Fluorescence measurements of free Ca2+ concentration in human erythrocytes using the Ca2+-indicator fura-2

Leucocyte Rheology at Baseline and after Activation in Post-Phlebitic Syndrome

Objective:

To evaluate leucocyte rheology, expressed as leucocyte filtration, polymorphonuclear leucocyte (PMN) membrane fluidity and cytosolic Ca2+ concentration in subjects with post-phlebitic leg syndrome (PPS).

Methods:

In 22 subjects with PPS we determined leucocyte filtration [unfractionated, mononuclear (MN) and PMN cells], employing the St George Filtrometer, PMN membrane fluidity using the fluorescent probe 1-[4-(trimethylamino)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) and PMN cytosolic Ca2+ concentration using the fluorescent probe Fura 2-AM. Subsequently we determined the same PMN parameters after in vitro activation with 4-phorbol 12-myristate 13-acetate (PMA) and N-formyl-methionyl-leucyl-phenylalanine (fMLP).

Results:

At baseline we observed a difference in the filtration parameters of unfractionated and MN cells and an increase in PMN cytosolic Ca2+ concentration. After activation, a significant variation in PMN filtration parameters was evident both in normals and in PPS subjects, although in subjects with PPS this variation, especially with PMA, was significantly greater. We found a decrease in PMN membrane fluidity and an increase in PMN cytosolic Ca2+ concentration only in subjects with PPS.

Conclusion:

These results suggest that there is a functional alteration of systemic leucocytes in PPS, in which the mechanisms are not yet clear.

Functional significance of the intermediate conductance Ca2+-activated K+ channel for the short-term survival of injured erythrocytes

Increased cytosolic Ca(2+) concentrations activate Gardos K(+) channels in human erythrocytes with membrane hyperpolarization, efflux of K(+), Cl⁻, and osmotically obliged H₂O resulting in cell shrinkage, a phenomenon referred to as Gardos effect. We tested whether the Gardos effect delays colloid osmotic hemolysis of injured erythrocytes from mice lacking the Ca(2+)-activated K(+) channel K(Ca)3.1. To this end, we applied patch clamp and flow cytometry and determined in vitro as well as in vivo hemolysis. As a result, erythrocytes from K(Ca)3.1-deficient (K(Ca)3.1(-/-)) mice lacked Gardos channel activity and the Gardos effect. Blood parameters, reticulocyte count, or osmotic erythrocyte resistance, however, did not differ between K(Ca)3.1(-/-) mice and their wild-type littermates, suggesting low or absent Gardos channel activity in unstressed erythrocytes. Oxidative stress-induced Ca(2+) entry and phospholipid scrambling were significantly less pronounced in K(Ca)3.1(-/-) than in wild-type erythrocytes. Moreover, in vitro treatment with α-toxin from Staphylococcus aureus, which forms pores in the cellular membrane, resulted in significantly stronger hemolysis of K(Ca)3.1(-/-) than of wild-type erythrocytes. Intravenous injection of α-toxin induced more profound hemolysis in K(Ca)3.1(-/-) than in wild-type mice. Similarly, intra-peritoneal application of the redox-active substance phenylhydrazine, an agent for the induction of hemolytic anemia, was followed by a significantly stronger decrease of hematocrit in K(Ca)3.1(-/-) than in wild-type mice. Finally, malaria infection triggered the activation of K(Ca)3.1 and transient shrinkage of the infected erythrocytes. In conclusion, K(Ca)3.1 channel activity and Gardos effect counteract hemolysis of injured erythrocytes, thus decreasing hemoglobin release into circulating blood.

TRPC6 contributes to the Ca(2+) leak of human erythrocytes

Human erythrocytes express cation channels which contribute to the background leak of Ca(2+), Na(+) and K(+). Excessive activation of these channels upon energy depletion, osmotic shock, Cl(-) depletion, or oxidative stress triggers suicidal death of erythrocytes (eryptosis), characterized by cell-shrinkage and exposure of phosphatidylserine at the cell surface. Eryptotic cells are supposed to be cleared from circulating blood. The present study aimed to identify the cation channels. RT-PCR revealed mRNA encoding the non-selective cation channel TRPC6 in erythroid progenitor cells. Western blotting indicated expression of TRPC6 protein in erythrocytes from man and wildtype mice but not from TRPC6(-/-) mice. According to flow-cytometry, Ca(2+) entry into human ghosts prepared by hemolysis in EGTA-buffered solution containing the Ca(2+) indicator Fluo3/AM was inhibited by the reducing agent dithiothreitol and the erythrocyte cation channel blockers ethylisopropylamiloride and amiloride. Loading of the ghosts with antibodies against TRPC6 or TRPC3/6/7 but neither with antibodies against TRPM2 or TRPC3 nor antibodies pre-adsorbed with the immunizing peptides inhibited ghost Ca(2+) entry. Moreover, free Ca(2+) concentration, cell-shrinkage, and phospholipid scrambling were significantly lower in Cl(-)-depleted TRPC6(-/-) erythrocytes than in wildtype mouse erythrocytes. In conclusion, human and mouse erythrocytes express TRPC6 cation channels which participate in cation leak and Ca(2+)-induced suicidal death.

Changes in blood cell membrane properties in HIV type-1-infected patients

To evaluate the possible HIV-1 infection-induced changes in cell membrane properties and in calcium signaling, membrane fluidity, acetylcholinesterase (AChE, a glycosylphosphatidylinositol-anchored protein) activity, and intracellular calcium concentration ([Ca2(+)](int)) were evaluated in lymphocytes and erythrocytes of infected individuals, previous to their engagement in antiretroviral therapy. Membrane fluidity was assessed by fluorescence spectroscopy measurements, using the fluorescence probes 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[4-(trimethylamino)-phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). AChE activity was determined by the colorimetric Ellman's method and [Ca2(+)](int) using the fluorescent fura-2 acetoxymethyl ester. When compared with the control group, lymphocytes of infected patients presented significantly decreased membrane fluidity, decreased AChE activity, and increased [Ca2(+)](int). Erythrocytes from HIV-infected patients presented decreased [Ca2(+)](int) when compared with the control group and decreased membrane fluidity near the lipid/water interface. Our data show that HIV-1 infection leads to biochemical and biophysical changes in the membrane itself and in membrane protein activity in lymphocytes (average of infected and noninfected subpopulations) and even in erythrocytes. The present observations are in agreement with a process of facilitated propagation of the infection to new cells, stimulation of virion production, and maintenance of a reservoir of erythrocyte-bound infectious virus.

