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K Krishnan

Publications and source records attributed to K Krishnan.

At least 37 records · Page 2Linked to original sources
PubMed2000

Validation of a physiological modeling framework for simulating the toxicokinetics of chemicals in mixtures.

The objective of this study was to investigate the usefulness of a physiologically based toxicokinetic (PBTK) modeling framework for simulating the kinetics of chemicals in mixtures of varying complexities and composition. The approach involved the simulation of the kinetics of components in two situations: (i) when one of the mixture components was substituted with another (i.e., benzene in the benzene (B)-toluene (T)-ethyl benzene (E)-m-xylene (X) mixture was substituted with dichloromethane (D)), and (ii) when another chemical was added to the existing four-chemical mixture model (i.e., when D was added to the existing BTEX mixture model). In both cases, differing compositions of mixtures were used to obtain simulations and to generate experimental data on kinetics for validation purposes. Since the quantitative and qualitative mechanisms of interaction among B, T, E, and X have already been established, the mechanisms of binary interactions between D and the BTEX components (e.g., competitive, noncompetitive, or uncompetitive metabolic inhibition) were investigated in the present study. The analysis of rat blood kinetic data (4-h inhalation exposures, 50-200 ppm each) to all binary combinations (D-B, D-T, D-E, and D-X) investigated in the present study was suggestive of competitive metabolic inhibition as the plausible interaction mechanism. By incorporating the newly estimated values of metabolic inhibition constant (K(i)) for each of these binary combinations within the five-chemical PBTK model (i.e., for the DBTEX mixture), the model adequately predicted the venous blood kinetics of chemicals in rats following a 4-h inhalation exposure to various mixtures (mixture 1:100 ppm of D and 50 ppm each of T, E, and X; mixture 2: 100 ppm each of D, T, E, and X; mixture 3: 100 ppm of D and 50 ppm each of B, T, E, and X; mixture 4: 100 ppm each of D, B, T, E, and X). The results of the present study suggest that the PBTK model framework is useful for conducting extrapolations of the kinetics of chemicals from one mixture to another differing in complexity and composition, based on mechanistic considerations of interactions elucidated at the binary level.

Administration, Inhalation↗
PubMed2000

Estimation of rat blood:air partition coefficients of volatile organic chemicals using reconstituted mixtures of blood components.

The objective of the present study was to estimate the rat blood:air partition coefficients (PC) of some volatile organic chemicals (VOCs) using reconstituted mixtures of blood components. Based on previous observations, three blood components (water, lipid, hemoglobin) should be necessary in the case of lipophilic VOCs (e.g. bromoform (BF), chlorobenzene (CB) chloroform (CF), and ethylbenzene (EB)) whereas a mixture of oil (lipid surrogate) and water should be adequate to estimate the blood:air PC (P(b:a)) of other VOCs (e.g. butyl methyl ether (BME), t-butyl methyl ether (tBME), diethyl ether (ETH), isooctane (ISO), methyl ethyl ketone (MEK), and alpha-pinene (PIN)). Vial equilibration studies showed that the matrix:air PCs for the oil+water samples were similar or greater than those of rat blood (mean+/-S.E., n=7-8) for BME (11.1+/-2.0 vs. 6.64+/-1.4), tBME (15.0+/-4 vs. 15.0+/-2), ETH (9.50 +/-1.16 vs. 9.24+/-0.75), ISO (2. 88+/-0.5 vs. 1.92+/-0.4), MEK (159.3+/-8 vs. 139+/-6), and PIN (20. 5+/-2.7 vs. 16.9+/-1.8), whereas they were significantly lower for BF (19.0+/-3.4 vs. 161+/-5), CF (3.4+/-0.75 vs. 16.9+/-1.1), CB (8. 3+/-2.35 vs. 61.8+/-2.8), and EB (7.13+/-1.6 vs. 50.8+/-1.3). These results suggest that additional consideration of solubility/binding in blood proteins is essential in order to adequately determine rat P(b:a) of BF, CB, CF, and EB. The PCs determined using whole blood were comparable to those obtained using a reconstituted mixture of n-octanol (lipid surrogate), water and hemoglobin (mean+/-S.E., n=3-4) for BF (154+/-1.5), CB (55+/-6), CF (15+/-0.87), and EB (30+/-1.5). The results of the present study suggest that VOC partitioning into three blood components, namely, water, lipids and hemoglobin determines to a large extent the magnitude of their blood:air PCs.

