, in choline Cl medium, Na(+)-independent Mg(2+) efflux can be performed by non-selective transport via the choline exchanger. This was supported through inhibition of Mg(2+) efflux by hemicholinum-3 (HC-3), dodecyltrimethylammonium bromide (DoTMA) and cinchona alkaloids, which are inhibitors of the choline exchanger. Increasing concentrations of HC-3 inhibited the efflux of choline and efflux of Mg(2+) to the same degree. The K(d) value for inhibition of [(14)C]choline efflux and for inhibition of Mg(2+) efflux by HC-3 were the same within the experimental error. Inhibition of choline efflux and of Mg(2+) efflux in choline medium occurred as follows: quinine>cinchonine>HC-3>DoTMA. Mg(2+) efflux was reduced to the same degree by these inhibitors as was the [(14)C]choline efflux.

Chimeric analysis of Na(+)/Ca(2+) exchangers NCX1 and NCX3 reveals structural domains important for differential sensitivity to external Ni(2+) or Li(+)

Externally applied Ni(2+), which apparently competes with Ca(2+) in all three isoforms of Na(+)/Ca(2+) exchanger, inhibits exchange activity of NCX1 or NCX2 with a 10-fold higher affinity than that of NCX3, whereas stimulation of exchange by external Li(+) is significantly greater in NCX2 and NCX3 than in NCX1 (Iwamoto, T., and Shigekawa, M. (1998) Am. J. Physiol. 275, C423-C430). Here we identified structural domains in the exchanger that confer differential sensitivity to Ni(2+) or Li(+) by measuring intracellular Na(+)-dependent (45)Ca(2+) uptake in CCL39 cells stably expressing NCX1/NCX3 chimeras or mutants. We found that two segments in the exchanger corresponding mostly to the internal alpha-1 and alpha-2 repeats are individually responsible for the alteration of Ni(2+) sensitivity, both together accounting for approximately 80% of the difference between NCX1 and NCX3. In contrast, the segment corresponding to the alpha-2 repeat fully accounts for the differential Li(+) sensitivity between the isoforms. The Ni(2+) sensitivity was mimicked, respectively, by simultaneous substitution of two amino acids in the alpha-1 repeat (N125G/T127I in NCX1 and G159N/I161T in NCX3) and substitution of one amino acid in the alpha-2 repeat (V820A in NCX1 and A809V in NCX3). On the other hand, the Li(+) sensitivity was mimicked by double substitution mutation in the alpha-2 repeat (V820A/Q826V in NCX1 and A809V/V815Q in NCX3). Single substitution mutations at Asn(125) and Val(820) of NCX1 caused significant alterations in the interactions of the exchanger with Ca(2+) and Ni(2+), and Ni(2+) and Li(+), respectively, although the extent of alteration varied depending on the nature of side chains of substituted residues. Since the above four important residues are mostly in the putative loops of the alpha repeats, these regions might form an ion interaction domain in the exchanger.

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.

Differential inhibition of Na+/Ca2+ exchanger isoforms by divalent cations and isothiourea derivative

We compared the properties of three mammalian Na+/Ca2+ exchanger isoforms, NCX1, NCX2, and NCX3, by analyzing the effects of Ni2+ and other cations as well as the recently identified inhibitor isothiourea derivatives on intracellular Na+-dependent 45Ca2+ uptake into CCL-39 (Dede) fibroblasts stably expressing each isoform. All these NCX isoforms had similar affinities for the extracellular transport substrates Ca2+ and Na+. Ni2+ inhibited 45Ca2+ uptake by competing with Ca2+ for the external transport site, with 10-fold less affinity in NCX3 than in NCX1 or NCX2. Ni2+ and Co2+ were most efficient in such discrimination of NCX isoforms, although their inhibitory potencies were less than those of La3+ and Cd2+. The monovalent cation Li+ stimulated 45Ca2+ uptake rate by all NCX isoforms similarly with low affinity, although the extent of stimulation was somewhat smaller in NCX1. On the other hand, the isothiourea derivative KB-R7943 was threefold more inhibitory to NCX3 than to NCX1 or NCX2. Thus distinct differences in the kinetic and pharmacological properties were detected between NCX3 and the other two isoforms.

Characterization of sodium-calcium exchange in rabbit renal arterioles

Experiments were performed to test the hypothesis that renal arterioles exhibit Na-Ca exchange capability and that this process is regulated by protein kinase C (PKC). Glomeruli with attached arterioles were dissected from rabbit kidney and loaded with fura-2 for measurement of intracellular calcium concentration ([Ca2+]i) using microscope-based photometry. In tissue bathed in Ringer's solution containing 150 mM Na+ and 1.5 mM Ca2+, afferent and efferent arteriolar [Ca2+]i averaged 136 +/- 6 and 154 +/- 7 nM, respectively. Removal of extracellular Na+ increased afferent arteriolar [Ca2+]i by 70 +/- 7 mM, while efferent arteriolar [Ca2+]i only increased by 39 +/- 5 nM (P < 0.01 vs. afferent arteriole). These responses were inhibited by 6 nM Ni2+ and required extracellular Ca2+, but were unaffected by 10 microM diltiazem. After incubation in 500 microM ouabain, 5 microM monensin, and 5 microM nigericin, [Ca2+]i responses to removal of extracellular Na+ were exaggerated significantly, averaging 174 +/- 50 nM in afferent arterioles and 222 +/- 82 nM in efferent arterioles (NS vs. afferent arterioles). Moreover, responses to removal of extracellular Na+ were enhanced by 100 nM phorbol 12-myristate 13-acetate, an affect which was blocked by PKC inhibition (25 nM K252b). These data indicate that both afferent and efferent arterioles express the Na-Ca exchanger, and that PKC activity impacts on exchange capacity in these vessels.

