Stopped-flow kinetics of the interaction of the fluorescent calcium indicator Quin 2 with calcium ions

Protein-specific localization of a rhodamine-based calcium-sensor in living cells

A small synthetic calcium sensor that can be site-specifically coupled to proteins in living cells by utilizing the bio-orthogonal HaloTag labeling strategy is presented. We synthesized an iodo-derivatized BAPTA chelator with a tetramethyl rhodamine fluorophore that allows further modification by Sonogashira cross-coupling. The presented calcium sensitive dye shows a 200-fold increase in fluorescence upon calcium binding. The derivatization with an aliphatic linker bearing a terminal haloalkane-function by Sonogashira cross-coupling allows the localization of the calcium sensor to Halo fusion proteins which we successfully demonstrate in in vitro and in vivo experiments. The herein reported highly sensitive tetramethyl rhodamine based calcium indicator, which can be selectively localized to proteins, is a powerful tool to determine changes in calcium levels inside living cells with spatiotemporal resolution.

Distinct mechanisms of calmodulin binding and regulation of adenylyl cyclases 1 and 8

Calmodulin (CaM), by mediating the stimulation of the activity of two adenylyl cyclases (ACs), plays a key role in integrating the cAMP and Ca(2+) signaling systems. These ACs, AC1 and AC8, by decoding discrete Ca(2+) signals can contribute to fine-tuning intracellular cAMP dynamics, particularly in neurons where they predominate. CaM comprises an α-helical linker separating two globular regions at the N-terminus and the C-terminus that each bind two Ca(2+) ions. These two lobes have differing affinities for Ca(2+), and they can interact with target proteins independently. This study explores previous indications that the two lobes of CaM can regulate AC1 and AC8 differently and thereby yield different responses to cellular transitions in [Ca(2+)](i). We first compared by glutathione S-transferase pull-down assays and offline nanoelectrospray ionization mass spectrometry the interaction of CaM and Ca(2+)-binding deficient mutants of CaM with the internal CaM binding domain (CaMBD) of AC1 and the two terminal CaMBDs of AC8. We then examined the influence of these three CaMBDs on Ca(2+) binding by native and mutated CaM in stopped-flow experiments to quantify their interactions. The three CaMBDs show quite distinct interactions with the two lobes of CaM. These findings establish the critical kinetic differences between the mechanisms of Ca(2+)-CaM activation of AC1 and AC8, which may underpin their different physiological roles.

QZ1 and QZ2: rapid, reversible quinoline-derivatized fluoresceins for sensing biological Zn(II)

QZ1, 2-[2-chloro-6-hydroxy-3-oxo-5-(quinolin-8-ylaminomethyl)-3H-xanthen-9-yl]benzoic acid, and QZ2, 2-[6-hydroxy-3-oxo-4,5-bis-(quinolin-8-ylaminomethyl)-3H-xanthen-9-yl]benzoic acid, two fluorescein-based dyes derivatized with 8-aminoquinoline, have been prepared and their photophysical, thermodynamic, and zinc-binding kinetic properties determined. Because of their low background fluorescence and highly emissive Zn(II) complexes, QZ1 and QZ2 have a large dynamic range, with approximately 42- and approximately 150-fold fluorescence enhancements upon Zn(II) coordination, respectively. These dyes have micromolar K(d) values for Zn(II) and are selective for Zn(II) over biologically relevant concentrations of the alkali and alkaline earth metals. The Zn(II) complexes also fluoresce brightly in the presence of excess Mn(II), Fe(II), Co(II), Cd(II), and Hg(II), offering improved specificity for Zn(II) over di(2-picolyl)amine-based Zn(II) sensors. Stopped-flow kinetic investigations indicate that QZ1 and QZ2 bind Zn(II) with k(on) values of (3-4) x 10(6) M(-1) s(-1), compared to (6-8) x 10(5) M(-1) s(-1) for select ZP (Zinpyr) dyes, at 4.3 degrees C. Dissociation of Zn(II) from QZ1 and QZ2 occurs with k(off) values of 150 and 160 s(-1), over 5 orders of magnitude larger than those for ZP probes, achieving reversibility on the biological (millisecond) time scale. Laser scanning confocal and two-photon microscopy studies reveal that QZ2 is cell-permeable and Zn(II)-responsive in vivo. Because of its weaker affinity for Zn(II), QZ2 responds to higher concentrations of intracellular Zn(II) than members of the ZP family, illustrating that binding affinity is an important parameter for Zn(II) detection in vivo.

