A review of the health impacts of barium from natural and anthropogenic exposure.

There is an increasing public awareness of the relatively new and expanded industrial barium uses which are potential sources of human exposure (e.g., a shale gas development that causes an increased awareness of environmental exposures to barium). However, absorption of barium in exposed humans and a full spectrum of its health effects, especially among chronically exposed to moderate and low doses of barium populations, remain unclear. We suggest a systematic literature review (from 1875 to 2014) on environmental distribution of barium, its bioaccumulation, and potential and proven health impacts (in animal models and humans) to provide the information that can be used for optimization of future experimental and epidemiological studies and developing of mitigative and preventive strategies to minimize negative health effects in exposed populations. The potential health effects of barium exposure are largely based on animal studies, while epidemiological data for humans, specifically for chronic low-level exposures, are sparse. The reported health effects include cardiovascular and kidney diseases, metabolic, neurological, and mental disorders. Age, race, dietary patterns, behavioral risks (e.g., smoking), use of medications (those that interfere with absorbed barium in human organism), and specific physiological status (e.g., pregnancy) can modify barium effects on human health. Identifying, evaluating, and predicting the health effects of chronic low-level and moderate-level barium exposures in humans is challenging: Future research is needed to develop an understanding of barium bioaccumulation in order to mitigate its potential health impacts in various exposured populations. Further, while occupationally exposed at-risk populations exist, it is also important to identify potentially vulnerable subgroups among non-occupationally exposed populations (e.g., elderly, pregnant women, children) who are at higher risk of barium exposure from drinking water and food.

Bioavailability of barium to plants and invertebrates in soils contaminated by barite.

Barium (Ba) is a nonessential element to terrestrial organisms and is known to be toxic at elevated concentrations. In this study, the bioavailability and toxicity of Ba in barite (BaSO4) contaminated soils was studied using standard test organisms (Lactuca sativa L. "Great Lakes", Eisenia fetida). Contamination resulted from barite mining activities. Barium concentrations in contaminated soils determined by X-ray fluorescence were in the range 0.13-29.2%. Barite contaminated soils were shown to negatively impact both E. fetida and L. sativa relative to control soil. For E. fetida, pore-water concentrations and acid extractable Ba were linearly related to % body weight loss. In L. sativa, pore-water Ba and exchangeable Ba were both strongly related to shoot Ba and shoot biomass production. A negative linear relationship was observed between shoot Ba content and shoot weight (P < 0.0004, R(2) = 0.39), indicating that Ba accumulation is likely to have induced phytotoxicity. Plant weights were correlated to % weight loss in earthworm (r = -0.568, P = 0.028). Barium concentrations in pore-water were lower than predicted from barite solubility estimates but strongly related to exchangeable Ba, indicating an influence of ion exchange on Ba solubility and toxicity to E. fetida and L. sativa.

Relationship between gastrointestinal transit time and anesthetic fasting protocols in the captive chimpanzee, Pan troglodytes.

BACKGROUND: Lengthy social separation and prolonged fasting time contribute to increased risks associated with anesthesia in captive primates. This study is an initial attempt to identify a safe pre-anesthetic fasting procedure by identifying gastric emptying time (GET) and gastrointestinal transit time (GTT) of captive chimpanzees, Pan troglodytes. METHODS: Seven adult chimpanzees at the North Carolina Zoo immobilized for annual physical examinations were fed barium-impregnated polyethylene spheres to measure GET. Eleven animals were individually fed a color dye marker and fecal passage was observed to determine GTT. RESULTS: Gastric emptying time (GET) was approximated to be >3 hours but <16 hours. The mean GTT was 16.5 hours. CONCLUSIONS: This study indicates that a fasting time of 3 hours would allow for complete gastric emptying and could potentially replace the current overnight fast (≥16 hour) to help minimize complications associated with pre-anesthetic fasting in captive primates.

A Scalable Database of Organ Doses for Common Diagnostic Fluoroscopy Procedures of Children: Procedures of Historical Practice for Use in Radiation Epidemiology Studies.

