Function Of Calcium Concentration Research Articles

Controls on the Precipitation of Carbonate Minerals Within Marine Sediments

The vast majority of carbonate minerals in modern marine sediments are biogenic, derived from the skeletal remains of organisms living in the ocean. However, carbonate minerals can also precipitate abiotically within marine sediments, and this carbonate mineral precipitation within sediments has been suggested as a third major, and isotopically distinct, sink in the global carbon cycle, particularly important earlier in Earth history. Here we present a global compilation of pore fluid data and compare the sulfate, calcium, phosphate and magnesium concentrations with pore fluid alkalinity to explore the emerging relationships and explore what drives carbonate mineral precipitation in sediments. Our data compilation shows that the gradient of pore fluid sulfate concentrations correlates strongly with the gradient of alkalinity as well as with the gradient of calcium, and that these correlations improve dramatically in sediments where methane is present. We also note that sedimentary pore fluids that are high in phosphate concentration are also high in alkalinity, which may indicate suppression of carbonate mineral precipitation in the presence of sedimentary phosphate. Our data can be used to highlight sediments where both dolomite formation and dolomitization of previously deposited calcium carbonate minerals is occurring. We explore how carbonate mineral saturation state changes as a function of calcium concentrations, alkalinity, and pH, and suggest a reason why calcium concentrations are never fully depleted in sedimentary pore fluids. We conclude that carbonate minerals precipitate in sediments with methane, where the anaerobic oxidation of this methane helps promote particularly high saturation states for carbonate minerals.

Negative charge of the AC-to-Hly linking segment modulates calcium-dependent membrane activities of Bordetella adenylate cyclase toxin

Two distinct conformers of the adenylate cyclase toxin (CyaA) appear to accomplish its two parallel activities within target cell membrane. The translocating conformer would deliver the N-terminal adenylyl cyclase (AC) enzyme domain across plasma membrane into cytosol of cells, while the pore precursor conformer would assemble into oligomeric cation-selective pores and permeabilize cellular membrane. Both toxin activities then involve a membrane-interacting ‘AC-to-Hly-linking segment’ (residues 400 to 500). Here, we report the NMR structure of the corresponding CyaA411–490 polypeptide in dodecylphosphocholine micelles and show that it consists of two α-helices linked by an unrestrained loop. The N-terminal α-helix (Gly418 to His439) remained solvent accessible, while the C-terminal α-helix (His457 to Phe485) was fully enclosed within detergent micelles. CyaA411–490 weakly bound Ca2+ ions (apparent KD 2.6 mM) and permeabilized negatively charged lipid vesicles. At high concentrations (10 μM) the CyaA411–490 polypeptide formed stable conductance units in artificial lipid bilayers with applied voltage, suggesting its possible transmembrane orientation in the membrane-inserted toxin. Mutagenesis revealed that two clusters of negatively charged residues within the ‘AC-to-Hly-linking segment’ (Glu419 to Glu432 and Asp445 to Glu448) regulate the balance between the AC domain translocating and pore-forming capacities of CyaA in function of calcium concentration.

Human Connexin 26 (Cx26) N14K Mutant Alters Hemichannel Calcium and Voltage Sensitivity

