Active Calcium Transport and Ca2+-Activated ATPase in Human Red Cells

Erythrocyte oxidative damage in rat treated with CCl4

Owing to the presence of hemoglobin and polyunsaturated fatty acids, erythrocytes are a convenient model to understand membrane oxidative damage induced by various xenobiotic pro-oxidants. This study investigated the antioxidant activity of vanillin, a naturally occurring food-flavoring agent, against carbon tetrachloride (CCl4)-induced erythrocyte damages in Wistar albino rats. A single injection of CCl4 (1 ml/kg, intraperitoneally [i.p.]) caused a significant induction of oxidative damage as evidenced by increased thiobarbituric acid reactive substances, protein carbonyl levels and osmotic fragility accompanied with a significant decrease in Na+/K+-ATPase and Ca2+-ATPase activities. Furthermore, catalase and superoxide dismutase activities were significantly elevated, while glutathione levels, glutathione- S-transferase and glutathione peroxidase activities were markedly reduced in the erythrocytes of CCl4-treated rats. Pretreatment of rats with vanillin (150 mg/kg/day, i.p.) for 3 consecutive days before CCl4 injection protected erythrocytes against the increase of lipid peroxidation and degradation of membrane proteins compared to CCl4-treated rats and exhibited marked prevention against CCl4-induced oxidative stress, alterations of membrane-bound enzymes as well as erythrocyte osmotic fragility. Our results suggest that vanillin plays a protective and curative role against the harmful effects of CCl4 on erythrocytes, thus ensuring membrane cell integrity.

The haemolytic effect of verapamil on erythrocytes exposed to varying osmolarity

The haemolytic effect of verapamil on red blood cells (RBCs) exposed to varying osmolarity was investigated. The experimental approach used a modified red cell haemolysis assay with concentrations of verapamil ranging from 50-1500 microM compared to drug free controls. The time-course of haemolytic effects was also investigated. We also briefly determined the haemolytic effects of verapamil in Ca2+-free conditions (with added EGTA). In conditions representing decreasing osmolarity (dilution from 140-0 mM NaCl) there was a significant increase in erythrocyte haemolysis that was also dependent on verapamil concentration (ANOVA, p<0.05). The red cells also showed a significantly increased rate of haemolysis over 5 h with increasing verapamil concentration (ANOVA, p<0.05). The degree of RBC hypotonic haemolysis was significantly increased in a Ca2+-free medium (+EGTA) compared to normal saline and this effect was exacerbated by additions of verapamil (ANOVA, p<0.05). Overall the data suggested that verapamil can cause haemolysis of RBCs in a predictable time- and concentration-dependent manner, and that verapamil increases the fragility of the erythrocytes further during hypotonic osmotic stress and Ca2+-free conditions. The mechanism of verapamil-dependent haemolysis could be directly related to the observed biphasic concentration-effect and could consequently involve several ion transport pathways.

Transmembrane Ca2+ gradient is essential for high anion transport activity of human erythrocytes

The role of a transmembrane Ca2+ gradient in anion transport by Band 3 of human resealed erythrocyte ghosts has been studied. The results show that a transmembrane Ca2+ gradient is essential for the conformation of erythrocyte Band 3 with higher anion transport activity. The dissipation of the transmembrane Ca2+ gradient by the ionophore A23187 inhibits the anion transport activity. The extent of this inhibition approaches 90% as the Ca2+ concentration on both sides of the ghost membrane is increased to 1.0 mM and half-maximum inhibitions is observed at 0.25 mM Ca2+. Addition of ATP (0.4 mM) to the resealing medium can partly reestablish the transmembrane Ca2+ gradient by activation of Ca(2+)-ATPase and alleviate the inhibition to some extent. N-ethylmaleimide, an inhibitor of erythrocyte Ca(2+)-ATPase, prevents such restoration. Electron micrographs reveal that numerous larger intramembranous particle can be observed on the P-faces of freeze-fractured resealed ghosts in the absence of a transmembrane Ca2+ gradient.

