Ana Isabel Ruiz

Voltage inactivation of Ca2+ entry and secretion associated with N‐ and P/Q‐type but not L‐type Ca2+ channels of bovine chromaffin cells

1 In this study we pose the question of why the bovine adrenal medullary chromaffin cell needs various subtypes (L, N, P, Q) of the neuronal high‐voltage activated Ca2+ channels to control a given physiological function, i.e. the exocytotic release of catecholamines. One plausible hypothesis is that Ca2+ channel subtypes undergo different patterns of inactivation during cell depolarization. 2 The net Ca2+ uptake (measured using 45Ca2+) into hyperpolarized cells (bathed in a nominally Ca2+‐free solution containing 1·2 mM K+) after application of a Ca2+ pulse (5 s exposure to 100 mM K+ and 2 mM Ca2+), amounted to 0·65 ± 0·02 fmol cell−1; in depolarized cells (bathed in nominally Ca2+‐free solution containing 100 mM K+) the net Ca2+ uptake was 0·16 ± 0·01 fmol cell−1. 3 This was paralleled by a dramatic reduction of the increase in the cytosolic Ca2+ concentration, [Ca2+]i, caused by Ca2+ pulses applied to fura‐2‐loaded single cells, from 1181 ± 104 nM in hyperpolarized cells to 115 ± 9 nM in depolarized cells. 4 A similar decrease was observed when studying catecholamine release. Secretion was decreased when K+ concentration was increased from 1·2 to 100 mM; the Ca2+ pulse caused, when comparing the extreme conditions, the secretion of 807 ± 35 nA of catecholamines in hyperpolarized cells and 220 ± 19 nA in depolarized cells. 5 The inactivation by depolarization of Ca2+ entry and secretion occluded the blocking effects of combined ω‐conotoxin GVIA (1 μM) and ω‐agatoxin IVA (2 μM), thus suggesting that depolarization caused a selective inactivation of the N‐ and P/Q‐type Ca2+ channels. 6 This was strengthened by two additional findings: (i) nifedipine (3 μM), an L‐type Ca2+ channel blocker, suppressed the fraction of Ca2+ entry (24 %) and secretion (27 %) left unblocked by depolarization; (ii) FPL64176 (3 μM), an L‐type Ca2+ channel ‘activator’, dramatically enhanced the entry of Ca2+ and the secretory response in depolarized cells. 7 In voltage‐clamped cells, switching the holding potential from ‐80 to ‐40 mV promoted the loss of 80 % of the whole‐cell inward Ca2+ channel current carried by 10 mM Ba2+ (IBa). The residual current was blocked by 80 % upon addition of 3 μM nifedipine and dramatically enhanced by 3 μM FPL64176. 8 Thus, it seems that the N‐ and P/Q‐subtypes of calcium channels are more prone to inactivation at depolarizing voltages than the L‐subtype. We propose that this different inactivation might occur physiologically during different patterns of action potential firing, triggered by endogenously released acetylcholine under various stressful conditions.

The performance of natural clay as a barrier to the diffusion of municipal solid waste landfill leachates

In this paper, the diffusion of solutes in natural clay from a concentrated solution consisting primarily of ammonium, sodium and chloride ions at a pH level of 8 was studied and was based on an existing 20-year-old landfill. Contaminant transport through clay liners was predicted using transport and reaction geochemical codes to help explain the experimental data. The model predicted the chloride anion diffusion and cation exchange processes for three different experiments: (1) small-scale interactions in compacted clay, (2) 1:1 European Union (EU) Directive demonstration experiments (0.5-m-thick clay barrier), and (3) analysis of a bore hole with core recovery drilled in an old landfill located above a similar type of clay as that studied in (1) and (2). Orders of magnitude between 10(-10) and 10(-9) m(2) s(-1) were used for the apparent diffusion coefficient to fit the chloride profiles at the different scales; however, at larger space and time scales, diffusion was retarded due to the presence of more consolidated, non-mechanically disturbed clay materials at large depths in a natural clay-rock emplacement.

