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

Characterization of postsynaptic Ca2+ signals at the Drosophila larval NMJ

Postsynaptic intracellular Ca(2+) concentration ([Ca(2+)](i)) has been proposed to play an important role in both synaptic plasticity and synaptic homeostasis. In particular, postsynaptic Ca(2+) signals can alter synaptic efficacy by influencing transmitter release, receptor sensitivity, and protein synthesis. We examined the postsynaptic Ca(2+) transients at the Drosophila larval neuromuscular junction (NMJ) by injecting the muscle fibers with Ca(2+) indicators rhod-2 and Oregon Green BAPTA-1 (OGB-1) and then monitoring their increased fluorescence during synaptic activity. We observed discrete postsynaptic Ca(2+) transients along the NMJ during single action potentials (APs) and quantal Ca(2+) transients produced by spontaneous transmitter release. Most of the evoked Ca(2+) transients resulted from the release of one or two quanta of transmitter and occurred largely at synaptic boutons. The magnitude of the Ca(2+) signals was correlated with synaptic efficacy; the Is terminals, which produce larger excitatory postsynaptic potentials (EPSPs) and have a greater quantal size than Ib terminals, produced a larger Ca(2+) signal per terminal length and larger quantal Ca(2+) signals than the Ib terminals. During a train of APs, the postsynaptic Ca(2+) signal increased but remained localized to the postsynaptic membrane. In addition, we showed that the plasma membrane Ca(2+)-ATPase (PMCA) played a role in extruding Ca(2+) from the postsynaptic region of the muscle. Drosophila melanogaster has a single PMCA gene, predicted to give rise to various isoforms by alternative splicing. Using RT-PCR, we detected the expression of multiple transcripts in muscle and nervous tissues; the physiological significance of the same is yet to be determined.

Ca2+ dynamics along identified synaptic terminals in Drosophila larvae

Changes in intracellular Ca2+ concentration ([Ca2+]i) play an important role in the function and plasticity of synapses. We characterized the changes in [Ca2+]i produced by action potentials (APs) along two identified motor terminals found on separate muscle fibers in Drosophila larvae and examined factors that influence the amplitude and duration of the residual Ca2+ signal. We were able to measure Ca2+ transients produced along terminals by both single APs and AP trains using Oregon Green 488 BAPTA-1 and streaming images at 20-50 Hz. The decay of [Ca2+]i after single APs or AP trains was well fit by a single exponential. For single APs, the Ca2+ transient amplitude and decay rate were similar at boutons and bottleneck regions and much smaller at the axon. Also, the amplitude of single-AP Ca2+ transients was inversely correlated with bouton width. During AP trains, the increase in [Ca2+]i became more uniform: the difference in boutons and axons was reduced, and the increase in [Ca2+]i was not correlated with bouton width. The [Ca2+]i decay tau was directly correlated with bouton width for both single APs and AP trains. For one terminal, distal boutons had larger single-AP Ca2+ transients than proximal ones, probably attributable to greater Ca2+ influx for distal boutons. Pharmacological studies showed that Ca2+ clearance from these synaptic terminals after single APs and AP trains was primarily attributable to Ca2+ extrusion by the plasma membrane Ca2+ ATPase (PMCA). Immunostaining of larval muscle fibers showed high levels of the PMCA at the neuromuscular junction.

Autocrine action and its underlying mechanism of nitric oxide on intracellular Ca2+ homeostasis in vascular endothelial cells

