Studies on the cation permeability of human red cell ghosts. Characterization and biological significance of two membrane sites with high affinities for Ca

Single Ca(2+)-activated K+ channels in human erythrocytes: Ca2+ dependence of opening frequency but not of open lifetimes

Using the patch-clamp technique single-channel properties of Ca(2+)-activated K+ (CaK) channels were investigated in inside-out membrane patches of human erythrocytes. In a physiological K+ gradient (5 mM K+ externally: 150 mM K+ internally) the single CaK channel conductance is 15 pS in the membrane potential range of -40 to +40 mV. The channel open probability, opening frequency and open and closed time distributions are voltage-independent. The open probability and the opening frequency of the CaK channel depend on [Ca2+]i and increase between 0.5 and 60 microM Ca2+ from approx. 10% to 90% of the maximum value obtained at 115 microM. The relation between open probability and [Ca2+]i can be described by a sigmoid concentration-effect curve with an EC50 of 4.7 microM and a slope factor of 1. Independent of [Ca2+]i open time distributions yield two time constants of 5.3 and 22 ms. The relative amplitudes of the fast and slow components of the open time histogram as well as the maximum open probability and the maximum opening frequency of CaK channels vary considerably. In addition, CaK channels in multiple channel patches are highly interdependent. It is concluded that the Ca(2+)-dependence of CaK channels in human erythrocytes is due to the modulation of opening frequency by internal Ca2+. The results are consistent with a classical receptor-agonist model in which ligand interaction kinetics are much faster than channel gating.

Membrane sidedness and the interaction of H+ and K+ on Ca2(+)-activated K+ transport in human red blood cells

The sided effects of H+ on Ca2(+)-stimulated K+ transport (the Gardos channel) were studied in human red blood cells. Cells were loaded with Ca2+ during energy depletion with the internal pH adjusted to desired levels prior to treatment with the anion-exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which inhibits pH equilibration across the membrane. This treatment provides a "pH clamp" whereby the internal and external H+ (H+i and H+o) concentrations can be varied separately. Channel activity was evaluated by measuring either net K+ loss or unidirectional 42K+ efflux from cells where SO2(-4) replaced Cl- on both sides of the membrane. When pHi was set at 7.4, decreasing pHo from values of 8.0 to 5.0 inhibited K+ efflux. This effect of H+o could be overcome by increasing K+o at all values of pHo. In addition, this effect of K+o could be separated from its effects on altering the membrane potential, indicating an interaction between K+o and H+o on the channel. A similar interaction was shown to occur between H+i and K+i. K+o is known to be required for activation of Ca2(+)-stimulated K+ transport, since the channel in cells preincubated in the absence of K+o (prior to exposure to Ca+i) becomes refractory to subsequent activation by Ca2+i and K+o. We found that H+o would not substitute for K+o in this regard nor would H+o inhibit the protective effect of K+o; in addition, H+ was not transported inward in exchange for K+i. Thus it would appear that there are two external sites where K+o interacts with the channel. One site is antagonized by H+o, whereas the second site is required for channel activation independent of H+ in the range studied. The inside of the channel would have, by an analogous argument, at least one site where K+i and H+i interact.

Calcium binding capacity of erythrocyte membrane in human hypertension

The cell membrane calcium binding capacity of genetically hypertensive rats is reduced when measured in the presence of the submicromolar calcium concentrations proper of intracellular environment. The present work, performed as an ancillary study to an epidemiological survey on an entire population, aimed to investigate the existence of a similar abnormality in human hypertension. Calcium binding to the erythrocyte membrane was measured in clinically healthy normotensive (n = 12) and hypertensive individuals (n = 24). For this purpose, a filtration technique was used, based on the determination of 45Ca bound to the erythrocyte membrane in the presence of free calcium concentrations (40 nmol/l and 1 mumol/l), which are similar to those of the intracellular environment. The intra-assay technical error was determined on 35 duplicate samples and, when expressed as percent of the mean, was 24.1 at the 40 nmol/l concentration and 16.8 at the 1 mumol/l concentration. Membranes of untreated hypertensive patients, at both calcium concentrations, bound significantly less calcium than the control group. Treated and untreated hypertensive individuals had comparable values of membrane calcium binding capacity. Membranes of the treated hypertensive group bound less calcium than those of the normotensive group at both calcium concentrations, but the difference was statistically significant only in the presence of 40 nmol/l free calcium. A significant positive correlation was observed between the calcium binding capacity at 40 nmol/l concentration and that at 1 mumol/l in the treated and untreated hypertensive groups (r = 0.73 and 0.75, respectively; 0.51 for the normotensive group). These findings support the hypothesis that a cell membrane abnormality is detectable in some hypertensive patients.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Evidence for the involvement of calmodulin in the operation of Ca-activated K channels in mouse fibroblasts