Effect of the ionic strength and prostaglandin E2 on the free Ca2+ concentration and the Ca2+ influx in human red blood cells

Human red blood cells (RBCs) were loaded with the Ca(2+)-sensitive fluorescent dye fura-2 to investigate the effects of media ionic strength and prostaglandin E2 (PGE2) on the intracellular free Ca2+ concentration ([Ca2+]i). [Ca2+]i of intact RBCs in a Ca(2+)-containing physiological (high) ionic strength (HIS) solution was 75.1 +/- 8.3 nM after 5 min incubation, increasing to 114.9 +/- 9.6 nM after 1 h. In Ca(2+)-containing low ionic strength (LIS) solutions, [Ca2+]i was significantly lower than in the Ca(2+)-containing HIS solution (p = 0.041 or 0.0385 for LIS solutions containing 200 or 250 mM sucrose, respectively), but, as in HIS solution, an increase of [Ca2+]i was seen after 1 h. In Ca(2+)-free (0 Ca2+ plus 15 microM EGTA) media, [Ca2+]i decreased (ranging from 15 to 21 nM), but were not significantly different in HIS or LIS, and did not change following 1 h incubation. The effect of the ionic strength and PGE2 on passive Ca2+ influx was investigated on ATP-depleted RBCs. Ca2+ influx was faster during the initial 10 min in comparison with the subsequent time period (10-45 min), both in HIS and LIS media, decreasing from 20.3 +/- 1.9 to 12.9 +/- 1.3 micromol/(lcells x h) in HIS, and from 36.7 +/- 5.3 to 8.6 +/- 1.2 micromol/(lcells x h) in LIS. Prostaglandin E2 (PGE2; 10(-7)-10(-11) M), dissolved in deionised water or in ethanol, did not affect [Ca2+]i in either normal or in ATP-depleted RBCs suspended in Ca(2+)-containing HIS medium. Finally, the addition of carbachol (100 microM) did not affect [Ca2+]i. The present findings suggest that stimulation of the Ca(2+)-activated K+ channel by PGE2, reported in [J. Biol. Chem. 271 (1996) 18651], cannot be mediated via increased [Ca2+]i.

Patch-clamp analysis of the "new permeability pathways" in malaria-infected erythrocytes

The intraerythrocytic amplification of the malaria parasite Plasmodium falciparum induces new pathways of solute permeability in the host cell's membrane. These pathways play a pivotal role in parasite development by supplying the parasite with nutrients, disposing of the parasite's metabolic waste and organic osmolytes, and adapting the host's electrolyte composition to the parasite's needs. The "new permeability pathways" allow the fast electrogenic diffusion of ions and thus can be analyzed by patch-clamp single-channel or whole-cell recording. By employing these techniques, several ion-channel types with different electrophysiological profiles have been identified in P. falciparum-infected erythrocytes; they have also been identified in noninfected cells. This review discusses a possible contribution of these channels to the new permeability pathways on the one hand and their supposed functions in noninfected erythrocytes on the other.

Effects of nitric oxide on red blood cell deformability

In addition to its known action on vascular smooth muscle, nitric oxide (NO) has been suggested to have cardiovascular effects via regulation of red blood cell (RBC) deformability. The present study was designed to further explore this possibility. Human RBCs in autologous plasma were incubated for 1 h with NO synthase (NOS) inhibitors [N(omega)-nitro-l-arginine methyl ester (l-NAME) and S-methylisothiourea], NO donors [sodium nitroprusside (SNP) and diethylenetriamine (DETA)-NONOate], an NO precursor (l-arginine), soluble guanylate cyclase inhibitors (1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one and methylene blue), and a potassium channel blocker [triethylammonium (TEA)]. After incubation, RBC deformability at various shear stresses was determined by ektacytometry. Both NOS inhibitors significantly reduced RBC deformability above a threshold concentration, whereas the NO donors increased deformability at optimal concentrations. NO donors, as well as the NO precursor l-arginine and the potassium blocker TEA, were able to reverse the effects of NOS inhibitors. Guanylate cyclase inhibition reduced RBC deformation, with both SNP and DETA-NONOate able to reverse this effect. These results thus indicate the importance of NO as a determinant of RBC mechanical behavior and suggest its regulatory role for normal RBC deformability.

Platelet-erythrocyte interactions enhance alpha(IIb)beta(3) integrin receptor activation and P-selectin expression during platelet recruitment: down-regulation by aspirin ex vivo

Activated platelets release biologically active compounds, which then recruit additional platelets into an evolving thrombus. We studied activation of alpha(IIb)beta(3) and exposure of P-selectin on platelets recruited by releasates obtained from collagen-treated platelets and evaluated modifications in prothrombotic effects of releasates induced by platelet-erythrocyte interactions and aspirin treatment. Releasates from collagen-stimulated platelets induced alpha(IIb)beta(3) activation and P-selectin exposure (monitored by flow cytometry using fluorescein isothiocyanate-PAC-1 and phycoerythrin-CD62 antibodies). These responses were markedly amplified by releasates from combined platelet-erythrocyte suspensions. This finding demonstrates a novel mechanism(s) by which erythrocytes intensify platelet aggregability and mediate increased platelet recruitment. Because P-selectin and alpha(IIb)beta(3) are potential sites for platelet-leukocyte interactions, erythrocytes may also modulate leukocyte recruitment. Following aspirin ingestion both the recruiting capacity of platelet releasates and erythrocyte-induced amplification of platelet recruitment were down-regulated. These events represent an additional antithrombotic property of aspirin. We also examined the possibility that arachidonic acid, or eicosanoids derived therefrom, can induce a prothrombotic activity of erythrocytes. The TXA(2)-analog U46 619 and free arachidonate, but not PGI(2) or 12-HETE, induced increases in cytosolic Ca(++) and promoted phosphatidylserine (PS) exposure on a subpopulation of erythrocytes. PS exposure and increases in erythrocyte [Ca(++)](i) are associated with enhanced procoagulant activity, increased endothelial adhesion, and reduced erythrocyte deformability. Our findings, therefore, suggest that TXA(2) and arachidonic acid, derived from activated platelets, induce a prothrombotic phenotype on erythrocytes in proximity. We conclude that by these mechanisms, erythrocytes can actively contribute to platelet-driven thrombogenesis and microvascular occlusion.

Biochemical, biophysical and haemorheological effects of dimethylsulphoxide on human erythrocyte calcium loading

The studies using dimethylsulphoxide (DMSO) and/or the 4-bromo-calcium ionophore A23187 (Br-A23187) often neglect the precise knowledge of some of their biochemical, biophysical and haemorheological effects. The aim of the present study was to evaluate these effects on erythrocytes after whole blood incubations with DMSO or Br-A23187 dissolved in DMSO. There were no significant differences between the different aliquots in the values of P(50), pH, erythrocyte deformability, erythrocyte membrane fluidity, haemoglobin and intracellular Ca(2+) concentrations ([Ca(2+)](i)). Aliquots with DMSO (independently of the presence of Br-A23187 or added Ca(2+)) had lower erythrocyte aggregation indexes and higher plasma concentrations of K(+)], Na(+)] and Ca(2+) than the aliquots without DMSO (independently of the presence of added Ca(2+)). Aliquots with added calcium (without the presence of Br-A23187 in DMSO) had a significantly higher erythrocyte acetylcholinesterase activity. Our data shows that calcium loading, the usual objective of Br-A23187 incubations, cannot be fulfilled with the studied experimental conditions. The coherence between our results and those obtained by other authors with different biological systems and different modulators of the rise on [Ca(2+)](i) suggests a non-specific effect of DMSO, disabling the action of the modulator. It can be reasoned that the decreased erythrocyte aggregation (without significant changes on the deformability or membrane fluidity) can result either from the decrease of the hydrogen bonding contribution to erythrocyte aggregation or the increased ionic strength influence on the erythrocyte membrane surface.

Erythrocyte calcium influx is related to severity of ventricular arrhythmias in uraemic patients

BACKGROUND

Myocardial disorders are a remarkable cause of morbidity and mortality in chronic haemodialysed patients (HD). They could be favoured by alteration of cell Ca(2+) handling. In previous studies we characterized an erythrocyte Ca(2+) influx, sensitive to membrane potential and inhibited by Ca(2+) antagonists. Since its maximal influx rate was decreased in HD patients, this study investigates if Ca(2+) influx alterations are related to myocardial disorders in HD patients.

