Equilibrium binding of calcium to fragmented human red cell membranes and its relation to calcium-mediated effects on cation permeability

Interaction of p-aminobenzoic acid with erythrocyte membrane: photoaffinity labeling of the binding sites

p-Aminobenzoic acid (PABA) was found to prevent eichinocytosis of red cells in vitro. Equilibrium binding studies with right-side-out membrane vesicles revealed a similar number of binding sites and Kd values for both normal and sickle cell membranes. A [14C]Azide analog of PABA was synthesized as a photoaffinity label to probe its sites of interaction on the erythrocyte membranes. Competitive binding studies of PABA with its azide indicated that both the compounds share common binding sites on the membrane surface. The azide was found to covalently incorporate into the membrane components upon irradiation; 52-35% of the label was associated with the proteins and the remaining with the lipids. Electrophoretic analysis of photolabeled membranes revealed that the azide interacts mainly with Band 3 protein in the case of intact erythrocytes and right-side-out sealed vesicles; however, if unsealed ghosts are used, other membrane proteins besides Band 3 are photolabeled. PABA was found to inhibit both high and low affinity calcium-binding sites situated on either surface of the membrane apparently in a non-competitive manner. However, calcium binding stimulated by magnesium and ATP was only slightly affected. Calcium transport into inside-out vesicles was inhibited by PABA, but it did not affect the calcium ATPase activity.

Intracellular calcium content of human erythrocytes: relation to sodium transport systems

To study the possible role of intracellular Ca (Cai) in controlling the activities of the Na+-K+ pump, the Na+-K+ cotransport and the Na+/Li+ exchange system of human erythrocytes, a method was developed to measure the amount of Ca embodied within the red cell. For complete removal of Ca associated with the outer aspect of the membrane, it proved to be essential to wash the cells in buffers containing less than 20 nM Ca. Ca was extracted by HClO4 in Teflon vessels boiled in acid to avoid Ca contaminations and quantitated by flameless atomic absorption. Cai of fresh human erythrocytes of apparently healthy donors ranged between 0.9 and 2.8 mumol/liter cells. The mean value found in females was significantly higher than in males. The interindividual different Ca contents remained constant over periods of more than one year. Sixty to 90% of Cai could be removed by incubation of the cells with A23187 and EGTA. The activities of the Na+-K+ pump, of Na+-K+ cotransport and Na+/Li+ exchange and the mean cellular hemoglobin content fell with rising Cai; the red cell Na+ and K+ contents rose with Cai. Ca depletion by A23187 plus EGTA as well as chelation of intracellular Ca2+ by quin-2 did not significantly enhance the transport rates. It is concluded that the large scatter of the values of Cai of normal human erythrocytes reported in the literature mainly results from a widely differing removal of Ca associated with the outer aspect of the membrane.

Guanine nucleotides reduce the free calcium requirement for secretion of granule constituents from permeabilized human neutrophils

Human neutrophils can be permeabilized with the cholesterol complexing agent digitonin and then induced to secrete lysosomal constituents by increases in free Ca2+ alone. In order of increasing requirements for Ca2+, vitamin B-12 binding protein, lysozyme and beta-glucuronidase were released. A variety of guanine nucleotides were examined with respect to their abilities to modulate this response. GTP, along with its analogues 5'-guanylylimidodiphosphate (Gpp[NH]p) and guanosine-5'-O-[3-thio]-triphosphate (GTP[gamma S]) decreased the Ca2+ requirements for secretion of all three granule constituents by one third to one order of magnitude. This synergy was dependent upon the concentration of guanine nucleotides employed. The effects of Gpp[NH]p could be blocked with the inactive derivative GDP[beta-S]. The active guanine nucleotides, particularly GTP, served as stimuli in their own right. At high concentrations of Ca2+ and GTP, degranulation was strikingly inhibited; inhibition was also achieved with high concentrations of guanylyl[beta, gamma-methylene]diphosphate (Gpp[CH2]p). Both GDP and GMP were without any effect. When neutrophils were pretreated with pertussis toxin, granule discharge induced by fMet-Leu-Phe was almost completely blocked, as reported by others. If the neutrophils pretreated with pertussis toxin were then permeabilized with digitonin, the synergy between Ca2+ and the stimulatory guanine nucleotides was maintained. These data suggest the involvement of G-proteins in secretion induced by Ca2+; however, this response either uses a different G-protein or a different pool of G-proteins from those responses triggered by fMet-Leu-Phe.