The role of biomineralization in the origin of sepiolite and dolomite

Sepiolite is an important industrial mineral whose mechanism of formation and detailed origin are still not clear. Sepiolite is frequently found with dolomite, Mg-smectites or palygorskite in the Intermediate Unit of Miocene sediments of the Madrid basin. Herein, we describe mineralogy, texture and mineral chemistry of sepiolite-rich layers (up to 90% concentration) in marl-clay sediments, either with gypsum, chert (micro-crystalline quartz), or dolomite, that are up to 2 to 3 m thick. The materials studied were formed in shallow lake environments or mudflats. These sediments contain dolomite aggregates resembling mineralized microorganisms (biomorphs) with ovoid morphologies and spherical to tubular internal voids. The regular size and relatively uniform shape of the aggregates are likely controlled by the shape and size of the microorganisms. The paper describes the presence of organic debris, silicification processes and potassium silicate depletion related to the basal sediments that preceded sepiolite rich layers. Potassium depletion, as a nutrient, in detrital components and the occurrence of organic and reduced sediment (U and sulfide concentration) at the interface with sepiolite also can be linked to biosignatures related to the origin of sepiolite. In addition, the occurrence of dolomite with carbon that has δ13C PDB values of < −7.5 permil confirms the possibility of an organic carbon source. High concentrations of sepiolite are observed where the amount of dolomite is small. The remaining dolomite in virtually pure sepiolite layers is present as partially dissolved biomorphs or fully recrystallized isolated crystals. Thus, a general process for dolomite dissolution and recrystallization is associated with sepiolite differentiation. In addition, the formation of sepiolite related to the biomineralization of dolomite is likely. The potential role of biomineralization in the formation of these high-grade sepiolite deposits depends on two factors: (1) specific organic-inorganic interactions and (2) the highly porous nature of the sepiolite precursor dolomite during the diagenetic evolution of these sediments. The confirmation of these processes awaits the future development of sepiolite synthesis strategies that incorporate the appropriate biogeochemical reactions.

Calcium signalling mediated through α7 and non-α7 nAChR stimulation is differentially regulated in bovine chromaffin cells to induce catecholamine release

BACKGROUND AND PURPOSE Ca2+ signalling and exocytosis mediated by nicotinic receptor (nAChR) subtypes, especially the α7 nAChR, in bovine chromaffin cells are still matters of debate.

Containment and attenuating layers: An affordable strategy that preserves soil and water from landfill pollution

The performance of a widely distributed natural clay to attenuate contaminants released from an old landfill was investigated. The objective is to evaluate its potential use as a barrier for waste containment systems. Core samples of the natural clay were collected below the landfill and their parameters distribution with depth was determined. Partition coefficients, retardation factors and percentage values of pollutants concentrations, revealed a rapid decrease of contaminants with depth. The background values of the pollutants were below the maximum limits for drinking and irrigation water and with no need of reactors, collectors, aeration or recirculation systems. Impermeable waste capping is discouraged in order to decrease leachate toxicity, decomposition time and conservative species, and in order to avoid high-reducing conditions that would mobilize redox-sensitive contaminants. A review on leachate-composition evolution and on natural-attenuation processes was undertaken to understand the interactions leachate-substratum, which is essential to properly estimate the leachate transport and implement the attenuation strategy. This strategy complements the traditional containment one regarding (1) the susceptibility of engineering liners to fail, (2) the inevitable diffusion of contaminants through them, (3) the remaining high number of old landfills before the requirements of liner systems and (4) the low-cost and feasibility for developing countries.

Formation of C-A-S-H phases from the interaction between concrete or cement and bentonite

Abstract Concrete and bentonite are being considered as engineered barriers for the deep geological disposal of high-level radioactive waste in argillaceous rocks. Three hydrothermal laboratory experiments of different scalable complexity were performed to improve our knowledge of the formation of calcium aluminate silicate hydrates (C-A-S-H) at the interface between the two materials: concretebentonite transport columns, lime mortar-bentonite transport columns and a portlandite- (bentonite and montmorillonite) batch experiment. Precipitation of C-A-S-H was observed in all experiments. Acicular and fibrous morphologies with certain laminar characteristics were observed which had smaller Ca/Si and larger Al/Si ratios with increasing temperature and lack of accessory minerals. The compositional fields of these C-A-S-H phases formed in the experiments are consistent with Al/(Si+Al) ratios of 0.2- 0.3 described in the literature. The most representative calcium silicate hydrate (C-S-H) phase from the montmorillonite-cement interface is Al-tobermorite. Structural analyses revealed a potential intercalation or association of montmorillonite and C-A-S-H phases at the pore scale.

Processes and impacts of acid discharges on a natural substratum under a landfill.

Analyses of substratum samples under a landfill were performed to assess the pollution impact of waste over a clay-sand material after nine years of exposure. These samples presented different illite/kaolinite ratios and an acid pH, especially low near the waste/soil contact in a 1-1.5 m soil thickness with low density and despite the basic pH of the collected actual methanogenic leachate. This study has raised the effects of a presumably acid stage in the waste leachate on the substratum final quality of clay and its physical-chemical properties as an attenuation buffer. These effects were the dissolution of carbonate minerals, decrease of dry density, increase of hydraulic conductivity, release of metals and formation of clays with low cationic exchange capacity (CEC) as kaolinite. The large presence of H(+) and Al(OH)(3-x)(x+) depleted the neutralizing capacity of the substratum and occupied exchangeable sites, decreasing therefore the available sites for retaining leachate pollutants, which traveled further than the first-meter depth of the substratum. In order to combat and prevent pollution as well as to preserve the good barrier properties under new landfills it is proposed to select illitic materials better than kaolinitic substratum, to avoid acid landfilling and if not possible to add lime.

