Studies on the mechanism of regulation of the red-cell Ca2+ pump by calmodulin and ATP

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Elevated intracellular Ca2+ reveals a functional membrane nucleotide pool in intact human red blood cells

Elevated intracellular calcium generates rapid, profound, and irreversible changes in the nucleotide metabolism of human red blood cells (RBCs), triggered by the adenosine triphosphatase (ATPase) activity of the powerful plasma membrane calcium pump (PMCA). In the absence of glycolytic substrates, Ca(2+)-induced nucleotide changes are thought to be determined by the interaction between PMCA ATPase, adenylate kinase, and AMP-deaminase enzymes, but the extent to which this three-enzyme system can account for the Ca(2+)-induced effects has not been investigated in detail before. Such a study requires the formulation of a model incorporating the known kinetics of the three-enzyme system and a direct comparison between its predictions and precise measurements of the Ca(2+)-induced nucleotide changes, a precision not available from earlier studies. Using state-of-the-art high-performance liquid chromatography, we measured the changes in the RBC contents of ATP, ADP, AMP, and IMP during the first 35 min after ionophore-induced pump-saturating Ca(2+) loads in the absence of glycolytic substrates. Comparison between measured and model-predicted changes revealed that for good fits it was necessary to assume mean ATPase V(max) values much higher than those ever measured by PMCA-mediated Ca(2+) extrusion. These results suggest that the local nucleotide concentrations generated by ATPase activity at the inner membrane surface differed substantially from those measured in bulk cell extracts, supporting previous evidence for the existence of a submembrane microdomain with a distinct nucleotide metabolism.

Ca-ATPase of human syncytiotrophoblast basal plasma membranes

In the present work, a Mg(2+)-dependent, Ca(2+)-stimulated ATPase activity was determined and characterized in purified preparations of syncytiotrophoblast basal (fetal facing) plasma membranes, and its characteristics were compared to those of the active Ca(2+)-transport already demonstrated in this tissue. Similar to the active Ca(2+)transport, the Ca-ATPase is Mg(2+)-dependent, is stimulated by calmodulin, and is inhibited by vanadate. The K(m) for Ca(2+)activation is 0.25+/- 0.02microM, a value near to that described for calcium active transport in this tissue. Consequently, the Ca-ATPase activity of human syncytiotrophoblast basal plasma membrane described in this paper could be responsible for the active extrusion of calcium from the syncytiotrophoblast toward the fetal circulation.

The plasma membrane calcium pump--a physiological perspective on its regulation

This review focuses on the physiological role of the plasma membrane Ca(2+)+ Mg(2+)-dependent adenosine triphosphatase (PM Ca(2+)-ATPase) in cellular signalling. Particular attention has been paid to the regulation of the PM Ca(2+)-ATPase (PM Ca2+ pump) by calmodulin, proteases, protein kinases, acidic phospholipids and oligomerization in intact cells. We also review recent work investigating the possible regulation of the PM Ca2+ pump by G proteins and agonists. The source of adenosine triphosphate (ATP) and Ca2+ in fueling and activating the Ca2+ pump is discussed, as well as the possible role of the PM Ca(2+)-ATPase in subplasma membrane Ca2+ regulation. The physiological implication of the localisation of the PM Ca2+ pump in caveolae is also considered.

Inhibition of erythrocyte membrane (Ca2+ + Mg2+)-ATPase by the organophosphorus insecticides parathion and methylparathion

Organophosphorus insecticides parathion and methylparathion non-competitively inhibited the activity of (Ca2+ + Mg2+)-ATPase bound to and solubilized from pig erythrocyte membrane. Both enzyme preparations exhibited biphasic substrate curves displaying the existence of two functional active sites with low and high affinity to ATP. Also, the relationship between the activity of bound enzyme and Ca2+ concentration was biphasic. The activity reached maximum at 20 microM then dropped progressively as the Ca2+ concentration was raised. The inhibition of the activity was more pronounced for parathion than for methylparathion and the solubilized enzyme preparation was more affected than the bound one. The inhibition constants (Ki) for parathion for bound enzyme were 55 and 158 microM for high- and low-affinity active sites, respectively; for methylparathion these values equalled 74 and 263 microM, respectively. Ki values for parathion were 36 and 118 microM for solubilized enzyme (high- and low-affinity sites, respectively), for methylparathion -62 and 166 microM, respectively. The magnitude of the effect was greater for a low Ca2+ concentration, which could arise from different conformational states of the enzyme at different calcium concentrations. The results of the experiment suggest that the insecticides inhibited the ATPase by binding to a site on the enzyme rather than by the interaction with associated lipids, although lipids could weaken the action of the compounds due to the strong affinity of organophosphorus insecticides to lipids.

