Predicting oral relative bioavailability of arsenic in soil from in vitro bioaccessibility

Several investigations have been conducted to develop in vitro bioaccessibility (IVBA) assays that reliably predict in vivo oral relative bioavailability (RBA) of arsenic (As). This study describes a meta-regression model relating soil As RBA and IVBA that is based upon data combined from previous investigations that examined the relationship between As IVBA and RBA when IVBA was determined using an extraction of soil in 0.4 M glycine at pH 1.5. Data used to develop the model included paired IVBA and RBA estimates for 83 soils from various types of sites such as mining, smelting, and pesticide or herbicide application. The following linear regression model accounted for 87% of the observed variance in RBA (R(2) = .87): RBA(%) = 0.79 × IVBA(%) + 3. This regression model is more robust than previously reported models because it includes a larger number of soil samples, and also accounts for variability in RBA and IVBA measurements made on samples collected from sites contaminated with different As sources and conducted in different labs that have utilized different experimental models for estimating RBA.

Evaluation of small arms range soils for metal contamination and lead bioavailability

Although small arms ranges are known to be contaminated with lead, the full extent of metal contamination has not been described, nor has the oral bioavailability of lead in these soils. In this work, soil samples from ranges with diverse geochemical backgrounds were sieved to <250 microm and analyzed for total metal content. Soils had consistently high levels of lead and copper, ranging from 4549 to 24 484 microg/g and 223 to 2936 microg/g, respectively, while arsenic, antimony, nickel, and zinc concentrations were 100-fold lower. For lead bioavailability measurements, two widely accepted methods were used: an in vivo juvenile swine relative bioavailability method measuring lead absorption from ingested soils relative to equivalent lead acetate concentrations and an in vitro bioaccessibility procedure which measured acid-extractable lead as a percent of total lead in the soil. For eight samples, the mean relative bioavailability and bioaccessibility of lead for the eight soils was about 100% (108 +/- 18% and 95 +/- 6%, respectively) showing good agreement between both methods. Risk assessment and/or remediation of small arms ranges should therefore assume high bioavailability of lead.

Estimation of relative bioavailability of lead in soil and soil-like materials using young Swine

In this article we summarize the results of a series of studies that measured the relative bioavailability (RBA) of lead in a variety of soil and soil-like test materials. Reference material (Pb acetate) or Pb-contaminated soils were administered orally to juvenile swine twice a day for 15 days. Blood samples were collected from each animal at multiple times during the course of the study, and samples of liver, kidney, and bone were collected at sacrifice. All samples were analyzed for Pb. We estimated the RBA of a test material by fitting mathematical models to the dose-response curves for each measurement end point and finding the ratio of doses that gave equal responses. The final RBA for a test material is the simple average of the four end point-specific RBA values. Results from 19 different test materials reveal a wide range of RBA values across different exposure materials, ranging from 6 to 105%. This variability in RBA between different samples highlights the importance of reliable RBA data to help improve risk assessments for Pb in soil. Although the RBA value for a sample depends on the relative amounts of the different chemical and physical forms of Pb present, data are not yet adequate to allow reliable quantitative predictions of RBA from chemical speciation data alone.

Bioavailability of lead to juvenile swine dosed with soil from the Smuggler Mountain NPL Site of Aspen, Colorado