Localization and characterization of the inhibitory Ca2+-binding site of Physarum polycephalum myosin II

A myosin II is thought to be the driving force of the fast cytoplasmic streaming in the plasmodium of Physarum polycephalum. This regulated myosin, unique among conventional myosins, is inhibited by direct Ca2+ binding. Here we report that Ca2+ binds to the first EF-hand of the essential light chain (ELC) subunit of Physarum myosin. Flow dialysis experiments of wild-type and mutant light chains and the regulatory domain revealed a single binding site that shows moderate specificity for Ca2+. The regulatory light chain, in contrast to regulatory light chains of higher eukaryotes, is unable to bind divalent cations. Although the Ca2+-binding loop of ELC has a canonical sequence, replacement of glutamic acid to alanine in the -z coordinating position only slightly decreased the Ca2+ affinity of the site, suggesting that the Ca2+ coordination is different from classical EF-hands; namely, the specific "closed-to-open" conformational transition does not occur in the ELC in response to Ca2+. Ca2+- and Mg2+-dependent conformational changes in the microenvironment of the binding site were detected by fluorescence experiments. Transient kinetic experiments showed that the displacement of Mg2+ by Ca2+ is faster than the change in direction of cytoplasmic streaming; therefore, we conclude that Ca2+ inhibition could operate in physiological conditions. By comparing the Physarum Ca2+ site with the well studied Ca2+ switch of scallop myosin, we surmise that despite the opposite effect of Ca2+ binding on the motor activity, the two conventional myosins could have a common structural basis for Ca2+ regulation.

Improvement and biological applications of fluorescent probes for zinc, ZnAFs

The development and cellular applications of novel fluorescent probes for Zn2+, ZnAF-1F, and ZnAF-2F are described. Fluorescein is used as a fluorophore of ZnAFs, because its excitation and emission wavelengths are in the visible range, which minimizes cell damage and autofluorescence by excitation light. N,N-Bis(2-pyridylmethyl)ethylenediamine, used as an acceptor for Zn2+, is attached directly to the benzoic acid moiety of fluorescein, resulting in very low quantum yields of 0.004 for ZnAF-1F and 0.006 for ZnAF-2F under physiological conditions (pH 7.4) due to the photoinduced electron-transfer mechanism. Upon the addition of Zn2+, the fluorescence intensity is quickly increased up to 69-fold for ZnAF-1F and 60-fold for ZnAF-2F. Apparent dissociation constants (K(d)) are in the nanomolar range, which affords sufficient sensitivity for biological applications. ZnAFs do not fluoresce in the presence of other biologically important cations such as Ca2+ and Mg2+, and are insensitive to change of pH. The complexes with Zn2+ of previously developed ZnAFs, ZnAF-1, and ZnAF-2 decrease in fluorescence intensity below pH 7.0 owing to protonation of the phenolic hydroxyl group of fluorescein, whose pKa value is 6.2. On the other hand, the Zn2+ complexes of ZnAF-1F and ZnAF-2F emit stable fluorescence around neutral and slightly acidic conditions because the pKa values are shifted to 4.9 by substitution of electron-withdrawing fluorine at the ortho position of the phenolic hydroxyl group. For application to living cells, the diacetyl derivative of ZnAF-2F, ZnAF-2F DA, was synthesized. ZnAF-2F DA can permeate through the cell membrane, and is hydrolyzed by esterase in the cytosol to yield ZnAF-2F, which is retained in the cells. Using ZnAF-2F DA, we could measure the changes of intracellular Zn2+ in cultured cells and hippocampal slices.