Calcium pump in the disk membranes isolated from bovine retinal rod outer segments

The existence of a Ca2+ pump in rod outer segment disks of bovine retina is strongly suggested by the isolation on sodium dodecyl sulfate polyacrylamide gel electrophoresis of a hydroxylamine-sensitive phosphorylated intermediate (E-P) of molecular mass of about 100 kDa as well as by measurements of active calcium transport and adenosine 5'-triphosphate (ATP) hydrolysis. Active Ca2+ uptake by disks was dependent on the presence of Mg(2+)-ATP, was inhibited by vanadate or lanthanum and appeared poorly sensitive to calmodulin. ATP hydrolysis by disk membranes was a function of free Ca2+ concentration in the absence of exogenous Mg2+. The presence of a Ca2+ pump on disk membranes is discussed in terms of its possible role in Ca2+ ion buffering during photoreceptor cell functioning.

Inhibition of the calcium pump by high cytosolic Ca2+ in intact human red blood cells

1. The inhibitory effect of high intracellular calcium on the saturated Ca2+ efflux through the Ca2+ pump (Vmax) was investigated in intact human red cells. Cells were loaded with Ca2+ by exposure to the calcium ionophore A23187, at different external Ca2+ concentrations ([Ca2+]o). Ca2+ extrusion by the pump was followed after either ionophore removal or Co2+ addition. 2. fifty per cent inhibition of Vmax was obtained with total intracellular calcium ([CaT]i) of approximately 3 mmol/l cells. For any given initial Ca2+ load, Vmax showed no tendency to increase as [CaT]i was progressively reduced during Ca2+ efflux. This suggests that the pump Vmax was determined by the magnitude of the initial [Ca2+]i. 3. To estimate [Ca2+]i from [CaT]i in Co(2+)-loaded cells, the possible competition between Co2+ and Ca2+ for the known cytoplasmic Ca2+ buffers (alpha-buffers) was investigated first. Comparison between Ca2+ efflux after either Co2+ exposure or ionophore wash-out showed that the efflux patterns were essentially identical, down to the lowest measurable [CaT]i. This indicates that Co2+ does not compete with Ca2+ for the alpha-buffers. Hence, since [Ca2+]i = alpha [CaT]i, and alpha approximately 0.15-0.35, the initial [Ca2+]i load for 50% Vmax inhibition was between 0.4 and 1.1 mM. 4. Ancillary new findings demonstrated that, unlike the situation with alpha-buffers, Co2+ displaced Ca2+ from the cell-incorporated calcium chelator benz-2, and that benz-2 incorporation had no effect on Co(2+)-exposed Ca2+ pump desaturation. This validates the use of benz-2 to study Ca2+ pump kinetics in intact cells.

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.

The [Ca2+ + Mg2+]-dependent adenosine triphosphatase of SV40 transformed WI38 lung fibroblasts

The Ca2+-stimulated, Mg2+-dependent ATPase of SV40 transformed WI38 lung fibroblast homogenates exhibits a high affinity for Ca2+ (K0.5 = 0.20 microM) and moderately high affinity for ATP (Km = 28.6 microM) and Mg2+ (K0.5 = 138.5 microM). This activity was NaN3, KCN and oligomycin insensitive but very sensitive to vanadate (I50 = 0.5 microM) suggesting its being neither mitochondrial or microsomal but plasma membrane in origin. Under optimal conditions of protein, hydrogen ion and substrate concentration, 16-19 nmoles phosphate was released per min per mg protein. Hill plot analysis indicated no cooperativity to occur between Ca2+ binding sites. Nucleotides other than ATP and dATP were ineffective as substrates. The trivalent cation, lanthanum (La3+) completely inhibited hydrolysis of ATP at approximately 70 microM (I50 = 25 microM). Calmodulin antagonists trifluoperazine and calmidazolium inhibited ATP hydrolysis in a dose dependent fashion.

