Na-Ca exchange in circulating blood cells

Extracellular Ca2+ uptake by T cells might help to make a diagnosis of acute rejection

To more non-invasively diagnose acute rejection, we focused on the uptake of extracellular Ca2+ by T cells as a result of the activation of Ca2+ release activated Ca2+ channels. A full thickness of the skin allograft model was established using BN rats as the donors and LEW rats as the recipients, and similar LEW rats as both donors and recipients in the control group. After transplantation, the grafts were staged histopathologically in both rats. The uptake of extaracellular 45Ca pre T cell was measured in the macrophage-treated and non-treated groups, and the ratios between the two groups were calculated and the results were compared according to the post-operative day. No histopathological findings of acute rejection were observed in the control group. The allograft model group showed acute rejection histopathologically beginning on day 2 and increased through day 5. The macrophage-treated model/non-treated model 45Ca uptake ratio (CAR) was significantly higher in the allograft rats on day 2. No significant difference was observed on day 4. Measuring the uptake of extracellular Ca2+ by recipient T cells using donor macrophages might be useful for making a diagnosis of acute rejection.

Enhancement of calcium signalling dynamics and stability by delayed modulation of the plasma-membrane calcium-ATPase in human T cells

In addition to its homeostatic role of maintaining low resting levels of intracellular calcium ([Ca2+](i)), the plasma-membrane calcium-ATPase (PMCA) may actively contribute to the generation of complex Ca2+ signals. We have investigated the role of the PMCA in shaping Ca2+ signals in Jurkat human leukaemic T cells using single-cell voltage-clamp and calcium-imaging techniques. Crosslinking the T-cell receptor with the monoclonal antibody OKT3 induces a biphasic elevation in [Ca2+](i) consisting of a rapid overshoot to a level > 1 microM, followed by a slow decay to a plateau of approximately 0.5 microM. A similar overshoot was triggered by a constant level of Ca2+ influx through calcium-release-activated Ca2+ (CRAC) channels in thapsigargin-treated cells, due to a delayed increase in the rate of Ca2+ clearance by the PMCA. Following a rise in [Ca2+](i), PMCA activity increased in two phases: a rapid increase followed by a further calcium-dependent increase of up to approximately fivefold over 10-60 s, termed modulation. After the return of [Ca2+](i) to baseline levels, the PMCA recovered slowly from modulation (tau approximately 4 min), effectively retaining a 'memory' of the previous [Ca2+](i) elevation. Using a Michaelis-Menten model with appropriate corrections for cytoplasmic Ca2+ buffering, we found that modulation extended the dynamic range of PMCA activity by increasing both the maximal pump rate and Ca2+ sensitivity (reduction of K(M)). A simple flux model shows how pump modulation and its reversal produce the initial overshoot of the biphasic [Ca2+](i) response. The modulation of PMCA activity enhanced the stability of Ca2+ signalling by adjusting the efflux rate to match influx through CRAC channels, even at high [Ca2+](i) levels that saturate the transport sites and would otherwise render the cell defenceless against additional Ca2+ influx. At the same time, the delay in modulation enables small Ca2+ fluxes to transiently elevate [Ca2+](i), thus enhancing Ca2+ signalling dynamics.

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.

Ouabain-induced enhancement of rat mast cells response. Modulation by protein phosphorylation and intracellular pH

The digitalic glicoside ouabain induces potentiation of rat mast cell histamine release in response to several stimuli, which is mediated by Na+/Ca2+ exchanger. In this work, we studied the effect of ouabain on cytosolic calcium, intracellular pH and histamine release with Ca2+ ionophore A23187 in conditions designed to maximize ouabain-induced potentiation of rat mast cells response. The effect of protein kinase C (PKC), cAMP and phosphatase inhibition was also tested. Ouabain induced an enhancement in histamine release, cytosolic calcium and intracellular pH. The adenylate cyclase activator forskolin reduced the effect of ouabain on histamine release and intracellular pH, but enhanced the effect on cytosolic calcium. PKC activator PMA enhanced the effect of ouabain on histamine release and cytosolic calcium, without affecting intracellular pH. A PKC inhibitor, GF-109203X, reduced ouabain-induced enhancement of histamine release and intracellular pH, but increased the enhancement on cytosolic calcium. Finally, inhibition of protein phosphatases 1 and 2A with okadaic acid, increased the effect of ouabain on histamine release and intracellular pH, but reduced cytosolic calcium in presence of ouabain. This result suggest that ouabain-induced potentiation of rat mast cell histamine release with A23187 is modulated by kinases, and this modulation may be carried out by changes in intracellular alkalinization. However, the mechanism underlying cellular alkalinization remains to be elucidated.

