Low Barium Concentrations Research Articles

GEOCHEMICAL MARKERS OF THE JOINT STRUCTURAL AND COMPOSITIONAL EVOLUTION OF THE BASEMENT AND COVER (NORTHERN LADOGA SWECOFENNIDES, RUSSIA)

Detailed studies of the deeply metamorphosed Early Precambrian rocks of the Northern Ladoga region allowed us to distinguish three deformation stages of the Svecofennian tectogenesis during which there occurred significant structural and compositional transformations of the "cover (Paleoproterozoic) – basement (Archean)" system. In addition to the structural-paragenetic analysis, which allowed to allocate transversal structural paragenesis in both floors, there are some other opportunities in the recognition of their-hosted granitoid veined bodies with a positive Eu anomaly. The rock varieties with this anomaly are always high in barium and do not show a direct correlation between the Eu anomaly and (La/Yb)n, Ca and Sr. This is contrary to the ideas about the occurrence of a positive Eu anomaly due to the substitution of divalent strontium by Eu++ and suggests that the formation of such rocks took place under the influence of deep reduced fluids. It was found that granitoids with a positive Eu anomaly were formed during the first and last stages of the structure evolution, with a predominance of brittle deformations and a deep-reduced fluid breakthrough. At the second stage, with the dominant manifestation of plastic deformations, when such fluids could be "blocked" within the system, there was a formation of granitoids with low barium concentrations and a negative Eu anomaly.

Somatic and dendritic GABAB receptors regulate neuronal excitability via different mechanisms

GABA(B) receptors play a key role in regulating neuronal excitability in the brain. Whereas the impact of somatic GABA(B) receptors on neuronal excitability has been studied in some detail, much less is known about the role of dendritic GABA(B) receptors. Here, we investigate the impact of GABA(B) receptor activation on the somato-dendritic excitability of layer 5 pyramidal neurons in the rat barrel cortex. Activation of GABA(B) receptors led to hyperpolarization and a decrease in membrane resistance that was greatest at somatic and proximal dendritic locations. These effects were occluded by low concentrations of barium (100 μM), suggesting that they are mediated by potassium channels. In contrast, activation of dendritic GABA(B) receptors decreased the width of backpropagating action potential (APs) and abolished dendritic calcium electrogenesis, indicating that dendritic GABA(B) receptors regulate excitability, primarily via inhibition of voltage-dependent calcium channels. These distinct actions of somatic and dendritic GABA(B) receptors regulated neuronal output in different ways. Activation of somatic GABA(B) receptors led to a reduction in neuronal output, primarily by increasing the AP rheobase, whereas activation of dendritic GABA(B) receptors blocked burst firing, decreasing AP output in the absence of a significant change in somatic membrane properties. Taken together, our results show that GABA(B) receptors regulate somatic and dendritic excitability of cortical pyramidal neurons via different cellular mechanisms. Somatic GABA(B) receptors activate potassium channels, leading primarily to a subtractive or shunting form of inhibition, whereas dendritic GABA(B) receptors inhibit dendritic calcium electrogenesis, leading to a reduction in bursting firing.

Binding and leakage of barium in alginate microbeads.

Microbeads of alginate crosslinked with Ca(2+) and/or Ba(2+) are popular matrices in cell-based therapy. The aim of this study was to quantify the binding of barium in alginate microbeads and its leakage under in vitro and accumulation under in vivo conditions. Low concentrations of barium (1 mM) in combination with calcium (50 mM) and high concentrations of barium (20 mM) in gelling solutions were used for preparation of microbeads made of high-G and high-M alginates. High-G microbeads accumulated barium from gelling solution and contained higher concentrations of divalent ions for both low- and high-Ba exposure compared with high-G microbeads exposed to calcium solely and to high-M microbeads for all gelling conditions. Although most of the unbound divalent ions were removed during the wash and culture steps, leakage of barium was still detected during storage. Barium accumulation in blood and femur bone of mice implanted with high-G beads was found to be dose-dependent. Estimated barium leakage relevant to transplantation to diabetic patients with islets in alginate microbeads showed that the leakage was 2.5 times lower than the tolerable intake value given by WHO for high-G microbeads made using low barium concentration. The similar estimate gave 1.5 times higher than is the tolerable intake value for the high-G microbeads made using high barium concentration. To reduce the risk of barium accumulation that may be of safety concern, the microbeads made of high-G alginate gelled with a combination of calcium and low concentration of barium ions is recommended for islet transplantation.