Unpaired hemichannels, those not forming gap junction channels, play an autocrine/paracrine role by releasing transmitters, such as glutamate and ATP, to the extracellular medium. Hemichannels are gated by changes in extracellular Ca+2 concentration and membrane voltage. Several human connexin mutations that produce diseases increase hemichannel opening (e.g., gain of function due to decreased sensitivity to extracellular Ca+2). Using Xenopusoocytes and two-electrode voltage clamp techniques, we studied the voltage and calcium sensitivity of a connexin 26 (Cx26) mutation, N14K, located in the N-terminal domain (NT). The NT is thought to fold into the pore and form part of the pore lining; position N14 is approximately at the “hinge” region at which the NT bends toward the pore. We observe a significant leftward shift in the voltage-dependence N14K (midpoint at - 6.7 mV ± 0.7) mutants with respect to wild type channels (midpoint at + 9.6 mV ± 0.9). Rearrangements of the NT are associated with voltage dependent sub-conductances in wild type Cx26 hemichannels, however, N14K mutant channels did not display voltage dependent sub-conductances. The apparent calcium affinity was also significantly reduced in N14K (KD = 0.74 mM) compared to wild type channels (KD = 0.34 mM). Deactivation kinetics in wild type channels show single mono-exponential relaxations that are accelerated as a function of calcium concentration, but not voltage. The Cx26 mutant N14K displays two slower components. One of these components is both Ca+2 and voltage dependent but the other is not. Overall, our data indicate that under physiological conditions the N14K mutation stabilizes the open state relative to wild type channels by reducing both calcium and voltage sensitivity. R01GM099490/R01GM101950.

Solid-State NMR and FTIR Study of a Neuronal Calcium Sensor (NCS) Protein, Recoverin

Recoverin is a peripheral protein present in retinal photoreceptors. This protein is one of the fourteen members of the neuronal calcium sensor protein (NCS) family. These proteins have different functions such as phototransduction in the case of recoverin. The conformation of these proteins varies as a function of calcium concentration. At low Ca2+ concentration, recoverin is in a cytosolic form with its myristoyl group sequestered into a hydrophobic cavity and the binding of 2 Ca2+ results in the extrusion of the myristoyl group. This is the so-called Calcium Myristoyl Switch. The study of the structure and membrane binding of recoverin in the absence and presence of calcium is therefore of great interest. On the other hand, the membrane composition of the rod outer segments is more than 60% of polyinsatured lipids. This parameter could therefore modulate the membrane interaction and binding of recoverin. We have investigated in the present study the conformation of recoverin in solution with and without calcium and myristoyl. We have also investigated the interaction of recoverin with phospholipids with different polar headgroups and acyl chains using a combination of solid-state NMR and FTIR spectroscopies as well as the effect of cholesterol at a 10% molar ratio in membranes. Our results demonstrate that protein aggregation is favored in the absence of calcium and for the non myristoylated form of the protein. In addition, 31P solid-state MAS NMR results suggest an important role of the myristoyl group and of the hydrophobic pocket of recoverin in its interaction with lipids. The investigation of the membrane binding of recoverin with a deuterated myristoyl group by 2H solid-state NMR at 900 MHz clearly demonstrates a Calcium Myristoyl Switch for recoverin for specific lipids.

Interfacial Shear Rheology of Calcium Naphthenate at the Oil/Water Interface and the Influence of pH, Calcium, and in Presence of a Model Monoacid

In this article, the interfacial shear rheological properties of calcium naphthenate with a model tetraacid at the chloroform/xylene-water interface has been investigated as a function of aqueous pH, calcium concentration and monoacid concentration. The experiments are carried out using an interfacial rheology system with an electro commutated motor, direct strain oscillation and a biconical bob geometry. The model tetraacid used, BP10, has previously been shown to have similar bulk and interfacial properties as a narrow group of tetraprotic, so-called Arn acids, and these acids are known to be responsible for formation of hard deposits during oil recovery. A great increase in the elastic modulus was observed around pH 6.2, which is in agreement with observations from oil fields with calcium naphthenate deposition problems. The gel strength and elastic nature is highest around the gelation onset, believed to be due to a bilayer-like conformation of the tetraacid generating a densely packed interface with high cross-linking density and possible film growth. As a function of calcium concentration, both a reduction of the gel strength and slower gel formation was observed when decreasing the calcium concentration from 10 to 4 mM. Myristic acid, a linear C14 fatty acid, was employed as a model for indigenous monoacids and the influence onto the viscoelastic properties of the Ca2+-TA film was studied as a function of myristic acid concentration at pH 8.0 and 6.5. A great reduction of both the gel strength and elasticity was obvious in the range of 100 to 1000 higher monoacid than tetraacid concentration. This is however typical indigenous acid concentrations for an acidic crude oil, and may indicate that indigenous monoacids have the ability to act as indigenous inhibitors towards formation of calcium naphthenate. This could explain why some Arn-containing acidic crude oils have deposition problems while others do not. Moreover, all parameters should be taken into account when predicting the deposition risk for a given crude oil, such as concentrations of Ca2+, Arn, monoacids and other indigenous acids.