Air↗
PubMed2000

Concentration dependency of rat blood: air partition coefficients of some volatile organic chemicals.

The rat blood:air partition coefficient (PC) of lipophilic volatile organic chemicals (VOCs) cannot be predicted with the sole consideration of their solubility in blood water and lipids, suggesting an important role of blood proteins. The possible concentration dependency and the quantitative nature of VOC binding to blood proteins [i.e., association constant (Ka), number of binding sites (n)] have not been investigated previously. The objectives of this study were therefore (1) to determine the concentration dependency of the blood:air PC (P(b:a)) of four VOCs, bromoform (BF), chloroform (CF), chlorobenzene (CB), and ethylbenzene (EB), hypothesized to display binding to rat blood proteins; and (2) to derive Ka and n values for these chemicals in rat blood. In vitro studies were conducted using 0.1-0.5 ml whole blood, or an equivalent mixture of water and n-octanol exposed to varying amounts of VOCs (BF, 0.11-11.4 micromol; CB, 0.11-24.6 micromol; CF, 0.11-186.6 micromol; and EB, 0.11-20.2 micromol) in sealed glass vials. The P(b:a) of CB, CF, and EB decreased significantly at higher amounts added, with no significant change in their n-octanol + water mixture:air PC. For each in vitro exposure situation, the concentration of free chemical (Cfree) in rat blood was calculated with the PC for the n-octanol + water mixture, whereas the concentration of bound plus free chemical (Ctot) was calculated from knowledge of the PC determined experimentally with whole blood. The respective values of Ka and n for hemoglobin binding estimated by linear regression of a plot of the reciprocal of the molar ratio of bound chemical versus 1/Cfree were: BF, 0.8, 4; CB, 2.8, 1.4; CF, 1.8, 1.2; and EB, 2, 1.4. The results of this study suggest that the concentration-dependent nature of P(b:a) need not be considered for modeling rat inhalation exposures to these VOCs for up to several thousand parts per million.

1-Octanol↗
PubMed2000

Evaluation of the pharmacokinetic interactions between orally administered trihalomethanes in the rat.

The blood kinetics of trihalomethanes has recently been reported to differ between an oral administration of any single trihalomethane (0.25 mmol/kg) [THMs: chloroform, bromoform, bromodichloromethane (BDCM), dibromochloromethane (DBCM)] and a combined administration of 0.25 mmol/kg of each of the 4 THMs. The significant increase in blood concentrations of THMs could be a consequence of pharmacokinetic interactions between two or more of the THMs present simultaneously. The objective of the present study was to characterize the blood kinetics of THMs following oral administration singly or as binary mixtures in order to assess the relative contribution of each THM to the kinetic interferences observed with the quaternary mixture. A single dose of each THM (0.5 mmol/kg) alone or of a binary mixture containing 0.5 mmol/kg of each THM was administered by gavage to male Sprague-Dawley rats. The venous blood concentrations of unchanged THMs were measured for up to 720 min postadministration by headspace gas chromatography. Results showed that, compared to single administration, each binary mixture caused a significant increase in the blood concentrations of both THMs present and this effect increased with time. The impact, however, was not similar for each mixture, especially during the first hour following administration of the compounds (bromoform and DBCM). Among the four THMs, bromoform and DBCM kinetics appeared to be more sensitive to the mixture effect and to exert the greatest impact on the kinetics of the second THM present in the mixture. Simulation exercises conducted with physiologically based toxicokinetic models suggest metabolic inhibition as the possible mechanism of the interaction between THMs. In conclusion, the results of this study show that, at the dose level investigated, every binary combination of THMs, when orally administered, resulted in a significant modulation of their pharmacokinetics and suggest that this is probably the consequence of a mutual metabolic inhibition between the THMs.