The rise in cytosolic Ca(2+) concentration (Ca(2+)(i)) in vascular endothelial cells (ECs) activates the production and release of nitric oxide (NO). NO modifies Ca(2+)(i) homeostasis in many types of nonendothelial cells. However, its effect on endothelial Ca(2+)(i) homeostasis at basal and excited states remains unclear. In the present study, to elucidate the effect of NO on basal Ca(2+)(i), inositol 1,4,5-trisphosphate-induced Ca(2+)(i) release (IICR) was blocked by expressing an antisense against type-1 inositol 1,4,5-trisphosphate receptors or by microinjecting heparin to individual ECs, and the effects of NO that was released by and diffused from adjacent IICR-intact ECs were recorded. After ATP or bradykinin stimulation, IICR-inhibited ECs showed a marked reduction of basal Ca(2+)(i), which was abolished by N(G)-monomethyl-l-arginine monoacetate pretreatment. The reduction disappeared in sparsely seeded ECs. Exogenous NO gas mimicked the effect of ATP or bradykinin to reduce basal Ca(2+)(i). Blocking plasma membrane Ca(2+)-ATPase (PMCA), but not Na(+)-Ca(2+) exchange or sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase, suppressed the reduction, indicating that the reduction resulted from a NO-dependent potentiation of PMCA. To elucidate the effect of NO on elevated Ca(2+)(i), ATP-, bradykinin-, or thapsigargin-evoked Ca(2+)(i) response in the presence and absence of NO production was compared in adjacent IICR-intact ECs. NO was found to potentiate PMCA, which, in turn, greatly attenuated agonist-evoked Ca(2+)(i) elevation. NO also potentiated Ca(2+) influx, which markedly increased the sustained phase of Ca(2+)(i) elevation and possibly NO production. NO did not affect other Ca(2+)(i)-elevating and Ca(2+)(i)-sequestrating components. Thus, NO-dependent potentiation of PMCA is crucial for Ca(2+)(i) homeostasis over a wide Ca(2+)(i) range.

Probing the extracellular release site of the plasma membrane calcium pump

The plasma membrane Ca(2+) pump is known to mediate Ca(2+)/H(+) exchange. Extracellular protons activated (45)Ca(2+) efflux from human red blood cells with a half-maximal inhibition constant of 2 nM when the intracellular pH was fixed. An increase in pH from 7.2 to 8.2 decreased the IC(50) for extracellular Ca(2+) from approximately 33 to approximately 6 mM. Changing the membrane potential by >54 mV had no effect on the IC(50) for extracellular Ca(2+). This argues against Ca(2+) release through a high-field access channel. Extracellular Ni(2+) inhibited Ca(2+) efflux with an IC(50) of 11 mM. Extracellular Cd(2+) inhibited with an IC(50) of 1. 5 mM, >10 times better than Ca(2+). The Cd(2+) IC(50) also decreased when the pH was raised from 7.1 to 8.2, consistent with Ca(2+), Cd(2+), and H(+) competing for the same site. The higher affinity for inhibition by Ni(2+) and Cd(2+) is consistent with a histidine or cysteine as part of the release site. The cysteine reagent 2-(trimethylammonium)ethyl methanethiosulfonate did not inhibit Ca(2+) efflux. Our results are consistent with the notion that the release site contains a histidine.

Calcium pump kinetics determined in single erythrocyte ghosts by microphotolysis and confocal imaging

The activity of the plasma membrane calcium pump was measured in single cells. Human red blood cell ghosts were loaded with a fluorescent calcium indicator and either caged calcium and ATP (protocol A) or caged ATP and calcium (protocol B). In a suitably modified laser scanning microscope either calcium or ATP were released by a short UV light pulse. The time-dependent fluorescence intensity of the calcium indicator was then followed in single ghosts by repetitive confocal imaging. The fluorescence intensity was converted into calcium concentration, which in turn was used to derive the kinetic parameters of the calcium pump, the Michaelis-Menten constant Km, and the maximal transport rate vmax. Km and vmax values derived in this manner were 24 +/- 14 microM and 1.0 +/- 0.6 microM/(ghost s) for protocol A, and 4 +/- 3 microM and 1.0 +/- 0.6 microM/(ghost s) for protocol B, respectively. The difference between A and B is presumably caused by calmodulin, which is inactive in the experiments with protocol A. The possibilities to extend the new method to living nucleus-containing cells transiently transfected with mutants of the plasma membrane calcium pump are discussed.