Kinetics of Formation of Ca(2+) Complexes of Acyclic and Macrocyclic Poly(amino carboxylate) Ligands: Bimolecular Rate Constants for the Fully-Deprotonated Ligands Reveal the Effect of Macrocyclic Ligand Constraints on the Rate-Determining Conversions of Rapidly-Formed Intermediates to the Final Complexes

The apparent bimolecular rate constants, k(1) (M(-)(1) s(-)(1)), for the formation of Ca(2+) complexes of a series of acyclic (edta, egta, cdta) and macrocyclic (dota, teta, do3a) poly(amino carboxylate) ligands were determined in the pH range 7-13 using the fluorescent ligand quin2 in a stopped-flow apparatus to monitor the ligand competition reaction. The k(1) values are observed to reach maximum constant values at high pH, characteristic of reactions involving the fully-deprotonated ligand species. Bimolecular formation constants k(Ca)(L), k(Ca)(HL), and k(Ca)(H)()2(L), characteristic of the reaction of the fully-deprotonated and mono- and diprotonated ligands, respectively, were derived from the pH dependence of the k(1) values. The k(Ca)(L) values of the acyclic ligands are edta, 4.1 x 10(9) M(-)(1) s(-)(1); egta, 2.1 x 10(9) M(-)(1) s(-)(1); and cdta, 2.3 x 10(9) M(-)(1) s(-)(1), while the corresponding values for the macrocyclic ligands are dota, 4.7 x 10(7) M(-)(1) s(-)(1); teta, 1.1 x 10(7) M(-)(1) s(-)(1); and do3a, 1.0 x 10(8) M(-)(1) s(-)(1). The smaller values for the macrocyclic ligands are consistent with ligand-dictated constraints imposed on the conversion of a stable intermediate to the final complex, a process which involves the simultaneous stripping of several water molecules from the first-coordination sphere of the Ca(2+) ion.

Ca2+ release-induced inactivation of Ca2+ current in rat ventricular myocytes: evidence for local Ca2+ signalling

1. Inactivation of Ca2+ current (ICa) induced by Ca2+ release from sarcoplasmic reticulum (SR) was studied in single rat ventricular myocytes using whole-cell patch-clamp and indo-1 fluorescence measurement techniques. 2. Depolarizing pulses to 0 mV elicited large Ca2+ transients and ICa with biexponential inactivation kinetics. Varying SR Ca2+ loading by a 20 s pulse of caffeine showed that the fast component of ICa inactivation was dependent on the magnitude of Ca2+ release. 3. Inactivation of ICa induced by Ca2+ release was quantified, independently of voltage and Ca2+ entry, using a function termed fractional inhibition of ICa (FICa). The voltage relation of FICa had a negative slope, resembling that of single-channel Ca2+ current (iCa) rather than the bell-shaped current-voltage (I-V) relation of macroscopic ICa and Ca2+ transients. 4. Intracellular dialysis of myocytes with 10 mM EGTA (150 nM free [Ca2+]) had no effect on ICa inactivation induced by Ca2+ release, despite abolition of Ca2+ transients and cell contraction. Dialysis with 3 or 10 mM BAPTA (180 nM free [Ca2+]) attenuated FICa in a concentration-dependent manner, with greater inhibition at positive than at negative potentials, consistent with more effective buffering of Ca2+ microdomains of smaller iCa. 5. Spatial profiles of [Ca2+] near an opened Ca2+ channel were simulated. [Ca2+] reached submillimolar levels at the mouth of the channel, and dropped steeply as radial distance increased. At any given distance from the channel, [Ca2+] was higher at negative than at positive potentials. The radii of Ca2+ microdomains were significantly reduced by 3 or 10 mM BAPTA, but not by 10 mM EGTA. 6. In conclusion, the distinctive voltage dependence and susceptibility of Ca2+ release-induced ICa inactivation to fast and slow Ca2+ buffers suggests that the process is mediated through local changes of [Ca2+] in the vicinity of closely associated Ca2+ channels and ryanodine receptors.