The oscillation of membrane potential in fibroblastic L cells is known to result from periodic stimulation of Ca2+-activated K+ channels due to the oscillatory increase in the intracellular Ca2+ concentration. These repeated hyperpolarizations were inhibited by putative calmodulin antagonists, trifluoperazine (TFP), N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and promethazine (PMZ), and the concentrations required for half-maximal inhibition were 25, 30 and 300 microM, respectively. These doses were lower than those for reducing the membrane resistance due to nonspecific cell damages. Another calmodulin antagonist, chlorpromazine (CPZ), was also effective, but CPZ-sulfoxide was not. Intracellular pressure injections of calmodulin-interacting divalent cations, Ca2+, Sr2+, Mn2+ and Ni2+, elicited slow hyperpolarizations, whereas Mg2+ and Ba2+, which are known to be essentially inert for calmodulin, failed to evoke any responses. The injection of purified calmodulin also brought about a similar hyperpolarization. Quinine, an inhibitor of Ca2+-activated K+ channels, abolished both Ca2+- and calmodulin-induced hyperpolarizations. TFP prevented Ca2+-induced hyperpolarizations. The TFP effect was partially reversed by the calmodulin injection. It is concluded that calmodulin is involved in the operation of Ca2+-activated K+ channels in fibroblasts.

Effects of intracellular calcium on lens membrane permeability

The present investigation was designed to assess whether lens membrane permeability is affected by changes in levels of intracellular calcium. Lanthanum, an inhibitor of Ca-ATPase, affected an increase in the concentration of intracellular calcium (Cai) measured in cortical fiber cells. Preculture of lenses in lanthanum (1.0mM) caused an accumulation of 36Cl during subsequent culture at a rate three-fold higher than control lenses. Changes in calcium levels, however, were not responsible for the observed flux changes because a 40mV depolarization was observed to occur prior to a significant increase in calcium levels. The non-specific effects of lanthanum and other potential inhibitors of calcium transport were avoided by preculturing lenses in an ion-HEPES medium containing 20mM calcium chloride. In lenses with a six-fold increase in calcium levels there resulted only a 10% increase in 36Cl uptake over a 3 hr period. 86Rb efflux was also measured and the rate constant was unchanged compared to control lenses. Calcium accumulation did lead to a small (8mV) depolarization which may account for the small increase in chloride accumulation. By light microscopy, morphology of cortical lens fibers and the epithelium appeared unchanged in the calcium-loaded lens. The results provide little evidence that an increase in Cai leads to acute changes in lens membrane permeability.

The effects of terbium (III) on the Ca-activated, K channel found in the resealed human erythrocyte membrane

Incorporated terbium, Tb3+, activates the Ca-activated K channel found in the resealed erythrocyte ghost membrane and allows the net efflux of K. As in the case of Ca activation, low levels of external K stimulate the net efflux of K. The action of incorporated Tb3+ seems to be analogous to that of incorporated Ca. Externally applied Tb3+, however, inhibits the net efflux of K after either Ca or Tb3+ activation. The net-efflux of K can be inhibited by Tb3+ even after partial digestion of the channel by low levels of incorporated trypsin. Furthermore, the channel when incorporated into planar bilayers, can be inhibited by Tb3+. Externally applied Tb3+ does not seem to inhibit net K efflux indirectly via inhibition of the pathways for the co-transported anions. In addition because of the low concentrations of Tb3+ required for inhibition, it seems unlikely that an alteration of surface potential is responsible for the observed effects.