The effect of verapamil and diltiazem on alveolar type II cells during warm and cold metabolic ischaemia

The alteration of cytosolic free calcium concentration is an important event during cellular ischaemia. Calcium channel blockers have been shown to be beneficial during experimental ischaemic organ protection. To investigate the mechanisms of this protection, the behaviour of type II pneumocyte cultures, subjected to warm and cold metabolic ischaemia (6 h), was studied. The cells were incubated in electrolytic solutions and treated with high doses of verapamil (10 mg/l) or diltiazem (100 mg/l). Alveolar type II epithelial cells were removed from adult rat lungs using the modified Dobbs' method. Cell viability was determined by analysis of the total protein content, and from the rate of protein synthesis as indicated by the [35S]methionine uptake assay. The results show that verapamil does not have a direct cytoprotective or cytotoxic effect on the incubated cells, but diltiazem seems to be toxic to the cells, especially during cold ischaemia when the toxicity is significant (P < 0.05). Thus, the protection from ischaemia previously attributed to calcium channel blockers is ascribed to action on the blood vessels resulting in vasodilatation, rather than to a direct influence on cytosolic free calcium homeostasis.

Assay of the Ca pump ATPase activity of intact red blood cells

An assay for the Ca pump ATPase of intact human red blood cells (RBCs) was developed. The assay utilized a small volume (typically 10 microliters) of packed RBCs in 1 ml of a buffer of known composition. The assay was based on the exposure of intact RBCs to the ionophore, A23187, in the presence of Ca. Such exposure caused a rapid degradation of ATP in RBCs. This degradation process is modeled in a numerical simulation in a companion paper (Vincenzi, F. F. and Hinds, T. R. (1992) Biochim. Biophys. Acta 1105, 63-70). The loss of ATP followed pseudo-first-order kinetics, and the rate constants for ATP degradation was taken as a measure of the capacity of the Ca pump ATPase. A number of variables were examined to optimize the activity of the ATPase. These variables included the concentrations of Ca and A23187. Because A23187 can promote loss of cellular Mg, it was necessary to include MgCl2 in the incubation medium to optimize ATPase activity. Likewise, it was determined that inclusion of iodoacetic acid optimized the rate of ATP loss, presumably by preventing the resynthesis of ATP from ADP and inorganic phosphate. Cobalt inhibited the ionophore-dependent loss of ATP by apparent competition with Ca for binding to A23187. Results of many assays demonstrated substantial differences in the rate constant for ATP loss in RBCs from different individuals. RBCs were selected according to density. Density associated loss of Ca pump ATPase activity was observed both by the intact RBC assay, and by assay of Ca pump ATPase activity in saponin lysates of RBCs. The correlation coefficient between the two assays was 0.93. It is suggested that the rate constant for ATP loss in intact RBCs exposed to A23187 and Ca can be taken as a measure of the Ca pump ATPase activity. This may be useful when isolated membrane ATPase assays fail (e.g., dog RBCs). The intact cell assay can also be carried out on very small volumes of cells and may be of particular value when RBC volumes are limited.

Energy metabolism during damaging contractile activity in isolated skeletal muscle: a 31P-NMR study

Creatine kinase (CK) release in response to excessive electrically stimulated contractile activity has been studied in isolated rat soleus muscles. The exacerbation of CK release induced by contractile activity was found to be directly related to the length of time for which the muscle was stimulated and indirectly related to the recovery of force following the end of stimulation. 31P-NMR studies were undertaken using a recirculating superfused muscle preparation and demonstrated that muscles subjected to two different stimulation protocols (stimulation for 0.5 s every 2 s in oxygenated medium or for 1.5 s every 2 s in anoxic medium) had similar falls in ATP content and pH despite a substantially greater release of CK from the muscles stimulated under anoxia. However, stimulated muscles under anoxia showed a more rapid fall and reduced recovery of phosphocreatine and a greater sustained elevation of inorganic phosphate than muscles in oxygenated medium. It is concluded that only part of the increased loss of CK from muscles stimulated in anoxic medium can be explained by release from cells which have lost energy supplies and therefore that other mechanisms must exist which allow release of CK and other cytosolic enzymes from muscle cells.