Trypsin-induced increase in cyclic AMP concentration in rat thymocytes. An effect independent of calcium and calmodulin

Trypsin produces a dose-related increase in cellular cyclic AMP concentration in rat thymocytes [Shneyour, Patt & Trainin (1976) J. Immunol. 117, 2143-2149; Segal & Ingbar (1983) Clin. Res. 31, 277A]. In the present study, I examined whether this effect of trypsin requires Ca2+ and whether it is modified by calmodulin. In fresh thymocytes suspended in standard medium (containing 1 mM-Ca2+), trypsin produced a concentration-dependent increase in cytoplasmic free Ca2+ concentration, which was evident at a concentration of 50 micrograms of trypsin/ml and reached maximal values at about 1 mg/ml. This effect of trypsin was very prompt in onset, almost immediate, and reached maximal values within 2-3 min. But in cells suspended in essentially Ca2+-free medium (6 nM free Ca2+), trypsin had no effect on cytoplasmic free Ca2+ concentration, which indicates that trypsin acted by increasing Ca2+ uptake rather than Ca2+ release from an intracellular pool. However, the increase in thymocyte cyclic AMP concentration produced by trypsin was independent of extracellular Ca2+ and was not influenced by calmodulin, because it was the same in the presence or absence of Ca2+ and was not changed by the calmodulin inhibitor trifluoperazine. I therefore suggest that in rat thymocytes the trypsin-induced increase in cyclic AMP concentration does not require Ca2+ and is not influenced by calmodulin.

Calcium-induced transient potassium efflux in human red blood cells

Human red blood cells pretreated with low-ionic-strength solutions and resuspended in saline respond biphasically to extracellular Ca. At first, addition of Ca causes a large transient K efflux of as much as 600 mM . liter cell H2O-1 . h-1; this is followed by a decrease in K flux below control levels. The first phase (phase I) resembles the Gardos effect in several respects. It is inhibited by oligomycin, by external K, and by increased exposure time to Ca. Further, the K permeability of phase I is similar to that of the Gardos effect (5 X 10(-8)-9 X 10(-8) cm/s), and the cells hyperpolarize in a low-K medium when Ca2+ is added. However, phase I is not identical to the Gardos phenomenon. For example, La, which prevents the Gardos response, is ineffective on phase I. Moreover, external Ba prevents the development of phase I but not the Gardos response, whereas internal Ba prevents the Gardos response. Attempts to demonstrate a Ca leak or pump failure during phase I have failed; passive Ca movements of both treated and normal cells are similar. The results suggest that low-ionic-strength solution exposes Ca-sensitive sites to the external medium; these sites are maintained when the cells are returned to saline.

A method for estimating free Ca within human red blood cells, with an application to the study of their Ca-dependent K permeability

Murphy, Coll, Rich and Williamson (J. Biol. Chem. 255:6600--6608, 1980) described a null-point method for estimating intracellular free Ca in liver cells. They used digitonin to lyse the cells in solutions of varying Ca concentration. This method has been adapted for use with human red cells. The values found are about 0.4 micron or micrometer Ca in fresh cells, and from 0.4 to 0.7 micron or micrometer Ca in blood-bank cells, at pH 7.2 and 37 degrees C. These are likely to be overestimates, and the errors and limitations of the method are discussed. Red cells may be loaded with Ca by metabolic depletion in Ca-containing solutions. Such cells have an elevated K permeability, and the relationships between free Ca, total Ca and K permeability were investigated, using 86Rb as a tracer for K. 86Rb flux studies show that the affinity of the K channel for Ca is the same in cells as in resealed ghosts where intracellular Ca can be controlled with Ca buffers, but the rate of tracer equilibration is 3-6 times faster in ghosts than in cells.