Modulation of Ca2+-mediated K+-gating of erythrocyte ghosts by external Ca-EGTA

Using 86Rb+ as a marker for K+ permeability, we find that extracellular Ca-EGTA influences the rate of 86Rb+ efflux from erythrocyte ghosts preloaded with 86Rb+ and "buffered" Ca2+. At an internal free Ca2+, where the rate of 86Rb+ efflux is minimal and uninfluenced by either external EGTA or external Ca2+, external Ca-EGTA at 0.2-0.5 mM can raise the flux rate to as high as can be attained by raising internal Ca2+, in the presence of an excess externally either of Ca2+ or of EGTA. Higher concentrations of Ca-EGTA (up to 1-2 mM) diminish the flux rate. External Ca-EDTA or Mg-EDTA can substitute for Ca-EGTA in enhancing and suppressing flux rate. The peak rate is insensitive to external free Ca2+ but depends on internal Ca2+; internal Mg-EDTA does not substitute for internal Ca-EGTA. Thus, the erythrocyte membrane is asymmetric with respect to its interaction with Ca2+ and Ca-EGTA. Also, 22Na+ does not substitute for 86Rb+. The peak rate of 86Rb+ flux produced by external Ca-EGTA is diminished by chlorpromazine (0.1 mM) and augmented by 1-propranolol (25 microM), in the same way as the rate produced by increasing internal Ca2+. The results suggest that external Ca-EGTA enhances the affinity of internal Ca2+ for its receptor(s) which operate the K+-gate at the inner surface of the membrane. At external concentrations of Ca-EGTA above 1-2 mM, 86Rb+ flux rate again rises with increase of Ca-EGTA. This phenomenon does not depend upon internal Ca2+, is not affected by chlorpromazine or by 1-propranolol, and is associated with an enhanced permeability to 22Na+, inulin, and haemoglobin.

The spontaneous activation of a potassium channel during the preparation of resealed human erythrocyte ghosts

Resealed erythrocyte ghosts prepared under conditions which deplete the cell of its endogenous chelators and metabolites are found to be selectively permeable to potassium. The net efflux of potassium is stimulated by low concentrations of external potassium and can be inhibited by oligomycin. The effect is not expressed when resealed ghosts are formed by hemolysis in the presence of chelators or magnesium. The spontaneously activated pathway is actually the calcium-activated potassium channel, first discovered by Gardos in 1958. In the intact cell, the combined actions of the calcium pump and endogenous chelators maintain the calcium concentration below the threshold for activation. Current observations indicate that the channel is spontaneously activated by traces of calcium originating from the cell itself or from the unavoidable background of calcium found in the media. The channel in ghosts depleted of endogenous chelators exhibits its high affinity for calcium. Channel activation occurs during hemolysis and persists throughout subsequent washings.

The influence of electrochemical gradients of Na+ and K+ upon the membrane binding and pore forming activity of the terminal complement proteins

The hemolytic activity of the terminal complement proteins (C5b-9) towards erythrocytes containing high potassium concentration has been reported to be dramatically increased when extracellular Na+ is substituted isotonically by K+ (Dalmasso, A.P., et al., 1975, J. Immunol. 115:63-68). This phenomenon was now further investigated using resealed human erythrocyte ghosts (ghosts), which can be maintained at a nonlytic osmotic steady state subsequent to C5b-9 binding: (1) The functional state of C5b-9-treated ghosts was studied from their ability to retain trapped [14C]-sucrose or [3H]-inulin when suspended either in the presence of Na+ or K+. A dramatic increase in the permeability of the ghost membrane to both nonelectrolytes - in the absence of significant hemoglobin release - was observed for C5b-9 assembly in the presence of external K+. (2) The physical binding of the individual 125I-labeled terminal complement proteins to ghost membranes was directly measured as a function of intra- and extracellular K+ and Na+. The uptake of 125I-C7, 125I-C8, and 125I-C9 into membrane C5b-9 was unaltered by substitution of Na+ by K+. (3) The binding of the terminal complement proteins to ghosts subjected to a transient membrane potential generated by the K+-ionophore valinomycin (in the presence of K+ concentration gradients) was measured. No significant change in membrane binding of any of the C5b-9 proteins was detected under the influence of both depolarizing and hyperpolarizing membrane potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca-induced K transport in human red blood cell ghosts containing arsenazo III. Transmembrane interactions of Na, K, and Ca and the relationship to the functioning Na-K pump