Effects of dietary salt and angiotensin-converting enzyme inhibitor on ATP-driven Ca pump and Na/Ca exchange in aortic rings of Dahl rats

We examined effects of dietary salt and cilazapril on ATP-driven Ca pump and Na/Ca exchange system in arterial smooth muscle of Dahl salt-sensitive (DS) rats. Twenty-four DS rats were assigned to 4 groups. H- and H+ rats were fed a high-salt diet. L- and L+ rats were fed a low-salt diet. H+ and L+ were administered cilazapril. Aortic rings were superfused with physiological saline, and isometric tension was measured. Relaxation rates of low-Na(+)-induced contractions were promoted by the removal of external Ca. Cilazapril significantly decreased blood pressure in both the high- and low-salt diet groups. Salt loading reduced relaxation rates in 1.2 mmol/l Na+, and enhanced the increase in relaxation rates from 1.2 mmol/l to normal Na+. A negative correlation was observed between Ca extrusion by the Ca pump and blood pressure, and a positive correlation was observed between Ca extrusion by Na/Ca exchange and blood pressure. The results suggest that the decrease of Ca2+ extrusion by the ATP-driven Ca pump resulting from a high-salt diet might lead to an elevation in the concentration of cellular Ca2+.

Calcium homeostasis in guinea pig type-I vestibular hair cell: possible involvement of an Na(+)-Ca2+ exchanger

In type-I vestibular hair cells (VHCs), the mechanisms involved in intracellular calcium homeostasis have not yet been established. In order to investigate the involvement of an Na(+)-dependent ionic exchanger in the regulation of cytosolic free calcium concentration, we analyzed the effect of the removal of external sodium on the cytosolic concentration of calcium ions ([Ca2+]i), sodium ions ([Na+]i), and protons (pHi). These concentrations were measured in type-I VHCs isolated from guinea pig labyrinth, using Fura-2, sodium benzofuran isophtalate (SBFI), and 1,4 diacetoxy-2,3 dicyanobenzol (ADB) respectively. Complete replacement of Na+ in the superfusion solution with N-methyl-D-glucamine (NMDG+), reversibly increased [Ca2+]i by 276 +/- 89% (n = 46) and decreased [Na+]i by 23 +/- 6% (n = 14). Both responses were prevented by removing external Ca2+ or chelating internal Ca2+. This suggests the presence of coupled Ca2+ and Na+ transport. The [Ca2+]i increase evoked by Na(+)-free solution was reduced by about 55% with the application of amiloride derivatives and was totally abolished in the presence of high [Mg2+]o. No pHi variation was detected during [Na+]o reduction. In the absence of external K+, the Na(+)-free solution failed to induce [Ca2+]i increase; the readmission of external K+ restored the [Ca2+]i response. These results are consistent with a Na(+)-Ca2+ exchanger operating in reverse mode. An K+ dependence of this exchange is also suggested.

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.

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Na(+)-Ca2+ exchange in intact endothelium of rabbit cardiac valve

A new method of measuring cytoplasmic free Ca2+ ([Ca2+]i) of individual intact cardiovascular endothelial cells by using imaging fluorescence microscopy was designed. Application of agonist to the aortic or pulmonary valve of the rabbit triggered an increase in [Ca2+]i, which depended on the existence of endothelium on the surface of the valve. Under resting conditions, sudden reversal of the Na+ gradient by substituting external Na+ with N-methyl D-glucamine (NMDG) resulted in a [Ca2+]i spike, which then returned toward the resting level. Increasing intracellular Na+ concentration ([Na+]i) by application of ouabain or monensin induced a sustained [Ca2+]i increase. Na+ substitution by NMDG during the agonist- or monensin-induced [Ca2+]i increase gave rise to a further [Ca2+]i spike, which subsequently declined to a level higher than that before removal of external Na+. A selective inhibitor of Na(+)-Ca2+ exchange, 3',4'-dichlorobenzamyl (DCB), abolished the transient [Ca2+]i increase induced by Na+ substitution, and Mg2+, an inorganic inhibitor of Na(+)-Ca2+ exchanger, markedly reduced this transient [Ca2+]i increase. On the other hand, the selective Na(+)-H+ exchanger blocker 5-(N,N-hexamethylene)amiloride (HMA) did not abolish the transient [Ca2+]i increase caused by Na+ substitution. In summary, decreasing the Na+ gradient of the endothelial cells through either receptor stimulation (agonist), Na(+)-K+ pump inhibition (ouabain), pretreatment with Na+ ionophore (monensin), or reversing the Na+ gradient through Na+ substitution (NMDG) all increased [Ca2+]i. This raised [Ca2+]i was antagonized by agents such as DCB or Mg2+, which are thought to inhibit Na(+)-Ca2+ exchange, but not by HMA, an inhibitor of Na(+)-H+ exchange.(ABSTRACT TRUNCATED AT 250 WORDS)

Lanthanum can be transported by the sodium-calcium exchange pathway and directly triggers catecholamine release from bovine chromaffin cells