IDENTIFICATION OF POST‐DEPOSITIONAL CHEMICAL ALTERATION OF CERAMICS FROM THE NORTH COAST OF PAPUA NEW GUINEA (SANDUAN PROVINCE) BY TIME‐OF‐FLIGHT–LASER ABLATION–INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (TOF–LA–ICP–MS)*

The measured chemical composition of archaeological ceramics can result from a variety of geological, cultural and taphonomic factors. In the present study, we evaluate the likelihood that elevated barium concentrations in ceramics from the archaeological site of Wom/Aiser, located on the northern coast of Papua New Guinea, result from post‐depositional enrichment. Using time‐of‐flight–laser ablation–inductively coupled plasma–mass spectrometry, we mapped chemical compositional profiles across cut sections of ceramic sherds from four archaeological sites, all previously chemically characterized. We identify the formation of pronounced concentration gradients in samples from Wom/Aiser relative to sherds with low barium concentrations, consistent with post‐burial uptake.

Involvement of inwardly rectifying K+ channels in secretory responses of human ileal mucosa.

In acute secretory diarrhoea the primary event driving fluid secretion is a transcellular, electrogenic, serosal to mucosal transport of chloride ions. Such transport requires the maintenance of an electrically negative cell membrane voltage, which is achieved through a basolateral outward leakage of potassium ions. The aim of this study was to investigate the nature of K(+) channel involvement in facilitating secretory processes in the human ileum. Muscle-stripped mucosal preparations of human ileal mucosa were set up in Ussing chambers for recording short-circuit current and transmucosal conductance. Escherichia coli heat-stable toxin and vasoactive intestinal peptide (VIP) produced concentration-dependent increases in short-circuit current. Responses to the heat-stable toxin were unaffected by basolateral application of 4-aminopyridine (5 mM), glibenclamide (10 microM) or a combination of charybdotoxin (0.3 microM) plus apamin (0.3 microM). However, basolateral barium (0.2-5 mM) caused a concentration-dependent inhibition. Responses to VIP were similarly affected by barium (0.05-1 mM). These results suggested that electrogenic chloride transport by human ileal mucosa required the presence of basolateral K(+) channels. The use of selective K(+)-channel inhibitors and low concentrations of barium suggested that the channels involved might be of the inwardly rectifying type.

The origin and production geochemistry of radioactive lead (210Pb) in NORM-contaminated formation waters

Radioactive produced formation waters in Wytch Farm, a Triassic fluvial reservoir, onshore southern UK, are unusual in that they are enriched in NORM (naturally occurring radioactive material: in this case 210Pb). 210Pb belongs to the 238U decay chain. Key issues include the origin and distribution of radiogenic lead in sub-surface waters and the absence of the more common type of NORM (226Ra). The sandstones are unusually enriched in uranium with concentrations of 0–70ppm; 210Pb was thus probably sourced internally within the reservoir. Stratigraphic and aerial distribution of high levels of subsurface uranium correlates well with the highest level of NORM-contamination in produced waters. Triassic formation waters have very high sulphate and very low barium concentrations and are at saturation with respect to barite. Radium is not a NORM problem presumably because its concentration is limited by coprecipitation with barite. While the sulphate concentration is very high, the sulphide concentration is below routine detection limits. Geochemical modelling indicates that Triassic formation waters are undersaturated with respect to PbS (galena) thus allowing radiogenic lead to remain in aqueous solution.