Calmodulin activation of the Ca2+ pump revealed by fluorescent chelator dyes in human red blood cell ghosts

Ca2+ transport in red blood cell ghosts was monitored with fura2 or quin2 incorporated as the free acid during resealing. This is the first report of active transport monitored by the fluorescent intensity of the chelator dyes fura2 (5-50 microM) or quin2 (250 microM) in hemoglobin-depleted ghosts. Since there are no intracellular compartments in ghosts and the intracellular concentrations of all assay chelator substances including calmodulin (CaM), the dyes, and ATP could be set, the intracellular concentrations of free and total Ca [( Cafree]i and [Catotal]i) could be calculated during the transport. Ghosts prepared with or without CaM rapidly extruded Ca2+ to a steady-state concentration of 60-100 nM. A 10(4)-fold gradient for Ca2+ was routinely produced in medium containing 1 mM Ca2+. During active Ca2+ extrusion, d[Cafree]i/dt was a second order function of [Cafree]i and was independent of the dye concentration, whereas d[Catotal]i/dt increased as a first order function of both the [Cafree]i and the concentration of the Ca:dye complex. CaM (5 microM) increased d[Catotal]i/dt by 400% at 1 microM [Cafree]i, while d[Cafree]i/dt increased by only 25%. From a series of experiments we conclude that chelated forms of Ca2+ serve as substrates for the pump under permissive control of the [Cafree]i, and this dual effect may explain cooperativity. Free Ca2+ is extruded, and probably also Ca2+ bound to CaM or other chelators, while CaM and the chelators are retained in the cell.

Cromakalim inhibits contractions of the rat isolated mesenteric bed induced by noradrenaline but not caffeine in Ca(2+)-free medium: evidence for interference with receptor-mediated Ca2+ mobilization

The effects of the K+ channel opener cromakalim on phasic contractions induced by noradrenaline and caffeine were studied in the rat isolated mesenteric bed. In the presence of 1.4 mM Ca2+, 1-s pulses of noradrenaline increased the perfusion pressure of the preparation concentration dependently (midpoint at 92 +/- 10 microM noradrenaline). Cromakalim (0.3 and 1 microM) inhibited these contractions in a non-competitive manner. Contractions elicited by 1-s pulses of noradrenaline (100 microM) were inhibited by the dihydropyridine Ca2+ antagonist isradipine by maximally 24 +/- 1%, indicating that only a minor component of this contraction depended on Ca2+ entry via dihydropyridine-sensitive Ca2+ channels. Cromakalim was a much more effective inhibitor of these contractions (maximum inhibition by 80%, midpoint of the inhibition curve at 171 +/- 15 nM). The effect of cromakalim was stereoselective, inhibited by the sulphonylurea glibenclamide, and abolished in partially depolarizing media (KCl = 35 and 50 mM). In Ca(2+)-free medium, cromakalim inhibited the contraction induced by noradrenaline (100 microM) by maximally 69 +/- 4%, with a midpoint at 58 +/- 14 nM. The effect of cromakalim was again stereoselective, inhibited by glibenclamide, and abolished in the presence of 50 mM KCl. Contractions induced by caffeine (10 and 100 microM) were not affected by cromakalim (1 microM). The results indicate that, in rat mesenteric resistance vessels, cromakalim interferes with the ability of noradrenaline, but not caffeine, to mobilize Ca2+ from intracellular stores. The antivasoconstrictor effect of cromakalim against noradrenaline is inhibited by glibenclamide and appears to be linked to the ability of cromakalim to hyperpolarize the cell membrane.

The effect of pH on rate constants, ion selectivity and thermodynamic properties of fluorescent calcium and magnesium indicators

Fluorescent calcium indicators fluo-3, fura-2 and indo-1, and fluorescent magnesium indicators mag-fura-2 (FURAPTRA) and mag-indo-1 were evaluated for the effects of pH on their association and dissociation rates, ion selectivity and thermodynamic properties. Calcium indicator affinities for Ca and Mg were reduced and the discrimination between Ca and Mg decreased in fura-2 and indo-1 at acidic pH. Alterations in apparent dissociation constants were caused primarily by reduced association rates. Magnesium indicators did not show these changes. The enthalphies of the calcium indicators' Ca complex were 1-3 kcal/mole and magnesium indicators' Mg complex were 7-9 kcal/mole. The potential effects of a biexponential dissociation rate of fluo-3 and of Ca interactions with magnesium indicators were examined.

