Uptake phases

When exposed to mixtures, chemicals in the exposure medium may affect each other s uptake by humans in a manner that is analogous to some of the bioavailability effects outlined here for environmental species. This was, for instance, shown for the neurotoxicity of EPN (O-ethyl-O-4-nitrophenyl phenylphosphono-thionate), which was enhanced by aliphatic hydrocarbons due in part to increased dermal absorption (Abou-Donia et al. 1985). It was also shown that dietary zinc inhibits some aspects of lead toxicity, which could in part be explained by decreasing dietary lead absorption (Cerklewski and Forbes 1976). Other examples of interactions of chemicals at the uptake phase in humans, which may in part be related to bioavailability interactions, are summarized in Table 1.3. 

Changing of test-chemical concentration in water during the uptake phase... 

The ambient chemical concentrations in the water must be below their water solubility and should be measured during the uptake phase. 

After a predetermined time interval (e.g., 1, 3, 5, and 21 h postdrug removal) perform steps 4 through 7 of Uptake phase. 

The measurement of bioconcentration is difficult because the water concentration must remain constant during the run and contact must be maintained until equilibrium is reached in the organism. Equilibration, signalled by a plateau in the concentration vs time plot, may take several days. This entails, particularly in the case of relatively hydrophobic compounds of low water solubility, dosing in a flow-through chamber at levels well below the toxic threshold. A complete experiment involves analysis of samples during the exposure, or "uptake phase", and also following transfer to a clean environment where release (depuration) occurs. Both the parent chemical, from which the bioconcentration factor is calculated, and known metabolites are analyzed (15). 

Table 1 also lists amount of O2 adsorption uptake, phase observed by X-ray diffractograms (XRD), and apparent oxidation number of the Mo, after reduction. O2 uptake for the catalyst reduced at 773 K was similar to that for the catalyst reduced at 673 K. The higher activity of the catalyst reduced at 773 K, seems to be related to the higher metallic Mo content. However, reduction at 873 K resulted in the formation of large amount of metallic Mo, leading to the catalyst producing hydrocarbons predominantly. This is in agreement with the effect of Mo precursors (ref. 7) and the order of... 

An experimental approach commonly used to determine is summarized in Figure 5.17. Fish are exposed under controlled conditions to a constant concentration for sufficient time to establish [Cfl s dCi/dt = 0) and since C is known, ATb can be calculated. A limitation with this approach is the problem of knowing how much time is required to achieve the steady state. During the uptake phase of the study, the rate of change in Cf is defined by both fei and k2-... 

Dietary uptake, retention, and tissue distribution of Mn, Co, and Cs in the rainbow trout have been studied, because this fish is widely distributed in European waterways. These radionuclides present a special interest from a biological standpoint, because Cs is biochemically analogous to K and Mn and Co are classified among the ten vital elements for life. The theoretical values of the steady-state trophic transfer factor (TF) were significantly lower than one for Mn and Co, indicating that these radionuclides are not being biomagnified in their transfer to trout, while Cs showed the opposite. At the end of the uptake phase (42 days), the... 

Differences in growth phase can influence metal uptake in batch-cultured bacteria. Germanium uptake by Pseudomonas putida occurs in a biphasic pattern in a catechol-enriched medium the second uptake phase corresponds to catechol degradation, products of which facilitate germanium transport into cells [35]. 

SCHEME 2 Two-compartment model for the uptake (accumulation) and clearance of chemicals in fish. = concentration of phenol in water, Cp = concentration in fish, t = time, k., = rate constant for the uptake phase, = rate constant of the clearance phase, t = time for the end of the uptake phase, BCF = bioconcentration factors. After Blau et al. (1975). 

The non-linear regression program supplied with the Hewlett-Packard 9845 minicomputer was used to solve the kinetic uptake phase equation for a one-compartment open model operating under first order kinetics ... 

Time to Steady-State Tissue Burdens. The time required to reach steady-state tissue concentrations in the uptake phase can be determined by measuring the contaminant elimination rate k. The formula (TSS50) = 0.693/At2 for time to 50% of steady state, also used to calculate tissue half-life, was used to determine when 50% of the steady-state tissue concentration would be expected if constant uptake is assumed. The time to 95% of steady state can be determined with 2.99/k2, or 4.3 times the time to 50% steady state. In Fig. 11, plotted tissue burden versus time to show how ki elimination rates affect the time to steady state. The elimination rate defines the shape of these curves and determines when tissue concentration will achieve steady state, because under the conditions of constant uptake and no elimination, tissue concentration would continue to increase indefinitely. The rate of uptake determines the steady state tissue concentration for a given rate of elimination and has no effect on the time to steady state. Many studies give elimination constants that can be used to compute the time to steady state. Table 2, which lists tissue half-lives for various PAH compounds, may also be used to indicate the time needed to achieve 50% of the maximum tissue burden in the uptake phase. The majority of ki values reported for PAH elimination in marine organisms occur between... 

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