A comparison of the effectiveness of the trivalent cation, lanthanum (La3+) relative to Ca2+ in causing catecholamine release from bovine chromaffin cells has been made, together with a determination of the pathway by which La3+ enters these cells. In chromaffin cells maintained in tissue culture and permeabilised with digitonin, both La3+ and Ca2+ caused 3H release from cells preloaded with [3H]-noradrenaline; La3+ and Ca2+ caused similar maximal release but the EC50 for La3+ was an order of magnitude less than that for Ca2+. At maximal release caused by either La3+ or Ca2+ (approximately 14% of cell 3H content in 15 min), the other cation caused a small, but significant, further release. At submaximal effective concentrations the effects of the two cations were exactly additive. Using 3H release as an indicator of cytosolic La3+, its route of entry into intact chromaffin cells was investigated. With La(3+)-containing medium there was no release evoked by nicotine or by K(+)-depolarisation indicating that La3+ does not enter either via the nicotinic receptor linked ion channel or via voltage-sensitive (Ca2+) channels. However, in sodium-loaded chromaffin cells (ouabain incubation in Ca(2+)-free medium for 15 min) exposure to bathing media containing either Ca2+ or La3+ caused 3H release. La3+ (0.1 mM) caused a release similar in magnitude to that caused by Ca2+ (about 1 mM). La3+ at low concentrations had an additive (0.1 mM La3+) or synergistic (0.25-0.45 mM La3+) action with Ca2+ (< 3.6 mM) on 3H release. At higher concentrations (> 0.9 mM) the effects of La3+ predominated and prevented the expected effects of Ca2+. In other experiments, La3+ (1 mM) blocked export of 45Ca2+ via both Nao-dependent and independent pathways, i.e. sodium-calcium exchange and the calcium pump. The results indicate that La3+ can enter bovine chromaffin cells via the Nai/Cao exchange pathway independently of, or together with, Ca2+ but, that concentrations above 0.9 mM block the influx or efflux of Ca2+. However, Ca2+, even at 3.6 mM, did not block the influx of La3+. The results further indicate that, within chromaffin cells, La3+ is at least as effective as Ca2+ in triggering catecholamine release and maintaining prolonged release. La3+ also appears to act cooperatively with Ca2+ at the release pathway.

Atrial natriuretic peptide and cGMP inhibit Na+/H+ antiporter in vascular smooth muscle cells in culture

The aim of the present paper was to study the mechanisms of the inhibitory effect of atrial natriuretic peptide (ANP) on the sustained contraction phase of vascular smooth muscle cells (VSMC). Specifically, the potential role of ANP on the Na+/H+ antiporter and Na+ transport systems was investigated. Both ANP and 8-bromo cGMP inhibited 22Na+ uptake and decreased intracellular Na ([Na+]i) in VSMC, an effect that was mimicked by the specific Na+/H+ antiporter inhibitor, hexamethylen amiloride (HMA). The effect of ANP was not additive with HMA, therefore suggesting that both inhibit the same 22Na+ transport pathway. On the other hand, the inhibition of 22Na+ accumulation by ANP was additive with the inhibition by furosemide or bumetanide, thus suggesting that both drugs act on different Na+ exchange systems. In HEPES-buffered medium, ANP, cGMP, and HMA significantly inhibited the AVP-induced intracellular alkalinization, an effect which was associated with significant inhibition of the AVP-induced shape change. In bicarbonate buffered medium, ANP and cGMP decreased the pH level below the baseline after application of AVP, and an inhibition by ANP and cGMP of AVP-induced VSMC shape change was also observed. The recovery of cellular pH after three different types of acid load, namely, ammonium chloride pulse, nigericin clamp and lowering of extracellular pH, was significantly decreased by ANP and cGMP. Taken together, these results indicate that ANP/cGMP inhibit the activity of the Na+/H+ antiporter in VSMC, either in hormone- or pH-stimulated conditions.

Extracellular N-methyl-D-glucamine leads to loss of hair-cell sodium, potassium, and chloride

The organic cation N-methyl-D-glucamine (NMDG) is often used to replace extracellular sodium in experimental studies. Replacing 100 mM of Na+ with NMDG+ in the fluid bathing isolated goldfish hair cells led to a rapid loss not only of cell sodium, but also of cell potassium and chloride. The loss of inorganic cell solutes was accompanied by acidification of the cells. Cell volume did not change significantly. These results are consistent with passage of the cationic form of NMDG, a titratable amine with a pKa of 9.6, across the hair-cell membrane. These results should have bearing in interpreting results of experiments in which this cation is used to replace extracellular sodium, particularly for periods of time longer than 3 min.

Stable expression of the cardiac sodium-calcium exchanger in CHO cells

A line of Chinese hamster ovary (CHO) cells called CK1.4 was produced by transfection with the gene for the bovine cardiac Na(+)-Ca2+ exchanger. CK1.4 cells stably expressed substantial exchange activity and exchanger protein as shown by immunoprecipitation. Exchange activity was quantified as 45Ca2+ influx that depended on both increasing intracellular Na+ and lowering the concentration of external Na+. Replacing external Na+ with K+ slightly increased 45Ca2+ uptake by CK1.4 cells with basal Na+ and greatly increased 45Ca2+ uptake by Na(+)-loaded cells. Neither exchange activity nor exchanger protein was detected in the nontransfected parental line. By contrast to CK1.4 cells, replacing external Na+ with K+ decreased 45Ca2+ uptake in the nontransfected cells whether or not they were Na+ loaded. Changes in cytosolic free Ca2+ determined with fura-2 were consistent with the 45Ca2+ uptake data. Analysis of poly(A)(+)-RNA by Northern blot confirmed that CK1.4 cells, but not the parental line, expressed the exchanger. Expression of the exchanger was also observed in aortic myocytes and a renal epithelial cell line (LLC-MK2) but not in other lines of renal epithelial cells (MDCK, LLC-PK1) or human dermal fibroblasts. The cardiac exchanger produced substantial 45Ca2+ efflux from CK1.4 cells in response to hormone-evoked release of stored Ca2+. CK1.4 cells are an attractive model for studies of the regulation of the cardiac exchanger because they stably express sufficient exchanger for biochemical and immunological analysis.