METHODS

Voltage-sensitive erythrocyte Ca(2+) influx was measured in 30 healthy controls and in 53 patients (47 HD patients and six patients with left ventricular hypertrophy and normal kidney function), using fura 2. In 29 HD patients and in six healthy subjects Ca(2+) influx was also determined in the presence of parathyroid hormone (PTH) in vitro. Patients were classified according to Lown's ventricular arrhythmias classification after 24-h Holter electrocardiograph (ECG) monitoring. Forty-six patients underwent echocardiography.

RESULTS

Voltage-sensitive erythrocyte Ca(2+) influx was significantly reduced in HD patients. Maximal influx rate was significantly higher in HD patients of Lown's classes 3 and 4 (0.789 +/- 0.156 nmol/s, n = 8; P < 0.01) than in patients of classes 1 and 2 (0.499 +/- 0.055 nmol/s, n=15), or without ventricular arrhythmias (0.400 +/- 0.041 nmol/s, n = 24). Maximal influx rate was directly correlated to left ventricular mass index (LVM) (r = 0.353, P < 0.05). Subjects with left ventricular hypertrophy and normal kidney function displayed erythrocyte Ca(2+) influx similar to that of normal subjects. Multiple regression indicates that LVM and Ca(2+) influx were independently related to severity of arrhythmias. When added to the influx assay, PTH increased the maximal influx rate only in patients with ventricular arrhythmias.

CONCLUSION

Myocardial dysfunction and altered ventricular excitability could be related in uraemic HD patients to alterations of calcium transport, as found in the erythrocyte model. Reduced resistance to PTH could contribute to this phenomenon.

Polymorphonuclear integrins, membrane fluidity, and cytosolic Ca(2+) content after activation in essential hypertension

The purpose of this research was to obtain further information about the role of polymorphonuclear leukocytes in essential hypertension. These cells could be involved in the pathogenesis of organ injury. Thirty subjects (14 men and 16 women) with essential hypertension were enrolled. In these subjects we determined, at baseline and after in vitro activation with 4-phorbol 12-myristate 13-acetate and N:-formyl-methionyl-leucyl-phenylalanine, the polymorphonuclear leukocyte membrane fluidity, obtained by labeling the cells with 1-[4-(trimethylamino)phenyl]-6-phenyl-1,3, 5-hexatriene, cytosolic Ca(2+) concentration, obtained by marking the cells with Fura 2-AM, and integrin pattern (CD11a, CD11b, CD11c, and CD18), by using the indirect immunofluorescence with a flow cytometer. At baseline there was no difference in membrane fluidity between normal subjects and hypertensives, whereas hypertensives showed an increase in cytosolic Ca(2+) content and an increase of the phenotypical expression of CD11a, CD11b, and CD18. In normal subjects and in hypertensives, after activation, no variation was found in membrane fluidity and cytosolic Ca(2+) content. In normal subjects, after activation, we observed a significant increase of the expression of all adhesion molecules, whereas in hypertensives we found an increase of the expression of CD11b, CD11c, and CD18 but also a decrease of CD11a. The behavior of the polymorphonuclear leukocyte integrin profile may have several explanations, and in particular, the trend of CD11a after chemotactic activation may be related to its cleavage or to an altered integrin phosphorylation/dephosphorylation balance hypothetically present in this clinical condition.

Acute ischemic stroke : polymorphonuclear leukocyte membrane fluidity and cytosolic Ca(2+) concentration at baseline and after chemotactic activation

BACKGROUND AND PURPOSE

Several reports have considered the role of systemic leukocytes in acute ischemic stroke (AIS). Initially, greater attention was focused on the leukocyte count and subsequently on their adhesiveness, aggregation, rheology, and activation. The aim of this study was the evaluation of certain polymorphonuclear leukocyte (PMN) parameters, reflecting their rheology and activation, in subjects with AIS.

METHODS

In a group of 19 subjects with AIS and in a control group of 18 subjects with asymptomatic vascular atherosclerotic disease, we evaluated the PMN membrane fluidity and cytosolic Ca(2+) concentration at baseline and after in vitro chemotactic activation with 4-phorbol 12-myristate 13-acetate (PMA) and N-formyl-methionyl-leucyl-phenylalanine (fMLP).

RESULTS

From the obtained data, it is evident that at baseline only PMN membrane fluidity distinguishes control subjects from AIS subjects. After PMN activation with PMA and fMLP, prolonged for 5 and 15 minutes, we found an increase in PMN cytosolic Ca(2+) concentration and a decrease in PMN membrane fluidity only in subjects with AIS.

CONCLUSIONS

These findings emphasize that in subjects with AIS a functional alteration of systemic PMN cells is clearly expressed during chemotactic activation, although the mechanism of this abnormality is not yet explained.

Leukocyte rheology before and after chemotactic activation in some venous diseases

OBJECTIVE

to evaluate leukocyte rheology, polymorphonuclear leukocyte (PMN) membrane fluidity and cytosolic Ca2+ concentration in subjects with post-phlebitic leg syndrome (PPS) and acute deep-venous leg thrombosis (DVT).

SUBJECTS

twenty-two subjects with leg PPS and 14 subjects with leg DVT.

METHODS

we evaluated the leukocyte filtration (unfractionated, mononuclear cells (MN) and PMN), the PMN membrane fluidity and the PMN cytosolic Ca2+ concentration. Subsequently, we evaluated the same PMN variables after in vitro chemotactic activation with 4-phorbol 12-myristate 13-acetate (PMA) and N -formyl-methionyl-leucyl-phenylalanine (fMLP).

RESULTS

at baseline we observed a significant difference in the filtration variables of unfractionated and MN cells and in PMN cytosolic Ca2+ concentration. After activation, in normal subjects and subjects with PPS and DVT, a significant variation in PMN filtration at 5 and 15 minutes was evident. In normal subjects, no variation was present in PMN membrane fluidity or cytosolic Ca2+ concentration after activation. In subjects with PPS and DVT, we found a decrease in PMN membrane fluidity and an increase in PMN cytosolic Ca2+ concentration. After PMN activation (at 5 and 15 min) Delta% of IRFR distinguished normal subjects from subjects with PPS and DVT, while no difference was found in Delta% of membrane fluidity or cytosolic Ca2+ concentration.

CONCLUSIONS

there is a functional alteration of leukocytes in these patients whose mechanisms are not yet clear.

Erythrocyte voltage-dependent calcium influx is reduced in hemodialyzed patients

BACKGROUND

Uremia displays increased cytosolic free calcium ([Ca2+]i) in many different cell types, supporting the hypothesis of an altered Ca2+ transport modifying the functional activity of calcium signaling pathway.

METHODS

Thirty-five hemodialyzed patients and 20 age-matched subjects were studied. Erythrocyte resting [Ca2+]i and Ca2+ influx were measured by the fluorescent Ca2+-sensitive dye fura-2.

RESULTS

We found an increase of resting [Ca2+]i in erythrocytes from uremic hemodialyzed patients compared with matched healthy controls (103 +/- 2.5 nM, N = 20, vs. 90 +/- 4, N = 20, P < 0.01). Moreover, we found an altered voltage-dependent Ca2+ influx showing a reduced transport rate (0.42 +/- 0.03 nM/second vs. 0.74 +/- 0.08, P < 0.01). High levels of plasma parathyroid hormone (PTH) were related to augmented Ca2+ entry (r = 0.511, P < 0.05), contributing to maintain a high level of [Ca2+]i. Hemodialysis had no effect on cell calcium level and Ca2+ influx indices. The therapy with Ca2+ antagonists did not modify the values of resting [Ca2+]i or Ca2+ influx indices, but the correlation between PTH and influx indices was lost.