High-affinity calcium binding in rat synaptosomal plasma membranes: decrease in young, genetically hypertensive rats

A large number (about 4--5 nmol/mg of protein) of high-affinity (apparent dissociation constant at 37 degrees C: KD37 degrees C = 5 x 10(-8) M) calcium binding sites was characterized in synaptosomal membrane fractions enriched in plasma membranes that were isolated from rat brain. These sites were studied simultaneously in membranes from spontaneously hypertensive young rats (SHR) and their normotensive controls. No difference was observed between whole synaptosomes from these two substrains. However, plasma membrane-enriched fractions from SHR exhibited a reduced calcium binding capacity without a significant change in affinity. This decrease which averaged 15--20% was not due to any variation in the accessibility of calcium to its binding sites, as similar results were obtained in the presence of the calcium ionophore A23187. The reduction found in calcium binding is very similar to that previously described in erythrocyte membranes. It is envisaged that such an abnormality at nerve endings might play a role in the pathogenesis of genetic hypertension.

Effects of divalent cations, trypsin, and phospholipases on the passive permeability to sodium of inside-out vesicles from human red cells

Inside-out vesicles (IOV) were prepared from human red blood cells. Steady-state uptake of 23Na was observed to generally follow an exponential time course with a rate constant of 1.57 +/- 0.09 h-1 (SE). One week of cold storage (0-4 degrees C) increased the rate constant to 2.50 +/- 0.12 h-1 (SE). Mg2+, Ca2+, or Sr2+ decreased the rate of 22Na uptake with no observable differences between the three divalent cations when tested at concentrations of 50 microM. Mg2+ was shown to decrease the rate of 22Na uptake at concentrations as low as 5 microM with maximal effect at 50 to 100 microM. The decrease in rate of 22Na uptake induced by Mg2+ could be enhanced by exposure of IOV to Mg2+ for longer periods of time. Trypsin treatment of OIV increased the rate of uptake of 22Na and was dependent on the concentration of trypsin added between 5 to 25 micrograms/ml (treated for 5 min at 25 degrees C). The ability of Mg2+ (50 microM) to decrease the rate of 22Na uptake was still observed after maximal trypsin treatment. Phospholipase A2 or phospholipase C treatment of IOV increased the rate of 22Na uptake and was dependent on the amount of phospholipase A2 (0.1 to 1.0 units/ml) or phospholipase C (0.25 to 2.5 units/ml) added (treated for 5 min at 25 degrees C). After phospholipase A2 treatment, the observed decrease in the rate of 22Na uptake induced by Mg2+ (50 microM) was generally greater than controls. After phospholipase C treatment, the observed decrease in rate of 22Na uptake induced by Mg2+ (50 microM) was less or absent when compared with controls. Phospholipase C treatment was less effective in preventing the Mg2+ effect the longer IOV were exposed to Mg2+. The results suggest that Mg2+ binds to phospholipid headgroups to reduce Na permeability perhaps by inducing a change in bilayer structure or phospholipid association.

Influence of EGTA on the apparent Ca2+ affinity of Mg2+-dependent, Ca2+-stimulated ATPase in the human erythrocyte membrane

The apparent Ca2+ affinity of Mg2+-dependent, Ca2+-stimulated ATPase (Mg2+ + Ca2+)-ATPase) in human erythrocyte membranes increased with increasing concentrations of EGTA used to buffer free Ca2+. The shift in apparent Ca2+ affinity was seen in membranes prepared by hypotonic hemolysis and in membranes depleted of endogenous activators by EDTA treatment. The effect of EGTA differed from that of calmodulin, as it increased Ca2+ affinity without increasing V. EGTA also increased the apparent Ca2+ affinity when calmodulin was present in the assay medium. ATP-stimulated calcium binding to membranes was greater at 1 mM EGTA than at 0.1 mM EGTA. Similarly to ATPase activation, whereas binding decreased as Ca2+ was raised above 35 microM at 1.0 mM EGTA, binding progressively increased up to 100 microM or more free Ca2+ at 0.1 mM EGTA. EGTA also increased the Ca2+ affinity of Triton X-100-solubilized (Mg2+ + Ca2+)-ATPase, indicating that its effect did not depend on an intact membrane. Analysis of the kinetic data by a computerized nonlinear curve fitting procedure showed that a low Ca2+ affinity state of the enzyme was converted to a high Ca2+ affinity state in the presence of EGTA. The species associated with the enzyme interconversion appeared to be [CaEGTA]2-.