Biochemical analysis of the interaction of calcium with toposome: A major protein component of the sea urchin egg and embryo

We have investigated the biochemical and functional properties of toposome, a major protein component of sea urchin eggs and embryos. Atomic force microscopy was utilized to demonstrate that a Ca(2+)-driven change in secondary structure facilitated toposome binding to a lipid bilayer. Thermal denaturation studies showed that toposome was dependent upon calcium in a manner paralleling the effect of this cation on secondary and tertiary structure. The calcium-induced, secondary, and tertiary structural changes had no effect on the chymotryptic cleavage pattern. However, the digestion pattern of toposome bound to phosphatidyl serine liposomes did vary as a function of calcium concentration. We also investigated the interaction of this protein with various metal ions. Calcium, Mg(2+), Ba(2+), Cd(2+), Mn(2+), and Fe(3+) all bound to toposome. In addition, Cd(2+) and Mn(2+) displaced Ca(2+), prebound to toposome, while Mg(2+), Ba(2+), and Fe(3+) had no effect. Collectively, these results further enhance our understanding of the role of Ca(2+) in modulating the biological activity of toposome.

The binding of HIV-1 gp41 membrane proximal domain to its mucosal receptor, galactosyl ceramide, is structure-dependent

The peptide of HIV-1 envelope gp41 (a.a 628–683), referred to herein as P5, contains P1, a conserved galactose-specific lectin domain for binding the mucosal HIV-1-receptor, galactosyl ceramide (GalCer), as shown earlier, and a potential calcium-binding site (a.a 628–648). P1 contains contiguous epitopes recognized by the broadly neutralizing antibodies 2F5, 4E10, Z13. However, similar neutralizing antibodies could not be raised in animal model using immunogens based on these epitopes. We now show that the structure of both P5 and P1 peptides, as measured by circular dichroism, differs according to their environment: aqueous or lipidic, and as a function of calcium concentration. P5, but not P1, binds to calcium with a low binding affinity constant in the order of 2.5 × 10 4. Calcium binding results in a conformational change of P5, leading in turn to a decrease in affinity for GalCer. Hence, the affinity of the gp41-lectin site for the galactose harbored by the mucosal HIV-1 receptor GalCer is modulated by the peptide secondary and tertiary structure and the local environment. Therefore, definition of the conformation of this novel extended gp41 membrane proximal region, containing the conserved peptide P1 and the Ca 2+ binding site, could help designing an immunogen efficient at inducing neutralizing anti-HIV-1 antibodies.

Water Hardness as a Photochemical Parameter: Tetracycline Photolysis as a Function of Calcium Concentration, Magnesium Concentration, and pH

The environmental photochemical kinetics of the antibiotic compound tetracycline were investigated. The aqueous speciation of tetracycline over a range of natural pH and water hardness values is dominated by association with Ca2+ and Mg2+ ions. The association constants necessary to calculate tetracycline aqueous speciation given knowledge of pH, [Ca2+], and [Mg2+] were measured by spectrophotometric titrations and matrix deconvolution of a series of UV-vis absorption spectra into individual component species. A series of photolysis experiments was performed under simulated sunlight, and quantum yields for the solar photolysis of each environmentally relevant species were calculated. The results indicate that the pseudo-first-order rate constant for tetracycline photolysis at varied Mg2+ and Ca2+ concentrations relevant to natural conditions can vary by up to an order of magnitude. A self-sensitization effect was observed and was accounted for by varying the initial tetracycline concentration under each set of photolysis conditions.