Calcium and pH in anoxic and toxic injury

The critical events that lead to the transition from reversible to irreversible injury remain unclear. Studies are reviewed that have suggested that a rise in cytosolic free Ca2+ initiates plasma membrane bleb formation and a sequence of events that leads ultimately to cell death. In recent studies, we have measured changes in cytosolic free Ca2+, mitochondrial membrane potential, cytosolic pH, and cell surface blebbing in relation to the onset of irreversible injury and cell death following anoxic and toxic injury to single hepatocytes utilizing multiparameter digitized video microscopy (MDVM). MDVM is an emerging new technology that permits single living cells to be labeled with multiple probes whose fluorescence is responsive to specific cellular parameters of interest. Fluorescence images specific for each probe are collected over time, and then digitized and stored. Image analysis and processing then permits quantitation of the spatial distribution of the various parameters within the single living cells. Our results indicate the following: (1) formation of plasma membrane blebs accompanies all types of injury in hepatocytes; (2) cell death is a rapid event, initiated by rupture of a plasma membrane bleb, and is coincident with the onset of irreversible injury; (3) an increase of cytosolic free Ca2+ is not the stimulus for bleb formation or the final common pathway leading to cell death; (4) a decrease of mitochondrial membrane potential precedes loss of cell viability; (5) cytosolic pH falls by more than 1 pH unit during chemical hypoxia. This acidosis protects against the onset of cell death.

Density increase and ageing of erythrocytes in stored blood

An increase in the density of erythrocytes was observed after storage of whole blood for 30 days at 4 degrees C in either acid citrate-dextrose or citrate-phosphate-dextrose-adrenaline. Glucose-6-phosphate dehydrogenase activity in unfractionated red blood cell lysates did not vary with the storage time. Enzyme activity in the lighter fraction separated by density gradient centrifugation was higher than that in heavier fractions. The decline in glucose-6-phosphate dehydrogenase activity with density was less marked after storage of whole blood for 30 days. It is suggested that density modifications are not related to the ageing of erythrocytes and additional mechanisms may be involved.

Characterization of a (Ca2+ + Mg2+)-ATPase system in the osteoblast plasma membrane

A high affinity, calmodulin-sensitive (Ca2 + Mg2+)-ATPase was demonstrated in the plasma membrane preparation of three different osteosarcoma cell lines previously demonstrated to respond to parathyroid hormone with an increase in cytosolic calcium and a decrease in pH. The maximal velocity of the enzyme activity in the membrane preparations ranged from 0.83 to 2.42 nmol Pi released per min per mg protein with half-saturation constants of 26 nM of free Ca. The enzyme activity was not affected by Na+, K+, ouabain and azide, and exhibited an absolute requirement for Mg2+ ions. These results suggest a possible role for a membrane Ca2 + Mg2+-ATPase in initiating and perpetuating the ionic control of osteoblastic function.

Reperfusion injury

Several lines of evidence point to a major role of oxygen free radicals in the pathogenesis of cell death or dysfunction in a variety of disease processes. Recent studies from this as well as other laboratories have demonstrated that oxygen free radicals play a major role in the pathogenesis of post-ischemic reperfusion injury in the heart. We have recently developed methods for direct measurement of radical species and/or specific byproducts of radical injury. Timely administration of oxygen radical scavengers reduced the quantity of free radicals generated following reperfusion and in addition improved recovery of post-ischemic ventricular function and metabolism. In a regionally ischemic model the free radical scavenger recombinant human superoxide dismutase also administered at the time of reflow was shown to limit infarct size. In this article we review the biophysical and molecular mechanisms of oxygen free radical generation that are viewed as contributing to post-ischemic reperfusion injury. We also discuss the mechanisms by which the body defends against free radical attack and the interrelationships of free radical injury to other mechanisms of tissue injury.