CONCLUSIONS

In conclusion, we found evidence for an alteration of erythrocyte Ca2+ influx caused by uremic toxicity that could be related to some organ disorders in uremia. The chronic increase of cellular calcium may contribute to influx derangement.

Effect of pravastatin on erythrocyte rheological and biochemical properties in poorly controlled Type 2 diabetic patients

AIMS

The rheological properties of erythrocytes are impaired in diabetes mellitus, especially because of changes in their membrane lipid composition. In hypercholesterolaemic patients, lowering plasma cholesterol is associated with an improvement of the erythrocyte rheological parameters. The aim of this study was to investigate the relationship between erythrocyte deformability, plasma lipids, lipid membrane composition and cytosolic cations in poorly controlled Type 2 diabetic patients and to test the effects of a cholesterol-lowering treatment on these parameters.

METHODS

We compared 37 poorly controlled Type 2 diabetic patients with 26 controls. In 22 of the diabetic patients who showed an impairment in erythrocyte deformability (filtration index >10.5 on the Hanss' haemorheometer), a double-blind randomized trial compared the effect of the inhibitor of HMG CoA reductase pravastatin 20 mg per day for 4 months vs. placebo on the erythrocyte parameters.

RESULTS

Compared with controls, diabetic patients had higher filtration index (FI), erythrocyte sodium and calcium contents and lower free cholesterol-phospholipids ratio in erythrocyte membranes. Erythrocyte sodium content correlated positively with the FI and the membrane free cholesterol-phospholipids ratio. In the pravastatin-treated group (11 patients), fibrinogen decreased significantly, FI reached a normal value (<10) in six patients. Four of the five other patients who still had abnormal FI after 4 months of treatment had either a high plasma triglycerides (> or =4.60 mmol/l) or a high plasma fibrinogen (> or =4 g/l) level at baseline. Only two of the 11 placebo-treated patients achieved a normal FI.

CONCLUSION

These data suggest that in poorly controlled Type 2 diabetic patients there is a link between the chemical composition and the rheological properties of erythrocytes. Erythrocyte deformability may be improved by lowering plasma cholesterol with a statin.

Nisoldipine improves the impaired erythrocyte deformability correlating with elevated intracellular free calcium-ion concentration and poor glycaemic control in NIDDM

AIMS

To explore the mechanisms underlying the impaired erythrocyte deformability (RBC-df) in diabetic patients, the relationship between erythrocyte intracellular free calcium-ion concentration ([Ca2+]i) and RBC-df, and the effects of Ca2+-channel blocker on [Ca2+]i and RBC-df were evaluated.

METHODS

Forty-eight patients with NIDDM and 24 control subjects were enrolled in this study. [Ca2+]i was determined using fura-2, and RBC-df by filtration method expressed as Deformability Index (DI). Erythrocytes were treated with nisoldipine to evaluate the effects of a Ca2+-channel blocker.

RESULTS

[Ca2+]i was significantly higher (82.6 (78.0-87.2) vs 76.6 (74.3-81.2) nmol lRBC-1, P<0.001), and DI was significantly lower (0. 14 (0.09-0.28) vs 0.22 (0.16-0.28), P<0.01) in NIDDM than in controls. There was a significant correlation between HbA1c and [Ca2+]i (r=0.38, P<0.01), between HbA1c and DI (r=-0.51, P<0.01), and between [Ca2+]i and DI (r=-0.42, P<0.01). Stepwise multiple regression analysis revealed HbA1c and [Ca2+]i as independent determinants for the impaired RBC-df. Nisoldipine treatment in vitro significantly decreased [Ca2+]i, and significantly improved RBC-df.

CONCLUSIONS

These data indicate that the impaired RBC-df in NIDDM may at least partly be attributed to the elevated [Ca2+]i and poor glycaemic control. In addition, favorable effects of a Ca2+-channel blocker on both [Ca2+]i and RBC-df have been demonstrated.

Chronic renal failure: leukocyte rheology and polymorphonuclear cytosolic Ca2+ concentration

In a group of subjects with chronic renal failure (CRF), we evaluated the leukocyte rheological activity, expressed as the filtration rate, the polymorphonuclear (PMN) membrane fluidity and the PMN cytosolic Ca2+ content. Using the St. George's Filtrometer, we determined the filtration rate of unfractionated, mononuclear and PMN leukocytes. Using the fluorescent probe 1.4-(trimethylamino)-phenyl-4-phenyl-hexatriene (TMA-DPH), we examined the PMN membrane fluidity and, using the Fura 2-AM, the PMN cytosolic Ca2+ content. From the results obtained, it is evident that only the initial relative flow rate of unfractionated leukocytes was significantly reduced in subjects with CRF, while the filtration parameters of mononuclear and PMN cells did not distinguish normals from CRF subjects. No variation was evident for the PMN membrane fluidity, while the PMN cytosolic Ca2+ content was significantly increased in CRF subjects. In these subjects no correlation was found between leukocyte filtration parameters, PMN membrane fluidity, PMN cytosolic Ca2+ content and plasma parameters (blood urea nitrogen and serum creatinine), reflecting the degree of the CRF. In conclusion, in CRF subjects the abnormality of the leukocyte flow properties was restricted to the initial flow rate of unfractionated leukocytes, whereas the increase of PMN cytosolic Ca2+ content might be attributed to PMN activation.

Cytosolic pH and calcium in Dahl salt-sensitive and salt-resistant rats: the relationship to plasma lipids

OBJECTIVE

To search for alterations of cytosolic pH and cell calcium handling in platelets and erythrocytes of Dahl rats susceptible and resistant to salt-induced hypertension.

DESIGN AND METHODS

Blood pressure, plasma lipids, platelet cytosolic calcium concentration ([Ca2+]i) and pH (pHi) together with thrombin-induced changes in these parameters as well as erythrocyte [Ca2+]i and 45Ca influx were determined in Dahl salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats aged 9, 15 and 24 weeks, which were fed a low-salt diet (0.3% NaCl), and in animals fed high-salt diet (4% NaCl) for 5-10 weeks since weaning.

RESULTS

With a low salt intake platelet pHi was lower in SS/Jr than it was in SR/Jr rats, whereas basal platelet [Ca2+]i was similar in rats of both strains. The difference in basal pHi between SS/Jr and SR/Jr rats increased progressively with age of animals. A high salt intake from youth did not influence platelet [Ca2+]i in rats of either strain but it caused an earlier decrease in pHi in SR/Jr than it did in SS/Jr rats. Thrombin stimulation induced similar elevations of pHi and [Ca2+]i in rats of both strains, irrespective of age, salt intake and response of blood pressure to salt intake. Erythrocyte 45Ca influx and [Ca2+]i were greater for SS/Jr rats but only the latter parameter was correlated positively to blood pressure. Both regulation of platelet pHi and erythrocyte Ca2+ handling were significantly related to plasma lipid levels.

CONCLUSIONS

Platelets of SS/Jr rats fed a low-salt diet were characterized by a lower basal cytosolic pHi but unchanged [Ca2+]i relative to those of SR/Jr rats. Hypertension induced by high salt intake was associated with increased erythrocyte [Ca2+]i but not with elevation of platelet [Ca2+]i or alteration of response to stimulation with thrombin.