Powder X-ray diffraction and Rietveld analysis of La1−xCaxMnO3(0<X<1)

Single-phase samples of La1−xCaxMnO3compounds have been synthesized by solid-state reaction. Detailed Rietveld analysis of powder X-ray diffraction data showed that La1−xCaxMnO3undergoes a structural phase transformation fromR-3ctoPnmaatX=0.05–0.075. There is a linear variation of lattice parameters as a function of calcium concentration. The slope of this linear variation changes atX=0.6. A correlation between observed Mn-O bond lengths, corresponding to different compositions, and possible magnetic phases is discussed.

Preparation and effect of Ca 2+ on water solubility, particle release and swelling properties of magnetic alginate films

Magnetic natural films composed of alginate and maghemite nanoparticles are studied. A surface treatment by citrate ions of the magnetic nanoparticles is first required to limit interactions with carboxylate functions of alginate and to stabilize them in neutral pH. Sodium alginate films, with or without nanoparticles, are immersed in a calcium chloride bath to convert them into mixed sodium/calcium alginate films. The ion exchange process is quantified by the degree of substitution (DS) deduced from sodium and calcium content obtained by atomic absorption spectroscopy. The magnetic nanoparticles content is also analysed to correlate the release of the particles to the amount of calcium present in the film. Nanoparticles do not significantly change the ion exchange process. Water insoluble films, which do not release magnetic nanoparticles, are obtained for a complete conversion of sodium alginate into calcium alginate (DS value is thus close to the stoechiometric ratio equal to 0.5). Such increase in water resistance of alginate films is caused by the formation of a dense network by crosslinking of the alginate polymer with Ca ions which prevents the alginate from going out of the film. Swelling properties of the mixed films are then investigated as a function of calcium concentration and nanoparticles content. It is found that the swelling ability in water of the films decreases by increasing the immersion time in CaCl 2 bath, the swelling ratio is thus a measure of the extent of crosslinking

Effect of calcium on rat intestinal alkaline phosphatase activity and molecular aggregation

Two fractions of rat intestinal alkaline phosphatase (IAP) were detected by Western blot: 168 ± 6 and 475 ± 45 kDa. The low molecular weight fraction constitutes 43% of the isolated proteins exhibiting 82% of the enzymatic activity, and a heavier fraction constitutes 57% of the isolated proteins and has 18% of the enzymatic activity. Calcium produced an increase of the 475-kDa form to the detriment of the 168-kDa form. This work also describes the kinetic and structural changes of IAP as a function of calcium concentration. With [Ca2+] < 10 mmole/L, the Ca2+-IAP interaction fitted a binding model with 7.8 ± 4.4 moles of Ca2+ /mole of protein, affinity constant = 19.1 ± 8.4 L/mmole, and enzymatic activity increased as a linear function of [Ca2+] (r = 0.946 p < 0.01). On the other hand, with [Ca2+] >10 mmole/L the data did not fit this model and, the enzymatic activity decreased as a function of [Ca2+] (r = − 0.703 p < 0.05).

A Flagellum-specific Calcium Sensor

The flagellar calcium-binding protein (FCaBP) of the flagellated protozoan Trypanosoma cruzi associates with the flagellar membrane via its N-terminal myristate and palmitate moieties in a calcium-modulated, conformation-dependent manner. This mechanism of localization is similar to that described for neuronal calcium sensors, which undergo calcium-dependent changes in conformation, which modulate the availability of the acyl groups for membrane interaction and partner association. To test whether FCaBP undergoes a calcium-dependent conformational change and to explore the role of such a change in flagellar targeting, we first introduced point mutations into each of the two EF-hand calcium-binding sites of FCaBP to define their affinities. Analysis of recombinant EF-3 mutant (E151Q), EF-4 mutant (E188Q), and double mutant proteins showed EF-3 to be the high affinity site (Kd approximately 9 microM) and EF-4 the low affinity site (Kd approximately 120 microM). These assignments also correlated with partial (E188Q), nearly complete (E151Q), and complete (E151Q,E188Q) disruption of calcium-induced conformational changes determined by NMR spectrometry. We next expressed the FCaBP E151Q mutant and the double mutant in T. cruzi epimastigotes. These transproteins localized to the flagellum, suggesting the existence of a calcium-dependent interaction of FCaBP that is independent of its intrinsic calcium binding capacity. Several proteins were identified by FCaBP affinity chromatography that interact with FCaBP in a calcium-dependent manner, but with differential dependence on calcium-binding by FCaBP. These findings may have broader implications for the calcium acyl switch mechanism of protein regulation.