Analysis of calcium handling in erythrocyte membranes of genetically hypertensive rats

Calcium handling by erythrocyte membranes was compared in genetically hypertensive (SHR) and normotensive (WKR) rats by direct measurement of calcium binding, passive influx, and adenosine triphosphate (ATP)-dependent extrusion. The SHR erythrocyte membranes exhibited the following abnormalities: 1) the binding capacity of the high affinity Ca2+-binding sites located on the inner side of the membrane was 0.84 +/- 0.07 nmole/mg protein compared with 1.17 +/- 0.08 nmole/mg protein in WKR, 2) ATP-dependent Ca2+ extrusion, measured as the Ca2+ influx into inside-out vesicles, was also lower than the WKR, as was the La3+ -sensitive, Ca2+ -dependent hydrolysis, indicating reduced activity of the calcium pump; 3) the passive calcium influx into ATP-depleted red blood cells was slightly accelerated. these abnormalities in Ca2+ binding and transport probably enhanced intracellular Ca2+ concentration, and were observed under both prehypertensive an hypertensive conditions, in 3-week-old and adult SHR respectively. Similar membrane defects in excitable cells may help to explain the pathogenesis of hypertension, since they may increase vascular tone and/or catecholamine release.

Calcium binding alteration in plasma membrane from various tissues of spontaneously hypertensive rat

Calcium binding properties of plasma membrane enriched fractions from various tissues were studied in young spontaneously hypertensive rats (SHR) and their normotensive controls (WKY). In all tissues tested (heart, liver, nerve endings, erythrocytes), high affinity calcium binding sites were characterized. Their KD values were in the range of the cytosolic free calcium concentration : (10(-8)-10(-7)). Whatever the tissue, plasma membrane enriched fraction from SHR exhibited a reduced calcium binding capacity but no significant change in affinity. This decrease which averaged 15-25% was also observed in the presence of the calcium ionophore A23187 and thus does not result from a modified accessibility of calcium to its binding sites. It is suggested that this abnormality may be of genetic origin and is possibly implicated in the pathogenesis of hypertension.

Vanadium increases selective K+-permeability in human erythrocytes

In human erythrocytes that had been depleted of ATP by incubation with iodoacetate and adenosine, vanadate induces a 10-15-fold increase of K+-permeability. The effect is similar to that produced by calcium ions. Like the calcium-induced permeability change, the vanadate-induced effect is preceded by a lag period. Preincubation without substrates for ATP synthesis reduces the length of the lag period following the addition of either vanadate or calcium. The selective change of K+-permeability was brought about by vanadate anions (+5 oxidation state) as well as by vanadyl cations (+4 oxidation state). In both cases, the presence of EDTA prevented the permeability change. Blocking of the anion-transport system of the human erythrocytes by H2DIDS was used to discriminate between the unstable forms of vanadate anion and vanadyl cation in producing the potassium loss. The observation that H2DIDS had little if any effect on the efficiency and the previously reported fact by Cantley, L.C. and Aisen, Ph. (J. Biol. Chem., 254 (1979) 1781) that vanadate appears mostly as vanadyl in the cell interior suggests that, similar to Ca2+, Mg2+ or Pb2+, vanadyl (VO2+) can open the "potassium channel" in the erythrocyte membrane.

Does the conformation of Mg-ATPase (spectrin-dependent ATPase) influence the passive permeability to K+?

The processing of human erythrocytes disclosed changes in Mg-ATPase activity following action of Pb2+ and Nile blue, and changes of permeability of K+ after treatment with Nile blue. The obtained results and those from previous papers can be summarized as follows : Substances decreasing the activity of stimulated membrane Mg-ATPase (spectrin-dependent ATPase) in red blood cells increase the passive permeability to K+, and substances increasing the stimulated Mg-ATPase activity decrease the passive permeability to K+. A hypothesis is proposed that the conformation of Mg-ATPase is secondarily reflected in the state of the proper path for K+ transport through the membrane; thus the rate of passive permeability to K+ is influenced.