Activation of the endoplasmic reticulum Ca2+ pump of pancreatic acini by Ca2+ mobilizing hormones

Stimulation of the pancreatic acinar cells with Ca2+ mobilizing hormones increased the ATP-dependent Ca2+ uptake into the ER of permeabilized cells. Activation of the ER Ca2+ pump resulted in increased apparent affinity for Ca2+ from 0.26 to 0.09 uM and Vmax from 2.68 to 5.74 nmoles/mg prot./min. The apparent affinity of the pump for VO4 = was dependent on [Ca2+]. Activation of the pump also decreased apparent affinity for VO4 = from 12 to 32 uM at [Ca2+] of 0.138 uM. These findings suggest that pump activation is due to acceleration of the rate of the conformational transition between the VO4 = (E2) and Ca2+ (E1) sensitive forms of the pump.

Glucose and erythrocyte ATP: distinctive effects of dipyridamole and of ticlopidine

This experiment suggests the following points: 1. Erythrocytes in control patients and in atherosclerosis patients seem to have a variable grade of affinity for adenosine and for plasma glucose. This variable grade seems to fix the level of the adenosine triphosphate (ATP) reserves and induces the erythrocytes' deformability. 2. The drop in the level of ATP reserves that induces the poor deformability of the erythrocytes in atherosclerosis patients would appear to be caused by two consecutive shortages: first a shortage that seems to be related to a deficiency of erythrocyte adenosine as the ATP shortage disappears with dipyridamole treatment and then a shortage induced by the lack of erythrocyte glucose and suppressed by the addition of ticlopidine to the dipyridamole treatment.

Effects of Ca2+, Mg2+ and calmodulin on the formation and decomposition of the phosphorylated intermediate of the erythrocyte Ca2+-stimulated ATPase

Formation of the phosphorylated intermediate (ECaP) of the human erythrocyte Ca2+-stimulated ATPase (Ca2+-ATPase) was more rapid and reached steady state sooner at 400 microM-Ca2+ than at 1 microM-Ca2+. Calmodulin increased the apparent rate of ECaP formation at 1 microM-Ca2+, whereas at 400 microM-Ca2+, calmodulin decreased the steady-state level of the ECaP without affecting its apparent rate of formation. Removal of endogenous Mg2+ with trans-1,2-diaminocyclohexane-NNN'N'-tetra-acetic acid, which decreased both the velocity and Ca2+-sensitivity of the Ca2+-ATPase, did not alter the Ca2+-sensitivity or the apparent rate of formation of ECaP. ECaP formation at high Ca2+ concentrations was not affected by Mg2+ concentrations as high as 1 mM, and the ECaP could be dephosphorylated by ADP and ATP along either the forward or reverse pathways. The results suggest that high Ca2+ concentrations inhibit Ca2+-ATPase activity by preventing dephosphorylation of the E2P complex, rather than by inhibition of the transformation from E1CaP ('high-Ca2+-affinity' ECaP) to E2CaP ('lower-energy' ECaP).

Fast reversal of the initial reaction steps of the plasma membrane (Ca2+ + Mg2+)-ATPase