Nickel-induced increases in gap junctional communication in the uterine cell line SK-UT-1

Previous studies have suggested that gap junctions may have a role in various uterine functions, including parturition. Because nickel has been demonstrated to increase uterine contractility in vitro, the effect of nickel (II) chloride on gap junctional communication was assessed in a tumorigenic uterine cell line, SK-UT-1 (ATCC HTB 114). Cells were exposed in vitro to 25 and 50 microM NiCl2 for 24 h or 100 microM NiCl2 for 3, 12, and 24 h, then functional gap junctional communication was measured as the transfer of Lucifer yellow dye from microinjected donor cells to their primary neighbor cells. Dye transfer was significantly increased only in cell cultures exposed to 100 microM NiCl2 for 24 h, compared to untreated controls, lower doses, and shorter exposure periods. This response was inhibited by the simultaneous co-treatment of SK-UT-1 cells with magnesium by adding 100 microM MgSO4 to the dosing medium. Possible mechanisms and implications for these findings are discussed.

Vascular smooth muscle cell calcium fluxes. Regulation by angiotensin II and lipoproteins

This study examined 45Ca uptake, 45Ca efflux, and the distribution of exchangeable 45Ca in confluent, quiescent cultures of aortic smooth muscle cells (VSMCs) from normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). These parameters were investigated under basal conditions and after addition of angiotensin II (Ang II) and low (LDL) and high (HDL) density lipoproteins. Basal 45Ca uptake was approximately 50% greater in VSMCs from SHRs (p < 0.005 versus WKY). Calcium antagonists (diltiazem or nifedipine) abolished this difference. The 45Ca uptake response to Ang II was approximately twofold greater in SHR than in WKY VSMCs (p < 0.05), and Ang II-induced increments of 45Ca uptake were weakly inhibited (by approximately 15-25%) by calcium antagonists. Lipoproteins also stimulated 45Ca uptake in VSMCs, and the apparent affinity of this process was approximately fivefold greater for LDL than for HDL. Calcium antagonists did not inhibit either LDL- or HDL-induced 45Ca uptake. SHR and WKY VSMCs did not differ with respect to 45Ca uptake induced by either LDL or HDL. The initial size of the slowly exchangeable pool of intracellular Ca2+ was approximately 35% greater in SHR VSMCs (p < 0.05 versus WKY). Ang II-induced mobilization of intracellular calcium (measured as the decrease in 45Ca content of the slowly exchangeable pool) was threefold greater in SHR VSMCs (p < 0.005 versus WKY). LDL and HDL marginally stimulated 45Ca efflux from this pool (< or = 20% above control) and to comparable extents in both SHR and WKY VSMCs.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of preconditioning. Ionic alterations

The mechanism by which preconditioning (brief intermittent periods of ischemia and reflow) improves recovery of function and reduces enzyme release after a subsequent 30-minute period of ischemia was investigated in perfused rat hearts. Specifically, it was hypothesized that ischemia after preconditioning would result in a decreased production of H+ and therefore a smaller rise in [Na+]i and [Ca2+]i via Na(+)-H+ and Na(+)-Ca2+ exchange. To test this hypothesis we measured pHi, [Na+]i, [Ca2+]i, and cell high-energy phosphates during ischemia and reflow, and we correlated this with recovery of contractile function and release of creatine kinase during reflow. 31P nuclear magnetic resonance (NMR) was used to measure pHi and cell phosphates. [Na+]i was measured by 23Na NMR using the shift reagent thulium 1,4,7,10-tetraazacyclododecane-N,N,'N",N"'-tetramethylenephosph onate to distinguish intracellular from extracellular sodium. [Ca2+]i was measured by 19F NMR using hearts loaded with 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid, termed 5F-BAPTA. Basal time-averaged levels of pHi, [Na+]i, and [Ca2+]i were 7.07 +/- 0.08, 9.4 +/- 0.8 mM, and 715 +/- 31 nM, respectively. After 30 minutes of ischemia, in preconditioned hearts, pHi was 6.5 +/- 0.06, [Na+]i was 2.09 +/- 4.4 mM, [Ca2+]i was 2.1 +/- 0.4 microM, and ATP was negligible. In untreated hearts, after 30 minutes of ischemia, pHi was 6.3 +/- 0.08, [Na+]i was 26.7 +/- 3.8 mM, [Ca2+]i was 3.2 +/- 0.6 microM, and ATP was undetectable. During reperfusion after 30 minutes of ischemia, preconditioned hearts had significantly better recovery of contractile function than untreated hearts (71 +/- 9% versus 36 +/- 8% initial left ventricular developed pressure), and after 60 minutes of ischemia, preconditioned hearts had significantly less release of the intracellular enzyme creatine kinase (102 +/- 12 versus 164 +/- 17 IU/g dry wt). We also found that unpreconditioned hearts arrested with 16 mM MgCl2 (to inhibit calcium entry via calcium channels and Na(+)-Ca2+ exchange) before 30 minutes of ischemia recover function on reflow to the same extent as preconditioned hearts with or without magnesium arrest. Thus, preconditioning has no additional benefit in addition to magnesium arrest. In addition, in hearts that received 16 mM MgCl2 just before the 30-minute period of ischemia, preconditioning had no effect on the rise in [Ca2+]i during the 30-minute period of ischemia. These data support the hypothesis that preconditioning attenuates the increase in [Ca2+]i, [Na+]i, and [H+]i during ischemia, most likely because of reduced stimulation of Na(+)-H+ and Na(+)-Ca2+ exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of ouabain on muscle tension and intracellular Ca2+ level in guinea-pig aorta