Bioavailability of lead (Pb) has become an issue in quantifying exposure of sensitive populations and, where necessary, establishing cleanup levels for contaminated soil. Immature swine were used as a model for young children to estimate the degree to which Pb from two fully characterized composite samples from the Smuggler Mountain Superfund Site in Aspen, Colorado may be bioavailable to resident children. The composite soils contained 14,200 and 3870 micrograms Pb/g of soil. Relative and absolute enteric bioavailabilities of Pb in soil (oral dose groups of 75,225, and 675 micrograms Pb/kg body wt/day) were estimated by comparison with an orally administered soluble Pb salt (lead acetate = PbAc2.3H2O) (dose groups of 0, 75, and 225 micrograms Pb/kg body wt/day) and an intravenously administered aqueous solution of Pb (100 micrograms Pb/kg/ day) from the same trihydrate salt administered daily for 15 days to 50 juvenile swine. The biological responses (area under the blood Pb concentration-time curve, and the terminal liver-, kidney-, and bone-lead concentrations) produced by Pb from PbAc2.3H2O and lead-contaminated soils were determined. This study revealed Pb from soil containing 14,200 micrograms Pb/g of soil had a bioavailability relative to Pb from PbAc (RBA), ranging from 56% based on the area under the blood lead concentration-time curve (AUC) versus dose, to 86% based on calculations from liver-Pb loading versus dose. Similarly, Pb from soil containing 3870 micrograms Pb/g of soil had an RBA ranging from 58% based on the AUC versus dose, to 74% based on calculations from liver- and kidney-Pb loading versus dose. Bioavailability of Pb in soils may be more or less than EPA's default RBA of 60%, therefore, measuring site-specific RBAs provides a basis for improved exposure and risk assessment.

Erythrocyte cytosolic free Ca2+ and plasma membrane Ca2+-ATPase activity in cystic fibrosis

The properties of the Ca2+, Mg2+-ATPase of erythrocyte membranes from patients with cystic fibrosis (CF) were extensively compared to that of healthy controls. Following removal of an endogenous membrane inhibitor of the ATPase, activation of the enzyme by Ca2+, calmodulin, limited tryptic digestion or oleic acid, as well as inhibition by trifluoperazine, were studied. The only properties found to be significantly different (CF cells vs controls) were calmodulin-stimulated peak activity (90 vs 101, P less than 0.02) and trypsin-activated peak activity (92 vs 102, P less than 0.02). No significant difference could be measured in the steady-state Ca2+-dependent phosphorylation of CF and control erythrocyte membranes indicating similar numbers of enzyme molecules per cell. The functional state of Ca2+ homeostasis in intact erythrocytes was investigated by measuring the resting cytosolic free Ca2+ levels using quin-2. Both CF and control erythrocytes maintained cytosolic free Ca2+ between 20 to 30 nM. Addition of 50 uM trifluoperazine resulted in an increase in erythrocyte cytosolic free Ca2+ to about 50 nM in both CF and control cells. Estimates of erythrocyte membrane permeability using the steady-state uptake of 45Ca into intact erythrocytes revealed no differences between CF and control cells. These results confirm that there is a small decrease in the calmodulin-stimulated activity of the erythrocyte Ca2+, Mg2+-ATPase in CF. However, this deficit is apparently not large enough to impair the ability of the CF erythrocyte to maintain normal resting levels of cytosolic free Ca2+.

Pathological mechanisms in carbon tetrachloride hepatotoxicity

Liver cell injury induced by carbon tetrachloride involves initially the metabolism of carbon tetrachloride to trichloromethyl free-radical by the mixed function oxidase system of the endoplasmic reticulum. It is postulated that secondary mechanisms link carbon tetrachloride metabolism to the widespread disturbances in hepatocyte function. These secondary mechanisms could involve the generation of toxic products arising directly from carbon tetrachloride metabolism or from peroxidative degeneration of membrane lipids. The possible involvement of radical species such as trichloromethyl (.CCl3), trichloromethylperoxy (.OOCCl3), and chlorine (.Cl) free radicals, as well as phosgene and aldehydic products of lipid peroxidation, as toxic intermediates is discussed. Data do not support the view that an increase in cytosolic free calcium is important in the toxic action of carbon tetrachloride or bromotrichloromethane. In addition, carbon tetrachloride-induced inhibition of very low density lipoprotein secretion by hepatocytes is not a result of elevated levels of cytosolic free calcium.