The loss of enzyme activity from erythroid cells during maturation

Erythrocyte enzyme activities in patients with reticulocytosis or transient erythroblastopenia show that loss of age-dependent enzyme activity is not a simple exponential process occurring throughout the life-span of the cell. In vivo studies of reticulocyte maturation in rabbits indicate that there are multiple mechanisms of enzyme decay, and that proteolysis continues after the maturation of (morphologically recognisable) reticulocytes into young erythrocytes. Most reticulocyte hexokinase is degraded by lysosomal proteolysis, apparently triggered by an initial attack by lipoxygenase.

On the dissociation constants of BAPTA-type calcium buffers

We have determined or redetermined the calcium dissociation constants of seven BAPTA-type buffers with KD's in the range from 0.4 microM to about 20 mM in 300 mM KCl. These include four newly synthesized ones: 5-nitro BAPTA; 5,5'-dinitro BAPTA; 5-methyl-5'-nitro BAPTA; and 5-methyl-5'-formyl BAPTA. Moreover, we tabulate dissociation constants or KD's for BAPTA and eleven BAPTA-type buffers, compare most of them with an empirical curve based upon so-called Hammett values, and predict KD's for several still unsynthesized but potentially valuable buffers.

Calcium buffer injections block fucoid egg development by facilitating calcium diffusion

The polarity of fucoid eggs is fixed either when tip growth starts or a bit earlier. A steady flow of calcium ions into the incipient tip is thought to establish a high calcium zone that is needed for its localization and formation. To test this hypothesis, we have injected seven different 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-type calcium buffers into Pelvetia eggs many hours before tip growth normally starts. Critical final cell concentrations of each buffer prove to block outgrowth (as well as cell division) for up to 2 weeks. This critical inhibitory concentration is lowest for two buffers with dissociation constants or Kd values of 4-5 x 10(-6) M and increases steadily as the buffers' Kd values shift either below or above this optimal value to ones as low as 4 x 10(-7) M or as high as 9.4 x 10(-5) M. To analyze these results, we have derived an equation (based on the concept of facilitated diffusion) for the effects of diffusable calcium buffers on steady-state calcium gradients. The data fit this equation quite well if it is assumed that cytosolic free calcium at the incipient tip is normally kept at about 7 microM and, thus, far above the general cytosolic level.

Kinetics of calcium binding to fluo-3 determined by stopped-flow fluorescence

The kinetics of Ca2+ dissociation from fluo-3 was measured using stopped flow fluorimetry. Analysis of dissociation revealed, in contrast to other commonly used fluorescent Ca2+ indicators, a biexponential behaviour with two distinct dissociation rates of 550 s-1 and 200 s-1 at physiological pH and room temperature. The dissociation rate constant of the fast phase increases to 700 s-1 at physiological temperature, whereas that of the slow phase does not change markedly. While the rate constants do not depend on pH between 6.6 and 7.8, the dissociation turns out to be monoexponential at pH 5.86. The association rate of Ca2+ to fluo-3 could not be measured within the mixing dead time and is estimated to be above 10(9) M-1 s-1. Since the rate constants of fluo-3 are larger than those of other fluorescent Ca2+ indicators, fluo-3 is well suited for investigations of Ca2+ oscillations in biological systems.

Voltage-dependent and calcium-dependent inactivation of calcium channel current in identified snail neurones