The Effects of Mutation on the Regulatory Properties of Phospholamban in Co-Reconstituted Membranes

Reconstitution into proteoliposomes is a powerful method for studying calcium transport in a chemically pure membrane environment. By use of this approach, we have studied the regulation of Ca(2+)-ATPase by phospholamban (PLB) as a function of calcium concentration and PLB mutation. Co-reconstitution of PLB and Ca(2+)-ATPase revealed the expected effects of PLB on the apparent calcium affinity of Ca(2+)-ATPase (K(Ca)) and unexpected effects of PLB on maximal activity (V(max)). Wild-type PLB, six loss-of-function mutants (L7A, R9E, I12A, N34A, I38A, L42A), and three gain-of-function mutants (N27A, L37A, and I40A) were evaluated for their effects on K(Ca) and V(max). With the loss-of-function mutants, their ability to shift K(Ca) correlated with their ability to increase V(max). A total loss-of-function mutant, N34A, had no effect on K(Ca) of the calcium pump and produced only a marginal increase in V(max). A near-wild-type mutant, I12A, significantly altered both K(Ca) and V(max) of the calcium pump. With the gain-of-function mutants, their ability to shift K(Ca) did not correlate with their ability to increase V(max). The "super-shifting" mutants N27A, L37A, and I40A produced a large shift in K(Ca) of the calcium pump; however, L37A decreased V(max), while N27A and I40A increased V(max). For wild-type PLB, phosphorylation completely reversed the effect on K(Ca), but had no effect on V(max). We conclude that PLB increases V(max) of Ca(2+)-ATPase, and that the magnitude of this effect is sensitive to mutation. The mutation sensitivity of PLB Asn(34) and Leu(37) identifies a region of the protein that is responsible for this regulatory property.

Modeling Rheological Characteristics and Calcium Content of Mozzarella Cheese

ABSTRACT: Correlation and regression analyses of calcium concentration and rheological characteristics in Mozzarella cheese were studied. Part‐skim and fat‐free Mozzarella cheeses were manufactured on 4 separate occasions. Calcium concentration in both types of cheeses was manipulated at 4 different levels. Rheological characteristics such as melt area, softening, melting, and flow were studied on days 1, 7, 15, and 30. The results revealed that 57%, 52%, 66%, 74%, 67%, and 53% variation, respectively, in melt area, softening temperature, softening time, melting time, flow rate, and extent of flow could be explained as a function of calcium concentration of part‐skim Mozzarella cheese. Calcium concentration explained 50%, 61%, and 43% variation, respectively, in melting time, flow rate and extent of flow, but poor correlation existed between calcium concentration and melt area, softening and melting time‐temperatures of fat‐free Mozzarella cheese. Calcium concentration, among all compositional parameters, was highly correlated with rheological characteristics of Mozzarella cheeses. Calcium concentration was also one of the major predictor variables in regression models, which were developed to decide the rheological characteristics of Mozzarella cheese. Based on the above correlation the cheese industry can develop a strategy to obtain specific characteristics in Mozzarella cheese for varied applications in the food systems.

Calcium-binding site beta 2, adjacent to the "b" polymerization site, modulates lateral aggregation of protofibrils during fibrin polymerization.