The effects of ouabain on muscle tension and the intracellular Ca2+ level ([Ca2+]i) were examined in guinea-pig aorta loaded with fura-2. Ouabain caused a gradual and sustained increase in both [Ca2+]i and muscle tension. There was a positive correlation between these two parameters. In Ca(2+)-free solution, ouabain did not affect either [Ca2+]i or muscle tension, suggesting that the ouabain-induced increase in [Ca2+]i was not due to Ca2+ release from storage sites. The ouabain-induced increase in [Ca2+]i and muscle tension was inhibited by Ni2+, which inhibits the Na+/Ca2+ exchanger, but not by verapamil. Furthermore, anionic and cationic amphiphiles were used as modulators of the Na+/Ca2+ exchanger. Sodium dodecyl sulfate accelerated the responses to ouabain, whereas dodecyltrimethylammonium bromide inhibited them. These results suggest that in the guinea-pig aorta, ouabain induces contraction by increasing the Ca2+ influx through the Na+/Ca2+ exchanger on the plasma membrane, but not through verapamil-sensitive Ca2+ channels.

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.

Endothelial-dependent sexual dimorphism in vascular smooth muscle: role of Mg2+ and Na+

1. In isolated aortae of the male rat [Mg2+]o withdrawal and concomitant reduction in [Na+]o (to 84 mM) induced significant increases of basal tone, but, surprisingly, this did not occur in intact aortae removed from female rats. Such tension development, however, was observed in endothelium-denuded aortic preparations from both sexes. These observed gender-related differences were not dependent on animal strain or types of tissue preparations. 2. No tension development was observed in aortae obtained from castrated males treated with oestradiol. Aortic tissues of sexually-immature male and female rats exhibited marked tension development when exposed to 0 mM [Mg2+]o and low [Na+]o. 3. Tension development in Mg(2+)-free, low-Na+ media was not tachyphylactic and completely dependent on extracellular Ca2+; addition of 1.2 mM Mg2+ to the Mg2+ and Na(+)-deficient incubation media relaxed the increase in tension to a normal basal level. 4. Two known endothelial-derived relaxant factor (EDRF) inhibitors, methylene blue and haemoglobin, induced tension development in female aortae with intact endothelium exposed to Mg(2+)-Na+ deficient media, while use of a specific inhibitor of EDRF-derived nitric oxide, viz., NG-monomethyl-L-arginine (L-NMMA), resulted in potentiation of tension development in male, but not in female, aortae. This effect of L-NMMA was antagonized by L-arginine. 5. The Ca ionophore, A23187, partially relaxed contractile responses in male aortae (with intact endothelium) which were followed by potentiated contractions. Endothelium-dependent vasodilator responses to A23187 (10(-10)-10(-6) M) of aortic rings from male or female rats in normal Krebs-Ringer bicarbonate solution were not different.6. These results suggest that: (a) as in vascular smooth muscle cells, Mg2+ plays an important role in Ca2 + homeostasis in endothelial cells, probably via Na+-Ca2+ exchange; and (b) sex steroid hormones, probably the female sex hormone, 17-beta-oestradiol, may regulate contractile responses of intact vascular smooth muscle by modifying endothelium functions through such Mg2 '-regulated internal Natdependent Ca2+ entry. These data may help to explain why female subjects, despite Mg deficiency, unlike male subjects, are protected against ischaemic heart disease and cerebrovascular disease until menopause.

Multiple effects of ryanodine on intracellular free Ca2+ in smooth muscle cells from bovine and porcine coronary artery: modulation of sarcoplasmic reticulum function