Erythrocyte membrane ATPase and calcium pumping activities in porcine malignant hyperthermia

To investigate possible abnormalities in erythrocyte membrane enzyme activities in the pharmacogenetic disorder MH, membrane ATPase activities have been examined in erythrocyte ghosts prepared from red blood cells of MHS and normal swine. While no differences were noted in Mg2+-ATPase activities, the (Na+, K+)-ATPase activity of MHS erythrocyte ghosts was less than that of normal ghosts. Ca2+-ATPase activity exhibited low- and high-affinity Ca2+-binding sites in both types of erythrocyte ghost. While the Km for Ca2+ was greater for normal than for MHS erythrocyte ghosts at the high-affinity Ca2+-binding site, the reverse was true at the low-affinity Ca2+-binding site. Irrespective of the type of calcium binding site occupied, the Vmax for normal erythrocyte ghost Ca2+-ATPase activity was greater than that for MHS ghosts. In the presence of calmodulin, there was now no difference between MHS and normal erythrocyte ghosts in either the Km for Ca2+ or the Vmax of the Ca2+-ATPase activity. To determine if the calcium pumping activity of intact MHS and normal pig erythrocytes differed, calcium efflux from the 45Ca-loaded erythrocytes was determined; this activity was significantly greater for MHS than for normal erythrocytes. Thus, the present study confirms that there are abnormalities in the membranes of MHS pig red blood cells. However, we conclude that these abnormalities are unlikely to result in an impaired ability of MHS erythrocytes to regulate their cytosolic Ca2+ concentration.

Clinical disorders of the red cell membrane skeleton

The lipid bilayer of the adult red cell is supported on its inner surface by a complex arrangement of proteins known as the membrane skeleton. This filamentous network, a major component of which is a multifunctional protein called spectrin, has an essential role in determining the shape, structural integrity, and deformability of the red cell. A significant achievement of modern biochemistry and hematology has been the elucidation of the organization of the components of the membrane skeleton and their relationship to other membrane proteins and lipids. This article reviews current concepts of membrane skeleton structure and function and emphasizes recent advances which have been made in characterizing and classifying molecular defects of the skeleton which manifest clinically with changes in the shape and stability of the red cell. The pathobiology of hereditary skeletal defects associated with hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and hereditary pyropoikilocytosis (HPP) are comprehensively discussed. Secondary defects of the membrane skeleton occurring in glucose-6-phosphate dehydrogenase deficiency and sickle cell anemia are also briefly considered.

Evidence that the platelet plasma membrane does not contain a (Ca2+ + Mg2+)-dependent ATPase

The present study was designed to determine the subcellular distribution of the platelet (Ca2+ + Mg2+)-ATPase. Human platelets were surface labeled by the periodate-boro[3H]hydride method. Plasma membrane vesicles were then isolated to a purity of approx. 90% by a procedure utilizing wheat germ agglutinin affinity chromatography. These membranes were found to be 2.6-fold enriched in surface glycoproteins compared to an unfractionated vesicle fraction and almost 7-fold enriched compared to intact platelets. In contrast, the isolated plasma membranes showed a decreased specific activity of the (Ca2+ + Mg2+)-ATPase compared to the unfractionated vesicle fraction. This decrease in specific activity was found to be similar to that of an endoplasmic reticulum marker, glucose-6-phosphatase, and to that of a platelet inner membrane marker, phospholipase A2. We conclude, therefore, that the (Ca2+ + Mg2+)-ATPase is not located in the platelet plasma membrane but is restricted to membranes of intracellular origin.

Abnormal calcium and magnesium stores in erythrocytes and granulocytes from patients with inflammatory connective tissue diseases. Relationship to inflammatory activity and effect of corticosteroid therapy

The mass fraction of Ca and Mg in isolated erythrocytes and granulocytes was measured using the nuclear microprobe technique. Conspicuous abnormalities were observed in cells from patients with rheumatoid arthritis and other inflammatory arthritides. Compared with the normal cellular content, total Ca was increased an average of 3 times in erythrocytes and 5 times in granulocytes. Total granulocyte Mg was increased about 3 times, whereas erythrocyte Mg was reduced to as much as 60% of normal. These abnormalities were less prominent or were absent in scleroderma patients, except for levels of granulocyte Ca, which were increased more than 3 times beyond normal in this patient group. A significant positive correlation was found between serum haptoglobin and erythrocyte or granulocyte Ca content among these patients, but not between haptoglobin and erythrocyte or granulocyte Mg values. During corticosteroid treatment, a significant increase in erythrocyte Mg and a significant reduction in erythrocyte Ca were noted, but normalization of these levels was not achieved. Granulocyte Ca was also significantly reduced, while granulocyte Mg remained unaltered. Serum levels of Ca and Mg were within normal ranges and were not influenced by corticosteroid therapy. The results indicate that at least Ca abnormalities in erythrocytes and granulocytes are associated with the intensity of the inflammatory process and that the amounts of Ca and Mg in these cells are influenced by potent antiinflammatory therapy.