Calmodulin-depleted red cell membranes catalyse a Ca2+, Mg2+-dependent ATP-[3H]ADP exchange at 37 degrees C. The Ca2+, Mg2+-dependent exchange, measured at 20 microM CaCl2, 1.5 mM MgCl2, 1.5 mM ADP and 1.5 mM ATP, is comparable to the (Ca2+ + Mg2+)-ATPase activity, between 0.3 and 0.8 mmol/litre original cells per h. EDTA-washed membranes present a Ca2+-dependent ATP-ADP exchange whose rate is not more than 7% of that found in a Mg2+-containing medium, while their Ca2+-dependent ATPase is essentially zero. Addition of 1.5 mM MgCl2 to the medium restores both activities to the levels found with membranes not treated with EDTA. Calmodulin (16 micrograms/ml) produces an eight-fold stimulation of the Ca2+-dependent ATP-ADP exchange, slightly less than it stimulates the Ca2+-dependent ATP hydrolysis. The effect of 1.5 mM MgCl2 on the exchange is greater in the presence than in the absence of calmodulin. It is proposed that the reversal of the initial phosphorylation of the Ca2+ pump, occurring at a fast rate at 37 degrees C, involves a conformational change in the phosphoenzyme. Thus, it would be an ADP-liganded phosphoenzyme of the form EP(ADP) that would experience the fast conformational transition at 37 degrees C. The great difficulty in producing an overall reversal of the Ca2+ pump should then be due to one or more reaction steps later than and including Ca2+ release and dephosphorylation.

Mechanism of the stimulation of cardiac sarcoplasmic reticulum calcium pump by calmodulin

Calmodulin has been shown to stimulate the initial rates of Ca2+-uptake and Ca2+-ATPase in cardiac sarcoplasmic reticulum, when it is present in the reaction assay media for these activities. To determine whether the stimulatory effect of calmodulin is mediated directly through its interaction with the Ca2+-ATPase, or indirectly through phosphorylation of phospholamban by an endogenous protein kinase, two approaches were taken in the present study. In the first approach, the effects of calmodulin were studied on a Ca2+-ATPase preparation, isolated from cardiac sarcoplasmic reticulum, which was essentially free of phospholamban. The enzyme was preincubated with various concentrations of calmodulin at 0 degrees C and 37 degrees C, but there was no effect on the Ca2+-ATPase activity assayed over a wide range of [Ca2+] (0.1-10 microM). In the second approach, cardiac sarcoplasmic reticulum vesicles were prephosphorylated by an endogenous protein kinase in the presence of calmodulin. Phosphorylation occurred predominantly on phospholamban, an oligomeric proteolipid. The sarcoplasmic reticulum vesicles were washed prior to assaying for Ca2+ uptake and Ca2+-ATPase activity in order to remove the added calmodulin. Phosphorylation of phospholamban enhanced the initial rates of Ca2+-uptake and Ca2+-ATPase, and this stimulation was associated with an increase in the affinity of the Ca2+-pump for calcium. The EC50 values for calcium activation of Ca2+-uptake and Ca2+-ATPase were 0.96 +/- 0.03 microM and 0.96 +/- 0.1 microM calcium by control vesicles, respectively. Phosphorylation decreased these values to 0.64 +/- 0.12 microM calcium for Ca2+-uptake and 0.62 +/- 0.11 microM calcium for Ca2+-ATPase. The stimulatory effect was associated with increases in the apparent initial rates of formation and decomposition of the phosphorylated intermediate of the Ca2+-ATPase. These findings suggest that calmodulin regulates cardiac sarcoplasmic reticulum function by protein kinase-mediated phosphorylation of phospholamban.

The effect of Mg2+ and calmidazolium on the calmodulin binding to inside-out vesicles of the human red cell membrane

Calmodulin binds to inside-out vesicles of the human red cell membrane in a Ca2+-dependent manner. Mg2+ was found to be essential to this binding, half maximum binding occurring at decreasing Ca2+ concentration with increasing Mg2+ concentration. The calmodulin antagonist calmidazolium strongly reduced the Ca2+-dependent binding of calmodulin to the inside-out vesicles.

Platelet hyperaggregability, blood prostacyclin and dipyridamole

This study demonstrates the presence of PGI2 in blood and its influence on platelet retention tests, possibly by the intermediate of a releasing system in the columns, which is followed by a proximate recuperation on the erythrocyte sites after the passage. The presence of prostacyclin on the erythrocyte sites seems to depend upon the red cell deformability in relation to the good condition of their erythrocyte ATP reserve. The load of the erythrocyte sites increases with the daily dose of dipyridamole. The maximum load of the sites appears to be reached with a daily dose of dipyridamole 450 mg. Approximately 10% of the atherosclerosis patients who have been treated by dipyridamole keep their platelet hyperaggregability and their abnormally lowered prostacyclin level.