Improvement of attenuation functions of a clayey sandstone for landfill leachate containment by bentonite addition.

Enhanced sand-clay mixtures have been prepared by using a sandstone arkosic material and have been evaluated for consideration as landfill liners. A lab-scale test was carried out under controlled conditions with different amended natural sandstones whereby leachate was passed through the compacted mixtures. The compacted samples consisted of siliceous sand (quartz-feldspar sand separated from the arkose sandstone) and clay (purified clay from arkose sandstone and two commercial bentonites) materials that were mixed in different proportions. The separation of mineral materials from a common and abundant natural source, for soil protection purposes, is proposed as an economic and environmentally efficient practice. The liner qualities were compared for their mineralogical, physicochemical and major ions transport and adsorption properties. Although all samples fulfilled hydraulic conductivity requirements, the addition of bentonite to arkose sandstone was determined to be an effective strategy to decrease the permeability of the soil and to improve the pollutants retention. The clay materials from arkose sandstone also contributed to pollutant retention by a significant improvement of the cation exchange capacity of the bulk material. However, the mixtures prepared with clay materials from the arkose, exhibited a slight increase of hydraulic conductivity. This effect has to be further evaluated.

Nature of C-(A)-S-H Phases Formed in the Reaction Bentonite/Portlandite

Reactions between bentonite/montmorillonite and portlandite have been studied in the context of the engineered barriers of a purpose built repository for the deep geological disposal of spent fuel and high-level radioactive wastes. Portlandite was selected as the representative material of cement leaching in the early alkaline stage expected in a repository when conventional Ordinary Portland Cement (OPC) is used. Eight different batch experiments were performed for a reaction time near to two months, including bentonite or montmorillonite at montmorillonite/portlandite molar ratios of 2 : 1 and 3 : 1 under hydrothermal conditions. Temperatures of reactions were maintained constant at either 60 or 120°C. Calcium silicates hydrates with limited substitution of Al for Si (C-(A)-S-H phases with Al/Si <0.3) were formed with different structures and compositions as a function of the reaction conditions. Orthorhombic 11 A-tobermorite-type phase was detected in experiments at 120°C while a more disordered monoclinic tobermorite formed at 60°C. These results are useful for the interpretation of experimental data in more complex experiments using concrete or cement pastes and bentonite, where C-(A)-S-H phases of variable compositions can precipitate, in addition to the characteristic cement hydrates and other secondary minerals carbonates.

Resistant cellulose-derivative biopolymer templates in natural sepiolite

Sepiolite is a fibrous sheet silicate with a composition of Si12Mg8 (H2O)4(Si6O15)2(OH)4 · 8H2O and has significant physicochemical and rheological properties due to the presence of zeolitic channels in its structure. The highest concentrations of sepiolite worldwide are found in the Madrid Miocene Basin within the edges of saline lakes and are associated with dolomitic and siliceous microbialites. It is remarkable that monomineral sepiolite sediments up to 6 m thick are composed of 6 nm crystallite-sized fibers. The aim of this study is to gather information concerning the organomineralization of sepiolite. The mineral paragenetic sequences associated with this mineral were characterized by XRD in representative samples. The texture was studied by polarized light microscopy and by SEM-, TEM- and HRTEM-EDX. The remains of mineralized biomass are commonly found mixed with sepiolite fibers. Lipidic biomarkers reveal the prevalence of non-branched derivatives of short-chain fatty-acids (C9 to C18), which are characteristic of cyanobacteria and microalgae and support the presence of microorganisms within the studied sediments. The textural organization of sepiolite fibers is similar to the fabric of cellulose fibers produced by microorganisms. There are two types of fiber associations: short and straight fibers (<2 μm length) intertwined in compact bundles and long curved fibers (>10 μm length) forming thin tissues that mimic cellulose and other exopolymers produced by bacteria (for example, acetobacter, cyanobacteria). The similarity is extended to the nanometric scale. Sepiolite is composed of 6 to 10 nm-thick fibers that form mesocrystals (oriented nanocrystal aggregates) that are 50 to 200 nm in length. Based on an XRD analysis of the crystal size distribution, this average fibril thickness is the same as that of cellulose. The growth of sepiolite is limited to the size of cellulose crystallites. The cellobiose unit size is similar to the size of the zeolitic channels of sepiolite (1.04 nm) in the β-[1.4] linkage propagating direction, in which a 180° rotation is produced, as well as to the SiO4 tetrahedra in sepiolite. Its formation is controlled by the orientation and repeating distance of polar groups (OH and derivatives), with numerous defects and disruptions that coincide with amorphous phases of cellulose. The maintenance of polar or ionic characteristics in the organic reticulum and its orientation is essential for the nucleation of sepiolite. This fact makes its recrystallization difficult, which, in turn, proves the high geochemical stability of sepiolite in spite of its nanometric nature.