Comparison of the membrane-bound (Ca2+ + Mg2+)-ATPase in erythrocyte ghosts from some mammalian species

The properties of the membrane-bound calcium-pumping protein, the (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) were compared in erythrocyte ghosts isolated from five mammalian species--human (Homo sapiens), bovine (Bos taurus), porcine (Sus scrofa melitensis), ovine (Ovis aries crassicandus) and caprine (Capra hircus syriaca). The specific activity of the enzyme in porcine erythrocytes is one order of magnitude higher than in the other species. It was also stimulated to various extents by the regulator protein, calmodulin, and by phosphatidylinositol in all the species. Analysis of membrane proteins revealed a number of differences which seem to suggest that the molecular architecture of the red cell membrane influences the activity of the enzyme.

Ca-ATPase activity in bovine parathyroid cells

Ca-ATPase is thought to function as a calcium extrusion pump that may regulate cytosolic calcium concentration. Because the parathyroid gland is among the few tissues that are directly regulated by extracellular calcium and because cytosolic calcium may be a mediator of the effects of extracellular calcium on parathyroid hormone secretion, we have investigated the presence of this enzyme in homogenates of parathyroid cells. High performance liquid chromatography (HPLC) was used to quantify the formation of ADP from ATP following incubation of ATP with cellular homogenate in a buffer containing ethylenedioxy- (diethylenedinitrilo) tetra acetic acid (EGTA), ouabain, and calcium. Enzyme activity was calcium-dependent, with Ca-ATPase showing two Km (Ca) values, 31 and 853 nM. High affinity Ca-ATPase activity was reduced by the calmodulin inhibitor, trifluoperazine (TFP), with half-maximal inhibition occurring at 7 X 10(-5) M. Monovalent cations stimulated high affinity Ca-ATPase activity (K+ greater than Na+ greater than Rb+ greater than Li+) in the presence of calcium. Magnesium (0.8 mM) also stimulated cleavage of ATP. Sodium increased Ca-dependent ATPase activity by 82% but had no significant effect on Mg-stimulated activity. Furthermore, azide, an inhibitor of mitochondrial ATPase(s), had a significantly greater inhibitory effect on Mg-dependent than on Ca-dependent activity. In summary, a high affinity Ca-ATPase is present in bovine parathyroid cells which has a Km in the range of the cytosolic calcium concentration that is found in other cells. Ca-ATPase(s) may be of importance in regulating the cytosolic calcium concentration and, therefore, hormonal secretion in this cell type.

A high affinity, calmodulin-responsive (Ca2+ + Mg2+)-ATPase in isolated bone cells

Although acute alterations in Ca2+ fluxes may mediate the skeletal responses to certain humoral agents, the processes subserving those fluxes are not well understood. We have sought evidence for Ca2+-dependent ATPase activity in isolated osteoblast-like cells maintained in primary culture. Two Ca2+-dependent ATPase components were found in a plasma membrane fraction: a high affinity component (half-saturation constant for Ca2+ of 280 nM, Vmax of 13.5 nmol/mg per min) and a low affinity component, which was in reality a divalent cation ATPase, since Mg2+ could replace Ca2+ without loss of activity. The high affinity component exhibited a pH optimum of 7.2 and required Mg2+ for full activity. It was unaffected by potassium or sodium chloride, ouabain or sodium azide, but was inhibited by lanthanum and by the calmodulin antagonist trifluoperazine. This component was prevalent in a subcellular fraction which was also enriched in 5'-nucleotidase and adenylate cyclase activities, suggesting the plasma membrane as its principal location. Osteosarcoma cells, known to resemble osteoblasts in their biological characteristics and responses to bone-seeking hormones, contained similar ATPase activities. Inclusion of purified calmodulin in the assay system caused small non-reproducible increases in the Ca2+-dependent ATPase activity of EGTA-washed membranes. Marked, consistent calmodulin stimulation was demonstrated in membranes exposed previously to trifluoperazine and then washed in trifluoperazine-free buffer. These results indicate the presence of a high affinity, calmodulin-sensitive Ca2+-dependent ATPase in osteoblast-like bone cells. As one determinant of Ca2+ fluxes in bone cells, this enzyme may participate in the hormonal regulation of bone cell function.

