Na+-Ca2+ exchange activity in rabbit lymphocyte plasma membranes

Aldosteronism and peripheral blood mononuclear cell activation: a neuroendocrine-immune interface

Aldosteronism eventuates in a proinflammatory/fibrogenic vascular phenotype of the heart and systemic organs. It remains uncertain whether peripheral blood mononuclear cells (PBMCs) are activated before tissue invasion by monocytes/macrophages and lymphocytes, as is the case for responsible pathogenic mechanisms. Uninephrectomized rats treated for 4 weeks with dietary 1% NaCl and aldosterone (ALDOST, 0.75 microg/h) with or without spironolactone (Spi, 100 mg/kg per daily gavage) were compared with unoperated/untreated and uninephrectomized/salt-treated controls. Before intramural coronary vascular lesions appeared at week 4 of ALDOST, we found (1) a reduction of PBMC cytosolic free [Mg2+]i, together with intracellular Mg2+ and Ca2+ loading, whereas plasma and cardiac tissue Mg2+ were no different from controls; (2) increased H2O2 production by monocytes and lymphocytes together with upregulated PBMC gene expression of oxidative stress-inducible tyrosine phosphatase and Mn2+-superoxide dismutase and the presence of 3-nitrotyrosine in CD4+ and ED-1-positive inflammatory cells that had invaded intramural coronary arteries; (3) B-cell activation, including transcription of immunoglobulins, intracellular adhesion molecule-1, and CC and CXC chemokines and their receptors; (4) expansion of B lymphocyte subset and myosin heavy chain class II-expressing lymphocytes; and (5) autoreactivity with gene expression for antibodies to acetylcholine receptors and a downregulation of RT-6.2, which is in keeping with cell activation and associated with autoimmunity. Spi cotreatment attenuated the rise in intracellular Ca2+, the appearance of oxidative/nitrosative stress in PBMCs and invading inflammatory cells, and alterations in PBMC transcriptome. Thus, aldosteronism is associated with an activation of circulating immune cells induced by iterations in PBMC divalent cations and transduced by oxidative/nitrosative stress. ALDO receptor antagonism modulates this neuroendocrine-immune interface. The full text of this article is available online at http://www.circresaha.org.

Na(+)/Ca(2+) exchange inhibitors modulate thapsigargin-induced Ca(2+) and Na(+) influx in human lymphocytes

Thapsigargin has been shown the elevate intracellular Na(+) concentration in human lymphocytes, but mechanisms underlying thapsigargin-induced Na(+) entry are little understood. In the present study we investigated thapsigargin-induced changes in cytosolic free Na(+) and Ca(2+) concentration in human lymphocytes after inhibition of the Na(+)/Ca(2+) exchange with two structurally unrelated compounds, dimethylthiourea ad bepridil. The intracellular Na(+) increase induced by 5 microM thapsigargin was significantly enhanced in the presence of 5 mM dimethylthiourea or 40 microM bepridil. In contrast, both compounds significantly decreased the thapsigargin-induced intracellular Ca(2+) elevation. No effect of dimethylthiourea or bepridil on thapsigargin-induced Ca(2+) influx was observed in the absence of extracellular Na(+). These observations are consistent with the hypothesis that thapsigargin stimulates Na(+)/Ca(2+ )exchange in human lymphocytes. However, Na(+)/Ca(2+) exchange does not mediate Na(+) influx in human lymphocytes.

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.

Na-Ca exchange in circulating blood cells

The experiments with peripheral lymphocytes raise two provocative questions: is SDCI composed of Ca influx via both a Ca channel and Na-Ca exchanger?, and what is the role of Na-Ca exchange in lymphocytes? In regard to the first issue, the potential for this dual Ca influx pathway exists, inasmuch as both Ca store depletion (by exposure of cells to EGTA) and TG-treatment initiated Ca influx that was enhanced following reversal of the Na gradient. These data could be interpreted to suggest a role for Ca influx via the exchanger during lymphocyte activation. However, our ability to demonstrate Na-Ca exchange activity was facilitated by the removal of Ca sequestering or extrusion mechanisms, including SERCA Ca pumps and forward mode Na-Ca exchange. Thus, it seems likely that under physiological conditions the primary function of the exchanger is to mediate Ca efflux. In this regard, it might play a role in lymphocyte activation by limiting net Ca entry during the sustained phase of Ca mobilization. Since sustained Ca entry is critical for Ca-dependent processes including interleukin-2 production, exchange activity would be an important modulator of this process. Changes in membrane potential, intracellular [Na] and cytosolic pH could therefore regulate Cai through its effects on Na-Ca exchange activity. Future challenges include defining the role of the Na-Ca exchange in Cai homeostasis and characterizing its function in lymphocyte populations.