Influence of barium on rectification in rat neocortical neurons

The effect of low concentrations of barium on the membrane properties of rat neocortical neurons was studied in vitro. Potassium currents were examined using single-electrode current- and voltage-clamp techniques. Neurons responded to bath application of barium (10–100 μM) with a membrane depolarization associated with an increase in input resistance. Under voltage clamp conditions, an inward shift in holding current was observed. The effects of barium were rapidly reversible upon washing and persisted in the presence of TTX. The equilibrium potential for the barium-induced inward current was near −110 mV, suggesting that barium inhibited a tonically active potassium conductance. Measurements of current voltage relationships indicated an inward rectification of this conductance between −50 and −130 mV. These results provide strong evidence that barium blocks a persistent potassium ‘leak’ current in neocortical neurons that contributes to the resting potential of these cells.

Muscarinic agonists activate an inwardly rectifying potassium conductance in medial pontine reticular formation neurons of the rat in vitro

Intracellular recordings were obtained from neurons in pontine reticular formation slices of the rat to characterize a cholinergic-gated increase in conductance. The conductance increase was associated with a hyperpolarization of the membrane potential and with an outward current under voltage-clamp conditions. Current-voltage relations and potassium substitution experiments indicated mediation by a change in permeability, primarily to potassium. This potassium conductance exhibited inward rectification at membrane potentials negative to resting potential, a novel finding for cholinergic actions in CNS neurons. Further characterization of this inwardly rectifying potassium conductance revealed marked sensitivity to low concentrations of barium. Cholinergically evoked currents were relatively unaffected by the presence of extracellular cesium. Cholinergic effects persisted in TTX. The outward currents elicited by carbachol or methacholine were blocked only by high concentrations of pirenzepine, a selective antagonist of the M1 muscarinic receptor. The interaction between these agents is quantitatively consistent with cholinergic action at postsynaptic muscarinic receptors of the non-M1 subtype.

Voltage-activated potassium, but not calcium currents in cultured bovine aortic endothelial cells

The electrophysiological properties of cultured bovine aortic endothelial cells were characterized using the patch clamp technique. Resting potentials were measured on passing to the whole cell recording configuration and were close to--65 mV in healthy cells. In cell-attached recordings with a high potassium pipette solution, inward single channel currents were observed with zero applied pipette potential. A linear slope conductance of 25 pS was found for a wide range of hyperpolarizing patch potentials and also for depolarizing patch potentials of up to 50-60 mV. A pronounced inward rectification was apparent as no reversal of these currents was seen for larger depolarizations. Whole cell recording in physiological solutions revealed the presence of a hyperpolarization-activated inward current with strong inward rectification and no voltage-dependent ionic current was observed upon depolarization in this subset of cells. Substitution of potassium for external sodium resulted in a shift in the zero current potential consistent with potassium being the main permeant ion. Together with the characteristic voltage-dependent blocking actions of external sodium ions and low concentrations of barium and caesium ions, our results indicate that this current is very similar to the classical inward rectifier as originally described in skeletal muscle and in tunicate eggs. In a second population of cells, a depolarization-activated outward current displaying characteristics of the fast transient A-type potassium current as first reported in molluscan neurones was also observed. No evidence for inward voltage dependent sodium or calcium currents was found.

Automaticity of ventricular fibers induced by low concentrations of barium

The effects of low concentrations of Ba have been studied on dog Purkinje fibers and on guinea pig left ventricular epicardium. In both tissues, Ba induces diastolic depolarization at a membrane potential more negative than -80 mV. This effect becomes clearly detectable at concentrations as low as 10-20 microM in Purkinje fibers bathed in 5.4 mM K Tyrode solution and around 100 microM in ventricular epicardium. In both tissues the Ba-induced depolarization is blocked in a dose-dependent manner by low concentrations of Cs (0.5-5 mM) as is the normal pacemaker current (if) in Purkinje fibers. In ventricular muscle the Ba-induced depolarization develops at the same rate whether the tissue is at rest or depolarized by repetitive stimulation. The maximum (initial) slope of diastolic depolarization is insensitive to D 600, La, and 4-aminopyridine, increased by tetraethylammonium chloride, decreased by tetrodotoxin, and almost entirely suppressed by Na removal. Experimental data and reconstruction of electrical activity suggest that in ventricular muscle the Ba-induced diastolic depolarization results to a large extent from the unmasking of a small pacemaker inward current, mainly carried by Na ions and similar to the pacemaker current of Purkinje fibers. Unmasking of the depolarizing effect of the pacemaker current is very likely due to the decrease by Ba of the inward going rectification. Steady-state inward Na current flowing through the Na window can increase the diastolic depolarization. Residual Cs-insensitive, Ba-induced diastolic depolarization may result from a time-dependent blockade of the background K conductance like the one described in skeletal muscle.