Apparent Ca2+ dissociation constant of Ca2+ chelators incorporated non-disruptively into intact human red cells

1. A recently developed method of measuring cytoplasmic Ca2+ buffering in intact red cells was applied to re-evaluate the intracellular Ca2+ binding properties of the Ca2+ chelators benz2 and BAPTA. Incorporation of the free chelators was accomplished by incubating the cells with the acetoxymethyl ester forms (benz2 AM or BAPTA AM). The divalent cation ionophore A23187 was used to induce equilibrium distribution of Ca2+ between cells and medium. 45Ca2+ was added stepwise to cell suspensions in the presence and absence of external BAPTA. To induce full Ca2+ equilibration, the plasma membrane Ca2+ pump was inhibited either by depleting the cells of ATP or by adding vanadate to the cell suspension. 2. The properties of the incorporated chelators were assessed from the difference in cytoplasmic Ca2+ buffering between chelator-free and chelator-loaded cells, over a wide range of intracellular ionized calcium concentrations ([Ca2+]i), from nanomolar to millimolar. 3. Under the experimental conditions applied, incorporation of benz2 and BAPTA into the red cells increased their Ca2+ buffering capacity by 300-600 mumol (340 g Hb)-1. The intracellular apparent Ca2+ dissociation constants (KDi) were about 500 nM for benz2 and 800 nM for BAPTA, values much higher than those reported for standard salt solutions (KD) of about 40 and 130 nM, respectively. These results suggest that, contrary to earlier observations, the intracellular red cell environment may cause large shifts in the apparent Ca2+ binding behaviour of incorporated chelators. 4. The possibility that the observed KD shifts are due to reversible binding of the chelators to haemoglobin is considered, and the implications of the present results for early estimates of physiological [Ca2+]i levels is discussed.

Characterization of voltage-dependent calcium influx in human erythrocytes by fura-2

Thus far, the methods used to determine erythrocyte Ca2+ influx have not allowed the assessment of the kinetics of ion uptake. To overcome this drawback, we studied a new method, using the fluorescent Ca2+-chelator fura-2, which directly quantifies intracellular Ca2+ changes in human erythrocytes. This method has the advantage over previous techniques that it monitors continuously cellular Ca2+ levels. The Ca2+ influx is modulated by cellular membrane potential in the presence of a transmembrane Ca2+ concentration gradient and exhibits a first slow increase of the intracellular Ca2+ concentration, followed, after the reachment of a threshold value of 125 +/- 13 nM Ca2+, by a faster increase until a plateau is reached. The influx rate is inhibited by dihydropyridines in the micromolar range. These findings support the hypothesis that erythrocyte Ca2+ influx is mediated by a carrier similar to the slow Ca2+ channels and is dependent on membrane depolarization.

Ionic calcium content of light dense human red cells separated by Percoll density gradients

In this paper we have compared the adequacy of two methods using Percoll density gradients to separate light and dense erythrocytes from fresh human blood. After measuring the distribution of some classical age-markers such as haemoglobin, potassium and creatine contents, it was found that preformed gradients generated more stringent conditions for age-related density separations. Employing such gradients the free Ca2+ content of above sub-populations was assessed with Fura-2, under conditions where the viscosity effect was abolished. In five experiments, the free Ca2+ content (mean value +/- 1 S.D.) was 8.4 +/- 2.82 nM and 31.2 +/- 13.0 nM for the 7-10% lightest and densest cells, respectively. These results are discussed in connection to red cell senescence.

Cell calcium handling and intracellular pH regulation in hereditary hypertriglyceridemic rats: reduced platelet response to thrombin stimulation

Multiple cell membrane alterations have been described in humans and animals with various genetic forms of hypertension and/or dyslipidemia. The aim of our study was to characterize some properties of platelets and/or erythrocytes (cytosolic calcium handling, intracellular pH regulation and thrombin responsiveness) in a new model of genetic hypertension associated with hyperlipidemia-Prague hereditary hypertriglyceridemic (HTG) rats. There were no differences in basal cytosolic Ca2+ values in platelets or erythrocytes of HTG rats and control Wistar rats. Ca2+ influx into erythrocytes was also similar in HTG and control rats. In both strains Ca2+ influx correlated positively with plasma triglycerides. The slope of this relationship was less steep in HTG than in Wistar rats. Cytosolic Ca2+ response to thrombin stimulation was smaller in HTG platelets, which were also characterized by a major reduction of thrombin-induced Mn2+ entry through receptor-operated Ca2+ channels. Platelets of HTG rats had the same basal intracellular pHi values and similar buffering capacity as control rats but their pHi response to thrombin stimulation was substantially reduced. It can be concluded that reduced responsiveness to thrombin stimulation is a major alteration found in platelets of hypertensive hereditary hypertriglyceridemic rats.

Leukocyte flow properties, polymorphonuclear membrane fluidity, and cytosolic Ca2+ content in subjects with vascular atherosclerotic disease with and without noninsulin-dependent diabetes mellitus

The aim of this research was the evaluation of white blood cell (WBC) filtration, reflecting WBC flow properties, polymorphonuclear cell membrane fluidity, and cytosolic Ca2+ content in subjects with vascular atherosclerotic disease (VAD) and in VAD subjects with noninsulin-dependent diabetes mellitus (NIDDM), in good hemodynamic balance. The authors examined WBC filtration (unfractionated, mononuclear [MN], polymorphonuclear [PMN] cells), using the St. George Filtrometer and considering, respectively, the initial relative flow rate (IRFR) and the clogging rate (CR); the PMN membrane fluidity, employing the fluorescent probe TMA-DPH and calculating the fluorescence polarization degree; and the PMN cytosolic Ca2+ content, adopting the fluorescent probe Fura 2-AM and considering the ratio between the Fura 2-Ca2+ complex and the unchelated Fura 2 fluorescence intensity. The obtained data showed that only the filtration parameters (IRFR, CR) of unfractionated WBCs discriminated normal subjects from VAD groups, whereas the filtration parameters of MN and PMN cells did not demonstrate any distinction. PMN membrane fluidity did not distinguish normal subjects from VAD groups, whereas PMN cytosolic Ca2+ content was significantly increased in VAD groups in comparison with normal subjects. No relationship was evident between WBC filtration and plasma metabolic parameters; the correlations obtained between PMN filtration and other PMN parameters need further investigation.

Analysis of a cation-transporting ATPase of Plasmodium falciparum

We have cloned and characterised one gene, PfATPase4 which encodes a P-type ATPase containing all the primary sequence motifs characteristic of this class of transmembrane ion transporters, and also a fragment of a second P. falciparum P-type ATPase pseudogene (PfATPase5). Analysis of conserved domains and motifs of specific ATPases reveals that PfATPase4 is most analogous to Ca2+ ATPases of the endoplasmic reticulum. The PfATPase4 gene gives rise to a transcript of 8 kb shortly after erythrocyte invasion. Although this mRNA is not detected in later stages, the protein detected immunologically at 190 kDa persists throughout and is detected in free merozoites. Immunofluorescence microscopy reveals that the PfATPase4 protein is concentrated in discrete compartments at the periphery of the parasite. Detailed sequence and structural analyses of these and the other P-type ATPases of P. falciparum described previously, reveals that they comprise an unusual family in several respects. Firstly, the large number of non-homologous genes so far characterised reflects the complexities of ionic regulation in the diverse environments encountered by the parasite. Secondly, the plasmodial P-type ATPase family may be classified both at primary sequence and structural levels into two distinct groups-those typical of P-type ATPases (including PfATPase4) and those which are much more divergent. A third complexity is illustrated by the fact that one of the other members [1] here termed PfATPase6, has an even greater similarity to the sarcoplasmic reticulum Ca2+ ATPases than does PfATPase4, which raises questions about the possible functional relationship between these two members.