The nature of the Ca2+-pump defect in the red blood cells of patients with cystic fibrosis

The reduction in (Ca2+ + Mg2+)-ATPase activity in the cystic fibrosis red blood cells can be attributed to a reduction in the number of active Ca2+ pumps per red blood cell and an altered interaction of calcium ions with the pump. Despite this, the normal free intracellular [Ca2+] is preserved due to a lower rate of passive calcium entry.

Cooperative calcium binding and calmodulin regulation in the calcium-dependent adenosine triphosphatase purified from the erythrocyte membrane

Calcium binding to calcium-dependent ATPase purified from erythrocyte membrane was assessed by measurements of the ATPase intrinsic fluorescence. Calcium-binding isotherms obtained by fluorescence titration are identical to curves representing the Ca2+-concentration dependence of ATPase activity, and demonstrate that cooperativity is in fact a feature of the binding mechanism rather than an apparent effect of enzyme kinetics. Loss of cooperativity and a reduction of the ATPase affinity for calcium is observed at very low enzyme concentrations. This effect of enzyme dilution is prevented by calmodulin at 37 degrees C but not at 25 degrees C. It is suggested that calcium binding by erythrocyte-membrane ATPase is influenced by hydrophobic interactions of binding domains, exhibiting a dissociation constant between 10(-7) and 10(-8) M in the absence of calmodulin, at 37 degrees C and in a specific set of experimental conditions. The dissociation constant is decreased by calmodulin.

Two modes of inhibition of the Ca2+ pump in red cells by Ca2+

Two different and independent modes of inhibition of the Ca2+ pump by Ca2+ can be detected measuring active Ca2+ extrusion from resealed ghosts of human red cells: one requires extracellular and the other requires intracellular Ca2+. Ki for inhibition by extracellular Ca2+ is about 10 mM. Extracellular Mg2+ replaces Ca2+ in inhibiting Ca2+ transport but with an apparent affinity for inhibition about 3-times less than that for Ca2+. Inhibition by external Ca2+ is not affected by Na+ or K+ at both surfaces of the cell membrane, external EGTA, internal Ca2+ or ATP. The apparent affinity for external Ca2+ progressively raises as pH increases. The effects of extracellular Ca2+ and Mg2+ are consistent with the idea that for Ca2+ pumping to proceed, external sites in the pump must be protonated and not occupied by extracellular Ca2+ or Mg2+. Inhibition by intracellular Ca2+ takes place with a Ki of about 1 mM and is independent of external Ca2+. The inhibitory effects of intracellular Ca2+ can be accounted for if Ca2+ and CaATP were competitive inhibitors of the activation of the pump by Mg2+ and MgATP, respectively.

Synthesis of ATP catalyzed by the (Ca2+ + Mg2+)-ATPase from erythrocyte ghosts. Energy conservation in plasma membranes

The (Ca2+ + Mg2+)-ATPase from erythrocyte ghosts catalyzed the hydrolysis of ATP together with the synthesis of ATP or ATP in equilibrium 'Pi exchange. The modulation of the ATPase reaction cycle was controlled by high- and low-affinity calcium-binding sites asymmetrically located on the enzyme. Calmodulin accelerated the reaction cycle in both directions, stimulating the overall turnover of the enzyme. Calcium transport was achieved utilizing optimal conditions for the expression of the ATP in equilibrium Pi exchange system.

Two Ca2+-requiring p-nitrophenylphosphatase activities of the highly purified Ca2+-pumping adenosinetriphosphatase of human erythrocyte membranes, one requiring calmodulin and the other ATP