Na+/Ca2+ exchange-mediated calcium entry in human lymphocytes

Regulation of cytosolic Ca2+ and cytosolic Na+ is critical for lymphocyte cation homeostasis and function. To examine the influence of cytosolic Na+ on Ca2+ regulation in human peripheral blood lymphocytes, Ca2+ entry and cytosolic Ca2+ (measured with fura-2) were monitored in cells in which cytosolic Na+ was increased and/or the Na+ gradient was decreased by reduction of external Na+ concentration. Ouabain-treated cells (0.1 mM for 30 min at 37 degrees C), suspended in Na(+)-free medium, showed a 30-65% increase in Ca2+ uptake compared to cells in 140 mM Na+ medium. Enhanced Ca2+ influx was entirely dependent on ouabain pretreatment and reversal of the Na+ gradient. Na pump inhibition or Na ionophore addition and subsequent exposure to Na(+)-free medium resulted in a sustained elevation of cytosolic Ca2+. As preincubation of cells in Ca(2+)-free medium further enhanced the ouabain-dependent increase in cytosolic Ca2+, the effects of the microsomal Ca(2+)-ATPase inhibitor thapsigargin on Ca2+ influx and cytosolic Ca2+ were studied. Thapsigargin stimulated Ca2+ entry following ouabain pretreatment and reversal of the Na+ gradient; the effects of thapsigargin were retained in the presence of LaCl3, a potent inhibitor of store-dependent calcium influx pathways. These results show lymphocytes demonstrate Na+/Ca2+ exchange activity and suggest the Na+/Ca2+ exchanger modulates cytosolic Ca2+ following intracellular Ca2+ store depletion.

Effect of dietary magnesium supplementation on intralymphocytic free calcium and magnesium in stroke-prone spontaneously hypertensive rats

The effects of dietary magnesium (Mg) supplementation on intralymphocytic free Ca2+ ([Ca2+]i) and Mg2+ ([Mg2+]i) were examined in the stroke-prone spontaneously hypertensive rats (SHRSP) at the age of 10 weeks. After 40 day Mg supplementation (0.8% Mg in the diet), systolic blood pressure (SBP) was significantly lower in Mg supplemented group (Mg group) than the control group (0.2% Mg). [Ca2+]i was significantly lower and [Mg2+]i was significantly higher in Mg group than in the control group. Further, [Ca2+]i was positively and [Mg2+]i was negatively correlated with SBP. These results suggest that dietary Mg supplementation modifies [Ca2+]i and [Mg2+]i, and modulates the development of hypertension.

Intralymphocytic free calcium and magnesium in stroke-prone spontaneously hypertensive rats and effects of blood pressure and various antihypertensive agents

1. Free Ca2+ ([Ca2+]i) and Mg2+ ([Mg2+]i) were measured in peripheral lymphocytes from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY) at the age of 5, 7 and 17 weeks, from various antihypertensive agents-treated SHRSP, and from secondary hypertensive WKY. 2. At the age of 5 weeks, no difference was observed in systolic blood pressure (SBP), or lymphocyte [Ca2+]i and [Mg2+]i between SHRSP and WKY. At the age of 7 or 17 weeks, SBP and [Ca2+]i of SHRSP were significantly higher than in WKY, and at the age of 17 weeks, [Mg2+]i of SHRSP was significantly lower than in WKY. Further, [Ca2+]i or [Mg2+]i was positively or negatively correlated to SBP, and [Mg2+]i was negatively correlated to [Ca2+]i. 3. SBP of SHRSP fell significantly after antihypertensive treatment with calcium antagonist, angiotensin-converting enzyme (ACE) inhibitor or hydralazine for 40 days. [Ca2+]i was significantly lower in calcium antagonist and hydralazine groups, and tended to be low in ACE inhibitor group. These four groups showed no difference in [Mg2+]i. 4. After 40-day administration of NG-nitro-L-arginine (L-NNA), WKY developed severe hypertension, but there were no significant differences in lymphocyte [Ca2+]i and [Mg2+]i between the L-NNA treated and non-treated groups. 5. These results suggested that increased lymphocyte [Ca2+]i and decreased [Mg2+]i observed in SHRSP are not only secondary to hypertension but possibly related to a basic genetic abnormality of divalent cation handling.