The influence of blood pressure on intracellular Ca2+ content in erythrocytes: effects of cadmium chloride and nifedipine

Cellular abnormalities associated with elevated Ca2+ concentrations have been postulated to be involved in the pathogenesis of hypertension. The present study was undertaken to investigate the effects of blood pressure changes on cytosolic Ca2+ levels in erythrocytes. Cadmium, which has been implicated in the etiology of hypertension was used as the hypertensive agent and the classical blocker of voltage-operated calcium channels nifedipine was used to treat hypertension. 10 weeks old male rats were divided into four groups; control, CdCl2, CdCl2 and nifedipine, nifedipine groups. CdCl2 caused elevations in blood pressure and in the cytosolic erythrocyte Ca2+ levels both of which were reduced after nifedipine administration. After nifedipine alone, cytosolic Ca2+ levels were increased. These findings suggest that cytosolic Ca2+ content decreasing action of nifedipine in the CdCl2 and nifedipine applied group could be secondary to the antihypertensive action.

Evaluation of triple-band filters for quantitative epifluorescence microscopy

The performance characteristics of two sets of triple-band epifluorescence filters have been evaluated for use with digitally enhanced fluorescence microscopy. Use of such filters, at most, requires movement of the excitation filter, while the dichroic and emission filters remain fixed, allowing multi-wavelength imaging to be performed on standard microscopes. The dyes appropriate for use with these particular filters include Texas Red (TR), Bodipy (BD), FITC and Cascade Blue (CB), four fluorophores now commonly conjugated to both immunochemical probes and other proteins and lipids of biological interest. Good spectral separation existed for most experimental conditions allowing accurate localization of the different fluorophores during multi-wavelength imaging. Anomalous responses were observed during near-UV excitation at high concentration for some dyes. Scanning spectrofluorometry demonstrated that concentration-dependent spectral shifts occurred, resulting in large increases in near-UV absorbance. Despite the complexity of concentration and dye-interaction effects, quantitative measurements of dye concentration could be made, even in regions of multiple dye co-localization. Therefore, multi-band pass filters are an additional valuable approach for performing quantitative fluorescence microscopic imaging.

Isradipine affects histamine-induced cytosolic Ca2+ movements in human endothelial cells

Although endothelial actions of dihydropyridines remain controversial, isradipine has been observed to exert anti-atherosclerotic actions in which endothelium could be involved. This study was designed to investigate the direct effects of isradipine on cytosolic Ca2+ concentration in cultured human umbilical vein endothelial cells. Isradipine (from 10 nM to 1 microM) had no effect on unstimulated cells but dose-dependently decreased both the transient [Ca2+]i peak and the sustained increase induced by histamine. Its maximal effects were reached at 0.1 microM. In the absence of Ca2+ influx or in depolarized cells, 1 microM isradipine still significantly decreased the transient [Ca2+]i peak (by 23 +/- 8% and 42 +/- 11%). Ca2+ influx induced by re-establishment of transmembrane Ca2+ gradient was also inhibited by isradipine, as was that induced by 1 microM thapsigargin. These results demonstrate that isradipine is able to reduce both Ca2+ release from internal stores and the consequent Ca2+ entry in stimulated human endothelial cells.

Refilling state of internal Ca2+ stores is not the only intracellular signal stimulating Ca2+ influx in human endothelial cells

To further analyse the role of the refilling state of internal Ca2+ pools in the stimulation of Ca2+ influx in human endothelial cells, we investigated the combined effect of thapsigargin (TG) and histamine on cytosolic Ca2+ concentration ([Ca2+]i) and inositol polyphosphate production. At normal extracellular Ca2+ levels, TG induced a progressive and sustained elevation in [Ca2+]i which was dose-dependently prevented by pretreatment with 1-10 microM histamine. Similarly, pretreatment with 0.1 and 1 microM TG suppressed histamine-induced Ca2+ transients partially and totally, respectively. TG pretreatment did not alter the inositol triphosphate (IP3) level liberated by histamine, but modified IP3 metabolism by decreasing inositol biphosphate (IP2) and increasing inositol monophosphate (IP1) contents. In the absence of Ca2+ influx, 1 microM TG only induced a small transient increase in [Ca2+]i whereas the Ca2+ mobilization evoked by 10 microM histamine was unchanged. In both cases, the absence of any additional effect of either TG, histamine or 2 microM ionomycin indicated the complete depletion of Ca2+ stores. The re-establishment of the transmembrane Ca2+ gradient induced a transient rise in [Ca2+]i. Its amplitude differed between histamine- and TG-treated cells. It was imposed by cell pretreatment and was selectively affected by changes in the membrane potential. At 5 mM external K+, the transient rise in [Ca2+]i was more marked in histamine- than in TG-stimulated cells; this difference was suppressed by TG pretreatment. The presence of 130 mM external K+ increased Ca2+ entry in TG-treated cells but reduced it in histamine-stimulated cells. These results indicate that the refilling state of internal Ca2+ stores does not constitute the single regulator of Ca2+ influx. TG and histamine seem to activate Ca2+ influx through distinct but interdependent pathways regulated by membrane potential.

Insulin activation of red blood cell Na+/H+ exchange decreases the affinity of sodium sites

We have previously reported increased activity of Na+/H+ and Na+/Li+ exchanges in red blood cells (RBC) of patients with hypertension and diabetic nephropathy. The presence in human red blood cells (RBC) of insulin receptors has led us to examine the effects of this hormone on the kinetic parameters of Na+/H+ exchange as a first approach to define its mechanism of action. The antiporter activity was measured as net Na+ influx driven by an outward H+ gradient in acid-loaded, Na-depleted RBCs preincubated with or without (w/wo) insulin (0 to 100 microU/ml) for different time periods. The effects of insulin on the H+ and Na+ activation kinetics of Na+/H+ exchange were examined in RBCs of normal subjects fasted for 12 hours. Insulin (50 microU/ml for 1 hr) increased the Vmax from 28 +/- 6 to 49 +/- 8 mmol/liter cell x hr (N = 10, P < 0.0005) and the Km for Na+ from 72 +/- 10 to 142 +/- 19 mM (N = 4, P < 0.05) but did not change the Km for intracellular H+. Insulin also increased the Vmax of Na+/Li+ exchange at pHi 7.4 (0.34 +/- 0.03 to 0.45 +/- 0.04 mmol/liter cell x hr, N = 9, P < 0.005) as well as the Km for Na+ (31 +/- 3 to 6 +/- 10 mM, P < 0.0003). Therefore, insulin can modulate Na+ sites of Na+/Li+ or Na+/H+ exchanges independent of the occupancy of H+ sites to favor the release of bound Na+ into the cytoplasm. Insulin stimulation of Na+/H+ exchange required endogenous cytosolic Ca2+ levels. The kinetic effects of insulin on Na+/H+ and Na+/Li+ exchanges were imitated by okadaic acid (300 microM), an inhibitor of protein phosphatases which dephosphorylate serine-threonine residues. Okadaic acid increased the Vmax of Na+/H+ and Na+/Li+ exchanges and the Km for Na+ as insulin did. In conclusion, insulin stimulation of the Na+/H+ antiporter occurs by a novel kinetic mechanism leading to a decreased affinity for external Na+ without changes in the affinity for Hi. On the basis that insulin effects were imitated by okadaic acid, we hypothesize that this hormone may increase the phosphorylated state of serine-threonine residues of this antiporter protein.