Halomethane-induced inhibition of protein synthesis in isolated hepatocytes

The effect of several halomethanes on protein synthesis has been studied in isolated hepatocytes. When cells are added to medium preequilibrated with CCl4 or CBrCl3, protein synthesis is inhibited after a lag period of 4 to 10 min. The concentrations of CBrCl3, CCl4, and CHCl3 which cause a 50% inhibition of protein synthesis are about 6 microM, 400 microM, and 4 mM, respectively. This order of potency parallels the rate at which these compounds are metabolized by the hepatic mixed function oxidase, suggesting that metabolism is required for toxicity. The inhibitory effect caused by 18 min of exposure to CBrCl3 is not reversed when the toxin is removed, indicating that inhibition involves some irreversible modification of cellular material. Unexpectedly, the inhibitory effect caused by 18 min of exposure of CCl4 is about 30-40% reversed when the toxin is removed. This suggests that CCl4 causes inhibition not only by a metabolism-dependent (irreversible) pathway, but by a metabolism-independent (reversible) mechanism as well. Extracellular Ca2+ is not required for CCl4 inhibition of protein synthesis.

Effect of halomethanes on intracellular calcium distribution in hepatocytes

Exposure of isolated rat hepatocytes to hepatotoxic halomethanes results in a 40-60% decrease in intracellular Ca2+ content. The order of halomethane potency (CBrCl3 CCl4 CHCl3) suggests that this effect requires halomethane metabolism by the hepatic mixed function oxidase system. Although the Ca2+ sequestering ability of the endoplasmic reticulum is destroyed by CBrCl3 and CCl4, it appears that much of the Ca2+ lost from the cell is mitochondrial in origin. Paradoxically, saturating concentrations of CCl4 cause a marked increase in cell Ca2+. CCl4 also causes an acute increase in cytoplasmic free Ca2+ (from about 60 nM to about 90 nM), but this effect does not appear to require CCl4 metabolism and is probably a result of direct action of CCl4 on the plasma membrane.

Cytosolic free calcium concentration and intracellular calcium distribution in lymphocytes from cystic fibrosis patients

Lymphocytes prepared from normal individuals and patients with cystic fibrosis (CF) were compared with regard to intracellular Ca2+ concentration, distribution, and handling. No difference between control and CF was found in the concentration of cytosolic free Ca2+ (98 +/- 5 vs 102 +/- 7 nM), and no difference was observed in the kinetics with which control and CF cells restored cytoplasmic Ca2+ toward normal following a perturbation induced by cold-exposure. However, total intracellular Ca2+ is about 25% higher in CF lymphocytes than in control. Of this excess Ca2+, about 50% appears to be sequestered in mitochondria. This suggests that some difference in Ca2+ handling does exist, but the significance of this in cystic fibrosis remains to be determined.

Carbon tetrachloride-dependent inhibition of lipid secretion by isolated hepatocytes. Characterization and requirement for bioactivation

Secretion of lipid as very low density lipoprotein (VLDL) by isolated hepatocytes was studied in a system in which the cells were exposed to a constant concentration of carbon tetrachloride (CCl4) throughout the duration of the incubation. Inhibition of secretion was characterized in terms of CCl4 concentration and duration of incubation. Half-maximum inhibition of VLDL secretion occurred at 80 microM CCl4. At 390 microM CCl4, VLDL secretion was inhibited 40% in 2 min and was suppressed completely in 5 min. No CCl4-dependent release of cellular glutamic oxaloacetic transaminase occurred under the conditions studied. Evidence is presented indicating that the metabolism of CCl4 is required for the expression of CCl4-dependent inhibition of lipid secretion. With respect to the parameters studied, the isolated hepatocyte system closely mimics the hepatic response of the intact animal to CCl4.

Evidence against involvement of calcium in carbon tetrachloride-dependent inhibition of lipid secretion by isolated hepatocytes

Carbon tetrachloride (CCl4)-induced inhibition of very low density lipoprotein (VLDL) secretion was studied in isolated hepatocytes. The hypothesis that inhibition of secretion is due to altered calcium homeostasis following CCl4-dependent inhibition of endoplasmic reticulum calcium sequestration was investigated. Inhibition of VLDL secretion by CCl4 was not dependent on extracellular calcium, since inhibition occurred when extracellular calcium was reduced to 0.1 microM. CCl4 inhibited hepatocyte VLDL secretion more rapidly than it inhibited microsomal calcium sequestration. Further, the concentration of CCl4 that produced half-maximal inhibition of VLDL secretion was about one-half the concentration required to produce half-maximal inhibition of microsomal calcium sequestration. The calcium ionophore A23187 did not mimic the action of CCl4 in inhibiting VLDL secretion under conditions in which A23187 altered cellular calcium homeostasis. The results that an alteration of calcium homeostasis is not involved in inhibition of VLDL secretion by carbon tetrachloride.