Assessment of health effects from low-dose radiation exposures in patients undergoing diagnostic imaging is an active area of research. High-quality dosimetry information pertaining to these medical exposures is generally not readily available to clinicians or epidemiologists studying radiation-related health risks. The purpose of this study was to provide methods for organ dose estimation in pediatric patients undergoing four common diagnostic fluoroscopy procedures: the upper gastrointestinal (UGI) series, the lower gastrointestinal (LGI) series, the voiding cystourethrogram (VCUG) and the modified barium swallow (MBS). Abstracted X-ray film data and physician interviews were combined to generate procedure outlines detailing X-ray beam projections, imaged anatomy, length of X-ray exposure, and presence and amount of contrast within imaged anatomy. Monte Carlo radiation transport simulations were completed for each of the four diagnostic fluoroscopy procedures across the 162-member (87 males and 75 females) University of Florida/National Cancer Institute pediatric phantom library, which covers variations in both subject height and weight. Absorbed doses to 28 organs, including the active marrow and bone endosteum, were assigned for all 162 phantoms by procedure. Additionally, we provide dose coefficients (DCs) in a series of supplementary tables. The DCs give organ doses normalized to procedure-specific dose metrics, including: air kerma-area product (µGy/mGy · cm2), air kerma at the reference point (µGy/µGy), number of spot films (SF) (µGy/number of SFs) and total fluoroscopy time (µGy/s). Organs accumulating the highest absorbed doses per procedure were as follows: kidneys between 0.9-25.4 mGy, 1.1-16.6 mGy and 1.1-9.7 mGy for the UGI, LGI and VCUG procedures, respectively, and salivary glands between 0.2-3.7 mGy for the MBS procedure. Average values of detriment-weighted dose, a phantom-specific surrogate for the effective dose based on ICRP Publication 103 tissue-weighting factors, were 0.98 mSv, 1.16 mSv, 0.83 mSv and 0.15 mSv for the UGI, LGI, VCUG and MBS procedures, respectively. Scalable database of organ dose coefficients by patient sex, height and weight, and by procedure exposure time, reference point air kerma, kerma-area product or number of spot films, allows clinicians and researchers to compute organ absorbed doses based on their institution-specific and patient-specific dose metrics. In addition to informing on patient dosimetry, this work has the potential to facilitate exposure assessments in epidemiological studies designed to investigate radiation-related risks.

Phytoremediation in flooded environments: Dynamics of barium absorption and translocation by Eleocharis acutangula.

Macrophytes are widely used in water treatment and have potential for remediation of flooded soils. Many techniques have been proposed to increase the phytoextraction of metals by macrophytes, however, the knowledge of periods of maximum absorption and translocation is essential and is a gap in the management of phytoremediation. To evaluate the absorption and translocation of Ba over time by Eleocharis acutangula, a greenhouse experiment was conducted and the dry matter production of plants, Ba content in the roots and aerial parts, mass of Ba accumulated in plants, translocation factors and removal coefficients of Ba, and Ba content in two layers of the soil (0.0-0.1 m and 0.1-0.2 m) were determined. The highest translocation rates were observed after 105 days of cultivation, when the plants reached a state of hyperaccumulation. The maximum accumulation of barium occurred in the aerial parts of the plants at 105 days and in the roots at both 120 and 180 days. The barium content was reduced up to 120 days, as a result of an increase in available barium content in the soil layer of 0.0-0.1 m up to 105 days and in the layer 0.10-0.20 m up to 120 days, favoring the intense accumulation of Ba during this period. After 120 days of cultivation, the accumulation in the roots maintained a high coefficient of removal of Ba from the soil to the plant. After 180 days the available barium in the soil was depleted due to this high rate of removal by the roots.

Localized secretion of ATP and opioids revealed through single Ca2+ channel modulation in bovine chromaffin cells.

In bovine chromaffin cells, the Ca2+ channels involved in exocytosis are effectively inhibited by ATP and opioids that are coreleased with catecholamines during cell activity. This autocrine loop causes a delay in Ca2+ channel activation that is quickly removed by preceding depolarizations. Changes in Ca2+ channel gating by secreted products thus make it possible to correlate Ca2+ channel activity to secretory events. Here, using cell-attached patch recordings, we found a remarkable correlation between delayed Ca2+ channel openings and neurotransmitter secretion induced by either local or whole-cell Ba2+ stimulation. The action is specific for N- and P/Q-type channels and largely prevented by PTX and mixtures of purinergic and opioid receptor antagonists. Overall, our data provide evidence that exocytosis, viewed through the autocrine inhibition of non-L-type channels, is detectable in membrane patches of approximately 1 microm2 distributed over 30%-40% of the total cell surface, while Ca2+ channels and autoreceptors are uniformly distributed over most of the cell membrane.