1. The effects of ryanodine and caffeine on intracellular free Ca2+ concentration ([Ca2+]i) were studied by use of fura-2 microfluorometry in single smooth muscle cells freshly dispersed from bovine and porcine coronary artery. 2. Bovine and porcine cells demonstrated similar sensitivities to 10 min of exposure to ryanodine in physiological salt solution (PSS), as determined by comparable dose-dependent decreases in the subsequent [Ca2+]i transient induced by 5 mM caffeine. 3. Ryanodine (10 microM) caused a significant increase in [Ca2+]i to a plateau level 27 +/- 3% and 38 +/- 4% above baseline [Ca2+]i (baseline [Ca2+]i = [Ca2+]i at 0 min) in porcine and bovine cells, respectively, when bathed in PSS. In bovine cells the time required to reach 1/2 the plateau level was only 3 min versus 6 min for porcine cells. 4. The ryanodine-induced plateau increase in [Ca2+]i was 35 +/- 5% above baseline for bovine cells bathed in 0 Ca PSS (PSS including 10 microM EGTA with no added Ca2+), but only 7 +/- 3% above baseline in porcine cells during 10 min exposure to 10 microM ryanodine. In bovine cells [Ca2+]i showed proportional increases when extracellular Ca2+ was increased from the normal 2 mM Ca2+ PSS to 5 and 10 mM. 5. Cells pretreated with caffeine in 0 Ca PSS, which depleted the caffeine-sensitive sarcoplasmic reticulum Ca2+ store, showed no increase in [Ca2+]i when challenged with 10 microM ryanodine. The ryanodine-associated increase in [Ca2+]i, which was sustained in 0 Ca PSS during the 10 min ryanodine exposure in cells not pretreated with caffeine, suggests that ryanodine releases Ca2+ from the sarcoplasmic reticulum, but also inhibits Ca2+ efflux.6. Intracellular free Ba2+ ([Ba24],) was measured with fura-2 microfluorometry to define further the Ca2" efflux pathway inhibited by ryanodine; specifically, Ba2+ is not transported by the Ca2" pump, but will substitute for Ca2" in Na+-Ca24 exchange. In porcine cells pretreated with caffeine in 0 Ca PSS to deplete the caffeine-sensitive sarcoplasmic reticulum Ca2+ store, depolarization with 80 mM K4 in 2 mM external Ba24 caused a 100 +/- 6% increase in fura-2 fluorescence ([Ba2+]j). During the 17.5 min 0 Ca PSS recovery from depolarization, exposure to 10 microM ryanodine inhibited the removal of [Ba24]i by 69 + 3% when compared with control (0 Ca PSS without ryanodine).7. It was concluded that in bovine and porcine smooth muscle cells: (a) ryanodine (> 10 microM) releases Ca24 from the sarcoplasmic reticulum; (b) ryanodine ( 10O microM) decreases Ca24 efflux, probably by inhibition of Na+-Ca2+ exchange; (c) the sarcoplasmic reticulum Ca24 store may be larger in bovine than in porcine smooth muscle cells; thus, porcine cells have a relatively greater reliance on Ca24 influx to increase [Ca2+]i.

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.

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.

Extracellular Na+ dependency of free cytosolic Ca2+ regulation in aortic vascular smooth muscle cells

This study examined contribution of Na(+)-dependent processes to the regulation of free cytosolic calcium (Ca2+i) in cultured vascular smooth muscle cells (VSMC) using fura-2. Removal of Na+ from superfusate (replacement with choline) resulted in an increment of Ca2+i that was greatly augmented by pretreatment with ouabain. Under both conditions, Ca2+i increase was followed by partial recovery to a new steady state that was still significantly higher than that seen before removal of external Na+ (Na+o). In ouabain-pretreated cells lowering of Na+o caused progressive increases in Ca2+i. Addition of NiCl2, a Na(+)-Ca2+ exchange inhibitor, completely blocked the increase in Ca2+i produced by removal of Na+o, indicating that the Na(+)-Ca2+ antiporter was responsible for observed Ca2+i changes. Ca2+i increase produced by reduction of Na+o was also seen after depletion of inositol trisphosphate-sensitive Ca2+ stores with repeated pulses of angiotensin II or after blockade of sarcoplasmatic reticulum Ca2+ release with TMB-8 but was not observed in the absence of external Ca2+. These observations indicate that the source of Ca2+i increase in response to changes in the transmembrane Na+ gradient is largely external, and potentiation of the Ca2+i surge by ouabain suggests Ca2+ influx via the Na(+)-Ca2+ exchanger operating in the reverse mode. The relative contribution of a Na(+)-dependent and -independent component of Ca2+i recovery was investigated by superfusing cells with ionomycin in a Na(+)-free medium and later adding Na+ to the medium. This Ca2+ ionophore increased Ca2+i to a peak, and this was followed by a rapid but partial recovery to a new steady state. Readdition of varying amounts of Na+ to the superfusate, in the continued presence of ionomycin, resulted in concentration-related decline in Ca2+i, thereby uncovering a substantial contribution of a Na(+)-dependent mechanism of Ca2+i regulation. Decline of Ca2+i produced by readdition of Na+ was blocked by addition of NiCl2 to the superfusate. Our findings thereby provide evidence for Ca2+i regulation in VSMC via a Na(+)-dependent mechanism, consistent with a Na(+)-Ca2+ exchanger, which acts as a Ca2+ efflux mechanism when Ca2+i is elevated. Na(+)-Ca2+ exchanger acts as a Ca2+ influx mechanism when intracellular Na+ is elevated by prior exposure to ouabain.

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.

Calcium export by sodium-calcium exchange in bovine chromaffin cells

Calcium efflux from bovine chromaffin cells in tissue culture has been examined after loading them with small amounts of Ca2+ by brief depolarization in media containing 20 mumol/l to 1 mmol/l Ca2+ and 45Ca2+ in trace amounts. In the presence of normal external Na+ and Ca2+ concentrations cells depolarized in media containing up to 200 mumol/l Ca2+ exported nearly 100% of their accumulated Ca2+ loads within 10 min and 20% within the first 5 s. In the absence of external Na+ and Ca2+ the proportion of a small (i.e., depolarization in 20 mumol/l calcium) Ca2+ load exported at any time point in the range to 10 min was approximately two thirds of the total efflux measured in their presence indicating that under these conditions the external Na+/Ca(2+)-dependent and Na+/Ca(2+)-independent mechanisms both contribute significantly to the export of calcium. At higher cellular loads of calcium (i.e., depolarization in 200 mumol/l to 1 mmol/l calcium) the Na+/Ca(2+)-dependent mechanism exported a progressively greater proportion of the accumulated Ca2+. Both sodium and calcium alone promoted a component of Ca2+ efflux; Ca2+ (i.e. calcium-calcium exchange) was as effective as Na+ (i.e. sodium-calcium exchange). The Km for Na+ stimulation of Ca(2+)-efflux (KNa) was approximately 65 mM. Increased external Mg2+ (from 1.2 to 10 mmol/l) increased the apparent KNa to 90 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Inhibition of sodium-calcium and sodium-proton exchangers by amiloride congeners in arterial muscle cells