1. The dependence of Ca2+ current inactivation on membrane potential and intracellular Ca2+ concentration ([Ca2+]i) was studied in TEA-loaded, identified Helix neurones which possess a single population of high-voltage-activated Ca2+ channels. During prolonged depolarization, the Ca2+ current declined from its peak with two clearly distinct phases. The time course of its decay was readily fitted by a double-exponential function. 2. In double-pulse experiments, the relationship between the magnitude of the Ca2+ current and the amount of Ca2+ inactivation was not linear, and considerable inactivation was present, even when conditioning pulses were to levels of depolarization so great that Ca2+ currents were near zero. Similar results were obtained when external Ca2+ was replaced by Ba2+. 3. In double-pulse experiments, hyperpolarization during the interpulse interval served to reprime a portion of the inactivated Ca2+ current for subsequent activation. The extent of repriming increased with hyperpolarization, reaching a maximum between -130 and -150 mV. The effectiveness of repriming hyperpolarizations was considerably increased when Ca2+ was replaced by Ba2+. 4. A significant fraction of inactivated Ca2+ channels can be recovered during hyperpolarizing pulses lasting only milliseconds. If hyperpolarizing pulses were applied before substantial inactivation of Ca2+ current, Ca2+ channels remained available for activation despite considerable Ca2+ entry. 5. The relationship between [Ca2+]i and inactivation was investigated by quantitatively injecting Ca2+-buffered solutions into the cells. The time course of Ca2+ current inactivation was unchanged at free [Ca2+] between 1 x 10(-7) and 1 x 10(-5) M. From 1 x 10(-7) to 1 x 10(-9) M, inactivation became progressively slower, mainly due to a decrease of the amplitude ratio (fast/slow) of the two components of inactivation, which fell from about unity to near zero at 1 x 10(-9) M. In double-pulse experiments, recovery from inactivation was enhanced in neurones that had been injected with Ca2+ chelator. 6. We conclude that inactivation of Ca2+ channels in these neurones depends on both [Ca2+]i and membrane potential. The voltage-dependent process may serve as a mechanism to quickly recover inactivated Ca2+ channels during repetitive firing despite considerable Ca2+ influx. 7. The results are discussed in the framework of a model which is based on two states of inactivation, INV and INCA, which represent different conformations of the inactivating substrate, and which are both reached from a lumped state of activation (A). Inactivation leads to high occupancy of INV during depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

(Ca2+ + Mg2+)-ATPase activity associated with the maintenance of a Ca2+ gradient by sarcoplasmic reticulum at submicromolar external [Ca2+]. The effect of hypothyroidism

The formation and maintenance of Ca2+-filling levels by sarcoplasmic reticulum vesicles from euthyroid (control) and hypothyroid skeletal muscle were investigated using the Ca2+-indicator quin-2, at [Ca2+] in the medium [( Cao2+]) of 0.05-0.3 microM. Rapid ATP-dependent Ca2+ uptake resulted in a steady-state Ca2+-filling level, Cai2+, within one minute. This Ca2+ gradient was maintained for at least three minutes, during which less than 20% of the ATP was consumed. Cai2+ was maximal (120 nmol/mg) for [Cao2+] greater than 0.3 microM and decreased to 40 nmol/mg at [Cao2+] of 0.05 microM. Preparations from both experimental groups showed qualitatively and quantitatively the same relationship between Cai2+ and [Cao2+] at steady state, despite a significantly lower Ca2+-pump content of hypothyroid sarcoplasmic reticulum, which resulted in a 25% lower maximal (Ca2+ + Mg2+)-ATPase activity. Maintenance of the steady state, at all levels of Cai2+, was associated with net ATP consumption by the Ca2+ pump and cycling of Ca2+, which processes were 30% slower in the hypothyroid group as compared to the control group. Determination of the passive efflux of Ca2+, as well as the fraction of leaky or unsealed sarcoplasmic reticulum fragments, excluded either of these possibilities as an explanation for the relatively high (Ca2+ + Mg2+)-ATPase rates at steady state. On the basis of these and previously reported results, it is concluded that the maintenance of a Ca2+ gradient by sarcoplasmic reticulum under physiological conditions with respect to external [Ca2+] and the concentrations of ATP, ADP and Pi, is associated with the cycling of Ca2+ coupled to net ATP hydrolysis. Using the obtained data it is calculated that the sarcoplasmic reticulum may account for 20% of the resting metabolic rate in skeletal muscle. Consequently, together with the previously reported lower sarcoplasmic reticulum content of skeletal muscle in hypothyroidism, we calculate that about one third of the decrease in basal metabolic rate in this thyroid state can be related to the alterations of the sarcoplasmic reticulum.