Structural analysis of recombinant fibrinogen fragment D revealed that the calcium-binding site (beta2-site) composed of residues BbetaAsp261, BbetaAsp398, BbetaGly263, and gammaGlu132 is modulated by the "B:b" interaction. To determine the beta2-site's role in polymerization, we engineered variant fibrinogen gammaE132A in which calcium binding to the beta2-site was disrupted by replacing glutamic acid at gamma132 with alanine. We compared polymerization of gammaE132A to normal fibrinogen as a function of calcium concentration. Polymerization of gammaE132A at concentrations of calcium <or=1 mM exhibited an uncharacteristic 2-3-fold increase in lateral aggregation and fiber thickness compared to normal fibrinogen, while polymerization of variant and normal were indistinguishable at 10 mM calcium. These results suggest that the beta2-site controls the extent of lateral aggregation. That is, when the calcium anchor (beta2-site) is eliminated before "B:b" interactions occur then lateral aggregation is enhanced. We solved structures of fragment D of gammaE132A fibrinogen (rfD-gammaE132A) with and without Gly-His-Arg-Pro-amide (GHRPam) and found no change to the global structure. X-ray diffraction data showed GHRPam binding in the "a" and "b" polymerization sites and that calcium could still bind to the beta2-site of gammaE132A fibrinogen at 70 mM calcium. We found that the gamma2 calcium-binding site (in loop gamma294-301) did not have calcium bound in the structure of fragment D of gammaE132A fibrinogen with GHRPam bound (rfD-gammaE132A+GH). Analysis of structures rfD-gammaE132A+GH and rfD-BbetaD398A+GH indicated that differences in calcium occupation of the gamma2-site resulted from minor conformational changes provoked by crystal packing and GHRPam binding to the "a" site did not directly modulate calcium binding to this site.

Conformational Changes in Guanylyl Cyclase-activating Protein 1 (GCAP1) and Its Tryptophan Mutants as a Function of Calcium Concentration

Guanylyl cyclase-activating proteins (GCAPs are 23-kDa Ca2+-binding proteins belonging to the calmodulin superfamily. Ca2+-free GCAPs are responsible for activation of photoreceptor guanylyl cyclase during light adaptation. In this study, we characterized GCAP1 mutants in which three endogenous nonessential Trp residues were replaced by Phe residues, eliminating intrinsic fluorescence. Subsequently, hydrophobic amino acids adjacent to each of the three functional Ca2+-binding loops were replaced by reporter Trp residues. Using fluorescence spectroscopy and biochemical assays, we found that binding of Ca2+ to GCAP1 causes a major conformational change especially in the region around the EF3-hand motif. This transition of GCAP1 from an activator to an inhibitor of GC requires an activation energy Ea = 9.3 kcal/mol. When Tyr99 adjacent to the EF3-hand motif was replaced by Cys, a mutation linked to autosomal dominant cone dystrophy in humans, Cys99 is unable to stabilize the inactive GCAP1-Ca2+ complex. Stopped-flow kinetic measurements indicated that GCAP1 rapidly loses its bound Ca2+ (k-1 = 72 s-1 at 37 degrees C) and was estimated to associate with Ca2+ at a rate (k1 > 2 x 10(8) M-1 s-1) close to the diffusion limit. Thus, GCAP1 displays thermodynamic and kinetic properties that are compatible with its involvement early in the phototransduction response.

Mapping Subdomains in the C-terminal Region of Troponin I Involved in Its Binding to Troponin C and to Thin Filament

Troponin I (TnI) is the inhibitory component of troponin, the ternary complex that regulates skeletal and cardiac muscle contraction. Previous work showed that the C-terminal region of TnI, when linked to the "inhibitory region" (residues 98-116), possesses the major regulatory functions of the molecule (Farah, C. S., Miyamoto, C. A., Ramos, C. H. I., Silva, A. C. R., Quaggio, R. B., Fujimori, K., Smillie, L. B., and Reinach, F. C. (1994) J. Biol. Chem. 269, 5230-5240). To investigate these functions in more detail, serial deletion mutants of the C-terminal region of TnI were constructed. These experiments showed that longer C-terminal deletions result in lower inhibition of the actomyosin ATPase activity and weaken the interaction with the N-terminal domain of troponin C (TnC), consistent with the antiparallel model for the interaction between these two proteins. The conclusion is that the whole C-terminal region of TnI is necessary for its full regulatory activity. The region between residues 137 and 144, which was shown to have homology with residues 108-115 in the inhibitory region (Farah, C. S., and Reinach, F. C. (1995) FASEB J. 9, 755-767), is involved in the binding to TnC. The region between residues 98 and 129 is involved in modulating the affinity of TnC for calcium. The C-terminal residues 166-182 are involved in the binding of TnI to thin filament. A model for the function of TnI is discussed.