Increasing free intracellular Ca (Cai) from less than 0.1 microM to 10 microM by means of A23187 activated Ca-stimulated K transport and inhibited the Na-K pump in resealed human red cell ghosts. These ghosts contained 2 mM ATP, which was maintained by a regenerating system, and arsenazo III to measure Cai. Ca-stimulated K transport was activated 50% at 2-3 microM free Cai and the Na-K pump was inhibited 50% by 5-10 microM free Cai. Free Cai from 1 to 8 microM stimulated K efflux before it inhibited the Na-K pump, dissociating the effect of Ca on the two systems. 3 microM trifluoperazine inhibited Ca-stimulated K efflux and 0.5 mM quinidine reduced Na-K pumping by 50%. In other studies, incubating fresh intact cells in solutions containing Ca and 0.5 microM A23187 caused the cells to lose K heterogeneously. Under the same conditions, increasing A23187 to 10 microM initiated a homogeneous loss of K. In ATP-deficient ghosts containing Cai equilibrated with A23187, K transport was activated at the same free Cai as in the ghosts containing 2 mM ATP. Neither Cao nor the presence of an inward Ca gradient altered the effect of free Cai on the permeability to K. In these ghosts, transmembrane interactions of Na and K influenced the rate of Ca-stimulated K efflux independent of Na- and K-induced changes in free Cai or sensitivity to Cai. At constant free Cai, increasing Ko from 0.1 to 3 mM stimulated K efflux, whereas further increasing Ko inhibited it. Increasing Nai at constant Ki and free Cai markedly decreased the rate of efflux at 2 mM Ko, but had no effect when Ko was greater than or equal to 20 mM. These transmembrane interactions indicate that the mechanism underlying Ca-stimulated K transport is mediated. Since these interactions from either side of the membrane are independent of free Cai, activation of the transport mechanism by Cai must be at a site that is independent of those responsible for the interaction of Na and K. In the presence of A23187, this activating site is half-maximally stimulated by approximately 2 microM free Ca and is not influenced by the concentration of ATP. The partial inhibition of Ca-stimulated K efflux by trifluoperazine in ghosts containing ATP suggests that calmodulin could be involved in the activation of K transport by Cai.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-dependent 86 Rb efflux and ethanol intoxication: studies of human red blood cells and rodent brain synaptosomes

Effects of ethanol on calcium-dependent potassium efflux were investigated in red blood cells (RBC) from humans and brain synaptosomes from rats and mice. 86 Rb was used as a tracer for potassium. Synaptosomes and RBC were lysed and resealed with 86 Rb and calcium-EGTA buffers to regulate intracellular levels of ionized calcium. In vitro addition of ethanol (100 mM) stimulated the calcium-dependent 86 Rb efflux of synaptosomes. This stimulation was blocked by apamin, an inhibitor of the calcium-dependent potassium current of nerve cells. In addition, intracerebroventricular injection of apamin inhibited ethanol-induced narcosis in mice, providing behavioral evidence for the importance of calcium-stimulated potassium efflux in alcohol intoxication. In vitro addition of ethanol, propanol or butanol increased calcium-dependent 86 Rb efflux of human RBC at low concentrations of free calcium, but did not change the calcium-independent efflux of 86 Rb. These results suggest that the calcium-dependent 86 Rb efflux of nerve endings may have an important role in the pharmacological and toxicological effects of ethanol.

Effects of various cations on the slow K+ conductance increases induced by carbachol, histamine and dopamine in Aplysia neurones

A study was made of the effects of various cations other than K+ on three K+ conductance increases induced by carbachol, histamine and dopamine in an identified group of Aplysia neurones: the 'A' neurones of the cerebral ganglion. The 3 responses were sensitive to alterations of both the extracellular and the intracellular concentrations of Na+ and Ca2+. In particular, they could be reduced markedly by: (a) lowering [Na]0 (replacing NaCl by either Tris-HCl, glucosamine chloride, MgCl2 or sucrose); (b) increasing [Na]i (by intracellular injection of Na+, or by blockade of the Na+-K+ pump); (c) increasing the extracellular divalent cation concentration; or (d) increasing [Ca]i4. Some of the effects of Na+ and divalent cations appear to occur on reaction steps common to the three K+ responses, while others probably imply reaction steps specific to one of the systems, since they differ according to the agonist used. The sensitivity to Na+ and Ca2+ of slow inhibitory responses due entirely to an increase in K+ conductance must be taken into account in the interpretation of some slow hyperpolarizing responses previously assumed to involve changes in Na+ conductance.