Irreversible effects of calcium ions on the plasma membrane calcium pump

The calcium pump of human red cells can be irreversibly activated by preincubation of the membranes in the presence of calcium ions, with a pattern reminiscent of that produced by controlled trypsin attack. With 1 mM Ca2+, the activity of the basal enzyme increases three to fourfold over 30 to 60 min, to levels about half those obtained in the presence of calmodulin. On the whole, the effect occurs slowly, with a very low Ca2+ affinity at 37 degrees C and is unaffected by serine-protease inhibitors. The activation caused by 1 mM Ca2+ is little affected by leupeptin (a thiol-protease inhibitor) and that obtained at 10 microM Ca2+ is not inhibited. Preincubations at 0 degrees C also lead to activation, to a level up to half that seen at 37 degrees C, and the effect is not affected by leupeptin or antipain. No activation is observed by preincubating soluble purified Ca,Mg-ATPase in Ca(2+)-containing solutions at 37 degrees C. Instead, calcium ions protect the detergent-solubilized enzyme from thermal inactivation, the effect being half-maximal between 10 and 20 microM Ca2+. We conclude that the activation of the membrane-bound Ca,Mg-ATPase by Ca2+ should result from an irreversible conformational change in the enzyme and not from attack by a membrane-bound protease, and that this change presumably arises from the release of inhibitory particles existing in the original membrane preparations.

Effects of deoxygenation on active and passive Ca2+ transport and cytoplasmic Ca2+ buffering in normal human red cells

1. The effects of deoxygenation on cytoplasmic Ca2+ buffering, saturated Ca2+ extrusion rate through the Ca2+ pump (Vmax), passive Ca2+ influx and physiological [Ca2+]i level were investigated in human red cells to assess whether or not their Ca2+ metabolism might be altered by deoxygenation in capillaries and venous circulation. 2. The study was performed in fresh human red cells maintained in a tonometer either fully oxygenated or deoxygenated. Cytoplasmic Ca2+ buffering was estimated from the equilibrium distribution of 45Ca2+ induced by the divalent cation ionophore A23187 and the Vmax of the Ca2+ pump was measured either by the Co(2+)-exposure method or following ionophore wash-out. The passive Ca2+ influx and physiological [Ca2+]i were determined in cells preloaded with the Ca2+ chelator benz-2 and resuspended in autologous plasma. 3. Deoxygenation increased the fraction of ionized Ca2+ in cell water by 34-74% and reduced the Vmax of the Ca2+ pump by 18-32%. 4. To elucidate whether or not these effects were secondary to deoxygenation-induced pH shifts, the effects of deoxygenation on cell and medium pH, and of pH on cytoplasmic Ca2+ binding and Ca2+ pump Vmax in oxygenated cells were examined in detail. 5. Deoxygenation generated large alkaline pH shifts that could be explained if the apparent isoelectric point (pI) of haemoglobin increased by 0.2-0.4 pH units in intact cells, consistently higher than the value of 0.15 reported for pure haemoglobin solutions. 6. In oxygenated cells, the fraction of ionized cell calcium, alpha, was little affected by pH within the 7.0-7.7 range. Ca2+ pump Vmax was maximal at a medium pH of about 7.55. Comparison between pH effects elicited by HCl-NaOH additions and by replacing Cl- with gluconate suggested that Vmax was inhibited by both internal acidification and external alkalinization. Since deoxygenation alkalinized cells and medium within a range stimulatory for Vmax, the inhibition observed was not due to pH. 7. There was no significant effect of deoxygenation on passive Ca2+ uptake, or steady-state physiological [Ca2+]i level. 8. The deoxygenation-induced reduction in Ca2+ binding capacity may result from the increased protonation of haemoglobin on deoxygenation and from binding of 2,3-diphosphoglyceric acid (2,3-DPG) and ATP to deoxyhaemoglobin; inhibition of the Ca2+ pump may result from shifts in the [Mg2+]i/[ATP]i ratio away from a near optimal stimulatory value in the oxygenated state.

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.

