In vitro modeling of the post-ingestion mobilization and bioaccessibility of pesticides sorbed to soil and house dust.

Soils and dusts can act as sinks for semivolatile lipophilic organic compounds and children ingest relatively large amounts of both soils and dusts. Following intake, sorbed chemicals may desorb (mobilize) and become available for intestinal absorption (bioaccessible). When chemicals are not degraded in the digestive tract, mobilization can approximate bioaccessibility. Alternatively, when gastrointestinal degradation of mobilized chemicals does occur, it can be useful to separate mobilization from bioaccessibility. In this study we used synthetic digestive fluids in a sequential, three-compartment (saliva, gastric, and intestinal) in vitro assay to construct mobilization and bioaccessibility models for 16 pesticides (log Kow 2.5-6.8) sorbed to 32 characterized soils and house dusts. To address the potential loss of mobilized pesticides due to absorption, the assays were repeated using a solid phase sorbent (tenax) added to the digestive fluid immediately after addition of the intestinal fluid components. We found that pesticide mobilization was predicted by pesticide log Kow and the carbon content of the soils and dusts. Pesticide loss measurably reduced the bioaccessibility of most pesticides, and bioaccessibility was largely predicted by log Kow and pesticide loss rate constants. Introduction of the sink increased mobilization by x̄ = 4 ± 6% (soil) and x̄ = 9 ± 7% (dust) while bioaccessibility increases were x̄ = 41 ± 21% (soil) and x̄ = 24 ± 12% (dust). The physicochemical properties of the soils, dusts, and pesticides used in this study successfully predicted the in vitro mobilization and bioaccessibility of the pesticides. This suggests that modeling of pesticide mobilization and bioaccessibility could reduce uncertainty in exposure and risk assessments.

Is food type important for in vitro post ingestion bioaccessibility models of polychlorinated biphenyls sorbed to soil?

Soils are sorbents for many organic compounds and children consume relatively large amounts of soil. To improve the estimated health risks from this exposure pathway, we examined the role of co-ingested foods in determining the post-ingestion bioaccessibility (mobilization) of 18 polychlorinated biphenyls (PCBs) sorbed to 10 characterized soils. The bioaccessibility test system (DIN 19738, 2004) was an in vitro, 3-compartment, digestive tract containing salts, protein, and bile. Each soil was fortified with PCBs, then, digestive fluids appropriate to each compartment, were added sequentially. Next, digestive fluid and soil were seperated and PCB concentrations in both media were measured. This complete test system was then reduced to assess contributions of individual endogenous digestive fluid constituents (water, salts, pancreatin, bile, and mucin) and representative foods: protein (bovine serum albumin (BSA)), sugar (glucose), and fat (oleic acid). Then, the influence of increasing concentrations of BSA, glucose, and oleic acid was evaluated (individually) complete test systems. In a subset of the samples, solid phase microextraction (SPME) was used to measure freely dissolved PCBs. Across all treatments, percent soil organic carbon was the most influential bioaccessibility determinant, accounting for ≥87% of the explained variation. When evaluated individually, pancreatin, mucin, BSA, bile, and oleic acid each effectively increased PCB bioaccessibility and reduced freely dissolved PCB concentrations. This suggests competitive sorption of PCBs by organic constituents of the digestive constituents. Without sink material, intra-PCB mobilization differences were observed as freely dissolved PCB concentrations inversely correlated (p < 0.05) with their respective log Kow's. When added to the complete digestive fluid, increasing oleic acid mass increased PCB bioaccessibility (p < 0.05), while adding more BSA or glucose did not (p > 0.05). This indicates that fat intake may be the sole consideration needed when modeling dietary contributions to bioaccessibility of soil sorbed PCBs.

Effects of a reward downshift on the consummatory behavior and flower choices of bumblebee foragers.

Insect foragers often exhibit flower constancy, the tendency to visit single flower types rather than sample alternative flowers that provide equal or higher levels of reward. We employed a negative incentive contrast procedure to examine whether a decrease of sucrose concentration in a regularly visited flower type affects bumblebee (Bombus impatiens) consummatory or choice behavior. Subjects were trained to enter a test arena where they foraged on a single, red, artificial flower that contained 140 microl of sucrose solution. Subjects were reinforced with a concentration of either 0.50 or 0.20 sucrose solution in 20 trials and in 12 subsequent test trials subjects were given a choice between a red and yellow flower that each contained 140 microl of 0.20 sucrose solution. Subjects that experienced a downshift of reward showed an abrupt disruption in consumption of sucrose solution from the red flower. These subjects were also significantly more likely to visit the novel yellow flower than subjects that were reinforced with 0.20 sucrose solution in red flowers in all trials. However, the effects of the downshift of reward were transient and appeared to disrupt consummatory behavior more strongly than flower preferences. These results support the idea that bumblebee foragers form expectations of reward in flowers--as is implied by the results of studies of honeybees--and suggest that unrealized expectations of reward may cause foragers to sample alternative flowers or to fly considerable distances in search of particular types of flowers.