Calcium ions, drug action and the red cell membrane

Intracellular calcium regulates a number of membrane functions in the erythrocyte, including control of shape, membrane lipid composition and cation permeability. Measurement of total red cell calcium has yielded values between 5 and 15 nmol/ml cells, and these low values in part reflect the absence of Ca2+ -containing organelles. Most intracellular Ca2+ is bound and the low cell ionized Ca2+ concentration (approximately 0.2 microM) is maintained by a combination of low membrane permeability and a powerful Ca2+ -pump. This pump has been identified with a (Ca2+ + Mg2+)-stimulated ATPase, and both Ca2+ transport and ATP splitting are stimulated by calmodulin, a low molecular weight protein which binds Ca2+ avidly and activates many Ca2+ -dependent enzymes. Both high and low affinity kinetics for Ca2+ pumping have been demonstrated, depending on the extent of binding of calmodulin to the pump. A stoichiometry of either 1 or 2 Ca2+ ions pumped per ATP molecule split has been shown, and the value may vary with the level of intracellular Ca2+. Phenothiazines, such as chlorpromazine inhibit the Ca2+ -pump by antagonizing the increment in activity produced by calmodulin. The passive inward leak of Ca2+ into erythrocytes can be quantitated by 45Ca2+ uptake into red cells whose Ca2+ -pump has been inhibited. Estimates of the Ca2+ permeability, based on unidirectional influx, yield values many orders of magnitude lower than for nucleated cells. Influx of Ca2+ into human erythrocytes occurs by a facilitated diffusion process, which can be inhibited by phenothiazines and the cinchona alkaloids. Calcium affects many membrane functions including cation permeability, lipid composition and some cytoskeletal interactions which may determine cell shape. Any rise in intracellular Ca2+ activates a specific K+ channel which normally makes little contribution to K+ fluxes. Kinetic studies of this process demonstrate either high or low affinity Ca2+ -activation of K+ efflux, with low affinity of the channel to Ca2+ being the probable state in vivo. Propranolol is the best known activator of Ca2+ -stimulated K+ efflux, although the mechanism of stimulation is unclear. Like other tissues, red cells possess a Ca2+ -activated phosphoinositol phosphodiesterase. Although it has been suggested that the echinocytic shape change induced by Ca2+ is due to the hydrolysis of polyphosphoinositides, it seems more likely that this shape change results from an effect of Ca2+ on the macromolecular interactions of the cytoskeleton. Abnormal Ca2+ permeability may contribute to red cell destruction in a variety of diseases. For example, in sickle cell anemia a large Ca2+ influx occurs when cells are sickled under deoxy conditions, and moreover, the ability of the Ca2+ -pump to extrude the increment of cell Ca2+ is impaired. Thus, red cell Ca2+ is increased 3-7-fold above normal and this may contribute to the short survival of sickle red cells...

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.

The influence of insulin, cAMP and the calcium ionophore X 537 A on the growth of the cartilage analagen of limb buds in vitro

Limb buds of 11-day-old mouse embryos were cultured for 6 days with insulin, dibutyryl cAMP and X 537 A. The cartilage anlage was reduced by insulin and enlarged by dibutyryl cAMP and X 537 A. The effects are due to changes in the amount of intercellular substance.

Unmasking of a potassium leak in resealed human red blood cell ghosts

A selective potassium leak is observed in resealed, human red blood cell ghosts when hemolysis is performed with distilled water at pH 6.5, 0 degrees C. The leak, which has a maximum near pH 6.7, is suppressed when either magnesium or a chelating agent is present in the hemolysing medium. The potassium leak has the additional property that it can be suppressed after resealing by washing the ghost membranes in a medium containing a low concentration of ATP or EDTA. The data suggest that through the dilution of endogenous chelating agents at hemolysis a potassium leak may be unmasked.