Administration, Oral↗
PubMed2000

Changes in HOXB6 homeodomain protein structure and localization during human epidermal development and differentiation.

HOX homeodomain proteins are master developmental regulators, which are now thought to function as transcription factors by forming cooperative DNA binding complexes with PBX or other protein partners. Although PBX proteins exhibit regulated subcellular localization and function in the nucleus in other tissues, little data exists on HOX and PBX protein localization during skin development. We now show that the HOXB6 protein is expressed in the suprabasal layer of the early developing epidermis and throughout the upper layers of late fetal and adult human skin. HOXB6 signal is cytoplasmic throughout fetal epidermal development, but substantially nuclear in normal adult skin. HOXB6 protein is also partially nuclear in hyperproliferative skin conditions, but appears to be cytoplasmic in basal and squamous cell carcinomas. Although all three PBX genes are expressed in fetal epidermis, none of the three PBX proteins exhibit nuclear co-localization with HOXB6 in either fetal or adult epidermis. RNA and protein data suggest that a truncated HOXB6 protein, lacking the homeodomain, is expressed in undifferentiated keratinocytes and that the full-length protein is induced by differentiation. GFP-fusion proteins were used to demonstrate that the full-length HOXB6 protein is localized to the nucleus while the truncated protein is largely cytoplasmic. Taken together, these data suggest that during epidermal development the truncated HOXB6 isoform may function by a mechanism other than as DNA binding protein, and that most of the nuclear, homeodomain-containing HOXB6 protein does not utilize PBX proteins as DNA binding partners in the skin. Published 2000 Wiley-Liss, Inc.

Adult↗
PubMed2000

Relative lipid content as the sole mechanistic determinant of the adipose tissue:blood partition coefficients of highly lipophilic organic chemicals.

The adipose tissue:blood partition coefficient (PCat:b) refers to the ratio of chemical concentration or solubility in adipose tissue and blood. The solubility of a chemical in adipose tissue or whole blood is equal to the sum total of its solubility in lipid and water fractions of these matrices. For highly lipophilic organic chemicals (HLOCs, i.e., chemicals with log n-octanol:water partition coefficients (PCo:w) greater than four), their solubility in the water fractions of both tissue and blood is negligible, and therefore their solubility in lipid fractions of tissue and blood alone determines PCat:b. Since the numerical value representing chemical solubility in lipids is likely to be the same for both blood lipids and adipose tissue lipids, the PCat:b values should be hypothetically, equal to the ratio of lipid content of adipose tissue and blood. The objective of the present study was therefore to verify whether the PCat:bs of HLOCs (volatile organics, dioxins, PCBs, PBBs, DDT) are equal to the ratio of adipose tissue and blood lipid levels. The data on lipid content of rat and human blood and adipose tissues were obtained from the literature. The calculated tissue:blood lipid ratios were comparable to the human and rat PCat:b of volatile organic chemicals, dioxins, PCBs, PBBs and/or DDT obtained from the literature. These results then suggest that, regardless of the identity and PCo:w of HLOCs, their PCat:b is equal to the ratio of lipid in adipose tissues and blood.

Adipose Tissue↗
PubMed2000

Distinct patterns of cytokine gene suppression by the equivalent effective doses of cyclosporine and tacrolimus in rat heart allografts.