Cytosolic calcium staircase in cultured myocardial cells

To elucidate the mechanism of the tension staircase, chick embryonic myocardial cell aggregates were loaded with the fluorescent cytosolic calcium indicator, indo 1. Fluctuations in indo 1 fluorescence were compared with recordings of cell edge movement during spontaneous beating and during stimulation by intracellular current pulses. Indo 1-loaded aggregates exhibit fluorescence transients during each transmembrane action potential. The rising phase of the transients is rapid, but the decaying phase is slow (several hundred msec) and is similar in time course to the pandiastolic relaxation seen in the optical recordings of cell edge movement. Acceleration of beat frequency by brief depolarizing current pulses produces an ascending staircase in both edge movement and systolic [Ca2+]i. There is a similar staircase in the diastolic [Ca2+]i, which is also reflected by diastolic cell edge movement. The existence of a diastolic [Ca2+]i staircase may provide new insight into the mechanism of the force-frequency relation in the heart.

The kinetics of calcium binding to fura-2 and indo-1

The kinetics of Ca2+ dissociation from fura-2 and indo-1 were measured using a stopped-flow spectrofluorimeter. The dissociation rate constants were 84 s-1 and 130 s-1, respectively, in 0.1 M KC1 at 20 degrees C. The rate constants were insensitive to pH over the range 7.0 to 8.0. The second order association rate constants were estimated indirectly to be in the region of 5 X 10(8) M-1 X s-1 and thus approach the diffusion-controlled limit. The results demonstrate that these new generation indicators are well-suited to measure rapid changes in concentration of intracellular Ca2+.

Ca2+-translocation activities of phosphatidylinositol, diacylglycerol and phosphatidic acid inferred by quin-2 in artificial membrane systems

Ca2+-translocating activities of phosphatidylinositol, diacylglycerol and phosphatidic acid were investigated in phosphatidylcholine liposomes. Using a fluorescent indicator of Ca2+ concentration, quin-2, release of encapsulated Ca2+ from egg yolk phosphatidylcholine liposomes containing 2 mol% of one of these lipids was measured at 37 degrees C. The rate of Ca2+ translocation across the liposomal membrane mediated by phosphatidic acid was about 3-fold larger than those mediated by phosphatidylinositol and diacylglycerol. The result implies that phosphatidic acid has Ca2+-ionophore activity in the agonist dependent metabolism of inositol phospholipids. The ionophoretic activity depended on the degree of unsaturation of the fatty acyl chains. The Ca2+ translocation rate was smallest in dipalmitoylphosphatidic acid, and it increased in the order of dioleoyl-, dilinoleoyl- and dilinolenoyl-phosphatidic acid. Ca2+ mobilization of a stimulated cell is discussed in the light of Ca2+-ionophore activity of phosphatidic acid converted from inositol phospholipids.

Kinetics of calcium dissociation from calmodulin and its tryptic fragments. A stopped-flow fluorescence study using Quin 2 reveals a two-domain structure

The kinetics of calcium dissociation from bovine testis calmodulin and its tryptic fragments have been studied by fluorescence stopped-flow methods, using the calcium indicator Quin 2. Two distinct rate processes, each corresponding to the release of two calcium ions are resolved for calmodulin at both low and high ionic strength. The effect of 0.1 M KCl is to accelerate the slow process from 9.1 +/- 1.5 s-1 to 24 +/- 6.0 s-1 and to reduce the rate of the fast process from 650 s-1 to 240 +/- 50 s-1 at 25 degrees C. In the presence of 0.1 M KCl it was possible to determine activation parameters for the fast process: delta H# = 41 +/- 5 kJ mol-1 and delta S# = -63 +/- 17 J K-1 mol-1. These values are in good agreement with those obtained by 43Ca NMR. Studies of the tryptic fragments TR1C and TR2C, comprising the N-terminal or C-terminal half of calmodulin, clearly identified Ca2+-binding sites I and II as the low-affinity (rapidly dissociating) sites and sites III and IV as the high-affinity (slowly dissociating) sites. The kinetic properties of the two proteolytic fragments are closely similar to the fast and slowly dissociating sites of native calmodulin, supporting the idea that calmodulin is constructed from two largely independent domains. The presence of the calmodulin antagonist trifluoperazine markedly decreased the Ca2+ dissociation rates from calmodulin. One of the two high-affinity trifluoperazine-binding sites was found to be located on the N-terminal half and the other on the C-terminal half of calmodulin. The affinity of the C-terminal site is at least one order of magnitude greater.