Assessment of the role of calcium ion in halocarbon hepatotoxicity

Halogenated hydrocarbons (CCl4, BrCCl3, 1,1-dichloroethylene, bromobenzene) cause a wide spectrum of dysfunction and injury in liver cells. An early effect of CCl4, BrCCl3, and 1,1-dichloroethylene is destruction of the Ca2+-sequestering ability of the endoplasmic reticulum, and it has been suggested that this lesion leads to subsequent disruption of other cell functions. Work to test this hypothesis has begun in this and other laboratories. While it appears that redistribution of intracellular Ca2+ does occur following these agents, the importance of this in cell injury is not fully resolved. Current results suggest Ca2+ redistribution may be involved in some cases (e.g., surface blebbing caused by bromobenzene), but not in others (e.g., inhibition of lipid secretion by CCl4).

Evidence against a role for disturbed hepatocellular calcium homeostasis in the fatty liver of carbon tetrachloride hepatotoxicity

A consensus does not exist regarding the nature of mechanisms linking the initial events of CCl4 metabolism to emergence of the classical indices of CCl4 liver cell injury. The possibility that a CCl4-dependent disturbance of intrahepatocellular calcium homeostasis might be a linking mechanism was investigated with isolated hepatocytes in suspension. CCl4-dependent inhibition of very low density lipoprotein secretion was studied. On the basis of kinetic data, dose-response data, and failure of elevated cytosolic calcium levels to inhibit lipid secretion, it was concluded that disturbed intracellular calcium homeostasis probably is not important in CCl4-dependent inhibition of secretion of very low density lipoproteins.

Calcium inhibition of rat liver microsomal calcium-dependent ATPase

Measurement of the inward rate of Ca2+ transport by rat liver microsomes under conditions of varying free intravesicular Ca2+ (1 microM to 5 mM) revealed that inward transport rate is maximum at low intravesicular Ca2+, and that transport rate decreases with an apparent inhibition constant of about 250-350 microM as intravesicular Ca2+ accumulates. This relationship is confirmed by measurement of Ca2+-dependent ATPase activity; activity is greatest when intravesicular Ca2+ is 1 microM, is lower when intravesicular Ca2+ is 60 microM, and is minimum when intravesicular Ca2+ is 5 mM. Unexpectedly, the ratio of Ca2+ transport rate to Ca2+-dependent ATP hydrolysis rate appears to be significantly greater than 2:1.

Analysis of microsomal calcium sequestration by steady state isotope exchange. Enzyme kinetics and role of membrane permeability

ATP-dependent calcium sequestration by rat liver microsomes has been analyzed by steady state isotope exchange. Liver microsomes display high affinity for Ca2+; the half-maximal concentration of free Ca2+ is 0.10 microM, and intravesicular steady state concentrations of 7-8 mM Ca2+ are achieved under optimal conditions. The uptake system displays multiphasic kinetics with respect to both Mg-ATP and free Mg2+, suggesting that microsomal preparations contain two distinct Ca2+-sequestering systems. Measurement of the kinetics of Ca2+ sequestration permits independent assessment of the activity of the calcium active transport system(s) and of the permeability of the membrane to Ca2+ backflux. Addition of ionophore A-23187 to microsomes renders them more permeable, and this is reflected in a more rapid equilibration of isotope. Conversely, low levels of free Ca2+ lead to a decreased rate of active transport, and this is reflected in a lower initial rate of isotope exchange. This system should be useful for investigating the mechanisms by which hormones, hepatotoxins, and other agents influence Ca2+ fluxes in cells.