Error count of radiopaque markers in colonic segmental transit time study.

OBJECTIVE: The objective of our study was to evaluate the feasibility and efficacy of a radiologic technique in increasing colon visibility in colonic transit time studies. Three radiologists counted segmental colonic radiopaque markers in two patient groups, based on classic criteria in the first group and also on a colonic barium trace in the second. Agreement between marker counts was assessed using method comparison analysis. CONCLUSION: With the barium trace technique, the anatomic conspicuity of colonic segments is improved, a correct segmental marker count can be obtained, and colonic inertia can be more easily distinguished from distal constipation.

Hybrid spectral CT reconstruction.

Current photon counting x-ray detector (PCD) technology faces limitations associated with spectral fidelity and photon starvation. One strategy for addressing these limitations is to supplement PCD data with high-resolution, low-noise data acquired with an energy-integrating detector (EID). In this work, we propose an iterative, hybrid reconstruction technique which combines the spectral properties of PCD data with the resolution and signal-to-noise characteristics of EID data. Our hybrid reconstruction technique is based on an algebraic model of data fidelity which substitutes the EID data into the data fidelity term associated with the PCD reconstruction, resulting in a joint reconstruction problem. Within the split Bregman framework, these data fidelity constraints are minimized subject to additional constraints on spectral rank and on joint intensity-gradient sparsity measured between the reconstructions of the EID and PCD data. Following a derivation of the proposed technique, we apply it to the reconstruction of a digital phantom which contains realistic concentrations of iodine, barium, and calcium encountered in small-animal micro-CT. The results of this experiment suggest reliable separation and detection of iodine at concentrations ≥ 5 mg/ml and barium at concentrations ≥ 10 mg/ml in 2-mm features for EID and PCD data reconstructed with inherent spatial resolutions of 176 µm and 254 µm, respectively (point spread function, FWHM). Furthermore, hybrid reconstruction is demonstrated to enhance spatial resolution within material decomposition results and to improve low-contrast detectability by as much as 2.6 times relative to reconstruction with PCD data only. The parameters of the simulation experiment are based on an in vivo micro-CT experiment conducted in a mouse model of soft-tissue sarcoma. Material decomposition results produced from this in vivo data demonstrate the feasibility of distinguishing two K-edge contrast agents with a spectral separation on the order of the energy resolution of the PCD hardware.

Hematological, biochemical, and histopathological impacts of barium chloride and barium carbonate accumulation in soft tissues of male Sprague-Dawley rats.

The present study was designed to investigate the hematotoxicity, sero-biochemical and histological changes due to the accumulation of BaCl2 and BaCO3, the most important barium salts in our daily lives, in different soft tissues including the liver, kidney, heart, and spleen of adult rats after an oral exposure for 30 consecutive days, and to explain the different mechanisms by which this metal can exert these impacts. For this purpose, adult male rats were divided into three main groups of 15 animals each: group I, serving as controls, group II, receiving BaCl2 orally in a dose of 179 mg barium/kg b.wt, and group III, receiving BaCO3 orally in a dose of 418 mg barium/kg b.wt. for 30 consecutive days. Obviously, normocytic normochromic anemia was evident in both barium groups. Serum biochemical analysis revealed significant declines in glutathione peroxidase, catalase, superoxide dismutase, and urea with significant elevations in malondialdehyde, lactate dehydrogenase, and creatine kinase levels. Hyperphosphatemia, hypokalemia, hypocalcemia, and hypochloremia were also evident in both barium groups. Besides, residual analysis of both barium salts in different body organs revealed significantly abundant barium residues in the liver, spleen, heart, and kidney, respectively in both barium salts groups. Moreover, splenic tissue showed hemosiderosis, peritubular congestion, and necrotic glomeruli with intratubular hemorrhage. Sever subepicardial congestion with intramuscular edema was evident in the heart. In conclusion, BaCl2 and BaCO3 were able to deliver mortalities, antioxidant enzymes exhaustion, and a sort of normocytic normochromic anemia, as well as marked disturbances in cardiac, hepatic, and renal functions due to the accumulation of these two salts in the soft tissues. Therefore, these results demonstrate the unrecognized toxicity of those two barium salts due to their accumulation in various soft tissues of the body and so, this needs to reconsider about barium exposure.

Differential regulation of CaV2.1 channels by calcium-binding protein 1 and visinin-like protein-2 requires N-terminal myristoylation.