In vitro studies of the mode of action of cyclosporine (CsA) and tacrolimus have indicated that both drugs produce immunosuppression by a quite similar cellular and molecular mechanism to block T cell receptor emanated transcriptional activation of interleukin(IL)-2 and other cytokine genes. Herein, we show that there are distinct patterns of cytokine gene expression in rat heart allografts under equivalent effective doses ("optimal dose") of CsA and tacrolimus. The optimal doses of CsA (10 mg/kg/day) and tacrolimus (3.2 mg/kg/day), which induce similar mean graft survival time (MST), were administered in LEW recipients with ACI heart grafts from day 0 after grafting until sacrifice. Heart grafts were harvested at days 3, 5, and 7. The expression of various cell surface markers, cytokines, and cytotoxic factors was determined by immunohistology and reverse transcriptase-polymerase chain reaction (RFT-PCR). Cell populations that stained positively in the heart tissues of allograft control increased through day 7 for CD4+ and CD8+ T lymphocytes, NKR-Pla+ natural killer (NK) cells, and ED2+ macrophages. CsA and tacrolimus have comparable activity to block these cell local infiltrations. The mRNA levels of the majority of the factors were dramatically up-regulated in the allografts over time, peaking at day 5. The optimal doses of CsA and tacrolimus had similar inhibitory effects on Th1 type cytokine IL-2 and interferon [INF]-gamma), inflammatory cytokine (IL-1beta and tumor necrosis factor [TNF]-alpha), and cytotoxic factor (granzyme B and perforin) mRNA expression. However, the drugs had different effect on Th2 type cytokines (IL-4 and IL-10). Whereas IL-4 expression was not affected by tacrolimus and was enhanced by CsA, IL-10 expression was more significantly suppressed by tacrolimus than CsA. Differences in the suppression of Th2 type cytokine gene expression indicate that the in vivo molecular networks by which CsA and tacrolimus exert their full immunosuppressive activity are not necessarily the same.

Animals↗
PubMed2000

Right atrial myxoma with extracardiac manifestations.

Right atrial myxoma is a rare intracardiac tumor that is often difficult to diagnose. Pulmonary embolism from tumor fragments originating from the tumor mass is a potentially fatal complication. Early diagnosis of cardiac myxoma is important since surgical treatment leads to resolution with low rates of recurrence and good long-term survival. The presence of a cardiac myxoma can be heralded by nonspecific constitutional symptoms as well as by disturbances in the clotting mechanism.

Abdominal Pain↗
PubMed1999

Physiological modeling of the toxicokinetic interactions in a quaternary mixture of aromatic hydrocarbons.

The available data on binary interactions are yet to be considered within the context of mixture risk assessments because of our inability to predict the effect of a third or fourth chemical in the mixture on the interacting binary pairs. Physiologically based toxicokinetic (PBTK) models represent a framework that can be potentially used for predicting the impact of multiple interactions on component kinetics at any level of complexity. The objective of this study was to develop and validate an interaction-based PBTK model for simulating the toxicokinetics of the components of a quaternary mixture of aromatic hydrocarbons [benzene (B), toluene (T), ethylbenzene (E), m-xylene (X)] in the rat. The methodology consisted of: (1) obtaining and refining the validated individual chemical PBTK models from the literature, (2) interconnecting all individual chemical PBTK models at the level of liver on the basis of the mechanism of binary chemical interactions (e.g., competitive, noncompetitive, or uncompetitive metabolic inhibition), and (3) comparing the a priori predictions of the interaction-based model to corresponding experimental data on venous blood concentrations of B, T, E, and X during mixture exposures. The analysis of blood kinetics data from inhalation exposures (4 h, 50-200 ppm each) of rats to all binary combinations of B, T, E, and X was suggestive of competitive metabolic inhibition as the plausible interaction mechanism. The metabolic inhibition constant (K(i)) for each binary combination was quantified and incorporated within the mixture PBTK model. The binary interaction-based PBTK model predicted adequately the inhalation toxicokinetics of all four components in rats following exposure to mixtures of BTEX (50 ppm each of B, T, E, and X, 4 h; 100 ppm each of B, T, E and X, 4 h; 100 ppm B + 50 ppm each of T, E, and X, 4 h). The results of the present study suggest that data on interactions at the binary level alone are required and sufficient for predicting the kinetics of components in complex mixtures.