Glucocorticoid modulation of Ca2+ homeostasis in human B lymphoblasts

1. We determined the effect of cortisol (200 nM for 48 h) on the intracellular Ca2+ concentration ([Ca2+]i) and parameters of Ca2+i signalling in 19 lymphoblastoid cell lines (LCLs). 2. Using the fluorescent dye fura-2, the basal [Ca2+]i in Ca2+-containing medium was 63.5 +/- 2.4 nM in vehicle (ethanol)-treated LCLs and 55.7 +/- 2. 6 nM (mean +/- s.e.m.) in cortisol-treated LCLs. 3. Ca2+i signalling following platelet-activating factor (PAF, 100 nM) addition was enhanced by cortisol treatment, with LCLs having small PAF responses showing the largest percentage increase after cortisol treatment. Mean peak [Ca2+]i responses to PAF were enhanced 67.0% and 55.7% in Ca2+-free and Ca2+-containing medium, respectively. 4. The endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin (100 nM) caused a transient increase in [Ca2+]i in Ca2+-free medium in which the peak change was increased in cortisol-treated cells (98.5 +/- 5.8 vs. 79.8 +/- 4.5 nM). Peak changes in the freely exchangeable Ca2+ in response to 5 microM ionomycin were also enhanced in cortisol-treated cells (923.7 +/- 113.9 vs. 652.2 +/- 64.5 nM) and correlated to the PAF-evoked [Ca2+]i response. 5. Cortisol-treated LCLs exposed to thapsigargin to empty intracellular Ca2+ stores (10 min treatment in Ca2+-free medium) and exposed to CaCl2 or MnCl2 had a greater rate of Ca2+ entry (18.6 +/- 1.8 vs. 13.8 +/- 1.5 nM s-1) and higher rate constant for Mn2+ entry (0.0345 +/- 0.0029 vs. 0. 0217 +/- 0.0020) than vehicle-treated cells. Peak [Ca2+]i in cells exposed to CaCl2 was also enhanced (869.4 +/- 114.7 vs. 562.6 +/- 61.7 nM). Parameters of divalent cation influx were highly correlated to the peak [Ca2+]i elicited by thapsigargin or ionomycin. 6. Inclusion of RU 486 (a glucocorticoid antagonist) with cortisol prevented the decrease in basal [Ca2+]i and stimulatory actions of cortisol on all Ca2+i parameters. RU 486 alone had no apparent effects on basal [Ca2+]i or Ca2+i signalling. 7. Based on data obtained over a wide range of responses (in the presence and/or absence of cortisol or RU 486), the results show that cortisol stimulation of glucocorticoid receptors decreases basal [Ca2+]i and enhances PAF-evoked [Ca2+]i signalling, most probably through its effects on intracellular Ca2+ stores. In turn, the extent of Ca2+ entry via store-operated plasma membrane Ca2+ channels is closely linked to the size of the Ca2+ stores.

Parameters of lymphocyte Na+-Ca2+ regulation and blood pressure: the gender effect

Alterations in cellular Ca2+ and Na+ regulation play a role in the pathogenesis of essential hypertension. Using peripheral lymphocytes from 68 normal persons, we observed the following relationships for major cellular Ca2+ regulatory parameters. Among men and women, Na+-Ca2+ exchanger activity was positively correlated with the resting cytosolic free Ca2+ ([Ca2+]c) (r=0.43, P=0.0003), and the resting [Ca2+]c was positively correlated with cytosolic Na+ ([Na+]c) (r=0.50, P=0.0001). For men only, store-operated Ca2+ entry was positively correlated with Na+-Ca2+ exchanger activity (r=0.63, P=0.0001). In addition, systolic and diastolic blood pressures were positively correlated with [Na+]c in men (r=0.53, P=0.001, and r=0. 41, P=0.017, respectively) but not in women (r=0.30, P=0.088, and r=0.24, P=0.17, respectively). Some of the relationships between cellular and blood pressure parameters were confounded by serum triglycerides. These observations indicate a gender effect on cellular Ca2+-Na+ regulation and its relationship with blood pressure.

Protein kinase C modulates cytosolic free calcium by stimulating calcium pump activity in Jurkat T cells

Although protein kinase C (PKC) activation has been shown to inhibit Ca2+ influx in T lymphocytes, the role of PKC on Ca2+ sequestration or extrusion processes has not been fully explored. We examined the effect of CD3 stimulation and PKC activators on cytosolic Ca2+ (Ca2+i) extrusion and 45Ca2+ efflux in human leukemic Jurkat T cells. Treatment of Fura-2 loaded cells with phorbol 12-myristate 13-acetate (PMA) or thymeleatoxin (THYM) resulted in a decrease in Ca2+i both in the presence and absence of extracellular Ca2+, whereas inactive phorbol esters had no effect. PKC activators added at the peak of a Ca2+i transient induced by anti-CD3 mAb, ionomycin or thapsigargin (TG) stimulated the rate and extent of return of Ca2+i to basal levels by 17-53%. PKC stimulation of the Ca2+i decline was not enhanced by the presence of Na+, indicating that PKC activators increase Ca2+ pump activity rather than a Na+/Ca2+ exchange mechanism. As CD3 receptor activation enhanced the Ca2+i decline in TG-treated cells, antigen-mediated activation of phospholipase C (PLC) signaling includes enhanced Ca2+ extrusion at the plasma membrane. The effect of PKC activators on parameters of Ca2+i extrusion were further explored. PMA significantly increased the rate of Ca2+ extrusion in TG-treated cells from 0.28 +/- 0.02 to 0.35 +/- 0.03 s-1 (mean +/- SEM) and stimulated the initial rate of 45Ca2+ efflux by 69% compared to inactive phorbol ester treated cells. The effects of PKC activation on the Ca2+i decline were eliminated by PKC inhibitors, PKC down regulation (24 h PMA pretreatment), ATP-depletion and conditions that inhibited the Ca2+ pump. In contrast, pretreatment of cells with okadaic acid enhanced the PMA-stimulated response. We suggest that Jurkat T cells contain a PKC-sensitive Ca2+ extrusion mechanism likely to be the Ca2+ pump. In lymphocytes, receptor/PLC-linked PKC activation modulates Ca2+i not only by inhibiting Ca2+ influx but also by stimulating plasma membrane Ca2+i extrusion.