P/Q-type Ca2+ currents through presynaptic CaV2.1 channels initiate neurotransmitter release, and differential modulation of these channels by neuronal calcium-binding proteins (nCaBPs) may contribute to synaptic plasticity. The nCaBPs calcium-binding protein 1 (CaBP1) and visinin-like protein-2 (VILIP-2) differ from calmodulin (CaM) in that they have an N-terminal myristoyl moiety and one EF-hand that is inactive in binding Ca2+. To determine whether myristoylation contributes to their distinctive modulatory properties, we studied the regulation of CaV2.1 channels by the myristoyl-deficient mutants CaBP1/G2A and VILIP-2/G2A. CaBP1 positively shifts the voltage dependence of CaV2.1 activation, accelerates inactivation, and prevents paired-pulse facilitation in a Ca2+-independent manner. Block of myristoylation abolished these effects, leaving regulation that is similar to endogenous CaM. CaBP1/G2A binds to CaV2.1 with reduced stability, but in situ protein cross-linking and immunocytochemical studies revealed that it binds CaV2.1 in situ and is localized to the plasma membrane by coexpression with CaV2.1, indicating that it binds effectively in intact cells. In contrast to CaBP1, coexpression of VILIP-2 slows inactivation in a Ca2+-independent manner, but this effect also requires myristoylation. These results suggest a model in which nonmyristoylated CaBP1 and VILIP-2 bind to CaV2.1 channels and regulate them like CaM, whereas myristoylation allows differential, Ca2+-independent regulation by the inactive EF-hands of CaBP1 and VILIP-2, which differ in their positions in the protein structure. Differential, myristoylation-dependent regulation of presynaptic Ca2+ channels by nCaBPs may provide a flexible mechanism for diverse forms of short-term synaptic plasticity.

Bioaccessibility of barium from barite contaminated soils based on gastric phase in vitro data and plant uptake.

Barite contamination of soil commonly occurs from either barite mining or explorative drilling operations. This work reported in vitro data for barite contaminated soils using the physiologically based extraction test (PBET) methodology. The existence of barite in plant tissue and the possibility of 'biomineralised' zones was also investigated using Scanning Electron Microscopy. Soils with low barium (Ba) concentrations showed a higher proportion of Ba extractability than barite rich samples. Barium uptake to spinach from soil was different between short term spiking studies and field weathered soils. Furthermore, Ba crystals were not evident in spinach tissue or acid digest solutions grown in barium nitrate spiked soils despite high accumulation. Barite was found in the plant digest solutions from barite contaminated soils only. Results indicate that under the conservative assumptions made, a child would need to consume extreme quantities of soil over an extended period to cause chronic health problems.

alpha(1E) subunits form the pore of three cerebellar R-type calcium channels with different pharmacological and permeation properties.

R-type Ca(2+) channels cooperate with P/Q- and N-type channels to control neurotransmitter release at central synapses. The leading candidate as pore-forming subunit of R-type channels is the alpha(1E) subunit. However, R-type Ca(2+) currents with permeation and/or pharmacological properties different from those of recombinant Ca(2+) channels containing alpha(1E) subunits have been described, and therefore the molecular nature of R-type Ca(2+) channels remains not completely settled. Here, we show that the R-type Ca(2+) current of rat cerebellar granule cells consists of two components inhibited with different affinity by the alpha(1E) selective antagonist SNX482 (IC(50) values of 6 and 81 nM) and a third component resistant to SNX482. The SNX482-sensitive R-type current shows the unique permeation properties of recombinant alpha(1E) channels; it is larger with Ca(2+) than with Ba(2+) as charge carrier, and it is highly sensitive to Ni(2+) block and has a voltage-dependence of activation consistent with that of G2 channels with unitary conductance of 15 pS. On the other hand, the SNX482-resistant R-type current shows permeation properties similar to those of recombinant alpha(1A) and alpha(1B) channels; it is larger with Ba(2+) than with Ca(2+) as charge carrier(,) and it has a low sensitivity to Ni(2+) block and a voltage-dependence of activation consistent with that of G3 channels with unitary conductance of 20 pS. Gene-specific knock-down by antisense oligonucleotides demonstrates that the different cerebellar R-type channels are all encoded by the alpha(1E) gene, suggesting the existence of alpha(1E) isoforms with different pore properties.

Critical dependence of cAMP response element-binding protein phosphorylation on L-type calcium channels supports a selective response to EPSPs in preference to action potentials.