Alterations of membrane properties in erythrocytes of salt hypertensive Sabra rats

This study was designed to investigate the effects of a hypertensive stimulus, high salt intake, in hypertension-prone (SBH) and -resistant (SBN) Sabra rats on erythrocyte Na+ content (Na+i), Ca2+ influx and cytosolic Ca2+ concentration ([Ca2+]i). The relationships of these parameters to plasma lipids, circulating digoxin-like immunoreactivity and membrane microviscosity, determined by the fluorescence anisotropy of trimethylamino-diphenylhexatriene (TMA-DPH) and diphenylhexatriene (DPH), were also evaluated. Erythrocytes of SBH rats were characterized by increased [Ca2+]i, unchanged Ca2+ influx and reduced Na+i. There were no significant differences in the plasma digoxin-like immunoreactivity between the two strains. High-salt intake decreased membrane microviscosity (DPH anisotropy) in SBH rats but did not alter the above parameters. Erythrocyte [Ca2+]i correlated positively with diastolic blood pressure and negatively with erythrocyte Na+i. Membrane dynamics evaluated by the two fluorescent probes did not correlate with [Ca2+]i, Ca2+ influx or Na+i whereas DPH anisotropy was inversely related to blood pressure. These relationships were independent of plasma cholesterol or triglycerides. It can be concluded that 1) similarly to earlier observations in essential hypertension and spontaneously hypertensive rats, erythrocyte [Ca2+]i correlates positively with blood pressure in salt-dependent hypertension, and 2) increased erythrocyte Na+ content need not be a hallmark of hypertension.

Erythrocyte Ca2+ handling in the spontaneously hypertensive rat, effect of vanadate ions

Cytosolic Ca2+ concentration ([Ca2+]i) and 45Ca2+ influx were investigated in erythrocytes from conscious spontaneously hypertensive rats (SHR) and their normotensive controls Wistar-Kyoto (WKY). [Ca2+]i was evaluated with fura-2 and intra- and extra-cellular calibration parameters were compared. Irrespective of the calibration parameters used, erythrocyte [Ca2+]i was always significantly higher in SHR than in WKY and Wistar rats (by 25 and 40%, p < 0.01 and 0.001). A rise of the external Ca2+ concentration from 1 to 2 mmol/l increased less [Ca2+]i in SHR than in WKY erythrocytes (17 vs 37%, p < 0.01). SHR erythrocytes incorporated more 45Ca2+ than those from WKY, with an initial rate of 45Ca2+ uptake higher by 57% than that of WKY erythrocytes (p < 0.05). Vanadate ions, after corrections of their quenching effect on red cell and fura-2 fluorescence signals, increased [Ca2+]i by 19% in WKY erythrocytes (p = 0.05), but did not modify the SHR values. They also increased 45Ca2+ accumulation and the initial rate of 45Ca2+ influx in WKY erythrocytes only (p < 0.01). This study indicates that, when compared to WKY rats, erythrocytes from SHR are characterized by higher [Ca2+]i values, higher initial rate of Ca2+ influx and low sensitivity to vanadate ions.

Solitary calcium spike dependent on calmodulin and plasma membrane Ca2+ pump

Resealed human red cell ghosts were loaded with Fura-2, ATP, Mg2+, and either calmodulin (CaM) or, to prevent CaM activation of the Ca2+ pump, a synthetic peptide that antagonized endogenous CaM (an analogue of the CaM binding domain of protein kinase II, referred to as 'antiCaM'). The ghosts reduced the cytosolic concentration of ionized calcium ([Ca2+]i) to 193 +/- 60 nM (SD, n = 15) in a medium containing 1 mM Ca2+ and to 30 +/- 27 nM (SD, n = 62) in a medium without Ca2+ addition. Without ATP, i.e. no fuelling of the Ca2+ pump, the [Ca2+]i remained high (approx. 5 microM or higher). The simultaneous addition of the ionophore A23187 and Ca2+ rapidly increased the Ca2+ influx, which in the CaM loaded ghosts caused a solitary spike of [Ca2+]i, reaching maximum around 2 microM within 24 +/- 6 s (SD, n = 40). On the contrary, in the ghosts loaded with antiCaM, the addition of A23187 with Ca2+ raised [Ca2+]i during the first 2 min to a high level (2-4 microM) with no preceding spike. Pre-incubation of CaM-ghosts with Ca2+ diminished the height of the Ca2+ spike, and treatment with trypsin even removed the Ca2+ spike. The trypsin treatment activated the Ca2+ pump prior to the rise of [Ca2+]i, making the time-consuming CaM activation unnecessary. In conclusion, the Ca2+ spiking is dependent on a delayed CaM activation of the plasma membrane Ca2+ pump in response to a rapid increase of Ca2+ influx.

Calmodulin activation of the Ca2+ pump revealed by fluorescent chelator dyes in human red blood cell ghosts

Ca2+ transport in red blood cell ghosts was monitored with fura2 or quin2 incorporated as the free acid during resealing. This is the first report of active transport monitored by the fluorescent intensity of the chelator dyes fura2 (5-50 microM) or quin2 (250 microM) in hemoglobin-depleted ghosts. Since there are no intracellular compartments in ghosts and the intracellular concentrations of all assay chelator substances including calmodulin (CaM), the dyes, and ATP could be set, the intracellular concentrations of free and total Ca [( Cafree]i and [Catotal]i) could be calculated during the transport. Ghosts prepared with or without CaM rapidly extruded Ca2+ to a steady-state concentration of 60-100 nM. A 10(4)-fold gradient for Ca2+ was routinely produced in medium containing 1 mM Ca2+. During active Ca2+ extrusion, d[Cafree]i/dt was a second order function of [Cafree]i and was independent of the dye concentration, whereas d[Catotal]i/dt increased as a first order function of both the [Cafree]i and the concentration of the Ca:dye complex. CaM (5 microM) increased d[Catotal]i/dt by 400% at 1 microM [Cafree]i, while d[Cafree]i/dt increased by only 25%. From a series of experiments we conclude that chelated forms of Ca2+ serve as substrates for the pump under permissive control of the [Cafree]i, and this dual effect may explain cooperativity. Free Ca2+ is extruded, and probably also Ca2+ bound to CaM or other chelators, while CaM and the chelators are retained in the cell.

Control of the erythrocyte free Ca2+ concentration in essential hypertension

Since Ca2+ ions seem to directly participate in the control of erythrocyte membrane structure and deformability and because cell Ca2+ metabolism has been repeatedly proposed to be modified in hypertension, the intracellular calcium ion concentration ([Ca2+]i) was investigated in red blood cells from hypertensive and normotensive subjects. [Ca2+]i was measured by using the fluorescent Ca2+ chelator fura-2. Red blood cell [Ca2+]i was increased in hypertensive compared with normotensive subjects in the whole population and further increased when hypertensive were compared with age-matched normotensive subjects. An inverse relation between age and [Ca2+]i was observed when calculated with blood pressure adjusted. In hypertensive patients, high [Ca2+]i values were associated with a reduced erythrocyte deformability. The initial rate of 45Ca2+ uptake did not differ between the two blood pressure groups. Similarly, when the extracellular Ca2+ concentration was elevated from 1 to 2 mmol/l, [Ca2+]i increased by 16 +/- 4% (p less than 0.03) in red blood cells from both groups, thus maintaining a significant difference between hypertensive and normotensive subjects. Under these conditions, the addition of 10(-7) mol/l nicardipine, a dihydropyridine Ca2+ antagonist, decreased [Ca2+]i by 15 +/- 4% (p less than 0.05) and 7 +/- 5% in erythrocytes from hypertensive and normotensive subjects, respectively, thereby reducing the difference in [Ca2+]i observed between these two groups. This nicardipine effect was positively correlated to the initial [Ca2+]i. In the presence of 5 mumol/l W7, a calmodulin antagonist, [Ca2+]i increased significantly only in erythrocytes from hypertensive patients (26 +/- 6%, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Simultaneous measurement of intracellular pH and Ca2+ using the fluorescence of SNARF-1 and fura-2