The inhibitory potencies of several amiloride congeners towards Na(+)-Ca2+ and Na(+)-H+ exchange were compared in rat aortic myocytes. N-(2,4-Dimethylbenzyl)amiloride (DMB) was 10 times more potent towards Na(+)-Ca2+ than Na(+)-H+ exchange. Amiloride and ethylisopropylamiloride were about 5,000 and 10,000 times more potent toward Na(+)-H+ than Na(+)-Ca2+ exchange respectively. N-(3,4-Dichlorobenzyl)amiloride was almost equipotent towards both exchangers. About 40 nM ethylisopropylamiloride inhibited Na(+)-H+ exchange by 50%. Ethylisopropylamiloride (10 microM) had no effect on basal or angiotensin-evoked 45Ca2+ efflux or net Ca2+ efflux. In contrast to ethylisopropylamiloride, 25-50 microM DMB, which strongly inhibits Na(+)-Ca2+ exchange, markedly decreased both 45Ca2+ efflux and net Ca2+ efflux produced by angiotensin. Replacing extracellular Na+ with N-methyl-D-glucamine inhibited angiotensin-evoked 45Ca2+ efflux similarly to DMB. Neither DMB nor Na+ placement had any effect on basal or angiotensin-evoked production of [3H]inositol phosphates. These findings suggest that Na(+)-H+ exchange has no major influence on short-term Ca2+ regulation and provide evidence that Na(+)-Ca2+ exchange is a major pathway of rapid Ca2+ efflux in stimulated arterial muscle cells.

Extracellular magnesium-dependent sodium efflux in squid giant axons

Experiments were designed to determine whether the putative Na(+)-Mg2+ exchanger previously demonstrated to mediate Mg2+ efflux (R. DiPolo and L. Beagué. Biochim. Biophys. Acta 946: 424-428, 1988) could also mediate the efflux of Na+ (presumably a Na+ efflux-Mg2+ influx exchange) in squid giant axons. The effects of external Mg2+ (Mg(o)) on 22Na efflux were measured in internally dialyzed, ATP-fueled axons in which the contribution to Na+ efflux by other pathways was inhibited. To facilitate measurement of Mg(o)-dependent Na+ efflux, the intracellular concentration of Na+ was increased. To prevent Na(+)-Na+ exchange, external Na+ was replaced by tris(hydroxymethyl)aminomethane. To assess the effect of Mg(o) on Na+ efflux without altering the total divalent cation concentrations, Mg(o) was replaced mole-for-mole by external Ba2+ (Ba(o)). This manipulation produced reversible reductions in Na+ efflux. These reductions were neither due to membrane hyperpolarization nor to a direct effect of Bao but were due instead to the reduction in Mg(o). The Mg(o)-dependent Na+ efflux was inhibited by external amiloride but was spared by bumetanide. In the absence of external Na+, the Mgo-dependent Na+ efflux increased as a function of external Mg2+ with Michaelis-Menten kinetics. These results indicate that the Na(+)-Mg2+ exchange can mediate the efflux of Na+ (operate in Na+ efflux-Mg2+ influx mode of exchange).

Effect of dietary magnesium on development of atherosclerosis in cholesterol-fed rabbits

The effect of dietary magnesium (Mg) on the development of atherosclerosis in cholesterol-fed rabbits was investigated. Male New Zealand White rabbits (n = 31) were placed on five kinds of diets: regular, 1% cholesterol, and 1% cholesterol diets supplemented with either 300, 600, or 900 mg (as Mg) of Mg sulfate. The regular and 1% cholesterol diets contained 400 mg of Mg per 100 g. Each rabbit received 100 g daily of the appropriate diet. Additional Mg was well tolerated and did not affect blood pressure or body weight. The rabbits were sacrificed after 10 weeks, and the oil red O-positive atherosclerotic area that covered the aortic intima and the cholesterol content of the aorta was measured. Additional Mg decreased both the area of the aortic lesions and the cholesterol content of the aortas in a dose-dependent manner. The 1% cholesterol diet significantly increased plasma cholesterol and triglyceride concentrations and decreased high density lipoprotein (HDL) cholesterol concentration. Additional Mg had no further effect on cholesterol and HDL cholesterol concentrations, but it slightly decreased the rise in triglyceride concentration. These results indicate that dietary Mg prevents the development of atherosclerosis in cholesterol-fed rabbits by inhibiting lipid accumulation in the aortic wall.