Calcium metabolism of the rabbit lens

The Ca concentration in the rabbit lens is less than that in its bathing aqueous and vitreous humors. This Ca is not distributed evenly in the tissue; it has a higher concentration in the outer peripheral than the inner parts of the organ. About 60% of the total lenticular Ca is present in a layer extending about 0.5 mm down from the outer surface (25% of the total tissue in the lens). When incubated in a solution containing 45Ca about 40% of the lenticular Ca readily exchanges with the isotope in 3 hr; no further change occurred over the next 21 hr. This exchange appeared to occur across either the anterior or posterior side of the lens. The accumulated 45Ca was found to be present in a surface layer about 1 mm deep. The remaining 60% of the lenticular Ca was difficult to displace. Incubation for 5 hr in a Ca-free solution containing EDTA or following freezing and thawing resulted in a loss of only an additional 20% of tissue Ca. The remaining 'immobilizable' Ca (40%) was present in both the nuclear and peripheral zones of the lens. When the Ca concentration of the external medium is raised, lens Ca rises also, and when the external concentration is reduced the tissue level declines. In the presence of metabolic inhibitors (iodoacetate plus cyanide) lenticular Ca rises. This effect is slow; little change was seen in 5 hr. Ca accumulation in the lens was enhanced by the Ca ionophore A23187 but an effect was not detectable in 5 hr. Incubation in Na-free media or ouabain did not influence Ca accumulation suggesting that a Ca-Na exchange is not involved in regulation. Quercetin and orthovanadate, which can inhibit Ca-activated ATPase in some tissues were without effect on lens calcium. The phenothiazine trifluoperazine (TFP), however, enhanced accumulation of Ca. Efflux of accumulated 45Ca from the lens was rapid; 60% in 10 min and 90% in 3 hr. Lanthanum reduced the latter to 60% loss. Efflux of Ca from the lens was unchanged, under experimental conditions, by metabolic inhibitors, by EGTA, verapamil, TFP and vanadate.

Studies of factors regulating the ageing of human erythrocytes--V. The role of vesiculation in the loss of membrane-bound sialic acid

1. Erythrocytes aged in vitro in protein-free media with and without leukocytes, release glycopeptides and vesicles. 2. Addition of soybean trypsin inhibitor (SbTI) blocks almost completely the release of glycopeptides but not the vesiculation. 3. Vesicles appear as a heterogeneous population with a diameter of 125 +/- 20 nm and with a lower content of N-acetylneuraminic acid (NAN) than ghosts of the original erythrocytes. 4. The NAN:phosphorus and NAN:cholesterol ratios in the vesicles are lower than in intact membranes. 5. The results indicate a redistribution of membrane components during in vitro erythrocyte incubation and the major importance of sialopeptides release in comparison with vesiculation.

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.

Effect of indomethacin on Ca2+-stimulated adenosine triphosphatase in the synaptic vesicles of rat brain in vitro

1. Indomethacin inhibits calcium-stimulated adenosine triphosphatase (Ca2+-ATPase), calcium, magnesium-stimulated adenosine triphosphatase (Ca2+,Mg2+-ATPase) and magnesium-stimulated adenosine triphosphatase (Mg2+-ATPase) activities in rat brain synaptic vesicles in vitro. 2. The Ca2+-ATPase activity is most strongly affected by this drug all of the activities of ATPases tested. 3. The decrease of Ca2+-ATPase activity by addition of indomethacin is due to a decrease of Vmax. 4. The Ki values for this drug for ATP and Ca2+ in Ca2+-ATPase were 1.13 mM and 0.68 mM, respectively.