Activity-dependent gene expression in neurons shows a remarkable ability to differentiate between different types of stimulation: orthodromic inputs that engage synaptic transmission are much more effective than antidromic stimuli that do not. We have studied the basis of such selectivity in cultured hippocampal neurons in which nuclear cAMP response element-binding protein (CREB) phosphorylation is induced by synaptic activity but not by action potential (AP) stimulation in the absence of EPSPs, although spikes by themselves generate large elevations in intracellular Ca(2+). Previous work has shown that Ca(2+) entry through L-type Ca(2+) channels plays a dominant role in triggering calmodulin mobilization and activation of calmodulin-dependent kinases that phosphorylate CREB, raising the possibility that L-type channels contribute to the selective response to EPSPs rather than APs. Accordingly, we performed voltage-clamp experiments to compare the currents carried by L-type channels during depolarizing waveforms that approximated APs or dendritic EPSPs. The integrated current generated by L-type channels was significantly less after mock APs than with EPSP-like depolarizations. The difference was traced to two distinct factors. Compared with other channels, L-type channels activated at relatively negative potentials, favoring their opening with EPSP stimulation; they also exhibited relatively slow activation kinetics, weighing against their contribution during an AP. The relative ineffectiveness of APs as a stimulus for CREB phosphorylation could be overcome by exposure to the agonist Bay K8644, which potentiated the AP-induced influx through L-type channels by approximately 10-fold. Under normal conditions, the unique biophysical properties of L-type channels allow them to act as a kinetic filter to support spike-EPSP discrimination.

Discrete Ba2+ block as a probe of ion occupancy and pore structure in the high-conductance Ca2+ -activated K+ channel.

In this study, high-conductance Ca2+-activated K+ channels from rat skeletal muscle were incorporated into planar phospholipid bilayers, and discrete blockade of single channels by Ba2+ was studied. With 150 mM K+ held constant in the internal solution, increasing external K+ over the range 100-1,000 mM raises the rate of Ba2+ dissociation. This "enhancement effect," which operates at K+ concentrations 3-4 orders of magnitude higher than those required for the "lockin" effect described previously, depends on applied voltage, saturates with K+ concentration, and is not observed with Na+. The voltage dependence of the Ba2+ off-rate varies with external K+ in a way suggesting that K+, entering the channel from the external side, forces Ba2+ dissociation to the internal solution. With K+ held fixed in the external solution, the Ba2+ off-rate decreases as internal K+ is raised over the range 0-50 mM. This "lock-in" effect is similar to that seen on the external side (Neyton and Miller, 1988), except that the internal lock-in site is of lower affinity and shows only a fivefold preference for K+ over Na+. All the results taken together argue strongly that this channel's conduction pathway contains four sites of very high affinity for K+, all of which may be simultaneously occupied under normal conducting conditions. According to this view, the mutual destabilization resulting from this high ionic occupancy leads to the unusually high conductance of this K+-specific channel.

Potassium blocks barium permeation through a calcium-activated potassium channel.

Single high-conductance Ca2+-activated K+ channels from rat skeletal muscle were inserted into planar lipid bilayers, and discrete blocking by the Ba2+ ion was studied. Specifically, the ability of external K+ to reduce the Ba2+ dissociation rate was investigated. In the presence of 150 mM internal K+, 1-5 microM internal Ba2+, and 150 mM external Na+, Ba2+ dissociation is rapid (5 s-1) in external solutions that are kept rigorously K+ free. The addition of external K+ in the low millimolar range reduces the Ba2+ off-rate 20-fold. Other permeant ions, such as Tl+, Rb+, and NH4+ show a similar effect. The half-inhibition constants rise in the order: Tl+ (0.08 mM) less than Rb+ (0.1 mM) less than K+ (0.3 mM) less than Cs+ (0.5 mM) less than NH4+ (3 mM). When external Na+ is replaced by 150 mM N-methyl glucamine, the Ba2+ off-rate is even higher, 20 s-1. External K+ and other permeant ions reduce this rate by approximately 100-fold in the micromolar range of concentrations. Na+ also reduces the Ba2+ off-rate, but at much higher concentrations. The half-inhibition concentrations rise in the order: Rb+ (4 microM) less than K+ (19 microM) much less than Na+ (27 mM) less than Li+ (greater than 50 mM). The results require that the conduction pore of this channel contains at least three sites that may all be occupied simultaneously by conducting ions.