Aqueous solutions of acrylamide-acrylic acid copolymers: stability in the presence of alkalinoearth cations

The interactions between acrylamide-acrylic acid copolymers and alkalinoearth cations were studied by many different techniques, as functions of various parameters: composition of the copolymer and distribution of the acrylic units along the chain; pH; temperature; ionic strength. In a first step, the phase diagrams were established and reveal ranges of polymer and salt concentrations where phase separation occurs. In a second series of experiments, the change in the chain conformation and the aggregation phenomena were investigated by viscosimetry and light scattering. It sorts out that at low enough polymer and salt concentrations (inside the solubility domain), it is possible to get a variation of the intrinsic viscosity of the polymer as a function of calcium concentration and charge parameter of the polymer consistent with previous literature results obtained in the presence of sodium if a binding term depending on the nature of the ion is taken into account. At higher polymer and calcium concentration, high Huggins constants and large increases of scattered intensity reveal the formation of aggregates, which are attributed to intermolecular bridges. The third series of experiments (conductimetry and densimetry) were carried out to characterize the ion-polyion interactions. Finally, the phase separation is discussed in light of the other results, using some simple association models.

A MATHEMATICAL MODEL FOR THE SPATIO-TEMPORAL DYNAMICS OF INTRINSIC PATHWAY OF BLOOD COAGULATION. I. THE MODEL DESCRIPTION

We developed and analyzed the mathematical model of the intrinsic pathway based on the current biochemical data on the kinetics of blood coagulation individual stages. The model includes eight differential equations describing the spatio-temporal dynamics of activation of factors XI, IX, X, II, I, VIII, V, and protein C. The assembly of tenase and prothrombinase complexes is considered as a function of calcium concentration. The spatial dynamics of coagulation was analyzed for the one-dimensional case. We examined the formation of active factors, their spreading, and growth of the clot from the site of injury in the direction perpendicular to the vessel wall, into the blood thickness. We assumed that the site of injury (in the model one boundary of the space segment under examination) becomes a source of the continuous influx of factor XIa. In the first part, we described the model, selected the parameters, etc. In the second part, we compared the model with experimental data obtained in the homogeneous system and analyzed the spatial dynamics of the clot growth. Copyright © 1996 Elsevier Science Ltd

Cross-bridge movement in rat slow skeletal muscle as a function of calcium concentration.

A single fibre bundle from rat soleus muscle was chemically skinned with saponin and the transfer of myosin heads from the thick filaments to the thin filaments at a sarcomere length of 2.4 microm was measured as a function of Ca2+ concentration using an x-ray diffraction method at 4-7 degrees C. In the relaxed state, the 1,0 spacing was 42.08 nm. The spacing showed no significant decrease when the Ca2+ concentration was below the threshold (-log10 [Ca2+] or pCa 5.8). No significant transfer of the myosin heads occurred when the Ca2+concentration was below the threshold (pCa 5.8). When the muscle was maximally activated at pCa 4.4, the spacing decreased to 40.35 nm. During the maximum isometric contraction at pCa 4.4, 54. 9 +/- 6.5% (+/-SE of the mean) of the myosin heads were transferred to the thin filaments. The transfer of the myosin heads was approximately proportional to relative tension. These results suggest that myosin heads of both fast-twitch and slow-twitch skeletal muscles transferred on the common movement as a function of Ca2+ concentration.