Administration, Inhalation↗
PubMed1999

Quantitative relationship between steady-state blood concentrations and structural features of aliphatic hydrocarbons.

The objective of this study was to investigate the quantitative relationship between steady-state blood concentrations and structural features of aliphatic hydrocarbons. The literature data on steady-state blood concentrations (Cb(ss)) of 11 C6 to C10-aliphatic hydrocarbons (five n-alkanes, three alkenes, and three iso-alkanes) obtained in rats after exposure to 100 ppm of these chemicals were analyzed using a commercially available software (QSAR-PC). Based on a multiple linear regression analysis, the contribution values of (i) the basic structure [=C1H2-CH2-CH2-CH2C5H2-, + 6.8009], (ii) the substituents at carbon position 1 [(-H)2,=CH2, and (-CH3)2, associated with values of - 3.25, + 7.44 and - 2.02, respectively) and (iii) the substituents at carbon position 5 (CH3, C2H5, C3H7, C4H9 and C5H11 associated with values of - 1.20, - 1.69, +0.71, + 1.26 and + 2.82, respectively) were quantified. This analysis explained 98.8% of the variability in rat Cb(ss) among the hydrocarbons investigated. The present work represents the first attempt to characterize the quantitative contributions of specific molecular fragments to the toxicokinetic behavior of aliphatic hydrocarbons.

Animals↗
PubMed1999

The toxicokinetics of pyrene and its metabolites in rats.

Five experiments were conducted in male Sprague-Dawley rats regarding the kinetic of urinary excretion of 1-hydroxypyrene (1-OHP) following i.v., oral and dermal exposure to 0.5-50 micromol/kg pyrene either as a single substance or as mixture of various polycyclic aromatic hydrocarbons (PAH). Frequent urine collections over 48 h after exposure and a tissue versus time distribution experiment using [14C]pyrene allowed to define the kinetic profile of both pyrene and 1-OHP. For all exposure routes, there is a linear relationship over two orders of magnitude between the dose of pyrene and the urinary excretion of 1-OHP. Differences in biliary/urinary 1-OHP excretion ratio in canulated rats (3) versus faecal/urinary 1-OHP excretion ratio in non-canulated rats (0.6) indicate major enterohepatic recirculation of the metabolite. Half-lives of both pyrene and 1-OHP in all measured tissues were all comprised between 3.1 and 5.4 h, and 5.2-6.7 h, respectively, so that no long term accumulation would be predicted from these values for any tissue. Binary and ternary mixtures involving naphthalene and benzo(a)pyrene in addition to pyrene has no influence on the urinary excretion profile of 1-OHP. All these observations led to the proposal of a dynamic compartment model of pyrene and metabolite flows indicating that following rapid initial distribution to fatty tissues, pyrene is rapidly biotransformed into various metabolites and undergoes major enterohepatic recycling. Part of the initially formed and part of the recirculated 1-OHP eventually undergoes urinary excretion such that close to 60% of pyrene is eliminated as metabolites in urine by 24 h after injection while 20% is excreted in the faeces over the same period.

Administration, Oral↗
PubMed1999

A modeling approach to account for toxicokinetic interactions in the calculation of biological hazard index for chemical mixtures.