Ca2+ control of electrolyte permeability in plasma membrane vesicles from cat pancreas

The influence of Ca2+ and other cations on electrolyte permeability has been studied in isolated membrane vesicles from cat pancreas. Ca2+ in the micromolar to millimolar concentration range, as well as Mg2+, Sr2+, Mn2+ and La3+ at a tested concentration 10(-4) M, increased Na+ permeability when applied at the vesicle inside. When added to the vesicle outside, however, they decreased Na+ permeability. Ba2+ was effective from the outside but not from the vesicle inside. When Ca2+ was present at both sides of the membrane, Na+ efflux was not affected as compared to that in the absence of Ca2+. Monovalent cations such as Rb+, Cs+, K+, Tris+ and choline+ decreased Na+ permeability when present at the vesicle outside at a concentration range of 10 to 100 mM. Increasing Na+ concentrations from 10 to 100 mM at the vesicle inside increased Na+ permeability. The temperature dependence of Na+ efflux revealed that the activation energy increased in the lower temperature range (0 to 10 degrees C) when Ca2+ was present at the outside or at both sides, but not when present at the vesicle inside only or in the absence of Ca2+. The results suggest that the Ca2+ outside effect is due to binding of calcium to negatively charged phospholipids with a consequent reduction of both fluidity and Na+ permeability of the membrane. The Ca2+-inside effect most likely involves interaction with proteins with consequent increase in Na+ permeability. The data are consistent with current hypotheses on secretagogue-induced fluid secretion in acinar cells of the pancreas according to which secretagogues elicit NaCl and fluid secretion by liberating Ca2+ from cellular membranes and by stimulating Ca2+ influx into the cell. The increased intracellular Ca2+ concentration in turn increases the contraluminal Na+ permeability which leads to NaCl influx. The luminal sodium pump finally transports Na+ ions into the lumen.

Properties of (Mg2 + Ca2+)-ATPase of erythrocyte membranes prepared by different procedures: influence of Mg2+, Ca2+, ATP, and protein activator

Erythrocyte membranes prepared by three different procedures showed (Mg2+ + Ca2+)-ATPase activities differing in specific activity and in affinity for Ca2+. The (Mg2+ + Ca2+)-ATPase activity of the three preparations was stimulated to different extents by a Ca2+-dependent protein activator isolated from hemolysates. The Ca2+ affinity of the two most active preparations was decreased as the ATP concentration in the assay medium was increased. Lowering the ATP concentration from 2 mM to 2-200 microM or lowering the Mg:ATP ratio to less than one shifted the (Mg2+ + Ca2+)-ATPase activity in stepwise hemolysis membranes from mixed "high" and "low" affinity to a single high Ca2+ affinity. Membranes from which soluble proteins were extracted by EDTA (0.1 mM) in low ionic strength, or membranes prepared by the EDTA (1-10 mM) procedure, did not undergo the shift in the Ca2+ affinity with changes in ATP and MgCl2 concentrations. The EDTA-wash membranes were only weakly activated by the protein activator. It is suggested that the differences in properties of the (Mg2+ + Ca2+)-ATPase prepared by these three procedures reflect differences determined in part by the degree of association of the membrane with a soluble protein activator and changes in the state of the enzyme to a less activatable form.

Localization and role of calcium in the erythrocyte coat: effects of enzymes and storage

The effects of various treatments on erythrocyte shape, surface, cell coat and calcium binding sites have been investigated by means of high voltage electron microscopy (HVM), scanning electron microscopy (SEM) and conventional electron microscopy (TEM). Papain caused the formation of small blisters within the cellular surface as well as crenation and 'budding' of the erythrocytes. After neuraminidase treatment, long filaments were observed to radiate from the surface of the erythrocyte. The other enzymes investigated, RNA'se DNA'se, phospholipase, protease and trypsin, produced no demonstrable effect on the cellular structure, nor (with the possible exception of trypsin) on the cell coat as seen by subsequent staining with ruthenium red. Putative calcium binding sites on and in the erythrocyte membrane were demonstrated. Following incubation with radioactive calcium, activity was found in the erythrocyte membranes. Calcium binding could be reduced by prior treatment of the erythrocyte with EDTA, neuraminidase, and to a lesser extent, by papain and trypsin. Other enzymes had no demonstrable effect. Stored erythrocytes showed a progressive diminution in calcium binding over a period of up to 4 weeks.

Changes of intracellular Ca++ as measured by arsenazo III in relation to the K permeability of human erythrocyte ghosts

A Ca-sensitive dye, arsenazo III, has been incorporated into resealed human erythrocyte ghosts and calibrated to monitor continuously micromolar concentrations of intracellular ionized Ca ([Ca++]i). When the external concentration of Ca is much greater than [Ca++]i, [Ca++]i increases because of a net balance between Ca influx and efflux. Dynamic changes in [Ca++]i regulate K efflux, which in turn may influence the rate of Ca influx. A procedure for purifying arsenazo III is also described.