Laser microirradiation of cerebellar neurons in culture. Electrophysiological and morphological effects

Electrophysiological and ultrastructural effects of focused laser radiation on neurons from neonatal rat cerebellum in tissue culture are reported. Action potentials were elicited by an extracellular current pulse train. The stimulator voltage required for half-maximum response frequency was measured as a function of the energy delivered by a single laser pulse. Above a "threshold" laser energy, the cell response to stimulation became negligible for all stimulator voltages. Electron micrographs of cells revealed that the mitochondria are preferentially damaged at an energy comparable to the electrophysiological threshold. The damaged mitochondria showed swollen matrix space and disrupted cristae membranes. Higher laser energies resulted in damage to other cytoplasmic structures. The results are consistent with a model that assumes that light interaction with the nerve cells proceeds by local heating of the mitochondria and nearby structures and leads to an increased conductance of the membrane to some ionic species.

Effect of pentoxifylline on single red cell deformability

In the present investigation the effect of pentoxifylline on single red cell deformability was determined. The deformability of human red blood cells were measured in a Singlepore Erythrocytes Rigidometer (SER). The new method determines optoelectronically the passage of each individual cell through a singlepore membrane (6 micrometers diameter and 50 micrometers length) under a driving pressure gradient (delta p = 100 Pa, tau = 3 Pa). Venous human blood samples were taken, prepared and divided into three samples. Sample A served as a physiological control, while to sample B and C CaCl2-solution was added. After Ca2+ stress sample C was incubated with 0.36 mmol/l pentoxifylline. The medium passage time (MPT) for sample A was 43.3 +/- 6.5 ms and for sample B 87.5 +/- 14.7 ms. The addition of pentoxifylline to the Ca2+-treated red cell suspension (sample C) reduced the MPT to 51.6 +/- 11.3 ms.

The effects of vanadate on calcium transport in dialyzed squid axons. Sidedness of vanadate-cation interactions

(1) Vanadate (VO3-) fully inhibits the ATP-dependent uncoupled Ca efflux (Ca pump) in dialyzed squid axons. (2) Vanadate inhibits with high affinity. The mean apparent affinity (K 1/2) obtained was 7 microM. (3) Inhibition by vanadate is dependent on Cao. External Ca lead to a release of the inhibitory effect. (K 1/2 congruent to 3 mM). This antagonistic effect can be reverted by increasing the vanadate concentration. Internal K+ increases the affinity of the intracellular vanadate binding site. External K+ has no effect on the inhibition. (4) Vanadate has no effect on the Nao-dependent Ca efflux component (forward Na-Ca exchange) in the absence of ATP. In axons containing ATP vanadate modified this component.

Activation and deactivation kinetics of Ca transport in inside-out erythrocyte membrane vesicles

The kinetics of active Ca2+ transport in inside-out vesicles of human erythrocyte membranes has been studied. Hemolysate or purified calmodulin increased the apparent affinity of the Ca2+ transport system for Ca2+ and increased the apparent maximum velocity of Ca2+ transport. However, as Ca2+ concentration was increased above 90 microM in the presence of hemolysate or calmodulin, the extent of activation of Ca2+ fluxes decreased. This deactivation was less prominent if the Mg2+ concentration was increased. These results may be explained by postulating that calmodulin has a site which binds either Mg2+ or Ca2+ and if Mg2+ occupies this site calmodulin activates Ca2+ fluxes, while if Ca2+ occupies this site, calmodulin is unable to activate the transport system.

Active Ca2+ transport by membrane vesicles from pigeon erythrocytes. Stimulation by amino acids, ATP, GTP, Pi and some other cell constituents

Pretreatment of pigeon erythrocyte membrane vesicles with amino acids, ATP, GTP, Pi and some other simple cell constituents (singly and in combination) causes an increase in ATP-dependent Ca2+-uptake activity of vesicles upon subsequent incubation with 45Ca2+ after removal of the above agents from the 'i' face. Amino acids augment the stimulation by all stimulatory agents and are required for stimulation by Pi. The effects of amino acids, ATP, GTP and Pi all occur at physiological concentrations. Many if not all of the effects of the mixture of amino acids that occur naturally in the cells can be accounted for by the group transported by the 'ASC' transport system of Christensen (Christensen, H.N. (1975) Biological Transport, 2nd edn., W.A. Benjamin, Inc., Reading, MA), but not by any single amino acid at its physiological concentration. The effects of ATP and GTP are not mimicked by their non-hydrolysable beta,gamma-imido analogues not by the corresponding 3',5'-cyclic monophosphates. None of the effects described appears to involve calmodulin. We suggest that amino acid transport plays a role in metabolic regulation through effects on cell [Ca2+]. Analogous effects on cell [Ca2+] may be involved in the action of the many hormones which augment amino acid accumulation by the 'A' amino acid transport system.