Energy dependence of sodium-calcium exchange in vascular smooth muscle cells

Three different types of mitochondrial poisons (oligomycin, antimycin A, and dinitrophenol) strongly inhibited Na(+)-Ca2+ exchange in aortic myocytes. Exchange activity was assayed as 45Ca2+ uptake that depended on inverting the Na+ gradient and was inhibited by 25 microM dimethylbenzamil. Glucose markedly decreased the inhibition of exchange activity by these three poisons. Glucose also prevented rotenone from inhibiting exchange and depleting cellular ATP. In the absence of glucose, rotenone decreased ATP and exchange activity with half-times of 0.8 and 0.9 min, respectively. Almost eliminating cellular ATP with rotenone maximally inhibited exchange by 80%. Repletion of ATP with glucose substantially restored Na(+)-Ca2+ exchange activity. Ca2+ uptake by organelles, subsequent to entry via exchange for Na+, does not appear to contribute significantly to exchange activity as assayed in intact myocytes. The specific activity of Na(+)-Ca2+ exchange was approximately 30 nmol.min-1.mg protein-1. These findings suggest that ATP modulates exchange activity and that there are approximately 150,000 Na(+)-Ca2+ exchangers per cell, assuming that the turnover number is 1,000 s-1.

The role of calcium and magnesium in the development of atherosclerosis. Experimental and clinical evidence

Based on the findings presented in this study, we propose the hypothesis that calcium could be a mediator for the development of atherosclerosis. Figure 8 shows a schematic illustration of the hypothesis. The presence of risk factors such as hypertension, hyperlipidemia, and smoking may increase the influx of calcium into vascular ECs. We have shown that reactive oxygen species, which are considered to be a risk factor for the development of atherosclerosis, actually increase [Ca++]i in vascular ECs. Increased intracellular calcium may damage the function of ECs, resulting in platelet aggregation at the damaged site. Increased intracellular calcium may also increase uptake of macromolecules in plasma such as fibrinogen and LDL, eventually forming atherosclerotic plaque. We have also shown that the influx of calcium into vascular ECs is associated with LDL transport across vascular ECs. The pretreatment by nifedipine inhibited both the increase in [Ca++]i and the increase in LDL transport, suggesting that intracellular calcium modulates LDL transport across ECs. Growth factors released from platelets may provoke migration and proliferation of medial SMCs in the aterial intima. It has been reported that migration of SMCs from arterial media to intima is enhanced by the presence of calcium, and can be inhibited by the pretreatment of calcium antagonist. As demonstrated in this study, calcium also plays an important role in the proliferation of SMCs provoked by some kinds of growth factors such as EGF. On the other hand, we found that an increased amount of dietary Mg suppressed the development of atherosclerotic lesions in the aorta of cholesterol-fed rabbits without affecting plasma total cholesterol and HDL-cholesterol concentrations. The mechanism of action might also be related to the calcium entry blocking action. The clinical and nutritional implications of these phenomena should be investigated further. The evidences presented in this study, however, would not be sufficient to fully explain the etiological role of calcium in atherogenesis. Further studies are required to elucidate the mechanism of the contribution of calcium to atherogenesis. The efficacy of calcium antagonist for the prevention of atherosclerosis in humans should also be investigated further.

Effects of polyvalent cations and dihydropyridine calcium channel blockers on recovery of CNS white matter from anoxia

The effects of anoxic injury on the functional integrity of mammalian central white matter were studied electrophysiologically using the rat optic nerve model. Previous studies on this model have shown that extracellular Ca2+ is critical to the production of irreversible anoxic injury, and suggest that during anoxia Ca2+ crosses the membrane to enter the intracellular compartment. We attempted to elucidate the mechanism by which this damaging Ca2+ influx occurs. The inorganic Ca2+ channel blockers Mn2+ (1 mM), Co2+ (1 mM) or La3+ (0.1 mM) had no effect on recovery of the area under the compound action potential after a standard 60 min period of anoxia; only Mg2+ (10 mM) significantly improved recovery (54.9 +/- 8.9% vs. 28.7 +/- 10.1%, P less than 0.005). Treatment with organic Ca2+ channel blockers of the dihydropyridine class, nifedipine (1-10 microM) or nimodipine (1-40 microM), also had no effect on recovery from anoxia. We conclude that Ca2+ influx during anoxia does not occur via conventional Ca2+ channels sensitive to polyvalent cations or dihydropyridines.

Cadmium enhances potassium conductance in cultured renal epitheloid (MDCK) cells

The kidney is a main target organ for cadmium toxicity. The present study has been performed to test for effects of cadmium on electrical properties of cultured subconfluent kidney (MDCK) cells. Cadmium leads to a rapid, sustained and reversible hyperpolarization of the cell membrane, paralleled by an increase of the potassium selectivity and a decrease of the resistance. Thus, cadmium increases the potassium conductance of the cell membrane. The half maximal effect is elicited congruent to 0.2 microM, a concentration encountered during chronic cadmium intoxication. At extracellular calcium concentration reduced to less than 0.1 microM, 5 microM cadmium leads to a transient hyperpolarization, which can be elicited only once. High concentrations (50 microM) of cadmium lead to a sustained hyperpolarization even at extracellular calcium concentrations of less than 0.1 microM. According to fluorescence measurements cadmium leads to an increase of intracellular calcium activity, which is sustained at 1 mM and transient at less than 1 microM extracellular calcium activity. In conclusion, cadmium at low concentrations enhances the potassium conductance in a calcium dependent way. The observations suggest that cadmium enhances intracellular calcium both by recruitment from intracellular stores and by modification of calcium transport across the cell membrane. At high concentrations cadmium enhances the potassium conductance independently from enhanced intracellular calcium activity.

Role of Mg in the activation of Na-H exchange in dog red cells

Depleting dog red cells of Mg prevents activation of Na-H exchange by cell shrinkage but has no effect on activation by acidification of the cytosol. Replacing cell Mg restores the activation of Na-H exchange by cell shrinking. We conclude that Mg is required, not for the transport process per se, but for the transduction of the volume stimulus.