Barium-induced spontaneous activity in sheep cardiac Purkinje fibers

We used intracellular microelectrodes to study the electrophysiological effects of low barium concentrations (1 − 4 × 10 5 m) on sheep cardiac Purkinje fibers. The main effect of barium was an increase in action potential duration (APD) both at −60 mV (APD −60) and at 100% repolarization (APD 100). The prolongation of APD was greater at a lower (30/min) than at a higher driving rate (120/min). Barium significantly modified the normal linear relationship between driving rate and APD. The effects of barium on APD were enhanced by lowering [K +] 0 and antagonized by increasing [Ca 2+] 0. Barium caused a slowing of phase 3 repolarization, a steepening of diastolic depolarization and induced spontaneous activity in the resting potential range during the interruption of the drive. The first spontaneous action potential was usually preceded by an oscillatory potential. By means of several procedures (lowering [K +] 0, increasing [Ca 2−] 0, increasing the driving rate) it was possible to identify two separate mechanisms underlying the initiation of spontaneous activity: (1) enhancement of normal diastolic depolarization and (2) induction of oscillatory afterpotentials. Finally, barium induced repetitive activity through early afterdepolarizations. We conclude that in Purkinje fibers low barium concentrations cause a lengthening of APD and can induce spontaneous activity by means of at least three different mechanisms. The main factor underlying the barium effects seems to be a reduction in potassium conductance.

Depression of calcium-dependent potassium conductance of guinea-pig myenteric neurones by muscarinic agonists.

Intracellular recordings were made from myenteric neurones of the guinea-pig ileum. Muscarinic agonists acetylcholine (ACh) and oxotremorine reduced membrane potassium conductance (gK). Calcium carried into the neurone by one or more action potentials increased membrane potassium conductance (gK, Ca). The time course of the muscarinic changes in gK was compared to that of the change in gK, Ca following an action potential. The time course of conductance decrease was similar in both cases, and both time courses had the same temperature coefficient. Concentrations of ACh (100 nM) which were too low to cause a detectable reduction in resting gK shortened the duration of the gK, Ca increase which followed an action potential. Low concentrations of barium (10-100 microM) had the same effect as ACh. This was not due to a reduction in calcium entry during the action potential. Higher concentrations of ACh and barium also reduced resting membrane conductance. The conductance changes during the muscarinic action and the action potential after-hyperpolarization did not add linearly. It is proposed that muscarinic agonists and barium may act by reducing the availability of calcium ions at a site within the membrane which controls gK.

The distribution of rubidium, cesium and barium in the oceans

Rubidium, cesium and barium have been determined in several oceanic profiles by a neutron activation procedure based on the extraction of salt from 100 ml. samples by freeze-drying. Assuming a constant strontium-chlorinity ratio of 0.0425, strontium was used as an internal flux monitor. For 19.0% chlorinity the average concentrations of these three elements are 125 μg Rb/1. (10 samples), 0.30 μg Cs/1. (28 samples), and 13 μg Ba/1. (6 samples) with very little variation with depth or geographic location. Rubidium and cesium apparently are not efficiently transported to great depths by organisms, and cesium at least is not in exchange equilibrium with deep-sea clays. The barium data are for six stations only, mainly from areas of low productivity and low barium concentrations in associated deep-sea sediments. High oceanic barium values might be expected in areas of high productivity and associated high barium in deep-sea sediments.