Upon cell stimulation with hormones and other mitogens, a variety of biochemical and physiological responses occur within the first few minutes. Changes in both intracellular pH (pHin) and intracellular Ca2+ concentration ([Ca2+]in) are prominent and play a major role in the signal transduction mechanism leading to the physiological response, i.e., secretion, neurotransmission, proliferation, or differentiation. However, it is not clear whether these ions work independently in the activation pathway leading to a particular physiological response. The fluorescence characteristics of most Ca2+ indicators are pH sensitive, and quantitative estimates of [Ca2+]in cannot be made without knowledge of pHin. Thus it is desirable to have a technique to simultaneously monitor these two ions with relatively high time resolution. Here we have developed experimental conditions that allow us to use optimum emission conditions for a pH fluorescent indicator SNARF-1 and optimum excitation conditions for the Ca2+ indicator fura-2. The fluorescence spectra of these compounds are sufficiently different to allow simultaneous measurement of pH and Ca2+ both in vitro and in situ. We have observed simultaneous changes in both pHin and [Ca2+]in in BALB/c 3T3 cells on treatment with the nonfluorescent Ca2+ ionophore 4-bromo-A23187. This temporal relationship between pHin and Ca2+ gives further credence to the interrelationship between these two second messengers in the expression of physiological responses.

Modulation by external Ca2+ and nicardipine of Ca2+ influx and cytosolic concentration in human erythrocytes

Variations of Ca2+ influx (evaluated by the initial rate of 45Ca2+ uptake) and cytosolic free Ca2+ concentration ([Ca2+]i, measured with fura-2) were investigated in human erythrocytes. When external Ca2+ concentration ([Ca2+]o) rose from 1 to 2 mM, the initial rate of Ca2+ influx nearly doubled whereas [Ca2+]i increased only by 15%. Nicardipine dose-dependently decreased both initial rate of Ca2+ influx and [Ca2+]i (up to 53 and 18%. respectively at 10(-6) M). The less marked changes in [Ca2+]i than in Ca2+ influx indicate a partial adjustment of the Ca2+ extruding-pump activity to of Ca2+ influx. In vivo administration of nicardipine reduced [Ca2+]i only when its initial value exceeded 80 nM and prevented the rise in [Ca2+]i induced by the increase in [Ca2+]o. Our results indicate that nicardipine may reduce Ca2+ influx in human erythrocytes and participate in the control of [Ca2+]i when elevated.

Measurement of intracellular calcium and pH in avian neural crest cells

1. Intracellular pH (pHi) and calcium (Cai2+) were studied in freely migrating neural crest cells and in closely packed non-migrating cells derived from avian neural tubes in vitro, using the fluorescent dyes 2,3-dicyanohydroquinone (DCH) and Indo-1 to measure pHi and Cai2+ respectively. 2. In freely migrating crest cells the pHi was approximately 0.2 pH units more alkaline and Cai2+ 90 nM lower than in closely packed cells. 3. Experiments to establish the cellular mechanisms regulating pHi in isolated neural crest cells demonstrate the presence of Na(+)-H+ exchange in 66% of the cells and Na(+)-HCO3(-)-dependent pHi-regulating mechanisms in all cells examined. 4. Interactions between pHi and Cai2+ were examined. pHi was altered using either NH4Cl pulses resulting in small changes in Cai2+ or using a weak acid and base (propionate and trimethylamine), which produced a fall and a rise in Cai2+ respectively. 5. Exposure to Ca2(+)-free media caused a lowering of Cai2+ and induced a transient acidification. 6. Application of BAPTA-AM (50 microM), a cell-permeant analogue of EGTA, resulted in a fall in Cai2+ and an intracellular acidification. 7. Co2+ and La3+ (2 mM) each induced a reversible fall in Cai2+ that was accompanied by intracellular acidification. These data suggest the presence of a transmembrane flux of Ca2+ in the resting cells. 8. It would appear that the mechanisms influencing Cai2+ and pHi are linked. This idea is discussed in terms of possible mechanisms and roles for Ca2+ and pH as modulators of neural crest cell behaviour.

Multiple sites of methyl esterification of calmodulin in intact human erythrocytes

Aspartyl and asparaginyl residues are susceptible to spontaneous chemical degradation reactions that result in the formation of isomerized and racemized aspartyl residues. At least a subset of these abnormal residues are recognized by a widely distributed protein D-aspartyl/L-isoaspartyl methyltransferase (EC 2.1.1.77) that can participate in their conversion to normal L-aspartyl residues. We have used this methyltransferase as a probe to identify modified aspartyl and asparaginyl residues in peptides and proteins. In purified calmodulin from bovine brain, major sites of methylation were found to originate from the Asp-2 residue near the amino terminus and the Asp-78 residue in the alpha-helix that connects the two globular calcium-binding domains. When purified calmodulin was incubated at physiological temperature and pH in the absence of calcium, additional methylation sites were found in three of the four calcium-binding sites. In this work we have analyzed the methyl esterification of human calmodulin catalyzed by this enzyme in intact erythrocytes. On the basis of results from peptide mapping studies, Asp-2, Asp-78/80, and residues in calcium-binding domains III and IV appear to be methylated. Methylation of sites in the calcium-binding regions appears to reflect the low concentration of free calcium in human erythrocytes. We also found that calmodulin isolated from erythrocytes and methylated in vitro contains major methylation sites at Asp-2 and Asp-78/80 but not in the calcium-binding sites. Comparison of the number of available methylation sites of calmodulin in intact cells and in material aged in vitro supports the hypothesis that repair processes can occur in erythrocytes.

Inositol trisphosphate induces calcium release from Neurospora crassa vacuoles

Inositol 1,4,5-trisphosphate is known to release calcium ions from intracellular stores thought to be parts of endoplasmic reticulum in animal cells. In Neurospora crassa, however, inositol 1,4,5-trisphosphate acts on vacuoles stimulating a calcium efflux with a Km of 5.28 microM. The calcium release is inhibited effectively by dantrolene. These results were obtained by applying two independent methods, measuring calcium binding to fura-2 and loading vacuoles with 45Ca.

Calcium content of the erythrocytes: a sensitive and easy handling method for measuring free calcium ions, and modulation of the Ca2+ ion concentration by the calcium antagonists nifedipine and pentoxifylline

1. A method for determining free Ca2+-ions in the erythrocyte is described, using a commercially available ORION-Ca-electrode and calomel reference electrode assembly, where changes in free Ca2+-ion concentration upon addition of 0.01% digitonin could be measured. 2. The average value found for fresh cells from 20 healthy donors at 37 degrees C (pH = 7.4) was 0.20 +/- 0.04 mumol/L referred to a haematocrit of 10%. 3. Decrease of the simultaneously determined adenosinetriphosphate (ATP) concentration indicates that ATP is presumably needed to activate the Ca-ATPase. 4. In vitro addition of the calcium antagonists pentoxifylline and nifedipine, respectively, induced a normalization of the intraerythrocytic Ca2+-ion concentration after previous increase with the ion carrier ionophore A23187. 5. The advantages and possible clinical applications of this method are discussed.