Biological hazard index (BHI) is defined as biological level tolerable for exposure to mixture, and is calculated by an equation similar to the conventional hazard index. The BHI calculation, at the present time, is advocated for use in situations where toxicokinetic interactions do not occur among mixture constituents. The objective of this study was to develop an approach for calculating interactions-based BHI for chemical mixtures. The approach consisted of simulating the concentration of exposure indicator in the biological matrix of choice (e.g. venous blood) for each component of the mixture to which workers are exposed and then comparing these to the established BEI values, for calculating the BHI. The simulation of biomarker concentrations was performed using a physiologically-based toxicokinetic (PBTK) model which accounted for the mechanism of interactions among all mixture components (e.g. competitive inhibition). The usefulness of the present approach is illustrated by calculating BHI for varying ambient concentrations of a mixture of three chemicals (toluene (5-40 ppm), m-xylene (10-50 ppm), and ethylbenzene (10-50 ppm)). The results show that the interactions-based BHI can be greater or smaller than that calculated on the basis of additivity principle, particularly at high exposure concentrations. At lower exposure concentrations (e.g. 20 ppm each of toluene, m-xylene and ethylbenzene), the BHI values obtained using the conventional methodology are similar to the interactions-based methodology, confirming that the consequences of competitive inhibition are negligible at lower concentrations. The advantage of the PBTK model-based methodology developed in this study relates to the fact that, the concentrations of individual chemicals in mixtures that will not result in a significant increase in the BHI (i.e. > 1) can be determined by iterative simulation.

Animals↗
PubMed1999

Influence of oral administration of a quaternary mixture of trihalomethanes on their blood kinetics in the rat.

Trihalomethanes (THMs; chloroform, bromoform, bromodichloromethane, dibromochloromethane), formed as by-products of chlorine disinfection, are found to occur in combination in drinking water supplies. THMs are metabolized by cytochromes P-450 and are likely substrates of CYP2E1. Therefore, it is possible that mixed exposure results in toxicokinetic interactions among THMs. The toxicokinetics of THMs during mixed exposures has not been investigated previously. The purpose of this study was to characterize the blood kinetics of the four THMs administered singly or in combination in the rat. A single dose of 0.25 mmol/kg or 0.5 mmol/kg b.w., of each THM alone, or of a quaternary mixture containing 0.25 mmol/kg of each THM (total dose of 1.0 mmol/kg) was administered by gavage. The venous blood concentrations of the THMs were measured by headspace gas chromatography (GC) at 20, 40, 60, 120, 180, 270 and 360 min post-administration. Results showed a nonlinear relationship between the area under the blood concentration versus time curves (AUCs) and administered doses of THMs, suggesting that metabolism is saturated in this dose range. The venous blood concentrations of THMs following administration of the quaternary mixture were significantly higher compared to single exposures. The altered kinetics of THMs during combined exposures is consistent with the occurrence of mutual inhibition of their hepatic metabolism. Simulation exercises conducted with physiologically based toxicokinetic models support metabolic inhibition as the possible mechanism of the interaction among THMs. The data reported in this study provide the starting point for evaluating the significance of interactions among THMs in the risk assessment process.

Administration, Oral↗
PubMed1998

Identification of amino acid residues critical for the Src-homology 2 domain-dependent docking of Stat2 to the interferon alpha receptor.

The interaction between Src-homology 2 domains (SH2) domains and phosphorylated tyrosine residues serves a critical role in intracellular signaling. In addition to the phosphotyrosine, adjacent residues are critical mediators of the specificity of this interaction. Upon treatment of cells with interferon alpha (IFNalpha), the IFNaR1 subunit of the IFNalpha receptor becomes tyrosine phosphorylated at position 466. The region surrounding phosphorylated tyrosine 466 subsequently acts as a docking site for the SH2 domain of Stat2, facilitating phosphorylation of the latter and, thus, the transduction of the IFNalpha signal. In this report site-specific mutagenesis was employed to analyze the nature of the interaction between the SH2 domain of Stat2 and the region surrounding tyrosine 466 on IFNaR1. Mutation of the valine at the +1 position carboxyl-terminal to tyrosine 466 or of the serine at the +5 position inhibits the association of Stat2 with phosphorylated IFNaR1. Moreover, receptors mutated at either of these two positions act in a dominant manner to decrease IFNalpha signaling, as assayed by both Stat2 phosphorylation and expression of an IFNalpha-responsive reporter. The demonstration that these two residues are critical in mediating the interaction between Stat2 and IFNaR1 suggests that STAT proteins might utilize a structurally distinct subset of SH2 domains to mediate signal transduction from the cell surface to the nucleus.

CD4 Antigens↗