The highly purified Ca2+-pumping ATPase from human erythrocyte membranes displays two p-nitrophenylphosphatase (NPPase) activities: one of these requires calmodulin and low concentrations of Ca2+, while the other requires ATP and higher Ca2+ concentrations. The free Ca2+ concentrations required for the expression of the two NPPase activities differed very substantially. Both activities required high free Mg2+ concentrations and displayed simple hyperbolic kinetics toward p-nitrophenyl phosphate (NPP) with a Km in the range of 5-20 mM. Study of the dependence of the calmodulin-stimulated NPPase on Mg2+ and NPP indicated that the Mg-NPP complex is not the substrate of the enzyme. Under conditions optimal for ATP-requiring NPPase (1 mM free Ca2+), the Ca2+-ATPase displayed simple hyperbolic kinetics with a low Km for ATP. NPP competitively inhibited this activity, and the apparent Ki for NPP was less than 1 mM, much lower than the Km for NPP as a substrate. If NPP were inhibiting the ATPase by binding at the same site at which NPP is hydrolyzed, the apparent Ki for NPP as inhibitor would be the same as the Km for NPP as substrate. (Under these circumstances, the apparent Ki and the Km can be directly compared, since NPP was being hydrolyzed under both circumstances.) Since Ki was much lower than Km, NPP must have been inhibiting at another site; thus, these data show the existence of two types of NPP sites on the enzyme, one at which NPP is hydrolyzed and the other at which it inhibits ATP hydrolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the involvement of (Cu-ATP)2- in the inhibition of human erythrocyte (Ca2+ + Mg2+)-ATPase by copper

The effects of copper on the activity of erythrocyte (Ca2+ + Mg2+)-ATPase have been tested on membranes stripped of endogenous calmodulin or recombined with purified calmodulin. The interactions of copper with Ca2+, calmodulin and (Mg-ATP)2- were determined by kinetic studies. The most striking result is the potent competitive inhibition exerted by (Cu-ATP)2- against (Mg-ATP)2- (Ki = 2.8 microM), while free copper gives no characteristic inhibition. Our results also demonstrate that copper does not compete with calcium either on the enzyme or on calmodulin. The fixation of calmodulin on the enzyme is not altered in the presence of copper as shown by the fact that the dissociation constant remains unaffected. It may be speculated that (Cu-ATP)2- is the active form of copper, which could plausibly be at the origin of some of the pathological features of erythrocytes observed in conditions associated with excess copper.

Calmodulin and the target size of the (Ca2+ + Mg2+)-ATPase of human red-cell ghosts

An average target size of 251 kDa has been obtained for the (Ca2+ + Mg2+)-ATPase of calmodulin-depleted erythrocyte ghosts by radiation inactivation with 16 MeV electrons. This is close to twice the size of the purified calcium-pump polypeptide. When calmodulin was included during the ATPase assay, a component of about 1 MDa appeared in addition to the activated dimer.

Partial purification and characterization of the (Ca2+ + Mg2+)-ATPase from squid optic nerve plasma membrane

A membrane fraction enriched in axolemma was obtained from optic nerves of the squid (Sepiotheutis sepioidea) by differential centrifugation and density gradient fractionation. The preparation showed an oligomycin- and NaN3-insensitive (Ca2+ + Mg2+)-ATPase activity. The dependence of the ATPase activity on calcium concentration revealed the presence of two saturable components. One had a high affinity for calcium (K1 1/2 = 0.12 microM) and the second had a comparatively low affinity (K2 1/2 = 49.5 microM). Only the high-affinity component was specifically inhibited by vanadate (K1 = 35 microM). Calmodulin (12.5 micrograms/ml) stimulated the (Ca2+ + Mg2+)-ATPase by approx. 50%, and this stimulation was abolished by trifluoperazine (10 microM). Further treatment of the membrane fraction with 1% Nonidet P-40 resulted in a partial purification of the ATPase about 15-fold compared to the initial homogenate. This (Ca2+ + Mg2+)-ATPase from squid optic nerve displays some properties similar to those of the uncoupled Ca2+-pump described in internally dialyzed squid axons, suggesting that it could be its enzymatic basis.

Effects of calmodulin antagonists on the active Ca2+ uptake by rat liver mitochondria

The mechanism of Ca2+ transport by rat liver mitochondria was investigated with respect to the possible involvement of calmodulin in this process. We studied the action of exogenous calmodulin isolated from brain tissue on the Ca2+-transport system, as well as the effect of two types of calmodulin antagonists; the phenothiazine drugs trifluoperazine and chlorpromazine and the more specific substance compound 48/80. Our results show that Ca2+ transport by mitochondria and mitochondrial ATPase activity are insensitive to exogenous calmodulin, although they can be inhibited by the phenothiazines. Since no effect of compound 48/80 was observed, we believe that the phenothiazines act through a mechanism that does not involve calmodulin. This is in accord with our inability to locate significant quantities of calmodulin in mitochondria by radioimmunoassay analysis. Our results further show that trifluoperazine and chlorpromazine also inhibit the electron-carrier system of the respiratory chain, and this effect may mediate their inhibitory action on Ca2+ transport when it is energized by respiration instead of ATP hydrolysis.