The Ca(2+)-transport ATPases from the plasma membrane

The initial studies on the plasma membrane (PM) Ca(2+)-transport ATPases were made in the erythrocyte, a structure that can not be taken as representing a typical eukaryotic cell. In other cell types however, the study of the PM Ca(2+)-transport ATPase is complicated by the simultaneous expression of related Ca(2+)-pumps in intracellular stores. Whereas there are as yet no known specific inhibitors for the PM Ca(2+)-transport ATPase, a number of selective inhibitors for the endo(sarco)plasmic reticulum Ca2+ pumps have been described: thapsigargin, cyclopiazonic acid and 2,5-di-(tert-butyl)-1,4-benzohydroquinone. With the recent introduction of the molecular biological approach, it became quickly obvious that a family of at least 5 different PM Ca(2+)-transport ATPase genes govern the tissue-dependent expression of PM Ca2+ pumps. Moreover alternative splicing of the primary gene transcripts was found to further enhance the number of pump variants. The PM Ca(2+)-transport ATPase are subject to modulatory control by calmodulin, by acidic phospholipids, and by the known families of protein kinases. Each of the ensuing effects are mutually related and interdependent. The wide variety PM Ca2+ pump isoforms and their regulation by such an intricate modulatory network allows the distinct tissues to adapt most adequately to the prevailing tissue and stimulus specific requirements.

Maximal calcium extrusion capacity and stoichiometry of the human red cell calcium pump

1. The uphill calcium efflux through calcium-saturated pumps in intact red cells was investigated with the aid of a new method, in initial conditions of uniform ionophore A23187-induced calcium distribution among the cells. The method is based on findings by Tiffert, García-Sancho & Lew (1984) which show that cobalt can suddenly arrest passive calcium transport by the ionophore and expose, without noticeable interference, uphill calcium extrusion by the pump. The results comprise methodological aspects and questions concerning interactions between inner pump sites, ATP and Ca2+, and the calcium: ATP stoichiometry of the calcium-saturated pump. 2. Ionophore-induced calcium influx was set to be far in excess of the maximal calcium pump capacity. This secured a uniform calcium distribution among the cells, and Ca2+ equilibration by 2 min or less of calcium permeabilization. Cobalt was added between 15 s and 5 min after ionophore addition. The calcium and ATP content of the cells was followed during ionophore-induced influx and cobalt-exposed efflux. 3. The external cobalt concentrations required to block completely ionophore-mediated calcium transport were similar or only marginally higher than those of calcium. 4. The reproducibility of independent cobalt-exposed calcium efflux measurements from single blood samples was within an 8% range. 5. During cobalt-exposed calcium efflux, the calcium content of subpopulations of cells, with and without active Ca2+-sensitive K+ channels, investigated by post-incubation of samples in low-K+, thiocyanate (SCN-) media (modified from García-Sancho & Lew, 1988a), was similar. This is consistent with the maintenance of a uniform calcium distribution among the cells during uphill calcium extrusion. 6. Cobalt-exposed calcium efflux was similar in the interval from 15 s to 5 min after calcium permeabilization although cell ATP levels had fallen by over 50% in that period. Therefore, cell ATP concentrations within the physiological range do not seem to be regulatory for calcium-saturated pumps in the intact red cell. 7. All cobalt-exposed calcium efflux curves were linear in time, at least until total cell calcium contents reached levels below 100 mumol/l cells. This suggests that internal calcium is not inhibitory for calcium-saturated efflux in intact cells in the 0.1-1 mmol/l cells range. 8. The cobalt-exposed calcium fluxes were in the range from 4 to 24 mmol/(1 cells.h) for fresh cells and from 10 to 18 mmol/1 cells. h) for samples from the Blood Bank.(ABSTRACT TRUNCATED AT 400 WORDS)

Asymmetric effects of divalent cations and protons on active Ca2+ efflux and Ca2+-ATPase in intact red blood cells