Dibutyryl cyclic AMP-induced phosphorylation of specific proteins in adenohypophysial cells

The objective of this work was to identify the natural substrates of cyclic AMP-dependent protein kinase in pituitary cells. Studies were performed using 2 systems: intact pituitary cells stimulated with dibutyryl cyclic AMP (DBC) after preincubation with [gamma-32P]. Phosphorylation of proteins was analyzed by two-dimensional gel electrophoresis, followed by autoradiography. In intact cells, the only clear and reproducible effect of DBC stimulation is increased phosphorylation of 3 proteins (termed A, B, and C), each with a molecular weight of about 20 000 dalton. The time-course and dose-dependence of phosphorylation of A, B and C are generally similar to that for DBC-induced hormone secretion, which is consistent with a role for these proteins in the secretory mechanism. When [gamma-32P]ATP is added to cell extracts, proteins A, B, and C are not measurably phosphorylated, either in the absence or presence of cyclic AMP. This observation suggests that proteins A, B and C may not be directly phosphorylated by cyclic AMP-dependent protein kinase, but may be phosphorylated indirectly by a second kinase. On the other hand, growth hormone and prolactin are readily phosphorylated in cell extracts by cyclic AMP-dependent protein kinase (although they are not phosphorylated in vivo). This finding makes clear the need for caution in interpreting results from broke cell systems.

Effect of glucocorticoid levels in vivo on growth hormone biosynthesis

The rate of growth hormone (GH) biosynthesis in pituitary cells prepared from rats with different histories of glucocorticoid exposure was analyzed by a dual-labeling acrylamide-gel technique. Glucocorticoid deficiency, produced by adrenalectomy, reduces GH synthesis by approximately 50%. GH synthesis adrenalectomized rats treated with either natural or synthetic glucocorticoids, is restored to normal or above normal levels. Acute exposure (1 h) of cells to corticosterone in vitro does not produce a significant increase in GH synthesis. Adrenalectomized rats treated with steroid for one day show a small but detectable increase in GH synthesis, while treatment for 3--6 days results in progressively larger stimulatory effects. All of these results are similar to previous findings regarding glucocorticoid stimulation of GH synthesis in vitro.

Effect of cordycepin on CRF stimulation and steroid inhibition of ACTH secretion by rat pituitary cells

Isolated pituitary cells prepared from adrenalectomized rats secrete ACTH in response to CRF, and this response is inhibited by corticosterone. Both the stimulation of release by CRF and the inhibition of release by corticosterone are antagonized by cordycepin (3'-deoxyadenosine). Inhibition of CRF-stimulated secretion by cordycepin is apparently not related to inhibition of RNA synthesis, since high doses of actinomycin D do not affect ACTH secretion. More likely, cordycepin's inhibition of secretion stems from its inhibition of adenylate cyclase activity. Inhibition of corticosterone action by cordycepin is qualitatively similar to that previously reported actinomycin D. This effect of both drugs is probably due to inhibition of RNA synthesis. Significantly, a low dose of cordycepin has a greater inhibitory effect on corticosterone action than on total cellular RNA synthesis. Cordycepin is reported to preferentially inhibit messenger RNA synthesis, and low dose preferentially inhibits appearance of cytoplasmic RNA in pituitary cells. These data suggest that corticosterone-induced RNA is a cytoplasmic (messenger) RNA.

Bovine liver glutamate dehydrogenase: tentative amino acid sequence; identification of a reactive lysine; nitration of a specific tyrosine and loss of allosteric inhibition by guanosine triphosphate

A tentative but almost complete amino acid sequence for the subunit peptide chain of bovine liver glutamate dehydrogenase indicates a minimal size of 506 residues with a molecular weight of 56,100, in accord with the physical size of the subunit of 55,900. Inactivation with pyridoxal 5'-phosphate, followed by reduction with sodium borohydride, has permitted identification of the essential lysine as residue 97. Nitration of tyrosine-412 is accompanied by loss of the allosteric inhibitory effect of guanosine triphosphate. Comparison of the sequences of glutamate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase has indicated that only two 12-residue sequences are similar in the two enzymes; this sequence includes reactive lysine-97 of the former enzyme.