Hysteretic activation of the Ca2+ pump revealed by calcium transients in human red cells

The enzymatic basis for the Ca2+ pump in human red cells is an ATPase with hysteretic properties. The Ca2+-ATPase shifts slowly between a ground state deficient in calmodulin and an active state saturated with calmodulin, and rate constants for the reversible shifts of state were recently determined at different Ca2+ concentrations (Scharff, O. and Foder, B. (1982) Biochim. Biophys. Acta 691, 133-143). In order to study whether the Ca2+ pump in intact red cells also exhibits hysteretic properties we have analysed transient increases of intracellular calcium concentrations (Cai), induced by the divalent cation ionophore A23187. The time-dependent changes of Cai were measured by use of radioactive calcium (45Ca2+) and analysed with the aid of a mathematical model, based partly on the Ca2+-dependent parameters obtained from Ca2+-ATPase experiments, partly on the A23187-induced Ca2+ fluxes determined in experiments with intact red cells. According to the model a delay in the activation of the Ca2+ pump is a prerequisite for the occurrence of A23187-induced calcium transients in the red cells, and we conclude that the Ca2+ pump in human red cells responds hysteretically. It is suggested that Ca2+ pumps in other types of cell also have hysteretic properties.

The site of action of La3+ in the reaction cycle of the human red cell membrane Ca2+-pump ATPase

Lanthanum (La3+) inhibits the Ca-pump of the red cell by arresting the protein in a phosphorylated form (PI). Similar La3+ concentrations are required to increase the amount of PI and to stop PI-decay. In the presence of La3+ phosphorylation becomes insensitive to Mg2+. PI made in the presence of Mg2+ is not prevented from decaying by subsequent addition of La3+, whereas that made in the absence of Mg2+ is. Taken together, these findings seem to indicate that La3+ blocks the transition between a 1st and a 2nd form of PI.

A genetic approach to reveal the action of the opiate receptor in selected neuroblastoma-glioma cells. Interaction with alpha-adrenoceptors, calmodulin and Ca2+-ATPase

Three clones of neuroblastoma-glioma cells that contain low amounts of calmodulin were selected from the NG108-15 cells after several treatments with high concentrations of chlorpromazine. Purified membranes of the three clones had decreased numbers of both alpha-adrenergic and opiate receptors, monitored with [3H]yohimbine and [3H,D-Ala2]methionine encephalinamide, respectively. No changes were observed in the affinity of these radioactive ligands to the receptors of the selected cells as compared to the parent cells. Addition of bovine brain calmodulin did not affect the binding of [3H,D-Ala2]methionine encephalinamide to the membranes of the selected cells and they had the same number of acetylcholine receptors, determined with 1-quinuclidinyl-[phenyl-4-3H]-benzilate, as the parent NG108-15 cells. The basal ATPase activity in the membranes of the selected cells was 35-50% of the parent cells, with a decreased V value and no significant change in the affinity constant Ka to ATP. Addition of Ca2+ to the purified membranes increased the V of the ATPase in the selected as well as the parent cells but the V of the selected cells remained lower than that of the parent cells. Ca2+ had no effect on the Ka to ATP in either cell type. The Ca2+-dependent ATPase activity of both the parent and the selected cells was also calmodulin-dependent dependent since it was blocked in vitro by chlorpromazine. The co-regulation of opiate and adrenergic receptors and their interaction with calmodulin and Ca2+-ATPase is discussed in view of recent observations indicating biochemical and physiological association between opiates, Ca2+ and adrenergic compounds.