The influence of the asymmetric addition of various divalent cations and protons on the properties of active Ca2+ transport have been examined in intact human red blood cells. Active Ca2+ efflux was determined from the initial rate of 45Ca2+ loss after CoCl2 was added to block Ca2+ loading via the ionophore A23187. Ca2+-ATPase activity was measured as phosphate production over 5 min in cells equilibrated with EGTA-buffered free Ca2+ in the presence of A23187. The apparent Ca affinity of active Ca2+ efflux (K0.5 = 30-40 mumol/liter cells) was significantly lower than that measured by the Ca2+-ATPase assay (K0.5 = 0.4 microM). Possible reasons for this apparent difference are considered. Both active Ca2+ efflux and Ca2+-ATPase activity were reduced to less than 5% of maximal levels (20 mmol/liter cells.hr) in Mg2+-depleted cells, and completely restored by reintroduction of intracellular Mg2+. Active Ca2+ efflux was inhibited almost completely by raising external CaCl2 (but not MgCl2) to 20 mM, probably by interaction of Ca2+ at the externally oriented E2P conformation of the pump. Cd2+ was more potent than Ca2+ in this inhibition, while Mn2+ was less potent and 10 mM Ba2+ was without effect. A Ca2+: proton exchange mechanism for active Ca2+ efflux was supported by the results, as external protons (pH 6-6.5) stimulated active Ca2+ efflux at least twofold above the efflux rate at pH 7.8 Ca2+ transport was not affected by decreasing the membrane potential across the red cell.

The reaction of Mg2+ with the Ca2+-ATPase from human red cell membranes and its modification by Ca2+

Media prepared with CDTA and low concentrations of Ca2+, as judged by the lack of Na+-dependent phosphorylation and ATPase activity of (Na+ +K+)-ATPase preparations are free of contaminant Mg2+. In these media, the Ca2+-ATPase from human red cell membranes is phosphorylated by ATP, and a low Ca2+-ATPase activity is present. In the absence of Mg2+ the rate of phosphorylation in the presence of 1 microM Ca2+ is very low but it approaches the rate measured in Mg2+-containing media if the concentration of Ca2+ is increased to 5 mM. The KCa for phosphorylation is 2 microM in the presence and 60 microM in the absence of Mg2+. Results are consistent with the idea that for catalysis of phosphorylation the Ca2+-ATPase needs Ca2+ at the transport site and Mg2+ at an activating site and that Ca2+ replaces Mg2+ at this site. Under conditions in which it increases the rate of phosphorylation, Ca2+ is without effect on the Ca2+-ATPase activity in the absence of Mg2+ suggesting that to stimulate ATP hydrolysis Mg2+ accelerates a reaction other than phosphorylation. Activation of the E1P----E2P reaction by Mg2+ is prevented by Ca2+ after but not before the synthesis of E1P from E1 and ATP, suggesting that Mg2+ stabilizes E1 in a state from which Mg2+ cannot be removed by Ca2+ and that Ca2+ stabilizes E1P in a state insensitive to Mg2+. The response of the Ca2+-ATPase activity to Mg2+ concentration is biphasic, activation with a KMg = 88 microM is followed by inhibition with a Ki = 9.2 mM. Ca2+ at concentration up to 1 mM acts as a dead-end inhibitor of the activation by Mg2+, and Mg2+ at concentrations up to 0.5 mM acts as a dead-end inhibitor of the effects of Ca2+ at the transport site of the Ca2+-ATPase.

Investigation of (Ca2+ + Mg2+)-ATPase phosphoprotein formation in erythrocyte membranes of patients with cystic fibrosis

The (Ca2+ + Mg2+)-ATPase present per mg of protein in erythrocyte membranes of controls and patients with cystic fibrosis (CF) was determined by estimation of the levels of its phosphoprotein. In the presence of 10 mM free Ca2+, which inhibits phosphoprotein decomposition, significantly less phosphoprotein intermediate, ECaP, was found in erythrocyte membranes from CF patients than in age- and sex-matched controls; this correlated with a significant decrease in (Ca2+ + Mg2+)-ATPase activity. These observations indicate a decrease in the number of functional (Ca2+ + Mg2+)-ATPase molecules in erythrocyte membranes from CF patients or an alteration in either the structure of the pump protein or the composition of its environment.