Extracellular sodium interacts with the HERG channel at an outer pore site.

Most voltage-gated K(+) currents are relatively insensitive to extracellular Na(+) (Na(+)(o)), but Na(+)(o) potently inhibits outward human ether-a-go-go-related gene (HERG)-encoded K(+) channel current (Numaguchi, H., J.P. Johnson, Jr., C.I. Petersen, and J.R. Balser. 2000. Nat. Neurosci. 3:429-30). We studied wild-type (WT) and mutant HERG currents and used two strategic probes, intracellular Na(+) (Na(+)(i)) and extracellular Ba(2+) (Ba(2+)(o)), to define a site where Na(+)(o) interacts with HERG. Currents were recorded from transfected Chinese hamster ovary (CHO-K1) cells using the whole-cell voltage clamp technique. Inhibition of WT HERG by Na(+)(o) was not strongly dependent on the voltage during activating pulses. Three point mutants in the P-loop region (S624A, S624T, S631A) with intact K(+) selectivity and impaired inactivation each had reduced sensitivity to inhibition by Na(+)(o). Quantitatively similar effects of Na(+)(i) to inhibit HERG current were seen in the WT and S624A channels. As S624A has impaired Na(+)(o) sensitivity, this result suggested that Na(+)(o) and Na(+)(i) act at different sites. Extracellular Ba(2+) (Ba(2+)(o)) blocks K(+) channel pores, and thereby serves as a useful probe of K(+) channel structure. HERG channel inactivation promotes relief of Ba(2+) block (Weerapura, M., S. Nattel, M. Courtemanche, D. Doern, N. Ethier, and T. Hebert. 2000. J. Physiol. 526:265-278). We used this feature of HERG inactivation to distinguish between simple allosteric and pore-occluding models of Na(+)(o) action. A remote allosteric model predicts that Na(+)(o) will speed relief of Ba(2+)(o) block by promoting inactivation. Instead, Na(+)(o) slowed Ba(2+) egress and Ba(2+) relieved Na(+)(o) inhibition, consistent with Na(+)(o) binding to an outer pore site. The apparent affinities of the outer pore for Na(+)(o) and K(+)(o) as measured by slowing of Ba(2+) egress were compatible with competition between the two ions for the channel pore in their physiological concentration ranges. We also examined the role of the HERG closed state in Na(+)(o) inhibition. Na(+)(o) inhibition was inversely related to pulsing frequency in the WT channel, but not in the pore mutant S624A.

Voltage-dependent and odorant-regulated currents in isolated olfactory receptor neurons of the channel catfish.

The electrical properties of olfactory receptor neurons, enzymatically dissociated from the channel catfish (Ictalurus punctatus), were studied using the whole-cell patch-clamp technique. Six voltage-dependent ionic currents were isolated. Transient inward currents (0.1-1.7 nA) were observed in response to depolarizing voltage steps from a holding potential of -80 mV in all neurons examined. They activated between -70 and -50 mV and were blocked by addition of 1 microM tetrodotoxin (TTX) to the bath or by replacing Na+ in the bath with N-methyl-D-glucamine and were classified as Na+ currents. Sustained inward currents, observed in most neurons examined when Na+ inward currents were blocked with TTX and outward currents were blocked by replacing K+ in the pipette solution with Cs+ and by addition of 10 mM Ba2+ to the bath, activated between -40 and -30 mV, reached a peak at 0 mV, and were blocked by 5 microM nimodipine. These currents were classified as L-type Ca2+ currents. Large, slowly activating outward currents that were blocked by simultaneous replacement of K+ in the pipette with Cs+ and addition of Ba2+ to the bath were observed in all olfactory neurons examined. The outward K+ currents activated over approximately the same range as the Na+ currents (-60 to -50 mV), but the Na+ currents were larger at the normal resting potential of the neurons (-45 +/- 11 mV, mean +/- SD, n = 52). Four different types of K+ currents could be differentiated: a Ca(2+)-activated K+ current, a transient K+ current, a delayed rectifier K+ current, and an inward rectifier K+ current. Spontaneous action potentials of varying amplitude were sometimes observed in the cell-attached recording configuration. Action potentials were not observed in whole-cell recordings with normal internal solution (K+ = 100 mM) in the pipette, but frequently appeared when K+ was reduced to 85 mM. These observations suggest that the membrane potential and action potential amplitude of catfish olfactory neurons are significantly affected by the activity of single channels due to the high input resistance (6.6 +/- 5.2 G omega, n = 20) and low membrane capacitance (2.1 +/- 1.1 pF, n = 46) of the cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional properties of cardiac L-type calcium channels transiently expressed in HEK293 cells. Roles of alpha 1 and beta subunits.