Dynamic and structural properties of the calcium binding site of bovine serine proteases and their zymogens. A multinuclear nuclear magnetic resonance and stopped-flow study

The combined use of 43Ca and 113Cd nuclear magnetic resonance (n.m.r.) has provided information on the structural and dynamic properties of the calcium binding site located in homologous positions in bovine beta-trypsin, alpha-chymotrypsin and their zymogens. The 43Ca and 113Cd n.m.r. chemical shifts are consistent with an octahedral symmetry of the binding site and with the substitution of one of the two carboxylate ligands present in trypsin(ogen) with a neutral ligand in chymotrypsin(ogen). The constancy of the 113Cd n.m.r. chemical shifts upon binding of the pancreatic trypsin inhibitor and/or the dipeptide Ile-Val to trypsinogen confirms that structural changes in the activation domain do not affect the calcium binding site. The exchange between bound and "free" (solvated) Ca2+ is slow on the 43Ca n.m.r. time-scale for trypsin(ogen), but falls in the intermediate exchange region for chymotrypsin(ogen). In trypsin, the Ca2+ off-rate was measured by stopped-flow making use of the calcium indicator 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid and was found to be 3(+/- 1) s-1. In chymotrypsin(ogen) the off-rates calculated from the 43Ca n.m.r. data are 70 s-1 and 350 s-1, respectively. The dynamic properties of the calcium binding site of serine (pro)enzymes have been related to the flexibility of the binding site itself and have been compared to those of other extracellular and intracellular calcium binding proteins.

Measurement of free Ca2+ changes and total Ca2+ release in a single striated muscle fibre using the fluorescent indicator quin 2

The fluorescent Ca2+ indicator, quin 2, has been used in isolated striated muscle fibres. There is a distinct quin 2 fluorescence peak at lambda 500 nm upon excitation at lambda 339 nm after axial injection of the potassium salt of quin 2, pH 7.1. Single voltage-clamp or current clamp electrical stimulation resulted in a distinct transient change in the fluorescence at lambda 500 nm which was not observed at lambda 400 nm, the peak of the fibre autofluorescence. Ca2+ buffering is marked at high quin 2 concentrations (greater than or equal to 400 microM) producing a slow decay of force and fluorescence. At lower concentrations (8-30 microM) of quin, the decay of force is within the range observed in non-injected control fibres. A Kd of 457 nM at 5 mM free Mg2+ suggests an upper resting free Ca2+ concentration of 310 nM at 12 degrees C.

Kinetic investigations in single muscle fibres using luminescent and fluorescent Ca2+ probes

The Ca2+-sensitive photoprotein aequorin and the Ca2+-dependent fluorescent indicators quin 2 and TnCDANZ have been used to investigate contractile processes in single crustacean muscle fibres. The investigations with quin 2 indicate that the free Ca2+ rises to a maximum value before peak force as with aequorin light (approximately 200 msec delay at 12 degrees C) and subsequently decays more slowly, unlike the majority of the aequorin signal, although an aequorin 'tail' signal remains. The resting quin 2 fluorescence from the cell suggests an upper limit of 348 nM for the resting calcium concentration. Experiments with TnCDANZ indicate that this fluorescence response rises rapidly but then the rate of rise slows to reach a maximum value at a time when peak force is achieved and then the fluorescence signal decays more slowly than force. The latter result implies that Ca2+ is attached to the Ca2+-specific sites of TnC when externally recorded force is small.

Lack of evidence for voltage dependent calcium channels on platelets

Intracellular calcium was measured in human platelets using the fluorescent calcium indicator Quin 2. A concentration dependent increase was observed with thrombin. Depolarisation induced by high KCl concentrations did not alter [Ca++]i. The calcium agonist Bay K 8644 did not affect resting levels or thrombin stimulated elevation of intracellular calcium. The calcium antagonists diltiazem, verapamil and PN 200-110 did not inhibit the thrombin stimulated elevation in [Ca++]i. Pretreatment of platelets with adenylate cyclase stimulants reduced the rate and magnitude of the maximal [Ca++]i elevation due to thrombin. In addition, thrombin stimulation of 45Ca++ influx was insensitive to Bay K 8644, verapamil, diltiazem and Pn 200-110. We conclude that functional voltage sensitive calcium channels are not present on human platelets.