Kinetics of calcium transport across the lymphocyte plasma membrane

We have investigated plasma membrane Ca2+ transport by monitoring the fluorescence of human peripheral T-lymphocytes loaded with fura 2. Thapsigargin (TG) was utilized the block the Ca(2+)-ATPase of the endoplasmic reticulum and elevate the cytosolic Ca2+ (Ca2+i). Ca2+ influx was inhibited by chelating extracellular Ca2+ with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). The rate of decline in the Ca2+i signal of TG-treated lymphocytes after exposure to EGTA was used to assess Ca2+ extrusion across the plasma membrane. Initial rates of Ca2+i decline were examined in cells suspended in Na(+)-containing and Na(+)-free solutions; initial rates were linearly related to the [Ca2+]i at the onset of the Ca2+i decline and were unaffected by varying the extracellular Ca2+. Extracellular Na+ increased the rate of Ca2+ extrusion and decreased the threshold [Ca2+]i for extrusion, indicating a substantial role for the Na(+)-Ca2+ exchange in Ca2+i homeostasis. Both decreased temperature and calmodulin inhibition significantly slowed the Ca2+i decline in Na(+)-free HEPES-buffered solution, suggesting Ca2+ extrusion under these conditions was mediated by the Ca2+ pump. Protein kinase C (PKC) activation or inhibition did not affect the Ca2+i decline parameters. However, Ca2+ accumulation and Mn2+ (a Ca2+ surrogate) uptake were significantly and Mn2+ (a Ca2+ surrogate) uptake were significantly inhibited by activators of PKC. Cyclic nucleotides altered neither the parameters of the Ca2+i decline nor Mn2+ uptake. Thus human T-lymphocytes exhibit Na(+)- and Ca(2+)-dependent transporters characterized as the Na(+)-Ca2+ exchanger and Ca2+ pump. The main effect of PKC in these cells is the modulation of Ca2+ entry across the lymphocyte plasma membrane.

Aldosterone influences free intracellular calcium in human mononuclear leukocytes in vitro

Mineralocorticoid receptors have been detected in human mononuclear leukocytes (HML) and a physiological effector mechanism was demonstrated subsequently by which aldosterone is able to prevent the loss of intracellular sodium, potassium and cell water during incubation in an aldosterone-free medium. In the present paper, free intracellular calcium, [Ca2+]i, was measured in HML from normal subjects by Quin-2 and Fura-2 fluorescence after incubation for 1 h at 37 degrees C in RPMI-1640 medium. In fresh HML, [Ca2+]i was 54 +/- 15 nM (Fura-2, mean +/- SD, n = 26). After incubation without aldosterone, [Ca2+]i in HML was 118 +/- 27 nM (Quin-2, n = 11) and 50 +/- 13 nM (Fura-2). After incubation with 1.4 (Fura-2) or 2.8 nM (Quin-2) aldosterone, [Ca2+]i was 139 +/- 38 nM (Quin-2, P less than 0.05 compared with value after incubation without aldosterone) and 57 +/- 11 nM (Fura-2, P less than 0.00001). The Kd-value for dose-response curve was 0.4 nM. The effect of aldosterone was antagonized by N-ethyl-isopropylamiloride, but not by canrenoate, canrenone, cycloheximide and actinomycin D. It was absent in a sodium-free buffer. Corticosterone and hydrocortisone were active as agonists. These results show that aldosterone exerts an effect on the [Ca2+]i in HML in vitro which could be involved in hemodynamic responses to mineralocorticoids if also present in cardiovascular tissues.