Human erythrocyte membranes exhibit a cooperative calmodulin-dependent Ca2+-ATPase of high calcium sensitivity

It is thought that the ionized Ca2+ concentration in the cytosol of healthy erythrocytes is in the range of 0.01--0.1 microM (ref. 1) and that this low concentration is maintained by an ATP-driven calmodulin-dependent Ca2+ pump in the plasma membrane. The Ca2+-stimulated ATPase which is the enzymatic expression of this pump varies in its calcium sensitivity between different preparations of erythrocyte ghosts, with activation generally occurring in a concentration range between approximately 1 and 10 microM Ca2+ (refs 2--5). This is a higher range of Ca2+ concentrations than might be anticipated for activation of a pump that sustains intracellular concentrations of Ca2+ below 0.1 microM, and recent reports have suggested activation in some membrane preparations at Ca2+ concentrations in the range 0.1--1.0 microM (refs 6, 7). We report here a simple method for preparing human erythrocyte membranes in 2.5 mM HEPES/1 mM EGTA at pH 7.0 (see Fig. 1 legend) in which the activation of the Ca2+-ATPase by Ca2+ and intracellular concentrations of calmodulin is highly cooperative and is complete by approximately 1 microM Ca2+. Unlike other available erythrocyte membrane preparations, the pattern of activation by Ca2+ and calmodulin is not complicated by partial resealing of the ghosts during and after isolation. We suggest that this cooperative activation of the Ca2+ pump may explain how healthy erythrocytes maintain their normal cytosol Ca2+ concentration at a threshold value at or below approximately 0.1 microM. We also note that several other calmodulin-dependent enzymes display similar cooperative activation kinetics.

Plasma membrane Ca2+ transport: antagonism by several potential inhibitors

Inside-out vesicles prepared from human red blood cells took up Ca2+ by an active transport process. Membranes from the same red blood cells displayed Ca2+-activated, Mg2+-dependent adenosine triphosphatase activity. Both the initial rate of Ca2+ transport and the (Ca2+ + Mg2+)-adenosine triphosphatase activity were increased approximately twofold by the calcium binding protein, calmodulin. Activities in the absence of added calmodulin were termed basal activities. Calmodulin-activated Ca2+ transport and adenosine triphosphatase activities could be antagonized in a relatively selective fashion by the phenothiazine tranquilizer drug, trifluoperazine. High concentrations of trifluoperazine also inhibited basal Ca2+ transport and adenosine triphosphatase activity. By contrast, calmodulin binding protein from beef brain selectively antagonized the effect of calmodulin on Ca2+ transport with no inhibition of basal activity. Ruthenium red antagonized calmodulin-activated and basal activity with equal potency. The results demonstrate that although phenothiazines can act as relatively selective antagonists of calmodulin-induced effects, other effects are possible and cannot be ignored. Calmodulin-binding protein may be a useful tool in the analysis of calmodulin functions. Ruthenium red probably interacts with Ca2+ pump adenosine triphosphatase at a site not related to calmodulin.

Laser action spectrum of reduced excitability in nerve cells

The change in excitability of unstained nerve cells from neonatal rat cerebellum was measured as a function of the energy flux and wavelength of incident laser light. The energy flux was in the range of 0 to 100 microJ/sq. microns. 6 wavelengths between 490 and 685 nm were used. Laser pulses above a threshold energy flux significantly reduced the cells' excitability as measured by extracellular stimulation. The sensitivity of the cells, defined as the inverse of the threshold energy density, increased by an order of magnitude toward the shorter wavelengths. These results are consistent with primary absorption of the light by mitochondrial enzymes, resulting in local heating followed by mitochondrial calcium release into the cytoplasm.