The cardiac dihydropyridine-sensitive calcium channel was transiently expressed in HEK293 cells by transfecting the rabbit cardiac calcium channel alpha 1 subunit (alpha 1C) alone or in combination with the rabbit calcium channel beta subunit cloned from skeletal muscle. Transfection with alpha 1C alone leads to the expression of inward, voltage-activated, calcium or barium currents that exhibit dihydropyridine sensitivity and voltage- as well as calcium-dependent inactivation. Coexpression of the skeletal muscle beta subunit increases current density and the number of high-affinity dihydropyridine binding sites and also affects the macroscopic kinetics of the current. Recombinant alpha 1C beta channels exhibit a slowing of activation and a faster inactivation rate when either calcium or barium carries the charge. Our data suggest that both an increase in the number of channels as well as modulatory effects on gating underlie the modifications observed upon beta subunit coexpression.

Rapid inactivation of depletion-activated calcium current (ICRAC) due to local calcium feedback.

Rapid inactivation of Ca2+ release-activated Ca2+ (CRAC) channels was studied in Jurkat leukemic T lymphocytes using whole-cell patch clamp recording and [Ca2+]i measurement techniques. In the presence of 22 mM extracellular Ca2+, the Ca2+ current declined with a biexponential time course (time constants of 8-30 ms and 50-150 ms) during hyperpolarizing pulses to potentials more negative than -40 mV. Several lines of evidence suggest that the fast inactivation process is Ca2+ but not voltage dependent. First, the speed and extent of inactivation are enhanced by conditions that increase the rate of Ca2+ entry through open channels. Second, inactivation is substantially reduced when Ba2+ is present as the charge carrier. Third, inactivation is slowed by intracellular dialysis with BAPTA (12 mM), a rapid Ca2+ buffer, but not by raising the cytoplasmic concentration of EGTA, a slower chelator, from 1.2 to 12 mM. Recovery from fast inactivation is complete within 200 ms after repolarization to -12 mV. Rapid inactivation is unaffected by changes in the number of open CRAC channels or global [Ca2+]i. These results demonstrate that rapid inactivation of ICRAC results from the action of Ca2+ in close proximity to the intracellular mouths of individual channels, and that Ca2+ entry through one CRAC channel does not affect neighboring channels. A simple model for Ca2+ diffusion in the presence of a mobile buffer predicts multiple Ca2+ inactivation sites situated 3-4 nm from the intracellular mouth of the pore, consistent with a location on the CRAC channel itself.

Divalent cation conduction in the ryanodine receptor channel of sheep cardiac muscle sarcoplasmic reticulum.

The conduction properties of the alkaline earth divalent cations were determined in the purified sheep cardiac sarcoplasmic reticulum ryanodine receptor channel after reconstitution into planar phospholipid bilayers. Under bi-ionic conditions there was little difference in permeability among Ba2+, Ca2+, Sr2+, and Mg2+. However, there was a significant difference between the divalent cations and K+, with the divalent cations between 5.8- and 6.7-fold more permeant. Single-channel conductances were determined under symmetrical ionic conditions with 210 mM Ba2+ and Sr2+ and from the single-channel current-voltage relationship under bi-ionic conditions with 210 mM divalent cations and 210 mM K+. Single-channel conductance ranged from 202 pS for Ba2+ to 89 pS for Mg2+ and fell in the sequence Ba2+ greater than Sr2+ greater than Ca2+ greater than Mg2+. Near-maximal single-channel conductance is observed at concentrations as low as 2 mM Ba2+. Single-channel conductance and current measurements in mixtures of Ba(2+)-Mg2+ and Ba(2+)-Ca2+ reveal no anomalous behavior as the mole fraction of the ions is varied. The Ca(2+)-K+ reversal potential determined under bi-ionic conditions was independent of the absolute value of the ion concentrations. The data are compatible with the ryanodine receptor channel acting as a high conductance channel displaying moderate discrimination between divalent and monovalent cations. The channel behaves as though ion translocation occurs in single file with at most one ion able to occupy the conduction pathway at a time.