Intralymphocytic sodium and free calcium and plasma renin in essential hypertension

Intracellular sodium, potassium, and free calcium concentrations were investigated in lymphocytes of 30 patients with essential hypertension and 30 normotensive controls. All subjects were placed on a diet containing 8 to 10 g of sodium chloride per day. Lymphocyte sodium concentration was higher in hypertensive patients than in normotensive controls (19.8 +/- 1.8 vs 18.4 +/- 1.8 mmol/kg wet weight; p less than 0.01), whereas lymphocyte potassium concentration was similar in both groups. Lymphocyte free calcium concentration was also higher in hypertensive patients than in normotensive controls (134.6 +/- 13.2 vs 120.2 +/- 16.4 nmol/L; p less than 0.01). There was a positive correlation between lymphocyte sodium and free calcium concentrations in normotensive controls, in hypertensive patients, and in the subjects combined (r = 0.59, p less than 0.01; r = 0.71, p less than 0.001; and r = 0.70, p less than 0.001, respectively). Lymphocyte potassium concentration was not related to lymphocyte sodium or free calcium concentration in each group. In patients with essential hypertension, intracellular sodium and free calcium concentrations were negatively correlated with plasma renin activity (r = -0.66, p less than 0.001; r = -0.60, p less than 0.001, plasma norepinephrine concentration. These results suggest that a considerable relationship exists between intracellular sodium and free calcium in lymphocytes and that, in essential hypertension, the alteration in cellular metabolism of sodium and calcium may be linked to the renin system but not to blood pressure, age, or adrenergic activity.

Role of cellular calcium in salt sensitivity of patients with essential hypertension

The mechanism by which excessive sodium chloride intake raises blood pressure has not been fully clarified. The present study was therefore undertaken in patients with essential hypertension to investigate the possible role of an intracellular calcium-dependent mechanism in salt sensitivity. The difference in mean blood pressure between a week of low sodium chloride diet (3 g/day) and a week of high sodium chloride diet (20 g/day) was studied in relation to the intracellular free calcium concentration in lymphocytes and an acute hypotensive response to a 10-mg sublingual dose of nifedipine in 12 inpatients. Sodium chloride loading induced significant increases in mean blood pressure (from 111 +/- 12 to 122 +/- 11 mm Hg; p less than 0.01), intracellular free calcium in lymphocytes (from 133 +/- 13 to 145 +/- 9 nmol/L; p less than 0.01), and the hypotensive response to nifedipine (from 19 +/- 6 to 31 +/- 10 mm Hg; p less than 0.01). In addition, serum total calcium concentration was decreased while urinary calcium excretion was increased. The elevation of mean blood pressure was closely and positively correlated with the increase in intracellular free calcium concentration (r = 0.71, p less than 0.05) and the increase in the hypotensive effect of nifedipine (r = 0.91, p less than 0.01) after sodium chloride loading. However, changes in these values had no relation to the change in serum concentration or urinary excretion of calcium. These data suggest that change in the cellular calcium-dependent vasoconstriction mechanism may be associated with salt sensitivity of patients with essential hypertension.

Is serum the optimal source for HLA antibodies?

The cytotoxic reactivity of 18 predefined class I HLA serum antibodies was compared with that of antibody preparations containing anticoagulants. ACD-A, EDTA, 4% citrate and heparin plasmas all showed lower cytotoxicity than serum antibodies. Recalcification of both platelet-rich and platelet-poor ACD-A plasma did not fully restore the antibody reactivity, suggesting a detrimental effect of calcium chelation. This effect was exclusive of volume or of any platelet or plasma protein involvement. The changes in pH contributed to the lower reactivity and to the increased lympholytic effect, whereas adjusting the pH toward the serum value improved the reactivity. Heat-inactivated antibodies showed only a slightly reduced cytotoxicity. Heparin had the least effect of all anticoagulants on the reactivity, although in heparin there was a definite dose-dependent decline in cytotoxic titer which was probably related to anticomplementary activity. Calcium chelators, such as EDTA and citrate, showed marked cytotoxic inhibition at half the usual complement concentration. This effect was more pronounced when the anticoagulant and lymphocytes were incubated prior to cytotoxicity testing. At the complement concentrations used, the inhibitory effects of the citrate anticoagulants appeared to be primarily calcium-related. Inhibition tests, serial titrations and testing of varying calcium concentrations confirmed the superiority of serum as antibody source.