Physiological [Ca2+]i level and pump-leak turnover in intact red cells measured using an incorporated Ca chelator

The physiological actions of Ca2+ as a trigger and second messenger depend on the maintenance of large inward resting Ca2+ gradients across the cell plasma membrane. An ATP-fuelled Ca-pump, originally discovered and still best characterized in human red cells, is now believed to mediate resting Ca2+ extrusion in most animal cells. However, even in red cells, the truly physiological pump-leak turnover rate and cytoplasmic free Ca2+ level are unknown. Previous estimates were only very imprecise upper limits because normal intact red cells have a minute total pool of exchangeable Ca of less than 1 mumol 1 cells; Ca fluxes could not be measured without artificially increasing that pool with ionophores or disrupting the membrane to incorporate Ca buffers. Both procedures leave the membrane considerably leakier than in intact cells. Here, we have increased the exchangeable Ca pool by non-disruptively loading a Ca-chelator into intact cells, using intracellular hydrolysis of a membrane-permeant ester. The trapped chelator made the free cytoplasmic calcium concentration, [Ca2+]i, an easily defined function of directly measurable total cell Ca. We were then able to establish the physiological steady-state [Ca2+]i and pump-leak turnover rate of fresh cells suspended in their own plasma. If [Ca2+]i was lowered below the normal resting level, the Ca pump rate decreased according to the square of [Ca2+]i, and the inward Ca leak increased. The increase in leak did not develop if the cells were depleted of ATP and ADP.

Activation of human erythrocyte Ca2+-dependent Mg2+-activated ATPase by calmodulin and calcium: quantitative analysis

The effect of Ca2+ and calmodulin on (CaM) on the activation of Ca2+-dependent Mg2+-activated ATPase (Ca2+,Mg2+-ATPase; ATP phosphohydrolase, EC 3.6.1.3) has been carried out because of the finding that the CaM dependence of the activation varies with the concentration of free Ca2+, similarly to brain phosphodiesterase and adenylate cyclase. The study was carried out in the absence of chelating agents because they strongly interfere in the enzyme kinetics. Three main conclusions can be drawn (i) CaM-Ca3 and CaM-Ca4 together are the biochemically active species in vitro. (ii) These species bind in a non-cooperative way to the CaM-binding site of the enzyme with a dissociation constant of 6 x 10(-10) M or 1.1 x 10(-8) M, depending on whether Ca2+ saturates the substrate binding site of the enzyme or not. (iii) The binding of CaM-Ca3 to the enzyme lowers the dissociation constant of the enzyme for Ca2+ at the substrate binding site from 51.5 to 2.8 microM. Contrary to general belief, CaM does not induce pronounced positive cooperativity in the binding of Ca2+ to the enzyme. Such a cooperativity is seen only when the enzyme is incompletely saturated with the activator, but it disappears in the presence of saturating concentrations of CaM-Ca3. The rate equation proposed here accurately predicts the extent of enzyme activation over a wide range of Ca2+ and CaM concentration. In healthy erythrocytes the concentrations of Ca2+ and CaM are such that the Ca pump works with a minimal dissipation of energy, but a small increase in the intracellular Ca2+ concentration leads to a strong amplification of the pumping activity.

Regulation of the Ca2+-pump by calmodulin in intact cells

ATP-enriched human red cells display high rates of Ca2+-dependent ATP hydrolysis (16 mmol . litre cells-1 . h-1) with a high Ca2+ affinity (K0.5 approximately 0.2 microM). The finding suggests a mechanism for regulation of cell Ca2+ levels, involving highly-cooperative stimulation of active Ca2+ extrusion following binding of calmodulin to the (Ca2+ +Mg2+)-ATPase.

On the substrate specificity of the red cell calcium pump

ATP-dependent active calcium transport in inside-out human red cell membrane vesicles is stimulated by magnesium essentially parallel with an increase in MgATP concentration. At a constant, low (1 microM) calcium concentration, increasing ATP and magnesium increase the maximum calcium transport rate irrespective of the constant or decreasing concentrations of CaATP present. KCa for calcium pumping is practically unchanged at variable ATP and magnesium concentrations. Free magnesium above 1-2 mM inhibits active calcium transport, probably through a direct interaction with the transport enzyme. Based on the experimental findings reported we suggest that the true, physiological substrate of the red cell calcium pump is MgATP.