Na+/Ca2+ countertransport in plasma membrane of rat pancreatic acinar cells

The presence of a coupled Na+/Ca2+ exchange system has been demonstrated in plasma membrane vesicles from rat pancreatic acinar cells. Na+/Ca2+ exchange was investigated by measuring 45Ca2+ uptake and 45Ca2+ efflux in the presence of sodium gradients and at different electrical potential differences across the membrane (= delta phi) in the presence of sodium. Plasma membranes were prepared by a MgCl2 precipitation method and characterized by marker enzyme distribution. When compared to the total homogenate, the typical marker for the plasma membrane, (Na+ + K+)-ATPase was enriched by 23-fold. Markers for the endoplasmic reticulum, such as RNA and NADPH cytochrome c reductase, as well as for mitochondria, the cytochrome c oxidase, were reduced by twofold, threefold and 10-fold, respectively. For the Na+/Ca2+ countertransport system, the Ca2+ uptake after 1 min of incubation was half-maximal at 0.62 mumol/liter Ca2+ and at 20 mmol/liter Na+ concentration and maximal at 10 mumol/liter Ca2+ and 150 mmol/liter Na+ concentration, respectively. When Na+ was replaced by Li+, maximal Ca2+ uptake was 75% as compared to that in the presence of Na+. Amiloride (10(-3) mol/liter) at 200 mmol/liter Na+ did not inhibit Na+/Ca2+ countertransport, whereas at low Na+ concentration (25 mmol/liter) amiloride exhibited dose-dependent inhibition to be 62% at 10(-2) mol/liter. CFCCP (10(-5) mol/liter) did not influence Na+/Ca2+ countertransport. Monensin inhibited dose dependently; at a concentration of 5 X 10(-6) mol/liter inhibition was 80%. A SCN- or K+ diffusion potential (= delta phi), being positive at the vesicle inside, stimulated calcium uptake in the presence of sodium suggesting that Na+/Ca2+ countertransport operates electrogenically, i.e. with a stoichiometry higher than 2 Na+ for 1 Ca2+. In the absence of Na+, delta phi did not promote Ca2+ uptake. We conclude that in addition to ATP-dependent Ca2+ outward transport as characterized previously (E. Bayerdörffer, L. Eckhardt, W. Haase & I. Schulz, 1985, J. Membrane Biol. 84:45-60) the Na+/Ca2+ countertransport system, as characterized in this study, represents a second transport system for the extrusion of calcium from the cell. Furthermore, the high affinity for calcium suggests that this system might participate in the regulation of the cytosolic free Ca2+ level.

Calcium transport mechanisms in basolateral plasma membrane-enriched vesicles from rat parotid gland

Ca2+ transport was studied by using basolateral plasma membrane vesicles from rat parotid gland prepared by a Percoll gradient centrifugation method. In these membrane vesicles, there were two Ca2+ transport systems; Na+/Ca2+ exchange and ATP-dependent Ca2+ transport. An outwardly directed Na+ gradient increased Ca2+ uptake. Ca2+ efflux from Ca2+-preloaded vesicles was stimulated by an inwardly directed Na+ gradient. However, Na+/Ca2+ exchange did not show any 'uphill' transport of Ca2+ against its own gradient. ATP-dependent Ca2+ transport exhibited 'uphill' transport. An inwardly directed Na+ gradient also decreased Ca2+ accumulation by ATP-dependent Ca2+ uptake. The inhibition of Ca2+ accumulation was proportional to the external Na+ level. Na+/Ca2+ exchange was inhibited by monensin, tetracaine and chlorpromazine, whereas ATP-dependent Ca2+ transport was inhibited by orthovanadate, tetracaine and chlorpromazine. Oligomycin had no effect on either system. These results suggest that in the parotid gland cellular free Ca2+ is extruded mainly by an ATP-dependent Ca2+ transport system, and Na+/Ca2+ exchange may modify the efficacy of that system.